THE COMPLEX INTERACTION OF FLUVIAL PROCESSES AND TECTONICS AS RECORDED BY THE HOLOCENE STRATIGRAPHIC ARCHIVE OF SYLHET BASIN, BANGLADESH

Sylhet Basin, an actively subsiding basin within the Ganges-Brahmaputra-Meghna delta (GMBD) in northeastern Bangladesh, presents a unique opportunity to explore the interaction of fluvial processes and tectonics in a natural setting that contains many of the desirable characteristics of a laboratory tank experiment. The basin is confined on all sides by topographic boundaries and contains two well-defined sediment inputs and one output. Estimates of spatially variable subsidence rates throughout Sylhet Basin have been obtained and are of an appropriate scale to influence river path selection. Modern values of water and sediment discharge are fairly well constrained, and provenance indicators contained within the bulk geochemistry allow the discrimination of major sediment sources to the basin. We use a dense network of over 400 shallow (up to 90 m) boreholes with a robust radiocarbon geochronology to examine the Holocene stratigraphic and tectonic evolution of Sylhet Basin. The early Holocene is characterized by fine-grained material sourced by small, local catchments of the Shillong Massif and the Tripura fold belt. During the mid- and late-Holocene, a series of at least 3 avulsions of the Brahmaputra River into the basin focused sediment delivery along two pathways associated with antecedent topography inherited from Pleistocene lowstands. Stacked channel sands (20-30 m thick) confined to these 15-25 km wide paleovalleys are the predominant facies within the upper delta, and are of a comparable scale to the modern braidbelt of the Brahmaputra River. As a result of downstream fining, facies grade to more isolated sand lenses and interbedded muds in the distal reaches of Sylhet Basin, indicative of a shift from a highly mobile braidbelt to a less mobile distributary system as bedload is extracted to deposition. Rapid (up to 7 mm/yr) subsidence in Sylhet Basin has not acted as a strong attractor for channel steering during much of the Holocene. Instead, the Brahmaputra has followed the steepest descent path associated with flexural loading downstream of the hinge zone along the modern braidbelt.