Petrography and Geochemistry of Sandstones of Eocene Kopili Formation, Shillong Plateau: Implications on Paleo-weathering, Provenance and Tectonic Setting

Abstract

Geochemical and mineralogical studies were carried out on the sandstones of the Eocene Kopili Formation, Shillong Plateau to evaluate paleo-weathering, provenance and tectonic setting. The study area is located on the bank of the Lubha River along the Badarpur-Jowai road section. The Kopili Formation is represented by a sequence of alternation of shales and sandstones. The sandstone beds are usually thin, fine grained, ferruginous and mostly parallel laminated. The thickness of the sandstone beds increases towards the top with decreasing numbers of shale beds. Petrographically the sandstones are dominated by monocrystalline and polycrystalline quartz followed by feldspar, mica and rock fragments. The matrix content does not exceed 15% and hence the sandstones can be classified as arenite. The Kopili sandstones are classified as quartz arenite to sublitharenite, and arkose to sublitharenite based on their petrographic and geochemical parameters respectively. Arenitic composition and high SiO2/Al2O3 ratio indicate that the Kopili sandstones are highly mature. The weathering indices mostly indicate low to moderate (except 49D sample) degree of chemical weathering under arid to semi humid climatic conditions. The major and trace element-based diagrams and their ratios indicate that the Kopili sandstone received sediments from felsic dominated source with minor contribution from basic source rocks and are deposited in tectonically passive margin setting.