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Abstract

Abstract: Deepwater mass-transport deposits (MTDs) are associated with Upper Quaternary seafloor leveed-channel complexes at the mouth of a large canyon at the base of slope of the offshore Niger delta. They make excellent analogs for interpreting older subsurface features and reservoirs, and for geohazard analysis. These leveed-channel complexes and mass-transport deposits are assessed within a 3D seismic survey, using detailed images of seafloor maps and stratal surfaces, artificially digitally colored, and vertically exaggerated to create optimal imaging. A large canyon head incises the present-day shelf margin of the Niger delta and traverses down the upper and lower slope for 45 km towards the southeast. At the canyon mouth, a large apron of leveed-channel complexes covers the slope and basin plain for a distance of 30 km within the seismic survey. Large sediment waves occur on outer levees of channel bends, attaining heights of 200 m. In some areas, synclinal limbs of individual sediment waves have been deformed by numerous rotated blocks along small listric faults to form small mappable MTDs.

Other mass-transport deposits occur associated with and above the leveed-channel complexes. Lengths of the MTDs range from 1 km to over 16 km, and thicknesses commonly range from 100 m to 200 m. Headward escarpments are well imaged in both map and cross-section views. Proximal facies of the MTDs includes rotated blocks and large angular glide blocks. These pass distally into smaller glide blocks and chaotic seismic facies inferred to be debrites. Intermediate parts of the MTDs have longitudinal linear features parallel to inferred flow direction. Distal patterns consist of transverse compressive ridges. Other MTDs too large to be completely imaged within the 3D survey show internal facies, consisting of large angular glide blocks in a matrix of seismically visible smaller blocks and chaotic facies inferred to be debrites. Multiple causes of MDTs in this area are probable. In possible order of importance, these include tilting and oversteepening of sediments because of tectonic uplift, high sedimentation rates at the mouth of the canyon, and eustatic falls of sea level.

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