ROLE OF MASS MOVEMENT IN SHELF CLINOFORM GROWTH: THE AMAZON AND GANGES-BRAHMAPUTRA EXAMPLES

Shelf clinoforms off the Amazon and Ganges-Brahmaputra Rivers reveal have some striking similarities, as well as differences, that provide clues on the genesis of these large-scale deposits. In both cases, rapid progradation of the foreset beds is accomplished, in part, by gravity driven sediment flows. In the case of the Amazon, fine-grained fluid muds formed in the topset region flow seaward and are dispersed episodically down the foreset gradient. Fluid muds have been shown to be generated by a number of physical mechanisms which trap fine sediment and limit its consolidation. Sediment gravity flows across the foreset region also characterizes the Ganges-Brahmaputra shelf clinoform, however, in this case the flows consist of fine sand and silt. The sandy silt layers appear as m-thick acoustically transparent beds in high-resolution seismic, and in sediment cores appear as massive/unsorted bedding. One interpretation of the genesis of these sandy-silt layers is that high density flows are generated during frequent cyclones by wave resuspension, and cascade seaward across the foreset beds where they contribute to high progradation rates. Thus it appears that, although the physical mechanism may be distinct for the Amazon and Ganges-Brahmaputra, in both cases gravity-driven flows contribute directly to clinoform growth.