No abstract

The North West Shelf is an ocean‐facing carbonate ramp that lies in a warm‐water setting adjacent to an arid hinterland of moderate to low relief. The sea floor is strongly affected by cyclonic storms, long‐period swells and large internal tides, resulting in preferentially accumulating coarse‐grained sediments. Circulation is dominated by the south‐flowing, low‐salinity Leeuwin Current, upwelling associated with the Indian Ocean Gyre, seaward‐flowing saline bottom waters generated by seasonal evaporation, and flashy fluvial discharge. Sediments are palimpsest, a variable mixture of relict, stranded and Holocene grains. Relict intraclasts, both skeletal and lithic, interpreted as having formed during sea‐level highstands of Marine Isotope Stages (MIS) 3 and 4, are now localized to the mid‐ramp. The most conspicuous stranded particles are ooids and peloids, which 14C dating shows formed at 15·4–12·7 Ka, in somewhat saline waters during initial stages of post‐Last Glacial Maximum (LGM) sea‐level rise. It appears that initiation of Leeuwin Current flow with its relatively less saline, but oceanic waters arrested ooid formation such that subsequent benthic Holocene sediment is principally biofragmental, with sedimentation localized to the inner ramp and a ridge of planktic foraminifera offshore. Inner‐ramp deposits are a mixture of heterozoan and photozoan elements. Depositional facies reflect episodic environmental perturbation by riverine‐derived sediments and nutrients, resulting in a mixed habitat of oligotrophic (coral reefs and large benthic foraminifera) and mesotrophic (macroalgae and bryozoans) indicators. Holocene mid‐ramp sediment is heterozoan in character, but sparse, most probably because of the periodic seaward flow of saline bottom waters generated by coastal evaporation. Holocene outer‐ramp sediment is mainly pelagic, veneering shallow‐water sediments of Marine Isotope Stage 2, including LGM deposits. Phosphate accumulations at ≈ 200 m water depth suggest periodic upwelling or Fe‐redox pumping, whereas enhanced near‐surface productivity, probably associated with the interaction between the Leeuwin Current and Indian Ocean surface water, results in a linear ridge of pelagic sediment at ≈ 140 m water depth. This ramp depositional system in an arid climate has important applications for the geological record: inner‐ramp sediments can contain important heterozoan elements, mid‐ramp sediments with bedforms created by internal tides can form in water depths exceeding 50 m, saline outflow can arrest or dramatically slow mid‐ramp sedimentation mimicking maximum flooding intervals, and outer‐ramp planktic productivity can generate locally important fine‐grained carbonate sediment bodies. Changing oceanography during sea‐level rise can profoundly affect sediment composition, sedimentation rate and packaging.

No abstract

The Great Australian Bight (GAB), the largest sector of the southern Australia continental margin, is a site of cool-water carbonate sedimentation throughout, ranging from locally warm-temperate inboard to cool-temperate outboard. Surficial sediments are a mixture of calcareous Pleistocene skeletal and lithic intraclasts (relict grains), and Holocene biofragments, with minor amounts of quartz inboard. The inner shelf is an area of abundant macrophytes and seagrasses, active carbonate sediment production and accumulation, and little relict sediment. The huge middle portion is a ''shaved shelf'' with active sediment winnowing and mostly relict sediment. The outer shelf and upper slope is a variably productive sediment factory characterized by prolific calcareous epibenthic growth on hard substrate subaqueous ''islands'' shedding particles into surrounding sands and muds.Patterns of Holocene sedimentation are linked to modern oceanographic parameters in this high-energy setting characterized by overall downwelling. Prolific rhodoliths occur on the NW inner shelf, where shallow summer waters are the warmest in the GAB. These warm, saline, nutrient-depleted waters then drift eastward across the shelf, suppressing heterozoan carbonate production on the central and eastern mid-shelf. This arrested production in the eastern GAB is countered locally by summer coastal upwelling along western Eyre Peninsula, with bryozoan-rich sediment extending well inboard onto the midshelf. The outer shelf and upper slope is an area of prolific bryozoan growth, likely linked to upwelling, except in the central GAB, a region of year-round downwelling, where the area is one of off-shelf fine sediment transport and carbonate mud deposition. These patterns, in the central GAB at least, are present in the underlying Holocene and Pleistocene, suggesting that the general modern oceanographic dynamics and resultant carbonate sedimentation have persisted throughout the Quaternary.

The seafloor beneath these cold, stormy waters is the site of prolific temperate water carbonate sediment production and accumulation.