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

N. Gladwin Gnana Asir ¹, S. Ramasamy ², J. S. Armstrong-Altrin ³, J. Madhavaraju ⁴, V. Stephen Pitchaimani ²

Affiliations
1 People's Action for Development, 2/29, Sebastian Kovil Street, Vembar - 628 906, IN
2 Sedimentology Lab, Department of Geology, University of Madras, Chennai - 600025, IN
3 Centre de Investigaciones en Ciencias de la Tierra, Universidad Autdnoma del Estado de Hidalgo, Ciiidad Universitana, Carretera Pachuca-Tulancingo km. 4.5, Pachuca, Hidalgo, 42184, MX
4 Universidad Nacional, Autonoma de Mexico, Institute de Geologia, Estacion Regional del Noroeste, Hermosillo, Sonora, MX

Abstract

An 8.8m of consolidated core obtained below 20m of unconsolidated marly sediments near the Tuticorin coast was subjected to detailed stratigraphic, petrographic and geochemical analyses. A lithological succession with eight minor Litho-Units in ascending order has been established: Shell limestone, lower sandy shell limestone, calcareous conglomerate, middle sandy shell limestone, algal limestone, upper sandy shell limestone, calcareous sandstone and calcrete. The petrographic types are mostly the variants of wackestone, boundstone, arenite and calcrete. CaO is high in shell limestone, algal limestone and sandy shell limestone and proportionately SiO₂ is low in these litho-units. Algal limestone is enriched in MgO content, The conversion of high Mg-calcite to low Mg-calcite, aragonite inversion as well as mixed zone diagenetic setting resulted in depletion of both δ¹³C and δ¹⁸O, The petrographic evidence and assemblage of the juvenile gastropods, pelecypods, bryozoans, algae and benthic foraminifers suggest a fluctuating inner neritic to a moderately high saline lagoonal depositional environment. The absence of sparry calcite indicates an early marine to mixed zone diagenetic settings and the shell limestone unit experiences a moderate burial diagenesis while the upper units experience a shallow burial diagenesis. A Late Tertiary age is assigned to the shell rich limestone unit.

Keywords

Depositional And Diagenetic Environments, Petrography, Stable Isotopes, Shallow Core, Tuticorin Coast, Tamil Nadu.

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J. S. Armstrong-Altrin ¹, J. Madhavaraju ², S. Ramasamy ², N. Gladwin Gnana Asir ²

Affiliations
1 Centro de Investigaciones en Clencias de la Tierra, Universidad Autonoma del Estado de Hidalgo, Ciudad Universitaria, Carretera Pachuca-Tulancingo km 4 5, Pachuca, Hidalgo- 42184, MX
2 Department of Geology, School of Earth and Atmospheric Sciences, University of Madras, Guindy Campus, Chennai - 600 025, Tamil Nadu, IN

Abstract

Quartz grains from Kudankulam Formation were studied using Scanning Electron Microscope(SEM), which brought out eighteen microtextures. The quartz grams of Kudankulam Formation show different types of conchoidal fracture, arcuate steps, V-shaped patterns, pits and sub angular to Sub-Rounded outline. The presence of conchoidal fracture and arcuate steps suggest that these quartz grains were derived from the nearby crystalline source. The microtextures such as V-shaped patterns, pits, straight and curved scratches are characteristic of marine environment.

Keywords

Quartz Grains, Microtextures, Kudankulam Formation, Provenance, Depositional History, Tirunelveli District, Tamil Nadu.

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J. Madhavaraju ¹, S. Ramasamy ¹, S. P. Mohan ¹, S. M. Hussain ¹, N. Gladwin Gnana Asir ¹, V. Stephen Pitchaimani ¹

Affiliations
1 Department of Geology, School of Earth and Atmospheric Sciences, University of Madras, Guindy Campus, Chennai- 600 025, IN

Abstract

Petrography, fluid inclusions and surface texture studies have been carried out on the Nimar Sandstone, which have provided fruittful results on pravenance and depositional environment The detrital omposition of sandstones plotted in the Q-F-L diagram suggests cratonic orrgin The modal percentages of different types of quartz plotted in the diamond diagram Indicate that these sediments were derived from the low-grade metamorphic rocks Quartz grains show randomly distributed primary inclusions, small trails of pseudosecondary inclusions and needle-shaped solid phases also support that the source rocks are mainly low-grade metamorphc rocks The surface textures on quartz grains reveal the presence of twenty-two distinct surface textures Based on their origin, these textures are grouped into three types viz I) thirteen features that show mechanical origin, II) five features that exhibit mechanical and/or chemical origin and III) four features that reveal chemical (precipitational)origin The quartz grans exhibit conchoidal fractures, arcuate steps, and fractured plates/planes, which resulted during the liberation of these grains from the crystalline formations They also exhibipt petransportational features such as pedological overgrowths, silica globules, flower and pellicles, which are indicative of short transportation and rapid deposition of these sediments Vs, straight and curved scratches are more common on the quartz grains from the upper Nimar Sandstone (whtch are characteristic features of marine environment), whereas the lower frequency of these features on the quartz grains from the lower Nimar Sandstone suggest fluvia depositional environments.

Keywords

Petrography, Quartz grains, Fluid inclusion, Surface textures, Nimar Sandstone, Provenance, Depositional environment, Bagh Beds, Madhya Pradesh.

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J. Madhavaraju ¹, S. Ramasamy ¹, S. P. Mohan ¹, N. Gladwin Gnana Asir ¹

Affiliations
1 Department of Geology, School of Earth and Atmospheric Sciences, University of Madras, Guindy Campus, Chennai 600 025, IN

Abstract

An ooid-rich limestone band of Sillakkudi Formation is encountered in a well section near Ariyalur. The ooids are believed to be deposited in a marine regime. They contain berthierine as the major mineral and haematite, siderite and quartz as minor minerals. They exhibit alternate rings of berthierine and haematite which shows that these minerals were developed under changing physico-chemical conditions. In the present paper, the petrographical characters, mineralogy, origin and depositional environment of the ooids are discussed.

Keywords

Ooids, Berthierine, Sillakkudi Formation, Depositional Environment, Ariyalur, Tamil Nadu.

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