Constraining the erosional response of deep-water channel systems to growing folds and thrusts, Niger Delta.

Abstract:

Gravity-driven folds and thrusts often characterize the slope and deep-water settings of passive margins. These structures exert a significant control on sediment gravity flows because they determine the location and configuration of sediment depo-centres and transport systems. Here we exploit 3D seismic data in the outer toe-thrust region of the deep-water Niger Delta to analyse the interaction between Plio-Pleistocene channel systems and actively-growing folds and thrusts. We first map folds and thrusts from the seismic data and we use this data to reconstruct the history of fold growth in detail. We then make quantitative measurements of the geomorphic response of submarine channels to growing tectonic structures in order to provide new constraints on their long-term erosional dynamics. This information is used to infer morphodyanamic processes that sculpted the channel systems through time, and to estimate the bed shear stresses and fluid velocities of typical flow events.

 The bathymetric long profiles of these channels have concavities that range from -0.08 to -0.34, and an average gradient of ~1^o. Thrusts are associated with a local steepening in channel gradient of up to 3 times, and this effect extends 0.5 – 2 km upstream of the thrust. Within these knickzones, channel incision increases by approximately by a factor of 2, with a corresponding width decrease of approximately 25%. Channel incision across growing structures is achieved through enhanced bed-shear stress driven incision (up to 200 Pa) and flow velocity (up to 5 ms^-1) assuming typical bulk sediment concentrations of 0.6%. Comparison of structural uplift since 1.7 Ma, and channel incision over an equivalent period, shows that many of these channels are able to keep pace with the time-integrated uplift since 1.7 Ma, and may have reached a bathymetric steady-state. Generally, bed-shear stresses of ~150 Pa are sufficient to keep pace with structural strain rates of 10^-15 s^-1. More widely, our data demonstrates that submarine channel systems dynamically adjust their geometry and basal gradient in order to keep pace with growth of tectonic structures and our results suggest that these factors must be incorporated into models to fully predict the downslope pathways of sea-bed channels in structurally complex areas.