PRELIMINARY PALEONTOLOGICAL DATA FOR FIVE CRYOGENIAN SILICICLASTIC UNITS

Diversity compilations of Proterozoic organic-walled microfossils indicate a precipitous drop before the Sturtian glaciation and limited diversity during the interglacial interval. However, few synglacial and interglacial units have been included in such diversity compilations, thus it is not clear to what extent limited diversity reflects limited sampling rather than a genuine representation of depressed diversity. Assemblages of this time are typically depauperate, dominated by small, unornamented sphaeroids and the pluricellular form Sphaerocongregus variabilis. Preliminary study of five Cryogenian units from the Amadeus and Adelaide Rift Basins of Australia and the Elbobreen Formation in Svalbard support the view of low eukaryotic diversity.

Infrequent, complete leiosphaerid acritarchs of varying vesicle thickness have been found from mudstones in the upper portion of the synglacial (Sturtian correlative) Areyonga Formation, Amadeus Basin (BR05DD01 drill core). The thin-walled nature of some fossils suggests that this biota is autochthonous rather than the result of reworked sediment.

Shales of the interglacial Tapley Hill Formation, Adelaide Rift Basin, (SCYW-79-1a) and Aralka Formation, Amadeus Basin (Wallara-1 drill core) host fossil assemblages of unornamented, but morphologically variable leiosphaerid acritarchs (15 to 120µm, diameter), uniseriate filaments, and a variety of colonial aggregates (cf. Ostiana sp and Stictosphaeridium sp) that differ in cell and colony size (cells ~8 to 20 µm diameter, colonies ~100 to 300µm, diameter), vesicle thickness and texture, the geometry of cell packing, and the presence of colonial envelopes. These morphological variations suggest the presence of multiple taxa of colonial aggregates.

Ten shale samples from two localities of the interglacial MacDonaldryggen Member, Elbobreen Formation, Svalbard have been found to be barren of fossils, hosting only an abundance of pyrite framboids. Shales of the interglacial Tindelpina Shale Member of Adelaide Rift Complex (Blinman-2 drill core) were also found to be barren.

Although preliminary, these data support the view of limited organic-walled microfossil diversity in the Neoproterozoic glacial interval.