Large shore-parallel sand bodies on the lower shoreface of the embayed and cliffed coast of central New South Wales, Australia, provide indirect evidence of downslope transport of sand. These sand bodies are 10 to 30 m (33 to 100 ft) thick, extend discontinuously for 40 km (25 mi) along the coast, and add a pronounced convexity to the shoreface profile.

Evidence from surface samples, deep borings, and side-scan sonar and high-resolution seismic reflection profiling from the sand bodies indicates that deposition occurred by downslope transport of sand from the upper shoreface and surf zone. The sand bodies consist entirely of locally derived quartz sand intermixed with marine mollusk fragments and microfauna. The internal acoustic structure of the sand bodies indicates growth by seaward progradation; the seaward face, or foreslope, locally attains a slope greater than 5°. Surface morphology indicates coalescing of individual sediment lobes and damming of bedload sediment against bed-rock ridges at the toe of the sand body in water depths of 70 to 80 m (230 to 262 ft). Channels and individual sediment lobes oriented normal to the horeline also indicate seaward transport.

On this coast, the sea level reached its present position 6,000 years ago, and these sand bodies were produced by high-energy conditions on a steep shoreface during the stillstand period. Average waves here are 1 to 2 m (3 to 6.5 ft) and storm waves are much higher. Radiocarbon dates from analogous sand bodies elsewhere in New South Wales suggest continuous seaward progradation and upbuilding since the cessation of sea level rise. No direct observations or measurements of downslope transport are available, but we infer that sand derived from along the coast or the cliff face is gradually reworked seaward on the steep upper shoreface during fair-weather conditions. During storm periods, sand is flushed seaward by returning bottom flows onto and across the surface of the sand body.

Figure

In addition to downslope transport of sand to deeper water, there is strong evidence of concurrent modification of the sand bodies by processes acting parallel to the shoreline. This evidence consists of textural trends, shore normal sand waves, and the overall alongshore continuity of the sand bodies.

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