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Comparison of Suprasalt and Subsalt Depositional and Halokinetic History of Patawarta Diapir, Flinders Ranges, South Australia

Abstract

Outcrops of mixed carbonate/siliciclastic strata comprise the Neoproterozoic Wonoka Formation and Patsy Hill Member of the Bonney Sandstone at Patawarta diapir in the Flinders Ranges, South Australia, which is a ramping allochthonous salt sheet flanked by suprasalt and subsalt minibasin strata. Lithofacies distribution, thicknesses, and stratal geometries are described and analyzed to correlate suprasalt and subsalt Wonoka and Patsy Hill strata within a depositional and halokinetic sequence stratigraphic framework. Regionally, Wonoka and Patsy Hill strata represent a regressive sequence deposited in a storm-dominated carbonate shelf environment. The subsalt minibasin at Patawarta diapir contains upward-shallowing outer wave-dominated shelf to coastal plain facies in the Wonoka Formation and tidally-dominated barrier bar facies in the Patsy Hill Member, which form, respectively, 3rd order highstand and lowstand systems tracts. The Patsy Hill Member records a major shift in depositional systems from the underlying carbonate-dominated Wonoka Formation to the overlying siliciclastic-dominated Bonney Sandstone. Wonoka strata correlate laterally between the suprasalt and subsalt minibasins in terms of facies distribution and depositional setting. In contrast, Patsy Hill facies show significant lateral variation between suprasalt and subsalt positions, suggesting deposition was strongly controlled by the bathymetric high of the diapir. This is especially evident from the lateral distribution of quartzite pebble conglomerates in the Patsy Hill, which record different periods of exposure and erosion of the allochthonous salt sheet during deposition. Subsalt Wonoka and Patsy Hill strata compose a Previous HittaperedTop composite halokinetic sequence that displays upturn over a distance of 560 m and stratal thinning (1100 to 110 m) toward the diapir. Suprasalt Wonoka and Patsy Hill strata display significant but gradual thinning (1,638 to 19 m) over a distance of 4-5 km forming a broad, open fold with local upturn at the salt-sediment interface. The geometric and depositional elements from the suprasalt and subsalt minibasins at Patawarta diapir were evaluated to generate a depositional model that accounts for lateral and vertical salt migration and salt-modified bathymetry in a shallow marine environment characterized by allochthonous salt.