DETRITAL HISTORY OF PLIOCENE-PLEISTOCENE SIWALIK-EQUIVALENT DUPI TILA FORMATION, BENGAL BASIN, BANGLADESH

The Pliocene-Pleistocene Dupi Tila Formation of the Bengal basin is composed of yellow, light brown, and pink, coarse to very fine-grained, moderately hard to loose sandstone, siltstone, silty clay, mudstone and shale with some pebble beds. These syn-orogenic sediments crop out in northern (foothills of Garo hills and Shillong Plaetau), eastern (Sylhet trough and Chittagong Hills), and central Bangladesh (Comilla and vicinity) and occus in the subsurface in most other areas of Bangladesh, including the northwest Indian Platform area. This aquifer-bearing siliciclastic unit blankets thicker gas-rich Neogene strata in Bangladesh. The Dupi Tila was deposited primarily in alluvial fan and fluvial environments in eastern and northwestrn Bangladesh but the occurrence of a few authigenic glauconite-bearing marginal-marine outcrops have been reported from western and central Bangladesh.

Dupi Tila sandstones consist predominantly of quartz, feldspar and lithic fragments. Quartz grains are mostly monocrystalline, and feldspars are overwhelmingly dominated by potassium feldspar. Lithic fragments include roughly equal amounts of sedimentary and metamorphic fragments. Upper grade metamorphic lithic fragments dominate. These sands contain no obvious volcanic lithic fragments. Quartzolithic to quartzofeldspathic sandstone modes of the Dupi Tila Formation indicate a “recycled orogenic” provenance from the Himalayas and Indo-Burman ranges.

The presence of significant potassium feldspar and upper-grade metamorphic lithic fragments suggests a felsic plutonic and deep-crustal metamorphic source, most likely in the eastern syntaxial region of the Himalayas, and delivered by the paleo-Brahmaputra. However, sediments also have been contributed from crystalline rocks of the Indian shield via the paleo-Ganges. The proximal Shillong Plateau could not have served as a source as its uplift post-dates the deposition of the Dupi Tila Formation. Ongoing work on detrital and authigenic geochronology and microprobe work will provide better constraints on the depositional and detrital history of the Dupi Tila Formation.