290-3 The Origin of Geomorphic and Stratigraphic Transitions on the Ganges-Brahmaputra-Meghna Delta

Session: Delta Evolution from Rivers to the Shelf: Past, Present and Future Perspectives for Society

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Abstract:

In the Ganges-Brahmaputra-Meghna Delta (GBMD), Holocene sediments preserve signals of autogenic and allogenic processes as rivers respond to internal, fluvial and tidal dynamics, as well as to changes in tectonics, climate, and relative sea level. Over the past 15 years, the collection and analysis of hundreds of sediment cores from the delta has developed a dataset that yields valuable insights into the Holocene delivery, transport, and dispersal of sediment across the delta. Here we identify emerging patterns from these data based on the reconstruction of a comprehensive sediment budget and the introduction of new data from the lower fluvio-tidal reach of the delta. Two prominent geomorphic and stratigraphic transitions emerge from these data. First, a distinct geomorphic slope break that corresponds with the transition from fluvial- to fluvio-tidal dominated processes. This slope break coincides with a downstream fining of the river’s sand load below the bedload transport regime into the suspended-load-only regime. We hypothesize that the slope break originates from this disappearance of bedload, through a reduction in riverbed aggradation and fluvial transport slope that allows tides to propagate up to 100 km inland of the coast, infilling accommodation away from the main rivermouth with tide-transported muds. The second prominent transition, which occurs just downstream of the slope break, is a stratigraphic transition where the fraction of sand in Holocene strata changes from ~90% in the upper delta to <50% across the lower fluvio-tidal deltaplain. This change is relatively abrupt, occurring over a distance that is just 10% of the delta length. The position of this stratigraphic transition also shows a landward shift 12-9 ka, suggesting transgression, but it then maintains a largely uniform position through most of the Holocene.  Together these geomorphic and stratigraphic transitions separate the contiguous delta into distinct zones. Initial observations motivate our hypothesis that these transitions arise from mass extraction and distribution of the sediment supply across the delta, leading to feedbacks within the governing channel, overbank, and tidal processes that build the delta. Our initial hypotheses will be tested as part of ongoing field, modeling, and data-analysis efforts. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9581

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