EARLY PERMIAN GLACIAL ADVANCE-RETREAT CYCLES RECORDED IN THE WYNYARD FORMATION OF TASMANIA, AUSTRALIA

Strata of the Early Permian Wynyard Formation were deposited along the eastern margin of Gondwana during the late Paleozoic ice age (LPIA). Study of the Wynyard Formation can provide a more refined understanding of the LPIA, because Tasmania was glaciated during the main phase of the LPIA and correlates with the acme of glaciation in eastern Australia. Interpretations of the glacial thermal regime and geographic setting will help determine the intensity of glaciation at that time and suggest how quickly the change from icehouse to greenhouse conditions occurred.

The Wynyard Formation, the basal unit of the Parmeener Supergroup contains diamictite deposited by debris flows and rain-out from floating ice and meltwater plumes, conglomerate and sandstone deposited by glacial outwash and glacimarine effluent flow, and pebbly mudrock deposited from suspension settling and rain-out from floating ice. The overall succession consists of interbedded conglomerates, sandstones, and diamictites that grade upwards into thinly bedded fine sandstones and pebbly mudrocks, which then pass up into interbedded conglomerates and diamictites. The lower portion was deposited by glacial outwash, and deposition may have occurred rapidly, as suggested by the occurrence of cross-bedded sandstones and conglomerates and dewatering structures. Additionally, vertically repeated intervals of thrust-faulted strata suggest ice-proximal ice-shove deformation, possibly due to seasonal advance and retreat cycles. The middle portion of the formation is finer-grained and likely represents a more substantial retreat of the ice front. However, dropstones and rippled sandstones suggest ice deposition from ice rafting and underflow currents. Within these rocks, mud/sand volcanoes up to 1 m across occur in massively bedded, 3-8 cm thick beds and were likely deposited from debris flows. Finally, the coarser-grained upper portion of the formation indicates glacial advance with ice shove features similar to those in the basal beds.