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Basinward propagation of submarine channels and their seafloor expression
[Thesis]. Manchester, UK: The University of Manchester; 2020.
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Abstract
Submarine channels are spectacular geomorphological records of sediment transport and deposition across the Earth's continental slopes and basin floors. They are the conduits through which vast volumes of sediment are transported by sediment gravity flows to build the largest sedimentary deposits on earth, submarine fans. However, key questions remain regarding their evolution: 1) What is the relative impact of external and internal processes which build or affect submarine fans? 2) How do submarine channels propagate into their receiving basins? 3) How does submarine channel evolution vary across hierarchical scales? These questions are addressed using three distinct approaches: 1) a 3D flume tank experiment; 2) an outcrop study of the Shannon Basin, Western Ireland; 3) a subsurface study with 3D seismic and core from offshore Tanzania. Three-dimensional flume tank experiments suggest that external controls such as waxing-to-waning sediment supply have the greatest impact upon submarine channels whilst basin floor deposits are more influenced by internal processes. Results indicate that external signals can be preserved in the depositional record through the progradation of basin floor deposits, but internal processes progressively obscure this signal through time. The Gull Island Formation at Doonbeg, Western Ireland, features a Carboniferous-age intraslope lobe that was incised by a submarine channel on a muddy and unstable slope. It is proposed that submarine channels may incise more progressively in ponded intraslope environments, and more rapidly on steeper slope sections. The bottom current-influenced 'Lavani' channel system, offshore Tanzania, exhibits fundamentally different cross-sectional geometries along its profile. This system is interpreted to have evolved as a frontal lobe which was progressively incised by a channel, leading to basinward propagation of the channel system. Results indicate that increased hydrodynamic and topographic complexity on the slope may result in more complex system architecture but that this does not necessitate fundamental changes to existing channel propagation models. The results of this project suggest that despite some differences, submarine channels evolve similarly across hierarchical scales, from individual flow events in flume tanks to large channel belts on continental slopes. It is suggested that this similarity of expression is reflective of fundamental external and internal controls that have consistent and predictable effects on submarine channels at all scales.