Integrated sedimentology, diagenesis, and geochemical provenance of the Eocene Rusayl Formation in the Oman Mountains

This study examines the lithological and compositional variations within an interbedded sequence of multicolored shales, marl, coal, and limestone from the Lower to Middle Eocene Rusayl Formation, exposed in the Oman Mountains. A section was measured and described from an outcrop at the Old Al Khod Village road-cut to characterize the formation’s depositional features. Fifteen fresh rock samples were collected from the carbonate and shale intervals for compositional and microfacies studies. Eleven carbonate samples were analyzed through petrographic thin sections and classified, while four shale samples were studied using XRD and XRF analyses. The Rusayl Formation is divided into two lithofacies associations: (i) carbonate lithofacies association, comprising marly limestone, wackestone, dolosparite, packstone, and grainstone, and (ii) shale lithofacies association, consisting of carbonaceous clay/coal, red shale, green shale, and grey shale. Six microfacies were identified: peloidal worm tubes grainstone (MF1), brachiopod and red algae packstone (MF2), miliolid packstone (MF3), brachiopod wackestone (MF4), miliolid wackestone (MF5), and non-fossiliferous micro-sparitic limestone (MF6), representing an inner restricted platform depositional environment. The multicolored shales are composed of minerals like gypsum, kaolinite, smectite, calcite, natrojarosite, quartz, and were deposited in arid to semi-arid climatic conditions. The shales were sourced from the Ophiolite rocks which were uplifting during the deposition of the formation. The variation in colors of the shale beds was caused by cyclic redox reactions influencing mineral composition during deposition. Key diagenetic processes, including micritization, compaction, dissolution, fracturing, neomorphism, and secondary calcite precipitation, were identified, with a paragenetic sequence reflecting sea-level fluctuations and meteoric diagenesis prior to burial. The findings of this study may serve as an analog for understanding the deposition of multicolored shale intervals in similar successions in the Arabian Platform.