dune sequences, western Australia

Sedimentary Geology, 32 (1982) 301--328

301

Elsevier Scientific Publishing Company, Amsterdam--Printed in the Netherlands

RECENT AND PLEISTOCENE AUSTRALIA

BEACH/DUNE

SEQUENCES,

WESTERN

V. SEMENIUK and D.P. JOHNSON

21 Glenmere Road, Warwick, WA 6024 (Australia) Dept. of Geology, James Cook University of North Queensland, Townsville, QLD 4811 (Australia) (Received April 14, 1981; revised and accepted February 23, 1982)

ABSTRACT Semeniuk, V. and Johnson, D.P., 1982. Recent and Pleistocene beach/dune sequences, Western Australia. Sediment. Geol., 32: 301--328. Wave-dominated sandy shores occur along much of the coast of Western Australia. Despite local variations there is a characteristic distribution of lithofacies (corresponding to different geomorphic zones). Five lithofacies are recognised: (1) trough-bedded sand/ gravel; (2) laminated sand; (3) laminated/bubble sand; ( 4 ) l a m i n a t e d / d i s r u p t e d sand; and (5) aeolian cross-stratified sand. The trough-bedded sand/gravel lithofacies is being deposited in the shallow shoreface below LWL. The laminated sand and laminated/bubble sand lithofacies are sands with gravel layers being deposited on the foreshore swash zone; extensive bubble (or vesicular) sand is common towards HWL especially in berms. The laminated/disrupted sand lithofacies is being deposited on the backshore between HWL and storm water levels and consists of horizontally layered to homogeneous sands with storm debris, especially wood, weed and floatable skeletons (e.g. Sepla and Spirula). The aeolian cross-stratified sand lithofacies is forming in beach ridge/dune areas and consists of fine sands with large-scale, generally landward-dipping forests; soils and rootlets are common. Recognition of these lithofacies within a sedimentary sequence enables reconstruction o f gross shoreline conditions in terms of wave and eolian environments, tidal and storm heights, and palaeogeography. Each of these lithofacies with their characteristic features is recognised in Pleistocene sequences in Perth Basin. The Pleistocene sequences fit a model of coastal progradation with the trough-bedded sand/gravel lithofacies at the base and the aeolian sand lithofacies at the top. The value of such a stratigraphic sequence, however, extends beyond the Pleistocene.

INTRODUCTION

Coastal processes are effective agents in sorting sediments, shaping sedimentary bodies and generating sedimentary structures. Deposition of coastal sedimentary bodies occurs under a wide range of conditions and environments, including high- to low-energy environments, wave
302

dominated conditions, clastic or biogenic materials, temperate to tropical climates. The dynamics of these processes have been documented and summarised by many authors (e.g. Manohar, 1955; Ingle, 1966; Meyer, 1972; Hails and Cart, 1975; Komar, 1976). The distribution of sedimentary structures also has been well reported (e.g. Thompson, 1937; Land, 1964; Clifton, 1969; Clifton et al., 1971; Howard and Reineck, 1972; DavidsonArnott and Greenwood, 1976; Howard, 1976). Generally each paper has concentrated either on textures and structures from a limited zone, or on particular types of structures. Integrated views of the coastal stratigraphic and structural sequences developed are n o t so well described. Kumar and Sanders (1975) documented the sequence deposited by a migrating tidal inlet. The works of Kraft (1971), Beall (1968), Curray and Moore (1964) and Davies et al. (1971), are further examples where gross coastal stratigraphy has been described and explained. This paper describes the stratigraphic sequence of sedimentary structures of beach coasts with aeolian hinterland. Widely-separated studies of beach/dune systems have emphasised the essential similarities of such sequences. The results presented here extend the work of Thompson (1937) and Clifton et al. (1971) in that subtidal and foreshore (Komar, 1976) facies of the beach environment are placed within a broader stratigraphic framework of a prograded sequence in which there are overlying storm and aeolian deposits. The paper is in three sections. Firstly the coastal setting and sequences of structures on the modern beach to dune areas are described; secondly a Pleistocene sequence is described; and thirdly conclusions are drawn to aid the understanding of ancient sequences. METHODS

Five main areas were studied on the coast of Western Australia at Albany, Bunbury, Carnarvon, Dampier and Perth (Fig. 1). In addition, reconnaissance surveys were made at numerous other beaches that include Busselton, Yanchep and Lancelin (Fig. 1). At each location a profile was surveyed from shallow subtidal (and within the zone of breaking waves) to the beach ridges and dunes. Bedforms across the profile were d o c u m e n t e d with respect to both the tidal level and the swash zone for the wave train on the particular day that the profile was established. Surface sediments, i.e. only upper layers that were visibly mobilised by wave action, were collected for standard sieve analyses. Areas were revisited over several seasons to d o c u m e n t summer vs. winter profiles. Bunbury and Perth areas also were revisited after major storms to d o c u m e n t the modified profile and the sedimentary material that had been deposited. Sedimentary structures were studied by splitting intact blocks and cores of sediments collected with box corer or plastic tubing. Nine sections of indurated Pleistocene sediment were studied at wave-

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304

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REGIONAL SETTING

The study areas lie on exposed portions o f the Western Australian coastline in climatic settings that range from subtropic-humid to tropical and semi-arid (Gentilli, 1972}. The areas are either small embayments or extensive sandy coasts and face a continental shelf that slopes to water depths in

Coastline borders Precambrian rock hinterland; beaches face SW, S and SE and abut Precambrian rock, or line small embayments

Coastline borders an extensive coastal plain; beaches face N, N W or S W and abut Pleistocenee sediments and limestone, or line small embayments, or form part of a tombolo system

Coastline borders a prograding complex of the Gascoyne river delta; beaches face W

Coastline borders Precambrian rock hinterland; beach aspect is developed for all compass bearings; beaches abut Precambrian rock, or are developed in embayments, or form part of tombolo systems which connect various islands

Albany

Perth and Bunbury

Carnarvon

Dampier Archipelago

Subtropical humid

Subtropical humid to subhumid

Subtropical semi-arid

Tropical arid

Deposition with embayments and tombolos; locally shoreline is stationary or retreating. Holocene deposition is occurring within the Perth Basin Deposition occurs within a prograding deltaic system in the Carnarvon Basin Deposition occurs within embayments but best developed sequences are formed in tombolo settings

Climate

Deposition within embayments cut into Precambrian shield; progradation of shoreline locally occurs

Geological setting

Depending on aspect and position within the Archipelago (inner or outer), receives swell and/or locally generated wind/ waves

Locally generated wind waves

Depending on absence or presence of offshore islands and rocky ridges, receives swell and/or locally generated wind waves

Oceanic swell and locally generated wind waves

Types of waves impinging on shore

* Data from MacArthur and Bettenay, 1960; Seddon, 1972; Johnson, 1974; Woodside Petroleum Development, 1978; Semeniuk and Meagher, 1981; and personal observations,

Physiographic setting of beaches/dunes

Area

Regional setting of main study sites *

TABLE I

c~ c~ c~

From W and SW (1.5--8.0 m/sec) in winter; from E, SE S and SW (3.0--11.0 m/sec) in summer

Winter storms (2--4/yr) from NW to SW produce winds up to 20 m/ sec; relatively light wind (<:10 m/sec) during remainder of winter; NW to SW seabreezes {up to 15 m/sec) are dominant in summer

Pattern similar to Perth/Bunbury but with more southerly summer seabreezes

Winds originate from E and SE in winter and from W and SW in summer; on annual average basis only 30% of wind exceeds 7 m/sec

Albany

Perth and Bunbury

Carnarvon

Dampier Archipelago

Wave heights are small, generally ~1.3 m

Waves 'enerated by seabreezes are small, mainly ~1.5 m height [

Wave heights average 2 m during winter and 1 m during summer

Wave heights up tolm

Long-period swell originates from N and NW; exposed portions of outer islands receive much of this swell

Protected from swell by offshore islands and barriers

Near 80% of max. swell wave heights are 1.5--4.0 m with periods 5--8 see; deep water max. wave heights often reach 6--7 m with periods of 8--10 sec. during winter storms

Swell

Wind generated waves

d

I

Mixed tidal pattern; neap tides are diurnal (0.3 m) spring tides are semidiurnal (0.9 m) Macrotidal, semidiurnal; spring range is 3.6 m; neap range is 1 m

#

Microtidal with irregular diurnal tides (max. 1.4 m)

Tides

Cyclones generated in tropical areas further north in summer months, periodically pass through these areas

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Other oceanographic/meteorologic features

* Data from Easton, 1970; Logan and Cebulski, 1970; Seddon, 1972; Johnson, 1974; Bureau of Meteorology, 1975; Steedman, 1977; Woodside Petroleum Development, 1978; Steedman and Craig, 1979; Semeniuk and Meagher, 1981, pers. commun. Harbour Master Albany, 1979; Dept. of Defence, 1980; and personal observations.

Wind

Area

TABLE II. Summary of oceanography/meteorology of main study sites * ¢,o O O~

307

excess of 100 m. They encompass sedimentary environments that are regionally prograding to those that are only locally prograding to those that are retreating. All locations receive swell and/or locally generated wind waves. The incident wave character depends on the presence or absence of islands and offshore submerged r o c k y ridges. Ocean swell m a y pass largely unhindered into shallow water and impinge on the coast. However, in many localities (e.g. between Carnarvon and Bunbury) there is a line of shallowly submerged rocky ridges or islands situated up to several kilometres offshore which dissipate the energy of swell. Thus, waves that impinge on these shores are dominantly those generated b y local wind systems and wave direction follows wind direction; the energy regime is lower than that of coasts exposed to swell. Bathymetric charts and aerial photography indicate that 40% of the coast between Geraldton and Bunbury offers no hindrance to swell. Lengthy tracts of coast have lines of unbroken offshore ridges for up to 70 km. A b o u t 60% of the coast has offshore rocky ridges and thus locally generated wind waves b e c o m e more important. However, in the c o n t e x t of these beaches the differentiation merely is a spectrum of energy types within essentially a wave-dominated environment. Tidal influence ranges from microtidal (Perth and Bunbury) to macrotidal (Dampier). The areas are described in Tables I and II in terms of geologic setting, winds and oceanography to give a perspective of the range of coastal settings studied. MODERN BEACH/DUNE SEQUENCES

Modern sandy coastlines of Western Australia m a y be subdivided into four geomorphic zones generating five lithofacies. While the regional stratigraphy in each area varies according to the depositional system (e.g. coastal sandplain at Bunbury--Perth vs. deltaic at Carnarvon), the structures generated and their stratigraphic sequence are the same. Comparison with material described in the literature substantiates this view. This study concentrates on the stratigraphic sequence of structures, since textural and compositional features (e.g. grainsize) vary according to depositional setting and sediment source in each area (Tables III and IV). The shoreline area is considered in four zones: (1) the subtidal shoreface below mean low-water level; (2) the intertidal beach foreshore between mean low-water and mean high-water levels; (3) the beach backshore b e t w e e n mean high-water and storm highwater levels; and (4) the beach ridge~dune landward of the beach (Figs. 3 and 4). The relative width normal to the shoreline and extent parallel to the shoreline of each zone are dynamic. Beach profiles and widths change due to repetitive alternation of storm and calm conditions, which m a y be cyclic and seasonal (Shepard, 1963; Zenkovitch, 1967; King, 1972; and others), or be irregularly spaced in time and location.

Fine and medium quartz sand dominant; coarse sand skeletal debris and shell gravel also present

Fine, medium and coarse quartz and skeletal sand; shell and lithoclast gravel locally present

Medium and coarse quartz sand is dominant; locally shelly

Medium and coarse quartz and skeletal sand; locally shell and lithoclast gravel

Perth and Bunbury

Carnarvon

Dampier Archipelago

Trough-bedded sand/gravel

Lithofacies

Albany

Area

Grain types of beach/dune sediments

TABLE III

Fine and medium quartz sand; coarse sand skeletal debris and shell gravel

Laminated/disrupted sand

Medium and coarse quartz and skeletal sand

Medium and coarse quartz sand; locally shelly

Fine, medium and coarse quartz and skeletal sand is dominant; shell and lithoclast gravel is minor

Fine and medium quartz sand dominant

Laminated sand and laminated bubble sand

Fine and medium quartz and skeletal sand

Fine and medium quartz sand

Fine and medium quartz and skeletal sand

Fine and medium quartz sand

Aeolian crossstratified sand

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• Vertical polychaete tubes near LWL

• Heavy mineral layers, often

• Soils and rootlets

on minor disconforn

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• Ridge-and-runnel,

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• Corroded skeletal

parallel lamination

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• Gravel mounds and

debris

• Ocypode burrows

layers

• Bubble sand, especially in berm and near HWL

Fig. 4. Sedimentary structures across an idealised, modern beach to beach-ridge transect.

SHOREFACEZONE

Morphology The term shoreface is used here to denote those environments always below the low-water level (Emery, 1960). The zone extends d o w n to depths in excess o f 3 m. The form of the shoreface is variable: it may drop from the low-tide level steeply into deeper water with or w i t h o u t bars and longshore troughs (Fig. 3); alternatively, there may be a gently sloping sand flat with a steeper outer edge. The b o t t o m usually is covered with ripples or small megaripples o f parallel, lunate or interference forms. Plane beds normally occur under

313 the surf zone (cf. Clifton et al., 1971). Bivalves, other skeletal debris and gravel are c o m m o n l y present in a longshore trough or scattered on the sandflat. Skeletal debris is derived mainly from in~itu infauna, but nearby rock platform or offshore assemblages also contribute.

Sedimentation The shoreface environment is dominated by wave action and littoral currents and is the environment of the trough-bedded sand~gravel lithofacies. Intense winnowing removes fine sediment and leaves lags of gravel, and the trough lithofacies consists of m e d i u m and coarse sand, with small pockets and layers of gravel, or coarse sandy gravel (Fig. 4). Gravelly lenses 0.3--0.5 m thick, are oriented parallel to shore. Small-scale trough crossstrata are the dominant bedding type and cross-strata m a y be oriented landward or seaward. Biogenic structures were not found. Intense physical reworking would destroy burrows which do form. Clifton et al. (1971) and Davidson-Arnott and Greenwood (1976) also described trough cross,strata in the immediately subtidal zone from near-shore environments. FORESHOREZONE

Morphology The foreshore slopes evenly to low-tide level where there c o m m o n l y is a break in slope and a small trough. The surface is smooth apart from features such as drainage rills, rhomboid marks and scours tapering downslope of large objects such as shells or seagrass clumps. Ridge and runnel forms also m a y occur. The substrate is often spongy due to bubble sand (Hoyt and Henry, 1964) which is best developed in the berm along the upper edge of the foreshore.

Sedimentation Foreshore deposits, termed the laminated-sand lithofacies and laminated/ bubble sand lithofacies are bedded and laminated sand, with gravel and shell in layers oriented parallel to layering. Large-scale structures consist of wedge-shaped sets of subhorizontal continuous parallel lamination (cf. Thompson, 1937). Each set has a low dip (Fig. 4 and 5D), generally seaward, and it truncates the underlying set (Fig. 4). Within the laminated-sand lithofacies laminae are defined by variation in composition (quartz, skeletal grains, shell gravel and dark minerals) and grain-size (medium, coarse and rarely, fine sand). Shell and lithoclast gravel layers form as lags after winnowing or accumulate during storms. Gravel bodies, up to 0.5 m thick and several metres across, trailing into thin layers, are developed periodically, generally located on cusp horns. Biogenic struc-

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315 tures are rare except near low-tide level; here worms form vertical burrows 1--2 mm across with a thin m u d or mucous-lining and with small d o w n w a r d depressions in surrounding laminae that distinguish them from air-escape vents. Swash bars occur parallel to the shoreline; they have a steeper face upslope and the downslope face merges with the parallel laminations. These bars originate on the low-tide terrace during periods of decreasing wave activity, and migrate over several tidal cycles up the foreshore to the berm. Generally, the bar is dispersed by later swash activity b u t it m a y be preserved (cf. Pleistocene sequences). A laminated~bubble sand lithofacies is developed in the upper part of the foreshore and consists of interbedded (centimetre to decimetre thick) sheets and lenses of laminated sand and bubble sand (or 'vesicular' sand). Air bubbles trapped in the sediment form r o u n d e d holes 1--4 mm across which disrupt lamination and develop vesicular structure (cf. Emery, 1945); there also are 1--4 mm diameter pipes, up to 1 cm long, that are air-escape vents (Palmer, 1928). Bubble sand also forms in the lower part of the swash zone b u t tends to be preserved only in the u p p p e r part, probably because: (1) sediment lower d o w n is more often saturated and reworked; and (2) air migrates through the sand during a rising tide and accumulates at the berm. The gently dipping sets of parallel lamination which form in the foreshore swash zone have long been recognised as characteristic of this environment (Thompson, 1937; Clifton, 1969}. The laminae follow the beach surface; on cusped beaches large festoons characteristically form between the cusps (Thompson, 1937; Clifton, 1969). Clifton ( 1 9 6 9 ) c o n c l u d e d the laminae formed by differentiation in a high
Morphology The backshore extends up to the slope break at the f o o t of the beach ridge. The zone is c o m m o n l y marked b y a linear deposit of storm debris, mainly wood, seaweed and marine skeletons. The seaward edge of the backshore zone is marked b y a change in slope or b y a small cliff (<1 m high). The surface of the backshore generally is flatter than the foreshore and may slope landward as a washover into a small depression in front of the beach ridge. Alternatively the backshore may contain one or more older berms modified b y later storm or wind action. The backshore surface is generally dry and the sand soft since it is inundated only during storms above normal high tide level. Thus it is subject to more continuous wind action and c o m m o n l y is covered b y aeolian ripples. There m a y be scattered tussocks of dune vegetation. Crab burrows (Ocypode sp.) occur, particularly near the f o o t of the beach ridge.

316

Sedimentation The backshore deposits, termed laminated~disrupted sand lithofacies consist of fine to coarse sand with irregular gravel deposits derived from storms. Parallel bedding and lamination and small-scale cross-strata are c o m m o n b u t disrupted and homogenous structures also occur. Bedding is generally horizontal in contrast to stratigraphic units above and below. Minor disconformities and laminations marked b y heavy minerals are c o m m o n (Fig. 4); deformation structures are present locally (Fig. 5C); bubble sand is absent; Ocypode sp. burrows (up to 1 m deep) occur. The backshore is an environment of intermittent storm and high springtide deposition. It is subject to extended periods of wind action which winnow sediment, form aeolian bedding structures and pile sand drifts around obstacles such as storm debris. Erosion during storms or by winds truncate earlier strata, and subsequent deposition forms minor erosional interfaces. Storm debris is incorporated into the backshore deposits (Fig. 4). The debris characteristically is material which floats or is easily carried in the water, and is left behind as storm waters rapidly retreat. There is w o o d brought down by rivers, seagrasses and algae, and marine skeletons which include sponges, mollusks, cuttlefish (Sepia) and the planktonic cephalopod Spirula spirula. Other skeletal debris attached to seagrasses and driftwood includes encrusting bryozoa, coralline algae, foraminifera and goose barnacles (Lepas anatifera). In the aerated beach environments most of the organic materials decay and produce disrupted bedding in the process; concentrations of skeletal epibionts, bivalves, cuttlefish and Spirula spirula shells are left, and these are characteristic of the backshore deposits. Similar deposits occur also at lower beach levels b u t are subject to reworking and remobilisation and hence are rarely incorporated into the sediment. BEACH RIDGE/DUNE ZONE

Morphology The beach ridge/dune zone rises 3--10 m above storm-tide level. The seaward face of the ridge may be smooth or rippled. Ocypode sp. burrows are common. The ridge behind the present beach is generally bare though those further inland become progressively vegetated. Where there have been blowouts, active dune fields m a y extend inland for 5 km. In the Perth Basin and Albany areas beach ridges pass immediately into dunes. In the Carnarvon area the beach ridges are overlain by alluvial muds of the Gascoyne delta (Johnson, 1974).

Sedimentation Beach ridge and dune deposits, termed aeolian cross-stratified sand lithofacies, consist of fine and medium sand, generally finer than that of the

317

beach. These form thick wedges of large-scale cross-strata dipping landward and seaward, and large scale festoons (Fig. 5B) where sand filled a beach ridge hollow. Bounding surfaces observed immediately above beach deposits are planar. Cliff sections and trenches exposing internal sections of large landward moving dunes show curved bounding surfaces (cf. McKee, 1966) and intercalated incipient soil sheets. Ocypode sp. and ant burrows occur. Rootlets and soils occur when the ridge has been stabilised for sufficient time. Skeletal grains near the surface are commonly corroded (e.g. gastropods reduced to 'corkscrews') or completely leached from humic soils. SEQUENCE GENERATED

Holocene sedimentation has resulted in a prograding sequence in embayment and tombolo setting in the Albany, Bunbury and Perth areas. Prograding sequences have developed in tombolo settings in the Dampier Archipelago area. In the Carnarvon area the beach/dune sequences are part of a regionally prograding deltaic system. Elsewhere beach/dune sequences are thin, or ephemeral (seasonal, or 5--10 yr cyclicity), or stratigraphically incomplete. The sequence generated by shoreline progradation has been established by trenching and coring (Fig. 3B). An idealised sequence is shown in Fig. 6. A complete sequence forms only after many episodes of deposition and erosion. The base of the sequence has small-scale trough cross-strata with gravel concentrations, deposited subtidally. Above this are the wedge-shaped sets of laminated sediment deposited in the swash zone followed by bubble sand interlayered With laminites. Mean low-water level is at the boundary between the trough bedded sand and laminated sand lithofacies. Locally the parallel laminations may extend lower or the trough cross-strata higher (cf. Howard and Reineck, 1972). The laminated/bubble sand lithofacies is overlain by parallel-bedded sand occasionally with ripple cross-lamination and generally showing shell and heavy mineral concentrations (formed by aeolian winnowing) and minor disconformities. Concentrations of storm debris (particularly wood, cuttlefish and mollusk shells) occur at the upper edge. The top of the sequence is composed of thick wedges of steeply dipping large-scale cross-strata of the aeolian sand lithofacies. The thickness of each lithofacies depends on several factors. Normal-tide and storm-tide ranges determine the thickness of the foreshore and backshore lithofacies. Storm tides vary in their height (up to 2 m above MSL) and intensity so their effect is also variable. Commonly, the backshore has up to 1 m relief, and deposits up to 1.0 m may develop (Table V). The thickness of the beach ridge/dune deposits may be up to 10 m but is mostly 3--6 m.

~

Small-scale trough cross-strata

Inclined wedges of parallel lamination

Horizontal bedding to crudely laminated or homogeneous

Large-scale foresets

BEDDING

• Gravel layers

• Scours

• ?Gravel mounds and layers

• Erosional cliffs of old berms

• Shelly gravels

• Shell abundance increases down section

• Marine skeletal debris common

• Bubble sand

• Storm debris

• Floatable marine debris (Sepia sp. and Spiru/a sp.) weed, and wood

• Debris corroded

• Non-marine skeletons eg. land snails • Fine marine debris along single laminae

SKELETAL OEBR!S

• Heavy mineral layers

• Soil sheets intercalated

OTHER FEATURES

• Rare vertical tubes of polychaetes, often with depressed laminae

• Rare Ocypode burrows

• Ocypode burrows common

• Insect burrows -+ larval cases

• Rare Ocypode

BURROWS

• None

• Minor

• Abundant to scattered

PLANTS ROOTLETS

• Depends on aspect and energy regime

• 1-3 m commonly

• Intertidal deposits, thickness depends on tidal range

depending on normal v. storm tide range

• 0.5-1.0 m

•3-10m depending on dune sizes

THiCKNES6

Fig. 6. Idealised b e a c h t o b e a c h r i d g e / d u n e regressive s e q u e n c e b a s e d o n m o d e r n a n d P l e i s t o c e n e i n f o r m a t i o n .

TROUGH - BEDDED SAND / GRAVEL

LAMINATED SAND

LAMINATED / BUBBLE SAND

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LITHOFAClES

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1.3

0.9

1.4

5.3

Albany

Bunbury

Perth and environs (8 beaches)

Carnarvon

Dampier Archipelago

1.0 (neap) 3.6 (spring)

0.3 (neap) 0.9 (spring)

0.4

0.5

0.4

Mean tidal range in metres

ca. 2.0

0.8

0.6--1.1

1.1--1.3

1.0

Thickness (in m) of laminated sand lithofacies

1.8

0.4

0.3 b

0.3 b

0.2

Tidal range (in m) above MSL

1.0--1.5

0.4

0.3--0.5

0.3--0.5

0.2

Thickness (in m) of laminated bubble sand lithofacies

a Barometric pressure may exert a larger influence on height of tide than astronomical influences. b Heights estimated from data in Public Works Department (1979).

Max. tidal range in metres

Area

Relationship of tide and storm water levels to lithofacies thickness

TABLE V

1.0 1.0---2.0

1.3 2.0

0.9--1.0 1.0

0.6--1.0

1.0 0.7

0.4--0.8

Thickness (in m) of laminated/ disrupted sand lithofacies

Height (in m) of storm water above MHW a

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to laminite to disrupted lithofacies. C: Pleistocene beach to dune section Muderup Rocks, showing stratigraphic relationships between trough, laminite, laminite/bubble, laminite/disrupted aeolian cross-stratifiedlithofacies. Note orientation of bedding in aeolian cross-stratified lithofacies. Line diagram traced from

faceA--A'in(A)

Fig. 7. Pleistocene beach to dune sections, illustrating stratigraphic relationships between the lithofaeies. Line diagrams traced from composite photographs. A: Ocean Reef area; B: Mullaloo area. Note the low-angle, seaward-dipping discordances in the laminated sand lithofaeies and interdigitation with trough-bedded sand/gravel lithofacies. Bedding in the aeolian cross-stratified lithofacies is almost completely destroyed by rootlets leaving vertical "grain" in deposit. Inter-

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PLEISTOCENE LIMESTONE BENEATH UNCONFORMITY

LARGE- SCALE CROSS- STRATIFIED FINE AND MEDIUM SAND W I T H ROOT STRUCTURE OVERPRINT

~

03 t~ I--L

322 PLEISTOCENE SEQUENCES

The Pleistocene Coastal Limestone (= Tamala Limestone; Playford et al., 1976) is well exposed in the Perth Basin in Western Australia (Fig. 2). Additionally the limestone has been extensively quarried to reveal stratigraphic sections up to 10 m thick. The formation mainly is an indurated cross-bedded aeolianite and developed in several dune-building episodes. Accordingly, it contains unconformities, palaeosols, and buried karst features. There are also intercalated marine beds (generally less than 5 m thick) and buried, marine-eroded platforms. This study has been limited to those marine units with recognisable beach/ dune sequences that are well exposed either on rocky shores or on quarry walls and does not include the numerous marine units that appear to have formed in gulf, embayment, estuarine or seagrass bank environments. The main study sites are located at Muderup Rocks, Mullaloo, Ocean Reef and Spearwood (Fig. 2); other supplementary areas of study are also located on Fig. 2. The Pleistocene sections exhibit all the modern beach/dune lithofacies. Each of the lithofacies occurs as a sub-horizontal unit in a stratigraphic sequence (Fig. 7). The sediments in the formation range from unlithified to weakly lithified to well-indurated sand and shelly sand. Rather than referring to the suite as limestones they will be described utilising sedimentary structure terms to convey their important structural attributes. The units are: Top: (5) Root-structured, poorly cross-laminated and large-scale crossbedded fine and medium sand (3--6 m thick); top eroded away and overprinted by calcrete. (4) Bioturbated and root-structured, fine, medium and coarse shelly sand (0.5--1.0 m thick), with Sepia, Spirula spirula and scattered gravel in mixed orientation; abundant erosional interfaces (Figs. 8C, 8D). (3) Laminated medium, coarse and fine sand (0.3--0.5 m thick) with oriented shell layers alternating with locally preserved bubble sand (Fig. 8B). (2) Laminated and bedded medium, coarse and fine sand alternating with oriented shelly layers (0.5--1.5 m thick); Fig. 8A. Base: (1)Trough-bedded, medium and coarse sand alternating with laminae, beds and festoons of coarse shell and lithoclast gravel (1--2 m thick); gravel lenses 1--2 m wide, 30 cm high, and up to 6 m long and locally exhumed and exposed on bedding surfaces; base rests on older limestone or rock platform unit; Fig 8A. The entire section is 7--11 m thick. The profile is a direct analog of Holocene beach/dune deposits and indicates progradation. Sub-tidal trough stratified sands, pass into laminites deposited in a swash zone. Storm deposits are bioturbated and crudely laminated sands bearing floatable skeletons and debris. Sedimentary lamination within the profile is inclined to seaward indicating direction of beach slope; the sequence is capped by beach ridge to aeolian units in which cross-bedding and lamination are inclined to land (Fig. 7C).

323

Fig. 8. Sedimentary structures in Pleistocene sequences. A. Trough-bedded sand/gravel passing upward into laminated sand; scale is marked in 30-cm intervals. B. Laminated/ bubble sand showing vesicular structure. C. Bioturbated, disrupted sand with Sepia skeletons (arrowed). D. Bioturbated, disrupted sand showing (Ocypode) burrow (arrowed).

The thickness of individual lithofacies in Pleistocene sections is much the same as in modern deposits. In the more extensive outcrops showing downdip sections, individual laminae can be traced across facies boundaries (Fig. 7A). For instance, a laminite of alternating bubble sand and laminated sand can be traced u p , l i p into a bioturbated sand bearing Sepia andSpirula skeletons, and can be traced down
324

coset boundaries vs. gentle inclination of lamination (Fig. 7A). Generally, these lithofacies occur within stacked, wedge-like units (beach cross-bedding type D of Thompson, 1937). The base of a wedge has a relatively steep interface inclined at 15 ° discordantly cut into gently inclined laminites (dip of 5°). The laminites overlying the interface have steep dip, harmonious with the interface, b u t they thicken slightly, down-dip such that within 10 m or so the dips again are gentle. The series of gently inclined laminites are then truncated by the base of the next wedge. Since the beach face generates the sedimentary lamination in the swash, the slope of the foreshore can be inferred from the inclination of laminations. Pleistocene strata indicates foreshore profiles had slopes varying from 5--15 °. The steep interfaces at the bases of a wedge, are interpreted as an erosional interface developed when beaches were being cut back and swash zone slopes were generally steep. Strata deposited on this slope in the following depositional phase are initially sympathetically steeply inclined but in time the slope moderates as deposition occurs. Thus each wedge of strata indicates alternating beach erosion and deposition with net accretion. Intermittent beach erosion also is evidenced by small buried cliffs. The strike sections of beach units generally exhibit sheet-like parallel array of facies, b u t there may be large-scale festoons indicative of beach cusp sedimentation. Locally, within the laminated/bubble sand lithofacies, are found swash bars and 'hoof' structures; the former indicates preservation of a wave
Many of the features within the stratigraphic sequence are diagnostic of a lithofacies within the beach/dune environment. Thus a fully preserved, or even partially preserved sequence, offers a powerful tool for the sedimentologist because it provides not only a recognisible sequence to identify beach/dune units, b u t also an indication of relative levels of tides and storms. The characteristic features of the facies of prograded Holocene and Pleistocene beach/dune units are summarised below: Trough-bedded sand lithofacies: Diagnostic features are the trough bedding and an abundance of (shell and lithoclast) gravel. The lithofacies is well preserved and readily recognised in modern and Pleistocene sequences. Laminated sand lithofacies: The evenly laminated sands with shells oriented parallel to inclined lamination are well preserved and recognisable in prograded sequences and in Pleistocene sections. Laminated/bubble sand lithofacies: The bubble structures are very diagnostic. Their preservation, however, is unpredictable. Holocene and Pleistocene sections show that bubble sand may be preserved with burial. Bubble sand disrupts lamination and thus any destruction of bubble (crushing by burial) leads to development of thin homogenous sand sheets interlayered

325

with laminite; the former presence of bubble sand then may be inferred from centimetre- to decimetre-thick homogenous (not-laminated) sand layers. Laminated/disrupted lithofacies: Horizontal bedding, disrupted, bioturbated to homogenous structure, incipient r o o t structuring and floatable skeletons (cuttlefish and Spirula spirula) are diagnostic. Floatable skeletons remain even when sediments are strongly bioturbated b y roots and animals. In sequences where cuttlefish and Spirula are n o t elements of the fauna, w o o d and other cephalopods m a y be used as indicators. Aeolian cross-stratified sand lithofacies: Large scale cross-stratification and humic soil sheets separating large wedges and lenses of (fine and medium) sand are the diagnostic features. The occurrence of abundant r o o t structures and ant burrows also are diagnostic. DISCUSSION

The progradational sedimentary sequence of the Holocene wave-dominated sandy coastlines of Western Australia occur in a wide range of regional, depositional and climatic settings. Yet they display a unifying similarity which suggests that they m a y provide a valuable tool to identify similar shoreline deposits in the stratigraphic column. Depositional areas vary as to materials, and the stratigraphic sequence of structures can be expected to form in various mineralogical suites. These m a y include quartz sands, skeletal quartz sands with local shell (as typified by most of our study sites), dominantly carbonate sands, sand with locally derived l~thoclast gravel, and so on. The essence of the sequence is that, given a range of sand and gravel-grain types then the material will be transported, sorted, deposited and shaped by the shoreline processes. Under prograding conditions the ideal sequence of sedimentary structures will be generated. Recognition of such a sequence should enable interferences to be drawn a b o u t the h y d r o d y n a m i c and aeolian conditions of a shoreline deposit. (1) Coastal setting. The sequences represent environments that were wave-dominated sandy coasts with onshore aeolian environments. The palaeogeographer can infer extensive progradation b y combination o f beach accretion and aeolian deposition. Palaeocurrent studies of the laminite and aeolian cross-stratified sand lithofacies will enable strandline and wind directions to be determined. (2) Tide and storm levels. Relative thickness of laminated sand, laminated/bubble sand, and laminated/disrupted sand lithofacies can give an indication of relative tide- and storm-water levels (Fig. 3A and Table V). (3) Relative sea-level changes. The various lithofacies are accurate indicators of tidal levels. T h e y can be used to establish relative sea level changes in Holocene and Pleistocene units. For example laminite lithofacies in Pleistocene sequences of the Perth area have been measured 2 m above modern laminite lithofacies levels and precisely indicate the former sealevels. (4) Cyclicity of deposition. The geometric array of wedges within the

326

laminated sand and laminated/bubble sand lithofacies (or conversely, lack of wedges) gives insight into the dynamics of beach profiles. Wedge units indicate alternating erosion and deposition with net deposition. Lack of wedge-like units would tend to indicate a more stable prograding beach where foreshore profiles did not periodically fluctuate from gently inclined to steep. (5) Offshore area types. The nature of storm deposits gives an indication of marine conditions offshore. Sandy offshore areas will yield sand and resident shelly benthos which will be incorporated into the backshore deposits. Offshore areas comprised of rock-reefs and islands will yield encrusting rockdwelling organisms and lithoclasts. (6) Palaeoceanography. Beaches facing open oceans will receive and incorporate into storm deposits planktonic skeletons and other floating debris typical of oceanic waters. In contrast, restricted, sheltered or barred embayments will not reflect oceanic contributions in their storm deposits. Within the Pleistocene sections studied, therefore, wedge stratification indicates that, in a net progradational setting, the beach foreshore alternated from gentle profiles to steep profiles, in much the same way they do today. The composition of the storm deposits indicate the beaches fronted open oceanic waters. The skeletal and trace fossil component (mollusks, Ocypode burrows, etc.) are essentially similar to modern sequences. The tidal and storm level indicators within the sequence show similar ranges to those of today but displaced 2 m higher. ACKNOWLEDGEMENTS

We thank R.K. Steedman for providing oceanographic data, the Harbour Master at Albany for data on sea states there and the Bureau of Meteorology in Perth for climatic data. Data from Carnarvon, and preliminary thoughts for the first part of this paper come from Ph.D. experience (D.J.) funded by Australian Postgraduate Research Award and a University of Western Australia Research Grant. We also thank John Ngai of the Cartographic Centre, James Cook University, who supervised the drafting, and Anne Archer, Kerry Vickers and Lyn Stott for typing. REFERENCES Beall, A.O., 1968. Sedimentary processes operative along the Western Louisiana Shoreline. J. Sediment. Petrol., 38: 869--877. Bureau of Meteorology, 1975. Climatic averages for Western Australia. Aust. Govt. Publ. Serv., Canberra. Clifton, H.E., 1969. Beach lamination -- nature and origin. Mar. Geol., 7: 553--559. Clifton, H.E., Hunter, R.E. and Phillips, R.L., 1971. Depositional structures and processes in the non-barred high-energy nearshore. J. Sediment. Petrol., 41: 651--670. Curray, J.R. and Moore, D.G., 1964. Holocene regressive littoral sand, Costa de Nayarit, Mexico. In: L.M.J.U. Van Straaten (Editor), Deltaic and Shallow Marine Deposits. Develop. Sedimentol., 1: 76--82.

327

Davidson-Arnott, R.G.D. and Greenwood, B., 1976. Facies relationships on a barred coast, Kouchibouguac Bay, N e w Brunswick, Canada. In: R.A. Davis and R.L. Ethington (Editors), Beach and Nearshore Sedimentation. Soc. Econ. Palaeontol. Mineral., Spec. Publ., 24: 149--168. Davies, D.K., Ethridge, F.G. and Berg, R.R., 1971. Recognition of barrier environments. Bull. A m . Assoc. Pet. Geol., 55: 550--565. Department of Defence, 1980. Australian National Tide Tables 1980. Aust. Hydrogr. Publ. 11. Aust. Govt. Publ. Serv., Canberra. Easton, A.K., 1970. The tides of the continent of Australia. Res. Pap. Horace L a m b Centre Oceanogr. Res., 37: 1--326. Emery, K.O., 1945. Entrainment of air in beach sand. J. Sediment. Petrol., 15: 34--49. Emery, K.O., 1960. The Sea off Southern California. Wiley, N e w York, N.Y., 366 pp. Gentilli,J., 1972. Australian Climate Patterns. Nelson, Melbourne, 285 pp. Hails, R.R. and Cart, A., 1975. Nearshore Sediment Dynamics and Sedimentation. Wiley, N e w York, N.Y., 316 pp. Howard, J.D., 1976. Trace fossils as criteria for recognising shorelines in stratigraphic record. In: J.K. Rigby and W.D. Hamblin (Editors),Recognition of Ancient Sedimentary Environments. Soc. Econ. Palaeontol. Mineral., Spec. Publ., 16: 215--225. Howard, J.D. and Reineck, H.E., 1972. Georgia coastal region, Sapelo Island, U.S.A., Sedimentology and biology. IV. Physical and biogenic sedimentary, structures of the nearshore shelf. Senckenbergiana Mar., 4: 81--123. Hoyt, J.H. and Henry, V.J., 1964. Development and geologic significance of soft beach sand. Sedimentology, 3: 44--51. Ingle, J.C., 1966. The Movement of Beach Sand. Dev. Sedimentol., 5: 1--221. Johnson, D.P., 1974. Sedimentation in the Gascoyne River Delta, Western Australia. Unpubl. Ph.D. Thesis, Univ. Western Australia. King, C.A.M., 1972. Beaches and Coasts. Arnold, London, 470 pp. Komar, P.D., 1976. Beach Processes and Sedimentation. Prentice-Hall,Englewood Cliffs, N.J., 429 pp. Kraft, J.C., 1971. Sedimentary facies patterns and geologic history of a Holocene marine transgression. Geol. Soc. A m . Bull., 82: 2131--2158. Kumar, N. and Sanders, J.E., 1975. Inlet sequence formed by the migration of Fire Island Inlet, Long Island N e w York. In: R.M. Ginsburg (Editor), Tidal Deposists. Springer, Berlin, pp. 75--83. Land, L.S., 1964. Eolian cross-bedding in the beach-dune environment, Sapelo Island, Georgia. J. Sediment. Petrol., 34: 389--394. Logan, B.W. and Cebulski, D.E., 1970. Sedimentary environments of Shark Bay. Western Australia. In: B.W. Logan (Editor), Carbonate Sedimentation and Environments, Shark Bay, Western Australia. Mere. A m . Assoc. Pet. Geol., 13: 1--37. Manohar, M., 1955. Mechanics of bottom sediment movement due to wave action. Beach Erosion Board, Tech. Mem., 75: 1--121. McArthur, W.M. and Bettenay, E., 1960. The development and distribution of the soilsof the Swan Coastal Plain, Western Australia. Aust. C.S.I.R.O. Soil. Publ., 16. McKee, E.D., 1966. Structures of dunes at White Sands National Monument, N e w Mexico (and a comparison with structures of dunes from other selected areas). Sedimentology, 7: 1--69. Meyer, R.E., 1972. Waves on Beaches and Resulting Sediment Transport. Academic Press, N e w York, N.Y., 462 pp. Palmer, R.H., 1928. Sand holes of the strand. J. Geol., 36: 176--180. Playford, P.E., Cockbain, A.E. and Low, G.H., 1976. Geology of the Perth Basin, Western Australia. Geol. Surv. West. Aust. Bull., 124: 1--311. Public Works Department, 1979. Tidal Information, Western Australian Coast. Public Works Dept. Western Australia, Plan No. P W D W A 47574/2, Perth. Seddon, G., 1972. A Sense of Place. Univ. Western Australia Press, 214 pp.

328 Semeniuk, V. and Meagher, T.D., 1981. The geomorphology and surface processes of the Australind--Leschenault Inlet coastal area. J. R. Soc. W.A., 64: 33--51. Shepard, F.P., 1963. Submarine Geology. Harper and Row, New York, N.Y., 557 pp. Steedman, R.K., 1977. Report on Oceanographic and Marine Environment Studies, Shire of Wanneroo, Mullaloo Marina; Section I -- Physical Oceanographic Studies. Report to Shire of Wanneroo, Western Australia. Steedman, R.K. and Craig, P.D., 1979. Numerical model study of circulation and other oceanographic aspects of Cockburn Sand. Report to Cockburn Sand Study Group., Dept. of Conserv. and Environment, Perth, W.A. Thompson, W.O., 1937. Original structures of beaches, bars and dunes. Bull. Geol. Soc. Am., 48: 723--752. Woodside Petroleum Development, 1978. Northwest Shelf Development: Environmental Review and Management Programme. Prep. by Woodside Petroleum Development, Perth. Zenkovitch, V.P., 1967. Processes of Coastal Development. Oliver & Boyd, Edinburgh, 738 pp.