Offshore Transport and Sand-Body Formation: Evidence from a Steep, High-energy Shoreface, Southeastern Australia

Abstract

ABSTRACT Elongate, shore-parallel sand bodies lie on the lower shoreface of the embayed and cliffed coast of central New South Wales, Australia. These sand bodies are 10 to 30 m thick, extend almost continuously for 40 km parallel to the coast, and display a pronounced convexity on the sea floor in profile. Sediment sampling and detailed seismic reflection and side-scan sonar profiling provide evidence that the upper part of these large lobate sand bodies consists chiefly of sand transported downslope from the upper shoreface and surf zone. The sand bodies consist entirely of marine sand probably derived locally from retreating embayed beaches and in part from sandstone cliffs. Channels and sediment lobes oriented normal to the shoreline and internal structure indicate growth by seaward progradation, and surface morphology indicates coalescing of lobes and damming of bed-load sediment against bed-rock ridges in water depths of 70 to 80 m. The precise mechanism, or mechanisms, of seaward transport on the shoreface is unresolved. We surmise that during storm events, disturbance by large waves and seaward transport by downwelling bottom currents result in sand movement onto and across the sand-body surface at depths of 40 to 80 m. Concurrent modification of the sand bodies by shore-parallel processes is suggested by textural trends, bed forms, and the overall alongshore continuity of the sand bodies.