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An Outcropping Salt-Withdrawal Mini-Basin Filled with Deepwater Sediments – the Donkey Bore Syncline, South Australia

Tobias Payenberg, Simon C. Lang, Mark Reilly, Blaise Fernandes, and Carmen Krapf
Australian School of Petroleum, University of Adelaide, Adelaide, Australia

Salt withdrawal mini-basis filled with deepwater sediments are significant exploration targets around the world, including in the Gulf of Mexico and along the west coast of Africa. Reservoir and seal facies distribution within such mini-basins are highly variable and difficult to predict. As a result, many development programs face higher compartmentalization than anticipated, often leading to higher costs and/or lowered reserves.

Outcrop analogues are one way of gaining a generic understanding of compartmentalization in salt-withdraw mini-basins. The Cambrian Donkey Bore Syncline, Flinders Ranges, South Australia is one such analogue exposing a complete mini-basin fill with over 400m of section next to the Wirrealpa Diapir. The doubly-folded syncline exposes shallowly-dipping sediments along its three limbs over an area of approximately 20 km2.

The fill comprises basal shallow marine carbonates and Archaeocyatha reef complexes. Towards the top of the limestone slumping occurs frequently, and reef complexes are found as allochthonous blocks. This mass wasting shows the initiation of the mini-basin. Overlying the carbonates are thick, massive densite and debrite sandstone beds separated by mudstones – the initial clastic fill of the basin. Up section the densites and debrites decrease in thickness and frequency, and turbidite beds start occurring as a result of a decreasing slope through filling of the mini-basin. The overall transgressive deepwater succession ends with thin turbidite and thick mudstone packages overlain by chloritic siltstones. Sandbody continuity within the basin is mainly dependent on the type of flow mechanism and the input location relative to the diapir.