Glauconites from the Bahariya Oasis: An evidence for Cenomanian marine transgression in Egypt

Abstract

Although the Cenomanian sediments at the Bahariya Oasis, Egypt have been extensively studied, the origin of glauconites in these sediments is still questionable. In addition the paleoenvironmental significance of these glauconites was not discussed in these studies. In this paper, the glauconites that occur in the upper most part of the Bahariya Formation were subjected to petrographical, mineralogical, and geochemical studies to assess their origin and depositional environment.Glauconite appears as oval, rounded, fine to medium-grained (100–200μm), moderately- to well-sorted, pale green to yellowish green pellets in clayey and iron oxides matrix. X-ray diffraction analysis indicated that bulk samples are composed of glauconite, while clay fractions (<2μm) are mainly smectite. K2O contents (4.8–6.1%) as well as the distance between (001) and (020) reflections (11.3–11.8cm) suggested that the studied glauconite can be classified as slightly evolved or of low to moderate maturity. The occurrence of clayey matrix and patches of diffuse green glauconitic pigmentation in the matrix may support the autochthonous origin of the studied glauconite. On the other hand, the occurrence of glauconite as rounded, moderately- to well-sorted and well-defined grains along with the abundance of detrital quartz grains may suggest a detrital or extrasequential origin of these glauconites. This apparent controversy can be solved by considering the studied glauconite as parautochthonous glauconite in which the autochthonous glauconites have been transported for short distance from their place of origin. The abundance of smectite may also suggest the formation of these glauconites from detrital smectite precursor through diffusion of K from the seawater–sediment interface. Average ΣREE content in the studied glauconites is 484ppm and chondrite-normalized REEs patterns exhibit LREE enrichment relative to HREE as shown by (La/Yb)N ratios, that vary from 5 to 8 and slightly negative Eu anomalies are pronounced with Eu/Eu∗ from 0.7 to 0.8. The difference in the REE concentrations and patterns between the Cenomanian glauconites of the Bahariya Formation and the typical seawater can be attributed to the overprint of the smectite precursor on the REE contents and patterns in the studied glauconites. These glauconites are considered to represent a Transgressive System Tract (TST) in the upper part of the Cenomanian Bahariya Formation.