Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Origin of some ellipsoidal cavities by carbonate cement dissolution in the Jizera Formation sandstones, Kokořín area
Abstract
The origin of some regular (ellipsoidal, spherical) cavities, tens of centimetres to metres in size, in the Jizera Formation sandstone, Kokořín area, was proved to be connected with precursor calcareous concretions. Evidence is based on 1. uniform azimuths of cavity elongation, suggestive of their former lithological control, 2. the presence of dense systematic fracturing in cavity walls, passing not far to the ambient sandstone, 3. the presence of corroded remains of calcareous concretions in some of the cavities. Dense networks of tensional fractures limited to carbonate concretions embedded in sandstones have been reported from many sites worldwide. They form in response to far-field compressive stresses in the lithosphere and strike parallel to the maximum principal stress. After the concretion has been removed, either completely dissolved or partly dissolved and gravitationally detached, fractures in the cavity walls become modelled into a set of vertical ribs and grooves. This contribution is to stimulate a discussion as to what proportion of cavities in the sandstones of the Bohemian Cretaceous Basin are true tafoni formed principally by salt weathering.
