Rhaetian strata, Wombat Plateau: analysis of fossil communities as a key to paleoenvironmental change

Rhaetian fossil reef was penetrated on the Wombat Plateau. A study of fossil populations and associated sediment types showed a vertical transition from a sponge-dominated to a coral-dominated community, with associated hydrozoans-tabulozoans. The first colonization of the mobile ground was undertaken by sponges, whereas the coral community assemblage constituted the main core of a pinnacle reef complex, dominated by Retiophyllia in the lower part and by Astreomorpha associations upward. This change in fossil communities reflected shallowing of the environment of deposition. The density of frame-building organisms at Site 764, the fossil community organization, and the geometry and spatial arrangement of the organic buildups, as reconstructed by the seismic-reflection profiles, are all instrumental in defining the Rhaetian pinnacle assemblage offshore the Exmouth Plateau as a low-energy, bank-margin "reef complex'.