Unidirectionally migrating deep-water channels: Architectural styles and flow processes

Monday, 15 December 2014
Chenglin Gong1, Ronald J Steel1, Yingmin Wang2 and Qiang Xu3, (1)Department of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin, TX, United States, (2)State Key Laboratory of Petroleum Resources and Prospecting (China University of Petroleum, Beijing), Beijing, China, (3)CNOOC Research Center, Beijing, China
3D seismic data are used to investigate flow processes and sedimentation in deep-water slope channels of an alternate type characterized by short and straight channel courses, a lack of levees, and absence of any coeval fans. The study allows a picture of unusual flow processes in submarine channels. The studied channels can be divided into two discrete segments:

(1) Upper segments are characterized by low aspect ratio(W/T), little lateral offset (Lm), and low migration/aggradation ratios (Lm/Va). These upper segment channels build vertically-stacked channel-complex sets (CCSs), each of which is characterized by a facies transition from fine-grained sands in the lower part overlain by debris flow deposits and then shale drapes. Energetic sediment density flows triggered by fluid escape and/or strong wave action were well able to bypass sediment and to mask relatively weak bottom currents, yielding deep-water channels characterized by little lateral offset and dominantly aggradational stacking patterns.

(2) Lower segments are characterized by higher W/T, wide lateral offset (Lm), and high Lm/Va. They consist of laterally-migrated CCSs, each of which consists of fine-grained reworked sands in the lower part overlain by debris flow deposits and, finally, shale drapes. Bottom currents restricted within the channels would have induced a tilt of the interface between turbidity currents and the overriding bottom currents (Wedderburn number > 1). This would have deflected turbidity currents downward and back toward the gentle channel bank, thus causing channel migration (the steep bank) by ~2° to 15°, and yielding a helical flow circulation composed of a high-velocity zone along the steep bank and a low-velocity zone along the gentle bank. This bottom current-induced helical flow circulation promoted deposition on the gentle bank, but it favored erosion on the steep banke, yielding deep-water channels exhibiting wide lateral offset and dominantly laterally-offset stacking patterns.

Sediment supplied to the studied channels was therefore sorted, winnowed and swept along the steep bank by the high-velocity zone along this bank, yielding bottom current-reworked sands that that are preferentially disturbed along the steep bank of the studied channels.