Proglacial deposition and deformation in the Upper Carboniferous to Lower Permian Wynyard Formation, Tasmania: A process analysis

Abstract

The Wynyard Formation of Tasmania, Australia, provides the youngest evidence of grounded ice during the Late Carboniferous–Early Permian in the Tasmania Basin of southeastern Gondwana during the late Paleozoic ice age. Within the Wynyard Formation, four facies associations are recognized: 1) massive diamictite, 2) stratified diamictite, 3) conglomerate and sandstone, and 4) deformed mudstone and fine sandstone. A detailed facies analysis was performed in order to interpret the depositional processes and environments. The massive diamictite facies association contains massive diamictite and sandstone and was deposited primarily by iceberg rain-out in a subaqueous morainal bank setting. The stratified diamictite facies association is composed of stratified diamictite, sandstone, and conglomerate, and was deposited by debris flows and iceberg rain-out, and deformed by glacial pushing. The conglomerate and sandstone facies association is made up of cross-stratified, channelized conglomerate and pebbly sandstone that were deposited by glacial outwash on grounding-line fans. The deformed mudstone and fine sandstone facies association is composed of pervasively deformed pebbly mudstone and fine-grained sandstone that exhibit volcano, flame, and dyke structures. This facies association was deposited by a combination of suspension settling, iceberg rain-out, and soft sediment deformation, in a quiet distal proglacial setting. The facies analysis supports the interpretation that the Wynyard Formation was deposited by a wet-based, tidewater glacier or glaciers. The depositing glacier(s) occupied a ~ 40 km-wide trough in northwest Tasmania. Based on the distribution of the Wynyard Formation and the presence of glacial deposits in other eroded troughs in Tasmania, glaciation occurred in the region within broadly eroded valleys. The Wynyard Formation and overlying mudstone of the Inglis Formation provide a glacial–postglacial signature similar to those in South Africa, Antarctica, and eastern Australia during the latest Carboniferous–Early Permian, therefore the climate warmed in southern Gondwana following this glacial interval. The continued glaciation in eastern Australia was a result of regional topography and oceanographic processes.