The Biogeography of the Australian North West Shelf || Intertidal Rocky Shores

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<ul><li><p>The Biogeography of the AustraC H A P T E R3</p><p>Intertidal Rocky Shores</p><p>Intertidal rocky shores provide diverse habitats and generallysupport species-rich biota. Many rocky shore species that inhabit theupper-littoral and lower-littoral zones are narrowly confined to thosezones and comprise distinctive intertidal assemblages. On the other hand,most rocky shore mid-littoral invertebrates are also found on rockysubstrata in the adjacent shallow sub-tidal zone.</p><p>The most critical factors that determine the ecological characteristics ofrocky shores are their level (height) within the intertidal zone, their degreeof exposure towave action, themagnitude of tidal range, and the rugosityof the surface, i.e., the complexity of the surface in terms of loose stones,ledges, pools, crevices, and gutters. There is great variation in the inverte-brate assemblages of rocky shores along the coast and islands of the NorthWest Shelf as a result of varied ocean conditions and the varied geomorphicforms and shore profiles that different rock types produce. Of the latter,rock platformsnear horizontal surfaces in the intertidal zoneprovidethe most diverse habitats and the most varied biota.</p><p>This section considers rock types, rock platforms and their geomorphicfeatures, and the characteristic species assemblages that inhabit rockyshores of the North West Shelf.3.1 ROCK TYPES</p><p>The rock types along the coast of the NorthWest Shelf are varied, form-ing shores of many different profiles, surfaces, textures, rugosities, andother habitat characteristics that affect the lives of the invertebrates andfishes that inhabit them. While there is no evidence to suggest that rocktype is a major factor in the geographical distribution of rocky shore spe-cies (with the exception of certain taxa that bore only in carbonate rocks), itcertainly affects their local ecological distribution and relative abundance.</p><p>There are two sections of the North Shelf coast where Proterozoicrocky shores are dominantthe northwest Kimberley (Kimberley and59lian North West Shelf 2013 Elsevier Inc. All rights reserved.</p></li><li><p>60 3. INTERTIDAL ROCKY SHORESBonaparte Bioregions) and the Dampier Archipelago where stretches ofrocky shore are interrupted by short beaches and mangals. Quaternarylimestone shores are common on the Pilbara coast but rare in the KimberleyBioregion except for superficial biogenic encrustations over the surfaces ofrock platforms.</p><p>Shores of the Canning Bioregion are characterized by long stretches ofsandy beach, sometimes with mangal habitats, and rocky shores are repre-sentedbyonlyoccasional rockyheadlandsofQuaternaryorMesozoic rocks.The shore of the Eighty Mile Beach Bioregion, between Cape Jaubert andCape Keraudren, is a stretch of beach around 230 km long without rockyshore habitat andwithout offshore islands. The importance of that coastlineas a biogeographic barrier to obligate rocky shore species is discussed later.3.1.1 Rocky Shores of Precambrian Igneousand Metamorphic Rocks</p><p>The Kimberley Bioregion fromCape Londonderry to Cape Leveque is amacrotidal, ria-coast. The shores are dominantly rocky (between shortbeaches and mangal habitats) comprising mainly Proterozoic igneousrocks andmetamorphosed bedded sandstone. In the Bonaparte Bioregion,east of Cape Londonderry along the western side of Joseph BonaparteGulf, the coastline is also predominantly of Proterozoic sandstones butit is not a ria-coast and has a low profile of cliffs and sloping rocky shores.</p><p>Onshores of these kinds, the intertidal zone is generally steepandnarrowwith a highly irregular profile, prolific growth of macroalgae, and a richcryptic intertidal invertebrate fauna. The Proterozoic sandstones (mainlyKing Leopold andWarton Sandstones) are flat-bedded and strongly jointedand generally feature angular ledges and blocks (Figures 3.1 and 3.2). Theyoften form terraced intertidal rock platforms with wide mid-littoral andlower-littoral zones. In the northKimberley, there are also basalt shores thatmay have smooth, sloping ramps (Figure 3.3) or fields of rounded bouldersor cobbles (Figure3.4). In thenorthernBonaparteArchipelago,many islandshave a cap of Tertiary laterite that produce scree slopes and rocky shores ofirregularly shaped laterite boulders (Figure 3.5).</p><p>Rocky shore habitats of Archaeozoic igneous and metamorphic rocksoccur on the shores of the peninsulas and islands of the central Pilbaracoast, where the Pilbara Craton outcrops along the coast between CapeKeraudren and Cape Preston and in the Dampier Archipelago. Sectionsof this coast are also drowned and ria-like and characteristic shore profilesare of massive basalt headlands and boulders tumbling into the sublittoralzone (Figure 3.6). Rock platforms are not developed on the Archaeozoicrocky shores of the Dampier Archipelago (but do occur on Pleistocenelimestone shores that form the perimeter of the archipelagosee below).</p></li><li><p>FIGURE 3.1 West Montalivet Island, Bonaparte Archipelago, north Kimberley. Flat-bedded and strongly jointed Warton Sandstones form rock platforms and angular blocksand ledges on a seaward shore. Photo: Natalie Rosser.</p><p>FIGURE 3.2 Flat-bedded Warton Sandstone at high tide in Sampson Inlet, CamdenSound, North Kimberley. During the Last Glacial Maximum, this long, winding, narrow inletwas a river canyon incised into the western margin of the Kimberley Plateau but it is nowflooded by the sea. Photo: Barry Wilson.</p><p>613.1 ROCK TYPES</p></li><li><p>FIGURE 3.3 A smooth basalt (Carson Volcanics pillow larva) shore at North MaretIsland. Note the overlying laterite cliff and fallen boulders. The basalt surface representsthe unconformity with the laterite cap. It outcrops at about the intertidal zone in many placeson the eastern shore of North Maret Island. Photo: Barry Wilson.</p><p>FIGURE 3.4 A shore of basalt cobbles of Carson Volcanics in St. George Basin, KimberleyBioregion. Note the flat-topped Mt. Trafalgar mesa (King Leopold Sandstone) in the back-ground. Photo: Barry Wilson.</p><p>62 3. INTERTIDAL ROCKY SHORES3.1.2 Rocky Shores of Mesozoic Sedimentary Rocks</p><p>In thenorthernpart of theCanning coast, fromCapeLeveque toCapeMis-siessy, many rocky headlands are of early Cretaceous sandstones that varygreatly in lithology. They include hard, sometimes silicified, fine to coarse</p></li><li><p>FIGURE 3.5 Brunei Bay, NorthMaret Island. Tumbled laterite boulders from the Tertiarylaterite cap often form rocky shores in the Bonaparte Archipelago. Photo: Barry Wilson.</p><p>FIGURE 3.6 Tumbled Archaean basalts at Dolphin Island, with a mangrove fringe,typical of the rocky shores of the Dampier Archipelago. Photo: Barry Wilson.</p><p>633.1 ROCK TYPESquartzitic sandstones, soft mudstones and siltstones, and conglomeratedeposits. Accordingly, the rocky shores of those areas are of varied profile.Theymay be cliffed, e.g., Emeriau Point (Figure 3.7) and Gantheaume Point.Rock platforms are poorly developed on shores of that kind.</p></li><li><p>FIGURE 3.7 A cliffed headland in Mesozoic sandstone at Emeriau Point on the DampierPeninsula. Photo: Barry Wilson.</p><p>64 3. INTERTIDAL ROCKY SHORES3.1.3 Rocky Shores of Quaternary Limestone</p><p>On the north Kimberley coast, there are no Pleistocene rocky shores,perhaps because of ongoing subsidence.1 However, throughout the biore-gion there are extensive intertidal fringing coral reefs (Chapter 4) withrock platforms built of Holocene coralgal material over Proterozoic rocksthat provide hard calcareous substrate for burrowing invertebrates.There are also several examples in the southern part of the bioregionof massive limestone platforms in the intertidal zone whose age andorigins are undetermined (Chapter 4).</p><p>Limestone rocky shores, built of Late Pleistocene limestone ofmarine oraeolian origins, are a feature of shores and islands in the Pilbara as far eastas Cape Keraudren (Figure 3.8). These shores may be of low cliffs, com-monly with a supralittoral bench 36 m above mean sea level and a widerock platform (Figure 3.9) or they may form limestone barriers alongsandy shores. The limestone barriers often shelter lagoons and mangalsbehind them like that at Cape Keraudren.</p><p>Intertidal limestone rock platforms are characteristic of seaward shoresthat are exposed to moderate or severe wave action and are discussed inmore detail later (Section 3.1.2). Such limestone rocky shores occur on thewestern side of Exmouth Gulf, the inner shelf islands of the West Pilbara,and around the outer perimeter of the Dampier Archipelago (where theyrepresent the remnants of the Late Pleistocene shore line). The limestone isgenerally massive and hard.</p></li><li><p>Tidalmud flats</p><p>CootenbrandCreek</p><p>BlazeBay</p><p>CapeKeraudren</p><p>0 1 2</p><p>km</p><p>L/S rock platform</p><p>L/S ro</p><p>ck</p><p> pla</p><p>tform</p><p>W</p><p>N</p><p>E</p><p>S</p><p>FIGURE 3.8 Cape Keraudren, the most easterly of the Quaternary limestone headlandson the Pilbara coast. It is typical of the limestone coastal barriers of the Pilbara (nearshore)Bioregion with embayments, mangals, and tidal creeks developed behind it. The intertidalrock platform in front has a rich invertebrate fauna including a diversity of corals but thereis no reef-building. Source: Landgate, W.A. Government.</p><p>FIGURE 3.9 A typical West Pilbara Late Pleistocene limestone rock platformwith supra-littoral bench on a seaward shore at Kendrew Island, Dampier Archipelago. There is aspecies-rich fringing reef along the reef-front with a diverse assemblage of reef-buildingcorals (not visible). Photo: Barry Wilson.</p><p>653.1 ROCK TYPES</p></li><li><p>66 3. INTERTIDAL ROCKY SHORESThe limestone shores of western and southern Exmouth Gulf and someof the offshore islands are remnants of Late Pleistocene coral fringing reefswith well-preserved fossil corals. Along the mainland shores of the WestPilbara, some Late Pleistocene limestone shores are of dunal origin withcrossbedding and may contain fossil terrestrial snails (camaenids) whileothers are of lagoonal origin and contain fossil bivalves and gastropodscharacteristic of shallow inshore marine habitats.</p><p>There are also limestone shores, thought to be of Pleistocene age, in thenorthern part of the Canning Bioregion. They are of varied lithology andhave been referred to the Bossut Formation.13 However, a review of thebiostratigraphy of the Canning coast4 has shown that the Bossut Forma-tion may be an artificial mixture of lithologies of different Pleistoceneand Holocene ages. Whatever their age, these limestone shores oftenexhibit well-developed intertidal rock platforms.</p><p>On the northwestern coast of the Dampier Peninsula in the southernKimberley (not to be confused with the Dampier Archipelago in thePilbara), finely bedded or crossbedded limestone, attributed to the Bos-sut Formation, outcrops along the coast at several locations.5 Such lime-stone shores occur along the seaward side of Packer Island (Figure 3.10)and between Middle Lagoon and North Head (Figure 3.11). This forma-tion is referred to as the North Head Limestone and is described as abioclastic and quartzose fine calcarenite.4 On the shore, it weathersand presents intertidal habitats that are similar to the typical LateFIGURE 3.10 An algal-dominated rock platform of Pleistocene NorthHead Limestone atPacker Island (near Lombadina) Dampier Peninsula, southern Kimberley. The upper part ofthe upper-littoral zone is etched; the lower part with rock-oysters (Saccostrea cucullata). Thehigh supralittoral bench suggests an early or middle Pleistocene age. Photo: Barry Wilson.</p></li><li><p>FIGURE 3.11 A rock platform of North Head Limestone on the southern shore ofMiddleLagoonon theDampier Peninsula, southernKimberley. Theuppermid-littoral flat in the fore-ground is pot-holedwith raised algal rims andalgal turf. Thehigh rocks in the backgroundareremains of a limestone headland that has been eroded away. Photo: Barry Wilson.</p><p>673.1 ROCK TYPESPleistocene limestone rock platforms of the Pilbara and West coastswith high supralittoral benches, wide intertidal rock platforms, andstrongly etched surfaces in the supralittoral zone (Figure 3.10). Anoyster-zone with its associated invertebrate fauna is moderately welldeveloped in the lower part of the upper-littoral zone. Fronting thecliffs, there is a wide, algal-dominated rock platform with an array ofmid-littoral and lower-littoral microhabitats and diverse assemblagesof invertebrates including scleractinian corals although the latter arenot reef forming.</p><p>On the north-facing cliffs at the southwestern entrance of Pender Bay,there are patches of another limestone formation, called the Chimney RockOolite, also considered to be the part of the Bossut Formation lying uncon-formably over Mesozoic sandstone. It is described as a calcreted, well-cemented oolitic limestone.4 It does not form intertidal rock platformsbut is strongly etched in the high supralittoral zone and there are sectionsof the shore comprising fallen limestone boulders. Its rocky shore inver-tebrate fauna is impoverished.</p><p>Further south, on the Pindan coast south of Broome, there are cliffs ofmassive limestone, up to 13 m high (above MSL) along the western shoresof Gourdon Bay and southwest of Cape Latouche-Treville as far Cape</p></li><li><p>FIGURE 3.12 A limestone (calcilutite) cliff, assigned to the Pleistocene Bossut Formation,at Cape Latouche-Treville, Canning Bioregion. This rocky shore has a very restricted inver-tebrate fauna. Photo: Barry Wilson.</p><p>68 3. INTERTIDAL ROCKY SHORESBossut that are also attributed to the Bossut Formation.5 At CapeLatouche-Treville, the limestone is a fine, horizontally bedded (sometimesslightly folded) calcarenite, and calcilutite with shelly bands (Figure 3.12).There is a high supralittoral bench and the cliff face and edge are smooth(i.e., not etched). There is little or no development of the upper-littoraloyster-zone (Saccostrea cucculata) with its characteristic invertebrate asso-ciates. A rock platform is poorly developed and the mid-littoral zonebelow the cliff face comprises a steep sand ramp. There are areas withboulder fields (of limestone and Cainozoic laterite) in the mid andlower-littoral zones that provide habitats for invertebrates but the rockyshore fauna is generally rather sparse.</p><p>Holocene limestone known as beachrock commonly occurs as a slopingstony ramp on upper-littoral beaches (Figure 3.13). It is formed by shallowsubsurface cementation and consolidation beneath the sand and may beexposed by erosion. Because of its slope and exposure to air, sun, waves,and sand scouring, it is an inhospitable habitat for invertebrates but theremay be ledges and shallow pools that provide refuge and some of theupper-littoral rocky shore gastropods may be found there, especiallylittorinids, nerites, and siphonarians.</p></li><li><p>FIGURE 3.13 Holocene beachrock slope in the upper intertidal at South Beach, SouthMaret Island, Bonaparte Archipelago. The rocky shore above the beach is of laterite and thereis a wide fringing coral reef of theAcropora bank kind in themid- to lower-littoral zone. In theMaret Islands, beachrock deposits like this are often fossiliferous. Photo: Barry Wilson.</p><p>693.1 ROCK TYPES3.1.4 Limestone Rocky Shores of Pre-Pleistocene Age</p><p>At the western end o...</p></li></ul>


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