Uvahy o puvodu konkavnich tvaru na nekterych lokalitach karbonskych arkoz ve strednich a v zapadnich Cechach

Arkose sandstones that crop out along southern erosive margins of the Carboniferous basins in central and western part of the Czech Republic display a variety of concave and cavernous cavities somewhat similar in appearance to tafoni and honeycomb weathering. These features, however, differ in that they include combinations of spectacular rising wall channels, rising sets of coalesced copulas, ceiling half tube channels, separate ceiling copulas, ceiling domepits and half-spherical upward-convex arches (Fig. 1). We argue that these particular combinations of the forms which are clearly arranged in spatially related groups represent components of transitional and outlet features of the morphologic suite of rising flow (MSRF; Klimchouk 2011) which, in turn, provide diagnostic evidence for hypogene speleogenesis.
On top of that, the walls of the cavities, prominent bedding planes and sub-vertical fractures that cut through the arkose sequences are locally covered by massive ferruginous crusts composed of hematite, goethite, authigenic quartz and gypsum. Immediately below this crust, 2-10 cm thick alteration zone of loose sandy sediment is commonly developed in which feldspars and secondary clay minerals were extensively dissolved while the primary quartz grains remained essentially unaltered. We speculate that these corrosive alterations may have been provided by aggressive, warm, CO₂-saturated Na-Cl brines that are known to occur in deeply buried Carboniferous strata further to the north and north-east of the area studied. The fluids apparently ascended upward through the system of sub-vertical north-south-striking tectonic fractures and migrated laterally outward along permeable bedding planes (Fig. 3). Similar type of fracturing and hydrothermal alterations has also affected the overlying Cretaceous sandstone formations.
As indicated by the apatite fission track analysis (AFTA) data, the alteration probably occurred beneath the surface but at relatively shallow depth, upon the stage of uplift that affected the Bohemian Massif since the Neogene period (Fig. 2). In this environment, the corrosion of the sandstone host may have been accelerated by the effects of mixing corrosion by deeply derived fracture-conduit fluids that interacted with meteoric waters. When the uplifted sandstone sequences eventually reached the surface, the hypogene cavities were subjected to the subaerial weathering processes including preferential weathering and fluvial and biogenic erosion the effects of which were superimposed on older hypogene features

References

Adamovič, J. - Cílek, V., ed.: Železivce české křídové pánve. - Knih. Čes. speleol. Společ. 38, 172 str. Praha.

Brophy, J. G. - Lahann, R. W. - Rupp, J. A. (2010): Selective Feldspar Dissolution in Response to Elevated CO2 Pressure. - AAPG Search and Discovery Article #90116, AAPG Eastern Section Meeeting, Kalamazoo, Michigan, 25-29 September 2010.

Bruthans, J. - Soukup, J. - Vaculíková, J. - Filippi, M. - Schweigstillová J. - Mayo, A. L. - Mašín, D. - Kletetschka, G. - Řihošek, J. (2014): Sandstone landforms shaped by negative feedback between stress and erosion. - Nat. Geosci. 7(8), 597-601. View article

Dublyansky, Y. V. (2014): Hypogene speleogenesis - discussion of definitions. In: Klimchouk, A. - Sasowky, I. D. - Milroie, J. - Engel, S. A. - Engel, A. S., ed.: Hypogene Cave Morphologies, 1-3. - Karst Water Inst. Spec. Publ. 18.

Glasmacher, U. A. - Mann, U. - Wagner, G. A. (2002): Thermotectonic evolution of the Barrandian, Czech Republic, as revealed by apatite fission-track analysis. - Tectonophysics 359, 381-402. View article

Groom, K. M. - Casey, D. A. - Mol, L. - Paradise, T. R. - Hall, K. (2015): Defining tafoni: Re-examining terminological ambiguity for cavernous rock decay phenomena. - Progress Phys. Geography 39/6, 775-793. View article

Jetel, J. - Rybářová, L. (1986): Nové poznatky o hydrogeologii permokarbonu roudnické a mnichovohradišťské pánve. - Zpr. geol. Výzk. v Roce 1985, 80-82.

Klimčuk, A. B. - Timochina, E. I. - Ameličev, G. N. - Dubljanskij, Ju. V. - Špetl, K. (2013): Gipogennyj karst Predgornogo Kryma i ego geomorfologičeskaja rol´. - 205 str. DIAJPI, Simferopol.

Klimchouk, A. B. (2011): Hypogene Speleogenesis: Hydrogeological and Morphogenetic Perspective. - 106 str. Nat. Cave Karst Res. Inst., Carlsbad. 2nd Edition.

Klimchouk, A. - Tymokhina, E. - Amelichev, G. (2012): Speleogenic effects of interaction between deeply derived fracture-conduit flow and intrastratal matrix flow in hypogene karst settings. - Int. J. Speleology 41 (2), 161-179. View article

Zeman, A. - Suchy, V. - Stejskal, M. - Janku, J. - Cermak, J. - Turek, K. (2000): Migration of fluids controlled by equidistant fracture systems: an example from Central Europe (Czech Republic, Slovakia and Austria). - J. Geochem. Explor. 69/70, 499-504. View article