IS PLIOCENE-PLEISTOCENE DUPI TILA FORMATION OF THE BENGAL BASIN, BANGLADESH, EQUIVALENT TO UPPER SIWALIK SEDIMENTS OF THE WESTERN HIMALAYAN BASINS?

The upper Siwalik (700-2300 m) Pliocene-Pleistocene deposits of the western Himalayan basins show similarity in petrological and sedimentological characters with the Dupi Tila Formation of the Bengal Basin, Bangladesh (eastern Himalayan basin), which exhibits typical freshwater molassic deposits laid mostly by the ancestral paleo-Brahmaputra. Consisting primarily of fining-upward fluvial deposits of yellow, light brown, and pink, medium to fine grained, poorly indurated sandstone, siltstone, silty clay, mudstone and shale with some pebble beds of petrified wood, the 300-2500 m Pliocene­-Pleistocene Dupi Tila Formation records collisional history between the Indian and Eurasian continents. These orogenic sediments crop out and are drilled in most of the eastern and northern Bengal basin.

A systematic comparison of available detrital modes for sandstones from the Sylhet Trough, Lalmai Hills, Garo Hills, Stable Platform, and Chittagong Hills include an array of sublithic to subfeldspathic quartz arenites. Framework composition of forty-five samples range from 60% to 90% quartz, 3% to 30% feldspar, and 5% to 20% lithic fragments. Quartz grains are mostly monocrystalline, and feldspars are overwhelmingly potassium types. Lithic fragments include roughly equal amounts of sedimentary and metamorphic fragments, but low-­ to intermediate­-grade metamorphic lithic fragments are more common. The heavy mineral assemblages of the Dupi Tila Formation contain in average 0.8% heavy minerals, comprising zircon, garnet, sillimanite, staurolite, kyanite, epidote, rutile, biotite, chlorite, and chloritoid. The opaque fraction includes magnetite, hematite, ilmenite, pyrrhotite, and rarely pyrite. The heavy mineral data suggest a wide range of metamorphic as well as granitoid source areas.

The Dupi Tila sediment shows fining-upward sequences whereas the upper Siwalik sediments are coarsening-upward and relatively proximal to the orogenic belt. Both the upper Siwalik and Dupi Tila Formation sourced from the deeper part of the orogenic system. Like the upper Siwaliks, the sandstone modal composition and detrital heavy mineral suites reflect that the sources of the Dupi Tila Formation are orogenic, and most likely located in the eastern Himalayas, and the Indo­-Burman ranges.