Issuu

GEOLOGICAL SOCIETY OF AUSTRALIA INCORPORATED

SPECIAL PUBLICATION No. 6 Dynamic metamorphism; processes and products n n a t Ipv rnpk"Q Ciinrnnff R a s i n mDLypVvVnUnl il ljCi fl lil pv al irl hUV71IU 1 vvIYijj V ^ U i l l l U ^ o \X/pcfprti A notraliQ VV G M t l l l /1lU311(111CI

B. W. LOGAN and V. SEMENIUK

Edited by B. D. WEBBY

SYDNEY AUGUST, 1976

1976 THE GEOLOGICAL SOCIETY OF AUSTRALIA INCORPORATED

Office Bearers 1975-1976 (Elected 13 May 1975)

President: G. M. PHILIP Vice-Presidents: DOROTHY HILL, S. W. CAREY Hon. Secretary: R. E. WASS Hon. Treasurer: J. N. CRAMSIE Hon. Administrative

Officer: K. G. MOSHER

Hon. Editor: D. A. BROWN

GEOLOGICAL SOCIETY OF AUSTR, INCORPORATED

SPECIAL PUBLICATION No. 6

E. FRANKEL

Dynamic metamorphism; processes an(?Apr<51MWs in Devonian carbonate rocks, Canning Basin, Western Australia By B. W. LOGAN and V. SEMENIUK

Edited by B. D. WEBBY

SYDNEY AUGUST, 1976

©THE GEOLOGICAL SOCIETY OF AUSTRALIA INC. 1976 Reference: LOGAN, B. W . , & SEMENIUK, V . , 1976. Dynamic metamorphism; processes and

products in Devonian carbonate rocks, Canning Basin, Western Australia. Spec. Pubis geol. Soc. Aust., 6, 138 pp., 11 tables, 97 figs.

Registered in Australia for transmission by post as a book. National Library of Australia card number and ISBN ISBN 0 909869 08 1

Issued August 1976.

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CONTENTS Abstract Introduction Development of Research Project Terminology The Term Iden Regional Setting Stratigraphy Structure Host Sediments Pillara Formation Sadler, Napier and Virgin Hills Formations Pressure Solution and Shear Fracture Pressure-Solution Surfaces (Stylolites) .... Pressure-Solution Residues Stylocumulate Reactate Stylocumulate and Reactate Idens Fabrics Structures Stylobedding Stylolamination Stylonodular Structure Stylomottled Structure Stylobreccioid Structure Rock Types Stylolaminites Stylonodular Rocks Stylomottled Rocks Stylobreccias Rock Associations and Petrogenesis Analysis of Pressure-Solution Features; Stress General Conditions and Stylolitic Structures Origin of Stylolaminites Origin of Stylonodular Rocks Origin of Stylomottled Rocks Origin of Stylobreccias Fate of the Solute Dolomitization Rock Types Fenestral Dolomites Massive Dolomites Dolomitic Stylolaminites Dolomitic Stylobreccias Dolomitic Stylomottled Rocks Dolomitic Stylonodular Rocks Petrogenesis Review; Origin of Dolomite Replacement Dolomitization Pressure-Solution Reactate Dolomitization Pressure-Solution Stylocumulate Dolomitization Paragenesis of Dolomite Tensional Fracture, Solution and Emplacement Structures Veins and Vein Structures Secondary Vugs and Vugular Structures

Page 1 3 5 5 7 8 8 9 10 11 11 11 11 13 13 16 18 21 21 21 21 23 23 23 23 23 26 26 31 32 45 52 53 55 56 57 57 58 58 58 58 59 59 60 60 60 60 64 67 68 68 68 68 68 70

Fabrics Cavity-Filling Materials Calcite Crystals and Crystal Aggregate Types Internal Sediments Rock Types Veined Rocks Vugular Rocks Calcite-Cemented Breccias Petrogenesis Gradations Origin of Cavities Origin of Cavity-Filling Materials .... Origin of Rock Types Pods and Megabreccia Sheets Pods Vugular Pods Breccia Pods Skeletal-Boundstone Pods Megabreccia Sheets Origin of Pods and Megabreccia Sheets .... Origin of Vugular Pods Origin of Breccia Pods Origin of Skeletal-Boundstone Pods .... Origin of Megabreccias Areal Geology General Napier Range and Fairfield Valley Idens of Pillara Type Behn Conglomerate Iden Idens of Napier Type 1 Idens of Napier Type 2 Iden of Napier Type 3 Idens of Napier Type 4 Interidenic Boundaries Pillara Range Pillara Iden Sadler Iden Iden Boundaries Horse Spring Range Pillara Iden Virgin Hills Iden Iden Boundaries Laidlaw Range Metamorphism Metamorphic Zonation Metamorphic Grade Conclusions Metamorphism, Compressive-Stress Environment Metamorphism, Tensional-Stress Environment Metamorphic Zonation and Grade .... Field Relations Acknowledgments References Appendix I: Interpreting Volume Loss after Pressure Solution Appendix II: Nomenclature of Coarse Crystalline Calcite; Crystals and Aggregates

Page 71 71 71 78 78 78 78 80 83 83 83 86 95 96 96 98 98 98 103 103 103 105 106 110 Ill Ill Ill 113 113 113 114 117 118 118 125 125 125 125 126 128 128 128 128 128 128 129 130 131 132 133 133 134 134 137

137

DYNAMIC METAMORPHISM; PROCESSES AND PRODUCTS IN DEVONIAN CARBONATE ROCKS, CANNING BASIN, WESTERN AUSTRALIA By BRIAN W. LOGAN and V. SEMENIUK ABSTRACT

The majority of Devonian carbonate rocks cropping out along the northern margin of the Canning Basin are metamorphic products which have resulted from chemical solutions acting in a stressed environment. Metamorphic processes identified are: (1) pressure solution, (2) fracture, (3) dolomitization, (4) solution, (5) emplacement, (precipitation and infiltration), and (6) recrystallization. The interaction of these proceses yielded complex products. Analysis of metamorphic features is complicated by the extreme scalar range of rock components, the wide range of lithotypes reacting to metamorphism, and the gradations from unaltered rocks to metamorphosed products. The term iden is defined to aid in the description of metamorphic rock bodies and components. An iden is a body of material that behaves as a statistically homogeneous unit under a set of specific physical and chemical conditions; it is a finitely extended body independent of scale, geometry and composition. Processes of metamorphism react with, or produce idens. Three types of interfaces or structures are described on the basis of their relationship to idens: (1) interidenic, (2) circumidenic and (3) intraidenic. Pressure solution and shear fracture occur under compressive stress and the two processes are frequently penecontemporaneous; pressure-solution interfaces become shear fractures and vice versa. Structures produced by pressure solution and shear fracture are: (1) stylolites, (2) stylobedding, (3) stylolamination, (4) stylomottled structure, (5) stylonodular structure, and (6) stylobreccioid structure. These structures are superimposed on earlier sedimentary or metamorphic rock features. There is an order of relative pressure-solubility and less soluble minerals therefore accumulate at pressure-solution interfaces. The accumulated material is termed stylocumulate. Common stylocumulate minerals are quartz, feldspar, micas, clay minerals, dolomite and Fe-oxides. Calcareous components also are ordered in relative pressure-solubility and occur as stylocumulate. Reactions occur at pressure-solution interfaces and involve growth of minerals stable in the PT field; these minerals are termed reactate. Dolomite, mica, quartz and calcite are reactate minerals. Stylocumulate and/or reactate idens are disposed in pressure-solved rocks according to the disposition of stylolitic structures, as laminar sheets and lenses, as irregular patches and as anastomosing networks; they are usually bedded and are internally laminated because of numerous superimposed stylolites. Intergradational fabrics developed by pressure solution are: (1) iden support (pictorially analogous to grain-support fabric), (2) condensed, (3) fitted, (4) stylocumulate-support and (5) reactate-support. Fitted fabric, characterized by idens that are bounded by circumidenic stylolites is the penultimate product which evolves with continued pressure solution. Stylocumulate- and reactate-support fabrics are characterized by isolated idens which 'float' in stylocumulate or reactate ground. An intergradational suite of rock types is produced by pressure solution and shear fracture: (1) stylobedded rock, (2) stylonodular rock, (3) stylomottled rock, (4) stylolaminite and (5) stylobreccia. Stylolaminite is the penultimate product being characterized by subparallel stylolite sets and abundant stylocumulate (or reactate) sheets and lenses. Stylonodular rocks, stylomottled rocks and stylobreccia are transitional to stylolaminite, under sustained stress. Dolomitization is an additional process in rock alteration and may take place under directed stress. Dolomitic rocks are abundant and most contain evidence that indicates a metasomatic origin penecontemporaneous with pressure solution and shear.

Dolomitization probably proceeds by reaction of Mg++ with calcite in the calcite pressure-solution field but as dolomite is less pressure-soluble than calcite, dolomitization can proceed further via passive accumulation at pressure-solution surfaces. Strain cavities which are produced by both dilation and rotational deformation under tensional stress include veins, vugs and interfragment voids in breccias. Straincavity shape and orientation may be independent of structures and fabrics in the host but more commonly these exert an influence and cavities are aligned along surfaces such as stylolites. The formation of interconnected networks of cavities permits free flow of intrastratal fluids and leads to further metamorphism via processes of: (1) solution, (2) precipitation and (3) infiltration (internal sedimentation). Strain cavities are enlarged and solution cavities further develop by dissolution of host-rock components. Thus strain cavities and solution cavities are intergradational and their development is frequently penecontemporaneous. Precipitate crystals (aragonite or calcite) nucleate on cavity walls and grow inwards, eventually filling cavities. Infiltration results in emplacement in cavities of internal sediment, generated by solution stoping or mechanical stoping. Internal sediments vary greatly in composition, but common components are silt or sand sized grains or crystals of calcite, dolomite, quartz, mica and Fe-oxides. Lamination, current and graded bedding and micro-unconformities indicate deposition of internal sediment under conditions of free fluid flow. Formation of strain and solution cavities and emplacement of precipitate and infiltrate generates new structures and fabrics. Vein structures include: (1) parallel sheet, (2) rectangular to rhomboid and (3) breccioid. Vugular structures include: (1) irregular, (2) lacy, (3) stromatactid and (4) breccioid. Fabrics developed are: (1) host-support and (2) fill-support. New rock types are: (1) veined rock, (2) vugular rock and (3) calcite-cemented breccia. Discrete idens, termed pods, embedded in the stylobedded rocks are oval, lensoid, equidimensional to quadrate and range up to tens of metres in size. Three basic pod types are distinguished on composition: (1) vugular, (2) breccia and (3) skeletal boundstone. Vugular pods formed in local tensional zones within compressional-stress environments. Breccia pods, composed of stylobreccia developed along low angle dislocations. Simple (monoidenic) skeletal-boundstone pods are remnants of once larger boundstone idens; many skeletal-boundstone pods are composite (polyidenic) masses composed of smaller skeletal-boundstone pods. These polyidenic pods are formed by agglomeration of resistant idens as intervening host-rock idens are pressure-solved. Skeletal-boundstone pods and breccia pods are intergradational. Megabreccia commonly occurs in sheets and lenses separating major rock units. Megabreccia is dominantly stylobreccia developed by large scale fracture along major dislocations. Megabreccia sheets pinch out laterally and grade into zones of skeletalboundstone pods. Major structural elements in the northern Canning Basin are: (1) a block of Lamboo Complex composed of crystalline metamorphic rocks, flanked or overlain by gently dipping Pillara Formation stylobeds and (2) an off-dipping homocline of Napier, Virgin Hills or Sadler Formation stylobeds. Variations on the basic structural plan mainly relate to configuration of the homocline and the Lamboo-Pillara block: (1) linear homoclines, (2) semicircular antiforms with a central block of Pillara rocks and an off-dipping sequence of homocline rocks and (3) lensoid antiforms with a central Pillara block flanked by off-dipping homoclines. Boundaries between all formations and rock units are major sinuous strike faults that dip in the general direction of the homocline. Numerous subsidiary strike and oblique faults are abundant adjacent to the contact between Lamboo-Pillara rocks and the homocline. Belts of deformed rocks increase in metamorphic grade toward the Lamboo/ Pillara-homocline abutment. The rocks have responded to shear stress mainly by pressure solution and fracture, with movement taken up by stylobedding. Metamorphism under compressive stress has produced three broad rock suites locally gradational but more commonly separated by faults: (1) highest grade, dominated by dolomitic rocks (stylolaminite, massive dolomite and stylobreccia), (2) quartzose stylolaminite, stylobreccia and large pods and (3) least metamorphosed, of stylobedded limestone and stylobreccia. Metamorphism under tensional stress has produced a simple zonation: veined rocks, vugular rocks and calcite-cemented breccia are most common near the Lamboo/Pillara-homocline abutment and tail away in abundance. The development of chlorite, micas and dolomite and the coexistence of

dolomite and quartz points to low metamorphic grade (moderate pressure, low temperature). Idens responding to deformation are micron- to formation-sized bodies, and the structural and fabric relationships between small bodies is a microcosm of that which exists at formation scale. The peculiar stratigraphic-structural relationship of many field features such as the block of Pillara Formation, the abutment zone, antiform structures and lensoid and sinusoidal breccias are the result of deformation and pressure solution; they are the large scale expression of phenomena observed on outcrop and in thin section. Brian W. Logan and V. Semeniuk, Department of Geology, The University of Western Australia, Nedlands, Australia 6009; MS received 12 December 1975. shears are modified by pressure solution. In the INTRODUCTION The high susceptibility of carbonate sedi- Devonian rocks the most obvious expression of ments to diagenetic alteration is generally pressure solution (and shear), is ubiquitous recognized (Ham & Pray, 1962). Thermody- subparallel stylolites that impart a strongly namic considerations also suggest that bedded appearance to the rocks but pressurecarbonate sediments should be profoundly solution surfaces occur at all scales—separating altered in conditions of low metamorphic grade tabular rock masses in apparently conformable where other sediments such as sandstone and sequence, circumscribing and truncating disshale, and other sedimentary mineral assem- concordant bodies of variable size (block to blages are stable, but there has been little docu- pebble) and truncating fossil fragments and mentation of metamorphic processes operating non-skeletal carbonate grains. in ranges of P and T above normal or of carSince pressure-solution surfaces relate to bonate rock types, structures and textures stress it is possible to analyse their disposition produced. As will be shown herein, it is in terms of stress mode and application. Simple probable that many metamorphic features in analytic models provide a theoretical base for carbonates and metamorphic rock types have a range of structures and fabrics that are been interpreted as sedimentary or early dia- observed in pressure-solved carbonate rocks. genetic. This applies particularly to carbonates There also is potential for use of pressure-soluthat occur in structurally complex areas, faulted tion surfaces in structural analysis but developterrains, or those known to have been deeply ment of such techniques lies beyond the scope buried. of this paper. Abundant pressure-solution surfaces imply Middle to Upper Devonian carbonates cropping out along the northern Canning Basin, volume reduction. Consequent upon this is the Western Australia (Fig. 1) have been meta- principle that modifications are not only at the morphosed under conditions of medium scale of grains, textures and structures but pressure and relatively low temperature possibly also at larger scales of stratigraphic (following Turner, 1968). The main metamor- and structural relationships. Given that rocks and rock components have phic processes identified are: (1) pressure solution, (2) fracture, (3) dolomitization, (4) variable resistance, as units in a pressure-solusolution, (5) emplacement, and (6) recrystal- tion field (depending on mineralogy and other lization. Some of these processes apparently properties) another potentiality of the pressureoperated simultaneously, interacting with solution process becomes clear—more suscepvariable intensity to yield complex products. tible units will tend to be removed, less soluble Detailed petrologic study and field documenta- units will tend to remain. Fundamentally then, tion of gradations indicate a basic metamorphic the pressure-solution process concentrates residual materials at pressure-solution interpattern that reflects stress gradients. Processes of pressure solution, shear fracture faces. It follows that: (1) residual materials and dolomitization are linked through the form bodies of shape and dimension related to gradational relationships evident in their pro- the disposition of pressure-solution surfaces in ducts. Compression is a requirement for both the host rocks, and (2) discrete insoluble pressure solution and shear fracture and these bodies agglomerate into larger masses by processes have frequently been contempor- pressure-solution loss of intervening host. In the rocks of this study, parallel pressureaneous. Thus pressure-solution surfaces localize shear movements and alternatively shears solution surfaces (sets) result in layered become pressure-solution surfaces; drag folds, sequences of tabular units composed of residual tension fractures and breccia associated with materials and corroded remnants of host rocks;

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

Fig. 1. Map showing location of study area along northern margin of the Canning Basin, and the principal geographic/geologic, locations. the scale of layering extends from beds to very fine lamination. Pressure-solution layering (stylobedding) as with shear surfaces may parallel original bedding but its orientation does not always coincide, and stylobedding may be at steep angles to sedimentary layering. The pressure-solution agglomeration process is responsible for'the formation of large pod-like masses of skeletal boundstone and other rock types.

Metasomatism and recrystallization, penecontemporaneous with pressure solution and shear fracture are additional factors in rock alteration under compressive stress. The potential for generation of new minerals depends on availability of suitable reacting ions and the PT environment. Reactive ions likely to be most abundant in a pressure-solving carbonate sequence are Ca++, Mg++, HC0 3 ~ and Fe++. Dolomite is stable in PT fields of low

INTRODUCTION grade metamorphism (Turner, 1968) and it is therefore probable that dolomitization by reaction with calcite occurs in the calcite pressuresolution field. Dolomite is less pressure-soluble than calcite, and dolomitization may proceed further via passive accumulation of the mineral at pressure-solution surfaces. Dolomitic rocks are abundant in the Devonian sequence and most of them contain evidence that indicates a metasomatic origin, penecontemporaneous with pressure solution and shear. Crystallization in adjustment to the PT conditions of the calcite pressure-solution field affects calcite, Fecalcite, clays, micas and Fe-oxides. Processes of tensional fracture, solution and emplacement are linked through gradational relationships evident in their products. Tensional fracture and solution are primary mechanisms that create cavities in host rocks. Emplacement, as used here, is a general term for processes of precipitation and infiltration (internal sedimentation) that result in filling of cavities. Cavities are filled mostly with crystalline carbonate (precipitate) but internal sediments also were introduced. Some rocks contain up to 50% cavities and some have been opened, and re-opened to the extent that little remains of the precursor; the product consists mainly of precipitate and internal sediment. DEVELOPMENT OF RESEARCH PROJECT

This paper comes from a research project that was started in February, 1971 with encouragement and support from West Australian Petroleum Pty Ltd. The main objectives were reconstruction of Middle to Upper Devonian palaeo-environments and palaeogeography along the northern margin of the Canning Basin (Fig. 1). The Devonian sequence as described in earlier works (Teicher, 1943, 47; Guppy et al, 1958; Playford & Lowry, 1966) appeared to possess such a wealth of material for basic research in carbonate sedimentation, palaeoecology and diagenesis that the research programme was seen as long-range investigation involving staff and post-graduate students. The Pillara Formation (Guppy et al, 1958) was selected as the first unit for detailed sedimentologic analysis on the criteria that: (1) the unit contained lithologies, e.g. cryptalgal limestone, useful in stratigraphic analysis, (2) it was the oldest unit which other units overlie, abut or grade into, and (3) there was much continuous exposure. The Pillara study was completed with publications by J. F. Read (1973a, b). It was shown that the Pillara For-

mation was characterized by sedimentary cycles with tidal-fiat facies interspersed among shallow subtidal facies; interpretations were possible using modern analogues. Investigations of the Virgin Hills, Napier and Sadler formations were a second study segment which was started in June, 1972 by the present authors. Initial objectives were similar to those of the Pillara study and early procedure was measurement of stratigraphic sections to determine lithofacies, sequence and geographic variations. This procedure was followed for some weeks during the initial field season but it became increasingly difficult to account for many rock types and stratigraphic relationships in terms of sedimentary models. Furthermore, heavy imprints of post-depositional processes were evident and there was a realization that many rock types might also have been generated by these processes. Consequently, section measuring procedures were abandoned in favour of tracing lithologic gradations and seeking localities where stages in petrogenesis could be observed. As a result of the above effort and the obvious importance of metamorphism, the project objectives were amended to the documentation of metamorphic products and interpretation of processes. Pressure solution, shear fracture (as a metamorphic process), dolomitization, solution, tensional fracture, emplacement and recrystallization were identified as major factors in generating structures, textures, fabrics and rock types that were ubiquitous throughout the formations. The programme continued in 1973 with detailed field mapping in selected areas to establish relationships between metamorphic products and major structural elements. This revealed a zonation that reflects stress gradients. TERMINOLOGY

Realization of the role of metamorphism in producing lithologic features led to numerous problems in terminology and nomenclature describing the rocks. Firstly, most existing terms in the lexicon of sedimentologists and stratigraphers had too strict sedimentary or early diagenetic connotations. Secondly, there was a need to make distinction between metamorphic and sedimentary features which were sometimes intergradational and/or superficially analagous, e.g. sedimentary lamination versus lamination produced by pressure solution. Therefore, we have been compelled to define some new terms. These new terms are defined

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

where introduced in the text and a summary is given below. Where conventional terminology has been adequate it has been retained (e.g. stylolite, conjugate) and many of the proposed terms have involved combinations of known terms (e.g. stylolite, bedding, to give stylobedding). Observations in other carbonate sequences and literature review suggest that the terminology proposed has wide applicability to carbonate rocks. CALCITE-CEMENTED BRECCIA. A breccia with voids which are filled with carbonate precipitate. CIRCUMIDENIC STYLOLITE. Stylolite that circumscribes idens. CONDENSED FABRIC. A fabric where idens are overclosely packed, far tighter than a normal idensupport sedimentary packing; grains are no longer in point contact but contact over a surface. EMPLACEMENT. Processes of precipitation and infiltration (internal sedimentation) that result in the filling of open space structures (tensional fractures and solution cavities). FILL-SUPPORT FABRIC. This fabric is characterized by supporting framework of void-fill material. Infiltrate-support fabric is one in which idens are supported by a medium of internal sediment. Precipitate-support fabric is one in which idens are supported by a medium of precipitate; generally results when idens in an iden-support frame are embayed and marginally replaced by precipitate. FITTED FABRIC. A fabric where idens are in contact with neighbours along their entire margins. IDEN. Body that behaves as a statistically homogeneous entity to physical or chemical stimuli. Drawn from Latin: idem . . . the same, similar; ens . . . entity. It is also a finitely extended body independent of scale, geometry and composition. IDEN-SUPPORT FABRIC. Fabric where idens form a self-supporting frame and are in contact at points. When idens are sedimentary particles, synonymous with Dunham's (1962) grain-support fabric. INFILTRATE ( = INTERNAL SEDIMENT). Silt-, sandand gravel-sized sediment which commonly floors cavities; may also be interlayered with precipitate. INTERIDENIC STYLOLITE. Planar stylolite developed between idens. Idens commonly are stylobeds, stylolaminae, sheets of stylocumulate or reactate. INTRAIDENIC STYLOLITE. Stylolites developed within larger idens. Idens are commonly metamorphosed carbonate rocks. MEGABRECCIA. A breccia containing abundant randomly oriented limestone and dolomite blocks, a metre to several metres in size and, in some cases, tens of metres in size. POD. Discrete carbonate iden larger than cobblesize that occurs in a stylobedded or stylolaminated sequence. Composition of pods may be: (1)

skeletal boundstone, (2) vugular rock, or (3) breccia.

PRECIPITATE ( = COARSELY CRYSTALLINE CARBONATE). Mineral that has crystallized from

solution in void space. Precipitates are commonly acicular carbonate with long axes of crystals oriented normal to void walls, or equant calcite. Voids may be interidenic space, solution cavities or tensional fractures. The term cement is inadequate as it has connotations of 'cementing' or 'lithifying'; moreover, precipitates in solution cavities are not cements but void-filling materials. PRESSURE SOLUTION. Solution of mineral commonly at iden to iden contacts; solution is enhanced by increased load and hydrostatic pressure. REACTATE. Material which has grown at pressuresolution interfaces under PT conditions of pressure solution. Examples of reactate are dolomite, and syntaxial overgrowths on calcite and quartz. REACTATE-SUPPORT FABRIC. Fabric in which large idens are supported by finer-grained reactate; pictorially analogous to wackestone fabric of Dunham (1962). STYLOBEDDING. A structure imparted to a rock due to stylolitic parting. Stylolites and associated thin stylocumulate seams are recessively weathering and the rocks have a flaggy or 'bedded' appearance. Stylobedding is parallel to original layering where stylolites follow lithological contacts, but stylobedding may also be developed at low to high angles to original sedimentary layering. STYLOBRECCIA. Breccia in which fragments are bound by stylolites. The rock formed where carbonate rocks subjected to shearing stress have fractured; fracturing was accompanied by pressure solution between fragments of the breccia; some stylobreccia formed by intersection of irregular and peaked stylolites in stylolaminites and stylomottled rocks, accompanied by rotation of stylolite-bound fragments. STYLOCUMULATE. Material that passively accumulated as insoluble residue along a pressure-solution interface. Composed mostly of relatively insoluble mineral idens such as quartz, mica, dolomite and Fe-oxides, or of the relatively more insoluble calcitic idens such as crinoid and brachiopod fragments. STYLOCUMULATE-SUPPORT FABRIC. Fabric in which larger idens are supported by finer-grained stylocumulate; pictorially analogous to wackestone fabric of Dunham (1962). STYLOLAMINITE. Rock laminated due to the abundance of low amplitude stylolites; the insoluble residue (stylocumulate) in the stylolite seams is important in determining the final rock composition : those with abundant sand-sized quartz stylocumulate are quartzose stylolaminite; those with quartz silt, micas and Fe-oxide are quartzose-micaceous stylolaminite; where dolomite is abundant, the rock is dolomitic stylolaminite; stylolitically laminated limestone that

INTRODUCTION GRAINS A N D C R Y S T A L S

STYLOBEDS AND STYLOLAMINAE OF SEDIMENTARY AND METAMORPHIC ROCKS

SKELETONS

CALCITE CRYSTALS

STYLOCUMULATE/REACTATE MOTTLES

BLOCKS

SKELETAL BOUNDSTONE^ POD

BRECCIA BLOCK

VR^SJMOTTLE

•y&r

O HOST

LIMESTONE BLOCK

FORMATION

Fig. 2. Types of idens that respond as homogenous units to stress. contains minor insoluble residue or that contains VUG. Cavity generally formed by solution and filled with precipitate or infiltrate. Vugs may be residue of relatively insoluble calcitic grains is irregular, lacy (thin, laterally extensive and calcareous stylolaminite. sheetlike with irregular roofs), or stromatactid STYLOLITE. Interdigitating, irregular to smooth (platy to lensoid, flat floors, highly irregular pressure-solution interface that is developed at roofs). microscopic (grain to grain), mesoscopic and VUGULAR ROCK. Rock with abundant vugs megascopic levels. Can be further subdivided on arranged in irregular, lacy, stromatactid or brecbasis of configuration into columnar, peaked, cioid vugular structures. irregular, hummocky and smooth types. Stylolites may be interidenic, circumidenic and inTerm Iden traidenic on the basis of their relationship to TheAnalysis of metamorphic features is compliidens. STYLOMOTTLED ROCK. These are carbonate rocks cated by: (1) extreme range of scalar properwith irregular mottles of stylocumulate and reac- ties exhibited by rock components, (2) wide tate that developed where pressure solution was range of reacting lithotypes, sedimentary and restricted to patches; the mottle may be metamorphic, and (3) gradations from quartzose, quartzose-micaceous, dolomitic, or an unaltered rocks to metamorphic products. Our aggregate of relatively insoluble calcitic com- main problem has been terminology for entities ponents. reacting to, or resulting from, pressure solution. STYLONODULAR LIMESTONE. Rock that is transi- These include all types of sedimentary grains, tional between stylobedded limestone and stylo- matrix particles and coarse calcite crystals; laminite; lenses and nodules of limestone idens skeletons of sessile, encrusting (frame-building) are separated by sheets and lenses of stylolaminite (mainly quartzose-micaceous stylo- and vagile benthos; breccia blocks; laminae and beds; discordant bodies such as reefal mounds; laminite). VEIN. Straight to irregular tensional fracture that components of metamorphic carbonates, has been filled with precipitate or infiltrate. blocks, laminae and pressure-solution residues. Where walls of the fracture are modified by Conventional sedimentary terms such as grain, solution, veins grade into vugs. clast and fragment have restricted connotations VEINED ROCK. Rock with abundant veins arranged and the metamorphic literature provides in parallel sheet, rectangular, rhomboidal or nothing suitable. breccioid structures.

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN idenic where they are sheetlike (and separate tabular idens of host-rock) or circumidenic where they form rectangular to breccioid structures; idens bound by these larger vein structures may contain smaller (pre-existing) veinlets—these are intraidenic. REGIONAL SETTING

Fig. 3. Arrangement of interidenic and circumidenic stylolites in relation to various idens. Heavy lines denote stylolites. Similar problems exist in description of the products of fracture and solution. All varieties of host rock are broken or dissolved into bodies with a vast range of composition, size, shape and other textural parameters. Furthermore, these fragments are in place or much disturbed and have been subjected to later phases of fracture and solution. We propose to use the term 'iden' drawn from Latin, meaning body of the same. An iden is a body of material that behaves as a statistically homogeneous unit under a set of specific physical and chemical conditions; it is a finitely extended body independent of scale, geometry and composition. Thus, an iden can range in size from the smallest particle to the massif of the structural geologist and beyond; in shape from regular geometric to irregular. In composition an iden may be essentially homogeneous (e.g. reefal mound, bed, etc.); in petrogenetic terms it may be sedimentary, metamorphic or igneous (Fig. 2). Reaction interfaces can be described on the basis of their relationship to idens. Three types of relationships are distinguished: (1) interidenic, (2) circumidenic, and (3) intraidenic. In the Devonian rocks that have been subject to pressure solution, stylolites form boundaries between idens and three basic types are distinguished. Interidenic stylolites are subplanar surfaces that form contacts between neighbouring idens; circumidenic stylolites circumscribe idens (Fig. 3). Idens bounded by large stylolites may contain smaller pressure-solution surfaces—these are intraidenic stylolites. Processes of fracture, solution, dolomitization and recrystallization also produce or react with idens. In tensional fracture, idens are bounded by veins that can be termed inter-

Stratigraphy Carbonate rocks of Devonian age occur throughout most of the Canning Basin but they are exposed only along the northern margin (Fig. 1). The carbonate sequence that crops out rests on Precambrian crystalline rocks (Lamboo Complex), Ordovician rocks (Prices Creek Group) or ?Devonian conglomerates and is overlain at various localities by Carboniferous and Permian sediments (Guppy et al., 1958). The conglomerates crop out between Precambrian rocks and Devonian limestones, and have been given a variety of formation names, e.g. Behn Conglomerate, Van Emmerick Conglomerate, mainly dependent on geographic location (Guppy et al., 1958). There has been much interest in the Devonian limestones because of their potential as petroleum reservoirs or as hosts for base metals. Regional stratigraphic studies began with Teichert (1943, 47) and have been continued by the Commonwealth Bureau of Mineral Resources (Guppy et al., 1958; Veevers & Wells, 1961), the Geological Survey of Western Australia (Playford & Lowry, 1966) and West Australian Petroleum Pty Ltd ( W A P E T ) . Further detailed work has been carried out by Seddon (1970) and Read (1973a, b). Mapping by Guppy et al. (1958) carried out in the 1950s established the main stratigraphic framework and was an outstanding contribution for its time. These geologists defined numerous rock units, highlighted the peculiar structural/stratigraphic relationships and developed further concepts of reef growth that had been started by Wade (1938) and Teichert (1947). Playford and Lowry (1966) interpreted the sequence as a reef complex and considerably modified stratigraphic nomenclature to fit this conceptual model. Eight formations corresponding to notions of reef, forereef, back-reef and inter-reef facies were proposed. Subsequent studies by WAPET geologists resulted in further alterations and a framework in keeping with age determinations based on conodonts. The WAPET studies have not been published and we have been obliged to use the stratigraphic terminology of Playford & Lowry

INTRODUCTION

TABLE I

Stratigraphic

Name

Units, Devonian, Northern

Canning Basin

Lithologies (Guppy et al., 1958; Playford and Lowry, 1966; Read 1973a, b) Thickness

Age based on conodont data from W A P E T

Occurrence

NAPIER FORMATION (Guppy et al., 1958)

Red to gray calcareous siltstone and sandstone, calcarenite, bioherms, dolomitic limestone and dolomite, calcirudite and megabreccia

300 m (type section)

?Frasnian I7Fammenian III-VI

Napier Range, Oscar Range, Fairfield Valley

VIRGIN HILLS FORMATION (Guppy et al., 1958)

Red silty limestone, calcareous siltstone, shale, some arenite, small bioherms, some calcirudite and calcarenite

210 m (type section)

Frasnian I7Fammenian III

Virgin Hills, Horse Spring Range, Bugle Gap, Old Bohemia, ?Napier Range

SADLER LIMESTONE (Guppy et al., 1958; Playford and Lowry, 1966)

Calcarenites, sandy and silty limestone, bioherms

370 m (max.)

Frasnian la

Emanuel and Pillara Ranges, Bugle Gap, Hull Range

MENYOUS MEMBER (Read, 1973b)

Cycles of stromatoporoid biostromes, pellet and intraclast limestone, minor skeletal-fragment limestone (see Table II)

420 m (max.)

PILLARA FORMATION (Guppy etal., 1958)

RED BULL MEMBER (Read, 1973b)

Interbedded terrigenous sediment, limestone and marl

270 m (max.)

BIG SPRING MEMBER (Read, 1973b)

Terrigenous cycles; carbonate beds

200 m (max.)

(1966); such usage does not, however, imply our acceptance of the 'reef' interpretation. Read (1973a, b) carried out detailed investigations of Pillara Formation rocks and, as a result, we follow the sub-divisions (members) defined by him. Four Devonian limestone formations were studied: (1) Pillara Formation, (2) Sadler Limestone, (3) Virgin Hills Formation, and (4) Napier Formation. Characteristics of these formations are summarized in Table I. Structure The Devonian limestones crop out in narrow west-northwest to northwest trending belts that form the Napier and Oscar Ranges and Oscar Plateau and discontinuously in a series of low hills—Pillara, Emanuel, Hull, Horse Spring, Horseshoe Ranges and Red Bluffs (Fig. 1). There is a gross pattern in distribution and attitude of formations that applies with minor modification at all localities. The basic geometry is: ( 1 ) a block composed of Precambrian basement rocks ( L a m b o o Complex) and subhorizontal to shallow dipping Pillara Formation strata, and ( 2 ) a homocline of Napier, Virgin Hills or Sadler formation rocks in abutment with

GivetianFrasnian la

Emanuel Range, Bugle Gap, Horse Spring Range, Hull Range, Pillara Range, Geikie Range, Napier Range, Oscar Range-Oscar Plateau

the Lamboo-Pillara block. The homocline is defined by strong stylobedding (see definition, p. 6 ) , and dips vary f r o m 10 to 60° (Fig. 4 ) .

Variations on the basic structural plan mainly relate to configuration of homocline and block (Fig. 4). Linear homoclines are exemplified by the Napier and eastern Pillara Range structures. Semi-circular antiform structures with a central block of Pillara rocks and an off-dipping sequence of Sadler and Virgin Hills formations are a common structural type, exemplified by the Horse Spring and Horseshoe Ranges. Lensoid antiforms are typified by the Laidlaw and Lawford Ranges (Fig. 84); here, a central Pillara block is flanked by off-dipping Sadler and Virgin Hills rocks that are folded and faulted along the flanks of the block. This relatively uniform structural pattern has been a major factor in interpretations of the sequence as a platform margin (Guppy et al., 1958) and/or reef complex (Playford & Lowry, 1966). The term abutment is apt for the contact between rocks of the Lamboo/ Pillara block and the formations in the homocline. This abutment and associated features are described in reference to specific localities in greater detail elsewhere in this paper (see

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

Fig. 4. Diagrammatic cross-sections illustrating Lamboo/Pillara block and homocline relations: A—linear homocline. B—semi-circular and elongate, lensoid antiform. p. 111-128) but a brief general description is given here as background for ensuing sections on petrology and structure. Frequently, the abutment is a surface or very narrow zone (of a few metres width) across which there is an abrupt transition in lithology. The surface is subplanar to irregular, and inclined at variable angle; inclination may be as much as 80° truncating dips in the homocline, or it may be as low as 10°; commonly the abutment surface dips subparallel to the homocline at 40 to 50°. Irregular abutment surfaces have low amplitude undulations of variable scale and/or steps that are related to distinct layers in units on either side. In many locations the abutment cannot be clearly defined at a surface, rather there is a zone, up to tens of metres wide, characterized by displaced blocks and breccia derived from formations on either side of the abutment, veins, internal sediment and dolomite.' Rocks in the abutment

zone also contain numerous stylolites. This melange of material has been mapped as Windjana Limestone by Playford & Lowry (1966). As will be shown, the abutment is a zone of dislocation characterized by intense cataclastic metamorphism and metasomatism. HOST SEDIMENTS

Pillara Formation carbonate units are extensive sheets bounded by macrostylolites, representing an unknown volume reduction, but pressure solution has apparently been confined to these larger interfaces and the rapid lensing of layers and abundant small-scale stylolites characteristic of other Devonian formations are absent; nor, in general, have Pillara sediments been subject to other metamorphic processes. However, Pillara sediments are metamorphosed adjacent to the abutment and similarly against small faults that dislocate Pillara units. Gradations from Pillara sediments to metamorphic

11 PRESSURE SOLUTION AND SHEAR FRACTURE carbonates are easily followed, and for this P R E S S U R E S O L U T I O N A N D S H E A R reason, sediment descriptions by Read (1973a, F R A C T U R E b) are valuable reference standards. PRESSURE-SOLUTION SURFACES (STYLOLITES) Most rocks in the Sadler, Virgin Hills and Pressure-solution interfaces have varied conNapier formations are metamorphosed and it figuration from the familiar stylolite with interis difficult, therefore, to return, with exactitude penetrating teeth or columns to smooth planar to original sedimentary features. Pressure solu- surfaces (Bushinsky, 1961, Trurnit, 1968a, b). tion along macrostylolites has modified and/or Five intergradational surface types (Figs 5, 6) obliterated sedimentary structures, e.g. bed are recognized: forms, while solution along microstylolites also (1) Hummocky to smooth surfaces with low has modified depositional textures, often to the amplitude undulations, 0.1 mm to about 5 cm, extent that designations such as 'grainstone', (2) Irregular, 'packstone' and 'boundstone' (Dunham, 1962) (3) Peaked, low amplitude, are highly interpretive. We are therefore (4) Peaked, high amplitude, limited to reconstruction of sediment lithotype (5) Flat-topped columnar. on bases of composition-grain types, fossil to smooth surfaces and irregular assemblages with frame-building potential, and Hummocky types are most common but peaked types also remnants of cement and matrix. Problems arise are abundant; configuration changes gradationwith this procedure because of differential solu- ally from one type to another along the surface. bility of various grain types under stress. Fracvary in length and breadth from a ture, dolomitization, recrystallization and Stylolites millimetres to many tens of metres; they solution further compound problems of estab- few terminate: (1) gradually into coarse equant lishing a sedimentary baseline. Millimetre- to calcite (2) by diminution of peaks, centimetre-size remnants of relatively unaltered and (3)aggregate, against rock unit junctions; sediment in metamorphic groundmass and they also abruptly end against cross-cutting stylolites background data from Recent carbonate sedi- and fractures (Fig. 6). ments were useful in this reconstruction. Stylolites bound bodies of rock or rock comPillara Formation ponents that have behaved as essentially homoPillara Formation limestones have been des- geneous units (idens). Circumidenic stylolites cribed by Read (1973a, b) and abbreviated circumscribe idens; they may be isolated or summaries drawn from his publications are interconnected with adjacent circumidenic or presented in Table II. Pillara lithofacies occur interidenic stylolites. Interidenic stylolites are in two types of carbonate cycles (Read, subplanar surfaces between neighbouring idens; 1973a): these idens are normally tabular to lensoid in form. Idens bounded by large scale stylolites Type A Carbonate Cycle: (4) Pellet, intraclast limestone commonly with also contain smaller scale pressure-solution surcryptalgal and fenestral fabrics; minor ooid faces—the intraidenic stylolites. Circumidenic limestone, stylolites form ramifying networks, isolating (3) Amphipora limestone, idens of sizes from grain to the largest blocks (2) Stachyodes limestone, and of all shapes. Interidenic stylolites occur in (1) Subspherical-stromatoporoid l i m e s t o n e ; subparallel orientation or sets (Figs 5, 6); conoverlies pellet limestone of previous cycle. jugate interidenic stylolite sets are frequent but Type B Carbonate Cycle: usually one set is dominant; it is this set that (5) Pellet limestone with cryptalgal and fenes- imparts a strongly layered or bedded appeartral fabrics, ance to formations. (4) Stachyodes limestone, In Pillara rocks there is a dominant inter(3) Subspherical-stromatoporoid limestone, idenic set that is subhorizontal to shallow dip(2) Tabular-stromatoporoid limestone, (1) Skeletal-fragment limestone; overlies pellet ping (5 to 10°) and subparallel to sedimentary limestone of previous cycle. layering. Conjugate interidenic sets occur mainly in proximity to the abutment and small Sadler, Virgin Hills and Napier Formations and as many as 3 sets may be present. Host rocks in the Sadler Limestone include faultsbedding set dominates and weaker sets are skeletal and ooid limestone (Table III). Host The oriented at 40 to 90° to this. rocks in Virgin Hills and Napier formations The dominant interidenic set in Sadler, Virinclude grainstones, pellet limestone with gin Hills and Napier rocks is inclined at 10 ?cryptalgal structures, and four types of skele- to 60° from the horizontal and away from the tal boundstone (Table IV). f

TABLE I I

Host Rocks, Menyous Member, Pillara Formation (after Read, 1973a, b) Lithology SKELETALFRAGMENT LIMESTONE

Structure Fabric Homogeneous, local layers of corals and algal Grainstone and packnodules stone

TABULARSTROMATOPOROID LIMESTONE SUBSPHERICALSTROMATOPOROID LIMESTONE STACHYODES LIMESTONE

Massive limestone, 3 to 10 cm thick; with abundant in place large stromatoporoids and interlayered skeletal-fragment limestone Massive limestone generally less than 3 m thick

AMPHIPORA LIMESTONE PELLET AND PELLET INTRACLAST LIMESTONE

OOID LIMESTONE

Massive, with abundant flat-lying cylindrical stromatoporoids (Stachyodes and Amphipora) Massive to crudely layered; Amphipora sticks commonly flat-lying, locally concentrated on bedding planes, and may be aligned Massive, poorly bedded to cross-bedded. Cryptalgal and fenestral fabrics common; 3 types— (1) colour-mottled pellet limestone with fine tubular fenestral fabric (2) pellet intraclast limestone with irregular fenestral fabric (3) pellet limestone with laminoid fenestral fabric Laminated to cross-laminated; locally with algal-nodule layers

I£

Texture and Composition Coarse sand-sized skeletal fragments (mollusk, dendritic corals, crinoid ossicles) and minor pellets; locally contains coral gravel and nodules (Girvanella, Sphaerocodium and encrusting o stromatoporoids) Boundstone: skeletal frames mainly of tabular stromatoporoids (up to 10 cm thick); a sheets in growth position parallel to bedding, scattered large cerioid corals and locally abundant cylindrical corals Packstone to wacke- Subspherical stromatoporoids, gravel-sized fragments of cylin- o stone drical stromatoporoids and corals in matrix of skeletal or £ pellet packstone and wackestone Wackestone and pack- Gravel and sand-sized fragments of cylindrical stromatopor- w stone oids, small subspherical stromatoporoids and locally, thin § tabular stromatoporoids; grains of calcispheres, dasyclad algae, ostracods, segmented or branching spicules; packstone contains >H W pellets and intraclasts of lime mudstone ^ Packstone and wacke- Amphipora sticks and fragments, sand-sized skeletal grains o (dasyclad algae, calcispheres, Amphipora), pellets in patches g stone or layers, intraclasts, fine skeletal debris and lime mud yx Mainly packstone, but Fine pellets, sand- to gravel-sized intraclasts (pellet and ooid grainstone, w a c k e - limestone) and minor skeletal grains (abraded sand-sized stone, mudstone and Amphipora, mollusks, calcispheres, dasyclad algae, ostracods, boundstone fabrics spicules); variable amounts of lime mud >—i also occur O w> (Z5 Grainstone Sand-sized ooids, lesser intraclasts, pellets and algal nodules; intraclasts are lumps of ooid or pellet packstone

PRESSURE SOLUTION AND SHEAR FRACTURE

TABLE III

Host Rocks, Sadler Limestone Lithology BRACHIOPOD LIMESTONE

NAUTILOID LIMESTONE STROMATOPOROID LIMESTONE

AGAL NODULE LIMESTONE

Structure Fabric Brachiopod valves orien- Valves in grain-support ted in layers or scattered; fabric with interstitial articulated valves partly matrix, or supported by to completely filled with matrix; matrix is skeletalgeopetal sediment similar fragment pellet packstone, to surrounding host. locally grading to grainstone Nautiloid cones oriented Nautiloids supported by in layers skeletal-fragment pellet packstone, locally grading to grainstone Frame of stromatoporoids Stromatoporoids typically in subparallel, encrusting form frame (boundstone), to domed layers, with with interframe sediment thin sheetlike to tabular of skeletal-fragment packform; some cylindrical stone grading to grainstone; stromatoporoids less commonly stromatoporoids are discrete colonies in packstone or grainstone Homogeneous, or crudely Algal nodules are mainly in grain-support framework; layered interstitial sediment is skeletal-fragment intraclast grainstone

abutment; it has variable inclination to original sedimentary layers, from subparallel to as much as 30°. Evidence for angular relations between pressure-solution layering and original sedimentary layering is provided by: (1) primary geopetal fabrics (Guppy et al. 1958; Playford & Lowry, 1966; Playford & Cockbain, 1972), (2) orientation of fossils, and (3) remnants of sedimentary bedforms (Fig. 7). Peaked surfaces in the dominant set usually have peaks and columns perpendicular to the median plane of the surface but in some localities there are stylolites with peaks inclined at 40 to 60° to this plane. Weakly developed conjugate interidenic sets are at variable angles of 30 to 90° to the attitude of the dominant set. The dominant set imparts a strongly bedded appearance to the formations, but between these 'stylobeds' there usually are interconnecting networks of small-scale hummocky to smooth circumidenic stylolites. Circumidenic stylolites also bound pod masses, breccia blocks and other large discordant bodies and at smaller scales they occur around skeletal grains, algal nodules and at junctions between laminar skeletons (Fig. 3). Interidenic stylolite surfaces are frequently disf

Texture and Composition Brachiopod valves 1 to 2 cm in size; matrix comprised of fine sand-sized pellets and fine to coarse sand-sized skeletal debris (brachiopods, crinoids, mollusks, trilobites, ostracods). Nautiloid cones 3 to 10 cm long; matrix similar to above Interframe and supporting matrix similar to above. Stromatoporoid sheets are a few millimetres thick; cylindrical forms are up to several centimetres long Sphaerocodium - Girvanella nodules are 0.5 to 4 cm diam., commonly 2 to 3 cm diam.; interstitial sediment is fine to coarse sand-sized quartz, comprises up to 5% of the sediment; gravelsized, whole brachiopods and mollusks are locally abundant

located by cross-cutting fractures and veins and deformed into a variety of fold types. Stylolites are exceptionally abundant in rocks adjacent to the abutment where ramifying circumidenic networks bound rock components from block to grain dimension. In the Devonian rocks, stylolite spacing is at all scales between widely spaced surfaces that bound major rock units with separations of many metres, to close-spaced interfaces that truncate individual grains, with separations of microns. Stylolite spacing probably is a function of many factors but it must ultimately be determined by volume reduction. An end point is reached when: (1) interfaces merge, (2) host-rock idens are reduced to insoluble residues, (3) idens are stabilized by alteration to less soluble minerals, e.g. dolomite, or (4) soluble grains are protected by development of a supporting framework of insoluble mineral particles. PRESSURE-SOLUTION RESIDUES

Stylocumulate Stylocumulate is pressure-solution residue which results from passive accumulation ..of

TABLE I V

Lithology

Structure

OOID GRAINSTONE

SKELETAL GRAINSTONE

LITHOCLAST GRAINSTONE

Sediments homogeneous or with textural and compositional layering; platy grains and elongate grains are oriented in the lamination; crinoid stems are linearly oriented

PELLET GRAINSTONE

LIMESTONE WITH 7CRYPTALGAL STRUCTURES

Cryptalgal structures are developed in sheetlike rock 30 to 60 cm thick. Structures include digitate columns (2 to 3 cm wide) and laterallylinked hemispheroids (4 to 10 cm wide, vertically continuous for at least 30 cm)

Host Rocks, Virgin Hills and Napier Formations Fabric Texture and Composition Cement Abundant ooids, minor skeletal, pellet, lithoclast and quartz grains; ooids mostly medium to coarse sand-sized, some are granule-sized where envelopes are on lithoclasts; ooids exhibit radial and concentric structures Well preserved grainstones have interstitial Abundant skeletal grains; sand- fine-grained calcite (averto gravel-sized, whole and frag- age size 0.01 mm); in mented crinoids, brachiopods, some aggregates crystals gastropods, pelecypods, stro- marginal to voids are matoporoids, calcareous algae columnar, while crystals (Renalcis, Girvanella) and calGrainstone to voids are cispheres; pellets, lithoclasts and central equant, this aggregate ooids are minor may grade locally (within the same thin secAbundant lithoclasts, minor tion) into coarse equant skeletons, ooids, quartz, pellets; poikiloblastic calcite lithoclasts are sand- to gravelsized grains derived mainly from (average size 0.3 mm) ooid limestone Abundant fine sand-sized pellets, lesser amounts of calcispheres, and minor medium to coarse sand-sized, debris of thin-walled skeletons One of two types of fabrics exist— 1. a laminated microspar and pseudospar; minor Skeletal-fragment and pellet sedimentary grains occur grains comprise the grainstone in the lamination, and packstone laminae; Sphaerocodium occurs within some 2. a laminated microspar laminations with scattered to abundant laminae of mediumgrained grainstone grading to packstone

Remarks

£ 1. These four end-member sediment types are intergradational, and most sediments contain varying mixtures of ooids, skeletons, lithoclasts, pellets and quartz 2. Quartz and minor felspar generally comprise less than 10% of the sediment

o *

nd Ei—i (Z) o £

§> a

H ffl *O O* yi

9 HH O

Closely spaced stylolites form lamination

TABLE IV—(cont.)

Lithology SKELETAL BOUNDSTONE (RenalcisGirvanella boundstone)

Structure Light gray, massive to crudely layered to crudely laminated

Fabric Texture and Composition Renalcis forms layers up to several millimetres thick and is intergrown and interlayered with Girvanella, Sphaerocodium, encrusting foraminifers, some ostracods; microspar. Patches of microspar occur throughout the rock. Receptaculites is rare to common

SKELETAL BOUNDSTONE (laminar stromatoporoidRenalcis boundstone)

Gray, crudely layered

Stromatoporoids are sheetlike and a few to tens of millimetres thick; Renalcis is rare to common, interlayered with stromatoporoids, encrusting top and/or undersides of the coenostea. Primary cavities occur where undulating stromatoporoids are domed; these are filled with skeletal pellet grainstone and packstone

SKELETAL BOUNDSTONE (StachyodesRenalcis boundstone)

Gray to light red, massive interlocking frame of cylindrical stromatoporoids

A frame of Stachyodes is encrusted by Renalcis; interframe sediment is pellet limestone

Cement

Remarks 1. Some skeletal boundstones are inter-layered with some sediment (mainly grainstone or brachiopod limestone) 2. The limestones commonly contain irregular to tubular cavities which are filled with columnar and equant calcite and green, buff, brown and red internal sediment (siltstone and grainstone) 3. Renalcis and Stachyodes commonly project into any cavity 4. Columnar calcite commonly replaces Girvanella, leaving Renalcis surrounded by this aggregate

•tf*

W co co

w O r A H o >

CO 1

Z A CO

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

B COLUMNAR PARALLEL S E T S

PEAKED HIGH AMPLITUDE IRREGULAR, ANASTOMOSING SETS

PEAKED LOW AMPLITUDE

IRREGULAR

HUMMOCKY CONJUGATE SETS SMOOTH

Fig. 5. A—Configuration of pressure-solution surfaces. B—Classification of pressure-solution (stylolite) sets. relatively insoluble material as more soluble components are lost to intrastratal fluids; stylocumulate occurs in sheet and lensoid to irregular idens along pressure-solution interfaces (Figs 8, 9). Stylocumulate is variable in composition, depending on host-rock mineralogy. Detrital quartz, felspar, clay minerals and mica are common. Dolomite also is a common component but this may be partly stylocumulate (Glover, 1968) and partly a product of reactions at the interface. In addition, more resistant calcitic components occur in the residue (Fig. 9), particularly coarse crystals and coarsely crystalline skeletal grains (e.g. crinoid ossicles). Fe-oxide is a minor but ubiquitous component in stylocumulate, usually occurring as thin films around other components and it imparts a red coloration.

Recognition of stylocumulate is based on the following criteria: (1) close relationship to stylolites and truncation of earlier rock features by stylocumulate idens (Fig. 9), (2) gradation from host rock (with low percentages of insolubles) to stylocumulate with corroded relict idens (from host rock) and abundant insolubles (Fig. 9), and (3) characteristic condensed, fitted and stylocumulate-support fabrics, described more fully on p. 21.

Reactate

Reactate is defined as material that has crystallized at stylolite interfaces under the PT conditions of pressure solution. Reactate includes: dolomite, calcite, quartz, mica and chlorite (Fig. 10). Some reactates nucleate on pre-existing mineral crystals, for example over-

Fig. 6. Stylolite types: A—Irregular and peaked stylolites in grainstone host (upper part of photo); hummocky to smooth stylolite separates layers of different lithology. Napier Formation, MacSherrys Gap Napier Range. White part of pen for scale is 10 cm. B—Large scale, smooth pressure-solution surfaces form layered structure of lensoid, sheet-like and irregular idens. Napier Formation, Geikie Gorge, Geikie Range. C—Pressure-solution interface (arrows) separating Renalcis-Girvanella boundstone (above) from stylolitically-laminated grainstone (below). Note that the pressure-solution interface truncates stylolite sets in rock below. Napier Formation, near 97-mile Creek, Oscar Range. Scale is 10 cm long.

PRESSURE SOLUTION AND SHEAR FRACTURE

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

Fig. 7. Relationship of stylobedding to sedimentary layering: A—Stylobedding parallel to original sedimentary layering as indicated by palimpsest bedding and orientation of fossils. B—Stylobedding at acute angle to sedimentary layering as indicated by palimpsest bedding and primary geopetal fills. C—Same as B; stylobedding sub-perpendicular to original sedimentary layering. growths on quartz but reactates can also be new minerals, e.g. dolomite and micas, that have crystallized in the pressure-solution field. Dolomite. Euhedral reactate dolomite occurs as scattered crystals or as sheets along stylolites that are developed in non-dolomitic host rocks (Fig. 10). Dolomite probably results from reaction between Mg++, Ca++ and HC0 3 ~ at the pressure-solution interfaces. Dolomite also replaces calcite idens (skeletons, ooids, sparry crystals) immediately adjacent to interidenic and circumidenic stylolites. Replacement is indicated by: (1) relict ooid lamellae, (2)

skeletal microstructure in dolomite crystals, and (3) palimpsest sedimentary fabrics in dolomite aggregates. Calcite. Reactate calcite grows on crinoids, other coarsely crystalline skeletal grains and smaller equant crystals. When nucleated on skeletal grains it forms syntaxial overgrowths which poikiloblastically enclose other carbonate grains neighbouring the nucleus. Reactate calcite also grows from interstitial blocky calcite crystals, and gradations can be observed from an aggregate of small calcite crystals (average size 0.01 mm), through larger blocky crystals, finally to 1 mm poikiloblastic crystals. Coarse equant-calcite crystals have zig-zag, straight, to irregular boundaries. Individual reactate-calcite crystals commonly have a sutured or stylolitic contact with neighbouring stylocumulate idens and sedimentary grains. Aggregates of reactate calcite form 30 to 50% of some rocks as a supportive frame in which sedimentary grains float. Interidenic stylolites (some with up to 1 mm thick stylocumulate seams) commonly pass into the intercrystalline boundaries of these calcite aggregates. It is interesting to note that reactate calcite may be ferroan, particularly when syntaxially overgrown on crinoid plates; in this case the skeletal foundation also may be ferroan calcite. Quartzy micas and . chlorite. Reactate quartz occurs as syntaxial overgrowths and is common in stylocumulates where individual quartz idens are in stylolitic contact. Large muscovite, biotite and chlorite crystals occur in small quantity in many stylocumulates. Textural relationships suggest that the micas and chlorite have grown in the pressure-solution environment; the criteria are: (1) chlorite replacing calcite in dolomitic rocks, (2) occurrence of micas in calcite-crystal aggregates, and (3) large size of mica crystals compared to detrital grains from the host. Stylocumulate and Reactate Idens Similar considerations as discussed for stylolite spacing apply to the thickness of stylocumulate and reactate idens which range from fractions of millimetres to tens of metres in the extreme case. Stylocumulate idens thicken either by accumulation of material at a single

Fig. 8. Pressure-solution residues; stylocumulate: A—Stylocumulate seams and mottles (arrows) in limestone; stylocumulate here is mainly Feoxide, quartz silt, mica and less soluble fossils. Pillara Formation, Cowan Hills. Scale is 10 cm long. B—Fine sand-sized quartz occurring as stylocumulate along hummocky stylolites. Note that crests of the hummocky stylolites are superimposed.

PRESSURE SOLUTION AND SHEAR FRACTURE

FIG. 8

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

Fig 9. A—Stylocumulate along stylolite, photomicrograph. B—Close-up of stylocumulate showing quartz and minor calcite, along stylolite shown in A.

PRESSURE SOLUTION A N D SHEAR FRACTURE

pressure-solution interface or by merging of adjacent interfaces and related stylocumulate. The second case results in stylocumulate-reactate idens that are bedded or laminated, reflecting variations in composition of confluent stylocumulate sheets, variations in stylocumulate fabric, and the presence of multiple stylolite surfaces. FABRICS

Idens, circumidenic stylolites and associated stylocumulate and reactate are the main textural elements in rocks that have been subject to pressure solution. Fabric types (Fig. 11) are intergradational but convenient descriptive categories are: (1) iden-support, (2) condensed, (3) fitted, (4) stylocumlate-support, and (5) reactate-support. Since idens are nonscalar, identification of a fabric type rests on the larger elements observed. lden-Support Fabric. Iden-support fabric (Figs 11, 12A) is characterized by a supporting framework of idens arranged in contact at points. The volume of interidenic space is a function of iden shape and size—for spherical idens it is 26 to 47% (Graton & Fraser, 1935); for tabular and irregular idens it may be as much as 70 to 80%. Iden-support fabric is analogous to 'grain-support depositional texture' (Dunham, 1962) and in relatively unaltered sedimentary rocks where idens are grains or clasts the two terms are synonymous. Interidenic space is filled by a range of materials: relicts of small sedimentary grains, matrix, cement, stylocumulate and reactate. Condensed Fabric. This fabric (Figs 11, 12B) is typified by idens in contact along surfaces formed by stylolites. Packing is far tighter than that exhibited by iden-support fabric in cubic and hexagonal packing modes; interidenic space is accordingly reduced. In rocks with subspherical idens it averages 10 to 20%. The space is most commonly filled by stylocumulate or reactate, cement and matrix. Fitted Fabric. Fitted fabric (Figs 11, 12C) is one in which idens are in contact along their entire margins and there is marginal truncation of internal microstructures and embayment of more soluble idens; iden boundaries are circumidenic stylolites. A small volume of interidenic space is occupied by stylocumulate or reactate and corroded relicts of smaller idens. Stylocumulate-Support Fabric. Stylocumulatesupport fabric is characterized by dominance of stylocumulate over remnant idens (Figs 11, 12D). Idens are not in contact and 'float' in the

groundmass of stylocumulate. Their boundaries are defined by circumidenic stylolites which are interconnected through an anastomosing network in the supporting stylocumulate. Reactate-Support Fabric. This fabric (Fig. 11) is similar to stylocumulate-support fabric, but supporting material is formed by chemical reaction under conditions of pressure solution. Dolomite, calcite, mica and clay are probable reaction products. STRUCTURES

Pressure-solution surfaces, host-rock idens and idens of stylocumulate and reactate are the main structural elements in rocks that have been subject to pressure solution and shear. Structural types (Fig. 13) are intergradational but convenient descriptive categories are: (1) stylobedding, (2) stylolamination, (3) stylonodular structure, (4) stylomottled structure, and (5) stylobreccioid structure. Stylobedding Carbonate rocks in the Pillara, Sadler, Napier and Virgin Hills formations occur in flaggy, sheet-like to lensoid idens, 1 cm to several metres thick, that are bounded by strongly developed interidenic stylolites (Figs 14, 15A). Idens consist of relatively unaltered sedimentary rock, metamorphic carbonate rock, stylocumulate or reactate. Stylolitic contacts give a flaggy to strongly bedded appearance which is termed here stylobedding (Fig. 14C). This term is appropriate recognition of the role of stylolites in forming rock-unit contacts and layering. Stylobedding may be parallel to sedimentary layering, or at an oblique angle to it (Fig. 7). The Pillara Formation is characterized by thick stylobedding that is subparallel to sedimentary layering (Fig. 14A). Idens between stylolite sets retain many sedimentary features —primary lamination defined by grain size, laminoid, irregular and tubular fenestrae and depositional textures (Table II). Stylobedding structure is strongly developed in rocks located in the homocline, separating tabular idens of metamorphic rock, residual material and some layers that retain remnant sedimentary features (Fig. 15B). Stylolite separation varies from tens of metres and grades down through beds of 1 to 5 cm into laminated rocks (stylolaminated). Stylolamination Stylolamination is defined as a lamination (scales after McKee & Weir, 1953) due to the presence of closely spaced stylolite sets and

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 10

23 PRESSURE SOLUTION AND SHEAR FRACTURE clays, dolomite and Fe-oxide along with calFABRIC T Y P E S citic grains in various stages of pressure solution. Mottles contain abundant intraidenic stylolites and are either sharply bounded by a circumidenic stylolite or grade through anastomosing stylolites into the host rock. Frequently the mottles are isolated to interconnected, and randomly dispersed and oriented. Many mottles, however, are localised: (1) along interidenic stylolites, (2) in areas between coalescent interidenic stylolites, or (3) in troughs and crests of irregular and peaked interidenic stylolites. Some tabular mottles are oriented subparallel to host stylobedding. Stylobreccioid Structure Stylobreccioid structure is where stylolites are mostly in an irregular circumidenic array (Fig. 13). This structure results from: (1) intersecting and anastomosing networks of irregular and peaked stylolites, (2) intersecting networks of conjugate irregular stylolites, (3) intersecting randomly oriented stylolites, and Fig. 11. Classification of pressure-solution fabrics. (4) stylolites around fragments of a pre-existing breccia. Stylobreccioid structure is mostly associated sheets of residue (Fig. 13). Rock formed by irregular, peaked and hummocky units characterized by stylolamination are stylolite types. Stylolites tend to isolate hostabundant in Sadler, Napier and Virgin Hills rock elements into discrete idens. The structure formations as sheet-like to lensoid idens, is common in Napier and Virgin Hills rocks several centimetres to tens of metres thick. and is less common in the Sadler and Pillara formations. The fabric may be: (1) condensed, Stylonodular Structure (2) fitted, (3) stylocumulate-support, and (4) Stylonodular structure is one in which iso- reactate-support. lated to interconnected, lenticular to rounded idens of limestone are surrounded by lamin- ROCK TYPES ated stylocumulate (Fig. 19). The limestone Stylolaminites idens, reminiscent of boudins, are generally Stylolaminites are rocks with stylolaminar >1 cm in thickness and separated by several structure formed by a close-spaced set of planar centimetres of laminated stylocumulate or reac- to undulating interidenic stylolites, hence they tate: more commonly the thickness of both consist sheet-like to lensoid idens of sedilimestone idens and intervening residue is about mentaryof and metamorphic carbonate, styloequal. cumulate or reactate. Undulations where present tend to be superimposed and crest and Stylomottled Structure stylolites tend to be vertically perStylomottled structure is characterized by troughs ofLimestone idens in the interlaminar isolated idens of stylocumulate or reactate that petuated. are either sharply bound by stylolites are scattered through a host-rock groundmass sequence through altered limestone, with con(Fig. 13). The structure is mainly developed in or gradefabric into stylocumulate or reactate grainstone and packstone precursors but it densedfitted fabric. Stylolaminites are subalso occurs in dolomitic hosts. The mottles are with divided on basis of composition into four endred to brown, tabular equant to irregular idens, member types: (1) calcareous, (2) quartzose, 1 to 5 cm in size composed of quartz, micas, CONDENSED

STYLOCUMULATE/REACTATE SUPPORT

STYLOCUMULATE V^OfiOQAd

Z p

REACTATE/>^VC'

Fig. 10. Pressure-solution A—Stachyodes inresidues; styloliticreactate: ground. Note remnant patch of host rock (arrow) and corroded B—Iden of host rock (pelletal and skeletal grainstone) circumscribed by stylolites (outlined). C Stylolitic part of rock: skeletons in fitted fabric; reactate dolomite along stylolites (arrows). 6

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

Fig. 12. Pressure-solution fabrics: A—Iden-support fabric; idens are ooids. B—Condensed fabric; ooids contact over a surface. C—Fitted fabric; ooids in contact along entire margins. D—Stylocumulate-support fabric: ooids and skeletons float in a ground of stylocumulate. Photomicrograph of a stylocumulate sheet along a stylolite.

PRESSURE SOLUTION AND SHEAR FRACTURE PRESSURE SOLUTION

STRUCTURES

STYLOBEDDED

STYLOLAMINATED

STYLONODULAR

STYLOMOTTLED

STYLOBRECCIOID

sheets are internally laminated along planar to hummocky intraidenic stylolites (Fig. 17). Limestone idens in these rocks are sharply bounded by stylolites or grade through altered limestone, with condensed fabric into stylocumulate. Limestone idens also exhibit crosscutting stylolites conjugate to the main set; where these conjugate stylolites localize stylocumulate, limestone idens become isolated in laminated stylocumulate (Fig. 17). The proportion of quartz stylocumulate to limestone idens is highly variable through a range that extends from initial calcareous stylolaminite, as described above, into rocks dominated by quartz in fitted fabric, where carbonate idens are observed as rare floaters in a stylocumulate ground, and small corroded remnants in a condensed or fitted framework of quartz grains (Fig. 18C).

(3) quartzose-micaceous, and (4) dolomitic stylolaminite. Calcareous Stylolaminite. Rocks composed of thin limestone idens with sheet form that alternate with interidenic stylolites. The limestone idens are composed of relatively pure carbonate sediment and skeletal-fragment, ooidpellet and intraclast grainstone are common lithotypes with stromatoporoid, brachiopod, mollusk limestone and minor lime mudstone (Tables II, III and IV). Stylolite surfaces are usually marked by Fe-oxide and discontinuous sheet ideas of stylocumulate or reactate (quartz, dolomite and clay) but stylocumulate layers are very thin and composed of corroded host idens (Fig. 16).

Quartzose-Micaceous Stylolaminite. This rock consists of interlaminated red-brown stylocumulate idens, interidenic stylolites and relict limestone idens. Stylocumulate idens are composed of quartz silt particles, micas, clay minerals, and Fe-oxides along with larger detrital quartz grains, corroded sedimentary carbonate grains and small quantities of reactate including calcite crystals, micaceous clays and dolomite. Stylocumulate-silt particles occur in condensed and fitted fabric or in Fe-oxideclay ground and the sheets are internally laminated along closely spaced intraidenic stylolites. Limestone idens in these rocks are sharply bounded by stylolites or grade through altered limestone with condensed fabrics into stylocumulate. The proportion of quartzose micaceous stylocumulate to limestone idens is highly variable through a range that extends from initial calcareous stylolaminite into rocks dominated by quartzose micaceous stylocumulate where relict limestone idens are observed as rare corroded floaters in stylocumulate ground.

Quartzose Stylolaminite. This is quartzose rock with stylolaminar structure defined by interlayered stylocumulate idens, interidenic stylolites and variable numbers of relict limestone idens (Fig. 17). Stylocumulate idens which dominate the rock are composed of quartz grains, corroded sedimentary carbonate particles and minor felspar, clay and Fe-oxide; reactate minerals occurring in the sheets as minor constituents include calcite crystals (ferroan and non-ferroan types), dolomite and micas. Stylocumulate materials occur in condensed to fitted fabric (Fig. 18) and the

Dolomitic Stylolaminite. This is red to yellow dolomitic rock with stylolaminar structure of dolomite stylocumulate or reactate idens, interidenic stylolites and relict limestone idens. Dolomite iden sheets are composed of aggregates of dolomite, rare large reactate calcite crystals, Fe-oxide, minor quartz and rare corroded sedimentary carbonate sheets and grains. There is an internal fine wavy lamination that is an expression of numerous closely spaced, hummocky to planar stylolites. The character of dolomitic stylolaminites is discussed further on p. 59.

Fig. 13. Classification of pressure-solution structures.

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

Occurrence. Stylolaminites occur as tabular to lensoid idens interlayered in stylobedded carbonate rocks. The thickness of stylolaminite idens is variable: Calcareous 1cm to 1.5 m Quartzose 3 cm to 6 m Quartzose-Micaceous 1 cm to 1 m Dolomitic 3 cm to 60 m They extend for distances up to many metres along the stylobedding strike before passing gradationally into stylobedded limestone, stylomottled rock, stylonodular rock or stylobreccia. Contacts between stylolaminite idens and neighbouring rocks are either sharply defined by interidenic stylolites or gradational through rocks containing numerous stylolites, corroded host-rock idens, stylocumulate and reactate. Stylolaminites are abundant in Napier, Virgin Hills and Sadler rocks located in the homocline; they are rare in the Pillara rocks except in sections adjacent to the abutment and faults which cut the formation. Stylonodular Rocks Stylonodular rocks have lenticular to rounded host-rock idens separated and surrounded by stylolaminite (Fig. 19). Nodules are 1 to 10 cm thick, isolated to interconnecting idens of host limestone, and are bound by interidenic and some circumidenic stylolites. Contacts of limestone idens with surrounding stylolaminite are generally sharply defined by stylolites but contacts also are locally gradational (Fig. 20). Stylolaminite intervening

between host-limestone idens is mostly 1 to 10 mm thick, or rarely up to 50 mm thick. Stylolaminites are quartzose, quartzose-micaceous and calcareous types, and are similar to those described above. With increase in stylolites, stylonodular rocks pass gradationally into stylolaminite and with decreasing stylolite abundance into stylobedded limestone. Occurrence. Tabular bodies of stylonodular rock are interlayered with stylobedded rocks (Fig. 20). They vary from about 5 cm up to 6 m in thickness, and are laterally extensive for tens of metres. Stylonodular rocks pass gradationally into stylobedded and stylolaminated rocks, and commonly are the transitional rocks between stylobeds and stylolaminites. Locally stylonodular rocks also pass into stylobreccia. Stylonodular rocks are abundant in the Napier and Virgin Hills formations. They are less abundant in the Pillara and Sadler formations except in areas adjacent to the abutment and other faults. Stylomottled Rocks Stylomottled rocks are those with idens of stylocumulate and/or reactate developed as mottles in a host-rock groundmass (Figs 21, 22). Mottle-idens vary from 1 mm to 3.5 cm though most are in a size grouping of 2 mm x 10 mm; shapes are mainly equant to ellipsoidal to tabular, but rhomboidal, pyramidal and irregular shapes also occur. Numerous intraidenic stylolites ramify through the mottle-

Fig. 14. Stylobedding: A—Stylobedding in Pillara Formation approximates sedimentary layering. Windjana Gorge area, Napier Range. Height of cliff is 70 to 100 m. B—Aerial view of inclined stylobedding in Napier Formation homocline; Copley Valley. Height of cliffs in foreground approximately 6 to 10 m. C—Inclined stylobedding in Napier Formation: stylobeds pinch and swell. Brooking Gorge. Fig. 15. A—Conjugate stylolite sets; either set could be viewed as stylobedding; but the more closely spaced set is usually dominant. The weaker set is outlined. Some peaks along the stylolites are inclined to the median plane of the pressure-solution interface. Napier Formation, Copley Valley. Scale (arrowed) is 10 cm long. B—Strong stylobedding; tabular idens of limestone and metamorphosed carbonate rock; a close-up of this cliff face is illustrated in Figure 20. Napier Formation, Sheep Camp Yard, Geikie Range. Scale is marked in 30 cm units. Fig. 16. A—Sheets of calcareous stylolaminite interlayered with host grainstone. Pillara Formation. B—Photomicrograph of calcareous stylolaminite; host rock was cylindrical stromatoporoid limestone with a wackestone/packstone matrix. Menyous Member, Horse Spring Range. Fig. 17. Stylolaminite: A—Gradation of stylobedded limestone (1) through stylonodular rock (2) into calcareous stylolaminite (3). Menyous Member, Menyous Gap, Pillara Range. Scale is 10 cm long. B—Quartzose stylolaminite (1) interlayered with and grading into thin idens of host limestone (2), isolated corroded remnants of host limestone occur as discrete nodules (3). Napier Formation, Dingo Gap. Scale (arrow) is 10 cm long. C—Quartzose stylocumulate along closely spaced stylolites, and development of quartzose stylolaminite; note gradational relationship of stylolaminite to host and also corroded nodular remnants of host limestone (arrows). Napier Formation, Dingo Gap.

PRESSURE SOLUTION AND SHEAR FRACTURE

FIG. 14

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

B FIG. 15

PRESSURE SOLUTION AND SHEAR FRACTURE

FIG. 16

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 17

PRESSURE SOLUTION AND SHEAR FRACTURE idens, most of these terminate abruptly at the circumidenic stylolite bounding the mottle-idens but others pass out into the host rock and fade into intercrystal boundaries of reactate-calcite aggregates (Fig. 21C, 22B). The composition of mottle-idens is variable and can be related to host-rock composition, degree of development and whether they are dominated by stylocumulate or reactate materials. In grainstone hosts (Tables II, III and IV) stylocumulate mottle-idens range through an intergradational series of compositional types between idens composed mainly of sedimentary carbonate grains in fitted fabric to idens composed mainly of quartz, mica, clay and Fe-oxide with minor corroded carbonate grains. The colour of the mottle-idens passes from light red to dark red to brown through this range (Fig. 22A). Most grainstone host rocks contain abundant stylolites and large reactate-calcite crystals. These calcite crystals which form 30 to 50% of the rock occur interstitial to primary carbonate grains and additionally form local reactate-support frames (Fig. 21B). Mottles have sharp to gradational contacts with the host rock (Fig. 21). Many contacts are sharply defined by a stylolite; contacts are gradational where host rock grades via anastomosing circumidenic stylolites into mottles. Some mottles have a sharp contact on one margin which passes into a gradational contact on another. Gradations from host to mottle pass from sediment with iden-support fabric to condensed and fitted and finally to a residue of Feoxide-stained stylocumulate. Relatively insoluble crinoid stems and brachiopod valves lie across the gradation (Fig. 22C). In dolomite and dolomitic host rocks, mottle-idens are composed mainly of brown to red stylocumulate or reactate dolomite crystals along with calcite crystals and Fe-oxide films. Occurrence. Stylomottled rocks occur in tabular to lensoid idens interlayered in stylobedded carbonate rocks (Fig. 21 A). The thickness of stylomottled units is variable, but mostly in the range 5 to 30 cm; rarely some are up to 2 m thick. Stylomottled rocks are laterally extensive for tens of metres along strike of stylobedding. Contacts with other rocks are sharp (defined by interidenic stylolites) to gradational; stylomottled rocks pass gradationally into host limestone or dolomite, stylolaminite and stylobreccia. Stylomottled rocks are developed mostly in the Napier and Virgin Hills formations, and are rare in the Pillara and Sadler formations.

Stylobreccias Stylobreccias are polymict breccias composed of idens (fragments) in condensed and fitted fabric or stylocumulate and reactate-support fabric. Idens are bounded by circumidenic stylolites and thin laminated sheets of stylocumulate and reactate; these sheets contain abundant small-scale interidenic and circumidenic stylolites. Idens range greatly in size, shape and other textural parameters (Fig. 23, 24) but in any one breccia there often is a relative uniformity; textural parameters are: (1) Size: range 1 cm to many metres (megabreccia). (2) Size Sorting: variable, poor to moderate. (3) Iden Shape: variable; elongate-tabular, lensoid and ellipsoidal, irregular, subrounded, prismatic, tetrahedral. (4) Orientation: Elongate idens frequently subparallel to boundaries of breccia body but in these cases some fragments lie at steep angle to the general orientation; in other bodies the fragments are randomly oriented. The composition of the idens in stylobreccia is of significance in interpreting the genesis of this rock type. In general terms the idens are of similar lithology to rocks adjacent to the breccia body. Thus stylobreccia units in the homocline of Sadler, Napier and Virgin Hills formations contain idens composed both of sedimentary rock, and rocks bearing imprints of metamorphism prior to brecciation. Stylobreccia units in the abutment zone contain idens from both the formations in the homocline (as above) and formations in the Lamboo-Pillara block (see p. 9). Stylobreccia units in the block contain idens only of Pillara Formation and Lamboo Complex lithotypes. Compositional types include all of the sedimentary rocks listed in Tables II, III and IV. These idens of sedimentary lithotype simulate lithoclasts but they lack the encrustations of marine organisms, pelletal rinds and borings typical of marine breccias; they also lack calcrete envelopes, solution voids and other features which characterize terrestrial sedimentary breccia (Read, 1974). Grainstone idens have rare preservation of sedimentary lamination and sedimentary grain-support texture (Dunham, 1962) but most commonly they contain intraidenic stylolites and grains are in condensed to fitted fabric. Idens composed wholly of skeletal material are frequent; in coarse breccia these are stromatoporoid, coral, Renalcis-Girvanella fragments, algal nodules, crinoid ossicles, and brachiopod valves. Idens of metamorphic lithotypes are composed of all rock types generated by pressure

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN 32 solution, fracture, dolomitization, solution and mite. Stylocumulate or reactate seams are emplacement, plus partly altered limestones finely laminated, reflecting abundant small that bear imprints of these processes. Table V scale hummocky to planar stylolites in the summarizes compositional types. Special note ground. Contacts between breccia idens and should be made that stylobreccia fragments interstitial material are either sharply defined frequently are composed of stylobreccia, stylo- by circumidenic stylolites or are gradational. laminite types and stylomottled rock along with Occurrence. Stylobreccia occurs as: (1) conrocks related to fracture including vein calcite, cordant to semi-concordant sheets and lenses internal sediment/vein calcite admixtures, cal- in stylobedded sequences (Fig. 25), (2) podcite-cemented breccia, and vugular rocks (Figs like to irregular masses in discordant relation23, 24) (see later page). Dolomitic stylo- ship with containing stylobedded carbonates laminite and part-dolomitized sedimentary (see p. 98) and (3) wedge to irregular masses, limestone also occur in the assemblage. Idens trapezoidal in cross-section surrounding pods derived from Lamboo Complex basement are composed of skeletal boundstone. Breccia composed of mica schist, quartzite and other bodies vary in thickness from several centihigh-grade rocks; those derived from con- metres to several tens of metres. Contacts with glomerate formations are fragments of con- surrounding rocks are either sharply defined by glomerate with interstitial felspathic sandstone, subplanar surfaces or are gradational from angular quartz and mica schist pebbles. breccia into stylobedded host rock through a Material between breccia idens is usually red, zone where stylobeds are fragmented and brown to buff stylocumulate, composed of moved from original positions (Fig. 26). quartz sand and silt, clay minerals and Fe-oxide with floated and corroded sedimentary car- ROCK ASSOCIATIONS AND PETROGENESIS bonate grains, small corroded fossil fragments, Stylolaminites, stylomottled rock, stylonodue.g. crinoid ossicles and brachiopods, and lar rocks and stylobreccia form 50 to 60% cement crystals. Reactate may, however, of the Napier, Virgin Hills and Sadler formaoccupy interiden space; this is usually dolo- tions. They occur in stylobedded sequences Fig. 18. Quartzose stylolaminite: A-C—Photomicrographs; gradation from grainstone with dispersed quartz to quartzose stylolaminite in thin section in which quartz is in condensed to fitted fabric; remnant carbonate grains (arrows) occur interstitial to the quartz frame. This gradation is observed over a distance of several millimetres within the same thin section. Napier Formation. Fig. 19. Stylonodular rock: A—Interlayered tabular host-limestone idens and stylocumulate (quartzose-micaceous stylolaminite) which locally grade into stylonodular rock. Scale is marked in inches (2.5 cm units). Note remnant, oval host-limestone iden. Napier Formation, Dingo Gap area. B—Host limestone (light gray) gradational into stylonodular rock, with progressive isolation of limestone idens by pressure solution Note also fractured and rotated tabular iden of host (arrow 1). Uppermost argillaceous unit (arrow 2) is quartzose-micaceous stylolaminite which grades down into stylonodular rock. Napier Formation, Sheep Camp Yard, Geikie Range. C—Photomicrograph of limestone nodules from stylonodular rock—note stylolite with stylocumulate of quartz, Fe-oxides, micas and clays. Fig. 20. Stylonodular rock: A—Sheet-like development of stylonodular rock interlayered with limestone idens. Note progressive disappearance of a limestone layer (arrow). Napier Formation, Sheep Camp Yard, Geikie Range. B—Gradation from host limestone (white) to stylonodular rock (dark) by progressive isolation of idens by stylolites. Napier Formation, Sheep Camp Yard. C—Stylonodular rock: lenses and nodules of limestone separated by calcareous stylolaminite. Base of Menyous Member, Menyous Gap, Pillara Range. Scale is 10 cm long. D—Stylonodular rock with lensoid remnants of host limestone (arrow 1); note palimpsest sedimentary layering (arrow 2). Napier Formation, Sheep Camp Yard. Fig. 21. Stylomottled rock: A—Sheetlike iden of stylomottled rock (bracketed) interlayered with stylonodular rock and host limestone. Napier Formation, Sheep Camp Yard, Geikie Range. B—Photomicrograph showing stylomottles (arrow) interspersed with host grainstone and coarse Teactate calcite. Note gradation of host into mottles. C—Stylolites within the stylomottles (arrow) passing out into aggregates of coarse equant reactate calcite. Note that the stylomottle has sharp to gradational contact with the host grainstone.

PRESSURE SOLUTION AND SHEAR FRACTURE

FIG. 18

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

2mm

FIG. 19

A FIG. 20

D B

PRESSURE SOLUTION AND SHEAR FRACTURE

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 21

PRESSURE SOLUTION A N D SHEAR FRACTURE

imposed on stylobreccia leads towards stylolaminite (Fig. 32). The origin of the rock types relates primarily to shear and the formation of stylolites in precursor limestone. Compressive stress is required for pressure solution but the role of shear in generating many structures associated with pressure-solution features must be evaluated. Shear directed mainly along stylobedding planes frequently is expressed in circumidenic stylolites, conjugate stylolites, drag folds, rotated idens (breccia) and displaced vertical markers. Stress modes thus probably varied from simple normal compression (no shear component) to oblique compression (with normal and shear components). There has been controversay about stylolite origin with contraction-pressure (Shaub, 1939) and pressure-solution hypotheses (Stockdale, 1922, 1943; Dunnington, 1954; and many other authors). However, abundant criteria support pressure solution as the process responsible for stylolite formation, and among most workers today there is general acceptance that stylolites form when rocks are subject to stress (Bathurst, 1971). In the Devonian sequence, pressure solution affects all rock types including those formed by other metamorphic processes and thus it is concluded that stylolites here, have formed in indurated rocks. The literature reveals that major uncertainties about stylolites are: (1) stress conditions under which pressure solution occurs, (2) degree of lithification of host sediments, and (3) time of stylolite formation. To this list can be added the dearth of information on factors that either promote or along with carbonates that have been formed inhibit pressure solution, e.g. temperature. Strangely an extensive literature on stylolites by other metamorphic processes, and partly altered limestones. In the field it is possible to (Trurnit, 1967, cites 700 references) has been mainly concerned with geometry and classificatrace along strike or in sequence, transitions from one category to another over distances tion of pressure-solution surfaces. There is a general acceptance that stylolites imply subranging from centimetres to tens of metres. Thus, stylobedded rocks can be traced through stantial loss of sediment; implications of this in terms of petrogenesis and stratigraphy stylomottled and stylonodular rocks into stylo- loss been explored only by Dunington (1967) laminite; stylobedded rocks can be traced have and Glover (1968). Logic suggests that it is through stylobreccia into stylolaminite, and possible to pressure-solve limestones to resistylolaminite grades into stylobreccia. Grada- duals of insoluble components. tions are illustrated in Figures 27-32, inclusive. Pressure-solution theory and mechanisms The gradational relationships indicate that were reviewed by Bathurst (1971) with inforthese rock-type categories are members of a consanguineous suite and ordered in a spectrum mation drawn from basic papers of Thomson (1862), Reicke (1895), Weyl (1959), Becker extending from stylobedded limestone to stylolaminite, as the first and near-final products of & Day (1916) and Taber (1916). Much has pressure solution (Fig. 32). Stylobreccia and been made of Reicke's principle based on presmelting but this is an inexact analogy stylomottled rock are special parts of this spec- surepressure solution taking place between cartrum for their origin involves not only pressure to bonate idens. Thermodynamic data are of basic solution but also relief of stress by fracture and movement of resulting idens. Pressure solution importance in understanding pressure solution TABLE V

Composition of Iden Types in Stylobreccia I. LIMESTONE 1. Pillara Formation lithotypes (Table II) 2. Brachiopod limestone 3. Ooid, skeletal and lithoclast grainstone 4. Skeletal boundstone (Table IV) II. FOSSILS 1. Algal nodules 2. Corals 3. Stromatoporoids 4. Brachiopods 5. Crinoids III. ROCKS FORMED BY PRESSURE SOLUTION AND SHEAR 1. Stylonodular rock 2. Stylomottled rock 3. Quartzose stylolaminite 4. Quartzose-micaceous stylolaminite 5. Calcareous stylolaminite 6. Dolomitic stylolaminite 7. Stylobreccia IV. DOLOMITIC ROCKS 1. Dolomitized Pillara Formation lithotypes 2. Dolomitized skeletal boundstone 3. Massive dolomite 4. Dolomitic stylomottled rock 5. Dolomitic stylobreccia V. ROCKS FORMED BY TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT 1. Veined rock 2. Vugular rock 3. Calcite-cemented breccia 4. Vein calcite 5. Internal sediment VI. NON-CARBONATE ROCK TYPES 1. Conglomerate 2. Quartz pebbles 3. Schist pebbles 4. Precambrian metamorphic rocks

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

in carbonate rocks but data available on carbonate minerals mainly relate to stability and solubility at N T P , or in high T conditions where mineral assemblages are indices of higher grade metamorphic conditions. We can summarise what is known only in very general terms. Pressure increases calcite solubility through its effect on C 0 2 solubility in intrastratal fluids (Ellis, 1959; Miller, 1952). U n d e r stress, calcite idens dissolve at contact points with adjacent idens and it follows that: (1) intrastratal fluids become charged with solute, (2) less soluble residues accumulate along pressure-solution interfaces, (3) reactions involving intrastratal fluid and mineral phases occur in adjustment to PT, and hydrochemical conditions, and

(4) contact points between idens become contact surfaces and link with adjacent stylolites to form larger interfaces. As pressure solution continues, residues of less soluble materials, including carbonate idens, accumulate along pressure-solution interfaces. Trurnit (19686) orders minerals in a sequence of relative pressure solubility (Table V I ) but of great significance is the pressure solubility order among carbonate materials (related to heterogeneity, crystal size, orientation and composition) which can be deduced f r o m fabric relationships across stylolites. This order is: Most Soluble: pellets, intraclasts, lithoclasts, ooids, Intermediate: skeletons of brachiopods, corals and stromatoporoids; calcispheres,

Fig. 22. Stylomottled rocks, polished slabs: A—Irregular stylomottles; there is gradation in colour from light red (arrow 1) through to dark red (arrow 2) that reflects increasing loss of calcite and increase in Fe-oxide. B—Close-up of stylomottle showing stylolites (arrows) that pass out of the mottle into the host grainstone. C—Close-up of stylomottle showing fossil (arrow) partly enclosed by the mottle. D—Large stylomottles composed mainly of calcitic idens and only minor insoluble residue. Fig. 23. Stylobreccia features: A—Rotated tabular idens of grainstone in stylobreccia. B—Blocks of stylobreccia (1), quartzose stylolaminite (2), and vugular rock (3). The block of stylobreccia (margin is outlined) is composed of vugular limestone and limestone fragments. Napier Formation, Dingo Gap area. Scale is 10 cm long. Fig. 24. Stylobreccia idens: A—Block of vugular rock (1) and Renalcis-Girvanella boundstone (2); Napier Formation, Dingo Gap area. B—Fragments with parallel sheet vein structure. Napier Formation, MacSherrys Gap. C—Large fragment with breccoid vugular structure, Napier Formation, Sheep Camp Yard. Scale (arrow) is 10 cm long. D—Veined limestone fragment (arrow). Scale marked in inches (2.5 cm units). E—Fragment with conjugate stromatactid vugular structure; Sadler limestone, Pillara Range. F—Fragment of calcite-cemented breccia; Napier Formation, MacSherrys Gap. Scale marked in inches (2.5 cm units). Fig. 25. Stylobreccia geometry and relationships: A—Semiconcordant, thick body, in stylobedded limestone. Napier Formation, Wire Spring. Thickness of breccia is 4 m. B—Discordant body truncating stylolaminite; contact is outlined; 6 km SE of Dingo Gap. C—Lensoid body (32 cm thick) in stylolaminite. Napier Formation, 6 km SE of Dingo Gap. D—Discordant stylobreccia adjacent to small reverse fault in stylobedded rocks. Napier Formation, Dingo Gap. Fig 26. Stylobreccia origin: A—Lensoid body of stylobreccia partly concordant, with thicker part discordant to containing stylobedded limestone. Block-sized fragments are disoriented along top of dislocation zone. Napier Formation, MacSherrys Gap. Hammer (arrow) for scale. B—Close-up of above: discordant and gradational relationship between stylobedded limestone and stylobreccia. Stylobeds break off (arrow) and merge with stylobreccia. C—Drag fold along the base of a stylobreccia. Drag-folded stylobeds break off and are incorporated into the overlying breccia. Napier Formation, 6 km SE of Dingo Gap. Fig. 27. Stylobreccia features and gradations: A—Strong set of conjugate stylolites cutting across stylobreccia. B-D—Gradation from stylomottled rock to stylobreccia: isolation of stylomottles by pressuresolution surfaces. Compare the incipient stylobreccia of C with stylomottled rock illustrated in Figure 22D.

PRESSURE SOLUTION AND SHEAR FRACTURE

FIG. 16

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

B FIG. 23

PRESSURE SOLUTION AND SHEAR FRACTURE

FIG. 24

42 METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 25

PRESSURE SOLUTION AND SHEAR FRACTURE

FIG. 26

44 METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 27

PRESSURE SOLUTION A N D SHEAR

Least Soluble: skeletons of calcareous algae— Renalcis, Girvanella and Sphaerocodium; crinoid plates and ossicles; equant and columnar calcite crystals. Differential pressure solubility has important implications in the composition of pressuresolved carbonate rocks because it implies more rapid or selective removal of more soluble components. TABLE

Order of Pressure Most soluble

Least soluble

Solubility

(after Trurnit,

1968b)

1. halite and potassium salts 2. calcite 3. dolomite 4. anhyrite 5. gypsum 6. amphibole and pyroxene 7. chert 8. quartzite 9. quartz, glauconite, rutile, hematite 10. felspars, cassiterite 11. micas, clay minerals 12. arsenopyrite 13. tourmaline, sphene 14. pyrite 15. zircon 16. chromite

Relative pressure solubility between idens of carbonate rock (as in breccia) is far more difficult to determine because solution occurs at circumidenic stylolites and also may occur to a lesser extent at intraidenic stylolites. These intraidenic stylolites are unlikely to be contemporaneous with the circumidenic stylolites. Pressure solubility of rock idens relates to the solubility order of components given above, and differences in textures and fabrics. It is possible to generalize: sedimentary grainstone and packstone idens are most pressure soluble while dolomitic rock and stylolaminite are least soluble. Between these extremes, in a general order of overlapping stages are: (1) grainstone, packstone, (2) packstone, wackestone, mudstone, (3) mudstone, cryptalgal laminite with fenestrae; fenestral limestone, (4) fenestral limestone, skeletal boundstone (tabular stromatoporoid, Renalcis-Girvanella, stromatoporoid-Renalcis), (5) vugular limestone, veined limestone, calcite cemented breccia, lacy-calcite structures, (6) dolomite, dolomitic stylolaminite, (7) quartzose stylolaminite. This order has important implications in pressure-solved carbonate sequences and polymict carbonate breccias because it implies selective removal of some lithotypes and persistence of

FRACTURE

others as stylocumulate residue. This order of pressure solubility applies to carbonate rocks in the Devonian sequence, Canning Basin margin; other sequences in different settings may reveal different orders. Stages in pressure solution and volume reduction are thus represented by a range from rocks with recognizable depositional fabrics and textures through rocks with pressuresoluble components organized in fabrics and textures imprinted by the pressure-solution process, to rocks that are dominantly composed of residues (stylocumulate and reactate) low in the pressure-solubility order (Table V I ) . The quantity and organization of these residual components, related to the variable arrangement of pressure-solution surfaces is the basis for formation of new structures (stylobedding, stylolamination, stylomottles and stylonodules) and fabrics (condensed, fitted, stylocumulateand reactate-support). Analysis of Pressure-Solution Features; Stress Compressive stress produces strain features in carbonate rocks; however, in these rocks under appropriate T and chemical conditions the earliest and probably dominant response is pressure solution. Stress modes for pressure solution must extend from simple lithostatic stress produced by overburden pressure, through normal stress (no rotation) to shear stress (with rotational strain), common in tectonic deformation. Analyses of stylolite types and related pressure-solution phenomena in terms of stress modes can therefore be attempted. In rocks, internal adjustments to pressure solution are complex because of textural variability and differential solubility. Thus the genesis of pressure-solution structures and fabrics is best examined in terms of idealized models. The simplest models are based on spherical idens of equal radius and pressure solubility in supporting frameworks with either cubic or hexagonal (rhombohedral) packing modes (Graton & Fraser, 1935) (Fig. 33). A response model based on rhombohedral packing is of special interest since this packing is approximated in clastic-textured rocks (Pettijohn, 1957); furthermore, it is the likely packing mode first assumed by an iden framework under shear stress where there is no rotational strain. Other variable specifications, described below, are based on variations in stress application, heterogeneous idens with insoluble compounds and reactate development at pressure-solution interfaces.

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

Model 1, Cubic Packing. Under conditions of uniform lithostatic stress with a 1 = a 2 = o-3, pressure solution starts at contact points in the framework and stylolites develop at these loci in a three-dimensional interidenic array (Fig. 34). Contact points become contact surfaces as idens dissolve. At an intermediate volume reduction stage (Fig. 34B) the idens are cubes with rounded corners, remnant after the original sphere, and void space defined by these surfaces lies between 0 and 47% (void volume in original cubic packing). This is the simplest form of condensed fabric. Continued diminution of idens along stylolites results ultimately in an array of cubic idens bounded by stylolites, and no voids (Fig. 34C). This is the simplest fitted fabric. Transition from condensed to fitted fabric can be taken as the stage when vestiges of original curvature are lost. In the cubic iden-support framework under conditions of unequal triaxial compressive stress, o-i ><j2 there are differential solution rates with highest rate localised at the interidenic stylolites normal to the maximum stress direction. This results in diminution of spheres to prism idens with long axes normal

to the maximum stress direction (Fig. 35). Intermediate condensed fabric stage is here characterized by prismatic idens with rounded corners remnant after the sphere (Fig. 35B); fitted fabric is characterized by prisms bounded by stylolites and zero void space (Fig. 35C, D). Stylocumulate-support fabric develops when the dissolving idens are heterogeneous and contain relatively insoluble compounds. These insolubles accumulate at pressure-solution interfaces as bulk volume is reduced and form 'walls' against which idens continue to dissolve. Stylocumulate 'walls' thicken as idens diminish and remnant idens then 'float' in stylocumulate ground (Figs 11, 12). Reactate-support fabric is developed in a similar way to stylocumulatesupport but growth of new minerals is involved as an adjustment to PT and geochemical conditions (Fig. 11). Under unequal stress conditions ((ri>o- 2 >o- 3 ), formation of stylocumulate and reactate seams and strong interidenic stylolite sets develops a stylolamination oriented normal to the maximum stress direction (Fig. 35D). Model 2, Rhombohedral Packing, Normal Stress. Rhombohedral packing is characterized by stability and the smallest volume of possible

Fig. 28. Gradations; stylomottled rock to stylobreccia: A—Stylomottled rock with strongly developed interidenic stylolites grade into B. B—Incipient stylobreccia formed by pressure-solution isolation both of mottles and lensoid idens of host rock, grades into C. C-D—Stylobreccia: idens are of host limestone and stylomottled rock. These slabs are serial sections separated by 5 mm and illustrate the variety of iden shapes and irregularity in 3-dimensions. Fig. 29. Gradations; stylobreccia to stylolaminite: A—Megabreccia fragments grading into dolomitic stylolaminite. Napier Formation, Sheep Camp Yard, Geikie Range. B—Stylobreccia underlain by quartzose stylolaminite; breccia idens float in a quartzose ground which grades down into stylolaminite. Napier Formation, Dingo Gap area. C—Stylobreccia grading into dolomitic stylolaminite. Breccia idens in (1) are in fitted fabric with dolomite in circumidenic stylolites; this grades into a reactate-support fabric (2) which grades into stylolaminite (3). Napier Formation, Sheep Camp Yard, Geikie Range. Fig. 30. Stylobreccia origin: gradations from stylobedded limestone through to stylobreccia by interpenetrating and anastomosing stylolites. A—Stylobedded limestone; minor interpenetration of stylolites, Napier Formation, east of Wire Spring. B—Stylobedded limestone, Napier Formation, Wire Spring area (scale is 10 cm long); development of incipient stylobreccia by interpenetration of stylolites across tabular limestone idens, grades into C. C—Stylobreccia, Napier Formation, Wire Spring area; extreme isolation of once continuous tabular stylobeds; more continuous stylobeds are in varying degrees of fragmentation. Note conjugate stylolites (arrow) that cut across tabular idens. Fig. 31. Stylobreccia origin: gradation of stylobeds into stylobreccia observed over a vertical distance of 2 m. Napier Formation, Wire Spring area. A—Stylobedded limestone grades within 1 m into B. B—Limestone with conjugate stylolite sets; some rhomboidal limestone idens bound by circumidenic stylolites are slightly rotated; grades within 1 m into C. C—Concordant stylobreccia; individual fragments are bound by circumidenic stylolites and are rotated up to 45° from original positions; fragments are tabular, rhomboidal to equidimensional.

PRESSURE SOLUTION AND SHEAR FRACTURE

FIG. 28

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 29

PRESSURE SOLUTION AND SHEAR FRACTURE

FIG. 30

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 31

PRESSURE SOLUTION AND SHEAR FRACTURE void space, approximately 26 per cent. Under uniform lithostatic stress, o^ = <r2 = (Fig. 36), pressure-solution starts at contact points in the framework; there are 12 contacts per iden and with the development of interidenic stylolites, idens tend to become rhombdodecahedral in form (Fig. 36B). With triaxial compressive stress, o-1>o-2^>o-3, there are differential rates of pressure solution in the iden array with highest rates localized at pressure-solution surfaces most nearly normal to the maximum stress direction; pressuresolution surfaces approximately normal to intermediate and minimum stress directions have weaker development, respectively. Idens reduced through distorted rhombdodecahedral form towards a monoclinic polyhedron because of differential solubility rates along the intersecting surfaces (Figs 36C, D). Iden shapes developed are simulated in rocks by conjugate sets of interidenic stylolites—compare Figures 36C, D with Figure 31. Formation of stylocumulate and reactate seams and a strong interidenic stylolite set develops stylolamination oriented normal to the maximum compressive stress direction. Model 3, Rhombohedral Packing, Shear Stress, No Rotational Movement. Under the specifications of this model (Figs 37A, 38A), conjugate stylolite sets develop, one set at 30 to 40° to the direction of stress application, and another set normal to it (Fig. 38A). Pressure solution along these sets results in the formation of a layered structure in which step-like interidenic stylolites define the boundaries of tabular idens (Fig. 37B, C). The layered structure equivalent to stylobedding or stylolamination is inclined at about 60° to the direction of stress application. There is also potential for opening of tension fractures across the tabular iden layers (thin dashes, Fig. 37B) but these fractures will open only if gliding occurs along the plane of the tabular idens (Fig. 37B). Model 4, Rhombohedral Packing, Dominantly Shear Stress, Intermittent Movement. Development of stylolamination (or stylobedding) as an intermediate stage of volume reduction, inclined in the direction of shear as outlined in model 3 above, leads on to potential for movement or gliding along the pressure-solution interfaces. Gliding potential is probably enhanced when layers of fifie stylocumulate or reactate (silt, clay, dolomite) are deposited along the interfaces and would probably occur with momentary increases in rate of stress application. With strongly stepped pressure-solution

surfaces (Fig. 37B), movement resolves into shear along a set, normal to the x-z plane (thick lines, Fig. 37B) and tension at the conjugate set (thin lines, Fig. 37B). The conjugate stylolites then tend to part as rhombohedral tension fractures. However, stress is concentrated at step-juncion loci and hese junctions will: (1) be obliterated by pressure-solution, (2) fracture into isolated fragments, and (3) localize mottles of stylocumulate and/or reactate. Similarly, tensional stress at lower step junctions localises voids and/or areas of relatively low stress characterized by growth of calcite crystals and other reactates. Rotational gliding along inclined layers generates drag folds and fractures in stylocumulate and reactate. Local retardation (on stylolite steps) in gliding layers results in growth* of stylolites perpendicular to the main pressure-solution-glide surfaces (Fig. 37B). Fracture of inclined iden layers is consequent on rapid movement and/or changes in stress conditions; it is followed by rotation and formation of breccia in which fragments (rotated idens) are bounded by circumidenic stylolites. Model 4 conditions probably apply in rocks proximal to high angle reverse faults and along bedding surfaces in folds. Model 5, Rhombohedral Packing, Dominantly Shear Stress, with Movement of Idens. Model 5 specifications are dominantly shear stress parallel to iden layers and involve interlayer adjustment from the onset of stress application by finite glide between layers, i.e. layers behave as idens (Fig. 39). This leads to volume loss by pressure solution plus distortion of the 'unit cell' as shown in Figures 39B, C. Strong interidenic stylolites developed at 30 to 40° to the direction of stress application and there is potential for tensional partings at 60° to the layer plane (Fig. 38B). Under conditions of interlayer glide this potential probably will not be realized because stress is concentrated at step junctions and these will either be obliterated by pressure solution and fracture, or will localise mottles of stylocumulate and reactate, as in model 4 above. Glide along layers with local retardation results in local growth of stylolites, perpendicular to the main glide layers. Formation of stylocumulate and reactate occurs at the glide-pressure-solution surfaces; this may be in discontinuous patches (mottles), irregular elongate patches (equivalent to stylonodular structure) or continuous sheets (equivalent to stylobedding or lamination). Momen-

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

STYLONODULAR

STYLOBEDDED

c=C>

STYLOBEDDED

STYLOLAMINATED

STYLOMOTTLED

STYLOBRECCIOID

A Fig. 32. Gradational relationships between various rock types generated by pressure solution and shear fracture. tary increases in stress may exceed the capacity UNIT CELL UNIT CELL of the rock to deform by pressure solution, RHOMBOHEDRAL PACKING CUBIC PACKING strain becomes dominant and is followed by fracture (or folding). Fracture and rotation of fracture-idens is the basic process in formation of breccia in which fragments (rotated idens) are bounded by circumidenic stylolites. Model 5 conditions apply generally in proximity to reverse faults, thrusts and other fracture surfaces with a dominant shear-stress component. General Conditions and Stylolitic Structures Interidenic stylolite sets are a response to compressive stress and they develop with median planes normal to the maximum principal compressive stress direction (Fig. 31A). Rocks are usually heterogeneous so that there is resolution along discontinuities, this leads to formation of weakly developed stylolites at various angles to the predominant set. Stress also resolves around bodies such as fossils to form weak circumidenic stylolites. cells with spherical idens showing Conjugate sets of interidenic stylolites, Fig. 33. Unit cubic and rhombohedral packing (after bounding rhomboidal idens are formed in Graton and Fraser, 1935).

PRESSURE SOLUTION AND SHEAR FRACTURE

(

IDEN-SUPPORT FABRIC CUBIC PACKING

A •

\ /

•

IDEN-SUPPORT FABRIC CUBIC PACKING

•

v yV y

roro

CONDENSED FABRIC

•

FITTED FABRIC

Fig. 34. Development of condensed and fitted fabric by pressure solution under uniform lithostatic stress = <r = <r. Spherical idens are in cubic packing. Unit cell is shown in A. response to shear stress under conditions where there has been no movement or rotation. One interidenic set commonly predominates with more weakly developed sets at angles to this. Rhomboidal idens bounded by the sets are not disturbed from position in relation to neighbouring idens (Fig. 3IB). Circumidenic stylolites also develop, being localised at discontinuities such as boundaries of large fossil fragments, but these are weakly developed in comparison to the interidenic stylolites. Yield point conditions with no rotational movement are transitional into shear accompanied by rotational movement. Once the yield point is exceeded, the overall stress distribution becomes markedly heterogeneous and circumidenic stylolites are generated; these are localized at iden boundaries and interconnect through irregular hummocky to smooth interidenic stylolites. Movement occurs along stylolites bedding planes and other discontinuities and idens are rotated from their original positions in relation to other idens (Fig. 31C). Stylolite surfaces become shear fractures, and alternatively, shear-fracture surfaces become p r e s s u r e - s o l u t i o n interfaces with continued stress. Intrastratal folding, through bedding2

i : i : i

FITTED FABRIC

Fig. 35. Development of condensed and fitted fabric by pressure solution under lithostatic stress >02>03. Application of is vertical. Spherical idens are in cubic packing. Final diagram (D) also illustrates development of stylolaminar structure. Unit cell is shown in A. plane slip and drag along fractures also occurs and these folds are later modified by pressure solution. Interidenic stylolite sets are folded and dislocated. Origin of Stylolaminites Stylolaminites are penultimate products of pressure solution and, as shown in the discussion of stylolites and stress modes, they can form under normal or shear stress. Stylolamination is a reflection of parallel stylolite sets, continued volume reduction along these surfaces and formation of stylocumulate or reactate. The range of stylolaminite types reflects mainly the composition of the precursor rocks. Quartzose Stylolaminite. This rock type originates from parent limestones that contain sand-size detrital quartz grains. The percentage of detrital quartz and mode of occurrence in limestones is so highly variable that it is necessary to limit the discussion of petrogenesis to two general cases: (1) parent grainstone with detrital quartz dispersed in a supporting frame-

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

Fig. 36. A—Unit cell, rhombohedral packing. B—Fitted fabric developed by uniform lithostatic stress = 0*2 = a sC-D—Fitted fabric developed by triaxial compressive stress 0i><r2><r3; application of <f\ is vertical; note idens are reduced to distorted polyhedra. The principal pressure-solution surfaces are heavy lines, Ax and Ay indicate increments of volume reduction along x and y axes. work of carbonate grains, and (2) detrital quartz layers (sedimentary sandstone or stylocumulate) interlayered with relatively pure limestone. In case 1, quartz grains accumulate along stylolite sets or come into contact through pressure solution of intervening calcitic grains and cement (Fig. 18 B). At an intermediate stage, quartz grains embay adjacent calcitic idens as a result of differential pressure solubility but the residual quartz grains eventually form a supporting framework with quartz to quartz contacts and corroded calcitic idens in interframe space (Fig. 18C); the packing mode for the quartz framework is likely to be approximately rhombohedral as the state of minimum interframe volume, about 26% is equivalent to the residual carbonate.

Fig. 37. A—Unit cell, rhombohedral packing under condition of shear stress applied parallel to x-axis; no movement of idens; development of pressure-solution interfaces is illustrated in Figure 3 8A. B—Development of stylolaminar structure inclined to direction of shear, and development of fitted fabric. Heavy lines between idens are the main pressuresolution interfaces; dashed lines are potential tensional openings. Ax and Ay indicate increments of volume reduction along x and y axes. C—Final result with continuing pressure solution is stylolaminar structure oriented approximately normal to the stress direction. The quartz framework is resistant in the field of carbonate-pressure-solution and the process of volume reduction in this field is here terminated. However if pressure solution of quartz takes place, it leads on to development of sutured contacts, disappearance of relict intergranular calcite and finally a fitted fabric with only quartz, i.e. a typical quartzite. The response to compression of an interlaminated limestone-sandstone sequence (case 2) is complex. If sandstone layers contain calcitic grains and limestone layers contain quartz, the internal response follows the pattern of case 1, above. Eventually, the rock is reduced

PRESSURE SOLUTION AND SHEAR FRACTURE

Fig. 38. A—Detail of stress resolution within unit cell of Figure 37 where idens are resistant to movement. The principal pressure-solution surfaces at iden contacts are marked. B—Detail of stress resolution within unit cell of Figure 39 where horizontal layers of idens glide over each other under shear stress conditions. The principal surface of pressure solution inclined to the layer is outlined by a heavy line at iden contacts; a potential tensional parting outlined by thin double line. to interlayered stable quartz (stylocumulate) and limestone layers with stylolite sets at interlaminar boundaries. From this stage, pressure solution continues to remove limestone, and quartz layers begin to come into juxtaposition. An intermediate stage is when limestone layers are truncated by cross-cutting stylolites to form residual discrete carbonate idens in a quartz ground (Fig. 17A). Quartzose-Micaceous Stylolaminite. This rock type develops from parent carbonates that contain clay, mica and silt-size particles of quartz. These relatively insoluble minerals may have been dispersed and/or in distinct layers in the precursor; commonly, the source of stylocumulate is carbonate lithoclast grains. The percentage of insolubles in the parent is indeterminate (Appendix I) because of the unknown volume loss involved. Stylonodular rocks with quartzose micaceous idens are an intermediate stage of volume-reduction where sheets of stylocumulate are interlayered with irregular sheets and layers of discrete limestone idens in various stages of corrosion (Fig. 19). Calcareous Stylolaminite. This rock type is formed by growth of subparallel stylolite sets in relatively pure limestone, e.g. massive evengrained lime mudstone and wackestone of the Pillara Formation. Stylolite sets impart a strong lamination of alternate stylolites and tabular to

lensoid idens of the parent rock and stylocumulate layers (Fig. 16). Because of the lack of insoluble minerals in the parent there is very little stylocumulate development along the interfaces; stylocumulate is usually distinguished by small quantities of Fe-oxide and clay minerals and corroded grains from the host (Fig. 16B). Reduction of the tabular parent-rock idens leads to confluence of stylolites into stacked sets and perpetuation of hummocks and other irregularities in the surfaces. The irregular laminae simulate certain types of cryptalgal lamination. Dolomitic Stylolaminite. This stylolaminite is dominated by sheets of reactate and stylocumulate dolomite. It formed in dolomitic rocks as stylocumulate sheets in a manner similar to that described for quartzose stylolaminites; and in limestone as reactate sheets. Origin of dolomitic stylolaminite, however, is dealt with in greater detail on p. 67. Origin of Stylonodular Rocks Stylonodular rocks are the intermediate rock types as stylobedded limestone is reduced to stylolaminite, representing the stage where nodular and lensoid remnant idens of host limestone are separated by sheets and lenses of stylolaminate (Fig. 19). These remnatfts of host rock may have been: (1) relatively insoluble parts of tabular limestone idens, or (2)

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

^^-L \ \

-AV=* \

\ \

Fig. 39. A—Unit cell, rhombohedral packing under conditions of shear stress applied parallel to x-axis. Idens move and glide from the onset of stress application. Resolution of stress within unit cell is illustrated in Figure 38B. B-C—Development of stylolaminar structure parallel to direction of stress application, and development of fitted fabric. Heavy lines are the main pressure-solution interfaces. Double lines are potential tensional openings. Fine dashed lines are potential conjugate stylolites that develop if a gliding layer is locally retarded; Ax and Ay indicate increments of volume reduction along x and y axes. thicker portions of tabular limestone idens produced by hummocky and irregular stylolites. Continued pressure solution along the thin extremities of limestone idens, eventually eliminates and reduces them to stylocumulate (stylolaminite). In some cases, discrete remnants of host remain as isolated stylolite-bound nodules (which may resemble pebbles) in a stylolaminite ground (Fig. 19A). Stylolaminae commonly bow under and over these isolated nodular bodies. Origin of Stylomottled Rocks Stylomottled rocks are pressure-solution products where stylocumulate and reactate form

isolated idens in the host rock. The genesis of stylocumulate and reactate is similar to that outlined for stylocumulate and reactate in stylolaminite sheets and other pressure-solution fabrics. Composition of the stylomottle-idens depends on the type of insoluble minerals and potential reactate in the host. Quartz-bearing limestones have quartzose stylocumulate mottles. Rocks in which reactate dolomite was growing during pressure solution have reactate and stylocumulate (dolomitic) stylomottles. Dolomite rocks yield stylomottles composed of Fe-stained dolomite aggregates. Stylomottle idens composed of carbonate grains in fitted and condensed fabrics form in pure limestones. Two mechanisms are envisaged to have formed stylomottles: (1) uniform lithostatic compression, and (2) triaxial compression. In the first case, isolated portions of the rock more susceptible to pressure-solution are dissolved, forming isolated stylocumulate and reactate patches. The overall geometry of the body is maintained by continual internal adjustments; if one susceptible part of the limestone is undergoing rapid pressure solution, internal stresses are set up that caused another susceptible portion of the rock mass to dissolve and equilibrate the overall rate of pressure solution. This results in stylomottles that are in all stages of development. The second case involves compression with a component of shear stress leading to small finite movements along interidenic stylolites (e.g. models 4 and 5, p. 51). Irregular, stepped and hummocky stylolites have the potential for development of stylomottle idens if movement is initiated along the stylolite surface (Figs 37, 39). Stepped surfaces or irregularities along interidenic stylolites that face the direction of principal resolved stress either fracture or pressure solve; insoluble minerals in the host therefore accumulate at the irregularity or stepped junction, since this is an area of localised stress. Formation of dolomite-reactate also is possible at this locus. The result is irregularly distributed patches of stylocumulate and/ or reactate along smooth, hummocky and irregular stylolites. The process has gone to completion where irregularities and stepped surfaces have been obliterated and interidenic stylolites have become smooth. In grainstone hosts, areas adjacent to mottles exhibit growth of coarse equant calcite at the expense of both polycrystalline particles (ooids, pellets, lithoclasts) and unfavourably oriented calcite crystals. The equant calcite crystals are large enough to form a reactate-

PRESSURE SOLUTION AND SHEAR FRACTURE support frame in which sedimentary particles 'float'. Reactate-calcite crystal boundaries also acted as pressure-solution interfaces, evidenced by: (1) solution and penetration of sedimentary grains by these crystals, (2) stylolitic contact of crystals and stylomottles, and (3) passing of stylolites into boundaries of calcite aggregates. It is probable that while susceptible parts of the host rock were developing stylomottles and interidenic stylolites, coarse calcite-reactate was growing, founded on crinoid grains and favourably oriented calcite crystals. Solute from pressure solution of the host was transferred to these growing crystals. Origins of Stylobreccias Stylobreccias are formed by several intergradational mechanisms; these include: (1) internal adjustments due to differential pressure solution, (2) internal adjustments by pressure solution followed by fracture, and (3) fracture coupled with, and followed by, pressure solution. Internal Adjustments by Differential Pressure Solution. Formation of stylobreccia by differential pressure solution commences with: (a) isolation of tabular to lensoid parent-rock idens by intersection of undulating interidenic stylolite sets across stylobeds and laminae; the structure is stylonodular to stylobreccioid, or (b) isolation of polygonal to rounded parentrock idens by formation of a 3-dimensional array of conjugate stylolite sets (models 1 and 2, p. 46) or anastomosing circumidenic stylolites; the structure is stylobreccioid. At this stage the parent rock-idens show no evidence of rotation. Further compression leads on to: (c) selective pressure solution of susceptible parent-rock idens and development of isolated stylocumulate or reactate idens remnant after these; the structure is stylomottled, superimposed on stylobreccioid. Loss of idens involves internal adjustments and rotation of remaining parent-rock and stylocumulate idens in the stress field. The rock is a stylobreccia characterized by parent-rock idens, idens of stylocumulate and/or reactate. Internal Adjustments followed by Fracture. Development of stylobreccia by these mechanisms follows essentially the same sequence as above but at a late stage the rate of stress application exceeds the capacity of the material *

to respond by pressure solution alone; shear fracture occurs along stylolites and other surfaces of weakness and there is rotation of idens. The product is a stylobreccia characterized by idens of parent rock, stylocumulate and reactate, plus idens of stylomottled rock, stylobreccia and stylonodular rock. Pressure solution, after shear fracture, continues to reduce idens to stylocumulate or reactate. Fracture and Pressure Solution. Formation of stylobreccia by fracture with subordinate pressure solution occurs under compressive stress when, at an early stage, the rate of stress application exceeds the capacity of the material to respond by pressure solution alone. The first stage is development of stylobedding or lamination or conjugate sets as stress is applied (Figs 31 A, B) but this is followed by development of shear fractures which, in some cases, have been observed to be parallel with or at a relatively low angle (10 to 30°) to the stylobedding. Movement along the shears results either in drag folding or fracture of those stylobeds truncated by shear surfaces (Fig. 31C). Stylobed and stylolaminite layers are detached and incorporated into a breccia body along this surface. Pressure solution continues after movement, reducing stylobreccia fragments and modifying drag folds and shear fractures. The stylobreccia contains parentrock idens, stylocumulate and reactate idens, plus idens of stylolaminite, stylomottled and stylonodular rocks, and stylobreccia. Fate of the Solute Pressure solution results in release of solute Ca++ and HC0 3 ~ to intrastratal fluids which become charged to saturation or supersaturation. Weyl (1959), in a theoretical model for pressure solution, predicts precipitation from these fluids and overgrowths on existing calcite idens in the direction of least stress. Weyl's model is based, however, on specifications of: (1) normal stress, (2) absence of other reacting ions in intrastratal fluids, and (3) hydrostatic conditions. The first two (1, 2) are approximated in nature but hydrostasis is improbable under compressive stress, since fluid migration is impelled by reduction of pore space.* Fluids migrate to loci of relative low compressive stress or negative (tensional) stress. Mostly such loci will be in rocks distant from the pressure-solving source rocks. Precipitation will occur in tensional fractures, inter-

. . flow can occur completely outward . . . merely because the reference space itself is contracting" (in our case, the dissolving rock) . . . "while the volume of water remains constant" (Hubbert, 1953, p. 1969).

M E T A M O R P H I S M O F C A R B O N A T E ROCKS, C A N N I N G

granular and intragranular voids and solution voids. Loci of relative low stress may occur also in the solid medium of a pressure-solving host rock either as a result of resolution of stress into tensional components (models 3, 4, 5 on p. 51), or temporal variations in stress mode. It is here that precipitation occurs as overgrowths on existing calcite-crystal idens. DOLOMITIZATION ROCK TYPES

The dolomitic rock suite is complex and the main basis for classification is structure: fenestral, massive, stylolaminar, stylobreccioid, stylomottled and stylonodular. Some dolomitic rocks are further characterized on whether they are pure dolomite, or whether dolomite forms a significant proportion of the rock along with calcareous idens. The following rock types are recognized (Table VII): In all dolomitic rocks, dolomite occurs in equigranular, medium to coarsely, crystalline aggregates that are buff, brown, red or yellowbrown. Crystals are euhedral to anhedral, clear, and some are rimmed by or contain zones of Fe-oxides. Staining techniques (Dickson, 1965) and X-ray diffractometry show the dolomite to be mainly non-ferroan; crystals show undulatory extinction but deformation twins are uncommon. Dolomite is euhedral when in contact with calcite; the latter occurs in re-entrants of dolomite aggregates or supports dolomite euhedra. Quartz and dolomite are commonly in stylolitic contact. Dolomite aggregates that are internally stylolitic are composed of anhedral dolomite and have Fe-oxide localized along crystal boundaries. FenestraI Dolomites Fenestral dolomites have cryptalgal lamination and fine laminoid, irregular and tubular fenestral fabrics (Logan, 1974; Read, 1973a, b); also scattered through the dolomite are mollusk molds and irregular solution vugs, floored with internal sediment (cf. Read, 1973a, b). Most fractures, solution vugs and skeletal moulds are

BASIN

filled with coarse equant calcite. Lamination is due to colour differentiation. All components such as pellets, lime mud, skeletal debris and intergranular sparry calcite of the precursor limestone are replaced by equigranular dolomite. Transitional rocks are dolomitic fenestral limestone with scattered irregular patches, stringers and sheets of dolomite a few centimetres to tens of centimetres in size; sedimentary structures of the host persist into these dolomite patches. Occurrence. Fenestral dolomite and transitional rocks occur in extensive sheets concordant with stylobedding in the Pillara Formation, proximal to major faults. These dolomites are thickly stylobedded but the rocks within the stylobed have undergone little pressure solution. Stylolites are rare and the geometry of original sedimentary fabric is unaltered. Massive Dolomites Massive dolomites mostly are structureless; they are composed of equigranular anhedral dolomite crystals and there are minor occurrences of detrital quartz and calcite crystals. A variety of rock types are transitional into massive dolomite and gradations can be traced from undolomitized precursors through carbonate host-rocks with irregular patches, stringers and sheets of dolomite to massive dolomites with palimpsest sedimentary and metamorphic features. Some examples of sedimentary precursors are Stachyodes packstone (Pillara Formation) and Renalcis-Girvanella and stromatoporoidRenalcis boundstone (Napier Formation). Massive dolomite after Stachyodes packstone contains coarse equant calcite casts after Stachyodes in dolomite ground while boundstones are represented by thin encrusting skeletal layers. The geometry and distribution of skeletal casts in the dolomitic rocks are directly comparable with depositional fabrics in precursor limestones. Dolomitic stylolaminites, stylomottled and stylonodular rocks and dolomitic stylobreccia

TABLE VII Dolomitic Rock Types Structure Fenestral Massive Stylolaminar Stylonodular Stylomottled Stylobreccioid

Calcareous Idens +

Dolomite

(rare) (rare) Dolomitic stylolaminite Dolomitic stylonodular Dolomitic stylomottled limestone Dolomitic-calcareous stylobreccia

Pure Dolomite Fenestral dolomite Massive dolomite Dolomitic stylolaminite (rare) Stylomottled dolomite Dolomitic stylobreccia

59 DOLOMITIZATION also grade into massive dolomite by progressive stylolites. Dolomite is restricted to areas along and near stylolites and is absent in the limeloss of calcitic idens and stylolitic structures. Occurrence. Massive dolomite with Stachyodes stone idens. Limestone idens commonly are casts occurs as stylobedded units interlayered grainstone with interstitial fine grained sparry with fenestral dolomite, concordant with Pillara calcite or coarse equant reactate calcite. Occurrence. Stylolaminated dolomite stylobeds stylobedding, in proximity to major faults. to 60 m in thickness occur in the lower Massive dolomite with palimpsest features up Napier Formation. Massive structureless doloof skeletal boundstone occurs in large pods in interlayered calcareous idens occurs in the dolomitic megabreccia in lower parts of the lower to middle Formation where doloNapier Formation. Massive structureless dolomite occurs in aNapier sheetlike concordant body up mite occurs in a sheetlike concordant body up dolomitic stylolaminites are interlayered with to 30 m thick, at the base of the Napier Forma- sheets of quartzose stylolaminite and limestone. tion. In the abutment zone, massive dolomite stylobeds 30 to 60 cm thick, are inter- Dolomitic Stylobreccias layered with dolomitic stylobreccia (composed Two types of dolomitic stylobreccia are of Pillara limestone fragments and skeletons). recognized: (1) breccia with calcareous fragMassive dolomite also occurs locally in cores ments, and (2) breccia with dolomite fragof anticlines; here, its distribution is irregular. ments. Massive dolomite occurs along faults and Dolomitic-Calcareous Stylobreccia. This is fractures both in the Pillara Formation and the stylobreccia ranging up to megabreccia, comhomocline sequence near the abutment zone posed of idens of limestone, fossils and meta(Fig. 40). It forms dyke-like bodies 30 cm morphosed carbonate rock; dolomite occurs as to 1 m thick along fault planes and in fault interidenic (reactate) material (Fig. 42). Idens zones (Fig. 40B). Massive dolomite occurs as are mainly in fitted to condensed fabric; dolothin sheets and patches (up to several centi- mite is restricted to thin seams around breccia metres thick) along fracture systems (Fig. idens (Fig. 42A). These breccias grade into 40C); these fractures are concordant to dis- rocks with reactate- and stylocumulate-support fabrics, where calcareous idens are supported cordant to host-rock stylobedding. by a dolomite ground (Figs 42B, C) and finally Dolomitic Stylolaminites Dolomite commonly occurs as stylobeds that into massive dolomite, where all calcareous are internally stylolaminated (Fig. 41). Closely idens have been pressure-solved or replaced by spaced interidenic stylolites impart lamination dolomite. Thick dolomitic calcareous stylobreccia and to the rock and the strength of lamination depends mainly on the degree of stylolite megabreccia occurs: (1) in Pillara Formation near the abutment, (2) as concordant bodies development. Stylolaminae are planar parallel sets of low amplitude, hummocky stylolites; in lower parts of Napier Formation, and (3) in crests and troughs of hummocky stylolites tend the abutment zone. In the Pillara Formation, to be superimposed, thereby forming domed dolomitic stylobreccia occurs in a narrow zone structures and micro-unconformities reminis- up to 15 m wide immediately adjacent to the abutment. In lower parts of the Napier Formacent of cryptalgal structures and lamination tion and in the abutment zone, dolomitic stylo(Fig. 41B). The rock consists mainly of an- breccia and megabreccia are interlayered with hedral dolomite and minor quartz in stylolitic of massive and stylolaminated dolocontact. There is little preservation of pre- stylobeds mite. Contacts vary from sharp cursor rock features except for minor brachio- and discordantwithto interlayers gradational. Stylobreccia pods oriented parallel to stylobedding, and rare units range in thickness from 15 to 60 cm and stylolite-bound stromatoporoids. Dolomitic stylolaminite also is composed of megabreccia units are up to 30 m thick. Dolomitic Stylobreccia. This is stylobreccia thin stylolaminated sheets of dolomite a fraction of a millimetre to a few centimetres thick, composed entirely of dolomite idens. Idens are that are interlayered with sheets and lenses of stylolaminated to massive and they are mostly corroded limestone idens up to 1 cm thick pebble-sized, though there is a range from (Fig. 41 A). This type of dolomitic stylo- granule- to cobble-size. Idens are in fitted fablaminite often grades into stylolaminite com- ric and locally there is development of styloposed entirely of dolomite (as described above) cumulate-support fabric where larger dolomite and into limestone. The dolomite sheets are idens 'float' in coarse ground of dolomite fragseparated from limestone idens by interidenic ments and crystals.

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

Dolomitic stylobreccia commonly occurs as sheetlike to lensoid bodies up to 3 m thick in fault zones that truncate massive dolomite and stylolaminite units (Fig. 43). Stylobreccia units have sharp contacts with surrounding rocks but containing strata are often drag-folded against the contact. Dolomitic Stylomottled Rocks Two types of dolomitic stylomottled rocks are recognized: (1) rock with dolomitic stylomottles in a calcareous host, and (2) stylomottles of dolomite in a dolomite host (stylomottled dolomite). Dolomitic Stylomottled Limestone. Dolomite occurs in stylomottle idens up to 3 cm in diameter in host limestone. Stylomottle idens are aggregates of dolomite with minor calcite, quartz and Fe-oxides that are in stylolitic contact with host rock. The host is commonly grainstone with interstitial to supporting coarse, equant (reactate) calcite. Where mottles are abundant, the rock grades into massive dolomite with scattered remnants of calcareous idens, or grades into stylolaminated dolomite with crude layering. This type of dolomitic stylomottled rock is typical of the Napier Formation where dolomitic rocks grade into calcareous rocks. Stylomottled Dolomite. These are rocks with Fe-oxide stained stylomottle idens of dolomite in a host of dolomite (Fig. 44A). Host rocks are buff, yellow-brown, and brown, composed either mostly of dolomite with minor quartz and calcite, or of calcite-dolomite aggregates. Stylomottle idens are equant, tabular, ellipsoidal to irregular in shape and mostly 2 mm to 2 cm in size. They consist of brown to red aggregates of stylocumulate dolomite, and in

some cases of dolomite and quartz. Dolomite in the stylomottles is finer-grained than in the host; it is anhedral and in stylolitic contact with neighbouring crystals; Fe-oxides are concentrated along crystal boundaries. The anhedral form of dolomite in stylomottle idens contrasts with the euhedral form which commonly occurs in host rocks. Stylomottled dolomites are sheetlike bodies up to 1 m thick, concordant with stylobedded dolomite in the lower Napier Formation. Dolomitic Stylonodular Rocks Dolomitic stylonodular rocks are characterized by lensoid to rounded idens of limestone, 1 to 10 cm in size, embedded in dolomitic stylolaminite (Fig. 60). Limestone idens are sharply bound by stylolites, or in some cases, grade into the stylolaminite. Dolomitic stylonodular rocks occur as lensoid to sheetlike units up to 10 cm thick, interlayered with stylobedded limestone, stylobreccia and dolomitic stylolaminite; they commonly occur as transitional rock types between dolomitic stylolaminites and calcareous rocks in the lower Napier Formation. PETROGENESIS

Review; Origin of Dolomite Numerous massive, bedded and laminated dolomites are reported from ancient carbonate sequences. Literature review reveals that the majority of dolomites occur in: (1) bedded and laminated deposits interlayered with limestone and/or evaporites (Pettijohn, 1957; Braun & Friedman, 1969; Friedman & Sanders, 1967; Laporte, 1967; Matter, 1967), (2) reef cores (Fairbridge, 1957; Schlanger, 1963),

Fig. 40. Dolomite: A—Massive dolomite (arrows) within fractures of skeletal-boundstone pod in megabreccia. Abutment zone, Dingo Gap area. B—Massive dolomite in fault; contacts with host rock outlined; stylobedding of host rocks is arrowed. Dingo Gap area. C—Dolomite in fractures and stylolites (arrows). Sadler Limestone, Lloyd Hill, Bugle Gap area. Fig. 41. Dolomitic stylolaminite: A—Reactate dolomite developing along stylolites in host-rock grainstone (length of scale is 30 cm); note corroded remnant sheets and lenses of host (arrows); grades into dolomitic stylolaminite of B. B—Dolomitic stylolaminite. MacSherrys Gap. Fig. 42. A—Dolomitic stylobreccia with idens in fitted fabric and dolomite in circumidenic stylolites (lower part) grading into reactate-support fabric (which then grades up into stylolaminite, see Figure 29C). Napier Formation, Sheep Camp Yard. B—Stylobreccia with reactate dolomite in interiden spaces. Fabric is condensed. Hammer is arrowed. Napier Formation, MacSherrys Gap area. C—Stylobreccia with reactate-support fabric; calcareous idens reacting along margins forming additional dolomite. Napier Formation, MacSherrys Gap area.

DOLOMITIZATION

FIG. 40

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 41

DOLOMITIZATION

FIG. 42

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

tion in MacSherrys Gap. (3) unconformity surfaces (Fairbridge, 1957; Sonnenfeld, 1964), (4) hypersaline lakes, playas (Alderman & Skinner, 1957; Graf et al, 1961) and supratidal flats (Shinn et al, 1965; Logan, 1974). (5) hydrothermal ores, commonly replacing calcareous wall rock (Hewett, 1928; Meyer & Hemley, 1967; Stanton, 1972), (6) association with large thrusts, faults and fractures (Sonnenfeld, 1964; Ham, 1951; Friedman & Sanders, 1967; Hewett, 1928, 1931; Calvert, 1964), and (7) regionally metamorphosed carbonate rocks (Turner, 1968; Vance, 1968; Bowen, 1940; Goldsmith et al, 1955; Hewitt, 1973). Laboratory synthesis of dolomite was successful only under elevated T and P conditions (for reviews see Siegel, 1961; Friedman & Sanders^ 1967). Dolomite is formed by: (1) hydrothermal reaction between calcite and Mg++ (Rosenberg et al, 1967), (2) hydrothermal alteration of Mg-calcites or Ca-Mg carbonate gels (Graf & Goldsmith, 1955; Land, 1967), and (3) 'dry' solid-state alteration of Mg calcite (Graf & Goldsmith, 1955). These results account for dolomite in hydrothermal associations, in faults and in areas of regional metamorphism. Experimental results suggest that dolomite should form readily after burial and where temperatures are in excess of 100°C (Lippmann, 1973); it is then stable into conditions of greenschist metamorphism (Turner, 1968).

The stability of dolomite in regard to the range of promoting and inhibiting factors (e.g. T, P, p C 0 2 , pH, other ions) has not been fully explored. Uncertainties also exist as regards dolomite solubility, although limited data suggest that dolomite is less soluble than calcite and that its solubility increases with pressure but decreases with temperature. Dolomite is supposedly the stable carbonate phase in the system C a C 0 3 - M g C 0 3 at NTP (Hsu, 1967), but reported Recent dolomite is relatively rare, and these occurrences (Alderman & Skinner, 1957; Shinn et al, 1965; Logan, 1974) should be considered insignificant in terms of genesis of most dolomitic rocks, in view of their limited occurrence and specific lithologic associations. Dolomite and dolomitic rocks form up to 20% of carbonate rock types exposed along the northern Canning Basin. The majority of these rocks occur in the homocline, adjacent to the abutment but they also occur in the Pillara Formation near the abutment and in proximity to faults which cut the Pillara. Dolomitic rocks occur in three basic geological settings: (1) along pressure-solution interfaces, (2) in fault zones, and (3) in compressional zones of anticlines. Transitions from limestone to dolomite and from one type of dolomite rock to another are traceable along strike and in sequence. These transitions provide vital data on dolomite genesis and from them it is possible to postulate three basic modes of dolomite formation: (1) replacement, (2) pressure-solution reactate, and (3) pressure-solution stylocumulate. Replacement Dolomitization Dolomite replaces calcite in a variety of limestone and carbonate-rock precursors and these dolomites retain palimpsest structures, textures and fabrics of the parent rock. Furthermore, gradations are traced from parent rock to massive dolomite, through rocks that contain increasing amounts of equigranular dolomite crystals. In the Pillara Formation, common precursors are pellet and stromatoporoid limestone; these rocks are transformed to fenestral dolomite and massive dolomite (Fig. 45).

Fig. 44. Metamorphic structures in host dolomite: A—Dolomite with stylomottled structure. B—Dolomite with stylolaminar structure. Fig. 45. A—Host-rock Stachyodes limestone from Pillara Formation. Scale in cm. B—Replacement dolomite: Stachyodes as coarse calcite casts (black); overall structure and fabric of rock is retained. C—Reactate dolomite: Stachyodes in fitted and condensed fabric, locally reactate-support fabric; matrix between Stachyodes pressure-solved. Note gradation from Stachyodes-packed rock into dolomitic stylolaminite where there are only a few corroded Stachyodes remnants. Thin sections of this rock are illustrated in Figure 10.

DOLOMITIZATION

FIG. 44

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 15

DOLOMITIZATION Partial dolomitization has been observed in tabular- and subspherical-stromatoporoid limestones; dolomite replaces matrix first and replaces stromatoporoid skeletons along growth lamellae. In the Napier Formation, skeletal boundstone is a common precursor of massive dolomite. Mechanisms of Replacement. The preservation of palimpsest structures and textures in rocks transitional to massive dolomite indicates that replacement dolomitization was probably molecule for molecule, involving small scale solution of calcite and penecontemporaneous precipitation of dolomite. Replacement dolomitization is probably due to the activity of Mg-bearing fluids which originated in adjacent formations where calcite, Mg-calcite and dolomite? were undergoing pressure solution. Alternatively, the fluid may have been introduced from hydrothermal sources. The PT conditions of replacement dolomitization are not known in any quantitative way. The occurrence of 'replacement-dolomites' adjacent to faults suggests high P, and the lack of pressure-solution features in these dolomites leads on to the thesis that T conditions may also have been high, at least high enough to inhibit pressure solution of calcite. It is probable that T was around 100°C to 300°C, but lower than the range where talc would have formed by reaction of dolomite with available Si0 2 . Pressure-Solution Reactate Dolomitization Primary criteria for the formation of dolomite as reactate during pressure solution and the consequent generation of dolomitic rocks with structures and fabrics after pressure solution are the transitions that can be traced from calcitic precursors. In the transitional rocks, dolomite is limited to pressure-solution interfaces and is disposed in the rocks according to the orientation of pressure-solution structures (Fig. 45). Typically, dolomite-reactate is in juxtaposition with corroded calcitic host-rock idens and the fitted and condensed fabrics characteristic of pressure-solved rocks grade into reactate support where calcitic idens are scattered in dolomite ground (Fig. 45C). Dolomitic Stylolaminite. Generation of reactate dolomite along stylolite sets produces a stylolaminar structure with interlayered sheets of laminar dolomite and precursor limestone (Fig. 44). With continued pressure-solution: (1) calcitic sheets dissolve against dolomitic sheets, (2) dolomite continues to form as reactate along pressure-solution surfaces, and (3) earlier formed dolomite becomes stylocumulate. As in-

tervening calcitic idens are lost, dolomite sheets come into superposition along irregular surfaces. The final product is a finely laminated dolomite with numerous pressure-solution laminae. Stylomottled Dolomite. The genesis of stylomottled rock has been discussed in earlier pages, for the general case of calcitic rocks. If dolomite forms as reactate along the irregularly dispersed pressure-solution surfaces then patches or mottles of this material begin to appear. With continuing pressure solution these mottle-idens continue to grow as reactate dolomite is formed at their margins. Dolomitic mottles also remain as pressure-insoluble idens in calcitic host. Mottles therefore begin to coalesce moving towards the formation of massive dolomite with palimpsest stylomottled structure. Dolomitic Stylobreccia. Stylobreccia forms by a combination of pressure solution and shear fracture. Formation of reactate dolomite in the anastomosing circumidenic stylolites leads on to dolomitic stylobreccia in which calcitic breccia idens are bounded by dolomite. The initial fabric is fitted or condensed, but if pressure solution and reactate dolomitization is continued, reactate-support fabric evolves with corroded calcitic idens supported in dolomite ground. Conditions for Pressure-Solution Reactate Dolomitization. The overall conditions for formation of reactate dolomite are compressive stress with pressure solution of calcitic idens in the presence of Mg-bearing solutions. Quantitative pressure limits for pressure solution of calcitic idens in these geochemical conditions are not available from Experimental results. Basic thermodynamics and our observations concerning the geological occurrence of reactate dolomite in pressure-solved rocks, indicate that high stress conditions are necessary. Experimental results indicate that moderate temperate, c. 200°C to 300°C, is necessary for dolomite precipitation from a Ca-Mg solution (Rosenburg et al, 1967) and that in higher ranges (300°C ranging up to 800°C depending on p C 0 2 ) , dolomite thermally decomposes (Graf and Goldsmith, 1955) or yields talc by reaction with available quartz (Metz & Trommsdorff, 1968). Since talc is not formed in association with the reactate dolomite of this study we are able to suggest an upper limit of about 300°C to 400°C. Mechanisms for the formation of reactate dolomite are not clear but it is suggested that Mg-bearing fluids react with calcitic idens

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN 68 across the pressure-solution interface. It is probable that the strained calcite lattice yields Ca to the fluids and dolomite, the more stable phase is precipitated. There may also be replacement across the interface, but the distinction between solution-precipitation and replacement at this level is trivial. Pressure-Solution Stylocumulate DoJomitization Dolomitization can proceed by accumulation of relatively pressure-insoluble dolomite, i.e. as stylocumulate, along pressure-solution surfaces in dolomitic precursor rocks. The precursors range from calcitic rocks with low percentages of dolomite to rocks that are dominantly dolomite. Responses to pressure solution and fracture Fig. 46. Classification of vein structures. in dolomites are similar to those observed in dulating. Contacts with host-carbonate rocks calcitic rocks. Stylolaminar, stylonodular, stylo- are mostly sharp but embayed margins due to mottled and stylobreccioid structure is deve- solution and/or replacement of host rock also loped (Fig. 44) with dolomite idens in fitted are common. Some contacts are marked by and condensed fabrics. host-rock and vein-fill idens in breccioid structure. Paragenesis of Dolomite Multiple veins are common with as many as The paragenesis of dolomitic rocks is varied and complex. It has been shown above that: five cross-cutting generations, each with its own (1) there are several processes of dolomitiza- distinctive fill and some veins are refractured. tion, (2) many types of sedimentary and meta- Single vein sets cut host rocks into tabular morphic precursor rocks yield dolomite, and idens and multiple conjugate sets produce a (3) dolomite is host to later metamorphism. structure of host-rock idens, interconnected or Thus dolomitization cannot be viewed as a isolated and of variable shape. This is the basis simple, single metamorphic event; rather it of three intergradational structural types: (1) occurred at various stages in the ongoing meta- parallel-sheet, (2) rhomboidal to rectangular, morphism of the Canning Basin carbonate and (3) breccioid (Fig. 46). rocks. Parallel-Sheet Vein Structure. This vein structure is formed by a single subparallel vein set TENSIONAL FRACTURE SOLUTION that separates tabular host-rock idens (Figs 46, AND EMPLACEMENT 47). Idens are interconnected at vein terminaSTRUCTURES tions or isolated by vein bifurcation. Vein spacVeins and Vein Structures ing is variable, from millimetres to several Veins are fissures filled with coarsely crystal- metres and rarely, tens of metres. The sheet line calcite or internal sediment. Veins mostly structure usually dips at 60 to 90°, and trunare tabular bodies but rhomboidal and lensoid cates stylobedding and stylolamination at varishapes also are common and some bifurcate able angle depending on the local attitude of irregularly. Vein width ranges from millimetres this layering. Vein sets frequently are subto over a metre, strike length is up to several parallel to stylobedding strike but dip at steeper hundred metres or more, and veins extend in angles in the same or opposite direction; vein dip length for at least tens of metres. Vein sets parallel to stylobedding also occur (Fig. walls are subparallel, planar to irregularly un- 47). Fig. 47. Vein structures: A—Large rhomboidal vein filled mostly with internal sediment. This vein cuts across earlier parallel-sheet vein structures, and is itself cut by later fractures. Napier Formation, Geikie Gorge. B—Parallel-sheet vein structure. Napier Formation, Geikie Gorge. C—Parallel-sheet vein structure parallel to stylobedding; veins (white) locally have irregular margins" and grade into lacy-type vugs. Napier Formation, Copley Valley. Scale is 10 cm long. D—Rectangular to rhomboidal vein structure in upper part of photograph; rectangular to breccioid vein structure in lower part. Virgin Hills Formation, Bugle Gap. Scale is 10 cm long. VEIN

PARALLEL SHEET

RHOMBOIDAL

STRUCTURES

RECTANGULAR

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT

FIG. 47

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN VUGULAR STRUCTURES LACY

IRREGULAR

WMMM Mmm\ STROMATACTID STROMATACTID

z^iigipjii

I m B I I S s

Fig. 49. Classification of vugular structures.

Fig. 48. Classification types.

intergradational

vug

Rhomboidal to Rectangular Vein Structure. This vein structure (Figs 46, 47D) is formed by conjugate vein sets that are either cogeneric or of separate age (as indicated by dislocation). Host-rock is cut into connected idens which are of shape determined by the angular relations between vein sets; commonly iden shape is prismatic with rhomboid to rectangular cross-section; iden size ranges from a few millimetres to several metres. Breccioid Vein Structure. This vein structure is formed when anastomosing, irregular or numerous conjugate vein sets cut host-rocks into irregular isolated to interconnected idens (Figs 46, 4 7 D ) ; iden size ranges from a few millimetres to several decimetres. Secondary Vugs and Vugular Structures Vugs are holes that are filled with a similar range of materials as veins, above. The secondary nature of vugs is indicated by: (1) truncation of earlier sedimentary and metamorphic structures and fabric elements, and (2) occurrence in host-rocks that are metamorphic—including stylolaminite, stylonodular and stylomottled rocks, stylobreccia, dolomitic rocks and vugular rocks. Three intergradational vug types are distinguished on shape: (1) irregular, (2) lacy, and (3) stromatactid (Fig. 48). In some host-rocks, vugs (filled)

form 40 to 70% of the bulk volume and they are organized in definite patterns. This gives rise to four intergradational vugular structures: (1) irregular, (2) lacy, (3) stromatactid, and (4) breccioid (Fig. 49). Irregular Vugs and Vugular Structure. Irregular vugs (Fig. 49) are equidimensional, tabular to irregular cavities ranging from millimetres to about 20 cm in height and length; they are isolated or interconnected. These vugs are randomly orientated, forming between 5 and 50% of the bulk volume. Vugs truncate stylobedding but also tend to follow stylolites; they are developed around stylocumulate mottles and follow stylolitic contacts between large skeletons (e.g. brachiopod shells) and host rock (Figs 50B, C). Lacy Vugs and Vugular Structure. Lacy vugs are sheetlike to lensoid in form and typically are separated by millimetre- to centimetre-thick sheets of host rock, to give lacy vugular structure (Figs. 49, 51, 53A). Lacy vugs vary from a few millimetres to metres in length and from a millimetre to 20 cm in height. Shape varies from irregular, defined by irregular floors and roofs to regular with smooth floors and roofs. Contacts between fill and host rock are either sharp or gradational; where gradational, crystalline calcite filling the vug has replaced interstitial sparry calcite and to a lesser extent, grains in the host-rock. In dolomitic rocks, contacts between crystalline calcite and host are sharp except where undolomitized host remnants are intersected; here gradational, replacement contacts occur. Stromatactid Vugs and Vugular Structure. Stromatactid vugs are platy to lensoid cavities that have greater height:length ratios than lacy

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT

graded or equigranular, and columnar crystals also are oriented with long axes normal to an interface. The basic aggregate types occur alone or in combination, e.g. a columnar aggregate may contain a central or an off-centred equantcrystal aggregate. Columnar Crystals. The length:width ratio for columnar crystals ranges from 8 to 2. Crystals are slightly tapered, narrower at proximal ends and have pyramidal terminations at distal ends (Fig. 54). Most columnar crystals contain inclusions along their length but innermost columns in aggregates have inclusion-free zones. Some crystals contain bands alternately rich and poor in inclusions. These bands, up to 500 jxm wide, are traceable from crystal to crystal and are parallel to the aggregate margin (Fig. 54C). Inclusions are mostly spherical to tubular cavities (less than 1 JJLm in size) that may have been filled with gas or fluid; they are now empty; some inclusions are minute dark aggregates less than 1 ^m in size. Inclusions in columnar crystals are aligned and impart a FABRICS strong linear fabric; they outline the margins of The organization of veins, vugs, interfrag- a former tight aggregate of oriented subradiatment voids (in breccioid rocks) and host-rock ing to normal acicular crystals, 1 to 4 ^m wide idens give rise to two basic and intergradational and 150 to 450 ^on long (Fig. 54C). Each of fabric types: (1) host-support, and (2) fill-sup- these palimpsest acicular crystals had slightly port. Host-support is when host-rock idens are different lattice orientation that varied systeminterconnected and would therefore support atically, as indicated by the inclusions. This has voids and void-fill. Fill-support fabric is charac- imparted an undulose extinction to the large terized by a supporting framework of void-fill columnar crystals that replaced these precurFurther description of columnar crystals material crystalline calcite and/or internal sedi- sors. ment); host-rock idens are isolated or in point is given in captions for Figure 54. contact. Fill-support is the fabric of rocks with Equant Crystals. These crystals are in a size breccioid-vein or breccioid-vugular structure. range 150 to 3000 JJLm but the average range is 300 to 1500 jim. The crystals are characterized CAVITY-FILLING MATERIALS straight extinction and inclusions are rare; Veins, vugs and interfragment voids (of by breccia) are filled by aggregates of coarse cal- twins are common in some larger crystals. cite crystals and by internal sediment particles. Columnar-Crystal Aggregates. Columnar crysInternal sediment is interlayered with coarse tals occur in tight, graded to equigranular crystalline calcite aggregates and some cavities aggregates with columns arranged in orientedcontain up to four generations of internal sedi- normal to subradiating patterns (Fig. 55); the ment interspersed with the calcite; rarely, in- aggregate width is 400 to 2000 ^m. In graded ternal sediment dominates and completely fills columnar aggregates, crystal size increases from cavities in which there is no coarse crystalline 30 to 1500 |xm towards the centre (Fig. 54A). calcite. Descriptive terminology for the habit of Growth stages are marked by inclusion zones crystalline calcite and for aggregates of these that are traceable from crystal to crystal across the aggregate, paralleling the growth surface. crystals is discussed in Appendix II. In equigranular columnar aggregates, crystals Calcite Crystals and Crystal Aggregate Types to 10 000 ^m wide and crystal-growth Aggregates composed of coarse calcite crystals are up are marked by Fe-oxide films, internal are of two types, characterized by crystal stages sediment and/or inclusions that give the aggreshape: (1) columnar, and (2) equant. The distribution of crystal sizes in these aggregates is gate a banded appearance. Contacts between

vugs and typically have flat bases and highly irregular to digitate roofs (Figs 49, 52). Vugs vary in length from a few millimetres to tens of centimetres but in any one exposure there usually is a general size grouping. Roof contacts between vug-fill and host rock are gradational as a result of replacement and solution. In many exposures, vugs are aligned parallel to stylobedding and their bases are bounded by stylolites and stylocumulate (Fig. 53B). Where conjugate stylolite sets occur, stromatactid vug groups parallel these sets; this produces a stromatactid structure that is inclined to the stylobedding at angles of 30 to 45° (Fig. 52). Stromatactid vugular structure is characterized by parallel to subparallel vug groups that form 5 to 30% of the bulk volume; rocks with more abundant vugs have breccioid structure. Breccioid Vugular Structure. This structure is characterized by irregular three-dimensional networks of interconnected vugs that isolate host-rock idens (Fig. 49). Host-rock idens are mostly a few millimetres to a few centimetres in size.

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

adjoining crystals are irregularly subplanar, stepped to indented. Columnar aggregates commonly are uniform in their internal features, but there are variations in aggregates that cut across growth zones. For instance, crystals with palimpsest outlines of acicular aggregates can grade over millimetres into columnar crystals with dispersed inclusions and curved twin lamellae (Fig. 54D), which in turn can grade into clear equant crystals. Columnar crystals in aggregates are epitaxial on cavity walls, rock idens in breccias and on skeletal idens such as echinoderm plates; the contacts between crystal and foundation are sharp (Fig. 56A). Some crystals are syntaxial on columnar crystals in mollusk, stromatoporoid and coral skeletons. Equigranular columnar-crystal aggregates are most commonly observed in veins (Fig. 55). The crystals contain abundant zoned inclusions and there has been some crystal growth by replacement of sparry calcite (cement) in the host rock adjoining the vein. Replacement leaves sedimentary grains projecting into the aggregate of crystals (Fig. 56). Columnar-crystal aggregates are developed in vugs, only where vugs are very small or conditions have been favourable for growth of very large columnar crystals. As vug-size increases, the aggregate becomes a combination type with marginal columnar-crystal aggregate and central equant-crystal aggregate. Equant-Crystal Aggregates. Equant crystals form tight equigranular aggregates; rarely there is some grading with smaller crystals at the periphery. Crystal boundaries in these aggre-

gates are straight to smoothly irregular (Fig. 55), indented to stepped; triple junctions are common and enfacial angles (Bathurst, 1971) are common in some cases and rare in others. Internal sediment particles, fossils and grains are poikiloblastically enclosed in the crystals. Contacts between aggregates and host rock are sharp and commonly stylolitic. Equant-crystal aggregate occurs in veins, vugs and breccia voids, typically in dolomite host. The crystals in many of these are very large and only a few form the whole aggregate. Combination Aggregates. Columnar-crystal aggregates and equant-crystal aggregates often occur in combinations as illustrated in Figure 55. The combinations are of a marginal zone of columnar crystals and a centred or offcentred equant-crystal aggregate. Combination aggregates with centred equant crystals are most common in vugs and in voids of breccia. In breccias, the crystals rim host-rock idens but are penetrative into wall rocks and breccia idens, with replacement of sparry calcite (cement). The columnar zone is succeeded abruptly, or with transition, by equigranular equant aggregate at the cavity centre. Offcentred combination aggregates have similar occurrence to the centred variety, but in these cases, some crystals of the columnar aggregate have been recrystallized to equant forms. Variation on Aggregates. One variation on crystal aggregate is marginal replacement of host rock surrounding a vug or vein, or of a breccia iden. Generally, marginal replacement occurs in a narrow zone as deduced from: (1) internal sediment that defines a floor, (2) poikiloblas-

Fig. 50. A—Vugs (filled with white calcite) localized around stylomottles and lithoclasts of the host rock, Napier Formation, Wire Spring area. B—Irregular vugs (filled with white calcite) localized along stylolites at stylolite junctions, and around sedimentary grains. ?Napier Formation, Geikie Gorge. C—Vugs (margins delineated by white calcite) localized around stylomottles. Napier Formation, Dingo Gap. Fig- 51. Vugs and vugular structure: A—Lacy vugs and vugular structure from a vugular pod. Napier Formation near Wire Spring. B—Lacy vugs in brachiopod limestone. Brachiopods were more insoluble than matrix. Sadler Limestone, Lloyd Hill, Bugle Gap area. Fig. 52. Vugs and vugular structure: A-—Stromatactid vugs. Napier Formation, Copley Valley. B—Stromatactid vugs and vugular structure parallel to stylobedding and stylolamination of host rock. Napier Formation, near Dingo Gap. Scale (arrow) is 10 cm long. C—Stromatactid vugs (locally grading into lacy vugs) and vugular structure discordant to attitude of host-rock stylobedding. Sadler Limestone, near Menyous Gap, Pillara Range. Fig. 53. A—Photomicrograph of lacy vugular structures. Arrows indicate stylomottles of host. B—Stromatactid vug in quartzose stylolaminite. Floor of vug is parallel to stylolaminar structure. Wall of vug truncates stylolaminar structure. Thin discontinuous vugs are aligned along some stylolites. Note Girvanella sheet (arrow) which has survived pressure solution of host rock.

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT

FIG. 50

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 51

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT

FIG. 52

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 53

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT

COMBINATION

AGGREGATES

OFF-CENTRED

EQUANT-CRYSTAL

Fig. 54. Features of columnar crystal aggregates: A—Tight, oriented subradiating, graded aggregate. B—Crystals with abundant inclusions and undulose extinction; on clockwise rotation, extinction shadows swing clockwise, anticlockwise, or form V-shaped bands that move toward the column centre and at the same time toward the distal end, simulating a 'feathery' extinction. C—Palimpsest outlines delineated by inclusions within columnar crystal aggregate of tight oriented subradiating acicular crystal aggregate. D—Lateral change in internal features within 2 mm: abundant inclusions in columnar crystals (left) grading to columnar crystals with curved twin lamellae and with lesser inclusions (right). tically enclosed sediment grains, arrangement of inclusion zones.

and

Fig. 55. Columnar

AGGREGATE

crystal aggregate fill in vugs

and veins. Combination aggregates with outer columnar zone and centred equantcrystal aggregate (left); and with offcentred equant-crystal aggregate (right). Equant-crystal aggregate fill in vugs and

COLUMNAR-CRYSTAL AGGREGATE — SHARP CONTACT

(3)

Breccia fragments are more extensively replaced and in some cases an entire fragment (up to 2.5 mm in size) is replaced; the only clues to its former presence are sharp initial crusts of oriented columnar crystals, palimpsest fabrics and scattered sedimentary grains now poikilitically enclosed in the aggregate. Replacement is selective and sparry calcite (of fine to medium crystallinity) is commonly affected, leaving grains such as fossils and ooids partly projecting into or, in many cases poikilitically enclosed by the inclusion-rich columnar aggregate. Another variation or modification of aggregates is where the original margin has been destroyed by pressure solution. Typically such margins are irregular interfaces, or if smooth, are discordant to the banding of internal sediment.

COLUMNAR-CRYSTAL AGGREGATE - THIS ZONE TRACEABLE WITHIN 1mm INTO SHARP CONTACT BETWEEN HOST ROCK AND COLUMNAR-CRYSTAL AGGREGATE ZONE REPLACEMENT OF HOST; GRAINS OF HOST FLOAT IN COLUMNAR-CRYSTAL AGGREGATE

HOST GRAINSTONE WITH INTERSTITIAL SPARRY CALCITE

Fig. 56. A—Sharp contact between columnarcrystal aggregate and host rock. B—Gradational contact between columnar-crystal aggregate and host rock; gradation zone is zone of replacement.

METAMORPHISM

OF C A R B O N A T E ROCKS, C A N N I N G

Internal Sediments Internal sediments vary in colour from red, green, brown, buff, orange to grey. The sediments may be laminated (colour, grainsize, compositional differentiation), cross-laminated, massive or graded (Figs 57B, D ) ; some deposits exhibit erosional or solutional microunconformities. Internal sediment constituents are silt- to coarse sand-sized; gravel-sized components are generally rare but are locally abundant (Figs 57A, C). Constituents of internal sediments are: calcite-crystal silt and sand, quartz silt and sand, ooids and ooid-fragments, pellets, intraclasts, lithoclasts, whole skeletons and fragments (crinoids, brachiopods, mollusks, calcispheres, calcareous algae), limestone fragments with stylolitic structures and fabrics, dolomite, micas, Fe-oxide, and fragments of columnar-calcite aggregates (Figs 57, 58). Internal sediment is usually dominated by one of the above constituents. Internal sediment mostly is layered, parallel to cavity floors, but in some cavities, particularly vugs, older layers are overlain by younger internal sediments with angular unconformity. Commonly, internal sediment overlies a flooring aggregate of columnar calcite crystals, but it also occurs in sheetlike to lensoid bodies interlayered with generations of columnar calcite (Fig. 58). In cavities which were filled only partly by internal sediment, the remaining space was filled with equant calcite-crystal aggregate. ROCK TYPES

Veined Rocks Veined rocks contain abundant vein-fill material organized in parallel-sheet or rectangular to rhomboidal structure, but are characterized by host-support fabric with interconnected remnants of host rock between vein sets. Veined rocks are transitional into calcitecemented breccia (p. 80) as host-rock remnants become isolated and supported by veinfill, and/or rotated from original positions. Veined rocks also are gradational into vugular rocks. All Devonian formations contain veined rocks, especially adjacent to the abutment and small faults. Lamboo Complex basement rocks also contain veins, similarly filled with calcite and internal sediment, where the basement is in abutment with the carbonate formations. The most common hosts for veins are stylobedded limestone, stylonodular and stylomottled rocks, stylobreccia and stylolaminite. Locally dolomite and dolomitic limestones also contain abundant vein structures. Veins truncate pres-

BASIN

sure-solution features, stylocumulate and reacate layers and shear fractures, and their formation must therefore post-date these metamorphic phenomena. Rocks composed of crystalline calcite and internal sediment ('vein rock') are host to later generations of veins as are vugular rocks and calcite-cemented breccia. Vugular Rocks Vugular rocks are those with abundant cavities filled with coarsely crystalline calcite and internal sediment. Cavities are organized in irregular, lacy or stromatactid vugular structure. Vugular rocks are characterized by hostsupport fabric, but are transitional into rocks with breccioid-vugular structure (fill-support fabric) and then into calcite-cemented breccia. Rocks that are host to vugs include those with pressure-solution structures, dolomites, other veined and vugular rocks and limestones of the Pillara Formation. Cavities indiscriminately truncate earlier fabrics, such as cryptalgal fenestral fabrics, pressure-solution structures and other cavities and veins, but more commonly, vug distribution and geometry is controlled by host-rock fabric. For instance platy, lensoid and sheet-like vugs are aligned along stylolites; thin discontinuous vugs along stylolites thicken into wider cavities within a few millimetres to a few centimetres (Fig. 53B). Some cavities are localized at lithological contacts {e.g. between Renalcis-Girvanella boundstone and grainstone). Irregular cavities are located at stylomottles, and partly enclose these. The irregular and digitate morphology of cavity walls and roofs is controlled by hostrock fabric, and in part by solution-embaymeint, replacement and later pressure solution. Finger to sheet-like projections of irregular walls correspond to solution embayments into interstitial sparry calcite of host rock; many sedimentary grains such as shells, clasts and ooids therefore project into the cavities. Some curved walls are along mollusk or brachiopod valves. Irregularity also is due to small finger to sheet-like extensions of cavities into and along stylolites. Replacement of host rock by crystalline calcite also causes some irregularity in cavity walls; in these cases, the calcite extends beyond the cavity and replaces interstitial sparry calcite and grains of the host. The transitional zone consists of coarsely crystalline calcite and poikilitically enclosed host-rock grains (Fig. 56B). Some irregular roofs and margins are caused by later columnar and peaked stylolites.

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT

Fig. 57. Internal sediments: A—Internal sediment in open space fracture, Geikie Gorge. B—Large fissures filled with laminated internal sediment. Abutment zone, South Oscar Range near 12-mile Bore. C—Close-up of A above showing angular fragments of rock in laminated fine-grained calcitic internal sediment. D—Laminated fine-grained calcitic internal sediment (arrow) overlying marginal columnarcrystal aggregate.

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN TABLE VIII Occurrence. Vugular rocks occur as irregular, lensoid to sheetlike bodies a few centimetres to Composition of Iden Types in Calcite-Cemented Breccia a metre thick (Fig. 59). In the Pillara, vugular rocks are restricted mostly to within a few tens of metres of the abutment; in the homocline I. LIMESTONE 1. Pillara Formation lithotypes (Table II) their occurrence is more widespread but they 2. Brachiopod limestone increase in abundance towards the abutment 3. Stromatoporoid limestone and near smaller faults. Irregular vugs and 4. Ooid, skeletal, lithoclast and pellet limestone (Table IV) vugular structures occur in stylolitic rocks, 5. Skeletal boundstone (Table IV) dolomites, Renalcis-Girvanella boundstone and II. FOSSILS Pillara Formation rocks. Lacy vugs and vugu1. Stromatoporoids lar structures occur in: (1) stylolitic rocks, 2. Corals dolomites and Renalcis-Girvanella boundstone, 3. Brachiopods 4. Receptaculites (2) fault zones, (3) monoclinal flexures, and FORMED BY PRESSURE SOLUTION (4) around skeletal-boundstone pods (Fig. III. ROCKS AND SHEAR 60B). Stromatactid vugs and vugular structures 1. Stylonodular rock occur in stylolitic rocks of the homocline. 2. Stylomottled rock 3. Quartzose stylolaminite Calcite-Cemented Breccias 4. Quartzose-micaceous stylolaminite Calcite-cemented breccias are defined as 5. Dolomitic stylolaminite 6. Stylobreccia rocks with either breccioid-vein or breccioidvugular structure (Figs 46, 49); they are IV. DOLOMITIC ROCKS 1. Dolomitized Pillara Formation characterized by a fill-support fabric. Interiden 2. Dolomitized skeletal boundstone voids of the breccia are filled with coarsely 3. Massive dolomite crystalline calcite, mainly in columnar and 4. Dolomitic stylolaminite 5. Dolomitic stylomottled rock equant habit (Figs 61, 62). This rock category 6. Dolomitic stylobreccia is gradational, however, into breccia in which FORMED BY TENSIONAL FRACvoids are filled with internal sediment. Voids V. ROCKS TURE, SOLUTION AND EMPLACEMENT may contain calcite alone, multiple generations 1. Veined rock of crystalline calcite and internal sediment, or 2. Vugular rock internal sediment alone. Breccia with internal 3. Calcite-cemented breccia 4. Vein calcite sedimentfillis, however, a minor rock category. 5. Internal sediment Calcite-cemented breccia varies greatly in ROCK TYPES textural parameters, but in any one breccia VI. NON-CARBONATE 1. Precambrian metamorphic rocks body there frequently is a general uniformity. Textural parameters are: (1) iden size: 0.5 cm to many metres The most distinctive feature of many idens is (2) sorting: poor to moderate strongly embayed margins (Fig. 61), related (3) shape: variable; tabular, lensoid, ellipsoidal to solution and/or recrystallization; iden marspherical, irregular, tetrahedral (4) orientation: mostly random but some gins truncate earlier structures and textures, e.g. stylolamination, parallel-vein structure and orientation of long axes parallel to margins skeletal structure (Fig. 62). of breccia body

Fig. 58. Internal sediments: A—Fragments of stylolaminite in internal sediment. B—Dolomitic internal sediment lying between columnar-crystal aggregate and centred equantcrystal aggregate. C—Interlayered internal sediment (arrow) and columnar-crystal aggregate in interfragment voids of breccia. Calcite crystal aggregate here is centred combination type. D—Internal sediment of brachiopods and other calcitic detritus. E—Internal sediment overlain and underlain by columnar-crystal aggregate. Internal sediment (arrow) consists of lithoclasts, skeletons and other grains freed from the host by solution. F—Close-up of internal sediment: whole ooids and fragments, fragments of columnar crystal aggregates (arrow), and other carbonate grains. Fig 59. Geometry of vugular rocks: A—Sheet to lensoid idens (white) of vugular rock. Napier Formation, Geikie Gorge. B—Wedge-like idens (arrows) of vugular rock flanking a skeletal boundstone pod. Napier Formation, Sheep Camp Yard. C—Irregular iden of vugular rock in host grainstone. Napier Formation, Dingo Gap.

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT

FIG. 58

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 59

TENSIONAL FRACTURE, SOLUTION A N D EMPLACEMENT

The composition of idens in calcite-cemented PETROGENESIS breccia is of significance in genetic interpreta- Gradations tions. In general terms they are of similar Veined, vugular rocks and calcite-cemented lithology to rock adjacent to the breccia body. breccia are intergradational; transitions from Thus calcite-cemented breccia units in the one category to another provide vital data on homocline of Sadler, Napier and Virgin Hills petrogenesis and indicate that these rock types formations contain idens comprised both of are categories of a consanguineous association. sedimentary rock and metamorphic rock. The association forms 10 to 30% of rocks in Idens of metamorphic lithotypes are composed the Napier, Virgin Hills and Sadler formations of the rock types generated by pressure solu- and also occurs in the Pillara Formation adjation, fracture, dolomitization, solution and em- cent to the abutment and small faults. placement, plus partly altered limestones that The veined to vugular rock gradation occurs bear imprints of these processes. Table VIII when parallel-sided veins become irregular with summarizes compositional types. Special note embayed margins and pass into increasingly should be made that breccia idens are most isolated vugs of irregular, lacy and stromatactid commonly stylobedded limestone, stylonodular, type (Fig. 65). The gradation of veined rock stylomottled rocks and stylolaminite. Fragments to calcite-cemented breccia, illustrated in of dolomite, dolomitic limestone, veined rock, Figures 65, 66 and 67, is continuously traceable vugular rock, vein-fill (coarsely crystalline cal- with passage from host rock with few veins, cite and internal sediment) also are frequent, through veined rock characterized by hostespecially in calcite-cemented breccia adjacent support but abundant vein-fill, to calciteto the abutment (Fig. 62). Idens derived cemented breccia with fill-support fabric. Idens from Lamboo Complex basement are com- of host rock become increasingly isolated, emposed of mica schist, quartzite and other high bayed and in the final stage are rotated as indigrade metamorphic rocks (Fig. 62D). cated by random attitude of stylolaminatiotl Calcite-cemented breccia bodies also occur and other directional features in the fragments. in contact with the Pillara Formation, either at Dilation of earlier vein structures is indicated the abutment or along small faults that cut the by crosscutting sets. The vugular to calcite-cemented breccia Pillara sequence. Here the breccia idens include fragments of stromatoporoid, pelletal and gradation, illustrated in Figure 68, is concryptalgal limestone, discrete fossil fragments, tinuously traceable with passage from host rock and metamorphosed carbonate bearing palimp- with few vugs (irregular, lacy or stromatactid), sest features indicative of derivation from the through rocks with host-support fabric but Pillara Formation; compositional types include abundant interconnected vugs and vugular all of the Pillara rocks listed in Table II. Idens structure, into calcite-cemented breccia with of sedimentary lithotype simulate lithoclasts fill-support and isolated, embayed idens of host but they lack the encrustations of marine rock. The idens are rotated as indicated by the organisms, pelletal rinds and borings typical of random attitude of directional features. marine breccias and features which characThe gradations described above indicate that terize terrestrial sedimentary breccias (Read, the origin of veined, and vugular rocks and 1974). calcite-cemented breccia is due to interaction of several processes: (1) tensional dilation and Note should also be made of the gradation fracture, (2) solution, (3) precipitation, (4) inbetween calcite-cemented breccia and some filtration, and (5) recrystallization. Tensional stylobreccia. The calcite is emplaced in stylo- fracture and solution are primary mechanisms breccia along the former position of circum- resulting in cavities; precipitation and infiltraidenic stylolites. tion result in emplacement of cavity-filling Occurrence. Calcite-cemented breccia forms materials. Recrystallization affects both cavity lensoid bodies a few centimetres to several fillings and host rock at cavity margins. metres in thickness and in extreme cases tens Origin of Cavities of metres thick; the units extend for metres to Tensional stress results in parting of host tens of metres along strike and in the downdip direction. Breccia bodies have variable attitude rock along surfaces to form cavities, termed herein strain cavities. Strain cavities can be probut usually are subparallel to stylobedding in by: (1) dilation, and (2) rotational dethe host; they become discordant downdip or duced formation. Dilation involves volume increase along strike; they step across stylobedding with no change in attitude of host-rock idens. (Figs 63, 64).

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

In the Devonian rocks, strain cavities and solution cavities are closely related. Thus solution commonly has enlarged pre-existing strain cavities and, alternatively, solution cavities have been dilated or deformed by tensional stress. The relationships are summarized below:

STYLOBEDDED GRAINSTONE >

-VUGULAR P O D V

Dilation^^^ Rotational Deformation Solution ^ ^

STYLOBEDDED ROCKS .

jffc'&r'iSKELETAL ? # B O U N D S T O N E POD

LACY V U G S

V U G U L A R PODS

150-800 metres

Fig. 60. Occurrence of vugular rocks: A—As a vugular pod in fault zone. B—As lenses and wedges of vugular rock flanking a skeletal boundstone pod. C—As vugular pods in a zone of monoclinal flexure.

Rotational deformation follows dilation, as stress is resolved into shear components, and is characterized by dislocation (faults), drag folding and rotation of idens. Solution cavities develop when host rocks are invaded by migrating fluids that are undersaturated with respect to mineral phases present (Weyl, 1958). Solution cavities are characterized by: (1) irregular embayed margins and in situ isolation of host-rock idens, (2) selective solution of crystalline material in host rock, and (3) selective solution of matrix with isolation of larger idens.

The spectrum of rock types (and structures) can be partitioned according to the dominant mechanism involved in their formation (Fig. 69). Veined rocks and structures relate mainly to dilation and rotational deformation. Vugular rocks have developed with solution as the dominant process, but dilation may have been a precursor mechanism and rotational deformation may have followed. Calcite-cemented breccia is a product of rotational deformation, with solution as an additional mechanism. Control of Strain Cavities. Strain-cavity shape and orientation are fundamentally controlled by stress mode and application, and can therefore be independent of host-rock structures and fabrics (Ramsay, 1967). Major systems of strain cavities, mainly veins, in rocks adjacent to the abutment, reflect this relationship. The veins cut across stylobedding and stylolamination with formation of parallel sheet, rhomboidal and rectangular patterns, and a dominant vein set parallels the abutment surface. Many vein systems are dilational but others are dislocations with associated dragfolding, subsidiary tensional cavities and brecciated margins. Large bodies of calcite-cemented breccia are associated with these discordant vein systems and are similarly disposed, discordant to stylobedding. Away from the abutment zone, strain cavities tend to become approximately concordant with earlier pressure-solution surfaces. Dilationalstrain cavities frequently occur along inter-

Fig. 61. Calcite-cemented breccia: A—Stylolitic rock as breccia idens: solutional embayments commonly follow stylolites (arrow) sometimes developing parallel system of embayments. Napier Formation, Dingo Gap area. B—Breccia (white) with fragments of relatively insoluble brachiopods and rock fragments. Virgin Hills Formation, Horse Spring Range. C—Development of irregular, ellipsoidal, equant idens by solution around stylomottles (arrows). Napier Formation, Dingo Gap. D—Dominantly tabular to equant idens where solution has followed parallel stylolitic structures (stylobedding and stylolamination) and stylomottle boundaries respectively. Same locality as C. E—Dommantly tabular idens where solution and dilation has followed laminar stromatoporoid/sediment interfaces. Sadler Limestone, Lloyd Hill, Bugle Gap area.

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT

FIG. 61

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

idenic and circumidenic stylolites and similarly, such stylolites localized numerous small scale rotational-strain cavities. Thus strain cavities commonly parallel stylobedding and lamination and opening along these precursor surfaces gives rise to parallel-sheet vein structure and lacy-vugular structure; resolution of tensional stress into shear components leads on to generation of calcite-cemented breccia bodies that are concordant to mildly discordant to stylobedding. Similar considerations apply in host rocks with conjugate stylolite sets (stylobreccioid structure). Dilation along these surfaces leads to formation of rhomboidal to rectangular vein structure, breccioid-vein and vugular structure and calcite-cemented breccia. Breccioid, rhomboidal and rectangular-vein structures also are produced by vein sets cutting across earlier vein generations. Irregular strain cavities also develop at boundaries of stylomottle and stylonodular idens and at the circumidenic stylolite conjunctions in stylobreccia. Solution Cavities. The order of solubility for carbonates at NTP is aragonite, calcite and dolomite but solubility products are much affected by T, P, C 0 concentration, other ions in solution and granulinity (Krauskopf, 1967). Petrographic data from vugular rocks gives few pointers to chemical conditions in the vugs. Corroded vug-fill materials are evidence of alternate solution and precipitation and that migrating fluids were alternately undersaturated and saturated. That solution occurred under tensional stress is indicated by the close association of solution features with strain cavities and breccias. There is evidence of selective solution in extension of cavities along more soluble components. Skeletons of brachiopods, crinoids and calcareous algae seem to have been less soluble than fine matrix; calcitic components in dolomitic limestones were selectively dissolved. Solution cavities truncate host-rock features but their shape and attitude also is frequently controlled by earlier fabrics and differential solubility of fabric elements. This relationship is most strikingly displayed when vugular structure has developed in dolomitic tabular-stromatoporoid boundstone—here stromatoporoid skeletons have been dissolved creating cavity frameworks and isolated cavities in dolomite hosts. In the rocks of this study, solution cavities mostly post-date pressure solution and dolomitization, but features created by these metamorphic processes played a major role in deter2

mining the superimposed vugular structure. In particular, the ubiquitous stylolites became channel-ways for migration of undersaturated fluids and most vug types and structures relate to the disposition of these earlier pressure-solution surfaces (Fig. 59B). The role of dilation, however slight, in opening cavities along former pressure-solution surfaces is difficult to assess but it is likely to have played an important part. The relationships between vug-structure types and pressure-solution structures is direct. Irregular vugs commence along stylolites, at boundaries of stylomottles, at conjunctions of conjugate stylolite sets or at conjunctions of circumidenic stylolites. Stromatactid vugular structure develops along stylocumulate sheets, which come to form vug floors; single stylolite sets give a single stromatoacid vug set; conjugate stromatactid vug sets relate to conjugate stylolite sets. Most lacy vugular structures also grew by cavity development along close-spaced interidenic stylolite sets, single or conjugate, but lacy vugular structure also developed by differential solution of calcitic layers in a host of interlayered dolomite and boundstone. While many lacy vugs can be related directly to zones of tensional stress (e.g. drag-folded zones near faults), there are some rocks with lacy vugular structure that occur in settings suggestive of gross compression. These vugular rocks occur as thin wedges, lenses and sheets flanking massive skeletal-boundstone pods (p. 98) and interlayered with stylobedded limestone, stylolaminite and stylobreccia which are generated by compressive stress. The structural setting (Fig. 60B) is reminiscent of pressure shadows' that flank porphyroblastic crystals (Ramsay, 1967, p. 181; Spencer, 1969). These 'pressure shadows' are inferred to have developed adjacent to large crystals in the direction of maximum extension. A similar mechanism is envisaged for the vugular rocks flanking pods. Rotation of resistant augens (pods) in the compressive stress field resulted in local tension and dilation of former pressuresolution surfaces in the rocks flanking the pod. Emplacement of coarsely crystalline calcite to form lacy vugular structure followed dilation and these structures then tended to remain as relatively insoluble idens in the compressive stress field. Origin of Cavity-Filling Materials Coarsely Crystalline Calcite. Interlayering of coarsely crystalline-calcite aggregates with internal sediment indicates that most crystalline

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT

Fig. 62. Calcite-cemented breccia; fragment types: A—Fragments of quartzose stylolaminite with interlayered calcite-cemented breccia (arrow 1), and fragment of calcite-cemented breccia (arrow 2). Napier Formation, MacSherrys Gap area. B—Fragments of fossils (Receptaculites, arrowed) skeletal boundstone and stylolitic rock. Note vein that truncates the calcite-cemented breccia and stylolites of the host sequence. Napier Formation, Dingo Gap area. C—Fragments of Renalcis-Girvanella boundstone (note stylolite, arrowed). Napier Formation, near 97-mile Creek, Oscar Range. D—Fragments of Lamboo Complex high grade metamorphic rock; note incipient brecciation. Abutment zone, near 97-mile Creek. Scale is 10 cm long.

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

calcite originated by precipitation in open cavities but petrographic data also suggest that there was limited crystal growth by recrystallization of host-rock idens in cavity walls and of internal sediment. Determination of the original precipitate is clouded by superimposed recrystallization fabrics. Palimpsest outlines delineated by inclusions indicate that precursors to the columnarcrystal aggregates were carbonate crystals which were laid down in acicular-crystal aggregates. These acicular crystals were zoned with numerous minute inclusions that may have been gas or fluid-filled.

The identity of the acicular precursor, whether aragonite or calcite, is an open question. Acicular habit favours the idea that it might have been aragonite.* An aragonite precursor also would explain the abundance of recrystallization fabrics evident in cavity fills, the evidence of syntaxial growth on mollusk, stromatoporoid and coral skeletons (which may have been aragonite) and epitaxial growth on calcitic skeletons, e.g. crinoids, brachiopods. Equant-crystals in centred composite aggregates apparently were precipitated in open cavities. Recrystallization, however, complicates the story because equant-crystal aggregates

* Although aragonite is typical of metamorphic rocks such as glaucophane schists (Brown, Fyfe, & Turner, 1962; Coleman & Lee, 1962), Vance (1968) has reported metamorphic aragonite in facies as low as prehnite-pumpellyite. Aragonite occurs as vein-fill in recrystallized limestone, replacement after skeletal calcite, and as vein-fill and amygdale-fill in metavolcanic rocks. Much of this aragonite shows faint dusty lines of tiny fluid inclusions incorporated on {110}; the aragonite gives parallel symmetrical extinction relative to these lines of inclusions (Vance, 1968). Fig. 63. Calcite-cemented breccia units: A—Lensoid bodies of calcite-cemented breccia (white) in dolomitic stylolaminite. Note pressure-solution interfaces along upper and lower margins of breccia. Napier Formation, MacSherrys Gap area. B—Large lensoid calcite-cemented breccia bodies bound by interidenic stylolites. Napier Formation, MacSherrys Gap area. C—Lensoid and sheet-like bodies of calcite-cemented breccia (white) in dolomite host rock; fragments of breccia are dolomite. Napier Formation, MacSherrys Gap area. Fig. 64. Calcite-cemented breccia units: A—Calcite-cemented breccia (30 cm thick, outlined) mainly discordant to host dolomite stylobedding but left and right extremities of breccia swing into concordance with stylobedding. Breccia therefore has locally 'stepped across' stylobedding. Napier Formation, Dingo Gap area. Hammer (arrow) for scale. B—Discordant calcite-cemented breccia (2 m) cutting through Pillara Formation. There is gradation into breccia from veined limestone. Note also drag-folding of Pillara stylobeds along breccia margin. Geikie Gorge, Geikie Range. Fig. 65. Gradations between veins, vugs, and breccia: A—Isolated stromatactid vugs located along a stylolite; grades along strike into B. B—Parallel-sheet vein structure concordant with host-rock stylolamination; veins are locally irregular; grades along strike into C. C—Parallel-sheet vein structure with local development of breccioid vein structure. Napier Formation, MacSherrys Gap area. D—Incipient calcite-cemented breccia developed along vein discordant to host-rock stylobedding. Napier Formation, 6 km SE of Dingo Gap. Scale is 10 cm long. Fig. 66. Gradations; veins to breccioid structure: A-C—Gradation occurring at same locality within 5 m: fractured host dolomite grades into calcite-cemented breccia. Napier Formation, MacSherrys Gap. Scale (arrows) is 10 cm long. D—Rectangular to rhomboidal vein structure, locally breccioid vein structure, with incipient development of calcite-cemented breccia. Geikie Gorge. Scale (arrow) is 10 cm long. Fig. 67. Gradations; origin of calcite cemented breccia: Series of photographs (A-C) to illustrate development of calcite-cemented breccia by veins. Virgin Hills Formation, Horse Spring Range. Fig. 68. Gradations; vugular structure to breccioid vugular structure: A—Development of calcite-cemented breccia by solution. Abutment zone, Tunnel Creek. Scale marked in inches (2.5 cm units). B—Gradation of stromatactid vugular structure (1) into breccioid vugular structure (2), that is, development of calcite-cemented breccia by solution. Abutment zone, Dingo Gap. Scale (arrow) is 10 cm long. C—Calcite-cemented breccia (to right of photo) developed by solution along circumidenic stylolites of a stylobreccia (to left of photo). Napier Formation, MacSherrys Gap.

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 64

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT

FIG. 65

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 66

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT

FIG. 67

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 15

TENSIONAL FRACTURE, SOLUTION AND EMPLACEMENT also have developed by recrystallization of columnar aggregates. This is indicated by discordant gradations of columnar aggregates into equant aggregates, and by the occurrence of palimpsest columnar and acicular features in some equant aggregates. The intermediate stage in recrystallization is a composite aggregate with off-centred equant aggregate. In summary, the history of coarsely crystalline calcite aggregates appears to be: (1) precipitation of acicular carbonate, possibly aragonite, followed by precipitation of equant crystals, (2) recrystallization of acicular aggregates to columnar aggregates, and (3) recrystallization of columnar aggregates to equant-crystal aggregates. Origin of Internal Sediment. The origin of internal sediment is closely connected with processes that formed the cavities in which it was deposited—solution and tensional fracture. The main mechanisms identified are solution stoping and mechanical stoping. Solution stoping is most important in vugular rocks. Here selective solution of host-rock components in cavity walls and roofs releases less soluble particles from supporting matrix. Differential solubility is a basic factor in selective solution so that host-rock composition plays a major part in determining the composition and quantity of internal sediments. Quartz, micas, dolomite and Fe-oxides are the main components in many of the host-rocks (stylocumulate and reactate) and therefore these minerals form a large part of the mineral suite in the internal sediments produced by dissolution. Calcareous components also freed by solution include ooids, pellets, lithoclasts and skeletal fragments (particularly brachiopod and crinoid skeletons). Mechanical stoping also generates internal sediment particles in strain cavities. Here particles are dislodged during movement along fracture surfaces. Grains and cement crystals are freed but by far the most common products are granule-to pebble-sized idens of host rock (Fig. 57A, C). Fragments of cavity-fill materials, crystalline calcite aggregates and internal sediment also are detached by later movements. In some large veins these idens range up to cobble-size and have been 'rounded' during movement. The origin of carbonate-crystal silt, a major component of some internal sediments, is indeterminate. It may originate as fine flocculent carbonate formed by rapid precipitation from pore fluids but other possibilities include solution stoping of fine host rocks, or hydraulic

erosion of precipitate-crystal aggregates (Dunham, 1969). Internal sediment particles are deposited on cavity floors; the larger particles are deposited adjacent to the generation site. Lamination, size grading, current bedding and micro-unconformities indicate complex depositional processes involving transport of finer particles in moving fluids and infiltration by gravitation through systems of interconnected cavities. Lamination and interlayering of internal sediment and precipitate-aggregates gives evidence of episodic influx. Angular unconformities separating generations of internal sediment evidence tilting during the depositional sequence, as does the occurrence of fragmental crystalline-calcite aggregates and earlier internal-sediment fragments in the internal sediment. Origin of Rock Types Veined and Vugular Rocks. The origin of veined and vugular rocks is obvious from preceding discussions. Strain and solution cavities were produced under conditions of tensional stress and emplacement of carbonate precipitate and internal sediment into the cavities followed. Calcite-Cemented Breccia. Veined rocks grade into breccias composed mainly of angular fragments; these breccias, located in the most intensely veined parts of the host were developed by rotational deformation. Vugular rocks also grade into breccia; brecciation mainly was by solution, with isolation and rotation of hostrock idens. More commonly, breccias appear to have formed by penecontemporaneous fracture and solution since fractured rocks and associated breccia display abundant solutional features. Where parallel-sheet or rectangular to rhomboidal structures were prominent prior to brecciation, iden types are dominantly tabular, rectangular to rhomboidal. Vein structures which were more irregular, tended to yield polyhedral idens. Solution brecciation was controlled mainly by the stylolitic fabric and, to a less extent, sedimentary fabric of the host. Initially, solution was along parallel stylolites, resulting dominantly in tabular idens (Fig. 61 A, D ) . Solution along the boundaries of laminar stromatoporoids in boundstone also generated tabular idens (Fig. 6 I E ) . Solution around stylomottle idens isolated those mottles as iden types (Fig. 61C). Solution along stylolitic contacts of lithocast pebbles or fossil fragments isolated these as fragment types. Calcite-cemented breccia that developed from stylobreccia by

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN FRACTURE D O M I N A N T

SOLUTION DOMINANT

FRACTURE A N D S O L U T I O N

Vt:7

c=>

J <1

Fig. 69. Summary diagram illustrating generation of calcite-cemented breccia by fracture and solution processes. solution along circumidenic stylolites, inherited idens from the precursor (Fig. 68C). Further solution and embayment along stylolites in the isolated fragments produced irregularly-shaped idens, or idens with parallel, sheet-like embayments (Fig. 61 A). The occurrence of idens of stylolitic rocks, dolomite and stylobedded limestone indicates that brecciation postdated dolomitization, pressure solution and shear fracture. The occurrence of fragments of veined limestone, internal sediment and crystal-calcite aggregates indicates vein formation before brecciation; re-fracture of veined limestone suggests movements may have been episodic (Fig. 70). The occurrence of idens of calcite-cemented breccia in calcite-cemented breccia indicates brecciation was a recurring event.

PODS A N D MEGABRECCIA SHEETS PODS

Pods are discrete carbonate-rock idens that occur in stylobedded and stylolaminated rocks. Pods range in size from a decimetre to tens of metres and vary greatly in shape—lensoid, ovoid, irregular, tabular, pyramidal, etc. Many pods are monolithic, but others are composed of several lithologic types, i.e. polylithic (Fig. 71). Large pod masses frequently are composite structures being composed of smaller pods agglomerated in fitted, condensed and, rarely, stylocumulate-support fabric. The internal structure of such composite pods is stylonodular to stylobreccioid. Composite pods are monolithic, i.e. all smaller elements are of one lithologic type, or polylithic. Pods are differentiated on the basis of dominant lithology into:

Fig. 70. A-B—Gradational sequence illustrating refracture of coarsely crystalline calcite veins and progressive breakup of vein calcite which becomes incorporated into the younger vein fill as breccia fragments. Napier Formation, Geikie Gorge. Scale is 10 cm long.

PODS AND MEGABRECCIA SHEETS

FIG. 70

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN 98 (1) vugular, (2) breccia, and (3) skeletal- on pages 31 and 80. Breccia pods occur in stylobedded and stylolaminated rocks as lensoid boundstone (Fig. 71). to equidimensional idens, a few metres to tens Vugular Pods metres in thickness and length. Vugular pods are idens composed of vugu- of Some lensoid pods are sigmoidal in crosslar rock with lacy and stromatactid vugular section thin margins parallel to stylobedstructure; irregular vugular structure is ding andwith the bulk of the pod mass discordant developed rarely in these idens and there are to it. Breccia pods normally are bounded by minor quantities of calcite-cemented breccia circumidenic stylolites that sharply define conand seams of stylocumulate. Monoidenic vugu- tacts with the host rocks. Stylobeds are draglar pods are ovoid to lensoid, a few decimetres folded against some pods and are thick; all are monolithic (Fig. 71). Polyidenic fractured, releasing fragmentstheto stylobeds the breccia (or composite) vugular pods (Figs 71, 72) are (Fig. 73). large lensoid to tabular bodies, up to 2 m thick Notable in breccia pods are skeletal-boundand 10 m long which contain smaller monoand vugular-rock idens, about 1 to 2 m lithic, ovoid to lensoid units of similar size to stone in but ranging up to 4 m. Transitions monoidenic vugular pods. Units in the poly- fromsizebreccia to the composite skeletalidenic masses are in fitted or condensed fabric boundstone podpodtype (see later page) occur or (rarely) stylocumulate-support (Fig. 72). where the skeletal-boundstone idens dominate Monidenic vugular pods occur in stylo- the assemblage of breccia fragments. bedded to stylolaminated rocks, discrete and sharply bound by a circumidenic stylolite; how- Occurrence. Breccia pods are typical of the ever, there are rare gradations wherein stylo- Napier and Virgin Hills formations, forming bedded rock with scattered vugs grades into less than 5% of the rocks in the homocline. small monoidenic pods through increases in Breccia pods occur as isolated idens separated abundance of vugs. Vugular structure in the by tens to hundreds of metres of stylobedded pod is generally aligned parallel to host-rock or stylolaminated rock but they map out in stylobedding. Polyidenic masses also are dis- zones along strike and in some localities are crete and bounded by a circumidenic stylolite strike equivalent to continuous megabreccia but frequently there is interdigitation of stylo- sheets. bedded rocks and pod, stylobeds tailing away Pods into thin stylocumulate seams within the pod Skeletal-Boundstone Skeletal-boundstone pods are idens com(Fig. 72). Polyidenic vugular pods lie in hostmainly of skeletons of encrusting algae rocks that also contain numerous small mono- posed stromatoporoids. Three distinctive organic idenic vugular pods. Widely scattered mono- and assemblages are recognized: (1) Renalcis-Giridenic pods are supported by the host, but there vanella, (2) laminar stromatoporoid-jRewa/c/.y, is a continuous gradation from host-support and (3) Stachyodes-Renalcis', boundstone types through iden-support to the typical condensed formed by these assemblages are described in and fitted fabric of the polyidenic pod. Table IV. Some skeletal pods also contain Occurrence. The occurrence and origin of layers of ooid and skeletal grainstone, and vugular rocks (and structures) has been dis- brachiopod packstone; the layers are centicussed on p. 95. Pods composed of these rocks metres to several tens of centimetres thick and occur scattered through the homocline. They lie between boundstone sheets that bifurcate also occur as detached idens in stylobreccia and and coalesce. Usually grainstones are unaltered megabreccia sheets and in calcite-cemented but in some cases they grade into dolomitic and breccia. In situ vugular pods occur in two quartzose stylolaminite sheets. structural positions: (1) in monoclinal flexures, Monoidenic skeletal-boundstone pods are and (2) within crests of drag folds in stylobeds ovoid, polygonal to pyramidal to irregular, 10 adjacent to faults (Fig. 52). The pods in mono- cm to 1 m in size and are monolithic being clinal locations are lenticular, ovoid to tabular composed of one boundstone type (Fig. 71). and range from 10 cm to 1 m in size; pods in skeletal-boundstone pods are large drag folds are lenticular to oval and up to 40 Polyidenic idens bounded by circumidenic stylolites and cm in size. are composed of a number of smaller monolithic pods (Fig. 71). Polyidenic pods are Breccia Pods Breccia pods are idens composed of stylo- variable in shape—equidimensional, ovoid, lenbreccia and/or calcite-cemented breccia. The soid, tabular and wedge-like—and there is an characteristics of these rock types are described extreme range in size from small metre-size

PODS AND MEGABRECCIA SHEETS

POLYIDENIC

MONOIDENIC

SKELETAL BOUNDSTONE

^ ^ ^ ^ ^

— - — ^

MONOLITHIC

SKELETAL BOUNDSTONE

VUGULAR

SKELETAL BOUNDSTONE VUGULAR U

X h

> -l

o0.

— BRECCIA

Fig 71. Classification of pods on internal structure and lithology. masses to immense bodies over 30 m in length Renalcis boundstone, brachiopod packstone and 10 m thick (Fig. 74). The small mono- and pellet limestone. lithic units which form the composite mass are Laminar skeletons, sedimentary layers and similar in size and/or shape to the monoidenic boundstone pods, above; they are bounded by primary geopetal fills* in intraskeletal voids are circumidenic stylolites and are in fitted fabric. useful indices of displacement from deposiIn some polyidenic pods, all smaller units are tional orientation. These directional features composed of Renalcis-Girvanella boundstone often parallel stylobedding in the host rocks but but in others the units are of a variety of litho- in many of the monoidenic skeletal pods they types. Renalcis-Girvanella and laminar stro- are at angles of up to 45° to the stylobedding matoporoid-Renalcis boundstone are common and, in some cases, are inverted. The direcwith grainstone; less common are Stachyodes- tional indicators vary from iden to iden in poly* Internal sediment similar to host rock. In contrast, secondary internal sediment fills solution cavities and veins, and is composed of Fe-oxides and other insoluble material (see p. 78) as well as carbonate particles.

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN 100 idenic pods, indicating rotation of idens relative to each other (Fig. 75). Contacts between skeletal-boundstone pods and host rocks are sharply delineated by circumidenic stylolites but some pods are faultbound on one side; high angle faults form pods with steep walls, low angle faults form the inclined face of wedge-shaped pods (Fig. 76). The fault contacts are characterized by sharp truncation of stylobeds in the host and frequently there is drag folding, indicating the direction of movement. Skeletal-boundstone pods are embedded in: (1) stylobedded limestone and stylolaminite, (2) stylobedded limestone, stylolaminite and Fig. 72. Polyidenic vugular pod; stylolites and vugular limestone, and (3) stylobreccia. Stylostylocumulate sheets separate sub-pod beds and stylolaminae flanking pods commonly elements. Napier Formation near Wire Springs. terminate at the circumidenic stylolite, and underlying and overlying beds bow around the pod (Fig. 76). Some of these pods have flat, unaltered skeletal boundstone to cases where smooth bases and some have stepped bases. only remnants of original features are preRarely, stylobeds and stylolaminae lateral to the served. The alteration sequence is summarized pod interdigitate with the boundstone mass, in Figure 77. The products of alteration are: (1) interidenic stylolite sets that truncate earlier trailing away into thin stylocumulate seams structures, producing a strong stylobedding (Fig. 75). Stylobeds near the polyidenic pod in the pod. The stylobeds are parallel or contain decimetre-size monoidenic skeletal pods oblique to stylobedding in the host rocks, that are scattered through the host, but adjacent (2) dolomite seams along stylolites and fracto the large mass they come to form iden-suptures, dolomitic stylomottles and styloport and condensed fabrics and finally merge in laminite, and fitted fabric. Many pods are flanked by thin (3) vugular structures in hosts of skeletalsheets and wedges of lacy vugular rock which boundstone, after the skeletal layers. is interlayered with stylobedded rock and stylo- Occurrence. Monoidenic skeletal-boundstone laminated rock. The contact between vugular pods are scattered through the stylobedded rock and pod is sharply delineated by the cir- limestone stylolaminites characteristic of cumidenic stylolite. Pods embedded in stylo- the Virgin and and Napier formations. They breccia are sharply separated from other rarely reachHills a metre in size and account for breccia idens by circumidenic stylolites. less than 1% of the total rock mass. These Alteration of Skeletal-Boundstone Pods. Many idens are more abundant, scattered in megaskeletal-boundstone pods have been altered by breccia sheets; locally in these sheets they are interacting processes of pressure solution, dolo- agglomerated in condensed and fitted fabric to mitization and solution. These processes form large composite masses. Polyidenic skeleobliterate original structures and fabrics but tal-boundstone pods are features of the homogradations are traceable from relatively cline, occurring in Napier and Virgin Hills Fig. 73. Breccia pods: A—Breccia pod, base discordant to stylobedded rocks of host sequence. Napier Formation, Wire Springs area. Scale marked in 30 cm units. B—Close-up of gradational contact showing stylobeds broken off and merging with breccia (arrows); stylolites defining stylobedding are disoriented at pod margin and become circumidenic stylolites around breccia idens. Fig. 74. Skeletal-boundstone pods: A—Large equidimensional skeletal pod bound by circumidenic stylolite. Napier Formation, Dingo Gap. B—Large rectangular skeletal pod; height of pod as outlined by arrows is 11 m. Another pod occurs to the upper right corner. Pods are bound by circumidenic stylolite. Napier Formation, Tunnel Creek. C—Large skeletal pod (height 6 m), triangular in cross section. Apex of the body rests on underlying stylobeds. Napier Formation, Brooking Gorge.

PODS AND MEGABRECCIA SHEETS

FIG. 73

101

102

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 15

PODS A N D MEGABRECCIA

formations and in megabreccia sheets. The composite bodies frequently are scattered but they also occur in zones, along with breccia pods. MEGABRECCIA S H E E T S

The term megabreccia is here applied to large-scale breccias that contain randomly orientated idens a metre to tens of metres in size (Table IX). Most megabreccias have a stylobreccioid structure with fragments in fitted and condensed fabric (Figs 78, 79) but some have open interiden voids with fills of coarsely crystalline calcite and internal sediment (equivalent to calcite-cemented breccia). The idens in megabreccia are of similar lithology to the rocks adjacent to the megabreccia unit and all of the rock types described on previous pages are represented in the suite. Megabreccias in the abutment zone contain idens of Pillara Formation and Lamboo Complex along with lithotypes typical of the homocline sequence. Megabreccia units in the homocline contain only idens of rock types from the formations in the homocline. These include stylobedded limestone, stylolaminite, stylomottled and stylonodular rock, and stylobreccia; veined and vugular rocks and calcitecemented breccia; dolomitic rocks and dolomite. The idens are rotated relative to each other, thus demonstrating that metamorphism took place prior to incorporation in the megabreccia. The most striking of idens in megabreccia sheets are the vugular pods and polyidenic skeletal-boundstone pod masses (Fig. 71). Iden-size in the megabreccia units varies greatly over distances of a few metres: cobbleto block-size idens 'float' in pebble-size grounds; large blocks form frames with fitted and condensed fabric. Skeletal-boundstone pods form large agglomerated masses which extend as tabular to equidimensional masses for hundreds of metres in extent and are limited in vertical extent only by the boundaries of the megabreccia sheets. These agglomerate bodies are composed of block-size idens in fitted and condensed fabric (Fig. 79). Occurrence. Megabreccia occurs as thick mappable sheets and lensoid bodies in the abutment zone and in the homocline (Fig. 80). The sheets commonly occur at junctions between major rock units such as a unit composed mostly of dolomite and one composed of calcareous rock, or between members of the Pillara Formation. In the Napier Range area, five distinct megabreccia units occur, separating major rock units at various structural-strati-

SHEETS

103

graphic levels from the Lamboo-Pillara block through the homocline. Elsewhere megabreccia is mainly a feature of the abutment zone or of rocks along the base of Virgin Hills or Napier formations. Megabreccia sheets and lenses are several metres to tens of metres thick; they thicken and thin along strike and down dip. Breccias are mainly concordant but in many localities they are markedly discordant where top and base of the body locally steps across the stylobedding (Fig. 80). These steps are in the order of tens of metres. Megabreccias commonly bifurcate and anastomose and contain local lenses (up to 30 metres thick) of stylobedded limestone and other metamorphosed carbonate rock. Some megabreccias pinch-out and pass along strike into zones containing isolated breccia pods and polyidenic skeletalboundstone pods (Fig. 80). The contact of megabreccia bodies with host sequence is sharp to gradational. Sharp contacts are planar to undulating and marked by large scale stylolites; stylobedded limestone and metamorphosed carbonate rocks may be dragged against the megabreccia. Gradational contacts occur where rocks grade into the megabreccia with increase in abundance of more randomly oriented stylolites or where stylobeds are folded and fractured (Fig. 78). ORIGIN OF PODS AND MEGABRECCIA SHEETS

Origin of Vugular Pods We are here concerned with the mode of formation of monoidenic and polyidenic (composite) pods composed of vugular rock; and the shaping of these pods. Vugular rocks were formed by solution (vugs) and emplacement (vug-filling) often localized in drag folds along faults and in the axes of monoclinal flexures. These zones of tensional stress, however, were developed in the stylobedded and stylolaminated rocks by movement penecontemporaneous with the pressure solution and shear fracture which generated the widespread stylolitic structures, fabrics and rock types. Stress conditions were thus grossly compressional. Solution and emplacement took place along stylolite sets to produce rocks that consisted of closely spaced vugs and interlayered host rock. These idens of vugular rock with fill of coarsely crystalline calcite remained as relatively insoluble 'augens' or knots as surrounding limestone was pressure-solved; thus small monoidenic vugular pods scattered in the host sequence were formed. The formation of polyidenic vugular pods is traceable in gradations from monoidenic vugular pods scattered in a stylobedded host. There

104

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

Fig. 75. Skeletal-boundstone pods: Photograph and line diagram of polyidenic skeletal-boundstone pod. Heavy lines outline stylolites circumscribing sub-pod elements. Stipples indicate grainstone, or stylocumulate/reactate after grainstone. Arrows indicate attitude of directional features. Sheep Camp Yard, Geikie Range.

105 PODS AND MEGABRECCIA SHEETS placement of monoidenic pods involves episodic solution and dilation of host rocks between scattered monoidenic pods. Evidence for this process is the superimposition of vugular structure on an earlier vugular-rock precursor observed in some polyidenic pods. Origin of Breccia Pods The origin of stylobreccia by combination of pressure-solution and fracture was discussed on earlier pages. We are here concerned with the formation of pods composed of this rock type m m m m and shaping of these breccia pods. Most breccia pods developed along shear fractures in stylobedded and stylolaminated Figure 76. Wedge-like skeletal-boundstone pod. rocks. The shears frequently were sub-planar Napier Formation, Dingo Gap area. to sigmoidally-curved, truncating stylobedding at low to high angle, but parallel to and grading are two paths, not mutually exclusive: (1) into stylobedding at the extremities (Fig. 81). agglomeration as stylocumulate, and (2) mul- Breccia fragments generated along the shear tiple emplacement. Agglomeration as stylo- surface 'piled-up' in the discordant sector and cumulate is based on the low pressure-solubility were less numerous at the concordant of vugular rock relative to grainstone host extremities. Thus the initial breccia body was a rocks (p. 38). Monoidenic vugular pods lensoid to sigmoidal-lensoid iden in a styloinitially scattered in the host, agglomerate with bedded host; the extremities were stratiform, pressure-solution loss of intervening host-rock the centre discordant. Continued pressure solucarbonate. The end product is a composite pod tion concentrated at the shear surfaces formed with smaller monoidenic pod elements in fitted a circumidenic stylolite around the breccia or condensed fabric and separated by thin iden. The stratiform margins were eliminated seams of stylolaminite (Fig. 72). Multiple em- leaving the central mass ('pile') as a lensoid to 5 metres

QUARTZOSE STYLOLAMINITE

STYLOBRECCIA

LIMESTONE. BRECCIA. STYLOLAMINITE

DECIMETER-SIZED VUG

RENALCIS-GIRVANELLA BOUNDSTONE

S T R O M A T O P O R O I D - R E N ALOIS BOUNDSTONE

B O U N D S T O N E INTERLAYERED W I T H SEDIMENT

UNALTERED PODS

1. PRESSURE S O L U T I O N

1. DOLOMITIZATION

2. S O L U T I O N OF BOUNDSTONE

2. SOLUTION OF STROMATOPOROIDS

1. PRESSURE SOLUTION AND DOLOMITIZATION MAINLY IN SEDIMENT LAYERS

ALTERED PODS

STROMATOPOROID

RENALCIS-GIRVANELLA

VUG

STYLOLITE

DOLOMITE

Fig. 77. Altered (metamorphosed) skeletal-boundstone pods.

2. SOLUTION OF B O U N D S T O N E A N D SOLUTION ALONG STYLOLITES

9. S E D I M E N T

INTERLAYERS

STYLOCUMULATE

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN 106 ovoid iden (Fig. 81). Development of equi- ships between mound and host sediment (Fig. dimensional and rectangular composite pods 82). from lensoid, ovoid and irregular forms was Precursor. There are three paths, due to pressure solution along pod margins. Biostromal mutually exclusive, in the modification of Pods localized stress at lateral margins, resolv- not a biostromal or lens. Pressure-solution ing shear into normal compressive stress. Low alone, leads tosheet development of a circumiangle dislocations across the top also modify denic stylolite atthebiostrome margins, internal pods to wedge-shapes; the inclined face of a adjustment and volume reduction, with eliminawedge-shaped pod being the fault surface. tion of thin margins. Thicker parts remain as It is important to note that skeletal-bound- corroded idens (Fig. 82). stone occurs as fragments in breccia pods. It Shear stress directed along the bedding of follows therefore, that if skeletal boundstone sheets results in both fracture and becomes dominant in the fragment assemblage boundstone pressure solution. Boundstone sheets are dis(either because there was an abundance of located and fragments are rotated; pressure boundstone along the shear fracture or because modifies the shape of these idens (Fig. of agglomeration by pressure solution) breccia solution The margins of the biostrome if composed pods grade into polyidenic skeletal-boundstone of82).interlayered boundstone and sediment yield pods. small pods composed of interlayered boundOrigin of Skeletal-Boundstone Pods stone and sediment; the main mass yields idens The most difficult pods to interpret are of skeletal boundstone. monoidenic skeletal-boundstone pods. The The third and most probable path involves question is whether these pods were originally biostromal sheet-like accumulations of neg- shear along stylobedding that is superimposed at an angle to the layering in the biostrome (Fig. ligible synoptic relief, or biohermal with signifi82). Shear causes separation of biostromes into cant synoptic/stratigraphic relief. Sheet-like small and pressure solution modifies the interlayers of skeletal boundstone and sediment shape idens of these. This path leads to skeletalin these pods suggests that synoptic relief may boundstone idens scattered in stylobedded host have been low but the evidence of supporting frames also suggests the potential for localized (Fig. 82). biohermal development. The question cannot Origin of Polyidenic Skeletal-Boundstone Pods. be resolved because of the superimposed struc- In accounting for the origin of polyidenic tures and fabrics and the unknown volume skeletal-boundstone pods attention is focused losses involved. Biohermal and biostromal pre- on: (1) gradations from breccia pods to polycursors are, however, useful models for an idenic skeletal-boundstone pods, (2) occuraccount of post-depositional modification of rence of the pods in megabreccia, (3) rotated skeletal-boundstone bodies by pressure solution structures and fabrics in pods, and (4) pod and fracture. The developmental sequence is margins. These gradations and the evidence of rotation indicate that the polyidenic pods are illustrated in Figure 82 with both precursors. Biohermal Precursor. Pressure-solution of a agglomerates. Agglomeration took place by biohermal precursor leads to modification of pressure solution and movement, but there are the margins and obliteration of stratigraphic several probable paths leading to a polyidenic interfingering and other stratigraphic relation- skeletal-boundstone pod (Fig. 83); these are: Fig. 78. Megabreccia: A—Drag folded rocks at base of megabreccia, Sheep Camp Yard. Scale (arrowed) is marked in 30 cm units. B—Drag folded rocks at base of megabreccia; stylobeds are fractured at contact and incorporated into breccia. Same locality as above, reverse side of cliff. C—Megabreccia with blocks of skeletal boundstone in reactate-support fabric (left of photograph) locally coming to form fitted fabric (right of photograph). Note also incipient stylobedding approximately at 45° to stylobedding of underlying rocks. Sheep Camp Yard, Geikie Range. Fig. 79. Megabreccia composed mainly of skeletal boundstone blocks in fitted fabric. Stylolitic structures (e.g. stylobedding) within blocks are randomly oriented. Fault (outlined) separates masses of skeletal boundstone blocks (upper part) from stylolaminite and skeletal boundstone. 4.5 km SE of Dingo Gap. Hammer (arrowed) for scale.

PODS AND MEGABRECCIA SHEETS

FIG. 78

107

108 METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 79

PODS AND MEGABRECCIA SHEETS

109

TABLE I X

Characteristics of Napier lden Types Lithology Intraidenic features Stylobedded and stylolaminated dolomite; lenses of dolomitic Numerous intraidenic faults; drag lden stylobreccia and megabreccia; some veined and vugular dolo- folds (up to 10 m amplitude) Type mitic and calcite-cemented (dolomite) breccia; these rocks grade associated with faults 1 toward structural top of the iden into dolomitic stylonodular and stylomottled limestone and into Iden 2 lithotypes Quartzose stylolaminite, stylobreccia, stylobedded limestone; stylo- Intraidenic faults; drag folds (up nodular rock and quartzose-micaceous stylolaminite locally abun- to 1 m amplitude) associated with Iden dant; calcite-cemented breccia lenses and vugular rocks in lower faults and stylobedding dislocations; Type portions; megabreccia and stylobreccia pods, lenses and wedges; breccia and skeletal-boundstone 2 large skeletal-boundstone pods; veined and vugular rocks pods discontinuous but map out in uncommon in middle and upper parts except as breccia com- zones parallel to stylobedding ponents trends Stylobedded limestone and vugular limestone; slightly dolomitic Intraidenic faults and associated and lenses of calcite-cemented breccia also common toward drag folds (up to 1 m amplitude) Iden structural base; middle parts have zones of locally abundant locally common Type vugular pods (up to 2 m size); upper parts are stylobedded 3 limestone with minor stylobreccia and skeletal-boundstone pods ( < 3 m size) Stylobedded limestone and stylobreccia sheets interlayered in Discordant faults uncommon but 30 cm to 1 m units; breccias are spaced 1 per 1.2 m of section; stylobedding dislocations extremely Iden minor skeletal-boundstone pods (1 to 1.5 m in size, rarely up common; randomly oriented folds Type to 3m), stylonodular rock and stylolaminite. Veined and vugular (up to 1 m amplitude) common 4 rock and calcite-cemented breccia absent except as breccia components (1) agglomeration of small scattered pods under conditions of shear stress, stylobedding plane movement or slip, and no fracture (Fig. 83A), or (2a) formation of a breccia pod (stylobreccia) in a host containing abundant skeletal boundstone; pressure solution removes more soluble breccia idens (Fig. 83B), (2b) post-fracture pressure solution of a breccia pod in which skeletal pods are scattered; this concentrates pods as stylocumulate as more soluble components dissolve (Fig. 83C), or (3a) pressure solution during development of megabreccia dissolves intervening stylobreccia and concentrates pods as large scale stylocumulate. This mass then tends to act as an obstacle upon which material continues dissolving and upon which other skeletal-boundstone pods agglomerate, (3b) post-fracture pressure solution of a megabreccia that contains large polyidenic skeletal pods (formed by 3a above) may completely remove more soluble stylobreccia around the composite skeletal mass leaving a large pod isolated in stylolaminite.

shear mechanisms (1 above) is amplified here. Specifications are: (1) small pods scattered in a stylobedded limestone, (2) shearing stress with accompanying glide along stylobedding, but no fracture, and (3) movement and rotation of small pods along stylobedding planes as movement takes place. Under these conditions, intervening rock (vertical and lateral) between pods is removed by pressure solution; pods are moved along the stylobedding planes and begin to agglomerate. A variety of skeletalboundstone types are brought to and welded upon the larger agglomerate; idens of pressuresolved host rock may be caught up between the sub-pod elements. Thus a composite mass develops in which pod elements are in random

The development of polyidenic skeletalboundstone pods as a type of breccia pod or by later pressure solution of breccia and megabreccia is easily envisaged (2 and 3 above). Fig. 80. Summary characteristics of megabreccia The formation of composite skeletal pods by sheets and lenses.

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN shapes; the inclined face of a wedge-shaped A B pod being the fault surface. Origin of Megabreccias Megabreccias are units produced by shear fracture as indicated by discordant relationships, drag-folding of adjacent stylobeds, and rotated fragments, many of which are rocks with metamorphic structures and fabrics. Megabreccias have an origin similar to stylobreccia, i.e. the fracture and pressure solution along dislocation zones and the only real difference between theme is scale: megabreccia sheets developed along major faults which split and coalesced, at times isolating lenses of host rock in the breccia. Brecciation probably Fig. 81. Development of breccia pods. occurred during periods of rapid relief. Pinch-out of megabreccia along strike may orientation and in condensed to fitted fabric. The margins of the pod interdigitate with stylo- be due to diminution of dislocation along stylobedded rocks of the containing sequence, but bedding, and/ or pressure solution of thinner these extensions represent smaller pods that portions of megabreccia. Isolated breccia pods, have been welded to the main mass (Fig. 75). strike equivalent to megabreccia sheets, Development of equidimensional and rect- probably represent locally thicker portions of angular composite pods from lensoid, sig- megabreccia sheets that survived later pressure moidally curved lensoid, ovoid and irregular solution; composite skeletal-boundstone pods forms is probably due to pressure solution along probably represent stylocumulate. In areas pod margins. Pods tend to localize stress at away from the location of major brecciation, lateral margins, resolving shear stress into nor- perhaps some several to tens of kilometres mal compressive stress. Low angle dislocations away where movement par ailed stylobedding, across the top also modify pods to wedge- the major dislocation zone is lost in stylobed110

DISCORDANT DISLOCATION

LOCAL THICKENING IN CONCORDANT BRECCIA

. A

A __

DEVELOPMENT Or BRECCIA

| REMOVAL OF THIN PORTIONS OF BRECCIA BODY BY PRESSURE SOLUTION

REMOVAL OF THIN PORTIONS OF •

BRECCIA BODY BY PRESSURE SOLUTION

PRECURSOR BOUNDSTONE BODIES stZsZTft^K

S M A L L BIOHERM

PROCESS PRESSURE SOLUTION MODIFICATION OF MARGIN

SHEET T O L E N S O I D B I O S T R O M E S PRESSURE-SOLUTION MODIFICATION: LOSS OF MARGINS

mmWH

SHEET T O LENSOID B I O S T R O M E S FRACTURE AND PRESSURE SOLUTION

SHEET T O LENSOID B I O S T R O M E S

PRESSURE- SOLUTION AND FRACTURE: SLIP ALONG STYLOBEDDING AND SUBSEQUEN1 PRESSURE-SOLUTION MODIFICATION OF POD SHAPE

Fig. 82. Paths which monoidenic skeletal-boundstone pods could develop.

111

AREAL GEOLOGY A

PRESSURE

SOLUTION, WITHOUT

AGGLOMERATION

FRACTURE

——r

—

"f' 'r-y--

MOVEMENT ALONG STYLOBEDDING UNDER S H E A R S T R E S S ;

AGGLOMERATION

ALONG

DISLOCATION, WITH

PRESSURE

SOLUTION

DOMINANT

FRACTURE

SUBORDINANT

AND

AGGLOMERATION

STYLOCUMULATE FROM PODS AND

BRECCIA

MEGABRECCIA

sssm^ RELATIVELY INSOLUBLE P O D S PILE

P R E S S U R E S O L U T I O N OF MORE

UP A T INCLINED FACE

S O L U B L E IDENS LEAVING

C O N COM MIT A N T S H O R T E N I N G IN

DISLOCATION AS SURROUNDING

VERTICAL A N D LATERAL DIRECTIONS;

S T R A T A ARE P R E S S U R E - S O L V E D

S T Y L O C U M U L A T E OF P O D S

P O D S R O T A T E , MOVE C L O S E R T O G E T H E R A N D BEGIN T O AGGLOMERATE

—

^ l a s s i s s

Fig. 83. Paths by which polyidenic skeletal-bounds tone pods develop. ding planes and only thin sheets and lenses of breccia are generated. However, if the dislocation moved out of the stylobedding for a short distance (i.e. 'stepped across' stylobedding) then sigmoidally-curved, lenticular breccia pods and skeletal-boundstone pods developed. This is equivalent on a large scale to what happens at stepped junctions at macro-and micro-scale where stepped pressure-solution surfaces are slip surfaces and stylomottles are formed (see p. 56). AREAL GEOLOGY GENERAL

Detailed mapping was carried out in selected areas mainly to document the distribution of metamorphic rock types in relation to the major structural elements, namely the homocline, abutment and Lamboo Complex-Pillara Formation block. The areas selected were: (1) Southeast Napier Range and Fairfield Valley, (2) Pillara Range, and (3) Horse Spring Range. These studies gave an overall coverage of relationships and they were supplemented by observations at other localities throughout the outcrop belt, including Windjana Gorge, Copley Valley, Brooking Gorge, Geikie Range, and southern parts of the Laidlaw and Lawford Ranges (Figs 1, 84).

Variations on the basic structural plan described on p. 9 mainly relate to configuration of homocline and block. Linear homoclines are exemplified by the Napier Range and Pillara Range structures and similar structures are seen in eastern parts of the Emanuel and Hull Ranges (Fig. 1). Semicircular antiform structures with a central block of Pillara rocks and an off-dipping sequence of Virgin Hills and Sadler formations are a common structural type. Horse Spring Range is a typical example; others include Horseshoe Range, Lloyd Hill, Teichert Hills and the northern part of the Lawford Range. Antiform structures, lensoid in plan are typified by Laidlaw and Lawford Ranges (Fig. 84). Here the central Pillara block is flanked by off-dipping Sadler and Virgin Hills rocks that are folded and faulted along the flanks of the block. A peculiarity of these structures is the narrow elongation of the antiform nose along the axis at the southern end. N A P I E R R A N G E AND FAIRFIELD V A L L E Y

Napier Range is a low (30 to 100 m) narrow NW-trending range that extends over a distance of about 180 km between Alexander Creek and Big Spring (Fig. 84). Idens of Napier Formation rocks form the greater part

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

112

i i r

MT. BEHN NAPIER FORMATION

+ + + -.+ + 44-4-4.

GAP

+ + + + -t -t- + 4- 4- 4- -t + + + + +. + •+ TUNNEL CREEK

PILLARA FORMATION T O O

1 « °° BEHN CONGLOMERATE ' ° O + + +

+ +

LAMBOO COMPLEX BASEMENT

MOROWN CLIFF

+ + + -r 4- + +

{

+ + + + +

+ -T

— SPRANG + + + 97 MILE CREEK STUMPYS SOAK

N I

++ + + + + ++ + + 4- + + + + +t + +

10 km

HOMOCLINAL ROCKS: SADLER, VIRGIN HILLS, BUGLE GAP FORMATIONS CADJEBUT

FAULT

PILLARA FORMATION A T T I T U D E OF HOMOCLINAL ROCKS; DIPS GENERALLY > 20°

Fig. 84. A—Simplified geological map of the Napier Range, Oscar Range, Fairfield Valley, showing location of areas mapped in detail. B—Simplified geological map of Emanuel Range,. Lawford Range, Laidlaw Range and Bugle Gap area: modified after WAPET map C-16, 257.

AREAL GEOLOGY 113 of the range and occur in homoclinal attitude rock unit mapped by previous workers as with stylobed dips of 10 to 60° SW. Idens of Napier Formation. It has been possible to subPillara Formation occur discontinuously along divide the Napier rock unit on the basis of the eroded northern margin of the range. Pil- metamorphic rock assemblages into 4 iden lara stylobeds have variable dip from hori- types (Table IX). Nine major iden types are zontal to 10 to 20°NE or SW and overlie distinguished in the Napier Range-Fairfield schist and gneiss of the Lamboo Complex Valley (Table X). (Guppy et al 1958). Idens of Pillara Type Between MacSherrys Gap and Big Spring, Isolated idens of Pillara Formation types the homocline becomes the northern limb of a occur discontinuously between the Lamboo broad synform structure which lies about a Complex and the homoclines in both the WNW-trending axis running through the Fair- Napier and Oscar Ranges (Figs 85, 87). The field Valley into the Oscar Plateau (Figs 84, idens are wedge-shaped to lensoid in plan and 85, 86). Homoclinal dips flatten southwards to stylobedding in them dips at 5 to 20° towards, the axis and flat-lying Napier Formation rocks or away from, the synform axis (Figs 85, 86B, crop out throughout the valley and much of 87, 88). The idens are bounded by sinuous the Oscar Plateau (Figs 84, 85, 86). NW-trending strike faults and also contain Oscar Range (Fig. 1) which forms the south- numerous intraidenic faults which truncate ern limb of the synform is a complex structure stylobedding; these intraidenic faults which dip dominated by a central, WNW-elongate core at high angle toward the synform axis or are comprised of Lamboo Complex schist and near vertical are frequently marked by dologneiss and Pillara Formation (Fig. 84). Pillara mitic rocks; dips in Pillara type idens in the stylobeds extend along the northern margin of NE Oscar Plateau steepen to 45° or vertical the core from Morown Cliff to Oscar Plateau near intraidenic faults. and dip at 1 to 12°NE. Homoclines of Napier Behn Conglomerate Iden Formation rocks flank the central Lamboo/ The Behn Conglomerate (p. 8) underlies Pillara ridge along the north side; Napier For- the foothills of Mt Behn to the NW of mation stylobeds dip at 10 to 40°NNE-N- MacSherrys Gap (Fig. 87). The iden is oval NNW from an abutment with Pillara rocks. in plan and is composed of interstratified conDips flatten towards the synform axis in the glomerate and felspathic sandstone (Table XI). Fairfield Valley and along strike to the ESE The rocks dip at 5°S and SW. The Behn Coninto the synform nose. On the southern flank glomerate crops out poorly in the Mt Behn of Oscar Range a homocline of Napier Forma- area and foothills are pebble-covered scree tion dips steeply at 20 to 40°S. Napier strata slopes; in situ exposure occurs only in gullies. are either in direct abutment with Lamboo Some of the unconsolidated scree has covered rocks or with Pillara units (Fig. 84). Devonian carbonate rocks that overlie the iden Rock units previously defined in this area of Behn Conglomerate and has obscured geo(Guppy et al, 1958; Playford & Lowry, 1966; logical contacts in low-lying areas. Read, 1973<z, b) are (1) Napier Formation, Idens of Napier Type 1 Tabular idens of Napier type 1 (Table IX) (2) Pillara Formation, (3) Behn Conglomerate, and (4) Lamboo Complex. Detailed are the lowest units in the homocline, abuting mapping, however, reveals a complex of meta- the complex of Pillara idens and Lamboo rocks morphic carbonate rocks, particularly in the in the Napier Range and along the northern y

TABLE X

Homocline Rocks

Summary of Iden Types in Napier Range-Fairfield Valley Lamboo-Pillara Block

Pillara Formation idens Menyous Member type Red Bull Member type Big Spring Member type Behn Conglomerate iden Iden type 3. Stylobedded limestone and vugular limestone Iden type 2. Stylolaminite and stylobedded limestone Lamboo Complex iden (Pre-Cambrian; crystalline metamorphic Iden type 1. Dolomitic rocks rocks)

Napier Formation Iden type 4. Stylobedded limestone and stylobreccia

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN 114 margin of the Oscar Range. These idens are faults are traceable for 5 km before either bounded by sinuous NW-trending faults and fading into stylobedding or being cut by extend over distances of several tens of kilo- oblique faults. Drag folds associated with faults metres (Figs 85, 87). Boundary-fault sinuosity (Fig. 89A) have fold axes oriented down the and abundant subsidiary fractures cause dip. Some drag structures are acute cuspate marked changes in thickness; as measured per- folds with axial planes normal to stylobedding. pendicular to stylobedding the average thickness is 70 m but there is a 200 m section in the Idens of Napier Type 2 Idens of Napier type 2 (Table IX) overlie iden north of MacSherrys Gap (Figs 85, 86, the type 1 idens in both the Napier and Oscar 87). Stylobedding and stylolamination in the Range. The type 2 idens are tabular to wedgeidens dips at 25 to 60° towards the synform shaped and bounded by sinuous NW-trending axis. Numerous intraidenic strike and oblique faults (Figs 85, 87). The sinuosity of the faults faults (Fig. 43) truncate stylobedding at low results in marked variation in thickness, as angles causing local wedging, lensing and dis- measured perpendicular to stylobedding. In the cordance so that beds are traceable along strike Napier Range, iden thickness ranges from 150 only for tens of metres. Large intraidenic strike to 180 m; in the Oscar Range, the iden wedges LEGEND FOR FIGURE 8 5 ALLUVIUM AXIAL BRECCIA S C A T T E R E D BRECCIA BODIES WITHIN NAPIER FORMATION

a A

MEGABRECCIA SHEET (AND ZONE) WITHIN NAPIER FORMATION DOLOMITIC MEGABRECCIA BRECCIA: BASE OF MENYOUS AND/OR RED BULL MEMBER IDEN 4 : LIMESTONE AND STYLOBRECCIA IDEN 3 : LIMESTONE AND VUGULAR LIMESTONE

NAPIER

IDEN 2 : STYLOLAMINITE AND LIMESTONE

FORMATION

IDEN 1 : DOLOMITIC ROCKS MENYOUS MEMBER

PILLARA FORMATION

RED BULL MEMBER

++ f+TTT

•J-J-MUj

LAMBOO COMPLEX

GEOLOGICAL CONTACT

BEDDING T R E N D S

S CONTACT DOLOMITIC R O C K S A N D OVERLYING U N I T

MAPPED ZONE OF MEGABRECCIA, BRECCIA PODS, S K E L E T A L P O D S

r o

Fig. 85. Geological map of the Napier Range, north Oscar Range, Fairfield Valley and Oscar Plateau in the Dingo GapWire Spring area. Locations of cross-sections (Figure 86) are marked A-B, C-D, etc.

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

116

150 metres '

.300 metres r

^ ^

1 D

F §w-r+ + H|

:-\'kL Mfyy. '; /'-Vi:

Fig. 86. A—Geological cross-sections of Figure 85. B—Detail of part of cross-section C-D above. Dingo Gap.

H 1

117

AREAL GEOLOGY DOLOMITIC BRECCIA

ALLUVIUM AND SCREE MEGABRECCIA BETWEEN IDENS 1 AND 2

LOWER BRECCIA (PILLARA FORMATION FRAGMENTS)

LAMBOO COMPLEX COWAN

HILLS

GRADATION-

1 kilometre

Fig. 87. A—Geological map of the Napier Range between MacSherrys Gap and Mt Behn. B Attitude of geological contacts, determined by plane table, of area in cross-section A-B; datum is alluvial flats at foot of range. . C Attitude of geological contacts determined by plane table of area in cross-section C-D. out from 150 m west of Wire Spring to zero 2 km south of Wire Spring (Fig. 85). Stylobedding in the idens dips at 30 to 40° towards the synform axis. Local drag folds occur, in the idens, mostly associated with low angle intraidenic faults and shears along stylobedding surfaces. The axes of the drag folds mostly plunge down the dip of the homocline, denoting strike-slip movement.

Iden of Napier Type 3

A tabular iden of Napier type 3 occurs in the homocline along the northern margin of the Oscar Range. This iden is about 250 to 360 m thick and has been traced along the homocline for distances up to 20 km. Stylobedding dips within the iden range from 20 to 35°N (areas west of Wire Spring) to NE (east of Wire Spring). There is a monoclinal fold or

118

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN TABLE X I

Contrast between Behn Conglomerate and Breccia at Behn Iden Boundary BRECCIA BEHN CONGLOMERATE Composed of interstratified conglomerate Megabreccia (mainly dolomitic stylobreccia) with crude layering or stylobedding centimetres to decimetres thick, defined by iden size and felspathic sandstone and composition; fragments of angular quartz and mica schist, Conglomerate beds: 15 cm to 2 m thick; boulders of conglomerate, also fragments and blocks of Napier and well rounded quartz, quartzite and schist Pillara formations, local lenses (1 m thick, 10 m long) of very thin pebbles and cobbles; interstitial to sup- to thin interbedded sandstone and siltstone (similar to Red Bull porting sediment of felspathic sandstone Member units) and interbedded sandstone, siltstone, dolomitized limestone, and thin stylobreccia; contact of lenses with breccia Felspathic sandstone beds mostly 15 to sharp: locally large blocks bow down or disrupt laminae in lenses 30 cm thick; laminated and crosslaminated Breccia passes up into stylobedded and stylolaminated dolomite that has abundant sand-, granule- and pebble-sized quartz as laminae and bands

simple flexure developed in middle portions of the iden at many localities; this is responsible for local steepening (30 to 35°) and flattening of dip (20°); locally there are abundant pods and lenses of vugular limestone in the crest of this flexure. There are numerous intraidenic faults, mostly low angle and approximately the same attitude as the homocline. These are very difficult to map since they commonly die out into stylobedding planes. Large mappable intraidenic faults (Fig. 85) are discerned because they discordantly truncate stylobedding trends. Some features associated with the faults include drag folds, cuspate drag structures (resembling tepees, Fig. 89B), vugular pods (Fig. 60C), calcite-cemented breccia and stylobreccia. In some localities, near strike faults, stylobeds are steeply dipping to recumbent. Idens of Napier Type 4 An iden of Napier type 4 (Table IX) forms the core of the Fairfield Valley synform structure. The iden is bounded by NW-trending strike faults that are slightly oblique to the synform axis; this results in an asymmetric volume distribution of rocks about the synform axis (Fig. 85). Stylobedding in the iden has a dip up to 10°, flattening towards the synform axis. The maximum thickness of stylobeds exposed is 100 m. In the axis of the synform, the stylobedded limestone grades into pebble-and cobble-sized stylobreccia (axial breccia of Fig. 85) which cuts discordantly across the strike of the stylobedding in the limbs of the structure (Fig. 90B). The breccia along the synform axis reaches a maximum exposed thickness of 12

m, but the top is not exposed. There is semiconcordance between the axial breccia and the strike of the stylobeds. Margins of the axial breccia consist of interlayered sheets of stylobreccia and stylobedded limestone (30 cm to 1 m thick) which grade laterally and vertically into thick stylobreccia sheets toward the axis, until breccia dominates (Fig. 90B). Locally, along the margins of the axial belt, drag-folded and recumbently folded stylobeds occur between stylobreccia sheets; the folds are commonly truncated by the breccia sheets. Discordant faults are rare in type 4, though stylobedding-surface dislocations are extremely common. Randomly oriented folds are common, particularly in the vicinity of the axis and the megabreccia at the base. lnteridenic Boundaries Major interidenic boundaries in the Napier Range-Fairfield Valley area (Figs 84, 85, 87) are: (1) the NW-trending sinuous strike faults that lie between Napier idens of type 1 (dolomitic rocks) and the Lamboo Complex rocks on the north and south sides of the synform, (2) sinuous NW-trending strike faults that form the boundaries between Napier idens of types 1, 2, 3 and 4 in the Fairfield synform and the Napier Range and northern Oscar Range homoclines. Isolated Pillara idens which lie in the fault zones between the Napier idens and the Lamboo Complex also are fault bound, being cut by sinuous strike faults subsidiary to the major dislocations. Contacts between iden types of Pillara Formation also are faults and marked

Fig. 88. A—Cross-section and isometric block diagram (upper) through line A-B of Figure 87. Heights of geological contacts and topography have been levelled by plane table above arbitrary datum. B-C—Cross-sections C-D (middle) and E-F (lower) of Figure 87. Heights of geological contacts and topography determined by plane table.

AREAL GEOLOGY

FIG. 88

119

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN 120 by breccia. For instance, megabreccia occurs becoming increasingly stylolitic and fragbetween Menyous and Red Bull idens in the mented. The most characteristic rocks of the fault NE Oscar Plateau (Fig. 85); it has the same general attitude (<15° dip) as Pillara stylo- zones are dolomitic megabreccia and massive bedding, and merges with dolomitic mega- dolomite which extend along the strike in 5 breccia that is developed between Menyous and to 35 m thick sheets. The main variation along Napier idens further to the SE (Fig. 85). the boundary is the degree of dolomitization. Breccia similarly is developed at contacts Breccias vary from undolomitized at their base between Pillara idens and Lamboo Complex to dolomitic at the top (Fig. 88B, C) but locally, dolomitization is more pervasive, i.e. the dolo(Fig. 90C). mite may be restricted to interidenic areas Napier Iden 1—Lamboo Complex Boundaries. between in condensed to fitted fabric, The boundaries between the idens of Napier or may blocks form reactate-support fabrics. Thus, type 1 rocks and Lamboo Complex rocks are megabreccia grades up section and along strike NW-trending faults. Faults are planar to highly into massive dolomite and veined and vugular undulating surfaces that dip at very variable massive dolomite. The developed along angles between 10 and 60° towards the syn- the faults are describedbreccias three main localities form axis. In plan, the fault traces are markedly —each illustrating the invariation lithology wavy to sinusoidal (Figs 85, 87); in local ver- and structural setting: (1) Dingoof Gap, (2) tical section, these surfaces exhibit sigmoidal MacSherrys Gap area, and (3) Mt Behn. curvature (Fig. 90A). Commonly, fault surThe megabreccia at Dingo Gap is composed faces are not clearly defined; the dislocation is in a broad zone of deformation and metamor- of two rock bodies separated by a thin sheet phism with numerous strike and oblique faults, (30 cm thick) of sheared dolomite (dipping at drag folds (Fig. 89A), variations in dip within 45°SW). The lower zone abuts a Pillara iden, fault-bound idens, and calcite veins. Isolated and an upper zone adjoins Napier iden 1. The idens of Pillara Formation lie between the lower part of the megabreccia is a sheet, 10 Napier and Lamboo rocks but in many places to 15 m thick (inclined 60°SW), discordantly along the zone, Pillara idens are faulted out cutting across Pillara stylobedding (Fig. 86B). and the Napier idens directly abut the Lamboo This breccia is a dolomitic-calcareous stylorocks (Fig. 87). Where the faults are in con- breccia composed of Pillara fragments and tact with Pillara idens, stylobedding in the blocks, stromatoporoid fragments and skeletalPillara is commonly truncated (Figs 85, 86B, boundstone pods. Megabreccia overlying the 90A) and drag folded; there also may be thin sheared dolomite is an approximately 20 gradation into breccia with Pillara rocks m-thick sheet and has stylobedding inclined at A

Fig. 89. A—Drag-fold in Napier iden 1; axis plunges down the dip of the homocline. Dingo Gap area. B—Drag-fold (resembling 'tepee'-like structure—arrow) along low angle dislocation in Napier iden 3. Note asymmetry in thickness of fold limbs. Wire Spring area. C—Lacy vugular rock, drag-folded and vertical, along the major dislocation between Napier idens 1 and 2. Hammer (arrow) for scale. MacSherrys Gap area. Fig. 90. A—Sigmoidal-curve of base of megabreccia. This megabreccia, composed of Pillara fragments, cuts discordantly across stylobedded Pillara Formation (Red Bull Member) to the right. Aerial view; height of tree on cliff aproximately 16 m. Cowan Hills area; photo courtesy WAPET. B—Stylobreccia along the axis of the synform at Oscar Plateau/Fairfield Valley; rocks in the foreground and lateral to the axial breccia are stylobedded limestone and stylobreccia of Napier iden 4. Aerial view toward NW along Fairfield Valley; width of photo in middle ground approximately 700 m; photo by courtesy WAPET. C—Breccia between Menyous Member and Lamboo Complex rocks (grassy slope in foreground) near Dingo Gap. Contact between breccia and Menyous Member is outlined; displaced block of Menyous is arrowed. Breccia varies from 7.5 to 12 m in thickness over 75 m strike distance, and has an undulating discordant contact (<10°) with Menyous iden stylobedding. Lithologically breccia is dolomitic stylobreccia, veined stylobreccia and calcitecemented breccia with fragments of dolomitized Menyous lithotypes. Height of cliff approximately 20 m. Fig. 91. A—Photograph of lensoid breccia iden, Dingo Gap area. Details of variation along strike of megabreccia between Napier idens 2 and 4: B—located at Dingo Gap. C—located on cliff face approximately midway between cross-sections A-B and C-D of Figure 85. This unit is the lower of the 2 breccia sheets. D—located on cliff face at cross section E-F of Figure 85.

AREAL GEOLOGY

121

122

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

FIG. 90

s r

V —H

^^^Tietre^j V= H

V= H STYLOBEDDED LIMESTONE AND STYLOLAMINITE

QUARTZOSE STYLOLAMINITE

STYLOBRECCIA

MEGABRECCIA K>

124

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

15 to 20° SW. The contact with Napier Formation is sharp and inclined at 20 to 30°SW. This upper megabreccia is dolomitic and composed of a melange of limestone blocks, metamorphosed carbonate blocks and wedges and lenses of metamorphic rock, including dolomite. In the MacSherrys Gap area the fault zone is a complex of fractured rocks. Approximately 1 km south of the Gap the breccia (6 m thick) is sandwiched between idens of Lamboo and Napier (Fig. 86A); no Pillara Formation occurs, but fragments of Pillara lithotypes are in the breccia. The contact with Napier iden 1 is undulating and discordant (Figs 87B-C, 88B) to stylobedding (in the Napier). The breccia is 'cut out' by fissile to massive dolomite (Napier iden type 1), which rests directly on Lamboo rocks approximately 250 m S of MacSherrys Gap. The breccia reappears beneath Napier iden 1 about 1 km north of MacSherry's Gap; from here further northwards it is composed of two closely related sheets (Figs 87, 88B): the lower part of the breccia is up to 30 m thick and typically contains fragments derived from the Pillara and the overlying Napier iden. Locally, lenses up to 5 m thick of Pillara Formation occur in the breccia; these are composed of sequentially intact but metamorphosed Pillara carbonate cycles (p. 11). There is much wedging caused by intraidenic dislocation surfaces in this lower breccia and several wedges may be present in 20 to 30 m of section (Fig. 90A). The breccia also becomes more dolomitic upwards, and passes abruptly or gradationally over several metres into an upper dolomite breccia. Contact between lower and upper breccia is mostly semiconcordant and hence the two roughly parallel each other along stylobedding strike; however, the dolomite breccia commonly truncates the lower breccia down-dip (Fig. 88B). The contact of the dolomite breccia with Napier iden type 1 varies from discordant to concordant and dolomite stylobeds locally are drag folded and abut against the breccia (Fig. 88B). To the far north the breccia swings to the NE and cuts out the underlying Pillara Formation; it then merges into a breccia that occurs between Behn Conglomerate and other units (Fig. 87). The fault between the Behn Conglomerate iden and idens of Pillara and Napier Formation in the Mt Behn area (Figs 84, 87) is a continuation of the dislocation at MacSherrys Gap. The exact attitude of the fault is not

known but outcrop distribution of fault breccia suggests low angles (<45 °). A lithologically distinctive breccia (Table XI) occurs in a SW-trending belt 1 km wide along the fault. Three zones are recognized in this belt. Nearest to the Behn Conglomerate iden (Fig. 87) the rocks are red-brown stylobreccia (Table XI) interlayered with minor dolomitic and quartzose stylolaminite; locally there also is conglomerate/ breccia up to 3 m thick composed of disoriented quartz and mica schist pebbles, fragments of mineralized and vugular dolomite and Pillara lithotypes. These pebbles (mostly rounded, less than 20% are angular) are in iden-support fabric and have interidenic dolomite. Total thickness of this zone is unknown though the exposure on hillsides is not more than 5 m thick. The next zone is well exposed and consists of megabreccia, approximately 7 m thick. The third zone, up to 10 m thick, is composed of interlayered (30 cm to 1 m units) stylobreccia (dolomitic and nondolomitic) and dolomitic and quartzose stylolaminite and passes gradationally into stylobedded and stylolaminated dolomite of idens of Napier type 1. Boundaries between Napier Idens. Boundaries between Napier idens 1, 2, 3, and 4 are major low angle faults that are planar to undulating to sinusoidal; the fault surfaces dip at similar angle to the stylobedding. These dislocations are commonly marked by megabreccia sheets and pods, stylolaminite and thick calcite veins. In many localities stylobeds, stylolaminites and vugular rocks are drag-folded against the megabreccia (Fig. 89C). Zones of megabreccia vary in thickness from 5 to 15 m, though individual breccia sheets are generally up to 10 m thick. Breccias are mostly concordant along the length of strike exposure but locally discordant to stylobedding trends in overlying and underlying rocks. Discordances cause marked thinning and thickening of adjacent units over relatively short distances. A good example of this occurs near Wire Spring where the breccia between idens 3 and 4 becomes discordant to both underlying and overlying stylobeds for approximately 4 km and then returns to an attitude that is semiconcordant with stylobeds in iden 3. Traced laterally, megabreccias pass into thick sheets of interlayered stylobreccia, calcitecemented breccia, skeletal-boundstone pods (up to 6 m high and 30 m wide), stylolaminite, thick calcite veins and megabreccia lenses and pods.

125 AREAL GEOLOGY The breccia zone between idens 2 and 4 in finally fades out into the stylobedding. Traced the Napier Range has the following variation: along its strike to the SE, the fault surface NW of Dingo Gap the zone contains two thick twists through vertical to a 20 to 25°NE dip megabreccia sheets (Figs 85, 86, 91B) which and is finally seen to merge with stylobedding merge into a single sheet to the SE. Around and lamination along the interidenic boundary Dingo Gap the zone is composed of several between the Pillara iden and the Lamboo Combreccia sheets, zones of breccia pods and thick plex, described below; locally conjugate stylosheet-like polyidenic skeletal-boundstone pods lite sets occur in Pillara rocks adjacent to the (Fig. 91). Further toward Oscar Plateau the fault (Fig. 93). Where the fault dips NE, zone becomes thinner and, at the SE extremity parallel to the stylobedding, it is marked by a of the mapped area (Fig. 85) consists of two narrow zone of stylobreccia and calcite veins; thin megabreccia and stylobreccia sheets with a broad wedge of stylobreccia and calciteintervening stylolaminite and discordant thin cemented breccia (Fig. 93D) marks the fault stylobreccia (Fig. 91C). A general summary of where it dips SW opposite to the stylobedding. changes along strike in a megabreccia sheet is Stylobeds become markedly lensoid adjacent to illustrated in Figure 80. the fault and individual beds 'pinch-out' adjacent to it; approximately 6 m of section is lost PILLARA RANGE distances of 10 m. A massive, non-bedded Pillara Range (Fig. 1) is a low NW- over appearance is commonly developed adjacent to elongated range about 27 km long and 2 to 3 the intraidenic dislocations. The massive struckm wide that rises out of alluvial plains. Rock ture is due mainly to increases in the number units previously defined in the range (Guppy of anastomosing stylolites which obliterate the et al.y 1958; Read, 1973a, b) are: (1) Lamboo stylobedding. Complex, (2) Pillara Formation, and (3) Sadler Limestone. The Lamboo Complex base- Sadler Iden The Sadler iden is composed dominantly of ment of gneiss and mica schist crops out poorly in the southern pediment and alluvial plains to vugular rock and calcite-cemented breccia. These rocks were developed in a brachiopodthe south of the range. The core of the Pillara Range is an iden of packstone precursor and partly altered remPillara Formation rocks (Fig. 92); this Pillara nants of this parent remain throughout the iden is elongate in a NW direction and rests section. A thick sheet of dolomitic megabrecon Lamboo Complex rocks. An iden composed cia containing blocks of Renalcis-Girvanella of Sadler Limestone stylobeds abuts and over- boundstone, vugular rock, brachiopod packlies the Pillara iden along the northern margin stone, and stylobreccia, is the uppermost unit in the Sadler iden extending to the limits of outof the range. crop at the level of the alluvial plain along the Pillara Iden range margin. The part of the Pillara iden which is exposed northern The Sadler rocks are strongly stylobedded on consists of stromatoporoid and pellet limestones scales up to a metre. The stylobedding dips at of the Menyous Member (Read, 1973a, b) 30 to 35°NE to form the homocline (Fig. 92) with a thickness of 420 m. The total thickness which abuts the Pillara iden along an irregular and shape of the Pillara iden is uncertain interidenic boundary (Fig. 93A). Locally the because of the difficulties in extrapolating iden stylobeds have been thrown into symmetrical boundaries into the subsurface. folds of 10 to 20 m wavelength axes that Rocks in the Pillara iden contain a strong are parallel to the general strikealong of the stylostylobedding that is subparallel to the original bedding. sedimentary layering and inclined at 20 to 25° NE (Fig. 92). Additionally, there are numerous Iden Boundaries intraidenic surfaces which strike N to NW and Pillara lden-Lamboo Complex. The boundary dip at variable attitude mainly from 20 to 60° between the Pillara iden and the Lamboo ComNE. These dislocate the stylobedding and have plex is exposed only along the southern face of associated drag folds, abundant joints, stylolites the range. Here it is a subplanar interidenic and stylobreccia. One of the more accessible surface that dips generally at 20 to 25°NE, of these faults was studied in detail to docu- parallel to the stylobedding in the Pillara iden. ment its attitude and associated metamorphic The boundary is marked by a 5 m thick zone, phenomena. Exposed on the faces of Menyous within the Pillara, of calcareous and minor Gap the fault surface dips at about 65°SW but quartzose stylolaminate, stylonodular rock and traced NW, the surface twists into a NE dip and stylobreccia. Corroded remnants of Amphipora

126

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN The contact of Sadler Limestone with boundary-zone rocks is locally gradational where calcite-cemented breccia and vugular limestone of the Sadler merge with similar rocks of the boundary zone; in other places the contact is abrupt and marked by large-scale stylolites. The contact of the Pillara Formation with the boundary-zone rocks is irregular to stepped, and generally abrupt (Fig. 93A). The contact is a large scale pressure-solution interface and there are commonly large penetrations (up to several metres) of boundary-zone rocks into stylobeds of the Pillara iden (Fig. 93A). The Pillara rocks at the abutment are heavily stylolitized. Stylobeds of tabular-stromatoporoid and coral limestone pass toward the abutment into a rock in which stromatoporoids and corals are in fitted fabric. Stylobreccia near the abutment is composed of stromatoporoid and coral fragments, with large stromatoporoids rotated up to 45° from the bedding position. vicinity of Menyous Gap.

and Stachyodes in the stylolaminates indicate that the precursors were probably Amphipora and Stachyodes packstones similar to those described by Read (1973a, b). Stylobeds in the boundary zone are crumpled into a series of low amplitude folds, 50 to 100 m wavelength along axes that plunge at 20°NE parallel to the stylobedding in the iden. Sets of strong shear joints (Fig. 93C) also cut the stylobedding, with the intersection of joint surfaces plunging down the stylobedding. The folds and associated pressure-solution features indicate that the interiden boundary is a shear zone with a strong strike-slip movement. Sadler-Pillara Iden Boundary. The abutment between the Pillara and Sadler idens is marked by a 3 to 10 m wide zone that contains layers, lenses and wedges of calcite-cemented breccia, stylobreccia, veins, and vugular rock. Breccia fragments consist of Pillara Formation lithotypes and Sadler Limestone lithotypes; veins and vugs in these breccias are filled with coarsely crystalline calcite and internal sediment.

HORSE SPRING RANGE

Horse Spring Range is a small semi-circular antiform structure about 4 km across, on the northern end of the Hull Range homocline. The structure was mapped in detail by Rogers (1971) and a modified version of his map is given in Figure 94. Stratigraphic units previously identified in the range include: (1) Lamboo Complex, (2) Pillara Formation, (3) Sadler Limestone, and (4) Virgin Hills Formation. The Lamboo Complex rocks are mainly schists that crop out weakly in alluvial plains to the south of the structure. The core of the structure is an iden of Pillara Formation rocks; this iden, sub-quadrate in plan (Fig. 94) probably rests on Lamboo Complex rocks but the contact is obscured by alluvium. Virgin Hills rocks crop out in a semicircular belt around the western, northern and eastern margins of the Pillara iden. The antiform structure is defined by a strong stylobedding in the Virgin Hills rocks that dips at 20 to 45° away from the central iden. The abutment between the Pillara and Virgin Hills

Fig. 93. A—Abutment zone, Pillara Range. Gently dipping, very thick stylobedded stromatoporoid biostromes to right; thick stylobedded iden of Sadler Limestone dips more steeply to the left. Abutment zone rocks outlined; note planar to sinusoidal contact with Sadler iden and planar to indented contact with Pillara iden. Locality 3 of Figure 92. B—Limestone with three conjugate stylolite sets overlying the sheared Lamboo/Menyous contact. Locality 1 of Figure 92. Scale is 10 cm long. C—Stylolaminite and cross-cutting shear joints in Menyous iden a few metres from its contact with Lamboo Complex. Locality 1 of Figure 92. D—Calcite-cemented breccia along the fault at Locality 2 of Figure 92. Note rotated fragments of fenestral' pellet limestone and Stachyodes limestone.

AREAL GEOLOGY

FIG. 93

127

128 METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN idens is marked by a megabreccia which con- The distribution of fragment types in the tains large displaced idens of Sadler Limestone. breccia is distinctive. Fragments of Pillara lithotype are common along the contact Pillara lden The part of the Pillara iden that is exposed between the megabreccia and the Pillara iden; consists of stromatoporoid and pellet lime- middle parts of the breccia contain fragments stones of the Menyous Member (Read, 1973a, of Pillara and Sadler lithotypes. Fragments of b) with a total thickness of 800 m. The shape Virgin Hills lithotypes are abundant along the of the iden is uncertain because of difficulties contact between megabreccia and that formain extrapolating iden boundaries into the sub- tion. Other abundant fragment types include stylobreccia, calcite-cemented breccia, vein calsurface. Rocks of the Pillara iden contain a strong cite and polyidenic skeletal-boundstone pods. stylobedding that is subparallel to the sedi- LAIDLAW RANGE mentary layering and inclined at 10 to 20° The Laidlaw Range (Fig. 84) is an elongate NE. Numerous intraidenic faults that strike N-trending low range about 8 km long. It is NW (Fig. 94) cause stratigraphic discrepancy triangular in plan, with a width of 0.5 km at and are characterized by narrow shear zones the southern, and a width of 5 km at the with stylobreccia and calcite-cemented breccia; northern end. Pillara stylobeds form the greater stylobeds adjacent to the faults are drag-folded part of the range; the stylobeds generally dip and there are many cross-cutting calcite-filled at less than 5°N, NE or NW. Sadler Limeveins. stone forms an anticlinal structure around most of the margin of the central Pillara iden. The Virgin Hills lden The Virgin Hills iden consists of red car- homocline of Sadler stylobeds dips at 15 to bonate rocks with strongly developed stylo- 30°W and SW on the western side of the range bedding and lamination. The main rock types and 12 to 35°E on the eastern side. The westare stylobedded limestone, calcareous and ern and eastern homoclines of Sadler meet in a quartzose-micaceous stylolaminite; stylobreccia faulted contact at the southern end of the and calcite-cemented breccia lenses are inter- range. The fault zone is characterized by layered with the stylolaminite. Monoidenic and breccia, dolomite, calcite veins, sulphide polyidenic skeletal-boundstone pods are scat- mineralization and intense stylolitization. Silttered throughout but increase in abundance stones of the ?Permian Grant Formation (Guppy et al., 1958) also are caught up in the towards the Virgin Hills-Pillara boundary. fault zone; these siltstones occur as disoriented lden Boundaries blocks, or steeply inclined to vertical, sheared Pillara-Virgin Hills Iden Boundary. The boun- lenses and wedges. dary between the Pillara and Virgin Hills idens is marked by megabreccia that crops out in a METAMORPHISM 30 to 200 m wide zone. The attitude and thick- METAMORPHIC ZONATION ness of the megabreccia unit is unknown i it The most impressive aspect of the Devonian passes into thin fault zones along the strike on carbonate sequence along the northern margin the western side of the structure and is ter- of the Canning Basin, is the abundance of faults, minated by another fault on the eastern mar- folds, dislocation structures and metamorphic gin. On the west the megabreccia bifurcates to rocks generated by shear stress and metasomaenclose large blocks of Sadler Limestone (Fig. tism. Major rock units are bounded by parallel 94). strike faults of major dimensions that have a The megabreccia unit truncates stylobedding, strong strike-slip component. Concordant intraidenic faults and joints in the Pillara iden breccias, drag structures, megabreccia and and stylobeds are drag-folded and fractured other breccia tailing away into layering indicate against it. Contacts between the megabreccia that much movement was parallel to, and taken and Virgin Hills rocks are usually sharply up by, pressure solution and shear along stylodefined and oblique to the strike of the Virgin bedding and lamination. Hills stylobedding; in places thick meandering This style of cataclastic metamorphism has veins cut the contact and become increasingly been pervasive from the structural base of the abundant, forming calcite-cemented breccia. Pillara block through to the structural upper The megabreccia is mainly stylobreccioid levels of the homocline (Sadler, Napier and and dolomitic; locally there are calcite- Virgin Hills formations). The abutment cemented breccias and in places calcite veins between the formations in the homocline and cut breccia fragments and interfragment voids. the Lamboo/Pillara block was the major

129 METAMORPHISM presented. Similar zonations are observed in Virgin Hills and Sadler formations. A much condensed but similar metamorphic zonation is present in the Pillara Formation near the abutment; here, however the metamorphic zone is usually only a few tens of metres wide. The metamorphic zonation is summarized in Figure 95 and is analysed below in terms of: (1) compressional stress, and (2) tensional stress. Compressional Stress, Metamorphic Zonation. The zonation of metamorphic features produced under compressional stress, from the abutment away, is: Zone 1: (a) Dolomitic rock suite: massive dolomitic stylolaminite, dolomitic stylobreccia, stylomottled dolomite, (b) Polyidenic skeletal-boundstone pods: stylocumulate agglomerates. Zone 2: (a) Quartzose and quartzose-micaceous stylolaminite, stylobreccia, stylobedded limestone, (b) Polyidenic skeletal-boundstone pods: stylocumulate agglomerates. Zone 3: Stylobedded limestone, stylobreccia. The gradation (Fig. 95A) reflects increasing stress conditions towards the abutment with an increasing dominance of dolomitization, pressure solution and shear fracture, at all scales. The highest metamorphic grade is represented Fig. 94. Geological map of the Horse Spring by the dolomitic rock suite and the agglomRange area (after Rogers, 1971). erated, rotated pod masses adjacent to the abutment. The formation of dolomite adjacent to deformational zone, with most intense meta- the abutment may also be related to T gradients morphism; there was a decreasing intensity of and availability of Mg pressure-solved from metamorphism away from the abutment, host rocks. expressed in parallel belts of characteristic rock Tensional Stress, Metamorphic Zonation. Caltypes. cite-cemented breccias, veined rocks and vuguThe zonal pattern has been complicated by: lar rocks are most abundant near the abutment (1) temporal variations between compressional (zones 1 and 2) and grade away into the homoand tensional stress along the abutment, (2) cline in decreasing order of abundance (Fig. variable response of precursor rocks to meta- 95B); these rocks are absent in zone 3 (above) morphism, and (3) local variations in structure except as components of breccia. The distribui.e. intensity of deformation. Temporal variation of vugular rocks is complicated by their tions in stress conditions are clearly expressed occurrence in local fault zones and flexures. in overprinting of metamorphic features on earlier metamorphic precursors and studies of METAMORPHIC GRADE The data that elucidate the probable grade paragenesis (of features) indicate a history of alternating compression and tension. Rocks of metamorphism are: (1) co-existence of dolomite-quartz-calcite, produced by combinations of pressure solution, (2) general lack of talc or tremolite shear fracture and dolomitization (under comthroughout dolomitic units, and pressional stress) are host to, or overprinted by, (3) development of dolomite. features formed by combinations of fracture, solution and emplacement under tensional The highest metamorphic grade is represented stress; these in turn have frequently been sub- by the dolomitic rock suite where the mineral jected to later compressional-stress metamor- assemblage is dolomite-quartz-calcite, with phism to give a complex paragenesis. Neverthe- minor chlorite and micas. This is a stable less a general zonation, based mainly on Napier assemblage of the greenschist facies (Turner, Formation idens in the homocline can be 1968). Hewitt (1973) has recorded dolomite J QUATERNARY

j MEGABRECCIA

j VIRGIN HILLS FORMATION ] SADLER LIMESTQNE

? SADLER LIMESTONE

130

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

Fig. 95. A—Summary of zonation of compression al stress features. B—Summary of zonation, tensional stress features. as one of the first minerals to appear in argillaceous limestone in response to low grade (greenschist) metamorphism. Data from laboratory studies (Rosenberg and Holland, 1964; Lippman, 1973) also indicate that dolomitization proceeds readily with T > 100°C. Reaction between dolomite, calcite and quartz to produce talc or tremolite is documented experimentally with T > 400°C (Metz & Trommsdorff, 1968). Magnesian silicate minerals also are recorded from higher-grade greenschist and amphibolite facies (Turner, 1968). Thus the general lack of talc and tremolite in the dolomitic rocks sets an approximate upper T limit for the pervasive metamorphism of the Canning Basin carbonates. Away from

the abutment, T probably was relatively lower as suggested by widespread pressure solution of calcite. However, crystallization of chlorite and micas indicates that alteration was still within the field of low grade metamorphism (Turner, 1968). CONCLUSIONS Carbonate sediments and rocks are profoundly metamorphosed under T and P conditions in which other sedimentary rocks and mineral assemblages remain relatively stable. Metamorphism of carbonate rocks produces new rock types in addition to new structures, fabrics and textures. The general lack of documentation of metamorphic features in car-

CONCLUSIONS bonate rocks, especially those located in deformed sequences is due to the basic assumption, made by many sedimentologists, that observed features are sedimentary or early diagenetic. This assumption is questionable. Metamorphism under compressive stress involves three main processes that often are penecontemporaneous: (1) pressure solution, (2) shear fracture, and (3) dolomitization. Recrystallization processes also are operative in compressive stress fields. Metamorphism under tensional stress involves: (1) dilation and rotational deformation with consequent formation of strain cavities, and (2) solution with formation of vugs. Emplacement by precipitation and internal sedimentation (infiltration) results in filling of strain and solution cavities. Metamorphism, Compressive-Stress Environment Compressive stress produces strain features in carbonate rocks but the earliest and dominant response is pressure solution. Stress modes for pressure solution range from uniaxial such as produced by overburden pressure and also shear, common in tectonic deformation. The rate of stress application probably is crucial. If it exceeds the capacity of rocks to respond by pressure solution, then shear fractures develop. Thus, shear fracture as a metamorphic process is intimately related to pressure solution and the two processes are frequently penecontemporaneous. This is expressed by pressure-solution interfaces becoming shear fractures and vice versa. Structures produced by pressure solution and shear fracture are: (1) stylolites, (2) stylobedding, (3) stylolamination, (4) stylomottled structure, (5) stylonodular structure, and (6) stylobreccioid structure. These structures are superimposed on earlier sedimentary or metamorphic rock features. In particular, stylobedding and stylolamination are frequently at an angle to sedimentary layering (Fig. 7). In the Devonian sequence of the Canning Basin, homoclines are defined by stylobedding and original sedimentary layering (often near horizontal) is palimpsest. Pressure-solution interfaces (and shear fractures) develop at all component levels in carbonate rocks—grain to grain, fragment to fragment, bed to bed, etc.; that is, iden to iden (Fig. 96). There is volume reduction as solids are lost to intrastratal fluids and reductions up to 80% can be demonstrated using components of known dimensions, e.g. fossils. However, in most cases it is not possible to estimate the

131

volume loss across any pressure-solution interface (Appendix I). The unknown volume loss has important implications in regard to structures, textures and fabrics in rocks and also to the interpretation of the stratigraphy of pressure-solved sequences. There is an order of relative pressure solubility that depends primarily on the stability of the mineral lattice in the given PT field. Materials composed of stable minerals therefore accumulate at pressure-solution interfaces as less stable (equivalent to more soluble) idens are dissolved. The accumulated mineral is termed stylocumulate. Common stylocumulate minerals in carbonate rocks are quartz, felspar, micas, clay minerals, dolomite and Feoxides. Calcareous components of carbonate rocks also are ordered in relative pressure solubility. This order is related to crystallinity and granulinity. More coarsely crystalline components are less pressure-soluble than finely crystalline granular components. Calcareous materials more resistant to pressure solution also form components of stylocumulate. Thus, because of the unknown volume reduction involved, all carbonate rocks with stylolites must be regarded as stylocumulate. Reactions occur at pressure-solution interfaces. These involve intrastratal fluids and mineral phases, and new minerals, stable in the PT field grow. These minerals are termed reactate. Dolomite is the most common reactate in pressure-solved carbonate rocks but mica, quartz, Fe-oxide and coarsely crystalline calcite also are reactate minerals. Stylocumulate and/or reactate are disposed in pressure-solved carbonate rocks according to the orientation of stylolitic structures, as laminar sheets and lenses, as irregular patches (stylomottles) and as anastomosing networks (stylobreccioid structure). Stylocumulate and reactate bodies are usually bedded or laminated because of the superposition of numerous stylolite sets. Intergradational fabrics developed by pressure solution are: (1) iden support, (2) condensed, (3) fitted, (4) stylocumulate-support and, (5) reactate-support. These terms are nonscalar and apply to relationships between all idens in pressure-solved sequences, e.g. grains, blocks, beds (Figs 3, 96). Fitted fabric is characterized by idens that are bounded by circumidenic stylolites and are in contact along their entire margins; more soluble idens are embayed. Fitted fabric is the penultimate product of pressure solution (the

132

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

ultimate product with idens of relatively high purity, is nothing). It evolves from earlier fabrics with continued volume reduction and pressure-solution loss of material, which formerly separated idens. Pressure-solution agglomeration is an apt term for the process whereby relatively insoluble idens are brought into fitted-fabric relationships. Stylocumulate- and reactate-support fabrics are characterized by isolated idens which float in stylocumulate or reactate ground. Stylocumulate-support evolves in rocks that contain large quantities of material low in the order of pressure solubility; reactate-support evolves when reactate minerals crystallize at the pressure-solution interfaces. An intergradational suite of rock types is produced by dynamic metamorphism involving pressure solution and shear fracture: (1) stylobedded limestone, (2) stylonodular rock, (3) stylomottled rock, (4) stylolaminite, and (5) stylobreccia. Stylolaminite is the penultimate or ultimate product of pressure solution, being characterized by subparallel stylolite sets and abundant stylocumulate (or reactate) sheets and lenses. Stylonodular rocks and stylomottled rocks and stylobreccia are transitional to stylolaminite, under sustained stress. Metamorphism, Tensional-Stress Environment In carbonate rocks the dominant response to tensional stress is the formation of strain cavities. These can be produced by both dilation and rotational deformation and include veins, vugs and interfragment voids in breccias which develop where tensional stress is locally resolved into shear components. Strain-cavity shape and orientation is fundamentally controlled by stress application so that the cavities may develop independently of structures and fabrics in the host rocks. More commonly, however, pre-existing structures and fabrics exert an influence, and cavities are aligned along pre-existing surfaces such as stylobedding and stylolites. The formation of interconnected networks of cavities permits free flow of intrastratal fluids through the host rocks and leads to further metamorphism. The processes are: (1) solution, (2) precipitation, and (3) infiltration (internal sedimentation). Solution occurs when migrating fluids moving through cavities are undersaturated with respect to carbonate mineral phases in the host. Strain cavities (and other cavities) are enlarged and solution cavities further develop by dissolution of host-rock components. Thus strain

cavities are intergradational with solution cavities and development of the two types is frequently penecontemporaneous. Solution is selective on more soluble hostrock components. This results in embayed margins and extensions of cavities along more soluble layers. It also results in release into cavities of less soluble host-rock components, as supporting matrix is dissolved. Precipitation occurs when migrating fluids are saturated. Precipitate crystals nucleate on cavity walls and grow inwards, eventually filling the cavities if fluid migration is sustained. In the carbonate rocks of this study, crystalline carbonate, aragonite or calcite, was the precipitate. Infiltration results in emplacement of internal sediment, generated by solution stoping or mechanical stoping in cavities. Internal sediments vary greatly in composition but common components are calcite-crystal silt and sand, quartz-silt and sand, dolomite, mica and Feoxide. Carbonate grains also are components of internal sediment and include ooids, ooid fragments, pellets, intraclasts, lithoclasts, skeletal fragments and carbonate-rock fragments. Solution stoping occurs at cavity walls and involves selective solution of host-rock components and release of less soluble components as supporting matrix is dissolved. Mechanical stoping is a process that occurs when particles are dislodged from the host by movement along cavity walls. This process is responsible for the generation of large particles such as rock fragments in the internal-sediment suite. Sedimentary structures such as lamination, current and graded bedding and micro-unconformities indicate transport and deposition of internal sediment under conditions of free fluid flow through cavities. The formation of strain and solution cavities and emplacement of precipitate and infiltrate within them, generates new structures and fabrics that are superimposed on the host-rock precursors. Structures include: (1) parallel sheet ^ (2) rectangular to rhomboid I vein structure (3) breccioid J (4) irregular (6) stromatactid j u g u l a r structure (7) breccioid J Fabrics developed by cavity formation and filling are: (1) host-support, and (2) fill-support.

CONCLUSIONS

133

The metamorphic imprints result in new rock types; these are: (1) veined rock, (2) vugular rock, and (3) calcite-cemented breccia. In the Devonian carbonates of this study these rock types were developed in precursor rocks that had been subjected to prior metamorphism in compressive stress environments. The host rocks include: stylobedded limestone, stylolaminite, stylomottle and stylonodular rock and stylobreccia; dolomitic rocks.

Fig. 96. Scale of metamorphism: A—Pressure-solution agglomeration: quartz in fitted to condensed fabric with interidenic remnant carbonate grains. B—Pressure-solution agglomeration: skeletal-boundstone pods in fitted to condensed fabric with interidenic rock. C—Lensoid and sigmoidally-curved stylomottle idens (stippled) and resistant skeletal idens (white) scattered in sheetlike stylolaminite idens. D—Lensoid and sigmoidally-curved stylobreccia idens and resistant skeletal-boundstone pod idens scattered in sheetlike and lensoid stylobedded rock idens. E—Formation idens: blocklike to sheetlike to lensoid.

Metamorphic Zonation and Grade Dynamic (cataclastic) metamorphism is not generally zoned in terms of increasing grade toward a shear zone but carbonate rocks are very susceptible to alteration and have responded both chemically and physically, producing graded belts of deformed rock. The zonation reflects stress (and temperature) gradients. Metamorphism under compressive stress has produced three broad rock suites locally gradational but more commonly separated by faults: Zone 1, a region of intense shear and dislocation and probable (relatively) high temperatures, is dominated by dolomitic rocks, v/z. stylolaminite, massive dolomite, and stylobreccia; Zone 2 is dominated by quartzose stylolaminite, stylobreccia, breccia and polyidenic skeletal-boundstone pods and minor limestone; Zone 3 is least metamorphosed and composed of a thick suite of stylobedded limestone and stylobreccia. These zones reflect: (1) shear fracture through the sequence, (2) the eventual reduction of all sedimentary and metamorphic rocks to stylolaminites with increasing metamorphic grade, and (3) the development of dolomite in areas of relatively higher temperature. Metamorphism under tensional stress has produced a very simple zonation: veined rocks, vugular rocks and calcitecemented breccia are most common in regions of intense dislocation (zones 1 and 2) and tail away in abundance from such zones; metamorphic rocks formed by tensional stress are virtually absent in zone 3. The development of chlorite, micas and dolomite, the co-existence of dolomite and quartz, and the lack of talc and tremolite, points to low metamorphic grade (moderate pressure, low temperature) in the region. The highest grades were probably within greenschist facies. Field Relations The Devonian carbonate rocks outcropping along the northern margin of the Canning Basin are located in a shear zone of major

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

134

dimensions. T h e majority of carbonate rocks are metamorphic, the result of deformation under shear, compressional and tensional stress. T h e carbonate rocks have responded to dislocation in the shear zone, mainly by pressure solution and fracture, with movement taken up by layering. Idens responding to deformation are micronsized to formation-sized bodies, and the structural and fabric relationships between small bodies is a microcosm of relationships that exist at formation scale (Fig. 9 6 ) . Harker (1932) and Higgins (1971) are workers who recognized that metamorphic microstructure is a reflection of macrostructure. F o r instance, H a r k e r (1932, p. 167), in a discussion on lenticular structure in cataclastic rocks, stated: 'Normally stress has been relieved in the first place by the formation of oblique gently curved surfaces of discontinuity, along which slipping took place. The mass is thus broken into lenticular pieces, of larger or smaller dimensions, with one general orientation, and may become subdivided in the same way. The lens form is indeed highly characteristic of this type of dynamic metamorphism, and recurs on every scale of magnitude. It is seen on the geological map; it is seen in the field; and it is seen under the microscope.' T h e peculiar stratigraphic-structural relationships of m a n y features such as resistant idens of Pillara Formation, the abutment zone, lensoid breccias and pods, antiform structures and stylobedding folds, are the result of deformation and concurrent or subsequent large scale ALDERMAN, A . R . , & SKINNER, H . C . W . ,

BECKER, G. F., & DAY, A. L., 1916: N o t e on the

linear force of growing crystals. J. Geol. 24, p. 313-333.

BOWEN, N. L., 1940: Progressive metamorphism of siliceous limestone and dolomite. J. Geol. 48, p. 225-274. Carbonate

lithofacies and environments of the Tribes Hill Formation (Lower Ordovician) of the Mohawk Valley, N.Y. / . Sedim. Petrol. 39, p. 113-135. BROWN, W .

ACKNOWLEDGMENTS This paper is the result of research studies carried out in the D e p a r t m e n t of Geology, University of Western Australia, during the 3-year period 1971-74 by members of the sedimentology research group. V. Semeniuk played a m a j o r role in the research while supported on a Postdoctoral Fellowship provided by West Australian Petroleum Pty Ltd in the period January 1973 to M a y 1974. T h e study was f u n d e d by grants f r o m West Australian Petroleum Pty Ltd (WAPET) . Gratitude is due to R. G . Alexander, Jr, f o r m e r Chief Geologist of WAPET, who suggested the study programme, and M . H . Johnstone, Chief Geologist, who supported the project with advice and encouragement in later years. W A P E T geologists who were helpful with advice include D . A. Lyons, R. A. McTavish and E. Horstmann. T h e authors also thank C. Hughes and J. Lang f o r photography and drafting; Mrs. P. Whackett f o r typing; C. J. M u r r a y f o r thin sections; J. F . Read, T. Cooke and F . L. Billing f o r field assistance. J. F . Read, G . M. Hagan, R. Rezak and R. G. C. Bathurst provided m u c h helpful discussion. Special thanks go to J. J. E. Glover and R. G . Brown who critically read the manuscript. Views expressed herein are not necessarily those of West Australian Petroleum Pty Ltd.

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H . , FYFE, W . S., & TURNER, F .

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petal fabrics: important aide for interpreting ancient reef complexes (Abs.). Bull Am. Ass. Petrol Geol 56, 645. PLAYFORD, P . E., & LOWRY, D. C., 1966: Devonian reef complexes of the Canning Basin, Western Australia. Bull geol Surv. West. Aust. 118, 150 pp.

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21, p . 3 4 4 - 3 9 4 . READ, J. F., 1974: Calcrete deposits and

Quaternary Sediments, Edel Province, Shark Bay, Western Australia; in B. W. Logan et al., Evolution and Diagenesis of Quaternary Carbonate Sequences, Shark Bay, Western Australia. Mem. Am. Ass. Petrol. Geol. 22, p. 250-282. RIECKE, E . , 1895: Ueber das Gleichgewicht zwischen einem festen homogen deformierten Korper und einer fliissigen Phase, insbesondere iiber die Depression des Schmelzpunktes durch einseitige Spannung. Ann. Phys. 54, p. 731-738. ROGERS, K., 1971: Geology of the Horse Spring Range, Canning Basin. B.Sc. Hons, thesis, Univ. Western Australia. ROSENBERG,

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APPENDICES

137

APPENDIX I

taining relatively insoluble material: 1 cm of quartzose stylocumulate may represent residue from pressure solution of 1 m of limestone that had 10% quartz, or 10 m of limestone that had 1% quartz, or unknown thicknesses of pure limestone intercalated with quartzose limestone. In any hypothetical case, pure limestone is removed by pressure solution leaving no trace of stylocumulate; thin stylocumulate seams may represent the insoluble residue from only one bed. There also is the possibility that where shear has occurred along stylobedding planes, the observed rock sequence does not represent the original vertical distribution. Thus, unaltered limestones overlying a stylocumulate seam may not be representative of the original precursor rocks.

APPENDIX I I

erroneous use of 'bladed' to describe such forms. It is important to note that while fibrous habit (= fine thread-like strands) may be gradational into acicular, the two are not synonymous. Crystal Size Organization. Three main organizations are recognized: (1) equigranular: crystals are approximately similar in size; this is typified by equigranular aggregates of equant crystals or acicular crystals arranged as crust on a surface. (2) graded: crystals increase in size away from a surface; this is where equant or columnar crystals increase in size away from the margins of an aggregate. (3) inequigranular: where various sizes of crystal are randomly distributed. Crystal Orientation. Four categories are recognized : (1) oriented normal, where long crystal axes are oriented normal to a surface and commonly the surface is the foundation on which crystals nucleated; (2) oriented subradiating, where crystals are aggregated into subradiating groups; each group is juxtaposed against another, the group origin is on a surface and the plane of symmetry of the aggregate is normal to the surface; this surface is commonly a foundation on which the crystals nucleated (3) radiating or spherulitic, where crystals form spherulitic structures; (4) randomly oriented, where crystals have no orientation in the aggregate. Crystal Aggregate Packing. Packing refers to how closely crystals are arranged. It is tight where crystals adjoin and are in contact along most of their length. Packing is loose where crystals are scattered (Fig. 97); material between crystals in loose aggregates may be other types or host rock. Internal Features of Crystals. Internal features of crystals are: (1) straight extinction, (2) undulose extinction, (3) twinning, and (4) inclusions. Crystals contain combinations of the above fea-

Interpreting Volume Loss after Pressure Solution There are problems in estimating the amount of carbonate lost to intrastratal fluids due to pressure solution. Firstly, the distribution of potential stylocumulate in precursor host rocks is not known; furthermore stylocumulate such as mica, Fe-oxide and dolomite may have originated first as reactate. Secondly, the irregular distribution of stylocumulate along pressure-solution surfaces and the textural evidence of corrosion of quartz by carbonate idens, suggest that relatively insoluble stylocumulate also is dissolved under the appropriate chemical conditions. Thirdly, stylocumulate represents accumulation of residue from carbonates conNomenclature of Coarse Crystalline Calcite; Crystals and Aggregates Frequently it is difficult to decide from observation whether aggregates of coarse crystalline calcite in carbonate rocks are: (1) unaltered precipitate, (2) recrystallized precipitate, (3) recrystallized precipitate enlarged by marginal replacement of the host rock, (4) recrystallized host rock (neomorphic spar, Folk, 1965), and (5) recrystallized skeletal carbonate. In the veined, vugular rocks and calcite-cemented breccias of this study, petrographic data indicate that the aggregates are combinations of (2), (3), (4), and (5) above; in other words, are of multiple genetic types. This realization creates problems in application of the descriptive terminology used by students of carbonate diagenesis (Bathurst, 1971; Folk, 1965; Dunham, 1969) because most of these terms have strong genetic connotations. For example, the term cement, defined as a precipitate in a void (Bathurst, 1971), is too interpretive and misleading in that coarse-crystalline calcite in many cavities has no function in lithification. We, therefore, use a descriptive scheme based on: (1) crystal shape (geometry), (2) crystalsize organization, (3) crystal orientation, (4) crystal aggregate packing, and (5) internal features of crystals. This approach serves to describe most coarsely crystalline calcite aggregates. Features of this working scheme are summarized in Fig. 97. Note should be made that size of crystals was not brought into the scheme—this parameter is mentioned in descriptions. Crystal Shape. Common crystal shapes in carbonate crystal aggregates are: (1) equant ( = blocky, or equidimensional), (2) columnar ( = form resembling slender columns), and (3) acicular (== slender, needle-like crystals). Some columnar crystals may be slightly tapered at proximal or distal ends and are terminated by scalenohedral or rhombohedral faces. Two-dimensional views of columnar crystals have led to the

138

METAMORPHISM OF CARBONATE ROCKS, CANNING BASIN

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Fig. 97. Summary of descriptive terminology for single crystals and aggregates. tures, e.g. simple extinction and inclusions. Some crystals also contain inclusions in one part and not in another part. Crystals in aggregates do not all have similar internal features. For example,

crystals that have undulose extinction and numerous inclusions may occur in one part, while crystals that have straight extinction and no inclusions occur in another.