Multiple-proxy records of delta evolution and dispersal system behavior: fluvial and coastal borehole evidence from the Bengal Basin, Bangladesh

Abstract

A multiple-proxy study for borehole sediments from the Bengal Basin reveals stratigraphic, paleoenvironmental, and provenance data which suggest repeated event deposition, river avulsion and abandonment, and shifting provenance sources to the Ganges-Brahmaputra (G-B) margin during Late Quaternary delta evolution. Results indicate that the G-B delta system is not simply an amalgam of homogenous sediments, but, rather, is comprised of stacked units with distinct lithologies deposited from at least three major provenance sources. For the first time, trends in down-core strontium concentration are used to distinguish Brahmaputra, Ganges, and Eastern Himalayan provenance signatures within delta sediments. Major controls on delta development are reflected within two sediment cores, one dominated by fluvial processes and one by coastal marine processes. Results from this study introduce new, non-trivial mechanisms of G-B delta evolution including: 1) the significance of Eastern Himalayan tributaries to delta development, 2) direct borehole evidence for the apparent shut-off of Brahmaputra sediment flux to the margin, and 3) the deposition of discrete, anomalously coarse-grained sand and gravel pulses interbedded within typical delta sediments. This study suggests the filling, and subsequent draining, of previously documented glacially dammed lakes in the Himalaya may be the principle mechanism for temporary shut-off Brahmaputra sediment input to the margin and the deposition of anomalously coarse grained deposits within fluvial and coastal-dominated environments.