Interplay of Basement Tectonics, Salt Tectonics, and Sedimentation in the Kwanza Basin, Angola
Martin P. Jackson and Michael Hudec
University of Texas, Bureau of Economic Geology, Austin, TX
The Lower Congo Basin has the most prolific hydrocarbon resources in West Africa. However, the basin is atypical of the region because the dominant Congo Fan masks the effects of epeirogenic basement tectonics. In contrast, other parts of the West African margin reveal how differential basement uplift has influenced basin evolution. It seems likely that basement tectonics has also affected the Lower Congo Basin but has not yet been fully appreciated.
The adjoining Kwanza Basin evolved as two different subbasins. The onshore Inner Kwanza Basin (IKB) is an interior salt basin enclosed by basement highs. It is dominated by halokinetic troughs and fold belts formed by basement shortening and uplift. In contrast, the offshore Outer Kwanza Basin (OKB) is an open continental margin that deformed by gravity spreading, resulting in ~25 km of updip extension balanced by downdip shortening. From Late Cretaceous to Miocene, advance of the Angola Salt Nappe accommodated most shortening in the OKB. Then Plio-Pleistocene burial of the nappe toe blocked its advance, which compressed the entire deepwater salt basin.
Uplift of both basins varied greatly in space and time. Where the Tertiary cover is thickest, the IKB remained near sea level since the Eocene, which refutes the notion of a massively uplifted coastal plateau. Major uplift (2-4 km) of the Precambrian hinterland craton accelerated sediment input but had little Neogene structural effect on the deepwater OKB other than sedimentary loading. Instead, much smaller uplifts of the shelf drove spectacular Neogene deformation on a metastable slope, which radically changed basin kinematics and created or improved many structural traps.
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