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

Conodont Biofacies

[133]  The analysis of composition of the South Uralian conodont associations and of their predominant and attendant species provided the recognition of the Oepikodus, Periodon, and HDS biofacies.

The Oepikodus biofacies of the Middle Arenigian (= Oepikodus evae biofacies).
[134]  The Oepikodus evae biofacies is named for the predominant species Oepikodus evae Lindström and is considered as the equivalent of the Middle Arenigian Oepikodus biofacies.

[135]  The fauna of this biofacies includes dominating Oepikodus evae Lindström and Periodon flabellum (Lindström) and less common Protopanderodus rectus (Lindström) and Oistodus lanceolatus Pander. It is characteristic of the North Atlantic Realm. In the South Urals the fauna is recorded in the lower part of the Polyakovka Formation (Figure 2) and is associated with environments of an open oceanic basin.

[136]  The similar in composition and relationships of faunal components Middle Arenigian Oepikodus biofacies was described in the lower part of Bed 11, Green Point Formation [Johnston, 1987; Pohler, 1994], that was formed in situ in the lower part of the Western Newfoundland continental slope. This fauna is also dominated by O. evae Lindström and P. flabellum (Lindström) constituting about 80% of the total assemblage. Less numerous are Drepanodus arcuatus Pander, P. rectus (Lindström), and Paroistodus parallelus (Pander). The assemblage also includes rare Reutterodus andinus Serpagli and Bergstroemognathus extensus (Graves et Ellison) referred to the Juanognathus faunal component.

[137]  The absence of the Juanognathus component in the fauna of the Oepikodus biofacies in the South Urals (lower part of the Polyakovka Formation) and its low ratio in the same biofacies in the Western Newfoundland indicate the occurrence of different paleoenvironments in the discussed regions. In the former case it was a pelagic environment of the open ocean, far from land and relatively deep-water, probably below the thermocline. In the latter case the environment was restricted to the lower part of continental slope. Furthermore, the presence of the Juanognathus component in the fauna suggests the occurrence of conodonts above the constant thermocline level, since its species could not overcome this barrier [Dubinina, 1991, 2000]. However, both environments existed in open ocean conditions.

[138]  In the South Urals the synchronous conodont association was recorded in the lower part of the Kuragan Formation as well. It includes Bergstroemognathus extensus (Graves et Ellison) referred to the Juanognathus component (Figure 2) though the diagnostic species was not found there. The presence of the Juanognathus component indicates a more shallow, above the thermocline, and likely less remote from the shore marginal-marine environment. As in the Arenigian the Guberlya arc did not exist yet, the lower parts of the Kuragan and Polyakovka formations were accumulated in a single oceanic basin but, as shown above, in its different parts that varied in depth and distance from the shore.

The Periodon biofacies.
[139]  Six corresponding biofacies differing in composition and ratio of faunal components are recorded in the Upper Arenigian-Middle Caradocian interval. It is apparent that the distinction of six Periodon biofacies from the Upper Arenigian to mid-Caradocian presents difficulties in their interpretation, as for each biofacies a certain stratigraphic interval should be pointed out. In this paper we suggest to give them names for predominant species.

1.  Periodon biofacies of the Upper Arenigian (= Periodon flabellum biofacies).
[140]  The corresponding fauna includes the predominant Periodon flabellum (Lindström) and scarcer Protopanderodus rectus (Lindström) and Baltoniodus sp. It was encountered in the lower part of the Polyakovka Formation (Figure 2) and is associated with the open ocean environment.

2.  Periodon biofacies of the Lower Llanvirnian (= Periodon zgierzensis biofacies).
[141]  The predominant component of this fauna includes Periodon zgierzensis Dzik, Ansella jemtlandica (Löfgren), Paroistodus horridus (Barnes et Poplawski), and Walliserodus ethingtoni (Fahraeus). The attendant species are Drepanoistodus forceps (Lindström), Drepanodus arcuatus Pander, Strachanognathus parvus Rhodes, Protopanderodus sp. Scalpellodus viruensis (Löfgren), Dapsilodus viruensis (Fahraeus), and Baltoniodus medius (Dzik). The fauna (Figure 2) was studied in the mid-parts of the Polyakovka and Kuragan formations and in the lower portion of the Guberlya Formation, which correspond to an ocean basin, distal part of a marginal basin, and to the incipient Guberlya ensimatic island arc environments, respectively.

3.  Periodon biofacies of the Upper Llanvirnian (= Periodon aculeatus-Pygodus serra biofacies).
[142]  This fauna is dominated by Periodon aculeatus Hadding and Pygodus serra (Hadding). Significantly numerous faunal component is represented by Protopanderodus varicostatus (Sweet et Bergstrom) and Drepanoistodus suberectus (Branson et Mehl). Among attendant species are Ansella jemtlandica (Löfgren), Eoplacognathus robustus Bergstrom, and Dapsilodus viruensis (Fahraeus). The association was recorded in middle parts of the Polyakovka and Guberlya formations (Figure 2) and corresponds to an open ocean and Guberlya ensimatic island arc environments, respectively.

4.  Periodon biofacies of the uppermost Llanvirnian (= Periodon aculeatus-Pygodus protoanserinus biofacies).
[143]  The predominant component of the fauna involves Periodon aculeatus Hadding and Pygodus protoanserinus Zhang. Relatively numerous are Pygodus serra (Hadding), Pygodus anserinus Lamont et Lindström, Drepanoistodus suberectus (Branson et Mehl), and Spinodus spinatus (Hadding), whereas Ansella nevadensis (Ethington et Schumacher), and Protopanderodus cooperi Sweet et Bergstrom are attendant taxa (Figure 2). This fauna inhabited oceanic basin (mid-part of the Polyakovka Formation) and the ensimatic island arc area (mid-part of the Guberlya Formation).

5.  Periodon biofacies of the Lower Caradocian (= Periodon aculeatus-Pygodus anserinus biofacies).
[144]  The assemblage is dominated by Periodon aculeatus Hadding and Pygodus anserinus Lamont et Lindström. Common occurrence is characteristic of Protopanderodus varicostatus (Sweet et Bergstrom), Ansella nevadensis (Ethington et Schumacher), Panderodus gracilis (Branson et Mehl), and Spinodus spinatus (Hadding). The group of attendant species includes Drepanoistodus suberectus (Branson et Mehl), Drepanodus robustus Hadding, Protopanderodus cooperi (Sweet et Bergstrom), Dapsilodus similaris (Rhodes), and Walliserodus sp. (Figure 2). The fauna inhabited oceanic basin (middle-upper parts of the Polyakovka Formation), the ensimatic island arc area (upper part of the Guberlya Formation), and the island-arc foot area from the side of the marginal basin (Novokursk sequence of the Kuragan Formation) (Figure 6).

[145]  According to the relationship between faunal components, the third, fourth, and fifth Periodon biofacies of the South Urals are similar to the Pygodus-Periodon biofacies from northwestern China (Tarim, Kuruktag). This fauna that inhabited an open shelf (Tarim) and slope (Kuruktag) was referred [Wang and Qi, 2001] to relatively deep and cold-water environments of the North Atlantic Realm.

6.  Periodon biofacies of the "Middle'' Caradocian (= Periodon aculeatus to Periodon grandis biofacies).
[146]  The predominant component of this fauna includes the form transitional from Periodon aculeatus Hadding to Periodon grandis (Ethington). The following species are less common: Belodina compressa (Branson et Mehl), Spinodus spinatus (Hadding), Protopanderodus liripipus Kennedy et al., Ansella sp., and Baltoniodus sp. (Figure 2). The fauna inhabited oceanic basin (upper part of the Polyakovka Formation) and environment associated with the splitting of the island arc during riftogenesis (lower part of the Baulus Formation).

[147]  The Periodon biofacies of the South Urals belongs to that of the North Atlantic Realm, or more precisely to a part of this realm, still imperfectly studied [Zhen and Percival, 2003]. A similar in composition and relationship of faunal components Upper Arenigian Periodon biofacies is known from relatively deep-water environments around the North American continent and was described in many localities, for instance, from the lower part of the Western Newfoundland continental slope [Johnston, 1987; Pohler, 1994]. The Lower Llanvirnian Periodon biofacies was studied along the western margin of Baltica [Rasmussen, 1998] and in other open-ocean environments. The species characteristic of this biofacies are known in both open ocean environments of the Iapetus Ocean and on continental slopes of Laurentia, Baltica and other continents (South America, southern China). Rasmussen believes that this fauna does not indicate its attribution to a separate faunal province. However, the Periodon biofacies was suggested to be referred to a distinct subprovince of the North Atlantic Realm [Pohler, 1994]. We hold the former viewpoint.

The Hamarodus brevirameus-Dapsilodus mutatus-Scabbardella altipes or HDS biofacies of the Upper Caradocian-Ashgillian.
[148]  The fauna of the HDS biofacies includes (Figure 2): (1) the predominant species Hamarodus brevirameus (Walliser), Periodon grandis (Ethington), Protopanderodus liripipus Kennedey et al., Drepanodus robustus Hadding, and Drepanoistodus suberectus (Branson et Mehl) that are widespread in the North Atlantic Realm; (2) significant in number of specimens faunal component represented by Dapsilodus mutatus (Branson et Mehl), Scabbardella altipes (Henningsmoen), and Istorinus erectus Knupfer that are characteristic of the cold-water Mediterranean province of the North Atlantic Realm; (3) important though low in number component including Belodina confluens Sweet, Panderodus gracilis (Branson et Mehl), and Plectodina furcata (Hinde), i.e. species typical for the North American Midcontinent Realm; (4) few Yaoxianognathus ani Zhen et al. known from Australia and China.

[149]  The conodont fauna derived from the Upper Ordovician cherts and siliceous and tuffaceous shales in the South Urals, namely, from the uppermost Polyakovka, Kuragan, and Baulus formations (Figures 2 and 6), undoubtedly refers to the relatively deep-water and/or relatively cool-water HDS biofacies of the North Atlantic Realm [Sweet and Bergstrom, 1984] and bears rare, as a rule, synchronously redeposited, shallow, and likely more thermophilic elements of the North American Midcontinent type.

[150]  To date, it is found that the HDS biofacies was widely distributed in all three provinces, i.e. British, Baltic, and Mediterranean, of the North Atlantic Realm [Ferretti and Serpagli, 1999]. Moreover, the temperate (boreal) water masses are considered to be the most favorable for the HDS biofacies fauna [Stouge and Rasmussen, 1996]. The biofacies is known from a strongly diverse set of paleoenvironments [Armstrong and Owen, 2002; Ferretti and Barnes, 1997; Ferretty and Schonlaub, 2001; Ferretti and Serpagli, 1999; Stouge and Rasmussen, 1996; Sweet and Bergstrom, 1984; Zhen et al., 1999]. The depth of a basin evidently did not influence this fauna which is known from the deep shelf to marginal marine and open-ocean environments [Stouge and Rasmussen, 1996]. The HDS biofacies fauna of the South Urals is associated with the latter, namely, with an oceanic basin environment (upper part of the Polyakovka Formation), Guberlya ensimatic island-arc foot area from the side of a marginal basin (Novokursk sequence of the Kuragan Formation), and with environments associated with island arc splitting during riftogenesis (upper part of the Baulus Formation), which, as shown above, were also favorable for the black smokers formation.

[151]  Consequently, eight deep-water and/or relatively deep-water Lower-Middle-Upper Ordovician conodont biofacies of the South Urals are related to those of the North Atlantic Realm or, according to the new concept [Zhen and Percival, 2003], to biofacies of the biogeographic Open-Ocean (= Open-Sea) Realm of the Tropical Domain. We emphasize that the authors of the new concept on Ordovician conodont biogeography [Zhen and Percival, 2003] eliminated from consideration such terms as North Atlantic and North American Midcontinent realms, provinces, and faunal components. However, we retain in this paper these terms that were long used in the Ordovician conodont biogeography. Nevertheless, this is in no way to say that we ignore or disregard the new scheme [Zhen and Percival, 2003] of biogeographic units.


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.

Copyright 2008 by the Russian Journal of Earth Sciences

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