Petro-geochemical Signatures as Evidences for Tectonic Setting, Palaeoweathering and Provenance of Eocene Kopili Sandstones, Meghalaya, Northeast India

Abstract

Spectacularly preserved yet least studied middle to late Eocene Kopili Formation in parts of East Jaintia Hills district, Meghalaya has been documented to understand the provenance, tectonic setting and palaeoweathering employing petrological and geochemical attributes. The Kopili Formation is characterized by alternations of fossiliferous marl-splintery grey to black shalesiltstone and fine to medium grained ferruginous sandstone with occasional coal streaks. Petrographically, Kopili sandstone has been grouped into two categories i.e. subarkose and sublitharenite. Geochemically, Kopili sandstone possesses high SiO₂ and relatively low Al₂O₃ and Fe₂O₃ confirming to the modal composition Q_72.70 F_6.57 RF_7.01 CT_3.12 Mx_2.28 M_2.07 HM_1.82. These are quartz-rich (SiO₂ 75.96%) clean sandstone depicting strong negative correlation of SiO₂ with rest of the major oxide except for CaO which is relatively weak. High SiO₂ is in conformity with quartz enrichment (73.50%), does indicate a passive (atlantic type) continental margin set up for the deposition of Kopili Formation. Neo-development of quartz and feldspar in parts reflect high degree of diagenesis. Chemical indices like CIA, CIW, PIA and ICV reveal intense weathering under semi-arid to semi-humid climatic conditions. Besides minor contributions from continent interior, sediments were largely derived from recycled orogen involving rhyolitic / granitic sources containing plagioclase feldspars in the range of An₀-An₇₀ as evident from geochemical attributes.