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Evolution of Submarine Channel-Lobe Systems on a Salt-Influenced Passive Margin, Offshore Angola


Understanding the evolution of submarine channel-lobe systems on salt-influenced slopes is challenging as such systems often react to subtle changes in syn-depositional relief. Numerous studies document the impact of individual structures on channel-lobe systems, but at the regional scale the spatio-temporal evolution of such systems with a network of active structures is not well recorded. We use regionally-extensive, high-resolution 3D seismic reflection data to investigate the influence of salt-cored structures on the geometry and evolution of Miocene deep-water systems, offshore Angola. Advanced seismic attribute mapping (spectral decomposition) tied to standard seismic-stratigraphic analysis in a series of minibasins reveals numerous channel-lobe systems that record the interplay between relatively low rates of diapiric-rise and high sedimentation rates.

The Middle to Late Miocene interval contains five regional seismic units (SU1-SU5). SU1 contains channel-lobe systems blocked by salt-cored anticlines and confined to upslope minibasins, whereas SU2 contains oblique-to-structure flow of channel systems that are diverted to flow axially through a series of linked minibasins. Channel systems evolve from being relatively straight and orientated oblique-to-structure, to highly sinuous and transverse-to-structure between SU3 and SU4. Finally, SU5 contains compensationally stacked and backstepping lobes, as well as erosionally-confined channel systems with increasing meander belt widths. While a few high-relief salt stocks and walls have a strong influence on sedimentary delivery systems throughout the studied periods, the majority of salt structures gradually lose their topographic influence from SU1 to SU5 as the minibasins are gradually filled in a fill-and-spill manner. Autocyclic depositional controls became more significant than structural influences during the later stages of minibasin fill. However, the observation that avulsion nodes in SU5 are positioned along the elongated salt-structures and preferentially avulse towards the northeast, suggest some degree of local salt-tectonic control on sediment dispersal, possibly under the influence of more regional tilting. This study highlights the delicate topographic ‘tug-of-war’ between trains of salt-cored structures and submarine channel-lobe systems. Appreciating the often subtle topographical influences in active seafloor settings can aid in understanding stacking patterns for deep-water reservoirs.