4th International Submarine Canyon Symposium (INCISE2018)
5-7 November 2018, Shenzhen, CHINA
This is the detail of Flow processes and sedimentation in unidirectionally migrating deep-water channels
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Yingmin Wang, Ronald J. Steel, Michele Rebesco, Ehsan Ul Haq
Flow processes and sedimentation in unidirectionally migrating deep-water channels
A series of short and steep unidirectionally migrating deep-water channels, which are characterized by unidirectional channel-growth trajectories, asymmetrical channel cross-sections, and a lack of levees, are identified in the Pearl River Mouth and Lower Congo Basins, for the first time. Architecturally, unidirectionally migrating deep-water channels are composed of different channel-complex sets, within each of which reworked turbidites in the lower part grade upward into muddy debris-flow deposits and, finally, into shale drapes. Each of the channel-complex sets underwent three main stages of channel evolution: (1) early lowstand incision stage, during which intense turbidity flows mostly overwhelmed relatively weak bottom currents, resulting in basal erosional bounding surfaces and limited reworked turbidites; (2) the late lowstand lateral-migration and active-fill stage, during which the waned turbidity currents intensely interacted with bottom currents, resulting in substantial reworked turbidites and debris-flow deposits; and (3) the transgression abandonment stage, during which turbidity currents further waned, leading to extensive marine shales.
Inspired by the two-layer model of a stratified lake forced by wind stress, we introduce the concept of Wedderburn number (W) to quantify how turbidity and contour currents interacted to determine sedimentation in unidirectionally migrating deep-water channels (UCs). Bankfull turbidity flows in the studied UCs were computed to be supercritical [Froude number (Fr) of 1.11–1.38] and had velocities of 1.72–2.59 m/s. Contour currents with assumed constant velocities between 0.10 and 0.30 m/s flowing through their upper parts would result in pycnoclines between turbidity and contour currents, with amplitudes of up to 7.07 m. Such pycnoclines, in most cases, would produce Kelvin-Helmholtz (K-H) billows and bores that had velocities of 0.87–1.48 m/s and prograded toward the steep channel flanks by 4.0° to 19.2°. Their wavefronts with the strongest shocks and deepest oscillations would, therefore, occur preferentially along the steep flanks, thereby promoting erosion; on the other hand their wavetails with the weakest shocks and shallowest oscillations would occur preferentially along the gentle flanks, thereby promoting deposition. Such asymmetric intra-channel deposition, in turn, forced individual channels to consistently migrate toward the steep flanks, forming channels with unidirectional channel trajectories and asymmetrical channel cross-sections.
Special Session: Canyons and trenches in the South China Sea and West Pacific