LITHOFACIES AND STRATIGRAPHIC FRAMEWORK OF HIMALAYAN SEQUENCES IN ASSAM AND THE BENGAL BASIN
Paleogene sequences are much thicker in Assam. Eocene Disang strata (3+ km) in the Assam basin are almost nonexistent in the Bengal basin. Thick Oligocene Barail strata are more prominent in the Assam basin compared to the Bengal basin. Paleogene sediments are highly deformed and mildly metamorphosed in Assam, whereas deformation is very limited in the Bengal basin. Depositional sequences in Assam suffered syn-and post-tectonic deformation resulting in NW-verging thrusts that resulted in migration of depositional lobes. Paleogene sediments from Assam are orogenic, quartzolithic strata with diverse heavy minerals, whereas Bengal basin sequences are quartzarenitic strata with stable, mature heavy minerals.
Neogene strata, deposited over a regional unconformity observed in both Assam and the Bengal basin, are much more similar than the Paleogene deposits. Depocenters shifted throughout the early Miocene in Assam mostly toward the south while resulting in deposition of thicker sections (Surma) in the Bengal basin. In the upper Miocene, however, thicker units (Tipam) were deposited in the Schuppen belt compared to the Bengal basin due to channel shifting in a locally underfilled Assam basin resulting from tectonism. Depocenters shifted again (in the upper Miocene/Pliocene) toward the south as a result of encroaching mountain fronts close to the Assam basin. Compositionally, Neogene sediments in both areas are quartzolithic to quartzofeldspathic, presumably because of approaching orogenic fronts.
Overall, the Assam basin appears to represent a more proximal and earlier repository of deltaic detritus shed from the Himalayan collision, while the Bengal basin serving as a downstream and somewhat younger depocenter; with the offshore area south of Bangladesh representing the current, distal orogenic depocenter.