AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Late Cretaceous, Kilometric Scale Slides and Associated Slump Complexes, Orange Basin, SW Africa
(1) Department of Earth Sciences, Fault Dynamics Research Group, Egham, United Kingdom.
Interpretation of a recently acquired, depth migrated 3D seismic data set from the shelf-slope break portion of the Orange Basin on the passive margin of Namibia (SW Africa), shows that the Late Cretaceous post-rift succession underwent gravity-driven collapse that produced a series of, up to 2 km thick, more than 30 km long by ~20 km wide extensional slides that are overlain by a hundreds of metres thick chaotic slump complex. The slides are scoop-shaped with a distinct listric headwall fault system with associated 3D roll-over structures and synthetic basinward-dipping growth faults. The listric extensional faults sole out into a shale detachment layer that links down dip to imbricate fans of toe thrusts. Lateral segmentation of the slide complexes occurs along steep lateral ramp fault systems that display oblique extensional motion. Between the lateral ramps footwall horsts form long narrow ridges between individual slides. The slides are unconformably overlain by chaotic slump complexes hundred of metres thick. The slumps are characterised by seismically coherent slide blocks of the Late Cretaceous strata separated by seismically chaotic zones that are interpreted to be heterogeneous deformed slump units and debris flows. In the slump complexes, headwall scarps, thrust imbrications and longitudinal shear zones indicate dominantly down-slope movement on a number of different small collapse systems. The results of this detailed study give new insights into gravitational instabilities on continental margins and the resultant complex varieties of products that may be developed.