Origin and geochemical characterization of the glauconites in the Upper Cretaceous Lameta Formation, Narmada Basin, central India

This study presents geochemical characteristics of glauconites in estuarine deposits within the Maastrichtian Lameta Formation in central India. Resting conformably over the Bagh Group, the Lameta Formation consists of ∼4–5 m thick arenaceous, argillaceous and calcareous green sandstones underlying the Deccan Traps. The sandstone is friable, medium- to coarse-grained, well-sorted and thoroughly cross-stratified, and contains marine fossils. Detailed petrography, spectroscopy and mineral chemistry indicates unique chemical composition of glauconite with high K2O, MgO, Al2O3 and moderate TFe2O3. Glauconite is formed by the replacement of K-feldspars, initially as stringers in the cleavages and fractures of feldspars. Incipient glauconite subsequently evolves fully, appearing as pellets. Fully-evolved glauconite pellets often leave tiny relics of K-feldspar. XRD exhibits characteristic peak of 10 Å from basal (001) reflection of glauconite, indicating the “evolved” character. The K2O content of glauconites in the Lameta Formation varies from 5.51% to 8.29%, corroborating the “evolved” to “highly-evolved” maturation stage. The TFe2O3 content of glauconite varies from 12.56% to 18.90%. The PASS-normalized-REE patterns of glauconite exhibit a “hat-shape” confirming the authigenic origin of glauconites. The slightly-negative to slightly-positive Ce anomaly value and the moderate TFe2O3 content of glauconite agree well with a suboxic, estuarine condition. The replacement of K-feldspar by the glauconite contributes towards the high K2O content. Compositional evolution of glauconites in the Lameta Formation is similar to those observed in many Precambrian sedimentary sequences. Keywords: Glauconite, Estuary, Mineral chemistry, Cerium anomaly, Redox condition, Lameta Formation, Narmada Basi