Cenozoic history of the Himalayan-Bengal system: Sand composition in the Bengal basin, Bangladesh
Stratigraphic sequences preserved in the Bengal basin provide detrital information that documents a significantly older history of the eastern Himalaya than that available from Ocean Drilling Program and Deep Sea Drilling Project cores recovered from the Bengal fan. The Bengal basin, formed as a result of the Himalayan collision, is located at the juncture of the Indian craton to the west, the Shillong massif and the Himalayan belt to the north, and the Indo-Burman ranges to the east. Modal analyses of Eocene and Oligocene sandstones of the Cherra, Kopili, and Barail Formations document compositions that are dominated by subangular monocrystalline quartz grains with scarce to no feldspar grains, and few lithic fragments (Cherra and Kopili: Qt 99 F 1 L 0 ; Barail: Qt 90 F 3 L 7 ; Qt—total quartz, F—feldspar, L—lithic fragments). These compositions are similar to sands derived from the Indian craton, suggesting that they underwent intense chemical weathering, likely in a source with low relief and considerable transport. Himalayan tectonism during this time was probably significantly more distant from the Bengal basin than at present. Sandstones of the Miocene Surma Group (Bhuban and Boka Bil Formations) are rich in feldspar grains, argillite, and very low-grade metamorphic lithic fragments (Bhuban: Qt 66 F 15 L 19 ; Boka Bil: Qt 57 F 23 L 20 ) relative to older sandstones, suggesting onset of uplift and erosional unroofing in the eastern Himalaya, and initiation of stream systems supplying orogenic detritus to the Bengal basin. Sands of the upper Miocene to Pliocene Tipam Group and the Pliocene–Pleistocene Dupi Tila Sandstone contain abundant argillitic and low- to medium-grade metamorphic lithic fragments and feldspar grains (Tipam: Qt 61 F 19 L 19 ; Dupi Tila: Qt 70 F 13 L 17 ), suggesting continued orogenic unroofing. These younger sands are rich in potassium feldspar (P/F=0.30, 0.20) relative to plagioclase (P)-rich Bhuban and Boka Bil sandstones (P/F=0.66 and 0.48), suggesting a granitic source, probably the Miocene leucogranites of the High Himalayan Crystalline terrane. These results document for the first time contrasts in orogenic history recorded in the Bengal system vs. western Himalayan foreland basins. Sands deposited in the Bengal basin are generally more quartzose and less lithic than those from the western Himalayan foreland basins, and pre-Miocene strata in the Bengal system show little to no evidence of orogenic activity. In part, this probably reflects west to east progression of the Himalayan collision, but it probably also results from sedimentary systems propagating southward, ahead of the advancing mountain belt as it has been consuming the remnant ocean basin trapped between the Indian craton and the Burmese block.