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

P. Kotoky ¹, Baby Baruah ¹, R. Borgohain ²

Affiliations
1 Regional Research Laboratory, Jorhat-785 006, Assam, IN
2 Department of Applied Geology, Dibrugarh University, Dibrugarh-786 004, Assam, IN

Abstract

X-ray diffraction (XRD) and infrared (IR) spectral analyses of the Lakadong Limestone, Meghalaya indicate calcite as its dominant mineral phase. Association of dolomite at the base and presence of silica at the top of the sequence is evidenced from the spectra. A significant relationship between CaO content and the peak intensity of IR-spectra is found and needs standardisation before it can be utilised for rapid estimation of quality of limestone and can be an alternative for different time consuming analytical procedures. Stable isotope of δ¹³C {0.63 to 1.12‰( av.0.87±0.01‰)} points to enrichment of heavier C isotope and δ¹⁸O values {-7.34 to-10.55‰ (av.-9.06±0.01‰) ) to lighter O isotope enrichment. Isotopic grid analysis confirms the marine origin for the limestone. Biochemical fractionation along with diagenetic reorganisation might have played critical roles in controlling the isotopic ratios of this limestone.

Keywords

Geochemistry, Palaeocene, Limestone, Stable Isotope, Lakadong, Meghalaya.

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R. Borgohain ¹, D. J. Khound ¹, D. Bhuyan ¹, J. Phukan ¹

Affiliations
1 KDM Project, Department of Applied Geology, Dibrugarh University, Assam, IN

Abstract

The Therria - Lakadong unit forms a part of the Upper Assam basin explored by the Oil India Limited. Petrographic study on 47 subsurface samples drawn from 3548.5m to 4377m in five well sections has revealed that the sandstone type is dominantly wacke, minor arenite and calcareous sandstone with quartz content varying from 31.4 to 82.7%. The other constituents are feldspar, rock fragments, cement, matrix, mica, opaques, glauconite and fossils. The limestone comprises sparry calcite, micrite, bioclasts, peloids, quartz, opaques and glauconite. The heavy minerals recorded are zircon, rutile, titanite, epidote, garnet, hornblende, enstatite, hypersthene, augite and kyanite with traces of diopside, sillimanite, brookite and monazite. The heavy mineral assemblage suggests that the sediments were derived from igneous, metamorphic and sedimentary rocks. Based on IR and XRD studies, the clay minerals were identified as kaolinite, illite, montmorillonite and chlorite, which together with glauconite, suggest a nearshore depositional environment. Biosparite indicates high-energy conditions in the environment. Although some of the original porosity is lost at depth due to pressure effects, the sandstone in general has good porosity.

Keywords

Petrography, Sandstone, Limestone, Clay Minerals, Heavy Minerals, Therria - Lakadong Unit, Upper Assam Basin.

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