Petrological and geochemical evidence of hematite growth and dispersal in Permo-Triassic red beds under the Pangea mega-monsoon climate, Colorado Plateau, Utah

The palaeoclimatic significance of continental red beds (CRB) is the subject of a long-standing debate. The CRBs are strikingly often found in association with arid, aeolian sediments, humid fluvial sediments and palaeosols, reflecting variable climatic regimes. In this study, diffuse reflectance spectroscopy (DRS) is used in combination with facies analysis, petrology, bulk-rock and in-situ major and trace element geochemistry, and molybdenum stable isotopes to better understand the origin and distribution of the hematite pigment in a spectrum of Permo-Triassic aeolian, fluvial and pedogenic facies of the Colorado Plateau, Utah. Their red colour is characterized by high (> 30) percentage of red-band (625–700 nm) reflectance in the DRS spectra and high hematite-to-goethite peak height ratios (1.5 to 5.7) from their first derivatives. Most hematite occurs in form of submicronic to micron-sized, platy crystals in altered biotite grains, clayey grain coatings, clayey matrix, pyrite framboid pseudomorphs, and as pore-filling cement. Hematite is most abundant in palaeosols, but also present in aeolian, sand dune and damp interdune deposits, fluvial channel-fill, sandy and gravelly bars, and floodplain deposits. Iron for the hematitization was supplied from reductive release of Fe²⁺ from biotite and clay minerals. The hematitization itself occurred under oxic conditions, associated with formation of kaolinite, mobilization of V, Mo, As, U, and fractionation of rare earth elements and Mo isotopes, especially in lateritic palaeosols. Hematitization during weathering, pedogenesis, transportation and early diagenesis, and reworking and redeposition of the red material by rivers and wind is thought to be the principal mechanism of the formation of red beds in the Colorado Plateau. The processes of hematitization and the fluvial-aeolian dynamics were controlled by alternation of wetter and drier periods likely caused by the Permo-Triassic Pangean mega-monsoonal climatic regime.