The middle Eocene Márkushegy subbituminous coal (Hungary): Paleoenvironmental implications from petrographical and geochemical studies

• Published in: International journal of coal geology 2007-09, Vol.72 (1), p.33-52
• Main Authors: Bechtel, A., Hámor-Vidó, M., Sachsenhofer, R.F., Reischenbacher, D., Gratzer, R., Püttmann, W.
• Format: Article
• Language: English
• Subjects: Biomarkers; Carbon isotopes; Coal facies; Earth sciences; Earth, ocean, space; Exact sciences and technology; Maceral composition; Middle Eocene; Paleoecology
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2006.12.008

Samples from two coal seams of the Márkushegy underground mine (Hungary) were investigated for variations in maceral composition, petrography-based facies indicators and various geochemical parameters, using three seam profiles. Both seams originate from a topogenous mire and evolved within a peneplaned coastal area covered with eutrophic swamps. The presence of foraminifera and marine algae in the marly layers is indicative of marine ingressions. Coal facies characterisation from these parameters is combined with the results from biomarker analyses of soluble organic matter in order to reconstruct the depositional environment and to relate petrography-based indicators to the molecular composition. The contents of macerals of the liptinite group are positively correlated with soluble organic matter (SOM) yields and Hydrogen Index (HI). Consistent with maceral composition and high HI values, enhanced proportions of n-alkanes of intermediate molecular weight ( n-C 21–25), which are predominantly found in macrophytes, are obtained from samples of the marly shales. The final drowning of the mire is reflected by decreasing pristane/phytane ratios, due to the rise of the (ground)water table and the establishment of anaerobic conditions. The observed positive correlation between pristane/phytane ratios and inertinite percentages suggests that, at least in the Márkushegy deposit, both parameters reflect variations in redox conditions of the mire. The predominance of angiosperms in the peat-forming vegetation of the Márkushegy coal is evidenced by the high relative proportions of angiosperm-derived triterpenoids. The influence of the relative proportion of fossil wood on tissue preservation (TPI) is indicated by the positive relationship of the concentration of land plant-derived terpenoid hydrocarbons and TPI. The degree of gelification of plant tissue (GI) is governed by the microbial activity in the mire, as indicated by a negative relationship between GI and the steroids/hopanoids concentration ratios. The variation of the δ 13C values of coal is obviously governed by the relative contribution of plant lipids to the biomass. The obtained negative relationship between δ 13C of the coal and GI is explained by the effect of cellulose decomposition and microbial activity on the isotopic composition of the organic matter.