--> A Rare Insight Into the Sedimentology and Stratigraphy of a Salt-Withdrawal Minibasin: Outcrop Investigation of the Neoproterozoic Billy Springs Formation, South Australia

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A Rare Insight Into the Sedimentology and Stratigraphy of a Salt-Withdrawal Minibasin: Outcrop Investigation of the Neoproterozoic Billy Springs Formation, South Australia

Abstract

Salt-withdrawal minibasins form significant hydrocarbon reservoir-trap systems in some of the world's most productive regions, yet these features are rarely exposed in outcrop. We present an outcrop investigation that provides an opportunity to understand the stratigraphic architecture and sedimentologic processes in these environments at a scale not possible in the subsurface. Our focus is the Umberatana Syncline in the Adelaide Rift Complex of South Australia, an 11km x 15km structure exposed at the surface and interpreted as a Precambrian salt-withdrawal minibasin. Measured sections in the field reveal that minibasin fill is primarily comprised of the Ediacaran Billy Springs Formation, a fine-grained clastic unit dominated by silty mudstones and sands that have not previously been studied in detail. The formation is characterized by four distinct lithofacies: convolute-laminated slump deposits, rhythmically-laminated silty mudstones, diamictites, and fining-upward sandy turbidites. Lithofacies indicate the minibasin likely formed on a passive margin slope or ramp setting, representing one of the few deep water environments in the Adelaide Rift Complex. The formation is likely the distal portion of a highstand systems tract, overlain by progradational coarser sediments of the Pound Subgroup. Conglomeratic intervals consist of poorly sorted extrabasinal clasts and are interpreted to be the product of mass flow processes originating from nearby emergent diapirs that shed sediments brought up from deeper in the basin fill. This is in contrast to previous studies which interpreted the extrabasinal clasts as dropstones and invoked a glacial processes as being responsible for their deposition. Detrital zircon ages from turbidite sands have a dominant peak at 1112 Ma, suggesting a sediment source originating from a Grenville-age event and closely matching the Musgrave Province in central Australia. A Musgrave provenance implies long-distance transport of sands, and necessitates a review of existing models that show a more proximal source on the nearby Gawler Craton. This study thus contributes to the knowledge of the paleogeography of the Australian subcontinent during the Late Proterozoic, as well as providing an excellent outcrop analog for many oil and gas plays influenced by salt tectonics.