Sedimentary architecture of submarine lobes affected by bottom currents: Insights from the Rovuma Basin offshore East Africa

Abstract

The influence of bottom currents on submarine channels has been widely recognized, for instance, by the formation of asymmetric channel-levee systems and drifts. In contrast, it is often considered that submarine lobes can be only reworked by strong bottom currents and are not affected by bottom currents during their deposition. In this study, we analyse the potential effect of bottom currents on different hierarchical lobe architectures that formed during the lower Oligocene in the Rovuma Basin offshore East Africa. We characterize the stacking patterns, morphology and connectivity of different hierarchy lobes using well data and three-dimensional seismic data. We found no direct influence of bottom currents on the lobe complexes and single lobes that show a unidirectional stacking pattern that is opposite to the direction of bottom currents. Lobe elements in single lobes display vertical accretion with no obvious relationship with bottom currents. Additionally, the first deposited single lobe morphology presents an asymmetric shape, with a thicker lobe margin on the downstream side of the bottom currents, but this is due to an initial low topography on the downstream side rather than bottom currents. The architectural distribution reflects that the topography present before the depositions of the submarine lobes was controlled by previous asymmetrical channel-levee systems formed by the synchronous interaction of bottom currents and gravity flows. This asymmetric topography controls the subsequent deposition of lobes and results in the migration of single lobes in the upstream direction of bottom currents. Although weak to moderate bottom currents may not be able to substantially rework submarine lobes, our results demonstrate that they may control the geometry and evolution of submarine channels and thus indirectly affect the thickness and migration of lobes in more environments than previously thought.