Depositional environment of the Late Miocene Hausruck lignite (Alpine Foreland Basin): insights from petrography, organic geochemistry, and stable carbon isotopes

Abstract

Lignites and fossil wood from two boreholes in the late Miocene Hausruck district (Alpine Foreland Basin, Austria) were investigated with respect to organic carbon and total sulfur contents, ash yields, maceral composition, organic geochemistry, and stable carbon isotope ratios. The lignites from the two sampled profiles differ in sulfur contents and petrography-based facies indicators (gelification index [GI], tissue preservation index [TPI]). The results point to drier and more acidic conditions in the part of the mire sampled at drill site Lukasberg, whereas the lignites from Kalletsberg formed under near neutral conditions due to a raised (ground) water table. The different depositional environments are supported by the paleogeographic positions of the boreholes: borehole Lukasberg is located within a paleo-valley, whereas borehole Kalletsberg is located at the deeper part of a basin situated south and southwest of the Hausruck area. Environmental changes with time within the mire are confirmed by the molecular composition of the soluble organic matter (SOM). Differences in pristane/phytane ratio, isomerisation of hopanes at position 17, concentrations of hopanes, and in the ratio of hopanes to hop-17(21)-enes are related to the extent of gelification of plant tissue, sulfur content and pH values, caused by differences in (ground) water table. The variable contents of individual terpenoid hydrocarbons derived from land plants indicate that angiosperms and conifers contribute to peat formation in the Hausruck district in variable proportions. Overall, angiosperm biomarkers slightly predominate. The tissue preservation index (TPI) within the Kalletsberg profile is probably governed by the proportion of decay-resistant conifers, as indicated by the correlation between TPI and biomarker composition of the coals. Correlations between δ 13C values of the lignites (between −24.9‰ and −27.4‰) and their non-hopanoid terpenoid composition indicate the major influence of changes in peat-forming vegetation on the carbon isotopic composition of the coals in the Hausruck district. Carbon isotope data of macrofossils are consistent with their taxonomical classification as conifers ( δ 13C between −22.7‰ and −25.1‰) and angiosperms ( δ 13C between −25.5‰ and −26.6‰), respectively. The δ 13C data of the extracted cellulose reveal clear differences between fossil wood from gymnosperms (average δ 13C=−21.0‰) and angiosperms (mean δ 13C=−23.4‰).