Fate of sediments delivered to the sea by Asian large rivers: Long-distance transport and formation of remote alongshore clinothems

Abstract

Recent studies show that the global flux of river-derived sediment reaching the coasts and oceans is about 15-19 x 109 tons per year. New sediment budgets for the major Asian river systems (e.g., Yellow, Yangtze, Mekong, Ganges-Brahmaputra, etc.) suggest that 30-50% of their sediment load has been retained in the lower channel reaches to form an extensive subaerial delta plain, while the rest is discharged to the sea. Of the sediment load reaching the ocean, about half has been found to accumulate near the river mouth as a proximal subaqueous delta clinothem. However, the remaining sediment is found to be transported up to 600-800 km alongshore, ultimately being deposited as a shore-parallel middle-shelf clinothem. These clinoform deposits are generally <100 km in across-shelf width, 20-40 m thick nearshore, and pinch-out gradually seaward at 40-90 m water depth. A secondary nearshore depocenter can usually be found along the shelf away from the river mouth,with mud-lobe accumulation up to 40-50 m thick locally. Except for a few systems with shelf-indenting canyons (e.g., Ganges-Brahmaputra and Indus), most of Asian river- derived sediments are trapped on the inner and middle shelf, unable to reach the deep ocean (i.e.,>150 m) despite having been transported hundreds of kilometers from their mouths.