Bogra Graben

Bogra Graben located on the Western Foreland Shelf and contains discontinuous subcrops of rocks belonging to the Gondwana system. The succession encountered includes sedimentary and igneous rocks ranging from Permian to Lower Cretaceous in age. Damuda beds occur in the Bogra Graben where it is overlain unconformably by the Rajmahal Stage. The Bogra Graben also shows the rajmahal stage with reduced thickness, obviously as a result of erosional events. The thickness of the Sibganj Formation is reduced to 120m in Singra-1x well and to a mere 90m in Kuchma-x1, both located in the Bogra Graben. Subcrops of the tura formation are known from the graben feature near Jamalganj. Thicknesses of the Tura Formation recorded from Singra-1x well, Kuchma-x1 and Bogra-x1 wells all located from the Bogra Graben, are 360, 270 and 169m respectively. In Bogra-x1, a well drilled close to or within the boundary fault zone of the Bogra Graben, Oligocene strata were not identified by means of micropalaeontology; however, a palynostratigraphic re-examination of the samples originating from the nearby Kuchma-x1 well revealed the presence of Oligocene strata. In Bangladesh a portion of the Western Foreland Shelf deposits of the Middle Miocene age have been encountered in the wells Kuchma-x1, Bogra-x1 and Singra-1 in the Bogra Graben.

The Bogra Graben is assymetric in cross section with the steeply southeastwards dipping Main Boundary Fault located on the northwestern edge, while the southeastern fault or fault zone seems to be a flexure. Thus the structure is a SW-NE trending halfgraben with a Lower and Upper Gondwana fill of at least 1,300m thickness. Cyclic deposition of alternating sandstone, carbonaceous shales and coal beds indicates the intensity of faulting and the rate of subsidence. Bogra Graben is the down thrusted block of the bogra fault and shows thicker development of the Sylhet Limestone than the uplifted segment. This implies vertical movements along the fault and subsidence within the Bogra Graben during deposition of the Sylhet Limestone. The oldest deposits known within the bengal basin are the coal bearing strata of Lower Gondwana encountered in halfgraben structures on the Western Foreland Shelf of the trough, such as the Bogra Graben and the Jamalganj area. Numerous coal seams have been encountered in several boreholes in the Bogra Graben and in the Jamalganj area. [Sifatul Quader Chowdhury]