Colours of the Upper Neogene "poznan Clays" in the Light of Sedimentological, Mineralogical and Nuclear Methods

The Mio cene-Plio cene overbank fa cies "Poznan Clays" are par tic u larly well ex posed in large lig nite out crops in cen tral Po -land, for ex am ple, in the JoYwin IIB opencast mine. Dur ing their ac cu mu la tion the cli mate fluc tu ated from mod er ately warm and hu mid to cool and dry. In gen eral, the dark grey and the grey ish-vi o let colours come from mac ro scop i cally vis i ble or ganic mat ter and the ab sence of he ma tite and/or goethite. The or ganic mat ter also af fects the Fe3+/Fe2+ ra tio and, con se quently, the re dox con di tions. When the study area was poorly drained, there were fa vour able con di tions for plant veg e ta tion, re sult - ing in Histosols (hydromorphic palaeosols). In a re duc ing en vi ron ment, other sed i ments with "cold" colours (green ish to blu - ish shades) formed that in cluded py rite and/or gyp sum, though with few or no pig ments such as he ma tite, goethite or jarosite. At that time, el e men tal sul phur could also crys tal lize, oc ca sion ally giv ing a yel low ish shade to the sed i ment mot tles. When the area was well drained, most of the or ganic mat ter de cayed, and the Fe3+/Fe2+ ra tio was rel a tively high due to pro gres sive weath er ing. Un der such con di tions, Vertisols de vel oped with "warm" colours (from yel low to cherry-red), en riched in he ma - tite and goethite, and also con tain ing gyp sum and na tive sul phur.