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Interaction Between Deepwater Current Drifts (Contourites) and Canyon Fill-Slope Valley Turbidites, Cretaceous and Tertiary Sediments of Offshore West Africa


Current drifts (contourites) are common depositional features along the western continental margin of Africa and other margins. They are traditionally non-reservoir shales and silts, but they can be sandy, especially in distal toe positions. They can significantly influence the locations and depositional patterns of down-slope reservoir-quality turbidites such as canyon fills and slope valleys. Bedform geometries of current drifts are easily observed on good-quality 2D and 3D seismic data. Cretaceous and Tertiary current drifts in western Africa range in apparent dimensions from 6 km to over 35 km, averaging about 15 km; thicknesses vary from 100's of meters to over 1000 meters. The ratio of height to lateral dimensions varies widely. Current drifts in the study area typically have a prograding seismic pattern of toplap – downlap in the interpreted direction of migration, with bypass or little deposition on the up-current side and redeposition on the down-current side. Tops and bases of drifts form external boundaries limiting internal bed-form patterns. Current drifts are interpreted to form and migrate in the direction of the bottom current, which in western Africa is mostly from east to west. True current flow directions are difficult to determine, because current directions are rarely orthogonal to drift geometries. Current drifts commonly interact with down-slope turbidite canyon fill-slope valley systems, with successively younger turbidite systems being offset in the direction of drift development. Turbidites are commonly deposited in the topographic depressions between drifts. When an episodic period of turbidite deposition ends, the turbidite is covered by the continuously deposited current drift. The next turbidite deposit is then shifted in the direction of drift migration. This pattern may result in a long-term shift of successive slope valleys and canyons in the direction of drift development. Sediment waves, commonly associated with current drifts, are also deposited from currents and are often confused with drifts. They are an order of magnitude smaller than drifts, commonly 1 km or less from crest to crest, and may occur as small-scale components of current drifts. They appear to occur in a number of depositional settings, possibly under higher current flow regimes than drifts. They are thought to form in directions opposite to the current direction, commonly in anti-dunal wave trains not unlike climbing ripples at outcrop scales.