THE APPLICATION OF GEOCHEMICAL AND MINERALOGICAL STUDIES OF CRETACEOUS SHALES IN WEST SIBAIYA AREA, EGYPT TO INVESTIGATE THEIR TECTONIC AND SEDIMENTOLOGICAL ORIGIN WITH ENVIRONMENTAL IMPACTS

Abstract

Sibaiya area is located at the southwestern corner of the Qena bend, Nile Valley, Egypt. The area iscovered by Campanian to Maastrichtian rocks composing a number of shale layers in three formations. Thepresent investigation comprises insights in the geochemical composition and clay minerals included in theCretaceous shale–bearing sediments in Sibaiya west. Fifteen representative trench samples were collectedfrom a vertical lithostratigraphic section including all the geological formations (Qusir Shale Memberof Nubia Formation, Duwi Formaton and Dakhla Formation) in present area. They were subjected tomineralogical and chemical analyses using modern methods and application of the proper techniquesof data processing and analyses.The data were utilized to investigate the tectonic province of the source rock of Sibaiya shales bydisplaying the analyzed SiO2% against log (K2O/Na2O) on the tectonic discrimination diagram. Thisapplication obviously indicated that the source rock is of the arc tectonic province. In order to confirm thisorigin, trials were also made to construct an upper crust normalization pattern of Sibaiya shale–bearingsediments using the chemical analyses of major and trace elements. The diagram obviously indicates thatthe tectonic province of the source rock cannot belong to the upper continental crust (UCC). In addition,the display of the TiO2% against the Al2O3% diagram and correlation with the UCC, basalt and other globalshales, of known tectonic origin, has also indicated that the origin of Sibaiya shales is comparable withbasalt. This also supports a more basic source and infers an arc province origin. The immobile elementvalues, specially the relatively higher values of trace elements (Cr, V, Ni and Ti) provide a strongevidence that supports a basic origin of Sibaiya shales and also suggests a dominance of mafic volcanicsource in general, whilst, the high V/Cr and Ni/Cr ratios, in particular, may indicate that the investigatedsediments were deposited under oxidizing environment.Both the geochemical indices and the geochemical ratios reveal that there are no obvious differencesbetween the distribution of major and trace elements in most Campanian to Maestrichtian shales inSibaiya. This pattern of distribution suggests a single chemical clan and most of the investigated sedimentsare related mainly to the same rock source which is dominated by mafic volcanic rocks. Therefore ,thetectonic setting studies indicated that the investigated Sibaiya shales were derived mainly from maficvolcanic source rocks, most probably the Eastern Desert of basaltic and andesitic rocks.The study of the weathering trends and chemical mobility was also undertaken using the chemicaland mineralogical analyses. In this respect, the chemical Index of alteration (CIA) was calculated for allthe samples and revealed that Sibaiya shales describe low to moderate chemically weathered terrain.Beside, the fluviatile of post deposition contributions from local sources, such as the basement rocks in the Egyptian Eastern and Western deserts, can be considered as subordinate sources. This is representedby the relative abundance of kaolinite in the clay minerals.Additional environmental investigations were also undertaken in order to explore anyenvironmental radiation impact that may arise from the utility of these shales in any domestic purposesby the locals and/ industry. The calculation of geo-accumulation index in Sibaiya shales was used tostudy the pollution pattern and revealed that these shales describe a low to moderate polluted terrain.However the dilution factor when mixed with other material during domestic utility may make it anenvironmentally safe product.