The significance of large sediment supply, active tectonism, and eustasy on margin sequence development: Late Quaternary stratigraphy and evolution of the Ganges-Brahmaputra delta

Borehole data from the Ganges-Brahmaputra delta system unveil an intriguing Late Quaternary history controlled by immense sediment discharge, tectonic s, and eustasy. Sea level first intersected a major portion of the lowstand surface similar to 10,000-11,000 cal yr BP, marking the onset of sediment trapping and delta growth. Despite rapid sea-level rise (>1 cm/yr), sedimen t load was sufficient to maintain relative shoreline stability during this time. By similar to 7000 cal yr BP, accommodation was quickly infilled with slowed sea-level rise, leading to upstream channel migration and widesprea d dispersal of sands. This forced coastal progradation along the western de lta front where the Ganges was situated, and much of the river's fine-grain ed discharge bypassed the subaerial delta and formed a prograding deltaic c linoform on the shelf. Concurrently, Brahmaputra sediments were sequestered to an inland tectonic basin, thus starving the adjacent shoreline and lead ing to transgression along the eastern delta. By similar to 5000 cal yr BP, the two rivers had migated or changed course to discharge along the easter n delta shoreline which began to prograde into what is now the modern river -mouth estuary. Present delta configuration appears to have developed by si milar to 3000 cal yr BP. Evolution of the tectonically active Ganges-Brahma putra (G-B) system reveals important similarities and differences with othe r deltas. Overall facies succession follows that of basic models, progressi ng from an alluvial valley to coastal marine delta front to a prograding su baerial delta plain. However, the timing, thickness, and controls of these deposits differ. Immense sediment discharge from the Ganges and Brahmaputra rivers allowed initial delta growth 2000-3000 yr earlier than the global a verage, despite debouching onto a canyon-incised, high-energy margin. Subse quently, a thick (similar to 50 m) transgressive systems tract was formed d uring the early Holocene phase of delta aggradation, contrasting with exten sive shoreline retreat documented along most margins. Highstand progradatio n of coastal and shelf sequences ensued by the middle Holocene, but subsidi ng inland basins also favored accumulation of a thick (to 40 m) highstand s equence in the: lower floodplains. Unlike many other delta systems, subside nce in the subaerial G-B delta is not dominated by compaction, but rather b y plate-driven tectonic processes that generate rates up to 4 mm/year. Over all, the huge sediment load, tectonic subsidence, major seismic events, and a nearshore canyon system have led to widespread sediment dispersal and se quence formation across the subaerial delta, shelf, and deep-sea Bengal Fan throughout the Late Quaternary. (C) 2000 Elsevier Science B.V. All rights reserved.