Geochemistry and Depositional Environments of the Tertiary Clays in Makkah and Rabigh Quadrangles, West Central Arabian Shield, Saudi Arabia

Abstract

The present work deals with a detailed chemical study on the clay mineral assemblages characterizing some clay deposits, shales and mudstones, which differ in their texture, colour, lithology, and depositional environments. Samples selected from seven Tertiary formations located in Makkah and Rabigh quadrangles are chemically analyzed; some are used as environmental discriminators. The distribution of major and some trace elements in 77 samples has been investigated and the results obtained were tabulated and graphically illustrated. A wide range of variation is shown in the results of both major and trace constituents. This is due to their variable content of clay mineral constituents and the abundance or paucity of other admixtures, e.g. quartz, feldspar and minor heavy mineral grains. The local occurrence of minor gypsum and carbonate veinlets in some formations and ferrugination in others are also effective. A positive correlation exists in some formations between Fe2O3 and each of MnO, Cr, Ni, Cu & Zn; between each of Sr & Cu versus CaO; and in most cases between Ga and Al2O3. Whereas SiO2 shows a negative correlation with each of Al2O3 and L.O.I . Most of trace elements entered the depositional basin in close association with clay minerals and Ca-Fe-bearing detritus. The examined clay minerals and their trace elements are mainly source controlled (e.g. granitic and some pelitic rocks) and, an environmental illustration using Ga, B & Rb in a ternary diagram would 98 R.J. Taj et al. indicate transitional to marine depositional environments for the studied clay deposits.