9707067 Allison Funds are requested for a two-year collaborative study to examine the Holocene evolution of the Ganges-Brahmaputra delta. The Ganges-Brahmaputra (G-B) river system carries a billion tonnes of sediment annually, yet the fate of this material past the upstream gauging stations, as in most of the world's rivers, is not known. This study will test the hypothesis that at least half of the rivers' sediment load is being sequestered in the floodplain and delta plain, resulting in an evolution and facies architecture significantly different from classic deltaic models. Such magnitude of sediment sequestering, in the Ganges Brahmaputra as well as in other river systems, has important implications for our understanding of the global flux of riverine particulates to the world ocean. The specific objectives of this study are to: 1) Construct a sediment budget for the Holocene floodplain/delta plain: 2) Detail the stratigraphic architecture and history of infilling for the Sylhet, a large tectonic basin located along the previous course of the Brahmaputra: 3) Characterize the nature and magnitude of recent floodplain sedimentation in other portions of the Bengal Basin, including small sub-basins, and bil (wetland) areas; 4) Determine the stratigraphic architecture and magnitude of delta plain sedimentation, focusing on the tidal distributary channels of the Ganges; and 5) Link results from 1-4 above with our previous offshore studies to develop a holistic conceptual model for G-B delta evolution during the Late Holocene. Deep borings (~100 m) and cores will be collected from the floodplain and the delta plain during the dry season in order to construct the sediment budget and to constrain seismic observations. Water-based, high-resolution seismic-reflection studies would be conducted in the Sylhet Basin during the summer monsoon, when the basin is deeply flooded, in order to detail the stratigraphy of the Late Holocene basin fill. Geochronological and sedimentological studies of c ores collected from the floodplain and delta plain will enable us to determine the sediment process rates and stratigraphic architecture of extant sedimentary environments for the G-B system. The completion of the above tasks will enable us to construct a conceptual model of deltaic evolution which will represent an important end-member for deltaic styles, one for which floodplain/delta plain sediment sequestering likely plays a major role. In addition to the aforementioned geological and global implications, the understanding of recent deltaic evolution attained during this study would contribute to a rational strategy for development of one of the most densely populated, low-lying areas of the world
