James Et Al., 1992 - Modem Carbonate and Terrig Sediments, Cool Water, High Energy, Shelf, Australia

Sedimentology (1992) 39,877-903

Modem carbonate and terrigenous clastic sediments on a cool water, high energy, mid- latitude shelf: Lacepede, southern Australia

N O E L P . J A M E S * , Y V O N N E B O N E ? , C H R I S T O P H E R C . V O N D E R B O R C H t a n d V I C T O R A . G O S T I N ?

*Department of Geological Sciences, Queen’s University, Kingston, Ontario, Canada, K7L 3N6 ?Department of Geology and Geophysics, University of Adelaide, Adelaide, South Australia, Australia SO01

$School of Earth Sciences, Flinders University, Bedford Park, South Australia, Australia 501 I

ABSTRACT

The wide Lacepede Shelf and narrow Bonney Shelf are contiguous parts of the south-eastem passive continental margin of Australia. The shelves are open, generally deeper than 40 m, covered by waters cooler than 18°C and swept by oceanic swells that move sediments to depths of 140 m. The Lacepede Shelf is proximal to the ‘delta’ of the River Murray and the Coorong Lagoon. Shelf and upper slope sediments are a variable mixture of Holocene and late Pleistocene quartzose terrigenous clastic and bryozoa- dominated carbonate particles.

Bryozoa grow in abundance to depths of 250 m and are conspicuous to depths of 350 m. They can be grouped into four depth-related assemblages. Coralline algae, the only calcareous phototrophs, are important sediment producers to depths of 70 m. Active benthic carbonate sediment production occurs to depths of 350 m, but carbonate sediment accumulation is reduced on the open shelf by continuous high energy conditions.

The shelf is separated into five zones. The strandline is typified by accretionary sequences of steep shoreface, beach and dune carbonate/siliciclastic sediments. Similar shoreline facies of relict bivalve/ limestone cobble ridges are stranded on the open shelf. The shallow shelf, c.40-70m deep, is a wide, extremely flat plain with only subtle local relief. It is a mosaic of grainy, quartzose, palimpsest facies which reflect the complex interaction of modem bioclastic sediment production (dominated by bryozoa and molluscs), numerous highstands of sea level over the last 80 000 years, modem mixing of sediments from relatively recent highstands and local introduction of quartz-rich sediments during lowstands. The middle shelf, c.7CL140 m deep, is a gentle incline with subtle relief where Holocene carbonates veneer seaward- dipping bedrock clinoforms and local lowstand beach complexes. Carbonates are mostly modem, uniform, clean, coarse grained sands dominated by a diverse suite of robust to delicate bryozoa particles produced primarily in situ but swept into subaqueous dunes. The deep shelfedge, c. 14CL250 m deep, is a site of diverse and active bryozoa growth. Resulting accumulations are characteristically muddy and distinguished by large numbers of delicate, branching bryozoa. The upper slope, between 250 and 350 m depth, contains the deepest platform-related sediments, which are very muddy and contain a low diversity suite of delicate, branching cyclostome bryozoa.

This study provides fundamental environmental information critical for the interpretation of Cenozoic cool water carbonates and the region is a good model for older mixed carbonate-terrigenous clastic successions which were deposited on unrimmed shelves.

INTRODUCTION

Carbonate sediments on mid-latitude shelves outside the tropics are poorly understood compared to their warm water counterparts (Nelson, 1988). Such cool

water shelves are, nevertheless, sites of extensive carbonate production and accumulation and, with their distinctive composition and lack of reefs, are

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878 N . P. James et al.

useful modern analogues of many ancient limestones (Brookfield, 1988; James, 1990; James & von der Borch, 1991).

The southern, south-westem and south-eastern margins of the Australian continent comprise the world's largest modern cool water carbonate province. Most of the continental shelf receives negligible terrigenous sediment input, thereby favouring the production and preservation of sediments containing a high proportion of bryozoa, molluscs and foraminifera (Wass et al., 1970; Wilcox et al., 1988; Davies et al., 1989). Apart from the narrow shelf off southern Victoria and Tasmania (Rao, 1981 ; Jones & Davies, 1983) and off southern Western Australia (Collins,

1988) the extensive southern portion of the margin has remained virtually unsampled and unstudied, particularly with regard to Quaternary sedimentation.

This report documents present day sediments on the Lacepede Shelf and adjacent Bonney Shelf (Fig. 1) on the basis of bottom sediment samples. In contrast to most of the margin, this region is opposite a major drainage system, and so has terrigenous clastic and carbonate deposits. The paper is largely descriptive because it records the first such investigation of this large area. Little is known about cool water carbonates in general, and as such emphasis is placed on compositional attributes as well as facies differentia- tion. The shelves are potential modern analogues for

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Fig. 1. Generalized geology of the study area modified from Sprigg (1979) showing the location of the Lacepede Shelf and the Bonney Shelf. The area between the Fleurieu Peninsula and the Palaeozoic-Precambrian rocks in the east is the Murray Basin, filled with relatively flat lying Cenozoic rocks. The strata are gently tilted to the west because of uplift throughout the Pleistocene around an axis west of Mount Gambier. These are veneered with arcuate sets of Plic-Pleistocene palaeo beach dune complexes and sand sheets. Inset: the location of the shelves and drainage basin for the River Murray-Darling system.

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Cool water sediments, Lacepede SheK southern Australia 879

mixed carbonate-terrigenous clastic successions during periods in geological history when there were no reef-building metazoans.

METHODOLOGY

The study is based on information obtained during cruises FR3/89 and FR2/91 of CSIRO R.V. Franklin during March 1989 and January 1991. Navigation was by GPS (Global Positioning System), transit satellite, radar and dead reckoning. The accuracy of navigational fixes varied from metres to several tens of metres. Samples and bottom profiles were widely spaced in order to characterize the entire region. Bathymetry was determined using a precision depth recorder. Surface temperatures and salinities were recorded every 10 min and vertical temperature

profiles for 19 selected positions were documented by expendable bathythermography (XBT).

A total of 149 sediment samples was taken (Fig. 2); 136 dredges, using a simple bucket (Bleys Dredge) with a volume of approximately 20 litres, 10 grab samples using a Smith-McIntyre sampler, one gravity core and two piston cores. The closed dredge was set on the bottom and towed at a speed of 2 knots for 3-5 min, at which time the vessel was stopped and the dredge retrieved. All sediment samples are, therefore, a mixture of surface and subsurface material to a depth of 5-10 cm. Although a minor amount of the mud fraction may have washed out during retrieval, enough samples with significant amounts of mud were recovered to confirm that such loss was minimal. There are neither enough high resolution seismic profiles ,nor adequate vibrocores yet available to construct a three-dimensional picture.

Fig. 2. Bathymetry of the Lacepede Shelf and Bonney Shelf and adjacent deep water abyssal plain, based upon existing charts augmented by depth profiles produced during this study. Dots are sample sites on the shelf and numbered lines are detailed depth profiles and sample sites illustrated in Figs 3 & 4.

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880 N . P . James et al.

Bottom camera stations were located at 14 sites to a depth of 180 m using either a standard 35 mm camera with a strobe flash (four) or frame-mounted EG&G camera-flash unit (10). Sediment composition was determined by visual examination, under a binocular microscope where appropriate, of sample splits sieved into silt to fine grained sand, medium to coarse grained sand and very coarse grained sand and larger. Colour reported is based on comparison with the Munsell colour chart. Rock name equivalents to facies are noted to make results applicable to the geological record.

GEOLOGICAL SETTING

Location

The Lacepede Shelf (Figs 1 & 2), spanning approximately 130 x 190 km (25 000 km2), is a broad embay- ment into the otherwise relatively narrow continental shelf of south-eastern Australia. It is delimited by Kangaroo Island and Fleurieu Peninsula to the west and north and by the arcuate Younghusband Penin- sula/Coorong Lagoon complex, hereafter called the Coorong Strand, to the north-east. The contiguous, much narrower Bonney Shelf forms the margin south- eastward towards Bass Strait. The Great Australian Bight Abyssal Plain lies to the south in water depths of more than 5 km, a region typified by terrigenous and carbonate turbidites (Conolly & von der Borch, 1967). Major submarine canyons dissect the intervening continental slope (von der Borch, 1967).

Bedrock geology and tectonics

The region is a portion of the southern Australian passive margin, where the age of breakup between Australia and Antarctica has been estimated as 95 5 Ma (Cenomanian-Turonian) by Veevers (1986). The Lacepede Shelf is a seaward extension of the onshore Murray Basin (Brown, 1985), a broad, shallow depression to the north-east containing up to 1 km of mostly flat lying Tertiary limestone. Active Cenozoic faulting controls the western margin of both the onshore and offshore Murray Basin, and determines the geography of two north-south trending grabens, Gulf St Vincent and Spencer Gulf (Fig. 1). Investiga- tor Strait is a related east-west trending graben. Kangaroo Island and the Mt Lofty Ranges of Fleurieu Peninsula are horst-like features of elevated Protero- zoic and Palaeozoic basement. Strata immediately

beneath the Lacepede and Bonney shelves are Eocene- Miocene bryozoa-rich carbonates (Sprigg, 1952; James & Bone, 1989).

Quaternary geology and tectonics

Relatively flat lying Tertiary carbonates onshore are veneered by a thin succession of stranded Plio- Pleistocene beach dune ridges that extend inland for 200 km (Sprigg, 1952, 1958, 1979; Cook et al., 1977; Schwebel, 1983; Belperio & Bluck, 1990). Sprigg (1979) has identified 30 such linear palaeoshoreline systems, each one roughly parallel to the Coorong Lagoon and related to highstands and lowstands of sea level. Stranding has been due to the rising Gambier Arch (Fig. 1). Ridges are progressively younger south- westwards towards the sea (Cook et al., 1977). During glacial lowstands of sea level a northward shift of the ‘roaring forties’ climatic belt of low pressure systems resulted in increased rainfall and discharge of the River Murray (Belperio, 1992). Strong westerlies during such pluvial periods alsoled to aeolian transport of terrigenous sediments eastward from the River Murray and their deposition as extensive downwind sand sheets (Sprigg, 1979; Fig. 1).

The River Murray currently empties into a broad former estuary, now an artificial lake (Lake Alexan- drina), with only a single pass through the north- western end of the barrier system. Although the focal point of Australia’s largest drainage network (Fig. 1 ; inset), the river system is not currently contributing a significant amount of sediment to the shelf. Gradients are extremely low along the distal 600 km or so and flow is relatively sluggish, with the result that most of the bedload is trapped in upstream point bar systems. The relatively minor portion of the suspended load which bypasses Lake Alexandrina and reaches the ocean via the pass is widely dispersed by strong longshore and tidal currents. The river channel itself is graded to lower sea level stands, and is entrenched more than 50 m into Tertiary limestones and Quater- nary sediments of the Murray Basin.

MARINE ENVIRONMENT

Bathymetry

This area is best categorized as an open shelf (Ginsburg & James, 1974) without an elevated rim. The shelf is divisible into four depth-related segments, a steep shoreface, a wide shallow shelf (-40 to -70 m), a


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........... ...... \. ........... 800 Sprigg Canyon

& Tributaries ................. \- 600

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1100

Fig. 3. Detailed depth profiles across the edge of the Lacepede Shelf near the head of Sprigg Canyon (see Fig. 2 for location). The depth of recurring terraces and ridges is indicated by arrows. SE = shelf edge.

gently seaward dipping middle shelf (- 70 to - 120 m) and a steeper deep shelf (- 120 to - 200 m) leading to the shelf edge at about -200 m. Shelf margin bottom profiles are illustrated in Figs 3 and 4.

Shoreface

The Coorong Strand is a sand dune peninsula, the outermost of the succession of prograding Quaternary beach ridges and barriers. The adjacent sea floor is a series of offshore bars which form the top of a relatively steep shoreface that drops to the open shelf plain at - 40 m in less than 5 km. This steep shoreface passes south-eastwards into a broad, 12m deep bedrock and sediment veneered terrace (Sprigg, 1979) here called the Kingston Terrace. This arcuate terrace becomes more pronounced southward and merges with the offshore projection of Quaternary beach dune complexes and Cenozoic bedrock known as Margaret Brock Reefi

Shallow shelf

This is a wide, flat, 40-60m deep plateau on the Lacepede Shelf (Fig. 2), brokenonly by bedrock highs around the edge. The - 70 m contour is just inboard of the -80 m line. Seaward of the Coorong Strand the sea floor (- 40 to - 60 m) descends only 20 m in 160 km, a slope of 1 : 8000. The north-western part of the shelf has irregular bathymetry, reflecting the adjacent rugged topography of Kangaroo Island and Fleurieu Peninsula horsts. This basement topography is most dramatically expressed by Threshold Bank, Carters KnoN and the elongate Sanders Bank (Fig. 2) which rise 10-15 m above the surrounding sea floor. High resolution seismic profiles show that Threshold Bank is an uplifted basement block which marks the faulted western margin of the Murray basin. Backstairs Passage, between Kangaroo Island and Fleurieu Peninsula, is a scoured sea floor swept by strong tidal currents with little sediment accumulation.

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LINE 61

BONNEY SHELF I

LACEPEDE SHELF

SOUTH-EAST KANGAROO ISLAND

M = Mound T = Canyon tributary

The Bonney Shelf (Fig. 2) is comparatively narrow, 35-40 km on average, and characterized by linear ridges and depressions whichmimic adjacent, onshore Quaternary beach dune complexes and intervening corridors. The most distinctive ridges are (1) a wide complex which begins at - 100 to - 110 m and rises to - 75 to - 80 m, and (2) a narrow ridge which rises from - 50 to - 25 m (Fig. 4, lines 59,61).

Middle sheij

This zone varies from 10 to 15 km in width, except south of Kangaroo Island, where it is 40 km wide. In the central part of the region (Fig. 3, lines 48, 51, 52, 53 ; Fig. 4, lines 49,50), there is a zone of subaqueous dunes and prominent seaward-dipping cuestas between depths of 70 and 130m. These cuestas are interpreted on high resolution seismic profiles to be the truncated tops of prograding clinoforms.

Deep shelf

This outer part of the shelf, rarely more than 10 km wide, is a more steeply dipping and irregular slope.

There are prominent linear sea floor highs at - 130, - 140 to - 150 and locally - 170 m.

SheIfedge

The main break in slope is at a depth of 200 f 20 m. Whilst generally at a depth of 200 m, it is slightly shallower (- 180 m) along the south-eastern Lacepede Shelf and the Bonney Shelf, probably reflecting uplift in the Gambier region.

Slope

The sea floor immediately below - 200 m is relatively steep to a depth of about 300 m. This segment varies from an escarpment to a steep incline (c.70"). Deeper bottom topography is highly variable. In the central part of the shelf, near the heads of submarine canyons, the sea floor is steep and highly irregular. Arcuate reflectors seen on GLORIA profiles and the morphology (cf. Fig. 3, line 52) strongly suggest local slumping and mass wasting. The upper reaches of canyons and tributaries are typically slope-parallel before swinging seaward at the main canyon.


Other upper slope profiles to the east and west (Fig. 4, lines 49, 50) and those off the Bonney Shelf which lead to the Beachport Terrace (Fig. 4, line 59) are gentle inclines which dip seaward at 2-7". Isolated, laterally discontinuous sea floor highs with relief of 50 m are interpreted as carbonate buildups, either mounds or sediment drifts.

Oceanography

The shelf is swell-dominated. Davies (1980), Wright et al. (1982) and Short & Hesp (1982) have described the area as typified by high (> 2.5 m) modal deep water wave heights. Long period (> 12 s) swell waves are common, and wavelengths of 200 m have been

reported, with implications for sorting of sands by oscillatory motion to water depths in excess of 100 m.

General water movement is to the south-east throughout the year and is strongest in winter months, resulting in general downwelling and low nutrient levels. Physical structure of the waters is strongly seasonal, being stratified over the summer period and more vertically homogeneous through the winter months. Stratification is due to incursion of deep, cold, open ocean waters onto the shelf, largely because of periodic strong summer winds from the south-west which cause seasonal upwelling, especially along the Bonney Shelf (Schahinger, 1987). Our temperature profiles (Fig. 5, inset) taken during the summer months indicate : (1) a mixed surface layer (0-30 m) averaging

WATER TEMPERATURE ("C) 5 10 15 20

SURFACE TEMPERATURE _ __ __ -. SURFACE SALINITY

0 km I

Fig. 5. The temperature and salinity of surface waters in the summer (12-20 March 1989). Inset: the summer and winter temperature profiles for the area based on our observations in January 1991.


18°C; (2) a thermocline (30-80 m) dropping from 18°C to, on average, 13°C; and (3) deep water (below 80 m) decreasing about 1°C per 100 m to 600 m, the limit of our XBT measurements. During winter months most of the shelf to depths of 100 m is covered by waters of 17-1 8"C, with temperatures decreasing roughly 1°C per 50-100 m below that.

The River Murray does not appear to have much influence on shelf waters. In contrast, the generally eastward water movement along the shelf causes warm saline waters from Gulf St Vincent (Bye, 1976) to spill through Backstairs Passage and past Kangaroo Island onto the western part of the shelf. Our surface salinity measurements (Fig. 5) show a strong SE-NW gra- dient. Cold (14-15°C) waters with oceanic salinities (35.17,,) on the Bonney Shelf in the south-east grade across the Lacepede Shelf into relatively warm (19- 20°C) and saline (35.979 waters in the west and northwest. The pattern along the Lacepede Shelf edge was similar in January 1991 but surface waters on the Bonney Shelf were, on average, 1°C warmer and 0.03%, more saline inboard of the - 80 m contour.

SEDIMENTARY COMPONENTS

Sedimentary particles are terrigenous clastic, Holo- cene carbonate and relict carbonate. Terrigenous clastic grains are either clean or stained brown and since the River Murray is not presently supplying much sediment to the shelf, most are presumed relict. Holocene skeletal particles are white or buff in colour and their pores, although generally empty, locally contain high-Mg calcite cements. Relict carbonate grains, mostly late Pleistocene in age (see discussion below), are conspicuously stained brown or red- brown. The pores of relict skeletal grains are variably filled with high-Mg calcite cement, iron oxide and/or clay. Many particles in slope sediments are conspicuously dark grey in colour and intensively bored.

Terrigenous

Sediments are mostly sand-sized and quartz-rich. They vary from very fine to very coarse grained, with the finer particles generally angular to subangular and the medium to coarser fractions well rounded. Heavy minerals are prominent in the sediments from the Bonney Shelf. The composition of the terrigenous clastics is the topic of a separate study.

Carbonate (Holocene and relict)

All particles are skeletal. The most abundant components are bryozoa and a full spectrum of growth forms (Nelson et al., 1988) are present (Fig. 6). The most common are large, erect, robust branching Adeona sp., fenestrate (reteporiform), robust branching (adeonifom), foliaceous (eschariform), branching and articulated (cellariiform), delicate branching (vincu- lariiform), vagrant (lunulitiform) and unilaminar to multilaminar massive to branching celleporiform cheilostomes. Of particular importance in the finer sand size fractions are the singlets from extremely delicate, articulated catenicelliform cheilostomes. The most common cyclostomes are delicate branching and reteporiform.

Molluscs are locally as abundant as or more abundant than bryozoa in terrigenous-rich facies. The most common bivalves are the infaunal cockles Katelysia, Glycymeris and Venericardia. The most common gastropods are turitellids and the slit shell Siliquaria. All the calcareous algae are rhodophytes; corallines encrust, grow as branched forms (Metugo- niolithon) and form rhodolites ; peyssonnellids encrust and form sheets over loose sediment. Benthic foraminifera are ubiquitous, especially rotaliids, miliolids and textulariids, with encrusting red Homotrema especially conspicuous on particles in agitated environments. Serpulid worm tubes occur as clusters and single tubes. No hermatypic corals are present but the ahermatypic forms Caryophyllia and Flabellum are locally prominent. Brachiopods are omnipresent but rare. Only the spines of infaunal irregular and epifaunal regular echinoids were recovered. Coloured gorgonian spicules occur throughout. Planktonic foraminifera and ostracods, although present every- where, are commonest in muddy, deep water sediments. Crab claws are widespread but never abundant.

There are several types of limestone lithoclasts. Irregular to rounded pieces of skeletal wackestone to packstone composed of magnesium calcite, calcite and aragonite (confirmed by staining) and which contain large calcareous skeletons are interpreted to be Pleistocene/Holocene in age. Such clasts are usually intensively bored. Fine skeletal grainstone composed of rounded particles and cemented with calcite is identical to Pleistocene aeolianites onshore. Fine grained, muddy, friable to well cemented bryozoa limestone (locally with grey chert) is interpreted to be Cenozoic (Gambier Limestone and equivalents). Quite separate and distinct are chocolate-brown (5YR 314 to 4/4), iron-oxide impregnated, bivalve-rich


Fig. 6. A sample of bryozoa sand (sample number 71) from a depth of 130 m on the Lacepede Shelf. (a) The complete sample including finer sizes; prominent bivalve is Venericardiu sp. (b) The coarse grained fraction only, illustrating most of the major sediment producers: (1) delicate, branching cyclostome and cheilostome bryozoa; (2) reteporiform cheilostomes; (3) robust branching cheilostomes; (4) Adeonu sp. with encrusting unilaminar cheilostome ; ( 5 ) brachiopod (terebratulid); (6) Chlurnys sp.; (7) gorgonian spicules; (8) serpulid worm cluster; (9) Siliquaria sp. (c) Sea floor of small sand dunes textured by irregular ripples; numerous large sponges are 10-20 cm high. (d) Close up of the sea floor showing rooted Adeona sp. 20 cm high (lower centre).

calcarenite pebbles and cobbles, whose origin is uncertain. Relict detrital, clear to red-brown, zoned Pleistocene dolomite, in the form of single crystals and crystal aggregates, occurs in most shelf sediments (Bone et al., 1992).

AGE AND C O M P O S I T I O N OF R E L I C T S K E L E T O N S

Bryozoa

The age of relict particles is controversial. In this region they are generally thought to originate from nearby, poorly cemented, easily erodable, composi- tionally similar Cenozoic limestones (cf. Jones &

Davies, 1983). To confirm the antiquity of these particles, representative adeonid and delicate cheilostome rods were sorted under the binocular microscope and 250 mg dated at Isotrace (University of Toronto; Table 1). Particle mineralogy of 17 representative samples was determined by staining (Clayton Yellow; Choquette & Trusell, 1978) and electron microprobe analysis.

The mineralogy of Holocene and relict bryozoa is identical. Adeonids are aragonite with an axial core of high-Mg calcite (11-13 mol% MgC03). Cheilo- stomes are partly low-Mg calcite (3-5 mol% MgC03) and partly high-Mg calcite (9-12 mol% MgC03). Cenozoic bryozoa, in contrast, are all low-Mg calcite (James & Bone, 1989). Relict samples are bored by microendoliths and both those holes and the zooecia

886 N. P. James et al.

Table 1. 14C age (years BP) of bryozoan particles.

Adeonid Cheilostome rods

Living Coloured + 114.4f0.7 Clean, white ‘fresh’ 180f50 400 f 50

Holocene Abraded - 780 f 50 Light grey - 1430f 120

Relict Stained 21750k 150 17960+120

are variably filled with red/brown iron oxides and/or oxidized glauconite/berthierine.

The 14C age of the fresh or abraded ‘Holocene’ particles is less than 1000 years BP (Table 1) whilst grey cheilostome rods are 1400yearsBP. Relict bryozoa have 14C ages of 21 750 and 17 960 years BP. This is viewed as a minimum age because it is an integral of the age of the skeleton and the age of the oxide/clay infill.

In summary, the relect bryozoa particles are not Cenozoic but most likely late Pleistocene in age. They have not suffered any obvious diagenetic alteration except for filling of pores.

Bivalves

The 14C age of bivalves is variable, ranging from 9000 to 31 000 years BP. Regardless, like the bryozoa, they indicate a mixed late Pleistocene and Holocene assemblage.

DEPTH LIMITS OF LIVING BRYOZOA AND CORALLINE ALGAE

Coralline algae

As the only important calcareous phototrophs, coralline algae are useful universal indicators of shallow water ‘subtidal’ sedimentary environments in the rock record. Most living calcareous algae were determined from the samples where colour clearly indicated extant forms, while others were identified by B. Wommersley (University of Adelaide) from specimens preserved in formalin. Coralline algal rods, however, once dead, are difficult to determine in sediment, so the sediments from the outer shelf were analysed in thin section. Depth distribution plots for living coralline algae are given in Table 2. The number of samples containing living algae and the total number of samples taken at that depth is recorded for each 10 m interval.

Crustose corallines are living to a depth of 100 m,

Table 2. The presence of living coralline and peyssonnellid algae.

Water Encrusting Branching Encrusting depth corallines corallines peyssonnelids (m)

0 10 20 30 40 50 60 70 80 90

100 110 120 130 140 150

0-0. 0-0 3-9 3-10 3-13 3-14 8-15 1-6 1-5 5-17 2-6 0 4 0-4 0-3 0-2

0-0 0-0 1-9 1-10 2-1 3 1-14 1-15 0-6 0-5 0-1 7 0-6 0-4 0 4 0-3 0-2

0-0 0-0 2-9 1-10 4-13 2-14 1-15 1-6 1-5 1-17 1-6 0-4 0-4 0-3 0-2

* First number is the number of samples containing algae; second number is the total number of samples at that depth. Depth ranges: encrusting corallines, 0-100 m; branching corallines, 0-60 m; encrusting peyssonnelids, 0-100 m.


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as are peyssonnelids. Dead rhodoliths occur to a depth - 125 m. Living bryozoa can, with some overlap, be of 120 m. Robust branching corallines (Metugonioli- divided into four depth-related assemblages. thon) grow in waters shallower than 70m. Thin sections reveal that particles of branching corallines (1) Shelf (10-1 10 m). Small numbers of the large, occur in small numbers (2% to much less than 1% of conspicuous cheilostome Adeona sp. along with large the sediment) to depths of 240 m. These corallines are numbers of catenecelliform, membraniporiform, del- by all appearances fresh, not relict, but there is no way icate articulated branching, reteporiform, robust to determine this for certain. branching adeoniform and lunulitiforms.

SAMPLE CONTROL

RIGID DELICATE BRANCHING * 2 I

CATENECELLIIFORM (Singlets)* P -8 CELLEPORIFORM BRANCHING 8 CELLEPORIFORM MASSIVE % CELLEPORIFORM ROUND

P

8

MEMBRANIFORM (Encrusting)

VINCULARIFORM (Rods) * p

ADEONIFORM (Flatbranching)* 5 ADEONA SP.

' LUNULITIFORM (Free living)

- v 8

Bryozoa

A bryozoa particle or colony was considered living if it contained: (1) brown bodies; (2) an operculum, in the case of cheilostomes; (3) an outer organic sheath; and (4) the skeleton was coloured. Sea floor samples give a reasonable coverage to a depth of 425 m, but samples below this depth are sparse (Fig. 7). The next sample site below - 425 m is at - 560 m, and there are no living bryozoa at this deeper site or below. Since several forms live to depths of 425 m, their deepest occurrences lie somewhere between - 425 and -560 m.

All cheilostomes except celleporiform types are living at the shallowest depths sampled. Delicate catenecellids extend to depths of 425 m whilst all others except the celleporiforms do not seem to grow below - 250 m. Delicate, branching cyclostomes grow to at least -425 m in this study but it is uncertain whether they are extant in waters shallower than

(2) Shelfedge (1 10-200 m). The zone of most diverse growth comprising the ubiquitous delicate, branching cyclostomes and catenecellid cheilostomes together with large numbers of articulated delicate-branching fenestrate reteporiform, robust adeoniform cheilostomes and smaller numbers of membraniporiform, lunulitiform, membraniporiform and celleporiform cheilostomes.

( 3 ) Top of slope (200-250 m). Articulated, delicate, branching and catenecellid cheilostomes, delicate, branching cyclostomes and lesser celleporiform cheilostomes of variable morphology.

(4) Upper slope (250-?450 m). Delicate, branching cyclostomes and catenecelliform cheilostomes with variable multilaminar celleporiform cheilostomes.

The components in sediments generally correspond to these assemblages, with particles displaced by

1 @ 101 @ I @ ASSEMBLAGES 600 500 400 300 200 100 ODEPTH (m)


gravity downwards as much as 50 m on the deep shelf and upper slope.

Bivalves

There is a general trend in the distribution of infaunal bivalves. Donax is ubiquitous in the surf zone. Katalysia generally occurs across the shallow shelf to depths of about 80 m. Glycymeris is more common on the shelf between -60 and - 140 m. Venericardia is typical of the mid- to deep shelf, usually between depths of 80 and 140m and less abundant deeper. Epifaunal bivalve distribution is not as clear. They are most abundant on and around bedrock highs, palaeostrandlines, outer shelf and upper slope. Chla- mys occurs across the shelf to depths of 140 m and sporadically to depths of 350 m. Si!iquaria and Lima, both sponge dwellers, are most abundant on the deep, outer shelf and upper slope.

COMPOSITION AND TEXTURE OF HOLOCENE CARBONATE SEDIMENT

Composition and texture of bioclastic carbonate sediments is a function of the inherent size and articulation of skeletal hard parts (Ginsburg et al., 1963; James, 1984) locally modified by bioerosion. Modern carbonates on the Lacepede and Bonney shelves illustrate this axiom exceptionally well.

Carbonate in the silt and clay fraction has only been studied in a reconnaissance fashion. It is mostly whole and fragmented coccoliths and singlets of articulated catenecellid bryozoa. In life, the singlets, or single zooecia of catenecellids, are attached to one another at nodes by organic tissue (Fig. 8). This connective tissue disintegrates upon death and the colony produces hundreds of tiny particles. They vary from well calcified to very lightly calcified. Such delicate singlets are easily transported and fragmented. Ostracod valves and small planktonic foraminifera1 tests are also important, as are siliceous sponge spicules. Most mud is 15-20% terrigenous silt and clay.

Fine sand sized grains are the most ubiquitous components and are found in varying amounts from the strandline to the slope, regardless of facies. Delicate, branching cyclostome and catenecellid cheilostome bryozoa, and benthic foraminifera, together with siliceous sponge spicules, make up most of this fraction. The delicate, erect, rigid branching cyclostomes are preserved as single rods or bifurcating twigs. The singlets of articulated catenecellids are, however, more common. Medium and coarse grained sand is mostly large benthic foraminifera, bivalve fragments and pieces of cheilostome bryozoa. Unlike finer grains, these particles generally show signs of abrasion.

Whole cheilostome bryozoa and mollusc skeletons dominate the very coarse grained sand and cobble sized fraction. Bryozoa are larger pieces to branched segments of robust branching adeoniforms, large,

Fig. 8. Sample of a catenecellid cheilostome bryozoa (sample 6). The photograph to the right shows individual zooecia (singlets) attached to one another by organic tissue which disintegrates upon death, resulting in numerous fine grained sand sized carbonate particles.


semi-complete fenestrate reteporiforms together with complete multilaminar branching and massive celleporiforms, plates of Adeona sp. and whole small colonies of vagrant lunulitiforms.

SEDIMENTARY FACIES

Introduction

Sediments form six major facies (Table 3; Figs 9 & 10). Shelf deposits are grainy with variable amounts of quartz and relict particles. Calcareous quartz sands and quartzose bryozoalbivalve sands blanket much of the shallow shelf. The middle shelf is covered by clean bryozoa sands. Mixtures of these sediments, variable amounts of relict material and transition zones between major facies form subfacies of slightly different composition. Deposits below swellbase are

generally muddy, contain little quartz and have few obvious relict particles. The deep shelf, shelf edge and top of slope are covered by bryozoa muds and the slope proper is cloaked by pelagic muds. Coarse grained bivalve-coral gravels occur in deep water on the upper slope in the south-east.

(1) Calcareous quartz sand (Fig. 9)

Typically fine grained, quartz-rich sands, these sediments contain variable proportions of relict and Holocene carbonate, particularly bivalves and limestone cobbles.

1A. Molluscan quartz sand is fine to medium grained and generally very well sorted. Bivalves are mostly infaunal, mainly Donax sp. nearshore and Katelysia sp. below 40 m on the open shelf plain. The sea floor is smooth and monotonous; bottom photographs

PELAGIC FORAM

HOLOCENE CARBONATE I 0 I km 100 I

Fig. 9. Sedimentary facies on the shelf. Pie diagrams graphically depict the average composition of these facies whose percentage composition is tabulated in Fig. 10.

Tabl

e 3. M

odem

sed

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t fac

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n th

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lves

.

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350-

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Cool water sediments, Lacepede Shelf; southern Australia 891

I km 2 8 PUARTZOSE BRYOZOAN 0 10 20

SAND 3 ROBUST BRYOZOAN

Fig. 10. A schematic profile across the shelf edge of the region under study, distilled from the profiles in Figs 3 & 4. Pie diagrams graphically depict the average composition of the sedimentary facies (tabulated in the inset). Values are means and actual samples are generally f 10%. Dashed lines separate the facies; key to the pie diagrams is given in Fig. 9. Note the location of wave base.

(Fig. 11) reveal clean sediments with small straight- crested wave ripples (1&15 cm apart) and no large organisms other than burrowing anemones.

IB. Bivalve-cobble quartz sand along the Coorong strand is variably calcareous (Sprigg, 1952). Relict particles are worn bivalve and bryozoagrains, together with conspicuous limestone cobbles. Holocene grains are almost entirely fragments of the prolific surf clam Donax deltoides.

(2) Quartzose bryozoa/bivalve sand (Fig. 9)

These sediments are mixtures of facies 1 (calcareous quartzose sand) and facies 3 (robust bryozoa sand, described below) with variable, often high, proportions of relict particles and occur in areas intermediate and transitional between the two facies.

2A. Quartzose bryozoa-bivalve sand lies along the northern edge of the Lacepede Shelf near Kangaroo Island and across the Margaret Brock Reef -Kingston Terrace area. It also forms most of the sediment on the Bonney Shelf. Sediments are those of facies 2B (a mixture of robust bryozoa sand and relict grains) and facies 1A (calcareous quartz sand). Important modem bivalves are Glycymeris, Katelysia and Chlamys. There are also rare slit shells and a variety of small gastropods (olives, cones, periwinkles and cerithids).

2B. Quartzose bryozoa sand is a mixture of the robust bryozoa sand (facies 3 described below) and obvious brown relict particles, giving the sediment a speckled brown and cream or ‘salt and pepper’ appearance. Noticeably worn bivalve fragments are present in some areas while other regions contain worn small brown lithoclast pebbles. The most


Fig. 11. (a) Molluscan quartz sand facies; sea floor photograph at a depth of 45 m illustrating straight crested wave ripples orientated NW-SE and burrowing anemonies (compass is 8 cm in diameter). @) A sample of this facies composed of calcareous quartz sand and large particles of (1) relict lithoclast, (2) relict bivalve, (3) Circomphulus disjectu, (4) Nuculunu sp., (5) Solon uuginoides, and (6) Kutulysiu sp. (c) Bottom photograph of the relict limestone gravel facies at a depth of 57 m on the Lacepede Shelf illustrating coarse lithoclasts and bivalves at left and scattered rhodolites at centre (compass is 8 cm in diameter). (d) Sample photograph of the branching coralline algal gravel facies composed mostly of Metugoniolithon with scattered Glycymeris (1) and reteporiform bryozoa (2). Sample depth 40 m.

noticeable large bryozoa are Adeona sp., robust erect branching adeoniforms, and scattered large, multilaminar, globose to irregular Cellepora sp. The most abundant bivalves are the infaunal cockle Glycymeris sp. and the epifaunal scallop Chlamys sp., together with fragments of the epifaunal hammer oyster Malleus sp. and minor valves of the infaunal cockle Katelysia sp. The only gastropod of importance is the slit shell Siliquaria sp.

2C. Relict limestone gravel comprises granules to cobbles to scattered boulders of limestone (usually fine grained calcarenite), abundant bivalves and shelf sands of facies 2A and 2B. Clasts are stained chocolate brown (5YR 314 to 414) to yellow brown (10YR 5/4), and usually bored and/or encrusted by coralline or peysonnellid calcareous algae. Bivalves are mostly whole to very abraded Chlamys and/or worn Glycy-

meris, together with large and robust shells of the epifaunal forms Cleidothaerus and Ostrea, smaller and more delicate valves of the epifaunal nut shell Nuculana and the small epifaunal arc Barbatia, the hammer oyster Malleus as well as variable numbers of Katelysiu. The most common relict bryozoa are eschariforms. Bottom photographs (Fig. 11) reveal ridges to sheets of limestone pebbles and bivalve coquinas with bryozoa growing on many of the larger particles. These deposits occur in sublinear bands at depths of 40 and 65 m and are similar to sediments adjacent to the present shoreline. They are interpreted to represent palaeostrandlines.

2D. Branching coralline algal gravel (Fig. 11) is composed not only of the branching coralline Meta- goniolithon sp., but numerous bryozoa, especially large pieces of Adeona sp. It is localized to the northern part

Cool water sediments, Lacepede Shelf, southern Australia 893

of the shelf where the bedrock highs of Sanders Bank, Threshold Bank and Carters Knoll rise to within 30 m of the surface. Poor but useable bottom photographs and several bottom samples of these highs indicate surfaces veneered by a wide variety of encrusting and rooted (particularly eschariform) bryozoa, multiple layers of crustose coralline and peyssonneiid algae, coppices of branching coralline algae, small clusters of brachiopods and intergrowths of serpulid worms. Between and extending above this low level biota is an abundant and extraordinarily diverse fauna of demosponges. Rooted between all these organisms and rising into the water column in great profusion are fleshy red algae and kelp, which are locally the ephemeral substrate for other encrusting calcareous algae, bryozoa and foraminifera. Particles from this sediment factory are shed onto the surrounding sea floor where they locally dilute the relict carbonate and quartz grains.

(3) Robust bryozoa sands (- 80 to - 140 m) (Figs 9 &

Nearly pure bryozoa sands (Fig. 6), these sediments 10)

are typified by diverse but robust bryozoa which are variably abraded. Precision depth profiles indicate large subaqueous dune fields in the zone of seaward- dipping clinoforms. Bottom photographs reveal rock substrates covered by variable thicknesses of coarse grained sediment with environments ranging from rippled sands to starved ripples to an open hard sea floor dusted with sediment (Figs 12 & 13). Ripples to small subaqueous dunes have estimated spacings of 30-60 cm and heights of 10-30 cm, are symmetrical, straight-crested to bifurcating and their crests are either sharp and linear or textured by smaller ripples and always orientated NW-SE. Hard substrates are generally flat with centimetre scale irregular relief. Clasts recovered are similar in composition and mineralogy (confirmed by staining) to the loose sediments. Most photographs show starved small dunes to large ripples with abundant organisms growing on hard substrates in the troughs between bedforms. Open rippled sands are barren except for local areas where the surface is 2040% covered by organisms and sands between are rippled in a disorganized fashion. Open hard rock substrates are invariably 50-60% covered by sessile benthos. The

I HARD SUBSTRATE

I I n

CLINOFORMS AND SAND WAVES Fig. 12. Robust bryozou sund fucies; depth 120 m. A sketch of our interpretation of the carbonate sand-hard substrate setting of this environment with bottom photographs illustrating the different types of sea floor. (a) Numerous sponges ( S ) and catenecellid bryozoa shrubs about 20 cm wide (C) on a hard substrate. @) Starved linear wave ripple about 40 cm across on a hard substrate, populated at lower left by sponges and bryozoa. (c) Linear wave ripples with crests about 6 5 0 cm apart textured by smaller ripples. All wave ripples are orientated NW-SE.


Fig. 13. Robust bryozoa sandfacies. (a) Linear wave ripple about 40 cm across (S) and irregular hard limestone surface (H) populated by sponges and bryozoa; depth 120 m. @) Linear sand ripples spaced about 30 cm apart with heights of about 10 cm; depth 120 m. (c) Sea floor marked by small centimetre high ripples, pockmarked by a few burrow openings and colonized by scattered catenecellid bryozoa shrubs up to 10 cm high; depth 130 m. (d) Robust muddy sandfacies; open sea floor textured by diffuse ripples and dotted with burrow openings 6 cm in diameter; depth 140 m.

sea floor below 120 m is mostly rippled sediment with few large organisms.

The most obvious benthos are sponges, generally upright flat planar to digitate oscular forms (cf. Mycale, Zophon, Chondropis, Clathria) or globular types (cf. Ancorina; Fig. 12). They grow together with numerous hydroids and bryozoa, the most noticeable of which are catenecellids and Adeona sp.

Sediments contain no mud and virtually no quartz, and are poorly sorted and of greatly varying grain size. Grains and granules range from fresh to abraded to encrusted by bryozoa, corallines and foraminifera. Bryozoa grains are diverse and distinguished by large Adeona sp., robust rigid branching adeoniform, reteporiform and erect flexible cellariiform cheilostomes, and erect, rigid, delicate, branching cyclostomes. Unilaminar membraniporiform cheilostomes, which encrust algae and grasses, are not abundant,

but are ubiquitous. Broken fragments are encrusted by the foraminifer Homotrema sp. Massive and branching Cellepora sp. are also frequent. Large, vagrant, lunulitiforms are scattered throughout.

Although there is less than 10% Holocene mollusc particles by volume, large molluscs are diverse and conspicuous. The most important gastropods are infaunal turitellids and the sponge-inhabiting coiled slit shell Siliquaria sp. Cones, cowries, olives and volutes and the fragments of helmet shells are ubiquitous but never abundant. Bivalves include infaunal Venericardia, large and small Glycyrneris sp., and epifaunal Neotrigonia sp. and Chlarnys sp. Also present in lesser numbers are the small epifaunal arc Barbatia sp. and the jingle shell or saddle oyster Anomia sp. Other prominent components are irregular rhodolites (clumps of corallines and peyssonnelids), clusters of serpulids, large spines from regular echi-


noids and crab claws. Small ahermatypic corals such as Trematotrochus and Caryophyllia are attached to larger bryozoa and rhodolites.

(4) Bryozoa muds

These green and poorly sorted sediments cover the deep outer shelf, shelf edge and upper slope, becoming progressively depleted in bryozoa with increasing depth.

4A. Robust bryozoa muddy sand (- 140 to - 250 m) is fine to very fine grained, well sorted, muddy sand and contains variable numbers of large bryozoa. Bottom photographs (Fig. 13) of the upper parts of this zone ( - 140 to - 170 m) depict a gradual transition from shallower rippled coarse sands (facies 3) to these more muddy sediments; the sharp crested linear

ripples (10-15 cm height, 1&30 cm spacing) die out between - 140 and - 150 m and grade, with depth, into areas of subdued, more bifurcating ripples (5- 10 cm high, 2-5 cm spacing) textured by smaller confused and irregular ripples interspaced with regions with no obvious physical sedimentary structures, but pockmarked by numerous burrows. Areas of the seafloortodepthsof 170 m, althoughmostlyburrowed, are locally textured by subdued centimetre scale diffuse ripples with ladder ripples in their troughs that have been modified by burrowing.

The shelf edge between - 180 and - 200 m (Fig. 14) is a relatively flat to gently undulating surface dimpled with centimetre size depressions and bumps, pockmarked with vertical burrow openings and disturbed by occasional trails of the mobile benthos. Numerous, small, centimetre size or less, delicate, branching

Fig. 14. Robust bryozoa muddy sand facies at the shelf edge; depth 175 m. (a) Sea floor with clusters of branching bryozoa (arrows) and shrubs of catenecellid bryozoa (width of photograph=0.5 m). (b) Bryozoa growth similar to that in (a), illustrating high density of branching forms (width of photograph=c.lO cm). (c) Sediment sample with numerous branching bryozoa (1) and turitellid gastropods (2). (d) Bivalve-coral gravel facies; depth 25c300 m. Coarse grained fraction of sample consisting of (1) reteporiform bryozoa, (2) small globular celleporiform bryozoa, (3) free-living lunulitiform bryozoa, (4) lithoclasts, ( 5 ) crab claws, (6) regular echinoid spine, (7) ahermatypic coral, (8) Glycymeris, (9) Chlamys, (10) Karulysiu, and (11) turitellid gastropod.


bryozoa grow either scattered across the sea floor or clumped in patches 10-30 cm across. These clusters are dense intergrowths of bryozoa (especially adeonids and catenecellids), small sponges and hydroids that form a ‘ground-cover’ to heights of roughly 10 cm.

Sediments (Fig. 14) contain 10-20% mud, no quartz and roughly 10% relict particles. Branching adeonids, reteporids, unilaminar membraniporids and delicate, branching cheilostomes and cyclostomes are common throughout, but Adeona sp. is absent. A particularly noticeable bryozoa is a relatively thin, hollow tubular celleporiform. Scattered throughout are Venericardia, the epifaunal sponge-associated Lima sp., and the scallop Chlamys sp. Thin conical turitellid gastropods are present, but scarce. Other particularly noticeable fine particles are siliceous sponge and calcareous gorgonian spicules, serpulid tubes, echinoid pieces and pteropods.

4B. Delicate, branching bryozoa mud (-25 to - 350 m) mantles the base of the escarpment and the top of the slope. The coarse grained fraction is almost entirely branching cyclostome bryozoa (1 5% of material). Other scattered bryozoa are minor adeonids, reteporiforms and Cellepora sp. As in the shallower muddy facies, small hollow tubular bryozoa are common. Cellepora sp. is in the form of irregular encrusting sheets, free small particles, and subhemi- spheres up to 6 cm in diameter. The only bivalves are Chlamys sp. and Venericardia sp. while the sparse gastropods are the slit shell (Siliquaria sp.), turitellids, olives and whelks. Other components are solitary corals, regular echinoid spines, siliceous sponge spicules, pteropods and serpulid tubes.

4C. Bryozoa pelagic mud (-350 to -500m) is pelagic mud (facies 6 ) variably rich in catenecellid singlets with the only large bryozoa of importance being vagrant lunuliiforms. The only other scattered bryozoa particles are cyclostome branches. There are no living bryozoa. Agglutinated benthic foraminifera are conspicuous. The miniscule number of molluscs are tiny cerithid-like gastropods and pteropods. There are no bivalves.

(5) Bivalvecoral gravel (- 150 to - 300 m)

These grey (IOYR 6/2) to brown (IOYR 5/4), coarse grained sediments occur off the Bonney Shelf (Fig. 9). Particles are abraded, fragmented and bored. The sediments are difficult to summarize because of the wide depth range over which they occur, but in general they are coarse grained in shallow water and become finer with more planktonic elements in deeper water.

They contain 1045% quartz and 2040% black, old ‘relict’ grains. Bryozoa particles outnumber bivalves in the ratio of 2 : 1. Bryozoa are mostly catenecellids, adeonids, reteporids, articulated branching and numerous vagrant lunulitiforms. Cyclostomes are absent in shallow samples (1 50-1 75 m) and present in small numbers in deeper sediments. Bivalves are mostly worn Glycymeris sp. with rare Katelysia sp., Chlamys sp., Ostrea sp. and the infaunal intertidal shell Mactra- sp. Gastropods are turitellids and tulips. The most conspicuous aspect of these sediments (Fig. 14), besides their grey colour, is the ubiquity of solitary corals (particularly Flabellum sp.) and vagrant lunulitiform bryozoa. Other particles are limestone lithoclasts, siliceous sponge spicules and pteropods.

Aside from the ahermatypic corals, these sediments are most similar to the modem on-shelf facies 2A (quartzose bryozoa-bivalve sand), which forms most of the Bonney Shelf sediments between -40 and -70 m. Thus, this part of the shelf edge and upper slope has either been an area of non-deposition since the late Pleistocene or the overlying Holocene sediments have been removed by mass wasting.

(6) Pelagic muds (Foram-nanno ooze)

These sediments, which uniformly cloak most of the continental slope below - 500 m, are muds whose only coarse particles are fine sand to silt sized tests of benthic and planktonic foraminifera and hyaline fragments of ostracod, foraminifera and pteropods. Sponge spicules and faecal pellets are also conspicuous. Quartz silt is important locally.

SEDIMENTS AND LATE QUATERNARY SEA LEVEL HISTORY

Facies on this open shelf are the result of: (1) terrigenous clastic sediment input from land and carbonate sediment diagenesis during sea level lowstands; (2) strandline reworking during each sea level rise, stillstand and fall; (3) modem reworking of sediment deposited during previous highstands ; and (4) Holocene sea floor sediment production.

Late Pleistocene

Fluctuations of sea level over the last 150 000 years, as currently understood, are illustrated in Fig. 15.

Cool water sediments, Lacepede Shex southern Australia 897

GULF ST VINCENT SPENCER GULF

0

..... E w >

a -100

-50

!! w Ul

-1 50 0 25 50 75 100 125 150

Y E A R S x l d

Fig. 15. Generalized late Quaternary sea level curve (from Chappell & Shackleton, 1986) showing sea level highstands from Spencer Gulf (Hails et al , 1984) and Gulf St Vincent (Cann et al., 1988). The depth of the open shelf plain on the Lacepede Shelf (stippled) is superimposed on this curve to illustrate its relationship to fluctuating sea level. Note that Cann er al. (1988) reported significantly higher stage 3 sea levels than depicted by Chappell & Shackleton (1986).

When the depth of the extensive open shelf plain, between -40 and - 60 m, is superimposed on the sea level curve for southern Australia, it is clear that most of the shelf has been at or just below sea level for much of late Pleistocene time (isotope stages 3 and 4; Shackleton & Opdyke, 1976; Shackleton, 1986). Cann et al. (1988) recognized highstands at 40 ka (- 22.5 m) and 31 ka (-22 m) with an intervening lowstand at 36 ka during which sea level fell to - 28 m in Gulf St Vincent. The Lacepede Shelf during this period would have been a shallow, relatively low energy shelf with most wave action damped out by the shallow sea floor at the shelf edge. The strandline would still have been located close to its present position because of the steep shoreface. Shelf environments would have ranged from strandline to lagoon to shallow open shelf.

Sprigg (1979) identified a prominent nickpoint (the SAORI submerged coast) at - 100 to - 11 1 m along southern Australia, and interpreted it as a bench eroded during the major lowstand in sea level between 18 and 20 ka (isotope stage 2; Shackleton & Opdyke, 1976). The submarine topography between - 130 and - 110 m resembles the modern profile of the Coorong Strand, and is similarly interpreted here as a palaeo- coastal dune complex and correlated with the late Pleistocene lowstand at 18-20 ka. Lowstand features associated with this last major glaciation have been identified at similar depths off Queensland, Australia, and South Island, New Zealand (Carter et al., 1986).

The course of the lower reaches of the River Murray

is along the western side of the Murray Basin, because of uplift along the Mt Gambier axis. Seismic profiles confirm that the River Murray palaeochannel is buried beneath modern sediments along the north- western side of the Lacepede Shelf. Thus, during lowstands, the main water flow was across the shelf and into the Murray submarine canyon system. Whilst much sediment was probably funnelled into deep water via these canyons, if the past is any guide (Fig. l), sand was also blown out of the channel eastward across the shelf as extensive sand sheets. Such aeolian transport would have been enhanced by the northward shift of the ‘roaring forties’ low pressure systems and increased sediment load brought on by swollen runoff in the source area to the north. The lack of quartz in any sediments in waters deeper than 70 m also suggests that offshelf sediment transport was restricted to channels. The terrigenous sands which blanket the open shelf plain (facies 1A) are thus interpreted to be largely relict, spread across a flat exposed shelf by fluvial/aeolian action during shelf exposure.

Holocene transgression

The relatively level shelf sea floor and lack of much topographic relief, except locally on the Bonney Shelf, suggests that the Holocene transgression reworked much of the late Pleistocene sediment, leaving a ravinement surface and a bevelled sea floor. This flatness is enhanced by modern swells which continuously sweep the shelf plain. Relict terrigenous clastics swept out across the shelf plain during previous lowstands have been mixed with relict carbonate grains from interstadial highstands and together form the substrate for modern, largely infaunal bivalves. Palaeostrandline deposits (facies 2C) contain both late Pleistocene and Holocene bivalves and as such probably represent old nearshore complexes populated by younger Holocene communities. Coralline algae and bryozoa flourish only on bedrock highs above the sediment plain.

The outer shelf below a depth of 70m was submerged for most of late Pleistocene time. The proportion of obvious relict particles is low and the sea floor, subject to less agitated conditions than shallower environments, is the site of mostly bryozoa carbonate production. The sediments are actively moved by waves and swells and it is a zone of carbonate sand accumulation : any mud produced here is probably winnowed out and deposited in deeper water.

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DISCUSSION

Sedimentary facies

Shelf facies in water depths of less than 80 m are palimpsest. All sediments contain quartz and in most areas the carbonate is at least 50% relict. Since there appears to be little transport of quartzose sand onto the shelf today this means that at most these sediments have 45%, and usually less, ‘modem’ carbonate material. Linear zones and diffuse areas of lithoclast gravel at depths of c.40 and 65 m are interpreted as palaeoshoreline deposits. The largest amounts of Holocene carbonate are on the outer shelf in deeper water, which was submerged for proportionally longer periods during late Quaternary time compared to the mid- and inner shelf.

There is a dramatic change at a depth of about 80 m on the Lacepede Shelf where the outer shelf bryozoa facies, which contains insignificant quartz and less than 20% relict particles, is essentially modern. This facies can be thought of as the ‘outer shelf carbonate factory’, equivalent in style to a tropical barrier reef, but manifest here as a sand facies in much deeper water. This distinction is less obvious on the Bonney Shelf, where outer shelf sediments contain more quartz and relict grains, probably because the shelf is narrower and closer to land.

This deep water bryozoa carbonate factory is broken into high and low energy facies by swellbase at about -140m. Above this depth sediments are coarse grained, abraded and conspicuously rippled with robust bryozoa growing on hard substrates and in isolated patches. Below this depth sediments are distinctly muddy and delicate bryozoa are more common. The sea floor between - 350 and - 500 m is a transition zone with muddy bryozoa carbonates grading into pelagic carbonate muds.

This broad distribution of modem major carbonate grain types (bivalve-rich inner shelf, bivalve- and bryozoa-rich middle shelf and bryozoa-dominated outer shelf to upper slope) is like that on New Zealand shelves (Nelsonet al., 1981, 1988; Carter etal., 1985). Tnfaunal bivalves appear to be most prevalent in the terrigenous inner shelf facies whereas epifaunal bivalves are more common on the outer shelf. The most productive open shelf environments in terms of carbonate sediments are bedrock highs where coralline algae and bryozoa grow in abundance. The overall pattern is also similar to that off south-westem Australia, except that on the Rottnest Shelf an outer shelf band of coralline algal sand separates an inner

shelf terrigenous sand facies (related to the Swan River) from a shelf edge bryozoa facies (Collins, 1988). The facies pattern is unlike that summarized for cool water shelves and banks in the North Atlantic (Wilson, 1979; Scoffin et a[., 1980; Scoffin & Bowes, 1988) where barnacles are significant, particularly in shallow water; serpulids are abundant in mid-shelf depths; bryozoa, while present, are not important sediment producers across the environmental spectrum; Lophelia coral patches are present on the slope.

Wave energy

Apart from the lack of ‘reef-building’ skeletal metazoans, reflecting cool waters of the Southern Ocean, the high sea state is the most critical parameter determining sedimentary facies on this shelf. This seems to be most important in depths shallower than 70m, where sediments not actively colonized by grasses are moved constantly, constricting calcareous benthos to infaunal and rocky habitats. It is difficult to determine the depth of ‘wave abrasion’ as on the Rottnest Shelf (cf. Collins, 1988) because the sediments are terrigenous and not lithified carbonates. Regardless, sediment movement is important to depths of 140 m, as indicated by physical sedimentary structures and abraded particles. Although bryozoa grow throughout, their sharp increase in importance below a depth of 60 m suggests that - 60 m is a good approximation of the depth of wave abrasion.

It is the action of swells on the sea floor that appears to have partitioned the shelf into three major environments (Fig. 16), which, in a general way, are related to the shelf morphology: (1) the shallow sheK less than 60 m, a zone of minor carbonate sediment accumulation; (2) the middle shelf, -60 to - 140 m, a zone of active sediment accumulation in the form of coarse grained cross-bedded sands; and (3) the deep shelf edge, - 140 to -250 m, a zone of muddy carbonate accumulation. Thus, except for rocky submarine islands of high productivity, significant sediment accumulation begins below 60m as a region of carbonate sands, much deeper than is usually envis- aged for carbonate shelves. Similarly, muddy sediments are not important until depths of 140m or more. The critical interface here is not sea level, but the depth of wave abrasion; significant accretion of carbonate does not take place until this depth is exceeded. If carbonate sediment accretion rates are measured in shallow water they will be orders of magnitude less than those measured at intermediate depths.


I

Fig. 16. A summary cross-section of the shelves, illustrating the principal modem sediment facies, depth of swellbase and probable depth of wave abrasion, and fluctuations in late Quaternary sea level. Note that most of the shelf was not exposed but covered with shallow seawater throughout much late Quaternary time.

An implication of this concept is that the platforms are not ‘drowned’ (cf. Simone & Caranante, 1988). Sediment accumulation, but not production, is merely reduced or arrested in shallow water by high energy. High carbonate productivity is taking place in shallow water in nearby gulfs and embayments protected from Ocean swells (Gostin et al., 1988). Environments on the high energy shelf are very restricted for epifaunal organisms such as bryozoa. Even those grains that are produced are quickly abraded or swept landward. Whilst warm water platforms ‘drown’ because the depth of the shelf falls below the critical depth for carbonate phototrophs, cool water carbonate shelves, even at depths of several hundred metres, will still be productive because the organisms I are non-phototrophic. On open, high energy, cool water shelves it is probably shallow water, not deep water, that is inimical to accretion.

Holocene carbonates

The Holocene cool water carbonates in this region are produced largely by non-phototrophic organisms, namely bryozoa, foraminifera and molluscs with phototrophs such as coralline algae important only in shallow water (< 70 m). The volumetrically most important carbonate-producing organisms are bry-

ozoa which live in significant numbers to depths of 250m. These Holocene cool water carbonate sediments do not show as marked compositional variations as do warm water carbonates. This is probably why it has proven so difficult to assign detailed facies to, and reconstruct water depths for, Cenozoic cool water limestones in general.

The crux of the problem is that even the shallow shelf is relatively deep and sediments are produced largely by the same organisms across the environmental spectrum. Bryozoa seem to have a broad environmental tolerance and only in deep water’s are the separations into different types clear. Except for corallines in shallow water, and abundant planktonic elements in deep water, the differences between sediment types are subtle. Separation into grainy and muddy facies is a function of the extraordinarily high energy setting. The depth of wave base, and hence the depth of muddy sediments, in other modern and ancient settings will be a function of the meteorologi- cal/physical oceanographic state of the region, and could be much shallower.

Most abundant organism growth is localized to hard or coarse sediment substrates (cf. Nelson et al., 1988; Collins, 1988; Scoffin, 1988), highlighting the importance of antecedent topography and early lithification in carbonate environments. It is not yet


clear whether lithification is widespread but the presence of intergranular cements and overall high energy suggests it might be, especially in the zone of clinoforms between - 70 and - 140 m. Nevertheless, the abundance of living bryozoa in shifting sand and muddy substrates indicates that while not as obvious, these environments are also regions of high sediment productivity.

Terrigenous clastic sediments

The present outlet of the River Murray can be classified as a wave-dominated delta (cf. Galloway, 1975) in a region of microtidal range, whose front is formed by beach barrier shorelines of the Coorong Strand and whose delta plain is the strandplain of palaeodune complexes to the north (Fig. 1). The combination of Pleistocene uplift and arid climate in the lower reaches of the river system has resulted in a small sediment load. Meanwhile, the barrier beach system is as much constructed from offshore-derived carbonate as river-borne terrigenous sand. It is then, for all practical purposes, a ‘failed delta’ in which what little terrigenous sediment is delivered to the ocean is plastered against the shoreline and not transported offshore.

Nevertheless, the extensive relict terrigenous sand sheets on the shelf attest to the importance of the River Murray in delivering sediment to this region over the long term and confirm that during lower stands of sea level, terrigenous sands were spread across the now submerged delta plain. Even when sea level was low the same processes would be operative; terrigenous clastics would be largely trapped along the shoreline and only spill over the edge via submarine canyons. Thus, viewed in dynamic terms, it is a modern example of ‘reciprocal sedimentation’ (Meis- sner, 1972). In this case it is the energetic wave climate which keeps the terrigenous clastic sediments along the shoreline during highstands and the extensive lowstand shelf terrigenous clastics are probably as much a function of aeolian as fluvial processes (cf. Mazzulo et al., 1991).

Mixed relict and modem sediments

Relict carbonates, similar in composition to modern grains, but stained and impregnated with iron oxides, Mg-calcite cements and/or glauconite, are late Pleis- tocene in age and related to earlier sealevel highstands. The reason for the presence of mixed modern and relict sediments would seem to be a complex function

of mineralogy, wave energy, lack of reef-builders and shelf depth. The shelf is relatively deep (-40 to - 60 m) and so, with Pleistocene sea level fluctuations, has been partially to completely exposed and submerged at intervals of 5-10 ka for the last 125 000 years at least (Fig. 15). Thus, any period of carbonate deposition was short and followed quickly by exposure, which was, in turn, of relatively short duration, allowing for formation of only rudimentary calcrete and little meteoric cementation before renewed inun- dation. Furthermore, since the grains were mostly calcitic, there was little mineralogical contrast to drive cementation (cf. James & Bone, 1989). Lack of reef- builders kept the shelf margin ‘open’ and allowed high energy waves to sweep across the shelf, shifting the bottom sediments and mixing grains produced during the previous highstand with newly generated particles. Lack of such mixing on tropical shelves is because they are shallow and so have been sites of exposure and non-deposition since 80 ka, resulting in meteoric cementation and pedogenic alteration of the aragonite-rich sediments.

SUMMARY AND CONCLUSIONS

The Lacepede Shelf and Bonney Shelf are high energy settings with the depth of wave abrasion at approximately -60 m and swell base at about - 140 m. Sediments on these open shelves are palimpsest, a mixture of modern carbonate and relict terrigenous and carbonate particles. They are an integrated product of late Pleistocene and Holocene carbonate deposition, diagenesis under variable sea level conditions, terrigenous clastic deposition from the River Murray during sea level lowstands and Holocene reworking by waves and swells.

(1) Modern carbonate production, principally by bryozoa, bivalves and benthic foraminifera (and coralline algae on the shelf in less than 70 m of water), is taking place to depths of at least 400 m. Bryozoa live in abundance to - 350 m and occur conspicuously to -400m. They can be grouped into four depth- related assemblages useful in palaeoenvironmental interpretation : (1) Shelf-robust, Adeona-rich (< 110 m); (2)shelfedgeaiverse(ll0-200 m); (3)top of slope-delicate branching (200-250 m); and (4) upper slope-branching cyclostome (250-400 m +).

(2) Significant modem sediment accumulation is only taking place in protected gulfs and below depths of 60m. Sedimentation is arrested on the shallow, open shelf by high energy waves and swells that

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Cool water sediments, Lacepede SheK southern Australia 90 1

continuously move the grains, cause particle abrasion and disintegration and which transport grains away from the environments of production. The deep outer shelf and slope are together an active bryozoa factory which both produces sediment and probably receives sediment from the shallower shelf. These deeper zones are partitioned by swellbase into shallow water, cross- bedded, bryozoa grainstone facies and deep water, burrowed, bryozoa bafflestone to floatstone facies.

(3) The interaction of fluctuating Quaternary sea level, variable carbonate sediment production and transport of terrigenous clastics onto the shelf has resulted in four major depositional facies.

Open shelfpalimpsest calcareous and quartzose sands (40-60 m) . This vast, flat region has been subject to multiple periods of submergence and exposure over the last 80000 years. It is interpreted as presently being within the zone of wave abrasion. Sediments are dominated by a major lobe of quartzose sand in the centre of the Lacepede Shelf which was deposited during the last sea level lowstand and reworked during the subsequent transgression and is now continuously moved by strong waves and populated by scarce infaunal bivalves. Thislobe is surrounded by quartzose sediment rich in Holocene and relict carbonate, of primarily bryozoa, foraminifera and mollusc skeletons which extends southward along the Bonney Shelf. Palaeostrandlines form linear ridges throughout, especially at depths of 60 and 40 m. Bedrock highs are sites of prolific carbonate production, primarily coralline algae, which are shed onto the surrounding sea floor.

Outer shelf (60-140 m). This gently inclined sea floor is mostly above swellbase and so is of high energy. It is characterized by seaward-dipping clinoforms and sand waves, was exposed during the last major fall in sea level and is today the site of extensive bryozoa sediment deposition, with little admixed relict carbonate or terrigenous clastic material (except along the Bonney Shelf).

Deep shelf edge (140-250 m). Grainy sediments pass downward into muddy, poorly sorted, burrowed, diverse, bryozoa-rich sediments produced largely in situ; a deep water carbonate factory.

Upper slope (250-350 m). These deep settings are dominated by muddy, burrowed, cyclostome-rich sediments.

Slope ( > 350 m) . The upper part of this incline is covered by muddy, mixed bryozoa-rich and pelagic sediments which grade quickly, with increasing water depth, into fine grained pelagic muds. The slope is locally dissected by submarine canyons.

(4) The striking parallel between this region and the narrower Rottnest Shelf off south-westem Aus- tralia (Collins, 1988), in terms of general oceanographic setting, facies patterns and depositional history, points to similar facies models for this swell- dominated style of cool water shelf. The major difference seems to be the influence of the River Murray on the Lacepede Shelf which, by providing a blanket of terrigenous dastics during lowstands, altered the nature of the substrate for subsequent highstand deposition.

(5) There are several major implications, useful for the interpretation of ancient carbonates. For unrimmed platforms, apart from facies disposition, non- phototrophic, benthic carbonate production can be significant to a depth of 350 m. Cross-bedded carbonate sands can occur to depths of at least 140 m. The shallow shelf can be an area of little deposition, in spite of high potential productivity. If early cementation does not take place, mixing of shelf sediments formed during several sequential highstands can take place because of the high energy conditions.

Such cool water carbonate shelves may be good modem analogues for many ancient carbonate and carbonate-siliciclastic deposits. The controls on sedimentary facies are fundamental for the interpretation of Cenozoic cool water carbonates. For older carbonates the parallel lies not so much in the fact that the ancient carbonates may have originated in cool water, but that modem cool water carbonates are good proxies for periods in geological history when reef- builders were absent and shelves and platforms were unrimmed.

ACKNOWLEDGMENTS

This research is funded by the Australiap Research Council, Natural Sciences and Engineering Research Council of Canada and Flinders University Research Budget. We thank CSIRO Division of Oceanography and the captains and crew of the R.V. Franklin for enormous support during both cruises. E. Bamett, A. Belperio, T. Boreen, V. Drapala, K. Gaard, G. Heinson, M. Fuller, R. Rice and A. White provided invaluable shipboard help and expertise. E. Bleys designed and built the sampling dredges. Underwater cameras were kindly lent by R. Carter and J. Kean. K. Gowlett-Holmes identified some of the molluscs. B. Wommersley identified many of the algae. C. Bone and J. Rolefson-Ah1 aided in sample preparation and drafting. The manuscript was critically read by A. P. Belperio and T. Boreen.

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Cool water sediments, Lacepede ShelJ southern Australia 903

(Manuscript received 25 February 1992; revision received I1 June 1992)

James Et Al., 1992 - Modem Carbonate and Terrig Sediments, Cool Water, High Energy, Shelf, Australia

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