Upper Eocene glauconites from the Bahariya depression: An evidence for the marine regression in Egypt

•Microscopic investigations of the studied glauconite suggest a parautochthonous origin for such deposits.•XRD pattern structurally classified the Upper Eocene glauconite as ordered glauconite.•The Upper Eocene glauconite obtained from the Bahariya Oasis is classified as evolved glauconite with average 7.47% K2O.•The Upper Eocene glauconite formation was started with marine transgression and terminated by marine regression.

Glauconite deposits at the Bahariya Oasis are reported as Cenomanian and Upper Eocene deposits. The Upper Eocene glauconite deposits have received little attention in comparison with the Cenomanian counterpart. In the present study, glauconite deposits belonging to the Hamra Formation were investigated in terms of petrography, mineralogy and geochemistry to determine their origin and demonstrate their significance as proxies for the paleoenvironmental conditions.Petrographically, glauconite occurs as green to yellowish green, oval, sub-oval, rounded, fine to medium-grained (150–400 μm), moderately sorted pellets set in clayey matrix. Mineralogically, the studied deposit consists mainly of glauconite in association with quartz, feldspar, hematite, alunite, halloysite and calcite, whereas clay fractions (<2 μm) are composed essentially of glauconite with small amounts of illite-smectite mixed layer and kaolinite. Chemically, the studied glauconite contains K2O at average of 7.47%. Thus, it can be classified as evolved glauconite. The morphology of glauconite as oval, sub-oval, rounded pellets with deeply penetrating fractures on some grain surfaces, the moderate sort of these pellets and the occurrence of argillaceous matrix consisting of glauconitic plasma, illite-smectite mixed layer and kaolinite are considered criteria for the parautochthonous origin of the Upper Eocene glauconite. Moreover, the geochemistry of rare earth elements along with stratigraphy and the occurrences of many glauconitic ironstone horizons within the studied section proposed that the deposition of the studied glauconite, at depth of 100 m, started with marine transgression and terminated by marine regression during the Upper Eocene.