GeoScienceWorld
GeoRef Record 2009-010742

Color, mineralogy and composition of Upper Jurassic West Siberian glauconite; useful indicators of paleoenvironment

Color, mineralogy and composition of Upper Jurassic West Siberian glauconite; useful indicators of paleoenvironment

Latitude & Longitude N50°00′00″ – N70°00′00″ and E70°00′00″ – E112°00′00″

Abstract

Changes in color of Upper Jurassic glauconite of the Georgiev Formation, in the West Siberian Basin, in Russia, are related to changes in physicochemical conditions that caused glauconite maturation and alteration, driven by regional paleoenvironmental evolution. Maturation produces dark green (bluish) glauconite formed from Fe-rich smectite by increasing the content of Fe (super 2+) together with K. Alteration produces brown rims and cracks that are enriched in Al and depleted in Fe and K with respect to the glauconite cores. The change from a yellowish green to dark green color in progressively more mature glauconite is explained by light absorption induced by enrichment in octahedrally coordinated Fe (super 2+) relative to the total Fe, associated with the progressive decrease in the proportion of Fe (super 3+) -rich smectite interleaved with glauconite. The brown color in alteration rims is due essentially to light scattered by nanometric inclusions of Fe oxyhydroxides. These, together with residual Al-rich glauconite and a subordinate Fe-rich smectite, constitute reaction products formed by leaching of K and Fe (super 2+) , and by the oxidation of yellowish green glauconite cores. Berthierine formed later in the brown rims on Al-rich glauconite, and pyrite formed as a result of drowning of the platform during the latest Jurassic-earliest Cretaceous and sedimentation of black shales under increasingly reducing conditions.