Unusual seawater composition of the Late Cretaceous Tethys imprinted in glauconite of Narmada basin, central India

Abstract

AbstractA detailed investigation of a glauconite bed within the Late Cretaceous Bryozoan Limestone Formation of the Bagh Group in central India, as well as the study of existing records, reveals the existence of a ‘glauconitic sea’ along the margins of the Palaeo-Tethys Ocean during the Late Cretaceous Epoch. The authigenic green mineral formed abundantly on shallow seafloors unlike in its modern, deep-sea counterpart. We present an integrated petrographical, geochemical and mineralogical investigation of the glauconite within Late Cretaceous transgressive deposits to highlight its unique geochemistry with moderate Fe2O3 and high Al2O3, SiO2, MgO as well as K2O contents. X-ray diffractional parameters identify the ‘evolved to high evolved’ nature of the glauconite while Mössbauer spectroscopic study reveals the dominance of Fe3+ compared to Fe2+ in the atomic structure. The rare earth elements (REE) pattern of glauconite reveals moderate light-REE/heavy-REE (LREE/HREE) fractionation and weak negative Eu anomaly. The Ce anomaly of the glauconite indicates a sub-oxic diagenetic condition. We propose that Late Cretaceous glauconites formed within a shallow marine depositional setting across the Tethyan belt because of enhanced supply of K, Si, Al, Fe, Mg cations through continental weathering under the extant greenhouse climate.