Controls, Distribution, and Significance of Gravel in the Holocene Stratigraphy of the Brahmaputra River

Abstract

The origins of gravel bearing sediments in the upper Holocene stratigraphy of the Brahmaputra-Jamuna River in northern Bangladesh are explored in this thesis through sediment geochemistry analyses and characterization of gravel types sampled from 41 boreholes along a 120 km transect (BNGA) that crosses two major paleo-alluvial valleys in the Ganges-Brahmaputra-Meghna Delta. The modern Brahmaputra River (BR) is primarily a sand bedload system even though flow velocities are sufficient to transport significant amounts of larger sediments (i.e. gravel). However, gravels are common in the upper Holocene stratigraphy. Through the use of strontium concentration (Sr ppm) as a provenance indicator for the sands associated with gravel samples, we confirm that the gravels in question belong to the Holocene aged paleo-alluvial valley fill deposits of the BR. This thesis presents evidence that the previously defined range of Sr ppm for BR sediments (>140 ppm) should be revised to include sediments with >120 ppm Sr. The lower tail of the BR Sr ppm range (120-140 ppm) is interpreted as new evidence of variability in sediment mixing budgets throughout the Holocene between the BR and tributary systems with lower known Sr ppm signatures (e.g. Tista River). However, no direct correlation is found between gravel bearing strata and any specific range of BR Sr ppm values. It is determined that the model for the origin of gravels in the upper Holocene stratigraphy of the BR relies on the availability of gravel bearing sediments to be eroded by the BR through lateral bank migration, not direct input from any one particular tributary system.