Devonian succession in the East-Zilair zone: conodont-based subdivision and paleontological verification

Abstract

Research subject. Conodont-based biostratigraphic subdivision of the monotonous siliceous-terrigenous sequence stratigraphically underlying the Famennian Zilair Formation.Materials and methods. Conodont imprints on the bedding planes from our own collections along with the data from other researchers were used for dating and subdivision of siliceous formations.Results. Our study of conodonts from the deposits underlying the Zilair Formation at a number of intersections on the eastern side of the Zilair synclinorium showed that the stratigraphic scale of the straton, previously recognised as the “Betrinskaya Formation”, should be revised. Numerous collections from well-known and many new localities have proved that the terrigenous-siliceous deposits occurring stratigraphically below the graywackes of the Zilair Formation, in fact constitute the sequence of the entire Devonian section and include characterised by conodonts intervals of the upper Pragian and Emsian stages of the Lower Devonian, the Eifelian and Givetian stages of the Middle Devonian, and the Frasnian stage of the Upper Devonian. These deposits compose a lithologically homogeneous straton – the Novousmanovo succession with siliceous rocks prevailing in its section. The isolation of indicative siliceous members, in particular the Ibragimov horizon, is inexpedient due to the lithological uniformity of the succession as a whole, and its facies variability along the lateral. The limestone members included in it are dated by conodonts. They belong to different stratigraphic intervals and occur in situ in the section.Conclusions. The sediments stratigraphically underlying the Zilair Formation and fully comparable with the identified Novousmanovo succession are developed in the Kuragan-Sakmara zone (Akchura (D1), the remote facies of the Utyagulovo (D2 ef) and Sarbai (D2-D3f) Formations). In the West Magnitogorsk zone in the Voznesenka-Sakmara subzone below the Zilair Formation in a similar siliceous succession, a sequence in conodont zones of stratigraphic intervals of the Eifelian and Givetian stages (D2) and Frasnian stage (D3) united into the Aktau Formation are revealed.