Shelf-to-canyon connections: Transport-related morphology and mass balance at the shallow-headed, rapidly aggrading Swatch of No Ground (Bay of Bengal)

Abstract

The Swatch of No Ground (SoNG) canyon in the Bay of Bengal is a shelf-incising submarine canyon that is actively aggrading in its upper reaches despite regular gravity-driven transport and mass wasting. Although the canyon lies 150km downdrift of its main sediment source, the Ganges–Brahmaputra–Meghna (GBM) river mouth, high sedimentation rates (5–50cmyear−1) are sustained by both progradation of the subaqueous delta into the canyon head and the conveyance of shelf-generated hyperpycnal flows to the canyon floor. This rapid accretion appears to be largely balanced by mass failures triggered by regularly occurring storms, and less frequently by major earthquakes. Here we use high-resolution sub-bottom sonar data to elucidate dominant sediment-dispersal pathways and their transport-related morphology at the canyon head; these include: 1) a laterally prograding clinoform that intersects the canyon head at water depths of 20–120m; 2) several shelf-incising bypass gullies that originate in <20m water depth above the rollover point and connect the inner shelf to the canyon floor, and 3) numerous U-shaped slide valleys formed by deep-seated mass failures initiating at water depths >50m. The clinoform deposits reflect westward progradation of the GBM subaqueous delta into the upper canyon, where its axis-normal orientation leaves it dissected by cross-cutting gullies and mass failures. The morphology and acoustic stratigraphy of the gullies, coupled with strong bed shear and high suspended sediment concentrations on the inner shelf, suggest that these features are sustained by the regular conveyance of gravity-driven fluid muds that are formed in shallow water (<20m) where the gullies originate. The downslope termination of the gullies coincides with a break in slope at the canyon floor, indicating that gullies serve as sediment conduits linking shallow water processes with sediment deposition in the lower canyon. Together these locally interacting shelf, clinoform, and canyon features form a unique composite morphodynamic system that more broadly defines the highstand connection between this large fluvial delta system and its shelf-indenting canyon.