Systematics and biostratigraphy of Cambrian Small Shelly Fossils from East Antarctica and South Australia
The remote lower Cambrian Byrd group of sedimentary rocks from East Antarctica has been studied intermittently since its discovery over a century ago. Previous insights into the trilobites and archaeocyaths therefrom indicated a close correlation to the units within the sedimentary sequences of South Australia. The lowest unit of the Byrd Group is the fossiliferous Shackleton Limestone which overlies the Neoproterozoic metamorphic rocks of the Beardmore Group and is representative of a long period of carbonate shelf formation on a passive margin with the Palaeo-Pacific. This was truncated by marine transgression and the deposition of the deeper-water calcareous siltstones of the fossiliferous Holyoake Formation. This unit is overlain by the Starshot Formation and all three units are cross-cut by the Douglas Conglomerate, heralding the start of a collisional tectonic regime between the East Gondwana and palaeo-Pacific plates. The first systematically sampled and analysed sections through the carbonate Shackleton Limestone and argillaceous Holyoake Formation have yielded a new fauna of small primarily phosphatic and secondarily phosphatised shelly fossils. The abundant molluscs, brachiopods and tommotiids are reported here. These findings facilitate correlation of this section to the fossil biozones established for the lower Cambrian of South Australia, including the Dailyatia odyssei small shelly fossil Taxon Range Zone and the Pararaia janeae Taxon Range Trilobite Zone. Chemostratigraphic data from three sections preserve the profiles of two major stable carbon isotope excursions: the Mingxinsi Carbon Isotope Excursion and the Archaeocyathid Extinction Carbon isotope Excursion. The combination of these two lines of evidence are a strong indicator for Cambrian Series 2, early to mid-Stage 4 age. This is corroborated by newly described D. odyssei-P. janeae Taxon Range Zone small shelly fossils from the carbonate clasts from the Cambrian Stage 4 White Point Conglomerate of South Australia which bear strong similarity to the fauna of the Shackleton Limestone. Palaeobiogeographically the fauna recovered from the Byrd Group is remarkably similar to the East Gondwanan region of South Australia, with similar brachiopod assemblages to those recovered from the Xinji Formation of North China and molluscan assemblages to the Bastion Formation of North-East Greenland.