Himalayan Geology, Vol. 43 (1B), 2022, pp. 357-370, Printed in India
Lithostratigraphy, Sedimentary Facies, Petrography, Geochemistry and Depositional environment of the Permo-Carboniferous Lachi Formation, Sikkim Tethys Himalaya
V.C. TEWARI*, R.K. PRIYA, R.K. RANJAN
Department of Geology, Sikkim Central University, 6th Mile, Tadong, Gangtok -03592, Sikkim, India
*Email (Corresponding author): vctewari@cus.ac.in
Abstract: Lower Gondwana sequences of late Paleozoic Himalayan Gondwana is well preserved at the base of Tethyan Sedimentary Sequences (TSS) and known as Lachi Formation in northern Gondwana margin (Perigondwana), North Sikkim, northeast India. The main objective of the present study is to establish the depositional environment of the Late Paleozoic Lachi Formation on the basis of the detailed lithofacies analysis, petrographic and geochemical studies. Lithostratigraphy of the Lachi Formation of the Tethyan sedimentary sequence consists of massive diamictite at the base followed by alternate sandstone in middle and shales at the top of the succession. Massive diamictite and sandstones of Lachi Formation are predominantly composed of quartz grains (78-93%) with rounded to subrounded clasts. Matrix supported diamictites are mostly subangular to subrounded and of glacial origin. The diamictite and sandstone of Lachi Formation are mainly enriched with quartz grains and fall in the field of quartz arenites. Petrography of some grains reveal that they have occupied the field of sublithic arenites which is very close to the boundary of quartz arenites. Geochemistry of Lachi Sandstone reveals moderate to high SiO2 contents (80.48-93.09 wt %.) against all major elements which suggests that parent rock was enriched with silica minerals. The chondrite normalized REE pattern with high value of LREE/HREE ratio with negative anomalies also indicate that sediments were derived from felsic source. Dominance of Mn (171.08- 589.32ppm) and Ba (164.51- 671.23ppm) has strongly supported the shallow marine depositional environment. Chemical Index of Alteration (CIA) indicates a moderate to intense weathering under semi humid climate. Provenance and tectonic setting of the Lachi Sandstone Formation is interpreted on the basis of (major and traces elements) that they are derived from felsic source rocks and deposited in a passive continental margin. The weathering and sedimentary recycling of Lachi Sandstone was reflected by Th/U ratio which ranges between 3.17 to7.39 with an average of 4.8 ppm. Isotopic ratios of Lachi Shale varies between -24.6 ‰ and -26.5 ‰ (V-PDB) with low values of Total Organic Carbon (TOC) which varies from 0.13 to 0.19. The paleogeography and paleoclimate of the Indian subcontinent during Lower Gondwana has been discussed in the light of new results from the Tethyan Sikkim Himalaya, northeast India.
Keywords: Lower Gondwana, Lachi Formation, Glaciomarine, Geochemistry, Sikkim Tethys Himalaya, NE India
