Diagenetic phenomena in recent carbonate sediments from Shark Bay, Western Australia, are studied in terms of sedimentary units and diagenetic environments related to concentration gradients in embayment waters and groundwater brines.

The Hamelin Coquina comprises beach- and storm-ridge deposits, pelecypod coquina, and ooid grainstone. Three diagenetic environments influence these sediments. In the vadose soil profile, solution takes place in the upper layer, and solution and precipitation occur in the gravitational zone. Skeletal aragonite undergoes neomorphic replacement by blocky calcite; sparry calcite forms cement. The phreatic zone contains groundwater brine (31-184^pmil Cl); no diagenetic alteration occurs. The intertidal zone contains hypersaline (31-39^pmil Cl) waters; cementation is by cryptocrystalline aragonite.

The Intertidal Veneer includes sheet deposits in intertidal and supratidal zones; primary sediments of skeletal-fragment grainstone and ooid grainstone are altered to pellet grainstone and packstone in most diagenetic fields. The oceanic metahaline field (19-31^pmil Cl) is characterized by bioturbation structures and textures, by homogeneous to burrow-mottled structure and fragmented skeletal grains, and by tubular fenestral fabrics (root molds) in the supratidal zone. Features of the hypersaline field (31-39^pmil Cl) are pellets and pelletoid rinds, cryptocrystalline aragonite cement, cryptalgal fabrics, and intraclast pavements and breccia. The aragonite field (39-65^pmil Cl) is characterized by pellets and pelletoid rinds; cryptocrystalline aragonite cement; cryptalgal fabrics; and intraclast pavements, breccia, and grainstone. In the gypsum field (65-184^pmil Cl), diagenetic features are gypsum crystallization and disruption of earlier fabrics, intraclast breccia and grainstone, and porphyroblastic gypsum. The ephemeral brine-brackish field (0-184^pmil Cl) occurs in the vadose profile. Diagenesis results in pellets and pelletoids rinds; cryptocrystalline aragonite cement; intraclast pavements and breccia; dome, boxwork, and tent structures; tubular fenestral fabrics; and replacement of aragonite as a result of dolomitization.

The Sublittoral Sheet comprises sheet- to wedge-shaped bodies of grainstone (skeletal, ooid, lithoskel, and quartz) and coquina. Deposition takes place from low water to 6-m depth, in the emergent intertidal zone. The metahaline field is characterized by bioturbation structures and textures; the sediment is homogeneous to burrow-mottled. Diagenetic features of the hypersaline field are acicular aragonite cement and intraclast breccia. In the intertidal zone, hypersaline-field conditions result in pellets and pelletoid rinds, cryptocrystalline aragonite cement, and cryptalgal fabrics. Features of the ephemeral brine-brackish field are cement of microcrystalline aragonite and tubular fenestral fabric (root molds).

The Bank Unit contains sheets, wedges, and mounds of skeletal packstone, wackestone, and grainstone. Only in the metahaline field are bioturbation structures and textures present. These include homogeneous to burrow-mottled fabrics and fragmented skeletal elements. Pyritized grains also characterize the metahaline field.

The Basal Sheet, a sheet deposit formed on the deep embayment floor, includes transgressive deposits. Features of the oceanic to metahaline fields are bioturbation structures and textures, angular skeletal elements, and pyritized grains. The hypersaline field is characterized by acicular aragonite cement.