Abstract
The early Neoproterozoic Torrensian (= latest Tonian, ∼ 800-750 Ma) succession in the central and eastern Officer Basin was deposited in an intra-cratonic rift basin. The outcropping Pindyin Sandstone consists of basal lowstand conglomerate, and upward-fining transgressive sandstone and is conformably overlain by marine highstand shale of the Alinya Formation. In the Giles 1 well, the Pindyin Sandstone contains aeolian to fluvial deposits and is overlain by peritidal and sabkha sediments of the Alinya Formation. Seismic interpretation suggests that the succession is thin on the Murnaroo Platform and thickens toward the Munyarai Trough. The two sedimentary provinces are separated by the Ungoolya Hinge which probably marks the southeastern margin of the Fraser-Musgrave fold belt. Highstand organic-rich shale in the Alinya Formation is probably an important petroleum source rock. Acritarchs occur in both chert and shale in the Alinya Formation and their distribution was probably controlled by sedimentary environments. Similar assemblages from outcrop and subsurface provide a reliable evidence for regional correlation. The assemblages in Giles 1 are dominated by benthic coccoids and contain many distinct spinose acritarchs which are particularly abundant in the subtidal grey shale, whereas mainly phytoplankton is present in the chert. The assemblage, represented by Trachyhystrichosphaera vidalii-T. aimica-Cymatiosphaeroides kullingii, can be used for global correlation in the early Neoproterozoic. The abundant microfossils from the Alinya Formation and equivalent beds around the world probably indicate a microorganism explosion at 750–800 Ma, prior to the Neoproterozoic glaciations. Thirty-seven acritarchs/spheroids and seven cyanobacterial filaments are briefly described in this study, including 5 new species: Comasphaeridium tonium, Eoentophysalis gilesis, Goniosphaeridium alinyum, Gorgonisphaeridium pindyium and Gorgonisphaeridium torrensium and a newly combined species Archaeoellipsoides karatavicus comb. nov.
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