Sediment transport in the shelf canyon “Swatch of No Ground” (Bay of Bengal)

Abstract

The Swatch of No Ground is a shelf canyon that deeply incises the Bengal shelf near the Ganges–Brahmaputra river mouth, cuts the foreset beds of the subaqueous river delta and acts as temporary depocenter between river mouth and Bengal fan. Sedimentation rates in the Swatch of No Ground are highest near the canyon head at ∼50 cm a−1, decreasing to ∼15 cm a−1 in 600 m water depth. The canyon deposits consist of intercalated fine (silt–clay) and coarse (silt–sand) grained deposits. In seismic profiles, small-scale sedimentary structures and parallel-bedded layers reveal that sediment in the canyon is mainly deposited from suspension. During fair weather conditions tidal currents are the dominant mechanism that transports plumes of suspended river load towards the canyon, forming fine-grained silt–clay layers. During storm conditions, sediment is resuspended on the inner shelf and subaqueous delta east of the canyon. Storm-generated bottom currents transport the resuspended sediment alongshelf to the canyon where the particles are trapped and form graded coarse silt–sand layers. Channels and gullies in the Swatch of No Ground indicate active gravity-driven currents with an erosional character in the upper canyon and non-depositional character below ∼450 m, suggesting that persistent or ephemeral currents presently export sediment to the Bengal fan. Numerous slumps and slides observed in the canyon show small-scale acoustically transparent layers that indicate sediment liquefaction presumably initiated along the steep canyon margin. These mudflows move downslope and halt where the canyon gradient decreases. One widespread acoustically transparent layer with an age of approximately 140–160 years BP can be traced throughout much of the canyon and is probably the result of large-scale sediment remobilization triggered by a catastrophic event like an earthquake. This may represent decennial-to-centennial scale events that remove large quantities of sediment and prevent the canyon from rapid infilling.