Geological Setting, Marine Geomorphology, Sediments and Oceanic Shoals Growth History of the Kimberley Region

Abstract

The offshore sedimetary basins of the Kimberley region are becoming established as a major hydrocarbon province, but the region is also known for its marine wilderness values. Its position close to a plate boundary is reflected in significant rates of continental margin subsidence. In addition to the "normal" continental margin geomorphic units of shelf, slope and rise the offshore Kimberley region has well developed plateaux (e.g. Scott Plateau), terraces (e.g. Rowley Terrace), and banks (e.g. Sahul rise, Sahul bank) which interrupt the otherwise gentle sea slopes present, and provide foundations for the offshore reefs, including the Sahul shoals, Ashmore, Seringapatam, and Scott Reef and the Rowley Shoals. The continental shelf is a vast low gradient ramp with sandy bioclastic sediments reflecting both the modern biota and a history of past sea level and oceanographic changes, so that sediments are a mixture of modern bioclasts, particles stranded by sea level rise, and precipitated carbonate grains (ooids and peloids) which were dominant prior to Leeuwin Current onset some 12,000 years ago.Whilst little is known about the nearshore fringing reefs, in areas of macro-tides and significant sediment input, the morphology, internal architecture and growth history of reefs and shoals of the Oceanic Shoals Bioregion indicates that these are long-lived features which have survived despite relatively high rates of continental margin subsidence and oscillating sea levels of the Pleistocene glaciations. However, drowning by sea level rise was the fate of some of the reefs and shoals of the Sahul Shelf, situated at the leading edge of the downturning Australian plate, in contrast to continuing reef growth at Scott Reef and the Rowley Shoals to the south. In the morphological series provided by the three Rowley Shoals, differential subsidence is the primary control on rates of lagoon infill controlling platform morphology. This study demonstrates the resilience of reefs on the subsiding margin whilst linking reef morphology to the relative amount of pre-Holocene subsidence.