Reconstructing the Fluvial History of the Paleo-Ganges Delta Plain

The surface and shallow stratigraphy the western Ganges-Brahmaputra (G-B) delta provides an extensive yet complex archive of river channel activity and its role in delta plain construction. This region is characterized by abandoned channels that formed a scrollplain of pointbar deposits in the upper reaches and appear to have decreased in lateral mobility toward the tidal zone of the delta. Inferences about the behavior (e.g., migration rates, timing of activity) of these channels may be drawn from their planform morphology and from historical documents. Yet, there is presently no quantitative data to describe how the relict meandering channels operated during their activity, their role in constructing the delta plain, and their relationship to the predominantly-braided big rivers of the Ganges and Brahmaputra. For example, do the meandering channels represent the waning phase of paleo-Ganges distributaries? Or are they offtakes, siphoning fractions of suspended water and sediment from the main channel in a similar fashion to crevasse splays?
Here, we present the first quantitative assessment of relict channels of the western Ganges-Brahmaputra delta, informed by hand-drilled boreholes and optically stimulated luminescence (OSL) geochronology. We assess paired point-bar and outerbank deposits upstream and downstream along selected major channels, and also test the age of the abandoned landscape at sites far from the primary channels. The application of OSL dating to pointbar deposits provides the timing of channel activity and migration rates. Dating of the outerbanks that were cannibalized by the channels, and of sand within the distal floodplain, provides the timescale of landscape reoccupation by these channels- a first step toward defining avulsion frequencies within the Ganges-Brahmaputra delta.