Hamelin and Freycinet Basins are shallow (<15 m) landlocked embayments in Shark Bay, Western Australia, that represent successive stages in the development of a restricted marine basin. The historical evolution of these modern basins is delicately recorded in Holocene recent stratigraphic units and types of sediment bodies. Marine sedimentation commenced when the basins were flooded with oceanic waters (35-40^pmil salinity) by the transgressing Holocene sea at approximately 7,000-8,000 years B.P. A thin veneer of lithoclasts and lithoskels mixed with skeletal grains from resident communities was deposited as the Basal Sheet unit behind the advancing strand. During the ensuing recent interval (5,000 years B.P. to the present), regression has occurred and thick offlappin sequences of sediment units have formed in both basins. Freycinet Basin waters gradually reached metahaline concentrations (40-53^pmil salinity); sedimentation was dominated by seagrass communities, and burrow-mottled skeletal wackestone, packstone, and grainstone of the Bank Unit formed on sublittoral platforms; skeletal packstone and wackestone were deposited in embayment-plain environments. Seagrass banks grew from basin margins, intrabasinal islands, and shoals.

During the equivalent interval, Hamelin Basin passed through a transitory metahaline phase to its present hypersaline (56-70^pmil salinity) concentration. In the metahaline phase, sedimentation was dominated by seagrass communities forming sediments of the Bank Unit similar to those in contemporary Freycinet Basin. The gradual onset of hypersalinity marked the cessation of bank growth. As waters reached hypersaline concentrations, the biota was restricted to mollusks and algae. Coquina, microcoquina, ooid grainstone, intraclast breccia, pelletal sediment, and cryptalgal boundstone of the Sublittoral Sheet and Intertidal Veneer units were formed. Platforms of grainstone and coquina spilled over the relict bank structures into central basin regions. Storm ridges (Hamelin Coquina unit) o grainstone and coquina were deposited in the supratidal zone, and gypsum precipitation commenced in marginal ponds and tidal flats.

The increase in salinity in both basins has been due primarily to construction of large barrier seagrass banks at their northern extremities throughout Holocene-recent time. The barrier banks partitioned the landlocked basins from open, northern reaches of Shark Bay and thus acted as sills that progressively decreased tidal flushing. If future sea level is static, salinities will increase further in both basins, possibly culminating in phases of evaporite deposition.