Lateral Facies Variability Along the Margin of an Outcropping Salt-Withdrawal Minibasin, South Australia

Abstract

Many aspects of depositional processes adjacent to exposed evaporite diapirs arc not well understood, yet these processes are key to understanding sediments that are part of economically important hydrocarbon systems around the world. In the Adelaide Rift Complex of South Australia, diapirs intersected the seafloor and land surface during the Ediacaran-Cambrian at the same time as sediment was being deposited. Excellent exposures of these diapirs and their associated minibasins allow the character and distribution of these deposits to be studied in detail; this study examines the interaction between a diapir body and minibasin sediments from a sedimentological perspective. Numerous sections were measured along the minibasin margin, allowing the sedimentary facies, lateral sediment variability, and depositional processes to be determined. Deposition took place in a variety of environments, ranging from carbonate shelves to subaerially exposed tidal flats and alluvial fans. Minibasin sedimentation adjacent to the diapir is characterized by an abundance of gravity-flow deposits, including turbidites and debrites. These deposits often contain extraformational conglomerates brought to the surface by the diapir and redeposited into the minibasin depocenter. Within the minibasin fill, sedimentary facies are unevenly distributed, and sedimentary character is most affected by the diapir in areas where depositional thinning, onlap, and growth faulting are most common. Comparison of our observations with other localities where salt bodies intersect the surface show that syndepositional salt-sediment interaction results in a recurring set of features that can be useful in predicting the sedimentary character of these deposits. This study is one of relatively few to examine the sedimentology of an outcropping minibasin in detail and to describe lateral variability of sedimentary facies along an outcropping minibasin margin. The deposits discussed here therefore provide a valuable analogue for subsurface and seafloor deposits elsewhere where salt-sediment interactions cannot be studied in detail.