Mineralogical and geochemical imprints to determine the provenance, depositional environment, and tectonic setting of the Early Silurian source rock of the Qusaiba shale, Saudi Arabia

Abstract

The Early Silurian source rock of the Qusaiba shale from Saudi Arabia was analyzed for its mineralogical and geochemical compositions to investigate its provenance, depositional environment, and tectonic setting, which still has not been studied. The shale is approximately 85 m thick gray to dark gray to black shale intercalated with siltstone and sandstone. The non-clay minerals are quartz and dolomite, while the clay minerals are kaolinite, illite, and smectite in some samples. An intermediate igneous source is suggested for the Qusaiba shale based on the Al2O3/TiO2 ratio, calculated SiO2, and Zr–TiO2, Al2O3–TiO2, SiO2–Al2O3/TiO2 and Cen–La/Ybn binary plots. The Zr/Sc versus Th/Sc cross plot indicate that the Qusaiba shale was not subjected to sedimentary recycling. The abundance of kaolinite, C–values [Σ(Mn + Fe + Ni + Cr + Co + V)/Σ(Mg + Ca + Sr + Ba + Na + K)], Sr/Cu ratios and high chemical index of alteration (CIA) values indicate humid and warm conditions for the period of deposition. Geochemical proxies such as V–Al2O3 and P2O5–Al2O3 binary plots suggest a shallow marine depositional environment for the Qusaiba shale was prevailing during the Paleozoic in northeastern Gondwana. The Qusaiba shale represents a transgressive phase in the Early Silurian followed the Late Ordovician regressive event marked by the glacial non-marine sandstone of the Sarah Formation. The Qusaiba Formation shows another regressive event characterized by storm deposits at its top. The tectonic setting interpreted from major oxides, ratios of immobile–trace elements and rare earth elements (REE) parameters such as (La/Y)n, (La/Y)n, (Gd/Yb)n and Eu/Eu* depicts a passive margin matching with the regional setting for the northeastern Gondwana. Illite contents (13 %–33%) and IC values (0.52°–0.75° Δ2θ) suggest a late diagenesis stage for the Qusaiba shale.