SOURCE-TO-SINK PROCESSES RECORDED IN A DELTAIC GATEWAY: A POST-GLACIAL HISTORY OF THE BRAHMAPUTRA RIVER IN THE BENGAL DELTA

The Bengal delta is the gateway for the world’s largest sediment transport system from the Himalaya Mountains to the Bengal Fan. The key to reconstructing the climate, erosion, and transport history of this system is understanding how such processes are recorded in the stratigraphy of the deltaic margin. Here we present the stratigraphic record of a detailed coring transect across two fluvial valleys downstream from a prominent avulsion node of the Brahmaputra River. Sediment samples were acquired from 41 boreholes along a 120 km transect that spans the entire Holocene fluvial valley of the Brahmaputra. Preliminary core data suggests that geomorphic boundaries in the surface morphology are distinctly reflected in the downcore stratigraphy. The transect captures the stratigraphic architecture of these valleys at the transition zone between the Himalayan-sourced mountain streams and the low-lying, flat delta braidbelts. The stratigraphy of the transect is dominantly Holocene sands with gravel beds prevalent below 20 m. We note that the Brahmaputra does not currently transport gravels, which raises the question of how the source input and transport regime have varied throughout the Holocene. Very little mud is preserved below the surface, and those cores that do contain preserved subsurface mud are distal to the active channel locations. This pattern of poorly preserved mud stratigraphy contrasts with widespread mud deposition in the modern delta system. We hypothesize that avulsion and migration of the braidbelt offers little probability of floodplain preservation near the active channels, and those cores with a floodplain cap probably have not been occupied for several hundred years or more. This stratigraphic record of the proximal deltaic sink is an important element in understanding the Brahmaputra dispersal system from source to sink on a post-glacial timescale.