Abstract

The Bengal-Nicobar submarine fan complex is part of a linked sedimentary chain consisting of molasse, deltaic, and flysch deposits resulting from the sequential closing of a remnant ocean basin. Ultimate sources for turbidite sand from this fan complex are the uplifted gneissic, sedimentary, and metasedimentary terranes of the Himalayas. Detailed point-counts of lithic grains, as well as standard QFL percentages, of 22 Neogene sand samples from DSDP sites 211 and 218 reveal very uniform compositions. Typical QFL percentages are: 55-30-15. Plagioclase/total feldspar is typically near 0.7. Lithic types are dominated by quartz-mica tectonite. quartz-mica aggregate, polycrystalline mica, and other sedimentary and metasedimentary varieties. Andesitic volcanic lithic grains are absent. The indicated provenance ("tectonic highlands") for Bengal-Nicobar sands contrasts markedly with that of sand and sandstone derived from magmatic arcs and rifted continental margins. Lithic populations of magmatic arc sand and sandstone are dominated by volcanic grains. whereas lithic populations of rifted continental margin sand and sandstone are dominated by polycrystalline quartz and sedimentary grains. Two triangular plots of lithic grains distinguish the provenance of sandstone derived from major tectonic settings. Detailed point-counts of lithic grains from thick sedimentary accumulations of unknown tectonic setting are a powerful paleogeographic tool when used in the manner outlined here.

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