GLACIAL SEDIMENTOLOGY AND MICROMORPHOLOGY OF THE WYNYARD FORMATION, TASMANIA, AUSTRALIA: IMPLICATIONS FOR CLIMATE AT THE PEAK OF THE LATE PALEOZOIC ICE AGE

The sedimentology and micromorphology of the glacigenic deposits in the Wynyard Formation (Tasmania, Australia) provide information on glacial dynamics, glacier type, and ultimately climate at the peak of the late Paleozoic ice age (LPIA) in the Early Permian. The formation is composed of massive diamictite, stratified diamictite, conglomerate, sandstone, and deformed mudstone, and these deposits record ice rafting, gravity flows, glacial outwash, suspension settling, and water escape. The massive diamictite facies consists of massive and weakly stratified diamictite and sandstone pods, and the facies was deposited by a combination of rain-out from floating icebergs and meltwater plumes, and subaqueous debris flows. The stratified diamictite facies consists of stacked beds of interstratified diamictite, sandstone, and conglomerate, deposited predominantly by debris flows. The microstructures in Wynyard diamictites indicate that water-rich diamict experienced abundant ductile deformation rather than subglacial shearing. Rotational structures and well-sorted domains characterize the micromorphology of the diamictite and align well with the interpretation of deposition by iceberg rain-out and debris flow resedimentation. The skelsepic plasmic fabric formed as a response to the rotational structures, because clay and silt enveloped skeletal grains as they rotated, forming a fabric parallel to grain margins. The conglomerate and sandstone facies consists of sandy pebble conglomerate, cobble conglomerate, pebbly coarse-grained sandstone, and massive sandstone and was deposited by glacial outwash as parts of grounding line fans and/or a glaciofluvial system. The deformed mudstone facies displays profuse soft-sediment deformation, including flames, clastic dykes, mud volcanoes, loads, and convoluted bedding, indicating rapid sedimentation, water escape, and sinking of denser material into underlying water-saturated units. The facies analysis supports the interpretation that the Wynyard Formation was deposited by a wet-based, tidewater glacier/glaciers that occupied a trough cut through northwestern Tasmania. This study represents the first in depth analysis of the soft sediment deformation and micromorphology in the formation.