The smectite-bearing red bed series of the Palaeogene Ergeliin zoo Formation in Mongolia is part of an alluvial-fluvial fan prograding over prodelta/mudflat deposits. The series was investigated in the field (mapping and portable infrared (IR) spectrometry in the short wavelength (SW) range) and samples were analysed in the laboratory using thin sections, X-ray diffraction, X-ray fluorescence, cation exchange capacity and Fourier transform infrared spectrometry in the mid-IR range. Two reference sections are treated in more detail as to the environment of deposition and concentration of smectite. The deposits at Ulaan uul are representative of a distal fan section with prevalent mudflats, whereas Ulaan buur offers insight into a braided river drainage system. Metabasic rocks in the hinterland delivered mica and chlorite from which smectite originated during diagenesis under semi-arid climatic and surface-near conditions. Towards the basin edge, smectite is replaced by kaolinite. The prime area in which we were interested in the smectite is the most distal part of the alluvial-fluvial fan, which was the subject of a field-based IR survey. This exploration method is a valuable tool to obtain a quick overview of the mineral composition and reduce the number of samples in the field for follow-up analyses. Its weak and strong points are discussed.

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