Lower Cambrian carbonate microfacies and depositional environments on Yorke Peninsula, western Stansbury Basin, South Australia

The western Stansbury Basin is a largely subsurface early–middle Cambrian sedimentary succession centred on Yorke Peninsula, South Australia. The oldest carbonate units include the pervasively dolomitised Kulpara Formation, and the unconformably overlying nodular and bioclastic Parara Limestone, which includes archaeocyath–calcimicrobe bioherms of the Koolywurtie Limestone Member. General lithologies and sequence stratigraphy of these units have been outlined previously, but no detailed microfacies investigation has been undertaken. Petrographic and micro-XRF analyses facilitate microfacies descriptions, based on 67 oriented thin-sections collected from the Minlaton 1 and SYC 101 exploration drillholes on southern Yorke Peninsula, and surface outcrop at Horse Gully, near Ardrossan on northern Yorke Peninsula. A total of 13 microfacies occur through five facies sequences beginning with a semi-restricted inner carbonate shelf lagoon with periodic storm reworking of sediment, to a brief, open-shelf setting in the Kulpara Formation. A drowned sequence boundary manifests at Horse Gully by the growth of branched, ferruginous microstromatolites, forming a distinctive ‘red crust’ that marks the abrupt change from a mid to outer shelf setting. Deposition in the overlying lower Parara Limestone, initially dominated by upper-shelf-derived sediments and firmground surfaces, is replaced by in situ deposition of nodular carbonates and phosphatic horizons. A shallowing event in the upper Parara Limestone results in the deposition of the Koolywurtie Limestone Member with archaeocyathan–calcimicrobial bioherms and stromatolitic bindstones before conformably returning to outer shelf conditions in the uppermost Parara Limestone. Biostratigraphic control and taphonomic signals in the upper Kulpara Formation and Parara Limestone are reinforced by three absolute CA ID-TIMS dates from volcanic ash layers in the Parara Limestone in SYC 101 that tightly constrain the depositional history of this lower Cambrian carbonate shelf system.