Mid-Neoproterozoic biostratigraphy and isotope stratigraphy in Australia

Abstract

Palynomorph and stromatolite assemblages from mid-Neoproterozoic strata of Australia appear highly significant when collated with results obtained from isotope stratigraphy, and allow correlation across the Centralian Superbasin and Adelaide Rift Complex. The majority of palynomorphs from the lower mid-Neoproterozoic (∼830 Ma) have limited biostratigraphic value, although a few apparently have restricted ranges. Carbonates at this stratigraphic level contain the distinctive stromatolite Acaciella australica and other taxa of the Acaciella australica Stromatolite Assemblage. These carbonates have δ 13C carb values of between −4 and 0‰, and associated anhydrite has 87Sr/ 86Sr ratios of 0.7069. These depleted δ 13C carb values are immediately preceded by values of 6.1‰ in the western Officer and Amadeus Basins of the Centralian Superbasin. Similar C and Sr isotopic features are seen in the similarly aged upper Shaler Supergroup of Canada. Upper mid-Neoproterozoic (∼760 Ma) carbonates contain the distinctive stromatolite Baicalia burra and other stromatolites of the Baicalia burra Stromatolite Assemblage. Siliciclastic horizons in the same interval contain the acritarch Cerebrosphaera buickii, and have δ 13C carb values of between 0 and 8‰. The highest mid-Neoproterozoic δ 13C carb values of between 6.7 and 8.1‰ occur within the stratigraphic range of C. buickii in Australia. In the western Officer Basin and Adelaide Rift Complex the mid-Neoproterozoic δ 13C carb peaks are preceded by values of ca. 0‰. The apparently restricted range of C. buickii in Australia suggests the need for reinterpretation of the stratigraphic position of its occurrence in the Svanbergfjellet Formation, Akademikerbreen Group, Spitsbergen, which was previously considered a post-Sturtian-glacial deposit.