TIDAL SEDIMENTATION IN PART OF THE LATE SILURIAN GRAMPIANS BASIN, SOUTHEASTERN AUSTRALIA

Thick quartz arenite formations in the Upper Silurian Grampians Group of southeastern Australia have been previously interpreted as fluvial deposits, but detailed study of one of these formations, the Mount Dif ficult Sandstones, shows that shallow-marine conditions existed during their deposition in the southeastern part of the Grampians Basin. The Mount Difficult Sandstones are a succession of cross-bedded quartz ar enites with an overall fining-upward trend. Four lithofacies associati ons are recognized: the two lower associations record tidally dominate d sedimentation, whereas the two upper associations represent depositi on in wave-dominated environments. Boundaries are gradational, and the associations are interpreted as four spatially and genetically relate d environments within a tidal-inlet and barrier-beach system. The basa l tidal-inlet channel-fill complex overlies an erosion surface on tida lly influenced coastal-plain deposits of the Silverband Formation. Ver tically stacked fining-upward channel fills record lateral migration o f tidal channels during transgression. The overlying association was d eposited in mixed tide-wave conditions, and tidal-channel margin to eb b-tidal delta environments seaward of the inlet are proposed. The deve lopment of tidal-inlet channels and a tidal delta suggests deposition of the Mount Difficult Sandstones in a mesotidal setting (tidal range 2-4 m) with a time-velocity asymmetry dominated by ebb-tidal currents. The increasing influence of waves is recorded by barrier-beach deposi ts that migrated over the inlet-delta system. The barrier beach is rep resented by two lithofacies associations, interpreted as upper-shorefa ce and overlying foreshore deposits, an arrangement that signals a cha nge from transgressive to regressive (progradational) conditions. Thus , the Mount Difficult Sandstones represent a single transgressive-regr essive cycle (most likely third-order) that was controlled mainly by t he rate of subsidence in the tectonically active Grampians Basin, unde r conditions of a relative highstand of sea level and high rates of se diment supply.