Neogene gravity collapse of the eastern and western Niger Delta: Results from 2D forward kinematic structural modelling

Abstract

The Cenozoic Niger Delta displays a complex gravity collapse system underpinned by overpressured shale that forms a décollement for normal faults, detachment folds and imbricate-fold-thrust structures in a linked extensional-contractional system. To better understand the timing and dynamics of gravity-driven deformation in the eastern Niger Delta (END) and western Niger Delta (WND) since the late Cretaceous, we performed 2D forward kinematic structural restoration and backstripping of regional 2-D seismic sections using KronosFlow software. The restored cross-section, in the END, extends from the present-day onshore (the Oligocene-Tortonian extensional zone) to the abyssal plain, while that of the WND extends from the present-day continental shelf to the abyssal plain. A comparison of restored cross sections shows that the modern continental shelves of the END and WND are dominated by counter-regional and regional normal faults, respectively. Between the late Eocene (ca. 34 Ma) and the late Miocene (9.3 Ma), the END displays gravity-driven deformation, localised in the Oligocene-Tortonian extensional zone with relatively low deformation on the slope and the deep basin. However, a correlation of restored cross sections over the late Eocene-late Miocene, suggests that gravity-driven deformation in the WND was localised within the Oligocene-Tortonian extensional zone with little or no deformation on the slope and the deep basin. Between the late Miocene (ca. 9.3 Ma) and the early Pliocene (ca. 5.7-4.9 Ma), the Oligocene-Tortonian extensional zone prograded to the present-day continental shelf resulting in a coupling of extensional deformation to contractional deformation in the END at least since the late Miocene. In the WND, the Oligocene-Tortonian extensional zone prograded to the present-day continental shelf during the late Miocene (ca. 9.5 Ma) but there was no coupling between extension and contraction until the early Pliocene (ca. 4.9 Ma). While there is a general reduction in gravity-driven deformation in the END over the Pleistocene, there is an overall increase in gravity collapse of sedimentary wedge in the WND. The unique structural configuration of the present-day continental shelf in the END and WND exerted distinct control on the gravity collapse of the regions throughout the Neogene. The dominance of counter-regional normal faults on the END continental shelf facilitated a large-scale increase (x2) in regional subsidence and sediment storage on the shelf over the late Eocene-late Miocene. However, the dominance of regional normal faults on the WND continental shelf facilitates an overall progradation and sediment transfer to the deep basin since the late Miocene/early Pliocene. This study documents the long history of gravity-driven deformation of the eastern and western Niger Delta and could be applied in the reconstruction of other shale tectonic basins.