From River Mouth to Remote Depocenters: Coupled Oceanographic and Climatic Controls on Ganges-Brahmaputra Sediment Dispersal across the Bengal Margin

The Ganges-Brahmaputra delta is an end-member example of a high-volume river system that debouches onto a rapidly accreting, canyon-incised margin. Active deposition and widespread sediment dispersal across the delta plain, shelf, and deep sea have been well documented here. However, sediment transport mechanisms and pathways in the estuarine transition zone between these primary depocenters are poorly understood. Toward the goal of discerning how fluvial, oceanographic and climatic processes interact to control sediment fluxes at the subaerial-subaqueous transition, high-resolution subbottom sonar surveys were conducted on the inner shelf of the Bay of Bengal in March 2007. The data show shelf-wide acoustically transparent layers that extend across the topset-foreset boundary of the subaqueous clinoform to the canyon edge. Based on accretion estimates, the tops of these layers terminate at a depth that corresponds to the timing of a 19th century category 4 cyclone in the Bay of Bengal and could be the result of wave pumping during this storm event. Feeder gullies and failure surfaces are visible in the data along the eastern edge of the canyon, and may also be linked to storm activity. A follow-up survey in March 2008 was conducted to characterize the impact of the November 2007 category 4 cyclone (Sidr) on the subsurface facies of the subaqueous delta and the canyon. Correlation of canyon gullies and failure surfaces with acoustically transparent layers at the clinoform-canyon interface may lead to a more precise understanding of how storm events and related wave activity influence the dispersal of sediment to these depocenters.