An unusual paleokarst sedimentary rock in the Bohemian Karst (Czech Republic), and its regional tectonic and geomorphologic relationships

Abstract

An unusual type of paleokarst carbonate sedimentary rock has been found in the Bohemian Karst, Czech Republic. This well-layered coarse-crystalline limestone is reddish in color and occurs as horizontal or slightly inclined layers filling deep paleokarst cavities within karstified faults in Early Devonian marine-limestone host rocks. Field observations confirm that these sedimentary rocks are undoubtedly younger than the polyphased Variscan folding. Petrographic and geochemical studies indicate that the studied paleokarst limestone is quite different from the Silurian and Devonian limestones, from common cave speleothems, as well as from the typical clastic Late Cretaceous and Cenozoic karst sediments of the Bohemian Karst. Carbonate δ13C and δ18O stable isotopic ratios of the paleokarst limestone (δ13C from -4.0 to -6.2‰, δ18O from -6.2 to -8.4 ‰ PDB), differing both from marine Silurian and Devonian limestones and Cenozoic secondary cave carbonates of the area, correspond to a non-marine depositional environment. The paleomagnetic data suggest that magnetic properties of the studied rocks were acquired during the Early to Middle Triassic, although these ages have not yet been confirmed by finds of contemporaneous microfossils. The only microfossils that were extracted using dissolution methods consist of several younger plant and faunal remains including angiosperm pollen grain specimens belonging to the family Myricaceae, likely Tertiary in age. Their sporadic occurrence is interpreted as the result of secondary infiltrations into the millimeter-wide cracks and cavities, in correspondence with the thin-section data on diagenetic successions.