Linking process, morphology, and stratigraphy in the Ganges-Brahmaputra-Meghna delta

Abstract:

The Ganges-Brahmaputra-Meghna delta (GBMD) is characterized as a composite system with an upland fluvial fan delta, a lowland, backwater-reach delta, a downdrift tidal delta plain, and an offshore subaqueous-delta, reflecting the respective dominance of fluvial, tidal, and marine processes. Topographic transitions, coupled with surface morphology and underlying stratigraphy define the temporally and spatially integrated patterns of river behavior and sediment dispersal that characterize the delta system. These play important roles in the scale of natural hazards, such as flooding and storm surges, affecting the 150 million inhabitants of the GBMD.

Within the upland fan delta, aggradation of mobile braided channels within the active rivers support the wide-scale distribution of bed- and suspended-load sands that constitute nearly the entire underlying architecture of upper GBMD stratigraphy. Finer silt-dominated facies form on the floodplain from overbank deposition during waning stages of flow; however preservation is very low and localized because of the persistent lateral migration of braided channels. A differentiation in stream morphology and channel behavior is associated with a sharp decrease in stream gradient, channel avulsion and abandonment, and the transition across the backwater. Deposition and preservation of fine-grained mud and organic-rich successions are concentrated within broad interdistributary basins of the lowland fluvial plain or within tectonically subsiding Sylhet Basin. While ~15% of the 1 x 10⁹ t yr^-1 sediment load carried by the rivers is advected along shore and inland via tidal activity, a rapidly prograding subaqueous clinoform and the adjacent Swatch of No Ground canyon system offshore receive ~50% of the modern sediment load.

The overall stability of the GBMD landform, relative to many deltas, reflects the efficient, widespread dispersal of sediment by the large monsoon discharge and high-energy tides that affect this region. Understanding the processes that define inorganic and organic deposition and preservation is useful not only for delta development models, but also informing delta restoration efforts worldwide.