The present study aims to elucidate the depositional environment of Seam I in the Megalopolis Basin, Southern Greece, using existing geological data supported by maceral and mineralogical analyses, as well as by natural radioactivity determinations. Very high pyrite and uranium contents and a very low tissue preservation index usually suggest a marine influence during deposition. In Seam 1, however, these features are most probably the result of a karstic aquifer supplying the palaeomire with sulphate-rich, neutral to weakly alkaline water. The pyrite formation and the U precipitation are attributed to the anaerobic conditions, which prevailed during peat accumulation. The petrographic, mineralogical and geochemical data of the studied profile point to several changes in the surface runoff and the supply of clastic material during deposition. (C) 2009 Elsevier B.V. All rights reserved.