Graptolite assemblages and stratigraphy of the lower Silurian Mrákotín Formation, Hlinsko Zone, NE interior of the Bohemian Massif (Czech Republic)

Most of the graptolite biozones, previously recognized in the Llandovery succession of the classical Barrandian area of central Bohemia, are here identified in the heavily tectonized and poorly exposed, epizonal and contact metamorphic black slates and silicites of the Mrákotín Formation of the Hlinsko Zone. Graptolite material was collected by bulk sampling of loose boulders. The lower Llandovery (Rhuddanian) Cystograptus vesiculosus and Coronograptus cyphus biozones, Demirastrites triangulatus, D. pectinatus, “Monograptus” simulans, Pribylograptus leptotheca, Lituigraptus convolutus and Stimulograptus sedgwickii biozones of middle Llandovery (Aeronian) age and Rastrites linnaei, Spirograptus turriculatus, Streptograptus crispus and Monoclimacis griestoniensis biozones of late Llandovery (Telychian) age were identified in those loose boulders. The upper Telychian Torquigraptus tullbergi and Oktavites spiralis biozones were found in outcrop. The sedimentary succession of the Mrákotín Formation has been reconstructed, in particular by detailed correlation with the biostratigraphically well dated, graptolite-rich successions in the Barrandian area, Thuringia and elsewhere. 112 graptolite species are recorded; selected graptolites are figured and briefly discussed; Pseudorthograptus finneyi sp. nov. is described. There is little difference between the graptolite faunas of the Hlinsko Zone and the Barrandian area, but the lithological successions of the two areas differ. In the Hlinsko Zone the entire Llandovery is developed as anoxic black slates, siliceous slates and silicites. The black siliceous slates and silicites exhibit light coloured, quartz/chalcedony nodules such as characterize the basinal Silurian black shales and silicites in Thuringia and Sardinia. Barren greenish mudstone beds, which intercalate with the black graptolitic shales of the upper Llandovery Litohlavy Formation in the Barrandian area, are missing in the Hlinsko Zone. We assume that the Mrákotín Formation of the Hlinsko Zone was deposited near the depositional site of the coeval formations of Saxo-Thuringia and West Sudetes, rather than near the Barrandian area.