Abstract

Wirrealpa diapir, in the Central Flinders Ranges, South Australia, was a passive diapir located on the Arrowie basin slope during the Early Cambrian. Wirrealpa diapir is flanked by two secondary minibasins—the Donkey Bore and the Woodendinna—that contain different thicknesses and facies of Lower Cambrian carbonate strata. Because diapir rise rate must have been similar for both minibasins, this study allows for assessment of the role of sediment supply on the formation of composite halokinetic sequences (CHS). One minibasin contains tapered CHS, formed when sediment accumulation rate outpaces diapir rise rate. The opposing minibasin contains primarily tabular CHS, which form when diapir rise rate outpaces sedimentation rate. Three Cambrian depositional sequences are exposed adjacent to the diapir and display an overall deepening trend. The lower sequence consists of platform facies where diapir influence appears to be limited to minor facies changes and local shedding of diapir-derived detritus into the Woodendinna minibasin. The middle sequence is comprised of primarily slope facies into which the diapir created a bathymetric high. An isolated platform developed above the diapir and shed platform-derived grainflows onto the slope surrounding the diapir. The upper sequence begins with lowstand turbidite deposits, which are thick and amalgamated next to the diapir in the updip minibasin, but thinner and dispersed throughout the downdip minibasin. Thickness and facies variations between minibasins reflect differential sedimentation rates. The Donkey Bore minibasin sat above the Woodendinna minibasin on the slope, shielding it from upslope sediment sources and resulting in generally lower sedimentation rates and differing CHS styles. Banking of turbidites against the Wirrealpa diapir in the Donkey Bore minibasin mirrors the ponded deposition commonly observed in the Gulf of Mexico and other structured slope settings.