A profile within the Upper Freshwater Molasse (OSM) of eastern Switzerland was sampled. Particle-size analyses, bulk chemical analyses, cation exchange capacities and Fe fractions (oxalate- and dithionite-citrate-bicarbonate-soluble Fe, total Fe) were measured. The mineralogical composition was determined by X-ray diffraction and quantified with Rietveld analysis. The layer charge of selected fine-clay samples was determined with the alkylammonium method using chain nc = 12. The profile could be divided into a lower sequence (I) and an upper sequence (II) by a hiatus. Chemical data, particle-size distributions and calcite contents indicated that soil formation was essentially restricted to wetting-drying cycles associated with redox cycles evident from Feo/Fed and Fed/Fet ratios. A significant correlation between the d060 values of the dioctahedral smectite-illite mixed-layer phases and Feo/Fed was interpreted as the influence of microbial Fe3+ reduction, which affected both the Fe oxides and Fe-rich clay minerals. This is the first evidence that the Fe dynamics of both mineral groups may be linked. The amount of illitization also correlated with Feo/Fed, but only in the upper part of the profile. This indicates that wetting-drying cycles were necessary for the illitization process.

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