Abstracts 13 - Recent Sediments in Eastern Australia Marine through Terrestrial Syd Uni Feb 1985

Geological Society of Australia

ABSTRACTS Number 13

"RECENT SEDIMENTS IN EASTERN AUSTRALIA - MARINE THROUGH TERRESTRIAL"

ISSN 0 7 2 9

011

GEOLOGICAL SOCIETY OF AUSTRALIA INCORPORATED ,»

ABSTRACTS

NUMBER

RECENT

SEDIMENTS

MARINE

ft\ I

13.

IN E A S T E R N

THROUGH

AUSTRALIA

TERRESTRIAL

Abstracts of a Symposium organized by the New South Wales Division, in conjunction with the Australian Sedimentologists Group and the Australian Marine Sciences Association.

Held at Sydney University, February 14 - 15th, 1985.

Published by the Geological Society of Australia Incorporated, February, 1985.

ISSN 0729-011X

GEOLOGICAL SOCIETY OF AUSTRALIA INCORPORATED NEW SOUTH V7ALES DIVISION

SYMPOSIUM ORGANISING COMMITTEE

COMUEMOR:

DR. P.S. ROY N.S.W. DEPT. OF MINERAL RESOURCES

COMMITTEE:

DR. B.J. FRANKLIN DR. J.B. KEENE DR. C.R. WARD

NEW SOUTH WALES DIVISION COMMITTEE

19 8

4-85

CHAIRMAN:

Dr.

l/ICE-CHAIRMAW;

PROF. P.B. HOSTETLER

SECRETARY:

DR. C.R. WARD

ASSISTANT

SECRETARY

B.J. FRANKLIN

MR. A.E. WALTHO

TREASURER:

MR. G. McILVEEN

COMMITTEE:

DR. A.N. CARTER DR. J.B. KEENE DR. E. SCHEIBNER

»/

1.

INTRODUCTION

The N.S.W. Division Symposium for 19 85 has taken the form of an interdisciplinary conference on Quaternary sedimentary deposits in eastern Australia.

A total of

thirty eight papers are to be presented, describing processes that are taking place at present, or in the recent geological past/ in a wide range of continental, coastal and offshore environments, with emphasis, in many cases, on the relevance of these processes to the activities of man.

In keeping with the interdisciplinary nature of the program, the symposium has been co-sponsored by two other organizations, the Australian Sedimentologists Group and the Australian Marine Sciences Association, and the N.S.W. Division of the Society thanks the members of both these bodies for their co-operation.

Thanks are also due to the

individual authors of the papers presented, for responding to the call, to the Chairmen of the various Technical Sessions, and to Sydney University Geological Society for co-operation with the catering.

Lastly, but most

importantly, our thanks to Peter Roy, the Symposium Convenor, without whose dedication and enthusiasm this meeting would not have been possible.

Colin R. Ward

(Secretary)

for the N.S.W. Division Committee,

-

#

SYMPOSIUM PROGRAiMME AND LIST OF CONTENTS Thursday February 14.

8,00 8.50 -

8.50 9.00

Page No.

Registration - Carslaw Building, Sydney University.

Introduction & Welcome: BRENDA FRANKLIN, Chairman, N.S.W. Division, Geological Society of Australia.

MORNING TECHNICAL SESSION

Chairman:

BRUCE THOM.

- Theatre 12.

9.00 -

9.25

Wilma Blom & Jock Keene. of the Bass Basin.

9.25 -

9.50

Angus Gordon.

Sydney shelf.

Sediment features & processes of the

9.50 - 10.15

Harry Poulos. sediments.

Engineering properties of Bass Strait

MORNING TEA

Late Quaternary history

1.

4.

10.15 - 10.45.

10.45 - 11.35

KEYNOTE ADDRESS: Jim Bowler. Climatic & tectonic influences on sedimentation style in surficial deposits of southeastern Australia.

12.

11.35 - 12.00

Gerald Nanson. Holocene floodplain formation & reworking along N.S.W. coastal rivers.

17.

12.00 - 12.25

John Marshall & Peter Davies. Late Quaternary sedimentation on a rimmed continental shelf: northern Great Barrier Reef.

20.

LUNCH

12.25 - 2.00

CONCURRENT AFTERNOON TECHNICAL SESSIONS. SESSION A -

Chairman:

2.00 -

2.25

Rob Warner. Downstream variations in channel morTDholoav: Bellinger Valley, N.S.W.

22.

2.25 -

2.50

Gerald Nanson & Bob Young. Late Quaternary history of fluvial sedimentation on the Necean River near Penrith, N.S.W.

25,

2.50 -

3.15

Scott Lawson & Michael Knight. Potential land erodibility St effects on Lake Wollumboola during urbanization of Culburra, South Coast, N.S.W,

28.

AFTERNOON TEA

BOB LOUGHRAN.

-

Theatre 12.

3.15 - 4.00

4.00 -

4.25

David Outhet.

N.S.W,

4.25 -

4.50

Gresley Watson. Landform development i sedimentation in a delta in an ephemeral saline lake - Lake Buchanan, Queensland.

33,

4.50 -

5.15

Wayne Erskine & Mike Melville. Late Holocene valley fill stratigraphy of Dairy Arm, N.S.W,

36.

SESSION B

Chairman:

Sediment deoosition in Lake Wvanaala,

COLIN WARD

30.

rheatre 5.

2.00 -

2.25

Brian Lees & Annemarie Clements. Implications of chenier dates in northern Australia.

39,

2.25 -

2.50

Lex Nielson & Angus Gordon. on the inner shelf.

42.

2.50 -

3.15

Peter Roy. The origin of transgressive marine sands in southeastern Australia.

AFTERNOON TEA

Size of sand ripples

47.

3.15 - 4.00

4.00 -

4.25

Charles Phipps. Phosphatised volcanic ridge on the upper slope off central N.S.W.

51.

4.25 -

4.50

Bob Burne & G.W. Skyring. Anaerobic processes & the fate of organic carbon, Davies Reef, Great Barrier Reef.

55.

4.50 -

5,15

Barrie Bolton & G. Boadle. On aspects of the sedimentclogy & biologic interaction in intertidal environments near Red Bluff, Corner Inlet, Victoria.

SYMPOSIUM DINNER - Holme & Sutherland Room, Universitv Union.

7.on - 11.on

111.

Page No.

Friday February 15.

MORNING TECHNICAL SESSION

Chairman:

PETER ROY.

- Theatre 12

8.35 -

9.00

Armstrong Osborne.

9.00 -

9.25

Bob Loughran, Bryan Campbell & Greg Elliot. The use of the tracer caesium - 137 for studying sediment movement in drainage basins.

62.

9.25 -

9.50

Linda'Moore & Bob Burne. Genesis of lacustrine stromatolites in South Australia & Western Australia.

65.

9.50 - 10.15

John Pickett. Significance of faunas in interpreting Pleistocene geology, Richmond River valley. New South Wales.

67.

MORNING TEA 10.45 - 11.10

Sedimentation in caves - a review.

58.

10.15 - 10.45 Bruce Druery, Graham Hurrell & J. Floyd. The measurement of sediment transport in N.S.W. estuaries.

70.

11.10 - 11.35

Bruce Thorn, John Chappell & Colin Woodruffe.

Kolocene sedimentation in estuaries of northern & eastern Australia.

71.

11.35 - 12.00

Mai Jones & Andy Stephens. Evolutionary processes on a deltaic coast - evidence from the Barron Delta, Trinity Bay, Queensland.

72.

12.00 - 12.25

Alberto Albani, Peter Rickwood St J.W. Tayton. Acoustic stratigraphy & sediment structures of the continental shelf off Sydney.

LUNCH

75.

12.25 - 2.00

CONCURRENT AFTERNOON TECHNICAL SESSIONS. SESSION A -

Chairman:

2.00 -

2.25

Edward Drew. Sediment generation by Halimeda meadows in the northern Great Barrier Reefl

76.

2.25 -

2.50

Paula Douglas. Short-term sediment movement in the Woronora estuary.

77.

2.50 -

3.15

Jenny Hacker & Bill Ward. Holocene sedimentation :n the Brisbane River coastal plain.

30.

AFTERNOON TEA.

BRUCE DRUERY

-

Theatre 12

3.15 - 3.45

3.45 -

4.10

Brenton Grant, Peter Flood Edgar Frankel. The origin of 18-mile swamp: a modern back-barrier oeat-forming^ environment, north Stradbroke Island, S.E. Queensland.

83.

4.10 -

4.35

Brian Rust, Gerald Nanson & Graham Taylor. Mud braids in an arid-zone river. Cooper Creek, Central Australia.

84.

4.35 -

5.00

Tony Martin Palaeogeography, palaeolimnology, palynology & coastal environments - areas of^neclect in coastal studies.

86.

SESSION B -

Chairman:

2.00 -

2.25

Annemarie Clements.

Holocene-Pleistocene sands of the Fens Embayment, mid-north coast of N.S.W.

2.25 -

2.50

Marie Ferland. Barrier island morphology during rising sea level.

95.

2.50 -

3.15

John Hann. Late Quaternary geological evolution of Botany Bay.

96.

AFTERNOON TEA

ANDY SHORT.

- Theatre 5

92.

3.15 - 3.45

3.45 -

4.10

Peter Ccwell. Wave-induced sand mobility i deposition on the south Sydney inner continental shelf.

99.

4.10 -

4.35

Andy Short.

102.

4.35 -

5.00

Jack Davies. Longshore variability in nearshore sediments - north coast of Tasmania.

South-east Australia shoreface facies.

INFORMAL FAREWELL FUNCTION 5.00 - 7.00

106.

LATE QUATERNARY HISTORY OF BASS BASIN WILMA M. BLOM and J.B. KEENE ^

(Department of Geology and Geophysics, University of Sydney)

Bass Basin occupies a slight topographic depression in Bass Strait (see Figure 1) and is an active depositional basin for recent sediments which are in turn underlayen by rocks of Cainozoic and Late Cretaceous age. The sedimentary basin was first formed during rifting between Australia and Antarctica and it is bounded by older Palaeozoic and pre-Cambrian rocks. The pre-Quaternary rocks in the basin do not crop out either on land or on the sea floor however, 16 petroleum exploration wells have sampled the deeper strata. The maximum water depth in the basin today is 82 m and the sea floor is particularly smooth and near horizontal below 60 m. There is a shallow sill of Palaeozoic rocks to the east (maximum water depth 56 ni), and a less well-defined sill to the west (maximum water depth 70 m).

Thus there is relief of some 12 m below sill depth.

Very little is known about the Quaternary history of this depositional basin. The aims of the present study are: 1) to map the late Pleistocene to Recent stratigraphy in Bass Basin using ostracodes, foraminifera and radiocarbon dating to document the marine transgression ,following the last glacial, and in particular to establish basin morphology, water depths and depositional environments during lower sealevel stands; 2) to determine sedimentation rates in the various environments; and 3) to establish the facies pattern for bryozoal sand and calcareous mud sedimentation since this type of sedimentation is relatively rare on modern shelves but common in the geologic record.

A piston-coring programme was carried out in February 1984 on HMAS Kimbla, and in September 1984 on RV Sprightly.

In all, 29 grab samples and 29 cores of cumulative length of 54.4m were recovered.

Core recovery varied from less than half a metre in the coarser sediments of the basin edges, to nearly five metres in the finest sediments of the central and south-central basin. Sampling was carried out along a grid pattern (Fig. 1) and a fairly good coverage of the basin was obtained.

Preliminary analyses show that the modern basin margins in the east, north and west are covered by the coarsest sediments, as would be expected in shallower areas with higher energy regimes. Despite their proximity to granitic basement outcrops these sediments are predominantly bioclastic (bryozoans, echinoderms, bivalves and gastropods). The central and south-central basin areas are covered by calcareous sandy muds with up to 70% mud containing 80% CaCOa. The mud becomes more terrigenous towards the Tasmanian coast. Most of the longer cores show a distinct change in texture and composition at approximately Im below the seafloor. The central and south-central cores have sediments below this break which consist predominantly of laminated muds (with a CaCOs content less than 50%) infrequently

interbedded with thin, monospecific beds of small bivalves or gastropods. In some cases the finely laminated mud is only apparent with X-radiography. The cores from the eastern basin margins (close to the Bassian Rise) have well-sorted, well-rounded, iron-stained, bioclastic coarse sands, reminiscent of ancient beach deposits, either directly below the present-day shelf deposits or below the laminated mud interval.

1500 A radiocarbon date of 24,900 ± 1200 y^ ^-P' obtained from an in situ shell accumulation at the base of a 2.4m core (see Fig. 1).

This core consists of a modern open shelf muddy sand, followed downcore by 1) a bioturbated mud facies, 2) well-sorted, well-rounded, iron-stained sands, 3) vaguely laminated muds, 4) well-sorted, well-rounded, iron-stained sands, and 5) sandy mud with large numbers of bivalves and gastropods at the base. Thus, the date and facies reflect the environment before the last glacial maximum (^18,000 yr. b.p.) when open marine conditions prevailed. "We have selected additional samples for dating to relate the major environment changes with rising sea level.

Based on the sedimentary facies, together with palaeo-water depths and salinity indicated by ostracods and foraminifera, it seems that during the last glacial when sea level was low Bass Basin was a shallow lake surrounded by marshes and beaches and with the Tamar River flowing into it from the south. Seismic reflection lines show no evidence of an old Tamar river channel which would be expected if the river flowed across Bass Basin and into the sea in the west. The dark laminated muds probably represent a seasonally affected supply of sediment into an anoxic environment. The exact lake environment, including salinity of the water, is still to be determined. As sea level rose the lake and Bass Basin were flooded from the west to form a large embayment. The final change in the environment occurred when rising sea level flooded the eastern sill and the basin became part of the newly created strait.

This study is part of a Ph.D. project being carried cut by W. Blom and funded by Marine Sciences and Technologies grant to J. Keene.

4

f

3.

144®

145^30'

+

147°

+

— \

39°

^Wilsons Prom.

V u T

+

+ ^^ rtDeal Is.

X

^

\ X /king island )

^

^

FLINDER^

\

ISLAND^

X \

(

°616 \

\

40°

\ +

\ + \ \

\

+

-H

+

1 1

\ 4r

Figure 1.

X

-f o

Location of sampling stations in Bass Basin.

cores taken in February 1984 on RAMRL cruise: RANRL 4/84 cores taken in September 1984 on CSIRO cruise: SP7/84 core taken in 1982 radiocarbon date of 24,900 ± yr. b.p. obtained from the base of this core

SEDIMENT FEATURES AND PROCESSES OF THE SYDNEY SHELF ANGUS D. GORDON

1.

INTRODUCTION

Coastal and offshore engineering projects in the vicinity of Sydney have stimulated investigations of the sediment processes and distribution on the inner continental shelf. Over the past six years the author and his colleagues have undertaken a series of co-ordinated programmes aimed at mapping the shelf and obtaining current, wave and sediment transport data. Initially the effort has been concentrated between Broken Bay and Port Hacking. However the present programme of studies is extending the boiindaries north to The Entrance.'

2.

SHELF DATA

Some 5,000 km of side scan sonar and fathemei:er surveys have been carried out in the studv area using a Klein side scan system with 100 and 500 kHz towfish and a Deso 20 fathometer.

Typically the scan width for the Klein vias set at 150 metres either side of track with tracks spaced 200 metres apart giving full bottom coverage within the survey area. Verification of the records was undertaken ^jsing divers, underwater video equi pment, R.C.V.'s, seabed cameras and some 600 surface sediment samples. Accurate position fixing was obtained ^.ising a Miniranger system and a fixing interval of 30 seconds. Detailed maps are being prepared showing bath^/metry, surface sediments, bedrock and bedrock contours. An example is shown in Figure 1.

Table 1 summarises the instrumentation and data three sediment process study sites.

TA8LE Water

Period

depth

of

1.

STUDY

SITES,

INSTRUMENTATION

Instrumentation

data

Waves

collection

AND

and sampling

Wind

DATA

collected

at the

COLLECTED.

schedules

^

Sediments

Seabed morphology and b e d f o r m s

24m

24 months

W a v e n d e r buoy.

60m

6 months

Waverider i dynamic M a r s h MCBirney oressure sensor, X - Y current meter. t s X 'Sm X 4h I s X 3m X 4 h .

Lambrecht Surface s a m p l e s , Side-scan sonar, R C V anemometer. v i b r o and box coring, video rime iapse camera.

Continuous. suspended sedimenr samples. 3 2 s X 3m < 4 h .

80 m

6 months

Waverider 1 dynamic M a r s h M^Birney pressure sensor.

X - Y current meter. '/ts X 18m X 4 h .

I s X 8m X 4 h .

Lambrecht anemometer.

Continuous.

''2 s X 18m X 6 h .

*

Sampling

M a r s h M<^3irney X - Y current meter.

I s X ?0m X 7 h .

schedule :

Dynes Surface samples, anemograph d i v e r and v i b r o coring.

Continuous.

Surface samples, v i b r o and box coring^ suspended sediment somoies.

Side>$can sonar and diver observattons.

Side-scan sonar, RCV video A time loose camera.

32s X 3m X 4 h .

As x 3m x Ch a b u r s t sompiing at i n f e r v a i s of A seconds continuously for 8 m i n u t e s every C hours

5.

PORT JACKSON

MORPHOLOGY AND SEDIMENT DISTRIBUTION OF INNER SHELF SEDIMENT LOBES

$

N

coarse sediment on southern flank vand ridge 2m high rough seabed — 6 0 — d e p t h contour frm MID SHELF SAND RIDGES

100m

mm '

Interred sediment frcmport direction

Coogee Beach

:•:•:•;•;•;;•;•:•;•;.;.;.;.;.;.;:;>.• Cross- sec tion'.;-; SEDIMENT ACCUMULATION ON SOUTHERN SIOE OF VOLCANIC DIKE 2 km

KEY Seabed rock outcrop. rrrrrj Beach and nearshore !•••-i medium sands.

Medium and fine muddy sanas.

Shelly coarse sand and scattered reef.

Hummocked seabed.

BOTANY BAY MORPHOLOGY OF MID SHELF SEDIMENT BODIES

Very rough seabed. cQ2)

Sand ridge.

—

Volcanic dike.

- 1 0 - Depth contour {m, LSJ-.W)

Figure 1. SEA BED MORPHOLOGY, SEDIMENT DISTRIBUTIONS & MORPHOLOGICAL ANOMALIES

3.

RESULTS

A number of morphological anomalies associated with sediment features have been identified. These include convex upward sand lobes, cross shelf sand ridges and mid shelf sand bodies with rhythmic asymmetric morphology (Figure 1).

Wave induced ripples on the surface sediments were observed out to the limit of the study area at depths of 80 metres. Whilst rare at this depth they were quite prevalent at 40 metres, particularly in the coarser sediments.

Combined wave and current velocities near the sea bed were found to exceed 0.2 metres per second some of time in 24 metres deoth, 25? of time in 50 metres and 15% of time in 80 metres. Exceedence curves of near bed steady and oscillatory (gravity wave) currents at the three sites are shown in Figures 2, 3 and 4.

^ ^ ^

Uow.1 %

/

''v.,

i1

V i !

n

1

Uc i 1i

1

^ -v.

i1

5

l>V

!

O'-w 0®04-

i^C^WiBlMS- 7 A 7 2s Uw^iis

i

ui -ix.

•02

•01

0^1

^

VQ

Figure 2.

10^0

SOHO

Probobiliry of «xc««d«nc«

r

\

Probability of «xceed«nct % Figure 3. VELOCITY EXCEEDENCE R^ATIONSHIPS -60m SITE

90^0

%

VELOCITY EXCEEDENCE RELATIONSHIPS - 24m SITE

KEY Uc

«

steady current component.

Uvv

oscillatory component.

^c + w *

combined steady and oscillatory current.

To nJo sCRS gW" Probability of excMdcnce % Figure 4. VELOCITY EXCEEDENCE ^ ^ ^ ^ ^ RELATIONSHIPS - 80m SITE

990

Inner shelf currents were found to be dominated by wind action, particularly during storm events.

Net transport on the shelf is to the north as the process system is dominated by storm action in the Tasman Sea. Outside the surf zone, the net northerly movement results from the complex interaction of wave stirring and the shelf current transport system.

ENGINEERING PROPERTIES OF BASS STRAIT SEDIMENTS H.G, POULOS (University of Sydney)

1.

INTRODUCTION

Tests have been carried out to determine the strength and compressibility characteristics of calcareous sands from Bass Strait.

The soil was obtained from bores taken at the site of the Fortescue, Flounder and Cobra platforms. Because the objective of the research programme was to investigate the general characteristics of behaviour of these calcareous sediments, they were divided into three groups, designated AI, BI and BII, and tests were carried cut on reconstituted samples of these soils. The general characteristics of the three soild types are indicated in Table 1. iMicroscopic examination revealed that the soils contained predominantly bryozoa, with molluscs, foraminifera and echinoids also present.

TABLE 1 General Characteristics of Soils Tested

Soil Soil

Specific Gravity % CaCO. Min. Dry Unit Wt. Max. Dry Unit Wt.

Type

Property

G "7

Yjj^ij^ kN/m^ y, kN/m^ ' dmax

AI

BI

BII

2.72 62 11.20

2.73 78 11.70

2.72 88 11.34

14.50

*

•k

0.10

0.15

0.20

0.20

0.40

0.78

Coefficient of Uniformity

2.00

2.67

3.90

Coefficient of Concavity

1.13

0.81

1.08

* Not determined as test procedure damages particles.

2.

SHEAR STRENGTH CHARACTERISTICS

Conventional drained triaxial tests were carried out on the soils under different confining pressures. Typical stress- strain and volume strain versus axial strain relationships are shown in Fig. 1. Two impox'ta.n't chErs-cteristiics of bchs-vioux 3.r6 obscTvcd; i)

the angle of internal friction decreases with increasing confining pressure; this has been attributed to the crushing of the relatively soft biogenous particles;

ii) the soils exhibit a volume reduction during shearing, except at very low confining pressures; this behaviour is in contrast to that of normal terrestrial quartz sands, where volume increases (dilation) occur during shear at confining pressures less than about 1-2 MN/m .

The relationship between angle of internal friction and effective confining pressure aj (in kN/m ) can be approximated as: r

=

a - blog^^aj

CIJ

For soils AI, BI and BII respectively, tested at an initial relative density of about 50%, the values (for the peak value of (p-) are 55.3, 54.6 and 5 2,8, while corresponding values of b are 6.0, 5.6, and 4.1. In general, the value of for these calcareous sands is greater than for quartz sands of comparable relative density.

1300 -

11200 -

? 600 -

^ ! ? ! ! ! ' ! ' I^ I L-/ > .

U S 12 16 20 28 --C50 ^^.^loT 2UU

"Axial Strain

2 £OJ

0

>o

-2

100 5 0 ^ 1 '

1 1 ! 1 1 t

.

.

! i

f 1

FIG. 1 STRESS-STRAIN C^URVES FOR SOIL SIT

10.

3.

COMPRESSIBILITY CHARACTERISTICS

Standard one-dimensional oedometer tests and K consolidation tests have been carried out to examine the compressibility, cSefficient of consolidation and creep behaviour. The compressibility can be characterized by the compression index C^, the change in void ratio per log cycle of change of effective stress. C increases as the initial void ratio increases, as shown in Fig. 2. For a given initial void ratio, BI is the most compressible of the three soil types. For all three soils, the values cf Cj are much greater than for quartz sands and are similar to values for many clay soils.

The co-efficient of consolidation c tends to decrease with increasing effective stress, but generally lies within the range 2-15 mm /sec.

The one-dimensional creep characteristics are generally expressed in terms of the coefficient of secondary compression C , defined as the change in void ratio per log cycle of time. C^ is found to increase substantially as the effective stress a^ increases, and for Soil AI, can be roughly approximated as C^ = 0.00077 Ciog^Qj' - 1)

(2)

Previous tests on clays show that a relationship exists between C-^ and Cjy and the present tests indicate that the ratio C^/C^ generally lies between 0.01 and 0.05, which is typical of several inorganic clays. This ratio tends to decrease as the initial void ratio increases.

4.

ENGINEERING IMPLICATIONS

The engineering properties of calcareous sands differ from those of normal quartz-based terrestrial sands, and these differences arise because of the presence in the calcareous soils of intra-particle voids and the fact that the particles consist of relatively soft carbonate material (generally calcite) which is easily crushed and abraded. Differences in the engineering properties result in differences in the behaviour of foundations in calcareous and quartz sands. In particular, the following characteristics are to be expected, and in some cases, have been observed: i)

the bearing capacity of shallow foundations on calcareous soils is substantially less than on quartz sands, primarily because of the volume compression which occurs during shear of the calcareous soils.

ii)

piles in calcareous sands (particularly driven piles} develop much less skin friction than in quartz sands; because of the ability of the calcareous soil to reduce in volume, the lateral stresses developed between the pile and soil (on which the skin friction depends) may be very small.

IL

iii)

foundations in calcareous sands (particularly shallow foundations) will tend to suffer greater settlements than in quartz sands because of the greater compressibility of the calcareous sands;

iv)

long-term creep settlements of foundations will tend to be of more significance in calcareous sands than in quartz sands.

Recognition of the above factors may be of great importance in the design of foundations for offshore oil and gas platforms.

Other factors which have not been addressed herein may also have significant influence on foundation behaviour, in particular, the possible presence of cementation within the soil, and the response of the soil to the cyclic loading caused by wave action.

0.2

X

1; ^ C

0.1-

Soil Type

Symbol Oedomerer Ko Test Test

AT

1

0

•

BI

•

•

BI

A

•

Mean

line

for

31

\n (/) CJ u.

Q.

Mean

line

£ o

for

AI

Stress 278-575 0.8

0.9

Initial Void Ratio

1.0

Range kN/m^

1.1

e^

F)G. 2 SUMMARY OF ONE^OIMENSIONAL COMPPESSiBILiTY DATA

1.2

12,

CLIMATIC AND TECTONIC INFLUENCES ON SEDIMENTATION STYLE IN SURFICIAL DEPOSITS OF SOUTHEASTERN AUSTRALIA

J.M. BOWLER (Department of Biogeography & Geomorphology, Australian National University, Canberra)

LIMITS OF TIME, SPACE AND PROCESSES Continental sedimentary sequences of southeastern Australia are notable for the presence of distinctive facies changes in vertical sequence. Many such changes appear to be laterally extensive and restricted to discrete time intervals- Any general analysis of Che Quaternary components must take as its starting point, those environments and deposits which immediately predate the Quaternary and against which the younger records must be evaluated. Our initial time reference must extend to encompass surficial deposits of Neogene age within the southeastern highlands and surrounding plains.

These constraints exclude consideration of direct effects of volcanic activity. Additionally, we are not concerned with detailed assessment of marine sequences and eustacy. The two controlling processes remain tectonic and climatic influences.

NEOGENE CONTRASTS Murray Basin Large areas of the Murray Basin are characterised by a regional geochemical change from acidic weathering profiles of Mio-Pliocene age which gave way to alkaline facies by mid-Pleistocene. The change is registered both in the geochemical signature and sedimentary style.

It equates with the first appearance of aeolian landforms in the Murray Basin marking the progression, though not necessarily the initiation, of aridity. In both marine and non-marine sequences, alkaline soils overlie deeply weathered kaolinitic profiles sometimes with silcrete and ferricrete, the chemical and sedimentary features of the discontinuity representing the same evolutionary trend.

The changes that caused this major and widespread distinctive variation in weathering and depositional patterns were laterally extensive, independent of rock type and, within the terms of present resolution, apparently synchronous.

They can only be explained by variations in the hydrologic and climatic regime with the

13,

intensification of aridity ;culminating aeolian contribution from the southwest.

in a substantial calcareous

Gippsland Basin In the Gippsland Basin, an upper Tertiary marine regression of apparently the same age as that known in the Port Phillip and Murray Basins gave way to continental deposits of which the Haunted Hills Gravels are of particular importance. The origin of these younger coarse sediments has often been attributed to stream rejuvenation by uplift of the highlands.

However, within the catchments of the Mitchell, Thompson and Latrobe Rivers the uplift permitted within the limits of modern relief, without assistance from another factor, would have been quite insufficient to generate the extensive sheets of gravel involved. These are again best explained by rapid stripping of a deep regolith previously stabilised by thick vegetation cover throughout mid-Tertiary time, now rapidly de-stabilised by those same changes that produced the geochemical discontinuity in the Murray Basin.

Lake George In the stratigraphic sequence at Lake George, controlled by palaeomagnetic chronology, a phase of acidic weathering is registered in sediments dating to about 5-6 my ago.

Thereafter the sediments become less acidic to neutral eventually passing to calcareous alkaline clays only in the last million years. A thick sequence of poorly sorted, gravelly sediment of slopewash origin persisted between 5 to 2.5 my. It is tempting to correlate this episode with comparable events in the Gippsland and Murray Basins.

PLEISTOCENE COLD CLIMATES While the first appearance of cold climate processes in the record is difficult to specify, the pattern of glacial-interglacial oscillations most typical of Pleistocene time appears in the Lake George record only after 2.5 my and becomes most rhythmic in the last 700,000 years. In the inland, the oldest dunes at Nyah West and Lake Mungo, legacies of Pleistocene aridity, are dated to well in excess of 200,000 years. Throughout the Riverine Plain, the non-marine portion of the Murray Basin, a succession of stream channels, many with source-bordering dunes, testify there to the influence of Pleistocene bedload regimes of considerable but yet undated antiquity.

LAST GLACIAL PERIOD Events of Late Quaternary age, although limited in their expression through the vertical succession are most important in that their products often form the uppermost soil-sediment mantle and provide some of the youngest and best preserved landforms. More importantly, in falling within the range of available dating and being representative of previous glacial-interglacial changes, they provide

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a key to understanding of earlier, and as yet undatable, cyclic changes.

Such events are •registered in the streams, lakes and dunefields hydrologically influenced by the southeastern highlands.

Fluvial evidence The various streams draining the southwestern slopes of the southeastern highlands possess some distinctive patterns in common, the regional uniformity of which points towards a widespread hydrologic and therefore probably climatic influence.

Where radiocarbon data are available, the general pattern may be summarised.

1 In contrast to modern narrow, sinuous suspended load channels, ancestral courses possessing large meander amplitudes and sandy pointbars are found in the Darling, Lachlan, Murrumbidgee, Murray and Goulburn systems. Where dated, these persist from before 30,000 until soon after 15,000 B.P. The frequent association of source-bordering sand dunes with these systems points to the relative absence of channel-margin woodlands, a condition that extended from Mildura in the west to the Shoalhaven and Molonglo tableland catchments to the east.

2 Between 15,000 and 10,000 B.P. major changes occurred in the fluvial regimes.

Channels became smaller with diminished meander amplitude and carried a higher percentage of suspended load as reflected in the silty clay deposited in Holocene sediments throughout this region. Source-bordering dune developed ceased and the evidence of large trees in zones of burning within fluviatile sediments indicates the return of riparian woodlands.

Lacustrine evidence Perhaps the most dramatic and certainly the most productive evidence of Quaternary facies changes is recorded in the great variety of lake basins distributed throughout southeastern Australia.

From L . George to L . Mungo, from Keilambete to L . Tyrrell there are sufficient elements in common to suggest regional climatic controls.

High lake levels in early glacial time especially before 36,000 B.P. at Mungo and Tyrrell, persisted at reduced levels until about 25,000 B.P. Thereafter levels oscillated or fell reaching driest conditions in the interval approximately between 20,000 to 16,000 B.P. By 13,000 many lakes recovered reflecting more water in the landscape.

The early Holocene records indicate the availability of considerably more water than today. Playas such as Tyrrell and Frome, highly ephemeral lakes today, became permanent water bodies.

These hydrologic changes are reflected in a complex sequence of clastics, laminites and evaporites in the sediments of the inland basins.

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The evidence suggests that the present regime is almost as dry as any experienced throughout the past 10,000 years.

Aeolian evidence With progressive diminution of surface waters in late Quaternary time, aeolian processes were amplified, a combined result of water deficit and increased wind velocities. Dunes of both tranverse and longitudinal types were formed about 36,000 B.P. and especially in the interval 25-15,000 B.P. Of the various mechanisms involved in dune formation, the relevance of the hydrologic transition between regionally high, low salinity watertables to surface water deficit, concentration of salts, clay pelletization and deflation is particularly important. In the whole history of inland Australia, the groundwater influence is so often critical.

Salts With surface drying of the western plains, literally hundreds of small depressions as well as large playas became salt concentration pans and centres for deflation. The resultant dust fallout over the southeast with its large salt component would have been an order of magnitude greater than anything experienced under modern hvdrologic regimes.

The effects of chlorides, sulphate and carbonates contributed in this manner have hardly begun to be evaluated.

TECHTONIC V CLIMATIC EFFECTS Within this system of moderate relief and temperate climate, the climatic imprint has constantly to be evaluated against the tectonic signature.

While the latter is obviously an important factor providing energy for erosive processes and basin alluviation, identification of the tectonic influence seems often to have been over simplified.

In montane catchments it is the weathering and slope processes that set the initial conditions controlling ultimate depositional style. Tectonic instability is most important in regions where rates or magnitude of uplift are sufficient to bring summit levels into the zones of most intensive weathering, especially glacial or periglacial zones.

In southeastern Australia, the broad elements of physiographic pattern and tectonic relief were initiated by early Tertiary time.

The slow rates of movement have provided long periods for slopes and stream profiles to approach equilibrium conditions with soil-vegetative cover at successive intervals. If uplift since the lower Miocene in the highest areas near Kosciusko, was less than 1000 metres, as current evidence suggests, this rate (approx. equivalent to 40 metres/my) sets an upper limit on the new energy available for

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any phase of tectonic deformation.

If this were to occur at relatively constant rate, or as episodic pulses, its changing effects on stream energy over any one,million year period (40 m additional relief) would be infinitesmal in the present climatic regime where summit levels generally lie well below the zones of severe weathering processes.

CONCLUSIONS Many major variations in sedimentary and geochemical style through a wide area of southeastern Australia appear to be closely equivalent in time. In this context, the climatic imprint: over-rides tectonic effects in determining sedimentary facies of this region in Neogene time.

Within the Quaternary, early glacial regimes saw more bedload and high discharge fluvial systems developed synchronously with high levels in terminal lakes. During glacial maximum time surface water diminished, many depressions became groundwater discharge zones concentrating salts and contributing large amounts of suspended clastics and soluble salts to airborn dusts.

Aeolian activity generally was greatly intensified.

In post-glacial regimes, streams reverted to small meander, suspended - load systems, many lakes recovered partially though never reaching earlier levels, while aeolian activity was much reduced.

These conditions have persisted with minor changes through Holocene tine until the present day.

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CYCLES OF FLOODPLAIN STRIPPING AND RECONSTRUCTION ALONG COASTAL RIVERS OF NEW SOUTH WALES G . C . N A N S O N , (Geography Department, University of Wollongong, P . O . Box 1144, Wollongong. N . S . W . 2500)

INTRODUCTION Floodplains along high energy coastal rivers of New South Wales frequently exhibit very pronounced levees and a highly variable surface relief overall.

Consequently, the recurrence internals for flood inundation are spatially very variable on a single floodp l a i n , for levee crests may escape even the 100 year event. In p l a c e s , "terrace-like" alluvial residuals flank the valley w a l l s , yet their soils are barely distinguishable from those on surfaces definitely identifiable as contemporary floodplain. Sedimentologically and stratigraphically these floodplains appear, from initial inspection, to have been formed in response to channel migration. The basal floodplain sediments are coarse and could be interpretted as lateral accretion deposits, fining up to overbank deposits near the surface.

Yet radiocarbon dates and detailed stratigraphic analyses contradict any assumption of lateral accretion.

EXISTING THEORIES OF FLOODPLAIN FORMATION There presently prevails a very restricted view of the way floodplains form, a view based almost exclusively on work by Wolman and Leopold (1957). Their explanation proposes that erosion of the concave bank consumes a prior floodplain which is replaced by pointbar deposition (lateral accretion deposits) overtopped with a thin veneer of overbank sediment (vertical accretion deposits). So well established is this model that Allen (1965) and Douglas (1977) have stated categorically that, while some floodplains exhibit deposits of vertical accretion, lateral accretion deposits are common to all floodplains. Nanson and Young (1981) have demonstrated that small streams in the Illawarra form floodplains entirely from overbank deposits, but only in their low-energy downstream reaches.. They assumed the upstream floodplains were formed by channel m i g r a t i o n , but from evidence presented below and elsewhere, this assumption is probably incorrect.

THE CLYDE AND MANNING RIVER FLOODPLAINS These two floodplains were selected for detailed investigation for they are characteristic of many N . S . W . floodplains with highly variable surface relief. They are also widely separated geographically and are therefore probably broadly representative rather than

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a response to a set of locally unique conditions.. A t the study sites both floodplains are partially confined within mountain valleys and were constructed by relatively high-energy steep-gradient r i v e r s . The c h a n n e l patterns reflect strong structural c o n t r o l and the floodplains are formed of disjunct pockets of alluvium up to several kilometers in length and some hundreds of meters w i d e . The concave banks at each bend abut against bedrock valley sides^ hence there is little possibility of c h a n n e l migration; the v e r t i c a l accretion floodplains are formed between the rivers' convex banks and the adjacent valley w a l l s . Because there is abundant evidence of continued v e r t i c a l accretion w i t h each m a j o r flood, two important questions a r i s e .

Firstly how long have such laterally stable floodplains been in p o s i t i o n , and secondly, if these floodplains continue to accrete v e r t i c a l l y , how long w i l l it be before they exceed the height of a l l b u t the m o s t catastrophic floods?

Radiocarbon dates from b a s a l sediments near the valley w a l l along the C l y d e River at Yadboro showed the floodplain there to be. less than 1700 years o l d . Similar tests from the Manning floodplain at Charity Creek provided ages of less than 500 y e a r s . F u r t h e r m o r e , the Manning floodplain at that site was a t y p i c a l high-relief floodplain with a w e l l defined levee, u n t i l in 1955 a m a j o r flood stripped off the u p p e r 7-8 meters of fine alluvium exposing a b a s a l lag deposit of coarse g r a v e l . It would appear that enormous volumes of floodplain alluvium can be eroded by a single e v e n t , after which the floodplain is gradually rebuilt by v e r t i c a l accretion of sediment from a laterally stable c h a n n e l . Floodplain stratigraphies shows no evidence of lateral accretion; levee and back c h a n n e l sediments extend to the base of the floodplain demonstrating that their positions have been fixed over time, EXPLANATIONS W h y do some floodplains along the eastern margin of New South Wales present a different mode of formation to those described elsewhere?

F i r s t l y , these c o a s t a l rivers drain incised uplands and exhibit v e r y steep flood frequency curves w h e n compared to coastal basins, of equivalent size elsewhere in the world (Pickup 1976; N e l l e r , 1980).

Abrahams and C u l l (1978) emphasise the importance of magnitude rather than the frequency of flooding for accomplishing geomorphic work in these r i v e r s , for by world standards "catastrophic" events are relatively frequent. Consequently, eastern N . S . W . streams appear to be adjusted to a l l but the m o s t severe events within the extreme range that they experience, and moderate floods have very little e f f e c t .

Secondly, c h a n n e l migration is relatively unimportant because within the confining mountain v a l l e y s , concave river-banks abut against bedrock valley-walls. Without c h a n n e l migration or avulsion, floodplains constructed from v e r t i c a l accretion deposits are inevitable.

F i n a l l y , it is likely that the growth of floodplains by v e r t i c a l accretion in confined valleys leads inevitably to periodic floodplain

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destruction. Migrating rivers that form floodplains by lateral accretion keep their channel and floodplain geometry in equilibrium with slight changes in average flow regimen. In contrast, rivers that form floodplains by vertical accretion of alluvium in confined valleys act to progressively obstruct their extreme flood discharges.

Furthermore, high levee banks and deep backchannels and flood basins produce exceptional energy gradients for any flows that breach the levees. In valley reaches where the floodplain is fully developed, truely catastrophic discharges will be displaced across very steep floodplain surfaces, breaking through the vegetation cover and scouring the fine vertical-accretion deposits until only the gravel lag remains. This cycle will be repeated each time the floodplain reforms and obstructs the flow beyond some critical condition achieved by an exceptional flood.

By implication, the sediment transport regimens, for coastal N.S.W. rivers are likely to be extremely phasic, with large pulses of sediment locally released during catastrophic events. This is in contrast to the more even sediment flux associated with rivers that progressively erode their alluvial boundaries by channel migration. Furthermore, the interpretation of all "terrace-like" alluvial residuals as representative of widespread climatic change along coastal N.S.W. will be in error.

REFERENCES ABRAHAMS, A.D., & R.T. CULL (1979) The formation of alluvial landforms along New South Wales coastal streams - Search 10: 187-188.

ALLEN, J.R.L. (1965) A review of the origin and character of recent alluvial sediments - Sedimentology 5: 89-191.

DOUGLAS, I. (1977) Humid landforms - Aust. Nat. Univ. Press, Canberra, 288p.

NANSON, G.C. & R.W. YOUNG (1981) Overbank deposition and floodplain formation on small coastal streams of New South Wales • Zeit. fur Geomorph. 25: 332-347.

NELLER, R. (1980) Channel changes on the Macquarie Rivulet.

Zeit. fur Geomorph. 24: 168-179.

PICKUP, G. (1976) Geomorphic effects of changes in river runoff, Cumberland Basin, N.S.W. - Aust. Geogr. 13: 188-193.

WOLMAN, M.G. & L.B. LEOPOLD (1957) River flood plains: some observations on their formation - U.S. Geol. Survey Prof, paper 282C.

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LATE QUATERNARY SEDIMENTATION ON A RIMMED CONTINENTAL SHELF : NORTHERN GREAT BARRIER REEF JOHN F. MARSHALL AND PETER J. DAVIES (Bureau of Mineral Resources, GPO Box 373, Canberra City, ACT 2601

The 60km wide continental shelf off Cooktown is dominated by relatively large mid-shelf reefs and a semi-continuous line of shelf-edge ribbon reefs. A total of 625 km of high-resolution seismic reflection profiling and 69 vibrocores, as well as thirteen shallow (5-20m) drillholes on four reefs in the area, have helped to delineate late Pleistocene and Holocene processes on the shelf.

Boomer records define an incised late Pleistocene shelf surface with infilled river channels and topographic highs beneath the present reefs. At least three regional unconformities can be recognised in the boomer records, each presumably corresponding to a low sea level event. The uppermost unconformity, which corresponds to the late Pleistocene shelf surface, was penetrated at most vibrocore sites, and the cores show a distinct facies change across the shelf. On the inner shelf the Pleistocene sediments are represented by barrier sands and weathered, mottled equivalents of the modern marine sandy muds, whereas on the outer shelf they consist of leached limestones.

During the postglacial marine transgression a relatively thin (0.6-3.5m) blanket of dark, organic-rich muds was deposited on the shelf. This facies possibly represents time-transgressive mangrove deposits. This facies is absent in places, particularly on the outer shelf, where either the Pleistocene limestone is directly overlain by modern marine sandy muds or the mangrove deposits are replaced by an Operculina?-rich muddy sand. Sea level also transgressed the tops of the older reef knolls which became colonised by 8500 yrs B.P., and the Holocene reef grew vertically at rates of 3 to 14 m/1000 yrs. The windward margins of the ribbon reefs reached sea level by 6000 yrs B.P., soon after sea level stabilisat ion. This effectively lowered the energy regime of the shelf and a thin (0.2-3.5m), but continuous blanket of Holocene marine sandy muds began to be deposited over the entire shelf. In the relatively reef-free area directly behind the ribbon reefs, Halimeda-rich mounds or bioherms developed, possibly as early as 12000 yrs B.P., in water depths that are presently 40 to 60m below sea level, and which built up to heights of 10 to 25 m.

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Presently the shelf is experiencing deposit ion of both coarse and fine grained sediments. The coarse fraction is dominated by carbonates, very little of which is reef-derived, except in the immediate vicinity of the reefs themselves* This coarse fraction consists mainly of in-situ benthonic foraminifera, molluscs and Halimeda* The finer fraction contains a significant terrigenous component that is presumably discharged onto the shelf during monsoonal and cyclonic episodes.

22.

DOWNSTREAM VARIATIONS IN CHANNEL MORPHOLOGY IN THE BELLINGER VALLEY. NEW SOUTH WALES ROBIN F. WARNER (University of Sydney)

INTRODUCTION In this paper downstream changes in channel dimensions and discharges are described for the North Arm of the Bellinger River.

There is also an examination of the effects of these variations on the incidence of flooding.

BACKGROUND The catchment area is about 790 km^ with relief in excess of 1000 m. Bold relief dominates 610 km^ of the basin, particularly in Lower Palaeozoic volcanics and greywackes of the escarpment below the Dorrigo Plateau and in the mountains to the east. The remaining lowlands are mainly phyllites, with some younger granites. Rainfall varies from less than 1500 mm annually in sheltered lowlands away from the coast to probably more than 3000 mm on the mountains north of Bellingen. Flood runoff can occur at almost any time of the year.

This has caused major channel changes since 1946, after which the magnitude and frequency of flooding increased (at Bellingen, the mean annual flood (Q2,3 3) has increased from 350 to 800 mVsec).

On various excursions to the valley, downstream changes have been examined at seven main river sections, ranging from 157 km^ at Woods Creek junction to 671 km^ at Bellingen, and at six tributaries, ranging from 8 km^ at Twin Falls Creek to 97 km^ at Never Never Creek.

Gaugings, surveys, bed-load measurements and water analyses have been carried out to allow computation of discharge, bankfull dimensions and capacity, as well as other data. Gauging and stage-height records of the Water Resources Commission and the Public Works Department have been used to estimate post-1946 mean annual flood discharges.

RESULTS Channel widths at bankfull increase with increasing catchment area down to Thora at 450 km^ and then decrease. They increase again in the lower tidal channel, but this study only considered channel

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conditions to Bellingen. Cross-section areas and mean depths also vary in a similar fashion, while maximum depths increase throughout the 671 km^. Downstream fining of perimeter conditions could explain width reduction and depth increase in the phyllitic lowlands.

Velocities increase to Twin Falls (415 km^) and then decrease, probably reflecting the flattening of the gradient below 415 km^.

Area times bankfull velocity gives bankfull capacity. Not unexpectedly, this increases to 450 km^ and then decreases.

DISCUSSION AND IMPLICATIONS It is this reduced bankfull capacity which becomes important in the consideration of flooding. Discharge at any level or frequency increases with increases in contributing area, as a power relationship, with an exponent of less than 1. This is the case in coastal valleys where highest runoff is probably from steeper, smaller mountainous catchments. For contemporary gaugings, values of 0.80+ were obtained. Equations derived for post-1946 Q2,3 3 had exponents of about 0.85. With relatively good relations between discharge and catchment area, it has been possible to define Q^ ^^ for each section and then to compare this with bankfull capacity.

Some tributaries appear to have incised and eroded channels too large for Q2.33> while others have poorly developed flood plains near their junctions with the main river. These have a capacity lower than the present Q2,33 discharge. In the main channel, the upper three sections have'capacities less than Q2,3 3 but greater than Qi.58 (most probable annual flood). Downstream at Twin Falls and Thora, channel capacity exceeds Q^ ^ , particularly in the latter (by over 250 m^/sec). However, the two lowest sections again fall below the Q2.33 level and, at Bellingen, the present channel is smaller than the QI^^Q flood.

This means that local flooding can occur upstream when any flow gets near the Q2,3 3 level, but it would be insignificant with only very narrow contemporary flood plains at these levels. In the lower middle reaches, very little surcharging would occur until flows exceed Q2\,33 stages, but in the lowest parts, flooding can occur even before the QI^^Q stage is reached. This has been confirmed to some extent by the examination of another 35 sections between Bellingen and Fernmounto In some cases bank capacities are well below projected ^20 33 flo^s and in others, while high levee banks may contain flows, steep levee toes and adjacent flood basins are inundated by tributary back-up water, particularly where there are no mitigation works.

Thus flood frequencies vary along the valley, perhaps in some systematic fashion, and certainly with the incidence of flooding being far higher in the lower parts of coastal valleys. This tendency for channels to get smaller has already been noted for the short streams draining the Illawarra escarpment (Nanson and Young 1981a). They further suggested that overbank deposition would predominate in lower reaches (Nanson and Young 1981b). In larger rivers a similar process may be important for channel formation on

24 <

flat, post-Flandrian transgression gradients involving extensive filling of drowned valleys. High sediment-yielding streams have been able to fill much of the drowned areas behind barriers and to create extensive flood plains and deltas. Coarser sedimentation has prevailed close to the channels, while elsewhere vertical accretion of fines has been slower, leading to steep levees and low-lying flood basins.

These findings need to be further investigated in the Bellinger Valley, as well as elsewhere, because they may well help "explain" variable flood behaviour - something that has been apparent for a long time, but never fully investigated.

REFERENCES Nanson, Go Co and Y o u n g , R . W . (1981a) Downstream reduction of rural channel size with contributing urban effects in small coastal streams of Southeastern Australia. J . Hydro l. 52: 239-255.

N a n s o n , G.C. and Young, R . W . (1981b) Overbank deposition and floodplain formation on small coastal streams of New South W a l e s .

Zeit. f. Geomorph. 25: 332-347.

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LATE QUATERNARY HISTORY OF THE NEAPEAN RIVER NEAR PENRTIH, NEW SOUTH WALES NANSON, G.C. AND YOUNG R.W. (Dept. Geography, University of Wollongong, Australia.)

Introduction: The Nepean River and its associated Pleistocene terrace, the Cranebrook Formation near Penrith, have been the source of confusing interpretations in the scientific literature. This paper presents new information on the Quaternary history and sedimentology of the river and its alluvium.

The study reach extends from Penrith to 9km downstream at the Castlereagh Neck, the latter a bedrock base-level control and valley constriction. The present river is approximately 300m wide, 8-9m in average depth, and with a mean annual flood of about 2000m^ at a flood slope of 34cm km"^. With the exception of an abrupt s-bend at Penrith the river is nearly straight and abuts the Lapstone Monocline to the west, and its own Cranebrook terrace to the east. The origin and age of the Cranebrook Formation have been the focus of interest for a number of previous studies, but conclusions were reached in the absence of detailed stratigraphic or chronologic data. This previously unavailable information is presented here and related to what is known about Late Quaternary climatic change in eastern Australia.

The Cranebrook Formation: This alluvial unit lies unconforcnably on the eroded and weathered Ashfield Shale of the Wianamatta Group (Middle Triassic). The lower 5-8m consists of an uninterrupted, weakly cemented and partially weathered unit of coarse pebbles and cobbles (mean sizes 50-110mm).Clasts are largely quartz, quartzite, chert, porphyry, granite, hornfels, sandstone and silcrete^ and therefore consistent with derivation from the Lachland Fold Belt. About 50 per cent of the clasts are igneous and more weathered than the remainder. The upper 6-9m of the Formation is orange-stained fine sand, silt and clay. The gravels are horizontally bedded and show virtually no cross bedding, although trough and tabular sets of mediiom to coarse sand are present as isolated lenses and strata. The gravel-overburden contact is very abrupt but undulates gently over sections measured in 100s of meters.

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Chronology; The basal gravels contain numerous degraded logs; from 15 samples 7 were identified as Eucalyptus ^ 7 as Casuarina and one as probably Callistemon (pers. com. J . Ford, CSIRO). Radiocarbon ages of 6 samples range from 36700 ± 3300 to 41700 + 3000 - 2200 with an average age of 39000 B.P. Thermoluminescence dates from two sites in the gravel support the validity of the ^'^C dates, giving ages of 42100 ± 4800 and 41000±4500 yrs. There is, at present, no chronology for the fine overburden, but carbonate and T.L. dates are in train. The Clay Band, an abandoned channel of the Nepean that passes under Cranebrook Village, gives a basal ^^C date of 34200 ± 800 B.P. Weathering: Particularly striking is the extremely complex and variable weathering pattern throughout the gravels and fine overburden, apparently a response to variations in ground water and sediment texture. Intensely mottled sandy clays, large manganese and limonitic concretions, and completely weathered igneous clasts of gravel, all abound. Detailed analyses of the thin weathering rinds on hornfels and orthoquartzite cobbles give no systematic trend across the area, perhaps not surprising given the narrow range of ^^C ages and high degree of weathering variability.

Subsurface topography and palaeocurrent orientations: Extensive exploration drilling by aggregate mining companies located the gravel surface topography and revealed a multiple channel system depositing an extensive braidplain of coarse gravel. Gravel levees flank the base of the present Nepean and its abandoned channel, the Clay Band, leaving a backplain of relatively low gravel relief in between (Fig. 1). This backplain may have itself been a major channel at some time. Clast imbrications measured at 27 sites in gravel pits also suggest deposition by a number of low sinuosity channels wandering across the area.

Geomorphological Interpretation: The basal gravels of the Cranebrook Formation were deposited about 42000 yrs B.P. on a proximal braidplain probably extending from the mouth of the Nepean Gorge at M t . Portal^ to the Castlereagh Neck.

The absence of any lateral accretion deposits discounts previous interpretations of deposition by a high-energy meandering stream^ as do the palaeocurrent orientations determined from gravel imbrication.

Later these multiple channels appear to have contracted into two major channels, one still occupied by the present Nepean River, and the other now marked by the Clay Band. Both channels had well defined levees of coarse gravel sloping to a backplain with its low point aligned along the present location of Cranebrook Creek. The Clay Band was abandoned about 35000 yrs B.P. and gradually filled with fine clastic and organic matter.

Our present dates give no justification for the separation from

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the gravels of the Cranebrook Formation of a younger Lowlands Formation. Indeed gravels only a few meters from the present Nepean date at 37750 (+ 1500 -1250) yrs B.P. However, without an available chronology, interpretation of the fine overburden is problematic.

Most of it is definitely overbank in origin for it is not aeolean and there are no channel structures present over much of its extent.

However, in the areas presently marked as Lowlands Fomation we have observed channel infills, and surface patterns evident on aerial photographs also indicate reworking of at least part of the overburden.

Palaeoenvironmental Interpretations: The most dramatic change in the late Quaternary flow regime of the Nepean River appears to have been from a multiple channel bedldad to a single channel mixed load system between 42000 and 35000 yrs B.P. The early stage of this period approximately correlates with the highest water levels in the Willandra Lakes, as well as with a number of other environmental indicators of more effective precipitation here and across the continent. Of particular note is that the Nepean River system appears to have passed through the last glacial maximum (around 18000 yrs B.P.) without leaving any notable alluvial signature.

Consequently, it would appear that the glacial maximum was not a particularly important fluvial event in eastern New South Wales other than through the disruption caused by eustatic changes.

28.

POTEHTIAL LAND ERODIBILITY AND EFFECTS ON LAKE WOLLUMBOOLA DURING URBANIZATION OF CULBURRA. SOUTH COAST. N.S.W.

SCOTT J. LAWSON AND MICHAEL J. KNIGHT

(School of Applied Geology, University of New South Wales)

Culburra is SL small coastal village, with, a population of about 2,500, located approximately 150 kms. south of Sydney.

It is currently being considered as a potential area for major urban expansion.

This expansion is thought necessary to accommodate a population of over 12,000 by the turn of the century.

The development would take place in an area of about 5.3km.^ adjacent to and west of the present township.

The study area consists of gently undulating, forested land bounded by the Crookhaven River to the north, the present township and Lake I^ollumboola to the east, and rural lands to the south and west.

The yellow duplex soils which occupy most of the area are developed on Permian sandstones and siltstones of the Wandrawandrian siltstone, and on unconsolidated Quarternary alluvium.

They are characteristically very clayey and thus have poor permeability.

The potential for erosion of these soils is high due to the dispersible nature of the clays and to the hardsetting nature of the topsoils.

Approximately 75% of the village extension area lies within the catchment for Lake Wollumboola.

Lake Wollumboola is a 6.5km.^, shallow coastal lagoon which is almost permanently isolated from the sea by a sand barrier.

Due to its enclosed nature, the lake has accumulated large quantities of sediments and nutrients and is now in a eutrophic condition.

Whilst in this condition the lake occasionally releases large quantities of hydrogen sulphide gas from its mud sediments.

This gas is poisonous and very odourous and creates an uncomfortable living environment for residents of Culburra.

The bacteria associated with this condition pose a risk of infection to open wounds on swimmers.

Expansion of Culburra will not only expose many more people to the problems of the lake, but may also lead to a further rapid decline in its quality.

The highly erodible soils could contribute large amounts of sediment during construction stages of urbanisation, and polluted run-off may continually increase nutrient loads to the lake as the urban population grows.

To prevent accelerated eutrophication of the lake, planning of both construction and the eventual urban environment must aim to minimise sediment and nutrient inflows to the lake.

Furthermore, modifications

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to the lake may eventually have to be made to improve its condition.

Such modifications may involve the creation of a permanent lake entrance or dredging of the organic-rich mud sediments.

Before these undertakings, it is recommended that ecological relationships, nutrient inflows and lake sedimentation rates and patterns be established.

30.

SEDIMENT DEPOSITION IN LAKE WYANGALA NEW SOUTH WALES DAVID OUTHET (New South Wales Water Resources Commission)

INTRODUCTION The recent drought in New South Wales resulted in very low storage levels in Lake Wyangala. These low levels exposed bottom sediments, allowing the opportunity for studying their location, thickness and properties to determine total sediment volume, sediment sources and deposition processes.

Information on sediment deposition is an important contribution to the study of the drainage basin sediment system and sources. It is also important when determining erosion history from sedimentation.

METHODS Information on the sediment deposits was collected by means of large scale colour air photography, coring, pit excavation and seismic profiling (pinger). Normal stratigraphic analysis techniques were applied to the pits and cores as well as the samples obtained from them.

RESULTS The data revealed three main types of deposit. The following sections present more information about these three deposits.

Main Basin Deposit One of the pre-dam terraces found in several parts of the main basin is a representative site of the uniform main basin deposit. It is located well away from the pre-dam channel and side slopes.

Observations in this area were made at 260 points located at approximately 5 metre intervals along 3 transect lines. The mean sediment thickness is 150 mm with a standard deviation of 30 mm. The material is only faintly colour laminated (within Munsell group 1) with a uniform medium clay texture at all sites. Similar sediment properties and thicknesses were observed at all of the 10 other study sites in the main basin.

31.

Deposits on the reservoir side slopes thin out as elevation increases. The elevation at which sediment thickness thins to zero is called the "zero line" and is found at the same elevation at all sites in the main basin of the reservoir (351 m) . This elevation corresponds closely to the 50% submergence elevation above which erosion dominates and below which deposition dominates. The uniform and fine-textured nature of the main basin deposit indicates that it is formed by the process of fine particles settling slowly out of still water.

Tributary Arm Deposit In an upstream direction away from the main basin deposit along either of the two tributary arms, the zero line slopes upward, the sediment texture becomes coarser and colour becomes lighter (Munsell group 2 ). Colour laminations become more distinct. This is called the tributary arm deposit.

The laminated and downstream-fining texture of the tributary arm deposit indicates that it is formed by the process of particles settling out of moving water with decreasing velocity and turbulence (competence) in a downstream direction.

Channel Deposit The pre-dam channels of the tributaries contain a deposit quite different from the two previous ones. It is made up of many layers of contrasting texture which are interbedded clays, silts and sands.

This deposit is much thicker than the others (up to 7 m). It thins near the dam wall and near the upstream backwater limit. There are no deltas at the upstream limits of dam backwater on the tributary arms.

The stored water volume is highly variable and can be reduced from 100/^ of storage capacity to 10% in one year if catchment rainfall is low and water demand is high. For this reason, the upstream limit of backwater in the pre-dam channels can very by 22 km in distance and by 40-m in elevation.

The interbedded and laminated channel deposit indicates that it is formed in a highly variable depositional environment by both moving and still water with variable sediment loads. This variation is probably due to two additional processes. One is inflows moving along exposed pre-dam channels during low reservoir levels. The second is occasional density (underflow) or turbidity currents moving along submerged pre-dam channels and low flood plains during high water levels. One density current was measured at Burrinjuck Dam. It occurred in March 1983 and contained almost 2k% solids when sampled just upstream of the dam wall. Density currents are formed when an inflow has a higher sediment concentration, higher salinity or a lower temperature than the reservoir water.

32,

A DEPOSITIONAL MODEL All the preceding information forms the basis for a generalised model of the depositional system in Lake Wyangala. The system receives almost all its original sediment input as tributary inflows laden with gravel, sand, silt, clay and organic matter. As it travels down a tributary channel towards the reservoir backwater, an inflow may rework any channel sediment deposited by previous inflows at higher backwater levels. When it reaches the backwater, gravel and sand in the bed load settles out immediately over any previously-deposited sediment to form the channel deposit. As the inflow plume proceeds into the backwater zone it creates turbulent currents in the tributary arm which can carry fine sand and silt to elevations above the pre-dam channel (flood plains, terraces and side slopes) to form the tributary arm deposit.

Finally, the clay and buoyant organic matter is carried by slow currents caused by hydraulic, wind or thermal forces to the distant main basin. When the currents cease or when particles settle below the turbulent surface zone they accumulate on the bottom to form the main basin deposit.

MINOR PROCESSES Several subaerial processes have been observed to have minor effects on the three types of sediment deposit. These are: fluvial channel processes (inflows; wave action; sheet, rill and gully erosion.

These processes begin to modify any deposits as soon as they are exposed by low water levels and have the effect of moving the sediment downstream or downslope to the lower regions of the reservoir and concentrate it there.

CONCLUSION In a lake or estuary, the major sedimentation processes mentioned in this paper would produce the usual topset, foreset and bottomset bed sequence of deposits first described by G.K. Gilbert. However, the information gathered in this study has revealed that large water level variations produce a different depositional model and a very different set of deposits, namely a main basin deposit, a tributary arm deposit and a channel deposit. Part of the channel deposit and the tributary arm deposit are foreset and topset beds that have been distributed along tributary arms. Only the constantly-submerged part of the main basin deposit is similar to Gilbert's bottomset beds. In addition, some deposits are modified by subaerial erosion processes that tend to concentrate the sediment in the lower regions of the reservoir.

33,

LANDFORM DEVELOPMENT & SEDIMENTATION IN A DELTA IN AN EPHEiMERAL SALINE LAKE; LAKE BUCHANAN, QLD.

GRESLEY A. WATSON (University of Queensland, St. Lucia, Brisbane)

Lake Buchanan is a large ephemeral saline lake dominated by clastic rather than chemical sedimentation. It occurs in a closed drainage basin approximately 170km south of Charters Towers. The potential average evaporation of this area is roughly five times the average annual rainfall. Stockyard Creek is one of the many ephemeral creeks that enter the lake; it forms a delta 3.5km long and extending 1.5 km into the lake bed.

The object of this study is to determine the processes influencing the sedimentology and landform development of this delta.

Landforms are mapped in terms of three basic vegetational units, which are related to height: 1. The bush zone, occupying the crest of the landform; 2. The samphire zone, sloping down from and surrounding the bush zone; 3. The sandy zone, bare of vegetation, a gently concave slope to the base of the landform.

Low-lying flatlands, or lagoons and broad sandy channels, encircle the landforms.

GEOMORPHQLOGY The delta is divided into three lobes (Fig. 1): Lobe A, the active fluvial lobe. Landforms: Active and abandoned channels- of Stockyard Creek, with associated levee banks; an interdistributary zone between the active and abandoned channels, in which runoff becomes channelised in the distal area; and a system of islands, (lake-level) low flatlands, and (slightly higher) high flatlands.

Lobe B, the younger abandoned lobe, intermediate in morphology between Lobes A and C. Landforms: distally, small lagoons and extensive channels, bordered by crenulate lobate fringes; proximally, dominated by vegetated plain (see below).

Lobe C, the older abandoned lobe. Landforms: well-developed lobate islands and peninsulas, large lagoons, and a few small channels.

Vegetated plains extend along the boundaries between the lobes.

These are characterised by: greater elevation and areal extent, greater density of vegetation, greater species diversity, and a uniform morphology (does not show lobate morphology or obvious channel/levee systems).

34,

LANDFORMS & LAGOONS; GRAIN SIZE DISTRIBUTION Granuloraetric analyses indicate a strong correlation between landform zone, mean grain size, and sorting (Fig. 2). The bush zones on landform crests have the coarsest sediments and the best degree of sorting. Grain size decreases and sorting becomes poorer in successively lower zones. Using a model for sediment transport proposd by McLaren (1981), the trend suggests that fine sediments are being winnowed out of landform crests into lower zones, finally accumulating in lagoons.

Comparisons of larger and smaller lagoons/flatlands indicates that smaller lagoons contain coarser sediments with (generally) poorer degrees of sorting than larger lagoons. This suggests that coarser material is also being washed into the lagoons, increasing the range of grain sizes and decreasing the degree of sorting.

LANDFORM EVOLUTION In the delta, disection of the landforms by rills is occuring.

Rills initiated within the samphire zone become sites of local erosion, which causes ground surface lowering and headwards retreat of landform boundaries.

Early stages are crenulated landform boundaries or isolated knolls of bush within the samphire zone. In late stages a lobate island is isolated from its parent landform. Where erosion has proceeded to completion, a lagoon results.

CONCLUSIONS 1.

The morphology of the delta is strongly influenced by the ephemeral nature of the lake. Because the lake is dry for most of the year, the most common form of water flow is downslope runoff during seasonal rainfall. In Lobe A, the interdistributary zone does not develop a typical bay facies but becomes a channel for seasonal runoff. In Lobe C, runoff becomes an erosional influence on the delta.

2.

The most commonly accepted datum for the definition of delta elements is sea level, which is not applicable here. In this delta, vegetational zones provide an indication of the extent to which erosion has operated on the landform. The following analogues are suggested: Marine datum Stockyard Ck. datum delta front

basinal delta (sandy zones, lagoons)

below level of vegetation

lower delta plain

below sea level (basinal process predominates) within influence of tides, waves

lower delta plain

lobate islands and peninsulas

upper delta plain

beyond influence of tides, waves

upper delta plain

vegetated plains (no major erosion)

3.

This delta is affected by neither waves nor tides, yet it cannot be considered to display a fluvial-dominated morphology. It is suggested that in the classification of deltas in ephemeral lakes, a fourth basinal process (erosion) may be usefully considered.

McLaren, P., 1981. An interpretation of trends in grain size measures.

Journal of Sedimentary Petrology 51(2): 611-625

35,

Fig. The

Stockyard

Lobes A,B, - active

1

Ukc

Creek

& C.

delta.

Landforms: 1

& abandoned

channels

of Stockyard Gk.

2 - interdis-

tributary

of

Ck.,

zone

with

lines).

Stockyard

channels 3

flatlands.

4

(thick

islands

- small

and

lagoons

and crenulated channels of Lobe B.

5

- lobate

peninsulas.

vegetated plains. lobate

islands

6 - lagoons.

and 7

-

- typical

peninsula shown in Fig.

2.

BucKdnarv

Fig. 2 Relationship between elevation (landform zone), mean grain size (Mz), and sorting (Ot). * - not typical of trend; more typical response is dashed line. This samphire zone is unusually narrow and sparsely populated. bush

rone

^dnc/jj zone

(3

36,

LATE QUATERNARY HISTORY OF CHANNEL TRENCHING IN DAIRY ARM CATCHMENT. HUNTER VALLEY, N.S.W. WAYNE D. ERSKINE

M.D, MELVILLE

(River Management Branch) (N.S.W. Water Resources Commission)

(School of Geography) (University of N.S.W.)

IN-TRQDUCTION Field surveys, historical sources, stratigraphy and radiocarbon dating have been used to reconstruct the late Quaternary chronology of channel trenching on seven alluvial streams in the predominantly sandstone catchment of Dairy Arm in the extreme south of the Hunter Valley. The primary aim of the present study was to determine the number and timing of channel trenching episodes in order to assess the impact of European occupation on river stability in small catchments ( < 5 0 km'). The results of such a study have important implications for the design of river management plans to stabilise the river and flood plain.

HISTORIC CHANNEL TRENCHING Portion plans, oral histories and field evidence confirmed that there was no integrated channel network in the catchment before 1949.

Isolated large pools separated by ill-defined grassed depressions were present on lower Dairy Arm but no channel was recorded immediately upstream. A sand-bed channel did exist, however, in the upper reaches.

Larg^ scale trenching and channel widening were initiated by a catastrophic flood in June 1949. Up to 6 m of incision has occurred since then by the upstream migration of a system of knickpoints. This incision on the trunk channels progressively rejuvenated most tributaries. Although knickpoints are still present on some streams, waterfalls cut into massive quartzose sandstone mark the upstream limit of incision on others. Only the middle section of Dairy Arm has been trenched. The upper section lies above the rejuvenation head which has become stabilised by log dams and log steps.

The massive volumes of sediment removed from the upper catchment were largely deposited as channel-fill and overbank sands on the lower reaches of Dairy Arm and on Wollombi Brook downstream. Channel changes in this reach included aggradation, widening, lateral migration, avulsions and cutoffs.

37,

LATE QUATERNARY CHANNEL TRENCHING The number of channel trenching episodes varies not only between the streams investigated but also along the same stream in the downstream direction. On the upper section of Dairy Arm above the recent rejuvenation head there is only one alluvial fill with a channel and flood plain facies. No evidence of channel trenching since the deposition of this undated fill has been found. In the middle section of Dairy Arm and on the downstream reaches of most tributaries there are three alluvial fills with younger fills inset below older ones.

A radiocarbon date of 11 900 + 3100 yrs bp (A.R.L. 240) was obtained on -

2300

charcoal from 1 m above the base of the oldest fill on Cullys Arm and indicates that valley-fill sedimentation commenced in the late Pleistocene. This surface was abandoned by incision to form a high terrace and an inset alluvial fill was then deposited in the trench.

A radiocarbon date of 1100 ± 70 yrs bp (Beta 7^15) was obtained on charcoal from the basal gravels of the inset fill on an unnamed tributary of Olney Arm. The phase of incision that formed the high terrace occurred some time before 1100 yrs bp but after about 12 000 yrs bp. Further dates are being obtained to more precisely determine the time of trenching and valley-fill deposition throughout the catchment. Before the most recent phase of incision in 19^9 the surface of the inset fill at the Beta 7^15 site was cleared of all trees and the stumps burnt (there are 15 stumps in the lower 100 m of this gully). The surface was then buried by up to 0.3 m of stratified sands.

A contemporary flood plain with a fill of interbedded sand and gravel has formed in the wider downstream sections of the post-1949 trench.

Evidence of trenching during deposition of the oldest fill has been found on two streams. Discontinuous remnants of a channel facies comprising a 2-4 m thick coset of trough cross-laminated medium sands and granules has been found incised into the high terrace sediments.

This facies is buried by the upper members of the oldest fill and represents the earliest phase of trenching preserved in the valley-fill sediments. The three phases of trenching in the upper catchment appear to be matched by three episodes of valley-wide overbank deposition in the downstream reach of Dairy Arm. In this section the stratigraphic relations are reversed with the oldest sediments at depth, buried by younger deposits. No trenching has been recorded here.

CONCLUSIONS The results of the present study reinforce recent findings that different sections of the same channel in relatively small catchments can exhibit erosion, deposition or stability at the same time.

Stratigraphic evidence indicates that this situation has prevailed during the evolution of the valley-fill sequence. Base-level lowering therefore may not rejuvenate all channels throughout the drainage network upstream of the initiation point.

A strong association exists between the recent occurrence of a catastrophic flood and the initiation of an erosion episode. Catchment disturbance resulting from European occupation peaked in the middle

38.

of the n i n e t e e n t h century and' is not coincident in time w i t h t r e n c h i n g .

In fact it predates erosion inception by about 90 y e a r s . A l t h o u g h A b o r i g i n a l firing has been recently cited as an important control on v a l l e y - f i l l sedimentation in n e i g h b o u r i n g c a t c h m e n t s , it is important to note that the c o m m e n c e m e n t of sedimentation in Dairy Arm occurred at least 5000 years before major A b o r i g i n a l o c c u p a t i o n . It is not always possible on stratigraphic e v i d e n c e , h o w e v e r , to discriminate b e t w e e n large floods and climatic changes as the cause of v a l l e y - f i l l s e d i m e n t a t i o n . N e v e r t h e l e s s the variable number of alluvial fills present and significant changes in their stratigraphic r e l a t i o n s within the catchment suggests that the concepts of geomorphic thresholds and complex r e s p o n s e may a p p l y . Further r a d i o c a r b o n dating is r e q u i r e d to accurately determine the timing and duration of sedimentation throughout the c a t c h m e n t .

39,

SOME IMPLICATIONS OP CHMIEE DATES IN NOETHEEN AUSTEALIA

BRIAN G. LEES and ANNEMAEIE CLEMENTS ( GEO-EXEN, Geomorphological, Exploration & Environmental Services. c/o Box 309 Sutherland^ N,S.¥,2232 ) ( The Macleay Building, School of Biological Sciences.

University of Sydney. N.S.¥. 2000 )

INTRODUCTION Geomorphic systems, especially coastal depositional environments, are extremely sensitive indicators of variation in climatic conditions. Arid and semi-arid areas are especially sensitive to fluctuations in rainfall which may affect the fluvial system. If the receiving basin is an allochthonous shelf, then such fluctuations will be expressed in the coastal deposits.

Temporal variation in the character of coastal deposits along the eastern Australian coast, south of the Great Barrier Reef Province, has been tentatively ascribed to changes in the degree of storminess through time. Thom (1978) has suggested a relationship between degrees of storminess, coastal progradation and transgressive dune development on the New South Wales coast.

This has been supported to some extent by work in south-eastern Australia (Short & Hesp, 1982). The identification of a comparable pattern of coastal deposition on the comparatively well-protected coast of eastern Cape York at Temple Bay (Saenger, Lees &. Anderson, in prep) suggests that variation in the degree of storminess alone may not be the cause. An alternate hypothesis is that some of the changes in the form of coastal deposition are due to fluctuations in annual rainfall.

The problem of all such hypotheses is the separation of regional response from local factors.

In Rhodes' (1980) investigation of the coastline of the Gulf of Carpentaria he found that there were, at that time, insufficient data on upper Holocene climatic change for regional correlation of such events. Nevertheless he noted that there appeared to be a correlation between progradation of the beach-ridge plain in the Gulf of Carpentaria during the period 6,000 to 4,800 years B.P. and increased cyclogenesis accompanied by a pluvial in eastern and southern Australia.

Rhodes (1980) also felt that there was general agreement between a wetter period in southern Australia after 2,000 years B.P. and a long-stage of beach-ridge progradation in the Gulf of Carpentaria during the time 2,300 to 600 years B.P.

40,

RECEHT STUDIES Since Rhodes' study there have been a number of studies of coastal depositional features in northern Australia, many of them_ involving the dating of chenier plains. These include cheniers at Point Stuart on the delta of the Mary River, near Darwin (Clark, Wasson & Williams, 1979), Shoal Bay, also near Darwin (Hickey, 1981), the delta of the Victoria River (Lees, 1984), the delta of the Normanby River in Princes Charlotte Bay (Grindrod, 1984), and the delta of the South Alligator River ( Woodroffe, Chappell, Thorn & Wallensky, 1985).

HYPOTHESIS TESTING Chenier plains are particularly sensitive indicators of environmental change. Whilst the classic cheniers on the marginal deltaic plain of the Mississippi are generally agreed to have formed during periodic shifts in the river mouth location, delta switching is only one of several mechanisms which can cause the prograding of chenier plains.

The important feature of the environment in which chenier plains develop is a periodic variation in the balance between fluvial and marine forces. Given a fairly constant marine hydraulic environment, and a sedimentary environment in which cheniers could develop, fluctuations in alongshore sediment supply can result in the building of cheniers. Equally, given a fairly constant supply of sediment, an increase in the energy intensity of the marine hydraulic environment could lead to a change from mud flat deposition to chenier development (Otvos & Price, 1979).

The six studies of chenier plains in northern Australia are from widely separated marine environments. Unlike the eastern Australian coast, the northern Australian coast is compartmentalised into a series of marginal seas separated by major structural blocks. Thus the marine environment of each plain is a 'local' variable and the only important regional variable, as far as cheniers are concerned, is rainfall.

Assuming that the calibrated C14 ages (Klein et al., 1982) of the shells are reasonable estimates of the time at which the chenier ridges were laid down a null hypothesis, that chenier ridges are formed by random, local events, can be tested using a chi-squared test and a measure of 'dumpiness'.

The data set from Point Stuart (Clark et al., 1979) was not used in the computation due to the number of undated cheniers in the sequence.

There appear to be two 'clumps' of chenier ridge occurrence, between O.4 and 0.8K yrs and between 1.6 and 2.8K yrs.

In order to test this clumping effect for significance two statistics, a "Robinson" dumpiness test statistic, R, and chi-squared, are estimated, as the number of observations is small and the expected value, e, is less than 5, by 1,000 randomisations of the data. The number and duration of the observations are fixed during this process but the location, through time, is varied within the limits of the

4L

temporal range of the observations.

This permits the production of a statistic based on the data set against which the actual dumpiness of the data set can be compared and its probability calculated.

Using 'R', the probability of the peak at 1.6 to 2.8K yrs B.P." occurring as a random event is 0.16^.

Prom randomisation of the chisquared statistic a probability of 0.15^ that this clumping is a random event is estimated.

In order to test for the second clump at 0.4 to 0.8K yrs B.P., the data contributing to the first significant clump was removed and the procedure repeated. Using 'R', this event occurs at random with a probability of and from the less appropriate randomisation of the chi-squared statistic a probability of 31*9% that this clumping is a random event is estimated. This is not statistically significant, however this may be partly a consequence of missing data. Lees (1984) did not date the present beach/chenier ridge on the Victoria Delta and the data set from Point Stuart (Clark et al.,1979) was excluded from the calculations due to the number of missing observations.

The lack of a 'clump' in the mid-Holocene is partly a function of the lag between sea level rising to its present level and the sedimentary response to this geomorphic disequilibrium.

CONCLUSION In conclusion, a statistically significant regional change in conditions between 1.6 and 2.8K yrs B.P. is reflected in the chenier record of northern Australia.

It is probable that this is related to a reduction in the fluvial input to the coast indicating a period of relative aridity.

ACKNOWLEDGEMENTS We would like to acknowledge the considerable help of Associate Professor John Robinson in the preparation of this paper.

BIBLIOGRAPHY CLARK, M.P., WASSON, R.J. and WILLIAMS, M.A.J., 1979- SEARCH,10(3): 90-92.

HICKEY, S.H., 1981. N.T.GEOL.SVY., Tech. rep. GS81/I. GRINDROD, J., 1984. Ph.D. thesis, Australian National University, Canberra. Australia.

KLEIN,J., LERMAN, J.C., DAMON, P. & RALPH, E.K., 1982. RADIOCARBON 24(2). pp.103-150.

LEES, B.G. 1984. Ph.D. thesis. University of Sydney, pp. 36?.

OTVOS, E.G. & PRICE, W.A., 1979- MARINE GEOL. 31» pp.251-253 RHODES, E.G., 1980. Ph.D. thesis, Australian National University, Canberra. Australia, pp. 357. ' SHORT, A.D. & HESP, P.A., 1982. MARINE GEOL. 48. pp.259-284.

THOM, B.G., 1978. ^ 'DAVIES, J.L. & WILLIAMS, M.G. (eds.)

'TIME, SPACE AND LANDFORMS IN AUSTRALIA' A.N.U.Press 376pp.

WOODRUPFE, C.D., CHAPPELL, J., THOM, B.G., & WALLENSKY, E., 1985.

Proceedings of the Conference on Coasts and Tidal Wetlands of the Australian Monsoon Region. N.A.R.U. Monograph Series. Darwin.

42,

SIZE OF SAND RIPPLES ON THE INNER SHELF Alexander F. Nielsen and Angus D. Gordon (New South Wales Public Works Department)

INTRODUCTION This paper presents and examines f i e l d data on wave-generated ripples in nearshore sands. The work reported herein formed part of a larger study which examined the movement of sediment under wave and current action in an open coast embayment (Figure 1). To improve the accuracy of the sediment transport calculations care was taken to determine the fluid/sediment boundary roughness resulting from the presence of seabed ripples. The prediction of the size of these bedforms under natural wave motion was essential to this work.

FIELD DATA The f i e l d data comprise: diver observations, in particular measurements of bedform lengths,heights and crest orientations and assesments of sediment a c t i v i t y ; sea surface displacement data from waverider buoys; near-bed currents, both residual and o s c i l l a t o r y , and wave direction information from electromagnetic current meters; and seabed sediment characteristics assessed through a settling tube. The water depth at the four observation sites was approximately 24 metres and the sediments, comprising well sorted and rounded quartz sand, varied in grain size from 0.22mm to 0.45mm over the s i t e s . When the diver observations were made the s i g n i f i c a n t wave height was generally less than 1.5m and the spectral peak period was in the range 7s to 14s.

RESULTS To apply the existing understanding of sediment transport processes to natural f i e l d situations i t was f i r s t necessary to examine the appropriateness of characterising natural energy spectra in terms of displacement and frequency parameters. I t was considered that an examination of bedform response to near-bed velocity spectra could be used to i n d i cate and evaluate such characteristic parameters i f appropriate; there being considerable data available from flume and o s c i l l a t i n g water tunnel experiments concerned with bedform response to periodic, progressive, near-sinusoidal water movements.

43.

FIGURE

1.

STUDY

AREA

(after Nielsen and

Gordon, 1984)

44.

Laboratory data were examined and i t was found that the bedform curves of Mogridge and Kamphuis (1972) described and quantified well the existing understanding of bedform generation and degeneration under monochromatic wave conditions (Figure 2). Also, Nielsen (1984) ^ reviewed the f i e l d data of Inman (1957) and compared them to the bedform curves of Mogridge and Kamphuis. To obtain a good agreement between these data and the laboratory results i t was necessary to increase Inman's calculated significant hydraulic parameters by a factor of 1.5.

Inman's hydrodynamic data were based on fathometer records and that, coupled with considerations of depth effect on surface displacement energy, i s considered to have resulted in his over-estimation of nearbed velocities and displacements and under-estimation of wave periods.

To determine the characteristic spectral parameters, the f i e l d data were plotted i n i t i a l l y against the bedform curves of Mogridge and Kamphuis using s i g n i f i c a n t spectral hydraulic parameters with Tf peak (not presented). I t was found that the dimensionless wavelengths and wave heights of the f i e l d data points were considerably larger , on average by a factor of 2.15, than those of the laboratory data. The reason for the departure from the laboratory results was considered to lie in the choice of the velocity and displacement parameters used in the calculations of the data point co-ordinates. Consideration of the authors' diving observations while collecting the seabed data and the consistency with Inman's results suggested that maximum rather than s i g n i f i c a n t spectral parameters would be more appropriate.

Goda (1974) showed that the ratio of maximum to s i g n i f i c a n t wave height parameters has a mean value of 1.65 and a standard deviation of 0.26. He also showed that the ratio of the period parameter associated with the maximum wave to the period parameter associated with the peak spectral estimate lies in the range 0.5 to 1.4. Some early and limited results of studies carried out by Neil Lawson (Lawson and Treloar Pty. Ltd., Sydney, pers. comm.) indicate that this ratio has an average value of about 1.2, with a f a i r l y large standard deviation.

Adopting maximum spectral velocity (and displacement) parameters, i t was found by iterative procedures that a period parameter of value 1.3 Tpeak produced a reasonable agreement between the f i e l d data and the bedform curves of Mogridge and Kamphuis (Figure 3).

CONCLUSIONS The bedform investigation suggested that maximum spectral hydraulic parameters should be used when applying existing theories based on monochromatic wave studies to shelf bed response to natural wave action; the inference being that these maximum spectral parameters should be used also for sediment entrainment and transport calculations.

That the data were limited to a single water depth, a narrow band of s l i g h t wave conditions due to diver safety provisions, limited variation in quartz sand grain sizes and the d i f f i c u l t y in taking precise diver measurements, dictates that in the definition of these characteristic parameters, the constants 1.65 and 1.3 be seen as p r e l i minary figures only. Further, i t must be recognised that the overall aim was to develop a method of calculating sediment movements on the Continental Shelf for engineering projects.

I I I !

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3 4 5 6 7 9 10^ 2 3 4 567 SEDIMENT FROUDE NUMBER LEGEND

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Bedform height curves modified after Mogndge i Kamphuis {1972 )•, numbers to each curve are values

A = near-bed orbital displacement T = wave period = ripple length n = ripple height

0 = gram size U = near-bed, wave-induced, maximum ,horizontal orbital velocity p = density of water

^s - submerged specific weight of sediment

FIGURE 2 .

COMPARISON OF DATA FROM I N M A N 4 BOWEN (1963), KENNEDY & FALCON ( 1 9 6 5 ) , HORIKAWA & WATANA8E (1967), CARSTENS ETAL. (1969) & LOFQUIST ( 1 9 8 0 ) WITH BEDFORM CURVES MODIFIED AFTER MOGRIDGE & KAMPHUIS (1972) (after Nielsen & Gordon, 1984) .

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g 7u_ a 5 uu

0-14*.

007 \ 0-1 ( i n t e r p o l a t e d ) ^

CD 4

A0-21

^ 3 UJ -J ^ 2 (-0 t

1

1 1 1 1 i1

:

\ ^s^O.2 " \

\

0-08 \ 0^9 3 0-11

1 10°

O-CTZ 0-12 A

\

t

data assessment

= 0-2x10-^.

of field data points calculated using Um^joo and Tmax = 1-3T peak.

Numbers to each data point are values of

x

A = near-bed o r b i t a l

diameter.

llm,ioo = spectral maximum, near-bed, wave-induced ,maximum,horizontal orbital velocity.

T peak = the period associated with the frequency at which the spectral energy density distribution peaks.

FIGURE 3 .

SIZE DATA

\

1 2 3 4 5 6 7 910< 2 J3 ^ 41 51 6! \7 102 /Ou2m,ioo SEDIMENT FROUDE NUMBER 25 D

Bedform curves modified after Mogndge i Kamphuis (1972) for Co-ordinates

1 1 1 1 1 1 1

^'.^aOOS 0-20.°}.20 \

°0.11

7 c

T

0-18

0^7

i ^

3

COMPARISON OF DIMENSIONLESS FIELD BEDFORM W I T H BEDFORM CURVES OF MOGRIDGE & KAMPHUIS ( 1 9 7 2 ) (after Nielsen & Gordon , 1 9 8 4 )

46,

REFERENCES Carstens, M.R., Neilson, F.M. and Altinbilek, H.D. (1969). Bedforms generated in the laboratory under oscillatory flow: analytical and experimental study. TM-28, U.S. Army Corps of Eng., CERC, June, 1969.

Goda, Y. (1974). Estimation of wave statistics from spectral information Proc. Ocean Wave Msmt. Anal. A.S.C.E. New Orleans.Louisiana. Sept.

9-11, 1974.

Horikawa, K. and Watanabe, A. (1967). A study on sand movement due to wave action. Coast. Eng. Japan, Vol. 10, 1967. pp39-57.

Inman, D.L. (1957). Wave generated ripples in nearshore sands. Beach Erosion Board, TM-100.

Inman, D.L. and Bowen, A.J. (1963). Flume experiments on sand transport by waves and currents. Proc. 8th I.C.C.E., Mexico City.

Kennedy, J.F. and Falcon, M. (1965). Wave generated sediment ripples.

M.I.T., Dept. of Civ. Eng., Hyd. Lab. Rpt. No. 86, August,1965.

Lofquist, K.E.B. (1980). Measurements of oscillatory drag on sand ripples. Proc. 17th I.C.C.E., Sydney, Aust. March 24-28, 1980.

Mogridge, G.R. and Kamphuis, J.W. (1972). Experiments on bedform generation by wave action. Proc. 13th I.C.C.E., Vane., B.C., Canada, July 10-14, 1972.

Nielsen, A.F. (1984). Sand ripples under natural waves. M.Eng.Sc. thesis Univ. of New South Wales, Feb., 1980.

Nielsen, A.F. and Gordon, A.D. (1984). Sediment responses to natural waves. Proc. 19th I.C.C.E., Houston, Texas, Sept., 1984 (in press).

47.

THE ORIGIN OF TRANSGRESSIVE MARINE SANDS IN SOUTHEASTERN AUSTRALIA PETER S ROY (Geological Survey of N S W , Department of Mineral Resources)

Introduction Controversey has long surrounded the nature of coastal sand movements that occur during a marine transgression when the sea is rapidly rising and migrating across the continental shelf. On the tectonically stable, wave-dominated shelf of southeastern Australia, three distinct types of transgressive marine sand bodies have been identified which suggest that, here, contrasting modes of deposition operated during the postglacial marine transgression (PMT) c.l7 000 6 500 yrs B P . In this context "transgressive" refers to sand deposited in an onlapping stratigraphic relationship by nearshore marine processes. In most cases this occurred only during the PMT but, in some estuarine environments, it continued well into the stillstand (the last 6 500 yrs). The ages of the sand deposits are based on numerous radiocarbon dates, mainly on reworked shell fragments. Their origins have been evaluated using a simple model of shorefaces change.

The model is believed to have predictive capabilities.

The Model Changes that sandy barrier shorelines undergo during a marine transgression can be depicted in section by a concave equilibrium profile which is displaced upwards and landwards as sea level rises over a sloping surface. The shape of the profile depends on incident wave power and sand size. The trace that it subscribes as sea level rises usually follows the gross morphology of the continental shelf; temporary departures occur but cannot be prolonged because of sediment budget constraints. To maintain equilibrium, sand on the shoreface is redistributed: On relatively steep slopes, for a given rate of sea level rise, barriers shrink in size as sand is transported seawards onto the lower shoreface. On gentle slopes, the same rise in sea level induces rapid coastal retreat and the transfer of sand landwards onto and across the surface of a barrier island. An upper limit on the amount of sand that can be transported landwards is determined by the effectiveness of local waves, winds and currents. The amount of sand that can be stored behind the barrier is influenced by substrate gradient which, in turn, controls the size of the backbarrier estuary.

48.

Transqressive Marine Sand Bodies in SE Australia 1 Transgressive barrier and inlet sands form the basal parts of the present-day coastal sand complexes. They comprise poorly sorted, fine to coarse sand with mixed estuarine and marine shell assemblages that were deposited in shallow water environments in embayments with gently sloping substrates. The deposits reach thicknesses of 10 - 20 m , widths of 0.5 - 3.0 km and extend along shore for tens of kilometers. Bedding in the transgressive barriers is probably sub-horizontal to very gently landward dipping; in the inlets it is chaotic due to channel migration. While sea level was rising, sand reworked from the shoreface was carried landwards by storm overwash and inlet channel processes thus maintaining a barrier structure despite continual erosion of the shoreface. The resulting transgressive deposits were only preserved when sea level stabilised and regressive bay barriers began accreting at the present coast.

2 Transgressive tidal delta sands occur as subaqueous deposits in the mouths of deep, narrow, drowned river valley estuaries, These coast-transverse features reach thicknesses of 45 m , widths of 1.0 - 1.5 km and, with their associated stillstand deposits, extend up to 8 km upstream. The sands are medium to fine grained - they fine landwards and downwards - and overlie estuarine muds in the valley axes, which are very gently sloping. Transgressive deposits of stillstand age tend to be very shelly and have thick bedding sets with depositional slopes of 20-30^ dipping upstream; pre-stillstand deposits are less shelly and bedding is more gently dipping. Tidal delta sedimentation reflects a transition from barrier building to estuary infilling caused by an increase in estuary water depth and/or a reduction in the amount of sand transferred landwards. As proportionally more sand is deposited in the estuary, the subaereal barrier diminishes in size until it is permanently overtopped and estuarine sand transporting processes become dominant.

3 Transgressive shelf sands are found on the inner shelf in water depths of 30 - 80 m in areas where pre-Holocene surfaces are relatively steeply sloping. They occur in 10 - 30 m thick, 1 - 2 km w i d e , convex-up bodies of fine, well sorted nearshore sand that extend alongshore for distances of up to 30 km. Their upper parts are regressive and of stillstand age. Weakly defined bedding planes dip gently seawards, sub-parallel to their surface; in cores the sands are finely laminated with occasional, thin crossbedded sets. Storminduced, downwelling bottom currents are thought to be responsible for transporting sand offshore and depositing it on the lower shoreface below about -10 m . With continued rise in sea level, the sand becomes stranded below "wave base" on the inner shelf. As substrates steepen, erosion of the upper shoreface ceases to be the main source of sand which, alternatively, is supplied by littoral drift or erosion of relict dunes in the backshore.

49,

Discussion The shoreface model described here has been quantified using the following parameters: The idealised profile of equilibrium is concave and extends from the top of the storm berm, 3 km offshore to a water depth of 30 m . Its shape is typical of local open-coast shorefaces and encompasses the zone of nearshore sand. During successive increments of coastal change, the profile shape remains constant and the amounts of sand eroded and deposited are equal (changes to the profile shape and sediment budget are considered later). Sea level rises at 12 mm/year during the PMT which causes a marine transgression at rates depending on the shelf gradient.

Most shelf gradients in SE Australia are in the range 0.1^ - 1.0*^ and, for the stated sea level rise, correspond to theoretical shoreline retreat rates of 7.0 - 0.7 m/year. In some places, especially on the inner shelf, these slopes are exceeded and retreat rates were slower.

The model predicts that, on slopes steeper than about 0.6^ (= < 1.2 m/year coastal recession) the PMT induced a net offshore sand movement which led to the accumulation of transgressive shelf sand bodies.

Maximum rates of accumulation on the seabed of about 30 m^/m length of coast or c.lO mm/yr vertically occurred on gradients steeper than 5^. On slopes shallower than 0.6^, the model suggests that sand moved landwards. For example, on a slope of 0.23*^, the coast eroded at 3.0 m/year and about 65 m^ of sand/m length of coast was washed over and through the barrier. For an embayment 10 km long this represented a mass transfer of 650 000 m^ of sand each year. These estimates of coastal change assume that the equilibrium profile was maintained as sea level rose. However, there are at least three situations where this probably did not happen: 1 On steep substrates, an under-supply of sand would have led to over-deepening and over steepening of the shoreface and exposure of bedrock on the sea bed.

2 On extremely gentle slopes, very raped shoreface retreat may have imposed an excessive demand on sand transporting processes. A possible response would have been for only the upper part of the profile to translate landwards leaving the remainder on the inner shelf surface - the profile became shallower. The disequilibrium shelf may then have acted as an offshore sand source for coastal progradation during the stillstand.

3 On rapidly accreting shelf sand bodies, sedimentation was concentrated on the outer-most part of the equilibrium profile. This created excessively steep and unstable sea bed slopes near the base of the active zone. A tendency for fine, nearshore sand to move downslope under gravity progressively led to a modification of the outer part of the equilibrium profile. Even after sea level stabilised, this disequilibrium condition promoted offshore sand transport which resulted in the deposition of regressive sand lobes of stillstand age.

50,

Littoral drift has operated at all sea level stands on the SE Australian shelf, and in terms of sand transfer over very long distancesr may well have been more effective on the smooth, mid- and outer-shelf than on its irregular, embayed inner-most part. However, interrelationships between littoral drift and transgressive shoreline changes is unclear:to some extent they may be self-regulating.

Certainly, littoral drift has the potential to modify transgressive shoreline changes on both gentle and steep slopes: Net gains of sand to a coastal sector w i l l retard coastal recession and create thicker transgressive sand bodies. A net loss will do the reverse. Variations in shelf morphology may, in turn, modify littoral drift patterns: an example is where, as sea level rises, a littoral drift system encounters a steep coastal sector with an inadequate local sand supply. Here littoral sand on the upper shoreface is diverted seawards onto the inner shelf thus starving the coastal sand budget down-drift and accelerating shoreline erosion. It is suggested above that this phenomenon can carry—over into the stillstand period.

Conclusions In SE Australia, shelf m.orphology, mainly slope, emerges as an important factor in predetermining transgressive marine sedimentation styles at the present coast. Barriers, tidal deltas and shelf sand bodies began forming w e l l before sea level stabilised under conditions that probably also controlled sedimentation subsequently, even today.

In some cases, shelf sand bodies reflect a significant disruption to littoral drift during and after the PMT. Steep sections of the shelf surface thus act like headlands; areas downdrift experience long-term erosion. Permanent offshore sand losses caused in this way are undoubtedly a more widespread cause of contemporary coastal erosion than hitherto though.

Under no circumstances, on the SE Australian shelf, can barrier superstructure (beach and dune deposits) be expected to survive a marine transgression. While beach placer mineral deposits will not be found intact on the continental shelf, they may occur disseminated in shelf sand bodies located downdrift from mineralised areas.

Net seaward sand movements can occur during a marine transgression on surprisingly gentle slopes. These lead to the accumulation of linear bodies of fine, well sorted sand that have a relatively high preservation potential, especially if they formed over the shelf edge during very low stands of the sea. In ancient shelf sequences such sand bodies constitute prime petroleum targets.

5L

PHOSPHQRATISED VOLCANIC RIDGE ON THE UPPER CONTINENTAL SLOPE OFF CENTRAL N>S.W, CHARLES V.G.PHIPPS. Associate professor,Dept•Geology & Geophysics, University of Sydney.

Introduction.

The occurrence of a seamount on the upper continental slope off Sydney has been known for some time.(Phipps 1963).

In 1984,a cruise coring the slope off Port Macquarie ,traversed a flat topped ridge on this upper slope.

On subsequent cruises the ridge was dredged and mapped.

The survey used a mini-range position system to latitude 31®24'"S but due to erratic reception, radar range and bearings were used further to the south.

Morphology.

The ridge has a base on the west side of 220 to 230mm and on the eastern or seaward side to 260m. The shallowest portion has a depth of 160m.(Figure 1). The morphology is more clearly defined in the northern portion due to the more accurate and frequent fixing. South of 31°24*S contours are less specific and the topography appears to be less rugged. Minor irregularities in the southern section are indicated by spot depths,there being insufficient data to define the bottom.

In all cases,contour positions were obtained from echo records and not by interpolation between spot depths.

The feature can be traced for 10 nautical miles parallel to the shelf break and occurs as a prominent ridge for 7 miles. The width of the prominent part of the ridge varies from 1 - 2 nautical miles.

The most striking part of the ridge is the planar surface at around 160m,in the vicinity of 31^23'S (Figure This surface dips gently seaward.

Its depth corresponds to a prominent notch in the continental slope which has been recorded on many profiles along the N.S.W. slope at 150 - 160m. (Tuffin,Hons.Thesis,1984;Veeh & Veevers,1970 from the southern Great Barrier Reef;Jongsma,1970 in the Arafura Sea and Dill,1968 from several areas around Australia).It is considered to be due to the effects of a lower

52.

sea level s t a n d . ular.

(Table 2)

Below

165m the

t o p o g r a p h y is v e r y

irreg-

The rock d r e d g e d from the r i d g e , o f t e n with c o n s i d e r able d i f f i c u l t y , i s of a v e r y hard n a t u r e .

S a m p l e s were o b t a i n e d from the e a s t e r n slope and from the d e p r e s s i o n b e t w e e n the slope and the r i d g e .

The latter were in the form of c o b b l e s .

The m a t e r i a l d r e d g e d is a c h o c o l a t e b r o w n colour with a r o u n d e d c a v e r n o u s s u r f a c e . I n t e r n a l l y some p i e c e s showed f r a c t u r e f i l l i n g s of a fine g r a i n e d light b r o w n to y e l l o w ^.cday.

The m a i n b o d y of the rock c o n s i s t s of m o l l u s c s , f o r a m i n i f e r a and w o r m t u b e s in a fine c h o c o l a t e b r o w n m a t r i x .

No v o l c a n i c m a t e r i a l was found in any of the s a m p l e s e x a m i n e d .

S e d i m e n t d r e d g e d from b e t w e e n the slope and the ridge c o n s i s t s p r i n c i p a l l y of worm t u b e s .

Cores from the slope b o t h above and b e l o w the ridge c o n s i s t s of fine g r a i n e d grey-green unstratified mud.

C h e m i c a l a n a l y s e s of the s a m p l e s c o l l e c t e d show c o n s i d e r a b l e v a r i a t i o n in p h o s p a t e content from 3.91%P 0 to 8.94% P 0 .(Table 1) ZD z ID F u r t h e r down the slope a s m a l l e r feature was r e c o r d e d on the echo p r o f i l e s , a t 340m with a h e i g h t of 20m. This also c o n s i s t s of a c h o c o l a t e b r o w n p h o s p h o r a t i s e d m a t e r i a l .

The length of t h i s feature is u n k n o w n - b u t is -less , than six m i l e s .

Origin

of the

Ridge.

The ridge d e s c r i b e d is one of three f e a t u r e s k n o w n on the u p p e r s l o p e , b e t w e e n J e r v i s Bay in the south and Port M a c q u a r i e in the n o r t h .

One of these f e a t u r e s , a few m i l e s north of J e r v i s Bay was l o c a t e d on the N a t i o n a l M a p p i n g p r o f i l e s by L t . C o m m . M a t t h i a s at the Royal N a v a l C o l l e g e , Jervis Bay. T h i s has many f e a t u r e s s i m i l a r to the ridge off Port M a c q u a r i e in t e r m s of s i z e , d e p t h and seaward sloping p l a n a r t o p .

Off S y d n e y a feature k n o w n as Mount W o o l n o u g h is d e e p e r and has a more rugged t o p .

Samples d r e d g e d by M a c q u a r i e U n i v e r s i t y from this s t r u c t u r e include v o l c a n i c f r a g m e n t s (personal c o m m u n i c a t i o n ) .

The u p p e r surface of Mount W o o l n o u g h is also p h o s p h o r a t i s e d (Albani - p e r s o n a l c o m m u n i c a t i o n ) .

The p r e s e n c e of v o l c a n ica off S y d n e y and the g e n e r a l s i m i l a r i t y in size and shape s u g g e s t s that the three ridges have v o l c a n i c o r i g i n .

The J e r v i s Bay feature has yet to be s a m p l e d .

The three ridges have a v a l l e y b e t w e e n the p r e s e n t slope and r i d g e .

All f e a t u r e s are e l o n g a t e d p a r a l l e l to the shelf b r e a k and r e l a t i v e l y n a r r o w , s u g g e s t i n g that the o r i g i n a l igneous a c t i v i t y was c o n t r o l l e d by f r a c t u r e s p a r a l l e l to the shelf b r e a k .

It is s u g g e s t e d , t h e r e f o r e , that these r i d g e s r e p r e s e n t the r e s u l t of v o l c a n i c e r u p t ions along faults on the c o n t i n e n t a l m a r g i n a s s o c i a t e d with the b r e a k u p of the T a s m a n and are p r o b a b l y , t h e r e f o r e , o f mid to late C r e t a c e o u s age.

53.

JISiL

.I'ic^i'.V : J.' I !

• -^fsii

J* "itC-.v; • •

_21QnL

Mmx

Jim

2-Dirr .

- fi

-"Iv

WEST

EAST Fiq-ure

1

The

ridge

Cross

Section

extends

of

the

Port

approximately

10

Macquarie nautical

Ridge. miles

155:

16 0 -

EAST Figure Head

WEST 2. : Profile showing

of

Upper

a terrace

at

Continental 166m.

Slope

off

Norah

54,

TABLE

1.

2

CP 3

CP 4

Si02

CP

11.lA

18 .86

13.08

14.02

TiO^

0.21

0.24

0.23

0.22

AI2O3

3.13

3.31

3.80

3.13

28.00

28.33

36.09

30.38

Fe203(T)

1

CP

MnO

0.09

0.07

0.08

0.07

MgO

3.38

3.23

3.93

3.06

CaO

20.88

19.69

16.81

21.17

Na^O

0.95

0.89

0.82

1.11

1.08

1.02

0.63

0.73

3.91

5.34

7.00

8.94

2s SO3 Total

0.36

0.41

0.48

0.60

100.17

99.84

99.55

100.31

55,

ANAEROBIC PROCESSES AND THE FATE OF ORGANIC CARBON; DAVIES REEF, GREAT BARRIER REEF.

R V BURNE and G W SKYRING

(Baas Becking Geobiological Laboratory, PO Box 378

Canberra City

ACT 2601)

Coral Reef sediments generally do not contain significant amounts of organic carbon. The main producers of organic material in a reef eco-system are the corals, algal turfs and encrusting algae of the reef-slope, reef-crest and reef-flat. Many reefs are influenced by consistent current flows which transport organic matter from these areas of net organic production towards areas of net organic consumption in the back-reef and lagoon. Very little of this organic matter is preserved, and a study of the geomicrobiology of cores from the lagoon of Davies Reef was made in order to assess the diagenetic fate of the organic matter that did become incorporated into the sediments. This work was a contribution to the workshop on the Microbial Ecology of a Coral Reef (MECOR) held at the Australian Institute of Marine Sciences in July-August 1984.

Cores were obtained from three locations; in 6m of water immediately behind the reef flat, in 16m depth 800m from the reef flat, and in 26m depth 1500m from the reef flat. Cores were taken using plastic pipe and a jack-hammer operated by SCUBA divers.

Tudhope (1983) has drawn attention both to the major role of Calliariasids in reworking the lagoonal sediments of Davies Reef, and to the extent of bioerosion of sediment grains by microbial boring.

Both these processes will serve to reduce the organic content of the lagoonal sediments. Callianasid mounds and burrows covered the sea floor in the two shallower stations. At the deeper station Callianasid burrows were present, but not ubiquitous, and a fragile cyanobacterial mat had developed over areas of the sediment surface.

However all cores showed evidence of thorough bioturbation of the surface sediments. Despite the well aeriated nature of these sediments evidence of anaerobic microbial processes was detected in the surface layers.

Current knowledge of the role of micro-organisms in coral reef systems is not extensive, and anaerobic processes of fermentation, sulphate reduction, nitrate reduction, and methanogenesis are

56,

particularly poorly understood. Sulphate reduction is the most easily demonstrated of these anaerobic processes, and sulphate reduction rates were determined for the sediments of the cores (Skyring, In Press). Ra^es^were highest in the top 5cm of ea^h_(jore, increasing from 2mmol m d in the 0-1 cm layer to 3.5ramol m d in the l-5cm layer. Below 5cm rates were 1 to 2 orders of magnitude less, and below 30cm rates were very low, or beyond the limit of resolution of the method.

These rates are relatively low when compared with those determined for organic-rich coastal marine sediments, but considering the low levels of organic carbon incorporated into the sediments of the lagoon, they reflect an important aspect of the turnover of organic carbon within this environment.

References

SKYRING, G.W., In press - Anaerobic microbial processes in coral reef sediments. Abstract: 5th International Coral Reef Conference, Tahiti, 1985.

TUDHOPE, A.W., 1983 - Processes of lagoonal sedimentation and patch reef development, Davies Reef, Great Barrier Reef of Australia. Ph.D. Thesis. University of Edinburgh.

57.

ON ASPECTS OF THE SEDIMENTOLOGY AND BIOLOGIC INTERACTION IN INTERTIDAL ENVIRONMENTS NEAR RED BLUFE, CORNER INLET, VICTQRIA B.R. BOLTON and G. BOADLE (Department of Geology, La Trobe University)

ABSTRACT Corner Inlet, a large shallow marine embayment in southwestern Victoria, experiences a high tidal range, is affected by waves of moderate height, and is largely devoid of significant sediment input by stream discharge. The study area, near Red Bluff situated on the western margin of the Inlet, can be differentiated into beach, mangrove and open tidal flat environments.

Macrobenthos zonation is related to geomorphic features and can be defined in terms of species distribution and density. Dominant species characterize each zone: F. Nereidae (polychaete) is characteristic in the beach, mangrove Avicennia marina and crustacean, Carcinus maenas in the mangrove zone, and seagrass, Zostera muelleri and Posidonia australis, and anthoza, Anthopleura sp. in open tidal flats. Diversity is lowest in the beach and highest in the open tidal flat and in particular those areas dominated by Zostera.

In the study area mean grain size varies from very coarse sands to very fine silts and is mainly composed of quartz. The coarsest sands are restricted to the beach while fine sands and very fine sands are characteristic of the open tidal flat; very fine silt is restricted to the mangrove zone. Beach sands are usually massive or crudely bedded with low-angle truncations which gently dip seaward, while sediments in the mangrove and open tidal flat zones are generally massive. All areas are subject to varying intensity of bioturbation which may partially or completely destroy depositional structures.

Characteristic biogenic sedimentary structures are found in mangrove and open tidal flat zones. Vertical and horizontal molds of the pneiimatophores of Avicennia marina characterize the mangrove zone while the oblique to vertical burrows of Callianassa australiensis. the twisted oblique burrows of Carcinus maenas and the shallow, horizontal intrastratal trails of gastropod, Polinices can be used to define the open tidal flat environment.

58,

SEDIMENTATION IN CAVES r A B E V i m R.A.L OSBORNE (Department of Geology & Geophysics, University of Sydney)

Karst caves are among the smallest of terrestrial depositional environments, yet a wide range of sedimentary processes operate in them.

Caves act as sediment traps, and contain records of the terrestrial environment during periods of erosion. Since these have prevailed in much of Australia during the Quaternary, and for most of the Cainozoic, the study of cave sediments has much potential.

Caves not only contain non-clastic (speleothems) and organic (guano, and bone breccia) sediments but also a wide variety of clastics ranging from coarse fluviatile gravels to fine lacustrine muds, including slump deposits, collapse breccias, and turbidite sequences.

Due to the small size of caves, differing depositional mechanisms can be active in close proximity, leading to a very high degree of lateral facies change. Thus the stratigraphy of cave sediments is often complex.

Sedimentation in caves is controlled by distance from the cave entrance and relationship to the permanent karst water table. Five major depositional environments can be recognised in caves: 1 2 3 4 5

, upper vadose, entrance environment upper vadose, interior environment lower vadose environment dynamic phreatic environment nothephreatic environment

(Fig (Fig (Fig (Fig (Fig

1 A) 1 B) 1 C) 1 D) 1 E)

These five environments will not be found in all caves and their degree of development and significance will vary from cave to cave.

Upper Vadose, Entrance Environment The upper vadose, entrance environment comprises dry cave entrances and those parts of the cave in close proximity to them.

59,

Surface matter enters caves by rain-wash, and gravitational processes to produce an entrance talus cone (Fig 2 H&K). Entrance facies often contain a significant amount of aeolian material as well as locally-derived surface detritus.

Entrance facies may be rich in bone introduced by carnivores, roosting predatory birds, and by animals falling into cave entrances.

Bedding is often poor and inclined due to the geometry of talus cones.

Surface-derived infauna may infest and bioturbate entrance facies.

Speleothems are poorly developed in this environment with algal activity and evaporation playing a significant part in their deposition.

Upper Vadoser Interior Environment In the upper vadose, interior environment chemical and biological deposition dominate. Coarse clastics tend to be trapped in entrance talus cones so that only fines reach the dry cave interior. Coarse limestone fragments are deposited by breakdown (Fig 2 J) and movement of material in rockpiles.

Speleothems (Fig 2 G) are characteristic of this environment, with loss of carbon dioxide to the cave atmosphere being the dominant mechanism of deposition. Where water accumulates in pools cave pearls and a range of subaqueous crystalline sediments may be deposited.

Guano (Fig 2 I) is the most significant organic sediment of the upper vadose, interior environment. Guano is the major source of soluble phosphate in caves.

Lower Vadose, Interior Environment In the lower vadose, interior environment irregular flooding and the action of permanent streams (Fig 2 A) are the most significant depositional mechanisms. Other vadose processes, like the deposition of speleothems, continue in this environment, but are punctuated by flooding events.

Permanent streams deposit fluviatile sediments similar to those produced by surface streams, while floods can deposit laminated fines and coat cave walls with mud and surface-derived organic debris.

Dynamic Phreatic Environment The dynamic phreatic environment exists where water below the permanent karst water table is moving under pressure. This is a high energy environment with sand and gravel being deposited where the geometry of the water-filled passages results in a reduction in flow velocity.

60.

Nothephreatic Environment The nothephreatic environment exists where water below the permanent karst water table is moving slowly. This is a low energy environment, often distant from the surface.

Laminated fines (Fig 2 B) and subaqueous precipitation deposits (Fig 2 D) are typical sediments of this environment. Coarse material may reach the nothephreas by slumping and result in the deposition of turbidite sequences (Fig 2 C). If the nothephreatic water is saturated in calcium carbonate, floe calcite (Fig 2 E) will be deposited on its surface.

K

61,

FIGURE

1

DEPQSITIONAL ENVIRONMENTS

IN CAVES

WATER WATER TABLE

FIGURE 2

SOME TYPES OF CAVE

SEDIMENT

62,

THE USE OF THE TRACER CAESIUM-137 FOR STUDYING SEDIMENT MOVEMENT IN DRAINAGE BASINS, R.J.LOUGHRAN (Department of Geography, University of Newcastle) B.L^CAMPBELL (Australian Atomic Energy Comnission, Lucas Heights) G.L.ELLIOTT (Soil Conservation Service of N.S.W., Gunnedah Research Centre)

INTRODUCTION Caesium-137 (half-life 30y) is a product of atmospheric nuclear weapons tests. On reaching the earth's surface as fallout, the isotope becomes rapidly and firmly adsorbed on to soil fines.

Undisturbed soils that have undergone little or no erosion, accumulate Cs-137 in the upper part of the soil profile and can be regarded as "input" sites for a drainage basin. Sites that have been eroded will contain proportionally less Cs-137. Cultivated soils have Cs-137 redistributed within the ploughed layer, and levels of Cs-137 can indicate the degree of erosion that has occurred.

Sediments deposited over the past 30 years will contain Cs-137 directly from fallout, since Cs-137 was first detected in the environment in 1954, and from adsorbed Cs-137 on incoming sediments.

In a sedimentary sequence, layers with no adsorbed Cs-137 can be dated as pre-1954, while layers containing maximum Cs-137 concentrations may be related to 1964-65, the period of peak fallout.

Studies of Cs-137 redistribution on Australian soils have been carried out by Longmore et al. (1983) on the Darling Downs, Queensland, and by Campbell, Elliott, Loughran and co-workers in N.S.W. (eg. Campbell et al., 1982). The greater proportion of the work in N.S.W. has centered'on the drainage basin of Maluna Creek at Pokolbin, in the Hunter River valley. The use of Cs-137 as an indicator of geomorphic processes in a drainage basin can be demonstrated from results obtained in Maluna Creek catchment.

METHODS Soil samples were taken at depth increments of one to two centimetres, or by coring. The finer fraction (<2.4 mm) was analysed for Cs-137 activity by gamma spectroscopy using a Ge (Li) detector. Caesium-137 concentrations were expressed in millibecquerels per gram (mBq/g), or as areal activity (mBq/cm^).

Maluna Creek basin (1.7 km^) contains Carboniferous, Permian and Triassic sediments and volcanic rocks, and has a variety of soil types (black earths, podzolics, lithosols and coarsetextured earths). Forest occupies 60% of the basin, with 30% under

63.

grass and 10% under vines.

RESULTS AND DISCUSSION Forest soils have Cs-137 concentrated in their surface layers (0-6 cm). Per unit area, Cs~137 levels range from 60.0 to 214.7 mBq/cm% with an average of 150 mBq/cm^ (N=ll). Uncultivated sites under grass have Cs-137 concentrated in the upper profile, generally to a depth of 6-8 cm. Caesium-137 levels range from 26.0 to 137.1 mBq/cm% average 104.8 mBq/cm^ (N=ll). Hillsope sites under vines (N=25) contain least Cs-137, levels ranging from 3.3 to 97.9 mBq/cm^ (average 43.9 mBq/cm^).

The Cs-137 content of forest soils is significantly higher than that of the grazed soils, which in turn have significantly more Cs-137 than cultivated soils (Mann-Whitney U-test). These differences have been attributed to soil erosion by raindrop splash and runoff.

Sedimentary deposits in alluvial fans, flood plains and one farm dam have been sampled for Cs-137 analysis. Two typical profiles from a farm dam and an alluvial fan (Fig.l), show significant Cs-137 present to depths of 126 cm and 45 cm respectively. It was concluded that sediments above these depths had accumulated since the mid1950s. Maxim-jm Cs-137 concentations were found between 106-116 cm in the farm dam, and between 35-40 cm in alluvial fan no.4 (Fig.l).

These layers probably represent surfaces exposed at the time of greatest fallout (1964-65), plus deposits of additional sediment strongly labelled with Cs-137 and eroded from nearby slopes thereafter. The source of these sediments was in all probability the adjacent vineyards, first cultivated in 1970. This conclusion is supported by the presence of a sedimentary and colour -disconuity at the same depth, indicating a change in sediment dynamics.

CONCLUSION Caesium-137 as a tracer of sediment movement has universal application. Levels of erosion can be established against stable "Cs-137 input" sites, and sedimentation rates over the past 30 years can be determined. The tracer is capable of providing information on sediment movement that would otherwise be unavailable.

REFERENCES Campbell,B.L., Loughran,R.J. and Elliott,G.L., 1982 : Caesium-137 as an indicator of geomorphic processes in a drainage basin system, Australian Geographical Studies, 20, 49-64.

Longmore (McCallan),M.E., 0'Leary,3.M., Rose,C.W. and Chandica,A.L., 1983 : Mapping soil erosion and accumulation with the fallout isotope Caesium-137. Australian Journal of Soil Research, 21, 373-85.

64,

o

}o 20 ^o

+0 50

60 10

fiO 90 100 110

120

h

130 140 150 IW I70 /80^

P

' '—^—'—»—I—1—^—, o 5 IOIS 202S3C35-KD '^^CS COMC£NTRATIon IM

ilLT E, CLAY : mB<^

n—T—T—r . 1—I 2 8 10 IZ 14 16 18 20 22 ^^CS CCNCENTPATION IN SiLT CLAY m Bc^ g""^

FIGURE 1 Caesium-137 profiles for Maluna farm dam reservoir (left), and Maluna alluvial fan no. 4 (right).

65,

THE MORPHOLOGY AND GENESIS OF LACUSTRINE STROMATOLITES IN SOUTH AUSTRALIA AND WESTERN AUSTRALIA LINDA MOORE AND ROBERT BURNE (Department of Microbiology, University of Western Australia, and Baas Becking Geobiological Laboratory, Canberra)

Stromatolites are composite organosedimentary structures formed by the interaction between a microbial community and autochthonous sediments. Since their dominance of Proterozoic shallow water environs, they have decreased in diversity and abundance and are virtually absent from present-day holomarine environments. They are, however, an important component of some modern marine embayments, tidal marshes and lakes. Stromatolites occur in Lake Fellmongery, Lake Inneston and Sleaford Mere, in South Australia, and in Lake Clifton, Western Australia. They are the dominant ecological component of the shallow water margins of most of these lakes. The lakes differ widely in salinity, ionic composition and saturation indicies, sedimentological associations and faunal communities. The respective mineralogy of the stromatolites reflects some of these differences. The stromatolites in Lake Clifton and Sleaford Mere are composed of calcium carbonate, while those in Lake Fellmongery are monohydrocalcite, and in Lake Inneston are gypsum dominated.

The processes which initiate and continue to support stromatolite growth in these environments are, as yet, not completely understood. Stromatolite formation may be initiated by the preferential colonization of surface irregularities and innate substrates by microbial communities. Conversely, the growth of such a community may be stimulated by a combination of physico/chemical conditions under which the microbes proliferate, stabilize the sediment and eventually form their own substrate - a stromatolite.

Stromatolites are not simple structures. Their resultant morphologies reflect the varying interactions between physical, chemical and biological factors. These include differing rates of sedimentation, wave and current activity, chemical composition of the lake water with varying degrees of saturation and mineralization, and the effect of a number of biological associations. Variations in the relative importance of microbial species composition and activity, bioerosion and metazoan grazing contribute to the stromatolite morphology. The extent of microbial control over the precipitation of cements may also be significant in the formation and development of these lacustrine stromatolites. The importance of biologicallymediated precipitation as opposed to, or in addition to, inorganic precipitation and mechanical binding of sediments remains unresolved at present.

66,

Stromatolites represent intricate ecosystems, in some cases analogous to coral reef associations. They are the result of many subtle but complex interactive processes. Through a comparative study of contrasting stromatolite environments, the specialized nature of the ecosystem and the processes which initiate and support stromatolite growth can be recognized and documented. In Lake Clifton, for example, a lake of low salinity, the growth and distribution of stromatolites appears to be associated with a fresh, bicarbonate groundwater intrusion on the eastern shore of the lake.

In contrast. Lake Inneston is hypersaline, with gypsum saturated waters and no evident pattern of stromatolite distribution.

A great deal of emphasis has been placed upon the use of modern stromatolite environments for the interpretation of fossil structures and their developmental conditions. The demise of Proterozoic stromatolites has actually been attributed to the evolution of a grazing metazoan fauna. Current research on a variety of stromatolite environments indicates that these structures are not necessarily limited to extreme environments, where competition and predation are precluded. In fact, many modern structures are found to coexist with, and even maintain, a metazoan fauna as an integral part of the overall community. The study of living stromatolites is important, in its own right, as a highly successful ecological community that spans billions of years of evolutionary pressures and continues to encompass many diverse aquatic environments.

67.

SIGNIFICANCE OF FAUNAS IN INTERPRETING PLEISTOCENE GEOLOGY. RICHMOND RIVER VALLEY. NEW SOUTH WALES J. W. PICKETT (Geological Survey of New South Wales, Sydney)

An extensive drilling programme undertaken by the Water Resources Commission of N.S.W. has disclosed the presence of extensive clayey sediments with a fauna of marine invertebrates.

This unit has been called the Gundurimba Clay (Drury & Roman, 1983).

It underlies most of the valley floor, and extends from upstream points near Boatharbour on the Wilson Arm and at least Tatham on the main river (Drury & Roman, 1983, though marine fossils from bores near Greenridge indicate this as a more upstream point, Pickett, 1974), to beyond the present shoreline.

In the area near the coast it is overlain by the Woodburn Sand; this unit includes the ridges of the Pleistocene Inner Barrier, but extends much further inland than any identifiable beach ridges.

FAUNAS Corals The discovery of a fauna of scleractinian corals at a locality behind the Inner Barrier enabled Marshall & Thom (1976) to determine an age of 112-127 Ka for the deposit, and to suggest a sea level stand of 5 ± Im for this episode.

Later, when better data on extant coral faunas of N.S.W. became available, Pickett (1981) examined the coral fauna (20 species) and concluded a) that their occurrence was in accord with the sea-level suggested by Marshall and Thom, b) the assemblage suggested a warmer climate equivalent to 2^ of latitude and c) that "the estuary was wide open at some stage prior to barrier formation".

Ostracodes Ostracodes occur in most boreholes, up to the extremes of the Gundurimba Clay at Green Ridge and Boatharbour.

The assemblages are abundant and varied, even at the most upstream point. McKenzie & Pickett (in press) identified 44 species and subspecies in a study of these faunas. The assemblage most remote from the ocean, that at Boatharbour, comprised 7 ostracode species. This locality lies ca.

35 km from the probable entrance to the interglacial Richmond estuary.

In the modern Hawkesbury River at Getitlemans Reach, ca.

27 km from the entrance, the ostracode assemblage, while dominated by the same species, includes only six forms, while at locations

68,

35 km upstream the dominant, species Osticythere reticulata occurs alone- This evidence suggests that the interglacial Richmond estuary was at least as open, if not more open, than the present Hawkesbury River.

An analysis of the Pleistocene ostracode assemblages has produced significant results. Fig. 2 of McKenzie & Pickett (in press), reproduced here, shows the percentage composition of four major ostracode assemblages. The three genera Osticythera, Keijella and Trachyleberis are characteristic of estuarine environments, and these three heavily dominate the upstream faunas. Significantly also, Keijella, characteristic of the outer estuary, dominates in borehole 39145, but is progressively less well represented upstream, while Osticythere, characteristic of more brackish areas, shows the reverse pattern. 'All these forms together make up an insignificant proportion of the assemblage at the Evans Head coral locality, which is dominated by holomarine xestoleberidids. Thus there is a dramatic faunal discontinuity between borehole 39145 and the coral locality, plainly to be ascribed to the fact that the Boatharbour • 39135 former was outer estuary and the latter open ocean.

Foraminifera Foraminifera are also conspicuous in the assemblages from the drilling programme.

No detailed study of these has been undertaken to date, but an examination of the presence of planktonic species was made by Pickett (1981). Planktonic Foraminifera do not occur further upstream than the bores near Coraki. As in modern estuaries planktonic foraminifera are restricted to the outermost portions, this observation is in complete accord with the evidence from the ostracodes.

Fig. 1. Proportional representation of ostracode taxa in samples from a range of environments, Richmond River valley. Key to hachures: o = Osticythere; k = Keijella; X = xestoleberidids; L = loxoconchids; 3 = Bairdiacea; C = Cypridacea; diagonal lines = Trachyleberis; crosshatching = other groups.

14407

69,

CONSEQUENCES Open Estuary The evidence from all three groups of animals indicates quite clearly that the environment of the coral locality at Evans Head lay in an area of unrestricted marine circulation, and that the area around Coraki and beyond was estuarine. The ostracodes demonstrate further that Coraki lay in the outer estuary.

Inner Barrier Emplacement Sediments of the Inner Barrier (the Woodburn Sand) overlie the Gundurimba Clay, which was laid down at a time of higher sea level, and in an estuary which was at least as open to the ocean as the modern Hawkesbury River. The presence of an emergent barrier system in its present position, and of similar extent, would have reduced salinities so drastically that faunas of the types observed in the Gundurimba Clay could never have flourished. Quite clearly then, the barrier system was emplaced subsequent to the development of the high sea level represented by the Gundurimba Clay. As the theory of barrier emplacement recently presented by Roy & Thom (1981) implies shoreward advancement of the sand barrier during the advance of the ocean and its subsequent stranding with the regression, it is incompatible with the evidence from the faunas of the Richmond River valley. It is probable therefore that the barrier emplacement mechanism must be sought elsewhere.

REFERENCES Drury, L.W., & Roman, D., 1983. Chronological correlation of interglacial sediments of the Richmond River Valley, New South Wales, In Ambrose, W., & Duerden, P., eds. Archaeometry, an Australian perspective, Canberra, ANU Press, 290-296 (imprint 1982).

Marshall, J.F., & Thom, B.G., 1976. The sea level in the last interglacial. Nature 263, 120-121.

McKenzie, K.G., & Pickett, J.W., in press. Environmental interpretations of Late Pleistocene ostracode assemblages from the Richmond River valley, New South Wales. Proc, Roy, Soc, Vict, 96\ 227-242.

Pickett, J.W., 1974. Subrecent estuarine molluscs from Casino.

Rep, geol, Surv, N.5.W, GS1974/284, (Palaeont. 1974/24)(unpubl.)

Pickett, J.W., 1981. A Late Pleistocene coral fauna from Evans Head.

N.S.W. Alcheringa 5, 71-83.

Roy, P.S., St Thom, B.G., 1981. Late Quaternary marine deposition in New South Wales and southern Queensland - an evolutionary model.

J. geol, Soc, Aust, 28, 471-489.

70,

THE M E A S U R E M E N T OF S E D I M E N T T R A N S P O R T IN N 5 W E S T U A R I E S B , DRUERY - Supervising Engineer, Estuary Management Section, Public Works Department N.S.W. G. H U R R E L L - E n g i n e e r , E s t u a r y M a n a g e m e n t S e c t i o n J . FLOYD

- Engineer, Estuary Management Section

The a s s e s s m e n t of s e d i m e n t t r a n s p o r t rates varies from e s t i m a t e s of s e d i m e n t a c c u m u l a t i o n over thousands of y e a r s (geological) to intratidal and instantaneous m e a s u r e m e n t s associated w i t h c o n t e m p o r a r y p r o c e s s s t u d i e s . V a r i o u s m e t h o d s m a y assess o n l y the net t r a n s p o r t rate (accumulation models) or both the g r o s s and the net t r a n s p o r t r a t e . T h e y also vary from the d e s c r i p t i v e (aerial p h o t o g r a p h y and facies r e l a t i o n s h i p s ) to the fully q u a n t i t a t i v e .

Both the m e a s u r e m e n t and p r e d i c t i o n of s e d i m e n t t r a n s p o r t rates are still v e r y m u c h an a r t . It is c o m m o n for major c o n t e m p o r a r y s e d i m e n t a r y p r o c e s s e s studies to utilise a number of techniques to assess s e d i m e n t t r a n s p o r t rates and the interpretation and a n a l y s i s of the final results usually d e p e n d s upon the e x p e r i e n c e and judgement of the i n v e s t i g a t o r .

The Public W o r k s D e p a r t m e n t is c o n d u c t i n g a major study of the h y d r o d y n a m i c and s e d i m e n t a r y p r o c e s s e s of P o r t H a c k i n g , an e s t u a r y with a mixed tide and w a v e e n v i r o n m e n t . In c o l l a b o r a t i o n w i t h the D e p a r t m e n t of M i n e r a l R e s o u r c e s and the A u s t r a l i a n A t o m i c E n e r g y C o m m i s s i o n , a number of techniques are being used to assess s e d i m e n t transport rates in this e s t u a r y . The techniques are; Sedimentology Coring and r a d i o m e t r i c d a t i n g Bedform monitoring Simple surveys of b e d f o r m s R a d i o - a c t i v e sand tracing Delta front p r o g r a d a t i o n m e a s u r e m e n t s Photogrammetry The paper w i l l d i s c u s s the a p p l i c a t i o n of these

techniques.

7L

HOLQCENE SEDIMENTATION IN ESTUARIES OF NORTHERN AND EASTEEN AUSTRALIA B.G, THOM, J. CHAPPELL and C. WOODROFFE (Dept of Geography, University of Sydney; Dept of Biogeography and Geomorphology, A^N.U.; and Northern Australian Research Unit, A.N.U.)

Studies of Holocene sedimentation in drowned river valleys have suggested at least two basic models of deltaic-estuarine evolution.

The best known model is that associated with the progressive seawards growth of major river deltas like the Mississippi and the Yangtze.

There are many variants of this model.

The second model is well exhibited on the southeast coast of Australia. Flood-tidal deltas and/or sand barriers block the mouths of many embayments and drowned valleys. River deltas prograde into relatively deep estuarine basins so that infill takes place from both landward and seaward directions. Various stages of estuarine infill have been recognized along the N.S.W. coast.

In northern Australia a third type of estuarine sedimentation occurs. A number of large rivers flood annually in the wet season into macrotidal estuaries. Mangroves fringe these rivers with closed forests being restricted to shoaling mid-channel islands and point bars. Bare tidal flats or sedge/grass-covered alluvial plains dominate the estuarine-deltaic surfaces of Holocene age in these drowned valleys. Our research shows that mangrove forests were very much more extensive than present in mid-Holocene times, about 5500 to 6500 years ago. The magnitude of the change is illustrated by work in King Sound, the Ord delta, and by recent data from the Daly River and especially the South Alligator River. Mangroves were about 50 times more extensive in the mid-Holocene in the South Alligator area at this time, equal to about 8% of the present total for the whole of tropical Australia.

The model of deltaic-estuarine sedimentation inferred from morphostratigraphic work in northern Australia involves a phase of rapid progradation soon after sea level reached its present level 6500 years ago. Intertidal surfaces covering most of the drowned valley were extensively vegetated by mangroves at this time. About 5500 years ago a major change took place as the deltaic-estuarine surface accreted virtually up to and above the limit of high spring tides under conditions of a "stillstand" in relative sea level.

Sediment then became available for alongshore transport and accretion.

72.

EVOLUTIONARY PROCESSES ON A DELTAIC COAST - EVIDENCE FROM THE BARRON DELTA, TRINITY BAY, QUEENSLAND M,R. JONES & A.W, STEPHENS (Geological Survey of Queensland)

ABSTRACT Trinity Bay lies near the city of Cairns in tropical north Queensland. Along the western margin of the bay, coastal progradation during the Holocene has resulted from a continuing supply of sediments from the Barron River. The sediments have accumulated principally as two separate geomorphic units: the fluvial Barron Delta flanking the main distributary channel; and an extensive chenier plain in the Trinity Inlet embayment several kilometres to the south (Figure 1). The geological development of both areas has occurred simultaneously from sediments supplied by a single fluvial source. Despite the physiographic appearance of Trinity Inlet, there are no significant permanent streams supplying sediments directly to this embayment.

The onshore geological record at the Barron Delta comprises remnants of beach ridge barriers, estuarine back barrier deposits, and attached barrier-spit accumulations. Distributary mouth bar deposits presently occupy intertidal and near subtidal areas. A single distributary channel has supplied Barron River sediments to Trinity Bay during the Holocene sea level stillstand period and wave reworking around the flank of the delta has produced beach ridge barrier accretion. The oldest Holocene beach ridge barriers were developed on either side of the river channel where it entered Trinity Bay. In the initial stages, the river had not produced a significant sedimentary projection into Trinity Bay and wave energy levels on both sides of the entrance were similar, being sufficient to produce beach ridge barrier accretion. Continued development of the delta during the late Holocene produced a coastline bulge which caused differences in wave energy from north to south. The delta perimeter south of the mouth was subjected to progressively decreasing wave energy, while to the north, the wave energy remained relatively constant. A mangrove fringed shore evolved south of the entrance in contrast with the northern flank where sandy beaches continued to develop.

Since the early stages of delta progradation, the suspended sediments supplied by the river have accumulated not only in

73.

prodelta depositional environments on the bed of Trinity Bay but also at intertidal levels in the extensive Trinity Inlet embayment to the south. Trinity Inlet was sufficiently distant from the Barron River outflow for its sediment supply to comprise a high proportion of mud. With continued accumulation, the Trinity Inlet coastline prograded northwards becoming progressively closer to the Barron mouth and also subject to increasing wave energy. These factors led to a progressive change in accretionary forms from widely spaced cheniers to closely spaced cheniers and finally to a beach ridge barrier which extends to the modern shoreline on the northwestern side of the inlet (Figure 1). Further progradation of the beach ridge barrier has been prevented by a termination of the supply of sand sized sediments from the river. This has been caused by a progressive northward redirection of the delta distributary channel away from Trinity Inlet.

Channel movement in the lower reaches has played an important part in delta evolution. The depositional record indicates that numerous channel shifts have occurred near the present day lower reaches of the river to produce a distinctive accretionary unit comprising an extensive intertidal mangrove forest containing curvilinear sand ridges exhibiting cross cutting relationships. Historical changes provide a model for the development of such features. The curvi-linear ridges are interpreted as attached barrier-spits swept landwards by wave activity from the intertidal and near subtidal distributary mouth bar. The intervening mud deposits result from onshore transport of suspended terrigenous sediments. The build up of the barrier-spit and beach ridge accretion is due to a two stage sediment transport process: poorly sorted sediments are initially supplied during river floods to the intertidal and near subtidal distributary mouth bar; the sediments are then sorted by wave activity and sand is transported landwards to supply the littoral drift system along the delta flank.

During the mid and late Holocene, changes in the position of the distributary channel have been relatively localized and confined to the lower reaches. In historic times, a major channel shift has been initiated which could relocate the distributary mouth 5 km northwards. The Barron River has established a permanent connection with Thomatis Creek which provides shorter access to the sea than via the main channel (Figure 1). Increasing proportions of river flood flow are now being carried by the Thomatis channel. If this evolutionary trend is allowed to continue, Thomatis Creek will become the main channel, and a new distributary mouth bar will be developed in the position of the present Thomatis Creek entrance.

The current understanding of the geological evolution of the delta provides a basis for predicting the effects on coastline stability of such a major natural change. The impact of man made changes such as sand and gravel extraction from the Barron River can also be assessed.

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Figure I ^ Trinity Bay, North Queensland.

15.

ACOUSTIC STRATIGRAPHY AND SEDIMENT STRUCTURES OF THE CONTINENTAL SHELF OFF SYDNEY> A.D. ALBANI, P.C. RICKWOOD (School of Applied Geology, University of N.S.W,) J.W. TAYTON (School of Earth Sciences, Macquarie University).

The nature of the bedrock topography on the Continental Shelf from Broken Bay to Port Hacking and 50 km eastwards has shown that the many river valleys present along the inner part of the shelf are related to the Pleistocene low sea levels. Only the Hawkesbury River appears to have existed long before the others and a tentative age may be obtained by the relationship of its valley floor with the sedimentary structures present on the outer portion of the Shelf.

The sediment cover on the inner shelf is mainly limited to the infill of the river valleys and to a number of sediment lobes piled against the present rocky shores.

The Triassic rocks, often covered only by a thin"veneer of sediment, form most of the gently sloping sea bed and only occasionally form "reef" structures.

On the outer shelf a number of well defined acoustic interfaces can be tentatively correlated with the global sea level curve and thus they may give a tentative time scale to the maximum excavation of the Hawlesbury River System and to the deposition of the sediment wedges which form most of the unconsolidated cover.

One suspected volcanic centre, Mt. Woolnough, has been mapped in detail and some samples were dredged from it. In addition, a previously undetected volcanic centre has been found off Malabar; it is totally covered by sediment.

Many dykes observed on the coast have been traced seawards for up to 13 km and magnetic anomalies signify the presence of an even greater number that were not previously known.

76.

SEDIMENT•GENERATION'BY HALIMEDA MEADOWS IN THE NORTHERN GREAT BARRIER REEF

EDWARD DREW (Australian Institute of Marine Science, Townsville)

Abstract not available.

77.

oHORT

T E R n SEDIi^lENT nOUEPIENT IN THE U O R O N O R A

ESTUARY.

PAULA A . DOUGLAS ( D e p t . of G e o g r a p h y , U n i v e r s i t y

of

Sydney)

Introduction E s t u a r i e s of E a s t e r n A u s t r a l i a oue t h e i r p r e s e n t form to the last,- -post g l a c i a l rise in sea lev/el. G e o l o g i c a l l y , they are e p h e m e r a l f e a t u r e s i n f i l l i n g at v a r i o u s r a t e s u i t h f l u v i a l , m a r i n e and o i o g e n i c a l l y p r o d u c e d s e d i m e n t s . .

L o c a t e d a p p r o x i m a t e l y 20 k i l o m e t r e s s o u t h of S y d n e y , the U o r o n o r a R i v e r is the m a j o r t r i b u t a r y of the G e o r g e s R i v e r s y s t e m w h i c h d r a i n s in-to the s o u t h w e s t e r n c o r n e r of Botany Bay. C a t c h m e n t g e o l o g y is u n i f o r m l y H a u k e s b u r y s a n d s t o n e , c o n s i s t i n g p r e d o m i n a n t l y of u n i f o r m , m a s s i v e , t h i c k l y b e d d e d q u a r t z i t e and s a n d s t o n e uith some i n t e r b e d d e d l e n s e s of s h a l e and c o n g l o m e r a t e ( S t a n d a r d 1 9 6 9 ) .

T h e t e r r a i n is b r o k e n by a u e l l d e v e l o p e d set of tecton-ic j o i n t p a t t e r n s , g e n e r a l l y , t r e n d i n g n o r t h - s o u t h and e a s t - u e s t . T h e c a t c h m e n t h a s b e e n d e e o l y d i s s e c t e d by the U o r o n o r a R i v e r and its t r i b u t a r i e s i n t o the n o r t h w a r d t i l t i n g U o r o n o r a p l a t e a u . T h i s h a s r e s u l t e d in n a r r o w , s t e e p and r o c k y v a l l e y s , w i t h l i m i t e d f l o o d p l a i n d e v e l o p m e n t (Bird 1 9 8 4 ) . S t e e p t o p o g r a p h y and s a n d y p a r e n t m a t e r i a l h a v e r e s u l t e d in s a n d y s o i l s w h i c h a r e low in o r g a n i c c o n t e n t and highly erodible.

In the t i d a l U o r o n o r a , h u m a n a c t i v i t y h a s m o d i f i e d n a t u r a l p r o c e s s e s by the c o n s t r u c t i o n of a dam and w e i r in the h e a d w a t e r s , sand dredging,, bank r e c l a m a t i o n and urbanisation.

The S u r v e y and

Results

Q u a n t i f i c a t i o n of c h a n n e l c h a n g e o v e r the l a s t e i a h t y e a r s is p o s s i b l e due to an e a r l i e r ^ s t u d y w h i c h c o m p a r e d the c h a n n e l to a 1968 h y d r o g r a p h i c map ( U a r n e r and P i c k u p 1 9 7 8 ) .

I n c r e a s e s in c h a n n e l c a p a c i t y m a i n l y r e l a t e d to d r e d o i n o in the e a r l y 1 9 7 0 ' s w e r e r e c o r d e d . A r e ~ s u r v e y e i g h t y e a r s ^ l a t e r IS a u s e f u l i n d i c a t o r of r e c o v e r y r a t e s in a s t e e n , s a n d s t o n e catchment.

C o m p a r i s o n of 25 c r o s s s e c t i o n s a l o n g the 10,.5 k i l o m e t r e t i d a l c h a n n e l , s h o w e d an o v e r a l l d e c r e a s e in c a o a c i t y e s p e c i a l l y b e l o w I n d i a n S p r i n g s Low U a t e r . This' i n d i c a t e s a g e n e r a l i n f i l l i n g w i t h b o t t o m s e d i m e n t s . A n a l y s i s of v o l u m e s p e r s e g m e n t of c h a n n e l s h o w s an a l t e r n a t i n g p a t t e r n of a c c r e t i o n and e r o s i o n , with the g e n e r a l t r e n d for 1 9 7 6 - 1 9 8 4 b e i n g a r e v e r s a l of the 1 9 6 8 - 1 9 7 6 s e q u e n c e .

78.

A r e a s of n e t e r o s i o n in 1984 are r e l a t e d to tuo p r o c e s s e s . F i r s t l y , to c h a n n e l r e s t r i c t i o n s w h i c h i n c r e a s e v e l o c i t y , t h u s s h e a r s t r e s s and the c a p a c i t y of the r i v e r to e r o d e ; s e c o n d l y , .to the g e n e r a l r e v e r s a l in the p a t t e r n of e r o s i o n / d e p o s i t i o n i t s e l f . As the a r e a s of e r o s i o n c a u s e d - b y d r e d g i n g p r i o r to 1976 i n f i l l , they trap s e d i m e n t b e i n g m o v e d f r o m u p s t r e a m . T h i s l e a v e s the u a t e r m o v i n g d o u n s t r e a m of t h e s e s i t e s r e l a t i v e l y s e d i m e n t d e f i c i e n t and t h u s c a p a b l e of m o r e u o r k . The r e s u l t is b e d s c o u r i m m e d i a t e l y d o u n s t r e a m of the a r e a s b e i n g i n f i l l e d .

O v e r a l l a c c r e t i o n , as e v i d e n c e d by v o l u m e d e c r e a s e s , s i n c e 1976 is a b o u t 2 7 0 , 0 0 0 c u b i c m e t r e s , l/olume b e l o u IndiarrS p r i n g s Lou U a t e r is r e d u c e d by a p p r o x i m a t e l y 17% and a b o v e I n d i a n S p r i n g s L o u U a t e r by a p p r o x i m a t e l y T h e n a t u r e of b o t t o m s e d i m e n t s u a s c o m p a r e d to a 1976 sampling (N.S.U. S.P.C.C.). Results shoued a marked coarsening of s e d i m e n t s f o r all l o c a t i o n s . P e r c e n t a g e of sand in the 1976 s a m p l e s r a n g e d from 5 7 to 33;- in a d o u n s t r e a m d i r e c t i o n , u h e r e a s in 1934 the c o r r e s p o n d i n g f i o u r e s are 93 to 7 7 % .

L a n d u s e c h a n g e s u i t h i n tne c a t c h m e n t d u r i n g the same o e r i o d u e r e c o m o a r e d u i t h the use of a e r i a l p h o t o g r a p h s .

N i n e t y p e r c e n t of the t o t a l U o r o n o r a c a t c h m e n t o f " l 6 0 s q u a r e k i l o m e t r e s is u n d e r n a t u r a l v e g e t a t i o n . U r b a n i s a t i o n is r e s t r i c t e d to the n o r t h and m o s t of t h i s is c o n t a i n e d u i t h i n • the t i d a l s e g m e n t . T h e r e h a s b e e n an o v e r a l l i n c r e a s e in r e s i d e n t i a l d e v e l o p m e n t s i n c e 1976 of 51^5.

A ground survey shoued c o n s i d e r a b l e a m o u n t s of c o a r s e sand m o v i n g a o u n s l o p e from c l e a r e d a r e a s . T h i s s e d i m e n t c h o k e s up s m a l l u a t e r u a y s and is e v e n t u a l l y d e p o s i t e d in t r i b u t a r i es and the m a i n t i d a l c h a n n e l . In c o n t r a s t , u a t e r c o u r s e s in the u n d i s t u r b e d p a r t of the c a t c h m e n t s h o u l i t t l e sign of s e d i m e n t a t i o n , i n d i c a t i n g m i n i m a l e r o s i o n u n d e r natural conditions.

T o g e t h e r u i t h in — c h a n n e l r e u o r k i n o of s e d i m e n t s and some u p s t r e a m bank e r o s i o n , t h i s e x p l a i n s both the i n c r e a s e d g r a i n size and a m o u n t of b o t t o m s e d i m e n t s i n c e 1 9 7 6 .

Implications On a s h o r t term b a s i s , the i n c r e a s e d f l u s h i n o p o t e n t i a l a f f o r d e d by the l a r g e r d e c r e a s e in c a p a c i t y b e l o u ^ I S L U c o m p a r e d to t h a t b e t u e e n Lou U a t e r S p r i n g s and H i g h U a t e r S p r i n g s , m e a n s t h a t the r i v e r is b a s i c a l l y ' c l e a n e r ' . Any p o l l u t a n t s fed i n t o the s y s t e m are m o r e q u i c k l y f l u s h e d by the t i d a l p r o c e s s . S i m i l a r l y , the g e n e r a l c o a r s e n i n o of b o t t o m s e d i m e n t s m a k e s for a c l e a r e r u a t e r c o l u m n . As u e l l as i n c r e a s i n g the a e s t h e t i c v a l u e of the r i v e r , t h i s p r o m o t e s b o t t o m ^ l i v i n g o r g a n i s m s and sea g r a s s e s , u h i c h i n c r e a s e the e c o l o g i c a l v a l u e of the e s t u a r y .

79.

S u b a q u e o u s a c c r e t i o n r a t e s for the U o r o n o r a e s t u a r y o v e r the l a s t e i g h t y e a r s are c a l c u l a t e d at a r o u n d 30mm per y e a r .

Roy ( 1 9 8 4 ) h a s d e s c r i b e d r a t e s of a c c r e t i o n for f o u r N . S . U . e s t u a r i e s s i n c e the s t i l l s t a n d as r a n g i n g from 1 to 3mm p e r y e a r . On a m o r e long term b a s i s , t h i s w o u l d " i n d i c a t e t h a t r e s i d e n t i a l d e v e l o p m e n t in a g e o l o g i c a l l y and p e d a l o g i c a l l y f r a g i l e c a t c h m e n t , can i n c r e a s e n a t u r a l s e d i m e n t a t i o n r a t e s by u p t o one o r d e r of m a g n i t u d e . .

Hoy m u c h t h i s rate is d e p e n d e n t on the s e d i m e n t t r a p p i n g f u n c t i o n of the p a s t d r e d g e s i t e s c a n n o t be a s c e r t a i n e d , b u t e a r l i e r s t u d i e s of s e d i m e n t in B o t a n y 3ay ( n u n r o et al 1 9 6 7 , H a r u n g 1 9 7 6 ) i n d i c a t e t h a t t h e B o t a n y Bay e s t u a r y ' s m u d b a s i n e n v i r o n m e n t b e g i n s i m m e d i a t e l y doun s t r e a m of the U o r o n o r a R i v e r . T h i s w o u l d i m p l y t h a t the c h a n n e l of the t i d a l U o r o n o r a r e p r e s e n t s the l a n d w a r d end of the e s t u a r y , n a t u r a l l y c h a r a c t e r i s e d by f l u v i a l d e l t a i c d e p o s i t i o n . The p r e s e n t s t u d y i n d i c a t e s t h a t such d e p o s i t i o n h a s been a c c e l e r a t e d in d i r e c t r e s p o n s e to h u m a n a c t i v i t y w i t h i n the c a t c h m e n t .

References B i r d , N . and A s s o c i a t e s ( 1 9 8 4 ) U e s t Plenai R e l e a s e A r e a : E n v i r o n m e n t a l S t u d y , U o l . I i I I . R e p o r t and A p p e n d i c e s p r e p a r e d for S u t h e r l a n d S h i r e C o u n c i l .

H a r u n g , F.f'l. ( 1 9 7 6 ) O r i g i n and D i s t r i b u t i o n of S e d i m e n t s in P a r t of the G e o r g e s R i v e r , H . S c ( Q u a l ) T h e s i s . U n i v . of ^!•SU. N . S . U . State Pollution Control Commission (1976) Environm e n t a l C o n t r o l S t u d y of B o t a n y S a y , T e c h n i c a l R e p o r t BBS 2 .

R o y , P . S . ( 1 9 8 4 ) " H o l o c e n e s e d i m e n t a t i o n h i s t o r i e s of e s t u a r i e s in S o u t h E a s t e r n A u s t r a l i a " in H o d g k i n , E . P . ( e d ) E s t u a r i n e E n v i r o n m e n t s of the S o u t h e r n H e m i s p h e r e D . C . E . _ — B u l l e t i n N o . 161 .

S t a n d a r d , j . C . ( 1 9 6 9 ) " H a w k e s b u r y S a n d s t o n e " in P a c k h a m , G . H . (ed; The G e o l o g y of N . S . U . J . G e o l . S o c . A u s t . U . 1 6 , Pt 1 .

U a r n e r , R . F . and P i c k u p , G . ( 1 9 7 8 ) C h a n n e l C h a n g e s in the G e o r g e s R i v e r B e t w e e n 1959 and 1 9 7 3 / 7 6 and T h e i r I m p l i c a t i o n s R e p o r t p u b l i s h e d by B a n k s t o w n M u n i c i p a l C o u n c i l .

80.

HOLOCENE SEDIMENTATION IN THE BRISBANE RIVER COASTAL PLAIN

JENNIFER L.F. HACKER^^^

and

W.T. WARD ^^^

(1) Department of Civil Engineering, University of Queensland (2) Division of Soils, C.S.I.R.O., St Lucia, Queensland.

INTRODUCTION

Earthworks associated with the redevelopment of Brisbane International Airport have exposed a recent sequence of sedimentation.

The earthworks consisted of the excavation of a tidal floodway, two metres below minimxim tide level, 8200 m long, running in a NNE direction across the northern part of the Brisbane River coastal flood plain into Morston Bay (Figure 1). It was not possible to examine the sediments situ in the lower part of the floodway, below 5000 m, which was excavated by dredge and never de-watered.

THE SEQUENCE OF SEDIMENTATION The sections exposed in the upper portion of the floodway showed a xvidespread distribution of marine sediments, frequently with shells.

In the furthest inland portion, above 2700 m, the marine deposits are muddier and occur as a 3 to 4 m thick cover over earlier sediments which include alluvium and weakly consolidated Pleistocene deposits.

Sandy beach deposits were found in the earlier, deeper sediments at 2700 m. Between 2700 and 3200 m the marine sediments rest uncomfortably over earlier sediments including Pleistocene sands and gravels and tertiary sandstones of the Petrie formation. Around 5000 and 7000 m the marine sediments rest on weathered basalt considered to be of Tertiary age. Between 3200 m and 5000 m the marine sediments extend down through the full depth of the floodway.

From this sediment distribution three main stages may be recognised in the development of coastal plains in this area: a) an early coastal land-surface of low relief with alluvial channels and creeks; b) a period of marine inundation in which the waters of Moreton Bay extended over the study area; c) a period of consolidation as the waters of Moreton Bay retreated to their approximate present position.

8L

THE EARLY LANDSCAPE The facies distribution in the deeper sediments of the upper floodway indicates that coastal and near-shore conditions existed at the time of their formation. The local environment probably resembled the present day bay shore, but was approximately six kilometres further inland.

EXPANDED MQRETON BAY Sediments exposed in the floodway section show that the rising sea flooded the earlier landscape and reached a slightly higher level than at present. The evidence for this transgression is the 3 to 4 m cover of marine muddy sands which overlie the earlier sediments.

CONSOLIDATION OF THE LAND SURFACE AND FORMATION OF A REFLECTED LANDSCAPE After the sea-water retreated from the area of the floodway channel the sediments which it left behind became more consolidated.

Acid sulphate soils or 'cat clays' (mostly sulfaquepts and sulfic tropaquepts) developed on the marine sediments. The soft wet muds which infilled the streams and backswamps of the early landscape consolidated more than surrounding sands and gravels. As a consequence, the surface of the overlying marine sediments has assumed the topography of the underlying alluvium in such a way that the modern drainage lines follow old channels 3 to 4 m below the present ground surface. This effectively disguises the marine origin of the ground surface sediments, so much so, that in most places the marine origin was not recognised in surveys made before the present floodway was cut. The term 'reflected landscape' is adopted to describe this situation.

As the shore line adjusted to sea levels very similar to those of today, the present near-shore beach ridges developed. Their development may have been influenced by minor fluctuations in sea level. Further, the Brisbane River has intermittendly flooded the area as happened in 1893.

RADIOCARBON DATING Evidence from radiocarbon dating suggests that by 7230 BP the Holocene transgression was influencing sedimentation near the floodway. After 6800 BP Moreton Bay extended over most of the area and this condition remained until at least 4150 BP. The levels of the samples and their position within the sedimentary sequence suggest that the sea was approximately 1.5 m above its present level between 5280 and 4150 BP. The samples dated 1300 and 390 BP came from more recent beach formations close to the present shoreline.

82,

CONCLUSION The interpretation of the reflected landscape is apparently new evidence for the occurrence of a higher than present sea level during the mid Holocene in coastal plains. Similar conditions may be found'' to exist in other locations.

FIGURE 1. Brisbane River Coastal Plain

ACKNOWLEDGEMENT The assistance of the Commonwealth Department of Housing and Construction is gratefully acknowledged in allowing access to the site and for providing funds for the radiocarbon dating.

83.

The Origin of Eighteen M i l e Swamp; A M o d e r n Back-Barrier Peat E n v i r o n m e n t , North Stradbroke I s l a n d , S . E . Queensland

Forming

by Brenton P . Grayt Peter G. Floo^ Edgar Frankel

1

Department of Geology, University of New England, Armidale, N . S . W . 2351 Department of Geology and M i n e r a l o g y , University of Queensland, S t . Lucia 4067 ABSTRACT The origin of the barrier beach/dune sy^^em fronting North Stradbroke Island was unresolved until we obtained C dates on samples of freshwater peat collected by vibrocoring within Eighteen Mile Swamp.

In one core there was in excess of 3 m of fibrous peat resting on 1 m of muddy peat which in turn rested upon m u d d y , bioturbated sand.

Ages of 510 + 140 y r . B . P . and 610 + 310 y r . B . P . were obtained for the base of the peat and the muddy peat respectively. This is taken to indicate that about 600 years ago, a northward migrating spit joined the high dunes of the coastline near Point L o o k o u t .

Conditions behind the spit rapidly changed from marine to freshwater, and peat has been accumulating since that date. No sea level change is envisaged as the level of the junction of the muddy peat/bioturbated sand corresponds to the present level of the beach/subtidal open o c e a n .

A recent example of this type of event occurred in 1974 in North Queensland at Alva B e a c h , north of the Burdekin River where rapid longshore accretion of an offshore bar has finally joined the coastline producing a back-barrier swamp similar to Eighteen Mile Swamp.

Detailed analyses by one of us (B.P.G.) has shown that the freshwater peat of Eighteen Mile Swamp is similar in its composition, geochemistry e t c . to that described from the back-barrier peat-forming environments -of the eastern U . S . A . (An excellent opportunity exists for the study of modern sub-tropical back-barrier peat forming processes within Eighteen Mile Swamp).

84,

MUD BRAIDS IN AN ARID-ZONE RIVER; COOPER CREEK, SOUTHWEST QUEENSLAND RUST, B . R . , (Geology Department, University of Ottawa, O t t a w a , Canada)" N A N S O N , G . C . , (Geography Department, University of W o l l o n g o n g , Australia) TAYLOR, G . , (School of Applied Science, Canberra College of Advanced Education, Belconnen, Australia)

The surface sediments of Cooper Creek in Southwest Queensland are dominantly m u d , which overlies sand at depths of 2.5 to 6.5m.

Channel types are highly v a r i e d , but two basic patterns are recognisable: a low-sinuosity braided system, and an anastomosing system with sinuosity varying from almost straight to tortuous.

The anastomosing channels have low width:depth ratios (generally less than 10) and locally develop levees and crevasse-splays. Sand is restricted to the deeper channels, while sandy mud is deposited on levees, splays and floodplain. The channels show no evidence of lateral migration in the form of scroll b a r s , but relocate on the floodplain by avulsion. Initially highly sinuous, channels tend to decrease sinuosity by meander cutoff. Straight channels result from lateral constriction by bedrock or eolian dune remnants. The deepest channel segments intersect the underlying sand b o d y , and form permanent waterholes with mud seals.

The braided channel system occupies higher levels of the floodp l a i n , and was formerly interpreted as a relict of a braided pattern on the underlying sand b o d y . This interpretation is untenable because of the thickness of the mud cover, the divergence (up to 90^) between.paleocurrents in the sand and surface channels and evidence (lateral accretion structures) for formation of the sand body in a meandering-fluvial regime. The braided pattern is therefore regarded as contemporaneous with the anastomosing system, the former being active only during major floods, whereas the latter is active during moderate flow.

The existence of braids in mud is explained by the occurrence of the mud as sand-sized kaolinitic aggregates. Probably formed pedogenically in wet/dry cycles, the aggregates are highly resistant to disintegration. This was demonstrated by flume runs in which they were sufficiently stable to be formed into ripples, dunes, plane beds and antidunes. The surface mud is highly porous and easily entrained and worked into braids by flood flow. In contrast, deeper mud (below about 50cm) is compacted to a highly cohesive state which resists entrainment. As a result, d e e p , narrow channels of the anastomosing system are formed.

85,

Thermal luminescent dates suggest that the sand/mud boundary corresponds to a change in fluvial regime that occurred between 50,000 and 200,000 years ago. Interpretation of this change must await further dating, but the climate of the maximum Pleistocene glaciation and neotectonics are the obvious candidates.

86,

PALAEOECOLOGY, PALAEOLIMNOLOGY, PALYNOLOGY AND COASTAL ENVIRONMENT AREAS OF NEGLECT IN COASTAL STUDIES? A.R.H, MARTIN Botany Department, Sydney University The biota preserved in freshwater swamp, bog and lagoon sediments in coastal situations often reflect only short temporal sequences, interpretable through coastal dynamic processes, not or only secondarily in climatic terms. Such sequences yield insights into the nature of the geomorphic and hydrologic changes that have brought about their existence and preservation. Systematic differences in biotic composition at two sites may reflect standing differences in geochemistry between them.

Studies in this field are desirable in coastal research but seem to have been rather neglected in this country.

Some examples, published and unpublished, from work of the author and associates, are outlined in illustration of the palaeoecological approach.

1) Groenvlei, South Africa (Martin,1959,1968). This lake lies between lines of coastal dunes on the Knysna coast. The dunes, of Pleistocene age to the north and at least partly Holocene to the south, differ in weathering, the older ones being cemented, with spines of calcrete below highly leached acid soils, while the young mantling sands to the south are highly calcareous to the surface. The lake is fed entirely by runoff and ground flow, thus receiving varying contributions of acidic and calcareous water in its history. Stratigraphy of the adjacent fen shows underlying acidic lake mud with a rich acidophile diatom flora. This was succeeded by fern peat, then an incursion of estuarine mud due to rising Holocene sea level. A flora of marine diatoms shows warmer, apparently more tropical water followed by a more temperate phase. A freshwater lake was later re-established but this was floristically quite different from its predecessor, being fed by predominantly calcareous drainage. Char a,, Naias and a diatom flora of highly eutrophic forms now occur. Calcareous marl and sedge fen with Cladium were the final stages. The changed water chemistry can be attributed to the origin of the young calcareous dune sheet south of the lake.

2) Bombah Bog, Broadwater, N.S.W., (Martin, unpub.). Superficially resembling the preceding, this site shows very different history in detail (Fig. 1, Fig. 2). A shallow valley, graded to glacial sea level, is filled by estuarine clay of the Holocene rise to present level.

Peat, occurring intermittently along the floor of the valley, contains charcoal and pollen representing a well-wooded environment. A date of 19,660 + 240 years B.P.(SUA-1212) near the base of the peat profile indicates a late upper Pleistocene age. The marine incursion is undated except by two dates at the transitional stage to freshwater along its upper surface (SUA-1208:3810 ± 150; SUA-1210:4120 ± 110 years B.P.).

An impoverished diatom flora with Grammatophora oceanica is consistent with estuarine conditions. With stabilisation of sealevel, a levee appears to have been quickly thrown across the mouth of the embayment and a shallow lake established. With no source of calcareous water in the environment, the lake was acidic, fed mainly by superficial flow from the highly leached "Inner Barriersands to its north. The lake was rapidly colonised by the Water Shield, Brasenia schreberi, a plant

87, of acidic waters. The short-lived lake (dated by SUA-1209:3500 ± 90 B.P. and SUA.1207:2860 ± 90 B.P., perhaps suggesting slight diachronicity) was succeeded by reedswamp and mixed shrub swamp communities.

3) North Deewhy (Nartin, 1971). This site illustrates the replacement of a shallow lagoon stage, with algae (Desmids, Chara) by a paludic phase with swamp plants and heath, then a wetter phase of increasing Cyperaceae, Trig^opliip and other semi-aquatics. The increased wetness is attributed to drainage impedance by an approaching dune which at length terminated the existence of the swamp by over-riding it. Dated to a basal age of ca 3,900 years B.P., the site seems to belong to a group owing their existence to the mid-Holocene stillstand phase (Thom, Hails and Martin, 1969).

4) Kurnell Peninsula (Martin and Botany 3rd Year students, unpub.).

Belonging to the same group as North Deewhy, this site shows a similar sequence. At the base a juvenile lake seems to have existed, almost devoid of macrohydrophytes but an algal flora including Zygnemales, Oedogoniales and other freshwater Chlorophyceae is recorded. A stable heath environment seems to be recorded by the pollen (Fig. 3). Casuarina illustrates fringing woodland as at Deewhy and also Bombah Bog in its estuarine phase. This phase was followed by a dune-building phase (Acagia, Poaceae). Tree cover was sparse. As the dune-building phase passed its peak, the water table again rose. The sequence here suggests a floating stage in an acid bog or swamp with Sphagnum and acidophile diatoms and ferns (probably mainly Gleichenia). The final stage included a stabilisation with spread of Eucalvptus,. re-entry of Bapk^ia and a second peak of Casuarina probably from the still-extant fringing woodlands of the present mangrove swamps of Quibray Bay. The present flora of the site is very impoverished, due to the clearance of adjacent swamp for the nearby Kurnell Oil Refinery.

5) Fingal Bay (Macphail, 1973). This site too, formed behind a seaward sand barrier, subsequently eroded, leaving the site on the foreshore and partly below sea level, the origin being dated to before the last stillstand (Thom, Hails and Martin, 1969). Periodic phases of vegetational instability are indicated, when Eucalvptus and Angophora declined and were replaced by Paperbark and Ti-tree swamp.

It is clear that much valuable history is embedded in coastal swamps and that changes in the aquatic and swamp vegetation on the one hand and the vegetation of the surrounding land surfaces are interrelated, the evidence being complementary to other types of evidence of coastal evolution and often illuminating the purely geomorphic evidence.

REFERENCES Macphail, M.K., 1973. Pollen analysis of a buried organic deposit on the backshore at Fingal Bay, Port Stephens, New South Wales.

Proc. Linn. Soc. S i (4): 222-233.

Martin, A.R.H., 1959. The stratigraphy and history of Groenvlei, a South African coastal fen. Aust. J. Bot., 1 (2): 142-147.

Martin, A.R.H., 1968. Pollen analysis of Groenvlei sediments (Knysna), South Africa. Rev. Palaeobot. Palynol. 1 (2): 107-164.

88.

Martin, A.R.H., 1971. The depositional environment of the organic sediments on the foreshore at North Deewhy, New South Wales.

Proc. Linn. Soc. N.S.W. Si (4): 278-287.

Martin, A.R.H. (unpub. data).

History of Bombah Bog, Broadwater, N.S.W.

Martin, A.R.H. and Botany 3rd Year students (unpub. data) analysis of a freshwater swamp at Kurnell.

Pollen

Thorn, B.G., Hails, J.R. and Martin, A.R.H. 1969. Radiocarbon evidence against higher postglacial sea levels in eastern Australia.

Marine Geol. 1: 161-168.

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A

V >MI

liiJ ( (lUKMt felMuiMl#u• KM I.M *

-12-

BSa »I«im-» _ (Ml/IDIIM, [llll aww^sui #

uul fcsrj'ts

tJ

L]

-H-

Figure 1 Longitudinal section of Bombah Bog, Myall Lakes, showing : i. drowned valley graded to pre-llolocene sea level ii. estuarine phase iii. lake phase characterised by Brasenia seed layer.

00

VO

B

g :

p a U a J

Til) in r f ^ M ° tf distribution of tongue of estuarine clay van™ ? V ° U ' e P r «- | t o l«»®ne P ^ t and peaty sands in the limits n? tnh "e 1 c l a ' " " * * * * * * * l a k ° ( l l J ) closely to the ' J ? fh y ' P<»aibly reflecting the contour of the basin, rather than an ecological preference for a clay substrate.

Figure 3 . Preliminary pollen diagram of the swamp sediments of the freshwater site landward of Quibray Bay, near the Kurnell Oil Refinery. The regional spectra Indicate some dune movement in the period '1000-0 B.P. The local spectra indicate changes in the swamp - an early lake phase with little autochthonous vegetation; a later shrub swamp; a SpMgjioin-Fern stage,with acidophile diatoms; a post-Sphagnum phase. The modern phase (surface) is not seen.

92,

HOLOCENE AND PLEISTOCENE SAND OF THE FENS EMBAYMENT, MID NORTH COAST OF NEW SOUTH WALES ANNEMARIE CLEMENTS (The Macleay Building A12 School of Biolgical Sciences The University of Sydney)

ABSTRACT The vegetation of the Quaternary sands forms a mosaic of vegetation groups. Myerscough and Carolin (1985) in the Eurunderee Embayment found that vegetation types were indicators of geomorphological units and subunits of the area.

Geomorphologically, the Quaternary sediments of the east coast of Australia are considered to be laid down in at least two depositional phases in the form of sand barriers enclosing swamps and shallow lagoons and considered to be related to periods of high sea level.

The outer barrier, the most seaward is C ^ dated as of Holocene age (Thorn et al 1981 A) and formed from reworked sands of an earlier depositional period (Thom et al 198IB) under a rapid progradation at the end of the Holocene transgression which was succeeded by a period with a relatively stable coastline (Shepherd 1974).

If the inner barrier was formed under similar conditions to the outer barrier, then from the oxygen isotope records (Chappell 1983) it would have been formed 130-120 K years B.P. during the last Interglacial. Sea level oscillated, reaching an absolute minimum about 20-15 K years B. P. (Chappell 1983) with accompanying changes in climate (Beard 1982, Bowler 1976, Thiede 1979).

The Fens Embayment has been the focus of several studies (Thom 1965, Shepherd 1970 & 1974, Thom et al 1978, Bowman 1979). LongfordSmith and Thom (1969) regard the prograded barrier in this embayment as the site type for outer barrier formation in N.S.W. Thom et al (1981 A) describe the outer barrier beach-ridge plain as about 1 km wide and containing about 20 ridges, beir^ flanked on the seaward side by a high, partially mobile foredune and on the landward side, the innermost beach ridge abuts a low-lying swampy back barrier sand flat.

93.

The analysis of the distribution of plant species on the outer barrier beach-ridge shows four major plant groups. The most seaward being dominated by Bank^ia j^ntegrifoXia and Leotospermum laevigatum with Bapk?j.^ serrata behind partially vegetated incipient foredunes; the second being dominated by B. aemula and Melaleuca nodosa^ forming a heath about 2-3 m in height; the third dominated by Eucalyptus pilylarj.? and Angophora costata forest with B. serrata in the understorey and the fourth B. aemula and M. nodosa in wetter locations.

The first group is the vegetation of stabilised foredunes which have blown over the parallel beach ridges. Parallel beach ridges are clearly visible in the areas supporting the second vegetation group.

Forests of A., QOi^tata and E, pilularis occur on sands which are topographically higher than the areas supporting heath.

The fourth group, which include wet heath species, are located in hollows and areas adjoining the swamp forests of Melaleuca auinouenervia and Eucalvptus robusta of the former Interbarrier lagoon.

CONCLUSIONS The outer barrier beach-ridge plain of the Fens Embayment is not composed entirely of homogenous vegetation.

Roy and Crawford (1978) noted in the southern Botany Bay Kurnell region transgressive barrier dunes of 4-3.5 K years B.P., 21.5 K years B.P. and about .5 K years B.P. The occurrance of the third vegetation group and the first may be explained in terms of cooccurrant sand movements due to climatic effect similar to those experienced at Kurnell.

ACKNOWLEDGEMENT Special thanks to Assoc. Prof.

R. C. Carolin for help and encouragement; Drs. Tony Martin and Peter Myerscough for criticism and discussion; Barbara Tink and Belinda Pellow for field assistance, National Parks and Wildlife for permission to carry out investigation in Myall Lakes National Park; and Coastal Council of N.S.W. and The University of Sydney for financial support.

REFERENCES Beard, J.S. (1982). Late Pleistocene aridity and aeolian landforms in Western Australia, in Baker, W.L. and Greenslade, P.J.M. (eds).

Evolution of the Flora and Fauna of Arid Australia.

Peacock Publication, Frewville, S. Australia.

94,

Bowler, J.M. (1976). Recent developments in reconstructing late Quaternary environments in Australia. in Kirk, R.L. and Thorne, A.G. (eds). The Origin of the Australians. AustralianInstitute for Aboriginal Studies, Canberra/Humanities Press, New Jersey.

Bowman, G.M. (1979). Development of Podzol Soils in East Australian Coastal Sand Barriers, unpub. Ph.D. thesis, University of Sydney.

Chappell, J. (1983). years from PNG.

A revised sea level record for the last Search 14(3-4). 99-101.

Langford-Smith, T. and Thom B.G. (1969).

South Wales. J. Geol. Soc. Aust. 16.

300000

Coastal morphology of New 572-580.

Myerscough, P.J. and Carolin, R.C. (1985).

The vegetation of Eurunderee Sand mass, headlands and previous islands in the Myall Lakes area. New South Wales. Cunninghamia, in press.

Roy, P.S. and Crawford, E.A. (1979). Holocene Geological Evolution the Southern Botany Bay - Kurnell Region, Central N.S.W. Coast.

N.S.W. Geol. Surv. Res. 20 (2). 159-250.

Shepherd, M.J. (1970). Coastal Geomorphology of the Myall Lakes Areas. New South Wales, unpub. Ph.D. Thesis. Univ. of Sydney.

Shepherd, M.J. (1974). Progradation of a Holocene N.S.W, Search 5(5). 210-211.

Thiede, J. (1979). Wind regimes over the late Pacific Ocean. Geology 7. 259-262.

Thom, B.G. (1965).

Late Quaternary Stephens-Myall Lakes area, N.S.W.

Sand

Quaternary

Barrier

in

southwest

coastal morphology of Port J.Proc.R.Soc. NSW 98. 23-26.

Thom, B.G., Polach, H.A. & Bowman, G.M. (1978). Holocene age structure of coastal sand barriers in N.S.W., Australia. Geography Dept., Faculty of Military Studies, Duntroon.

Thom, B.G., Bowman, G.M., Gillespie, R., Temple, R. and Barbetti, M. (1981 A).

Radiocarbon dating of Holocene beach-ridge sequences in South-East Australia. Monograph No. 11, Dept. of Geography, U. N.S.W., Duntroon.

Thom, B.G., Bowman, G.M. and Roy, P.S. (1981B). Late Quaternary evolution of coastal sand barrier, Port Stephens-Myall Lakes area, central N.S.W., Australia. Quaternary Research 15.

345364.

95.

B A R R I E R ISLAND M O R P H O L O G Y D U R I N G R I S I N G SEA L E V E L

MARIE FERLAND (Department of G e o g r a p h y , U n i v e r s i t y of S y d n e y )

Abstract not available

96.

LATE QUATERNARY DEPPSITIONAL EVOLUTION OF BOTANY BAY JM HANN (Geological Survey of NSW, Department of Mineral Resources)

Bay and adjacent onshore Quaternary deposits in the Botany Basin of New South Wales represent the largest accumulation (89 sq km) of 'marine' sands on the New South Wales coast south of Newcastle.

Stratigraphic studies of these deposits have been carried out onshore

by Griffin (1963) and by Roy and Crawford (1981) in the Kurnell area.

No detailed studies have been made of the Botany Bay beach ridges.

Interpretation of samples from deep bore holes in the northern area of Botany Bay by Albani (1981) indicated a complex Quaternary history of estuarine and aeolian sedimentation. A peculiar feature of the stratigraphy revealed by Albani's studies is the thickness and extent of Pleistocene dune sands submerged beneath the bay and the paucity of Holocene sediments compared to many other New South Wales embayments.

This paper examines additional detailed stratigraphic evidence from a number of areas in Botany Bay (bay mouth, central bay, and western bay) and a broad model of depositional evolution is proposed, with particular attention given to Holocene sedimentological events.

The primary source of data is results from a drilling programme carried out in 1983 at the request of the Department of Public Works.

All work was funded by the Maritime Services Board, except for radiocarbon dating which was funded by the Department of Mineral Resources.

Botany Bay forms a large (6 km x 8 km) semi-enclosed bay (Roy 1984a), dominated by open ocean marine conditions. The bay has formed since the opening of the southeast Australian Continental Margin 40 to 60 m.y. ago by fluvial excavation in a minor structural depression of the Permo-Triassic Sydney Basin.

Sediments infilling bedrock valleys within Botany Bay are up to 90 m thick in the bay mouth (Johnson et al. 1977) and are commonly at least 10 m thick overlying bedrock highs. Four broad sediment units have been recognised that are believed to correspond to major depositional events in Botany Bay (figure 1). A model of deposition is proposed to account for the various lithologies present (figures 2 and 3).

STAGE 1 The earliest record of Quaternary sedimentation in Botany Bay indicates marine flooding of a deeply incised bedrock basin and deposition of estuarine sands and muds/clays (unit 1) in the deeper valleys (Albani 1981). These sediments probably partly overlie basal fluvial sands preserved in discrete channels.

97,

Figure 1 GENERALISED CROSS-SECTION,BOTANY BAY

WEST

, ^

.

REFERENCE ^ . > >.

Basal fluvial sands and gravels.esfuanne clays/mods and sands ( 2 / ^ar'y »o mid Holocene reworked bay sands Interbf^dded Pleistocene dune sands and freshwater/estuarine days ( J ) Late Holocene to Modern reworked bay sands

Figw^e 2

EVOLUTIONARY MODEL LATE QUATERNARY DEPOSITION IN BOTANY BAY STAGE 2 PLEISTOCENE LOW SEA LEVEL

STAGE 1 PLEISTOCENE HIGH SEA LEVEL

REFERENCE Estuarme deposits Dune deoosils (Pte»stocene) Barrier deoosits r ^ l

Biogenc deposits

i S

Headland dunes (Holocene)

S 3

/ STAGE 3 HOLOCENE RFSNG SEA LEVEC AND EARLY STILLSTAND

Beach ridges Barrier shoreline

,

Sediment transDort

•

Submarine rock outcrop

^^

Bay bed rewwor^tng

and Crawfortf.TMl)

STAGE I H I G H SEA LEVEL LATE HOLOCENE TO MODERN

98.

STAGE 2 A period of vigorous aeolian activity ensued resulting in the development of an extensive dunefield that covered much of the Botany Basin, It is proposed-that these dunes formed in response to the northerly transport of sand along a Pleistocene low sea-level barrier coast, becoming trapped by an inflection in the coastal alignment at Kurnell.

The dune sands are interbedded with clays as a result of both periodic marine incursions and freshwater swamp development.

These sediments (unit 2) dominate the subsurface stratigraphy, occuring throughout the bay (including the bay mouth), with a maximum known thickness of 25 m.

STAGE 3 Dunefields infilling the Botany Basin effectively prevented large quantities of transgressive sediments from moving into the partially drowned basin during the Postglacial Marine Transgression (PMT)• Initial Holocene deposition in Botany Bay (unit 3) resulted from reworking by waves of the sandy dune substrate during the late PMT and early stillstand (4 000 - 7 5000 yr BP, based on radiocarbon dates of shell).

During this period sediments were eroded from the central bay bed and southern part of the bay mouth (destroying unit 3 in these areas) and transported onshore (beach ridges) and possibly offshore (innershelf sand lobes, Roy 1984b)(figure 3 ) .

STAGE 4 Late Holocene to modern (<3 000 yr BP) reworking by waves of the bay bed is represented by the surficial sediments (unit 4 ) .

The presence of beer bottle glass fragments in this unit provides conclusive evidence for disturbance by waves at least 1 m'" below the central bay bed.

Offshore movement of sediment may be continuing.

Figure 3

PROPOSED HOLOCENE EVOLUTION OF SOTANY SAY EARLY HOLOCENE (RISING SL)

MID HOLOCENE - MODEfiN (STiLLSTANO) ACflETIOM '

EROSION

Prmmnt Mim^nt aurfac*

•

• • : yV/

UcRETION

o o « ® o iXi' Vy^^

by warn

REFERENCES Albani, A . D . , 1981 Geo - Mar. Letters Vol 1, 163-167.

Griffin, R . J . ,

1963 Geol. Surv. NSW Bull. No. 18.

Johnson, B . D . , Albani, A . D . , Rickwood, P . C . , and Tayton, J . W . , J . Geol. Soc. Aus., Vol 24, Pt 7, 403-408.

1977

Roy, P . S . , and Crawford, E . A . , 1981 Rec. Geol. Surv. NSW, Vol 20, Part 2, 159-250.

Roy, P . S . , 1984a in Coastal Geomorphology in Aust. Academic Press.

Roy, P . S . , 1984b Geol. Surv. NSW Report - GS 1984/158

(unpubl.).

99,

WAVE-INDUCED SAND MOBILITY AND DEPOSITION ON THE SOUTH SYDNEY INNER-CONTINENTAL SHELF PETER COWELL (Coastal Studies Unit, Sydney University)

INTRODUCTION The contribution of waves to both the evolution and present dynamics of the inner-continental shelf sand bodies off the south Sydney coast is assessed using the relationships developed by Nielsen and synthesised in Cowell and Nielsen (1984). These relationships are used to predict the mobility of sand averaged for all significant wave height — zero crossing period (H—T) combinations recorded synoptically by the NSW-MSB throughout the last 9 years. Occurrence frequencies for each H-T combination are used to obtain time-averaged concentrations of mobile sand. No spectral adjustment was made to near-bottom velocities (computed from linear theory) because of uncertainties about joint H-T distributions for the various conditions of swell and wind-driven sea to which the region is subjected. Sand mobility estimates are therefore conservative.

MOBILE-SAND CONCENTRATIONS The importance of waves in causing sand mobility is evident from the ubiquitous presence of wave-generated vortex ripples seen in sidescan sonar records and during diving operations. Predicted timeaveraged concentrations of mobile sand are given in Table 1. These concentrations are compared in Table 2 with deposition rates in terms of sediment fluxes and net flow rates required to produce the accumulations. The deposition rates were obtained from morphometric analysis of estuarine flood-tide deltas and stratigraphic evidence from the inner shelf. The predicted concentrations and measured accumulation rates imply advection of sediments by time-average (net) flows in the order of 10"^ m s"^^ or less.

Both tables indicate that sand mobility appears to be of significance over Holocene and historical time scales rather than at day to day, synoptic time scales. Even under the most extreme storms, the quantity of moving sand only corresponds to several millimetres of sand at rest on the bed.

100,

Table 1 Sediment concentrations (m^.m"'^ x lO"^) averaged over all time for the fine lobe sand, the coarse channel deposits and for fine sand entering the channels (see Fig. 1). ^The bracketed numbers are equivalent thickness of sand (m x lO""^) at rest on the bed assuming 30% porosity.

DEPTH (m)

FINE LOBE SAND (d = .25 mn) ON LOBES IN CHANNEL

COARSE CHANNEL SAND (d = 1.0 mm)

30

669.4

(937.2)

5181,3

(7523,8)

71.2

(99.7)

40

145.7

(204.0)

1144.0

(1601,6)

15.6

(21.8)

50

25.1

(35.1)

297.8

(416.9)

4.1

(5.7)

60

8.9

(12.5)

102.0

(142.8)

1,4

(2.0)

Table 2 Advection (or net flow) rates required to produce the measured deposition rates in relation to mobile-sand concentration averages in Table 1. The Holocene part of the inner-shelf sand body exists as a lense which characteristically thickens with water depth at a rate of 0.2 m.m attaining about 10 metres thickness in 60 metres water depth (Field and Roy, 1984).

Holocene Sand Thickness (m)

Water Depth (m)

Deposition Rate (m s~^)

Advection Rate (m s~

10"^^

1 . 8 1 X 10~®

Inner-

4

30

1.69 X

Shelf

6

40

2.54

1.22 X

lO""*

8

50

3.82 X

lO"^-

9.43 X

lO"*"

Body

10

60

4.23 X

10"^^

3.39 X 10"^

FloodTide Deltas

Botany Bay

30

-4.99 X

10"^^

5 . 3 2 X 10

Port Jackson

30

2.14 X

10"^°

2.28 X

Sand

'

.

X

-5

lO"""

MORPHOLOGICAL RELATIONSHIPS The large scale morphology of the upper 10 metres of the sand body shown schematically in Figure 1 indicates the existence of differential sand-transport patterns. Comparison of predicted sand mobility on the lobes and in the channels (Table 1) confirms that the wide-spread fine sand is by-passed rapidly due to winnowing whenever

101.

it enters the channels which contain coarse sand and transverse the sand body between successive lobes alongshore. This by-passing may provide an explanation for the development of the lobes during the Holocene evolution of the sand body.

Figure 1 Schematic representation of large-scale inner-shelf morphology off the south Sydney coast (based upon Gordon & Hoffman, 1985).

N

Deep Estuarine Embayment

KEY C o a r s e Sand Channel

Sand Lobe

Depth Contours in metres

Channel 10m-

-Slip-face-^^ —

B REFERENCES Cowell, P.J. and P . Nielsen, 1984, Coastal Studies Tech. R e p . N o . 84/2 Field, M.E. and P . S . Roy, 1984, J . Sed. Pet. Gordon, A . D . and J.G. Hoffman, 1985, Proc. 19th Int. Conf. Coastal Eng., (in press).

102.

SOUTHEAST AUSTRALIA SHOREFACE FACIES A.J). SHORT (Coastal Studies Unit, Department of Geography, University of Sydney, N.S.W. 2006)

INTRODUCTION The coastline of southeast Australia (Frazer Island to Cape Jervis) is dominated by sandy beaches and associated shorefaces. The beaches range from low energy reflective though moderate energy intermediate to dissipative on the east coast, to high energy dissipative on the south coast. Associated with the intra and inter regional variation in breaker energy and beach type are variations in shoreface morphology and dimensions (i.e. depth to modal wave base). The aim of this study is to systematically examine the morphology, bedforms, sediments and internal structures of shorefaces representative of the range of wave-beach systems in southeast Australia.

Field Sites and Methods The ten field sites and their environment characteristics are listed in Table 1. The sites are ranked from the lowest energy reflective beaches typified by a steep beach face and barless surfzone, through the rip dominated intermediate beach types, to the high energy dissipative with inner and outer parallel bars. The sites were chosen to represent the range of beach-surf zone types that exist around the southern Australian microtidal coast.

Each field site was surveyed using ground surveying across the beach surf zone, tied to echo sounding seaward. Sediments are collected across the beach-surf zone and at 2 m depth increments seaward.

Measurements and photographs of bedforms and box cores are taken across the surf zone and at selected depths seaward. The box cores were subsequently coated with araldite to make the core peels.

RESULTS; a.

Shoreface Dimensions:

The thickness or depth (beach to nearshore) of each shoreface increases with increasing wave energy from 5 to 10 m in low-energy reflective beaches, to 10 to 30 m in intermediate beaches and 30 m or more in high energy dissipative beaches. Width likewise increases from 100-200 m on reflective beaches to several kilometers in higher energy systems.

Beach face gradients tended to decrease with increasing wave energy and with decreasing grain size (Table 1). Seaward of the surf zone gradients are less predictable being related to wave energy and grain

103,

TABLE I Wave-Sediment Characteristics of beach sites

Location

Beach

n

Hb(m)

T

Gd (inm)

Grad.

Ws

Fishermans

R-R

10

0.3

10

.35

.05

.61

Pearl

R-R

17

0.5

10

.5

.076

.65

1.12

Bracken

I-LTT

11

0.8

10

.23

.03

2.6

1.16

Hawks Nest

I-LTT 14

1.0

10

.26

.035

2.85

1.13

Narrabeen

I-TBR

35

1.5

10

.3

.04

3.75

Marouba

I-RBB

7

1.6

10

Grants

I-RBB

21

1.6

10

.3

.04

4.0

1.29

M.Seven

I-LBT 25

1.6

10

.27

.036

4.4

1.2

Teewah

I-LBT

8

1.4

10

.21

.028

5.0

1.37

Goolwa

D-D

12

3

12

.2

.026

9.6

1.33

R - reflective I - intermediate D - dissipative LTT - low tide terrace TBR - transverse bar 5e rip RBB - rhythmic bar & beach LBT - longshore bar & trough

1

1.9

1.15 1.20

n - number of cores Hb - modal breaker height T - period Gd - mean grain size ^ Ws - mean full velocity (cm sec" ) Grad - subaerial beach gradient

size as well as bedrock geometry. Steepest gradients were associated with coarsest sediments rather than higher waves. b.

Facies occurrence and sequence:

The occurrence of individual facies (Table 2) (i.e. sedimentbedform-structure combinations) is dependent on the prerequisite morphodynamic coupling. Consequently, higher energy beaches will have features and structures not found on lower energy beaches and conversely.

The lowest energy Reflective beaches consist solely of a beach face, step and nearshore. Higher energy Reflective beaches have a berm crest, upper and lower beach face, step and deeper nearshore. The berm persists through the intermediate beaches becoming wider and lower in gradient. It is usually absent on dissipative beaches which widen and have more extensive upper and lower beach face deposits. Steps at first prominent become discontinuous longshore on rhythmic beach shorelines (absent on horns) and absent on fine-grained, high-energy intermediate and dissipative beaches. The bar-trough facies is initiated on a small scale in the low-to moderate energy low tide terrace/ridge and runnel beach state; it is prominent in the bar and rip states, with the trough increasing in depth to 3 m below MLW in the bar-trough state. The inner nearshore regions of intermediate and dissipative beaches are

104,

TABLE 2 Shoreface facies sequences

Beach State

Reflective

Intermediate

Dissipative

Wave energy

low (<1 m)

moderate (1-2-hn)

high >3 m

Facies:

beach face

berm upper beach face lower beach face

upper beach face lower beach face

Surf zone

step

step terrace/longshore trough bar/rip channel

inner trough inner bar outer trough outer bar

Nearshore

nearshore

inner nearshore outer nearshore

inner nearshore outer nearshore

Shelf

inner shelf^

inner shelf

inner shelf

Subaerial

* may

merge with adjacent higher energy nearshore systems.

dominated by lunate megaripples grading into cross-ripples. The outer nearshore to modal wave base, and the nearshore of reflective beaches, is dominated by asymmetrical wave ripples, at first very sinuous and discontinuous, but becoming increasingly parallel crested in deeper water. c.

Facies description: Beach face:

Planar surface with slightly seaward dipping parallel beach laminations. Generally coarse sediments with thicker discontinuous laminations on reflective beaches, grading into finer sediments and low angle thin continuous laminations on dissipative beaches. Lower beach face also contains high tide wave ripples and cross stratification.

Trough: Complex bedforms and structures with both wave ripples and shore normal current ripples producing trough cross-stratification. Seaward flowing currents in RIP CHANNELS produce seaward migrating megaripples with steep seaward dipping forset beds containing coarsest sediments.

m

105,

Bar C r e s t ; Planar surface with f i n e r grained p a r a l l e l laminations l a n d w a r d o v e r i n n e r edge of b a r .

dipping

Inner Nearshore; Moderate s c a l e l u n a t e m e g a r i p p l e s l i e i m m e d i a t e l y s e a w a r d of t h e b r e a k p o i n t , w i t h p r e d o m i n a n t l y s t e e p l y l a n d w a r d d i p p i n g beds ( c r e s t s ) over coarser p a r a l l e l t o cross bedding (trough).

The m e g a r i p p l e s g r a d e i n t o c r o s s r i p p l e s between l a n d w a r d d i p p i n g b e d s ( c r e s t s ) a l t e r n a t i n g with c r o s s - s t r a t i f i c a t i o n .

Outer Nearshore: A s y m m e t r i c a l wave r i p p l e s a r e i n i t i a l l y v e r y s i n u o u s and d i s continuoxis w i t h t y p i c a l c r o s s - s t r a t i f i c a t i o n .

With i n c r e a s i n g d e p t h t h e y become more c o n t i n u o u s and p a r a l l e l c r e s t e d .

At d e p t h t h e p r e s e n c e of l a d d e r - b a c k r i p p l e s s u g g e s t s low v e l o c i t y s h o r e p a r a l l e l c u r r e n t s may be p r e s e n t .

S t r u c t u r e s are predominantly c r o s s - s t r a t i f i c a t i o n , with b i o t i i r b a t i o n i n c r e a s i n g w i t h d e p t h . The d e p t h of wave d i s t u r b a n c e d e c r e a s e s s e a w a r d and may b e g i n t o a p p e a r as a d a r k e r r e d u c t i o n zone i n t h e 30 cm c o r e p e e l s i n d e p t h g r e a t e r t h a n 15 m.

Inner Shelf; In g e n e r a l a marked c o a r s e n i n g o c c u r s w i t h p o o r e r s o r t i n g and l a r g e p a r a l l e l wave r i p p l e s .

However t h e c o a r s e s e d i m e n t s and b i o t u r b a t i o n mask most s t r u c t u r e s .

106,

ALONGSHORE VARIATION IN NEARSHQRE SEDIMENTS: NORTH' COAST OF TASMANIA J.L. DAVIES (School of Earth Sciences, Macquarie University, Syciney) .

The Bass Strait coast of Tasmania is about 300 km long and has a tidal range at springs varying from about 1.8 m at each end to about 5.3 m in the middle.

It is sheltered from open ocean swell and the

major sources of wave energy are strong winds blowing from the northwest.

As a result, wave energy is generally low.

Median significant

wave height is about 0.5 m in the east, falling to about 0.3 m in the west.

The coast can be divided into three major geomorphic segments.

West of Rocky Cape, late Pleistocene and mid-Holocene sands form barrier systems conroosed of low sand beach ridges.

East of Point

Sorell the coast is dominated by extensive transgressive dune systems incorporating sands of similar age.

The difference between the two

areas in the form of the sediment bodies is related to a very big difference in onshore sand carrying capacity of the wind.

The central

segment in contrast is one of sediment deficiency, with true pocket beaches bounded seaward by rock and minimal development of dunes.

The three segments have contrasting hinterlands.

In the west

sediments are derived from catchments in quartzose rocks of Precambrian and Cambrian age whereas in the east they are largely derived from Devonian granodiorites.

The central catchments are dominated by

extensive Tertiary basalts. The Tamar system appears to stand apart because, in spite of its very large catchment, all evidence so far suggests that the Tamar trough acts as a great trap which has little sedimentological connection with the coast and the Bass Strait shelf.

Calculation of approximate ratios of tidal to river discharge in estuaries, together with data for magnitude, suggest that the rivers in the central segment are those most likely to be transmitting terrestrial sediment to the coast, now and in the Pleistocene.

107.

The contrasting late Quaternary sediment budgets on the three coastal segments, taken together with their geographical relation to energy and sediment sources would suggest that, whereas sediment might be expected to remain and accumulate within the western segment, there might have been a net export of material from the centre to the east.

A recent study of

nearshore sediments along the whole coast provides an opportunity of testing such a general h}T)othesis.

Swash zone sands were collected and analysed from 165 standard stations thought to be representative of the nearshore zone.

On this

coast the limit of the nearshore zone lies at 3-9 m depth or about 100-600 m distance offshore.

The sediment data indicate that at present

the whole coast is strongly compartmentalised. only within and not between embayments.

Littoral drift occurs

It also suggests that sediment

supply by rivers in the Holocene has been very small.

However the

question remains as to whether there has been movement at lower sea. levels in the late Pleistocene, in particular a high of -40 m which occurred at several times within the last 30 000 years.

Such movement

might have been stimulated by a coastline which was smoother in plan and had been supplied by abnormal amounts of sediment when the central rivers acted as distributors of glacial meltwater.

The sedimentological data confirm that sands in the western segment are very distinctive and highly characteristic of that stretch of coast.

The data from the other two segments are more equivocal.

The pronounced

accumulation of locally produced hornblendes and garnets in the east is in accord'with the expectation that there has been no movement from east to west.

The strong occurrence of rock fragments from basalts in the

centre and of feldspars from granites and dolerites in the east is in accord with the expectation that the recently produced sediment fraction has not moved far.

However the bulk of the sediment body and particularly

the quartz fraction shows little differentiation and does not invalidate the hypothesis of Pleistocene transport from centre to east.

108.

AUTHOR

INDEX.

ALBANI, A.

75

LAWSON, S.

BLOM, W.

1

LEES, B.

39

BOADLE, G.

57

LOUGHRAN, R.

62

BOLTON, B.

57

MARSHALL, J.

20

BOWLER, J.

12

MARTIN, A.

86

BURNE, R.

55, 65

MELVILLE, M.

36

CAMPBELL, B.

62

MOORE, L.

65

CHAPPELL, C.

71

NANSON, G.

17

CLEMENTS, A.

39, 92

NIELSEN, A.

42

COWELL, P.

99

OSBORNE, A.

58

DAVIES, J.

106

OUTHET, D.

30

DAVIES, P.

20

PHIPPS, C.

51

DOUGLAS, P.

77

PICKETT, J.

67

DREW, E.

76

POULOS, H.

8

DRUERY, B.

70

RICKWOOD, P.

75

ELLIOTT, G.

62

ROY, P.

47

ERSKINE, W.

36

RUST, B.

84

FERLAND, M.

95

SHORT, A.

102

FLOOD, P.

83

SKYRING, G.

55

FLOYD, J.

70

STEPHENS, A.

72

FRANKEL, S.

83

TAYLOR, G.

84

GORDON, A.

' 4, 42

TAYTON, J.

75

GRANT, B.

83

THOM, B.

71

HACKER, J.

80

WARD, W.

80

HANN, J.

96

WARNER, R.

22

HURRELL, G.

70

WATSON, G.

33

JONES, M.

72

WOODROFFE, C.

71

KEENE, J.

1

YOUNG, R.

25

KNIGHT, M.

28

28