Himalayan Geology, Vol. 43 (2), 2022, pp. 457-470, Printed in India

Clay Mineralogy, Petrography and Geochemistry of Cretaceous siliciclastic sedimentary rocks of Giumal Formation, Spiti region, Himachal Pradesh, Tethys Himalaya

HEENA¹*, SHAIK A. RASHID¹, JAVID AHMAD GANAI²

¹Department of Geology, Aligarh Muslim University, Aligarh, India

²Department of Earth Sciences, University of Kashmir, Srinagar, India

*Email (Corresponding author): heenakhan1591@gmail.com

Abstract: Geochemical and petrographic investigations of early to middle Cretaceous Giumal Formation (consisting of shales and sandstones) in the Spiti region, indicate that sandstones are subarkose to sublitharenite in composition and texturally poorly sorted, subrounded to subangular in shape with moderate sphericity. According to modal analysis, these sandstones originated from the craton interior and recycled orogenic region. Shale samples show more or less similar concentrations with that of PAAS (Post-Archean shales from Australia) with depletion of CaO and Na₂O as a result of intense weathering. Giumal sandstones show similar concentration of SiO₂ and TiO₂ relative to UCC (Upper Continental Crust) but are strongly depleted in Al₂O₃, Na₂O, Fe₂O₃, and MgO. The depletion of Na O relative to the UCC values suggests severe loss of plagioclase feldspar during chemical weathering of the source region. Weathering indices (chemical index of alteration, CIA and plagioclase index of alteration, PIA) show that the source rock in Giumal Formation has undergone moderate to strong chemical weathering. The scatter of Giumal Formation data on the A-CN-K ternary diagram suggests a granitic weathering”. X-ray Diffraction studies indicate that the prominent clay minerals in the Giumal shales are illite, chlorite and montmorillonite along with quartz, muscovite, alkali feldspar and calcite. The paleoclimate in the source area seems to be mostly semi-humid. The Giumal Formation indicate passive margin tectonic setting and dominant felsic provenance. Further, the occurrence of quartz grains with undulose extinction and polycrystalline quartz, high SiO₂, Al₂O₃, alkalis and overlapping compositions of Giumal shales with Vaikrita gneisses strongly substantiate the inference that the sediment of the Giumal Formation s may have been derived from a combination of felsic igneous rock (granitic) and associated metamorphic rocks, probably occurring south of the Spiti basin (i.e. Vaikrita Group of rocks).

Keywords: Tethys Himalaya; Giumal Formation; Sandstone; Black shales; Tectonic setting; Paleoweathering.