Late Quaternary Depositional Processes on Continental Rise of Western Equatorial Atlantic: Comparison with Western North Atlantic and Implications for Reservoir-Rock Distribution

Abstract

The latest Pleistocene continental-rise sediments of the western Equatorial Atlantic are composed of (1) hemipelagic clay rich in terrigenous silt and organic detritus, and (2) redeposited terrigenous silt/sand beds up to several meters thick. The abundance, thicknesses, structures, textures, and compositions of the redeposited silt/sand beds indicate that the continental rise has been shaped largely by deposition from turbidity currents and related mass flows. In contrast, previous investigators have demonstrated that the continental rise of the western North Atlantic has been shaped largely by deposition from geostrophic contour currents. Thus, although both the Equatorial and North Atlantic rises are on the western side of the ocean basin, receive large influxes of ter igenous sediment, and have similar late Pleistocene sedimentation rates, the two continental rises have very different sedimentary histories. The fact that contour-current deposition has not been a significant factor in shaping the continental rise of the western Equatorial Atlantic indicates that the contour-current model for continental rise formation which was derived from observations in the western North Atlantic cannot be utilized arbitrarily to explain formation of all continental-rise sequences, ancient or modern. The process which formed any particular continental rise may have an important bearing on the potential of that rise as a hydrocarbon province. Continental-rise sequences shaped by turbidity-current processes (such as the western Equatorial Atlantic) would seem to conta n both excellent source and reservoir rock: whereas good reservoir rocks apparently would be absent from continental-rise sequences shaped by contour currents (such as the western North Atlantic).