Early diagenetic controls on color variation in Eocene red beds: a case study from the Gercus Formation, Zagros Basin, Kurdistan Region, Iraq

ABSTRACT This study investigates the mechanisms controlling the coloration of red beds of the Gercus Formation, which was deposited in a deltaic environment during the Eocene, and focusses on the dynamic interplay between depositional setting and diagenetic processes in shaping the distinct hues. A comprehensive multi-proxy methodology was employed, including facies analysis, quantitative color assessment using diffuse visible spectral reflectance (DRS), optical and electron microprobe microscopy, bulk-rock geochemistry (XRF and XRD), and in-situ geochemical analysis via laser-ablation ICP-MS. The results reveal that the spectrum of sediment hues, from red, to yellow-brown, gray-green, and gray, arise from the formation and distribution of Fe oxy/hydroxides, which are largely determined by sedimentary lithology, redox conditions, and diagenetic transformations. Sedimentation rates play a crucial role in regulating redox conditions and determining sediment coloration. Rapid sedimentation restricts oxygen exposure, fostering suboxic conditions that inhibit Fe oxidation, leading to gray-green sediments. Conversely, slower sedimentation allows for prolonged oxygen exposure, facilitating formation of Fe oxy/hydroxide and hematite, and resulting in red and yellow-brown sediments. Chlorite clay minerals are pivotal in transporting and supplying the Fe necessary for Fe oxy/hydroxide formation. Their transformation to chlorite–smectite mobilizes Fe either predepositionally or during eodiagenesis. In oxic environments, enhanced smectitization releases more Fe, preserving Fe oxy/hydroxides and promoting development of red sediment. In contrast, anoxic conditions suppress smectitization, dissolve predepositional Fe oxy/hydroxides, and produce non-red layers. Additionally, sediments with low clay content are unable to develop red coloration, even under oxic conditions, due to insufficient clay-mediated hematite formation, thereby retaining their original hues.