Geology and depositional history of Cenozoic sediments of the Bengal Basin of Bangladesh

Abstract

The Bengal Basin lies on the eastern side of the Indian subcontinent and occupies the northern part of the Bengal geosyncline. Sedimentation started with the breakup of Gondwanaland. Cretaceous fluvial and deltaic sedimentation was slowly taken over by open marine and deep-sea fan sedimentation. By the Mid Eocene, the basin was witnessing the maximum marine transgression and most of the stable shelf was in carbonate regime, as part of the extended Tethyan carbonate shelf. The deeper part of the basin was dominated by deep-sea fan sedimentation fed by clastic turbidites from the northeast with limited carbonate turbidites from the shelf. The collision of the Indian plate with the Tibetan plate and with the Burmese plate in the Miocene resulted in a rapid switch in sedimentation pattern in the Bengal Basin. The basin structure and sedimentation were both strongly influenced by the collision pattern of the plates and by the uplift of the Himalayas. With the Miocene collision of the Indian Plate, there was a change from flysch to molasse sedimentation. The present-day basin configuration with the Ganges-Bramaputra delta on the north and the Bengal deep sea fan on the south was established in the Pliocene. The stratigraphic and tectonic framework of the Bengal Basin suggest that the deposition of hydrocarbon source rocks was associated with the flysch sedimentation, whereas formation of traps and deposition of most of the reservoir rocks in the basin was associated with Mid-Pliocene tectonic movements and with the molasse sedimentation. The pre-Mid-Miocene sediments now deeply buried in the deeper part of the Bengal Basin and in the eastern folded belt are the major source of gas of the eastern folded belt of the basin.