CHANNEL FILLS IN THE LOWER PERMIAN PEBBLEY BEACH FORMATION, SOUTHERN SYDNEY BASIN, AUSTRALIA: PRODUCTS OF LOWSTAND INCISION AND ESTUARINE SEDIMENTATION, NOT SHELFAL EROSION

The Lower Permian Pebbley Beach Formation of the southern Sydney Basin, NSW, Australia is a succession of terrigenous clastic deposits formed in an array of coastal and shallow marine environments under a cold but humid, sub-polar climate. Nine facies are recognized in this study on the basis of lithological and ichnological criteria. These facies represent deposition in offshore, offshore transition, lower and middle shoreface, estuarine channel, estuarine basin, and transgressive complex environments. The overall environment envisaged for the unit is a depositional coast and shallow marine setting with a highly indented coastline punctuated by large muddy estuaries. A series of cycles of relative sea-level change has been recognized, on the basis of vertical facies stacking patterns, and the identification of erosional surfaces interpreted as sequence boundaries. Three horizons in particular within the Pebbley Beach Formation show spectacular incision of channel forms up to 6 m deep and typically 100-250 m wide, filled by heterolithic sandstone-siltstone or massive fine siltstone. The channel fills preserve inclined heterolithic stratification, current ripples with bipolar palaeocurrent distributions together with wavy, lenticular and linsen bedding, abundant coalified plant debris and petrified log casts. The basal erosion surfaces show concentrations of basement gravel clasts, and some channel fills have a basal storey of trough cross-bedded, medium- to very coarse-grained sandstone. Some intervals show soft-sediment deformation structures and synaeresis cracks, A pronounced contrast in ichnofacies is evident across the basal erosion surface, even in cases where mud- grade channel facies overlie mud-grade offshore marine facies. The channels are interpreted by us as inshore coastal estuaries, cut during lowering of relative sea-level and lowstand, and filled progressively during the ensuing rise. This interpretation differs fundamentally from that of Eyles et al. (1998, in Sedimentology, 45, 121-161), who interpreted the same features as the result of scour in mid to outer shelf depths.