Preferential Downward Injection of Sandstone in the SW Karoo Basin, South Africa: Pre-Requiste Conditions and Trigger Mechanisms

Hodgson, David M.¹, Willem Van der Merwe¹, Carlos Oliveira¹ (1) University of Liverpool, Liverpool, United Kingdom

Clastic dykes and sills are widely exposed in Permian deepwater mudstones in the SW Karoo Basin, South Africa. The injectites are sourced from sand-rich basin-floor fan and submarine slope channel systems. The spatial and stratigraphic distribution of the injectites has been mapped using dGPS technology, and their orientations and morphological characteristics have been measured. Evidence suggests that the majority of the clastic dykes and sills have been injected downwards, including 1) tapering and bifurcation of dykes downwards, 2) identification of loci of injections in overlying parent sand body, 3) deflection around concretions and folding of mudstone cleavage, and 4) absence of underlying clastic sediment source.

The precise trigger mechanism responsible for the failure of sealed overpressure sands is not clear. The distribution of injectites indicates a close relationship to sharp-bases (erosionally-based) or sharp-topped (abrupt abandonment) sandstones. In particular, the margins of erosion surfaces (channel bases) are common loci of injection. The preferential injection direction indicates a decreasing downward pressure gradient. Seismic activity alone as a trigger mechanism does not explain this preferential direction. The mudstone was cohesive enough at the time of injection to allow the formation of hydroplastic lineations on the margins of the dykes, and for mudstone ‘clasts' to be entrained into the injected sand, so injection during channel-filling is not envisaged. As the majority of injections are found in a submarine slope setting, and failure through propagation of gravitationally-driven mode one fracture planes is inferred.