The influence of depositional conditions on the crystallo-chemical characteristics of Paleogene glauconites of Iraq

Glauconites are abundant in Paleocene Alaji Formation and Eocene Jaddala Formation.  This study highlights the influence of depositional conditions on mineralogical and chemical composition of Palaeogene glauconites in Iraq. Petrographical, textural, mineralogical, and chemical characterization were done on Paleogene green clays. The green clays were examined in marl and limestone, in proximal (Kirkuk section), medial (B 12/7 and Gharaf sections), and distal (B 15/7 sections) sections of the basin, formed in middle shelf, outer shelf, and upper slope to upper-middle slope conditions, respectively. The green clays are found in an upward fining sequence and condensed sections. The sedimentary aspects indicate a transgressive systems tract and maximum flooding surface condition of formation of the green clays. The green clays appear light to dark green under microscope and are ~30 µm to ~1000 µm long. The green clays occur as pellets, and as infillings in foraminifera, algae, showing radiating cracks. The medium to poor sorting of green clays, showing radiating cracks indicates their authigenic origin. Microstructurally, the green clays show well-developed flaky and rosette structures. The presence of well-developed lamellar texture indicates evolved character of the green clays. The green clays show ~10 Å (001), 4.53 Å (020), 3.32 Å (003) and ~1.511 Å (060) reflections, which are characteristic of glauconite. The distal glauconites show better-developed spectra with narrower and more intense reflections and a 10 Å (001) reflection (FWHM: 0.43 °2θ), compared to the proximal glauconites with a 10.5 Å (001) reflection (FWHM: 1.11 °2θ). Upon glycolation, the distal glauconites show no shift in the d-spacing of (001) reflection (10.02 Å), with hardly any increase in peak width (FWHM: 0.54 °2θ), while the proximal glauconites show some shift in the d-spacing of (001) reflection (10.15 Å) with minor increase in peak width (FWHM: 1.33 °2θ). These mineralogical observations indicate better-ordered crystal structure in the distal glauconites, compared to the proximal glauconites, with lower smectite content in the distal glauconites. Medial and distal glauconites contain > 15% to ~27.5% Fe₂O₃(total), ~6% to ~10% K₂O, ~2.5% to ~12.5% Al₂O₃, and ~4% to ~6% MgO, while the proximal glauconites show < 15% to ~10% Fe₂O₃(total), ~7% to ~8% K₂O, > 12.5% to ~15% Al₂O₃, and ~6% to ~8% MgO. The chemical nature of the medial and distal green clays indicates a glauconite affinity, while the proximal green clays are High-Mg, Al-glauconites. The medial and distal glauconites formed in a deeper setting, where sedimentation rates are low and detrital Fe-rich particles are available. While, the proximal glauconites formed in a shallower setting close to the Zagros ophiolite belt, where sedimentation rates are high and Mg-Al-rich detrital particles are available. Lower sedimentation rates favor higher evolution of glauconites. Thus, the medial and distal glauconites formed on a Fe-rich precursor like Fe-Al smectite, while the proximal glauconites formed on an Mg-Al rich precursor like Mg-Al phyllosilicate, with higher evolution of the medial and distal glauconites. The high abundance of the glauconites is attributed to the Paleocene-Eocene thermal maximum, as warm seas favor higher kinetics of the reaction.