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Diagenetic changes in red and white parts of the Skagerrak Formation (Triassic) from onshore welts in Denmark were analysed by scanning electron microscope, electron microprobe, and petrographic microscope in reflected and transmitted Light. The diagenetic sequences of the red and white parts of the Skagerrak Formation are in many ways the same: early infiltration clays, followed by precipitation of caliche calcite, dolomite, mixed-layer illite/smectite, quartz, feldspar, kaolin minerals, illite, chlorite and anhydrite with increasing burial. Differences between the red and white parts of the Skagerrak Formation comprise oxidation stage in detrital minerals (oxidation of glauconite in the red part and reduction of biotite in the white part), the presence of eogenetic authigenic minerals (pyrite and poikilotopic calcite in the white part, hematite and red coatings in the red part), the amount of mesogenetic authigenic kaolin minerals (larger in the white part), the chemistry of mesogenetic authigenic chlorite (Mg-rich in the red part and Fe-rich in the white paa), and the preservation stage of detrital opaque minerals (magnetite, hematite, chromite, ilmenite show none or minor alteration in the red part, whereas they have been almost completely altered in the white part). As the differences between red and white areas are found both in early eogenetic and in mesogenetic authigenic minerals, different oxidising conditions must have prevailed immediately after the deposition and continued into burial. Oxidising conditions were dominant in the arid Triassic climate and the widespread red colour of the Skagerrak Formation was probably formed during early diagenesis due to precipitation of iron oxides or hydroxides. Locally reducing areas were formed, probably by oxidation of organic material. Precipitation of oxidised iron minerals was thus prevented, and the original white colour was preserved. Reducing conditions prevailed until the reducing agent was exhausted, then an outer rim of the reduction spot might locally have been secondarily oxidised. These changing redox conditions explain the occurrence of hematized authigenic pyrite and crandallite group minerals in red areas. (C) 1998 Elsevier Science B.V. All rights reserved.