Abstract
The presented study deals with unusual geomorphological features discovered within the aeolian sand belt along the Morava River in southern Moravia (Czech Republic), situated on the northernmost margin of the Vienna Basin (Fig. 1). Geological substrate is represented by poorly permeable sandy clay belonging to the Upper Miocene (Bzenec and Dubňany formations).
The studied structures include 178 circular (n = 85) to semi-oval (n = 93) depressions with diameters ranging from 38 to 324 meters. The majority of these depressions are rimmed by significant ramparts.
In some cases the peat/swamp sedimentation occurred in the central part of these structures. Lithostratigraphic investigations carried out throughout the ramparts (Figs 1-3) revealed several distinct lithological units including (1) aeolian sand with thin Holocene soil on the top; (2) colluvial sediments (sand with layers of gravel), past thermal-contraction crack filled with aeolian sand has also been identified in this strata; (3) distinct layer of angular clasts with glassy surface lying on the boundary between colluvial deposits, and (4) Miocene substrate.
Mineral composition of these basal clasts consists almost exclusively of quartz; accessories include monazite, galena, clay minerals or K-feldspar. They are usually (micro)laminated and a lot of them comprise of organic remains (wood, stems, leaves, needles; Figs 4 and 5) radiocarbon dated to ~ 16.5 cal. kyr BP (Fig. 2).
Based on petrographic and geochemical investigations, we consider these rocks to represent remnants of the silica sinter precipitated under possibly hydrothermal conditions. Taking into account the presence of permafrost table in the area during the Late Weichselian, the origin of the ramparted circular depressions would be related to thermokarst processes.
We consider the studied landforms to represent remnants of the Late Pleiniglacial/Late Glacial hydraulic pingos and/or lithalsas. Such assumption is supported by comparison of morphometric and sedimentological characteristics of the studied rounded objects with pingo and lithalsa scars documented in the northern and north-western Europe as well as elsewhere in recent arctic areas.
In general, this study provides the first evidence of a largescale thermokarst activity in the Vienna Basin, and consequently brings up many further questions related to the Late Weichselian paleogeographic and paleoenvironmental development of this area.