Remobilisation Processes on the Submarine Slope: is the Degree of Deformation a Reliable Criteria for Transport Distance

Van der Merwe, Willem C.¹, David M. Hodgson², Steve Flint¹ (1) Liverpool University, Liverpool, United Kingdom (2) Livepool University, Liverpool, United Kingdom

Mass transport deposits (MTDs) are often identified in subsurface and seabed datasets, which can provide useful information on deposit bulk volume and spatial distribution. However, limited information on internal facies, architecture and transport distances exists. A rare example of deepwater kilometre-scale MTDs at outcrop is the Vischkuil Formation, Laingsburg Karoo Basin, South Africa, which allows detailed analysis of deformation processes and emplacement history. Down-dip facies transitions of correlated MTDs indicate a temporal evolution of individual events from proximal coherent slumps and slide bodies into distal disaggregated debrites over a down slope distance of approximately 55 km.

Detailed analysis of a single 50 m thick MTD, which is continuously exposed for 2 km down-dip at the Rietfontein locality was undertaken to understand the internal architecture and emplacement processes. In proximal localities the stratigraphic interval is undeformed. However, down-dip clastic dykes and low-angle shear surfaces are common indicating extensional movement where associated sediments have undergone ductile deformation at different scales. Further down-dip the orientation of the extensional shear surfaces spoon round and ramp-up under a compressional regime (mimicking ramp faulting). Above, and associated with the major compressional planes are large (10's metres amplitude) folds. The whole stratigraphic exposure at Rietfontein is unique in preserving the evolution of a slump from undeformed, through extensional (evacuating zone) to a compressional toe zone. This study also highlights that an intense zone of soft sediment deformation can develop over a short displacement distance.