Thin-section Petrography and Heavy Mineral Study of sandstones of the Barail Group of rocks occuring around Sonapur area of Jaintia Hills, Meghalaya, India

Rupjyoti Hazarika; Ranjan Kumar Sarmah; Jogendra Nath Sarma

doi:10.51710/jias.v39iII.193

Authors

Rupjyoti Hazarika Department of Applied Geology, Dibrugarh University, Dibrugarh, Assam- 786004
Ranjan Kumar Sarmah Department of Applied Geology, Dibrugarh University, Dibrugarh, Assam- 786004
Jogendra Nath Sarma Department of Applied Geology, Dibrugarh University, Dibrugarh, Assam- 786004

DOI:

https://doi.org/10.51710/jias.v39iII.193

Keywords:

Detrital modes, heavy mineral assemblage and provenance, Oligocene Barail Group

Abstract

The study area represents a part of the South Shillong Shelf, Meghalaya, where sandstones belonging to the Barail Group (Eocene-Oligocene) are well exposed. Thin section petrography and heavy mineral investigations have been undertaken to assess the geological significance of the sandstones. Framework grains of the Barail sediments comprise of quartz (56.98 to 71.91%), feldspar (1.87 to 7.35%), mica (1.07 to 9.72%), lithic fragments (2.20 to 8.41%) and matrix (9.20 to 20.43%). The detrital composition of the sandstones comprises primarily of quartz grains, angular to sub-rounded, fine to medium grained and moderately sorted in nature. The sandstones are classified as subarkose, feldspathic graywacke and lithic graywacke. Provenance discrimination triangular plots of QFL and Q_mFL_treflect that the detritus were mostly derived from cratonic interior sources. The Diamond diagram indicates that the Barail sandstones were derived from middle and upper rank metamorphic source.

Heavy mineral study of these sandstones indicates the presence of zircon, tourmaline, rutile, sillimanite, kyanite, staurolite, garnet, epidote, sphene, hypersthene, hornblende, chlorite, chloritoid, andalusite, apatite and opaque minerals. The percentages of most stable heavy minerals zircon, tourmaline and rutile vary from 6.11to 21.61, 2.60 to 6.56 and 2.42 to 7.57 respectively. The ZTR maturity index varies from 15.15 to 34.37, which reflects that the sandstones are mineralogically immature. The petrographic and heavy mineral study of the sandstones suggest that the detritus was probably derived from Shillong Massif where Precambrian metamorphic rocks of pelitic and arenaceous composition with plutonic bodies were exposed around the shelf margin. As the Shillong Massif comprises of Precambrian metamorphic rocks of pelitic and semitic (arenaceous) composition with intrusive plutonic bodies.

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References

Basu, A., Young, W. S., Suttner, J. L., James, C. W., and Mack, H. G., 1975, Re – evaluation of the use of Undulatory Extinction and Polycrystallinity in Detrital Quartz for Provenance Interpretation , Journal of Sedimentary Petrology, v. 45, no. 4, p. 873 – 882.

Dickinson, et., al., 1983, Provenance of North American Phanerozoic Sandstones in Relation to Tectonic Setting, Geological Survey of America Bulletin, v. 94, p. 222 – 235.

Dott, R. L., Jr., 1964, Wacke, Greywacke and Matrix – What Approach to Immature Sandstone Classification? Journal Sedimentary Petrology, v. 34, p. 625 – 632.

Hubert, J.F., 1962. A Zircon- tourmaline-rutile maturity index and the interdependence of the composition of heavy minerals assemblages with the gross composition and texture of sediments. Journal Sediment Petrol., v. 32, p. 440-450.

Ingersoll, R. V., Bullard, T. F., Ford, R. L., Grimm, J. P., Pickle, J. D., and Sares, S. W., 1984, The Effect of Grain size on Detrital Modes: A Test of the Gazzi-Dickinson Point-counting Method, Journal of Sedimentary Petrology, v. 54, p. 103–116.

Pettijohn, F. J., 1984, Sedimentary Rocks 3rd ed., CBS Publishers and Distributors, New Delhi, 628 p.

Rangarao, A., (1983): Geology and Hydrocarbon Potential of a part of Assam-Arakan Basin and its adjacent area: Petroleum Asia Journal, p. 127-158.

Sein, M.K. and Sah, S.C.D. (1974): Palynological demarcation of Eocene-Oligocene sediments in the Jowai-Badarpur Road Section, Assam: In: S.C.D. Sah and A.T. Cross (Eds.): Symposium Stratigraphy and Palynology, Lucknow 1971. Spec. Publ. 3, p. 99-105., Birbal Sahni Institute of Palaeobotany, Lucknow.

Singh, M. P., and Singh, A. K., 2000, Petrographic characteristics and depositional conditions of Eocene coals of platform basins, Meghalaya, India: International Journal of Coal Geology, V. 42, p. 315-356.