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A carbonate bank of Holocene age extends for 80 mi (129 km) along the mainland coast of Shark Bay, Western Australia. Development of the bank is attributed in part to the modifying influence of seagrasses on the physical environment. Bank formation under organic cover partly explains the origin of older mound-like bodies of carbonate sediment.
The bank is wedge shaped in cross section, has an average width of 5 mi (8 km) and a maximum thickness of 25 ft (7.6 m) at the seaward margin. For descriptive purposes, the bank is divided into 2 intergradational structural forms--the basal sheet and the submarine levees. More than 50 tidal channels cut across the bank. Seaward advance of the bank has been rapid; estimates of average rate of advance range from 528 ft (161 m) to 754 ft (230 m) per 100 years.
Salinity in sublittoral bank environments ranges from 38 ^pmil to more than 55 ^pmil (metahaline water type of Shark Bay). Substrates are covered in varying density by 3 seagrass communities.
Bank sediments are biogenic carbonates with admixtures of terrigenous clastics. Sediments deposited under seagrass cover are characterized by skeletal fragments of encrusting foraminiferids and articulate coralline algae from the seagrass epibiota. These sediments may contain up to 30% by weight of fine particles (<62µ), most of which are silt-size skeletal fragments of magnesium-calcite. Intertidal and sublittoral sand-sheet sediments are characterized by "micritization" of carbonate grains, whereas seagrass bank sediments become enriched in Fe and Mn.
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