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

A. B. Dutt ¹, Abhijit Mukhopadhyay ²

Affiliations
1 P481 Keyatola Road, Calcutta - 700 029, IN
2 12A, Bakul Bagan Row, Calcutta - 700 025, IN

Abstract

Gondwana basins of Peninsular India contain a thick pile of clastic sediments of predominantly fluvial origin. As expected in such a depositional milieu, the sedimentary pile is punctuated by numerous episodes of contemporaneous uplift, non-deposition and erosion. Most of these events are localised in nature and hence not correlatable from one basin to another. However, a few events of regional nature can be recognized. They comprise (a) basement surface with glaciated features like polishing, striations and frost wedging below the basal Talchir strata; (b) surface of angular and erosional unconformity between Permian/Triassic and Early Triassic/infra - Late Norian and (c) a major diastrophic episode resulting in reversal of palaeoslope between Late Jurassic and Early Cretaceous. Besides these, localized events represented by surfaces of contemporaneous subaerial exposure and oxidation showing characters of pedogenic (palaeosol) and lateritic (latosol) profiles have been recorded. These include Lower Barakar (Early Permian) of Hutar basin of Koel valley, Barren Measures (Late Permian) and Triassic/Jurassic contact of Rewa and Satpura basins, and Late Cretaceous Lometa Beds in Rewa Basin. Breaks represented by conglomerate beds occur in Early Permian Barakar strata of Talchir coalfield and Late Mesozoic Bagra Formation of Satpura basin. The palaeosols vary in thickness from 4 m to 25 m and are characterised by red brown clay with mottles, ischolar_mainlets, slickensides, ferruginous concretions and calcareous veinlets. The palaeosols are hydromorphic and are developed under poorly drained to well drained groundwater conditions. Even where the palaeosols are not present, the bedrock displays evidences of deep weathering. These probably represent the earlier topographic positive areas affected by physical weathering and intense groundwater activity.

Keywords

Gondwana Basins, Erosional Surfaces, Stratigraphic Gaps, Peninsular India.

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Abhijit Mukhopadhyay ¹

Affiliations
1 Geological Survey of India, Rajasthan Project-I, D-87, Meera Marg, Jaipur 302 006, IN

Abstract

The Jharia Coal Basin is composed of 2900 m of fresh-water Gondwana sediments deposited on the mature pediplained Archaean basement. The consistent fresh water features of the sediments indicate that the sedimentation kept pace with general subsidence. From the analysis of lithological attributes it can be postulated that there were longer intervals of rapid subsidence when fluviatile sediments were deposited; these alternated with shorter intervals of relative tectonic quiescence or slower rate of subsidence, favourable for the development of the regional coal seams. The subsidence was maximum during the Upper Barakar and Barren Measure times.

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Abhijit Mukhopadhyay ¹, P. N. Chaudhuri ², A. L. Banerji ²

Affiliations
1 Geological Survey of India, Rajasthan Project-I, 0-87, Meera Marg, Jaipur 302006, IN
2 Geological Survey of India, Coal Wing, 4, Chowringhee Lane, Calcutta 700016, IN

Abstract

An example of contemporaneous intrabasinal faulting in a Gondwana basin is described. The intrabasinal Jurabaga fault of Ib River Coalfield has been identified as contemporaneous fault on parameters followed in the Gulf Coast Province of U.S.A. namely, (a) abrupt increase in thickness of strata, and (b) increase of throw with depth on the downthrown side with respect to the upthrown side of the fault. The developmental history of Jurabaga fault in space and in time is reconstructed.

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S. K. Ghosh ¹, Abhijit Mukhopadhyay ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, IN

Abstract

Paraconglomerates, with pebble- or boulder-sized phenoclasts of laminated clay in a matrix of medium grained massive. sandstone, occur toward the top of a turbidite sequence in the Talchir Formation at the N. W. corner of the Jharia Coal Basin in eastern India. The soft-sediment folds within the shale phenoclasts are dominantly recumbent. An analysis of the geometry of the structures suggests that a mass of sand with interlayered clay units was liquefied or fluidized. This in its turn generated a plastic mass flow in the course of which tenacious clay yielded slabs which were subsequently rotated and deformed. When the down-slope movement of the sand body had slowed down or had come to rest on the basin floor, the piled up mass of soft sediments was flattened under its own load and the contortions of the clay phenoclasts acquired their recumbent character.

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