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.