The influence of depositional conditions on chemical and mineralogical composition of glauconite: Case study from the Late Cretaceous Dokan Basin in Kurdistan region of Iraq

Abstract

Although glauconite is a common authigenic mineral in Late Cretaceous marine deposits, the influence of depositional conditions on the composition of glauconite is poorly understood. This study documents the origin and significance of five green clay layers in the Kurdistan region of Iraq in three formations, viz. Gulneri, Kometan, and Shiranish, comprised of carbonaceous mudstone, marl, marly limestone, dolomite, and limestone with occasional phosphates. Two sets of green clays within the ∼255 m thick, primarily transgressive marine sedimentary succession formed in distinct depositional domains, viz., inner shelf (proximal), and outer shelf to the middle slope (distal). Both sets of green clays formed primarily as pellets and as infillings. Characteristic reflections of ∼10 Å (001) and ∼ 3.35 Å (003), with high K₂O (>7 wt%) content and lamellar structure exhibited by green clays indicated evolved to highly evolved glauconite. The proximal set of green clay was identified as evolved to highly evolved Al-glauconite with low Fe₂O₃(total) (<15 wt%) and high Al₂O₃ (>10 wt%). The distal set of green clay was highly evolved, containing high (>15 wt%) Fe₂O₃(total) and low (<10 wt%) Al₂O₃. The proximal set exhibited minor (<10 %) smectite interstratification, whereas the distal set of glauconite was almost entirely devoid of smectite interstratification. The compositional variation of glauconite across the basin was primarily dictated by substrate and depositional setting; the proximal set of glauconite formed by replacing Al-rich smectite and kaolinite, while the distal set formed in Fe-Al smectite. The phosphates within the glauconites formed under suboxic conditions during the initial stages of glauconitization.