Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Abstract

Significant data on the structure, tectonics and stratigraphy of Ganga basin have been obtained from aeromagnetic, ground magnetic, gravity and seismic surveys and the deep drilling conducted in the basin during the last fifteen years, Based on these data, the Ganga basin has been defined as a major platform depression and classified into seven tectonic zones, viz., Monghyr-Saharsa ridge, East Uttar Pradesh shelf, Gandak depression, Faizabad ridge, West Uttar Pradesh shelf, Sarda depression and Delhi-Hardwar ridge. This classification is based on the continuation of major tectonic trends from the Peninsular shield into the Ganga basin, the variations in the total thickness of the sedimentary cover, and the basement configuration as deduced from different surveys.

The sedimentary cover over most part of the Ganga basin is essentially composed of two main stratigraphic cum structural sequences representing the two main sedimentary stages in the geological evolution of the basin. The oldest, probably corresponding to the Vindhyans, is represented by stable to unstable shelf sediments composed of quartz-arenite-limestone-shale alternations. The younger sequence, unconformably overlying the Vindhyans, corresponds to the Neogene terrigenous clastics (Siwaliks). The structural and stratigraphic data of these sediments have been discussed. The presence of a profound unconformity between these two groups of sediments representing a considerable time gap ranging from (?) late Palaeozoic to Paleogene is an important factor in deciphering the tectonic evolution of the Himalaya. However, towards the northernmost depressed parts of the Ganga Basin, the age of the additional thickness of sediments intervening between the two above mentioned groups remains uncertain.

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Abstract

The Pranhita-Godavari graben, located in the eastern Indian Peninsula, is aligned NW-SE and covers an area of nearly 40,000 sq. km. The graben exposes sediments ranging in age from Proterozoic to Lower Eocene. The oldest, the Pakhal Series, are disconformably overlain by Sullavai Series and together comprise at 1.5-2.0 km thick sequence. The Sullavai Series are unconformably overlain by approximately 6.5 km thick Gondwana sequence which is in turn, unconformably overlain by infra-trappean beds. Deccan Trap effusives contain inter-trappean beds and are unconformable with the underlying infra-trappean beds.

Genesis of hydrocarbons in non-marine sediments containing predominantly vegetal constituents is feasible. The Lower Gondwana sediments of the Pranhita-Godavari graben contain coal seams, and dispersed organic matter. Sediments of Talchir Stage are poor in organic matter content and their hydrocarbon-generating potential appears doubtful. The Barakars contain coal seams and are rich in dispersed organic matter. The carbon ratios, computed from proximate analyses of coals from various localities, indicate that the stage of maturity of organic matter varies within such limits which do not exclude the Barakar sediments from being considered as potential source rocks of hydrocarbons. The Kamthi sediments are dominantly arenaceous and contain small amounts of dispersed organic matter. Lack of relevant analytical data makes it difficult to assess their hydrocarbon-generating potential.

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