Sedimentation and Tectonic Evolution of Cenozoic Sequences from Bengal and Assam Foreland Basins, Eastern Himalayas
Abstract
The Himalayan Mountains developed due to the collision of the Indian and Eurasian plates. There are two large foreland basins south of the eastern Himalayas: Assam Basin, located near the eastern syntaxis of the Himalayas, and the Bengal Basin further to the south. The Indo-Burman Ranges lie to the southeast of Assam and east of the Bengal Basin. The Indian craton is located to the west of the Bengal Basin. Mineralogical, geochemical, and geochronological studies provide critical information for the evolution of the Tertiary sequences of these basins. Sandstone modal analyses document that Eocene Disang (Qt56F5L40), Oligocene Barail (Qt59F7L34), Miocene Surma (Qt68F3L29), and Mio-Pliocene Tipam (Qt53F9L38) from Assam Basin, and Oligocene Barail (Qt83F3L14), Miocene Surma (Qt59F18L23), Mio-Pliocene Tipam (Qt57F14L29), and Pliocene Dupi Tila (Qt54F21L25) from the Bengal Basin plot in the “recycled orogenic” provenance field of Dickinson, indicating an orogenic source from the Himalayas and/or Indo-Burman Ranges. Studies of detrital heavy-minerals, including garnets, also suggest orogenic source terranes with input from low- to medium-grade metamorphic rocks. Chrome spinels from the Bengal Basin were probably derived either from Himalayan arc material or Indo-Burmese Alpine-type ophiolites, while those of Assam were mostly derived from the Indo-Burman Ranges. Oligocene muscovite grains from Assam give a cooling age range from 35 to 204 Ma, indicating that they likely sourced from the Gangdese batholith of Tibet and the Mogok Metamorphic Belt of Indo-Burman Ranges (Myanmar). Cooling ages of Miocene muscovites from Assam range from 28 Ma to 81 Ma, with noticeable modes at 28-50 Ma and 65-80 Ma. This suggests that source rocks for Assam Miocene units, most likely the Gangdese batholith of Tibet and the Mogok belt of Myanmar, are older than that from the Bengal Basin. Whole-rock chemistry data reveal that sediments from the Assam Basin and the post-Oligocene sediments from the Bengal Basin were derived from granitic source rocks. Oligocene samples from Bengal Basin, however, show high silica contents suggesting intense chemical weathering during deposition closer to the equator. Depocenters in the foreland basins of the eastern Himalayas progressed generally south and west through time (in Assam during the Paleogene and in the Bengal Basin during the Neogene). The right-lateral Kaladan fault may have brought these two depositional systems closer in the Miocene.