modern and pleistocene rocky shore sequences along carbonate coastlines, southwestern australia

37
Sedimentary Geology, 44 (1985) 225-261 225 Elsevier Science Publishers B.V., Amsterdam - Printed inThe Netherlands MODERN AND PLEISTOCENE ROCKY SHORE SEQUENCES ALONG CARBONATE COASTLINES, SOUTHWESTERN AUSTRALIA V. SEMENIUK and D.P. JOHNSON 21 Glenmere Road, Warwick, W.A. 6024 (Australia) Department of Geology, James Cook University of North Queensland, 7bwnsville, Qld. 4810 (Australia) (Received April 4, 1984; revised and accepted December 18, 1984) ABSTRACT Semeniuk, V. and Johnson, D.P., 1985. Modern and Pleistocene rocky shore sequences along carbonate coastlines, southwestern Australia. Sediment. Geol., 44: 225-261. Modern rocky shores on the coast of southwestern Australia commonly are cut into Pleistocene carbonate rocks. These rocky shores provide a wealth of data on morphology and stratigraphic products that are useful for studies in ancient sequences as well as studies in Quaternary sea-level history. Four types of rocky shore are recognised: Type 1 with platform and notch and a full range of morphologic features; Type 2 with sandy beach; Type 3 with breccia; and Type 4 with truncated profile. These shore types are intergradational in space and laterally equivalent along the coast. Macro- and micromorphology, biology and sediments of rocky shores are closely related to various geomorphologic settings and tidal levels, because of the distribution and dominance of a variety of physical, chemical and biological processes acting on these shores. As a result, many of the features on modern rocky shores are diagnostic of subenvironments and indicate types of processes which have been operating. Diagnostic morphologic, sedimentologic and biologic features of modern rocky shores were used to interpret palaeoenvironments of Pleistocene rocky sequences of the Perth Basin, suggesting that the results of this study may be used to interpret similar deposits elsewhere in Pleistocene and perhaps older formations. The information can be used to identify rocky shores in a gross context, as well as to identify components, environments and processes within ancient rocky shore systems, INTRODUCTION Rocky shores, which include boulder shores, cliff shores and shore platforms, may be cut into a wide variety of igneous, sedimentary and metamorphic rocks. These shore types, however, are particularly well developed on coasts where carbonate rocks are exposed because the lithologies are vulnerable to many styles of physical, biological and chemical processes of erosion. Carbonate sequences also offer a favourable setting for the development of rocky shores because these sediments frequently form coastal units which are lithified relatively early in their history. Thus within carbonate sequences which are subjected to frequent tectono-eustatic oscilla- 0037-0738/85/$03.30 © 1985 Elsevier Science Publishers B.V.

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Page 1: Modern and Pleistocene rocky shore sequences along carbonate coastlines, southwestern Australia

Sedimentary Geology, 44 (1985) 225-261 225 Elsevier Science Publishers B.V., Amsterdam - Printed inThe Netherlands

MODERN AND PLEISTOCENE ROCKY SHORE SEQUENCES ALONG CARBONATE COASTLINES, SOUTHWESTERN AUSTRALIA

V. SEMENIUK and D.P. JOHNSON

21 Glenmere Road, Warwick, W.A. 6024 (Australia) Department of Geology, James Cook University of North Queensland, 7bwnsville, Qld. 4810 (Australia)

(Received April 4, 1984; revised and accepted December 18, 1984)

ABSTRACT

Semeniuk, V. and Johnson, D.P., 1985. Modern and Pleistocene rocky shore sequences along carbonate coastlines, southwestern Australia. Sediment. Geol., 44: 225-261.

Modern rocky shores on the coast of southwestern Australia commonly are cut into Pleistocene carbonate rocks. These rocky shores provide a wealth of data on morphology and stratigraphic products that are useful for studies in ancient sequences as well as studies in Quaternary sea-level history.

Four types of rocky shore are recognised: Type 1 with platform and notch and a full range of morphologic features; Type 2 with sandy beach; Type 3 with breccia; and Type 4 with truncated profile. These shore types are intergradational in space and laterally equivalent along the coast. Macro- and micromorphology, biology and sediments of rocky shores are closely related to various geomorphologic settings and tidal levels, because of the distribution and dominance of a variety of physical, chemical and biological processes acting on these shores. As a result, many of the features on modern rocky shores are diagnostic of subenvironments and indicate types of processes which have been operating.

Diagnostic morphologic, sedimentologic and biologic features of modern rocky shores were used to interpret palaeoenvironments of Pleistocene rocky sequences of the Perth Basin, suggesting that the results of this study may be used to interpret similar deposits elsewhere in Pleistocene and perhaps older formations. The information can be used to identify rocky shores in a gross context, as well as to identify components, environments and processes within ancient rocky shore systems,

INTRODUCTION

Rocky shores, which include boulder shores, cliff shores and shore platforms, may be cut into a wide variety of igneous, sedimentary and metamorphic rocks. These shore types, however, are particularly well developed on coasts where carbonate rocks are exposed because the lithologies are vulnerable to many styles of physical, biological and chemical processes of erosion. Carbonate sequences also offer a favourable setting for the development of rocky shores because these sediments frequently form coastal units which are lithified relatively early in their history. Thus within carbonate sequences which are subjected to frequent tectono-eustatic oscilla-

0037-0738/85/$03.30 © 1985 Elsevier Science Publishers B.V.

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tions of sea level during development, transgressions often result in marine erosion of newly lithified carbonate rocks. In contrast, transgressive-regressive events in

siliciclastic sequences may result merely in erosion and redistribution of unlithified sediments.

It may be expected therefore that cyclic carbonate sequences probably will have passed through multiple phases of rocky shore development, and rocky shore sequences should be relatively common features within ancient carbonate forma- tions, particularly in the vicinity of basin margins or islands. The general lack of described rocky shore features from ancient carbonate formations may be due to a lack of width parallel to palaeoslope, i.e. they are too narrow a feature to be detected regionally or, alternatively, they may have been erased from the rock record. However, they may also have been overlooked.

Rocky shores have been described from various localities around the world (Bartrum, 1935; Wentworth, 1938; Teichert, 1947; Hills, 1949; Fairbridge, 1950; Ginsberg, 1953; Guilcher, 1953, 1958; Bird and Dent, 1966; Gill, 1967; Healy, 1968; Hodgkin, 1970; Stephenson and Stephenson, 1972). They have been the subject of widespread and often controversial debate regarding their usefulness as indicators of Quaternary sea-level fluctuations--the origin of the various platforms cut into the rocky shores has been germane to this problem. This paper is not a description with a view to presenting yet another interpretation of platforms in the context of Quaternary tectono-eustatic changes. Rather, we concentrate on documenting the erosion interface and sedimentary products, both of which are potentially preserva- ble and useful as indicators of the various parts of rocky shores. Within this context we view rock platforms merely as another component of a rocky shore.

The features of rocky shores described here, however, may assist interpretation of similar ancient units. A review of the literature (e.g., Baker and Gill, 1957; Read and Grover, 1977; Warme, 1977) suggests that many of the patterns and principles developed here have wide applicability. In this paper, we apply some of the data to interpret Pleistocene rocky shore sequences on the Swan Coastal Plain of the Perth Basin.

REGIONAL SETTING AND METHODOLOGY

The main study area is set in subtropical southwestern Australia along a coastline with a microtidal, wave-dominated oceanography and with parent rocks of karstified Pleistocene limestone. This setting is only one of a range available for study along the vast coastline of Australia. Other rocky shore types are set in tropical climates, macrotidal regimes, sheltered gulf/embayments, or are composed of pre-Quaternary limestones, quartz sandstone and igneous and metamorphic rocks; these shore types are outside the scope of this paper.

Three main regions containing well-developed modern rocky shores were studied

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in Western Australia (Figs. 1 and 2): (1) coastal plain of the Perth Basin (twenty study sites); (2) Yallingup Shelf of the Perth Basin (four study sites); and (3) Quobba Ridge area of the Carnarvon Basin (three study sites).

Our studies are restricted to rocky shores in microtidal ( < 2 m) regimes (Hodgkin and DiLollo, 1958; Easton, 1970). In all study locations the coast is subject to oceanic a n d / o r locally generated wave trains and the shore environment is viewed as wave dominated (Semeniuk and Johnson, 1982).

Study sites were profiled from the hinterland (of dunes or limestone) to the subtidal cliff that terminates the rocky shoreline. Along the profile the following were described: (1) morphology relative to tidal level; (2) sedimentary veneer (structures, texture and composition); (3) biota and its impact on the contemporary surface; and (4) underlying parent rock stratigraphy. The stratigraphic relationships between beach deposits, shoreline breccias and underlying rock were determined by coring, trenching and probing. Variability in the standard profile was mapped

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Fig. 1. Map showing study area and location of study sites mentioned in text. Additional study sites in the Perth area are shown in Fig. 2.

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Fig. 2. Geological map of the Perth area (after Playford et al., 1976) showing location of study sites.

laterally along the shore. Sedimentary structures were studied in intact blocks and cores of sediments. Subtidal portions of rock platforms were examined by diving.

Pleistocene rocky shore sequences were studied at twelve locations along trenches, quarries and wave-washed cliffs (Fig. 2). Stratigraphic profiles were photographed at various scales and lithological and morphological features were drawn onto photo- graphs.

Modern rocky shores

Modern rocky shores fringe islands and long stretches of the western Australian coastline (Fairbridge, 1950) and are best developed where the Quaternary Tamala Limestone of Playford et al. (1976) crops out (Fig. 2). This formation occurs along the coastal tract of the Perth and Carnarvon Basins (Fig. 1; Playford et al., 1974).

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Modern rocky shorelines exhibit a wide variation in profile and erosional features that are dependent on type of parent rock, induration of parent rock, aspect (i.e. orientation of coast with respect to wind and waves), wave climate, biological zonation and fresh-water influx. Most rocky shores are fringed by narrow rock platforms which are either awash or exposed only at low tide; they belong to the category of "low-tide platform" (after Bird, 1976). Generally, laterally extensive outcrops of Tamala Limestone are bordered by similarly extensive platforms. Coastlines composed of rocky headlands alternating with sandy beaches have platforms of limited extent around the headlands.

Prior to describing the various features of the modern rocky shore it is necessary to briefly outline the rock material that is host to the marine erosion.

Host rock

Modern rocky shores on the Western Australian coast are cut into various types of Pleistocene limestone which are only weakly indurated by sparry calcite (and/or calcrete) and frequently are friable. The main lithologies are:

(1) Quartz skeletal grainstone (eolianite); its structure is large-scale cross-bedding but may be bioturbated to structureless due to disruption by rootlets or to subaerial weathering, respectively; grainsize is medium and fine sand with well-rounded quartz, mollusc and calcareous algal fragments, lithoclasts and lithoskels.

(2) Quartz skeletal grainstone, locally shelly (marine and beach deposits); struc- ture is laminated, cross-laminated and festoon bedded, and locally bioturbated; grainsize is medium and coarse sand and gravel composed of rounded quartz, mollusc and algal fragments, lithoclasts and lithoskels.

(3) Shell and lithoclast conglomerate (strandline deposit); structure is stratified to festoon cross-bedded to homogeneous; grains are predominantly gravel-sized, rounded lithoclasts and fragmented to whole shells.

The limestones sequences also display variation in their disposition of lithology (Semeniuk, 1983). There are large-scale wedges and lenses of marine sediments (1-3 m thick) with small-scale cross-lamination and planar lamination, interlayered with thick (> 4 m) eolian cross-bedded units. Sheets of indurated calcrete and of yellow quartz sand or grey soil are common locally. Numerous vertical solution pipes (10-50 cm diameter) which are calcrete lined and filled with yellow quartz sand postdate lithification and truncate the layering (Seddon, 1972; Semeniuk, 1983). There also are abundant calcrete rhizoconcretions (0.5-5 cm diameter).

These large- and small-scale features influence the macro- and micromorphology of the rocky shore as well as the shape of lithoclasts eroded from the shore (Fig. 3). For instance: solution pipes on shore platforms help develop scour pools and depressions; indurated layers can develop residual knolls; well-stratified and cross- stratified units can develop structural platforms or small-scale "terraces"; reworked rhizoconcretions develop rod-shaped gravel.

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Geometry and surface features

233

In this study rocky shores are divided into four types, based on the character of high-tidal to supratidal morphology (Fig. 4): Type 1: rocky shore with p la t form/ notch (Figs. 5B and 6A); Type 2: rocky shore with sandy beach (Figs. 5C and 6B-D); Type 3: rocky shore with breccia (Figs. 5A, 7A and 8A, B); and Type 4: rocky shore with truncated profile (Fig. 6E). All types have fringing rock platforms developed to varying extent. Offshore there is commonly a breccia/block deposit overlying sand flats or rock pavements.

Type 1: rocky shore with platform~notch The Type 1 rocky shore is the best developed type and is the most variable of the

four types in that it has a large range of morphologic units and various levels of cliff. Following Fairbridge's (1950) descriptive scheme, there is a broad low-tidal, sub- horizontal platform or terrace (tens of metres to approximately 100 m wide) bordered to seaward by a subtidal cliff and locally by a raised outer rim (Fig. 4A); on its landward margin is a tidal notch backed by a high tidal seacliff, and a supratidal bench. Above the bench is a subaerial cliff. Situated on the platform may be stacks standing several metres above the general tidal level. These stacks may be residual knolls of in-situ limestone or displaced blocks derived by cliff collapse.

Four main variations occur on this pattern. Firstly, the bench may be absent and a cliff (3-6 m high) directly abuts the platform. Secondly, Pleistocene karst surfaces recently exhumed at tidal and storm-water levels, have a surface morphology not yet

fully overprinted or erased by contemporary processes and stand as residual surfaces unrelated to any contemporary processes. Thirdly, some platforms are backed to landward by a relatively moderate slope (10-15 °) termed a ramp, that is cut in bedrock and rises from high water to above storm level; in these situations the notch, cliff and bench are absent. Finally the shore platform may be traversed by narrow, deep channels and ruts, oriented normal to the shoreline extending to depths of 2-3 m below sea level.

The area seaward of the subtidal c l i f f / rock platform comprised of sand flats or rock pavements may be strewn with blocks and large slabs of rock ( < 1 m diameter).

Morphologic units of Type 1 rocky shores are described in Table 1. Micromor- phology is summarised in Fig. 9.

Type 2: rocky shore with sandy beach Type 2 rocky shores are similar to Type 1 above, but have a wedge, or pockets, or

a continuous ribbon of beach-dune sand overlying inner parts of the platform, notch, high tidal seacliff, supratidal seacliff and bench (Figs. 4B, 5C and 6B-D). The platform, outer rim and subtidal cliff are still present. The sand is a permanent feature but its seaward edge is subject to periodic redistribution.

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Type 3: rocky shore with breccia The Type 3 rocky shore is composed of a platform bordered to seaward by an

outer rim and subtidal cliff, while landward is a cliff in various stages of undercut- ting and collapse (Fig. 7A, B). A wedge or ribbon of breccia partly overlies the platform and buries the bench, seacliff and notch (Figs. 4C, 5A, 7A and 8A, B). The surface of the breccia is inclined 10-30 ° to seaward. The breccia is composed of a chaotic block framework which varies from block-supported in supratidal environ- ments, to sutured/f i t ted in tidal environments (Fig. 8C, D). Blocks are up to 3 m

a c r o s s .

Type 4: rocky shore with truncated profile The Type 4 rocky shore is similar to those described above but the seaward

portions of the profile have been markedly truncated by erosion (Fig. 6E). Conse- quently the outer rim is absent, the platform is narrow or absent and the subtidal

flats are strewn with blocks and slabs.

Relationship between the shoreline types The four rocky shore types are intergradational in space, are laterally equivalent

along the coast and occur in juxtaposition to one another (Figs. 10 and 6B, C). Erosion and transportation, proceeding at unequal rates, result in a large-scale scalloped coastline. Erosion of the Pleistocene limestone upland has produced an uneven to scalloped outline in plan; erosion at the subtidal cliff also has produced an irregular outline. The intervening rocky shore area is up to 100 m wide. Type 1 shores with pla t forms/notch and Type 2 shores are the most common. Erosion at the seaward edge modifies Type 1 shores such that the subtidal cliff is cut into the inner portions of the platform, or ultimately, it may be cut quite close to the notch (Type 4 shore). Undercutting and collapse of a rocky shore occurs at three levels: subaerial, at the notch and subtidal (Figs. 7A, B and 8A, B). Erosion at the landward edge results in a thick breccia wedge formed as the coast retreats (Type 3 shore). All types of cliff erosion can result in isolated collapsed blocks which are emplaced

Fig. 6. Features of rocky shores. A. Type 1 rocky shore showing platform, notch, bench and fallen blocks; waves are breaking along the subtidal cliff; inset shows the same platform on an extremely low tide and the hummocky platform surface and deep channels are evident. B. Type 1 rocky shore on small headlands (arrowed), alternating with Type 2 rocky shores with pocket beaches. C. Small Type 2 rocky shore with shoreline sandy beach located between headlands of Type 1 rocky shore. Note fallen blocks (arrowed) partly buried by sand. D. Type 2 rocky shore with shoreline ribbon of beach and dunes which totally bury the inner part of the platform as well as the notch and high tidal cliff. In the foreground the platform is narrow, it is traversed by metre-deep ruts and has potholes developed over solution pipes (see Fig. 14C). E. Type 4 rocky shore where there is a truncated narrow platform and high (greater than 4 m) cliff; there is no notch, high tidal cliff or bench.

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sporadically on lower tidal levels of the platform (Fig. 7C); these can become welded onto the platform to develop stacks or knolls.

Sand accumulates in upper reaches (or embayments) of deeply scalloped rocky coasts (Type 2 shore). The bench and notch (and locally, inner platform) may be covered by thick sand deposits and these rock platforms are backed by beaches and dunes.

Biology of rocky shores

The biotic assemblages of rocky shores are important for several reasons. Firstly, bioerosion (e.g. by urchins) degrades platforms and forms distinctive surface mor- phology (e.g. scalloped overhanging depressions). Secondly algal meadows and mussel beds trap, bind and bioturbate sediment. Thirdly, biota contribute skeletons, which in the absence of other sediment sources, become important as sediment particles and also serve to identify the rocky shore setting. Finally, the biota is zoned and if preserved in situ can be used to identify environments of a rocky shore.

Figure 11 summarises data on the biology pertinent to this study area from the literature (Fairbridge, 1950; Dakin, 1952; Hodgkin et al., 1957, 1959; Hodgkin, 1960; Marsh and Hodgkin, 1962; Smith, 1964; Phillips, 1969; Black et al., 1979) and from our own observations. The discussion here only deals with benthic members of the biotic assemblage that: (1) influence rock disintegration; (2) contribute skele- tons; or (3) trap and bind sediment.

On the subtidal cliff and seaward edge of the outer rim are algae (Ecklonia, Hormosira, Sargassum) and encrusting organisms (such as tunicates, sponges, barnacles, galeolarian worms, serpulid worms and calcareous algal crusts). Ecklonia functions as a wave baffle since it forms a dense cover with its holdfast embedded into bored and porous limestone. However, during storms it can be ripped from the rocky shore removing the piece of substrate held by its holdfast (cf. Edwards, 1951; Gill, 1971). This process aids in degradation of the outer rim and subtidal seacliff and also generates distinctive pebble-sized limestone lithoclasts and calcareous algal debris. The outer rim is colonised by a thin algal film and encrusting calcareous algae. Grazing molluscs, sessile barnacles, urchins and lithophagic organisms dominate this wave-washed environment.

Platforms have four important biotic elements: (1) destructive, boring and ex- cavating organisms such as urchins (e.g. Echinometra), pholads and abalone which are instrumental in developing irregular scalloped depressions and bored surfaces; (2) meadows of algae (Pterocladia, Ulva, Sargassum, Hormosira and the calcareous

Fig. 7. Features of cliff line collapse and its products. A. Cliffline of Type 3 rocky shore showing collapse of larger blocks onto the inner part of the platform; a block greater than 3 m in size is arrowed. B. Undercutting and collapse of a high tidal bench resulting in emplacement of large slabs onto the platform. C. Profusion of isolated corroded blocks ( l - 2 m in size) left stranded on platform by cliffiine collapse.

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various levels of a rocky shore showing multiple examples of vertical profiles of the distinct micromor-

phology. In all diagrams V = H. Arrows on diagrams of bored surfaces in D indicate area where there is a profusion of millimetre-sized borings which form a honeycombed network.

Fig. 8. Features of Type 3 rocky shore. A. Typical array of breccia wedge along collapsing cliffline: breccia buries the notch and bench. Height of cliff in middle ground is approximately 3 m. B. Wedge of

breccia overlying the bench along the foot of subaerial cliff. C. Close-up of breccia within intertidal zone

showing sutured to fitted framework. Width of photograph is 2 m. D. Close-up of breccia in supratidal zone showing grain-support framework. Hammer for scale.

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oZ~o ~ a'%..= PATELLOIDA ; ~ ' ~ z SIPHONA RIA ,=I u~(n - B R A C H I O O N T E S ">w(s" ~

, ~ LITTORINA I ~ AUSTRACOCHLEA •.,~,~ ~ . , ~ e (NOT SHOWN )

ARNACLES ( ~ ) BARBATIA ~nz°~

LCS

Fig. 11. Diagram summarising the essential features and zonation of the fauna and flora of rocky shores along a Type 1 profile.

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alga Jania) and beds of in-situ mussels (Brachidontes); these function in sediment- trapping and binding; (3) in-situ encrustations of serpulid worms, galeolarian worms, barnacles and sponges; and (4) a variety of sessile and vagile skeletal organisms (such as whelks and turbans) that contribute skeletal material that is distinctive of rocky shore assemblages.

The exposed notch environment is largely sediment-free and supports encrusting barnacles, mussels, limpets and vagile animals. The cryptic notch contains an assemblage of sponges, echinoderms and crabs (Leptograpsus). The seacliff environ- ment does not support abundant life. Its lower surface is encrusted by barnacles and limpets and grazed by small gastropods (Melaraphe and Nodilittorina). Fissures, crevices and channelways are micro-habitats which harbour crabs and clusters of sheltering gastropods.

The bench, breccia deposit and landward cliff biologically are relatively barren zones. Where local sand pockets occur there is terrestrial (halophyte) vegetation. High tidal sand beaches locally support ghost crabs.

Sedimentology

The rocky shoreline is an eroding or static coastline and not an environment of active (and thick) sediment accumulation. Nevertheless some sediment, due to geomorphological and biological factors, does accumulate. The sediment is diagnos- tic structurally and texturally and if preserved can be a useful environmental indicator.

Source of sediment There is variation in type and source of sediment on rocky shores (e.g. Fig. 3);

sediment grainsize varies from sand to boulders or blocks. Sand is quartz, skeletal grains, lithoclasts and lithoskels (Read, 1974) of medium, coarse and (rarely) very coarse size. Grains are abraded and rounded to subrounded. Sand sources are: (1) sandy shorelines elsewhere, with transport to rocky shores effected by longshore drift; (2) disaggregated Pleistocene limestone; (3) relict shelf carbonate sand and quartz sand; and (4) modern skeletons of littoral assemblages.

Gravel is generated locally. Several types are recognised (Fig. 3): shells from resident biota; lithoskels (shell and coral) eroded from the Pleistocene limestone; platy and bladed pebbles (i.e. wave-abraded, cemented laminae of limestone), rod-shaped pebbles (i.e. reworked, wave-abraded, calcrete rhizoconcretions) and wave-rounded lithoclasts (ovoid to equant in shape, derived from the main limestone body) reworked as lithoclasts. Locally, second cycle pebbles (Pleistocene lithoclasts) are reworked from the limestones.

Boulders and blocks are derived by collapse of limestone along cliffs. Collapse brought about predominantly by subaerial erosion brings limestone boulders and large blocks into the marine environment. All boulders and blocks are then subject

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to transport by storm waves, attrition and solution. Boulders and blocks that have collapsed from the subtidal cliff bear imprints of rock platform environments (e.g.

bored, scalloped surfaces; overhangs), whereas those recently derived from subaerial cliffs bear only diagnostic subaerial imprints).

Distribution and characteristics of sediment on rocky shores (Fig. 12) The outer rim is wave swept and sediment free. Its surface is locally covered by a

thin crust of coralline algae but this is subject to wave attack and is rarely preserved. Sediment on the surface of the platform is interspersed with (i.e. trapped by) thin

veneers of algal meadows and mussel beds; the sediments (sand, lithoclastic gravelly sand, and shelly sand) develop thicker pockets in depressions (Fig. 13A C). When all depressions and scours are filled with sediment and adjoining higher knolls are

veneered by a thin blanket of sand, the entire outer platform may appear as a uniform sandy or shell-encrusted surface. The sand trapped and bound by algae is bioturbated with little or no preservation of laminae (Fig. 13A, B). In section this sand veneer has a relatively smooth, flat top and a base that follows irregularities of the platform surface. The sand rests either on the platform or on an earlier deposited lag or gravel. Storm activity can, in the short term, disrupt and rework sediment veneers but the return of the algal meadow ensures that sand is trapped seasonally to accumulate.

Sediments on the extreme inner portions of platforms accumulate mainly in depressions. The sediment may be well-sorted gravel or composed of a lower section of gravel overlain by laminated to cross-laminated sand and shelly sand (Fig. 13D). Storms may temporarily erode and transport this upper sand and winnow the underlying gravels. The open notch environment is largely swept free of sediment, but the cryptic notch accumulates sediment in a manner similar to the platform.

Channelways, fissures, pools and pans in bench environments and cavities in the seacliff receive aeolian sand and, during storms, marine sediment (Figs. 12A and 13E). Much of the sand-sized sediment is flushed out later by marine and meteoric waters but locally the gravel may interlock and accumulate. Gravel blockages in interstices also assist in accumulating infiltrating sand. The resultant sediment body is disposed in the cavity or poo l / pan network of the limestone (Figs. 12A and 13E).

Solution pipes in the Pleistocene limestone are usually filled with uncemented yellow quartz sand. On platforms the sands in the pipes are scoured out by wave action to a depth of about 30 cm to form depressions (Fig. 14C and D). The sand or shelly gravel that fills these depressions consequently rests on yellow sand, a stratigraphic relationship that is common within solution pipes underlying Pleisto- cene platforms (Figs. 14D and 15B).

Beach-dune sands that occur in Type 2 shore settings are laminated sand, bubble sand and bioturbated shelly sand (Fig. 12B). They are typical high tidal, storm level and beach-ridge/dune deposits that exhibit similar stratigraphic and lithologic features to sediments described by Semeniuk and Johnson (1982).

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A. TYPE 1 PROFILE

~ BENCH DEPOSITS

l ~ 1 FO'STOGGRA;SZE• PLATFORM VENEER .~ i ~ 6 ~ ~ :' :~::::i":".::::::.!":::i~"

/ !i.!ii .: ~i:~.~!~:i~̀~!!~:.~!~:!~i~.~!~::!.;!~.F#!~i}~:~i~!i~i~!~!~2i~!~i~!~!:!)i::i!i~i~:~ i" .o .c . DEPOS.'TS %:!i~.i'i'I.'I~':~::!

....... l,J i = . i . . . . . . . . . . . . . . [ .... lt . . . . B. TYPE 2 PROFILE C, TYPE 3 PROFILE

• SHORELINE BEACH ~ . BRECCIA ~ .

SUMMARY OF CHARACTERISTICS OF ROCKY SHORE SEDIMENTS

SEDIMENT OCCURRENCE GEOMETRY DESCRIPTION TEXTURE COMMENTS ( HISTOGRAM )

PLATFORM BLANKET ON SHEET / BLANKET BIOTURBATED BIOMODAL ; SAND IS PLATFORM VENEER SAND AND MEDIUM AND BOUND AND VENEER GRAVELLYSANC FINE SAND WITH TRAPPED BY

LOCALLY WITH MUSSEL SHELL ALGAL TURF IN SITU GRAVEL MUSSELS

POTHOLES AND HOLLOWS IN PLATFORM

PLATFORM POTHOLE DEPOSITS NOTCH DEPOSITS

BENCH DEPOSITS

SHORELINE BEACH RIBBON

BRECCl A WEDGE

POTHOLES AND HOLLOWS AT BASE OF NOTCH

SOLUTION PIPES AND FISSURES ON BENCH

ALONG HIGH TIDAL SHORELINE. OVER~ING INNER PLATFORM AND NOTCH

ALONG HIGH TIDAL AND SUPRATIDA L SHORELINE

ISOLATED LENSES AND CYLINDERS AND FISSURE FILLS

ISOLATED LENSES CYLINDERS AND CAVITY FISSURE FILLS

CAVITY AND FISSURE FILLS

RIBBON

WEDGE

GENERALLY STRUCTURELESS GRAVEL WITH INTERSTITIAL SAND, MAYGRADE UPWARD TO LAMINATED SAND AND SHELLY SAND

STRUCTURELESS GRAVELLY SAND

LAMINATED BUBBLY TO BIOTURBATED SAND AND SHELLY SAND

MASSIVE BLOCK BRECCIA IN FRAME SUPPORT IN SUPRATIDAL AREAS AND IN FITTED FRAMEWORK IN TIDAL ZONES

UNIMODAL; PEBBLES, GRANULES AND VERY COARSE SANO, VARYING TO 81OMODAL: GRAVEL WITH INTERSTITIAL MEDIUM AND COARSE SAND

GENERALLY UNIMODAL: MEDIUM AND COARSE SAND, LOCAL GRAVEL

MEDIUM,COARSE AND VERYCOARSE SAND;GENERALLY UNIMOOAL

BLOCKS,BOULDERS AND COBBLES ARE DOMINANT; INTERSTITIAL SEDIMENT IS VARIABLE FROM GRANULES TO MEDIUM SAND

MEDIUM / COARSE SAND INTERSTITIAL TO THE COARSE UNIMODAL FRACTION OCCURS ONLY ON A SEASONAL BASIS

SEOIMENTS AREIN BEACH STRATIGRAPHIC SEQUENCE •

STRUCTURES IN INTERSTITIAL EXHIBIT BEACH STRAT- IGRAPHIC SEQUENCE

cf SEMENIUK AND JOHNSON, 1982 LCS

Fig. 12. Diagram summarising the essential features of the sedimentology of rocky shores.

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247

A . MUSSELS

Zcm I I

B . ~ MUSSEL

BURIED CLAST

2cm i i

__"~:~ (~ ~'~:~[;[~::~:¥'{~ "(" ' : ~ ' 5 . .: . . . . " .. ' . . . .~- SAND -27".~ " , . , < , .~!t., ~ ' . : . . . "

L i ~ i i ? i : i ; i ~ i ' : ; : ~ - . . . . . . ! !! ; : . ?,:i::.;::: • ? # : ' L ' . : : L. " - : :

':!:)~.. :'i . . . . . DROSSLAYERED MEDIUM ":':~:-:!'.':. ~ • AND Cl)ARSE SAND

:.".".'..\..:" :'.:::-2" ~ INTERSTIT IAL SAND • -J-I- I N T E R S T I "

°

O~:m'" " ") ' : : . : ; :-:~: ' : : ~ ~ ~ - ~ : io , ,i~!~= . : ' :

LCS

Fig. 13. Sketches of features of sediments from rocky shores. A, B. Sliced core showing structures within

the bioturbated, vegetation-structured sand veneer that overlies the rock platform. C. Profile through a sediment-filled pothole of middle part of platform. Upper part of profile was determined by box cores; lower part of profile was determined by excavation. D. Winnowed gravel fill in a pothole or inner part of

platform. E. Gravel and sand fill in fissure of bench. Profile of sedimentary fill was determined by excavation (air suction and trowelling).

The shoreline deposit of breccia (Fig. 12C) is commonly wedge-shaped. Initially there is a frame of boulders and blocks in a chaotic, poorly sorted array. Sand or sandy lithoclast gravel may occur interstitially. Partly filled interstices form potential geopetal structures. The lithoclast gravel and the sand fraction are emplaced partly by marine transport (toward the seaward edge of breccias) and partly by subaerial sheet wash from the land. Breccia deposits without interstitial sand/gravel in tidal environments appear to have adjusted internally through solution to form condensed

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and (progressively) fitted frameworks (Fig. 8C); this framework is similar to that

termed " impr isoned boulders" by Baker (1959). Breccia deposits without interstitial

sediment in supratidal environments remain in a block-supported framework (Fig.

8D). There is also a sheet of slabs, boulders and blocks left as a pavement strewn

across the subtidal sand flats in the wake of subtidal cliff retreat. These deposits with

their diagnostic rock platform imprint on the b o u l d e r / b l o c k components provide

valuable evidence of retreating shores.

Processes of erosion

The degradational processes The morphology of rocky shores is due largely to the interaction of several

degradational processes which include: (1) wave erosion; (2) bioerosion; (3) marine-

water solution; and (4) fresh-water solution.

Shoaling waves break at or near the subtidal cliff and become translatory as they

pass across platforms to the notch. The wave action, where water is charged with

sand and gravel particles, is a powerful abrasive agent and some workers consider

that the entire rock pla t form is developed mainly as a result of wave erosion (Hills,

1971 ; Bird, 1976). In this study area the extreme undercutt ing of cliffs is most likely

the effect of wave ac t ion on weakly lithified limestone because where the host

limestone is friable, collapsed blocks are abundant , suggesting that shore retreat was

rapid. The seaward part of the platform is more protected than the inner pla t form

and notch from such wave action by the veneer of algal-bound sand.

Irregular depressions and potholes, locally forming networks, are the result of

collapse of a surface hard crust through erosion of relatively softer limestone (to

develop "caverns") underneath. Circular depressions and potholes on platforms may

be exhumed solution pipes and erosion-emphasised solution pipes (Figs. 6D and

14C), or scour pools, and they form as a result of wave action. Depressions and

concavities are commonly filled with gravel and wave action keeps these winnowed

and in motion, thus enlarging the pools (Wentworth, 1944).

Bioerosion is an important mechanism in shoreline degradation and includes

Fig. 14. A. Fossil rocky shore sequence in Pleistocene limestone showing hummocky surface of platform (arrowed) overlain by gravel, boulders and shell. The shell assemblage is dominated by limpets, turbans and opercula of turbans. Coin (3 cm diam.) for scale. B. Fossil rocky shore sequence in Pleistocene limestone showing festoon-bedded sand overlying hummocky surface of platform (large arrows). The dominant shells infiltrating into cavities of the platform are turbans and their opercula which are typically inhabitants of modern rocky shores. Note lithophagic borings (small arrows). Coin (3 cm diam.) for scale. C. Potholes on modern rocky shore platform. These potholes are located over quartz-sand-filled solution pipes. D. Cliff section in a Pleistocene rocky shore sequence showing gravel ( 1 ) filling a pothole which has been excavated into a solution pipe, filled with quartz sand (2).

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abrasion (by urchins), boring (by pholads and other biota), chemical decomposition induced by organisms and feeding-abrasion by grazing organisms. Neumann (1968)

lists fourteen types of biota which contribute to bioerosion. Healy (1968) and Hodgkin (1970) concluded that biological erosion is a dominant and important mechanism in the formation of some elements of rocky shores. Hodgkin (1970) concluded that the notch was formed largely as a result of bioerosion.

Chemical corrosion (solution) by marine water is also an important process in shoreline degradation (Wentworth, 1939; Revelle and Emery, 1957). Seawater (and rainwater) perched on the bench develop solution pools (i.e. solution pits and pans of Wentworth, 1944) that have concave floors, slightly overhanging walls and rims. Such solution-induced corrosion also develops pinnacles and micropinnacles ("lapies" morphology of Guilcher, 1953), corroded shells, quartz grains in relief, pitting and rills on the limestone (Fig. 9).

Chemical corrosion by meteoric water is evident on the bench and seacliff which are normally out of reach of most waves and sea water. Cliffs and benches undergo alteration typical of subaerial environments. Meteoric water percolates down cavities and channelways enlarging them and developing solution rills, pittings and micro- pinnacles as in karst areas (Sweeting, 1972; and others).

Variation and interaction of processes of erosion The physical, ,biological and chemical processes of degradation depending on

location, aspect, availability of sedimentary particles and tidal level, can vary in intensity and can develop zones of distinctive macro- and micromorphology. For instance, within the zone of urchins which is behind the outer rim, bioerosion can result in relatively rapid and widespread degradation of the platform. In addition, the ongoing bioerosion effected by pholads, sponges, crabs and polychaetes serves to weaken overall the structure and coherence of the limestone. On the other hand, toward the inner platform where algal meadow cover is lacking and sediment mobility is more pronounced, physical erosion becomes important. On the bench and seacliff zones, meteoric, seaspray and storm processes dominate in sculpturing the surface morphology. In the notch environment the gradual recession of the notch eventually leads to an overhanging cliff which, in time, collapses. The cliff line retreats and a cavernous block is emplaced on the platform where it is subject to more wave erosion, bioerosion and chemical attack. Marked coastal retreat may completely remove the bench and then a high seacliff occurs above the notch. Rapid undercutting and retreat along the subaerial cliff results in a breccia wedge flanking the shoreline.

The bench morphology is interpreted by numerous workers to be a relict rock platform formed during a higher sea level (e.g. Fairbridge, 1950). Other workers suggest it may develop as a modern storm level or solution feature in Recent times (Bartrum, 1935; Wentworth, 1939, 1944; Guilcher, 1953; Hills, 1971). Our studies indicate that the distinctive morphology, bored surfaces and the characteristic

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sediment deposits of platforms are absent on the bench. However, there may be indurated Pleistocene layers within the stratigraphic sequence of the Tamala Lime- stone at the level of the bench. At many localities for instance there is a Pleistocene rocky shore horizon within the stratigraphic sequence of the Tamala Limestone at the same level as the bench, indicating that the bench in many localities is an exhumed indurated Pleistocene layer and therefore is a structural terrace. Locally at other sites, the bench is coincident with an indurated layer (which is cemented either by sparry calcite or calcrete) within the Pleistocene limestone sequence, again indicating that the bench itself is an exhumed indurated layer and therefore must be viewed as a structural terrace. The morphology of solution basins, pinnacles, micropinnacles and distinctive microkarst topography on the bench at all sites where it is developed further indicate the bench surface morphology can be developed, enhanced, or accentuated by modern high tidal, storm and meteoric processes. However, regardless of the origin of the bench, its morphology and micromor- phology do provide indication of its location within a rocky shore profile.

Preservable features

Rocky shores are products of erosion. The zone of erosion is progressively migrating, incising into new materials landward, and erasing itself at the seaward edge and several features such as the subtidal cliff and notch will rarely be preserved. When shoreline migration slows or stops there is potential for preserva- tion of diagnostic features associated with rocky shores. Many features, including geomorphic, sedimentologic and biologic types, may be used to identify ancient rocky shore sections. The main geomorphic features that are useful are (Figs. 4, 5 and 9): (1) a hummocky to undulating eroded surface which is extensive and subhorizontal; (2) cliffs, notches and seastacks, even if buried by breccia or beach deposits; (3) macromorphology such as overhangs, scour pools, pinnacles, structural benches and submarine caves; (4) irregular, scalloped depressions with overhanging walls excavated by urchins; (5) micromorphology such as micropinnacles, quartz grains in relief, truncated fossils; and (6) bench cavity systems.

Sediment accumulations on rocky shores can be identified by the criteria of geometry, internal structure, fabric and texture, sediment type, and stratigraphic relationships; these criteria indicate the location of a given sedimentary accumu- lation within one of the main depositional sites on rocky shores (viz. platform, cryptic notch, bench, shoreline breccia, shoreline beach/dunes and subtidal breccia). The main sedimentological units that are useful are (Fig. 12): (1) sediment veneers on the platforms; (2) lithoclast and skeletal gravels typical of scour pools on platforms, locally overlain by sands which are laminated and cross-laminated; in the Quaternary setting of this study these may be underlain by yellow sand within solution pipes; (3) sediments in cavities of seacliffs and benches; (4) breccia (wedge) deposits with their characteristic internal features such as open to sutured frame-

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works indicative of subaerial to tidal conditions, respectively; and (5) slabs, boulders and blocks with rocky platform imprints strewn across the subtidal sand flats seaward of the subtidal cliff.

The occurrence of a sheet or pavement of blocks and large rock slabs (5, above) within a subtidal marine unit may indicate a deposit left in the wake of a retreating rocky shore. A stratigraphic correlation between such block/s lab deposits and the p la t form/c l i f f /brecc ia suite onshore can provide indications of seaward to land- ward directions.

The biological features important to recognition of either rocky shores in a gross context or detailed geomorphic/habitat zones within rocky shores incluce: (1) skeletons of rocky shore assemblages; these indicate that rocky shore conditions either are present or occur sufficiently nearby to allow appreciable skeletal accumu- lation; (2) in-situ encrusting organisms (calcareous algae, barnacles and serpulid worms, etc.) on the subtidal cliff, outer rim, platform and notch; (3) excavations by urchins on the platform; (4) bioturbation of the sand veneer on the platform; and (5) boring structures produced by pholads and other biota. A summary of these diagnostic biological characteristics and their zone implications is given in Fig. 11. Some of these biological features have been used elsewhere by other workers to identify emergent Pleistocene platforms (Baker and Gill, 1957).

The examination of Pleistocene sections described below suggests that many of the above key features are preservable and these offer a tool for recognising both gross rocky shore conditions and the components of rocky shores.

PLEISTOCENE ROCKY SHORE SEQUENCES

Pleistocene rocky shores have been studied at twelve localities (Fig. 2). (We do not imply in this paper that the Pleistocene sequences described herein are time equivalent.) The localities are vertical cliff faces exposing lateral and down-dip expressions of Pleistocene rocky shores. However, the preservation is fragmentary in that the full profile is not exposed in the cliff. Thus relationships of platform to stacks or cliffs etc., could not be explored. Nevertheless it is common to find many of the features described from the modern and this section outlines the main features found useful for interpreting the ancient examples. In general, all localities exhibited features that enabled their environment of formation to' be interpreted, but for purposes of this paper four of the localities are described to illustrate typical features.

None of the Pleistocene rocky shore surfaces exhibit calcrete at the erosional interface, nor do they show subaerial erosional features at the interface; rather the erosional surface exhibits the suite of preserved rocky shore features as described on p. 252. This indicates that these Pleistocene rocky shore surfaces are not merely subaerial unconformities covered by later shoreline sediments.

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Muderup Rocks

This section exposes an erosional surface interpreted as a rock platform. The surface is well exposed along strike but traceable only to a limited extent inland.

Morphology: The large-scale morphology is broadly undulating with local over- hangs and cryptic spaces (Fig. 15F). The small-scale morphology is acutely scalloped with some development of pinnacles (Fig. 16A). The surface is bored (Fig. 16B,C).

Sediments: Coarse shell and lithoclast gravel overlie the platform surface and fill depressions and cavities; gravel is particularly abundant in scour pools.

Biota: Shells incorporated into the deposits are a typical rock platform assem- blage of turbans, limpels, barnacles and periwinkles similar to modern species; there are also in-situ encrustations of worm tubes, The rock surface of the fossil platform has been bored by a variety of lithophagic organisms (Fig. 16B, C) and sculptured by urchins (Fig. 16A).

Discussion: Lithified shallow subtidal and foreshore/swash zone sediments (il- lustrated in Fig. 7C by Semeniuk and Johnson, 1982) directly overlie this platform, indicating that it was covered by sand at a later date. The tidal levels indicated by the rock platform features and the beach sequence are compatible.

Spearwood quarry and Ocean Reef

The section at a Spearwood quarry is well exposed in three dimensions and exhibits a wide range of rocky shore morphology: platform, cliff, notch and bench (Fig. 17). The section at Ocean Reef was trenched and exposed by earthworks; the exposure exhibits a platform, sea stacks and subtidal sand with scattered blocks.

Morphology: A subhorizontal surface, buried seacliffs and eroded seaknolls are evident (Fig. 17A). Micromorphology is broadly scalloped on the subhorizontal surfaces (Fig. 17C) and acutely scalloped on higher parts of the sea knolls and cliffs. Encrusting coralline algae fill surface depressions of the stacks. There is development of overhangs and cryptic notches.

Sediments: Circular and cylindrical depressions filled with shell and lithoclast gravel occur locally and these are underlain by yellow sand indicating scoured-out solution pipes (Fig. 14D). A sheet of structureless (bioturbated) sand overlies the subhorizontal platform. Some parts of the platform are extremely cavernous and these cavities are filled with shell and rock gravel and sand. Locally, isolated collapsed and displaced blocks derived from an eroding subaerial cliff are embedded in the accompanying overlying beach /dune sediments which overlie the rocky shore section. Subtidal sands to seaward of the main exposed rocky shore section at Ocean Reef contain - 1 m-sized blocks of bored and encrusted limestone blocks derived from a rock platform.

Biota: Reworked rock platform biotic assemblages occur in the sediments above the platform. Locally in-situ worm tubes occur on overhangs and on blocks

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Fig. 16. Closeup of Pleistocene rocky platform illustrated in Fig. 15F. Compare these photos with Fig. 9D. A. Scalloped surface (arrowed) of fossil platform; this surface is typical of echinoid sculptured surfaces. B. Same fossil rock platform as above but with centimetre-sized borings. C. Underside of an overhang showing honeycomb network of small (millimetre-sized) borings (arrowed).

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i i i iiiiii!i!iii

Fig. 17. Features of a Pleistocene rocky shore sequence. A. Profile illustrates a bench, cliff, platform and small residual knolls. B. Closeup showing overhangs on cliff controlled by the eolian cross-bedding of host limestone into which the rocky shore was cut. C. Closeup of rock platform showing scalloped, stepped and overhanging morphology and micromorphology which are interpreted as echinoid sculpturing.

d i s lodged f rom subt idal seacliffs and there is deve lopment of acutely sca l loped

surfaces indicat ive of urchin excavat ions; the sand (2 -5 cm thick) overlying the

fossil rock p l a t fo rm is s tructureless to weakly b io tu rba t ed and is typical of the sand

sheet that is t r apped by algal meadows.

Marmion- Trigg Island sections

This area exhibi ts an upper t idal to s torm t idal level of a Pleis tocene rocky coast

which conta ined p la t form, cliff and breccia wedge.

Morphology: The p la t fo rm surface is highly kars t i f ied (Fig. 14B), cavernous,

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potholed and varies from undulating to stepped, with overhangs (Fig. 15A--E). Micromorphology is acutely scalloped to smooth. Locally a seacliff is preserved and this is flanked by breccia.

Sediments: Shelly and lithoclast gravels are dominant for up to a metre im- mediately above the platform surface (Figs. 14A and 15A, B). The gravel is structureless with platy grains oriented horizontally and it passes up into finer (granule-sized) planar bedded/ laminated and festoon-cross-bedded shelly sediment and then into laminated sand and shelly sand typical of beach (swash) sequences. The breccia is composed of boulder-sized limestone debris which locally forms a frame but also may be "floating" in a sandy matrix.

Biota: The biota is typically rock platform (Fig. 14A, B) and contains turbans, limpets, barnacles and periwinkles similar to modern assemblages. Notably common is the large limpet (Patellanax) which specifically colonises outer rim environments, indicating the presence of a nearby outer rim during the Pleistocene.

DISCUSSION AND CONCLUSIONS

The rocky shores cut into the wave-dominated limestone coastlines of southwest- ern Australia display a similarity which suggests they may provide a valuable tool to identify similar shoreline deposits in the stratigraphic column. This paper has documented many features of modern rocky shores and equivalent Pleistocene sequences in the Perth Basin that might be useful firstly in locating similar ancient features and secondly in interpreting ancient rocky shore sequences in detail in terms of their palaeoenvironments, palaeogeography, palaeobiology and palaeoceanogra- phy.

Although modern rocky shores along the southwestern Australian coastline are cut mainly into Pleistocene limestone, our experience elsewhere and the literature indicate that many of the processes and products described here are common to rocky shores cut into a wide variety of carbonate rock types. The essential principle is that given erosive marine conditions and a carbonate sequence exposed at the coast, then rocky shores will be developed because similar physical, chemical and biological processes can operate forming similar profiles and sediments.

Modern rocky shores and the Pleistocene equivalents in the Perth Basin contain very similar types of sediment grains. Such consistency could be expected along most rocky shores since earlier sequences are sources for later deposits. Thus in the ancient rock sequences lithologic factors such as changes in fauna or terrigenous content which alert researchers to the possibility of depositional breaks a n d / o r erosional phases, are absent. Rocky shore features m a y b e the only indications of such changes within otherwise continuous skeletal grainstone sequences. Careful examination of sediment types should show evidence of reworking (as lithoskels, lithoclasts and blocks) and an examination of sediment body geometry together with structural, fabric and textural criteria should serve to identify the sediment as to its site of formation on a rocky shore.

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the unconformity surface of the rocky shore at various locations normal to the shoreline. Note that the

various stratigraphic features across this profile can be used to indicate location within a rocky shore

environment and also landward-seaward directions. In this example the younger sediments that overlie

the unconformity are shallow subtidal /beach/dune sequences as described by Semeniuk and Johnson

(1982).

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In this study there is marked similarity between modern and Pleistocene biota in terms of skeletons and also in terms of sedimentary products such as bioturbation

and borings. Correlation between modern and Pleistocene fauna is relatively easy because many of the Pleistocene species are still extant. Thus robust whelks, turbans and limpets provide direct evidence of rocky shore conditions; the presence of other fauna and flora can be inferred from their products or imprints (for instance, rock borings and excavations by pholads, urchins and abalone and the bioturbation features generated by the algal meadow on the platform).

Many shallow-water carbonate sequences are marked by the development of cycles (e.g. Fischer, 1964; Schenk, 1969; Read, 1973; Semeniuk, 1973) consisting of offshore, nearshore and onshore sequences, disposed in regressive or transgressive arrays. Since carbonate sequences are prone to early lithification, either under submarine or subaerial conditions there is potential for development of rocky shores because a cycle with an erosional base may carve platforms or cliffs in lithified earlier deposits. Identification of rocky shores would then provide valuable indica- tions of depositional breaks within the sequence. Rocky shores form the edge of a basin for that time plane and hence sharply mark the boundary between marine and non-marine areas. Detailed mapping may indicate local seaward/landward direc- tions, since this study indicates there are also small- and medium-scale variations within rocky shores, both laterally and down-dip (Fig. 18). If rocky shore sections are found commonly within sedimentary sequences they may indicate an exposed environment of erosion alternating with shoreline progradation and cementation.

The deposits overlying rocky shore sequences, however may differ regionally or through time. The modern and Pleistocene examples of the Perth Basin are overlain by high energy beach /dune deposits (Fig. 18; Semeniuk and Johnson, 1982). This may not always occur. Growth of barriers or changes in relative sea level may cause rocky shores to be overlain by finer sediments, skeletal banks, reefs or soils.

Features described in this paper should enable recognition of rocky shorelines in ancient sequences and also should enable inferences to be drawn about the oceano- graphic and environmental conditions of the ancient or stranded shoreline. For instance, the sequences mostly represent environments that were wave-dominated eroding coasts and therefore give an indication of coastal setting. The various units of rocky shores are accurate indicators of tidal levels and sealevels. They can be used to establish relative sea-level changes in Holocene and Pleistocene units, and the relative height and thickness of morphologic units can give an indication of tide versus storm-water levels.

ACKNOWEEDGEMENTS

We thank E.B. Collins and D.J. Searle for critically reviewing the manuscript and P.N. Chalmer for providing some mollusc identification. J. Regazzo is thanked for typing and manuscript production.

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