Abstract

The coast of southeastern Australia has a high-energy embayed and cliffed coast that receives only minor amounts of modem sediment from streams. This setting, coupled with an apparently stable sea level for the past 6000 years, results in a thin erosional veneer of Holocenc sediment overlying truncated Pleistocene and older deposits on the shelf. In several locations, however, large sand bodies have accreted on the lower shoreface and these sand bodies are unlike those described from other shelves in the world. They arc 10 to 30 m thick, several kilometres wide, tens of kilometres long, and result in a convex shoreface profile. Radiocarbon dates indicate that most of the sand bodies were deposited in the past 6000 years after the sea level reached its present position. From the study of the texture and composition of long-core samples, it appears that the sand was derived from adjacent beaches and cliffs, and seismic-reflection profiles indicate progradation of sand bodies has occurred across the steep (2° to 5°) seaward flank into water depths of 60 to 80 m.

Three major factors appear to control formation of these sand bodies: an initially steep inner-shelf profile; local high-energy conditions; and a long time period of a stable sea level. Based on sediment texture, seaward dipping reflectors, and surface channels, we infer that sediment is transported seaward from the upper shoreface to the sand bodies by storm-induced downwelling. In areas of strong regional flows, such as off Cape Byron, where the East Australian current impinges on the coast, these flows play a major role in modifying the shape and textural character of the sand bodies.