Deposition of polar carbonates during interglacial highstands on an early Permian shelf, Tasmania

摘要

The early Permian Lower Parmeener Supergroup contains two bioclastic limestones that accumulated during the period of prolonged deglaciation of southern Gondwana as the region moved equatorward from 80 degrees S to 70 degrees S. The Darlington and Berriedale limestones formed in neritic environments, in areas where abundant ice-rafted debris testifies to numerous icebergs and glendonites indicating near-freezing seawater. These limestones consist of argillaceous and clean bioclastic floatstone, rudstone, and grainstone that contain a high-abundance, low-diversity heterozoan assemblage of calcareous invertebrates. The components are dominated by large, robust brachiopods, bryozoans, and Eurydesma bivalves. Sponge spicules and crinoids are also common, whereas coralline algae and conodonts are conspicuously absent. Carbonates were deposited on the middle shelf during sea-level highstands, below the iceberg grounding line, where the faunal communities remained undisturbed. In this setting the organisms were outboard of significant terrigenous sediment influx and brackish water, which were trapped on the inner shelf by bathymetry and icebergs. Strong bottom currents also prevented the deposition of fine-grained siliciclastics by continuous winnowing of the pure limestone facies. Upwelling of nutrient-rich water, inferred from the distribution of phosphate, promoted high primary productivity, which fueled this carbonate factory and inhibited dissolution of the biogenic carbonate.