Clay mineral compositions in surface sediments of the Ganges-Brahmaputra-Meghna river system of Bengal Basin, Bangladesh

Abstract Clay minerals are significant indicators that can be used to identify sources and transport patterns of both fluvial and marine sediments. The Ganges, Brahmaputra, and Surma-Meghna (SM) rivers are the most important rivers of the Bengal Basin (BB), loading a large amount of sediments from the Himalayas into the Bay of Bengal. Clay minerals of 132 surface sediment samples from the Ganges-Brahmaputra-Meghna (GBM) river system have been analyzed with X-ray diffraction (XRD) to show the variation of clay mineral assemblages in the sediments from upstream to downstream for the major rivers of the BB and to verify the validity of clay minerals as an important index for sediment provenance in the Bay of Bengal after these sediments are discharged into the neighboring seas. The results show that the clay mineral assemblage in the GBM river system consists dominantly of illite (average 66%), with smaller amounts of smectite, kaolinite, and chlorite (8%, 8%, and 17%, respectively). Smectite is diagnostic in the Ganges River sediments, with a higher content (23%) than that in the Brahmaputra (2%) and the SM River (1%); however, chlorite (21%) is high in the SM River sediments and the Brahmaputra River sediments have a high illite percentage (77%). Downstream, the smectite percentages in the Ganges River increase significantly, but the kaolinite percentages in sediments of the SM River increase slightly. Due to the continuous mixture of first the Brahmaputra River and then the SM River discharge, the clay mineral assemblages of this river system's sediments change greatly, e.g., smectite percentages decrease from 23% in the Ganges River to 8% in the GBM River via the stretch with 12% in the Ganges-Brahmaputra confluence river.