An integrated model of clastic injectites and basin floor lobe complexes: implications for stratigraphic trap plays

Injectites sourced from base-of-slope and basin-floor parent sandbodies are rarely reported in comparison to submarine slope channel systems. This study utilises the well-constrained palaeogeographic and stratigraphic context of three outcrop examples exposed in the Karoo Basin, South Africa, to examine the relationship between abrupt stratigraphic pinchouts in basin-floor lobe complexes, and the presence, controls, and character of injectite architecture. Injectites in this palaeogeographic setting occur where there is: (i) sealing mudstone both above and below the parent sand to create initial overpressure; (ii) an abrupt pinchout of a basin-floor lobe complex through steep confinement to promote compaction drive; (iii) clean, proximal sand beds aiding fluidisation; and (iv) a sharp contact between parent sand and host lithology generating a source point for hydraulic fracture and resultant injection of sand. In all outcrop cases, dykes are orientated perpendicular to palaeoslope, and the injected sand propagated laterally beneath the parent sand, paralleling the base to extend beyond its pinchout. Understanding the mechanisms that determine and drive injection is important in improving the prediction of the location and character of clastic injectites in the subsurface. Here, we highlight the close association of basin-floor stratigraphic traps and sub-seismic clastic injectites, and present a model to explain the presence and morphology of injectites in these locations.