|Nearly 2.5-km-thick Pliocene-Pleistocene Dupi Tila Formation of the Bengal basin is composed of yellow, light brown, and pink, coarse- to very fine-grained, moderately to loosely indurated sandstone, siltstone, silty clay, mudstone and shale with some pebble beds. These synorogenic sediments crop out in northern (foothills of Garo hills and Shillong Plateau), eastern (Sylhet Trough and Chittagong Hills), and central Bangladesh (Comilla and vicinity) and occur in the subsurface in most other areas of Bangladesh, including the northwest Indian Platform area. Detrital history from this area provides data pertaining to uplift and erosional history of the hinterland areas (i.e., Himalayas, Indo-Burma Ranges and Shillong Plateau).
A systematic study of available detrital modes of sandstones from the Sylhet Trough, Lalmai hills, Garo hills, Stable Platform, and Sitapahar anticline in Chittagong hills include an array of sublithic to subfeldspathic quartz arenites. Modal analyses of the sandstones of the Dupi Tila Formation from Sitapahar anticline (Qt64F10L27), Garo hills (Qt88F2L10), Northwest Stable Platform (Qt87F6L7), Sylhet Trough (Qt66F9L25) and Lalmai hills (Qt64F6L30) suggest that the sandstones have orogenic provenance signatures. Only samples from the Garo hills, which contain higher amounts of mono- and polycrystalline quartz, differ from the other area samples. The abundance of low- to intermediate-grade lithic fragments (Lm2) in all samples suggest unroofing of deep crustal levels of the orogens.
The Dupi Tila Formation samples contain an average 0.8% heavy minerals, comprising opaque minerals, garnets, sillimanite, tourmaline, kyanite, andalusite, epidote group minerals, chloritoid & chlorite, staurolite, etc., in order of decreasing in abundance. The opaque fraction includes magnetite, hematite, ilmenite, pyrrhotite, and rarely pyrite. The heavy mineral data suggest an orogenic provenance for the Dupi Tila Formation. The relative abundance of aluminosilicates and related heavy minerals in the Dupi Tila Formation throughout the Bengal basin reflect systematic unroofing of deeper crustal levels in the eastern Himalaya. Sillimanites (fibrolites) indicate the sediments were sourced from protoliths of high-grade regional metamorphic rocks.
Garnet chemistry data for the Dupi Tila Formation indicate a substantial amount of almandine suggesting provenance from amphibolite and granulite facies rocks. The presence of Mn-rich garnets (spessartine) in the Stable platform samples indicates provenance from pegmatite and low-grade metamorphic facies rocks. Tourmaline chemistry suggests derivation from Al-bearing metapelites, metasammites, calc-silicate rocks, Li-bearing pegmatites, granitoid pegmatites, and aplites. Epidote chemistry reveals sediment derivation from relatively high-grade metamorphic rocks of epidote-amphibolite facies. Chloritoid chemistry suggests that sediments originated from high-pressure blueschist metamorphic facies.
Whole rock geochemical data suggest that the majority of the sediments were derived from felsic igneous source terranes. Based on the chemical index of alteration, the intensity of weathering in the source area was moderate to high.
Sediments of the Upper Siwalik sequences in the Himalayan foreland basin are similar to the Dupi Tila Formation in terms of sandstone petrography and heavy mineral character. Hence, the Dupi Tila Formation of the eastern Himalayas may serve as the Upper Siwalik-equivalent extension of the western Himalayas. Future research projects employing detrital geochronology should provide additional information on the provenance history of the Dupi Tila Formation.