THE SUBMARINE DELTA OF THE GANGES-BRAHMAPUTRA - CYCLONE-DOMINATED SEDIMENTATION PATTERNS

The Ganges-Brahmaputra belongs to the world's largest rivers in terms of sediment discharge, but little is known yet about the processes con trolling sediment distribution on the shelf and the documentation of t hese processes in large- and small-scale sedimentary structures. For t his study the submarine delta of the river system was investigated by high-resolution seismic reflection profiles, and piston and gravity co ring. Logged physical properties of the cores were combined with visua l core description, grain-size analysis, and radiography. Sedimentatio n rates were estimated by Pb-210 and Cs-137 gamma spectrometry and cro ss-checked with evaluation of seismic profiles. The data reveal that t he 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 tot al riverine sediment load is deposited and thus cause an annual progra dation of the subaqueous delta front by about 15 m. The foreset beds c onsist of graded sand + silt layers which are thought to be deposited by sediment-laden flows generated during the surge of tropical cyclone s (tempestites) interbedded with silty clay layers. The head of a cany on, the Swatch of No Ground, that deeply incises into the shelf, forms a sediment trap along the westward cyclonic transport path where sedi mentation rate increases to about 50 cm a(-1). Episodically earthquake s or storms destabilize especially the rapidly accumulated foreset bed s in the eastern delta creating up to 8 m thick mass flows. On the out er shelf in water depth beyond 80 m no Holocene sediment is deposited. (C) 1998 Elsevier Science B.V. All rights reserved.