Discovery of the first Quaternary maar in the Bohemian Massif, Central Europe, based on combined geophysical and geological surveys

Abstract

Based on results of previous investigations of tephra-tuff volcaniclastic deposits and a geophysical survey in the surroundings of the Železná hůrka Quaternary volcano, West Bohemia, we performed detailed geophysical surveys using gravimetry, magnetometry and electrical conductivity techniques. Striking anomalies were revealed in a morphological depression near Mýtina, West Bohemia, as a strong evidence of the assumed maar-diatreme structure. The sharp isometric gravity low of − 2.30 mGal, as well as the corresponding positive magnetic anomaly of 200 nT with a negative rim on its northern side indicate a steeply dipping geological body of low density and containing magnetic rocks/minerals. Magnetic survey also showed pronounced local anomalies outside the depression that can reflect relicts of the tephra rim of the maar. This geophysical evidence was then proven by an exploratory drilling near the centre of the gravity anomaly. Macroscopic on-site evaluation of the core, and more detailed sedimentological, petrochemical, palynological and microbiological laboratory analyses further confirmed the existence of a maar structure filled by 84 m of lake sediments reflecting a succession of several warm and cold climatic periods. Results of palynological analyses confirm the presence of a continuous palaeoclimate archive, with at least three successive warmer periods of most probably interstadial character from the upper Quaternary Saalian complex. Therefore, the recovered sediment sequence holds strong potential for in-depth paleoclimate reconstruction and deep biosphere studies. At the bottom of the Mýtina-1 (MY-1) borehole (84–85.5 m), country rock debris was found, containing also volcanic bombs and lapilli. The discovered volcanic structure is considered to be the first known Quaternary maar-diatreme volcano on the territory of the Bohemian Massif. Because of hidden active magmatic processes in combination with earthquake swarm seismicity ca. 20–30 km north of the Mýtina maar, reconstruction of the palaeovolcanological evolution is important for evaluation of hazard potential of the NE and E part of the Cheb Basin.