Mineralogy, geochemistry, and depositional environment of the Beduh Shale (Lower Triassic), Northern Thrust Zone, Iraq

Authors: FARAJ TOBIA, SIRWA SHANGOLA

Abstract: Integrated mineralogical and geochemical methods are utilized to investigate the provenance, paleoweathering, and depositional setting of shale from the Lower Triassic Beduh Formation in the Northern Thrust Zone, Iraq. The ~64-m-thick Beduh Formation consists of calcareous shale and marl intercalations with thin calcareous sandstone interbeds. X-ray diffraction analysis revealed that clay minerals comprise illite, kaolinite, and chlorite, with a minor mixed layer of illite/smectite and illite/chlorite. Calcite and quartz are the main nonclay species with subordinate amounts of feldspar and hematite. The mineralogical and geochemical parameters of the shale (e.g., high content of illite and moderate illite crystallinity index, Al₂O₃/TiO₂, Th/Co, Cr/Th, and LREE/HREE ratios) indicate that they were derived from felsic and intermediate components. This is supported by the enrichment of LREEs, negative Eu anomaly, and depletion of HREEs. The discriminant function-based major element diagrams indicated that the origin of sediments was probably from passive (the Arabian Shield and the Rutba Uplift) and active (volcanic activity) tectonic environments. The source of sediments for the Beduh Formation was likely the Rutba Uplift and/or the plutonic-metamorphic complexes of the Arabian Shield located to the southwest of the basin. Paleoweathering indices such as the chemical index of alteration and chemical index of weathering, as well as the A-CN-K (Al₂O₃-CaO+Na₂O-K₂O) diagram of the shale of the Beduh Formation suggest that the source terrain was moderately to intensely chemically weathered. The Cu/Zn, U/Th, Ni/Co, and V/Cr ratios and negative Eu anomaly indicate the deposition of sediments under an oxygen-rich environment.

Keywords: Beduh Formation, clay mineralogy, provenance, tectonic setting, paleoweathering, paleoredox

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