Petrography, palynology and depositional environments of the early MioceneOberdorf lignite seam (Styrian Basin, Austria)

An early Miocene (Ottnangian) lignite seam up to 36 m thick occurs in the O berdorf trough at the northwestern margin of the Styrian Basin. It forms pa rt of the fluvial Koflach-Voitsberg Formation. Petrographic and palynologic data, together with ash yield and sulphur content of samples from two bore holes and two surface profiles have been used to study vertical and horizon tal coal facies variations. Numerous partings and a high ash yield show tha t the lignite originated in a topogenous mire. Petrographic indices and pal ynological data suggest a wet-forest swamp paleo-environment. A fluvial mai n parting with channel sediments at the eastern margin of the Oberdorf trou gh and overbank sediments in its central part splits the lignite into a low er 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 numb er and thickness of silty-clayey partings, interpreted as crevasse splay de posits, reflects the withdrawal of the fluvial sediment source. Coal compos ition is characterized by an upward increase in (detro-)xylitic coal and in tissue preservation. A good correlation between tissue preservation and pe rcentages of Taxodiaceae-Cupressaceae pollen suggests that this is mainly a result of an increasing contribution of decay-resistant gymnosperms (conif ers). The upper seam is also ash-rich. Abundant inertinite at the eastern m argin 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 upwar d increase in tissue preservation and in percentages of Taxodiaceae-Cupress aceae 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 Pal aeozoic and Raasberg-Sequence raised the pH value and controlled sulphur la nd ash) contents in those parts of the lower seam, which are uninfluenced b y the basal sulphur enrichment. Palynomorph spectra are dominated by Taxodi aceae-Cupressaceae pollen and pollen from plants living in a mixed mesophyt ic 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-temp erate subtropic and humid conditions. (C) 1999 Elsevier Science B.V. All ri ghts reserved.