RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES5001, doi:10.2205/2008ES000302, 2008

Introduction

[2]  In the South Urals the Paleozoic complexes of various structural-and-formational paleozones are tectonically juxtaposed. They make up a system of nappes with facially different sections. It is evident that in this case geodynamic and paleogeographical reconstructions are possible only on the basis of detailed stratigraphy of different-sized allochthons. This is particularly true for the Ordovician volcanogenic and volcanogenic sedimentary sequences that are widespread in the South Urals. The stratigraphic subdivision of sequences using conodonts and correlation of the recognized units were performed on the fine-resolution mapping. We studied 204 Ordovician to Devonian localities, 95 of which are Ordovician sites. Conodont elements were found in cherts, phthanites, siliceous siltstones, tuffites, tephroids, and in other rocks at bedding surfaces or inside the sediments. In the latter case they are visible in translucent chips. At times, to reveal a complete association the rock was sawn up to thinnest plates. The Ordovician conodont associations are generally abundant, taxonomically diverse, and are mainly represented by North Atlantic and, rarer, by Midcontinent species. Several thousands of conodont elements are included in the conodont collection no. 4876 deposited in the Geological Institute, Russian Academy of Sciences.

[3]  In this work we took into account the revision of Ordovician units [Fortey, 1995; Fortey et al., 1995a] in the historically type area, Anglo-Wales. It primarily concerns the expanded range of the Llanvirnian and the reduced Llandeilian which now is not a separate stage but is considered as the Llanvirnian upper substage. The Llandeilian is defined by the single graptolite teretiusculus Zone. The range of Caradocian is widened at the expense of decreased Llandeilian. The Arenigian and Tremadocian ranges have also changed, namely, a part of Arenigian is referred to the latter stage. This became possible owing to the evidence for corresponding of the Arenigian base, i.e. of the graptolite Tetragraptus approximatus Zone lower boundary, to the level within the conodont proteus Zone as demonstrated in the sections from the Hunneberg Mountains, southwestern Sweden [Maletz et al., 1996]. Only the lesser part, upper third of the proteus Zone, is now referred to the Arenigian. The rest two thirds of the zone extended the Tremadocian range.

[4]  Consequently, the traditional British Ordovician presently includes five units, namely, Tremadocian, Arenigian, Llanvirnian, Caradocian, and Ashgillian with the lower boundaries: at the base of the approximatus Zone for the Arenigian, at the base of the artus (= ` bifidus') Zone for the Llanvirnian, at the base of the gracilis Zone for the Caradocian, and only the Ashgillian lower boundary corresponds not to the base but to the middle of the linearis Zone [Fortey, 1995].

[5]  The lower series of the Ordovician includes the Tremadocian and lower half of the Arenigian; the Middle Ordovician embraces the Arenigian upper half and the Llanvirnian; Caradocian and Ashgillian are referred to the Upper Ordovician [Mitchell et al., 1997].

[6]  The first occurrence of conodont Iapetognathus fluctivagus Nicoll et al. defines the base of the Lower Ordovician. The following GSSP suggestions as to the Middle Ordovician base are now considered: (1) first occurrence of conodont Baltoniodus? triangularis (Lindström) in the Huanghuachang section in China and (2) first occurrence of conodont Cooperignathus aranda (Cooper) in the Niquivil section in Argentine. Finally, the Upper Ordovician base corresponds to the lower boundary of the graptolite Nemagraptus gracilis Zone (Ordovician News no. 23, 2006; no. 24, 2007).

[7]  In this paper we do not discuss stages of the general scale, which development has yet to be completed. However, the correlation between the unit boundaries of the traditional British scale and the general zonation is already outlined (International Subcommission..., 2006 (http://www.ordovician.cn)).

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Figure 1
[8]  The following three types of sections are recorded among the Ordovician volcanogenic and volcanogenic sedimentary sequences: siliceous basaltic, siliceous tuffaceous, and compound volcanogenic; they vary in stratigraphic completeness and are related to different elements of the Uralian paleoocean active margin (see below). In relation to tectonic zonality of the South Urals these types of Ordovician sections are distributed as follows (see inset map in Figure 1): the siliceous basaltic type is recorded in the Western Magnitogorsk zone; the other types, in the Sakmara and Sakmara-Voznesenka zones.

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Figure 2
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Figure 3
[9]  Taken as the basis the previously published description of the sections [Ryazantsev et al., 1999, 2000, 2005] is supplemented with new conodont localities and supported by fresh records. Among these are the data on composition and successive alteration of conodont associations in certain sections; recognition of stratigraphic range of every association in a section (Figure 2); on these grounds the distinction of additional biostratigraphic units, namely, "faunal beds''; the use of overlapped ranges of conodont assemblages for establishing the boundaries of faunal beds (see below); correlation of certain sections in order to compile a composite section of a formation, where the recognized faunal beds hold their places; elucidation, where possible, of major conodont morphophylogenetic lineages; and correlation of faunal-bed successions in three types of the South Uralian sections with the conodont zonation of Balto-Scandia and traditional British stages (Figure 3).

[10]  On the same basis we refined correlations and analyzed the conodont biofacies in the region. We illustrated some diagnostic species of Arenigian (see Appendix, Plate 1), Llanvirnian (see Appendix, Plate 2), Uppermost Llanvirnian (see Appendix, Plate 3), Uppermost Llanvirnian - Caradocian (see Appendix, Plate 4), and Upper Caradocian - Ashgillian (see Appendix, Plate 5) conodont associations of the South Urals.

[11]  Conodonts were photographed in reflected light using stereomicroscope Leica MZ 8 (with binocular tube assembly) in the Geological Institute, Russian Academy of Sciences. Prior to imaging, the rocks were moistened with water, whether it be cherts bearing conodonts visible in translucent chips or bedding surfaces of tuffaceous siltstones with conodonts.

[12]  In this work we took into account both the most detailed conodont zonation of Balto-Scandia [Cooper and Sadler, 2004] and the record on siliceous sequences of Central Kazakhstan [Dubinina, 1991, 1998, 2000; Tolmacheva et al., 2001, 2004]. The latter is especially important for comparison with the South Uralian conodont scale presented in this paper. Unlike paleoenvironments of Balto-Scandia, the deep-water Central Kazakhstan and South Urals' environments belong to a part of the North Atlantic Realm that was recently considered [Zhen and Percival, 2003] as a separate paleobiogeographic Open-Ocean Realm of the Tropical Domain. Stratigraphic range of certain species is there wider than in the Ordovician sections of Balto-Scandia. In addition, we considered the records on Western Newfoundland [Johnston, 1987; Pohler, 1994] and northwestern China (Kuruktag) [Wang and Qi, 2001] that are also referred to that realm.


RJES

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.

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