Subaqueous delta of the Ganges-Brahmaputra river system

Abstract

The Ganges-Brahmaputra is among the world's three largest river systems in terms of sediment load, but, until now, no high-resolution seismic data have been obtained to document the nature of the sediment deposit seaward of the rivers' mouths. The other two (Amazon, Huanghe) discharge into energetic coastal environments and form subaqueous deltas with characteristic clinoform stratigraphy. High-resolution seismic reflection profiles of the Bengal shelf reveal similar stratigraphy: topset beds dip gently (0.036 °) and diverge offshore; more steeply dipping foreset beds (0.190 °) converge farther seaward; and relatively thin, gently dipping bottomset beds (0.022 °) extend across the outer shelf, overlying an erosional surface presumed to be of Late Pleistocene age. Sediment accumulation rates are highest in the foreset region (≥ 5 cm/year) and reduced in the bottomset region (< 0.3 cm/year), corroborating the relative thickening and thinning of strata observed in seismic profiles. Taken together, these data indicate a subaqueous delta is actively prograding across the Bengal shelf. Volume estimates for the Holocene subaqueous delta reveal that about one third of the total load of the Ganges-Brahmaputra has accumulated on the shelf. The remainder is likely partitioned between the river floodplain/delta plain and off-shelf transport via the submarine canyon, Swatch of No Ground. The canyon incises the shelf in the area of highest sedimentation rates (foreset), and growth faults and slumping of modern sediments near the head of the canyon support the idea that significant off-shelf transport of sediments to the Bengal Fan is occurring.