Stratigraphic Evolution of a Tortuous Corridor From The Stepped Slope of Angola

Abstract

This paper documents the architecture of basin fill and depositional evolution along a stepped, above-grade profile in the Kwanza Basin, Angola Block 21. Detailed mapping of a well-imaged, near-seafloor seismic dataset reveals the influence on preserved depositional architecture of evolving slope gradient, along corridors flanked by pronounced lateral confinement. Sediment gravity flows entered the upper-slope to mid-slope basin system via an incised valley and interacted with the above-grade stepped topography. Effects of slope topography are most prominent during the early stages of deposition, such that flow contraction occurs over highs and flow expansion occurs in topographic lows. Evidence of sediment bypass, knickpoint migration, and erosion immediately outboard of areas of deposition supports the interpretation of an open stepped slope profile; there is only limited ponded accommodation in the study area. Initial flows into the system were deposited in both updip and downdip basins, suggesting a weakly confined stepped profile with flow stripping and/or bypass between basins. Ongoing deposition healed the slope profile, with subsequent flows bypassing to downdip basins. Preserved depositional architecture is dominated by distributary channel–lobe complexes that comprise offlapping, shingled lobes deposited downdip of slope breaks, progressively wrapping around the salt topography. Lobes are more numerous, decrease in size, and increase in degree of channelization up through the section. Bypass initially occurred through numerous narrow channel stories that converge to a wider, single through-going system. With time, and sediment progradation, the locus of deposition shifted progressively basinward, away from basinal highs. Fan apron stacking patterns are dominantly progradational, but fan packages are retrogradational prior to feeder-channel avulsion. Initial flows from each of the channels produced large-volume, highly channelized lobes. Prior to abandonment of the channel system, the flows are smaller and possibly muddier, and deposits are less channelized. Channel avulsion and local tectonism had a first-order control on the locus of deposition in the area, and caused an incomplete depositional cycle that deviates from predictions according to classic stepped-slope models.