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

P. Sanyal ¹, D. S. Bhattacharyya ¹, D. Dasgupta ¹

Affiliations
1 Department of Geology and Geophysics, IIT, Kharagpur, IN

Abstract

The Arkasani granite gneiss occurs as detached elongate outcrops close to the northern margin of the Chakradharpur granite gneiss. Both show imprints of the same deformation and same textural and structural characters. While the Chakradharpur gneiss ranges from tonalite - granodiorite - granite - alkali feldspar granite, the Arkasani gneiss is an alkali feldspar granite. Major element compositions of the two are either gradational or overlapping. The former has higher Na₂O values and the latter higher K₂O values. The two groups of gneisses are genetically related.

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Subhajit Sinha ¹, Sumit K. Ghosh ¹, Rohtash Kumar ¹, R. Islam ¹, P. Sanyal ², S. J. Sangode ³

Affiliations
1 Sedimentology Group, Wadia Institute of Himalayan Geology, Dehradun - 248 001, IN
2 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur - 721 302, IN
3 Department of Geology, University of Pune, Ganeshkhind, Pune - 411 007, IN

Abstract

The Neogene Siwahk fluvial succession (12 77-4 48 Ma) of the NW Himalayan foreland basin, east of Ravi River, are deposited under varied Tectono-Climatic conditions. Petrography and geochemistry of the sediments were carried out to understand the relative roles of tectonics and climate in this basin.

The sandstones are classified as sub-litharenites with subordinate arkosic-graywacke. The dental components particularly the rock fragments are consistent with the inferred source area presently disposed towards the north of the depocenter and suggest that the Higher, Lesser and Lower Tertiary formations supplied detntus since 12 77 Ma. The Th/Sc vs. Zr/Sc ratio indicates sediment recycling. The Cr and Ni, negative Eu anomaly, LREE enrichment and moderately flat HREE in the mudstones collectively suggest dominant contribution from felsic igneous rocks beside contribution from basic, sedimentary/metamorphic hithology. Near consistent nature of the dental modes and geochemical parameters through time suggest unvarying source since 12 77 Ma.The present study also infers that the source areas uplift owing to tectonic activity of Chad Thrust at least by 12 77 Ma with a perceptible activity of MBT after 8 Ma.

The 5¹⁸0 variations in soil carbonates reveals ongoing intense monsoon system since 12 77 Ma followed by a phase of aridity at 91 Ma. Presence of fresh and weathered feldspar, limestone, basic volcamcs and mica, both in humid and arid phase indicate rapid deposition and preservation. Despite the climatic turnover from humid to arid, distinctive changes in the framework mineral compositions over time is not discernable.

The petrographic and geochemical data reveals that the near similar source area, relatively rapid transport, moderate chemical weathering and sediment recycling controlled the composition of the sediments of the HFB in response to tectonics and climate.

Keywords

Climate, Provenance, Tectonics, Himalayan Foreland Basin, Siwahk, Kangra.

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