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

Ranjit K. Banerji ¹

Affiliations
1 Indian Institute of Technology, Kharagpur, IN

Abstract

The Lower Ariyalur Stage (Upper Cretaceous) cropping out in the north of Vridhachalam, has been sampled for detailed foraminiferal study. Nine species and subspecies and one new species of the genus Globotrunrana and several benthonic species are identified. On the basis of the vertical distribution of these species, the Lower Ariyalur Stage, earlier described as a [single uniform formation, is subdivided into five biostratigraphic zones and subzones, 'commencing from the? Late Turonian and continuing up to the Lower Maestrichtian. This sequence of strata is correlated with the well known Upper Cretaceous formations of North America, Trinidad, Middle East, Europe and Australia. The systematic description and evolutionary trends of the species and subspecies of Globutruncana are given. On the basis of percentage distribution of the planktonic forms, the depth of the basin is calculated. There is indication of a shallowing of the Upper Cretaceous sea at the end of the Globutruncana linneiana trlcarinata zone (Lower Maestrichtian).

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Ranjit K. Banerji ¹, Radha Mohan ¹

Affiliations
1 Institute of Petroleum Exploration, Oil & Natural Gas Commission, Dehradun, IN

Abstract

In this paper, an unified regional foraminiferal biostratigraphic zonation scheme for the Albian to Burdigalian sequence of the Cauvery Basin is presented. In all twenty-six zones and a number of sub-zones are delineated. Their typical sections, geographic extensions, important foraminifera and geologic age are described in detail. A number of faunal datum lines are also recognised. This type of zonation will serve as the basis of reference for the stratigraphic section, outcrop or sub-crop, of the entire Cauvery Basin as well as other sedimentary basins of India in terms of its precise and relative time-stratigraphic level.

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Ranjit K. Banerji ¹

Affiliations
1 Institute of Petroleum Exploration, Oil & Natural Gas Commission, Dehra Dun, IN

Abstract

The foraminiferal biostratigraphy of the marine Meso-Cenozoic sequence of Thanjavur sub-basin, situated in the south-western part of the Cauvery basin, is worked out in detail. On the basis of the distribution of well known planktonic and associated benthonic foraminifera, fifteen biostratigraphic zones ranging in age from Albian to Lutetian, are recognised. It is suggested that the Cretaceous-Tertiary boundary should be placed between Globotruncana gansseri - G. contusa assemblage Zone and Globorotalia velascoensis Zone in this sub-basin. The foraminiferal fauna of these zones are compared with those of the exposed marine sequence in the northern parts of the Cauvery basin. Certain aspects of inter-regional stratigraphic correlation and palaeoecology are discussed.

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Ranjit K. Banerji ¹, V. V. Sastri ²

Affiliations
1 Indian Institute of Technology, Department of Civil Engineering, P.O. Indian Institute of Technology, Powai, Bombay-400076, IN
2 Institute of Petroleum Exploration, Oil and Natural Gas Commission, Dehra Dun-248195, IN

Abstract

The paper deals with broad principles of quantified foraminiferal palaeoecology involved and their application in reconstructing the palaeoenvironments for different stratigraphic units (Upper Jurassic-Lower Palaeogene in age) exposed in the western part of the Cauvery Basin. The parameters used are the functions of structurally related characteristics and morphogenetic diversities of the foraminiferal fauna which are resultant of the environmental requirements, tolerance and preferences. The first sedimentary cycle started with fresh water deposits which soon gave way to brackish water and later continued under dominantly marine influence. The major transgressive phases during Neocomian-Albian, Senonian and Paleocene are better reflected in the quantitative bio-facies analysis. Along certain palaeomarginal highs, the environment favoured reef development where terrigenous influx was minimum. The strand-line progressively shifted since Lower Cretaceous times from west to east. The shift during Lower-Middle Eocene was more prominent and a delta front progressively started building with regression in the region bordering the western margin. The resultant sediments now cover a greater part of the exposed area and form the youngest exposed stratigraphic unit of this Basin.

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Ranjit K. Banerji ¹, R. M. Baruah ², S. C. Gupta ²

Affiliations
1 Indian Institute of Technology, Bombay 400076, IN
2 Oil and Natural Gas Commission, Sibsagar, (Assam) 785640, IN

Abstract

The exploration for hydrocarbons in the Bengal Basin has revealed the development of nearly 100m of a highly fossiliferous subsurface sequence near Galsi in West Bengal. This sequence has yielded a large number of planktonic and benthonic foraminiferal species. In all, twenty-two species belonging to seven genera and three families of planktonic foraminifera are identified and described. Based on their stratigraphical distribution, four assemblage Zones are distinguished which range in age from Zone N.4 to N.14 (Aquitanian to Langhian, Early to Middle Miocene). The microfaunal study suggests an oscillatory nature of the depositional basin varying from coastal marginal facies to short intervals of open marine inter-tongues within overall shallow marginal basin.

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Ranjit K. Banerji ¹

Affiliations
1 Geology Faculty, Indian Institute of Technology, Bombay 400076, IN

Abstract

The textural study of the sand grains of Sivaganga Formation indicates that sedimentation commenced in a fluvial environment passing through the lacustrine-marsh conditions and ending with the influence of tidal estuary. The clayey sediments in the upper part have yielded an arenaceous foraminiferal assemblage having a low dominance and diversity value belonging to the Neocomian-Aptian (Early Cretaceous) age. The sedimentary basin was formed as a result of downwarping of the crystalline basement along the prominent Eastern Ghat trend during Late Jurassic and Early Cretaceous. The basin established its fragmentary connections with the open sea situated at some distance towards the east and southeast.

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S. Ramasamy ¹, Ranjit K. Banerji ²

Affiliations
1 Department of Geology, University of Madras, Madras 600025, IN
2 Department of Earth Sciences, Indian Institute of Technology, Bombay 400076, IN

Abstract

A new litho unit-Terani Formation is proposed. The lower part of the Uttatur Formation is found to be homotaxially equivalent to the Dalmiapuram Formation. A new geological map based on these units is presented.

Keywords

Ariyalur Sequences, TiruchirapaJli Dist., Tamil Nadu, Petrography.

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