Structure, eruption and depositional history of two newly discovered Miocene maars in the western Bohemian Massif (Rohrloh and Bärnau, NE Bavaria, Germany)

Recent discoveries and studies of maars in the NE Bavaria-Czech Republic border region have significantly expanded our knowledge of Cenozoic volcanism in the western Bohemian Massif. Using geomorphological and geophysical investigations, two new circular geological structures with partly striking geophysical anomalies were discovered: the Bärnau Maar and the Rohrloh Maar. They were explored with two scientific drillings by the Geological Survey at the Bavarian Environment Agency. Both dry maars form shallow depressions from a few hundred to approximately 1000 m in diameter. In the Bärnau Maar, a 150 m thick sedimentary sequence consisting primarily of laminated, organic-rich clayey to silty sediments with numerous diatomite laminae of a lacustrine depositional environment was drilled, with frequent sand beds and soft-sediment deformation structures documenting repeated turbidity flows and slumps. The upper sedimentary sequence comprises soft-sediment deformed lacustrine sediments overlain by gravelly and sandy deposits. The 70 m thick sediment sequence drilled in the Rohrloh Maar similarly consists predominantly of fine-grained lacustrine sediments with intercalated sandy turbidite beds. Both sediment sequences are composed predominantly of quartz-, mica-, and kaolinite-rich sediments, representing the erosion products of neighbouring crystalline rocks; however, elevated smectite contents near both core bases suggest the contribution of juvenile material. The palynological data from the Bärnau Maar and Rohrloh Maar sediments advocate a Lower to Middle Miocene age and Lower Miocene age, respectively. During this time, the climate was predominantly subtropical to temperate, and swamp forests surrounded the maars, whereas mesophytic vegetation was present in the broader region.