RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES5001, doi:10.2205/2008ES000302, 2008
[152] The findings of Ordovician conodonts in volcanogenic and volcanogenic sedimentary sequences of different tectonic zones in the South Urals first provided a significant advancement of stratigraphy in the region. To do this, we studied three types of Ordovician sections, namely, siliceous basaltic, siliceous tuffaceous, and volcanogenic differing in stratigraphic completeness and being associated with various elements of the Uralian paleoocean active margin.
[153] The siliceous basaltic type of section (Polyakovka Formation) in the Western Magnitogorsk zone was formed at different stages of the ocean basin evolution. It makes up a system of nappes which represent a fragment of the accretionary prism in front of the Devonian Magnitogorsk arc. The formation embraces the Upper Tremadocian-Ashgillian stratigraphic interval. The other types of section are connected by gradual facies changes and, partly, by stratigraphic transitions and are recorded in structures of the Sakmara and Sakmara-Voznesenka zones.
[154] The siliceous tuffaceous type of section (Kuragan Formation) has been formed from the Arenigian to Ashgillian inclusive. The Arenigian-Llanvirnian interval, i.e. the lower part of the Kuragan Formation, is mainly composed of tuffaceous sandstones and siltstones, which in the Caradocian-Ashgillian were replaced by siliceous tephroids of the Novokursk sequence (upper part of the Kuragan Formation). The former were accumulated in the distal part of a marginal basin; the latter were associated with environments of the Ordovician Guberlya ensimatic island-arc foot area.
[155] The island-arc complex corresponding to the volcanogenic type of section is represented by the successive Guberlya and Baulus formations.
[156] The conodont studies provided the age estimate of the Guberlya Formation as Llanvirnian-lowermost Caradocian; its status as the Ordovician formation is regained; and its conformable contact with the Upper Caradocian-Ashgillian Baulus Formation is recognized.
[157] The section of the Baulus Formation includes a set of massive sulfide ore deposits associated with the black smokers activity. Directly in the supraore cherts of the Blyava and Komsomol'skoe deposits and in many localities along the supraore cherts strike the similar in composition Late Caradocian-Ashgillian conodont associations are recorded.
[158] From the conodont evidence we distinguished additional biostratigraphic units or faunal beds in the sections of the studied formations. The Ordovician sections of the Polyakovka and Kuragan formations include successions of eleven and five faunal beds, respectively. The section of the Guberlya Formation is characterized by six successive faunal beds embracing the upper Middle to lower Upper Ordovician. The Upper Ordovician Baulus Formation includes two faunal beds.
[159] The Ordovician conodont scale of the South Urals comprises eleven (11) units and is based on successive alteration of conodont associations with the use of stratigraphic successions of the Periodon, Pygodus, Ansella, and Protopanderodus species, which does not contradict the present notion of morphophylogeny of these genera.
[160] The stratigraphic succession of the Pygodus species is of particular importance; its middle member P. protoanserinus provided the distinction of fine-resolution units. In the uppermost Llanvirnian analogs the P. protoanserinus and P. anserinus Beds replace the P. protoanserinus and P. serra Beds (Figure 3). The former are distinguished in the Polyakovka Formation; the latter, in the Guberlya Formation. There are strong grounds to believe that both units will be recognized in the single siliceous basaltic (oceanic) type of section. Then the Ordovician stratigraphic scale of the South Urals will include twelve (12) conodont units.
[161] The Ordovician conodont scale of the South Urals represents one of still few variants of scales related to the biogeographic Open-Ocean (= Open-Sea) Realm (OSR) of the Tropical Domain [Zhen and Percival, 2003]. The realm was dominated by cosmopolitan and widespread taxa, whereas endemics were of minor importance. This fact prevents from distinction of a province, i.e. smaller biogeographic unit within the Realm. Recall that among the scales of this Realm are the Upper Cambrian-Lower Ordovician Sarykum scale [Dubinina, 1991, 1998, 2000] and the Upper Cambrian-Middle Ordovician Burubaital scale [Tolmacheva et al., 2001, 2004] of Central Kazakhstan.
[162] We identified eight deep-water and/or relatively deep-water biofacies, i.e.
(1) "Middle'' Arenigian Oepikodus evae; (2) Upper Arenigian Periodon flabellum;
(3) Lower Llanvirnian Periodon zgierzensis; (4) Upper Llanvirnian Periodon aculeatus-Pygodus serra; (5) uppermost Llanvirnian Periodon aculeatus-Pygodus protoanserinus;
(6) Lower Caradocian Periodon aculeatus-Pygodus anserinus; (7) "Middle'' Caradocian Periodon aculeatus
Periodon grandis; and (8) Upper Caradocian-Ashgillian Hamarodus brevirameus-Dapsilodus mutatus-Scabbardella altipes or HDS biofacies. The listed
Ordovician conodont biofacies of the South Urals refer to those of still imperfectly studied
part of the North Atlantic Realm that was recently considered
[Zhen and Percival, 2003]
as a distinct paleobiogeographic Open-Ocean Realm of the Tropical Domain.
[163] The South Uralian conodont biofacies were compared within the studied region to an extent that the fossil material permitted. For the most part, they were compared with biofacies of the North Atlantic Realm or of the Open-Ocean Realm of the Tropical Domain using the literature records.
Citation: 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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