PIETRAS, JEFFREY T., ROGER B. BLOCH, DON YING, KATHY A. KANSCHAT
Mobil Technology Company, Dallas, TX

Abstract: Evolution of Depositional Geometries in a Submarine Slope System using 3D Seismic, Pleistocene, Niger Delta

Seismic stratigraphic analysis of late Pleistocene slope sediments of the Niger Delta reveals an evolution through distinct stages of degradation and deposition. This produces systematic change in dimensions and geometries of depositional bodies. The base of the studied interval marks widespread mass-wasting that is most intense over a toe-thrust terrain and extends upslope, apparently due to progressive failure. This left depressions on the seafloor 5000 m wide and up to 300 m deep. The mass-wasting event coincides with retrogradation of the shelf edge following a significant period of progradation. Low amplitude draping reflections, interpreted as condensed section, overlie the mass-wasting surface. The condensed section is overlain by numerous channel-levee complexes that converge downslope into the depressions produced by mass-wasting of the toe-thrust terrain. The channels are 200-400 m wide and levees extend up to 1500 meters from the channel margin. This period represents the most significant influx of sediment by sediment-gravity flows. Most of the channels were abandoned at the onset of shelf progradation. Slope gully and lobe systems cap the progradational packages. The gullies are nearly straight, closely spaced laterally, and occur in distinct clusters on portions of the prograding clinoform. Gullies are 50-100 m wide, detectable (not resolvable) on seismic, and have "gull wing" geometry in their distal segments. They transition to lobes as slope angle falls below 1°. The lobes are roughly 1000 m wide in both strike and dip direction. Three cycles containing channel/levees and clinoforms with gullies/lobes occur above the mass-wasting horizon. The last cycle forms a channel/levee complex with seafloor expression that is now abandoned and degraded by channel-margin slumping.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas