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

Ajit Bhattacharyya ¹, S. K. Chanda ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta, IN

Abstract

Earlier studies on the geology of the Krol Belt in general have included little detailed information pertaining to the petrology and mineralogy of the Krol Group. The work reported herein represents a part of a comprehensive petrological study of the Krol Formations and provides information mainly about the petrology and probable environment of deposition of the basal unit of the Krol Group, namely the Krol sandstone, that overlies the Infra-Krols.

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S. K. Chanda ¹, Ajit Bhattacharyya ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta, IN

Abstract

There has been some disagreement between Medlicott and Matley as regards the stratigraphy of the Lameta-Jabalpur contact in the type area. On the basis of fieid evidences integrated with petrologic data, it is shown in the present work that there is a well-marked unconformity at this contact, and the paraconformity observed in the eastern part actually passes into an erosional unconformity of a greater magnitude towards the south-west. The "apparent presumption of an unconformity" by Medlicott is thus confirmed. Post-Jabalpur tectonism, visualised on the basis of this presumption of the unconformity, although a cause for Lameta sedimentation, did not effectively change the tectonic framework in the type area.

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S. K. Chanda ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta, IN

Abstract

The Lameta and jabalpur Groups, which are redefined and described in their type area, are separated by an unconformity at Dalpatpur. The unconformity grades to a paraeonformity of much lesser magnitude in the vicinity of jabalpur town. Local paleocurrents in the type area, directions of truncations and relative rates of change of stratigraphic hiatus across the unconformities and configuration of pre-trappean surface appear to be mutually related to each other. All these features suggest the existence of a westerly slope corresponding to Krishnan's' Narmada Trough' since the Jurassic period. While the Jabalpurs are the products of a typical flood plain in an interior basin, the Lametas appear to represent shallow marine sediments on the stable shelf. The terrigenous constituents of the Jabalpur Group are mainly derived from the pelitic metasediments in the east. The terrigenous constituents of the Lametas, on the other hand, represent a mixed assemblage of detritus derived from the local jabalpurs and pelitic metasediments. Diagenesis of the ]abalpur sediments mainly involves endogenic reconstitution. Exogenic cements in the lametas are mainly derived from the overlying Deccan Trap.

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S. K. Chanda

Abstract

No Abstract.

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S. K. Chanda ¹, Ajit Bhattacharyya ¹, Soumen Sarkar ²

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta 700035, IN
2 Geological Sciences Unit, Indian Statistical Institute, 203, B. T. Road, Calcutta 700035, IN

Abstract

An important outcome of recent research on carbonate diagenesis has been the increasing realization that compaction may be as important as cementation in the lithification of carbonate sediments. Absence of compaction-deformation features should no longer be relied upon as unique evidence against compaction in limestones. The paucity of such features arises from the fact that the load-bearing capacity of allochems (grains) is hardly exceeded in the normal course of lithification of carbonate sediments. Their load-bearing capacity seems to have been exceeded only in the presence of rigid bodies (nodules) or surfaces (hardgrounds or ernersion surfaces). Wherever this prerequisite is met, compaction-deformation may be set into motion in the adjacent sediments, irrespective of their environments of deposition.

So far as the mud-supported carbonate sediments are concerned, they may undergo autolithification entirely through solution-reprecipitation without the benefit of subaerial exposure. In the case of grain-supported carbonates too, there are indications that an appreciable amount of their cement may be derived from elsewhere in adjacent rocks undergoing deeper burial diagenesis within the same basin. While burial diagenesis seems normal, subaerial diagenesis is exceptional for carbonate sediments, other than those of the nearshore areas.

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