Mg-rich ferric illite in marine transgressive and highstand systems tracts: Examples from the Paleoproterozoic Semri Group, central India

This paper focusses upon two glauconitic sandstones in the Paleoproterozoic Deoland Formation and Chorhat Sandstone, both belonging to the Semri Group, central India. In both the cases glauconitic minerals occur in sandstones deposited in the marine realm, within a transgressive systems tract (TST) for the former unit and within a highstand systems tract (HST) for the latter. The proportion of glauconitic minerals increases in the paleo-offshore direction. Petrography reveals selective early glauconitization of detrital K-feldspars along their fringes, cleavages and the fractures created by volume expansion during progressive alteration, leading to the generation of peloids with small relics of the precursors. XRD and mineral chemistry reveal a structure typical of glauconite, and more akin to Mg-rich ferric illite. The mineral chemistry of the glauconitic mineral phases remains the same whether the glauconitization process was incipient or at an advanced stage.These findings contrast with the previously held belief that ferric illite is confined to terrestrial or marginal marine sediments, and concurs with recent observations that the mineral can form in the open sea, but with high Mg. Since there are no ferro-magnesian minerals in association with these Vindhyan glauconitic sandstones, the seawater appears to be the only source for the Mg and Fe required. The general low rate of sedimentation on Proterozoic open shelves would have permitted ferric illite formation even within a HST, as represented by the Chorhat Sandstone.