Cretaceous—Eocene sedimentation, tectonism and biofacies in the bengal basin, India

Abstract

The Bengal Basin, located in the eastern part of the Indian subcontinent, is one of the thickest sedimentary basins in the world. The basin started downwarping in the Early Cretaceous and the sedimentation has since been continuous. From an initial transgressive marine phase, lasting almost up to end of the Eocene, the basin has evolved through a subsequent regressive marine and a later continental deltaic phase. In the absence of detailed information available from the deeper basinal parts, an analysis of the sedimentary and tectonic history and biofacies of the regions forming the western and northwestern shelves of the Bengal Basin pertaining to the initial phase of its evolution is attempted. This is achieved through the synthesis of all the geological information available on the basis of recent exploration and research conducted on the sequences developed in West Bengal, Meghalaya, Mikir Hills and Assam, India. The subsidence of the Bengal Basin was the result of differential adjustments of the crystalline basement along certain well-established trends and was initiated soon after the Gondwana tectonism, sedimentation and volcanicity. The earlier sediments were not typically marine but were more paralic to estuarine in nature. The entire Bengal Basin, except the area now comprising Assam, came under the influence of the first regional marine transgression during the Late Campanian—Maestrichtian. The coarse clastic sediments were deposited under shallow coastal marine conditions. A more widespread marine transgression took place during the Early—Middle Eocene which covered almost the entire shelf area including Assam. During this transgression, the sediments belonging to the orthoquartzite—carbonate association were formed in the shelf. The sea started receding in the Late Eocene during which the entire Bengal shelf came under the influence of heavy unloading of fine clastics. The top of this fine clastic shaly sequence is an important sedimentary datum in the Bengal shelf which marks the end of the first phase in the evolutionary history of the basin.