Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
The discovery of the oldest Ordovician fauna in NE flank of the Prague Basin (at Praha-Troja and Praha-Vokovice)
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A unique assemblage of the Lower Ordovician brachiopods has been collected in the NE flank of the Prague Basin. A diversified association of obolid, elkaniid, acrothelid, acrotretid and siphonotretid brachiopods (Leptembolon, Elkanisca, Orbithele, Dactylotreta, and Celdobolus) was found in temporary excavation at Praha-Troja (Pod Havránkou street). The host rock, which belongs to the basal Ordovician sequence in this area, consists of red-purple tuffitic siltstones that are considered to belong to the variegated acid volcanics and pyroclastic rocks. Similarly preserved but very rare findings of organophosphatic brachiopods (Celdobolus) have also been collected at Praha-Červený vrch in red-purple basaltic tuffs and tuffites outcropping at the base of the Ordovician strata close above the angular disconformity with Neoproterozoic rocks. These findings enable safe age determination of lower parts of the “volcanic facies” sensu Králík et al. (1985) developed at the base of Ordovician strata in this area. This “volcanic facies” represents the Komárov Volcanic Complex that was assumed to be forming roughly the lower third of total thickness of the Šárka Formation (Middle Ordovician, Darriwilian) in the NE flank of the Prague Basin. It is obvious that at least the lower parts of the “volcanic facies” are Lower Ordovician in age (Floian). Based on lithological similarity and on the identical faunal content, these beds correspond well with the lower part of the “Olešná Member” of the Klabava Formation, which is present in the western part of the Prague Basin (in neighbourhood of the villages of Komárov, Těně, Točník, Strašice and Stradonice). So far known the easternmost occurrence of the Olešná Member with similar brachiopod faunas was found at Libečov near Chrustenice. Nutrient-rich environment of shallow sea, periodically supplied with tuffitic material was locally favorable to be inhabited by some taxa of brachiopods. A thorough search for the remains of other fauna (including non-brachiopod) in these beds could significantly upgrade our knowledge of the initial stage of extension of the Prague Basin in its NE flank.References
