The submarine delta of the Ganges–Brahmaputra: cyclone-dominated sedimentation patterns

Abstract

The Ganges–Brahmaputra belongs to the world's largest rivers in terms of sediment discharge, but little is known yet about the processes controlling sediment distribution on the shelf and the documentation of these processes in large- and small-scale sedimentary structures. For this study the submarine delta of the river system was investigated by high-resolution seismic reflection profiles, and piston and gravity coring. Logged physical properties of the cores were combined with visual core description, grain-size analysis, and radiography. Sedimentation rates were estimated by 210Pb and 137Cs gamma spectrometry and cross-checked with evaluation of seismic profiles. The data reveal that the broad delta topset (<20 m waterdepth) is formed by a thick layer of sand + silt. Highest sedimentation rates (about 10 cm a −1) occur at the centre of clinoform delta foreset beds where about 20% of the total riverine sediment load is deposited and thus cause an annual progradation of the subaqueous delta front by about 15 m. The foreset beds consist of graded sand + silt layers which are thought to be deposited by sediment-laden flows generated during the surge of tropical cyclones (tempestites) interbedded with silty clay layers. The head of a canyon, the Swatch of No Ground, that deeply incises into the shelf, forms a sediment trap along the westward cyclonic transport path where sedimentation rate increases to about 50 cm a −1. Episodically earthquakes or storms destabilize especially the rapidly accumulated foreset beds in the eastern delta creating up to 8 m thick mass flows. On the outer shelf in water depth beyond 80 m no Holocene sediment is deposited.