NEOGENE TO MODERN SEDIMENT ROUTING AND STORAGE ALONG THE BRAHMAPUTRA RIVER: A TALE OF TECTONICS, CLIMATE, AND GEOLOGICALLY INSTANTANEOUS EVENTS

The source to sink sediment routing system of the Himalayan highlands, Brahmaputra River, Ganges-Brahmaputra-Meghna Delta (GBMD), and the Bengal Fan contains an enormous sedimentary archive of climatic and tectonic influences on landscape evolution across a range of temporal and spatial scales. Using a suite of geochronologic, structural, sedimentologic, geochemical, and stratigraphic tools, we explore the Miocene to present evolution of this system in terms of sediment dispersal patterns, drainage basin organization, and source area changes. Neogene denudation of the emerging Himalayan orogen is recorded in the deltaic Barail, Surma, and Tipam Groups that crop out in the Indo-Burman Ranges (IBR) of eastern India. Detrital zircon provenance analyses indicate a shift from frontal Himalayan sources to Tibetan sources in the mid-Miocene, coincident with changes in monsoon intensity and tectonic activity. Basin thermal modeling of the Surma and Tipam Groups in the central IBR is consistent with 2-3 km burial within a GBMD-scale delta in the mid- to late-Miocene. Pliocene advancement of the deformation front, coupled with uplift of the Shillong Massif, forced a river avulsion from eastern India to its modern location in Bangladesh. Once established in this more western locale, the river has primarily occupied two pathways: one along the modern Brahmaputra/Jamuna valley and another on the western margin of Sylhet Basin in northeastern Bangladesh. Sediment sources (Ganges vs Brahmaputra vs local sources) can be discriminated using bulk geochemistry, and an extensive radiocarbon database aids in interpreting Holocene avulsion chronologies. The Holocene chronology suggests that large outburst flood dominate transfer zone deposition. Additionally, outsized turbidites in the Bengal Fan likely formed after flood sediment waves entered the marine system. Furthermore, these flood events appear to create antecedent topography which exerts a first order influence on river path selection over millennial timescales, in spite of tectonic processes that could also dictate sediment delivery pathways. Sediment budgets across varying Holocene monsoon and climate conditions indicate that the delta is sustainable, provided there is minimal sediment sequestration from upstream dams and diversions.