ABSTRACT

Clay mineral assemblages in relation to the chemical characteristics of Bajocian-Bathonian shales at G. Al-Maghara, North Sinai, Egypt, have been determined to infer the climatic conditions and the factors regulating their distribution. Mineralogical analyses using XRD revealed the dominance of kaolinite relative to illite and smectite. Quartz, calcite, goethite, feldspars and plagioclase represent the non-clay minerals encountered. Chemical data reflect the enrichment of chemically immobile elements (Al, Ti, Zr and Sc) and the depletion of mobile elements (Fe, Ca, Na, K and Mg). Major element concentrations reflect the dominant mineralogical composition and the weathering history of the source area. Moreover, the enrichment of trace elements (Sc, Zr and partially Cr) could be attributed to the source rock chemistry and its weathering history. High values of the Chemical Index of Alteration (CIA) (Al₂O₃/(Al₂O₃ + CaO + K₂O + Na₂O) × 100) were found. The high proportion of kaolinite associated with the enrichments of immobile particularly major elements (Al and Ti) and the high values of CIA suggest intensive chemical weathering of the source rock under tropical to subtropical humid climatic conditions. The abundance of kaolinite relative to smectite was also influenced by hydraulic sorting and relative sea level changes.