Geological and Geophysical Studies in Western Part of Bengal Basin, India

Abstract

Just beyond the western boundary of West Bengal, the great Indian shield disappears below a blanket of alluvium. The exposed part of the shield bordering the Bengal basin is marked by a row of intracratonic Gondwana basins, a series of thrust zones in Singhbhum, and extensive exposure of basic volcanics in the Rajmahal Hills. Intensive geophysical surveys and deep drilling in the alluvium-covered plains of West Bengal have revealed a thick section of Cretaceous and Tertiary sediments lying on a basement of basalt lava flows, presumably of the same age as the Rajmahal Group volcanics. An extension of the easternmost Gondwana basin farther east, below the Bengal alluvium, also is suggested. A series of buried basement ridges, marking the western margin of the Bengal basin, resumably kept the Gondwana continental basins isolated from the main Bengal basin through most of Tertiary time. Locally, during the late Tertiary, the sea transgressed over these basement ridges and onlapped parts of the Indian shield. Flanking the eastern margin of the buried ridges is a row of basin-margin en echelon faults and scarps, possibly the shallower expressions of some deep-seated movements in the basement. East of this marginal fault zone lies the stable shelf of West Bengal with a homoclinal dip toward the southeast. Seven seismic reflectors mapped in the Mesozoic and Tertiary sediments of the shelf indicate uniform increase of thickness (3,000 to 27,000 feet, approximately) of these sediments toward the southeast. Except for a few normal faults, the area is practically undisturbed structurally. An extensive unconformity between the Miocene and Pliocene has been recognized. Locally, weak evidence of another depositional break, at the top of the Oligocene, is present. Around Calcutta, the Eocene key horizon (Sylhet Limestone) shows a conspicuous basinward flexure (the hinge zone) at a depth of about 15,500 feet. East of this hinge, which traverses the whole Bengal basin, lies the deeper part of the basin with a greater rate of subsidence and a different lithofacies. Seismic interpretation suggests a sharp lithofacies change at this zone, from the Eocene nummulitic limestone of the stable shelf to a thick sequence of clay and shale in the deeper part of the basin. In the younger Tertiary sediments is a similar change of facies from the arenaceous sediments of the stable shelf to the dominantly argillaceous sediments downdip. Marine transgression on the West Bengal shelf occurred during the Late Cretaceous (locally), late Eocene (extensively), and Miocene (in the eastern parts only). Except for these periods of marine transgression, sedimentation took place under fresh-water, estuarine, or deltaic conditions. A summary of the tectonic and depositional history of the whole region, from the eastern margin of the Indian shield to the folded belt in Assam, is given in conclusion. This integrates the work done in West Bengal by the Indo-Stanvac Petroleum Project with that done in Assam by the Burmah Oil Company and its affiliates.