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

Kailas Dilli ¹, C. N. Rao ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, IN

Abstract

A thick (770m) pelitic section ranging in age from Quaternary to Middle Miocene encountered at the DSDP site 218 in the Bengal Fan has been investigated for the burial diagenesis of clay minerals based on x-ray diffractometry and partial chemical analysis. The clay minerals are dominated by illite, with minor amounts of montmorillonite, chlorite and kaolinite. All the clay minerals are descrete and detrital derived from the Himalayan hinterland. The clay mineral transformations are largely confined to compaction attendant on the escape of pore water and adsorption of K and Mg in the interlayer positions resulting in better organised illite, montmorillonite-illite and montmorillonite-chlorite mixed layers at depth. The sediments are considered to be in the early diagenetic stage of Dunoyer de Segonzac (1970).

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C. N. Rao ¹, K. Srinivasan ², Amit Roychowdhuri ³

Affiliations
1 Department of Geology and Geophysics, I.I.T., Kharagpur, IN
2 Knoxville, Tenesse, US
3 Oil and Natural Gas Commission, Agartala, IN

Abstract

Phyllites from the Cumbum Formation are exclusively made up of 2 M polymorph of illite. The 2 M polymorph, the most stable phase in the temperature range of 200-300°C, is comidered to be due to its heritage from the source area, having been derived from high-grade metamorphic Eastern Ghat and Dharwarian provenances. The Illite Crystallinity Jndex shows progressive improvement with depth as well as nearness to the Pb-Zn mineralised zone. The crystallinity Index averaging 0.4°2θ is consistent with anchizone of metamorphism. The progressive increase in the crystallinity and size with depth and in the proximity of the mineralized zone suggest that the mineral suffered post-depositional recrystallisation effected by the depth of burial, uplift and folding in the eastern margin of the Cuddapah basin and subsequent hydrothermal activity concomitant with the Pb-Zn mineralisation at Bandlamotu.

Keywords

Illite, Cumbum Formation, Cuddapah, Andhra Pradesh.

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Haile Michael Fentaw ¹, C. N. Rao ², D. K. Sengupta ²

Affiliations
1 Ethiopian Institute of Geological Survey, Addis Ababa, ET
2 Department of Geology and Geophysics, I.I.T., Khargpur, W. B, IN

Abstract

The clay deposits at Malti in Purulia District, West Bengal have formed on phyllites of probable Precambrian age.

X-ray diffraction, differential thermal and Infra-red spectral studies suggest that these clay deposits contain kaolinite as the predominant clay mineral. The kaolinite shows two distinct varieties of crystal perfection-one the highly crystalline with Hinckley Indices of well over 0.8 (8 samples) and the other of comparatively low crystallinity with Hinckley Indices of less than 0.8.

The poorly crystalline samples show strong low temperature endothermic peaks at 200°C in DTA curves suggesting the presence of some inter-layer water. The presence of hydrated halloysite has, however, not been conclusively established.

Keywords

Clay Minerals, Kaolinite, Malti, Purulia Dist, West Bengal.

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Siddhartha Sankar das ¹, C. N. Rao ¹

Affiliations
1 Department of Geology and Geophysics, I.I.T., Kharagpur-721 302, IN

Abstract

The Channuria limestone is characterized by grey-black colour, bedding structure, fine grain size and authigenic pyrite cubes. Petrographically a minimicrite, the rock exhibits a texture composed of dark crinkled and light coloured laminae of lime mud. Pyrite has discordant relationship with laminae, whereas silt-sized terrigenous particles are present in the interlaminar areas. Mineralogically the limestone is made up of low Mg-calcite and quartz. Trace element distribution tndicates that the original mineralogy at the time of deposition was calcitic in nature and its distribution in the rock was controlled by the depositional/diagenetic conditions as well as the detrital clastics supplied into the basin. The depositional environment of the limestone is interpreted as shallow marine lagoonal condition and the site of deposition was very near to shore. The dysaerobic to anaerobic condition which is attributable to the restricted circulation of water and the concomitant slow sedimentation rate were responsible for the development of authigenic pyrite and grey-black colour of the limestone.

Keywords

Sedimentary Petrology, Geochemistry. Proterozoic, Chhattisgarh Basin, Madhya Pradesh.

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I. K. Kaul ¹, C. N. Rao ¹, T. Sanyal ¹

Affiliations
1 Department of Geology and Geophysics, I.I.T., Kharagpur 721302, IN

Abstract

A new technique to determine the homogenization temperatures of fluid inclusions is described. The technique is based on the principle of optical density and involves no ambiguity in recording homogenization temperatures unlike in the standard heating stage method where because of problems of visual clarity of liquid phase disappearance, accurate measurement of homogenization temperatures becomes difficult and unreliable. A comparison of the two methods shows that the present technique gives somewhat higher values than those based on heating stage method.

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Kailas Dilli ¹, C. N. Rao ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur, 721 302, West Bengal, IN

Abstract

Drill core samples of DSDP sites 241 and 250 in the Somali and Mozambique Basins respectively have been examined to understand the nature and extent of burial diagenesis in the deep-sea pelitic sediments of the western Indian Ocean based on X-ray diffractometry and partial chemical analysis. The clay mineral suite is almost identical in both the basins with montmorillonite being the dominant mineral. Illite and kaolinite are present in minor amounts white chlorite is present only in traces. The sediments are in early diagenetic stage. The apparent breakdown of kaolinite and illite and the adsorption of K by the clay minerals has resulted in montmorilloniteillite mixed layers being the only phase present at depth. The mineral trends observed suggest that diagenesis is indeed initiated at much shallower depths as proposed by Matter.

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