Gondwana biostratigraphy and geology of West Bengal Basin, and its correlation with adjoining Gondwana basins of India and western Bangladesh

Abstract

Integrated biostratigraphic and geological studies on the drilled exploratory boreholes of West Bengal Basin have revealed the presence of very thick distinctive Permo-Triassic Gondwana successions in the subsurface, and widely recognized across the basin below the latest Jurassic–Early Cretaceous Rajmahal Traps. Precise geological age of identified Gondwana lithounits of this basin is inferred on the basis of associated palynofloral assemblages along with their correlation with the adjoining Gondwana basins. In addition, two Gondwanic grabens are recognized below the pericratonic set-up whose presence in this basin, till now, are poorly understood. Detailed palynological studies on the Gondwana successions in the key boreholes of this basin enable recognition of 12 established palynological zones ranging from Early Permian (Asselian) to Middle Triassic (Ladinian). Lower Gondwana (Permian) palynofloras of this basin resemble earlier recorded palynofloras from the Talchir, Barakar and Raniganj formations of Indian Gondwana basins, suggesting the occurrence of well-developed above Lower Gondwana lithounits in this basin and the absence of Karharbari and Barren Measures formations which are marked by the non-depositional hiatuses. Upper Gondwana successions (Triassic) of the basin are represented by the Early and Middle Triassic palynofloras only that resemble Panchet and Supra-Panchet (Molangdighi Formation) palynofloral assemblages, and indicate the absence of Late Triassic succession of Upper Gondwana. Newly acquired geological and Bouguer anomaly data, and spatial distribution of Gondwana bearing boreholes across the basin suggest that the boreholes of CHK-A, GB-A, HRP-A, AMD-A, PLS-A and MNG-A fall in a well-defined NNE–SSW aligned graben, defined as the ‘Chandkuri–Palasi–Bogra Gondwanic Graben’ which includes Chandkuri and Palasi Gondwana basins in Indian part and Bogra Gondwana Basin in western Bangladesh where thick Gondwana successions are recognised in the subsurface sections of Singra (Singra-1X, Kuchma-X1, Bogra-X1), Jamalganj (EDH-1, 6) and Barapukuria (GDH-40, 43) coalfields. Gondwana bearing boreholes in the western margins of West Bengal Basin (GLS-A, GLS-B, GLS-C) represent a quite separate Gondwana basin of ‘Galsi’ which is located in the southern part of the N–S trending ‘Purnea–Rajmahal–Galsi Gondwanic Graben’. Latest Jurassic–Early Cretaceous Rajmahal Traps cap the Gondwana successions in the above Gondwana basins of West Bengal Basin along with Bogra and Rajmahal Gondwana basins. Rajmahal Traps and its infra- and intertrappean beds are exlcuded from the Gondwana cycle as these beds are marked by the Early Cretaceous marine dinoflagellate cysts, and the Rajmahal volcanism took place after a very long time gap from Late Triassic to Late Jurassic (ca. 78 my). Rajmahal Traps and its infra- and intertrappean beds indeed represent the post-Gondwana syn-rift sequence in West Bengal, Mahanadi and Rajmahal basins, and closely related with the break-up of Indian Plate from the East Gondwanaland during Tithonian (ca. 150 Ma). The post-trappean Late Cretaceous successions mark the initiation of passive-margin phase in this basin with the deposition of marine Bolpur and Ghatal formations.