Glacial Advance and Retreat Cycles in the Permian Wynyard Formation, Tasmania, Australia
University of Wisconsin-Milwaukee, Department of Geosciences Milwaukee, Wisconsin; [email protected]
Strata of the Permian Wynyard Formation of Tasmania, Australia were deposited along the eastern margin of Gondwana in an area located between the better studied sections of eastern Australia and Antarctica during the late Paleozoic ice age. These glacial strata form the base of the Lower Parmeener Supergroup, which contains potential source, reservoir, and seal rocks. The Wynyard Formation contains diamictite deposited by debris flows and rain-out from ice, conglomerate and sandstone deposited by glacial outwash, and pebbly mudrock deposited from suspension settling and rain-out from ice.
The overall succession consists of interbedded diamictites, sandstones, and conglomerates that grade upwards into thinly bedded fine sandstones and mudrocks containing pebble-sized clasts, which then pass up into interbedded conglomerates and diamictites. The lower portion was deposited quickly, as suggested by the occurrence of cross-bedded sandstones and conglomerates and an abundance of 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 distal deposition from ice rafting and underflow currents. Within these rocks, mud/sand volcanoes up to 1 m across suggest possible seismic activity in the depositional basin at that time. Finally, the coarser-grained upper portion of the formation indicates glacial advance with ice shove features similar to those in the basal beds.
AAPG Search and Discovery Article #90083 © 2008 AAPG Foundation Grants in Aid