Original paper

Geochemistry of redox-sensitive trace elements and its implication on the mode of formation of the Upper Cretaceous oil shales, Central Jordan

El-Hasan, Tayel

Abstract

Geochemistry and paleo-depositional environments that existed during the accumulation of the Upper Cretaceous organic-rich oil shales of central Jordan were determined using trace elements, total organic matter and sulfur (TE-TOC-S) relationships, particularly using paleo-redox trace element indicators (i.e. Mo, V, Cd, Zn and U). By using XRF the samples collected from El-Lajjoun outcrop were analyzed for major and trace elements. Compared to the average shale the studied rocks show moderate degrees of trace element enrichments where Cd > Mo > U > Cr > Zn > Ni > Cu > V resembling those of Pennsylvanian organic-rich black shales. The studied samples present high TOC contents with an average of about 17.39 wt%, which exceeds the euxinic threshold of Algeo & Maynard (2004). However, trace elements show no covariance with the TOC, but have a positive covariance with sulfur. These results can be attributed to the euxinic depositional environment, where trace elements would co-precipitate as sulfide, selinide or oxihydroxide minerals rather than being incorporated within organic matter. The results of elemental/Al ratios and correlation coefficients indicate the authigenic or non-detrital source for the redox-sensitive elements. The vertical elemental/Al variation indicates a gradual change into a shallow sea environment.

Keywords

oil shalescretaceoustocredox-sensitiveeuxinicenrichment factor