Oxfordian neptunian dykes with brachiopods from the southern part of the Kraków-Częstochowa Upland (southern Poland) and their links to hydrothermal vents

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.