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 150 km downdrift of its main sediment source, the Ganges-Brahmaputra-Meghna (GBM) river mouth, high sedimentation rates (5-50 cm year −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 sedimentdispersal 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 20-120 m; 2) several shelf-incising bypass gullies that originate in b 20 m 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 N 50 m. 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 (b 20 m) 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.

Allogenic and Autogenic Signals in the Stratigraphic Record of the Deep-Sea Bengal Fan

“…The GB system is in this category as well: Holocene mud transport through the SoNG is well documented [ e.g. 60 , 61 ] , but there has been little if any concentrated sand delivery to the basin floor during this same period. Moreover, the latest Pleistocene to Holocene channel-levee of the Bengal Fan, from IODP 354 Site 1454, lacks distinct sandy turbidite beds 2 .…”

Where and Why Do Submarine Canyons Remain Connected to the Shore During Sea-level Rise? - Insights from Global Topographic Analysis and Bayesian Regression

“…We manually corrected canyon-head misclassifications during visual inspection. Although some canyons (Swatch-of-No-Ground, located 150 km off coast, Rogers et al, 2015;Indus canyon, 17 km, Li et al, 2018) presently receive terrestrial sediment through clinoform progradation, we did not classify these as shore-connected.…”

Connections Between Fluvial To Shallow Marine Environments and Submarine Canyons: Implications For Sediment Transfer To Deep Water

“…In contrast, the Indus and Ganges-Brahmaputra deltas have active, muddy, subaqueous clinoforms that reach the heads of their respective canyons (Currie et al 2002;Clift et al 2014;Rogers et al 2015), which have eroded headward and now penetrate 100 km or more into the shelf. Based on calculation of sediment flux, Goodbred (2003) estimated that two-thirds of the sediment load of the Holocene Ganges River is trapped in the delta and delta plain, and 1/3 passes down the ''Swatch of No Ground'' Canyon onto the Bengal Fan, which is known to have been active throughout the Holocene (Weber et al 1997).…”