Geochemistry of shales from the Upper Miocene Samh Formation, north Marsa Alam, Red Sea, Egypt: implications for source area weathering, provenance, and tectonic setting

The Upper Miocene shales of the Samh Formation, North Marsa Alam along the Egyptian Red Sea coastal plain were analyzed for major and selected trace elements to infer their provenance, weathering intensity, and tectonic setting. The Samh Formation consists of sandstone underlies by shale and marl intercalations. The Samh shales are texturally classified as mudstones. Mineralogically, these shales consist mainly of smectite and kaolinite, associated with non clay minerals (abundant quartz and trace of plagioclase, microcline, and halite). Compared to post-Archaean Australian shales (PAAS), the Samh shales are highly enriched in SiO2, Al2O3, and Fe2O3 and depleted in TiO2, P2O5, Na2O, MgO, and K2O contents. The K2O/Al2O3 ratio values indicate predominance of clay minerals over K-bearing minerals. Trace elements like zirconium (Zr), Cr, Pb, Sc, Rb, and Cs are positively correlated with Al2O3 indicating that these elements are likely fixed in K-feldspars and clays. The Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), and Chemical Index of Weathering (CIW) values indicate moderate to intense weathering of the source material in a semiarid climate. The geochemistry results suggest that the Samh shales were deposited in a passive margin of a synrift basin and derived from felsic (granitic) source rocks. The inferred tectonic setting for the Upper Miocene Samh shales in Marsa Alam is in agreement with the tectonic evolutionary history of the Eastern Desert of Egypt during the Upper Miocene.