The active channel–levee system of the middle Bengal Fan was studied by a combined analysis of Parasound echosounder and Hydrosweep swathsounder data. The channel is characterized by highly variable sinuosities. Compared to other mud-rich submarine fans, an exceptionally low channel slope is found. The system can be subdivided into inner and outer zones of significantly different depositional architecture. The inner zone consists of the active channel and sharply separated vertical blocks, which are characterized by parallel, distinct reflectors and planforms of bends. These blocks are interpreted as abandoned channel segments (cut-off loops).
The outer zones represent undisturbed levees, which are constructed of parallel and wedge-shaped sedimentary units. The wedge-shaped units, varying significantly in thickness and lateral extent, are found at the outer convex arcs of active and abandoned channel loops caused by overspilling of channelized turbidity currents at sharp bends. The parallel units are the deposits of turbidity currents, which spread their sediments over wide areas as their size significantly exceeds the cross-section of the channel. The complex vertical and horizontal distribution of partially small sedimentary units suggests a more complicated deposition in time and space as hitherto reported from other submarine fans.
Within the inner zone, more than 20 cut-off loops were identified over a channel length of 90 km. In contrast to most other large mud-rich submarine fans, channel avulsion within the active channel–levee system is a frequent process. In particular, a temporal succession of at least 4 cut-off loops was reconstructed in the southern study area, indicating channel avulsion on average every 750 years. Channel avulsion seems to be a repetitious process caused by erosion through turbidite currents in a highly sinuous channel. Compared to other submarine fans, no morphological parameter shows a remarkable difference except the channel slope, which is significantly smaller than, for example, on Amazon, Congo and Mississippi fans. The interaction between this low channel slope and the flow parameter of the turbidity currents is most likely the reason for the instability of the active channel planform, leading to an exceptionally large number of meander loop breaches and cut-off loops.