Abstract
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 (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 well-preserved morphology with sharp edges (Fig. 2). The morphology of Ztracený rybník was studied using the digital elevation model based on LiDAR 5th generation data provided by the State Administration of Land Surveying and Cadastre of the Czech Republic (Fig. 2).
The electric-resistivity tomography (ERT) and shallow boreholes (6-6.7 m in deep) were employed to study the inner structure, and the uppermost part of the sedimentary infill of the crater (Fig. 3). The ERT data reveal presence of ca 100 m thick sedimentary infill within the crater, under which a diatreme breccia was recognized (Figs 4 and 5).
The uppermost part of the sedimentary infill consists of Holocene peat and underlying lacustrine sediments. Radiocarbon dating of organic layer provides the age ~ 45.7 cal kyr BP, indicating the sedimentation to have taken place during the warmer phase of the last pleniglacial (interstadial MIS 3).
Nevertheless, this dating is not supported by the palynological results (Tab. 2), suggesting the Early Pleistocene age of the sediment. Ztracený rybník and Bažina maars represent the northernmost occurrences 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. Ztracený rybník with its sedimentary infill has a great potential for further research since it represents important paleoclimatic archive.
Another topic for further investigation appears to be a search for remnants of Ztracený rybník tephra in wider area.