Sedimentology, provenance, and salt-sediment interaction in the Ediacaran Pound subgroup, Flinders Ranges, South Australia

Abstract

Much of our understanding of the sedimentary character and stratigraphic architecture of subsurface sedimentary deposits is derived from field-based studies of similar depositional systems exposed in outcrop. In South Australia, excellent surface exposures in the Neoproterozoic-Cambrian Adelaide Rift Complex provide a unique opportunity to examine a series of clastic sediments deposited in an ancient fluvial-deltaic to deep marine setting. Through extensive field and laboratory work, this study documents the sedimentology, stratigraphy, provenance, facies distribution and salt-sediment interaction of the upper Bonney Sandstone and Billy Springs Formation. These sediments formed part of the margin of the Australian subcontinent during the Ediacaran, a key time in Earth history just prior to the development of multicellular life. Field investigations reveal that the Bonney Sandstone is primarily comprised of sands and shales, often in progradational parasequences that become progressively sand-dominated upward. The formation thickens significantly to the north through the preservation of additional sediments that contain abundant fluvial features, suggesting a northern depocentre in the basin. Zircon data indicate that sediments may be sourced from the distant Musgrave Province and enter the basin from a large deltaic system in the northwest. These results provide substantial new information as to the palaeogeography of South Australia during this time, and are the product of interpretation using multiple lines of evidence and the study of numerous localities. Throughout the Adelaide Rift Complex, salt diapirs penetrated the basin fill and formed adjacent rim synclines (minibasins) due to withdrawal of underlying salt. Salt-tectonized, passive-margin settings are significant components of hydrocarbon systems in some of the world’s most productive regions, yet these features are rarely exposed in outcrop as they are here. In the far northern Flinders Ranges, the Umberatana Syncline is interpreted as a salt-withdrawal minibasin that formed in a deeper-water setting. The map-view exposure of the structure allows the depositional processes and products in the minibasin interior to be studied in a way not possible elsewhere; deepwater minibasins are very rarely exposed at the surface. Field and petrographic work reveals a mud-dominated minibasin fill containing mass-flow deposits of varying stages of maturity, ranging from clastbearing convolute-laminated slumps to sandy turbidites. In a more proximal setting, numerous measured sections along the margin of the Mt Frome minibasin clearly show that sediment character is influenced by diapir activity and the shedding of diapir-derived clasts. Lateral facies variability is controlled by growth faulting and diapir topography, with increased abundance of diapiric material near faults and highs, as well as thinning, onlap, and rotation of sediment blocks. These results, as well as those from elsewhere in the basin, are highly applicable to the prediction of reservoir, source, and seal quality in similar geologic settings in the subsurface.