Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Calgary, Alberta, Canada, June 19–22, 2005
A Submarine Channel’s Response to an Evolving Seafloor Bathymetry: An Example from the Miocene Slope of Southern Gabon
Recent industry research has focused on the examination of submarine channels in the shallow subsurface to help understand and predict complexities that may exist in their more deeply buried, hydrocarbon-charged brethren. This study examines an upper Miocene slope channel complex and attributes patterns of erosion and deposition to local seafloor gradients that developed in response to (1) differential compaction of preexisting strata and (2) subtle salt withdrawal.
High-resolution seismic stratigraphy reveals a sequential, multistage history that included phases of (1) channel-lobe progradation, (2) downcutting, and (3) backfilling. Phase 1 deposits are mainly at-grade nonincised channels with an episode of avulsion that created a terminal sandy lobe that ponded within a small salt-withdrawal basin. Phase 1 deposits are strongly controlled by the position of earlier sandy channel and muddy interchannel regions. At the end of phase I, subtle diapiric movement created a local bathymetric depression that progressively captured a single meander loop of the main channel. During Phase 2 the system experienced downcutting. The now highly elongated meander loop began to entrench but was abandoned in favor of a more direct downslope path. The bulk of the flows then followed a straighter and widening erosional fairway. The downcutting phase records the channel's effort to achieve a new equilibrium profile. This resulted when the channel met an abrupt increase in gradient which kicked off a wave of upslope-migrating knickpoints. Backfilling occurred during phase 3 when the broadly incised fairway filled with a seismic facies consistent with underfit sandy channel fills. One noteworthy aspect of the system is that the phases appear to be transitional suggesting a more continuous depositional record rather than one punctuated by sequence boundaries.