Abstract:
Pleistocene sediments obtained by ODP Leg 182 from the middle slope region (site 1130A) of the Great Australian Bight was studied using a multi-method approach. Sediment interval from Unit I ( approximately 123-150.5 mbsf, subunits 1A and IB) were analyzed using petrography, grain size, XRD, point counts, and grain counts. Lithologic cycles consist of alternating bioclastic packstones (P1, P2, and P3) and wackestones (W1, W2, and W3). Grain size analysis corroborates the lithologic cycles. The most abundant lithology is W1 ( approximately 32%) followed by P1 ( approximately 30%), W2 ( approximately 17%) and P2 (10%) as second, W3 ( approximately 6%) and P3 ( approximately 5%) as the least abundant. The difference between the wackestone and packstone is the different proportion of grains and matrix as well as of differences in components. Grain count and mineralogical data confirms that packstones are mostly composed of skeletal grains and higher high-magnesium calcite values, while wackestones contain mostly planktonic foraminifera and the mineralogy is dominated by low-magnesium calcite. W3 and P3 are the anomalous lithologies; W3 is a white nannofossil interval with abundant planktonic foraminifera and P3 consists of both skeletal grains and diverse planktonic foraminifera and has a mixed mineralogical signal. W3 and also P3 have abundant forms of cooler water planktonic foraminifera suggesting deposition under lower sea level. The interpretation of the cycles is that the lithologies represent the alternation between offshore transport of neritic grains when donwelling currents dominate and deposition of planktonic foraminifera-rich interval when upwelling. In the Modern, the storm dominated shelf and oceanographic conditions have created an area of all year round downweling in the studied region. During this time sediment from the shelf is transported into the slope region. However, during lower sea levels, downwelling stops and cooler water upwell onto the slope. There are two scenarios deduced. In the first case, upwelling produced P3 (mixture of coarse planktonic foraminifera and coarse skeletal grains) due to the formation of bryozoan mound in the upper slope. In the second case (W3), sediment starvation of the upper slope do not produce coarse skeletal grains and the lithologies are dominated by coarse planktonic foraminifera derived from the water column.