Colours of the upper Neogene “Poznań Clays” in the light of sedimentological, mineralogical and nuclear methods

The Miocene–Pliocene overbank facies “Poznań Clays” are particularly well exposed in large lignite outcrops in central Poland, for example, in the Jóźwin IIB opencast mine. During their accumulation the climate fluctuated from moderately warm and humid to cool and dry. In general, the dark grey and the greyish-violet colours come from macroscopically visible organic matter and the absence of hematite and/or goethite. The organic matter also affects the Fe³⁺/Fe²⁺ ratio and, consequently, the redox conditions. When the study area was poorly drained, there were favourable conditions for plant vegetation, resulting in Histosols (hydromorphic palaeosols). In a reducing environment, other sediments with “cold” colours (greenish to bluish shades) formed that included pyrite and/or gypsum, though with few or no pigments such as hematite, goethite or jarosite. At that time, elemental sulphur could also crystallize, occasionally giving a yellowish shade to the sediment mottles. When the area was well drained, most of the organic matter decayed, and the Fe³⁺/Fe²⁺ ratio was relatively high due to progressive weathering. Under such conditions, Vertisols developed with “warm” colours (from yellow to cherry-red), enriched in hematite and goethite, and also containing gypsum and native sulphur.