Sediment dispersal and sequence development along a tectonically active margin: Late Quaternary evolution of the Ganges-Brahmaputra River delta.

摘要

Situated in the Bengal Basin, the Ganges-Brahmaputra river delta comprises one of the largest fluviodeltaic systems in the world, comprising ∼ 100,000 km2 of floodplain and delta plain and a 40,000 km2 subaqueous delta on the shelf. Sediment load of the Ganges-Brahmaputra river is 109 t/yr, and seasonal flooding may inundate >70% of the delta during large events. Active tectonic processes have resulted in both uplift and subsidence in this structurally complex region. These general characteristics suggest that the Ganges-Brahmaputra delta represents a heretofore undescribed delta end-member, forming along a high-yield, high-energy, tectonically active margin. To investigate this view, stratigraphic, sedimentologic, and geochronologic data are used to evaluate processes, controls, and development of the system over different spatial and temporal scales in the Late Quaternary.; Results of a century-scale sediment accretion study using radioisotope geochronology indicate that ∼ 30% of fluvial sediment load is sequestered to the delta and not reaching the coastal ocean as previously assumed. A Holocene-scale sediment budget generated from radiocarbon-dated stratigraphy also reveals ∼ 30% of sediment discharge was sequestered to the delta during this time. Considered with offshore data, these sediment budgets indicate contemporaneous highstand strata formation across floodplain, shelf, and deep-sea depocenters. Radiocarbon-dated stratigraphy was used to reconstruct the Late Quaternary history of delta formation. Growth of the Ganges-Brahmaputra delta began ∼ 10,000--11,000 cal yr BP, notably 2000--3000 years prior to most of the world's deltas. During subsequent rapid sea-level rise, the immense sediment discharge was sufficient to maintain relative shoreline stability while most margin systems experienced major transgression. Offset of radiocarbon dates from eustatic sea level indicate 2--4 mm/yr of subsidence in several areas of the delta, suggesting tectonic control on deltaic sediment trapping and sequence formation. Shallow vibracore stratigraphy from the delta reveals a cap of muddy sediments overlying largely sandy material, reflecting differences in preservation between floodplain and channel deposits. Over longer time frames, floodplain sediments are eroded through channel migration and avulsion, thus preferentially preserving channel sands. Overall, the role of tectonics in controlling deltaic processes and product in the Ganges-Brahmaputra river delta implies a fundamental distinction for deltas forming along active margins.