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

Tapan Chakraborty ¹, Asru K. Chaudhuri ¹

Affiliations
1 Geological Studies Unit, Indian Statistical Institute, 203, Barrackpore Trunk Road, Calcutta 700035, IN

Abstract

The Rewa Group of the Son Valley has been reclassified into five formations, i.e., Panna Shale, Asan Sandstone, Jhiri Shale, Drammondganj Sandstone and Govindgarh Sandstone in the ascending order. While two basal formations are developed only in the central and eastern part of the northern outcrop belt, the upper three are much more extensive than the underlying formations. Drammondganj Sandstone and Govindgarh Sandstone are newly defined formations, and the two constitute the 'Upper Rewa Sandstone' of earlier classifications. The scarp section at Drammondganj is the type area of the former, while the latter has its type area on the northern slope of the hills south of Govindgarh.

Analysis of palaeocurrent in Drammondganj Sandstone, trend of thickness variation as well as of the regional stratigraphic relations collectively point to a northeasterly slope of the marine Rewa basin. The basin was replaced by a westerly sloping fluvial basin during the late phase of Rewa sedimentation.

Keywords

Stratigraphy, Proterozoic, Rewa Group, Palaeogeography, Vindhyan Basin, Madhya Pradesh.

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Joydip Mukhopadhyay ¹, Asru K. Chaudhuri ²

Affiliations
1 Department of Geology, Presidency College, 86/1 College Street, Kolkata - 700 073, IN
2 Geological Studies Unit, Indian Statistical Institute, 203 B.T. Road, Kolkata - 700 108, IN

Abstract

Stratigraphic classification of apparently monotonous lithographic limestone of the Proterozoic Penganga Group leads to the understanding of the depositional setting and palaeogeography of a deep-water limestoneshale succession. The Penganga Group around Adilabad unconformably overlies granitic basement, and includes three formations, namely, the Pranhita Sandstone (c. 25 m), the Chanda Limestone (c. 250 m), and the Sat Nala Shale (>2000 m). The Pranhita Sandstone is a cross-stratified subarkosic sandstone deposited in a shoreface environment. The Chanda Limestone developed as a deep-water, below wave-base platform. It is essentially micritic, and includes a number of slope-related autoclastic limestone mass-flow deposits, a lentil of glauconitic sandstone emplaced as submarine fan deposit (c. 25 m), and two regionally persistent horizons of deep-water bedded chert interstratified with manganese ore. The Sat Nala Shale is a deep-water, brown shale without any siliciclastic coarser than mud. On the basis of stratigraphically-controlled variations in colour, bedding character and composition, the Chanda Limestone can be subdivided into a number of mappable (1:25,000) units, that can be organized into three formal members, the Bhimsari, Bilari and Ramai Members, and one informal Brown heterolithic member. The members were deposited in different parts of the platform, at different stages of its evolution, under varied state of oxidation and reduction. Detailed mapping of the units reveals the presence of two major thrust faults that divide the succession into a southern autochthonous package successively over-ridden by a para-autochthonous package and an allochthonous package, occurring in the northern and northeastern part of the area. The autochthonous package includes sandstone and shale of the Pranhita Sandstone deposited in shoreface and inner shelf environments. The para-autochthonous package is dominated by lithographic limestone of the Bhimsari and Bilari Members, and a thin horizon of the Sat Nala Shale. The allochthonous package is dominated by slope to basinal deposits of the Ramai Member and the Bhimsari Member, and a thick sequence of the Sat Nala Shale. This package also includes the stratified bedded chert and manganese ore indicating open ocean up welling circulation. The attitude of the thrust planes and the vergence of thrust-related folds suggest southerly propagation of thrust sheets. The allochthonous northern package was transported from a terrain beyond the northern limit of the study area, implying that the Penganga sea was open towards north. Northward opening of the basin along with upwellingrelated bedded chert and manganese deposits point to the existence of an open marine basin in the central part of peninsular India, north of the Dharwar craton.

Keywords

Proterozoic, Lithographic Limestone, Stratigraphic Classification, Thrust, Palaeogeography. Penganga Group, Adilabad, Andhra Pradesh.

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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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