Middle Permian Depositional Environments Inferred from Illite Crystallinity and Illite Chemistry Indices of X-Ray Diffraction Patterns, Saudi Arabia

Abstract The mineralogical composition along with the illite crystallinity and the illite chemistry indices are generally controlled by a combination of several factors such as provenance, diagenesis, and depth. These mineralogical parameters also can be used to reflect a short-term climate change. Knowing that depositional environments and sedimentary sequences are strongly affected by climate, the characterization of the Middle Permian sediments from Central Saudi Arabia is investigated using illite crystallinity and illite chemistry index to test whether this technique can be used to differentiate between different sedimentary facies. The upper Member generally consists of coastal/fluvial, fluvial, fluvial/aeolian and dune deposits representing a series of siltstones, argillaceous sandstones, and cross-bedded sheet sands with interbedded clean sandstones. The lithofacies along with the depositional environment of the aforementioned facies are readily distinguished using petrography. The illite chemistry index is determined by the ratio of peak intensities at 5 A to 10 A, indicating the degree of weathering, the provenance, and a proxy for indicating climate change. Highly weathered Al-rich illite (Chemistry Index >0.5), for example can reflect a strong acidic environment where hydrolysis is prevalent, while un-weathered Fe/Mg-rich illite (Chemistry Index <0.5) can reflect a dry arid alkaline environment. The illite chemistry index of the studied samples varies from 0.51 to 0.89, with the lower values recorded for coastal/fluvial facies and the highest values for fluvial/aeolian to dune facies. The crystal particle size of a clay mineral is generally determined by the Crystallinity degree parameter where higher degrees of crystallization are defined by a low crystallinity value. In general, the illite crystallinity is based on the measurement of the full width at half maximum (FWHM) at a 10 A diffraction peak which can be used to classify levels of crystallinity from excellent (FWHM<0.4), to good (0.4-0.6), to moderate (0.6-0.8) and finally poor (FWHM>0.8). In this study, the values vary from 0.39 to 0.95 with lower values that are identified for fluvial/aeolian to dune facies and the highest values recorded for fluvial sedimentary facies. Low values of Illite crystallinity could be indicative of low temperature and dry climatic conditions. Results indicate that illite crystallinity and illite chemistry indices are changing according to the depositional environments as determined by petrography. Illite crystallinity vs Illite chemistry diagram of typical samples shows the ability to differentiate between 3 zones, including fluvial/aeolian to dune, coastal/fluvial and fluvial facies groups.