Origin, Depositional Setting and Stratigraphic Implications of Palaeogene Glauconite of Kutch

A combined sedimentological, stratigraphical, mineralogical and geochemical investigation on Palaeogene glauconites of Kutch address some of the crucial issues related to origin of glauconite and its depositional and stratigraphic significance. Glauconite occurs within fossiliferous green shale units of Naredi, Harudi and Maniyara Fort Formation. It occurs primarily in two modes, either as an altered form of fecal pellets or as infillings within the bioclasts. The origin of glauconite may be best explained by direct precipitation of Fe-rich glauconitic smectite within the pores of bioclasts and fecal pellets followed by its maturation to glauconitic mica by the addition of K. Fecal pellet is a more favourable substrate than bioclasts for glauconitization because of easy percolation of pore water within and availability of relevant ions in pellets. Strong biostratigraphic control allows us to clearly establish that relatively high rate of sedimentation (>27 m/Ma) discourage glauconite formation in the Fulra limestone while low rate of sedimentation rate (<5m /Ma) allowed its formation. Occurrence of glauconite exclusively within the transgressive systems tracts and its absence within the highstand systems tracts corroborates the requirement of low rate of sedimentation. Formation of mature glauconite in shallow marine open as well as in protected lagoon suggests that glauconite may be a poor indicator of depositional environment, despite the fact that it forms in modern deep marine conditions. However, positive Ce-anomaly in all glauconites suggests that dys-oxic depositional setting encourages glauconite formation. Absence of glauconite within the unfossiliferous and emerged red shales, immediately overlying and underlying the glauconitic green shales suggest that oxic condition discourages glauconite formation.