--> Mineral Chemical Analysis of Glauconites Within the Upper Cretaceous Karai Shale Formation for Evaluation of Stratigraphic Condensation

AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

Mineral Chemical Analysis of Glauconites Within the Upper Cretaceous Karai Shale Formation for Evaluation of Stratigraphic Condensation

Abstract

This study investigates origin, evolution, compositional and structural variability of glauconites within the depositional and stratigraphic framework of the middle Cretaceous Karai Shale Formation in the Cauvery basin in the south India. The roughly 320 m-thick, shelf-originated Karai Shale comprises a transgressive systems tract at base containing abundant glauconitic shale, phosphorite nodules and calc-arenite beds and a highstand systems tract at top with predominant calc-arenite beds with subordinate glauconitic shale. A detailed mineral chemical investigation of glauconite within the condensed deposits of the Karai Shale Formation outcrops reflects wide spectrum in chemical composition related to its origin and evolution in different substrates, stratigraphic condensation and post-depositional alteration. Fe- and Mg-rich glauconite, comprising up to 60% of the sediments occurs as replaced form of fecal pellets, as infillings within pores and chambers of bioclasts including those of foraminifera, ostracoda, bryozoan and algae, and as altered form of mica exhibiting vermiforms. Authigenic precipitation of K- and Fe-poor glauconite, followed by addition of Fe and K into the lattice and concomitant release of Al and Si explains origin of glauconite pellets and infillings; glauconite vermiforms originates in partly degraded mica with minimal chemical alteration. Glauconite pellets and vermiforms exhibit sharply defined alteration zones along peripheries forming rims, and in proximity to cracks or cleavages with reduced K2O and Fe2O3 (total) and enhanced Al2O3 and SiO2, related to late stage meteoric water actions. Cores of glauconite pellets and unaltered zones of vermiforms reflect ‘evolved’ characteristics with >6% K2O, typical of a condensed section, while other glauconite varieties occurring at the same stratigraphic level exhibit ‘slightly evolved’ nature, not in consonant with condensation. Increasing abundance of glauconite pellets from the bottom to the top of the TST, accompanied by slight increase in K2O within their cores, reflects the effect of stratigraphic condensation on evolution of glauconite. High Fe2O3 (total) content of glauconite in the Karai Shale Formation is possibly related to upwelling. Detailed mineral chemical analysis enables us to distinguish stratigraphically significant glauconite within the Karai Shale Formation from the rest notwithstanding its wide compositional range.