Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Bidhan Chandra Mandal ¹, A. S. M. Woobaidullah ²

Affiliations
1 Geological Survey of Bangladesh, 153 Pioneer Road, Shegunbagicha, Dhaka 1000, BD
2 Department of Geology, University of Dhaka, Dhaka 1000, BD

Abstract

Depositional history and tectonic evolution of the Bengal Basin commenced with the collision between Asia and Greater India in Late Paleocene (55 Ma). Sedimentation within the basin was activated during the Eocene in response to the simultaneous collision of the Indian and Burmese plates in the east and the Indian and Tibetan plate in the north. The dominant compressional force for development of the eastern fold belt resulted from subduction of the Indian plate beneath the Burmese plate in eastern part of the basin. From Miocene onward, the Indian plate has been subducting beneath the Burmese plate with a counter clockwise rotational movement, while the Burmese plate was overriding the ' Indian plate with a clockwise rotation. Consequently, the overall trend of folded structures varies from NNW in the southern part to NNE in the northern part of the eastern fold belt. This variation in structural style along with the presence of longitudinal thrust faults and transverse faults suggests that the Indian plate initially pushed towards east and then turned northeastward during the Cenozoic. Structural style of the eastern fold belt is characterized by the presence of a series of elongated en-Echelon folds, which are tight and steep in the east, while towards the west they tend to be gentler and box shaped. Presence of longitudinal and cross faults along most of the folds suggests their hybrid origin. In seismic sections these faults appear to be heterogeneous in nature; and are believed to represent different time domain of their formation. Analysis of the stratigraphic succession in seismic profiles of selected structures suggests a close relationship between sedimentation style and tectonic framework within the eastern fold belt. Occurrence of overpressure zones in the lower Surma Group is believed to be primarily due to undercompaction caused by rapid accumulation of fine-Grained clastics and not tectonic deformation. Interpretation of seismic data and comparison of subsurface structural depth contour maps of different horizons reveals heterogeneous flexuring rates. The westward shift of the culminations and closures of different horizons from bottom to top reflects the imprints of time variant evolution of the folded structures. Hence it is thought that intensity and direction of the tectonic force responsible for development of compressional deformations within eastern fold belt of the Bengal Basin was time variant and contemporaneous with sedimentation.

Keywords

Sedimentary Tectonics, Subduction, Eastern Fold Belt, Surma Group, Bengal Basin, Bangladesh.

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Bidhan Chandra Mandal ¹, A. S. M. Woobidullah ², D. K. Guha ³

Affiliations
1 Geological Survey of Bangladesh, Dhaka, BD
2 Department of Geology, University of Dhaka, BD
3 Gana University, Savar, Dhaka, BD

Abstract

The Semutang anticline is located in the northwestern side of the eastern fold belt (EFB) of Bangladesh. The anticline is typically an elongated brachy anticline, 25 km long and 5 km wide. The fold is wide and gentle, of having dips of 4-5° and amplitude of about 600 m. The axis of the fold runs along NNW-SSE direction. The geological map of the area has been constructed using Remote sensing data like Landsat Imagery (TM), SPOT and aerial photos. The different Beta values of the different sections of the area show that the axial trace of the anticline is a curvilineat unit. The subsurface demarcation of different formations and members have been made from the geophysical well log data and seismic and gravity data analysis. Six stratigraphic conventional sequences and several prominent faults are identified in the subsurface. The faulting and bedding flexuring intensity of the Upper Bhuban is more complex than the Lower Bhuban. The southern plunge of the anticline is more faulted and the faulted blocks are occasionally down thrown than that of adjacent beds. On the basis of seismic data, it is identified that the core of the Middle Bokabil is slightly chevron type but in the eastern side, it is monoclinal. The nature of crisscross complex faults of both Upper and Lower Bhuban indicate the evidence of shale flow or diapiric activities. Two potential hydrocarbon zones are identified from the well log information data.

Keywords

Structural Geology, Semutang Anticline, Eastern Fold Belt, Stratigraphy, Bengal Basin, Bangladesh.

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