Abstract
The states of West Bengal and Bihar in northeast India are known to bear ‘A’-grade coal seams within Gondwana basins. An alluvium-covered area to the north of the Ajay river was considered to be prospective for the exploration of coal. The area was surveyed to determine the subsurface depressions in the basement (Archeans) underlying the Gondwana sediments. Gravity and magnetic surveys were mainly carried out over an area of 1900 km 2 with resistivity soundings taken at selected places. Geologically, the alluvium-covered area was known to be composed of rocks of different ages such as Archeans, Gondwana Triassic, Cretaceous (trap) and older alluvium in succession. Gondwana basins are formed over basement depressions or in the downthrown side of the faulted Archeans. Gondwana sediments lying over such basement subbasins are prospective areas for the exploration of coal. The gravity survey interpreted an 8-km wide and 20-km long depression, east of a linear boundary fault, known as the Pachami basin. This basin was drilled at several places and they intersected thick ‘A’-grade coal seams up to a maximum thickness of 150 m at the centre of the basin and at depths ranging from 15 to 887 m within the lower Gondwana sediments. The gravity survey broadly outlined the Pachami basin and several other basement depressions through gravity “low” contour closures. A forward modelling of a profile across the Pachami basin indicated a maximum depth of about 1.3 km to the basement at its deepest part. The basins are more perceptible on a residual gravity map. The magnetic map was vitiated through the presence of fluctuating high amplitude and high wavenumber anomalies due to a thick blanket of trap lying above the Gondwana sediments. Selected resistivity soundings could approximately interpret the shallow-basement depths, and the thickness and depths of the overlying trap. Further survey and drilling for coal is recommended in the area.
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