Palaeogeographical influence an Late Eocene biosiliceous sponge-rich sedimentation, southern Western Australia

Late Eocene nearshore shallow-marine environments within the Bremer and wes tern Eucla Basins of southern Western Australia were characterized by the t hick deposition of spongolite and spiculite deposits. Epibenthic sponge com munities dominated estuaries and topographically complex basin margin embay ments-archipelagos, while cool-water carbonates with up to 10% sponges accu mulated in open-shelf environments. The transition from a biosiliceous to c alcareous epibenthos was related to the degree of palaeogeographical 'prote ction'. Within basement-protected embayments there was an offshore gradatio n from shoreface spongolite and pure spiculite to a muddy spiculite facies towards central embayment areas. Calcareous fossils are rare throughout emb ayment facies, but rapidly increase in more open outer archipelago areas. T his depositional relationship occurred along 2000 km of the Late Eocene sou thern Australian coastline. Palaeogeographical protection from strong curre nts acted in concert with: (1) a planar, low-gradient inland topography wit h sluggish run-off, supplying fine-grained sediment, nutrients, and abundan t dissolved silica; and (2) a microtidal setting, weak to moderate swells a nd opposing wind and Coriolis surface current forcing, which inhibited wate r exchange between embayments-estuaries and the open shelf. This situation led to an embayment water chemistry that encouraged prolific sponge growth. Calcareous spiculites record the mixing front between these embayment wate rs and normal open-shelf waters supporting cool-water carbonates.