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Salt tectonic controls on deep-water turbidite depositional systems: Miocene, southwestern Lower Congo Basin, offshore Angola

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Abstract

Regionally extensive 3D seismic data from the Lower Congo Basin, offshore Angola, have been used to investigate the influence of salt-related structures on the location, geometry and evolution of Miocene deep-water depositional systems. Isochron variations and cross-sectional lap-out relationships have then been used to qualitatively reconstruct the syn-depositional morphology of salt-cored structures. Coherence and Red-green-blue-blended spectral decomposition volumes, tied to cross-sectional seismic facies, allow imaging of the main sediment transport pathways and the distribution of their component seismic facies. Major sediment transport pathways developed in an area of complex salt-related structures comprising normal faults, isolated diapirs and elongate salt walls with intervening intraslope basins. Key structural controls on the location of the main sediment transport pathways and the local interaction between lobe-channel-levee systems and individual structures were the length and height of structures, the location and geometry of segment boundaries, the growth and linkage of individual structures, and the incidence angle between structural strike and flow direction. Where the regional flow direction was at a high angle to structural strike, transport pathways passed progressively through multiple intraslope basins in a fill and spill manner. Segment boundaries and structural lows between diapirs acted as spill points, focusing sediment transport between intraslope basins. Channel–lobe transitions are commonly associated with these spill points, where flows expanded and entered depocentres. Deflection of channel-levee complexes around individual structures was mainly controlled by the length of structures and incidence angle. Where regional flow direction was at a low angle to structural strike, sediment transport pathways ran parallel to structure and were confined to individual intraslope basins for many tens of kilometres. Spill between intraslope basins was rare. The relative position of structures and their segment boundaries was fixed during the Miocene, which effectively pinned the locations where sediment spilled from one intraslope basin to the next. As a result, major sediment transport pathways were used repeatedly, giving rise to vertically stacked lobe-channel-levee complexes along the pathways. Shadow zones devoid of coarse clastics developed in areas that were either structurally isolated from the sediment transport pathways or bypassed as a result of channel diversion.

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