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

Abstract:

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 (L_m), and low migration/aggradation ratios (L_m/V_a). 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 (L_m), and high L_m/V_a. 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.