An early Miocene (Ottnangian) lignite seam up to 36 m thick occurs in the Oberdorf trough at the northwestern margin of the Styrian Basin. It forms part of the fluvial Koflach-Voitsberg Formation. Petrographic and palynologic data, together with ash yield and sulphur content of samples from two boreholes and two surface profiles have been used to study vertical and horizontal coal facies variations. Numerous partings and a high ash yield show that the lignite originated in a topogenous mire. Petrographic indices and palynological data suggest a wet-forest swamp paleo-environment. A fluvial main parting with channel sediments at the eastern margin of the Oberdorf trough and overbank sediments in its central part splits the lignite into a lower and an upper seam. The lower seam consists of a lower high-ash part with frequent splittings and a higher low-ash part. The upward decrease in number and thickness of silty-clayey partings, interpreted as crevasse splay deposits, reflects the withdrawal of the fluvial sediment source. Coal composition is characterized by an upward increase in (detro-)xylitic coal and in tissue preservation. A good correlation between tissue preservation and percentages of Taxodiaceae-Cupressaceae pollen suggests that this is mainly a result of an increasing contribution of decay-resistant gymnosperms (conifers). The upper seam is also ash-rich. Abundant inertinite at the eastern margin of the Oberdorf trough is related to fire episodes inside or outside the mire. Main features of the upper seam in the central basin are an upward increase in tissue preservation and in percentages of Taxodiaceae-Cupressaceae pollen. High sulphur contents in the fresh-water lignite are a result of a concentration of sulphate-rich waters above the impermeable floors of the lower and upper seams. Carbonate-rich surface waters from the Graz Palaeozoic and Raasberg-Sequence raised the pH value and controlled sulphur land ash) contents in those parts of the lower seam, which are uninfluenced by the basal sulphur enrichment. Palynomorph spectra are dominated by Taxodiaceae-Cupressaceae pollen and pollen from plants living in a mixed mesophytic forest. Pollen from aquatic and reed plants are rare. The ratio between arctotertiary and paleotropic pollen is close to one and supports the early Miocene age of the lignite. The detected pollen spectra point to warm-temperate subtropic and humid conditions.