Depositional environment of Upper Paleocene – Middle Eocene series of the Lesser Himalaya, Central Nepal

Abstract

Paleocene–Eocene epoch was the turning point when the Indian subcontinent experienced maximum isolation before it collided with Eurasia. Intense sedimentation occurred in late Paleocene in the foreland basins of young Himalaya, marking the beginning of a major transgression on the Indian subcontinent. In central Himalaya (Nepal), the synorogenic stratigraphy recorded the depositional environment changes from salt-water to brackish-water and even to fresh-water condition, indicating slight and gradual regressive phase. We report petrography, U–Pb detrital zircons ages, whole-rock geochemistry data from sandstone in Cretaceous–Paleocene, Eocene, and lower Miocene of the Lesser Himalaya, central Nepal. The Eocene and Miocene sequences have small Neoproterozoic to Cambrian age zircon populations indicating multiple source terrains. The whole-rock geochemistry supports the unroofing history documented by U–Pb zircon ages, the succession belongs to foreland basin, an area between the active and passive continental margins tectonic setting. Petrography data indicates a recycled orogenic provenance.For the first time, we identify the Eocene coal was deposited in tropical and humid conditions based on fauna and flora, geochemistry, fission-track, and Zircon-Helium (ZHe) age. The organic geochemical and organic petrological data suggests the depositional environment of Jhadewa coal of central Nepal in anoxic terrestrial condition signifying a peat-swamp flood basin environment. Therefore, we suggest that the studied coal is formed from ancient submerged peat forests in swampy environments subjected to the geological forces of heat and pressure over millions of years and the tectonic collision might have played an essential role for burial and formation of lignite to bituminous coal.