Acoustic facies and depositional processes in the upper submarine canyon Swatch of No Ground (Bay of Bengal)

The Swatch of No Ground (SoNG) is a canyon that incises the Bengal shelf deeply over more than 100km length landward of the shelf edge and similar to 100 km west of the Ganges-Brahmaputra-Meghna river mouth. It acts as conduit for the fluvially discharged sediment that bypasses the shelf and reaches the deep sea fan. What transport, deposition and mass-wasting processes contribute to the sediment export, and what sediment structures and main pathways are associated with these processes is studied by closely spaced bathymetric HYDROSWEEP swath sounder and PARASOUND sediment echosounder profiles over the upper SoNG. An analogue side-scan sonar profile completes the acoustic data set. From the PARASOUND sediment echosounder data three main acoustic facies types related to undisturbed normal deposition, mass wasting and erosive transport are defined and mapped with respect to the HYDROSWEEP bathymetry of the SoNG. Generally, nearly all facies types show an unusually high sediment echosounder signal penetration of similar to 80-100 m, indicating that unconsolidated, high-porosity sediments occur on the canyon floor, flanks and banks. Additionally, the acoustic facies distribution map yields that about half of the canyon floor is covered by undisturbed, well-stratified fine-grained, muddy and coarse grained, graded beds. They are deposited from a suspension plume transported to the canyon either during fine weather conditions by westward oriented tidal currents or by storm events that mobilize sediments on the inner shelf. Curved or inclined faults or growth faults are found at all steep flanks, indicating that active mass-wasting processes occur along all channels and gullies on both canyon flanks. Such mass-wasting processes are probably initiated by severe storms, earthquakes, high sediment load or excess pore pressure and lead to slides, slumps or mudflows. They stop in regions with low inclination angle and form acoustically transparent deposits of various extents on the channel and gully floors and on the southwestern canyon thalweg. Current marks identified on the canyon floor by high backscatter amplitudes in the side-scan sonar images indicate that low-density turbidity currents episodically flow down the canyon. They are generated in an area of very high sedimentation rates close to the canyon head by high-energy conditions like severe storms or earthquakes. Additionally, a small gravity-driven flow along the main channel(s) obviously continuously erode the steeper northeastern branches and inhibit sediment deposition in the more gentle southwestern channel below similar to 450-500m water depth. Together with larger-scale mudflows and low-density turbidity currents this gravity driven flow contributes to an active growth of the channel levee system in the deep-sea fan. (C) 2003 Elsevier Science Ltd. All rights reserved.