FROM FAN DELTA TO LOWLAND FLUVIAL DELTA: TRANSITIONS IN SURFACE MORPHOLOGY AND STRATIGRAPHY RECORDED BY MASS EXTRACTION IN THE GANGES-BRAHMAPUTRA MEGHNA DELTA, BANGLADESH

The Ganges-Brahmaputra-Meghna delta (GBMD) has followed two primary pathways throughout the Holocene: one along the modern Jamuna River braidbelt and a second route through the subsiding Sylhet Basin in northeastern Bangladesh. These 15-25 km wide pathways possibly formed as the result of erosion from megafloods sourced from ice dammed lakes in the Tsangpo Valley and have established the primary Holocene sediment delivery routes of the system. The stratigraphy of the upper delta is characterized by thick (25-35 m) amalgamated channel sands. These deposits represent mass extraction of bedload and suspended load of highly mobile channels near the apex of the delta and have constructed the steeper (slope angle ~10^-4) fan delta of the system. Further downstream along the Sylhet Basin pathway the stratigraphy transitions to isolated lenses of fine to very fine sand (5-15 m thick) within thicker (up to 15 m) layers of silt and mud, interpreted as overbank and splay deposits during relatively brief (~10³ years) occupations of the basin. This transition to lower net:gross deposits corresponds to a morphodynamic shift from highly mobile channels to more stable distributary networks in the lowland fluvial delta. An order of magnitude decrease in slope (slope angle ~10^-5) is also recognized across this transition zone. Downstream fining rates calculated along both of the major Holocene fluvial pathways illustrate more rapid fining in Sylhet Basin than along the modern Jamuna River braidbelt, likely a consequence of greater subsidence within Sylhet Basin. The transition from sandy to finer-grained stratigraphy is spatially coincident with the decreased slope angle, which is also coincident with the inflection point of downstream-fining models. Furthermore, when converted into scale-independent space, this transition occurs at a point where ~70% of the total mass of the system has been extracted, consistent with stratigraphic transitions noted in other depositional settings and experimental data. Extraction of coarse bedload appears to be a first order control on surface topography, channel mobility, and stratigraphic architecture of large fluvial systems.