Formation, Morphology and Preservation of High-energy Carbonate Lithofacies: Evolution of the Cool-water Recherche Archipelago Inner Shelf, South-western Australia

Cool-water carbonate environments may be responsible for up to one third of the carbonate sediment produced on continental shelves, and are useful modern analogues for many geologically ancient deposits. The extensive southern margin of the Australian continent is recognised as the world's largest modern example of a high energy, cool-water carbonate depositional realm. A number of studies have suggested that Quaternary sediment production here is largely influenced by oceanography, and that wave abrasion strongly limits sediment accumulation. Therefore, in this region the outer-shelf, below the storm wave base, is thought to be the focus of sediment accumulation. The inner shelf is considered a zone of active sediment production due to the proliferation of carbonate secreting organisms, although few studies have investigated sediment production or accumulation in this energetic and dynamic environment. The Recherche Archipelago, which sits at the western margin of the Great Australian Bight (GAB), was examined to better understand Quaternary shelf evolution and the importance of this type of inner shelf as a carbonate 'factory'. Surficial sediments, video, multibeam sonar data, cores and shallow seismics were collected. The present seabed of the archipelago features extensive areas where flat-lying limestones sit over the often irregular granite basement. The Pleistocene erosional surface is overlain by a coarse bivalve and rhodolith dominated gravel lag. Significantly, there are extensive Holocene deposits, up to 7 m thick, throughout the archipelago, particularly in association with granite islands. These deposits comprise cross-bedded gravelly carbonate sands dominated by fragments of calcareous algae (rhodoliths), molluscs and bryozoans. In contrast, the inshore and coast is dominated by terrigenous sediment. Seismic profiles and preserved palaeo-shoreline features suggest that slow but episodic aggradation of marine sediment has occurred on the inner shelf over successive Quaternary sea level cycles, although there are also extensive areas of non-deposition. This accumulation is partly attributable to the sheltering effect of high-relief granitic outcrops and cementation of subaerially exposed carbonate sediments.