ORIGINAL PAPER Depositional environment of Eocene oil shales of Wadi Shallala Formation from northern Jordan Mahmoud H. Al-Tamimi 1 & Mohammad Alqudah 1 & Mohammad S. Al-Atawneh 1,2 & Jamal Nazzal 1 & Saeb AlShraideh 1 Received: 1 June 2020 /Accepted: 16 January 2021 # Saudi Society for Geosciences 2021 Abstract Oil shale deposits of the mid-Eocene Wadi Shallala Formation in northern Jordan consist of organic-rich limestones and chalks. These organic-rich deposits are exposed at the surface near Wadi Ash Shajara, in a 12-m-thick outcrop. For this study, a total of 21 oil shale samples were analysed for bulk elemental geochemistry, inorganic geochemistry, organic geochemistry and microfacies analysis. Three microfacies types were recognised: pelagic mudstone/wackestone, foraminiferal packstone and bioclastic packstone/wackestone. The observed cyclicity of these microfacies in the study area indicates fluctuations in deposi- tional conditions in Neo-Tethys during the mid-Eocene. A shallow-water, calm, restricted marine environment with limited water circulation and low energy currents evolved into a deeper-water environment with open circulation at or just below the wave base. Wadi Shallala Formation in Wadi Ash Shajara composes mainly of chalk (CaCO 3 = ~92 wt%) with a TOC of up to 5 wt % and comprises abundant calcareous nannofossils, foraminifera and ostracods as well as non-clastic components like calcitic fragments. The sulphur content of the analysed samples is significant (1.24 wt %) and is associated with the organic matter. Redox-sensitive elements (Fe, Ni, Zn, S and Zr) are enriched in the oil shale samples, indicating an anoxic depositional environment fluctuating between euxinic and sub-euxinic. These fluctuations in depositional conditions of the basin are a result of many factors such as climate, and tectonisms affected deposition of oil shales during Middle Eocene time. Keywords Oil shales . Bituminous chalk . Eocene . Palaeoenvironment . Microfacies . Redox-sensitive elements Introduction High-quality oil shale deposits are present in south, central and north Jordan (Alali 2006; Dyni 2010) and in general con- sist of bituminous argillaceous limestones of Late Cretaceous- Cenozoic ages. The high TOC oil shales and other organic- rich sedimentary rock represent major disturbances in the global carbon cycle (Wagner 1998; Jenkyns 2010). Changes in carbon budge, depositional, palaeoecological and oceano- graphic settings are recorded in oil shale deposits. Palaeoenvironmental conditions can be reconstructed dur- ing precipitation of organic-rich sediments through the geological times based on investigation of their organic and non-organic components (Demaison and Moore 1980; Aqleh et al. 2013). Recent petrographical and geochemical observa- tions make it possible to understand many formerly mysteri- ous aspects of the genesis of organic matters (Demaison and Moore 1980; Ali Hussein et al. 2014; Aqleh et al. 2013; Alqudah et al. 2015). Organic matter accumulation and pres- ervation need special conditions. Many factors affect the de- position of the different periods. During these periods, a favourable combination of factors existed which led to the development of oxygen-deficiency in the Jordanian basins and thereby accumulation of organic matter deposits (Kolonic et al. 2005). It is appropriate to review the factors that influenced the accumulation and preservation of organic matters in Jordan and mainly in the north where studies are lacking. Factors responsible of influencing organic-matter ac- cumulation in sediments are both biologic and physical. Biologic factors include primary biologic productivity of the surface-water layers. Physical factors include shape of the basins and modes of transit of organic matter to depositional sites (Demaison and Moore 1980; Alqudah et al. 2014). Thus, Responsible Editor: Santanu Banerjee * Mohammad Alqudah mohammad.alqudah@yu.edu.jo 1 Department of Earth and Environmental Sciences, Faculty of Science, Yarmouk University, Irbid 21163, Jordan 2 Ministry of Education, Amman, Jordan Arabian Journal of Geosciences (2021) 14:209 https://doi.org/10.1007/s12517-021-06565-x