Provenance and depositional environment of the Middle-Late Jurassic shales, northern Iraq

Abstract

The Middle–Late Jurassic formations are regarded to be an important source rock in the Jurassic petroleum system of North Iraq. The major and trace element analysis on the Middle and Late Jurassic shales in the Imbricated Zone, Iraq was analyzed. This study investigated the shales provenance and depositional environment, in addition to evaluate the intensity of weathering in the source area. The chemical index of alteration (CIA), index of compositional variability (ICV) and the Al2O3-CaO + Na2O-K2O diagram of the shales suggest increasing in the intensity of chemical weathering from the Middle to the Late Jurassic due to the climatic change and/or tectonic activity. The elemental ratios: Al2O3/TiO2, Th/Sc, La/Th, Co/Th, (Gd/Yb)n, Eu/Eu*, and REE pattern; and tectonic discrimination diagrams indicated that they were derived from felsic source rocks in the crystalline basement of the northern fringe of the Arabian Shield and Rutba Uplift. The Sargelu, Naokelekan and Barsarin formations are also supplied from the intermediate source rocks from the Mid Oceanic Ridge and deposited in an active setting. The geochemical parameters suggest the lower part of the Middle Jurassic and the shales from upper part of the Late Jurassic were deposited in an oxic shallow marine environment; and other ages were deposited in an anoxic deep marine environment.