PETROGENESIS OF PERMIAN POLAR CARBONATES, TASMANIA

The Darlington (Sakmarian) and overlying Berriedale (Artinskian) limestones of the lower Parmeener Supergroup in Tasmania record deposition on a polar, cold-water ramp during the early Permian. At this time Tasmania was located adjacent to Antarctica and mainland Australia, at approximately 70-80°S. The lower Parmeener Supergroup preserves a record of southern hemisphere deglaciation, with glaciomarine facies at the base overlain by siltstone and limestone. Siltstone facies below the Darlington Limestone and those occurring stratigraphically between the two limestones are glendonitic, indicating deposition in very cold waters. Both the Darlington and Berriedale limestones consist of a heterozoan assemblage dominated by brachiopods and bryozoans, and contain Eurydesma bivalves consistent with the late Paleozoic Gondwana cold-water fauna. Dropstones occur in both carbonates, although they are larger and more numerous in the younger Darlington Limestone. Phosphate is present within the Darlington Limestone but becomes abundant and more widespread in the Berriedale Limestone. This increase is interpreted to reflect enhanced circulation of nutrient-rich seawater upwelling onto the ramp. Forty-eight brachiopods and eight Eurydesma bivalves were analyzed for stable carbon and oxygen isotopic and trace element compositions. Two diagenetic trends are recognized, diverging from values of approximately +1 for δO¹⁸ and +6.5 for δC¹³. These isotopic compositions are significantly different from values published by previous workers.