LAUSR

Abstract An integrated field, petrographical and geochemical investigation reveals a shallow marine dys-oxic depositional setting for the glauconite at the top of the Mesoproterozoic Lower Quartzite Member of the Vindhyan Supergroup. A thorough facies and paleogeographic analysis indicates a storm-influenced, shallow subtidal depositional condition for the glauconitic sandstone, occupying the mid-level of a transgressive systems tract (TST) deposit, which is capped by the maximum flooding zone (MFZ). The X-ray diffractional parameters confirm the evolved character of the glauconite. The mineral chemical analysis indicates the glauconite is enriched in K2O, MgO and Al2O3 and depleted in Fe2O3(total), compared to the Phanerozoic variety. It formed by the pseudomorphic replacement of the K-feldspar, by the addition of Fe2O3 at consistently high content of K2O, unlike most Phanerozoic examples. The elevated level of H3SiO4, Cl, K, Na, Mg and Fe, possibly derived by the continental weathering and/or contemporary volcanism, facilitated the glauconitization process. The high content of Mg in the glauconite corresponds to the Mg-rich, Mesoproterozoic seawater. A dys-oxic depositional condition, associated with a major marine flooding surface, favoured the formation of glauconite. While the onset of anoxic regime caused the disappearance of glauconite and the deposition of pyritiferous and carbonaceous shale in the overlying condensed zone deposits.