HEAVY MINERAL CONSTRAINTS ON PROVENANCE HISTORY OF OLIGOCENE BARAIL SEDIMENTS IN THE BENGAL BASIN AND SOUTHEAST SHILLONG, NORTHEAST INDIA

The predominantly arenaceous Oligocene deposits of the Barail Group in lower Assam (southeast Shillong) has a thickness of about 4650 m while in the Sylhet Trough of the Bengal Basin, which is no more than 100 km away from southeast Shillong, the equivalent unit has a thickness of approximately 800 m. Their depositional environments ranged from marginal marine to turbiditic at the lower Assam to estuarine deposits of the Bengal Basin. A distinct variation in lithofacies including the heavy mineral assemblages revealed the difference of these neighboring Oligocene deposits at the foreland basin of the eastern Himalayas.

The heavy-mineral suites of the Barail Group reported from both the study areas represent low weight percentage, ranging from 0.09% to 0.95%. Sediments from both areas contain around 50% of opaque minerals. A distinct variation is observed also in terms of stable mineral contents. The ZTR (zircon-tourmaline-rutile) index is higher (36.41%) for Sylhet Trough than those of the lower Assam area (23.83%) which reflects relatively higher maturity and more chemical weathering of the Barail sediments of Sylhet Trough. The other non-opaque minerals such as chlorite-chloritoid, garnet, topaz, amphibole, sillimanite, muscovite, and biotite are less abundant in Sylhet Trough than those of southeast Shillong. The mineralogical maturity and the overall mineral assemblage suggest that the Indian craton toward the west might have been a possible source area for the Oligocene Barail sediments of the Bengal Basin. On the other hand, the presence of well-rounded tourmaline and zircon grains in Oligocene sandstone of Shillong and the Sylhet Trough indicates a long transport and possible source from the early uplifts of the eastern Himalayas and the Indo-Burman ranges. The Bengal Basin perhaps was positioned farther south by the equator resulting in exposure to intense chemical weathering during the Oligocene which was later (Miocene) brought closer to the Shillong (lower Assam) area. Ongoing work on thin section petrography, mineral chemistry and detrital geochronology will provide better constraints on provenance history of these two neighboring Oligocene deposits.