Late Quaternary Sedimentary Record of Holocene Channel Avulsions of the Brahmaputra River in the Upper Bengal Delta Plain

Abstract

The first interpretation of the stratigraphic record of Holocene river-channel switching between the Brahmaputra-Jamuna and Old Brahmaputra paleovalleys is presented here in the context of Last Glacial Maximum (LGM) river incision and postglacial sea-level rise. Motivated by the historic diversion of the Old Brahmaputra channel into the present-day Brahmaputra-Jamuna course, we have obtained sediment and radiocarbon samples from 41 boreholes along a 120-km transect crossing these two braided-river valleys. The stratigraphy of this transect reveals two adjacent sand-dominated Holocene channel systems, each bounded by remnant, mud-capped Pleistocene stratigraphy. Using lithology and bulk strontium concentration as provenance indicators, we define the geometry and channel-occupation history of each paleovalley. The relatively steep valley margins and low width-to-thickness ratio (W/T: ~240) of the sub-valleys of the Old Brahmaputra indicate that it was filled primarily through vertical channel sand aggradation. Conversely, the gently-sloped valley margins and high width-to-thickness ratio (W/T: ~890) of the Brahmaputra-Jamuna valley suggest that it was filled primarily through lateral accretion and reworking of braidbelt sands. We attribute this disparity in valley geometry and fill processes to the previous occupation history of each valley. The much larger Brahmaputra-Jamuna was the principal lowstand river course, whereas the smaller Old Brahmaputra valley was first occupied during the early post-glacial transgression. We also demonstrate that the river has experienced 3-4 major avulsions since the LGM and occupies the Brahmaputra-Jamuna valley for longer time periods. Together these observations indicate that occupation history and antecedent morphology are important controls on river course mobility and avulsion behavior.