Abstract Originating in the Himalayan Mountains within distinct drainage basins, the Ganges and Brahmaputra rivers coalesce in the Bengal Basin in Bangladesh, where they form one of the world’s great deltas. The delta has extensive subaerial and subaqueous expression, and this paper summarizes the current knowledge of the Late Glacial to Holocene sedimentation from the upper delta plain to the continental shelf break. Sedimentation patterns in the subaerial delta are strongly influenced by tectonics, which has compartmentalized the landscape into a mosaic of subsiding basins and uplifted Holocene and Pleistocene terraces. The Holocene evolution of the delta also has been mediated by changing river discharge, basin filling, and delta-lobe migration. Offshore, a large subaqueous delta is prograding seaward across the shelf, and is intersected in the west by a major submarine canyon which acts both as a barrier for the further westward transport of the rivers’ sediment and as a sink for about a third of the rivers’ sediment discharge. Subaerial and subaqueous progradation during the Holocene has produced a compound clinoform, a feature which appears to be common for large rivers discharging onto an energetic continental shelf.

Abstract Sediment cores and high-resolution CHIRP seismic data were collected on the inner shelf adjacent to Atchafalaya Bay, Louisiana to examine the evolution of the newly forming mud delta associated with the Atchafalaya River lobe of the Mississippi deltaic plain. 210 Pb accumulation rates from sediment cores show maximum sedimentation rates (10-20 cm/yr) are concentrated on the innermost shelf (<6 m water depth) immediately seaward of the Pt. Au Fer shell reef at the bay mouth. Rates decrease rapidly offshore to 8-10 m water depth, where seismic profiles show modern deposits pinch out adjacent to shoals formed by erosional remnants of older Holocene deltaic deposits. Alongshore, rates remain relatively high to the west (along the chenier coast of West Louisiana) following the trend of coastal currents. The wedge-shaped prodelta reaches 2.5 m in thickness and is gas-charged adjacent to the Atchafalaya dredge channel on the shelf. In areas where accumulation rates exceed ~1 cm/yr, the prodelta muds take the form of cm-scale interlaminations of silty sands (proximal) or silts (distal) and clayey silt layers. At accumulation rates of ~0.5-1 cm/yr, primary fabric is partially destroyed by macrofaunal burrows. In seaward areas, 10-25 m water depth, where modern sediments are accumulating but rates are low, clayey silt and silty clay deposits are completely homogenized by burrowing activity. Sediment dispersal paths can be traced seaward of the bay mouth and westward, along the direction of prevailing coastal currents, using coarse silt content of the mud delta. This indicates that coarse silts behave as individual particles and are preferentially sorted, while fine-medium silts are contained within flocs with clay-size mineral grains, and show no preferential sorting downdrift from the source.