Architecture and evolution of submarine-fans, and coupling with shelf-edge processes in supply-dominated margins: example from Maastrichtian Washakie Basin
Koo, Woong Mo
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Along high-sediment supply margins, sediment delivered from shallow to deep-water by shelf-edge deltas is considered as the a main source for submarine-fans on the basin floor. As the consequence of direct feed, the architecture and spatio-temporal evolution of submarine-fans are closely related with processes on shelf margins. This thesis investigates 1) the linkage between shelf-edge architecture and evolution (stacking pattern) of basin-floor fans in two basin-scale clinothems (Clinothems 9 and 10) and 2) the lithofacies variability on the fringes of submarine-fans, using about 1,500 gamma well-logs and four cores in the Maastrichtian Washakie Basin, Wyoming. Based on the correlation of gamma-ray logs, submarine fan-lobe complexes in Washakie Basin are shown to develop through stages of initiation, progradation and/or aggradation, and retreat. The aggradational stacking pattern of Clinothem 9 lobe complexes and progradational stacking pattern of Clinothem 10 lobe complexes are shown to be linked to the coeval shelf-edge trajectory behavior. This direct linkage between shelf-edge trajectories and submarine fan-behavior was possible firstly because Washakie Basin was a sediment supply-dominated margin despite times of strong accommodation influence. Secondly, the sediment in shelf margins was transported quasi-continuously to deep-water via slope channels, which were continuously connected between shelf-edges and basin-floor during most of the sea-level cycle. In high sediment-supply systems, rivers commonly overcome the significant reworking power of waves and tides and are thus able to deliver sediments from the shelf edge to deep water. Cores in Washakie Basin show that there was variable lithofacies patterns in mapped submarine fan-fringes. Multiple transitions of lithofacies occur within the same bed, without significant erosion between high-concentration turbidites (e.g., structureless sandstones) and debrites (e.g., mud-clasts rich muddy sandstones) are identified in distal fringes of submarine fans. In contrast, in the lateral fringes of fans, there is a significantly shorter transition from turbidites to debrites. One explanation of facies variability is correlated with the run-out distance of flow that enhances transitions of flow. The longer axial run-out distance of flows results in deposition of mud-clast rich debrites. The shorter, transverse to lobe run-out distance causes only partial or non-transformed flows, resulting in deposition of muddy and sandy turbidites respectively. In the longitudinally elongated submarine fans in Washakie Basin, developed by long run-out distance of flow, the distal fan-fringes are thus significantly muddier than the lateral fringes, something of importance to hydrocarbon productivity on fan lobes.