Deep-water sinuous channels: their development and architecture
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The study combines an interpretation of 3D seismic and well-core dataset with laboratory experiments, process-based 3D numerical simulations and analysis of outcrop analogues to explore the varied architecture and formative processes of submarine sinuous channels. On the basis of their seismic imagery from a sector of the West African Neogene continental slope, deep-water channel belts are divided into four main categories and their origin is explained: (1) meandering non-aggradational channel belts, which form when the turbiditic system is near its potential equilibrium profile; (2) levéed aggradational channel belts, which evolve from incipient meandering conduits perturbed by system aggradation; (3) erosional cut-and-fill channel belts, which evolve by down-cutting of either moderately sinuous levéed or highly sinuous meandering conduits; and (4) hybrid channel belts, which result from a failed or incomplete transformation instigated by either aggradation or downcutting. The channel belts are typically stacked upon one another into fining-upwards valley-fill complexes, showing the turbiditic system’s evolution from a deep incision to transient equilibrium state − with the formation of coarse-grained lag deposits and nonaggradational meandering channel belts − and further to aggradation with the formation of levéed channel belts and eventual abandonment.
On the basis of laboratory experiments and numerical CFD simulations, the diversity of sinuous channel belts is attributed to four key factors that control the spatial pattern of sediment erosion and deposition in a conduit: (1) the relationship between the flow’s desired substrate equilibrium gradient and the host channel’s actual slope gradient; (2) the relationship between the length scale of the flow’s rotational helicoid and the channel’s preexisting curvature; (3) the relationship between the flow thickness and the channel depth; and (4) the relationship between the flow power and the channel bank strength. Channel meandering occurs uniquely when the flows are in hydraulic equilibrium with the channel slope, in phase with the channel curvature, in size or moderately undersized relative to the channel depth, and are modestly erosive with respect to the channel substrate. The diversity of channel-belt sedimentary architecture derives mainly from the formation of different intra-channel depocentres. Simulations indicate at least five different kinds of possible channel bars, including: classical point bars; bars formed in the channel-bend inflection zone at the inner- to outer-bank or outer- to inner-bank transition; and outer-bank bars formed directly upstream or downstream of the bend apex. Every bar type requires particular flow conditions, but some of them may form concurrently or alternate with one another in certain circumstance.
The study’s outcrop investigations are focused on the architectural diversity of deepwater point bars, which are volumetrically most significant and hence potentially most important as reservoir elements. Point bars vary greatly in: (1) their size, depending on the channel depth and extent of its lateral migration; (2) the geometry, facies and inclination of the component beds as well as the degree of bed basal erosion, depending on the variety of turbidity currents involved; and (3) the occurrence of internal erosional truncations, depending on the point-bar planform transformation. Apart from their major differences in this respect, the deep-water point bars have a number of key features in common, from which inferences can also be drawn about the meandering process as such. Their horizontal or gently inclined erosional bases indicate that the meandering channels undergo lateral migration in quasi-equilibrium slope conditions. Sparse levées indicate bypassing spill-out flows. The cohesive encasing deposits point to the importance of bank strength, as in meandering fluvial channels. The laterally accreted beds show updip fining and tractional oblique updip transport, which indicate a rotating flow helicoid rising against the inner bank, spreading its bedload over the point-bar flank and segregating laterally sediment grain sizes. The downdip parts of beds indicate a higher sediment concentration in the flow core part passing along the channel thalweg.
The study as a whole contributes significantly to an understanding of the diversity of deep-water sinuous channel belts and their sedimentary architectures, and also shedding new light on the variability of submarine point bars and the process of channel meandering.