The interaction between deepwater channel systems and growing thrusts and folds, toe-thrust region of the deepwater Niger Delta

The Niger Delta fold and thrust belt occurs in an area of tectonic shortening – caused by the thin-skinned gravitational collapse of large deltaic sediment wedges above a ductile overpressured shale. Syn-sedimentary processes such as down-slope flowing gravity currents interact with the deforming seafloor topography to produce growth packages that record the deformation history of the folds. The thesis documents the spatial and temporal interaction between Pleistocene to Recent submarine channels, and folds/thrusts that have been growing since 12.8 million years ago (Ma). 3D seismic reflection data and key stratigraphic/horizon ages are used to constrain and analyse the spatial and temporal variation in shortening of major folds having seabed relief. Geomorphic techniques were applied to quantify the geomorphic responses of submarine channels developing coevally with structural deformation. This thesis documents two types of structures (fault-propagation folds and a detachment fold) whose cumulative strain (shortening) varies spatially and through time. The maximum interval shortening rate occurred between 9.5 Ma and 3.7 Ma, and has reduced significantly from that time to present. Channels show a range of interactions with structures, from simple deflection to fold tips to complete diversion. However, channels are capable of crossing the actively growing fault-propagation folds in positions of recent strain minima and at interval strain rates that are generally less than 15 m/Myr. In contrast, channels have been completely diverted by the broad detachment fold albeit growing at comparably lower rates. This thesis emphasizes that careful fold displacement – distance measurements which bracket the time interval of channel system development are very important for predicting sediment pathways in deepwater settings. Detailed geomorphic analysis showed that the bathymetry longitudinal profiles of the active channels are relatively linear with concavity values that range from -0.08 to ̵ 0.34, with an average profile gradient between 0.9[degrees] and 1[degrees]. In contrast, channel systems that have been abandoned and buried for long period of time, have longitudinal profiles that are more convex. The profiles of both the active and buried channels are characterized by knickzones that are apparent near mapped structures – and implicitly record variations in substrate uplift rate. The recently active channels (the modern thalweg) show no systematic width change down-system but they do show an increase in incision depth/erosion of up to 70 % at structural locations. However, the channel system (made of several cut-and-fill sequences), shows clear width narrowing together with time-integrated incision and erosion in response to time-integrated structural uplift. Estimates of the down-system variation in channel bed-shear stress and flow velocity, using the thalweg-geometry of the active channels, suggests that near growing folds and thrusts, the enhanced bed-shear stress-driven incision is up to 200 Pa. and the flow velocity is up to 5 m s-1. In essence, the linear nature of the active channel profiles, in comparison to the convex nature of the buried channel profiles, suggests that the active channels are able to keep pace with the time-integrated uplift of folds and thrusts, and therefore appear to be in topographic steady-state with respect to structural uplift since at least 1.7 Ma. Facies analysis using the seismic data showed that the main seismic facies include: (i) channel axes sands and top-channel sands (ii) sheet-sands or crevasse splays (iii) slump deposits and (iv) pelagic drapes. The growth of structures with seabed relief has affected the location of channel avulsion, the locus and the deposition/distribution of sheet-sands (splays). These splays can spill over the growing fault-propagation folds in areas of lower fold growth rates, and absence of seabed scarps; but are completely blocked, and subsequently incorporated onto the limb of a broad detachment fold in the east of the study area as incoming channels are forced to divert through time. This thesis has contributed to the understanding of: (1) Deformation by thrust-related folds that have been growing since ca. 12 Ma, and attained maximum interval growth rates between 9.5 Ma and 3.7 Ma. These maximum growth rates have reduced significantly in the last 3.7 million years during which submarine channels that are generally less than 1.3 million years old also occurred. (2) How modern seabed channels (i.e., recently active channels) have responded to the time-integrated growth of structures along their paths; and the related effect on the positioning of channels pathways, which in-turn, governs the depositional system – especially the distribution of sands in the toe-thrust area of the deepwater Niger Delta. (3) The time-integrated channel system erosivity, the evolution of the channel system geometry and the channel system fill as these systems interact with active structures through time. (4) How submarine channels in the deepwater Niger Delta achieve, and maintain bathymetric steady-state over periods of approximately 1 – 1.3 million years.Open Acces