Lower Palaeozoic of the Bardo Mountains (Sudetes): A sequence of deep-sea pelagic sediments

Abstract

Lower Palaeozoic of the Bardo Mts. consists of deep-sea terrigenous and siliceous sediments, at least a hundred and tens metres thick. These sediments have been subdivided into the following lithostratigraphic units: Jodłownik Beds (Upper Ordovician), Lower Graptolitic Shales (Llandoverian-Lower Ludlowian), Grey-greenish Shales (Upper Ludlowian-Přidolian), Upper Graptolitic Shales (Uppermost Přidolian-Middle Pragian), and the Żdanów Shales (Middle Pragian-?).
Lower part of the Jodłownik Beds is interpreted as pelagic sediments deposited in oxygenated bottom-water conditions, and the higher part of the unit as the complex of sediments deposited by subaquaeous gravity flows, mostly turbidity currents, and deep-sea traction currents. The Lower Graptolitic Shales, Grey-greenish Shales and the Upper Graptolitic Shales represent pelagic sediments tens of metres thick, whose accumulation took place in mostly euxinic conditions (Lower and Upper Graptolitic Shales) or in oxygenated bottom-water conditions (Grey-greenish Shales). The Żdanów Shales are interpreted as pelagic or hemipelagic sediments with subordinate intercalations of the deposits of low density turbidity currents, accumulated in oxygenated bottom-water conditions.
The sedimentary contact between the Jodłownik Beds and the Lower Graptolitic Shales was ascertained. In the author's opinion changes of the sedimentation mode (vanishing of the turbiditic sedimentation, initiation of radiolarian ooze sedimentation and euxinization of bottom waters of the sedimentary basin) recorded in the transition zone between the Jodłownik Beds and the Lower Graptolitic Shales took place due to the glacieustatic rise of the World's ocean level in the Earliest Silurian.