Redox-driven clay mineral evolution associated with Fe redistribution and Mg availability during early diagenesis: Insights from Eocene red beds in the Kurdistan Region, Iraq

Constraining the relationships between redox-dependent element redistribution and clay mineral assembly during early diagenesis remains challenging, particularly in red-bed systems which are sensitive archives of paleo-redox conditions. This study investigates relationships among redox conditions, Fe redistribution, Mg availability, and clay mineral transformations in siliciclastics of the Eocene Gercus Formation, Iraq, using an integrated approach based on global- and clay-fraction X-ray diffraction, scanning electron microscopy, electron microprobe analysis, X-ray fluorescence, Mössbauer spectroscopy, and diffuse reflectance spectroscopy. The results revealed systematic variations in clay mineral assemblages across early-diagenetic redox-defined zones, marked by progressive alteration of serpentine derived from ophiolitic sources through chlorite intermediates and subsequent transformation into vermiculite and mixed-layer vermiculite–smectite. Under suboxic conditions, limited alteration restricts Mg and Fe release, resulting in vermiculite-rich assemblages and absence of palygorskite. In contrast, the more advanced alteration observed in S2, consistent with the influence of reducing-fluid overprint conditions promotes Mg release and initial palygorskite formation, while Fe follows contrasting pathways involving partial removal and retention within the altered mineral assemblage. Under persistently oxic pore-water conditions, Mg remains available for extensive palygorskite growth, whereas Fe is predominantly retained and stabilized in ferric phases, including hematite. These observations demonstrate that redox conditions influence mineral alteration intensity and the subsequent availability of Mg, while redox conditions directly govern Fe redistribution and stabilization. In this system, Mg availability controls Mg-rich secondary mineral formation, whereas Fe undergoes redox-dependent redistribution, retention, removal, or stabilization depending on pore-water conditions.