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

Subir Sarkar ¹, Partha P. Chakraborty ¹, Pradip K. Bose ¹

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta-700 032, IN

Abstract

Breccia and pebbly massflow products are present in abundance in basin slope deposits of an unnamed marine Proterozoic fonnation in Maharashtra. Seven distinctive genetic modes have been identified and attributed to slump, slide, spring sapping and evaporite dissolution. The clasts are tabular in form and intraclastic in origin. The observations suggest caution against the prevailing tendency to equate occurrence of tabular intraclasts with shallow depositional regime. In order to ascertain their sedimentological significance, a clear understanding of their genetic processes is thus warranted.

Keywords

Sedimentary Structures, Carbonates, Proterozoic, Sedimentology, Maharashtra.

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Partha Pratim Chakraborty ¹, Subir Sarkar ²

Affiliations
1 Department of Applied Geology, Indian School of Mines, Dhanbad - 826 004, IN
2 Department of Geological Sciences, Jadavpur University, Kolkata - 700 032, IN

Abstract

Detailed facies analysis in late Proterozoic Rewa Shale reveals three physically distinctive facies that differ in their paleogeography. Between shoreface and outer shelf the facies types are distributed as (I) shale with thin siltstone/Fine sandstone interbeds of outer shelf, (II) shale interbedded with HCS bearing sheet sandstone of inner shelf within storm wave base and (III) sandstone interbedded with silty shale of distal shoreface paleogeography. High resolution facies succession analysis reveals three different orders of depositional cyclicity, the lowest of which may be correlated with third order depositional 'sequence'. Thin (av 3 5 cm) discrete levels of emergence with their invariable associations and sharp transitions from inner shelf to shoreface facies mark the subaerial discontinuities, which punctuate the apparently monotonous. Rewa Shale succession Far from the coastline on a siliciclastic ramp that lacks shelf slope break, these features, even though cryptic, can provide important clues for sea level falls in shale dominated successions.

Keywords

Depositional Cyclicity Emergence, Sequence, Rewa Shale, Son Valley, Central India.

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Asru K. Chaudhuri ¹, Somnath Dasgupta ², Goutam Bandyopadhyay ², Subir Sarkar ², P. C. Bandyopadhyay ², K. Gopalan ³

Affiliations
1 Geological Studies, Unit, Indian Statistical Institute, Calcutta, IN
2 Department of Geological Sciences, Jadavpur University, Calcutta 700032, IN
3 National Geophysical Research Institute, Hyderabad, IN

Abstract

The Proterozoic sequence around Adilabad, Andhra Pradesh comprises a distinct lithostratigraphic unit of 'Group' status and has been given the name Penganga Group. The Penganga Group has been classified into three Formations which are, in order of younging, Pranhita Sandstone, Chanda Limestone and Sat Nala Shale. The sequence is dominated by limestones and shales which were deposited in relatively deep water environment and, thus, contrast sharply with the conglomerate, sandstone and shallow water limestones of the Pakhal Group. Radiometric dating of fresh glauconites from middle part of the Penganga Group gives an age of 770 ± 30 Ma. Thus, the Penganga Group is the youngest of all other Proterozoic sequences in Pranhita-Godavari Valley.

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