LATE QUATERNARY BRAHMAPUTRA RIVER SEDIMENT DISPERSAL AND STORAGE CONTROLLED BY REGIONAL TECTONIC UPLIFT

The Brahmaputra River (BR) is structurally confined in its upper Himalayan reaches, constructs a fan-delta that enters the Bay of Bengal, and ultimately feeds the Bengal turbidite fan. We analyze the stratigraphy of this system to explore the fluvial-tectonic interaction, highlighting implications for sediment storage in the transfer zone and impacts on signal propagation to the deepwater system. We characterized the Late Quaternary stratigraphy by determining fluvial provenance from 120 outcrops along the foothills of the Shillong Plateau (SP) and 143 boreholes from the subterranean Bengal delta. Timing of deposition on both sides of the deformed SP margin was established with radiocarbon and luminescence dating. The stratigraphy indicates 3 main depositional fairways, each controlled by a unique fluvial-tectonic regime: (1) the eastern fan-delta represents the path of steepest descent, forming a depositional fairway occupied during incipient uplift of the westernmost SP margin, before surface topography was sufficient to preclude the river from flowing through this region; (2) the western fan-delta preserves dominantly BR stratigraphy as far as 65 km west of the modern river, representing maximum westward deflection of the BR by the uplifting SP; (3) the central fan-delta stratigraphy comprises the relatively recent incision, widening, and fill sequence of the paleovalley, indicating that the Late Quaternary fluvial system overcame westward deflection, eroding back toward the SP margin. These results document regional fluvial-tectonic interaction for a major world river and imply that the interplay of tectonic forcing and the hydrologic regime of the system together control the course of the river, with tectonics dominating during periods of relatively low discharge and/or rapid rates of uplift. Over a hundred-thousand-year period, however, the tectonic deflection is overcome by the local river gradient, aided by increasing discharge. When dominant, tectonic deflection of the BR controls where sediment is preserved downstream in the fan-delta system, and sediment on the margins of the fan-delta are stored in floodplain terraces. These terraces control the volume of sediment buffered in the transfer zone, and their ages and relative locations control the time over which the sediments are stored before transport to the marine fan. This suggests that the relative dominance of allogenic controls on channel behavior in the transfer zone is an important control on sediment storage and propagation to the deep marine basin.