Evolution of subtidal sandbanks in Moreton Bay, eastern Australia

Abstract

Vibrocores, current meter data, boomer seismic profiling and sidescan sonar data provide an insight into the processes governing the formation of subtidal linear sandbanks in Moreton Bay, Australia. Bedforms observed on sidescan sonar images are predominantly 10–20 m wavelength dunes with flood-oriented slip faces within the section of the sandbank field studied. The sandbanks separate mutually evasive ebb- and flood-dominant sand transport pathways. Rates of sand transport, calculated from the current meter data, are greatest during flood tides, flowing between the interval low-low to high-high water (mixed tidal regime). Subordinate, ebb-directed transport is insignificant by comparison, which facilitates the preservation of flood-oriented foreset beds in high energy depositional settings (i.e. sandbank crests). Sediment peels obtained from cores indicate that foreset beds are preserved only in deposits of the sandbank crests; trough deposits show bioturbation. 14C dating establishes that sandbanks located further away from Moreton Island were formed prior to 3 k.y. B.P. and that the banks have not been rebuilt since this time. In contrast, material obtained up to 380 cm beneath the surface in sandbanks located inshore are of modern age, showing that these banks undergo regular rebuilding. The formation of linear sandbanks in Moreton Bay is mainly a consequence of the infilling of the bay mouth with sand delivered via littoral drift along the adjacent coastline.