Holocene and modern sediment budgets for the Ganges-Brahmaputra river system: Evidence for highstand dispersal to flood-plain, shelf, and deep-sea depocenters

The partitioning of fluvial sediment load across continental margins is an important control on strata formation and sequence development; however, fe w quantitative sediment budgets that encompass entire dispersal systems exi st. For the Ganges-Brahmaputra river system, sediment discharge is estimate d to be 10(9) t/yr at gauging stations similar to 300 km inland of the coas t, but little has been known of the downstream fate of this material. Geoch ronological, geophysical, and stratigraphic investigations of the lowland f lood plain, delta plain, and shelf help to delineate the extent of Holocene fill and allow calculation of a first-order sediment budget. Results revea l that 1500 x 10(9) m(3) of sediment fill has been sequestered within the f lood plain and delta plain since ca. 7000 yr B.P., or about one-third of th e annual discharge, The remaining load appears to be apportioned between th e prograding subaqueous delta (1970 x 10(9) m(3)) and transport to the deep sea Bengal fan via a nearshore canyon. Modern (<100 yr) budget estimates b ased on short-term accretion rates indicate a similar dispersal pattern and show that contemporaneous deposition continues within these disparate depo centers, The roughly equal partitioning of sediment among flood-plain, shel f, and deep sea settings reflects the respective influence of an inland tec tonic basin, a wide shelf, and a deeply incised canyon system. The findings also support new sequence stratigraphic models for these settings and indi cate the important insight that modern river deltas can provide for ancient margin systems. Furthermore, results affirm that values of riverine sedime nt flux to the oceans may be considerably overestimated by not accounting f or loss to the flood plains downstream of the gauging stations.