SEISMIC GEOMORPHOLOGY AND EVOLUTION OF SUBMARINE CHANNELS FROM THE ANGOLAN CONTINENTAL MARGIN

Abstract

Three-dimensional seismic data from the shallow subsurface of the continental margin offshore Angola reveal two end-member morphological styles of submarine channel: (1) high-gradient, low-sinuosity, narrow channels with gull-winged levees, and (2) lower-gradient, deeply incised systems with moderate- to high-sinuosity channel axes. A third, and rare, channel form has moderate incision, low to medium sinuosity, and a moderate long-profile gradient. Based on channel parameters (incision depth, long-profile gradient, channel-axis sinuosity) and crosscutting relationships, we suggest that the channels evolved from initially steep and straight, with low sinuosity, to highly sinuous and deeply incised with lower channel-axis gradients. Correlation of long-profile gradient with both incision and sinuosity suggests that incised channels appear to remove convex-up curvature from the original slope as the channel axis evolves toward an equilibrium profile. Localized changes in channel planform, gradient, sinuosity, and incision reflect the complex morphology of the slope associated with growth of salt-related structures. Linear, high-amplitude seismic features, which correspond to weakly incised striations, or rills, on the open slope are considered to be precursors of submarine channels.