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 shale-siltstone 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 Q72.70 F6.57 RF7.01 CT3.12 Mx2.28 M2.07 HM1.82. 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.