Coal deposition in the Noric Depression (Eastern Alps): raised and low-lying mires in Miocene pull-apart basins

Abstract

Pull-apart basins often form during the late stage of continental collision. Provided appropriate climatic conditions, these basins may contain thick coal seams. Two such examples are the Leoben and Fohnsdorf Basins in Austria, which formed in Miocene times during the post-collisional stage of the Alpine orogene. Both basins contain more than 10 m of sub-bituminous coal. Despite similar ages, tectonic settings, and similar stratigraphic position of the seams within the sedimentary sequence, the properties of the coals differ significantly. The Leoben coal is quite clean, whereas Fohnsdorf coal is rich in ash and sulfur. Petrographical and geochemical methods were applied to reconstruct the environmental factors that controlled coal properties in these basins. Coal formation in the Leoben Basin commenced in an eutrophic, low-lying mire. This was followed by deposition of the main body of coal, which formed in an ombrotrophic raised mire. Coal deposition was only shortly interrupted by deposition of volcanic ashes. Due to an acid paleo-environment, the volcanic material within the raised mire was altered to kaolinite. Thick vitrain bands representing large trees are found in the lower part of the seam only. Siderite is the main inorganic constituent in the main coal body. Lakes within the raised mire were present during late stages of peat accumulation and were filled with sapropelic coal. The high ash yields of this facies are due to abundant siliceous sponges, siliceous crusts, and tuffogenic minerals. Finally, high subsidence rates caused the flooding of the raised mire and sapropelic shales were deposited in the newly formed lake. The iron, which formed siderite in the raised mire, and biogenic and abiogenic silica present in the lakes within the raised mire, are derived from volcanic ashes. The high-ash Fohnsdorf coal was formed in a low-lying mire close to a brackish lake. The seam geometry is controlled by the progradation of a fluvio-deltaic system into the brackish lake, and its subsequent drowning. The roof of the seam is formed by coquinoidal limestones and sapropelic shales. The brackish influence reduced the acidity of the mire, which resulted in the transformation of volcanic glass into montmorillonite. The low acidity also promoted high bacterial activity, which together with the high sulfate content of brackish waters, explains the high sulfur and pyrite content of the coal. Its high content of biogenic (?) methane is probably another effect of bacterial activity. Low-ash, low-sulfur coal is restricted to thin layers. These might represent local raised mires, which formed after the withdrawal of a fluvial system.