Conodont stratigraphy and correlation of the Ordovician volcanogenic and volcanogenic sedimentary sequences in the South Urals

Geological Institute, Russian Academy of Sciences, Moscow, Russia

Abstract

[1]  Conodont studies of three types of the South Uralian Ordovician sections related to various tectonic zones and, accordingly, to different elements of the Uralian paleoocean active margin, have provided a significant advancement of the Ordovician stratigraphy in the region. The siliceous basaltic type of section (Polyakovka Formation) in the Western Magnitogorsk zone was formed at different stages of the ocean basin evolution. The stratigraphic range of the formation is extended to the Upper Tremadocian-Ashgillian owing to a finding of redeposited Late Tremadocian Loxodus cf. latibasis Ji et Barnes. The other section types are recorded in structures of the Sakmara and Sakmara-Voznesenka zones. Sediments of the siliceous tuffaceous type (Kuragan Formation) were accumulated from the Arenigian to Ashgillian inclusive. The basal, Arenigian-Llanvirnian part of the Kuragan Formation, was deposited in a distal area of the marginal basin and its top (Novokursk sequence) was associated with foot environments of the Ordovician Guberlya ensimatic island arc. The island arc complex corresponding to a third, volcanogenic type of section, is represented by the successive Guberlya and Baulus formations. The Llanvirnian-earliest Caradocian age of the Guberlya Formation is estimated from the conodont evidence; its status as the Ordovician formation is regained; and its conformable contact with the Upper Caradocian-Ashgillian Baulus Formation is revealed. A set of massive sulfide ore deposits is associated with the Baulus Formation sediments. Similar in composition, Late Caradocian-Ashgillian conodont associations are recorded in supraore cherts of the Blyava and Komsomol'skoe deposits, and in many of the deposits located along the supraore cherts strike. Conodont "faunal beds'' are first distinguished in sections of the reported formations. The Ordovician interval of the Polyakovka Formation is characterized by eleven successive faunal beds. Five faunal beds are recorded in the Ordovician of the Kuragan Formation. The section of the Guberlya Formation includes the succession of six faunal beds embracing the upper Middle to lower Upper Ordovician. Two faunal beds are recognized in the Upper Ordovician Baulus Formation. The proposed South Urals Ordovician conodont scale that includes eleven stratigraphic units, is based on the successive alteration of conodont associations with the use of species of the Periodon, Pygodus, Ansella, and Protopanderodus lineages. The scale represents one of still few variants of conodont scales developed for the Tropical Domain of the Open-Sea or Open-Ocean Paleobiogeographic Realm. Eight deep-water and/or relatively deep-water biofacies are identified and analyzed.

Received 19 January 2008; accepted 27 April 2008; published 1 June 2008.

Keywords: Stratigraphy, Ordovician, conodonts, formations, "faunal beds'', biofacies, volcanogenic sedimentary deposits, South Urals.

Index Terms: 0410 Biogeosciences: Biodiversity; 0459 Biogeosciences: Macro- and micropaleontology; 3030 Marine Geology and Geophysics: Micropaleontology; 4950 Paleoceanography: Paleoecology.

Citation: Dubinina, S. V., and A. V. Ryazantsev (2008), Conodont stratigraphy and correlation of the Ordovician volcanogenic and volcanogenic sedimentary sequences in the South Urals, Russ. J. Earth Sci., 10, ES5001, doi:10.2205/2008ES000302.