Graptolites, stratigraphy and depositional setting of the middle Llandovery (Silurian) volcanic-carbonate facies at Hýskov (Barrandian area, Czech Republic)


Authors: Štorch P

Published in: Bulletin of Geosciences, volume 76, issue 1; pages: 55 - 62;

Keywords: graptolites, stratigraphy, middle Llandovery, benthic fauna, volcanic-carbonate facies, shallow-water depositional setting,

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A graptolite assemblage comprising 13 species, and assigned to the lower to lowermost part of the Stimulograptus sedgwickii Biozone (middle Llandovery), is described from a tectonic block of volcanic-carbonate facies preserved along the Prague Fault at Hýskov near Beroun. The assemblage is considered to be a low-diversity relic fauna surviving the late convolutus (and/or early sedgwickii) extinction event. Pseudorthograptus insectiformis, Pristiograptus regularis solidus and Campograptus lobiferus cf. harpago, are the last survivors of the convolutus Biozone fauna. Petalolithus clandestinus sp.n. is confined to this level. Other species, namely Parapetalolithus praecedens?, Neolagarograptus tenuis, S. sedgwickii, Pristiograptus cf. variabilis and Torquigraptus magnificus, belong to the newly developing sedgwickii Biozone fauna. The graptolites are associated with a high diversity brachiopod-trilobite benthic fauna (Aegironetes-Aulacopleura Community) in which also dendroids, gastropods, tabulate and rugose corals, crinoids, bryozoans, conulariids and ostracods are common. Biostratigraphic correlation suggests that the sudden appearance of shallow water limestones (grainstones, packstones and lime mudstones) and bottom dwelling fauna in the otherwise anoxic, black-shale dominated lower Silurian of peri-Gondwanan Europe may be explained by rapid growth of local submarine volcano, simultaneous with eustatic sea-level fall. This setting developed in the sub-normally to normally oxygenated part of the middle-higher photic zone, influenced by both wave and current activity, in presumably subtropical conditions, similar to those that existed during the late Wenlock and early Ludlow in the Svatý Jan volcanic centre, as documented by the similar biodetrital microfacies and faunal communities.