Effects of Neotectonic Deformation on Channel Planform and Avulsion History of the Brahmaputra-Jamuna River, Bangladesh

Abstract:

Holocene sediments of the Brahmaputra-Jamuna River in the upper delta plain of the Bengal basin unconformably overlie latest Pleistocene alluvium. Previously drilled borehole data reveal that the unconformity is marked by a basal gravel unit of early Holocene age with boulders up to 30-cm in diameter atop a compacted but unlithified sand and gravel conglomerate (D50=0.6mm) of latest Pleistocene age. We tie these borehole data to a 2D marine multichannel seismic survey along a 255-km reach of the Brahmaputra-Jamuna River, the first survey to image the lowstand valley from the India-Bangladesh border to the confluence with the Ganges (Padma) River. A prominent reflector is imaged along the length of the survey demarcating the valley base and Holocene-Pleistocene unconformity, sloping from ~40m below water level upstream to ~90m below water level at the downstream confluence. The average slope of this paleosurface is ~3x steeper than that of the modern channel bed, which may be the result of lower base level and propagation of a low-relief knickpoint during the last glacial lowstand. Alternatively or in conjunction, the steeper channel gradient may be due to increased basinward subsidence. In addition, the lowstand reflector is tectonically deformed at multiple locations, defining 4 distinct tectonic domains along the length of the survey. We place these tectonic influences on the stream channel in the context of its avulsion history during the Holocene, with evidence that the most recent avulsion of the Old Brahmaputra into the modern Jamuna course ~200ya was induced by tectonic uplift along the western margin of the Shillong Massif (Meghalaya, India). We further propose locations of the modern river that may be affected by tectonically-induced planform changes.