STRUCTURE OF THE CRYSTALLINE BASEMENT IN THE WEST-BENGAL BASIN, INDIA, AS DETERMINED FROM DSS STUDIES

Deep seismic sounding data were acquired in the West Bengal basin, Ind ia, along two profiles: (i) Bishnupur-Palashi-Kandi, along a line abou t 227 km long in the north-south direction and (ii) Taki-Arambagh, alo ng a line about 120 km long in the east-west direction. Seismic refrac tion and wide-angle reflection data were recorded by continuous profil ing using two 60-channel digital seismic units (DFS-V) with an 80 m ge ophone group interval and 4 ms sampling rate. These data were interpre ted in order to delineate the basement configuration. The 2-D models o f the seismic data both indicate a five-layer velocity structure above the Archaean crystalline basement (5.9-6.2 km s(-1)). A low-velocity layer (4.0 km s(-1)) is inferred immediately above the basement in the shelf region of the basin corresponding to the Gondwana sediments (Up per Carboniferous to Lower Triassic) below the Rajmahal Traps (Upper J urassic to Lower Cretaceous) of 4.6 to 4.8 km s(-1) velocity, which is also confirmed from the nearby well data. The results along the Taki- Arambagh profile and the drilling results at the Jaguli (J-1) well are used to investigate whether Gondwana sediments and the Rajmahal Traps exist in the deep part of the Bengal basin. An additional layer of ve locity 5.2-5.3 km s(-1), delineated in the Palashi-Kandi profile overl ying the basement, may correspond to the Singhbhum group of rocks of t he Proterozoic. A structural contour map of the basement prepared from the present results indicates a south-easterly dip of the basement in general. The depth of the basement on the stable shelf of the basin g ently increases to about 8 km and dips steeply, plunging to a maximum depth of 14 km in the deep basin. No structural high that can be relat ed to the 'Calcutta gravity high' is found in the basement around the Hooghly River.