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Aseismic controls on in situ soft-sediment deformation processes and products in submarine slope deposits of the Karoo Basin, South Africa



Intervals of soft-sediment deformation features, including vertical fluid escape and load structures, are common and well-exposed in Permian lower slope deposits of the Tanqua Depocentre, Karoo Basin. The structures mainly comprise elongated flames and load structures associated with ruptured sandstones and structureless siltstones, observed over a range of scales. The presence of an upper structureless siltstone layer linked to the flames, interpreted as a product of the debouching of fine-grained material transported through the flame onto the palaeo-seabed, together with the drag and upward folding of lower sandstone layers is evidence that the flames were formed in situ by upward movement of sediment-rich fluids. Flames are oriented parallel to the deep-water palaeoslope in lateral splay deposits between two major slope channel complexes. Statistical correlation and regression analyses of 180 flame structures from seven stratigraphic intervals suggest a common mechanism for the deformation and indicate the importance of fluidization as a deformation mechanism. Importantly, deformation occurred in an instantaneous and synchronous manner. Liquefaction and fluidization were triggered by incremental movement of sediment over steeper local gradients that were generated by deposition of a lateral splay on an inherited local north-west-facing slope. Seismic activity is not invoked as a trigger mechanism because of the restricted spatial occurrence of these features and the lack of indications of earthquakes during the time of deposition of the deep-water succession. The driving mechanisms that resulted in the final configuration of the soft-sediment deformation structures involved a combination of vertical shear stress caused by fluidization, development of an inverse density gradient and a downslope component of force associated with the local slope. Ground-penetrating radar profiles confirm the overall north-east orientation of the flame structures and provide a basis for recognition of potential larger-scale examples of flames in seismic reflection data sets.