An extensive bank of carbonate sediment is forming along the eastern margin of Shark Bay, Western Australia. Development of the bank is attributed partly to the modifying influence of seagrasses on the physical environment. Bank formation under organic cover elucidates the origin of older moundlike bodies of carbonate sediment which lack an internal skeletal framework.

The bank covers 400 sq mi (1,036 sq km) and is 80 mi (129 km) long; average width is 5 mi (8 km). In cross section the bank is wedge-shaped, and the maximum sediment thickness of about 25 ft (7.6 m) is at the seaward margin. For descriptive purposes, the bank is divided into two intergradational structural forms--the basal sheet and the submarine levees. The basal sheet pinches out landward beneath younger intertidal and supratidal sand sheets, thus recording the progressive seaward advance (regression) of the bank. The submarine levees are forming at the margins of tidal channels, and are now the most active centers of deposition and seaward widening of the bank. More than 50 tidal channels cut across the bank.

Substrates in the sublittoral environments of the bank are covered in varied density by three seagrass communities: (1) the Posidonia community, (2) the Posidonia-algal community, and (3) the Cymodocea community.

Seagrasses influence sedimentation by (1) postmortem accumulation of skeletal carbonate from the epibiota and sheltered benthos, (2) trapping of particles and protection of substrates by the leaf baffle, (3) stabilization of sediment by the rhizome and root mesh, (4) colonization of new substrates by vegetative reproduction, and (5) modification of the chemical environment above and below the sediment-water interface.

Bank sediments are mainly biogenic carbonate sediments but include admixtures of terrigenous detrital grains (quartz). In the sublittoral environment, skeletal fragments of encrusting foraminifers and articulate coralline algae from the seagrass epibiota characterize sediments deposited under seagrass cover. Sediments below dense seagrass cover contain up to 30 percent by weight of fine particles (<62µ), most of which are silt-size skeletal fragments of magnesium-calcite composition.

Older carbonate mounds or "banks" may have been formed as a result of the influence of an external organic cover or baffle other than seagrasses. Where the organic cover, like seagrass, is not preserved in the sediment body, its former existence may be inferred by the presence of detached encrusting organisms and a large proportion of silt- and clay-size skeletal fragments.