Abstract
Neptunian dykes with abundant brachiopods, cf. Lacunosella sp. and fragments of echinoderms, occur in Oxfordian limestones in the southern part of the Krakow-Czestochowa Upland. The dykes fill fissures that have opened in the massive limestones due to local extension of the sedimentary basin located along the northern, passive margin of the Tethys Ocean. These fissures transmitted warm hydrothermal solutions that controlled the mass growth of free-living bacteria and microbial mats feeding the fauna, mostly brachiopods and echinoderms, settling the seafloor around the fissures. For some time, the fissures remained empty and their vertical walls were settled by stromatolites. Infilling of the fissures was an abrupt event related to faulting in the Oxfordian and to the rejuvenation of dislocations cutting through the Paleozoic basement. Then, in the Cretaceous, and primarily in the Cenozoic, tectonic discontinuities filled with neptunian dykes were penetrated by karst waters and by hydrothermal solutions, which partly silicified the carbonate material infilling the dykes. The formation of dykes is genetically related to the Late Jurassic, Pan-European stress-field reorganization caused by the opening of the Northern Atlantic and Tethys Oceans.
Highlights
Neptunian dykes are defined as fissures within rocks exposed on the sea bottom that have been filled with submarine sediments (Flügel 2004)
Two neptunian dykes with abundant brachiopods were exposed in the 1980s and 1990s at the operating Młynka Quarry (MQ)
The last-studied neptunian dyke was exposed for a short time in 2001 in the Grodzisko Rock (GR), located in the northern slope of the Pradnik River Valley, in an excavation dug for the construction of a house (Krajewski 2004)
Summary
Neptunian dykes are defined as fissures within rocks exposed on the sea bottom that have been filled with submarine sediments (Flügel 2004). In the southern part of KCU, the dominant form is L. cracoviensis, characterized by local morphotypes (Rózycki 1948) of mostly symmetric shells and average sizes significantly larger than those observed in the northern KCU, where asymmetric forms prevail (Rózycki 1948; Wierzbowski 1970; Heliasz and Racki 1980). This species predominates in sediments filling the studied neptunian dykes
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