Genesis of coexisting authigenic ferric illite and glauconite in the Deodongar Sandstones, Chattisgarh Basin, India: Unraveling redox dynamics of a Mesoproterozoic Sea?

This study investigates the co-occurrence of ferric illite and glauconite in the Deodongar sandstone of the Mesoproterozoic Chattisgarh Supergroup in India. Forming a Member within the stromatolitic Chandi Formation, these glauconitic arenites were deposited in a shallow marine setting, as revealed by field relationships. Electron Probe Micro Analyzer (EPMA) data shows that these ferric illites and glauconites are rich in magnesium and aluminum, although they exhibit considerable variation in iron and potassium content. Oxide cross-plots indicate that ferric illite and glauconite followed different geochemical pathways of formation. Textural observations indicate that these minerals formed over K-feldspar, quartz and chert fragments as authigenic phases. Mass balance calculations suggest that potassium released during the conversion of K-feldspar to glauconitic minerals may aid in transforming quartz/chert fragments to these minerals, though additional sources for iron, magnesium and aluminum ions are necessary. Thermodynamic calculations show that ferric illite formation is favorable than glauconite at 1 atmospheric pressure and 298 Kelvin temperature. Yet, occurrence of ferric illite and glauconite forming over adjacent substrates suggests that each substrate acted as a closed independent chemical system for mineral authigenesis.