Turbidites, topography and tectonics: Evolution of submarine channel-lobe systems in the salt-influenced Kwanza Basin, offshore Angola

Abstract

Understanding the evolution of submarine channel‐lobe systems on salt‐influenced slopes is challenging as these systems react to subtle, syn‐depositional changes in sea‐floor topography. The impact of large blocking structures on individual deep‐water systems is well documented, but our understanding of the spatial and temporal evolution of extensive channel‐lobe systems on slopes influenced by relatively modest salt structures is relatively poor. We focus on Late Miocene deep‐water depositional systems contained within a c. 450 ms TWTT thick interval imaged in 3D seismic reflection data from the contractional salt‐tectonic domain, offshore Angola. Advanced seismic attribute mapping, tied to seismic facies analysis and time‐thickness variations, reveal a wide range of interactions between structurally induced changes in slope relief, deep‐water sediment routing, geomorphology and sedimentology. Five seismic units record a striking tectono‐stratigraphic within eight minibasins. We observe gradual channel diversion through lateral migration during times of relatively high structural growth rate, as opposed to abrupt channel movement via avulsion nodes during times of relatively high sediment accumulation rate. Our models capture the response of deep‐water depositional systems to the initiation, maturation, and decay of contractional structures on salt‐influenced slopes. The initiation stage is defined by small, segmented folds with deep‐water depositional system being largely able to transverse multiple minibasins. In contrast, the maturity stage is characterised by large, now‐linked high‐relief structures bounding prominent minibasins leading to ponding and large‐scale diversion of channel‐lobe systems and the emplacement of MTCs derived from nearby highs. The decay stage is expressed by structures that are shorter and more subdued than those characterising the maturity stage; this leads to a more complicated array of channel‐lobe system, the evolution of which is still influenced by bypass, diversion and ponding. During the decay stage, remnant structures still exert a subtle but key control on the development and positioning of avulsion nodes.