Miocene syn-rift lacustrine sediments in the Mecsek Mts. (SW Hungary)

Abstract

The Pannonian Basin, a major back-arc basin in the Alpine–Carpathian orogenic belt, experienced its syn-rift phase during the Early–Middle Miocene. Studying coeval sediments can provide important information on the initiation of the extension. This paper investigates syn-rift deposits in the Mecsek Mts. in SW Hungary from a tectono-sedimentary aspect, using stratigraphy, palaeontology and structural observations to constrain palaeoenvironments and their tectonic background. Our study shows that in the Mecsek area the widespread Early Miocene fluvial sedimentation was not directly followed by inundation by the Central Paratethys sea, instead, a phase of lacustrine deposition in the Karpatian–Early Badenian (late Burdigalian–early Langhian) preceded the marine flooding. The lake sediments have a low-diversity but abundant, endemic mollusc and ostracod fauna, dominated by the bivalve Congeria boeckhi and the gastropod Ferebithynia vadaszi. Identical faunas at various sites indicate that “Lake Mecsek” was a single water body, covering the present-day Mecsek Mts. and their surroundings. Wedge-shaped clastic bodies along faults, fault scarp breccias and semi-soft sediment deformations suggest that extensional tectonic activity related to the rifting of the Pannonian Basin played a role in lake basin formation. The accumulation of lakes was probably also enhanced by increased precipitation during the Miocene Climatic Optimum. The Central Paratethys flooded the area in the Badenian (Langhian) and deposited normal marine sediments over the lacustrine ones. Considering the fauna, the sedimentary succession, the structural background and evolution history, the Mecsek area seems to be part of the Illyrian bioprovince and related to the Dinaride Lake System.