Organic geochemistry of the Lower Miocene Oberdorf lignite (Styrian Basin, Austria): its relation to petrography, palynology and the palaeoenvironment

Abstract

In the Oberdorf trough at the northwestern margin of the Styrian Basin (Austria), an up to 36-m thick Lower Miocene (Ottnangian) lignite seam occurs within the fluvial Köflach–Voitsberg Formation. Previous results indicate that the lignite originated in a topogenous mire within a wet-forest swamp palaeoenvironment. Differences in soluble organic matter (SOM) yield of borehole samples from the lignite seam are related to free lipids present in plant waxes or resins and the presence of thermally labile polymers indicated by the contents of liptinite macerals (i.e., resinite, suberinite, and liptodetrinite). The n-alkane distribution patterns are dominated by high-molecular weight lipids with marked odd over even predominance, originating from plant waxes. A contribution of algae to the even numbered short-chain lipids, observed in considerable amounts in the footwall sediments is proposed. Terpenoid biomarkers characteristic for conifers and angiosperms were detected, consistent with palynomorphic spectra which are dominated by forest swamp pollen (Taxodiaceae–Cupressaceae) and pollen from plants living in a mixed mesophytic forest. Excellent relationships between higher plant biomarker composition and palynologic data indicate that the organic geochemical results reflect the varying inputs of coniferales vs. angiosperms. The data further imply that cuparene may be used for taxonomical differentiation of coniferales families. Organic geochemical data provide evidence that the content of land plant-derived biomarkers is controlled by the input of resin-rich conifers. Concentrations of hopanoids, dominated by hop-17(21)-ene, generally increase towards the top of the lignite, a section characterised by high extents of aromatisation of triterpenoid biomarkers. The results suggest that the aromatisation of angiosperm-derived biomarkers may be governed by the activity of anaerobic rather than by aerobic bacteria. However, changes in the depositional environment and palynologic facies during peat formation probably influenced the origin and fate of angiosperm-derived triterpenoids. The results indicate how biomarker composition may contribute to the evaluation of environmental changes. Cross-correlations of organic geochemical parameters with palynological data and petrography-based facies indicators will provide further insights in the peat-forming vegetation and biogeochemical activities in the mire.