Speleothems in 6 sandstone caves in the Bohemian Paradise (Český ráj) were dated by means of 14C and U-series methods. Stable isotopes of C and O, FAAS, IR, XRD, XRF and SEM were used to characterize the carbonate material and its source. Stable isotopes (C and O) composition of speleothems in two caves corresponds to values characteristic for cave speleothems in Central Europe. In other caves they indicate evaporation and fast carbon dioxide escape during carbonate precipitation. The speleothems from the Krtola Cave were deposited between 8 and 13 kyr BP. Speleothems were deposited 5-8 kyr BP in the Sintrová, Mrtvé Údolí and U Studánky caves. Calcite coatings on smooth sandstone surfaces in studied caves demonstrate that cave walls did not retreat even a few mm in the last 5-8 kyr since speleothem deposition and are thus not evolving under recent climatic conditions. Most of the cave ceilings and walls are at present time indurated by hardened surfaces, which protect the sandstone from erosion. Sandstone caves probably intensively evolved either during or at the end of the Last Glacial period. There are two different erosion mechanisms which might have formed/reshaped the caves at that time: A) In the case of permafrost conditions: Repeated freeze/melt cycles affecting sandstone pore space followed by the transport of fallen sand grains by minor temporary trickles. We expect that heat was transmitted by air circulating between the cave and the surface; B) Seepage erosion of sandstone during the melting of permafrost, prior forming of case hardening., Jiří Bruthans, Jana Schweigstillová, Petr Jenč, Zdeňka Churáčková and Petr Bezdička., and Obsahuje bibliografii
Different types of rock crusts and the underlying unweathered sandstone were sampled in the Bohemian Cretaceous Basin, Czech Republic. Structure and mineral composition of the samples were studied using optical microscopy, scanning electron microscopy with EDAX, and X-ray diffraction. Pore parameters were determined using mercury intrusion porosimetry/ helium pycnometry. Principal salts identified in the rock crusts and in the efflorescences are gypsum and alums. Two types of rock crusts were distinguished on morphological basis: 1. patterned rock crusts with a variety of weathering forms (honeycombs, wandkarren), and 2. armoured rock crusts with a relatively smooth, hardened layer. Patterned rock crusts on medium- to coarse-grained quartzose sandstones show an increase in the size of macropores relative to unweathered sandstone, which mostly implies an increase in total effective porosity. This is explained by the subflorescent growth of salt crystals, the force of which leads to the loss of contact among grains, pore widening, and granular disintegration. Armoured rock crusts on fine-grained clayey sandstone show a reduced volume and size of macropores, as these are filled with clay mineral aggregates and gypsum crystals. A prominent increase in the volume of micropores is due to secondary porosity in kaolinite and corrosion of feldspar grains. Insufficient passability of macropores in the armoured layer for pore waters shifts the evaporation front deeper into the rock. This results in contour scaling as the main process of rock-surface degradation, as opposed to granular disintegration on patterned rock crusts., Jiří Adamovič, Radek Mikuláš, Jana Schweigstillová and Vlasta Böhmová., and Obsahuje bibliografii
Weathering and erosion of sandstone landscapes often results in many amazing landforms such as arches, alcoves (rock shelters), pedestal rocks and pillars. Long-term research has produced numerous contrasting ideas for the origin of these landforms. The presence of salt and/or occurrence of freezing water and/or similar potential weathering/erosion processes at site are common causes of these landforms. The effect of gravity loading stress has been overlooked or assumed to increase the landform’s weathering rate. Research at the Institute of Rock Structure and Mechanics is based on field observations of locked sands and cemented sandstones and on physical experiments, followed by a numerical modelling. This may be the first time that the landforms cited above were reproduced in laboratory experiments. The Institute interpreted its findings by a novel mechanical model called “the concept of locus of fabric instability.” The results clearly show that an increase in stress within the landform (fabric interlocking) reduces weathering and erosion. Material with insufficient loading is rapidly removed by that weathering process and the remaining load bearing landform structure is protected by the fabric interlocking mechanism. The Institute concludes that its research that planar discontinuities in sandstone and negative feedback between stress and weathering/erosion processes are sufficient conditions to create landforms. and Michal Filippi, Jana Schweigstillová.