Early Diagenetic Controls on Color Variation in Eocene Red Beds: A Case Study from the Gercus Formation, Zagros Basin, Kurdistan Region, Iraq

This study investigates the mechanisms controlling the coloration of the red beds of the Gercus Formation deposited within a deltaic environment during the Eocene climate, focusing on the dynamic interplay between depositional settings and diagenetic processes in shaping their 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 a spectrum of sediment hues, red, yellow-brown, gray-green, and gray arising from forming and distributing Fe oxy/hydroxides, processes largely governed 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 prolonged oxygen exposure, facilitating Fe oxy/hydroxide and hematite formation, 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 pre-depositionally or during eodiagenesis. In oxic environments, enhanced smectitization releases more Fe, preserving Fe oxy/hydroxides and promoting red sediment development. In contrast, anoxic conditions suppress smectitization, dissolve pre-depositional 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.