Acta Musei Moraviae

Lower Tournaisian microbialite-rich facies at the Mokrá quarries, Czech Republic

Microbial carbonates are autochtonous buildups produced by growth and metabolic activities of benthic calcimicrobes. Microbial carbonates, e.g., stromatolites, thrombolites, dendrolites, and leiolites, were abundant during Precambrian, and are rather rare from the beginning of Phanerozoic due to the boom of metazoan grazing activity. However, despite this trend, microbialites repeatedly became abundant during short times of biotic crises and their aftermath, which were typical by metazoan decline and microbial proliferation. The lower Tournaisian limestone succession from the Mokra-central quarry, Czech Republic, provided unique opportunity to study microbial-rich facies, deposited in the aftermath of Devonian-Carboniferous mass extinction (Hangenberg biotic crisis). The evidence of microbial-metazoan buildups is indirect, provided by allochthonous clasts of microbialites reworked within carbonate turbidites of a lower part of a carbonate platform slope. The most abundant are fragments of thrombolites revealing clotted fabric and cross sections of cyanobacteria (Girvanella and Ortonella-like), and rather rare are transitional forms between thrombolites and leolites. Common is occurrence of Frutexites and renalcids (Renalcis, Chabakovia). Micritic intraclasts, bryozoans, brachiopods, crinoids, calcareous foraminifers, and calcareous algae are also abundant. Very important is occurrence of bryozoans, which grew in co-operation with calcimicrobes. The bryozoan association evidences that the microbialites have been originally formed in the littoral zone. The lower Tournaisian marine microbialites are quite rare. The microbialites from the Mokra quarry show similarities with the well-studied shallow marine microbial-metazoan reefs from the lower Tournaisian Gudman Formation, Queensland, Australia. The occurrence of microbialites, oolites, and micritic limestones in the lower Tournaisian at Mokra represents association of the anachronistic facies, characteristic for aftermaths of biotic crises during the Phanerozoic.