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Carbonate Bank and Wave-Built Platform Sedimentation, Edel Province, Shark Bay, Western Australia1

By
James F. Read
James F. Read
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Published:
January 01, 1974

Abstract

Bank and wave-built platform deposits, which are superficially similar, are forming in inlets in the Edel province, Shark Bay, Western Australia. Fringing and barrier banks of skeletal grainstone have formed by accumulation of skeletal material from a seagrass (C/modocea) community on bank surfaces. Wave-built platforms and sills have formed by accumulation of grains eroded from Pleistocene sediments. The eroded grains are mainly skeletal fragments, termed “lithoskels” in this paper. Lithoskels are whole or fragmented skeletal grains reworked from unconsolidated or semiconsolidated sediments of an earlier sedimentary cycle.

The banks are biostromal wedge-shaped or lenticular bodies which have flat tops and marginal slopes of 5° or more. Wedge-shaped bodies form as fringing banks bordering inlet shores, whereas lenticular bodies are barrier banks which extend across inlets as shallow sills. Bank deposits are as thick as 6 m and they extend 2-5 m above the surrounding seafloor. Large areas of bank surfaces ore in water depths of 2 m or less and are sparsely covered by a seagrass (Cymodocea) community. Typical bank sediments are skeletal grainstones which contain abundant grains of mollusks, foraminifers, and coralline algae from the epibiota and sheltered benthos. Small percentages of matrix in bank sediments reflect the sparse seagrass cover and the limited effect of the leaf baffle. Sediments beneath seagrasses are stabilized by the binding action of the seagrass rhizomes and root mesh. Sea-grasses on barrier banks stabilize megaripples and help to maintain the relief of the banks against erosive tidal currents.

Wedge-shaped or sheetlike bodies of lithoskel grain-stone form as wave-built platforms bordering inlet shores; lens-shaped bodies of lithoskel grainstone extend across inlets as shallow sills. The grainstone bodies are 0-3 m thick at seaward margins. They are flat topped and have marginal slopes ranging from 5° to more than 30° in protected areas. Surfaces of wave-built platforms and sills are inhabited by molluscan communities that supply only small amounts of skeletal material to sediments. Seagrasses are generally absent, and sedimentation has been dominated by physical processes. Sediments are mainly lithoskel grainstones composed of rounded sand-size lithoskels of molluscan and coralline algal fragments, lithoclasts, detrital quartz grains, and small amounts of skeletal material from resident communities.

The grainstone bodies overlie skeletal packstone and wackestone (0-4.5 m thick) in offshore areas and pinch out landward on tidal terraces cut into Pleistocene limestone. Locally, banks of skeletal grainstone grade laterally into wave-built platforms of lithoskel grainstone, and they also are overlain by lithoskel grainstones following shoaling and removal of seagrasses. In tidal flats, lithoskel grainstones are overlain conformably by a thin sheet (0.5 5 m thick) of pellet grainstone and intraclast breccia.

Waves and currents have been important agents in the formation of banks and wave-built platforms. Material transported from surfaces of wave-built platforms and banks accumulates on sloping margins along with in situ skeletal material, causing the grainstone bodies to prograde as inclined foreset beds over skeletal packstones and wackestones. Barrier banks have formed by outbuilding and merging of fringing banks from opposing shores; other sills have formed by growth of wave-built platforms, and some are due to drowning of “highs” in the Pleistocene surface. Rapid outbuilding of sills takes place by deposition on submarine fans at mouths of channels. Most contacts between prograding grainstone bodies and underlying sediments are marked by mottled structure formed by burrowing organisms.

Skeletal-grainstone banks in the geologic record have probably formed where resident communities produced large quantities of skeletal carbonate but did not act as baffles. However, wave-built platform deposits can form by accumulation of skeletal material eroded from older formations in areas where resident communities have supplied only small amounts of skeletal material. Recognition of lithoskels may help in distinguishing bank and wave-built platform limestones.

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Contents

AAPG Memoir

Evolution and Diagenesis of Quaternary Carbonate Sequences, Shark Bay, Western Australia

Brian W. Logan
Brian W. Logan
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James F. Read
James F. Read
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Gregory M. Hagan
Gregory M. Hagan
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Paul Hoffman
Paul Hoffman
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Raymond G. Brown
Raymond G. Brown
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Peter J. Woods
Peter J. Woods
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Conrad D. Gebelein
Conrad D. Gebelein
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American Association of Petroleum Geologists
Volume
22
ISBN electronic:
9781629812175
Publication date:
January 01, 1974

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