Gradual Transition From Net Erosional to Net Depositional Cyclic Steps Along the Submarine Distributary Channel Thalweg in the Rio Muni Basin: A Joint 3-D Seismic and Numerical Approach

Abstract

We integrated measurements of cyclic steps and bankfull hydraulic geometry to estimate turbidity flow properties, yielding two main observations. First, three net erosional cyclic steps were recognized along a distributary channel thalweg based on (1) closed topographic depressions, (2) less asymmetrical cross-sectional morphologies, (3) disrupted and discontinuous internal architecture revealed from seismic data, (4) erosional basal contacts, (5) stronger up-current migration, and (6) relatively higher U1 (flow velocity just before each hydraulic jump), Q1 (bankfull discharge before hydraulic jump), and ΔEL (energy loss of hydraulic jump) of formative turbidity currents. In contrast, six net depositional cyclic steps were identified based on (1) open topographic depressions, (2) more asymmetrical morphologies, (3) well-bedded and continuous internal structures, (4) gradual basal contacts, (5) stronger vertical accretion, and (6) relatively lower U1, Q1, and ΔEL values. These observations may aid in distinguishing between net erosional and net depositional cyclic steps. Second, cross-sectional asymmetries (Ay) and width-to-depth ratios of the trough (W1/h1) of cyclic steps systematically vary with distance along the distributary channel thalweg from the point of overflow initiation (Di) and can be fit by a single relation, which, together with the fact that the two types of topographic depressions separate along these relations, demonstrate a gradual transition from net erosional to net depositional cyclic steps. U1, Q1, and ΔEL are proportional to the stoss-side slope angle of cyclic steps (а) but are inversely proportional to Ay and W1/h1, indicating that a progressive downstream decrease in U1, Q1, and ΔEL (i.e., decrease in sediment transport capacity) probably caused the gradual transition from net erosional to net depositional cyclic steps.