DISTINCTIVE GEOCHEMICAL CHARACTERISTICS AND PALAEOGEOGRAPHIC IMPLICATIONS OF PRECAMBRIAN GLAUCONY: EXAMPLES FROM PURANA BASINS IN INDIA

Authigenic, sand-sized glaucony pellet, forming close to the seafloor and containing more than 8% K₂O, is indicative of low rate of sedimentation, and hence, commonly tied-up with condensed deposits and transgressive systems tracts. Glaucony facies is most common around 200 to 300 m of depth in modern oceans and the same bathymetric interpretation is usually extrapolated for ancient glaucony. However, in absence of fecal pellets and bioclasts, the two most common substrates in Phanerozoic, glaucony has altogether different palaeogeographic implications in Precambrian. Detailed mineral chemical investigation of shallow marine originated authigenic glaucony from four Precambrian basins in India, Vindhyan, Chhatisgarh, Bhima and Pranhita-Godavari, reveals common geochemical characteristics of the mineral. The glaucony is characterized by high K₂O, Al₂O₃ and MgO and low total Fe₂O₃ content. Usually up to 20% of framework quartz and feldspar grains in sub-arksoic wave-reworked sandstones are replaced by glaucony. Textural evidence indicates that incipient glaucony forming within cleavages and fractures of detrital grains evolved to pellets by moderate to extensive alteration of the substrate. Geochemically each variety of glaucony is distinct from the other. K₂O content of all glaucony, including the incipiently-developed variety remains high (av. > 8%). XRD peaks of glaucony correspond to the highly evolved type notwithstanding its low total Fe₂O₃ content. Evolution of glaucony took place with addition of total Fe₂O₃ at a constant K₂O content. During the course of maturation Al₂O₃ was released while total Fe₂O₃, MgO and SiO₂ were added to the glauconite structure. Glaucony originated in a high a_K+ and high a_Si+ pore water environment by pseudomorphic replacement of feldspar and deformed quartz, aided by continental weathering-related supply of Fe, Mg, K and Si. Because of availability of sand-size sediments as suitable substrates and the general low rate of sedimentation inferred for Precambrian epeiric seas, glaucony formation was restricted in shallow marine depositional during the Precambrian. The geochemical characteristics and palaeogeographic implications of authigenic Precambrian glaucony are found to be completely different from those of their Phanerozoic counterpart.