2019 | Newly identified Pleistocene maar volcanoes in Western Bohemia (Czech Republic) used as palaeo-environmental and palaeo-volcanological archives

A set of two maar craters (Ztracený rybník and Bažina) was newly identified in Western Bohemia. The two new volcanic geomorphologic features are located on the western uplifted shoulder of the Cheb-Domažlice Graben (Fig. 1). With respect to other small preserved monogenetic volcanoes aligned along the Tachov Fault (Fig. 1), a Pleistocene age has also been ascribed to the two newly found maars. In contrary to the earlier discovered maars Mýtina and Neualbenreuth (Mrlina et al. 2009; Rohlmüller et al. 2017) which occur and had erupted in the phylites, Ztracený rybník and Bažina maars are confined to granites to granodiorites of the Smrčiny Pluton. The position of the craters in hard rocks results in a well-preserved morphology with sharp edges. The morphology of Ztracený rybník was studied using the digital elevation model based on LiDAR 5th generation data. The electrical resistivity tomography (ERT, Fig 1) and shallow boreholes (6-6.7 m in depth) were conducted to study the inner structure, and the uppermost part of the sedimentary infill of the crater. ERT data give evidence of c. 100 m thick fine-grained sedimentary infill within the crater, under which higher resistivities indicate coarser grained material or bedrock. The uppermost part of the sedimentary infill consists of Holocene peat and underlying lacustrine sediments. Preliminary radiocarbon dating of the organic layer in 6.6 m depth yields an age of approx. 45.7 cal kyr BP, indicating the sedimentation to have taken place during the interpleniglacial (MIS 3) characterised by millennial-timescale warming periods, known as Dansgaard-Oeschger cycles. Nevertheless, the chronostratigraphical position requires further validation as palynological results suggest an Early Pleistocene age of the sediment. Ztracený rybník and Bažina maars represent the northernmost occurrence of the Pleistocene volcanism associated with the Cheb-Domažlice Graben. Surprisingly, these two volcanoes located in a granitic pluton originated from maar- forming phreatomagmatic eruptions, whereas nearby Komorní hůrka, located in water-saturated sediments of the Cheb Basin, was formed mainly by dry Strombolian-type eruptions. The sedimentary infill of Ztracený rybník maar has a great potential for further research since it represents an important paleoclimatic archive and may provide evidence of climate change and landscape evolution on millennial to centennial timescales not only over the Early Pleistocene – the period from which a palaeoenvironmental information is quiet fragmental in this part of Europe.

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