Facies variability in the post-Marinoan cap carbonate Nuccaleena Formation on the Stuart Shelf, South Australia

The Marinoan Glaciation (~635 mya) marks one of Earth's most severe climatic excursions - a time period when sediments interpreted as glacial in origin were deposited at tropical paleo-latitudes. Carbonate sedimentary sequences that typically succeed these Marinoan low-latitude glacial deposits have been interpreted to record deglaciation and a transition to a severe greenhouse climate. The Nuccaleena Formation of South Australia is a reliable paleomagnetically-constrained Marinoan low-latitude post-glacial 'cap' carbonate that overlies the glacio-marine Elatina Formation in the Adelaide Rift Complex (ARC) and the periglacial-aeolian Whyalla Sandstone on the Stuart Shelf, west of the ARC. Facies within the Nuccaleena Formation overlying the Whyalla Sandstone are not well characterized and provide an opportunity to investigate cap carbonate deposition over non-marine rocks. Here we show facies variations of the Nuccaleena Formation across the Stuart Shelf using observations from drill-core analysis and outcrop study. Twelve drill-cores selected along a transect from north to south across the Stuart Shelf were measured and sampled for isotopic analysis. Measured core and digitally archived core logs provide the basis for stratigraphic correlations. The Nuccaleena Formation was found to consistently overlie the Whyalla Sandstone in all cores examined. Facies of the Nuccaleena Formation include buff to pink brecciated dolomite, sheet-cracked dolomite with isopachus cementation, chevroned pseudo-teepee structures, alternating bands of red and buff dolomitic silts, and interbedded coarse sand and dolomite ribbons. Dolomitic grainstones reported in the ARC were not observed on the Stuart Shelf. The thickness of the Nuccaleena Formation observed in core was variable between 10 cm to over 4 meters, with lower contacts ranging from gradational with underlying sand to abrupt. Despite a depositional setting on the Stuart Shelf that is more proximal than that in the ARC, the overall facies within the Nuccaleena Formation appear similar. The documentation of lateral facies variability in the Nuccaleena Formation above terrestrial sediments on the Stuart Shelf will provide further insight toward understanding the relationship between cap carbonate deposition and the severe ice-house conditions during the Marinoan Glaciation.