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Geological Society of Australia

ABSTRACTS Number 3

SEDIMENTS THROUGH THE AGES

Fifth Australian Geological Convention Perth 1981

FIFTH AUSTRALIAN GEOLOGICAL CONVENTION

THEME:

SEDIMENTS THROUGH THE AGES

ABSTRACT VOLUME

EDITED BY D.I. GROVES^ K. McNAMARA, R.G. BROWN & M.H. JOHNSTONE

GEOLOGICAL SOCIETY OF AUSTRALIA ABSTRACTS No. 3

PREFACE Abstracts of the papers to be delivered at the Fifth Australian Geological Convention are set out below in probable order of presentation within the sessions of the Convention. Abstracts of papers for sessions within the Convention Theme "Sediments Through The Ages" are presented first, and are followed by abstracts of papers for sessions of Specialist and other Groups. An abbreviated table of contents is presented below, followed by a table of contents listing all abstracts. An author index is presented at the rear of the volume. Minor changes have been made to abstracts to maintain uniformity and for clarity. All copy was typed by Mrs Jill Regazzo. pages

CONVENTION THEME "SEDIMENTS THROUGH THE AGES"

i - 64

KEYNOTE ADDRESSES MINERAL DEPOSITS IN SEDIMENTS HYDROCARBON GENERATION AND SEISMIC STRATIGRAPHY GLACIAL SEDIMENTATION EVAPORITES AND CHEMICAL SEDIMENTS

1-2 3-13 14-16 17-19 20-22

FLUVIAL, DELTAIC AND CLASTIC SHELF SEDIMENTS COAL MEASURES SEDIMENTATION SEDIMENTS OF MOBILE ZONES SEDIMENTS OF INTRACRATONIC BASINS TECTONICS AND DEP0SITI0NAL STYLE OF THE PRECAMBRIAN

23-26 27-29 30-34 35-37 38-43

STROMATOLITES THROUGH THE AGES REEFS AND OTHER CARBONATES CARBONATE AND SHELF SEDIMENTATION CAIN0Z0IC SEDIMENTATION GEOCHEMISTRY AND DIAGENESIS OF SEDIMENTS

44-45 46-53 54-56 57-61 62-64

ASSOCIATION OF AUSTRALASIAN PALAEONTOLOGISTS SYMPOSIUM

65-68

BIOSTRATIGRAPHY OF THE WEST AUSTRALIAN CONTINENTAL MARGIN

65 - 68

SPECIALIST GROUPS IN ECONOMIC GEOLOGY AND GEOCHEMISTRY & MINERALOGY

69-71

MINERALOGY AND GENESIS OF GOLD DEPOSITS

69-71

SPECIALIST GROUPS IN GEOCHEMISTRY AND MINERALOGY

72-82

GEOCHEMISTRY IGNEOUS ACTIVITY THROUGH TIME CONDITIONS AND PROCESSES DURING REGIONAL METAMORPHISM

72 - 73 74-76 77-79

GEOCHRONOLOGY/METAMORPHISM

80-82

ENGINEERING GEOLOGY GROUP SYMPOSIUM

83-89

SPECIALIST GROUP IN TECTONICS AND STRUCTURAL GEOLOGY

90-94

MODERN WORK IN CLASSICAL FOLD BELTS

90-91

GENERAL STRUCTURAL GEOLOGY

92-94

FILMS ON CARBONATE SEDIMENTATION

AUTHOR INDEX

96-97

CONTENTS * CONVENTION THEME "SEDIMENTS THROUGH THE AGES" KEYNOTE ADDRESSES R.G. WALKER Is There Life After Facies Models?

P.R. VAIL S J. HARDENBOL Interpreting Jurassic Unconformities, Palaeoenvironments and Sea-level changes from Seismic Stratigraphy and Biostratigraphy

ANDREW BUTTON S NOEL TYLER Unconformity Controls on Mineralization Through Precambrian Time with Special Reference to Southern Africa

MINERAL DEPOSITS IN SEDIMENTS E.F. STUMPFL Stratabound Base Metal Deposits - Neptune and Pluto

Re-united?

Sediment-hosted Ore Deposits of the Permian Metallogenic Epoch

JAN KRASON

IAN TEDDER & NIGEL J. ROWLANDS Stratiform Copper in the Late Proterozoic Sediments of the Ablah Graben, Kingdom of Saudi Arabia

BRIAN MARSHALL, S.R. SANGAMESHWAR, D.P.H. O'CONNOR, SHEN-SU SUN & M. BOUFFLER Genetic Aspects of the CSA and QTS Mineralizations, Cobar, N.S.W G.J. DREW & R.A. BOTH Petrological Studies of the Carbonate-hosted Silver-Lead Deposits of the Ediacara Mineral Field, South Australia

B. KRAPEZ & K.A. ERIKSSON The Sedimentology of a Palaeoplacer - The Ventersdorp Contact Reef at East Driefontein Gold Mine, Carletonvi1le, South Africa

WILLIAM E. GALLOWAY & DAVID K. HOBDAY Sandstone-type Uranium Deposits of Texas

JAN KRASON Metallogenic Environments in the Precambrian Basins of Northwestern United States and Southeastern Soviet Union

MARTIN K. NEUDERT Shallow Lacustrine Sedimentation in the Middle Proterozoic Mt Isa Group and its Implication to Ore Formation at Mt Isa, Queensland

C.W. ROBERTSON The Role of Pre-existing Sulphides and Sulphates in Primary Copper Orebody Formation at Mount Isa

* The papers in this volume are grouped in sessions. The order of presentation was not finalized when this volume was prepared, and does not necessarily follow the order shown here. The order of presentation is shown in the programme.

NEIL WILLIAMS S ROSS G . LOGAN Depositional Environments of the Sediments Hosting

the McArthur River Stratiform Pb-Zn Deposits

ROSS G . LOGAN & ROBERT W . DENNIS

Pb-Cu-Ag Mineralization in the H.Y.C. Deposit, McArthur River, Northern Territory

T . H . DONNELLY, J . KNUTSON & I.B. LAMBERT Recent Research in the South Australian Copper Province

NIGEL J . ROWLANDS Sedimentary Systems Classification of Copper in Sediments

in the Late Proterozoic of South Australia

D.E. ROBERTS & G.R.T. HUDSON

The Olympic Dam Copper-Uranium Deposit, Roxby Downs, South Australia

D.J. PERKIN The Economic Significance of Styles of Uranium Mineralization through Time - An Australian Perspective

S.R. SANGAMESHWAR, BRIAN MARSHALL & G.F.P. JONES

Trace Element and Sulphur Isotope Geochemistry of Sulphide Ores from the Numagee Mine, Cobar Area, New South Wales

P.W. GREGORY & J.S. HARTLEY

The Geology of the Thalanga Volcanogenic Sulphide Deposit, North Queensland

BRIAN OVERSBY

Preliminary Report on Concordant Barite and Base-Metal Sulphide Deposits in Proterozoic Einasleigh Metamorphics of the Werrington Area, Georgetown Inlier, Northeastern Queensland

S.R. SANGAMESHWAR Systematics of Supergene Alteration of Zinc-Lead-Silver Sulphides in Carbonate Terrains

HYDROCARBON GENERATION AND SEISMIC STRATIGRAPHY GERARD DEMAISON The Generative Basin Concept R. ALEXANDER, R.I. KAGI & G.W. W00DH0USE

Molecular Geochemistry - Biomarkers and Maturity Indicators

ALAN R. LIMBERT, DAVID M. MAUGHAN & A. JOHN MEBBERSON Def i nition and Development of the Mackerel Field, Gippsland Basin

STEVE CONLEY

Condensed Sequences in Mesozoic and Cainozoic Shelf Sediments Revealed by Detailed Sidewall Core Sampling R.G. VOS & C.M. McHATTIE

Upper Triassic Depositional Environments, Central Exmouth Plateau (Permit WA-84-P), Northwestern Australia

GLACIAL SEDIMENTATION P.J. BARRETT S B.C. McKELVEY Miocene and Pleistocene Marine and Glaciomarine Shelf Sedimentation, McMurdo Sound, Antarctica

CURT TEI CHERT Permian Glaciation in Pakistan

VICTOR A. GOSTIN S BRIAN R. RUST Late Quaternary Ice-Margin Sedimentary Complexes of the Ottawa-St Lawrence Lowlands

JOHN BAULD & MALCOLM R. WALTER Antarctic Saline Lakes: Analogue of Proterozoic EvaporiteStromatolite-Carbonate-Ti11ite Associations?

D.H. BELL Late Quaternary Sedimentation in Southern New Zealand

EVAPORITES AND CHEMICAL SEDIMENTS M.D. MUIR The Significance of Evaporites in Carpentarian Rocks of Queensland and the Northern Territory

I.H. CRICK & M.D. MUIR Evaporite Pseudomorphs and Their Origin in Early Proterozoic Carbonates, Pine Creek Geosyncline

PETER J. COOK & JOHN H. SHERGOLD Proterozoic and Cambrian Phosphorites

Y. BONE Lower Proterozoic Magnesites of the Rum Jungle Area of the Northern Territory

L. GEI DANS "Zebra Rock" of Western Australia

FLUVIAL, DELTAIC AND CLASTIC SHELF SEDIMENTS H.T. MOORS An Alluvial Fan-Fluvial Plain Depositional Model for the Devonian Willaraddie Formation and Munabia Sandstone of the Carnarvon Basin, Western Australia

N.R. KEMP, J.W. COLL INSON & J.T. EGGERT Fluvial Sequence in the Triassic of Tasmania

R.M. HOCKING Fluviatile Sedimentation in a Coastal Setting; The Tumblagooda Sandstone (Silurian), Western Australia

D.K. HOBDAY & M.B. EDWARDS Structural and Sedimentological Relationships in Ancient Shelf Deltas: Implications to Hydrocarbon Exploration

P.S. MOORE & R.M. HOCKING Wave-dominated Shelf Sedimentation - A Model from the Permian of the Carnarvon Basin, Western Australia

V.L. PASSMORE Depositional Environments of the Devonian Rocks of the Darling Basin - Prospects for Petroleum GRAHAM TAYLOR, KEN MAIDEN & WOLF MAYER Facies and Base-Metal Concentrations of Late Devonian Fluvial Deposits from the South Coast of New South Wales, Australia

COAL MEASURES SEDIMENTATION COLIN R. WARD Geology and Coal Resources of the Merit-Pi la Basin, Sarawak, East Malaysia

A. McMINN Palynostratigraphy of the Late Permian Coal Sequences of the Sydney Basin

P.J.G. FLEMING Coal-forming and Associated Environments in the Permian Denison Trough, Queensland

V.J. MORAN Identification of Collie Coal Seams by Maceral Analysis

P.G. FLOOD Application of Sedimentary Facies Studies in Coal Exploration and Mine Development

N.H.H. GODFREY The Lateral Migration of Sedimentary Channels and the Prediction of Strip Mining Overburden Conditions

SEDIMENTS OF MOBILE ZONES KEITH A.W. CROOK Flysch: Its Tectonic Settings and Implications for Continental Growth

D.A. FEARY The Depositional Record during Cessation of Subduction An Example from the Early Cretaceous of New Zealand

LIONEL CARTER Sedimentary Framework of the New Zealand Continental Shelf

R.J. KORSCH Sandstone Compositions from the New England Orogen: Indicators of Tectonic Setting

J.G. GEHLING Late Precambrian Ediacara Beds: Wilpena Area, South Australia

A Submarine Valley Fill, 3]

D.W. HAIG Upper Cretaceous and Lower Tertiary Deep-sea Sediments of the New Guinea Mobile Belt at Port Moresby ..

PATRICK L. ABBOTT Sedimentology of Eocene Conglomerates, Southern California and Northwestern Mexico

R.C. HORWITZ The Ashburton Trough

R.C. HORWITZ & R.E.T. HILL Origins of Large Olistoliths in the Ashburton Formation

BURTON MURRELL Sedimentation Patterns in the Adelaide Geosyncline

R.M. CARTER & LIONEL CARTER Conway Trough, A Transduction Basin within the Marlborough Shear Zone, New Zealand

SEDIMENTS OF INTRACRATONIC BASINS J.M. LINDSAY Global Sea-Level Cycles, Tectonism, Temperature, and Tertiary Sedimentation, St Vincent Basin, South Australia

G.D. POWIS & A.D. PARTRIDGE Mesozoic Sedimentation History of the Eyre Sub-Basin

BARRY J. COOPER Late Palaeozoic Sedimentation in the Arckaringa Basin, South Australia

B.G. JONES, K. FERGUSON & B. WYGRALA Tectonic Influence on Late Devonian Sedimentary Facies, Northern Amadeus Basin, Northern Territory

J. PINCHIN & B.R. SENIOR The Warrabin Trough, Western Adavale Basin

C.E. MAHER The Piper Oilfield, United Kingdom

TECTONICS AND DEPOSITIONAL STYLE OF THE PRECAMBRIAN K.A. ERIKSSON Archaean and Proterozoic Sedimentation Styles: for Progressive Cratonization

Evidence 38

J.A. HALLBERG Archaean Sedimentation in the Northeastern Yilgarn Block, Western Australia

K.A. ERIKSSON Archaean Iron-Format ions: Analogues of Holocene Pelagic Sediments

...

R . D . GEE Tectonic Framework of Proterozoic Sedimentation along the N o r t h e r n M a r g i n o f the Y i l g a r n Block W . C O M P S T O N , \.S. W I L L I A M S , M . T . M c C U L L O C H , J . J . F O S T E R , P.A. ARRIENS S A . F . TRENDALL 40

A R e v i s e d A g e f o r the H a m e r s l e y G r o u p P . C . M U H L I N G S A . T . BRAKEL Basement T e c t o n i c Control of S e d i m e n t a t i o n P r o t e r o z o i c Intracratonic Bangemall Basin

in the

R.C. MORRIS Some C o m p a r i s o n s b e t w e e n the Dales G o r g e M e m b e r a n d the M a r r a M a m b a Iron F o r m a t i o n of W e s t e r n A u s t r a l i a

JOHN A . BUNTING S e d i m e n t a r y E n v i r o n m e n t s a n d P a l a e o g e o g r a p h y o f the Earaheedy Group, Nabberu Basin, Western Australia

R.C. MORRIS & R.C. HORWITZ A S p e c u l a t i v e but G e o l o g i c a l l y - C o n s t r a i n e d M o d e l f o r the O r i g i n o f the H a m e r s l e y G r o u p M.J. JACKSON The Masterton Sandstone - A Mid-Proterozoic C l a s t i c Unit

Intracratonic 42

J . J . D R A P E R , D . E . M A C K E N Z I E & I.W. W I T H N A L L P r o t e r o z o i c Shelf S e d i m e n t a t i o n Northeastern Queensland

in the G e o r g e t o w n

Inlier, 42

W . V. P R E I S S A d e l a i d e a n S e d i m e n t a t i o n Styles in S o u t h e r n S o u t h A u s t r a l i a a n d T h e i r R e l a t i o n to L a t e P r o t e r o z o i c T e c t o n i c s

STROMATOLITES THROUGH THE AGES MALCOLM

WALTER

T h e S t r o m a t o l i t e S u c c e s s i o n T h r o u g h the A r c h a e a n : o f the E a r t h ' s E a r l i e s t B e n t h o s KATHLEEN

A Record 44

GREY

Studies of Some Precambrian Stromatolites

in W e s t e r n A u s t r a l i a

PHILLIP E. PLAYF0RD E n v i r o n m e n t a l C o n t r o l s o n the M o r p h o l o g y o f M o d e r n at Hamelin P o o l , Western Australia ROGER

Stromatolites 45

BUICK

Did L i f e E x i s t in the E a r l y A r c h a e a n ? Evidence from Sediments, North Pole, Western Australia

Shallow-water 45

REEFS AND OTHER CARBONATES P.G. FLOOD B i o c l a s t i c C a r b o n a t e S e d i m e n t s o f the G r e a t B a r r i e r R e e f , A u s t r a l i a

JOHN F. MARSHALL Submarine LithificatIon in Coral Reefs of the Southern Great Barrier Reef

SALEH M. BILLO Ancient Counterpart of the Modern Great Barrier Reef Environment of Australia

PETER J. DAVIES The Great Barrier Reef - High-Energy Analogue

E.G. RHODES & H.A. POLACH Radiocarbon Dating of Coral Reefs, Error Sources and Age Structure of a Mid-Shelf Reef, Great Barrier Reef

C.E. MAHER Intisar 'D' Oilfield, Libya

R.E. WASS Colonial Animals and Their Environmental Significance in Carbonates

A.E. COCKBAIN Distribution of Stromatoporoids in the Devonian Reef Complexes of the Canning Basin

NOEL P. JAMES Lower Cambrian Bioherms:

Pioneer Metazoan Reefs

ROBERT S. NICOLL & P.J. JONES Lower Carboniferous Conodont and Ostracod Biostratigraphy and the Lithofacies of the Bonaparte Gulf Basin, Northwestern Australia

PIERRE-ANDRE BOURQUE Late Silurian Carbonate Buildups of the Gaspe Basin, Quebec, Canada: Middle to Late Devonian-type Buildups

LLOYD C. PRAY Reinterpretation of Depositional and Diagenetic Processes, Capitan Reef Complex (Permian), Southwestern U.S.A

PHILLIP E. PLAYFORD Early Submarine Cementation and its Consequences in Devonian Reefs of the Canning Basin

LLOYD C. PRAY Submarine Erosion Surfaces and Retreat of Carbonate Bank Margins, (Permian), Southwestern U.S.A

NOEL P. JAMES Early Diagenesis at Modern Carbonate Platform Margins

J.F. READ Carbonate Platforms of Extensional Continental Margins

C.G.St.C. KENDALL & P.M. HARRIS Carbonate Reservoirs in Terms of Tectonics, Depositional Setting and Relative Sea-Level Change

CARBONATE AND SHELF SEDIMENTATION R.V. BURNE Zonation of Intertidal Carbonate Environment, Spencer Gulf: Regional and Global Significance

LINDSAY B. COLLINS Post-Glacial Non-Tropical Shelf Carbonate Sedimentation of the Rottnest Shelf, Western Australia

M. APTHORPE Tertiary Depositional History of the North West Shelf, Western Australia

J.F. READ Evolution of a Carbonate Continental Shelf, Cambrian-Ordovician, Virginia Appalachians, U.S.A

R.G. WALKER Shelf Sedimentation in the Cretaceous Seaway of Western Canada

WILLIAM J. STONE Transgressive/Regressive Sedimentation Style in the Late Cretaceous of San Juan Basin, New Mexico, U.S.A.

C. PRASADA RAO Cold-Water Carbonates: Tasmanian Examples

Recent, Last Glacial and Permian 56

CAINOZOIC SEDIMENTATION C.R.M. BUTT Residual-Colluvial Formation of Sandstones, Grits and Silcretes

JOHN A. DULHUNTY Quaternary Sedimentary Environments in the Lake Eyre Region, South Austral ia

A.P. BELPERIO A Comparison Between Temperate and Tropical Intertidal Depositional Environments

P.S. ROY & B.G. THOM Late Cainozoic Marine Sedimentation Patterns on the Southeast Australian Margin

JOHN L. BAXTER Heavy-Mineral-Bearing Si 1iciclastic and Bioclastic Barrier Sand-Deposits on the Swan Coastal Plain

A.D. ALLEN Late Tertiary and Quaternary Stratigraphy of the Swan Coastal Plain, near Perth, Western Australia

P.S. ROY Assessing Coastal Erosion Potential - A Geological Model

S.H. HICKEY An Investigation of Stranded Beach Ridges, Shoal Bay, Northern Territory: A Small Chenier Plain?

E.G. RHODES Depositional Model for a Chenier Plain, Gulf of Carpentaria

GEOCHEMISTRY AND DIAGENESIS OF SEDIMENTS C. PRASADA RAO & D.C. GREEN Oxygen and Carbon Isotopes of Cold-Water Carbonates (Recent, Last Glacial and Permian), Tasmania

MICHELLE SMYTH & M . CAMERON Organic Petrology and Source Rock Potential of Sediments in the Eromanga Basin, South Australia

M.F. MIDDLETON Coal ification Associated with the Folded Zone of the Bowen Basin

PETER J. COOK, L.A. PLUMB & P.A. TRUDINGER Early Diagenesis in Some Deep Sea Sediments, Northeast Indian Ocean

...

I.R. DUDDY Porosity and Permeability Reduction in Some Otway Basin Sandstones

ASSOCIATION OF AUSTRALASIAN PALAEONTOLOGISTS SYMPOSIUM BIOSTRATIGRAPHY OF THE WEST AUSTRALIAN CONTINENTAL MARGIN G.A. THOMAS Some Devonian and Permian Brachiopods from Western Australia: Their Biostratigraphical and Palaeogeographical Implications

I.H. LAVERING Carboniferous Macro-Invertebrate Biostratigraphy and Palaeoenvironments of the Carnarvon Basin; A Review of Data and Interpretations

...

N.W. ARCHBOLD The Lower Permian Sequences of Western Australia, Correlations with the Northern Margin of Gondwana and Implications for the Carboniferous-Permian Boundary in Northern Gondwana

N.W. ARCHBOLD The Development of the Suborder Chonetidina (Brachiopoda) in the Permian of Western Australia

M . APTHORPE & R. HEATH Late Triassic and Early to Middle Jurassic Foraminifera from the North West Shelf, Australia

JOHN BACKHOUSE Late Jurassic and Early Cretaceous Palynology of the Perth Basin

PATRICK G. QUILTY Objectives of an Oxygen Isotope Study in the Southern Indian Ocean

...

MICHAEL HANNAH & BRIAN McGOWRAN Late Cretaceous Foraminiferal Biofacies, Northeastern Indian Ocean

R. HEATH & M. APTHORPE Tertiary Foraminifera1 Biostratigraphy of the North West Shelf, Western Australia BRIAN McGOWRAN & MICHAEL HANNAH Oceans and Margins: and Tertiary Events

Biostratigraphy of Critical Late Cretaceous

SPECIALIST GROUPS IN ECONOMIC GEOLOGY AND GEOCHEMISTRY & MINERALOGY MINERALOGY AND GENESIS OF GOLD DEPOSITS P.J. MacGEEHAN Mineralogy, Geochemistry and Lateral Zonation within a 1.5 km long Gold-Bearing Fissure Vein System in the Campbell Red Lake and Dickenson Mines, Red Lake District, Ontario, Canada M.E. CLARK & G.N. PHILLIPS Conditions of Gold Deposition, Mt Charlotte, Kalgoorlie, Western Australia LEE Y. GOLDING & RE ID R. KEAYS Problems of Ore Genesis at Kalgoorlie S.D. GOLDING & ALLAN F. WILSON Geochemical and Stable Isotopic Studies of the No. k Lode, Kalgoorlie, Western Australia M. AHMAD Thermodynamics of Gold-Tel 1uride Deposits G.N. PHILLIPS & D.I. GROVES The Nature of Archaean Gold-Bearing Fluids

SPECIALIST GROUP IN GEOCHEMISTRY AND MINERALOGY GEOCHEMISTRY S.R. TAYLOR & SCOTT M. McLENNAN Rare Earth Element Patterns in Sedimentary Rocks as Indexes of Crustal Evolution J.C. VAN MOORT & C.J. SWENSSON The Oxidized Zone of the Broken Hill Lode, New South Wales T.H. GREEN & E.B. WATSON P 2 0 5 Systematics in Orogenic Igneous Rock Series Constraints Imposed by Apatite Crystallization ALLAN F. WILSON The Geochemical and Economic Significance of the Chemical Composition of Supergene Gold and of Gold Nuggets

xi i

IGNEOUS ACTIVITY THROUGH TIME M.E. BARLEY, G.D. BORLEY, G.C. SYLVESTER, J.R. DE LAETER, D.I. GROVES & N. ROGERS Archaean Ca1c-Alkaline Volcanism in the Pilbara Block, Western Australia

C.M. LESHER Concomitant Tholeiitic and Komatiitic Volcanism as an OreLocalizing Mechanism at Kambalda, Western Australia

SUZANNE Y. WASS & JULIAN HOLLIS Crustal Accretion in Eastern Australia - Evidence from Xenoliths in Basaltic Rocks

F.L. SUTHERLAND Pattern of Magmatic Evolution Through the Cainozoic Volcanism of Eastern Australia

R.A. DAY Jurassic to Tertiary Volcanism in Victoria - Tectonic Development and Geochemistry

CONDITIONS AND PROCESSES DURING REGIONAL METAMORPHISM N.J. ARCHIBALD & M.J. BICKLE Metamorphism of the Widgiemooltha-Norseman Area, Eastern Goldfields and Constraints on the Thermal Evolution of This Archaean Granite - Greenstone Terrain

M.E. BARLEY Hydrothermal Alteration and Low-Grade Metamorphism of Archaean Volcanic Sequences in the Eastern Pilbara Block

R.L. OLIVER The Reaction: Staurolite + Quartz t Cordierite + Sillimanite + H 2 0 , near Springton, South Australia

ROSEMARY ALLEN Metamorphic Reactions Involving Sillimanite in the Halls Creek Mobile Zone, East Kimberley

M.A. ETHER IDGE & J.A. COOPER Rb/Sr Isotopic and Geochemical Evolution of a Recrystal1ized Shear (Mylonite) Zone at Broken Hill

ALLAN F. WILSON Oxygen Isotopes as Indicators of Fluids Active in Some Mylonite Zones in Southwestern and Central Australia

GEOCHRONOLOGY/METAMORPHISM I.R. DUDDY Element Mobility and Accumulation During Diagenesis and Very Low-Grade Metamorphism

R.L. OLIVER Phase Relations in Aluminous Metasediments at Weekeroo, South Austral ia

W. COMPSTON, J. COLES & I.S. WILLIAMS Simultaneous Measurement of Pb and S Isotopes in Galenas from the New Lead Belt, Southeast Missouri, Using the Ion Microprobe IAN R. FLETCHER & K.J.R. ROSMAN Major Sm-Nd Age Differences Within the Yilgarn Block M.T. McCULLOCH & W. COMPSTON Sm-Nd Age Constraints on Kambalda and Kanowna Greenstones A.J.W. GLEADOW & I.R. DUDDY Geological Annealing of Fission Tracks in Detrital Apatites and Thermal Histories of Sedimentary Rocks

ENGINEERING GEOLOGY GROUP SYMPOSIUM R.P. MATHER Investigation for a Quarry for Breakwater Construction at Rocky Point, Western Australia D.H. BELL Railway Stability Investigations between Claverley and Oaro, Marlborough, New Zealand N.H.H. GODFREY A Field Recognition Guide to Swelling-Clay-Rich Weathering Profiles in Overburden Sediments in the Northern Bowen Basin, Queensland G. MARCOS Some Geotechnical Aspects of Weathering Profiles in the Darling Range - Perth Area, Western Australia G.J. MULES The Application of Electromagnetic Profiling to the Investigation of the Tarong Pipeline Route, Queensland P. WHINCUP, G.V. SMITH & E. McDONALD Dewatering of the Muja Open Cut Coal Mine, Western Australia

....

D.H. BELL Engineering Geology and Urban Development Practices in New Zealand G.S. GIBBONS & J.L. GORDON Sandstone as a Building Material C.A. KEY Detrimental Features of Ancient Pillow Basalts when used as a Source for Crushed Aggregate M.J. SANDY A Marginal Sedimentary Aggregate in Concrete R.B. SMITH Sample Pretreatment as an Indicator of Material Performance E.J. MINTY & G.K. KEARNS Rock Mass Workability L.J. WYLDE Northwest Western Australian Coastal Sands as Roadbase Materials

xi v

M. AHMAD S H. QUERESHI Hydrogeology and Hydrogeochemistry of Groundwaters from the Mage la Creek Area, Alligator Rivers Region, Northern Territory

IAN IRVINE Trace Metal Pollution in Sydney Harbour Sediments

D.H. STAPLEDON The Geotechnical Specialist and Contractual Disputes

SPECIALIST GROUP IN TECTONICS AND STRUCTURAL GEOLOGY MODERN WORK IN CLASSICAL FOLD BELTS H.L. DAVIES Convergent Tectonism in Western Papua New Guinea

DAVID R. GRAY Implications of Deformation Patterns in the Appalachian Foreland of Southwest Virginia, U.S.A

R.J. KORSCH Polyphase Acadian and Alleghanian Deformation in the Southeast Connecticut Part of the Appalachian Fold Belt

ERIC HE I DECKER Lineament Classification, Tasman Orogen, Northeastern Queensland

GENERAL STRUCTURAL GEOLOGY BRIAN MARSHALL & K.D. TUCKWELL Structure and Exploration in the Sherridon Mine District, Northern Manitoba, Canada

D.J. PATTERSON Microtextures of a Naturally-Deformed Pyrrhotite Ore

A.J. PARKER Structural and Metamorphic Controls on the Origin of Nephrite Jade in South Australia

S.H. WHITE Inter-Relationship between Deformation and Metamorphism in Fault Zones and Lineaments

A.J. PARKER, P.R. JAMES, V. MIELNIK & R.L. OLIVER Superposed Folding: A Classic Example in East Antarctica

S.H. WHITE, E.H. RUTTER & C.J. PEACH Experimental Study of Rock Deformation under Conditions Favouring Chemical Transformations

FILMS ON CARBONATE SEDIMENTATION

AUTHOR INDEX

CONVENTION THEME

" S E D I M E N T S THROUGH T H E A G E S "

KEYNOTE

ADDRESSES

IS THERE LIFE AFTER FACIES M O D E L S ? R.G. Walker McMaster University, Hamilton, Ontario, Canada A good facies model expresses the generalities of a given system, eliminating local variability. It will then act as a norm, for comparisons; as a predictor in new situations; as a guide for new observations; and as a basis for hydrodynamic interpretations. For each system, the model should summarize what is there (mineralogy, diagenesis), how it got there (processes, hydrodynamics), and where it is (depositiona1 environment, predictions in new examples). The concept of facies has been around for over 140 years; it was re-established in the sedimentological literature in about 1965, flourishing along with studies of recent sediments and sedimentary structures. By about 1972, models for fluvial, deltaic and submarine fan systems seemed well understood; a l l , however, need major revision now. No general models exist at all in shelf and slope environments, where more progress has been made in a study of ancient rocks than recent sediments. The "facies model" approach to sedimentology, then, is in no state of imminent demise. Whilst "newer and better models" will focus sedimentological studies along their present 1ines, the larger scale, basin-wide facies models wi11 better integrate basin tectonics and sedimentology. This will give yet more vigour to one of sedimentology's most useful concepts.

INTERPRETING JURASSIC UNCONFORMITIES, PALAEOENVIRONMENTS AND SEA-LEVEL CHANGES FROM SEISMIC STRATIGRAPHY AND BIOSTRATIGRAPHY P.R. Vail S J . Hardenbol Exxon Production Research Company, Houston, Texas Seventeen global unconformities and their correlative conformities (sequence boundaries) subdivide the strata of the Jurassic into 16 cycles. These 16 cycles comprise the Jurassic supercycle. Nine of the global unconformities are both subaerial and submarine and are believed to be caused by rapid falls o f e u s t a t i c sealevel. Eight of the unconformities are subaerial only and are believed to be related to slow falls of eustatic sea-level. In addition, over 12 marine-condensed sections (starved intervals) have been identified that are interpreted to be related to rapid rises of sea-level. Unconformity recognition is locally or regionally enhanced by periodic truncation of folded and faulted strata during sea-level lowstands and onlap onto topographic highs during sea-level highstands, but there is no evidence that the tectonics caused the global unconformities. The 16 cycles that subdivide the Jurassic supercycle are chronostratigraphic intervals that subdivide the Jurassic into a series of genetic depositional sequences, which are ideal for facies analysis. Highstand prograding deltas and lowstand submarine fans are associated with many of these cycles. The Jurassic unconformities and the stratigraphic facies patterns between them are caused by the interaction of basement subsidence, eustatic sea-level changes and varying sediment supply. Detailed analysis of the sediments with seismic stratigraphy and well data permit quantification of the subsidence history and reconstruction of palaeoenvironment and sea-level changes through time.

The integrated use of seismic stratigraphy and biostratigraphy provides a better geologic age history than could be obtained by either method alone. Palaeobathymetry, sediment facies, and relative changes of sea-level can be interpreted from seismic data and confirmed or improved by well control. Geohistory analysis provides a quantitative analysis of basin subsidence using geologic time-depth diagrams to visua 1ize the total basin subsidence. When this subsidence is corrected for compaction and sediment loading, the tectonic subsidence and long-term eustatic changes may be determined. Short-term, rapid changes of sealevel can be demonstrated from seismic and well data. The stratigraphic resolution of these changes rarely allows exact quantification of their magnitude, but a minimum rate of change of sea-level can often be determined. Applications of these procedures are demonstrated with examples.

UNCONFORMITY CONTROLS ON MINERALIZATION THROUGH PRECAMBRIAN TIME WITH SPECIAL REFERENCE TO SOUTHERN AFRICA Andrew Button S Noel Tyler Department of Geology, South Dakota School of Mines S Technology, Rapid City, South Dakota, U.S.A. A disproportionately large number of mineral deposits is associated with Precambrian weathering and erosion surfaces. In southern Africa, such surfaces range in ages from 3 200 Ma to 600 Ma, and are developed on rocks as diverse as granites, volcanics (both basic and acid), arenites, shales, carbonates and iron-formations. The sericitic arenites which host the auriferous/uraniferous conglomerates of the Witwatersrand, and other basins of comparable age were almost certainly derived from granitic palaeosols. There is some evidence to suggest that U was leached by palaeogroundwater during the formation of such palaeosols. It is suggested that some of this U may have become fixed in diagenetic titania skeletons in conglomerate palaeoaquifers. In addition to chemical concentration factors, it has long been recognized that erosive truncation and reworking of older conglomerates was an important way of creating mechanical concentrations of Au and U along early Proterozoic and late Archaean unconformities. In several areas in southern Africa, basic lavas were weathered during the early Proterozoic or earlier times to yield an alumina-rich (>30 percent A1 2 0 3 ) palaeosol. Such palaeosols represent a vast resource of non-bauxitic alumina. Unconformities that cut across Precambrian carbonate rocks are marked by palaeokarst surfaces, which have acted as emplacement loci for deposits as diverse as the fluorite (Pb-Zn) deposits of the western Transvaal to the Pb-Zn-Cu deposits in the Tsumeb Mine of South West Africa/Namibia. Manganese deposits are characteristically associated with erosion surfaces cut across early Proterozoic carbonate formations both in South Africa and in Western Australia. Apparently the palaeoatmosphere some 2 200-2 300 Ma ago contained enough oxygen to oxidize (and fix on the unconformity) both Mn and Fe that had previously been held in divalent form in carbonate minerals. Erosion surfaces cut across iron-formations are commonly mantled by iron-format.on clast conglomerates. These conglomerates are sometimes enriched, presumably by pa 1aeogroundwater, to high-grade, conglomerate-textured hematite ore. The vast period of geologic time represented by unconformities was probablv the most potent factor in mineral concentration. Given time, normal geologic processes can achieve a high degree of concentration of minerals. Other important factors identified include palaeotopography (due to differential erosion), solution by (and precipitation from) pa 1aeogroundwater, biological action, and porosity contrasts adjacent to unconformities. Continued study of Precambrian palaeoweathering and erosion surfaces promises to be an exceptiona11y fertile field for research T V t S S C i e n ? e ' a n d i n a d v a n c i n 9 our understanding of the fundamental questions of terrestrial atmospheric-hydrospheric evolution

MINERAL DEPOSITS IN SEDIMENTS STRATABOUND BASE METAL DEPOSITS - NEPTUNE AND PLUTO RE-UNITED? E.F. Stumpfl Institute of Mineralogy, Mining University, Leoben, Austria Evidence accumulated in recent years shows increasingly that much of the traditional "sedimentary vs hydrotherma1" controversy can be resolved by recognizing the significance for ore genesis of hydrotherma1 systems operating on the sea floor. Stratabound and cross-cutting mineralization occur in juxtaposition in major base metal deposits world-wide, irrespective of geological age, host rock lithology, mineralogical composition, or metamorphic grade. At Mt Isa, Queensland, Proterozoic stratiform Pb-Zn orebodies and Cu orebodies can be attributed to the same hydrothermal system. In the Eastern Alps of Austria, Palaeozoic volcano-sedimentary sequences of diffe rent metamorphic grade have originally been deposited in connected marine basins. They carry stratiform (and cross-cutting) base metal, Sb and W mineralization. New mineralogical and geochemical data are presented to stress the point that the format ion of many ore deposits depends on the interplay of tectonism, hydrothermal/ volcanic activity and sedimentation.

SEDIMENT-HOSTED ORE DEPOSITS OF THE PERMIAN METALLOGENIC EPOCH Jan Krason Geoexplorers International Inc., Denver, Colorado, U.S.A. On a global basis, sedimentary basins of Permian age are notable for their contained mineral deposits, in particular oil and gas, potassium salts and Cu deposits (with appreciable Ag, Pb, Zn, U, Mo, Rn, V, Bi, Ni and Co). The presence of evaporites is a significant indicator of the palaeogeographic and palaeoclimatic environment and the coincidence of hydrocarbons with metal sulphide concentrations is considered to have genetic connotations. Model studies of this metallogenic environment led to the discovery of the huge sediment-hosted polymetallic Cu deposits within the Lower Zechstein in western Poland some 20 years ago. The metallogenic environments of these deposits is compared with the "Kupferschiefer" type locality in the Mansfeld mining district and Cu deposits and occurrences in sediments of Permian age in the U.S.A. and U.S.S.R. In all three regions, mineralization is hosted by a wide variety of lithologies, including clastic, biogenic, and chemical (carbonate) sediments. Ore mineralization is clearly controlled by lithofacies and palaeogeography. Lagoonal and shallow marine environments appear to be most favourable for major ore concentrations, but mineralization is also found in fluvial and deltaic sediments. The importance of structure relates to the development of palaeohydrodynamic and hydrogeochemical systems. It is suggested that the metals are derived by weathering from a source area which, at some stage in the ore-forming process, is connected hydraulically with the depositional environment. However, such source-deposit relationships remain obscure for the deposits studied. Recent investigations of the Polish deposits have highlighted the importance of regional metasomatic ore mineral alteration.

STRATIFORM COPPER IN THE LATE PROTEROZOIC SEDIMENTS OF THE ABLAH G R A B E N , KINGDOM OF SAUDI ARABIA Ian Tedder & Nigel J. Rowlands Utah Development Co., Brisbane, Queensland The Ablah Group is composed of a sequence of clastic and volcaniclastic sediments of late Proterozoic age which is now preserved in a north-south trending, linear palaeotrough, located in the southern sector of the Saudi Arabian Shield.

The Group rests with unconformity o n , and is infolded with, the older Jiddah and Baish-Bahah Groups; it is unconformably overlain by the Proterozoic Ha 1 aban Group in the north. Stratiform Cu mineralization occurs at several levels in the Sarban Formation which is the basal Ablah unit in the southern sector of the palaeotrough. The Sarban Formation ranges from 500 m to 1 500 m in thickness and has been subdivided into four units (all components of a sedimentary shelf facies, laterally disposed to and underlying the vol caniclastic trough facies): (1) a Lower Clastic unit, characterized by pebble and boulder conglomerates, grits and arenites deposited as fluvial si1iciclastic fans wedging out from the graben s i des; (2) a Lower Marble unit, characterized by thin, strike-persistent carbonates at the pivotal point between an overall fining-up clastic sequence overlain by a coarsening-up sequence; this unit was laid down as a biogenic barrier bar system; (3) a Middle Clastic unit, which is characterized by matrix-rich rudites that were laid down by a series of deltas prograding across the Lower Marble from the north; (4) an Upper Marble unit, characterized by a fining-upwards clastic sequence, a pivotal carbonate, and topped by a coarsening-upwards sequence; this formed in shallow water as a biogenically influenced barrier-bar system. Copper mineralization is common in the Sarban stratigraphy, but best developed in the strike-vectored barrier-bar and sublittoral strandplain of the Lower Marble and Upper Marble.

GENETIC ASPECTS OF THE CSA AND QTS M I N E R A L I Z A T I O N S , C O B A R , N . S . W . Brian Marshall 1 , S.R. Sangameshwar 1 , D.P.H. O'Connor 2 , Shen-Su Sun 3 S M . Bouffler 1 1 2 3

The New South Wales Institute of Technology, Broadway, New South Wales CRA Exploration Pty Ltd, North Sydney, New South Wales CSIRO, Division of Mineralogy, North Ryde, New South Wales

The CSA Mine and QTS mineralizations are 11 km north of Cobar. QTS mineralization is stratigraphically lower than the CSA deposits and has been intersected by drilling from CSA Mine lower levels. Controversy exists over whether or not CSA Mine, and other mines in the tract south-southeast through Cobar to Nymagee, are located within deformed syn-sedimentary exhalative mineralization. Discovery of the Elura deposit north of CSA Mine extended the tract and intensified the controversy. The Pb-Zn QTS mineralization is stratiform in its meso-, and microscale relations with bedding (S Q ) Sulphides lie in S Q and are differentially elongated and flattened within Sj. 6 3 *S values [cp, 8.66 (s.d. 0.66); sp, 8.59 (0.46); g n , 6.84 (0.39)] suggest a seawa'ter source for sulphur. A S p - g n yields an equilibration temperature of 358 (19.0)°C. The mineralization is syn-sedimentary and exhalative. At CSA Mine, structural relationships and 6 3 * S data from selected samples of mineralization away from the orebodies again suggest syn-sedimentary exhalative formation. However, the ellipsoidal CSA orebodies are grossly discordant to bedding and mainly comprise vein systems paralleling and cross-cutting Sj. 6 3 * S values are similar to those from stratiform mineralization but, unlike those data, show evidence ot disequilibrium between coexisting sulphides. The simple stratiform geometry and equilibrated 6 3 I + S data from QTS and from ro hriumanfZ^,0^n "J <; k s rasts with the complex geometry and disequilibrium of the CSA orebodies. Published models invoking in situ ductile and/or chemical redistribution of stratiform exhalative mineralization do not fit the CSA ore-

PETROLOGICAL STUDIES OF THE CARBONATE-HOSTED SILVER-LEAD DEPOSITS OF THE EDIACARA MINERAL FIELD, SOUTH AUSTRALIA G.J. Drew 1

& R.A. Both

South Australian Department of Mines and Energy, Parkside, South Australia Department of Economic Geology, University of Adelaide, Adelaide, South Austral ia

The Ediacara mineral field is situated on the western margin of the Flinders Ranges, and consists of Ag-Pb and Cu deposits in lower Cambrian carbonate rocks that contain anomalous base-metal contents throughout the Adelaide Geosyncline. The main primary ore minerals of the Ag-Pb mineralization are galena and pyrite, with minor chalcopyrite and sphalerite. The gangue consists of chalcedony and quartz, with minor dolomite. The mineralization is stratabound and occurs in conformable zones. The Ag-Pb mineralization is mainly present in sandy cross-bedded dolomite and laminated algal dolomite, deposited in an environment ranging from sub-tidal to bar and channel and tidal flat respectively. Four types of mineralization have been recognized; disseminated sulphides of syngenetic and/or diagenetic origin and epigenetic concentrations along stylolites, in veins and as breccia fillings. Postdepositional solution activity has affected a large proportion of the carbonate sequence, the effects ranging from stylolites through stylobreccias to solution collapse breccias. The epigenetic concentrations of mineralization have apparently been formed by the remobi1ization of the disseminated sulphides during solution activity. The Ediacara Ag-Pb deposits have many features in common with Mississippi Valley-type Pb-Zn deposits and appear to have similarities in terms of genesis, in that the epigenetic mineralization has been formed as a result of post-depositional solution activity during diagenesis in a sedimentary basin.

THE SEDIMENTOLOGY OF A PALAEOPLACER - THE VENTERSDORP CONTACT REEF AT EAST DRIEFONTEIN GOLD M I N E , CARLETONVILLE, SOUTH AFRICA B. Krapez 1 & K.A. Eriksson 2 1 2

Department of Geology, University of Western Australia, Nedlands, Western Australia Programs in Geosciences, University of Texas at Dallas, Richardson, Texas, U.S.A.

The Ventersdorp Contact Reef, dated at 2 700 Ma, is an auriferous palaeoplacer that consists of several gravel types, each of which shows varying Au concentrations. Interpretations of these features is made possible by reference to Holocene sedimentary analogues. Stratigraphically, the Ventersdorp Contact Reef lies on an angular unconformity above the Witwatersrand Supergroup. The Ventersdorp Lavas conformably overlie the unit. A descriptive framework of gravel characteristics was evolved from interpretations based on underground observations and measurements of scalar properties recorded by photographs and thin-sections. Five gravel types were recognized from these observations. Type A gravels are interpreted as debris flood deposits formed on an alluvial fan. Type B gravels are interpreted as torrential streamflood deposits formed on the same alluvial fan. The dominant source for both types was from metasedimentary units of the Witwatersrand Supergroup and the Archaean basement, a 1 though both types have significantly different clast assemblages. Type C gravels were formed by bedload traction processes in a fluvial environment. Four sub-environments have been identified, corresponding to three topographical levels on a braided alluvial plain. The sub-environments were: (1) a deep incised channel of bare bedrock surfaces with lenticular coarse gravel lags; (2) a flood plain of coarse gravel lags; (3) a flood plain of coalesced, longitudinal gravel bars; and (4) isolated islands of bare bedrock or remnant gravel lags. Source rock terrains comprised hydrothermal quartz veins in a host of Archaean metamorphic and volcanic rocks. Type D gravels and sands were formed in a tributary braided stream consisting of solitary longitudinal gravel bars and adjacent, deep inter-bar channels. Thick

gravels and thin sands were deposited on the bar and thick sands and thin gravels were deposited in the channels. The source comprised a mixed terrain of Witwatersrand Supergroup and Archaean metasedimentary rocks and Archaean metamorphics with hydrothermal quartz veins. Type E gravels formed as channel fills by the coalescence of transverse bars. Bar development occurred during a rising flood stage within the confines of the incised channel on the braided alluvial plain. The bulk of the gravel and Au was derived by the reworking of types B and C , while minor amounts of sediment and Au were derived from a primary Archaean source. The palaeoenvironment consisted of an alluvial fan prograding from the east and a tributary braided stream flowing from the west onto a braided alluvial plain. Incision of the main channel was caused by the constriction of the alluvial plain by the tributary sub-environments. During flood conditions, coarse gravels were deposited on the eastern flood plain and reworked into longitudinal gravel bars. Channel-bottom gravels were reworked as transverse bars during a later ris ing flood stage.

SANDSTONE-TYPE URANIUM DEPOSITS OF TEXAS William E. Galloway & David K. Hobday Bureau of Economic Geology, The University of Texas at Austin, Texas, U.S.A. Texas currently ranks third nationally in uranium production. Mining is at present restricted to the Tertiary South Texas uranium province, where extraction procedures include open-pit excavations and in situ leach mining. Recently completed exploration programmes utilizing airborne surveys, rock, stream sediment, and water sampling, and detailed three-dimensional studies of outcrop and subsurface data have located a number of smaller U deposits in Upper Palaeozoic, Triassic, and Cretaceous strata elsewhere in the State. Upper Palaeozoic sediments reflect a gradation from foreland to intracratonic basin, with a thick succession of locally uraniferous Permian redbeds at the top. The Triassic Dockum Group was deposited in a closed basin with peripheral alluvial fans, fan deltas, and lobate deltas. Uranium is concentrated in channel lag and distal delta front facies. Basal Cretaceous fluvial sand axes in Northeast Texas are located over basement synclines. Overbank clays in immediate contact with the incised channel sands are highly radioactive, suggesting that U leached from contemporaneous ash-fall layers updip was transported in solution down the sand a x e s , and was scavenged by the adjacent organic-rich clays; larger U deposits may be located farther downdip. Critical requirements for South Texas U mineralization were (1) a framework of thick, integrated, highly-transmissive aquifers, (2) updip volcanic ash in stratigraphically-equivalent or overlying strata, (3) substantial groundwater recharge in a generally arid climate, and (4) downdip reductants including organic debris, sulphides, and gases migrating up growth faults. The depositional milieu comprises strandplain, barrier/lagoon, bedload channels, and overbank facies. Volcanic a s h , supplied in large volumes from northwest M e x i c o , was leached soon after deposition! Uranium was transported down dip-oriented channel sands. Mineralization is concentrated in the most permeable zones, particularly near hydrodynamic barriers.

METALLOGENIC ENVIRONMENTS IN THE PRECAMBRIAN BASINS OF NORTHWESTERN UNITED STATES AND SOUTHEASTERN SOVIET UNION Jan Krason Geoexplorers International Inc., Denver, Colorado, U.S.A. The stratiform ore deposits hosted within the Belt System of the northwestern United States and southeastern British Columbia and the Udokan Series of the southeastern part of the Soviet Union are confined to major sedimentary and geosynclinal basins which have been considered as major metallogenic provinces.

7 The metallogenic environments in these basins are controlled by a set of similar geological factors. Overall the tectonic features control most aspects of the palaeogeography, sedimentary environments, lithofacies, hydraulic systems, geochemistry, alteration and metamorphism. The Proterozoic strata in both geosynclinal basins are composed essentially of the flyschoid-type sequence with some typical redbed lithofacies. These rocks represent marine, deltaic, lagoonal and terrestrial sedimentary environments. Radiometric age dating indicates that in the Belt these rock systems range from 1 500 to 900 Ma, whereas Udokan Series has been dated at 2 150 to 1 100 Ma. The metalliferous mineralizations suggest similar ore-formation environments, but the stratigraphic factor is not always critical. However, the regional and local structural setting is of crucial importance. In spite of structural reorganization, considerable post-sedimentary alteration and regional metamorphism (usually of the greenstone metamorphic facies) there is evidence that the ore mineralization is also governed by porosity, permeability, hydraulic gradients, and hydraulic connections between metalliferous source and host rocks. The spatial relationships suggest that ore grade mineralization was introduced through penetration of the sedimentary sequence by solutions preferentially flowing along the conduits provided by the permeable rocks. However, metalliferous sources or source rock areas are not always clear. Generally similar metallic and non-metallic composition and mineral deposit host rock paragenesis provides the basis for comparison of the geochemical and overall analogous ore-formation environments.

SHALLOW LACUSTRINE SEDIMENTATION IN THE MIDDLE PROTEROZOIC MT ISA GROUP AND ITS IMPLICATION TO ORE FORMATION AT MT ISA, QUEENSLAND Martin K. Neudert Research School of Earth Sciences, Australian National University, Canberra, A.C.T. A new sedimentological study of the depositional environment of the Upper Mt Isa Group formations has revealed the occurrence of stromatolites, halite casts, large cross-bedded channel deposits and flat-pebble conglomerates associated closely with the well-known Mt Isa Cu-Pb-Zn deposit. These sedimentary structures provide strong evidence of shallow water deposition with intermittent hypersaline and emergent conditions during sedimentation of the Upper Mt Isa sequence. The assemblage of sedimentary structures is compatible with a shallow water lacustrine model involving intermittent streams and playa lake deposition, marginal ridge-ri11 structures and subsurface halite growth. Likely modern analogues are the 1 ake complex of the Dead Sea graben system and the lakes in the East African Rift Valley. The new discoveries have significantly improved understanding of ore formation at Mt Isa. Syngenetic or early diagenetic mineralization hypotheses must now conform witha sedimentary environment of limited water depth and high salinity. This constraint excludes the often postulated Red Sea model of deep submarine metal-sulphide exhalations as a modern analogue for the formation of the Mt Isa orebodies. Alternative mineralization models emphasizing the role of evaporite-derived brines should be cons idered.

THE ROLE OF PRE-EXISTING SULPHIDES AND SULPHATES IN PRIMARY COPPER OREBODY FORMATION AT MOUNT ISA C.W. Robertson Mount Isa Mines Limited, Mt Isa, Queensland The primary sulphide deposit at Mount Isa, Northwest Queensland is within a siltstone and shale sequence which has undergone little metamorphic transformation, but has been variably altered by hydrothermal events. Syngenetic fine-grained pyrite is interbedded with the sediments, and sulphates such as gypsum and anhydrite, now pseudomorphed, have been identified in the coarser dolomite beds.

Galena and sphalerite forming the Pb-Zn orebodies are interbedded with the sequence, but primary chalcopyrite accumulations appear to overprint. The Cu distribution in the largest orebody, the 1100 Orebody, shows a massive high-grade core developed in a stratigraphically equivalent horizon to high syngenetic pyrite accumulations, with a reduction in pyrite concentration in the core area. Sulphur isotope values for chalcopyrite lie within the range of the syngenet i c pyrite values, indicative of a common S source. Sulphurabundances in the syngenetic pyrite horizons do not increase in relationship to increased Cu grade. These factors support the view that the chalcopyrite has utilized S from the pre-existing sediments in its formation, and although the evidence for sulphate use is obscure, it is inferred by the disseminated nature of the lower grade Cu. Later structural and hydrothermal events have complicated and adjusted this distribution.

D E P O S I T I O N A L E N V I R O N M E N T S OF THE S E D I M E N T S

HOSTING

T H E M c A R T H U R RIVER S T R A T I F O R M P b - Z n D E P O S I T S Neil Williams & Ross G. Logan Carpentaria Exploration Company Pty Ltd, Brisbane, Queensland Stratiform Pb-Zn mineralization at McArthur River, Northern Territory, occurs in the Carpentarian H.Y.C. Pyritic Shale Member of the Barney Creek Formation. Sedimentological studies indicate that the Member formed in an evaporative marginalmarine or lacustrine environment in which water depths fluctuated widely. The main indicators of shallow-water evaporative conditions are distinctive units of dolomite nodules set in a matrix of distorted carbonaceous siltstone. The units closely resemble, and are interpreted to be Proterozoic examples of, the kinds of nodular carbonate- and anhydrite-bearing sediments that form today in shallow to emergent sabkha and sa1ine-1acustrine environments. However, shallow to emergent conditions did not prevail throughout the deposition of the Member because it also contains abundant slump breccias and graded turbidites that clearly formed under deeper-water conditions. Although actual water depths are indeterminate, they were sufficient to allow the formation of turbidite beds that are graded over thicknesses of up to several metres. Pb-Zn mineralization is associated with both the shallow- and deeperwater sediments, but is much better developed in the deeper-water intervals, indicating that the depositional environment of the host sediments was an important control on the formation of mineralization.

Pb-Cu-Ag MINERALIZATION

IN T H E H . Y . C . D E P O S I T ,

McARTHUR RIVER, NORTHERN

TERRITORY

Ross G. Loga n & Robert W. Dennis Carpentaria Exploration Company Pty Ltd, Brisbane, Queensland The stratiform H.Y.C. Zn-Pb-Ag deposit is hosted by the Carpentarian ( M 680 Ma old) H.Y.C. Pyritic Shale Member of the Barney Creek Formation. The deposit typically consists of lamina, up to several millimetres thick, of finely intergrown pyrite, sphalerite and galena, with accessory amounts of chalcopyrite. Recent drilling on the northern edge of the deposit, however, has revealed the presence of a small zone of atypical mineralization that is rich in Pb, Cu and Ag relative to the bulk of the H.Y.C. deposit. In this anomalous zone, the mineralization comprises sulphide bands, up to about 1 cm thick, rich in fine-grained chalcopyrite and/or galena, and containing minor pyrite and sphalerite. Freibergite is a minor, though important phase, and occurs as irregular-shaped grains in the chalcopyrite and galena. The banded sulphide mineralization is occasionally brecciated, being healed by relative coarsegrained galena, chalcopyrite, dolomite and barite.

The presence of chalcopyrite and freibergite, the higher galena content and the brecciation, suggest that the coarse sulphides formed closer to the source of the base metals than the sulphides in the bulk of the H.Y.C. deposit. The sulphidebarite rich matrix to the brecciated mineralization indicates that base-metal sulphides continued to be deposited after the formation of the host sediments. In addition, the presence of barite in the matrix, but not the bedded sulphides, suggests that the chemical composition of the solutions transporting base metals altered with time.

RECENT RESEARCH IN THE SOUTH AUSTRALIAN COPPER PROVINCE T.H. Donnelly, J . Knutson S I.B. Lambert Baas Becking Geobiologica1 Laboratory, Canberra, A.C.T. Stuart Shelf. At the Cattle Grid m i n e , near Mt Gunson, chalcocite, bornite, chalcopyrite, with some pyrite, carrolite, sphalerite and galena, infill fractures and vugs in the strongly brecciated palaeoweathering surface of the arenitic, early Adelaidean Pandurra Formation. The iron-rich halo in rocks overlying and adjacent to the Cattle Grid deposit, as well as textural evidence, indicate Cu-Fe sulphide minerals replaced earlier pyrite. Sulphur-isotope values indicate sulphur was most likely formed biogenically under conditions of unlimited sulphate supply and was probably introduced to its present site. The disconformably overlying Tapley Hill Formation, comprising mainly thinbedded dolomite and mudstone, contains widespread, but generally low grade, Cu (Pb, Zn) mineralization in which pyrite framboids have been partly or wholly replaced by chalcocite, bornite and chalcopyrite, with some remobi1ization of sulphide minerals into dewatering channels, desiccation cracks and fractures. There is vertical zonation from Cu to Pb and Zn inward from both upper and lower surfaces of the Tapley Hill Formation, with a relatively barren pyrite zone in the central portion of the thicker sections. Lateral zonation is indicated by a marked decrease in Cu relative to Pb and Zn outward from Mt Gunson. As in the Pandurra Formation there is an absence of hydrothermal alteration a n d , in addition to Cu, the mineralized rocks are enriched in Pb, Z n , Co, As and Th. A wide range of positive sulphur-isotope values implies closed-system bacterial sulphate reduction. Adelaide Geosyncline. At Kapunda, Cu mineralization is again present in the Tapley Hill Formation, which is considerably thicker than on the Stuart Shelf and has been recrysta11ized by low to middle greenschist facies metamorphism. Sulphideminerals occur along bedding laminae and in coarser grained carbonate-quartz-sulphide veins up to 30 cm thick. Chemical and stable-isotope data indicate that the veins formed by remobi1ization of the bedded mineralization. The suIphur-isotope results indicate bacterial sulphate reduction occurred under conditions similar to those for the Tapley Hill Formation on the Stuart Shelf. Copper mineralization at Copper Claim occurs in mildly metamorphosed carbonaceous dolomitic-sandstones and siltstones of the early Adelaidean Callanna Beds. Chalcopyrite and iron-sulphides are present as fine disseminations and in coarser veinlets. Sulphur, carbon and oxygen isotope results indicate that mineralization processes were similar to those at Kapunda, but the sulphur source was isotopically 1ighter. Mount Painter Block. This inlier at the northeast margin of the Adelaide Geosyncline comprises Early to Middle Proterozoic metasediments, metavolcanics and granites intruded by Early Palaeozoic granites and pegmatites. The results of isotopic, field and petrographic studies of massive granitic, hematitic and chloritic breccias within this block imply that rapid release of fluids from high-level, early Palaeozoic granitic magmas was more important in breccia formation than tectonic and sedimentary processes.

SEDIMENTARY SYSTEMS CLASSIFICATION OF COPPER IN S E D I M E N T S IN THE LATE PROTEROZOIC OF SOUTH A U S T R A L I A Nigel J. Rowlands Utah Development Co., Brisbane, Queensland The late Proterozoic Adelaidean Supergroup of South Australia consists of four 1ithostratigraphic units which, from oldest to youngest, are the Callanna, Burra, Umberatana and Wilpena Groups. Copper mineralization in sediments of the oldest three of these groups is assessed from the standpoint of the paragenesis of the host sedimentary facies. Traditionally, stratiform Cu deposits of the sensu-stricto type have been divided into ore-in-arenite and ore-in-shale subtypes. In this contribution, two criteria are used to reclassify stratiform Cu deposits. These two criteria are: (1) the facies association "lap" geometry which allows subdivision of host systems into "uplap" and "oblique lap" domains; and (2) the host sediment transport vector domain which is either dip-vectored or strike-vectored. These two criteria permit classification of stratiform copper deposits as fol1 ows: (1) Strike-vectored, uplap domain - ore-shale deposits of evaporite association, e.g. Kupferschiefer (Europe) and Callanna Beds occurrences. (2) Strike-vectored, oblique lap domain - ore-shale deposits of paralic (possibly lacustrine) affinity, e.g. Roan Antelope (Zambia), White Pine (U.S.A.) and Burra Group occurrences. (3) Dip-vectored, uplap domain - peneconcordant deposits of fluviati le paragenesis, e.g. Nacimiento (U.S.A.); this group was not investigated in South Australia. (4) Dip-vectored, oblique lap - ore-arenite deposits of deltaic paragenesis, e.g. Mufulira (Zambia), Udokan (U.S.S.R.) and Umberatana Group occurrences, plus some Callanna Beds occurrences. Investigation of the host facies associations for stratiform Cu anomalism the Callanna, Burra and Umberatana stratiqraphic units showed the following:

(1) A Callanna stratiform Cu model controlled by a clastic evaporative interface representing the change from stillstand sands and/or labile wedge arkoses to sabkhas. The host depositional association for stratiform Cu anomalism at this stratigraphic level is usually a strike-vectored one deposited during a biostasic phase and sourced by post-orogenic peneplains. The host association facies geometry can be both blanket-like and lentiform (for the rarer occurrences of Cu in Callanna palaeodeltas). The host association is characterized by stacked products of arid and humid palaeosabkha phases. (2) A Burra stratiform Cu model controlled by a clastic-chemical interface representing the change from wave-dominated sand-blankets to cyclic (non-saline) lacustrine highstands and lowstands (each exhibit confusing paralic overtones). The host depositional association for stratiform Cu mineralization in the Burra Group is a strike-vectored one, albeit a different strike system to the Callanna cupriferous event. The Burra stratiform Cu occurrences are fixed at a regional clastic-chemical interface interpreted as representing cessation of si 1iciclastic input from a hinterland that was in a decelerating uplift phase. The Burra host association for Cu mineralization is typified by blanket-1ike, multistoried, shallowing-up cycles. (3) An Umberatana stratiform Cu model occurring at a coarse clastic-laminite interface representing the change from river-dominated deltas to prodelta and shelf muds. This stratigraphic level reflects the surge of f1uvio-deltaic activity that followed the first great Adelaidean ice age. The host depositional association for stratiform Cu mineralization at this level is a dip-vectored one, sourced by a hinterland, which was in an accelerating uplift phase. The Umberatana host association for Cu is typified by a facies geometry characterized by straight, non-entrenched, multistoried and multilateral sands and gravels with a si1iciclastic overbank su i te.

This sedimentary-systems classification for Cu in sediments demonstrates that, although Cu fixation occurs across a spectrum of palaeogeographic conditions, there are unifying themes across the spectrum. These are the sedimentary transport vector; the host facies geometry; and the behaviour of the hinterland.

THE OLYMPIC DAM COPPER-URANIUM DEPOSIT,

ROXBY

DOWNS, SOUTH AUSTRALIA

D.E. Roberts S G.R.T. Hudson Western Mining Corporation, Unley, South Australia The Olympic Dam Cu-U deposit was discovered in 1975 during a Cu exploration programme in the Stuart Shelf region of South Australia. The deposit is spatially associated with large, circular gravity and magnetic anomalies and is unconformably overlain by about 325 m of flat-lying Adelaidean and Cambrian sediments. The Proterozoic sequence hosting the mineralization includes massive, coarsegrained, monomict and polymict sedimentary breccias with minor intercalated silty and arkosic bands, overlain by iron formation, altered felsic lavas and tuffs, volcanic breccias and conglomerates. The complete sequence is unknown but dri11ing has shown it to exceed 1 000 m in thickness and 15 km2 in area. Block faulting, the intrusion of dolerite dykes and erosion preceded deposition of the cover sediments. Stratabound Cu-U-REE mineralization extends throughout the breccia pile but ore zones are best developed in polymict, high hematite-matrix breccias. The bulk of ore grade mineralization consists of bornite-cha1copyrite-pyrite, overlain in places by a primary chalcocite-bornite assemblage. Locally, Au is associated with this overlying mineralization. Sericitic, hematitic and chloritic alteration are associated with mineralization and si1icification has occurred in some areas. Sulphides occur as disseminations in breccia matrices, massive sulphide clasts, and less commonly as thin lenses and veinlets. Very fine-grained Uand REE mineralsuraninite, coffinite, brannerite, bastnaesite and florencite - are broadly associated with sulphide zones. The gangue assemblage is dominated by hematite, fluorite, sericite, quartz, siderite, chlorite and barite. Cross-cutting fluorite, barite, carbonate and hematite veining occurs throughout and thin lenses of magnetite are present towards the base of the known sequence. Exploration of the deposit is continuing but insufficient data are available to develop a coherent genetic model.

THE ECONOMIC SIGNIFICANCE OF STYLES OF URANIUM MINERALIZATION THROUGH TIME - AN AUSTRALIAN PERSPECTIVE D.J. Perkin Bureau of Mineral Resources, Canberra, A.C.T. All of Australia's U production in the period 195^-1980, amounting to 13 330 tonnes of U 3 0 8 (11 300 tones U), has been mined from rocks of Early and Middle Proterozoic age. About 60 percent has come from the disseminated magmatic/metasomatictype deposits of Radium Hill, South Australia and Mary Kathleen, Queensland, and the remainder from the Proterozoic stratabound-polymetal1ic class of deposits of the Rum Jungle and South Alligator River fields in the Pine Creek Geosyncline, Northern Territory. A very small quantity (0.2 percent) has been derived from the volcanic/ sediment-hosted vein deposits (Middle Proterozoic) of the Pandanus Creek area, Northern Territory. In comparison, approximately 80 percent of Australia's demonstrated economic resources of U (which total 422 000 tonnes U 3 0 8 ) occurs in Early Proterozoic rocks, about 12 percent occurs in Quaternary deposits, with the remainder fairly evenly divided between Middle Proterozoic, Carboniferous, Cretaceous and Tertiary deposits.

In contrast with production to date, the disseminated magmatic/metasomatic type makes up only 1 percent of Australia's demonstrated economic resources; most of Australia's demonstrated economic resources belong to the Proterozoic strataboundpolymetallic class (79 percent), with the ca1crete-type making up 12 percent, sandstone-type 6 percent and volcanic/sediment-hosted vein deposits 2 percent. The relative proportions of the main classes of U deposits in Australia is different from those in other countries, although examples of all major types are recogn ized.

TRACE ELEMENT AND SULPHUR

ISOTOPE GEOCHEMISTRY OF S U L P H I D E

FROM THE NUMAGEE M I N E ,

COBAR AREA,

ORES

NEW SOUTH WALES

S.R. Sangameshwar, Brian Marshall & G.F.P. Jones The New South Wales Institute of Technology, Broadway, New South Wales The Nymagee Mine is located at about 80 km southeast of Cobar, N.S.W. The stratiform Cu-Zn-Pb sulphide mineralization occurs within a sequence of siltstones, sandstones and grits which are a part of the Late Silurian-Early Devonian Cobar Supergroup. Metamorphism within the Cobar Supergroup, and particularly in the vicinity of the Nymagee Mine, is of lower greenschist facies (slate-grade). The sulphides exhibit dominantly ductile deformation in response to metamorphism. Concentrations of Mn and Co in pyrrhotite and Mn and Cd in sphalerite show systematic variations from footwall to hanging wall of ore zones. Geothermometric estimates based on the partition of Mn and Cd in co-existing galena and sphalerite indicate anomalously high temperatures (average of ^90°C and 500°C respectively), in contrast to those obtained from partition of sulphur isotopes in co-existing galena, r z sphalerite and pyrrhotite (average ^3 *0°C). It is concluded that this discrepancy is due to lack of equilibrium partitioning of Mn and Cd in co-existing sulphides.

THE GEOLOGY OF THE THALANGA VOLCANOGENIC S U L P H I D E

DEPOSIT,

NORTH QUEENSLAND P.W. Gregory & J.S. Hartley Penarroya (Australia) Pty Ltd, Sydney, New South Wales The Thalanga Zn-Pb-Cu-Ag sulphide deposit is located approximately 60 km southwest of Charters Towers in northeast Queensland. The mineralization occurs within the Cambro-Ordovician Mt Windsor Vol canics, a sequence dominated here by rhyolitic and dacitic pyroclastics. Both the mineralization and host rocks have been metamorphosed to greenschist facies and have undergone at least two tectonic deformat ions. A bedded and locally slumped, large lens of massive to semi-massive sulphides constituting the main lens strikes east-southeast, dips steeply and pitches at about 12° west-northwest. The main lens mineralization occurs within hydrothermally altered rhyolitic pyroclastics close to their upper contactwith essentially unaltered dacitic pyroclastics. Several small lenses of sulphides and a barite-rich lens are also known. Carbonate, chlorite-actinolite, sericiteand barite are intimately associated with the sulphide lenses. Elsewhere these minerals may form essentially barren exhalites. The strike continuation of mineralized horizons is often represented by thin chert-magnetite exhalites. The stratiform mineralization (and gangue) is broadly zoned in a repetitive fashion from a pyrite-cha1copyrite base through to sphalerite-galena. Zoned units vary from 0.1 m to several metres in thickness and reflect different pulses of ore fluid. Rhyolitic pyroclastics including quartz "eye" tuffs, are intercalated with the sulphides in some places and have either diluted or swamped out the sulphide depos i t ion. Extensive wall rock a l t e r a t i o n is dominated by s i 1 i c i f i c a t i o n and s e r i c i t i z a t ion with associated disseminated and vein pyrite and chalcopyrite. Sericite, c h l o r i t e , a c t i n o l i t e , b i o t i t e , t a l c , epidote and carbonate with associated galena, sphalerite and pyrite a l s o occur.

The main lens of stratiform sulphides is thought to have deposited syngenetically under submarine conditions from several exhalative centres now represented by the silicified and sericitized wallrock alteration. The hydrothermal alteration of the rhyolitic pyroclastics was associated with the periodic ascent of the hot ore fluids to these sea-floor centres.

PRELIMINARY REPORT ON CONCORDANT BARITE AND BASE-METAL SULPHIDE DEPOSITS IN PROTEROZOIC EINASLEIGH METAMORPHICS OF THE WERRINGTON AREA, GEORGETOWN INLIER, NORTHEASTERN QUEENSLAND Brian Oversby Bureau of Mineral Resources, Canberra, A.C.T. Multiply-deformed amphibolite to granulite facies Einasleigh Metamorphics occur in the lowermost exposed Etheridge Group, being partly lateral equivalents of shallow-water Bernecker Creek and Robertson River Formations. At Werrington, the unit contains three intergradationa1 lithosomes, predominantly (l) biotite-rich, (2) hornblende-diopside-rich, and (3) quartz-feldspar-rich. Compositional bands probably reflect original stratification. Amphibolites, apparently mostly representing sills, are not abundant. Recrystal1ized barite lenses are concordant, and presumably antedate the first phase of deformation and metamorphism. They are concentrated near lithosome boundaries, and locally associated with disseminated to massive recrystal1ized sulphide (pyrite-pyrrhotite, with variable chalcopyrite, sphalerite, galena and molybdenite) concentrations, and quartz-magnetite or quartz-garnet rocks, all of comparable concordant- lensoidal form. No stringer-type sulphides, or conspicuous alteration zones are known. 6 3lf S contents of barite and chalcopyrite (about +11.5%o ar| d + ^«3°/oo, respectively) probably reflect metamorphic equilibration. These Werrington deposits are similar to others farther north in Einasleigh Metamorphics, although the type is apparently absent from laterally equivalent units. All have features suggesting a stratabound-stratiform nature, felsic volcanogenicexhalative origin, and distal position. Good evidence for an appropriate volcanic component in host rocks is lacking, and any centre of vol can ism was presumably east (or possibly west) of present outcrops.

SYSTEMATICS OF SUPERGENE ALTERATION OF ZINC-LEAD-SILVER SULPHIDES IN CARBONATE TERRAINS S.R. Sangameshwar Department of Applied Geology, New South Wales Institute of Technology, Broadway, New South Wales The stabilities and distribution of mineral assemblages formed during supergene alteration and enrichment of polymetallic sulphide ores in carbonates have been evaluated thermodynamica11y. Eh-pH relations are examined for assemblages of the Burgin Mine, East Tintic district, Utah at the 60°C of the local subsurface waters, and at 25°C for the Tynagh Mine, Ireland, and Matagente Deposit, Cerro-de-Pasco, Peru. The distribution of the supergene assemblages in these deposits is control 1ed by initial mineralogy, composition of groundwaters and permeability of the terrains. The supergene products at Burgin Mine exhibit simple vertical zonation. Both Tynagh and Matagente Mines exhibit complex zoning of oxidation products. The simple vertical zoning is caused by alteration of primary minerals at pHs fixed by carbonate buffering and Eh variable but decreasing with depth. Typical of initially pyrite-rich ores in permeable carbonate rocks are assemblages including anglesite, cerussite, smithsonite excluding hydrozincite, and hydrocerussite. Complexly-zoned assemblages result from oxidation of primary ore at alkaline pH values, above carbonate-buffered pHs in less permeable rocks. These are characterized by either of two assemblages: anglesite, cerussite and smithsonite or hydrocerussite, cerussite, hydrozincite and smithsonite. Cerargyrite and/or native silver is a common oxidation product of primary ore in both of these assemblages.

HYDROCARBON GENERATION AND SEISMIC STRATIGRAPHY

THE GENERATIVE BASIN CONCEPT Gerard

Demaison

Chevron Overseas Petroleum, San Francisco, California, U . S . A . Subsurface geochemical mapping can help grated

lower exploration

risk by being

inte-

into:

(1) Regional basin evaluation. Geochemistry, when mapped together with geology and g e o p h y s i c s , m a y be applied as a tool o f early g e o g r a p h i c a n d g e o l o g i c delineat i o n o f z o n e s o f h i g h s u c c e s s r a t i o s ( p e t r o l e u m " p l a y s " o r p e t r o l e u m z o n e s ) in s e d imentary basins. F o r i n s t a n c e , m a t u r a t i o n a n d o r g a n i c f a c i e s m a p s f o r t h e UpperJ u r a s s i c , in t h e N o r t h S e a ( U . K . a n d N o r w a y ) d e l i n e a t e t h e o i 1 g e n e r a t i v e b a s i n w h i c h b r o a d l y c o i n c i d e s w i t h t h e z o n e o f h i g h s u c c e s s r a t i o s (about 1 in 3 ) in f i n d i n g o i l . O t h e r examples from several geochemically d o c u m e n t e d basins a r o u n d the w o r l d , including A u s t r a l i a , s t r i k i n g l y d i s p l a y i d e n t i c a l d i s c r i m i n a t i v e p a t t e r n s in t e r m s o f r i s k r e d u c t ion. (2) Prospect appraisal. Measurable geological-geochemical parameters, such as generation timing, type of source b e d , effective (mature) source bed thickness a n d s i z e o f d r a i n a g e a r e a , p r o v i d e q u a n t i f i e d g e n e r a t i o n - m i g r a t i o n input f o r r e a l i s t i c p r e d i c t i o n o f p o t e n t i a l p e t r o l e u m r e s e r v e s p r i o r to d r i 1 1 i n g . A p r e v i o u s l y p u b l i s h e d e x a m p l e o f a p r o b a b i 1 i s t i c , c a l i b r a t e d p r o s p e c t a p p r a i s a l s y s t e m , t a k i n g into a c c o u n t t h e s e k e y g e o c h e m i c a l p a r a m e t e r s , is b r i e f l y r e v i e w e d . I n t e g r a t i o n o f g e o c h e m i c a l d a t a w i t h g e o l o g y a n d g e o p h y s i c s into a t o t a l h a b itat o f o i l m o d e l , w h e t h e r a t t h e e c o n o m i c p r o s p e c t a p p r a i s a l l e v e l p r i o r t o d r i l l i n g , o r by r e g i o n a l m a p p i n g o f g e n e r a t i v e b a s i n s , leads t o d e l i n e a t i o n o f l o w r i s k fairways for high-success exploration. Reducing risk, thus improving chances o f s u c c e s s in e x p l o r a t i o n , s h o u l d b e t h e u l t i m a t e e n d - p r o d u c t o f a l l s u b s u r f a c e g e o chemical investigations.

MOLECULAR GEOCHEMISTRY - BIOMARKERS AND MATURITY

INDICATORS

R. Alexander, R . I . Kagi £ G . W . Woodhouse Department of Chemistry, Western Australian Institute of T e c h n o l o g y , Bentley, Western Australia M o l e c u l a r t r a n s f o r m a t i o n s w h i c h o c c u r in o r g a n i c - r i c h s e d i m e n t s p r o v i d e a b a s i s f o r i n t e r p r e t i n g t h e t h e r m a l h i s t o r y o f h y d r o c a r b o n s f o u n d in s e d i m e n t s . The branched/cyclic hydrocarbon fraction of sediment extracts were isolated a n d analysed by g a s c h r o m a t o g r a p h y - m a s s s p e c t r o m e t r y to p r o v i d e i n f o r m a t i o n a b o u t t h e c o m p o s i tion o f their stranes a n d terpanes. The changing composition of these molecular types has been shown to reflect the increased depth o f burial (temperature) o f the sediments. Results which have been obtained using these techniques o n samples from the P a r i s B a s i n a r e r e v i e w e d a n d c o m p a r e d w i t h r e s u l t s f r o m s i m i l a r m e a s u r e m e n t s made o n samples from the Carnarvon Basin o f Western A u s t r a l i a .

DEFINITION AND DEVELOPMENT OF THE MACKEREL FIELD, GIPPSLAND BASIN 1

Alan R . L i m b e r t , David M . M a u g h a n 1 2

& A . John

Esso Australia L t d , Sydney, N e w South Esso Interamerica

Mebberson

Wales

T h e E s s o - H e m a t i t e M a c k e r e l o i l f i e l d is l o c a t e d a p p r o x i m a t e l y 7 0 k m o f f s h o r e in t h e G i p p s l a n d B a s i n , s o u t h e a s t e r n A u s t r a l i a . It is o n e o f a n u m b e r o f f i e l d s in t h i s b a s i n in w h i c h t h e r e s e r v o i r o c c u r s b e n e a t h a n u n c o n f o r m i t y a t t h e t o p o f t h e L a t r o b e G r o u p w h i c h is o f P a l a e o c e n e - E o c e n e a g e . T h e t r a p h a s b e e n f o r m e d w i t h i n a topographic erosional feature, sealed by calcareous shales a n d mudstones o f t h e Oligocene Lakes Entrance Formation.

15 This paper covers two topics. The first is the history of, and problems related to, the def inition of the irregularly eroded Latrobe surface. In particular, the use of migrated seismic data to define the top of Latrobe is discussed. The second topic of the paper is the use of seismic stratigraphy to help plan the development of the field. Progradationa1 seismic character in the Latrobe resservoir section is observed on the seismic data at Mackerel. Seismic stratigraphic work was integrated with the results from the four exploration wel1s to bui1da model of the reservoir section prior to field development. This model was then used to help plan the development well programme. Finally, the results of development drilling to date are discussed and an evaluation made of the pre-development geophysical and geological interpretation.

CONDENSED SEQUENCES IN MESOZOIC AND CAINOZOIC SHELF SEDIMENTS REVEALED BY DETAILED SIDEWALL CORE SAMPLING Steve Con ley Esso Australia Ltd, Sydney, New South Wales Detailed sidewall core sampling in offshore petroleum exploration wel 1 s dri1 led by Esso Australia Ltd over the past few years has shown that condensed sequences are common in the Mesozoic and Tertiary strata that underlie the continental shelves around Australia. The condensed sequences are most commonly (but not exclusively) found immediately above seismic sequence boundaries and major unconformities. Other unconformities or disconformities have been mistakenly identified, or thei r duration unnecessarily expanded through the non-recognition of the presence of a condensed sequence. The importance of good sampling to micropalaeontology and biostratigraphy is exemplified in the Deep Sea Drilling Project, the notable success story in biostratigraphy over the past decade. Its drilling programme has led to vast improvements in the microfossil zonal schemes, particularly for planktonic foraminifera, coccoliths and siliceous microfossi1s. The advances in biostratigraphy are partially due to the access to the stratigraphically more complete deep water sequences, repeated testing of zone schemes around the world, but the most important factor is the quality, quantity, and positioning of the wireline cores collected by the Deep Sea Dri11ing Project. The past twenty years has also seen extensive dri11ing of accessible continental shelves of the world by the petroleum industry. Unfortunately, in spite of all the exploration and drilling, micropalaeontological and palynological zonations that are generally available in petroleum wells do not compare favourably with the sophistication in biostratigraphy obtained from the Deep Sea Drilling Project. The reason is due to a lack of good quality samples. In petroleum wells, the most readily available samples, and often the only samples, are drill cuttings, while there are never enough conventional cores or sidewall cores. Of course, it is unlikely that the petroleum industry will ever cut large numbers of conventional cores for palaeontology, but it is possible to get equivalent results for micropalaeontology and palynology from extensive sidewall core programmes. In most cases sidewall cores are actually preferable to conventional cores because they are taken at the end of the electric logging runs when they can be positioned using the electric logs to solve specific problems. At Esso Australia Ltd, the palaeontologists have had significant input to the writing of the sidewall core programmes for exploration wells. The improved biostratigraphy resulting from the sidewall cores has enabled the palaeontologists to demonstrate, on many occasions, their usefulness and importance in solving geological problems. A consequence of this has been a progressive increase over the past ten years in the average number of sidewall cores taken in each exploration well. Illustrations are given of the type of biostratigraphic and geological problems that can be both recognized as well as solved. Ideas about the nature of unconformities and pal aeontological zonations which have evolved and been tested over several years by detailed sidewall core sampling are also discussed.

UPPER TRIASSIC DEPOSITIONAL ENVIRONMENTS^ CENTRAL EXMOUTH PLATEAU (PERMIT WA""8^"P)j NORTHWESTERN AUSTRALIA R.G. Vos £ C.M. McHattie Phillips Australian Oil Company, Perth, Western Australia Upper Triassic deposits within Permit WA-84-P on the Exmouth Plateau, off northwestern Australia, comprise a thick progradational f1uvial-deltaic clastic succession overlain by a thin transgressive sequence of nearshore and shelf fine-grained clastics and carbonates. The Carnian-Norian section, greater than 3 000 m thick, is dominated by deltaic plain facies including prodelta, distributary mouth bar, distributary/fluvial channel and levee, interdistributary bay, crevasse splay, and coal swamp. Upper deltaic plain/alluvial plain facies predominate in the eastern Permit area, and include point bar, levee, overbank, floodplain, and coal swamp facies. The thickness and nature of prodelta to coal-swamp sequences indicate deltaic progradation into low energy, shallow seas. The Rhaetian to Lower Jurassic transgressive sequence comprises 100-300 m of distal deltaic clastics overlain by 20-50 m of carbonates, demonstrating decreasing clastic sedimentation over the Permit area. Deltaic progradation was in a general northwesterly direction as reflected in the westward decrease of distributary/fluvial channel deposits and corresponding increase in distributary mouth bar and minor reworked sandstones. The marine transgression occurred in a general southeasterly direction as detrital input from the east diminished. This is documented by limestone development in Rhaetian time over the central Permit area, and in Hettangian time over the eastern Permit area.

GLACIAL

SEDIMENTATION

MIOCENE AND PLEISTOCENE MARINE AND GLACIOMARINE SHELF SEDIMENTATION, MCMURDO SOUND, ANTARCTICA P.J. Barrett 1 2

£ B.C. McKelvey

Victoria University of Wellington, New Zealand University o f New England, Armidale, New South Wales

S i t e d o n 2 m o f ice o v e r 196 m o f w a t e r , t h e M c M u r d o S o u n d S e d i m e n t a n d T e c t o n i c S t u d i e s rig p e n e t r a t e d 2 2 6 m o f f o s s i 1 i f e r o u s g l a c i o m a r i n e d i a m i c t i t e s a n d m a r i n e s a n d s t o n e s . M o s t o f t h e c o r e r e c o v e r e d is o f M i d d l e M i o c e n e a g e . A thin P l e i s t o c e n e sequence o v e r l i e s this d i s c o n f o r m a b l y . T h e petrology o f the core reflects c o n t e m p o r a n e o u s b a s a l t i c v o l c a n i s m ; t h e s t r i p p i n g in t h e T r a n s a n t a r c t i c M o u n t a i n s o f D e v o n i a n t o J u r a s s i c c r a t o n i c c o v e r b e d s ; a n d t h e u n r o o f i n g o f t h e late P r e c a m b r i a n a n d l o w e r P a l a e o z o i c b a s e m e n t c o m p l e x . T h e t e x t u r e o f t h e c o r e s u g g e s t s m u c h o f it reflects moderately rapid sedimentation below w a v e base from debris-laden melting ice. C o n s i d e r a b l e d o w n - h o l e v a r i a t i o n in t h e t y p e a n d d e g r e e o f s o f t s e d i m e n t d e formation indicates c o m p a c t i o n , slumping, a n d possible grounding o f expanded polarplateau derived glaciers. Lithification increases irregularly down hole with some highly cemented horizons forming prominent widespread seismic reflectors. Comparison o f the core w i t h the nearby 328 m marine (fjord) core recovered from t h e D r y V a l l e y D r i l l i n g P r o j e c t s i t e 11 s h o w s m a r k e d s t r a t i g r a p h i c a n d f a c i e s d i f f e r e n c e s a n d i n d i c a t e s c o n s i d e r a b l e c o m p l e x i t y in t h e M i o c e n e to P l e i s t o c e n e h i s t o r y of the East Antarctic ice-sheet.

PERMIAN GLACIATION IN PAKISTAN Curt

Teichert

Department of Geological Sciences, University of Rochester, Rochester, New York, U.S.A. E v i d e n c e f o r g l a c i a l c o n d i t i o n s in l a t e P a l a e o z o i c t i m e e x i s t s in t h e S a l t Range a n d the southern half o f the trans-Indus Khisor Range. T h e glacial beds o c c u r in t h r e e f a c i e s b e l t s t h a t s e e m t o b e o r i e n t e d in a g e n e r a l n o r t h - s o u t h to n o r t h e a s t southwest direction: (1) a t i l l i t e f a c i e s in t h e e a s t e r n m o s t S a l t R a n g e a l o n g a b e l t t h a t m a y h a v e b e e n 60-80 km w i d e , but w a s probably narrower; thicknesses are between 5 and 20 m , (2) a freshwater facies o f siltstone and shale, containing only scattered boulders o r n o n e a t a l l , f o u n d in t h e c e n t r a l S a l t R a n g e in a b e l t a t l e a s t 5 0 k m w i d e ; n o accurate figures for thicknesses are known; (3) a m i x e d f a c i e s o f d i a m i c t i t e , s a n d s t o n e , a n d b o u l d e r b e d s , d e p o s i t e d in a n a q u e o u s e n v i r o n m e n t a n d o c c u r r i n g in t h e w e s t e r n S a l t R a n g e a n d t h e K h i s o r R a n g e , in a b e l t t h a t m a y h a v e a m i n i m u m w i d t h o f 5 0 k m ; m a x i m u m e x p o s e d t h i c k n e s s is a b o u t

120 m . The lateral relationships between these facies belts are as yet unknown. The r o c k s o f t h e t i l l i t e f a c i e s a r e k n o w n a s T o b r a F o r m a t i o n , t h e r o c k un i ts o f t h e o t h e r two facies belts a r e as yet unnamed. Most p r o b a b l y , t h e g l a c i g e n e sediments w e r e d e r i v e d from a local c e n t r e o f glaciation that extended from the eastern Salt Range southward to the Kirana Hills between Lahore and Sarghoda. Palynological evidence and associated invertebrate faunas suggest an early Permian age of the glaciation.

LATE QUATERNARY ICE-MARGIN SEDIMENTARY COMPLEXES OF THE OTTAWA-ST LAWRENCE LOWLANDS Victor A. Gostln 1 S Brian R. Rust 2 1 2

Department of Geology, University of Adelaide, Adelaide, South Australia Department of Geology, University of Ottawa, Ontario, Canada

Continental glaciation of low-lying areas often shows facies indicative of marine or lacustrine deltaic outwash and sub-aqueous ice contact deposition formed during ice retreat. In the Ottawa region the withdrawal of the ice was accompanied by incursion of the Champlain Sea between 12 700 and 10 100 yrs B.P. Here, extremely variable ice melting rates periodically produced large volumes of meltwater which discharged through sub-glacial and englacial channels. These were aligned approximately normal to the ice front and were fairly stable in their position. At the subaqueous ice-channel exits,a variety of sediments was deposited rapidly and in pulses into a marine environment. These include thick crossbeds of pebble to boulder gravel, in places free of matrix, unstratified (massive) coarse sand up to 6 m thick, and sands with load, dish, and other soft sediment and liquifaction structures. As sedimentation rates decreased, stratified gravels and pebbly coarse sands, and thick sequences of climbing ripple sets were formed. Later, a succession of debris flows or flow tills slid down the melting surface of the ice front into the sea to cover parts of the earlier sediments. These diamictons are up to 10 m thick and now form the high parts of the landscape. After the main ice front had retreated, but before isostatic rebound dried the Champlain Sea, parts of the sequence were lowered over or against stagnant blocks of ice which melted slowly. The irregular topography caused by this sedimentary faulting was subsequently partly smoothed by wave action of the shallowing sea, and the modified or draped sedimentary complexes now stand as ridges surrounded by younger marine clays.

ANTARCTIC SALINE LAKES: ANALOGUE OF PR0TER0Z0IC EVAPORITE-STROMATOLITE-CARBONATE-TILLITE ASSOCIATIONS? John Bauld & Malcolm R. Walter Baas Becking Geobiological Laboratory, Canberra, A.C.T. The occurrence of dolomite and limestone beds in association with many late Proterozoic tillites has been a source of controversy because carbonate sediments (and stromatolites) are generally considered to be diagnostic of warm water environments. Two examples of stromatolitic dolomite occur, together with barite, at the top of late Proterozoic glacial sediments of the Amadeus and Ngalia Basins of central Australia. It is suggested that this apparent conflict can be resolved when such late Proterozoic glacial sediments are compared to those in Antarctic saline lakes. Carbonate sediments, sulphate evaporites, and even stromatolites are common components of cold arid-climate lakes. Antarctic lakes of high sal inities occupy enclosed glacial basins in both the Vestfold Hills and the Taylor and Wright Dry Valleys. Lake Bonney, the most comprehensively described example of a modern hypersaline lacustrine system in a cold arid climate, contains a large-scale cyanobacterial matevaporite association. These stromatol it ic mats and thei r construct ing cyanobacteria are comparable to those described from warmer climates. Modern hypersaline lacustrine systems from cold arid environments appear to be a useful analogue of Proterozoic ti11ite-carbonate-evaporite-stromatolite associations.

LATE QUATERNARY SEDIMENTATION

IN SOUTHERN NEW ZEALAND

D.H. Bell Department of Geology, University of Canterbury, Christch'urch, New Zealand The combined effects of climate change and active tectonism dominated Late Quaternary sedimentation patterns in southern New Zealand. The Southern A1 ps, whose elevation is a consequence of uplift accompanying transcurrent movements along the Alpine Fault zone, provided both a source of sediment and a snow accumulation area for Pleistocene valley glacier systems. In many of the modern valleys,remnant glacial, f1uvioglacial and glaciolacustrine deposits are preserved, and at least four major glacial episodes are recognized: loess deposits derived principally during glacial advances blanket many of the lowland areas to the east and south of the Main Divide. In a number of deeply incised valleys, extensive mass movement deposits have formed as a consequence of glacial over-steepening and large-scale bedrock failure: subsequent periglacial modification of many of these landslide deposits is also evident. Continuing uplift in many parts of southern New Zealand has preserved interglacial palaeo-shorelines, whilst marine sedimentation patterns have been strongly influenced by lower glacial sea-level stands. In this paper, data from the southern part of the South Island are discussed within a climatically-controlled framework, and the question of sediment supply as a function of uplift rate in the Southern Alps is reviewed. The contrasting sty les of sedimentation are compared for various stages of the glacial-interglacial "cycle", and the record of the last 30 000 years is discussed in some detail.

EVAPORITES AND CHEMICAL SEDIMENTS

THE SIGNIFICANCE OF EVAPORITES IN CARPENTARIAN ROCKS OF QUEENSLAND AND THE NORTHERN

TERRITORY

M . D . Muir C.R.A. Exploration Pty Ltd, Fyshwick,

A.C.T.

Evaporite minerals were abundant in the sediments hosting the major Pb-Zn deposits at McArthur River, Mount Isa, and Dugald River. These evaporites have been replaced by other minerals (dolomite, siderite, calcite, quartz, sericite, microcline, and other feldspars) at different stages during the basins' geological histories. Microscopic flakes and crystals of anhydrite and halite are still preserved in some cases, but morphological and sedimentological evidence provides the main source of information on the former evaporites. Gypsum, anhydrite, and halite pseudomorphs (suite 1) are particularly abundant in the McArthur Basin. Most of the sulphates precipitated displacively in early diagenesis, but replacement anhydrite indicates late burial diagenetic movement of sulphate brines. Two forms of halite occur: one originating in desiccating brine pools; and the other diagenetic. Pseudomorphs after shortite ( N a 2 C 0 3 • 2 C a C 0 3 ) and trona ( N a 2 C 0 3 - N a H C 0 3 - 2 H 2 0 ) (suite 2) occur in the McArthur Group, but are rare. In the Mount Isa a r e a , pseudomorphs after suite 1 and suite 2 minerals occur at intervals, but at Dugald River, suite 2 mineral pseudomorphs predominate. Suite 1 minerals are characteristic of hypersaline environments and could be either marine or lacustrine in origin. Suite 2 minerals are characteristic of alkaline lacustrine environments and can only be non-marine. Sedimentary structures confirm these environmental interpretations. Sabkhas and several types of lacustrine environments can be readily distinguished and indicate specific groundwater regimes at the time of deposition. Replacement of the evaporites on burial diagenesis made large volumes of sulphate available for incorporation into the ore deposits. The Dugald River orebody is sulphur-deficient because of the preponderance of carbonate suite 2 minerals in the total environment. Other differences in the mineralization at McArthur River, Mount Isa, and Dugald River can be explained by invoking differences in the primary and diagenetic waters which gave rise to the distinctive evaporite suites.

EVAPORITE PSEUDOMORPHS AND THEIR ORIGIN IN EARLY PR0TER0Z0IC CARBONATES, PINE CREEK GEOSYNCLINE I.H. Crick 1 S M . D . M u i r 2 1 2

Bureau of Mineral Resources, Canberra, A . C . T . C.R.A. Exploration Pty Ltd, Fyshwick, A . C . T .

Stromatolitic carbonates near the base of the Early Proterozoic succession contain abundant pseudomorphs after evaporite minerals. Pseudomorphs after gypsum are the most common and exhibit bladed and discoidal crystal forms, cruciform and swallow-tail twins. Carbonate pseudomorphs after anhydrite and halite are also present. The carbonates are predominantly magnesite with minor dolomite containing, in places, ghost pseudomorphs after discoidal gypsum, and possibly halite. The pseudomorph textures are generally well-preserved although the Early Proterozoic rocks surrounding the carbonates have been variously metamorphosed up to amphibolite grade. It is suggested that most of the evaporites survived as evaporites during regional metamorphism. Gypsum would have been dehydrated to anhydrite during burial diagenesis, and survived metamorphism as anhydrite before rehydration took place, probably during the Middle Proterozoic. Replacement by carbonate is the final stage in the history of the evaporites and may have taken place soon after rehydration of anhydrite to gypsum, or at some later time.

The presence of abundant evaporite pseudomorphs together with palaeontological and sedimentologica1 evidence suggest that the environment of deposition of the basal Early Proterozoic rocks containing the carbonates was analogous to a sabkha.

PROTEROZOIC AND CAMBRIAN Peter J. Cook 1 2

PHOSPHORITES

& John H. Shergold 2

Research School of Earth Sciences, Austral ian Nat ional Uni versi ty, Canberra, A.C.T. Bureau of Mineral Resources, Canberra, A.C.T.

Many phosphate deposits are believed to be spatially and temporally linked to oceanic upwelling systems and the generation of associated high organic productivity in the water column. The identification of very shallow water conditions in the Cambrian deposits of the Georgina Basin and other deposits provides difficulties for this association which are still to be fully resolved. Local conditions are clearly of fundamental importance in the format ion of Late Proterozoic and Cambrian phosphate deposits, whereas the development of regional phosphogenic conditions is the result of large-scale processes. The formation of the Georgina Basin and other Cambrian phosphate deposits may be in part a reflection of a high phosphorus content in the world ocean around the Precambrian-Cambrian boundary. Not only was this related to the abundance of phosphate deposits at this time but also to the rapid development of organic skeletonization that occurred at the base of the Cambrian. The Late Precambrian-Cambrian phosphogenic event was therefore of fundamental importance to the stratigraphic record.

LOWER PROTEROZOIC MAGNESITES OF THE RUM JUNGLE AREA OF THE NORTHERN

TERRITORY

Y. Bone Department of Economic Geology, University of Adelaide, Adelaide, South Australia The major U provinces of the Northern Territory show an association between the U-bearing sequence and magnesite. At Rum Jungle, the magnesite occurs within the Batchelor Group in the Celia and Coomalie Dolomites. The latter underlies the U and/or base metal-bearing carbonaceous metasediments of the Masson Formation, e.g. Woodcutters (Pb-Zn), Mt Fitch (U-Cu), Browns (Pb-Cu). Some of the magnesite was probably deposited as magnesite in primary carbonate in an evaporite environment, rather than as a gypsum-halite sequence. Diagenetic and metamorphic alteration produced stylolites and coarse recrystal1ization. Talc and chlorite formed by metamorphic reaction between magnesite and diagenetic quartz. Quartz replaced ubiquitous algal remains, e.g. Conophyton, domal and stratiform stromatolites. Concentration of sea water within such an environment would be one possible source of the U and base metals. Unpublished sulphur isotope studies (Donnelly) suggest a marine source for Woodcutters but a mixed marine-magmatic source for the Embayment deposits. Fluid inclusion data on the magnesite and the quartz point to mainly low temperatures and CaCl2~rich brines. The residence and/or passage of metal-bearing solutions through the magnesite are currently being investigated by thermoluminescence measurements, with a view to making comparisons with other Australian magnesites, e.g. Ranger, Balcanoona (Upper Proterozoic), Ski logal lee Dolomite (Middle Proterozoic) and Coorong (Recent).

22 "ZEBRA ROCK" OF WESTERN AUSTRALIA L. Geidans Minatine Aust. Pty Ltd, Subiaco, Western Australia The "Zebra Rock", first observed in the Kimberley Divi s ion of Western Australia, is grey claystone or mudstone with a spectacular regular pattern of red-brown spots and bands. The red spots on the face of the rock section in third dimension are discontinuous rods and the red bands are discontinuous lenses both of which may coalesce. There is no evidence of their chemical origin. Physical tests, chemical analyses and microscopic examinations of thin sections of the red and grey portions of the rock reveal that the horizons where the marking pattern occurs are of about uniform composition except for the presence of iron oxide in the red parts. Megascopic features lead to the conclusion that the origin of the regular pattern is due to gravitational separation of heavier (ferruginous) particles in the ripple-mark troughs of aluminous silty mud. It is demonstrated that variable patterns can be produced by changing the proportions of ferruginous and non-ferruginous particles during continuous growth of the ripple-marked sediment. The geometry of ripple marks and the associated features from world-wide servations are discussed.

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FLUVIAL DELTAIC AND CLASTIC SHELF SEDIMENTS

AN ALLUVIAL FAN - FLUVIAL PLAIN DEPOSITIONAL MODEL FOR THE DEVONIAN W I L L A R A D D I E FORMATION AND MUNABIA SANDSTONE OF THE CARNARVON B A S I N , WESTERN AUSTRALIA H.T. Moors Geological Survey of Western Australia, Perth, Western Australia The original concept of the Willaraddie Formation and Munabia Sandstone was of an unrelated sequence of marine clastic sediments. Furthermore, the coarse clastics were regarded as shoreline sediments which rapidly changed to an argillaceous basinal facies, severely restricting their potential as good, extensive reservoirs. Recent field work suggests that the two formations are in fact terrestrial deposits, genetically related, and at least in part lateral equivalents. The Munabia Sandstone consists of fining-upwards cycles of sand, wi th eros ional bases, completely devoid of all but plant fossils, and with sequences of sedimentary structures suggestive of a fluvial origin. The Willaraddie Formation consists of very immature sandstone beds and matrix-supported conglomerates, wi th sedimentary structures and organization suggestive of a mid- to distal-alluvial fan origin. Palaeocurrent directions in the two formations indicate a northerly palaeoslope. A combined depositional model for the two formations consists of a northerly-plunging graben containing a fluvial system (Munabia Sandstone), over which an alluvial fan complex (Willaraddie Formation) periodically prograded from the faulted eastern ma rg i n . This model makes the Munabia Sandstone distribution more extensive than viously believed, upgrading its importance as a reservoir target.

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FLUVIAL SEQUENCE IN THE TRIASSIC OF TASMANIA N.R. K e m p 1 , J.W. Collinson 2 S J . T . Eggert 2 1

T a s m a n i a n M u s e u m , Hobart, Tasmania 2 Department of Geology and Mineralogy, Ohio State University, Columbus, O h i o , U.S.A. The Triassic System in Tasmania consists of more than 1 000 m of fluvial sandstone and shale. These rocks were deposited by a major river system that flowed eastward across Tasmania toward the Pacific margin of Gondwana. The Triassic System can be divided into two sequences, a lower quartzo-feldspathic sandstone interbedded with greenish-grey to reddish-grey mudstone (e.g. Ross and Cluan Sandstones) and an upper volcaniclastic sandstone interbedded with carbonaceous shale and some coal {e.g. Brady and Tiers Formations). The lower sandstones are characterized by largescale trough and planar cross-stratification that consistently indicates dispersal toward the east. This lower sequence is interpreted as representing a s y s t e m o f low sinuosity streams that drained local highlands in western Tasmania and the contiguous Victoria Land segment of Gondwana. Abundant first cycle q u a r t z , microcline, orthoclase and plagioclase grains together with minor plutonic and schistose lithic fragments in these rocks suggest granitic and metamorphic source areas. The upper sequence, which is characterized by fining-upward cycles of sandstone to carbonaceous s h a l e , indicates meandering streams on a muddy floodplain with numerous lakes and marshes. Insufficient data were gathered in the upper sequence to determine palaeos l o p e , but abundant volcanic detritus of andesitic composition in sandstones suggests the presence of an additional source a r e a , possibly volcanoes, along the Pacific margin of Gondwana.

FLUVIATILE THE TUMBLAGOODA

SEDIMENTATION SANDSTONE

IN A C O A S T A L

SETTING;

(SILURIAN), WESTERN

AUSTRALIA

R.M. Hocking Geological Survey of Western Australia, Perth, Western Australia The Tumblagooda Sandstone is extensively and continuously exposed only in its 1 000 m thick type section, extending 50 km downdip in the lower Murchison River gorge, southern Carnarvon Basin. Here, deposition occurred in two major phases.^ Sedimentation commenced in a medium- to high-energy, sheet-braided fluviat i le envi ronment, with a southeasterly provenance. Both in time and away from the source (to the northwest), sediment supply decreased, marine influence increased, and a largely tidal, medium-energy marine environment with frequent emergence became dominant. Palaeocurrents were dominantly southerly, parallel to the shoreline, reflecting a long, narrow, north-south basin shape. This was abrupt 1 y stopped by the second phase, in which low sinuosity, sheet-braided fluviatile sandstones were deposited. They are characterized by fining-upward cycles 4 to 15 m thick andal so have a southeasterly source. Many cycles are topped by bioturbated beds and Skolithos is locally common, indicating a shoreline position with brief periods of marine dominance. This episode shows signs of waning at the (erosional) top of the type section. Cyclicity is present at two scales. Fining-upward cycles characterize both fluviatile episodes, and were formed by lateral shifting of sediment bars. Twomuch larger fining-upward cycles are formed by the two phases of sedimentation, and are interpreted as major progradationa1 episodes. These can be interpreted as deltaic, but differ from other delta models in that the sands prograded rapidly into a shallow, gently subsiding epicontinental sea as wide sheets rather than lobes. Conse1 1 quently fluviatile and tidal flat sediments interfinger , and prodel ta sediments did not develop.

STRUCTURAL AND SEDIMENT0L0GICAL DELTAS:

RELATIONSHIPS

IMPLICATIONS TO HYDROCARBON

IN A N C I E N T

SHELF

EXPLORATION

D.K. Hobday & M.B. Edwards Bureau of Economic Geology, University of Texas at Austin, Texas, U.S.A. The structural configuration of the receiving basin exercises fundamental control over delta morphology and sandstone reservoir geometry. Differences between delta systems located on a stable shelf or craton and those which are located near the shelf edge can mask the effects of energy flux in the receiving basin. Structural factors being equal, deltas fashioned by fluvial processes and subject to negligible remolding by marine processes tend to assume an elongate form. With progressive increase in the significance of marine reworking, a lobate, then arcuate shape is characteristic. A distinct geometry of framework sands is associated with each of these morphological types, and many ancient stable-shelf del ta deposits have been successfully interpreted in terms of this energy- or geomorphologically-based classification. Major Cainozoic units of Texas show, however, that shelf-edge delta deposits commonly do not conform to this scheme. The Eocene Rosita deltas in the upper Wilcox Group of south Texas comprise thick, strike-parallel sands in an expanded, unstable, shelf-edge setting. The prominent strike trend was imparted not only by growth faulting, but was accentuated by the greater effectiveness of marine processes on the outer platform. Similarly in the Oligocene Frio Formation, growth-faulting and associated diapirism were the dominant control over sandbody geometry along much of the Texas coast. Knowledge of the structural setting in ancient deltaic deposits, in conjunction with the established geomorphically-based models, can be particularly valuable in hydrocarbon exploration.

WAVE-DOMINATED SHELF SEDIMENTATION - A MODEL FROM THE PERMIAN OF THE CARNARVON B A S I N , WESTERN AUSTRALIA P.S. Moore 1

& R.M. Hocking

Geological Survey of Western Australia, Perth, Western Australia Present address: Delhi Petroleum Pty Ltd, Adelaide, South Australia

The Early Permian (Artinskian) Byro Group is a sequence of shale, siltstone and sandstone which crops out in the southeastern Carnarvon Basin, Western Australia. The sedimentology of the group can be expressed in terms of six facies which represent deposition on, and adjacent to, a marine shelf. The facies and their environments of deposition are: (1) black-shale facies (lower offshore environment); (2) grey-siltstone facies (upper offshore environment); (3) bioturbated-sandstone facies (transitional environment); (4) 1 aminated-to-burrowed facies (lower shoreface environment); (5) hummocky-cross-stratified sandstone facies (middle shoreface environment); and (6) trough-cross-stratified sandstone facies (upper shoreface environment). The facies reflect a balance between wave processes, which predominate in the shoreface zone, and biogenic processes, which predominate below effective wave base in the offshore and transition zones. The shoreface to offshore transition is very well developed in the 1 660 m thick Byro Group, and suggests that the sequence was developed in a wave-dominated environment, and that a great deal of sedimentation occurred as a result of storms. Laminated sandstone interbeds in the offshore facies lend support to the hypothesis suggested by some workers that distribution of sand below wave base is primarily by offshore-directed storm-induced dens i ty currents. No evidence has been found of tidal or oceanic current redistribution of sand in the zone below wave base.

DEPOSITIONAL ENVIRONMENTS OF THE DEVONIAN ROCKS OF THE DARLING BASIN - PROSPECTS FOR PETROLEUM V.L. Passmore Bureau of Mineral Resources, Canberra, A.C.T. The Darling Basin in western New South Wales contains up to 7 000 m of Late Silurian to Early Carboniferous clastic sediments. These sediments are preserved in a series of troughs that are largely concealed beneath a veneer of Mesozoic and Tertiary sediments. The results of recent Bureau of Mineral Resources drilling and source rock studies are incorporated with previous investigations to modify earlier depositional models for the Early Devonian sequence and to further evaluate the petroleum potential of the basin. Recent studies of sediments and fauna suggest that, during the Early Devonian, fine-grained clastic sediments (Amphitheatre Formation), derived from a westerly source, were deposited in an open epicontinental sea that extended from the Cobar Trough in the east to the Lake Wintlow and Wilcannia Highs in the west. Abasinwide regression, evidenced by a change from open mar i ne to nearshore and paralic sediments, occurred in the Pragian. The eastern edge of the basin was strongly deformed in the 1 ate Early Devonian, during the Tabberabberan Orogeny, but farther west there is evidence of only minor uplift, folding, and faulting, and of local discordance between units. Late Early Devonian to Early Carboniferous fluvial sediments, that were deposited during or after diastrophism, blanketed the basin, burying the earlier, marine sediments beneath a sedimentary pile up to 5 000 m thick in some parts of the basin. Geochemical analyses indicate a lack of source-rock potential for all units except the Early Devonian Amphitheatre Formation which contains sediments with a source-rock rating ranging from fair to lean. Maturation studies indicate that these rocks are over-mature to mature. Deep burial of this sequence by pre-Permian sediments and the lack of significant additional post-Carboniferous overburden or tectonism suggest the possibility of early maturation and migration of hydrocarbons.

FACIES AND B A S E - M E T A L C O N C E N T R A T I O N S OF LATE DEVONIAN

FLUVIAL

D E P O S I T S FROM THE SOUTH COAST OF NEW SOUTH WALES,, A U S T R A L I A Graham T a y l o r 1 , Ken Maiden 2 & Wolf Mayer 1 1

Department of Geology, Canberra College of Advanced Education, Canberra, A.C.T. 2 Department of Geology, University of the Witwatersrand, Johannesburg, South Africa The Late Devonian Worange Point Formation is a cyclic fluvial sequence, more than 420 m thick, consisting of rocks which range from conglomerates to mudstones. The conglomerates are largely composed of quartz pebbles and intraformational shale clasts. The sandstones are feldspathic with a high matrix content and with both siliceous and carbonate cements. The mudstones are composed of a mixture of quartz and clay. While the sand/mud ratio varies considerably (1.1-4.9), the nature of the facies present in this sequence does not change significantly. Six facies have been recognized, and most are present in all cycles: (1) A basal conglomerate, often containing intraformational shale clasts and quartz pebbles. The shale clasts are up to 25 cm long and occasionally imbricate. The base of each unit is irregular and erosional. (2) A coarse to medium sandstone, massive to plane laminated or with large-scale trough and/or planar cross-lamination which frequently shows evidence of slumping. Shale clasts and scattered quartz pebbles are sometimes present. Where facies 1 is absent, facies 2 has an erosional base. (3) A medium to fine sandstone with small-scale, trough cross-lamination passing upwards into plane and wavy and/or ripple cross-lamination. Ripple-drift cross lamination, slumped bedding and other wet-sediment deformation features occur occasionally. Facies 1-3 are mostly cream, greenish cream or purplish cream in colour. Facies 3 is largely colour banded parallel to the bedding. (4) A coarse siltstone to fine sandstone with a typically green colour. The facies is plane laminated with ripple-drift cross-lamination and deformation structures caused by dewatering of the sediments. (5) A red siltstone unit showing plane and wavy lamination and thin ripple crosslaminated beds which frequently contain sand-filled or calcite-fi1 led burrows. The facies occasionally grades vertically upwards into pa 1 aeoso 1 s which show typical mottling, clay coatings, root tubules? and, sometimes, calcretes. (6) Medium- to fine-grained sandstone similar in all aspects to facies 3 but curring within facies 5«

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The above sequence is interpreted as a mixed-load meandering stream facies with chute deposits (facies 6) in the overbank sediments, perhaps indicating large ranges of discharge. The green coloured facies 4 are interpreted as abandonedchannel fills or flood-basin lake deposits. The streams which depos i ted the sequence were large, with channel depths at times in excess of 12 m . Samples from the above facies have been chemically analysed for U, Cu, Pb, Zn, Mn, and V. High values for Cu are associated with facies 1 and, to a lesser'extent[ facies 4. Concentrations are highest at the base of quartz-pebble conglomerates (facies 1) where they rest on the floor of eroded channels. The distribution of the other elements analysed is less well defined.

COAL MEASURES SEDIMENTATION

GEOLOGY AND COAL RESOURCES OF THE MERIT-PILA B A S I N , S A R A W A K , EAST MALAYSIA Col in R. Ward Department of Applied Geology, New South Wales Institute of Technology, Broadway, New South Wales Potentiallv workable beds of low-ash, high-vitrinite sub-bituminous coal occur in an extensive belt of Miocene terrestrial sediments in the Merit-Pi la basin of Central Sarawak. Due to its geographic setting, exploration in this area has been based on somewhat different techniques to those commonly used in Australia. Field mapping of individual coal seam outcrops has been used, supported in pi aces by dr i 1 ling with man-portable rigs, to delineate the structure and distribution of major coal seams. The coal-bearing sediments have a relatively complex lithofacies pattern. They are interpreted from the data currently available to be essentially of fluvial origin, although marine beds are reported in units of similar age further to the north. The structure of the area is likewise variable, and appears in parts of the basin to control lithofacies trends and coal seam development. Geological studies in the area are continuing with a view to the possible establishment of mining operations.

PALYNOSTRATIGRAPHY OF THE LATE PERMIAN COAL SEQUENCES OF THE SYDNEY BASIN A . McMinn Geological Survey of New South Wales, Sydney, New South Wales Four palynological zones can be recognized in the Late Permian coal measure sequence of the Sydney Basin. These zones are based on the first occurrences of the spores Dulhuntyispora parvithola, Secarisporites bullatus and Microreticulatisporites bitriangularis and the acritarch Mehlisphaeridium fibratum. Recognition of these zones enables correlations to be made from south to north along the western margin of the Sydney Basin between Lithgow through Rylstone to Ulan and w e s t t o e a s t from Lithgow through the Hunter Valley to Cooranbong and Terrigal. This investigation also demonstrates that many of the major coal seams are inter-regiona11y diachronous whereas the major marine incursions are apparently isochronous across the bas in.

COAL-FORMING AND ASSOCIATED ENVIRONMENTS IN THE PERMIAN DEN I SON T R O U G H , QUEENSLAND P.J.G. Fleming Geological Survey of Queensland, Brisbane, Queensland The Geological Survey of Queensland is conducting an integrated bas in analysis in the Denison Trough, using data from outcrop and from 30 fully-cored boreholes. The facies relationships of coal and hydrocarbon occurrences in the marine and nonmarine succession of the Trough are being investigated. The lowermost, predominantly non-marine unit, the Reids Dome Beds, has so far been studied. The unit was deposited over irregular basement topography caused by faulting, and has a maximum thickness of more than 2 800 m . Twelve sedimentary environments have been determined by lithofacies analysis. The distribution of these environments indicates that the major depositional systems of alluvial fan, braided stream, meandering stream, lake, delta, and marginal marine are recognizable in the area.

Initial studies of the distribution and preservation of plant megafossils indicate broad correlations with depositional systems, although more detailed work on facies associations is proceeding. The biostratigraphy of the Reids Dome Beds is being provided by detailed analyses of plant microfossil assemblages. Coals formed in all depositional systems except lakes. All coal occurrences are being sampled, and sedimentary and palaeobotanical analyses of the coals indicate that their compositions can be correlated with sedimentary environments.

IDENTIFICATION OF COLLIE COAL SEAMS BY M A C E R A L A N A L Y S I S V.J. Moran Western Colliery Research Laboratories, Collie, Western Australia The Collie coal measures, although occurring in a well-defined and well-known granite basin, are topographically complex. The coal is remarkably rich in structured components and a study was made of the maceral analysis to determine whether that property could be used to assist correlations between boreholes. About 300 samples from boreholes and mines were analysed by maceral counting and it is shown that the oldest seams, those in the Collie member, are markedly different to the more recent ones in the Collieburn and Cardiff members, having mean vitrinite contents of 25% and 63% respectively. This has proved to be a useful and positive means of identification. Attempts to differentiate between seams within each of the three members were only partially successful where their occurrence is flat and extensive; but in one case where five seams occur in a localized depression, measuring 2 x 6 km, confident identifications could be made.

APPLICATION OF SEDIMENTARY FACIES STUDIES IN COAL EXPLORATION AND MINE DEVELOPMENT P.G. Flood Department of Geology, University of New England, Armidale, New South Wales During the past decade, marked advances have been made in our understanding and ability to visualize the conditions of coal measure sedimentation. Interest now is focusing on the nature and geological significance of the interseam sediments which are mainly coarse conglomerates to fine-grained sandstones, and occasional siltstone and carbonaceous mudstone intercalations. It is the apparently unpredictable spatial arrangement of these same sediments which produces difficulties during mine development when they are encountered by draglines removing coal overburden and which often produce roof and floor instabilities underground. Conceptual models have been formulated for the environments of deposition of coal measure sediments of Permian, Triassic and Jurassic ages which occur in various sedimentary basins in Queensland. These models are based upon detailed analyses of highwall and core information. Recognition criteria have been established to distinguish different depositional processes and the study of the vertical profiles illustrate the changing patterns of sedimentation with time. Used together with knowledge of the arrangement of facies associated with different sedimentary systems it has been possible to predict the spatial behaviour of the physical and chemical properties of both intra- and inter-seam sediments. It has also been possible to determine the dimensions of several of the fluvial palaeochannels which were responsible for the sedimentation, thereby enhancing the prediction of the trace Of the various morphological features and associated sedimentary deposits from information obtained from very few boreholes. Undoubtedly this approach represents a major cost-saving benefit amount of exploratory drilling can be reduced considerably.

because

the

T H E L A T E R A L M I G R A T I O N OF S E D I M E N T A R Y C H A N N E L S A N D T H E P R E D I C T I O N OF STRIP M I N I N G O V E R B U R D E N

CONDITIONS

N.H.H. Godfrey Utah Development Co., Moranbah, Queensland The Late Permian Moranbah Coal Measures at Goonyella Mine consist of volcanolithic channel and overbank deposits derived from low sinuosity stream systems in a fluvio-deltaic environment. Lateral migration of such channels over peat has involved a successive "roll down" of adjacent peat, in advance of channel encroachment, to a situation of partial compaction under channel sediments. Overbank sediments between peat top and channel flank accompany the peat surface down, to an increasing degree awayfromthe channel and are thus constrained to dip successively in the channel migrat ion di rection. The same bedding relationships are preserved and seen in the Goonyella mine highwall today. Thin, clayey overbank beds, subjected to the above process, act as potential basal failure planes for large highwall failures when dipping pitward. Their location, extent and attitude should be determined. Additiona1ly, blasting, bucketwheel excavation and spoil stability considerations favour downdip prediction of channel arenite location. The lithology and dip direction of the lutites is used as a guide to channel arenite proximity and location. A combination of all this information is then used to indicate "provinces" of lutite dip direction and, correspondingly, potential massi ve-highwal 1 -fa i lure areas. In addition, use of lateral migration guidelines reduces drilling requirements but enhances appreciation of downdip stripping conditions.

SEDIMENTS OF MOBILE ZONES FLYSCH: ITS TECTONIC SETTINGS AND IMPLICATIONS FOR C O N T I N E N T A L GROWTH Keith A.W. Crook Department of Geology, Australian National University, Canberra, A.C.T. Flysch is primarily an ocean-margin bathyal to abyssal facies which accumulates in diverse active and passive margin tectonic settings that are coupled in part to petrologically distinctive source terrains. Plate motions that lead to the growth of continental crust may transport flysch sequences into different tectonic settings so that the initial and ultimate strato-tectonic affiliation of a flysch unit may differ. As many as four distinct flysch-bearing strato-tectonic units may be present in the "stratigraphy" of a block of continental crust. Tectonic interpretations of flysch units require data on greywacke compositions, internal structures, and stratigraphic and structural relationships to adjacent units. Flysch and its metamorphic and anatectic derivatives constitute some twothirds of upper continental crust, which is primarilya sedimentary and tectonic construct. Flysch geochemistry is therefore an important factor in the chemical composition and evolution of continental crust.

THE DEP0SITI0NAL RECORD DURING CESSATION OF SUBDUCTION - AN EXAMPLE FROM THE EARLY CRETACEOUS OF NEW ZEALAND D.A. Feary Department of Geology, Australian National University, Canberra, A.C.T. The Jurassic to mid-Cretaceous rocks of the western Raukumara Peninsula display complex stratigraphic and structural relationships resulting from virtually continuous deposition during and immediately after cessation of the Rangitatan phase of subduct ion. This Permian to Early Cretaceous phase resulted in the rafting of a large submarine fan complex composed of quartzo-feldspathic material derived from Marie Byrd Land in western Antarctica, into a west-dipping subduction zone to form a thick accretionary prism - the Alpine Assemblage. Alpine Assemblage strata in western Raukumara Peninsula show a change in modal composition from dominantly quartzo-feldspathic to dominantly volcanogenic, reflecting increased proximity to a volcanic arc to the west. Often, complex structural styles reflect semi-continuous deformation of variably lithified sediments. Cessation of subduction in the midAlbian was followed by a period of tectonic quiescence as the locus of subduction marking the Australian/Pacific plate boundary shifted northward. As post-arc sedimentation in western Raukumara Peninsula continued, tectonic adjustment within the accretionary prism consequent upon cessation of subduction decreased in intensity and frequency. Strata deposited early in the post-arc sequence are moderately to locally intensely deformed and sedimentation patterns reflect block faulting of the margins of trench-slope basin depocentres. Later post-arc sediments are only gently folded, and have a more areally and temporally uniform sedimentation pattern.

SEDIMENTARY FRAMEWORK OF THE NEW ZEALAND CONTINENTAL SHELF Lionel Carter New Zealand Oceanographic Institute, Wellington, New Zealand New Zealand's position athwart 15° of latitude and a major plate boundary resul ts in significant climatic and tectonic influences on shelf sediments. Furthermore, the varied land geology, hydraulic regime and Pleistocene fluctuations of sea level have also played a role in shaping a sedimentary framework which, generally speaki ng, has a tripartite zonation either side of the plate boundary. (1) The northern and southern distal sectors of the shelf are tectonically stable, far removed from terrigenous sources, swept by strong currents, and proximal to nutrient upwelling. Consequently modern/relict, coarse biogenic sediment prevails.

(2) Closer to the New Zealand landmass the shelf remains tectonically stable but has a terrigenous supply. The prevailing along-shelf transport regime has fashioned sediments into (1) an inner shelf belt of modern terrigenous sand, (2) a middle to outer shelf belt of relict/palimpsest, terrigenous sand and gravel and (3) a shelf edge belt of modern to relict biogenic debris. In areas of weak currents, mi ddl e to outer shelf sediments are mantled by modern mud. (3) At the plate boundary, the shelf is inundated by modern terrigenous sediment derived f rom the tectonically active landmass. However, underlying older sediments may be exposed where the shelf is being tectonically deformed or eroded by powerful currents.

SANDSTONE COMPOSITIONS FROM THE NEW ENGLAND OROGEN: INDICATORS OF TECTONIC SETTING R.J. Korsch Department of Geology, Victoria University of Wellington, New Zealand Sandstones from the New England Orogen are mainly quartz-poor, lithic to feldspathic types derived from a predominantly volcanic arc terrain. The variation in detritus from Lower Devonian to Permian in the Tamworth Belt records a change in the nature of volcanism. The detritus becomes more felsic upwards from basaltic and andesitic in the Devonian to dacitic and rhyolitic in the Upper Carboniferous. Detrital quartz content is always low but increases upwards from <1 to about 15 percent. Detrital pyroxene is common in the lower part of the sequence, but in younger rocks hornblende is more common. Documentation of vertical changes in the Tamworth Belt sandstone compositions allows correlation with the virtually unfossi1iferous sandstones in the Tablelands Complex to the east of the Tamworth Belt. The Woolomin As sociation is correlated with the Devonian portion of the Tamworth Belt and the Sandon and Coffs Harbour associations are tentatively correlated with the Carboniferous portion. Most sandstones from the New England Orogen have been derived from an undissected volcanic arc provenance which evolved from mafic to more felsic in composition through time. Comparison with detrital sandstone compositions from known ancient and modern tectonic settings suggests that the sandstones were deposited in a forearc or backarc basin.

LATE PRECAMBRIAN EDIACARA BEDS: A SUBMARINE VALLEY FILL., WILPENA AREA/ SOUTH AUSTRALIA J.G. Gehling Hartley College of Advanced Education, Mag ill, South Australia Facies analysis of the Pound Quartzite has enabled a new environmental pretation of the Ediacara Beds within the Rawnsley Quartzite Member.

inter-

The red pelitic sandstones of the Bonney Sandstone Member are abruptly overlain by clean, current-bedded feldspathic sandstones of the Rawnsley Quartzite Member, which are interpreted as transgressive shallow marine and tidal sands. Enclosed within this monotonous thick sequence are the anomalous sediments of the Ediacara Beds. These comprise up to 400 m of section in the Wilpena Pound area. The Member consists of massive, amalgamated sandstones, lensing into massive and laminated siltstones and passing up into well-bedded fossi1iferous sandstones. South-east trending valleys, which cut 300 m into underlying sediments, were filled by this sequence of deepwater silts and grain-flow sands. The overlying, coarsening-up cycles of bedded siltstones and sandstones document shallowing-up to wave base, where stormsurge sands facilitated the preservation of animals of the Ediacara assemblage. Prograding shoreface and barrier sands capped the sequence, heralding, a widespread return to stable shallow marine conditions. Great thickness variations within the Pound Quartzite can be related to local synsedimentary tectonic movements within the basin.

UPPER CRETACEOUS AND LOWER TERTIARY DEEP-SEA SEDIMENTS OF THE NEW GUINEA MOBILE BELT AT PORT MORESBY D.W. Haig University of Papua-New Guinea, Port Moresby, Papua-New Guinea Maastrichtian to Eocene rocks of Port Moresby consist of a structurally complex succession of thin-bedded, laterally extensive, fine-grained carbonates and cherts. The 1ithostratigraphic column has been reconstructed mainly by recognition of planktic foraminiferal zones. The Maastrichtian and Palaeocene are represented by less than 200 m of argillaceous limestone and calcareous shale which contain abundant planktic foraminiferids. Maastrichtian rocks consist of salmon-pink limestone and shale. Two main lithofacies are represented in the Palaeocene: (1) cream- to mushroom-coloured limestone and shale with a predominantly pelagic microfauna; and (2) grey bioturbated shales with abundant benthic agglutinated foraminiferids. The Lower Eocene includes narrow units of fine-grained calcarenite (usually less than 30 m thick) interbedded with broader siliceous argil lite units. The calcarenite is composed mainly of fragmented bryozoans, echinoderms, calcareous algae, and foraminiferids, and contains a conspicuous terrigenous component of angularsubangular quartz, feldspar, glauconite, and lithic fragments. Cross-laminations and slump structures commonly occur in these thin-bedded rocks. The siliceous argil lite contains abundant radiolarian shells and sponge spicules. Middle and Upper Eocene calcarenites are coarser grained and usually contain abundant "larger" foraminiferids. Lithic fragments occurring in the calcarenite include reworked pelagic limestones of the Maastrichtian and Palaeocene as well as 1 ithic sandstone-si 1 tstone and intermediate-basic igneous and volcanic pebbles. Stratigraphic relationships are uncertain between the Maastrichtian-Eocene succession and Campanian orbitoidal limestone which occurs as small fault wedges in the area. The succession is overlain unconformably by Upper 01igocene-Lower Miocene sediments and was intruded during the Oligocene by a large gabbro batholith. The Maastrichtian and Palaeocene rocks are deep-sea carbonate-mud accumulations (2 000-4 000 m water depth) deposited in a quiet basin starved of terrigenous material. The Eocene was marked by deposition of deep-sea siliceous mud and ooze with periodic influx of current-transported shallow-water skeletal debris and 1ithic fragments. The increase in sediment grain size towards the top of the succession may indicate shallowing and reflects the increasing tectonic instability of the region during the Eocene.

SEDIMENT0L0GY OF EOCENE CONGLOMERATES, SOUTHERN CALIFORNIA AND NORTHWESTERN MEXICO Patrick L. Abbott Department of Geological Sciences, San Diego State University, San Diego, California, U.S.A. Eocene conglomerates in Baja California, southern California and the Channel Islands are dominated by exotic rhyolite clasts. Through statistical comparison of the trace-element chemical compositions of the rhyolite clasts, one long-distance dip-fed sedimentary system has been reconstructed from its bedrock source in the Imuris Volcanics of north-central Sonora, Mexico, to its terminal submarine fan deposits on the Channel Islands near Santa Barbara, California. Interpretations have been made of the depositional environments of the east to west flowing system.

Fluvial deposits perched on the Peninsular Ranges yield the following palaeohydrologic data: a 300 km-long river from a mountain source at 4 000 m elevation, sinu3 osity of 1.3, 100-year flood discharges up to 30 000 m / s e c , and seasonallydominated rainfalls of 50 to 80 cm annually. Going from east to west in San Diego, California are good exposures of fluvial, alluvial fan, paralic and submar i ne canyon facies. The submarine fan facies of this system are found 180 km to the northwest in the Channel Islands. The sequence of depositional systems forms an east-west oriented piercing point that has suffered two major offsets with the opening of the Gulf of California and major strike-slip faulting offshore from southern California.

THE ASHBURTON TROUGH R.C. Horwitz CSIR0, Division of Mineralogy, Floreat Park, Western Australia The Early Precambrian Ashburton Trough (Wyloo Trough, Ashburton Geosyncline) coincides with the Ashburton Regional Gravity Ridge and flanks the south and west of the Hamersley Shelf and its buried crystalline and metamorphic substratum. This buried substratum and its northerly outcropping extension form the Pilbara Block which occupies the Fortescue Regional Gravity Complex. Palinspastic maps and sections across this geological and gravitational boundary zone portray its existence at least as early as the time of Fortescue Group vol can ism and extending, with oscillations and modifications in the palaeogeography, up to the end of sedimentation of the Ashburton Formation. The osci 1 lat ions and thei r variations in amplitude have caused the migration of exhumed geanticlines and areas of unconformity during this whole period. The history of this area is presented as comprising separate tectonic entities of a geosynclinal evolution which can be compared to other classical geosynclines and considered as a branch of an ancestral Indian Ocean.

ORIGINS OF LARGE OLISTOLITHS IN THE ASHBURTON

FORMATION

R.C. Horwitz & R.E.T. Hill CSIR0, Division of Mineralogy, Floreat Park, Western Australia The Lower Proterozoic Ashburton Formation is a flysch-type sediment withinthe Ashburton Trough. Around the Wyloo Dome this formation is composed of proximal turbidites which contain olistoliths, some of which are several kilometres in strike length. They are associated with olistostromes, breccias and conglomerates at three general stratigraphic levels so far recognized over about 2 500 km 2 between Mt Clement and Red Hill. The lowest level appears to contain material whose provenance was nearby. Some units are still attached to their source and others exhibit various stages of evolution of decollements by gravity sliding. The next, where preserved, is separated from the first by over 2 km of greywacke. It contains, among other rocks, slabs of basic to acid tuffs and lavas, dolerite and BIF, which resemble the assemblages of the first level. The third level of these rocks, which truncates the second level at Mt Stuart in a manner similar to that of cross-bedding, is constituted by an interesting variety of rock types. These include chert and BIF, dolomite, mature conglomerates and quartzites as well as porphyritic tuffs and lavas. The petrography and geochemistry of the acid rocks does not rule out their source being part of the extensive Woongarra Volcanics. It is concluded that the olistoliths have been emplaced by successive gravi tational peeling-off of the succeedingly older units, in a process similar to the divert iculation of the U1trahelvetic Nappes of the Alps.

SEDIMENTATION PATTERNS IN THE ADELAIDE GEOSYNCLINE Burton Murrel1 C.R.A. Exploration Pty Ltd, Fyshwick, A.C.T. The Adelaide Geosyncline consists of three superimposed basin sequences deposited marginal to the Gawler Craton. Each sequence was deposited in a complex of half-grabens which deepened away from the craton, a tectonic pattern which persists to the present day. Sedimentation was initiated in a basin-and-range province with interconnected playa lakes (Callanna Group) followed by more extensive carbonate lakes with large fluctuations in level (Burra Group). A pattern of carbonate sedimentation along the craton (hinge) margins of the half-graben complex, opposite fluvial clastic sedimentation from the deeper downfaulted margins, becomes apparent during the Burra Group. The more extensive and better exposed Umberatana, Wilpena and Hawker Groups were deposited in an isolated sea within Gondwanaland. The distribution of thin chemical and fine clastic sediments versus the thick deposits of coarser clastic material shows the sediment to have been derived from the present positions of the Murray and Cooper Basins, with the Gawler Craton supplying mainly reworked material during phases of regression.

CONWAY TROUGH, A TRANSDUCTION BASIN WITHIN THE MARLBOROUGH SHEAR ZONE, NEW ZEALAND R.M. Carter 6 Lionel Carter University of Otago, New Zealand Oceanographic Institute, New Zealand The Conway Trough is a small, deep-marine basin situated between the southern head of the Hikurangi Trough and the emergent Lowry Peaks and Seaward Kaikoura mountain ranges. The basin is fault-bounded and of rectangular outline, being kO km long and 8-10 km wide, and occurs as a major indentation in the narrow continental shelf off Marlborough. 3.5 kHz profiler records and other marine geologic data show that the shelf into which the Conway Trough is incised is underlain by complexly deformed CretaceoTertiary (Kaikoura Sequence) sediments. Folding and faulting continue in the area today, as evidenced by the presence of deformed late Pleistocene strata and by the distribution of earthquake epicentres. The southern limit of the Marlborough Shear Zone is situated at the Motunau Fault, a previously unrecognized offshore extension of the Porters Pass-Ashley fault system. To the south of the Motunau Fault, the Kaikoura Sequence sediments that underl ie the Canterbury continental shelf are effectively undeformed and lie outside of the plate boundary zone. Though situated nearshore, the main Conway Trough today intercepts very 1ittle of the coarser, bedload sediment from eastern South Island rivers, but serves rather as a major mud-sink for suspended detritus from both south and north of Banks Peninsula. However, Holocene graded gravels have been recovered from over 1 000 m in the axis of the Kaikoura Canyon, suggesting that the canyon serves as a sedimentary conduit through to the Hikurangi Trough. The Conway Trough is therefore used as the basis for a sedimentary model of a transduction basin.

SEDIMENTS OF INTRACRATONIC BASINS

GLOBAL SEA-LEVEL C Y C L E S , T E C T O N I S M , T E M P E R A T U R E , AND TERTIARY S E D I M E N T A T I O N , ST VINCENT B A S I N , SOUTH AUSTRALIA J.M. Lindsay South Australian Department of Mines and Energy, Adelaide, South Australia This shallow rift-graben inherited an ancient tectonic framework between the Gawler Craton and the Mt Lofty Uplift, straddling the Torrens Hinge Zone. North Maslin Sand fluvial clastics, the first post-Permian sediments, commenced in the Middle Eocene when structural elements were rejuvenated consequent on nearby seafloor spreading events. Cainozoic faul ts general ly paral lei led old Delamerian trends of the Adelaide Geosyncline. Continuing basin subsidence, high Eocene sea levels, and warm moist climate, balanced against declining clastic input, produced the margina1-marine South Maslin Sand and the coal-bearing Clinton Formation in the Middle to Late Eocene. The Late Eocene marine succession (Aldingan Stage) from Tortachilla Limestone to the Aldinga Member of Port Willunga Formation, is apparently compressed into one third-order Vail sea-level cycle of 3 Ma; and is broken by a regressive episode (Chinaman Gully Formation) resulting from local tectonism and prograding clastics.

MESOZOIC SEDIMENTATION HISTORY OF THE EYRE SUB-BASIN G.D. Powis £ A.D. Partridge Esso Australia Ltd, Sydney, New South Wales Jerboa-1 is the first well drilled in the Eyre Sub-basin off the southern coast of Western Australia. Very detailed sidewall core sampling of the well has enabled confident age dating and determination of depositional environments. Detailed correlation is possible with other Australian Mesozoic sequences and their major eustatic events, resulting in a better understanding of the overall regional geology and enabling application of contemporary depositional models. These consisted of the oxic deep-lake deposits of Lake Baikal and the intimate relationship of the Black Sea with the Mediterranean. The detailed sampling revealed anomalies in palynomorph ranges and depositional regimes of the sequence penetrated by the well when compared with similar sequences in other Australian basins. Many stratigraphically important genera were noted to have abbreviated or disjunct ranges in Jerboa-1. For example, the genus Cicatricosisporites, whose first appearance is taken as the approximation to the base of the Cretaceous in Australia, was recorded in a sample near the base of a Neocomian section but was then absent in samples until its occurrence became consistent through the dark claystones of the Aptian to Cenomanian section. The genus Aequitriradites and Foraminisporis had marked delayed occurrences and the genus Pilosisporites, diagnostic in Aptian and younger Australian sediments, was not recorded at all. Confident age dating of the sequence therefore was only poss i ble because of the detailed sampling. This emphasizes the possible inaccuracies and misinterpretations inherent in dating by means of cutting samples and/or scattered core or sidewall core samples. It thus has implications in the way other Australian Mesozoic sections are viewed. Only by comprehensive sampling of sections can closer and more accurate correlations be obtained and Australian Mesozoic stratigraphy and the corresponding eustatic events be better understood.

LATE PALAEOZOIC SEDIMENTATION

IN THE ARCKARINGA B A S I N , SOUTH AUSTRALIA Barry J. Cooper,

South Australian Department of Mines and Energy, Eastwood, South Australia The Arckaringa Basin is an intracratonic basin containing sediments of Late Carboniferous and Early Permian age in northern South Australia. Tillites and glacially-derived sediments forming the base of the succession (Boorthanna Formation) are overlain by marine shales (Stuart Range Formation). Fluvial sands and silts with coals (Mount Toondina Formation) complete the sequence. The major controlling influence on sedimentat ion was the advance and subsequent retreat of a polar ice cap. No known sediments immediately predate this event. Continental glaciation produced a flood of sediments into the Arckaringa Basin and depressed the land sufficiently to allow a shallow sea to transgress and regress across the basin from the northwest and/or northeast with corresponding retreat and advance of the ice sheet. With the last retreat of the ice, the sea persisted in depressed areas during eustatic adjustment. Final marine regression to the northeast was followed by a brief period of fluvial deposition. Tectonic activity was also significant in basin development. It was a major factor in establishing a lowland with marginal half grabens in the region as well as in preserving the subsequent sedimentary record. Tectonics were also critical in the elevation of areas adjacent to the basin, which subsequently formed nucleii for ice-sheet development.

TECTONIC INFLUENCE ON LATE DEVONIAN SEDIMENTARY FACIES, NORTHERN AMADEUS BASIN, NORTHERN TERRITORY B.G. Jones 1 , K. Ferguson 2 S B. Wygrala 2 1 2

University of Wollongong, Wollongong, New South Wales Uranerz Australia, Subiaco, Western Australia

The delineation of sedimentary facies within the Late Devonian Pertnjara Group, and especially within the Brewer Conglomerate, has been enhanced over the past seven years by detailed mapping and extensive exploration drilling programmes. At the start of Late Devonian deposition, the northeastern portion of the Amadeus Basin was emergent and the main depocentre of lacustrine sediments was in the Mereenie Anticline area. The very extensive development of the braided stream facies of the Hermannsburg Sandstone was the result of widespread gentle uplift of the Arunta Block north of the present basin margin. A number of east-west folds within the basin were active at this time and influenced sedimentation. Uplift along the northern margin of the basin culminated in massive fanglomerate deposition (the Brewer Conglomerate) with several local unconformities. Three main loci have been identified (Tyler Pass, Ellery Creek and Ross River) and there is a complex interdigitation of the fans with pebbly sandstone and overbank facies (the Undandita Member). There appears to have been an eastward migration of fan loci with time suggesting that uplift in the Arunta Block was diachronous, starting in the west and migrating eastwards. The largely buried intrabasinal folds exerted controls on sedimentation, groundwater migration and diagenesis.

THE WARRABIN T R O U G H , WESTERN ADAVALE BASIN J. Pinchin & B.R. Senior Bureau of Mineral Resources, Canberra, A.C.T. The Warrabin Trough is a major intracratonic trough which, like the Cooladdi and Quilpie Troughs, forms part of the Devonian Adavale Basin in southwestern Queensland.

During 1980, a six-fold seismic traverse was recorded across the Warrabin Trough by the Bureau of Mineral Resources (BMR) as part of a multi-disciplinary study to investigate the structure, stratigraphy, geological evolution and petroleum potential of the central Eromanga Basin and underlying sedimentary basins. Good quality seismic data from the Warrabin Trough enabled the existing pre-1971 seismic data to be reinterpreted, allowed the basin structural framework of the Trough to be mapped and indicated that the area may be of considerable interest for petroleum explorat ion. Devonian sedimentary rocks were penetrated only by two wells on the flanks of the Warrabin Trough, but the BMR seismic data show a 3 000 m-thick sequence of these rocks within the Trough. These range in age from the lateral equivalent of the Buckabie Formation (Devonian-Carboniferous) and of the Cooladdi Dolomite (Middle Devonian) to possible early Devonian sedimentary rocks predating most of the sediments in the central part of the Adavale Basin. The Warrabin Trough is folded and faulted by high-angle reverse faults, and is now separated from the central Adavale Basin by the Canaway Ridge, which developed during the Carboniferous. LANDSAT studies provided additional information on the position of faults, which on seismic reflection data appear to be confined to the Warrabin Trough sequence. It is postulated that post-Cretaceous movements, due to differential compaction or slight tectonic rejuvenation, may have given surface-expression to some of these deep-seated faults. Petroleum geochemistry from the two exploration wells which penetrated the western flank of the Warrabin Trough provided contradictory results, with the sequence being post-mature in Chandos 1 and containing immature, mainly gas-prone kerogen in Bodalla 1. Another unnamed Devoni an trough 1ies in the general area of the Thomson Syncline, to the northwest of the Warrabin Trough. The relationship of this trough to the Warrabin Trough is not fully understood, although they appear to be connected by a thinner sequence of Devonian rocks. Hence Devonian to Carboniferous sedimentation west of the Canaway Fault was much more widespread than previously thought, providing an additional target for petroleum exploration below the already attractive Cooper and Eromanga Basin targets.

THE PIPER O I L F I E L D , UNITED KINGDOM C.E. Maher Australian Occidental Petroleum, Perth, Western Australia The Piper oilfield is offshore in the eastern end of the Moray Firth Basin, 200 km northeast of Aberdeen, Scotland. The field was discovered by Occidental of Scotland in December, 1972. Production commenced December 7, 1976 from a steel platform centrally located over the field in 146 m of water. In the Piper area, gentle folding, development of northeast-southwest and northwest-southeast fault trends and erosion preceded the deposition of a lower deltaic plain, interdistributory sequence of Callovian age on an upper Bathonian unconformity surface. A marine transgression in upper Oxfordian deposited a sandy, si1ty, bioturbated shale over the whole of the Piper area. Sedimentation in a high-energy, shallow-marine, shelf environment interrupted by seven minor transgressions followed. Kimmeridge Shale was deposited after a major transgression near the end of the Jurassic and is both a caprock and source rock. Subsidence and large vertical displacement along northwest-southeast trending faults in the Lower Cretaceous resulted in a field comprised of 3 parallel, folded, tilted fault blocks with erosion along fault scarps and overlain by 2 200-2 770 m of Mesozoic and Tertiary sediments. Upper Cretaceous marlstones provide the caprock along some fault scarps. Two northerly fault blocks cover 2 890 hectares, contain 99 percent of the oil, have gross sand thickness from 50 to 143 m , porosity averaging 2k percent, permeabilities from 200 md to more than 10 dareies, maximum reservoir column of 403 m and original oil water contact at 2 615 m subsea. A strong natural water drive is supplemented by water injection. Current production is 200 000 STB/day of 37° API, low-sulphur oil and cumulative production to May 1, 1980 is 358 million STB. Recoverable reserves are estimated to be 618 million STB.

TFCTONICS AND DEPOSITIONAL STYLE OF THE PRECAMBRIAN ARCHAEAN AND PROTEROZOIC SEDIMENTATION STYLES: FOR PROGRESSIVE CRATONIZATION

EVIDENCE

K.A. Eriksson Programs in Geosciences, University of Texas at Dallas, Richardson, Texas, U.S.A. Precambrian sedimentation occurred during subsidence of rifted continental margins, intracratonic grabens or complete cratons and reflects progressive cratonization through time. Archaean sedimentation in the Barberton Mountain Land, South Africa and Pilbara Block, Australia occurred initially in cratonic rift grabens and thereafter on the margins of small cratonic nucleii. Both steep rift and shelf rise continental margins developed; in the former, abrupt fluvial to submarine fan transitions occurred whereas, in the latter, a we11-developed shallow-marine assemblage of facies formed. Sedimentation was terminated by closure of ocean basins. Lower and middle Proterozoic sedimentation in South Africa, i ncl udi ng the Pongola, Witwatersrand and Transvaal Supergroups, occurred in epicratonic settings on the Kaapvaal Province which existed as a stable craton from 3 000 Ma onwards. Sediment was supplied by fluvial systems but the dominant mode of sedimentation in the epicratonic seas was tidal and occurred during prolonged periods of submergence of the craton. Deep-water facies equivalents are lacking but these may have been reworked in the younger high-grade Natal-Namaqua Province on the southern margin of the craton. A second style of Proterozoic sedimentation was associated with cratonic rifting or, more commonly, with rifting of high-grade metamorphic terrains between cratons. The Soutpansberg trough between the Kaapvaal and Rhodesian Provinces in southern Africa was a shallow feature in which volcanics and fluvial sediments accumulated. The Capricorn trough between the Pilbara and Yilgarn Provinces, Australia and the Damara trough in Namibia experienced considerable subsidence which enabled both shallow- and deep-water facies to develop. These troughs had sialic basements and may have taken the form of aulacogens. In the Damara and Capricorn geosynclines as well as in the Coronation Geosyncline of Canada, aulacogens can be traced into active continental margins floored by oceanic crust. Proterozoic continental margins resulted from rifting of progressively larger cratons which remained emergent during marginal sedimentation. Closure of passive margin oceans represents the completion of the Wi lson cycle. Examples of the resulting molasse sedimentation in foreland basins may be the lower Proterozoic Pongola Supergroup in South Africa, middle Proterozoic units in the Coronation and Capricorn geosynclines, and the upper Proterozoic Nama Group in Namibia. Progressive cratonization resulted from these different periods of ocean closure.

ARCHAEAN SEDIMENTATION

IN THE NORTHEASTERN YILGARN B L O C K ,

WESTERN AUSTRALIA J.A. Ha 11 berg CSIRO, Division of Mineralogy, Floreat Park, Western Australia An investigation of 20 000 km 2 of Archaean terrain in the Leonora-Laverton area has revealed several discrete, subaerial, andesite-dominated calc-alkaline volcanic centres associated with extensive quartz-poor, feldspathic epiclastic sediments which interdigitate distally with pillowed tholeiitic basalts. Sediments proximal to the volcanic centres consist of coarse, angular to subrounded, poorly-sorted, matrix-supported conglomerates which represent debris flows deposited in alluvial fans about the volcanic edifices. Alluvial-fan sediments pass distally into matrixsupported conglomerate, pebbly lithic wacke, pyritic feldspathic wacke and feldspathic shale and siltstone. Distal sediments show graded bedding, cross-bedding and flame structures, and display Bouma divisions ABCD and ABCE indicating thattheyare

39 turbidites. This lateral facies change is repeated in a vertical sense as fan conglomerates are overlain by turbidites. The abundance of epiclastic sediments and their distribution suggest that the andesitic centres were relatively high-relief features which were rapidly eroded. The picture of concomitant subaerial calc-alkal ine volcanism and subaqueous tholeiitic volcanism, with related sediments showing a rapid change from a fan environment to subaqueous turbidites, without evidence of a shallow-marine facies, is common in the Archaean of Canada. Andesitic volcanism in the southern part of the area (Edjudina Range) is predominantly submarine. Andesitic volcanic rocks and interdigitated debris flow deposits are laterally equivalent to, and overlain by, well-rounded boulder conglomerates and abundant shale, siltstone, and chert. The latter sediments lack the characteristics of turbidites, and may represent a shal low-mar ine facies, suggest ing a southerly change in sedimentary style in the area studied.

ARCHAEAN

IRON-FORMATIONS: ANALOGUES OF HOLOCENE PELAGIC SEDIMENTS K.A. Eriksson

Programs in Geosciences, University of Texas at Dallas, Richardson, Texas, U.S.A. In addition to occurring within basal-volcanic sequences, Archaean iron-formations in the Barberton Mountain Land, South Africa and Pilbara Block, Australia are also commonly associated with overlying terrigenous clastic intervals. The ironformations are exclusively lutitic and occur in a variety of palaeoenvironmental settings, namely; (1) at the base of progradational turbidite sequences; (2) enclosing inner-fan channel deposits; (3) capping Bouma turbidite beds; (4) intercalated within outer-fan to basin-plain shales and limestones; and (5) at the base of progradational shallow shelf-beach sequences. The above associations suggest that the iron-formations define diastems and that precipitation occurred in any environments basinward of, or between, events of terrigenous influx. Pelagic carbonates and cherts are developing in analogous deposit ional settings at the present time. Evidence of associated volcanism either as distinct flows or intercalated tuffaceous horizons points to a juvenile origin for the iron and silica in these Archaean sediments. Chert was probably the normal background precipitate in slightly acidic ambient waters. Iron minerals may have developed under temporary oxidizing conditions or during seasonal, more alkaline periods. Insofar as being removed from terrigenous dilution at the time of accumulation, any of the above iron-formations is a potential precious and base-metal target, especially if a volcanic association can be demonstrated.

TECTONIC FRAMEWORK OF PROTEROZOIC SEDIMENTATION ALONG THE NORTHERN MARGIN OF THE YILGARN BLOCK R.D. Gee Geological Survey of Western Australia, Perth, Western Australia Complex sedimentary sequences in the Nabberu Basin lie near the southern margin of the early Proterozoic Capricorn Orogen that developed latitudinally on a discontinuous Archaean sialic basement. Sedimentary history at the southern margin is controlled by a major east-west crustal fracture which now defines the partly buried northern margin of the Yilgarn Block. Sequences in this marginal zone record remnants of sediments deposited in extensive epeiric seas as well as trough sediments localized by incipient rifting and subsidence. The earliest sediments (beginning of the Glengarry Group - ca 1 800 Ma) are extensive mature marine sand and carbonate mud with evaporite features. Block faulting akin to graben formation produced a basin-and range province over the fracture zone. Thick arkose wedges developed over granite horsts, platform sediments were partly eroded and voluminous mafic sills and flows irrupted. North of the fracture zone a trough developed into which accumulated turbidites derived from the basalt and arkose. Chemogenic sediments and shallow-marine sand then formed in the filling trough. The earliest folding was by doming of basement at elevated temperatures north of the fracture zone.

A younger thick sequence (Earaheedy Group - ca 1 700 Ma) of marine sandstone, shale, iron-formation and dolomite marks the return to epeiric sea sedimentation. The old fracture zone did not influence sedimentation at this stage, but a fold belt formed to the north of its buried position, again by doming of basement.

A REVISED AGE FOR THE HAMERSLEY GROUP W. Compston1, I.S. Williams 1 , M.T. McCulloch 1 , J.J. Foster 1 , P.A. Arriens1 & A.F. Trendall 2 1

Research School of Earth Sciences, Australian National University, Canberra, A.C.T. 2 Geological Survey of Western Australia, Perth, Western Australia New isotopic age determinations that bear on the age of the Brockman I ron Formation are given by three different methods. The most precise are zircon U-Pb ages, which have been obtained from a shale horizon, considered to be a tuff band, within the Dales Gorge Member, and from the later Woongarra Volcanics. They are 2 490 ± 20 Ma and 2 470 ± 30 Ma respectively and closely limit the age of sedimentation to 2 500 Ma. This is substantially older than previous estimates of 2 000±100 Ma based on Rb-Sr results for the Woongarra Volcanics. There is no petrographic evidence that the shale zircons might have been weathered from an older igneous provenance, and no evidence in the isotopic Concordia pattern that zircons from either rock might contain inherited radiogenic Pb. The interpretation of the previous Rb-Sr data for the Woongarra Volcanics has been revised. The isochron method has been discarded in favour of the use of Rb-Sr model ages, in which a realistic, low value for the initial 8 7 Sr/ 8 6 Sr is assumed, in contrast to the extremely high values (up to 0.78) inferred from the previous isochron treatment. For 36 radiogenic samples in which the model age is not sensitive to small uncertainty in the initial 8 7 Sr/ 8 6 Sr, peaks in the model-age frequency distribution are found at ^2 370 Ma and 950 Ma. This bimodal distribution is consistent with an original age for the Volcanics of at least 2 370 Ma followed by resetting of the total-rock ages for many samples at 950 Ma, so that the Rb-Sr results can be understood in terms of the 2 470 Ma zircon age. Sm-Nd model ages for the Woongarra Volcanics are currently underway and will be reported at the Convention.

BASEMENT TECTONIC CONTROL OF SEDIMENTATION IN THE PR0TER0Z0IC INTRACRATONIC BANGEMALL BASIN P.C. Muhling1 & A.T. Brakel2 1 2

B.H.P. Exploration, Perth, Western Australia Bureau of Mineral Resources, Canberra, A.C.T.

The BangemalI Basin developed during the last phase of activity about 1 100 Ma ago in the orogenic belt which separates the Pilbara and Yilgarn cratons. The control of sedimentation by the tectonism of different basement segments is most evident in the lower part of the sequence. A complex sequence of mudstone, sandstone and subordinate chemical sediments was deposited during the advance of a barrier bar system onto a rifted mobile belt (Gascoyne Province). To the north, a simple sequence of carbonate and minor terrigenous clastic sediments was deposited on a gently sloping shelf formed by the stabi 1 ized Ashburton Fold Belt. East of themobile belt, a simple sequence of terrigenous clastic sediments was deposited on a mildly warped and faulted craton. As the basin developed during phases of stillstand, deepening and regression, contrasts in sedimentation on the different basement segments diminished. The basin formed by extension and sagging of continental crust, which was achieved by reactivation of faulted boundaries to basement blocks. Maximum extension occurred in the rifted mobile zone where dolerite sills showing chemical affinities with continental tholeiites intruded the cover sequence. Copper-Zn occurrences are located near former basement highs or hinge which reflect basement fault-block boundaries.

lines

SOME COMPARISONS BETWEEN THE DALES GORGE MEMBER AND THE MARRA MAMBA IRON FORMATION OF WESTERN AUSTRALIA R.C. Morris CSIRO, Division of Mineralogy, Floreat Park, Western Australia The best studied of the five major, laterally persistent, oxide-type banded iron formations of the Hamersley Group, the Dales Gorge Member, consists of a province-wide alternation of two types of macrobands. The change from precipitation of BIF-macrobands (oxide-type BIF) to deposition of S-macrobands (silicate-carbonate type BIF) is attributed to a rise in pH, triggered by reaction of sea-water with volcanic dust. Differences in mineralogy and texture between widely separated sites, despite detailed correspondence of fine banding and similarity in chemical composition, is the result of variations in diagenesis and low-grade metamorphism. Though laterally continuous, a province-wide continuity of the internal subdivisions of the Marra Mamba Iron Formation has not been firmly established. Cores of the upper BIF member show that it is also divided into distinctive lithological units with oxide-type BIF as one series. However the intervening thin beds are carbonate and shale, not BIF as in the Dales Gorge Member S-macrobands. Vulcanism has probably played only a minor role in this alternation, which may be related to periodic interruptions in the supply of BIF components.

SEDIMENTARY ENVIRONMENTS AND PALAEOGEOGRAPHY OF THE EARAHEEDY G R O U P , NABBERU B A S I N , WESTERN AUSTRALIA John A. Bunting B.H.P. Exploration, Kalgoorlie, Western Australia The Earaheedy Group (Early Proterozoic) was deposited ina broad, intracratonic basin along the northern margin of the Yilgarn Block, in environments that ranged from moderately deep marine, through shallow marine, to marginal, lagoonal and possibly lacustrine. Several periods of transgress ion and regress ion can be recognized. Lithofacies include granular and banded iron formations, sandstone, carbonate and shale. For most of its existence, the Earaheedy Sub-basin shallowed to the south and east, not the southwest as the present outcrop configuration might suggest. Evidence includes: (1) the basal Yelma Formation thickening to the north and west; (2) the distribution of shoal and deep-water facies in iron formations; (3) restriction of the Windidda-Wandiwarra disconformity to the southeast; (4) the northwestward progradation of the Princess Ranges Quartzite (and ambiguous palaeocurrent data); (5) lithofacies distribution in the Yelma, Frere, Windidda, Wandiwarra and Wongawol Formations; and (6) stromatolite elongation in the Kulele Limestone. Two important implications are the existence of an Early Proterozoic landmass in the area of the southern Officer Basin, and the late development of the Wiluna Arch (although a proto-Wiluna Arch possibly control led pre-Earaheedy sedimentation). The northern extent of the sub-basin is unknown, but it is unlikely that it linked up with the Hamersley Basin.

A SPECULATIVE BUT GEOLOGICALLY-CONSTRAINED

MODEL

FOR THE ORIGIN OF THE HAMERSLEY GROUP R.C. Morris & R.C. Horwitz CSIRO, Division of Mineralogy, Floreat Park, Western Australia With only minor exceptions, the sediments of the Hamersley Group are either (iron-formation, chert and carbonate), or pyroclastic/chemical ("shales")• Palinspastic reconstructions indicate that the sediments were deposited on a submarine, essentially volcanogenic (Fortescue Group), platform or bank, built on a sialic northwest-trending shelf protruding into, or marginal to, an ocean. chemical/biological

Upwelling of bottom currents resulted in precipitation of iron, silica and other components derived largely from oceanic rifts. The currents persisted throughout sedimentation of the Hamersley Group, diverted periodically due to various factors, such as the oscillating emergence and submergence of distant volcanic chains, whose dust emissions were largely responsible for the "shales" in the success ion, including those of the S-macrobands of the Dales Gorge Member. Individual mesobands represent periods of 2 or 3 to tens of years of relatively constant supply and depositional conditions. The transition from one mesoband to the next indicates an areawide change in these conditions. While no significant, lateral, depositional changes in chemical composition have been found in the BIFs, chert fragments in the lower part of the Turee Creek Group are interpreted as representative of a deep-water, iron-poor facies toward the southwest.

THE MASTERTON

SANDSTONE - A MID-PROTEROZOIC

INTRACRATONIC

CLASTIC

UNIT

M.J. Jackson Bureau of Mineral Resources, Canberra, A.C.T. The Masterton Sandstone (a provisional, informal redefinition of the Masterton Formation) is a 1ithologica11y distinct clastic unit at the base of the McArthur Group, in the McArthur Basin of the Northern Territory. It unconformably overlies subaerially extruded volcanics and is overlain by probable lacustrine and supratidal dolomitic siltstones and sandstones. Detailed measured sections (ranging from 50 to 900 m thick) from ten widely-spaced localities, over about 30 000 km 2 of the southern part of the basin indicate deposition in several different environments. Thick sections (several hundreds of metres) of coarse- to fine-grained sandstones, with interbedded cobble conglomerates and rare interstratified volcanics, are present in the north-central part. Thinner (up to about 150 m) sequences of sandstones and volcaniclastics are present on the western and eastern margins. In contrast, a thin sequence (around 50 m) of fine-grained, well-sorted rippled and cross-stratified sandstone is common in the south. A lateral facies change from a terrestrial, fault-bounded trough environment in the north-central to an intertidal to subtidal marine environment in the south, and flanked by volcano-fluvial/marine environments on the west and east, is indicated. This north-south facies change contrasts with the lateral uniformity of the overlying carbonate successions. In addition to this, a vertical profile analysis of the southern marine facies indicates an initial rapid deepening of the sea, followed by a gradual shoaling from subtidal, through intertidal, to supratidal sub-environments.

PROTEROZOIC

SHELF SEDIMENTATION NORTHEASTERN

IN T H E G E O R G E T O W N

INLIER,

QUEENSLAND

J.J. Draper 1 , D.E. Mackenzie 2 & I.W. Withnall 1 Q u e e n s l a n d Geological Survey, Brisbane, Queensland 2 Bureau of Mineral Resources, Canberra, A.C.T. Preliminary studies of primary sedimentary features indicate that the Proterozoic Etheridge Group of the Georgetown Inlier originated as mostly shallow-water sediments The Bernecker Creek and lower Robertson River Formations in the lowermost part of the Group consist of fine sandstone and siltstone probably deposited in shallow subtidal to intertidal environments, and have a D r o a r ^ H ^ i * e• marine basalts in the middle Robertson Rive^ Forn» J ^ T I n l £ ~ t ^ deeper subtidal conditions during deposition of sediments higher in the F o n S t l S . The overlying Townley, Heliman, and Candlow Formations consist n f m „ i , f siltstone that are commonly carbonaceous, and sandstone that L I

,?°ft:Sedimentdefrati°nStrUCtUreSare

^St^y^v

(subt.dal) sedimentation; however, minor intervals in the Candlow Formation contain

gypsum casts and possible stromatolites, which indicate periodic hypersalinity and slower sedimentation. Parts of the Candlow Formation, and the whole of the overlying Langdon River Mudstone consist of carbonaceous, commonly pyritic, mudstones probably deposited in deeper water. In the Langlo Vale Group, which unconformably overlies Etheridge Group, Malacura Sandstone consists of sublabile to labile sandstone deposited in fluviatile to deltaic environments, while the overlying Yarman Formation consists of shallow subtidal marine mudstones and several upward-fining sandy beds that probably represent episodes of deltaic progradation.

ADELAIDEAN SEDIMENTATION STYLES IN SOUTHERN SOUTH AUSTRALIA AND THEIR RELATION TO LATE PROTEROZOIC TECTONICS W.V. Preiss South Australian Department of Mines and Energy, Eastwood, South Australia The Adelaide Geosyncline evolved during the late Proterozoic as a rifted basin, possibly related to a major break-up of continents. In its initial (Willouran) stage, evaporitic carbonates and clastics were restricted to a north-northwesttrending trough, with more widespread basic vulcanism. In Torrensian time, a basal fluvial sand sheet spread eastward from the uplifted craton, overlapping Willouran rocks on to southern basement highs. The following repeated cycles of platform carbonates overlain by coarsening-upward clastic wedges indicate progradation into an eastward-deepening basin. In the Sturtian, a hiatus with local tectonism was followed by a first glacial phase, depositing tillites and ironstones in eastern basins. The second resulted in more widespread tillites, as well as basinal clastics in eastern troughs. Post-glacial rise in sea-level led to basin-wide deposition of fine carbonaceous clastics. Oolitic and stromatolitic limestones around basin margins resulted from late Sturtian regression. In the Marinoan, shel f and basinal areas were well differentiated, with western redbed facies, eastern basinal siltstones, and carbonates in the central Flinders Ranges. Marinoan glacials accumulated mainly in eastern basinal areas. Post-glacial transgression extended far on to the Stuart Shelf, and deltaic redbeds and sands prograded eastward. Late Adelaidean sediments tended to be laterally persistent, probably indicating the evolution of stable, broad continental shelves.

STROMATOLITES THROUGH THE AGES

THE STROMATOLITE SUCCESSION THROUGH THE ARCHAEAN: A RECORD OF THE EARTH'S EARLIEST BENTHOS Malcolm Walter Baas Becking Geobiological Laboratory, Canberra, A.C.T. Counted by major stratigraphic units, eleven occurrences of stromatolites are now known from Archaean sequences dating back to 3 500 Ma ago, although three of these are too poorly known to be confidently interpreted as stromatolites. More than half of the eleven occurrences were discovered or first described during the last decade, and with the increasing attention being given to Archaean sediments we can expect many more occurrences to be discovered. Many of the occurrences are very imprecisely dated, but it is apparent that stromatolites were an abundant component of the sedimentary environment from about 2 800 Ma ago, and in the late Archaean they formed in both marine and lacustrine environments. Archaean stromatolites are not markedly different from younger examples. Although most have some distinctive features, all can be interpreted by reference to Holocene analogues. What most limits the interpretation of Archaean stromatolites is our profound ignorance of the morphogenesis of Holocene stromatolites. It would be premature to equate the presence of stromatolites with the former presence of mats of cyanobacteria, rather than other kinds of bacteria. Furthermore, even if stomatol i tes are interpreted as cyanobacterial, it must be borne in mind that not all cyanobacteria use water as an electron donor; many use H2S and thus do not release oxygen to their environment. Only for the late Archaean stromatolites do several lines of evidence, when considered together, suggest that the constructing organisms were oxygenic cyanobacteria.

STUDIES OF SOME PRECAMBRIAN STROMATOLITES IN WESTERN AUSTRALIA Kathleen Grey Geological Survey of Western Australia, Perth, Western Australia Western Australian stromatolitic rocks range from early Archaean to Cambrian, providing a reasonably continuous record for testing empirical approaches to Precambrian biostratigraphy. Like other fossils, not all stromatolites have biostratigraphical potential. Stratiform types lack distinguishing features, limiting their utility. Some stromatolites, e.g. cones from Archaean vol caniclastic metasediments near Kalgoorlie, vary little from extant forms, and may show conservative morphological trends because of adaptation to specific ecological niches, e.g. hot spring environments. Others, however, are apparently time-restricted, showing sufficient variation for correlation purposes. Asperia aspera and Pilbaria perplexa occur in the Wyloo Group (approximately 2 000 Ma) and the Canadian Rocknest Formation (l 865 to 2 200 Ma). Twelve taxa occur in the approximately 1 700 Ma Earaheedy Group. Some belong to new groups, others to groups restricted to the Aphebian, but occurring in U.S.S.R., Canada and South Africa. One form is similar to Externia externa from the Canadian Et-then Format ion (1 300 to 1 865 Ma). Conophyton is particularly common in sequences of approximately 1 600 Ma and a small branching form is geographically widespread in rocks of approximately 1 500 M a . A low-diversity assemblage occurs in the 1 100 Ma Bangemall Group. Assemblages are as yet poorly known from younger rocks in Western Australia.

ENVIRONMENTAL CONTROLS ON THE MORPHOLOGY OF MODERN STROMATOLITES AT HAMELIN POOL, WESTERN AUSTRALIA Phi 11ip E. Playford Geological Survey of Western Australia, Perth, Western Australia Environmental factors are generally dominant in controlling the external morphology of stromatolites at Hamelin Pool, but biological factors a 1 so influence some external features, and they largely control the internal stromatolite fabrics. Many Hamelin Pool stromatolites are elongate parallel to the wave-translation direction. However, the prevailing winds are also believed to influence the growth direction of some forms. Columnar stromatolites that are inclined to the south and seif (linear) stromatolites that trend nearly north-south, are thought to have developed in response to the prevailing southerly winds. The seif stromatolites show characteristics suggesting that they have formed as a result of wind-induced paired helical vortices (Langmuir circulation) in water. The Hamelin Pool stromatolites generally require a hard substrate on which to grow. ^ This consists variously of indurated Pleistocene beach ridges, Tertiary quartzite, and calcrete over Cretaceous calcilutite or Pleistocene limestone. Where stromatolites have grown on Pleistocene beach ridges they tend to form curvilinear reefs, with parallel lines of stromatolites in each reef controlled by resistant beds in the underlying beach ridges. In other areas, large domal stromatolites have grown over eroded remnants of indurated solution pipes cutting Cretaceous calcilutite.

DID LIFE EXIST IN THE EARLY ARCHAEAN? EVIDENCE FROM SHALLOW-WATER SEDIMENTS, NORTH POLE, WESTERN AUSTRALIA Roger Buick Depar tment of Geology, University of Western Australia, Nedlands, Western Australia Recently, several reports cla imed that the oldest-known stromatolites and microfossil s occur i n ca 3 500 Ma old cherts at North Pole, P i 1 bara Block, Western Australia. However, further work shows that none are unequivocal rel icts of early Archaean life. Hollow carbonaceous microspheroids have such simple morphologies that a biogenic origin cannot be proved. Solid carbonaceous microspheroids may be aggregates of kerogen squeezed into shape by authigenic grain growth. Newly discovered filamentous microfossils are possibly post-depositiona1 contamination. An isolated nodular structure interpreted as a stromatolite contains no microstructures diagnostic of organosedimentary origin. Mesoscopic comparison with more modern stromatolites is inconclusive, because similar simple forms can be produced by many inorganic processes. A bed of pseudocolumnar structures with kerogenous microlaminae is more likely to be stromatolitic, though again there is no definitive microstructura1 evidence of such an origin. Conglomerates containing clasts with kerogenous laminae from which hollow carbonaceous microspheroids were isolated may be desiccated microbial mats. Some clasts have concentric kerogenous overgrowths, perhaps oncolitic. Sulphur isotope studies of coexisting barite and sulphides show no indications of bacterial sulphate reduction. Although several 1ines of evidence suggest that life may have existed ca 3 500 Ma ago at North Pole, there is, as yet, no conclusive proof of this.

REEFS AND OTHER CARBONATES B I O C L A S T I C C A R B O N A T E S E D I M E N T S OF THE G R E A T B A R R I E R REEF,, A U S T R A L I A P.G. Flood Department of Geology, University of New England, Armidale, New South Wales The variations which can be observed in the component compositions of the reefal sediments of the Great Barrier Reef result from differences in the percentage contributions made by five dominant skeletal types, namely: coral, coralline algae, Halimeda, foraminiferids and molluscs. The distribution within the reef-top environment is controlled partly by the nature of the reefal communities and partly by the production of specific skeleta 1-size modes which are preferentially transported to different depositional environments under a variety of high-energy and/or low-energy hydraulic regimes. The changing nature of the skeletal component composition of the reefal sediments associated with reefs at varying stages of their morphological evolution is clearly discernible. This allows relationships which exist between reef morphology, depositional environments, sediment types, sedimentary facies, and depositiona1 processes to be qualitatively and quantitatively assessed, thereby providing insight into the behaviour of reefal sediments in both a temporal and a spatial context. The commonly occurring sedimentary facies provide an extremely useful basis for pa 1aeoenvironmenta1 reconstructions of pre-Holocene reefs or ancient analogues especially as the Great Barrier Reef is one of the few modern carbonate terrains which have exact counterparts in the rock record (e.g. the Devonian "Great Barrier Reef" of the Canning Basin; the Permian Texas Reef, the Jurassic-Cretaceous buried reef off the Atlantic Coast, Devonian reef complexes of Canada, etc.).

SUBMARINE L I T H I F I C A T I O N

IN C O R A L REEFS OF THE

SOUTHERN GREAT B A R R I E R

REEF

John F. Marshal 1 Bureau of Mineral Resources, Canberra, A.C.T. Study of drill core from One Tree Reef, southern Great Barrier Reef, has shown varying degrees of submarine 1ithificat ion of the Holocene reef framework, the intensity of which appears to be partly environmentally controlled. Reefal environments characterized by a massive framework, consisting predominantly of head corals and thick coralline algal crusts and relatively slow growth rates (av. 3.2 m m y " 1 ) exhibit substantial cementation which is predominantly intraparticle. Environments consisting of an open framework of branching corals and with relatively high growth 1 rates (av. m m y " ) show only minor amounts of cementation. In contrast, those frameworks which are partly open, but which have had their cavities substantially infilled by internal sediments, have undergone extensive interpartic1e cementation. While the mineralogy of the cements is confined to two types (aragonite and Mg-calcite) the textures and fabrics of the cements are diverse. The combined action of endolithic borers, sediment infill, and cementation can transform the originally weak reef framework into a more durable rock, capable of withstanding the substantial impact of the external physical regime, and providing a sound substrate for additional reef growth.

A N C I E N T C O U N T E R P A R T OF THE M O D E R N G R E A T B A R R I E R

REEF

E N V I R O N M E N T OF A U S T R A L I A Saleh M . Billo Riyadh University, Riyadh, Saudi Arabia The Upper Permian pa 1aeogeography of west Texas and southeastern New Mexico provides a typical example of diverse facies formed under marine conditions. Along the margins of the Delaware Basin, a barrier reef at least 480 km long was developed which, at times, served as a boundary between a relatively deep basin and a broad

shelf. it may be comparable to the modern Great Barrier Reef of Australia which is more than 1 600 km long and is the world's best model of this type. The sedimentology of Castile evaporites in the Delaware Basin was effected by seasonal variations in evaporation rate and partial isolation of the basin from the open sea. Modern oceanographic studies of the circulation pattern in somewhat restricted lagoons or embayments show remarkable resemblance in the hydrodynamics of the system.

THE GREAT BARRIER REEF - HIGH-ENERGY ANALOGUE Peter J. Davies Bureau of Mineral Resources, Canberra, A.C.T. Biologic growth, sediment facies and overall reef morphology are controlled by two distinct and separate factors, substrate characteristics and the modern hydrologic regime. The shape and depth (-7 to -23 m) of earlier eroded Pleistocene reefs control initial colonization and growth to sea-level; the hydrologic regime controls growth at sea-level. Two growth phases are therefore recognizable: (1) Vertical growth to sea-level at rates of 1-8 m 10~ 5 years. Biological variations determined by progressively-shallowing water depths. In spite of modern heavy surf environment, algal ridges only incipiently developed. (2) Horizontal backward growth at rates of 5-40 m 10 3 years. Minimum or negative growth rates characterize windward margins while accretion dominates in leeward envi ronments. Early subtidal growth phases are characterized by distinct, juxtaposed sediment facies which change in the later intertidally dominated growth phases into laterally homogeneous sediment facies related to the backward spread of the highenergy focus across the reef top. Accretion on leeward margins increases as the reef evolves. Sequential reef development during a series of alternating transgressions and stillstands will accentuate the leeward growth concept. Developing sedimentary basins, which may be potential petroleum sources and/or reservoirs, will 1ie to leeward of the main reef front. Mass balance calculations for the potential leeward sediment accumulations are in accord with estimated basin thicknesses in the Great Barrier Reef. The growth characteristics of reefs in the Great Barrier Reef are also recognizable in modern shelf reefs of the Campeche Mexico, in the oceanic atolls of the Maldive archipelago, in the Miocene reefs off northwest Sarawak and in the Jurassic reefs of Europe.

RADIOCARBON DATING OF CORAL R E E F S ,

ERROR SOURCES AND AGE STRUCTURE

OF A MID-SHELF REEF, GREAT BARRIER REEF E.G. Rhodes 1 & H.A. Polach 2 Comalco Limited, Glenside, South Australia Radiocarbon Dating Laboratory, Australian National University, Canberra, A.C.T. Radiocarbon dating, the most commonly used method for determining ages of coral reefs, is subject to several major types of error or correction. Data from the stratigraphy of mid-shelf reef on the Great Barrier Reef show that corrections for oceanic reservoir effect impose some uncertainty on Holocene reef dates. Despite this uncertainty, 24 radiocarbon determinations from material beneath Britomart Reef show growth rates in the range 3.5 - 0 m 1 0 " 3 years under the reef front, with a higher range, 4 . 9 - 6 . 1 m l O " 3 years, under the reef lagoon. Although these rates are similar to those reported from other sites on the Great Barrier Reef, the observation of a higher rate beneath the lagoon than under the reef front is not inaccord with some existing models of reef development.

48 INTISAR # D # OILFIELD, LIBYA C.E. Maher Australian Occidental Petroleum, Perth, Western Australia The 1 D 1 oilfield is one of 11 Upper Palaeocene pinnacle reefs in the southern end of the Marsa Brega Trough, Sirte Basin, Libya. It was discovered by Occidental in October 1967 with the use of digitally recorded, six-fold seismic data. The recovery well tested 75,000 B/D. The Palaeocene of the Sirte Basin is characterized by carbonates and shales deposited in an epeiric sea. The Intisar reefs grew in an embayment surrounded on three sides by carbonate banks. The D reef is circular in plan, 5 km in diameter, flanks generally slope 18° and the maximum thickness is 388 m. The reef is coral/algal with grain- and mudsupported biomicrites and minor secondary dolomite. Porosity averages 22 percent and is mostly solution and intergranular. Measured permeability is as high as 500 md. The top reef was 245 m above the surrounding sea floor when growth was terminated by introduction of shale and interbedded marls in the Late Palaeocene. Two porous shoals grew successively around and over the reef before it was capped by Late Palaeocene and Early Eocene shales and marls. The D reef was filled to a spill-point of 2 900 m subsea with a maximum oil column of 293 m. The oil is 40° API gravity, paraffinic and low in sulphur. Original stock tank oil in place is estimated at 1.8 billion barrels. Cumulative production to May 1, 1980 is nearly 900 million barrels. Ultimate recovery efficiency is expected to approach 75 percent. Supplemental recovery was begun early with pressure maintenance by both gas and water injection. Reservoir pressure is maintained at k 000 psi, high enough for miscible gas displacement.

COLONIAL ANIMALS AND THEIR ENVIRONMENTAL SIGNIFICANCE IN CARBONATES R.E. Wass Department of Geology, University of Sydney, New South Wales Colonial animals, represented by both corals and Bryozoa, show a response to environment in the variation of their morphology. Bryozoa have been known for some time to exhibit a variety of growth forms, these forms being related to environmental parameters such as depth, sedimentation, current action and substrate. Their colonies, when fossilized, break into many fragments which, without any knowledge of the Recent organism, may be used incorrectly in environmental interpretation. This has been confirmed by a study of complete colonies of Recent Bryozoa. In those cases where growth forms have been incorrectly interpreted the determining factor is the nature of the attachment of the colony to the substrate. The attachment is rarely preserved in the fossil record. Some groups which appear to be erect and rigid and which were previously considered to be indicative of quiet waters are now known to be able to withstand strong current action due to the nature of their attachment. In some cases the attachment is partially calcified but in others it shows no calcification. Experimental manipulation of settlement panels in the Great Barrier Reef has given a very real idea of the type of colony growth in different environments. Panels were placed at four different localities with d i fferent envi ronmental parameters such as depth, sedimentation, currents and light intensity. Experimental studies on some genera of corals have revealed that morphology which is significant in taxonomy can vary depending on the environment in which the coral occurs. In the experimental studies, some of the environmental changes between localities are very subtle and are unlikely to be recognized in the geological record.

DISTRIBUTION OF STROMATOPOROIBS IN THE DEVONIAN REEF COMPLEXES OF THE CANNING BASIN A.E. Cockbain Geological Survey of Western Australia, Perth, Western Australia Twenty-three species of stromatoporoids have been identified from the Givetian and Frasnian reef complexes and two species from the Famennian reefs. Three species are new, the others being widespread European, Asian and North American forms. Stromatoporoids are almost confined to the platform facies and are most common at the platform margin. Clathrocoilona spissa, Hermatostroma n. sp. and Stachyodes costulata together with Actinostroma papillosum and A. n. sp. are common at the margin; in the platform interior the dominant species is Amphipora rudis with Actinostroma spp. as sub-dominants. Most stromatoporoids in the margina1-slope facies are fragmentary and derived; a few, mainly tabular forms, e.g. Stachyodes australe and Hermatostroma schleuteri, lived in the reefal-slope subfacies. Growth forms are tabular, hemispherical, bulbous and dendroid (in order of increasing surface area/volume ratio); dendroid forms are divided into amphiporiform and stachyodiform. Tabular forms characterize the marginal slope; large tabular and hemispherical ("massive") forms occur at the platform margin; bulbous and dendroid forms predominate on the platform with stachyodiform more common near the margin and amphiporiform abundant in the interior. A similar sequence of growth forms occurs in the Canadian reef complexes (e.g. at Miette). Patch reefs (e.g. Lloyd Hi 11) have a vertical zonation similar to the Belgian reefs with brachiopods and corals toward the base and large tabular and hemispherical stromatoporoids above. This is interpreted as a sere with the "massive" stromatoporoids dominating the climax community.

LOWER CAMBRIAN BIOHERMS:

PIONEER METAZOAN REEFS

Noel P. James Department of Geology, Memorial University of Newfoundland, St John's, Newfoundland, Canada Bioherms of Lower Cambrian age commonly contain a diverse population of organisms dominated by archaeocyathans and calcified algae. By late Lower Cambrian time these bioherms illustrate the complex pattern of biological accretion, internal sedimentation, early 1ithificat ion and biological destruction that characterizes both modern and other fossil reefs. Lower Cambrian bioherms exposed in southern Labrador, Canada, are composed of numerous stacked small mounds. Each mound comprises archaeocyathans and the calcified alga Renalcis in a matrix of lime-mud rich in sponge spicules, trilobite exoskeletons, hyolithids, and brachiopods. Numerous growth cavities roofed by pendant Renalcis are partly to completely filled with geopetal internal sediment. These cavities contain the remains of a relatively diverse fauna of sessi le invertebrates. Bioherms, but not interbioherm sediments, were pervasively cemented on the sea floor and the surfaces of mounds were commonly bored by endolithic organisms (Trypanites sp.). This subsea cement is now preserved as fibrous calcite, which on petrographic and geochemical evidence was probably Mg-calcite, and fans of acicular crystals which are, on similar evidence, interpreted to have been aragonite. These bioherms are similar in structure to the basal pioneer accumulations of much younger Lower and Middle Palaeozoic reefs. They did not develop as massive "ecologic" reefs because archaeocyathan skeletons never developed the necessary large, massive hemispherical skeletons. The spectrum of early cements found in these bioherms, together with the probable mineralogies of the skeletons themselves, suggests that the chemistry of ocean water in Lower Cambrian time was similar to that of today.

LOWER CARBONIFEROUS CONODONT AND OSTRACOD B I O S T R A T I G R A P H Y AND L I T H O F A C I E S OF T H E B O N A P A R T E G U L F B A S I N , N O R T H W E S T E R N

THE

AUSTRALIA

Robert S. Nicoll & P.J. Jones Bureau of Mineral Resources, Canberra, A.C.T. Investigations of new collections of surface and subsurface samples from Lower Carboniferous sediments of the o n s h o r e Bonaparte Gulf Basin, combined with material from previous conodont and ostracod studies provide a biostratigraphic framework of conodont and ostracod taxa, which can be used to interpret the palaeoenvironmental significance of the lithofacies. Two Tournaisian conodont biofacies (offshore and nearshore), and one Visean offshore conodont biofacies are recognized in the basin. Two major ostracod biofacies are recognized; one in Tournaisian shelf carbonates, and the other in Visean basinal shales. Lower Carboniferous deposition took place in two main areas, here called the Weaber Gulf and the Burt Range Embayment. The Weaber Gulf can be subdivided into a deep-water basin centred in the northwest, and a shallow-water shelf, narrow along the southwestern margin and broad on the southeastern margin. During the Tournaisian, the basin received predominately shales, while the Burt Range Embayment was the site of complex clastic-carbonate, shallow-water sedimentation. Widely distributed sand sheets and marine sandstone channels indicate that one or more major river systems entered the sea along the southern or eastern margin of the embayment. In Visean time, the basin continued to be the site of dominantly detrital sedimentation, with minor limestones deposited during periods of reduced clast ic influx. The Burt Range Embayment was significantly reduced in size and, with the rest of the shelf, was occupied by marine shales, siltstones and sandstones.

L A T E S I L U R I A N C A R B O N A T E B U I L D U P S OF T H E G A S P E B A S I N , Q U E B E C , C A N A D A : M I D D L E TO L A T E D E V O N I A N - T Y P E

BUILDUPS

Pierre-Andre Bourque Department of Geology, Laval University, Quebec, Canada Carbonate buildups of Early to Middle Silurian age are well-known in several places of the world, but very few are reported from the Ludlovian to Gedinnian (Late Silurian to Early Devonian) time interval. Moreover, most of the Silurian buildup 'descriptions come from stable shelf areas. Ludlovian-Pridolian carbonate buildups developed at the margin of uplifted blocks, on an unstable shelf area, in the Gaspe Peninsula, Quebec, Canada. In the southern Peninsula, a well-preserved buildup reaches a thickness of 600 mand is composed of three different superposed complexes: (1) a lower reef complex showing a shall owing-upward sequence, from basal colonial rugosan coral bioherms, to stromatactis mud mounds, and climaxing algal reefs; (2) a middle crinoid bank complex where cycles of progradation-rapid transgression are recorded and testify to synsedimentary tectonic activity; and (3) an upper reef complex, contiguous to a sabkha-type supratida I plain, where intertidal, lagoonal, back-reef, reef flat, reef margin and fore-reef facies are well-characterized. The stromatactis mud mounds of the lower reef complex conform to descriptions of the classical Middle to Late Devonian stromatactis mounds of the Dinant Basin in Belgium, some Late Devonian stromatactis mud mounds of Alberta, Canada, or even the Carboniferous Waulsortian reefs of Ireland. Stromatoporoid growth forms and algae are particularly significant for ecological zonation of the upper reef complex. This zonation is very similar to that established for Middle to Late Devonian complexes elsewhere in the world, such as the Miette carbonate complex of Alberta, the carbonate complexes of Central Europe, or the Frasnian reefs of the Canning Basin of Western Australia.

RE I N T E R P R E T A T I O N

OF DEPOSITIONAL AND DIAGENETIC

CAP ITAN REEF C O M P L E X

(PERMIAN), SOUTHWESTERN

PROCESSES, U.S.A.

Lloyd C. Pray Department of Geology and Geophysics, University of Madison, Wisconsin, U.S.A.

Wisconsin,

I n t e r p r e t a t i o n s o f t h e C a p i t a n R e e f C o m p l e x h a v e s h i f t e d w i d e l y in r e c e n t years. C l a s s i c i n t e r p r e t a t i o n s c o n s i d e r e d the C a p i t a n L i m e s t o n e an o r g a n i c r e e f , and backreef evaporites and carbonates products of subaqueous lagoonal processes. In t h e l a t e r 1 9 6 0 ' s , v a d o s e d i a g e n e s i s a n d e m e r g e n t p r o c e s s e s r e c e i v e d h o m a g e . The massive reef w a s reinterpreted as shelf-edge clastic sediment extensively modified by v a d o s e d i a g e n e s i s d u r i n g e p i s o d i c low s e a - l e v e l s t a n d s , t h e e v a p o r i t e a n d p i s o lite w a s r e i n t e r p r e t e d as sabkha p r o d u c t s and c a l i c h e , r e s p e c t i v e l y . The interpret e d pa 1 a e o g e o g r a p h i c h i g h s h i f t e d to t h e p i s o l i t e a n d t e p e e b e l t f r o m a s h e l f - e d g e (barrier reef) position. M o r e r e c e n t r e s e a r c h , p r i n c i p a l l y by U n i v e r s i t y o f W i s c o n s i n g r a d u a t e s t u d e n t s ( B a b c o c k , H u r l e y , N e e s e , S a r g , S c h w a r t z , a n d Y u r e w i c z ) , by E s t e b a n a n d P r a y , a n d o t h e r s (e.g. C y s , M a z z u l l o , S c h m i d t ) s w i n g s the interpretive p e n d u l u m back from v a d o s e to s h a l l o w s u b m a r i n e p r o c e s s e s . Sarg reinterprets the backreef evaporites as lagoonal. Esteban and Pray find little Permian c a l i c h e , believe most pisoliths p r i m a r y , f o r m e d by i s o p a c h o u s c e m e n t a c c r e t i o n a t , o r o n l y a f e w c e n t i m e t r e s b e l o w , the d e p o s i t i o n a l i n t e r f a c e , and c o n s i d e r v a d o s e fabrics m i n o r , late-stage pisolith o v e r p r i n t s . M o s t p i s o l i t e o c c u r s in i n t e r t e p e e d e p r e s s i o n s , p a r t o f u p w a r d s h o a l i n g sequences of a peritidal facies belt. Despite recognition of extensive organic boundstones (Upper C a p i t a n ) , Babcock, Y u r e w i c z , and others stress quantitative and g e n e t i c s i g n i f i c a n c e o f s u b m a r i n e c e m e n t in t h e C a p i t a n r e e f . T h e i r w o r k a n d b a c k r e e f e v i d e n c e o f p r i m a r y s e a w a r d d i p s into t h e C a p i t a n f a v o u r s r e e f g r o w t h o f o r g a n ic a n d / o r c e m e n t b o u n d s t o n e s a t a s u b m e r g e d s h e l f e d g e . Episodic shelf subsidence o c c u r r e d , but more than 1-2 m of sea-level fluctuation seems u n n e c e s s a r y . Research c o n t i n u e s , the pendulum may swing o n .

EARLY SUBMARINE DEVONIAN

CEMENTATION AND

P h i l 1ip E . Geological

ITS C O N S E Q U E N C E S

REEFS OF THE CANNING

BASIN

Playford

Survey of Western Australia, Perth, Western

Australia

T h e r e e f - m a r g i n , r e e f - f l a t , a n d r e e f a l - s l o p e s u b f a c i e s o f Devonian reef comp l e x e s in t h e C a n n i n g B a s i n w e r e s u b j e c t to w i d e s p r e a d e a r l y s u b m a r i n e c e m e n t a t i o n , s o t h a t t h e y f o r m e d r i g i d , w a v e - r e s i s t a n t r i m s to t h e l i m e s t o n e p l a t f o r m s . Submarine c e m e n t a t i o n w a s a l s o c o m m o n in t h o s e f o r e - r e e f d e p o s i t s w h e r e r a t e s o f s e d i m e n t a t i o n w e r e s l o w , b u t it w a s n o t w i d e s p r e a d in t h e b a c k - r e e f d e p o s i t s . E a r l y c e m e n t a t i o n a n d a s s o c i a t e d v o i d s e d i m e n t a t i o n r e s u l t e d in a l m o s t t o t a l d e s t r u c t i o n o f t h e h i g h i n i t i a l p o r o s i t i e s in r e e f l i m e s t o n e s , w h e r e a s m o s t b a c k reef limestones remained largely u n c e m e n t e d at the time of b u r i a l , retaining most of their primary pore space. Early void-filling cements now consist of banded, turbid, fibrous s p a r , w h e r e a s late (post-burial) cements form c l e a r equant s p a r . Fracturing of the rigid early-cemented limestones, probably resulting from e a r t h q u a k e s h o c k s , led t o t h e w i d e s p r e a d d e v e l o p m e n t o f n e p t u n i a n d y k e s in r e e f 1 imestones and some fore-reef limestones. S u c h f r a c t u r i n g a l s o led to t h e c o l l a p s e of s e c t i o n s o f t h e p l a t f o r m m a r g i n s , in t u r n o f t e n i n i t i a t i n g m a s s i v e d e b r i s f l o w s d o w n the m a r g i n a l s l o p e s . S t y l o l i t i z a t i o n a n d a s s o c i a t e d c o m p a c t i o n a f t e r b u r i a l w e r e g r e a t e s t in t h e most porous limestones, so that back-reef limestones are generally more intensely s t y l o l i t i z e d , and have c o m p a c t e d m o r e , than reef l i m e s t o n e s . This has resulted in the typical concave shape of many platforms. Cementation concomitant with stylolit i z a t i o n d e s t r o y e d m o s t o f t h e p r i m a r y p o r e s p a c e t h a t h a d r e m a i n e d in t h e r e e f c o m p l e x e s a f t e r e a r l y s u b m a r i n e d i a g e n e s i s , a n d t h e m a i n p o r o s i t y n o w f o u n d is s e c o n d a r y m o l d i c p o r o s i t y in d o l o m i t e .

52 SUBMARINE EROSION SURFACES AND RETREAT OF CARBONATE BANK M A R G I N S , (PERMIAN)v SOUTHWESTERN

U.S.A.

Lloyd C. Pray Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin, U.S.A. Bank margin erosional processes, other than down-slope channeling, are important but poorly understood. At the Northwestern Shelf-Delaware Basin transition area, as exposed along the Guadalupe Mountains escarpment, abundant submarine erosion surfaces occur within, or separating,parts of the mid-Permian Victoria Peak, Bone Spring, Cutoff, Grayburg, and Goat Seep Formations. Most occur at the outer 1 - 2 k m o f the shelf edge and along the basin margin slopes; some continue into the basin. Major types of erosion surfaces are basin-trending channels; two basinsloping undulose surfaces, considered submarine unconformities, that truncate 200 300 m of shelf-edge strata in 1 - 2 km; and two basin-sloping surfaces (20 - 50°), each abruptly truncating 100 m of shelf-edge (bank) carbonates that resemble slump scars in profile. The facies mosaic and abrupt truncation of flat-lying bank strata suggest that submarine erosion appreciably steepened bank margin depositional slopes and caused bankward retreat of 0 . 5 - 1 km or more. Density currents, saline or cold, originating on the shelf seem more likely agents of bottom scour than contour currents. Some bank carbonates were lithified prior to erosion; there is little evidence of siumping. Kilometre-scale shelfward retreat of the deeply submerged Blake Escarpment has been hypothesized by other workers. Modern and ancient carbonate bank margins need scrutiny to better understand submarine erosional phenomena occurring within their facies mosaics.

EARLY DIAGENESIS AT MODERN CARBONATE PLATFORM MARGINS Noel P. James Department of Geology, Memorial University of Newfoundland, St John's, Newfoundland, Canada The use of submersibles has dramatically altered our understanding of the depositional and diagenetic processes that take place on the deep margins of modern carbonate platforms. The margins of most Caribbean shelves and platforms consist of an upper steep marginal rim and a lower gently-inclined slope. The shallow rim is genera 1lya reef complex consisting of the reef proper fronted by a steep cliff (the reef w a l l ) , the base of which is in turn buried by a fore-reef accumulation of reef-derived sediment. This zone grades into the slope proper at a depth of ca 250 m . The slope, which may extend to depths of several kilometres, is mantled by fine-grained pelagic sediment (peri-platform ooze). The surface of the slope is often cut by arcuate slump scars and dissected by gullies which run downslope. These gullies are eroded by episodic transport of fore-reef material basinward and the resultant gravity flows are deposited near the toe of slope. Holocene reef limestones from the shallow marginal escarpment exhibit extensive rapid multigeneration cementation by micritic to fibrous Mg-calcite and acicular aragonite. Growth cavities are partly occluded by botryoidal aragonite. Depositional fabric is commonly altered to wackestone or mudstone by repeated generations of boring, internal cementation and rapid 1ithification. Slope and adjacent basinal peri-platform ooze remains soft unless pelagic cementation is arrested by erosion or continuous current action, in which case the fine-grained deposits at the sediment-water interface are rapidly lithified. Resultant limestones in the cool waters of the thermocline layer are mostly cemented by Mg-calcite which contains progressively less Mg with increasing water-depth and decreasing water temperature. Below about 1 000 m , where surrounding water temperatures are 4°C or less, 1 i thif ication results f rom recrystal 1 izat ion of a) 1 metastable components to calcite by solution-reprecipitation.

CARBONATE PLATFORMS OF EXTENSIONAL CONTINENTAL MARGINS J.F. Read Department of Geological Sciences, Vi rginia Polytechnic Institute and State University, Blacksburg, Virginia, U.S.A. Carbonate platforms of extensional margins may be grouped into several major categories. Homoclinal ramps have gentle slopes into deep water, with skeletal or ooid/pellet sand shoal complexes that grade without break in slope into deep ramp nodular limestone, and then into pelagic/hemipelagic basin facies; they generally lack significant slump and sediment gravity-flow deposits in the deeper water facies. Distally steepened ramps differ from the above in having a marked increase in slope at the seaward edge of the deep ramp, and abundant slumps, slope breccias and turbidites. However, clasts of shallow platform margin facies are generally absent from breccias. Rimmed shelves have linear trends of shelf-edge 1 ime sands and reefs, a marked increase in slope into deep water, and foreslope and slope sands, breccias (with clasts of platform margin rocks) and turbidites, grading seaward into basinmargin hemipe1agic/peIagic muds. They may be divided into accretionary, bypass and erosional margins. Isolated platforms are broad flat-topped shallow platforms surrounded by deeper water (few hundred metres to k km deep); most are bypass margins but accretionary and erosional margins also occur. Finally, drowned or open platfo rms may develop by rapid submergence of ramps, shelves or isolated platforms; platform margin facies are shifted landward and the earlier shallow-water platform is covered with transgressive lags, and deeper water blankets of hemipelagic or pelagic facies or open marine, whole-fossil wackestones. The various platform types may be recognized from continental margin sequences ranging from Proterozoic to Holocene in age.

CARBONATE RESERVOIRS IN TERMS OF T E C T O N I C S , DEPOSITIONAL SETTING AND RELATIVE SEA-LEVEL CHANGE C.G.St.C. Kendall 1 2

S P.M. Harris

Gulf Research, Harmaville, Pennsylvania, U.S.A. Gulf Research, Houston, Texas, U.S.A.

Carbonate reservoirs occur on interior shelves behind basin margins, on ramps or rims along margins, and within basins. They have three major tectonic settings: (l) intracratonic basins (sags and fault-bounded); (2) extensional margins (including isolated platforms); and (3) convergent margins (foreland and back-arc basins). Reservoir geometry and stratigraphy vary in response to relative sea-level changes. Thus original depositional geometry is preserved when carbonate upward growth is exceeded and terminated by relative sea-level rise. On ramps when carbonate sedimentation lags behind relative sea-level rise and then locally catches up, discrete thick buildups may form in shallow and downslope positions. Similarly, rimmed shelves may be partially drowned while the rim and/or local buildups on interior shelves continue to upbui Id. All buildups may eventually be enveloped by prograding coastal carbonates or basinal shales. When carbonate sedimentation keeps up with rising sea-level, ramps steepen into rimmed shelves and basins become starved. Carbonate accumulation is terminated by clastic influx and falling sea-level. Basin fills may enclose highstand buildups and prevent their regrowth. The most prospective tectonic settings are interior shelves back from the basin margin, e.g. Permian Basin, Texas and the Middle East. Field sizes are greatest in interior shelves with a history of compression, e.g. Gawar and Berri. Bui 1 dups formed during catch-up are some of the lowest risk prospects, e.g. pinnacles in Indonesia, the Michigan and West Canadian Basins.

CARBONATE AND SHELF SEDIMENTATION Z O N A T I O N OF

INTERTIDAL CARBONATE ENVIRONMENT, SPENCER REGIONAL AND GLOBAL

GULF:

SIGNIFICANCE

R.V. Burne Baas Becking Geobiological Laboratory, Canberra, A.C.T. The relationships between frequency of tidal inundation, topography, depositional environment, and sedimentary facies have been examined in four contrasting areas of the prograding carbonate shoreline of northeastern Spencer Gulf. Although exposed coasts and protected coasts give rise to different associations, bothshowa distinction between frequently inundated low intertidal environments in which bioturbated and homogenized sediments form, and high intertidal zones characterized by laminated facies. Spencer Gulf contrasts with other wel1-described areas of intertidal carbonate deposition in that it has a moderate tidal range (3-5 m), cool waters (10-26°C), and lies within a semi-arid climatic zone. Comparison with data from the Bahamas, Florida, and the Persian Gulf reveals close parallels with the environments and facies of Spencer Gulf mangrove tracts, cyanobacteria1 marshes, intertidal creeks, and low intertidal zones. However, the greater tidal range of Spencer Gulf allows the development of extensive intertidal flats previously described only from areas of terrigenous sediments. An idealized sedimentary sequence for Spencer Gulf peritidal sedimentation is intermediate in character between those derived from studies in the Bahamas and the Persian Gulf.

POST-GLACIAL NON-TROPICAL SHELF CARBONATE OF T H E R O T T N E S T S H E L F , W E S T E R N

SEDIMENTATION

AUSTRALIA

Lindsay B. Col 1ins Western Australian Institute of Technology, Bentley, Western Australia The Rottnest Shelf is a narrow, open, swell-wave dominated shelf in a passive continental-margin setting adjacent to a hinterland of low relief and sluggish drainage. Conditions of high physical energy, low nutrients in cool subtropical waters, and rapidity of the post-glacial transgression have combined to limit carbonate productivity, restrict grain types and rework the transgressed surface, so that only thin (<1 m) blankets of carbonate and relict sediment have been deposited in the absence of terrigenous influx. Subaerial weathering of the shelf surface during the Late Pleistocene emergence was followed by post-glacial transgressive drowning, erosional shoreface retreat, and generation of a basal transgressive lag sand sheet. Establishment of the contemporary foramal shelf biota, dominated by bryozoans and calcareous red algae, resulted in bioerosion of the shelf unconformity surface and generation of biohermal hardground veneers and blanket-like bodies of skeletal carbonate sediment. Linear topographic ridges composed of Pleistocene limestone partition the landward shallowing shelf into three discrete process regimes and sedimentary systems, which have varying physical energy, biota and sediment supply. Systems and sediments are: (1) Coastal Sys tem (shore zone to MSL - 20 m): beach ridge -dune barrier complex and allochthonous grainstone banks; (2) Inner Shelf System (20-60 m): blanket-like relict, residual and biogenic grainstone, and biohermal algal boundstone; (3) Outer Shelf System (60-250+ m): stone and skeletal wackestone.

blanket-like bryozoan grainstone to pack-

Sediments of the Rottnest Shelf comprise a Holocene analogue of non-tropical unconformity-bound, Cainozoic prograding shelf-carbonate cycles characteristic of ' the southwest and southern Australian continental margins.

T E R T I A R Y D E P O S I T I O N S . H I S T O R Y OF THE N O R T H W E S T S H E L F , W E S T E R N A U S T R A L I A M . Apthorpe Woodside Offshore Petroleum Development, Perth, Western Australia Tertiary sedimentation commenced with Early Palaeocene shales and limestones deposited diachronously on a sloping continental margin. Deepening throughout the Palaeocene culminated in the deposition of bathyal marls during the latest Palaeocene and earliest Eocene. Climatic and sedimentological differentiation produced a complex pattern of clastic and carbonate sediments during the Early to M M d l e Eocene. By Late Eocene time carbonate sedimentation had become widespread. A regional Early Oligocene hiatus coincided also with a period of shelf downwarping. This was followed by massive prograding over the edge of the shelf of dipping wedges of turbidites, and subsequently by the deposition of increasingly shallow-water carbonate facies as the shelf built upwards. The Middle Miocene shelf-building was terminated by a rapid shelf downwarp in the latest Miocene. After deepening of the continental m a r g i n , the latest Miocene to Pliocene were represented predominantly by m a r l s , deposited in deep shelf and slope environments. The paper outlines the sequence of these events as interpreted from palaeontological and lithological studies of offshore petroleum exploration wells and illustrates the sequence by means of facies maps and se i smi c sect ions. The influence of regional (plate tectonic) and local (climatic and sedimentological) factors on the sedimentary history of the area are tentatively assessed.

E V O L U T I O N OF A C A R B O N A T E C O N T I N E N T A L S H E L F , VIRGINIA APPALACHIANS,

CAMBRIAN-ORDOVICIAN,

U.S.A.

J.F. Read Department of Geological Sciences, Vi rginia Polytechnic Inst i tute and State Universi ty, Blacksburg, Virginia, U.S.A. The Cambro-Ordovician platform, Appalachians, eastern North America, is a prism of shelf carbonate 2 to 3 km thick that developed in a marginal (back-arc) basin setting between the North America craton and a Precambrian-Cambrian magmatic arc. It has a basal clastic phase overlain by a carbonate ramp phase that developed (in the Middle Cambrian) into a rimmed shelf with steep slopes, and abundant foreslope sands, megabreccias and turbidites. Inner shelf facies are cyclic, upward-sha 1 lowing peritidal carbonates. In the Upper Cambrian, a short-lived, intrashelf basin developed on the shelf, and was filled by storm-generated, shale, siltstone and limestone-conglomerate sequences. Drowned shelf phases occur in the Lower Cambrian and Lower Ordovician and are thick (80 to 300 m) sequences of subwave-base, thinbedded, pellet wackestone/mudstone and intraclastic layers that shallow upward into cyclic facies. During arc-continent collision, the continental shelf was uplifted and exposed, and then foundered; thick synorogenic clastics were deposited in a foreland basin on the old shelf during overthrusting. The similarity of evolutionary stages of this exposed Palaeozoic continental shelf to those of Mesozoic and Holocene shelves makes it a valuable ancient analogue for study of these geologic features.

SHELF S E D I M E N T A T I O N

IN T H E C R E T A C E O U S S E A W A Y OF W E S T E R N

CANADA

R.G. Walker McMaster University, Hamilton, Ontario, Canada A shallow cratonic seaway existed throughout the Cretaceous in western Canada, extending at times north-south from the Arctic to the Gulf of Mexico. T h e s o u r c e o f sediment was the actively rising Cordillera to the w e s t . Shallow marine sandstones at several horizons are dominated by hummocky cross stratification (HCS), a long wavelength, low amplitude undulating stratification believed to be formed by storm waves below fair-weather wave base. In many of these sandstones there is no evidence of shoreline facies, nor any evidence of fair-weather (tidal, longshore) currents in the form of preserved cross-lamination and cross-bedding. The model developing for

the seaway suggests a storm-dominated system, in which sediment is entrained into density currents by hurricane activity at the coast. These density currents flow down the palaeoslope (as did the Hurricane Carla density current in Texas in 1961), depositing turbidites below storm-wave base, and HCS above storm-wave base but below fair-weather wave base. Conglomerates up to 10 m thick are interbedded with the sandstones. These have traditionally been considered as beaches - the new stormdominated basin concept recognizes them also as offshore storm deposits. A correct interpretation of the conglomerates is important - they provide streaks of the best porosity and permeability for abundant gas and oil in stratigraphic traps.

TRANSGRESSIVE/REGRESSIVE SEDIMENTATION STYLE IN THE LATE CRETACEOUS OF SAN JUAN B A S I N , NEW M E X I C O , U.S.A. Wi11iam J. Stone New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico, U.S.A. An excellent record of the transgressions and regressions by the western shoreline of the shallow sea that bisected North America in Late Cretaceous time is preserved in the San Juan Basin. Analyses of field and subsurface data have shown that three general types of deposits accumulated along this shoreline: coal measures in coastal-swamp/wave-dominated delta environments, sandstones in beach/wavedominated delta-front environments, and mudstones in offshore environments. More specifically, delta-plain, estuarine, backshore, foreshore, shoreface, offshore-bar, offshore-shoreface transition, and offshore facies have been recognized. Knowledge of sedimentation style is important in evaluating the coal, petroleum, and water resources in these deposits. True transgressive deposits are rare; facies are usually arranged in progradational sequences regardless of whether they occur in regionally regressive or transgressive deposits. The predominance of progradation resulted from a high sediment-supply rate. Because of low relief in coastal areas, small changes in sealevel produced broad changes in shoreline position. Following a sea-level rise, offshore mud was often deposited on coal measures prior to eventual burial by prograding coastal sand. Sea-level fluctuations were triggered by regional subsidence and deposition as well as world-wide sea-level and climatic changes.

COLD-WATER CARBONATES:

RECENT, LAST GLACIAL AND PERMIAN

TASMANIAN

EXAMPLES

C. Prasada Rao Department of Geology, University of Tasmania, Hobart, Tasmania Modern carbonate in Tasmania is mixed with and grades into pre-Holocene glacial carbonates. Submarine calcite cements compose up to 90 percent by volume of bryozoan sand grains. The 6 0 1 8 and 6 C 1 3 values of fauna and cements are in equilibrium with cold, ambient marine waters and follow the linear-trend of cold, deep-sea lithified carbonates, thus differing from warm shallow-marine carbonates. Permian calcite cements are overlain by micrite, marine-mud, dropstones, and geopeta1 fillings of well-sorted internal marine sediments, and are cut by borings; all proving both synsedimentary ages and submarine origin of cements. These marine cements formed at temperatures <3°C. The mixing-zone cements formed, succeeding marine cements, are cloudy, inclusion-rich, coarse plates. The mixing zone cementation is preceded by erosion of early-formed crystals. The eroded crysta1s now occur as inclusions in mixing-zone cements. The Australian Permian fauna and whole rocks are characterized by heavy carbon and abnormally light oxygen. These oxygen values give us very unrealistic temperatures ranging between those of a boiling sea and a frozen sea. The calculated original o ^ g e n values, corresponding to marine carbon values from the linear-trend of 60 -6C in cold-water carbonates, give us reasonable Permian temperatures from 15 to -8°C with the coldest waters of k to -8°C around Tasmania. The Australian Permian sediments reacted with melt-waters.

CAINOZOIC

SEDIMENTATION

RESIDUAL-COLLUVIAL FORMATION OF SANDSTONES, GRITS AND SILCRETES C.R.M. Butt CSIRO, Division of Mineralogy, Floreat Park, Western Australia Sandstones, grits and silcretes overlie granitic and acid sedimentary rocks over much of semi-arid and arid western and central Australia. In the Barr-Smith Range of Western Australia, these sandstones are developed from kaolinized granitic rocks by the solution of kaolinite in upper parts of the profile and the consequent settling and compaction of the resistant quartz grains. The grits may also form columns or dykes, penetrating downward into the underlying saprolite. The grits are cemented by quartz, anatase and zircon (QAZ) to form silcretes, and/or by allophane, in places forming hard alumino-siIcretes. The profiles are characterized by low levels of alkalies and alkaline earths and most metals. The QAZ-silcrete horizons commonly contain over 3% Ti0 2 and 1 000 ppm Zr. The profiles evolved through at least four stages: (1) ^ Solution of kaolinite near the top of the profile, under very ac i d conditions, causing settling of resistant quartz grains, forming a sand grit-kaolinitic sapro1ite profile; (2) Precipitation of QAZ-cement, the Ti0 2 and Si0 2 being derived largely by lateral migration from upslope. Precipitation occurred mainly at the base of sand horizons, not penetrating the kaolinite, forming layers, columns and dykes of silcrete. Precipitation and hardening of this cement halted the development of the saprolite-grit profile; (3) Precipitation of allophane, in both the sandy grits and the saprolite. Additionally, continuing precipitation on the exposed face of breakaways has caused surface case-hardening; (4)

Erosion and exposure of the profiles by pedimentation.

The sandy grit-kaolinitic saprolite profile probably formed under humid conditions, as the equivalent of ferruginous laterites developed on more basic rocks nearby and of lateritic bauxites in the Darling Range. However, the sand was a surface horizon and there is no evidence that there was ever a ferruginous zone. The sequential precipitation of QAZ- and a 1lophane-cements, and later erosion, were a response to increasing aridity. A1lophane-cemented materials tend to disintegrate on exposure but they are probably more abundant than the more prominent silcretes.

QUATERNARY SEDIMENTARY ENVIRONMENTS IN THE LAKE EYRE REGION, SOUTH AUSTRALIA John A . Dulhunty Department of Geology, University of Sydney, Sydney, New South Wales A wet climatic phase between 45 000 and 25 000 years B.P. reduced aridity and filled Lake Dieri, the Pleistocene ancestor of Lake Eyre. Salinity layering produced a mixed fresh and salt water sedimentary record. Lake Dieri dried up with a return to aridity which reached a maximum some 14 000 years B.P. The water-table fell and Dieri sediments deflated. About 12 000 B.P., climate changed again to a wetter phase and the water-table rose, establishing ephemeral Lake Eyre in the deflated area, and Holocene sedimentation commenced. At about this time the lake bed tilted to a slope of some 3.9 cm per km, which developed three sedimentary environments: (1) the northern area of the lake, a saline playa environment without salt crusts, from which water drained before wholly evaporating; (2) the southern area, a terminal salina environment where final evaporation of brines left salt crusts overlying gypseous silt and clay; and (3) between these two areas, a slush zone formed with a saline flocculation environment where fine clay and colloidal organic

58 matter were deposited by flocculation, when muddy water from the northern playa met highly saline water of the southern salina. In all three environments, sedimentation was controlled by deposition-deflation equilibria in relation to water-table levels. A COMPARISON BETWEEN TEMPERATE AND TROPICAL DEPOSITIONAL

INTERTIDAL

ENVIRONMENTS

A.P. Belperio Department of Geology, University of Adelaide, Adelaide, South Australia Clastic intertidal environments around the world display enormous variety in form and physical characteristics. A classification sequence of tidal flats is proposed, based upon sediment reworking rates and the trap efficiency of the coast 1ine. Sedimentation models of Recent North Sea intertidal deposits are commonly used in the interpretation of ancient sequences. These tidal flats are characterized by effective sediment reworking, and they display specific grain attributes such as good sorting, rounding and mineralogical maturity. Distinct fining-upward sequences and equilibrium profiles may be generated. By contrast, tidal flats found along muddy tropical coasts display very immature grain characteristics and progradation generates highly variable vertical sequences. Trap efficiences of these environments are very much higher and sediment reworking is limited. Limitations for the proposed classification scheme are presently due to the paucity of accurate data on rates of gross-sediment reworking. Nevertheless, geologists interpreting ancient sedimentary deposits in terms of intertidal deposition should be aware of the complete spectrum of modern intertidal sedimentary sequences.

LATE CAINOZOIC MARINE SEDIMENTATION SOUTHEAST AUSTRALIAN P.S. Roy 1

P A T T E R N S ON

THE

MARGIN 2

S B.G. Thorn

Geological Survey of New South Wales, Department of Mineral Resources, Sydney, New South Wales Department of Geography, Faculty of Military Studies, University of New South Wales, Duntroon, A.C.T.

Coast and shelf sedimentation during the late Cainozoic in southeast Australia is characterized by increasingly frequent eustatic sea-level oscillations. These have been superimposed on a continental margin subsiding slowly due to thermal contraction and block faulting following continental breakup in the late Mesozoic. The coast and inner shelf in southern New South Wales (N.S.W.) has been an area of predominant marine erosion since the late Pliocene. As well, tectonic movements in the adjacent southern highlands may have influenced sedimentation patterns in late Miocene times. Increased wave and wind reworking in the Quaternary, compared with the preceding period, has produced a mature quartz sand population on the inner shelf and coast. Mass transfer of sand toward the north, mainly by littoral drifting at virtually all sea-level positions, is thought to be responsible for the increasingsize and stratigraphic complexity of marine sand deposits at the coast and for a progressive sand build-up (causing shallowing) on the northern shelf. Aeolian transport becomes more important in the north, especially during periods of low sea-level when the broad, shallow shelf is exposed, and has culminated in the formation of large dune islands on the southern Queensland shelf. Due to differing shelf morphologies, contrasting modes of late Quaternary sedimentation are proposed for the southern and northern parts of N.S.W. The southern shelf has been characterized by erosional transgressive stratigraphy with sediment tending to be lost seawards while the northern shelf is characterized by depositional transgressive stratigraphy that results in net sediment accumulation. Post-glacial (Holocene) deposition patterns reflect variations in coast and shelf morphology established earlier in the Quaternary. Coastal deposits in southern N.S.W. generally are small and occur in compartmented embayments on a mainly rocky coastline. In the north, embayments are larger and many are filled with Pleistocene (Last Interglacial) sand barriers; littoral bypassing is a common phenomenon in these areas. Only in central N.S w ' are large dual barrier systems of Last Interglacial and Post-glacial age well developed

HEAVY-MINERAL-BEARING SILICICLASTIC AND BIOCLASTIC BARRIER S A N D - D E P O S I T S

ON THE SWAN COASTAL PLAIN

John L. Baxter Western Australian Institute of Technology, Bentley, Western Australia The Swan Coastal Plain and adjacent Rottnest Shelf is covered by sand deposited unconformably on a pedogenic calcrete surface. The sand deposits are either alluvial deposits, lag sands or parts of barrier complexes. DSDP drilling in the South Pacific Ocean and Cainozoic stratigraphy in southern South Australia indicate that during the Cainozoic there have been numerous transgressive-regressive cycles. Many of these return to within 5 m of the present sea-level at transgressive peaks. Each transgression carries a shoreline sand prism, and this barrier-prism migrates landward as transgression proceeds. The Swan Coastal Plain has at least three shoreline sequences preserved and each is characterized by a complex group of beach and dune deposits. The two shorelines farthest inland, the Yoganup and Capel Shorelines, are si1iciclastic whereas those near the present coast contain bioclastic carbonates. Heavy minerals occur in all of the sequences but are generally better concentrated in the si1iciclastic barrier-sand deposits, where dilution by organic carbonate grains does not occur. The source of the heavy minerals and the si1iciclastic fragments is the Yilgarn Block, to the east of the Swan Coastal Plain. These materials are brought down by rivers and deposited on the coastal plain and near-shore areas more effectively during regression. However, most sediment from rivers draining low relief plateau areas is silt and clay. It is not until sorting occurs within the barrier-prism that heavy minerals and the sand grains are separated. The sand-sized grains remain in the barrier prism, while the silt and clay are dispersed in offshore deposits.

LATE TERTIARY AND QUATERNARY STRATIGRAPHY OF THE SWAN COASTAL P L A I N , NEAR P E R T H , WESTERN

AUSTRALIA

A.D. Allen Geological Survey of Western Australia, Perth, Western Australia Since 1972 about 250 exploratory bores for groundwater have been drilled on the Swan Coastal Plain about 70 km to the north and south of Perth. Data from these bores have provided new information about the Late Tertiary-Quaternary stratigraphy. The Late Tertiary-Quaternary formations are up to 150 m thick and rest on an irregular, seaward-sloping unconformity ranging from +30 m to -30 m M.S.L. The shallow-water marine Ascot Limestone (Pliocene) was deposited on the unconformity and subsequently unconformably overlain by the littoral and eolian Yoganup Formation (Early Pleistocene). Locally the Yoganup Formation is overlain by the estuarine Jandakot Beds (Early-Middle Pleistocene) and disconformably by the estuarine and littoral Guildford Clay and its lateral equivalent, the Gnangara Sand (Early Pleistocene). These formations are disconformably overlain by the eol ian Bassendean Sand (Late Pleistocene) which is unconformably overlain by the eolian and littoral Tamala Limestone (Late Pleistocene) along the coastal strip. The Safety Bay Sand (Holocene) overlies the Tamala Limestone and deposits of Holocene age were deposited in the lakes and estuaries of the coastal plain. The Late Tertiary-Quaternary sediments form a complex sequence deposited and eroded during eustatic changes in sea-level. Their deposition has also been affected by palaeogeography and possibly subsidence in the Vlaming sub-basin.

ASSESSING COASTAL EROSION POTENTIAL - A GEOLOGICAL MODEL P.S. Roy Geological Survey of New South Wales, Marine Geology Section, Sydney, New South Wales Indications of erosion are widespread along the exposed coast of New South Wales. Detailed studies in a few areas have identified various erosion mechanisms that have operated over geological time-spans. A general geological model is proposed that links the nature of the Holocene coastal sand deposits (mainly barriers) with their erosion potential. Two phases in barrier evolution are envisaged: a growth phase followed by a decayed phase. The growth phase is characterized by the transfer of sand from the inner continental shelf on to the coast (except in rare cases, rivers in southeast Australia have not contributed directly to this phase of barrier-build). Barrier building commenced as sea-level approached its present position (ca 6 000-7 000 years ago) and continued at rates and for durations that varied depending on the availability of sand, embayment size and orientation and the configuration of the inner shelf surface. These factors controlled initial barr ier size and stratigraphy. The decay phase is characterized by a loss of barrier sand due to sediment budgetary factors such as littoral drift, winds, tidal currents and offshore transport to deep water. Initiation of the decay phase and the rate barriers erode depends not only on the effectiveness of the various factors mentioned above, but also on the type of barriers that initially formed. Thus the nature of the present-day barriers is seen as a product of both early geological factors and later evolutionary changes. Four main barrier types have been recognized: (1) (2) (3) (4)

Prograded barriers Transgressive dune barriers (including composite barriers) Stationary barriers Receded barriers (including mainland beach)

(1) Prograded barriers have multiple beach ridges and a relatively thick sand sequence indicating an abundant sediment supply. They occur in deeply indented embayments (effective sediment traps) with a relatively broad and shallow offshore zone. In a number of cases these barriers are found at the downdrift end of a littoral drift coastal sector. Many have accreted up to the last 2 000 years and a few are still growing. Prograded barriers are believed to represent the most stable shoreline conditions on the New South Wales coast. (2) Transgressive dune barriers are composed of dune masses that have migrated inland from shoreline positions at or below present sea-level. They indicate past influxes of sediment that were presumably accompanied by shoreline recession. Usually several episodes of dune building can be identified. Where dunes overlie and postdate beach ridges, the barrier is a composite feature. Transgressive dune barriers occur in south-, or southeast-facing bays that have trapped an abundant sand supply; littoral drift losses from such embayments are thought to be quite small. Present-day changes depend on modern dune activity and especially on the rate sand is being lost from the beach through blowouts. Wei 1-developed vegetated foredunes thus indicate coastal stability in such embayments. (3) Stationary barriers are relatively narrow features that commonly block coastal lagoons. Beach ridges are poorly developed or absent and the barrier surface is usually overlain by a foredune complex. Offshore areas are relatively deep and steeply sloping. The barrier sand body is of intermediate thickness and there is little evidence of progradation in late Holocene times. This latter is probably due to a restricted sand supply which limited progradation, followed by slow erosion that destroyed the seaward-most beach ridges. Stationary barriers therefore indicate slow coastal recession. Receded barriers and mainland beaches are characterized by backbarrier or older deposits that either crop out on the beachface or are thinly overlain by beach and nearshore sediments. The barrier sand deposits are very thin. Originally barriers in these embayments were located further seawards but a 11 signs of progradation have been destroyed. However it is likely that the original sand supply for these embayments was quite limited and the barriers were never very large. This barrier type provides the clearest evidence of severe coastal erosion.

The model thus ranks long-term erosion potential according to the barrier types listed above: embayments with prograded barriers are most stable while those with receded barriers and mainland beaches are most erosion prone.

AN I N V E S T I G A T I O N OF S T R A N D E D B E A C H R I D G E S , S H O A L B A Y , NORTHERN TERRITORY:

A SMALL CHENIER

PLAIN?

S.H. Hickey Department of Mines and Energy, Darwin, Northern Territory Large chenier plains are commonly found along the coastline of the Northern Territory. I n contrast, the geology of the Darwin area inhibits such pla i n development, providing a monotonous sequence of Cretaceous cliff and beach embayment features with thick mangrove swamps along the lowlands. Camerons Beach at Shoal Bay provides a change to investigate a "chenier-1ike plain" in close proximity to Darwin. The beach is narrow, cut off at either end (east and west) by small but regularly flooding creeks. Behind the beach a small tidal flat has built up with narrow east-west trending, highly bifurcating small perched cheniers covering it. The number of cheniers varies considerably. An area containing five cheniers was chosen for investigation. The results indicate rapid progradation of the chenier plain seaward since 2 500 B.P. During this period a minimum of five cheniers were built on an area as small as 500 m, a lot narrower than the other plains found on the north coast of the Northern Territory. The results indicate a stable sea-level since 2 500 B.P., ample (though irregular at times) sediment supply from Micket and King Creeks, and moderate to high periodicity of storms attacking the Shoal Bay coastline. These parameters combine to provide the mechanism for chenier plain progradation, with this area undergoing more intense cycles, hence closer-spacing chenier ridges and asma11er chen ier pi ain.

D E P O S I T I O N A L M O D E L FOR A C H E N I E R P L A I N , G U L F OF

CARPENTARIA

E.G. Rhodes Comalco Limited, Glenside, South Australia The southern coast of the Gulf of Carpentaria, northern Austra1ia, consists of a broad chenier plain which is comparable in size and morphology to those reported on the coasts of Surinam and southwestern Louisiana. The Carpentaria chenier plain has, at some locations, prograded over 30 km since the middle Holocene by deposition of low-tide muds over subtidal muds during periods of increased sediment input by the rivers. Chenier ridges are formed during periods of reduced sediment flux by wave-induced sorting and redistribution processes. An alternation of increased and decreased sediment flux from fluvial sources has occurred several times during the upper Holocene. Such variation in sediment supply is believed to be related to temporal fluctuations in rainfall over the drainage basin which surrounds the southern and eastern Gulf.

GEOCHEMISTRY AND DIAGENESIS OF SEDIMENTS

OXYGEN AND CARBON ISOTOPES OF COLD-WATER CARBONATES (RECENT, LAST GLACIAL AND PERMIAN), TASMANIA C. Prasada Rao S D.C. Green Department of Geology, University of Tasmania, Hobart, Tasmania The 6 0 1 8 and 6C 1 3 values of micrites, lithified bryozoan sand grains and unlithified bryozoa of the present Tasmanian shelf grade into shallow to deep sea cold-water (5 to -10°C) carbonates and follow the linear trend of deep-sea lithified carbonates. The 60 1 8 variation with depth of sample collection shows that the modern cool-temperature carbonate in Tasmania is mixed with,and grades into,last glacial carbonate. Australian Permian brachiopods and molluscs and whole rocks are characterized by abnormally light 60 1 8 and much heavier 6C 1 3 than in modern and last glacial cooltemperate Tasmanian carbonates. The Permian fauna with the heaviest (+2.2% 0 ) and lightest (-16.9%O) $ 0 1 8 values have similar 6C 1 3 values ( ~ + 4 % O ) . This 6 0 1 8 - 6 C 1 3 trend in the Permian fauna and the occurrence of a uniform range of Sr contents through the entire range of 6 0 1 8 values in fauna and whole rocks argues against hot, subsurface diagenesis. Rather, it implies variable melt-water dilution adjacent to the Permian shoreline and requires growth of fauna in waters in contact with atmosspheric CO2. The whole rock 60 1 8 values fall at the edge of 'Normal Marine Limestone1 field and become appreciably lighter (up to -17°/a>) indicat ing that d iagenes is took place in the presence of light melt-waters. The 60 1 8 values of Permian fauna are unreliable indicators of palaeotemperatures because of variable melt-water dilution of the Permian sea. The calculated 18 60 values, corresponding to marine 6 C 1 3 values from the linear trend of 6 0 1 8 - 6 C 1 3 in cold-water carbonates, give reasonable Permian temperatures from 15to-8°C with the coldest waters of k to -8°C around Tasmania.

ORGANIC PETROLOGY AND SOURCE ROCK POTENTIAL OF SEDIMENTS IN THE EROMANGA BASIN, SOUTH AUSTRALIA Michelle Smyth 1 & M. Cameron 2 1 2

CSIRO, Institute of Earth Resources, North Ryde, New South Wales CSIRO, Division of Mathematics and Statistics, Lindfield, New South Wales

Cuttings and cores from the Poolowanna 1 well, South Australia (in which oil was discovered in Lower Jurassic reservoirs) and the Macumba 1 well (no oil) have been analysed petrographica1ly to assess the nature of the coals and dispersed organic matter (DOM) present. The Jurassic and Cretaceous coals have medium to high vitrinite contents, low to relatively high exinite, and medium to low inertinite contents. The DOM has comparatively less vitrinite, and more exinite and/or more inertinite than the associated coals. The microlithotype compositions of thecoals indicate that the original vegetation was largely woody in character and was buried before much oxidation had occurred. The depositional environment of the coal measure sequences was probably fluviatile and upper deltaic with some lagoonal facies. The Jurassic sediments contain up to 2 percent (v/v) DOM, a high proportion of which is exinite, including alginite. Vitrinite reflectances range from 0.5 to 0.7 percent. Where sufficiently mature, the Jurassic sediments are good potentia1 source rocks for hydrocarbons. Statistical testing of the analytical results for the Jurassic Poolowanna Formation shows that there is a significant association between the maceraIs incoal and DOM. The ratio of exinite to inertinite in DOM is reasonably well predicted by the corresponding ratio in the associated coal.

COALIFICATION ASSOCIATED WITH THE FOLDED ZONE OF THE BOWEN BASIN M.F. Middleton CSIRO, Division of Fossil Fuels, North Ryde, New South Wales High-rank coals (greater than 2% vitrinite reflectance) associated with the Folded Zone of the Bowen Basin have led various workers to suggest either uplift in excess of 2.5 km or extremely large geothermal gradients along the zone. These two hypotheses are examined in the light of simple kinetic models of coalification. Such kinetic models relate geothermal history to reflectance of vitrinite; observed vitrinite reflectance can be modelled assuming specific geothermal histories of the folded zone. A model entailing essentially uplift combined with local elevated temperatures due to frictional heating from faulting and folding is most cons istent with observed coalification.

EARLY DIAGENESIS IN SOME DEEP SEA SEDIMENTS, NORTHEAST INDIAN OCEAN Peter J. Cook 1 , L.A. Plumb 2 S P.A. Trudinger 2 R e s e a r c h School of Earth Sciences, Australian National University, Canberra, A.C.T. 2 Baas Becking Geobiological Laboratory, Canberra, A.C.T. Geochemi cal studies have been undertaken on a number of deep-sea cores from the northeast Indian Ocean. Most of the cores from the abyssal and bathyl zones show little or no evidence of bacterial sulphate reduction, utilizing alkalinity and sulphate content of the pore waters and reduced sulphur content of the sediments as a measure. However two sites, one in the Timor Trough and the other in the Bali Trough, showed abundant evidence of bacterial sulphate reduction and were studied in rather more detail. Sediment from the Timor Trough was obtained from DSDP Site 262. Some of the highest levels of pore-water alkalinity measured in marine sediments are found at this site. This, together with abundant framboidal pyrite and the extremely low concentrations of pore-water sulphate indicates vigorous sulphate reduction. The maximum alkalinity values occur at a depth of about 50 m below the sediment-water interface which is much deeper than normal. It is uncertain at this stage whether this is a reflection of present-day bacterial activity or is essentially a "fossil" peak frozen into the sediments by methane hydrates. <S3tfS values determined on pyrite are consistent with a biogenic origin; however, they vary down the drill hole and there appears to be no consistent pattern of variation. The Bali Trough investigations were undertaken on a gravity core. Abundant framboidal pyrite, depletion of sulphate, and increases in ammonia and total alkalinity again indicate abundant sulphate reduction. 6 31f S values for pyrite-sulphur are again consistent with a biogenic origin for the pyrite; but unlike the Timor Trough there is a strong correlation between increasing enrichment in 3 2 S and depth. This is an unusual trend for modern sediments. The only other site where it has been observed is the Santa Catalina Basin. The same trend has been noted for the McArthur River deposits and the Kupferschiefer. In both these cases the trend is ascribed to bacterial sulphate reduction in a closed system, but it is difficult to apply the same explanation to the Bali Trough.

POROSITY AND PERMEABILITY REDUCTION IN SOME OTWAY BASIN SANDSTONES I.R. Duddy Department of Geology, University of Melbourne, Parkville, Victoria The Otway Basin of southeastern Australia extends over 500 km across southern Victoria and contains a late Jurassic to Recent sedimentary pile that in places exceeds 9 km in total thickness. The early Cretaceous fluviatile volcanogenic Otway Group is the most widespread unit in the basin and attains a thickness approaching 3.5 km. Equivalent volcanogenic units occur in the adjacent Gippsland and Bass Basins (the Strzelecki and

"Otway" Groups respectively). Over much of these basins the volcanogenic units occur within the prime depth range for hydrocarbon generation and entrapment (2 to 5 km) at temperatures between ca 60 and 150°C. Otway Group sandstones when deposited were clean, well-sorted, fine- to medium-grained sands without a depositional matrix and composed ma inly of chemically unstable grains including volcanic glass, volcanic rock fragments, Ca-plagioclase and ferro-magnesian minerals. Most of the detritus was derived from contemporaneous pyroclastic volcanism associated with an early stage of continental rifting. Immediately following deposition these unstable grains began an alteration process which rapidly led to the occlusion of the sands' primary porosity and permeability. The stages are as follows: (1) Dissolution of volcanic glass and ferromagnesian grains (particularly pyroxenes) . (2) Precipitation of pore-lining clays - swelling chlorite and vermiculite. This significantly reduced permeability and occurred at essentially surface temperatures and at burial depths of ca I m. (3) Precipitation of Mn-bearing calcite cement as concretions and discontinuous layers. Concretion formation always followed clay linings and occurred at shallowburial depths, below erosion level but still at ca surface temperatures. Although this completely destroyed all primary porosity, it is a local feature only. (4) Precipitation of the Ca-zeolites, clinoptilolite and heulandite, as partial or complete pore fillings. This occurred at burial depths and temperatures considerably less than 2 km and 60°C respectively. (5) Pervasive alteration of detrital Ca-plagioclase and precipitated zeolites to produce a laumontite-albite assemblage. This occurred at temperatures not more than 60°C and burial depths not more than 2 km. Combined with some mechanical compaction, this alteration effectively occludes remaining porosity. The primary conclusion from these observations is that even though the Otway Group contains suitable hydrocarbon source rocks, significant accumulations are unlikely due to the very low temperature destruction of sandstone channel-ways and reservoi rs. Two other Otway Basin units are better petroleum prospects - the Pretty Hill Sandstone which is equivalent in age to the lower part of the Otway Group and the late Cretaceous Waarre Formation, the basal unit of the Sherbrook Group. Both units contain relatively pure quartzose sandstones derived from northern margin source areas and both contain hydrocarbon shows. The diagenetic effects in these units is not as marked as in the Otway Group, however pore-filling chlorite, kaolinite, silica and carbonates occur and decrease both permeability and porosity. Additionally, in the event of any hydrocarbon discovery in these units, the nature of these phases will effect the type of completion and production programmes used.

ASSOCIATION OF AUSTRALASIAN PALAEONTOLOGISTS SYMPOSIUM BIOSTRATIGRAPHY OF THE WEST AUSTRALIAN CONTINENTAL MARGIN

SOME DEVONIAN AND PERMIAN BRACHIOPODS FROM WESTERN AUSTRALIA: THEIR BIOSTRATIGRAPHICAL AND PALAEOGEOGRAPHICAL IMPLICATIONS G.A. Thomas Department of Geology, University of Melbourne, Parkville, Victoria The unusual 1 arge 1 ate Devonian rhynchonel 1 id Dzieduszychia occurs as a new species in the Bugle Gap Formation of the Canning Basin. The genus is known also from Morocco, Poland, the Urals of U.S.S.R., and Nevada, U.S.A., implying sha I low sea-way connections between these regions in the Famennian. Permian Spiriferinacea: Gjelispinifera sp. nov., Lamnaespina papilionata (Hosking) and Callispirina sp. nov. are fairly common in the early Permian Cal lytharra Formation, Carnarvon Basin. Gjelispinifera is otherwise known from the Gshelian (Late Carboniferous) of the Moscow Basin and the Basleo (middle Permian) of Timor. Lamnaespina is present also in the middle Permian Arthurton Group of New Zealand. Callispirina is world-wide. Implications are discussed. A large new species of the punctate orthotetacean Streptorhynchus is present in the early Permian Lyons Group and basal Callytharra Formation. It shows close affinity with a species from the early Permian of Tasmania.

CARBONIFEROUS MACRO-INVERTEBRATE BIOSTRATIGRAPHY AND PALAEOENVIRONMENTS OF THE CARNARVON BASINj A REVIEW OF DATA AND INTERPRETATIONS I.H. Lavering Esso Australia Ltd, Sydney, New South Wales Biostratigraphic subdivision of the macro-invertebrate faunas in the Carboniferous carbonate-clastic sequence of the onshore Carnarvon Basin is based on sparse, scattered data. Tournasian brachiopods of the Moogooree Limestone are the correlation "linchpin" within the sequence. St ratigraphic data show that the two carbonate units in the sequence (Moogooree Limestone and Yindagindy Formation) are separated by a non-marine fluviatile unit, the Williambury Formation. A hiatus separates this part of the sequence from the overlying Late Carboniferous Harris Sandstone. Poorly preserved plant debris provides the only internal evidence of age within the Harris Sandstone. The vertical succession of palaeoenvi ronments within the sequences ranges from mar i ne subt idal to intertidal, in the Moogooree Limestone and the Yindagindy Formation, to fluviatile and possibly alluvial fan in the Harris Sandstone and the Williambury Formation. It appears that the time interval represented by the sequence cannot be further subdivided on the basis of macro-invertebrates because of limited faunal data. Further, inter-basin correlation and palaeoenvironmental modelling may provide a better definition of the time-span represented by the sequence.

THE LOWER PERMIAN SEQUENCES OF WESTERN AUSTRALIA, CORRELATIONS WITH THE NORTHERN MARGIN OF GONDWANA AND IMPLICATIONS FOR THE CARBONIFEROUS-PERMIAN BOUNDARY IN NORTHERN GONDWANA N.W. Archbold Department of Geology, University of Melbourne, Parkville, Victoria The often incomplete and/or poorly known Early Permian sequences of Oman, Iran, Afghanistan, Pakistan, India and Southwest China (Tibet) can be compared with the well-known sequences of the Perth, Carnarvon and Canning Basins of Western Austral ia,

where faunas are well known and in sequence and stratigraphical relationships between units are clear, in an attempt to elucidate their age. Age assessments of the Western Australian faunas and implications for correlations with other regions of Gondwana as well as comments on recent palynological schemes for defining the Carboniferous-Permian boundary in Gondwana will be discussed.

THE DEVELOPMENT OF THE SUBORDER CHONETIDINA (BRACHIOPODA) IN THE PERMIAN OF WESTERN

AUSTRALIA

N.W. Archbold Department of Geology, University of Melbourne, Parkville, Victoria Representatives of the Chonetidina are often abundant within the diverse Permian faunas of the Western Australian Province, but they have received little attention since the description of Chonetes prattii by Thomas Davidson in 1859• The family Anopliidae is represented in the Early Permian (i.e. SakmarianArtinskian) faunas by up to four species of Tornquistia and one of Demonedys. The Rugosochonetidae is represented by species of Neochonetes, Svalbardia, Quinquenella and a possible Chonetinella in the Early Permian (i.e. Sakmarian to Kungurian) faunas and a poorly-known species of Neochonetes in the Late Permian ("Chhidruan") Hardman Formation fauna of the Canning Basin. Neochonetes in particular is a common genus and includes five species or subspecies in the Early Permian faunas. Chonetidine brachiopods are valuable for interbasinal correlations and are useful for precise correlations with foreign sequences; for example the burst of Svalbardia in Western Australia appears to be an accurate indicator of earliest Kungurian age. Most species are known from the Carnarvon Basin sequence, but important species enable correlation of the faunas of the Perth, Canning and Bonaparte Gulf Basin sequences with that of the Carnarvon Basin. The Chonetidina are unknown in the cold-water Early Sakmarian Lyons fauna, indicating temperature limitations for the group. They are useful for displaying links with other faunal regions (e.g. Thailand and the U.S.S.R.) and their mode of occurrence is useful palaeoecologically. The diversity of Chonetidine brachiopods in the Western Australian Permian Province offers a striking comparison with the low diversity or absence of thegroup in the Eastern Australian Permian Province.

LATE TRIASSIC AND EARLY TO MIDDLE JURASSIC

FORAMINIFERA

FROM THE NORTH WEST S H E L F , AUSTRALIA M . Apthorpe £ R. Heath Woodside Offshore Petroleum Pty Ltd, Perth, Western Australia Foraminifera have been found jn petroleum exploration wells from strata assigned to the Late Triassic Rhaetian-Norian stages, and in the Jurassic from the Sinemurian/Pliensbachian to Middle Bajocian inclusive. Two local (informal) zones are recognized in the poorly-known Norian (Nodosaria sp.-6/N. sp.-j/Discorbis sp.-3 zone, and Frondicularia sp.-10/F. brizaeformis zone). Six (formal) zones are recognized in the Jurassic. They are: the Sinemurian/Pliensbachian Lingulina tenera zone; the Early Toarcian Reinholdella crebra/Epistomina sp.-12 zone; the Late Toarcian to Aalenian Lenticulina d'orbignyi zone; the Late Aalenian to Early Bajocian Garantella aff. rudia zone; the (?Early) Bajocian Garantella ampasindavaensis zone; and the Middle Bajocian Lenticulina dictyodes/Citharina aff. inconstans zone. Most of the Jurassic foraminifera recorded in this study are known from temperate seas and embayments north and south of Tethys, i . e . Newfoundland, North West Europe and Britain, Portugal, Algeria, Madagascar and New Zealand. The ages suggested for the North West Shelf are based on age assignments from these a r e a s , and are supported by palynological age determinations.

LATE JURASSIC AND EARLY CRETACEOUS PALYNOLOGY OF THE PERTH BASIN John Backhouse Geological Survey of Western Australia, Perth, Western Australia The Yarragadee Formation and the Warnbro Group are a sandstone, siltstone and shale sequence of Late Jurassic to Early Cretaceous age. The two units are separated by a major Early Neocomian unconformity, representing the break-up of Gondwanaland in the Perth Basin area. Several thousand borehole samples have yielded assemblages of spores, gymnosperm pollen and dinoflagellate cysts. The upper part of the Yarragadee Formation is divided into the Retitriletes watherooensis, Aequitriradites acusus and Biretisporites eneabbensis Zones on the basis of spores which reach maximum diversity in the highest zone. Non-marine dinoflagel1 ate cysts dominate many assembl ages in the mid B. eneabbensis Zones. The dinoflagellates apparently inhabited lakes in the pre-break-up rift valley. The marine assemblages of the Warnbro Group in the Vlaming sub-basin and southern Dandaragan Trough are divided into four zones, largely on the basis of new taxa. Using these zones, a detailed correlation can be made between the offshore and onshore sections. Marine deposition in the Vlaming sub-basin and southern Dandaragan Trough commenced in the Valanginian and extended through most of the Dandaragan Trough by the Early Aptian.

OBJECTIVES OF AN OXYGEN ISOTOPE STUDY IN THE SOUTHERN

INDIAN OCEAN

Patrick G . Qui 1 ty Antarctic Division, Department of Science and Technology, Kingston, Tasmania The first steps have been taken to initiate an oxygen and carbon isotope programme on carbonate fossils from the Jurassic and younger of the eastern and southeastern Indian Ocean. Wherever possible, the plan is to use foraminifera but in sediments older than mid Cretaceous others, particularly belemnites and bivalves, will be used. The results should indicate the temperature history of the surface and bottom waters of the area which can be integrated with plate motion knowledge to detect gross changes in water mass characteristics. These results will in turn tell a great deal of the history of nutrient supply with consequent influence on organic content in sediments. Knowledge of sea temperature will also place limits on the type of climate and weathering patterns that can be expected on'nearby land. It is improbable that the temperature history will be complete because many serious gaps exist in the stratigraphic record, but the programme is intended to proceed on an opportunistic basis. Any assistance with well-dated, well-preserved fossil material will be appreciated.

LATE CRETACEOUS FORAMINI FERAL B I O F A C I E S , NORTHEASTERN

INDIAN OCEAN

Michael Hannah & Brian McGowran Department of Geology, University of Adelaide, Adelaide, South Australia Profiles of foraminiferal assemblages in the Late Campanian-Maastrichtian of the Carnarvon Basin and North West Shelf show several consistent changes in biofacies from section to section. Sharp fluctuations occur in the abundance of planktonics, agglutinated benthonics, Buliminids (mostly Praebulimina) and other benthonics. So far as can be determined with conventional biostratigraphic events as the main constraint in correlation, the biofacies changes are synchronous. Furthermore, conventional correlation indicates the "same" biofacies changes in environments that ought to contrast markedly with the Western Australian passive margin: oceanic carbonates of the Ninetyeast Ridge and deep water clays in a section in Pakistan, i . e . on the active margin of the Indian subcontinent.

There are five of those events centred on peaks in the abundance of agglutinated foraminifera. They are more frequent than are the Late Cretaceous transgress ionregression pulses plotted from seismic records or from patterns of marine invertebrates. However, it is likely that they are a response to the same kind of "eustatic" phenomenon; the distribution of Praebulimina reflects, in addition, changes in available oxygen. Biofacies changes are nonphyletic biostratigraphic events; they are analogous to chemostratigraphic events in that they can be more precise than classical biostratigraphy - once they have been sorted out by the latter.

TERTIARY FORAMINI FERAL BIOSTRAT16RAPHY OF THE NORTH WEST S H E L F , WESTERN

AUSTRALIA

R. Heath S M . Apthorpe Woodside Offshore Petroleum Pty Ltd, Perth, Western Australia Tertiary foraminifera from petroleum exploration wells on the North West Shelf have been extensively sampled and studied over the past decade. As a result of this effort, a sequence of foraminiferal events (base range, 'extinctions') have emerged as having significance for correlation within the palaeoenvironmental setting of the North West Shelf Tertiary sequence. These events are compared with foraminiferal events previously outlined in the 1iterature for the Tertiary of the Western Australian margin and the Indian Ocean region. A1 though many events are, understandably, similar, others are discussed here for the first time and some help to fill in several 'gaps' in the local Tertiary record.

OCEANS AND MARGINS:

BIOSTRATIGRAPHY OF CRITICAL LATE CRETACEOUS AND TERTIARY EVENTS Brian McGowran & Michael Hannah

Department of Geology, University of Adelaide, Adelaide, South Australia Palaeoceanography has been very successful in its strategy of constructing profiles through time. Mass processing of data has produced curves o f , for example, the palaeodepth of the carbonate compensation depth, of calcite dissolution gradients and the frequency of oceanic hiatuses, of sedimentation rates, and of the biogeographic fluctuations of assemblages of skeletonized protists. Curves based on single or composite sections include 6 X * C and 6 1 8 0 profiles. In parallel, passive continental margins have yielded transgression-regression curves and biofacies profiles, and curves of subsidence and sedimentation rates. A major preoccupation of palaeoceanography is now the analysis of events or shifts, rather than trends, in the transformation of the w a r m , salinity-dominated Mesozoic ocean to the cold, thermohaline modern ocean. The signals are mostly mic6 ropal aeon to logical and are perceived at a resolution of 10 years and less inoceanic and marginal sediments. The pre-Q.uaternary targets for detailed analysis can be grouped as: (1) the Santonian "anoxic event"; (2) Late Campanian-Maastrichtian perturbations preceding the boundary event; (3) Late Palaeocene-Early Eocene fluctuations foreshadowing the climatic deterioration at the Early/Middle Eocene boundary; and (k) late Middle to Late Eocene fluctuations before the earliest Oligocene crash; late Early and Middle Miocene perturbations before the deterioration into the Late Miocene.

SPECIALIST GROUPS IN ECONOMIC GEOLOGY AND GEOCHEMISTRY & MINERALOGY MINERALOGY AND GENESIS OF GOLD DEPOSITS

M I N E R A L O G Y , GEOCHEMISTRY AND LATERAL ZONATION WITHIN A 1.5 KM LONG GOLD-BEARING FISSURE VEIN SYSTEM IN THE CAMPBELL RED LAKE AND DICKENSON M I N E S , RED LAKE D I S T R I C T , O N T A R I O , CANADA P.J. MacGeehan Department of Geological Sciences, Queens University, Kingston, Ontario, Canada Present Address: Western Mining Corporation Ltd, Kalgoorlie, Western Australia The F-A (Campbell) -South C (Dickenson) vein system is composed of a sequence of metamorphogenic gold-bearing quartz-carbonate fissure veins, localized within a linear-deformed zone over 1.5 km in length and only 3 m in width, which transects a thick packet of Archaean mafic volcanic rocks bordering a sedimentary (dominantly exhalative) repository. Over most of its length, the vein system is comprised of from one up to eight sequentially emplaced cross-cutting en echelon unit veins,each composed of bilaterally symmetric outer rims of inward-growing comb-textured fibrous banded carbonate, and a central core of massive carbonate and/or fine-grained polygonal quartz after chert. At the east end, bordering the metasediments, the vein 1 system frays-out into a 'horse-tail of myriad ha i r-1 ine fracture-bound fissure veins. A strong lateral change in mineralogy and geochemistry is present in the vein system* from rich gold-bearing banded dolomite-ferrodolomite-quartz (recrystal1ized chert) vejns, flanked by magnetite-rich alteration selvages in the east, through lower-grade quartz-poor dolomite-ferrodolomite with successive zones of Hg (cinnabar) and W (scheelite) enrichment, to lower grade to uneconomic quartz-barren massive banded ferrpdolomite-siderite veins flanked by chlorite-rich alteration selvages in the west. Genesis of the vein system is consistent with derivation of gold-bearing hydrothermal fluid during dewatering of the adjacent sedimentary pile, and with formation of the present vein-filling mineral assemblages by the recrysta11ization of an earlier fissure-filling mud or gel.

CONDITIONS OF GOLD D E P O S I T I O N , MT C H A R L O T T E , K A L G O O R L I E , WESTERN AUSTRALIA M.E. Clark 1 S G.N. Phillips 2 1

Western Mining Corporation Ltd, KambaIda, Western Australia 2 Department of Geology, University of Western Australia, Ned lands, Western Australia The Mt Charlotte deposit, Kalgoorlie, Western Australia, is a stockwork of quartz veins with gold-bearing alteration selvages, in an Archaean greenstone belt. Fluid flow during mineralization and the properties of the ore fluid have been investigated from a study of vein structures, fluid inclusions in vein quartz and alteration assemblages. Mineralized veins developed as extension fractures during obi ique faulting and post-date regional folding and metamorphism. Veining is restricted to an Fe-rich granophyric zone of a differentiated tholeiitic metadolerite, the Golden Mi le Dolerite. Vein structures indicate fractures were produced by hydraulic fracturing which was limited to the relatively brittle granophyric zone. Three stages of vein development are recognized. Relative volumes and mineralogy of these stages are: Stage 1 (2%), quartz-ankerite-calcite-pyrite; Stage 2 (97%), quartz-pyrite (pyrrhotite)-sericite-calcite-scheelite; Stage 3 ( U ) , quartzpyrite chlorite-calcite. Stages 1 and 2 are associated with gold-bearing alteration assemblages of quartz-pyrite-sericite-ankerite-siderite-caleite.

Stages 1 and 2 were deposited from a supercritical fluid containing 20-30 mol % CO2 and less than 4 wt % dissolved salts. Homogenization temperatures for Stage 1 inclusions are 340-360°C and 295-320°C for Stages 2 and 3. Indicated fluid pressures of 1 . 2 - 2 kb imply that Stages 1 and 2 were deposited at 360-440°C. The ore fluid was near-neutral to slightly acid (pH • 5.3 ±0.6) and relatively reduced 27 7 (nv = 0.02 molal, a -10" , a ~10" ). LS O2 S2 The data are consistent with an ore fluid derived during low- to medium-grade metamorphism of a dominantly mafic pile. Gold transport as chloride complexes would be negligible under the indicated conditions but gold thio-sulphide complexes (AU(HS)2") could account for significant gold transport. It is proposed that increasing pore-fluid pressure during rapid post-metamorphic uplift induced hydraulic fracturing during faulting. Cooling of the ore fluid during uplift to temperatures around 400°C would tend to saturate the fluids in AU(HS)2~. Oxidation of the ore fluid by the wall-rock assemblages is the most 1ikely mechanism for precipitation of gold.

PROBLEMS OF ORE GENESIS AT KALG00RLIE Lee Y. Golding 1 & Reid R. Keays 2 1 2

Electrolytic Zinc Co. of Australasia Ltd, Bentley, Western Australia Department of Geology, University of Melbourne, P a r k v M l e , Victoria

The gold-telluride lodes at Kalgoorlie show characteristics of both eplgenetic and modified syngenettc mineralization. The main host rock, the Golden Mile Dolerite (G.M.D.), is a tholeiitic unit which is, at least in part, extrusive and which Is overlain by a sequence of shallowwater to subaerial sediments and pyroclastics called the Black Flag Beds (B.F.B.). Mineralization is localized about a synclinal Infold of the B.F.B. Into the G.M.D., known as the Boulder Dyke or Kalgoorlie Syncl!ne,and also within interflow sediments associated with volcanic rocks underlying the G.M.D. Gold, A g , T e , Se, A s , Sb, S, K and CO2 have been introduced into the lodes and stockworks. Anomalous levels of these elements in some Interflow sediments and B.F.B. shales suggest that the volcanic-sedimentary pile which hosts the ore deposits was the source of the ore constituents. As a tentative model, it is suggested that the Interaction of active volcanlsm with the shallow-water to subaerial environment of the B.F.B. In which organic-rich black shales, carbonates and evaporltebearing sediments were accumulating, provided ideal conditions for substantial localized concentration of metals. Deformation, thermal fluids and changes In physiochemical conditions were probably Instrumental In the transport and deposition of the ore-metals Into their present sites.

GE0CHEMICAL AND STABLE IS0T0PIC STUDIES OF THE NO, KALGOORLIE, WESTERN AUSTRALIA

LODE,

S.D. Golding S Allan F. Wilson Department of Geology & Mineralogy, University of Queensland, St Lucia, Queensland Gold-telluride mineralization from the Golden Mile Is the most Important historical source of gold In the Kalgoorlie area. The No. k Lode Is a persistent variably si 1IcI fled pyrltlc shear zone which Is located within steeply-dipping Golden Mile Dolerite on the western side of the Kalgoorlie Syncllne. There Is a gradual transition between chlor I tic country rocks and altered wall rocks which commonly contain quartz-serlclte-ankerlte-pyrlte assemblages. The core of the lode Is very siliceous with only minor carbonates and sulphides. Magnetite and barlte have also been Identified. Statistical analysis of major element data suggests local mobility of SIO2» CaO and MgO whereas Na20, A 1 2 0 3 and Fe20s were leached from the system. The wallrocks are significantly enriched In K2O, CO2, S, A s , Ba, Rb and the precious metals. Oxygen Isotoplc values of whole-rock samples within the lode shear are enriched In

0 relative to typical chloritic Golden Mile Dolerite, implying extensive water rock interaction. There is a positive shift in 6 1 3 C and 6 1 8 0 of carbonates across the lode which supports other evidence suggesting that the regional carbonation predated mineralization but that C0 2 was an important component of the ore fluid. Sulphur and carbon isotopic data indicate that the ore fluid was slightly alkaline and relatively reduced. The geochemical and isotopic data are consistent with a metamorphic derivation for the ore fluids.

THERMODYNAMICS OF GOLD-TELLURIDE DEPOSITS M. Ahmad Water Division, Department of Transport £ Works, Darwin, Northern Territory Physico-chemical environments of several gold-telluride deposits are summarized. Existing thermodynamic data on the aqueous Te species, and the tellurides of Au, Ag, Fe, Hg, Pb, Ni, As, Sb and Bi have been used to plot the phase boundaries of tellurides and sulphides of these elements as a function of fS 2 -fTe2, f02~pH, f02"T, and mETe-f02. Solubility of native gold and calaverite have been calculated in terms of gold-sulphur and gold-chloride complexes. Very low solubility of calaverite suggests the existence of gold tellurium complexes. The existing mineralogical and thermodynamic data suggest that most gold tellurides deposits are likely to have formed at temperatures = 200-350°C, p H ^ S i O . S f 2 3 f O2 = close to magnetite-hematite buffer, mES = 10~ to 10" , mCl and mETe • 10* to 10' 7 . Under these.conditions the solutions could have carried between 0.001 to 0.1 ppm Au. It appears that major Au precipitation may have been caused by (l) decreasing temperature, (2) increasing pH, (3) decreasing f02 and (4) decreasing S concentrations. It is suggested that in some cases these changes may be induced due to fluid boi1ing.

THE NATURE OF ARCHAEAN GOLD-BEARING

FLUIDS

G.N. Phi 11ips & D.I. Groves Department of Geology, University of Western Australia, Nedlands, Western Australia Epigenetic veins in metabasaltic host rocks from Archaean greenstone belts have been the major source of gold within the Yilgarn Block of Western Australia. Such deposits are important gold producers in the Eastern Goldfields, Murchison and Southern Cross Provinces and show a distinct correlation, both with areas of greenschist facies to greenschist-amphibolite transition metamorphic grade and with tholei i tic host rocks. Wall rock alteration, typically dominated by sericite (or biotite)-carbonatepyrite, is much more extensive than the quartz ±carbonate lodes themselves; alteration zonation is common. Fluid inclusion data suggest that many deposits formed from H2O-CO2 fluids of low salinity at temperatures only slightly below that of peak regional metamorphism. Available isotopic and fluid compositional data are compatible with solutions being of metamorphic derivation and transporting gold as bisulphide or bicarbonate complexes. The association of gold with specific host rocks and specific al terat ion types suggests an important control on gold precipitation by fluid-rock interaction. Fluid access was provided either by pre-existing shear zones or synchronous hydraulic fractures, whose localization was commonly controlled by host rock mechanical properties (combination of mineralogy and texture).

SPECIALIST GROUP IN GEOCHEMISTRY AND MINERALOGY

GEOCHEMISTRY

RARE EARTH ELEMENT PATTERNS IN SEDIMENTARY ROCKS AS INDEXES OF CRUSTAL EVOLUTION S.R. Taylor S Scott M . McLennan Research School of Earth Sciences, Australian National University, Canberra, A.C.T. The uniformity of REE patterns in clastic sedimentary rocks provides overall average upper crustal compositions for these elements. Effects of weathering, diagenesis and metamorphism are minor. Local provenance is recorded in first-cycle sediments, but is rapidly erased with sediment maturity. The REE patterns in Archaean sedimentary rocks indicate that the Archaean crust was not highly evolved. It appears to have been dominated by basaltic and Na-rich granitic rocks (tonalites and trondhjemites). The REE patterns in Proterozoic and later sedimentary rocks indicate a major episodic break at the Archaean-Proterozoic boundary. This is consistent with a change to a more differentiated upper crust, dominated by granodiorites, with negative Eu anomalies. These are inferred to result from intra-crustal melting, during which Eu is retained in a piagioclase-rich lower crust. Detailed descriptions of the change in REE patterns at the Archaean-Proterozoic boundary are given for the Huronian (Canada) and Pine Creek Geosyncline (Australia) successions. Evidence from these, the Hamersley basin (Australia) and the Pongola (South Africa) sequence indicate that the change in upper crustal composition was not isochronous, but extended over a period from about 3 200 to 2 500 Ma. A model for continental crust evolution suggests that the evolution of the present crust began in the Archaean and that most of the volume of the crust was formed from the mantle between 3 200 and 2 500 Ma. This was followed closely by intra-crustal melting which produced the post-Archaean upper crust.

THE OXIDIZED ZONE OF THE BROKEN HILL L O D E , NEW SOUTH WALES J.C. van Moort £ C.J. Swensson Department of Geology, University of Tasmania, Hobart, Tasmania A systematic description is given of the remaining outcrop of the Broken Hill lode together with geochemical data. The oxidized lode is characterized by a coronadite-rich gossan in the upper part followed by a complex heterogeneous cerussitic mineralization at depth. The cerussitic zone is in direct contact with the primary sulphide. Geochemically the gossans are characterized by an extreme enrichment i n M n a n d Pb.^ Significant features of the gossan of the original lode outcrop are a Pb/Fe ratio close to or more than 1 and a Pb/Mn ratio of 1-2 or more, also when free of lead carbonate. With increased depth of the ore-body, or across the strike, these values drop rapidly. The high Pb/Mn ratio of the gossan is caused by the abundance of a plumboan variety of coronadite containing 35-58% Pb. This Pb content exceeds the observed maximum of T]% Pb from coronadite whilst the d-spacings are also larger than those recorded previously. Complex X-ray diffraction data and crystal indexes are given. An additional new mineral, ramsdellite, is reported from Broken Hill.

P 2 0 5 SYSTEMATICS IN 0R06ENIC IGNEOUS ROCK SERIES CONSTRAINTS IMPOSED BY APATITE CRYSTALLIZATION T.H. Green & E.B. Watson 1 School of Earth Sciences, Macquarie University, North Ryde, New South Wales Permanent Address: Department of Geology, Rensselaer Polytechnic Institute, Troy, New York, U.S.A. Two separate series of experiments involving (1) imposing apatite saturation on a series of igneous rock compositions from basanite to rhyolite,and (2) crystallizing similar natural rock compositions progressively until apatite appears, demonstrate a close dependence between apatite saturation and silica content of the magma, and determine P 2 0 5 levels at a given silica value and temperature at which that composition may be expected to precipitate apatite. The P2O5 vs S10 2 relationships of the low-K island-arc suite, ca1c-a1ka1ine suite and high-K calc-a 1 ka 1 ine suite appear regular and characteristic for each suite, and when linked with the experimental work on apatite solubility, provide potential constraints on the genesis of the respective suites. Thus the low-K series may result from extended crystal fractionation of a basic parent, and does not achieve apatite saturation until rhyodacites are produced. The genesis of the calc-alkaline series is complex, necessarily involving varying water contents to depress the liquidi, and also varying degrees of melting of mafic source material. Most ca1c-a1ka1ine series reach apatite saturation when dacitic compositions are produced. Finally the high-K calc-a1kaline series is necessarily water-rich and appears to be saturated in apat i te throughout i ts compos i t iona 1 range, and may represent a series linked by crystal fractionation. Experimentally determined partition coefficients for REE distribution between apatite and coexisting liquid provide additional data and controls on observed geochemical patterns in the orogenic igneous rock series through geologic time.

THE GEOCHEMICAL AND ECONOMIC SIGNIFICANCE OF THE CHEMICAL COMPOSITION OF SUPERGENE GOLD AND OF GOLD NUGGETS A1 lan F. Wi1 son Department of Geology & Mineralogy, University of Queensland, St Lucia, Queensland The purity (fineness) of supergene or secondary gold is normally greater than gold found in unoxidized lodes beneath the water-table, especially if the primary gold is included in sulphides or arsenides. In alluvial deposits the Ag content of primary gold appears to be removed by electrochemical processes progressively with distance from its source. Variations in the non-Au components of the supergene gold alloy reflect the primary source of the primary Au, its chemical mode of secondary transport and the chemical environment of secondary deposition. Transport of supergene Au may be as thiosulphate- or chlorine-complexes, but the role of C02orhumic acids must not be ignored. Some very pure gold (>99 wt % Au) appears to have been accreted in a bauxitic matrix near Cloncurry, Queensland, and in "ironstone" environments of deeply weathered rocks near Kalgoorlie. Some nuggets and other detrital or eluvial gold which contain a range of alloyed Ag up to nine percent may be explained in terms of solution chemistry. Textural and probe studies of cross-sections of several quartz-free gold nuggets show that the gold commonly encloses pisolitic concretions of Fe-rich laterite or bauxite or other soil detritus. In some nuggets of mammillary form the ovoid inclusions of hematitic iron oxide become partly hydrated near the edge of nuggets, resulting in expansion cracks bursting through the apparently solid gold surface. In several regions where nuggets of this type are found there is no obvious vein or lode source for the accreted gold. For some, the source of the migrating gold appears to be finely divided gold (atomic or colloidal) or gold-bearing sul phides or arsenides disseminated throughout the country rocks.

IGNEOUS ACTIVITY THROUGH TIME

A R C H A E A N C A L C - A L K A L I N E V O L C A N I S M IN THE P I L B A R A BLOCK,, WESTERN AUSTRALIA M.E. Barley

1,l

\ G.D. Borley 2 , G.C. Sylvester 1 ' 5 , J.R. de Laeter 3 , D.I. Groves 1 S N. Roqers2

Department of Geology, University of Western Australia, Nedlands, Western Australia Department of Geology, Imperial College, London, U.K. School of Physics and Geoscience, Western Australian Institute of Technology, Bentley, Western Australia "Present Address: CSIRO, Division of Mineralogy, Floreat Park, Western Australia 5 Present Address: Noranda Australia Ltd, Subiaco, Western Australia

Archaean calc-a1kaline volcanics occur in two distinct geological settings in the Pilbara Block. Within the 3 500 to 3 300 Ma old Warrawoona Group, sequences of intermediate to acid pyroclastics derived from calc-alkaline volcanic centres are interlayered with an extensive sequence of komatiitic and tholeiitic lavas and shallow-water sediments. In the western Pilbara, a younger sequence of intermediate to acid lavas and pyroclastics is associated with shallow- to deep-water terrigenous sediments and minor mafic lavas. Both suites of volcanics are dominated by intermediate to acid 1avas and pyroclastics and have typical calc-alkaline mineralogy and geochemistry. The major and trace element chemistry (including REE) of the most mafic members of each suite (basalts and basaltic andesites) are consistent with derivation by partial melting of the mantle. A spectrum of high-level crystal fractionation trends (involving the observed phenocryst phases) relate more evolved members to parent magmas. Rb/Sr whole-rock systematics are consistent with this interpretation. The fractionation processes involved in the evolution of the ca1c-a1ka1ine suites operated at higher (although variable) PH2O than those involved in the development of the komati itic and tholeiitic lava sequences. Archaean calc-alkaline volcanics in the Pilbara are similar in many respects to modern calc-alkaline suites. Chemically they bear greatest resemblance to modern calc-al kal ine volcanics erupted through cont inental crust. However, more deta i led studies of both ancient and modern calc-alkaline volcanics are required before direct comparisons of petrogenetic processes and their relations to tectonism can be attempted with confidence.

C O N C O M I T A N T T H O L E I I T I C AND KOMATIITIC V O L C A N I S M AS AN ORELOCALIZING M E C H A N I S M AT K A M B A L D A , W E S T E R N

AUSTRALIA

C.M. Lesher Department of Geology, University of Western Australia, Nedlands, Western Australia The type-examples of komatiitic peridotite-hosted massive nickel sulphide ores occur at the base of an extrusive ultramafic sequence at Kambalda, Western Australia and are marginally confined to varying degrees by footwall tholeiitic metabasalts forming linear to elliptical re-entrant embayments. Previous stratigraphic studies and structural analysis indicate that the ore-bearing embayments not only predate the earliest recognizable deformations in the area, but also much of the overlying ultramafic komatiite sequence. This has been used to infer that topographic irregularities or subsiding grabens controlled the emplacement of initial ore-bearing lavas and were subsequently modified by deformation to produce the re-entrant geometry. Recent detailed underground mapping indicates that some of the ores and host komatiitic peridotite lenses were, in fact, conformably overlapped by the tholeiitic lavas. This is supported by contiguous stratigraphy, metabasalt facies and younging indicators in the enclosing metabasalt sequence, rare interflow metasedimentary horizons, relict igneous chromite distributions in the ores, compositions and tex-

tures of chilled komatiite margins compared with metasomatically-altered peridotites bordering unequivocal structural dislocations, geochemical fractionation trends in the lower part of the ultramafic pile, and general geometrical-structural incompatibilities. This suggests that the ore-bearing komatiitic peridotites marked the commencement of komatiitic volcanism in the area during waning, concomitant tholeiitic volcanism. Despite virtually complete exposure of the footwall during mining operations, no feeders to the ultramafic sequence have been recognized. Although stratigraphic relations may suggest very localized eruptive centres within the embayments, an alternative possibility is that the eruptive sites were at some distance from the present ore zones and that emplacement of the ore-bearing lavas was constrained by the locations of a few specific extrusion sites relative to local and/or regional topographic gradients.

CRUSTAL ACCRETION

IN EASTERN AUSTRALIA - EVIDENCE FROM

XENOLITHS IN BASALTIC ROCKS Suzanne Y. Wass

6 Julian Hoi lis

School of Earth Sciences, Macquarie University, North Ryde, New South Wales 2 Department of Mineralogy and Petrology, Australian Museum, Sydney, New South Wales Abundant xenoliths from the Anakie volcanic centre in southwestern Victoria provide evidence of the nature of the upper mantle and lower to middle crust, as well as past patterns of igneous activity. The xenoliths range from garnet-bearing granulites, through pyroxenites, amphibole-rich gabbroic rocks to felsic metamorphic rocks. Contact relationships between different rock types allow interpretation of age relationships of some intrusive events. Most xenoliths show evidence of highgrade metamorphism and/or metasomatic alteration. The mineralogy of the dominant amphibole-plagioclase bearing xenoliths is compatible with an original origin as either plutonic associations of island arc areas or cumulates and intrusives of tholeiitic continental magmas. Trace element discriminants may be able to distinguish these two origins. The mineralogy of certain xenoliths enables estimation of pressure and temperature conditions. Comparison with xenolith assemblages from other localities supports a generally basaltic composition for the lower crust in eastern Australia.

PATTERN OF MAGMATIC EVOLUTION THROUGH THE CAINOZOIC VOLCANISM OF EASTERN AUSTRALIA F.L. Sutherland Department of Mineralogy and Petrology, Australian Museum, Sydney, New South Wales The Nebo Province, northeast Queensland, had central volcano activity (34-31 Ma), then basaltic activity (28-21 Ma and 3 Ma). This may reflect the northward drift of Australia over anomalies in the mantle. The 28-21 Ma activity fal Is within dated activity younging southward from Mingela (31 Ma) through Monto (20.5 Ma) to Ipswich (16 Ma). One explanation for the activity is the passage of Australia over old spreading sites of the Coral Sea and southeast Papua regions. Compared with the central volcano basalts, the younger Nebo basalts include evolved mantle-derived magmas relatively enriched in K, Ti and P. The megacryst suites have common anorthoclase and/or kaersutite. This pattern occurs elswhere in eastern Australian activity, e.g. Newer Volcanics of Victoria. Preliminary investigations suggest that it may occur in some provinces with two periods of basaltic activity (McBride, Mingela and Monto). The development of K-enriched, mantle-derived basalts may reflect greater metasomatism and intrusion following the earlier magmatism. The Tasmanian basalts contain some examples, but they are more mafic types and lack anorthoclase or kaersutite megacrysts. Differences in the Tasmanian pattern may result from lack of central volcano activity, intensive intrusion by Jurassic dolerites and the presence of anomalous mantle.

JURASSIC TO TERTIARY VOLCANISM IN VICTORIA TECTONIC DEVELOPMENT AND GEOCHEMISTRY R.A. Day Department of Geology, Monash University, Clayton, Victoria Victorian Mesozoic and Cainozoic volcanism can be related to two continental splitting events: (l) the separation of the Lord Howe Rise and New Zealand from Australia; and (2) the separation of Australia and Antarctica. A protracted tensional tectonic regime explains the long duration of continental volcanism. Jurassic volcanics were erupted in a 'rift valley 1 sett i ng pr ior to the separation of Australia and Antarctica. They form a bimodal basa1t-phonolite assoc iat ion. All this volcanism pre-dates major continental separation. Tertiary volcanism. Australia-New Zealand separation had two ma i n consequences for eastern Victoria: (l) development of the Strzlecki and Gippsland Basins; and (2) uplift of the Eastern Highlands. Volcanism falls into three major groupings: Group 1 (95 to 39 Ma). Eruptions associated with formation of the Tasman Sea. Volcanism spread westward across southern Victoria. Group 2 (43 to 28 Ma). Volcanism associated with Mid-Cainozoic uplift in the Eastern Highlands. Major Palaeozoic faults control the location of provinces. Group 3 (28 to 18 Ma).

A large group of basalts in central Victoria.

The oldest basalts (Group 1) are alkaline and contain a high proportion of primitive compositions. Group 2 are primitive alkaline basalts and tholeiites. Group 3 comprises relatively fractionated hawaiitic and tholeiitic compositions. Trace element abundances of basalts over this age range imply a consistentlydepleted mantle beneath southern Victoria.

CONDITIONS AND PROCESSES DURING REGIONAL METAMORPHISM

M E T A M O R P H I S M OF THE WIDGIEMOOLTHA-NORSEMAN A R E A , EASTERN

GOLDFIELDS

AND CONSTRAINTS ON THE THERMAL EVOLUTION OF THIS ARCHAEAN GRANITE - GREENSTONE TERRAIN N.J. Archibald S M . J . Bickle Department of Geology, University of Western Australia, Nedlands, Western Australia Pelitic rocks sampled in the Widgiemooltha-Norseman area contain a number of significant mineral assemblages. Critical minerals include chloritoid, garnet, cordierite, staurolite, andalusite and sillimanite. Some of the rocks exh i b i t arrested reaction textures. By consideration of appropriate phase relat ions and by microprobe analyses of phases in critical assemblages, it is possible to determine 1imited sections of the pressure-temperature paths followed by individual rocks during their metamorphism. The significance of these pressure-temperature paths in relation to knowledge of the structural history of the area will be used to constrain thermal models for the metamorphism.

HYDROTHERMAL ALTERATION AND LOW-GRADE METAMORPHISM OF ARCHAEAN VOLCANIC SEQUENCES IN THE EASTERN PILBARA BLOCK M.E. Barley Department of Geology, University of Western Australia, Nedlands, Western Australia Present Address: CSIRO, Division of Mineralogy, Floreat Park, Western Australia The granitoid-greenstone terrain in the eastern Pilbara Block contains an extensive sequence of metamorphosed volcanics and sediments. Both the volcanics and overlying sediments have suffered metamorphism during regional deformation. However primary volcanic structures are still preserved in less-deformed volcanics and textural and mineralogica1 evidence clearly indicates that volcanics were extensively altered prior to the onset of the regional metamorphic episode. The extent of alteration reactions appears to have been markedly heterogeneous, and primarily conditioned by initial anisotropy in permeability of the volcanic sequence. For example almost complete mineralogical and chemical adjustment occurred in brecciated and pillowed tops of basaltic flows, whereas the less permeable massive portions of flows and sills exhibit a lower degree of alteration. This observation provides good evidence that these rocks have been in contact with an externa 1 circulating fluid; i . e . affected by hydrothermal alteration. The mineral assemblages developed and the intensity of alteration were a function of differing effective water/rock ratios. Hydrothermal alteration resulted in mineral assemblages typical of very low grade and low grade metamorphism and may locally have produced higher temperature assemblages. Mineralogica1 evidence suggests that the f 1 uid responsible was originally oxygenated, C0 2 -rich and Na-rich and became progressively reduced and Ca-enriched. This inferred geochemical trend is similar to that observed in modern sea-water derived hydrothermal brines. Comparison of samples from altered and least-altered domains within single N outcrops indicate that very few elements can be regarded as i m m o b i 1 e " . However, provided samples are carefully selected S i 0 2 , T i 0 2 , M n O , A 1 2 0 3 , M g O , P 2 0 5 , C r , N , Y , Zr and REE are close to original compositions in basic rocks. A 1 2 0 3 , T i 0 2 , P 2 0 5 and Y have been mobile during patchy sericitization of intermediate and acid rocks. It is considered that the drastic decrease in permeability caused by the generation of new minerals during hydrothermal alteration created unfavourable conditions for attaining re-equi 1 ibration during later low-grade metamorphism. Consequently in areas where low grade regional metamorphism was unaccompanied by penetrative deformation early hydrothermal mineral assemblages are unlikely to have been significantly affected.

THE REACTION:

STAUROLITE+ QUARTZ Z C O R D I E R I T E + SILLIMANITE+ H 2 0 , NEAR SPRINGTON, SOUTH AUSTRALIA R.L. 01iver

D e p a r t m e n t o f G e o l o g y and M i n e r a l o g y , U n i v e r s i t y o f A d e l a i d e , A d e l a i d e , South A u s t r a l ia In p e l i t i c rocks east o f S p r i n g t o n , in the M t Lofty R a n g e s , t h e r e a c t i o n : s t a u r o l i t e + q u a r t z ^ c o r d i e r i t e + si 11imanite + H2O

(1)

is a p p a r e n t in thin s e c t i o n . T e x t u r a l e v i d e n c e f o r this is the s t a u r o l i t e o c c u r r i n g as ragged remnants w i t h i n c o r d i e r i t e p o r p h y r o b l a s t s w i t h w h i c h a r e a s s o c i a t e d prism a t i c s i l l i m a n i t e . A n d a l u s i t e a l s o is p r e s e n t . T h e e q u i 1 i b r i u m c u r v e , r e p r e s e n t i n g the a b o v e r e a c t i o n , o n a PT d i a g r a m has been c a l c u l a t e d using a c t i v i t i e s derived from e l e c t r o n p r o b e - d e t e r m i n e d a n a l y s e s o f the c o r d i e r i t e a n d s t a u r o l i t e . Coexiste n c e o f s i l l i m a n i t e a n d a n d a l u s i t e in t h e rock s u g g e s t s a l s o that t h e P T o f c r y s t a l lization w a s in the p r o x i m i t y o f t h e e q u i l i b r i u m c u r v e f o r : Andalusite £ sillimanite

(2)

Using Holdaway's recommended curves ( H o l d a w a y , M . J . , 1 9 7 1 , A m . J . S c i . , 2 7 1 : 9 7 - 1 3 1 ) , the intersection o f reactions (1) a n d (2) a b o v e is at a p p r o x i m a t e l y 550°C and 3 k b . T h e a b o v e data c o r r e s p o n d s to a thermal g r a d i e n t o f a p p r o x i m a t e l y 50°/km assuming c o n s t a n c y o f this g r a d i e n t , passing t h r o u g h the o r i g i n .

METAMORPHIC REACTIONS INVOLVING SILLIMANITE IN THE HALLS CREEK MOBILE ZONE, EAST KIMBERLEY Rosemary A l l e n Department o f G e o l o g y , U n i v e r s i t y o f A d e l a i d e , A d e l a i d e , South A u s t r a l i a The v o l c a n i c a n d s e d i m e n t a r y rocks o f the T i c k a l a r a M e t a m o r p h i c s (high-grade e q u i v a l e n t s o f the Halls Creek G r o u p ) have u n d e r g o n e three fold e p i s o d e s w i t h associated m e t a m o r p h i s m . T h e thermal regime represents a c o n t i n u u m , m e t a m o r p h i s m c o m mencing syn F i a n d w a n i n g p r e F 3 . Peak t e m p e r a t u r e s o f u p p e r a m p h i b o l i t e g r a d e w e r e recorded during F 2 south o f the O r d R i v e r , a n d g r a n u l i t e g r a d e post F2 n o r t h o f t h e Ord. T e x t u r a l relationships actions : (1)

Staurolite

indicate s i l l i m a n i t e f o r m a t i o n by v a r i o u s p r o g r a d e re-

breakdown.

(a) Optically c o n t i n u o u s s t a u r o l i t e inclusions w i t h i n a l m a n d i n e garnet c o r e s w i t h rims including s i l l i m a n i t e needles provides t e x t u r a l e v i d e n c e f o r t h e equation Staur + Qtz

A i m + Sill + H2O

(b) M i n u t e s t a u r o l i t e droplets c l u s t e r in o p t i c a l c o n t i n u i t y in q u a r t z - f e l d spar mosaics. Intergrowths o f small s i l l i m a n i t e prisms c r o w d t h e g r a i n bound a r i e s . Ilmenite provides the Zn s i n k . (2)

P s e u d o m o r p h i c replacement o f a n d a l u s i t e .

(3)

Biotite r e p l a c e m e n t . M u c h o f the s i l l i m a n i t e a s s o c i a t e d w i t h biot i te crystallizes e p i t a x i a l l y , h o w e v e r b i o t i t e is s o m e t i m e s implicated in s i l l i m a n i t e formation.

(4)

M u s c o v i t e b r e a k d o w n . M u s c o v i t e d i s a p p e a r s n o r t h o f the O r d w i t h c o n c o m i t a n t increase in s i l l i m a n i t e a n d m i c r o c l i n e . S i l l i m a n i t e b r e a k d o w n at high g r a d e s (where g a r n e t - c o r d i e r i t e p a r a g e n e s e s bec o m e increasingly c o m m o n ) indicates t h e reactions Sill + B i o + Gt + M i c r o c l i n e + V and

Sill + Bio + Q t z + Cord + M i c r o c l i n e + V

. Retrograde reactions include euhedral andalusite replacing fibrolitic sillimanite, and fine euhedral staurolite prisms replacing and including fibrolite in the presence of biotite or garnet. Muscovite replacement of fibrolitic mats and silliman.te pseudomorphs after andalusite implies a drop in temperature, increase in f a n d / o r H20 V

R b / S r ISOTOPIC AND GEOCHEMICAL EVOLUTION OF A RECRYSTALLIZED SHEAR (MYLONITE) ZONE AT BROKEN HILL M.A. Etheridge S J.A. Cooper Department of Geology, University of Adelaide, Adelaide, South Australia Relatively narrow (1 to 1 000 m) planar zones of intense shear deformation and retrograde metamorphism (retrograde schist zones, RSZ) are common in the Willyama Complex around Broken Hill. A Rb/Sr isotopic study of one of these zones has revealed an unexpected isotopic pattern. All of the analysed points lie to the left of, and above the 1 665 Ma isochron of the Potosi Gneiss host. This result indicates adifferent isotopic composition of the zone as a whole rather than a chemical redistribution of Potosi Gneiss within a closed system. The isochron plot scatter is similar to that of ubiquitous metamorphic pegmatites in the basement of the Broken Hill region. The shear zone observation, however, is shown to be unrelated to the pegmatite segregation process and requires the introduction, by elemental exchange, of a more radiogenic strontium, whilst maintaining relatively fixed Rb/Sr ratios. Significant proportions of Si02, K 2 0 and Ba were removed at the same time. Na20, A 1 2 0 3 , T i 0 2 , MgO, P2O5, Zr, Ni, Sc, Y , Nb, Rb remain essentially constant after allowing for volume loss of the removed elements. CaO and Sr increase si ightly overal 1. This pattern of elemental exchange is consistent with the establishment of the chemistry of a micaceous mineral assemblage around the more immobile elements of an original ilmenite, garnet, feldspar assemblage by exchange with a fluid capable of carrying the more mobile elements. Large-scale fluid transport is required to achieve these results. The total rock isochron gives no indicat ion of the age of the retrograde events. However two biotite-total rock joins indicate that the last internal isotopic redistribution occurred 458 Ma ago, shortly after the time of a regional low-grade metamorphism. 8 7 S r / 8 6 S r vs 1 0 0 / 8 6 S r plots confirm that a mixing process took place at about this time and not at 1 490 Ma or the present day. The introduced fluid has an 8 7 S r / 8 6 S r ratio of about 0.794. If this figure still represents the original source value then it has come from what was long established mature crust at 458 Ma and not the mantle.

OXYGEN ISOTOPES AS INDICATORS OF FLUIDS ACTIVE IN SOME MYLONITE ZONES IN SOUTHWESTERN AND CENTRAL AUSTRALIA Allan F. Wilson Department of Geology and Mineralogy, University of Queensland, St Lucia, Queensland Some fault zone materials appear to have crystallized in response to fluids that have moved only short distances from surrounding rocks into fault zones, and 8 16 whole rock 1 0 / 0 ratios of these fault zone materials are generally buffered by the isotopic composition of the enclosing rocks. However, where a fault zone has been a well-established conduit for deep crustal fluids, its whole rock 1 8 0 / 1 6 0 ratio will commonly reflect this origin. These fluids differ substantially in both isotopic and chemical composition from those involved in earlier igneous, metamorphic or sedimentary development of the faulted rocks. Measurement of 1 8 0 / 1 6 0 of mineralized whole rocks and cogenetic minerals therein should be an important parameter in recognizing the source of mineralization in fault zones. Australian examples of these concepts are discussed.

GEOCHRONOLOGY/METAMORPHISM

ELEMENT MOBILITY AND ACCUMULATION DURING DIAGENESIS AND VERY LOW-GRADE METAMORPHISM I.R. Duddy Department of Geology, University of Melbourne, Parkville, Victoria The highly unstable volcanic constituents of the Otway Group make it an ideal sequence in which to study element mobility during diagenesis and very low-grade metamorphism. Mobility of a large range of major elements is obvious in the 1ight of precipitation of a variety of clay minerals, carbonates and zeolites in sandstone pore spaces and the replacement of swelling clays by illite with increasing burial in the interbedded mudstones. Significant minor element mobility and fractionation occurs at all stages and at several scales associated with these mineralogical changes. Early in the burial history, carbonate is precipitated as concretions in the sandstones, sealing that part of the sandstone from further contact with evolving pore solutions. This preserves inside the concretion the mineralogy and hence chemistry of the sandstone prior to deep burial. Analysis of concretions and adjacent laumontite-albite sandstones (very low-grade metamorphic rocks) reveals the following features: (l) The concretions preserve a suite of unstable minerals including Ca-plagioclase, pyroxene, amphibole, swelling diagenetic clay etc., that have been altered in the adjacent sandstones; (2) in absolute terms the concretions h a v e , a s a general rule, a higher content of rare-earth elements (REE), Ba, Sr and K and a lower content of Na. Lower Na is consistent with albitization of detrital Ca-plagioclase while the higher content of other elements is associated with the presence of diagenetic swelling clay which 'ages' to chlorite in the adjacent sandstones. Higher detrital apatite content of the concretions, as a consequence of its partial removal from the sandstones, may also contribute to the high REE content, as may the cementing carbonate itself. Barium and Sr removal from the laumontite-albite sandstones on a large scale is evident when they are compared with shallowly buried detrital Ca-plagioclaseheulandite bearing sandstones. It is particularly noticeable in some deep bore hole sections where the sandstone immediately above the transition to the laumontitealbite assemblage contains appreciable amounts of these elements. The explanation for enrichment of Ba and Sr at this mineralogical boundary 1 ies in the fact that these elements are not compatible with the structures of the newly-formed minerals, principally albite and laumontite, but occur significantly in their precursors heulandite and Ca-plagioclase. Ba and Sr thus released by the breakdown of these 1 ast-named minerals were expelled upwards in solution and captured by heulandite. In this way heulandite containing ca 5 wt % Ba and Sr was produced. Finally, increasing burial of the sedimentary pile produced illite at the expense of smectite in the mudstone. This involved the uptake of additional Kand the expulsion of Na and it appears that at least some of the K is provided bytheageing of K-bearing clays to chlorite and the breakdown of biotite and K-feldspar in associated sandstone. The progressively-released Na contributed to the [Na] of the sandstone pore solution and hence plagioclase albitization.

PHASE RELATIONS IN ALUMINOUS METASEDIMENTS AT W E E K E R 0 0 , SOUTH AUSTRALIA R.L. Oliver Department of Geology and Mineralogy, University of Adelaide, Adelaide, South Australia Muscovite schis ts on Weekeroo Station, north of Mannahi11, South Australia are an extension of the Broken Hill pre-Adelaide Supergroup basement complex. Andalusite porphyroblasts, up to 7 or 8 cm long, occur singly or in clusters. Strain has been responsible for the flattening of many of them parallel to the dominant schistosity. The latter is axial planar to conspicuous tight folding of layered arenaceous metasediments (pre-Adelaide Supergroup, F2).

In thin section, the large andalusites enclose earlier kyanite and are partially replaced by later fibrolite and sillimanite. Chloritoid and staurolite are abundant; there appears to be no reaction relation between them. Probe analyses indicate ZnO, up to 3.60%, in staurolite, inversely proportional to the total Fe content. The coexistence of sillimanite with chloritoid suggests disequilibrium. Coarse muscovite encloses fibrolitic sillimanite and appears to be 1ater. Later than all other phases, however, is a pervasive fine-grained sericite, the development of which probably accompanies post-Adelaide Supergroup folding (F3) in the area. The sericite has a Ti content distinctly lower than that of the coarse muscovite indicating a lower temperature of crystallization for the sericite.

SIMULTANEOUS MEASUREMENT OF Pb AND S ISOTOPES IN GALENAS FROM THE NEW LEAD BELT, SOUTHEAST MISSOURI, USING THE ION MICROPROBE W. Compston, J. Coles S I.S. Williams Research School of Earth Sciences, Australian National University, Canberra, A.C.T. A Sensitive High Resolution Ion Microprobe, SHRIMP, has been constructed at the Research School of Earth Sciences, ANU, for geological and meteoritical applications. Like the electron microprobe, samples are prepared as carbon-coated polished sections. Selected spots can be analysed isotopically by bombardment with a primary ion beam focussed down to ~15 ym, producing secondary ions of the sample which are then mass-analysed. One of the first applications is the simultaneous measurement of Pb and S isotopic compositions in galenas using PbS 2 " secondary ions. Pb results are within 0. ]% for Broken Hill galena and other standards, and within O . S 0 ^ for 3lf S/ 32 S. Analyses, using SHRIMP, of galenas from the Buick and F1etcherMines in the New Lead Belt, southeast Missouri, have confirmed a recently reported correlation between Pb and S isotopic compositions. This was for galenas collected on the scale of the Buick orebody and measured by orthodox methods. SHRIMP analyses show also that pronounced Pb and S isotopic variability exists on a millimetre scale within single polished sections. This result opens the way for detailed Pb and S isotope studies on orebodies, especially those like the Missouri deposits which show zoning, resolution and multiple generations of galena. The implications of the Pb and S isotope correlations and of Pb isotope systematics for tracing the sources of both elements will be out 1ined.

MAJOR Sm-Nd AGE DIFFERENCES WITHIN THE YILGARN BLOCK Ian R. Fletcher S K.J.R. Rosman School of Physics and Geoscience, Western Australian Institute of Technology, Bentley, Western Australia Early results from a major Sm-Nd geochronologica1 survey of gneisses and greenstone belts within the Yilgarn Block indicate ages of crustal differentiation spanning M 000 Ma. The oldest ages so far determined are model ages of 600 Ma for gneisses at Mt Narryer. This gives support to earlier arguments for the antiquity of the Western Gneiss Terrain, a region containing clear evidence of metasedimentary activity. In the Murchison Province, greenstones of the Warriedar Fold Belt yield ages of 900 Ma and a gneiss at Cue has given a model age of 800 Ma. Both of theseare significantly older than the ubiquitous 2 700 Ma Rb-Sr age. The greenstone sequence at Kanowna, in the Eastern Goldfields Province, gives an isochron age of ^2 750 Ma. It is striking that these age differences correspond to recognized structural sub-divisions of the Yilgarn, but the data are too few to claim that they characterize the sub-divisions. North from Mt Narryer, at the boundary of the Western Gneiss Terrain with the Gascoyne Province, ages of 000 Ma have been recorded.

S m - N d AGE C O N S T R A I N T S ON KAMBALDA A N D KANOWNA

GREENSTONES

M . T . M c C u l l o c h & W . Compston Research School of Earth S c i e n c e s , A u s t r a l i a n N a t i o n a l U n i v e r s i t y , C a n b e r r a , A . C . T . Sm-Nd data are presented for A r c h a e a n g r e e n s t o n e s a n d felsic intrusives from two a d j a c e n t areas w i t h i n the Eastern G o l d f i e l d s P r o v i n c e of the Y i l g a r n B l o c k . Results from the Kambalda a r e a , w i t h one replicated e x c e p t i o n , c l o s e l y fit a linear a r r a y , w h i c h suggests that the l lof Il d e r m a f i c - u l t r a m a f i c s e q u e n c e h e r e has e f f e c t i v e l y llf3 the same age a n d initial Nd/ * N d as intrusive felsic b o d i e s . T h i s j o i n t a g e is 2 7 9 0 ± 3 0 M a , equal w i t h i n errors to previous Pb-Pb and U - P b a g e d e t e r m i n a t i o n s f o r the g r a n o d i o r i t e a l o n e . If c o r r e c t , it implies that the m u l t i s t a g e d e v e l o p m e n t of crust from m a n t l e required only a short period in the Kambalda a r e a , c o m p a r a b l e to o r less than ± 3 0 M a . T h e above p r o c e d u r e of combining Sm-Nd data from m a f ic a n d fel sic rocks has been used e l s e w h e r e for all previous Sm-Nd age d e t e r m i n a t i o n s o f high precision. If only g r e e n s t o n e samples a r e e m p l o y e d , the Kambalda a g e becomes 2 9 1 0 Ma but w i t h a m u c h lower precision of ± 170 Ma due to t h e i r s m a l l e r range in S m / N d . The sample that deviates from the 2 790 Ma array is an a l t e r e d u l t r a m a f i c lava f r o m w i t h i n a m e t a s o m a t i c reaction zone d e v e l o p e d during regional m e t a m o r p h i s m at the contact between d i s s i m i l a r rock u n i t s . T h e high contents of Sm and Nd and low Sm/Nd o f this sample show that m e t a m o r p h i c t r a n s f e r of REE o c c u r r e d . 1

llf

Regardless of the question of the o r i g i n a l a g e , the initial ** Nd/ ^Nd of the Kambalda sequence is significantly g r e a t e r than the c h o n d r i t i c e v o l u t i o n c u r v e w i t h an eNd v a l u e o f + 3-4 ±0.6. For the g r a n o d i o r i t e a l o n e , the £Nd 3v a l u ex is independe n t l y set at + 3 - 9 , if the Pb-Pb a g e is u s e d . T h e high initial N d / ^ N d requires an e a r l i e r LREE depleted (high Sm/Nd) s o u r c e for the Kambalda v o l c a n i c s a n d therefore provides some of the first d e f i n i t i v e e v i d e n c e for long-lived heterogeneities in the A r c h a e a n m a n t l e . Results from the second a r e a , K a n o w n a , do not 1ie o n a s i n g l e linear a r r a y . In contrast to the Kambalda samples w h i c h are from deep d r i l l - c o r e s , the Kanowna samples are from surface o u t c r o p s so w e cannot e x c l u d e the p o s s i b i l i t y that their Sm/Nd has been disturbed by recent w e a t h e r i n g . One of the felsic r o c k s , p r o b a b l y an i n t r u s i v e , plots close to the Kambalda felsic intrusives a n d p r e s u m a b l y has a l l simi+3 lar a g e . The o t h e r , a d a c i t i c t u f f , is e i t h e r o l d e r , o r has a lower initial Nd/ llfif Nd o r has an a l t e r e d S m / N d . The interpretation o f an o l d e r a g e for the d a c i t i c tuff is consistent w i t h some regional interpretations w h i c h p l a c e the Kanowna rocks in an o l d e r v o l c a n i c cycle than those at K a m b a l d a .

G E O L O G I C A L A N N E A L I N G OF FISSION T R A C K S

IN D E T R I T A L A P A T I T E S

T H E R M A L HISTORIES OF SEDIMENTARY

AND

ROCKS

A . J . W . G l e a d o w & I.R. Duddy Department o f G e o l o g y , U n i v e r s i t y o f M e l b o u r n e , P a r k v i l l e , V i c t o r i a Fission tracks in m i n e r a l s are stable o v e r g e o l o g i c a l time o n l y b e l o w a chara c t e r i s t i c t e m p e r a t u r e range. Heating the m i n e r a l in this range c a u s e s a n n e a l i n g o f the radiation damage making up the t r a c k s , p r o g r e s s i v e l y resetting the f i s s i o n t r a c k a g e . A p a t i t e ages a r e the most s e n s i t i v e to thermal a n n e a l i n g , being reset b e t w e e n about 60 a n d 125°C o v e r 10 M a . Fission track ages have been m e a s u r e d on a p a t i t e s from v o l c a n i c l a s t i c sandstones o f the Early C r e t a c e o u s O t w a y G r o u p . In o u t c r o p s a m p l e s the a p a t i t e a g e s a r e c o n c o r d a n t w i t h s p h e n e and z i r c o n a g e s . T h e s e a p a t i t e s have t h e r e f o r e n e v e r been hot e n o u g h for track a n n e a l i n g to o c c u r , implying that z e o l i t e s and o t h e r diagenetic m i n e r a l s formed at relatively low t e m p e r a t u r e . A p a t i t e a g e s in Early C r e t a c e o u s samples from d e e p boreholes in the O t w a y a n d G i p p s l a n d Basins d e c r e a s e down the holes reaching z e r o a g e w h e r e present t e m p e r a t u r e s a r e a b o u t 125°C. T h e shape o f the p r o f i l e o f a p p a r e n t a g e w i t h d e p t h and the range of a n n e a l i n g t e m p e r a t u r e s can g i v e important information on the p r e s e n t a n d past d i s t r i b u t i o n o f t e m p e r a t u r e s in these h o l e s . Fission track lengths a l s o d e c r e a s e s y s t e m a t i c a l l y w i t h increasing d o w n - h o l e t e m p e r a t u r e . R e d u c t i o n in f i s s i o n track a g e and length in a p a t i t e p r o v i d e a p o w e r f u l new m e t h o d for investigating the thermal h i s t o r y of s e d i m e n t a r y rocks o v e r the t e m p e r a t u r e interval o f m a x i m u m o i 1 g e n e r a t i o n .

ENGINEERING GEOLOGY GROUP SYMPOSIUM

INVESTIGATION FOR A QUARRY FOR BREAKWATER CONSTRUCTION AT ROCKY P O I N T , WESTERN AUSTRALIA R . P . Mather Geological Survey of Western A u s t r a l i a , Perth, Western Australia Increasing environmental requirements and the special needs of modern breakwater construction are making increasing demands on the scope of quarry investigation. The steps leading to the selection of a quarry site at Rocky Point, such as land title search, regional and detailed geological m a p p i n g , seismic refraction geophysics, diamond drilling and the comparison of rock properties with those of existing quarries are described to give an example of the current a p p r o a c h . Some alternative ways of satisfying environmental requirements, such as rehabilitation, modification of attitude and shape of the quarry and the construction of screen banks, are discussed. Possible final solutions for a quarry at Rocky Point are compa red.

RAILWAY STABILITY INVESTIGATIONS BETWEEN CLAVERLEY AND O A R O , M A R L B O R O U G H , NEW ZEALAND D.H. Bell Department of Geology, University of Canterbury, Christchurch, New Zealand Engineering geological and geotechnical investigations were carried out on behalf of New Zealand Railways during 1978 and 1979 to evaluate instability problems along 6 km of the South Island Mai n North Line between Claverley and O a r o , in southern Marlborough. The permanent way follows the coastline through much of t h i s a r e a , where side-slopes are generally steeper than 30°. Bedrock lithologies include complexly folded "greywacke-argi11ite" basement rocks, exfoliating "soft" mudstones, and flat-lying limestones and marls of lower-mid Tertiary a g e . Major regolith (weathered mantle) instability was triggered by the passage of Cyclone Alison in March 1975, and since that time relatively frequent landsliding has occurred on to the rai1 tracks. Fill materials and slope deposits beneath the rail tracks have failed by landsliding, and the instability problems have been aggravated by flood scour and storm-wave attack. The railway also crosses the active Mikonui Earthflow, in which seasonally-control led movements occur over in situ bentonitic mudstones at a depth of about 20 m , and regular track maintenance is necessary. Investigations included engineering geological mapping at a scale of 1:500 over parts of the a r e a , diamond drilling and backhoe excavation to provide subsurface data and limited laboratory testing of samples from landslides and potentially unstable areas. Geotechnical assessment of the steep coastal slopes and catchments utilized back-analysis of the various types of instability recognized, and Railways' engineering consultants developed appropriate remedial options which ranged from tunnelling and bridging, to improved maintenance of the existing alignment. Major earthworks and culverting were carried out in one locality to remedy track instability, and recommended soil conservation measures (including retirement from grazing and the planting of many steep catchments with native and exotic species) are expected to be of considerable long-term importance in reducing the incidence of rai1way closure in the Claverley-Oaro area.

A FIELD RECOGNITION GUIDE TO SWELLING-CLAY-RICH WEATHERING PROFILES IN OVERBURDEN SEDIMENTS IN THE NORTHERN BOWEN BASIN, QUEENSLAND N.H.H. Godfrey Utah Development Co., Moranbah, Queensland Recurrent pitwall failures at Goonyella Mine led to a series of integrated studies to determine the likely causes. One of the major influences on spoi1 instability was found to be the dumping of rocks rich in smectite-swelling clays in basal spoil zones: the clays were also largely responsible for a minor proportion of highwal1 failures. Formed in severely weathered overburden, the clays occur both in lacustrine Tertiary deposits and in the Late Permian volcanolithic Moranbah Coal Measures sediments underneath. In general, such weathered Permian overburden occurs to greater depths under deeper Tertiary cover. The smectite clays are thought to have formed under humid tropical weathering conditions in the mid-to-late Tertiary period; deriving from i 11 ite and i11ite-smectite rich rocks produced via burial metamorphism from original volcanolithic arenites and lutites. In view of the problems which smectite clays can cause during stripping operations, it might be considered advisable to be able to recognize them as early as the initial exploration stage and then delineate them as i nvestigat ion proceeds. Accordingly, this paper presents a brief field guide to the identification of weathering profiles likely to harbour these clays in these and similar sediments in the Northern Bowen Basin.

SOME GEOTECHNICAL ASPECTS OF WEATHERING PROFILES IN THE DARLING RANGE - PERTH AREA, WESTERN AUSTRALIA G. Marcos Geological Survey of Western Australia, Perth, Western Australia The granitic rocks of the area are intruded by dolerite dykes and overlain by colluvium and alluvium. Their weathering profiles show major differences. Both rock types have a hard massive laterite underlain by ironstone gravel soil mixes and a gibbsite layer. The massive laterite's irregular thickness causes foundation problems. The gravelly soils have erratic engineering properties. The gibbsite soils on drying become hard and brittle and thus constitute main leakage paths in water-retaining structures. The interface between the laterite and completely weathered material is a main leakage path, causing piping in cuts. The mineralogical composition of the completely weathered granites varies: the lower parts are silty sand, rich in partly altered micas and feldspars; the middle parts are clayey sandy silts, composed of sand, kaolin and illite, and can have unsatisfactory properties; the upper parts are sandy silty clay, mainly quartz and kaolin, materials of desirable properties for embankments. Weathering of the dolerite results in a range of mixed material from soil to rock. Soils range from very plastic montmori1lonoid clays to slightly plastic mica silts, which lead to diverse engineering properties, thus necessitating careful investigation. The mineral halloysite occurs in both granitic and doleritic soils. If soils are to be used as fill it is essential to identify halloysite in the early stages of investigation to avoid compaction problems.

T H E A P P L I C A T I O N OF E L E C T R O M A G N E T I C

PROFILING TO THE

OF T H E T A R O N G P I P E L I N E R O U T E ,

INVESTIGATION

QUEENSLAND

G.J. Mules Queensland Water Resources Commission, Brisbane, Queensland A continuous trench 2 - 3 m deep and 96 km long is required to bury a pipeline connecting Boondooma Dam to the Tarong Power Station near Nanango, An undulating terrain of weathered granites, metamorphics and coal measures with a 11uvia1 deposits of sands and black clayey soils, must be negotiated. The investigation commenced in 1979 with a cont inuous electromagnet ic (conductivity) profile of the first 14 km using a hand-held Geonics EM 31. To this test section was also applied continuous 12 channel seismic, surface mapping, auger dri 1 ling, excavator trenching and resistivity soundings as an attempt to develop an understanding of EM response to geological conditions. The EM 31 was chosen for its in-field speed and ease of data processing along with the requirement for information at shallow depths. The results of the test section were successfully applied to electromagnetic profiles along the remainder of the line enabling a more efficient programming of detailed investigations at a considerable saving of time and cost. The correlation between EM response and investigated conditions was good, allowing a greater degree of certainty between sample points. The application of the technique was successfu1, however future investigations may be improved in light of experience gained here.

D E W A T E R I N G OF T H E M U J A O P E N C U T C O A L M I N E , W E S T E R N 1

P. Whincup , G.V. Smith 1 2 3

S E. McDonald

AUSTRALIA

Layton Groundwater Consultants, Perth, Western Australia Layton Groundwater Consultants, Bunbury, Western Australia Jacia Mine Management and Consulting Services, Perth, Western Australia

The Muja mine, operated by The Griffin Coal Mining Company Limited, is located near Collie in the southwest of Western Australia. A series of nine coal seams, extending down from Ate to lona is mined. The thickest seam, Hebe, was originally mined by underground method but uncontrollable flooding occurred when an open drillhole was intersected in 1965, and the Hebe mine was abandoned. Open cut operations were initiated soon afterwards. Groundwater control in the open cut is a major operation, made difficult by the cavernous nature of the sediment/coal contact. Sediments above Hebe are dewatered by bores which drain water into the old Hebe mine, to be removed by large capacity production bores. Conventional production bores are also employed. Removal of 130 m of overburden and coal has created significant groundwater pressures beneath the floor of the open cut. These pressures, to a depth of 150 m below the Hebe seam, are reduced to acceptable levels by installation of pressure-relief bores. The dewatering/depressurizing has proved effective by pursuing a programme of sound geological control, drilling technology and geophysics. At the peak of the programme, a total of 2h 000 kl/day of groundwater will be removed.

ENGINEERING GEOLOGY AND URBAN DEVELOPMENT PRACTICES

IN N E W

ZEALAND

D.H. Bell Department of Geology, University of Canterbury, Christchurch, New Zealand Geological hazards relevant to urban planning and development in New Zealand include mass movements of natural or filled ground by processes such as settlement and sliding; instability resulting from particle movements by processes such as soil shrinkage and dispersion/internal erosion; flooding or aggradation/scour problems associated with high-intensity rainstorms and fluvial processes; coastal erosion

resulting from storm-waves or tsunamis; ground shaking and/or surface deformation accompanying seismic activity; and volcanic eruptions. Selected case histories illustrate the wide range of urban problems resulting from such hazards i n New Zealand, and also the variety of site-specific geotechnical factors involved. Analyses of available records show that damaging floods and landslips o c c u r w i th the highest frequency, but that in economic and human terms a major earthquake centred near a large urban area is a much greater potential hazard. Urban planning and development practices in New Zealand are control 1ed by various legal statutes, and administered by numerous regional and local authorities. Selected examples highlight the present and potential roles of engineering geology at various stages of urban development from Regional Scheme Plan preparation to foundation design on individual sections. Planning and construction practices on the Port Hills, Christchurch, are reviewed in detail, with emphasis on techniques of chemical stabilization in dispersive loessial soils.

SANDSTONE AS A BUILDING MATERIAL G.S. Gibbons S J.L. Gordon New South Wales Institute of Technology, Broadway, New South Wales Many sandstone houses and other structures around Sydney date back over 150 years. New sandstone is used in paving, cladding, retaining walls, ornamental features and restoration. Some traditional craft skills have been lost over the past 50 years, along with the understanding (even by "experts") of t ime-honoured const ruction materials. In contrast, we now have better understanding of materials generally, and many new materials can be made compatible with traditional ones. As a result, the older materials can be more durable than ever before. Past problems with Sydney sandstone include fretting, colour variation, and structural failure. The processes of deterioration include leaching, clay degradation, and crystallization of water-soluble salts. On external exposed sandstone: (1) surface flow of rain-water is most important (often sheltered areas show most deteriorat ion); (2) joints behave differently in different locations, and two or three jointing materials may be appropriate on a single structure; (3)

different

horizontal top surfaces need to be waterproofed;

(4) damp-proof courses and waterproofing can frequently create serious problems if not properly designed; (5) the selection of building stone, in which geologists are frequently involved, requires an understanding of these factors, as well as the broad geological nature of the stone and its behaviour in laboratory tests.

DETRIMENTAL FEATURES OF ANCIENT PILLOW BASALTS WHEN USED AS A SOURCE FOR CRUSHED AGGREGATE C.A. Key Main Roads Department, Perth, Western Australia An altered Proterozoic pillow basalt outcrop in the arid Pilbara region of Weste rn Australia shows that the altered glassy margins of the pillows can weather to clay within a period of five years. The amount of deleterious pillow margin is seen to be approximately five percent by volume. The basalt is metamorphosed to low greenschist facies, has tremolite and epidote as the main alteration products, along with very fine-grained albite and zoisite and also has irregular areas with a fine-grained mixture of chlorite, quartz,

carbonate and prehnite, probably the remains of sheared-out amygdales. Fresh pillow margins are relatively hard and brittle and consist of a fine-grained foliated mixture of the same minerals. This is liable to produce crusher fines and dust mixture, which is too plastic for road-base. For the aggregate producer there is an obvious economic disadvantage when there is a definite possibility of producing up to five percent deleterious dust. The road authority will be unhappy if the aggregate needs to be re-screened and pre-coated, or, at worst, if five percent of the sealing aggregate can be lost by weathering within a period of five years.

A MARGINAL SEDIMENTARY AGGREGATE IN CONCRETE M.J. Sandy CSIRO, Division of Building Research, Highett, Victoria During an investigation into the poor performance of concrete in a dam structure, evidence of an unusual alkali-aggregate reaction was observed. Problems with the concrete included widespread surface cracking and tightening of gates used to control water flow through diversion tunnels. The aggregates used, local sandstone and siltstone, were shown to be dimensionally unstable with cycles of wetting and drying. This instability was a major factor in the concrete deterioration: however, an observed cement/aggregate reaction may also have contributed. Characteristics of this reaction in hand specimen are dark reaction rims on aggregate grains and infilling of cracks and voids, and coating of fractured aggregate by white powdery materia1. Investigation of phases by X-ray diffraction and scanning electron microscopy showed the white material to be a mixture of an unstable zeolite-A-1ike material and calcite. Associated phases are trona and an amorphous alkali-silicate gel. It appears these products have formed by reaction between micaceous minerals in the aggregate and Ca(0H) 2 in the cement. These results place serious doubt on the wisdom of incorporating sedimentary aggregates into structural concrete.

SAMPLE PRETREATMENT AS AN INDICATOR OF MATERIAL PERFORMANCE R.B. Smith Department of Main Roads, Haymarket, New South Wales At Wilcannia, New South Wales (N.S.W.) the road base proposed for use was a gravel consisting of concretionary limestone nodules, soft-rock fragments of lithic sandstone and silicified siltstone in a sandy-loam mix. The material, when tested in the deposit, conformed to the specification but when tested after compaction on the road did not conform to the specification requirements because of an increase in the plasticity indices. A laboratory investigation showed that the material was being broken down principally by the use of pulvi-mixers used to incorporate water prior to compaction. As these changes were not indicated by the usual test procedures a simple test was developed whichdid indicate breakdown would occur on the road. An investigation of samples of shale from nine localities in eastern N.S.W. indicated that subjecting a material to a series of compactions in the laboratory prior to testing more closely replicated field conditions than did subjecting it to ten cycles of wetting and drying. Follow-up testing of a shale pavement after twenty years indicated that the effects of traffic and weathering were minimal. At present three methods of pretreatment are used in the evaluation of Sydney breccia proposed for use with lime as lime-treated road base. Analysis of results obtained over the last six years indicates that the number of methods of pretreatment could be reduced to one, namely the repeated compaction procedure.

ROCK MASS WORKABILITY E.J. Minty & G.K. Kearns Main Roads Department, Sydney, New South Wales The authors consider that there is a need to improve the techniques currently used to evaluate the workability of rock masses. Data for sedimentary, igneous and metamorphic rocks are included in this paper for over 30 examples analysed in the course of our study. These examples were actual road construction or quarry sites for roadworks in N.S.W. This paper details an improved procedure uti1izing both geological and geophysical procedures to determine workability rather than adopting the oversimplified approach of making a "rippability assessment" based on the seismic velocity of the rock mass alone. Previous methods of assessment such as the "Caterpi 11 er Charts" do not fully allow for all the geological influences. The geological parameters which have been applied in this study are degree of rock weathering, rock strength, condition of discontinuities, spacing of discontinuities, orientation of discontinuities and groundwater conditions. Simi lar parameters have been used by Barton of the Norwegian Geotechnical Institute but for a different purpose. When rock is won by ripping or blasting it is necessary to take the nominal size of the product into account to obtain a real assessment of the effectiveness of the work done. Previous methods of assessment have ignored this factor.

NORTHWEST WESTERN AUSTRALIAN COASTAL SANDS AS ROADBASE MATERIALS L.J. Wylde Department of Geology, University of Western Australia, Nedlands, Western Australia Red sands are common surficial deposits of the Canning and Carnarvon Basins and in many areas offer an economically attractive alternative to natural gravel roadbase materials. Research on the behaviour and performance of these sands as roadbase materials is being carried at the Department of Geology, University of Western Australia, in conjunction with the Australian Road Research Board and the Main Roads Department of Western Australia. The sands being studied are typically finer than 1.18 mm and may have 15 to 20 percent of grains finer than 75 ym. They comprise rounded silica particles coated with a red mixture of iron oxides/hydroxides and clay minerals. The relationship between these constituents and the behaviour/performance of the sands in the road, and with certain standard test results such as Plasticity Index and compacted density are outlined as a progress report on the research. A connection between the nature of the sands and the geologica 1 and soi 1-forming conditions of their formation is also evident and allows the use of a geological basis for material search. Some observations on related aspects of Quaternary geology are also made.

HYDR0GE0L0GY AND HYDROGEOCHEMISTRY OF GROUNDWATERS FROM THE MAGELA CREEK A R E A , ALLIGATOR RIVERS R E G I O N , NORTHERN

TERRITORY

M. Ahmad & H. Quereshi Water Division, Department of Transport and Works, Northern Territory Hydrogeological studies suggest a two-aquifer system separated by adiscontinuous semi-pervious layer of extremely weathered bed rock. The upper aquifer is in the unconsolidated material and the lower aquifer is in the relatively fresh bed rock. The lower aquifer is influenced by rock lithologies and textures; granitic rocks and carbonate sediments appear to have larger yields.

89 Hydrochemical data suggest that most waters are bicarbonate type with Mg, Na and K as the dominant cations. They have near-neutral pH and redox potentials are 2+ s generally above the F e / F e + boundary. Total dissolved salts are usual ly below 300 mg/1. Tritium measurements suggest that most waters are pre-1952, but vertical variations are observed. Saturation indexes for calcite, dolomite, magnesite, gypsum, montmori1 Ionite and kaolinite have been calculated. These data suggest that most waters are unsaturated with respect to calcite, dolomite, gypsum and magnesite, but are probably in equilibrium with either montmori1 Ionite or kaolinite. It also appears that pH is buffered to near-neutral by water-rock interaction.

TRACE METAL POLLUTION

IN S Y D N E Y H A R B O U R

SEDIMENTS

Ian Irvine Dames and Moore, Crows Nest, New South Wales Sydney Harbour is a small drowned-valley estuary, characterized by narrow, winding channels and very irregular bathymetry. It has a maximum depth of 46 m , but tidal circulation is restricted by a bar at the entrance. The catchment is small and almost completely urbanized; freshwater input is minor. The harbour has had a long history of pollution, high organic loadings and fluctuating oxygen levels, but water quality has improved recently. Much of the harbour floor i s blanketed by black, organic-rich silts and muds - sands are found chiefly at the entrance and in the upper parts of tributary channels. Heavy metal and nutrient levels were studied from 227 surface-sediment samples, and 177 samples from short cores and boreholes. Elevated levels of Cu, Pb, Zn, Cr, Ni, Co, Mn, V, Cd and As were found. The most contaminated areas were the channel and shallow bays of the Parramatta River, and the bays near Darling Harbour. Enrichment factors of up to 100x(Cu), 40x(Pb), 30x(Cr) and 45x(Zn) occurred. The contaminated layer was normally 50-80 cm thick, due largely to bioturbation. The distribution of most elements was consistent with patterns of industrial discharge occurring during the past century - Pb and Zn levels were also due to dust fallout, and V levels to localized oil pollution. High levels of all elements in bay ends were attributed to urban runoff. Partial extract ion of selected samples revealed that all metals were mobilized significantly under reducing conditions - average extraction rates varied from 11 percent (Co) to 71 percent (Pb). Under oxidizing conditions, Fe and Pb were relatively immobile. For other elements the extract ion rate ranged from 10 percent (Cd) to 30 percent (Zn). Sediments may be alternately oxidized and reduced by processes such as bioturbation, dredging and water-quality variations, and there is some danger that heavy meta 1 s could be released from Sydney Harbour sediments by such processes.

THE GEOTECHNICAL SPECIALIST AND CONTRACTUAL

DISPUTES

D.H. Stapledon School of Applied Geology, South Australian Institute of Technology, Ingle Farm, South Australia Experiences as a consultant during four contractual disputes are described and the main features of the author's involvement in six other disputes are tabulated. The disputes were related to claims by contractors that adverse site conditions met during construction could not have been foreseen from essentially factual geotechnical data provided to tenderers. The author was retained in each case to make an independent assessment of the likely site conditions from the original data. From experience on all of these projects it is concluded that the main cause of disputes is inadequate site investigations which result in both the contractor and principal being surprised by some aspect of the site geological conditions during construction. The effects of poor site investigations are often aggravated by the poor standard of factual geotechnical data made available to tenderers. Some disputes had occurred despite successful site investigations and thepresentation of good quality factual data to tenderers. It is believed that these disputes should not have occurred if tenderers had received interpretive as wel1 as factual data, preferably provided by competent geotechnical specialists in the employ of the principal.

SPECIALIST GROUP IN TECTONICS AND STRUCTURAL GEOLOGY

MODERN WORK IN CLASSICAL FOLD BELTS

C O N V E R G E N T T E C T O N I S M IN W E S T E R N PAPUA NEW G U I N E A H.L. Davies Bureau of Mineral Resources, Canberra, A.C.T. In broad terms, the northern slopes of the central cordillera of Papua New Guinea are made up of igneous and metamorphic rocks with complex structure, while the southern slopes are made up of sediments of the Papuan Basin. The thrust belt which coincides with the southern slopes has long been regarded as an example of gravity tectonics. Current compilation of geology across the cordillera leads to the alternative hypothesis that the thrust belt was produced by north-south (or northeast-southwest) compression and crustal shortening, and is merely the most recent manifestation of a compressive regime which has persisted since the late Eocene. According to this hypothesis, thrusting was initiated in the aftermath of EoceneOligocene arc-continent collision. The locus of thrusting and of related uplift (and of related ?volcanic activity) has migrated progressively southward from what is now the Sepik depression to a present-day axis in the Star Mountains.

IMPLICATIONS OF D E F O R M A T I O N P A T T E R N S IN T H E A P P A L A C H I A N F O R E L A N D OF SOUTHWEST VIRGINIA, U.S.A. David R. Gray Department of Geological Sciences, Vi rginia Polytechnic Insti tute and State Univers ity, Blacksburg, Virginia, U.S.A. Patterns of internal distortion combined with the distribution, type and succession of minor structures within thrust sheets are diagnostic expressions of structural evolution in foreland zones. Little is known about these aspects in the Southern Appalachian Valley and Ridge Province. Characterized by interleaved thrust sheets stacked in imbricate fashion, the sector in southwest Vi rginia comprises f ive major sheets. Minor structures include wedge faults, contraction and extension faults, folds and spaced cleavage. Cleavage is sporadically developed and shows little correlation with southeasterly position in the thrust belt. However, terrain west of the Narrows Thrust (along the New River traverse) shows little penetrative deformation. To the east, layer-parallel shortening fabrics occur. Strains (R s ) from reduction spots (K .0 - 1.2) distorted fossils and mud rock polygons range from 1.2 to 2.0. Cleavage, although restricted to certain stratigraphic units (primarily Ordovician), has not been observed above the Ordovician-Si1urian boundary. Layerparallel shortening effects are also generally absent higher in the section. Folded faults and polydeformed strata (Cambrian and Cambro-Ordovician) of the Pulaski sheet occur adjacent to the "crystallines" of the Blue Ridge Province. Present work suggests that previously outlined east to west or west to east progressions of faulting may be too simplistic. Data collected to date are not sufficient to discriminate between gravity sliding, gravity spreading and "bulldozer" models of foreland evolution for this sector of the Appalachian Orogen.

POLYPHASE ACADIAN AND ALLEGHANIAN CONNECTICUT

DEFORMATION

P A R T OF T H E A P P A L A C H I A N

IN T H E FOLD

SOUTHEAST

BELT

R.J. Korsch Department of Geology, Victoria University of Wellington, New Zealand During the late Precambrian to late Permian, the New England sector of the Appalachian Fold Belt in eastern U.S.A. was subjected to at 1 east four major orogenic episodes, most of which produced polyphase deformation. The last episode, the Alleghanian, is post-Carboniferous and has affected Carboniferous rocks from Rhode Island producing east-west structures, high-grade metamorphics and granitic intrusions. In northern Connecticut and Massachusetts, older north-south structures and si 11imanite-grade metamorphics of Acadian age (mid-Devonian) occur. A major problem in southeast Connecticut is that all deformations have been regarded as Acadian but isotopic studies have given numerous Alleghanian ages as well. Using style, orientation and overprinting relationships of the mesoscopic structures in southeast Connecticut, at least four periods of deformation can be recognized. Di produced recumbent isoclinal folds, refolded by D2 isoclinal and often recumbent folds. D3 formed open to tight, upright to inclined folds and Di» is expressed as gent le upright warps. A layered granitic intrusion of early Permian age has been gently folded, confirming the presence of at least one Alleghanian deformation in southeast Connecticut. The complex interference pattern both in outcrop and map-scale has resulted from the north-south Acadian structures being overprinted by later east-west Alleghanian structures.

LINEAMENT CLASSIFICATION, TASMAN OROGEN, NORTHEASTERN

QUEENSLAND

Eric Heidecker Department of Geology and Mineralogy, University of Queensland, St Lucia, Queensland A structural map of northeastern Queensland has played a key role in detection and differentiation of lineaments. Through the removal of d i stract i ng petrolog ic and structural detail, this map brings out structural alignments. A remarkably regmatic array of four sets of structural trends is revealed. These sets are distinguished not only by azimuthal range, but also by structural characteristics. Kinematic analysis supports field evidence for association of easterly sinistral strike-slip shears, northeasterly tensional zones of intrusion, and northwesterly compressed and foliated zones. Tensional and compressiona1 disturbances appear to have been set off at the intersections of easterly shears and older crypt ic structures. These observations lead to the following classification of lineaments: Age Precambrian

Early Palaeozoic

Palaeozoic and younger

Trend N60°W N30°E

Cryptic structures. 11 11

N15°W

Tasman Zone early rifts.

N90°E

Sinistral shear zones, probably Tasman Zone transform faults.

N60°E

Tensional zones of intrusion.

N60°W - N^0°W

Foliated, compressed zones.

Trends as above

Resurgent lineaments,

GENERAL STRUCTURAL GEOLOGY

STRUCTURE AND EXPLORATION

IN T H E S H E R R I D O N M I N E

DISTRICT,

NORTHERN MANITOBA, CANADA Brian Marshall 1

& K.D. Tuckwell

Department of Applied Geology,. New South Wales Institute of Technology, Broadway, New South Wales Conzinc Riotinto of Australia Exploration Pty Ltd, Broken Hill, New South Wales

The Kisseynew Sedimentary Gneiss Belt comprises complexly deformed upper amphibolite facies rocks. It lies north of the lower grade Flin Flon Greenstone Belt and is probably its correlate since both yield 1 800-1 900 Ma ages (Rb/Sr, K/Ar and Pb isotope methods). Major volcanogenic Cu-Zn-Au deposits are exploited at many localities in the Greenstone Belt. In the Gneiss Belt, minor occurrences of stratiform Cu-Zn-Fe mineralization abound, but are only exploited at the Sherridon Mine. The Gneiss Belt is a prime exploration target, but metamorphism and deformation have resulted in contradictory interpretations of the position of the Sherridon Group (the ore-bearing sequence) within the regional 1ithostratigraphy. Previous interpretations of the Sherridon Group and other units are examined in relation to mesofabric constraints in selected parts of the Sherridon Mine District. Limiting situations are established and a new interpretation comprising three generations of folding and repeated thrusting is proposed. Part of the 1ithostratigraphy is ascribed to emplacement of granitoid sheets along the then recumbent F x axial surfaces. Despite the exploration premise that occurrences of minera1ization west of Cold Lake might represent intercalations of Sherridon Group rocks, there is no supporting evidence from structure or 1ithostratigraphy.

M I C R O T E X T U R E S OF A N A T U R A L L Y - D E F O R M E D

PYRRHOTITE

ORE

D.J. Patterson Research and Development Division, Mount Isa Mines Limited, Mount Isa, Queensland Pyrrhotite-rich ores at Renison Bell were deformed during mineralization, at temperatures of about 250 - 350°C and confining pressures of 1 - 2 kb. Pyrrhotite shows ductile deformation predominantly by kinking, with minor deformation twinning in strongly kinked samples. Recrysta11ization of kinked grains has produced finergrained aggregates of polygonal grains with a pronounced preferred orientation, which wrap older, kinked grains. Several episodes of deformation and recrysta11ization are discernible in occasional samples. Chalcopyrite exhibits minor development of polysynthetic deformation twinning accompanying "lensatic" twins. Arsenopyrite shows predominantly brittle failure, but contains rare polysynthetic ?deformation twins in strongly brecciated grains. Microtextures observed in these ores correspond closely to those produced in pyrrhotite and chalcopyrite deformed experimental ly at similar temperatures and confining pressures.

S T R U C T U R A L A N D M E T A M O R P H I C C O N T R O L S ON T H E O R I G I N OF JADE

IN S O U T H

NEPHRITE

AUSTRALIA

A.J. Parker South Australian Department of Mines and Energy, Eastwood, South Australia Some of the World's principal nephrite jade deposits are located near Cowel1 on Eyre Peninsula, South Australia. The jade occurs as lenticular pods wi thi n Early Proterozoic dolomitic marbles that have been complexly deformed and metamorphosed.

Struct ural and textural relationships clearly indicate that the nephrite was formed during a late-phase tectonic event involving cross-warping and fracturing of the highgrade metamorphic complex. Diffusion along these structures into and out from the adjacent country rock promoted local retrogression through a series of inter-related metamorphic reactions. In the dolomitic marbles, the addition of Si02 accompanied by the 1 oss of CaO and CO2 formed small pods of nephrite, whilst in the nearby gneisses the balance of the CaO facilitated the alteration of biotite to actinolite.

INTER-RELATIONSHIP BETWEEN DEFORMATION AND M E T A M O R P H I S M

FAULT ZONES AND LINEAMENTS S.H. White Department of Geology, Imperial College, London, U.K. Recent studies of fault rocks in minor zones have shown that the metamorphic processes which occurred during faulting combined to produce weak rocks. These in turn concentrated the deformation within the zone. In this contribution, it will be argued that similar effects occur in major fault zones (lineaments) and that these lead to repeated tectonic re-activat ion of such zones. The A1 pine Faul t in New Zealand is used as an example. The Alpine Fault Zone in the area between Haast and Hokitika is marked by a 1 km wide strand of fault rocks. A traverse across the zone, going from the Alpine schists to the present fault trace, reveals a laminated series of mylonites and cataclasites, the sequence being: garnet oligoclase schists - curly schists - schistose mylonites - green mylonites- augen mylonites - cataclasites - gouge. Apart from the gouge, each rock type in the sequence grades into the other. The curly schists and schistose mylonites are of a similar metamorphic grade to the Alpine schists - new garnets, biotite and oligolcase develop from the old schists. Analysis of the garnets and biotites indicate a temperature increase into the faul t zone. However, there is a sudden drop in grade at the green mylonites which conta in chlor i te, per ister i te and epidote with the grade decreasing into the augen mylonites and cataclasites. The stretching lineations and the quartz fabrics indicate that the fault rock sequence has developed to accommodate the present obiique reverse movements between the Indo-Australia and Pacific plates. There is no evidence for earlier mylonites which accommodated the strike-slip movements which occurred prior to late Miocene. This is a good illustration of the development of rock types to accommodate a specific deformation. Stress analysis indicates that high stresses existed during this re-orientation of the fault rocks which in turn led to shear heating and deep fracturing resulting in the juxtaposition of high- and low-grade mylonites. The fault-rock sequence is interpreted as reflecting uplifted - n o w inactive parts of the zone at depth and provides an insight into the softening processes that can occur during the movement of a major lineament.

SUPERPOSED FOLDING:

A CLASSIC EXAMPLE IN EAST ANTARCTICA

A.J. Parker , P.R. J a m e s 2 , V. Mielnik 2 & R.L. Oliver 2 1 2

South Australian Department of Mines and Energy, Eastwood, South Australia Department of Geology and Mineralogy, University of Adelaide, Adelaide, South Australia

Archaean granulite gneisses in the Vestfold Hills and the Rauer Group islands, East Antarctica record a complex history of multiphase deformation. In the Vestfold Hills, early-formed isoclinal and intrafolial folds are overprinted by broad, open, east-west trending structures and these in turn are overprinted by north-south trending mylonite zones and a number of narrow, pseudotachy1ite-veins, faults and fractures. In the Rauer Group islands, an additional, possibly Proterozoic, fold deformation overprints the "Vestfoldian" folds and has produced a number of classical

interference patterns. Although there is no di rect evidence, it appears superficially that the later "Rauerian" folds may be related to the formation of the mylonite zones in the Vestfold Hills.

EXPERIMENTAL STUDY OF ROCK DEFORMATION UNDER CONDITIONS FAVOURING CHEMICAL TRANSFORMATIONS S.H. White, E.H. Rutter & C.J. Peach Department of Geology, Imperial College, London, U.K. It is well known from the materials science literature that the occurrence of a phase change, mineral reaction or change in environment during deformation can lead to a marked softening of the deforming material. Similar effects should occur during syn-tectonic metamorphism. An experimental programme has been initiated to study these effects. Initial experiments concentrated on the effects of the introduction of water on the rheology of fault zones in Tennessee sandstones {85% quartz, 15% clays and oxides). A marked softening occurred due to water-assisted diffusive mass-transfer processes. Detailed electron microscopy studies related the softening to microstructural changes. It was found that new micas grew in the fault zones, and were aligned parallel to the fault zone edges. This, plus pressure solution of the quartz led to the development of mylonitic microstructure. This study is currently being extended by means of similar experiments on basaltic rocks, deformed both wet and dry at 600°C. Preliminary results indicate a softening effect during deformation of the wet rock, comparable to that observed with Tennessee sandstone. Electron microscopy is being used to identify the mineral ogical changes which accompany deformation.

FILMS ON CARBONATE SEDIMENTATION Three films have been provided for the Convent ion by courtesy of Mr E.A. Shinn of the United States Geological Survey (USGS). The films have been scheduled within the Convention programme, and details of times and venues should be consulted in the programme publication. The subject matter covers aspects of carbonate rocks pertinent to analyses of facies trends, delineation of stratigraphic traps, etc. The subject matter of the films is summarized below.

GEOLOGY OF THE BELIZE BARRIER REEF The film was produced for the USGS by the Fisher Island Station. The film is based on a core-drilling expedition by the USGS Fisher Island Station to the barrier reef off Belize, Central America. The film shows the thickness, cementation and facies changes associated with a more than 100-mile-long barrier reef, as well as the three-dimensional aspects of a typical lagoonal patch reef.

CARBONATE PETROLOGY The film was produced by Peter Scholle and E.A. Shinn at the USGS Fisher Island Stat ion. This film is based on Peter Scholle's American Association of Petroleum Geologists Memoir 27 titled A Color Guide to Carbonate Petrology. The film shows the basics of grain recognition, as well as their environmental significance, and discusses how such recognition can aid inthe search for, and mapping of, facies trends, which may contain oil and gas.

STRATIGRAPHIC TRAPS: THE TIDAL FLAT MODEL This is an American Association of Petroleum Geologists' film. The film, based on the Andros Island, Bahamas modern tidal flat model, shows how onlap and offlap sedimentation of carbonate tidal flats combined wi th subsidence can produce stratigraphic traps for oil and gas. Several ancient examples of oil fields developed in tidal flat stratigraphic traps are discussed and compared with the Andros model.

AUTHOR A B B O T T , P a t r i c k L.: 32 AHMAD, M.: 7 1 , 88 A L E X A N D E R , R.: 14 ALLEN, A.D.: 59 ALLEN, Rosemary: 78 APTHORPE, M.: 5 5 , 6 6 , 68 ARCHBOLD, N.W.: 65, 66 ARCHIBALD, N.J.: 77 ARRIENS, P.A.: 40 BACKHOUSE, John: 67 BARLEY, M.E.: 7 4 , 77 BARRETT, P.J.: 17 BAULD, John: 18 B A X T E R , John L.: 59 BELL, D.H.: 19, 8 3 , 85 BELPERIO, A.P.: 58 BICKLE, M.J.: 77 BILLO, Saleh M.: 46 BONE, Y.: 21 BORLEY, G.D.: 74 BOTH, R.A.: 5 BOUFFLER, M.: 4 BOURQUE, Pierre-Andre: 50 BRAKEL, A.T.: 40 BUICK, Roger: 45 B U N T I N G , John A.: 4l BURNE, R.V.: 54 BUTT, C.R.M.: 57 BUTTON, Andrew: 2 . CAMERON, M.: 62 CARTER, Lionel: 3 0 , 34 CARTER, R.M.: 34 CLARK, M.E.: 69 COCKBAIN, A.E.: 49 COLES, J.: 8l C O L L I N S , Lindsay B.: 54 COLL INSON, J.W.: 23 COMPSTON, W.: 4 0 , 8 l , 82 CONLEY, Steve: 15 COOK, Peter J.: 2 1 , 63 COOPER, Barry J.: 36 COOPER, J.A.: 79 CRICK, J.H.: 20 CROOK, Keith A.W.: 30 DAVIES, H.L.: 90 DAVIES, Peter J.: 47 DAY, R.A.: 76 DE L A E T E R , J . R . : 74 DEMAISON, Gerard: 14 DENNIS, Robert W.: 8 DONNELLY, T.H.: 9 DRAPER, J.J.: 42 DREW, G.W.: 5 D U D D Y , I.R.: 6 3 , 8 0 , 82 DULHUNTY, John A.: 57 EDWARDS, M.B.: 24 ERIKSSON, K.A.: 5 , 3 8 , 39 EGGERT, J.T.: 23 ETHERIDGE, M.A.: 79

INDEX FEARY, D.A.: 30 F E R G U S O N , K.: 36 FLEMING, P.J.G.: 27 F L E T C H E R , Ian R . : 81 FLOOD, P.G.: 28, 46 FOSTER, J.J.: 40 G A L L O W A Y , W i l l i a m E.: 6 GEE, R.D.: 39 GEHLING, J.G.: 31 GEI D A N S , L.: 22 GIBBONS, G.S.: 86 GLEADOW, A.J.W.: 82 GODFREY, N.H.H.: 2 9 , 84 G O L D I N G , Lee Y.: 70 GOLDING, S.D.: 70 GORDON, J.L.: 86 GOSTIN, Victor A.: 18 G R A Y , David R.: 90 GREEN, D.C.: 62 GREEN, T.H.: 73 GREGORY, P.W.: 12 GREY, Kathleen: 44 GROVES, D.I.: 7 1 , 74 HAIG, D.W.: 32 HALLBERG, J.A.: 38 HANNAH, Michael: 6 7 , 68 HARDENBOL, J.: 1 HARRIS, P.M.: 53 HARTLEY, J.S.: 12 H E A T H , R.: 6 6 , 68 HE I D E C K E R , E r i c : 91 HICKEY, S.H.: 61 HILL, R.E.T.: 33 HOBDAY, D.K.: 6 , 24 HOCKING, R.M.: 2 4 , 25 HOLLIS, Julian: 75 HORWITZ, R.C.: 3 3 , 4l HUDSON, G.R.T.: 11 I R V I N E , Ian:

JACKSON, M.J.: 42 J A M E S , Noel P.: 4 9 , 52 JAMES, P.R.: 93 JONES, B.G.: 36 JONES, G.F.P.: 12 JONES, P.J.: 50 KAGI, R.I.: 14 KEARNS, G.K.: 88 K E A Y S , Re id R . : 70 KEMP, N.R.: 23 KENDALL, C.G.St.C.: 53 KEY, C.A.: 86 K N U T S O N , J.: 9 KORSCH, R.J.: 3 1 , 91 K R A P E Z , B.: 5 KRASON, Jan: 3, 6 L A M B E R T , I.B.: 9 L A V E R I N G , I.H.: 65

LESHER, C.M.: 74 LIMBERT, Alan R.: 14 LINDSAY, J.M.: 35 LOGAN, Ross G.: 8 M a c G E E H A N , P.J.: 69 MACKENZIE, D.E.: 42 M c C U L L O C H , M.T.: 4 0 , 82 M C D O N A L D , E.: 85 McHATTIE, C.M.: 16 McGOWRAN, Brian: 6 7 , 68 M c K E L V E Y , B.C.: 17 M c L E N N A N , Scott M.: 72 M c M I N N , A.: 27 M A H E R , C.E.: 3 7 , 48 M A I D E N , Ken: 26 M A R C O S , G.: 84 M A R S H A L L , Brian: 4 , 12, 92 M A R S H A L L , John F.: 46 M A T H E R , R.P.: 83 M A U G H A N , David M.: 14 M A Y E R , Wolf: 26 M E B B E R S O N , A . John: 14 M I D D L E T O N , M.F.: 63 M I E L N I K , V.: 93 M I N T Y , E . J . , 88 M O O R E , P.S.: 25 M O O R S , H.T.: 23 M O R A N , V.J.: 28 M O R R I S , R.C.: 41 M U H L I N G , P.C.: 40 M U I R , M.D.: 20 M U L E S , G.T.: 85 M U R R E L L , Burton: 34 N E U D E R T , Martin K.: 7 N I C O L L , Robert S.: 50 O'CONNOR, D.P.H.: 4 OLIVER, R.L.: 7 8 , 8 0 , 93 O V E R S B Y , Brian: 13 PARKER, A.J.: 9 2 , 93 PARTRIDGE, A.D.: 35 PASSMORE, V.L.: 25 PATTERSON, D.J.: 92 P E A C H , C.J.: 94 P E R K I N , D.J.: 11 PHILLIPS, G.N.: 6 9 , 71 P I N C H I N , J.: 36 PLAYFORD, Phillip E.: 4 5 , 51 P L U M B , L.A.: 63 POLACH, H.A.: 47 P O W I S , G.D.: 35 P R A Y , LLOYD C.: 5 1 , 52 PREISS, W.V.: 43 QUERESHI, H.: 88 Q U I L T Y , Patrick G.:

RAO, C . Prasada: 5 6 , 62 R E A D , J.F.: 5 3 , 55 RHODES, E.G.: 4 7 , 61 R O B E R T S , D.E.: 11 R O B E R T S O N , C.W.: 7 R O G E R S , N.: 74 ROSMAN, K.J.R.: 81 ROWLANDS, Nigel J.: 3 , 10 R O Y , P.S.: 5 8 , 60 RUST, Brian R.: 18 RUTTER, E.H.: 3h SANDY, M.J.: 87 SANGAMESHWAR, S.R.: 4 , 12, SENIOR, B.R.: 36 SHERGOLD, John H.: 21 SMITH, G.V.: 85 SMITH, R.B.: 87 S M Y T H , Michelle: 62 STAPLEDON, D.H.: 89 STONE, Wi11iam J.: 56 STUMPFL, E.F.: 3 SUN, Shen-Su: 4 SUTHERLAND, F.L.: 75 SWENSSON, C.J.: 72 SYLVESTER, G.C.: 74 T A Y L O R , Graeme: 26 T A Y L O R , S.R.: 72 T E D D E R , Ian: 3 TEI CHERT, Curt: 17 T H O M , B.G.: 58 T H O M A S , G.A.: 65 T R E N D A L L , A.F.: 40 TRUDINGER, P.A.: 63 T U C K W E L L , K.D.: 92 T Y L E R , Noel: 2 V A I L , P.R.: 1 VAN M O O R T , J.C.: V O S , R.G.: 16

W A L K E R , R.G.: 1, 55 W A L T E R , Malcolm: 18, 44 W A R D , Col in R.: 27 W A S S , R.E.: 48 W A S S , Suzanne Y.: 75 W A T S O N , E.B.: 73 W H I N C U P , P.: 85 W H I T E , S.H.: 9 3 , 94 W I L L I A M S , I .S.: 4 0 , 8l W I L L I A M S , Neil: 8 W I L S O N , Allan F.: 7 0 , 7 3 , W I T H N A L L , I.W.: 42 W O O D H O U S E , G.W.: 14 W Y G R A L A , B: 36 W Y L D E , L.J.: 88