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ALBIAN (LOWER CRETACEOUS) DINOFLAGELLATE CYST BIOSTRATIGRAPHY OF THE LOWER GEARLE SILTSTONE, SOUTHERN CARNARVON BASIN, WESTERN AUSTRALIA

School of Earth and Geographical Sciences, University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia, e-mail: jbackhou{at}segs.uwa.edu.au

Rich and diverse assemblages of dinoflagellate cysts were recovered from the Gearle Siltstone in the Boologooro-1 stratigraphic well, Southern Carnarvon Basin, Western Australia. The cored interval through the Winning Group is proposed as a reference interval for the Canninginopsis denticulata, Endoceratium ludbrookiae, Dioxya armata and Xenascus asperatus dinoflagellate cyst zones of the Australian dinoflagellate cyst zonal scheme. The ranges of sixty-three dinoflagellate cyst species are documented in detail over this interval. Apart from the bioevents that define the widely-used dinoflagellate cyst subzones, the highest occurrences of Endoceratium turneri, Craspedodinium indistinctum and Litosphaeridium arundum, and the lowest occurrences of Nematosphaeropsis densiradiata, Canninginopsis denticulata, Leberidocysta chlamydata, Aptea? sp. cf. Aptea polymorpha, Litosphaeridium siphoniphorum, Balcattia cirribarbata and Stephodinium australicum are recognized as potential biostratigraphic datums in the Southern Carnarvon Basin. Species diversity increased in the highest Canninginopsis denticulata Zone at approximately the horizon where Diconodinium spp. become dominant. Foraminiferal data indicate the interval from 361.15 m to 312.12 m is marginally inner neritic and corresponds with an interval of abundant Diconodinium spp., and fewer Spiniferites spp. Diversity increased in the Xenascus asperatus Zone with an increase in Spiniferites spp., and a shift to middle neritic paleobathymetry. The age of the top of the Xenascus asperatus Zone is constrained by calcareous nannofossils and dinoflagellate cyst correlations to the latest Albian, or possibly the early Cenomanian. The age of the base of the Canninginopsis denticulata Zone is constrained by dinoflagellate cyst and foraminiferal data to the early Albian, above the earliest Albian. One new species, Chlamydophorella haigii, is erected and aspects of several other dinoflagellate cyst taxa are reviewed.

Key Words: Lower Cretaceous • Albian • dinoflagellate cysts • biostratigraphy • paleoecology • Western Australia