SEDIMENTOLOGY OF THE WANDRAWANDIAN SILTSTONE IN THE SOUTHERN SYDNEY BASIN, NSW, AUSTRALIA: RECORD OF A MIDDLE PERMIAN GLACIAL PERIOD?

The middle Permian Wandrawandian Siltstone forms a distinctive marine mudrock-rich interval within the succession of the southern Sydney Basin, New South Wales, Australia. The unit, <120 m thick, was deposited during a major marine transgression that can be correlated over 2000 km from south to north, and is enclosed by sandstone-prone formations of shallow marine to coastal origin. The unit comprises mainly gray siltstones rich in marine invertebrate fossils and containing a trace fossil assemblage that may be referred to the Cruziana ichnofacies. Outsized lonestones are abundant throughout the unit (interpreted as ice-rafted debris), and several horizons preserve concentrations of glendonite pseudomorphs after ikaite (a metastable, hydrated CaCO3 mineral that forms in the presence of waters close to freezing temperature). Some previous interpretations have therefore suggested that the unit records a temporary return to glacial conditions in the middle Permian of the Sydney Basin. In addition to the siltstones, deposited in quiet, offshore shelf environments, the Wandrawandian Siltstone also contains storm-emplaced sandstone beds that increase in number, grain-size and thickness upward into the overlying Nowra Sandstone, diamictite beds that are interpreted as debris flows, and an interval characterized by numerous slump sheets. The debris flows and slump sheets indicate instability of the submarine sediment surface, and are similar in some respects to a middle Permian horizon in Queensland that records the onset of foreland loading associated with the Hunter-Bowen contractional event. The various paleogeographic, paleoclimatologic and tectonic implications of these features will be discussed, along with their significance to understanding the scope and duration of the late Paleozoic Gondwanan glaciation in eastern Australia.