EVIDENCE FOR MID-PERMIAN GLACIATION RECORDED IN OFFSHORE-SHELFAL FACIES OF THE WANDRAWANDIAN SILTSTONE, SOUTHERN NEW SOUTH WALES, AUSTRALIA

The middle Permian (c. 270 Ma) Wandrawandian Siltstone consists of <150 m of heavily bioturbated siltstones and fine sandstones that record offshore-shelfal marine environments. Enclosed by shoreface sand-dominated units, the formation represents a marine transgression that can be correlated c. 2000 km from north to south. Outsized clasts up to 2 m in diameter, interpreted as ice-rafted debris, are prominent in the lower and upper parts of the formation. Examination of older strata and basement indicates that the clasts, which include blocks from the sandstone-dominated underlying formation, are locally derived. Glendonite pseudomorphs after ikaite, a metastable, hydrated carbonate mineral that forms at near-freezing temperatures, are particularly abundant in the clast-rich intervals in the lower and upper parts of the formation. The stratigraphic distribution of these climate-sensitive features suggests that the Wandrawandian Siltstone records two periods when cold, likely glacial, climate conditions developed. Throughout the formation, small-scale fluctuations in sea level are suggested by subtle changes in sand/silt ratios, bioturbation style, and firmgrounds. Within the lower half of the formation, a slumped interval of glendonite-bearing, offshore-shelfal facies indicates instability of the submarine sediment surface. This interval is similar to middle Permian strata in Queensland that record the onset of foreland loading associated with the Hunter-Bowen contractional event. Above the slumped interval, incised channel fills record rapid deposition following a major drop in base level. Tuff interbeds, which occur throughout the formation but increase in abundance and thickness upward, further imply a tectonic overprint on the sea level and climate signals described above. In the absence of direct evidence for a nearby ice sheet, we suggest that valley glaciers, perhaps emanating from more extensive ice masses in the continental interior, served as point sources for the delivery of outsized clasts to offshore regions. The apparent inconsistency between the marine transgression recorded by the Wandrawandian Siltstone and inferred periods of ice buildup is attributed to the interplay of isostatic loading during glaciation and tectonism, which resulted in deepening of the basin.