Quantifying the geomorphic response of modern submarine channels to actively growing folds and thrusts, deep-water Niger Delta

Abstract

The interaction between submarine channels and active seabed deformation controls sediment delivery to the deep sea. Here, we combined seismic and geomorphic techniques to investigate quantitatively how the gravity-driven growth of thrust-related folds in the deep-water Niger Delta has influenced the morphology of four Pleistocene to Holocene submarine channels with present-day geomorphic expression. We extracted the bathymetric long profile of each of these modern seabed channel systems, and we evaluated the down-system evolution of channel widths, depths, and slopes as they have interacted with growing seabed structures. This information was used to derive estimates of bed shear stresses and velocities, to infer morphodynamic processes that have sculpted the channel systems through time, and to evaluate how these channels have responded to actively growing structures in the toe of the delta. The long profiles of these channels are relatively linear, with concavity from −0.08 to −0.34, and an average gradient of ∼1°. They are characterized by small knickpoints that are apparent near mapped structures and therefore implicitly reflect variations in substrate uplift rate. Channel incised depths increase significantly near the active structures, leading to entrenchment, but there is little change in the down-system distribution of channel width, in contrast to rivers crossing active faults, or buried submarine channel complexes. Reconstructed bed shear stresses near faults are estimated to lie in the range of 100−200 Pa, which would be associated with turbidite flow velocities of 2−4 m/s. A comparison of the magnitude and distribution of structural uplift since 1.7 Ma and the distribution of channel incision over this time shows that three of these channels have been able to keep pace with the time-integrated uplift since 1.7 Ma and have likely reached a local topographic steady state. Entrenchment of the submarine channels upstream of growing folds helps to drive this process, and we estimate that bed shear stresses of >100 Pa are sufficient to keep pace with structural strain rates of ∼4 × 10 −3 m.y. −1 .