Deepwater West Africa Waterbottom Debris Flows Captured and Analyzed by 3-D Visualization Techniques

Jason P. Sutton and Robert M. Mitchum
Chevron, Houston, TX

3-D images of deepwater offshore West Africa Pleistocene debris flows and leveed channels occurring at and just below the waterbottom make excellent analogs for interpreting similar subsurface features. A large apron of multiple leveed channel systems and associated debris flows builds out from a canyon mouth directly onto the basin plain. Several avulsing channels and associated levees emerging from the canyon mouth are shown in spectacular detail. Sidewall failures and rotational slips produce debris flows in all stages of development.

Leveed-channel systems near the waterbottom surface occur in a complex complementary fill pattern. The leveed-channel systems are represented as meandering sets of channels occurring in straight, tightly constrained meander belts. Sediment waves occur selectively at major thalweg meander turns, modified by complex faulting. Sets of basal parallel high-amplitude reflectors represent attached frontal splays.

Broad shallow sheets of debris flows on the adjacent slope are developed from large slump scars with vertical displacements of 200 feet, with significant strike-slip movement. Images of these flows show patterns of distal pressure ridges and chaotic internal structures. An expansive boulder field represents large debris flows at the waterbottom showing details of blocks detaching from rigid bounding adjacent strata. Minor waterbottom transportation is triggered by faulting creating slope instability and fluid expulsions, and produced slide scours eroding v-notches into the slope.

The remarkable images of debris flows and channels are enhanced by 3D visualization techniques of color mapping, opacity/rendering of seismic volumes, and spectral decomposition for thin bed imaging.