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S U M M A R YDeep seismic sounding data were acquired in the West Bengal basin, India, along two profiles: (i) Bishnupur-Palashi-Kandi, along a line about 227 km long in the northsouth direction and (ii) Taki-Arambagh, along a line about 120 km long in the east-west direction. Seismic refraction and wide-angle reflection data were recorded by continuous profiling using two 60-channel digital seismic units (DFS-V) with an 80 m geophone group interval and 4 ms sampling rate. These data were interpreted in order to delineate the basement configuration. The 2-D models of the seismic data both indicate a five-layer velocity structure above the Archaean crystalline basement (5.9-6.2 km s-I). A low-velocity layer (4.0 km s-') is inferred immediately above the basement in the shelf region of the basin corresponding to the Gondwana sediments (Upper Carboniferous to Lower Triassic) below the Rajmahal Traps (Upper Jurassic to Lower Cretaceous) of 4.6 to 4.8 km s-' 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 velocity 5.2-5.3 km s-', delineated in the Palashi-Kandi profile overlying the basement, may correspond to the Singhbhum group of rocks of the 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 gently 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 related to the 'Calcutta gravity high' is found in the basement around the Hooghly River.

Characteristic latest Neoproterozoic and Early Paleozoic acritarchs and associated organic-walled microfossils are recorded from the sediments of Marwar Supergroup encountered in BGW-A well (Bikaner-Nagaur Basin) from 1123-481 m depth. Six distinct acritarch assemblages, broadly comparable with globally known Ediacaran (Vendian) and Cambrian assemblages are recognised. The recovered microfossils provide precise age for different units of the Marwar Supergroup whose ages, till now, were poorly understood due to absence or paucity of invertebrate and other mega and microfossils.Jodhpur Group (1123-1105 m), the basal unit of Marwar Supergroup, records abundant ornamented sphaeromorphs (Lophosphaeridium spp.) alongwith various species of Leiosphaeridia, suggesting Late Ediacaran age. Occurrence of small micrhystrids (Asteridium spp.) and appearance of Dictyotidium birvetense, Pterospermella solida and Annulum squamaceum in lower part of Bilara Group (1105-1081 m), suggests latest Ediacaran to early Early Cambrian age. Hanseran Evaporite Group (1068-907m) is marked by abundant Retisphaeridium dichamerum, Dictyotidium birvetense, Cristallinium cambriense, Comasphaeridium sp. cf. C. strigosum and Archaeodiscina umbonulata indicating late Early Cambrian to early Middle Cambrian age. Microfossil contents in Nagaur Group (907-727m) are very poor. The succeeding Upper Carbonate Sequence (727-481 m) shows abundant Cristallinium randomense, Cymatiosphaera crameri and Asteridium spp., along with other species of Cymatiosphaera and Cristallinium, and also includes the early Late Cambrian marker forms, such as Striatotheca loculifera and Dorsenidium (Veryhachium) minutum that suggest late Middle Cambrian to early Late Cambrian age. The recorded acritarch assemblages suggest Late Ediacaran to early Late Cambrian (ca 570-500 Ma) age for the Marwar Supergroup, with demarcation of Precambrian-Cambrian boundary within the lower part of Bilara Group. Associated microfossils indicate intertidal/foreshore depositional environment for Jodhpur and Nagaur groups, whereas inner neritic to subtidal for the Bilara, Hanseran and Upper Carbonate units. Occurrence of an Early Permian palynoflora in the succeeding sequence (481-427 m depth) indicates a major hiatus of ca 200Ma between the Marwar and overlying Bap/Badhaura sequences which involves the absence of middle Late Cambrian to Late Carboniferous sediments in this basin.Latest biostratigraphic data from Ujhani (Ganga Basin) and Bhander (Vindhyan Basin) sediments suggest that the above two sequences are closely comparable with the Marwar Supergroup, and show the prospective exploration challenges as do the Marwar Supergroup.

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