Resistivity imaging of strata and faults in Bangladesh
Abstract:The Ganges-Brahmaputra-Meghna Delta, the largest in the world, is subject to deformation by active tectonics and dynamic river systems. It lies near the juncture of the Indian, Eurasian and Burmese plates and is being overthrust by both the Shillong Massif and the Indo-Burman Ranges. There are multiple major and minor active faults in Bangladesh, many of which are buried by the sedimentation. For example, the Madhupur tract is a Pleistocene upland in the middle part of Bengal Basin. Whether it is a passive interfluve of the river system or a tilted and tectonically uplifted block has been debated for decades. The Tippera Surface, in Comilla at the eastern part of the basin, is composed of uplifted and oxidized Holocene strata and overlies buried anticlines of the Indo-Burman fold belt. Furthermore, the rivers are subject to migrations, avulsions and other changes in course. The last major avulsion of the Brahmaputra River was only ~200 years ago. During the sea level fall in the last glaciation the major rivers created large incised valleys. In much of the exposed uplands there was the development of a weathered clay surface. This now forms a clay layer separating the Pleistocene and Holocene strata in large parts of Bangladesh.
We use electrical resistivity surveying and hand-drilled borehole lithological data to better understand the subsurface discontinuities and structures. The resistivity system consists of an 84 electrode array powered by 2 car batteries and is capable of imaging lithologies to ~100m depth, similar to the depths of the boreholes used to calibrate the data. We extend our previous work on the western margin of the Madhupur Tract with additional lines on the eastern flank of Madhupur. Resistivity lines along the exposed Lalmai anticline in Comilla image the now tilted Holocene-Pleistocene clay layer. Additional lines along the subsurface continuation of the anticline provide additional information on the subsurface lithologies associated with the complex interplay of fluvial dynamics and active tectonics. We will present the latest images and interpretations.