Mineralogical, Geochemical, and Geochronological Evidences of Evolution of the Eastern Himalayan Foreland Basins

There are two large foreland basins south of the eastern Himalayas: Assam, located near the eastern syntaxis of the Himalayas and the Bengal to further 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 analysis documents that Eocene Disang (Qt₅₆F₅L₄₀), Oligocene Barail (Qt₅₉F₇L₃₄), Miocene Surma (Qt₆₈F₃L₂₉), and Mio-Pliocene Tipam (Qt₅₃F₉L₃₈)  from Assam 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, suggesting an orogenic source from the Himalayas and/or Indo-Burman Ranges. Detrital heavy-mineral studies, 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 the Himalayan arc material or Indo-Burmese Alpine-type ophiolites, and that of Assam were mostly from the Indo-Burman Ranges. The Bengal Miocene muscovite cooling ages range from 11Ma to 66 Ma, with modes at 17 Ma and that of Assam ranges from 28 Ma to 81 Ma, with modes at 39 Ma and 77 Ma, suggesting older source rocks for Assam Miocene units; most likely the Gangedese batholith of Tibet and the Mogok belt of Myanmar. Whole-rock chemistry data reveal that all the samples analyzed from the Assam Basin and the post-Oligocene sediments from the Bengal Basin were derived from granodiorite and granitic source rocks. The Bengal Oligocene samples however show high-silica content suggesting intense chemical weathering during deposition close to the equator.