RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES5001, doi:10.2205/2008ES000302, 2008
Conodont stratigraphy and correlation of the Ordovician volcanogenic and volcanogenic sedimentary sequences in the South UralsS. V. Dubinina, and A. V. Ryazantsev Geological Institute, Russian Academy of Sciences, Moscow, RussiaContents
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. 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)).
[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. Siliceous Basaltic Type of SectionPolyakovka Formation.[13] The siliceous basaltic complex in the northern Magnitogorsk zone is recognized as the Polyakovka Formation and is dated as the Ordovician from conodont evidence [Borisenok et al., 1998; Ivanov et al., 1989; Ryazantsev et al., 1999; Zonenshain et al., 1984]. Previously the Polyakovka Formation distinguished within the Uchaly region [Koptev-Dvornikov et al., 1940], was subdivided into two sequences, volcanogenic and volcanogenic sedimentary one. Its age was estimated as Silurian based on graptolite findings from the overlying siliceous clayey shales.
The first nappe.[15] On the western flank of the pericline, near the western margin of the Polyakovka Village, the serpentinite melange is overlain by a complex of parallel doleritic dikes ranging in thickness from 5 cm to 1 m. Among them the sets of the dike-into-dike type with chilled contacts, are recorded. In the basal part the screens are represented by serpentinites and pyroxenites; at the top, by aphyric pillow basalts. The overlying 30-m-thick basalt member yields single sills. The basalts are overlain by a 60-m-thick member of red-brown, grey-green, dark grey, and black chert beds. The red-brown bed nearby the base, at Point 25 (Figures 1, 2, 3, and 4), contains numerous Pygodus protoanserinus Zhang, P. anserinus Lamont et Lindström, scarce P. serra (Hadding), a full range of numerous elements of Periodon aculeatus Hadding, as well as Ansella nevadensis (Ethington et Schumacher), Panderodus sp., Protopanderodus sp., and the redeposited Late Tremadocian Loxodus cf. latibasis Ji et Barnes (see Appendix, Plate 3, fig. 14). This association is characteristic of the Pygodus protoanserinus and P. anserinus Beds of the Polyakovka Formation (Figure 3), which correspond to the lowermost anserinus Zone of Balto-Scandia [Cooper and Sadler, 2004], i.e. to the uppermost Llanvirnian of Britain [Fortey, 1995].[16] Upward from the base a 20-m-thick member of clayey siliceous shales bearing pale-yellow chert lenses and rare beds of carbonaceous siliceous shales, is recorded. In this member, recovered by a small quarry, at Point 24 (Figure 4) Pygodus anserinus Lamont et Lindström, Periodon aculeatus Hadding, and Protopanderodus varicostatus (Sweet et Bergstrom) were encountered. Stratigraphic range of the assemblage corresponds to the uppermost Llanvirnian-Lower Caradocian (Figure 2). It is characteristic of the Pygodus anserinus, Periodon aculeatus, and Ansella nevadensis Beds of the Polyakovka Formation (Figure 3), corresponding to the upper part of the anserinus Zone of Balto-Scandia, i.e. to the lower part of the Caradocian Stage (Upper Ordovician). [17] This example shows that the stratigraphic range of the conodont association does not coincide with the range of the recognized faunal beds. As noted in the Introduction, we use a method of overlapped ranges of conodont assemblages (Figure 2) to establish the boundaries of faunal beds (Figure 3). In this instance, owing to overlapping of adjacent intervals of conodont assemblage ranges, the base of faunal beds is shifted upwards (Figures 2 and 3). Thus the recognized Pygodus anserinus, Periodon aculeatus, and Ansella nevadensis Beds do not include the uppermost Llanvirnian analogs but are referred only to those of the lower part of the Caradocian.
[18] At 2 km southwards a member of grey-green coarsely flaggy cherts (Point 58/4)
(Figures 1 and 4) overlies the 15-m-thick bed of pyroxene-plagioclase porphyrites.
The cherts contain abundant Pygodus protoanserinus Zhang (see Appendix, Plate 3,
figs. 7-9, 11), few P. anserinus Lamont et Lindström, P. serra (Hadding), a full
range of numerous elements of Periodon aculeatus Hadding, along with Ansella sp. and
Baltoniodus sp. The association is characteristic of the Pygodus protoanserinus and
P. anserinus Beds of the Polyakovka Formation (Figure 3), which correspond to the lowermost
anserinus Zone of Balto-Scandia, i.e. to the uppermost Llanvirnian. We can say about
a part of the anserinus Zone or, more precisely, about its lowermost portion based on
the studies of the Pygodus morphophylogeny. The succession P. serra
[19] At the northern extension of the nappe, west of the serpentinite stripe recovered by the quarry, a sequence of homogeneous, massive, grey-green aphyric basalts bearing rare grey chert lenses and crowned by a 10- to 13-m-thick horizon of massive grey-green and red-brown bedded cherts, is recorded. The horizon extends for a distance of 1 km, from Point 11/9 to Point 12 (Figures 1 and 4). At the latter point Pygodus serra (Hadding) and Periodon aculeatus Hadding, typical for the Upper Llanvirnian beds of the same name, were gathered (Figure 3). [20] As a whole the interval from the Upper Llanvirnian analogs to Lower Caradocian, i.e. from the upper Middle to lower Upper Ordovician inclusive, is estimated in the first nappe. The second nappe (Figures 1 and 4).[21] The faults separating the second nappe are associated with serpentinite stripes and lenses. The nappe is composed of a 150- to 250-m-thick sequence of aphyric and, rarer, porphyritic basalts with grey chert lenses and members bearing Llanvirnian conodonts.[22] On the northern flank of the antiform at Point 14 (Figure 4) the rocks contain Pygodus cf. serra (Hadding) and Periodon aculeatus Hadding characteristic of the Pygodus serra and Periodon aculeatus Beds (Figure 3) corresponding to the Upper Llanvirnian. [23] On the southern flank of the antiform at Point 59 (Figure 4) Periodon zgierzensis Dzik, Ansella jemtlandica (Löfgren), Drepanoistodus forceps (Lindström), and Protopanderodus sp. were encountered. They are peculiar to the Paroistodus horridus, Periodon zgierzensis, and Ansella jemtlandica Beds corresponding to the Lower Llanvirnian analogs (Figure 3). [24] The chert beds at the southwestern extension of the nappe are referred to the Pygodus protoanserinus and P. anserinus Beds (Figure 3) that correspond to the uppermost Llanvirnian. At Point 129 located 2.3 km southwest of Point 59 (Figures 1 and 4) the cherts yield Pygodus cf. protoanserinus Zhang, Periodon aculeatus Hadding, Drepanoistodus suberectus (Branson et Mehl), and Drepanodus sp. The third nappe.[25] (Figures 1 and 4) The section embraces the interval from the "Middle'' Arenigian to Lower Caradocian on the northern flank, and to the Ashgillian, on the southern one (Figure 4).[26] On the northern flank of the antiform the 100-m-thick section is exposed on the left bank of the Ui River, along the road connecting the Komsomolskii and Starobairamgulovo villages, nearby the northwestern foot of the Karaul'naya Mountain. The following units are traced from the north to the south (Figure 4): [27] 1. Grey-green aphyric basalts. Thickness 15 m. [28] 2. Grey-green, compact cherts, at Point 15/1 bearing Periodon flabellum (Lindström) (see Appendix, Plate 1, figs. 1, 4) and Protopanderodus sp., corresponding to the Periodon flabellum Beds (Figure 3), i.e. to the "Middle''-Upper Arenigian analogs. Thickness 10 m. [29] 3. Grey-green aphyric basalts. Thickness 25 m. [30] 4. Yellow-green, siliceous flaggy siltstones, at Point 15/3 bearing Periodon zgierzensis Dzik, Ansella cf. jemtlandica (Löfgren), Paroistodus cf. horridus (Barnes et Poplawski), and Pygodus sp., and representing the Paroistodus horridus, Periodon zgierzensis, and Ansella jemtlandica Beds (Figure 3) referred to the Lower Llanvirnian analogs. Thickness 12 m. [31] 5. Grey-green aphyric basalts. Thickness 15 m. [32] 6. Pale-yellow, flaggy siltstones, at Point 15/2 yielding Pygodus anserinus Lamont et Lindström, Periodon aculeatus Hadding, Protopanderodus sp., and Baltoniodus sp., corresponding to the Pygodus anserinus, Periodon aculeatus, and Ansella nevadensis Beds (Figure 3), i.e. to the Lower Caradocian analogs. Thickness 6 m. [33] At 1.2 km northeast of Point 15/2, at Point 2c (Figures 1 and 4) the similar rocks bearing Pygodus cf. anserinus Lamont et Lindström, Ansella nevadensis (Ethington et Schumacher), Periodon aculeatus Hadding, Dapsilodus viruensis (Fahraeus), Protopanderodus varicostatus (Sweet et Bergstrom), and Drepanodus robustus Hadding are correlative to the same faunal beds of the Lower Caradocian (Figure 3). [34] On the southern flank of the antiform the Ordovician rocks make up a wider stripe of exposures in the form of a ridge along the Krasnokhty River left bank (Figures 1 and 4). They are represented by grey-green and black chert members alternating with aphyric basalts and hyaloclastites. The second and third nappes are separated by the fault, south of which the 50-m-thick member of grey-green aphyric basalts interbedded with grey-green, dark grey, and, rarely, white cherts, is recorded. The latter, at Point 56/4 (Figure 1) yield Periodon flabellum (Lindström) and Baltoniodus sp. characteristic of the Periodon flabellum Beds that correspond to the Upper Arenigian (Figure 3). Further southward, at the extension of this horizon, at Point 32 (Figure 1) the rocks bear Periodon flabellum (Lindström) and Oepikodus intermedius Serpagli attributed to the Oepikodus intermedius Beds, i.e. to the "Middle'' Arenigian analogs (Figure 3). The term "Middle'' Arenigian is informal and so is used in quotation marks. [35] The younger stratigraphic units are recovered at Points 51 and 55. Nearby the southeastern foot of the ridge, in bedrock outcrops, grey cherts are exposed. At Point 51/2 (Figures 1 and 4) they contain Pygodus serra (Hadding) and Periodon aculeatus Hadding, specific for the Pygodus serra and Periodon aculeatus Beds that correspond to the Upper Llanvirnian (Figure 3). Upward along the slope the grey cherts are replaced by hard-rock outcrops of clotted, grey- and white-colored cherts bearing at Point 51/1 (Figures 1 and 4) numerous Pygodus anserinus Lamont et Lindström (see Appendix, Plate 4, figs. 1, 5), Periodon aculeatus Hadding and scarce Pygodus serra (Hadding), Plectodina sp., and Panderodus sp. Despite the absence of Pygodus protoanserinus Zhang, the combined occurrence of the two other species, namely, P. anserinus Lamont et Lindström and P. serra (Hadding), which occur concurrently only in the lower part of the anserinus Zone of Balto-Scandia [Zhang, 1998], makes it possible to consider the association as a faunal characteristics of the Pygodus protoanserinus and P. anserinus Beds (Figure 3) corresponding to the uppermost Llanvirnian analogs. The member is about 50 m thick. In the westward direction it grades into the 200-m-thick sequence of monotonously alternated grey hornfels cherts and grey-green aphyric basalts.
[36] In the middle part of the ridge the section is terminated by the 3- to 8-m-thick chert
bed. At Point 55 (Figures 1 and 4) the massive, grey-green cherts yield Periodon aculeatus
Hadding, the advanced, transitional to P. grandis (Ethington) form (see Appendix, Plate 4,
figs. 6, 7, 9-12), Belodina compressa (Branson et Mehl), and Baltoniodus sp., i.e. the
association characteristic of the Belodina compressa, Periodon aculeatus
[37] At 200 m northeast of Point 55 the massive, grey-green cherts at Point 532 (Figures 1 and 4) bearing Pygodus protoanserinus Zhang, P. anserinus Lamont et Lindström, P. serra (Hadding), Periodon cf. aculeatus Hadding, Walliserodus ethingtoni (Fahraeus), Drepanodus cf. arcuatus Pander, and Panderodus sp. are referred to the Pygodus protoanserinus and P. anserinus Beds (Figure 3) that are correlative to the lowermost anserinus Zone of Balto-Scandia, i.e. to the uppermost Llanvirnian. [38] The grey cherts occurring among basalts at the southwestern extension of the nappe on the left bank of the Krasnokhty River (Points 127 and 128) (Figures 1 and 4) correspond to the Upper Llanvirnian Pygodus serra and Periodon aculeatus Beds (Figure 3). At Point 127 the cherts contain Pygodus cf. serra (Hadding), Periodon cf. aculeatus Hadding, and Ansella sp. and in Point 128, Pygodus serra (Hadding), Periodon cf. aculeatus Hadding, and Panderodus sp. [39] The repeated alternation of aphyric basalts and grey-green cherts is observed 2.3 km northeast of Point 128. Here, at Point 58/10 (Figures 1 and 4) the yellow-brown foliated cherts yield Periodon grandis bellus (Moskalenko), Ansella erecta (Rhodes et Dineley), Scabbardella altipes (Hanningsmoen), Yaoxinognathus ani Zhen, Webby et Barnes, and Panderodus gracilis (Branson et Mehl) (Figure 2). The assemblage is characteristic of the Hamarodus brevirameus, Scabbardella altipes, and Periodon grandis bellus Beds of the Polyakovka Formation (Figure 3) and ranges from the Upper Caradocian analogs to the Ashgillian. [40] Some facies changes are recorded at the southwestern extension of the Polyakovka assemblage distribution area referred to the third nappe, on the right bank of the Krasnokhty River. The 200-m-thick lenslike sequence including aphyric basalts, hyaloclastites, lava breccias, yellow and dark red cherts in places substituted by red jaspers, is exposed north of the Malomuinakovo Village (Figure 1). In the upper part of the section, in red cherts at Point 6175/1 Paroistodus horridus (Barnnes et Poplawski), Periodon zgierzensis Dzik, and Ansella sp. were collected. They are characteristic of the Paroistodus horridus, Periodon zgierzensis, and Ansella jemtlandica Beds (Figure 3) assigned to the Lower Llanvirnian analogs. Further westwards they are replaced by the 100-m-thick sequence mainly composed of carbonaceous siliceous shales and basalt lenses. At Point 6176/1 (Figures 1 and 4) the rocks contain the predominant Periodon aculeatus Hadding, along with Pygodus protoanserinus Zhang, P. anserinus Lamont et Lindström, Drepanoistodus suberectus (Branson et Mehl), and Panderodus gracilis (Branson et Mehl) that are peculiar for the Pygodus protoanserinus and P. anserinus Beds (Figure 3) referred to the uppermost Llanvirnian analogs. These faunal beds are also correlative to the chert strata at Point 511 located 1 km southwest of the previous point (Figures 1 and 4). They bear Pygodus cf. anserinus Lamont et Lindström, P. serra (Hadding), Periodon aculeatus Hadding, Protopanderodus cf. cooperi Sweet et Bergstrom, Drepanoistodus suberectus (Branson et Mehl), Spinodus cf. spinatus (Hadding), and Panderodus sp. At 440 m northwest of Point 511 the pillow basalts with hyaloclastites and lenses of carbonaceous siliceous rocks bearing Early Silurian graptolites, are recorded [Maslov and Artyushkova, 2000]. [41] West of Point 6176/1, in a 70-m-wide stripe, the serpentinite melange with lenses of basalts decoupled by grey-green cherts is exposed. The conodont association derived at Point 6180/1 (Figures 1 and 4) includes the predominant in the assemblage, transitional from Periodon aculeatus Hadding to P. grandis (Ethington) forms, as well as Pygodus cf. anserinus Lamont et Lindström, transitional forms from Protopanderodus varicostatus (Sweet et Bergstrom) to P. liripipus Kennedy et al., Drepanodus robustus Hadding, Drepanoistodus suberectus (Branson et Mehl), Panderodus gracilis (Branson et Mehl), and Yaoxianognathus sp. The association is characteristic of the uppermost Pygodus anserinus, Periodon aculeatus, and Ansella nevadensis Beds that are referred to the Middle Caradocian analogs, i.e. to the anserinus-tvaerensis zonal boundary of Balto-Scandia (Figure 3). [42] The uppermost Ordovician rocks are also recorded at the southwestern extension of the Polyakovka assemblage distribution area. At Point 44/1 located 1 km east of the Abzakovo Village (Figures 1, 2, 3, and 4), in a small roadside quarry the aphyric basalts are interbedded with grey, grey-blue, and black cherts bearing Hamarodus brevirameus (Walliser) (see Appendix, Plate 5, figs. 2, 8), Scabbardella altipes (Henningsmoen), Protopanderodus liripipus Kennedy et al., Periodon grandis bellus (Moskalenko), Plectodina aculeatoides Sweet, Besselodus borealis Nowlan et McCracken, Belodina cf. confluens Sweet, Drepanoistodus suberectus (Branson et Mehl), and Ozarkodina sp. At 4 km southeast of the Abzakovo Village, at Point 48 (Figures 1, 2, 3, and 4) Hamarodus brevirameus (Walliser), Periodon cf. grandis (Ethington), and Belodina sp. were encountered. The conodont assemblages at both points are peculiar to the Hamarodus brevirameus, Scabbardella altipes, and Periodon grandis bellus Beds (Figure 3) that are attributed to the Upper Caradocian-Ashgillian analogs. [43] Consequently, the rocks of the third nappe embrace an interval from the "Middle'' Arenigian to Ashgillian inclusive, i.e. from the uppermost Lower to Upper Ordovician. The fourth nappe.[44] The fourth nappe is represented on the northern flank of the antiform (Figure 1) and includes the Arenigian to Llanvirnian interval (Figure 4). The Arenigian complex, 80 m to 100 m thick, unlike that of the third nappe, is composed of red-colored cherts, siltstones, specular schists, and grey-green aphyric basalts. Scarce grey-green chert lenses are recorded as well.[45] At Point 2 (Figures 1, 2, 3, and 4) the middle part of the red siltstone member yields Periodon flabellum (Lindström) and Paroistodus parallelus (Pander) characteristic of the Paroistodus parallelus and Periodon flabellum Beds that correspond to the Lower Arenigian (Figure 3). At Point 2a, 60 m to the west, the red with grey-green spots siltstones contain rare Pygodus serra (Hadding) and Ansella jemtlandica (Löfgren) along with numerous Periodon aculeatus Hadding; and 10 m upward from the base, at Point 2b, the green siliceous siltstones bear Pygodus cf. serra (Hadding). Both points are referred to the Pygodus serra and Periodon aculeatus Beds corresponding to the Upper Llanvirnian (Figure 3). Further westwards, behind the fault, the grey and red specular schists bearing the Arenigian Periodon flabellum (Lindström), are exposed at Point 2e (Figure 4). [46] At Point 1/2 (Figures 1, 3, and 4) the red-colored member recovered on the hill, on the Balbukovka River left bank and referred to the Paroistodus parallelus and Periodon flabellum Beds corresponding to the Lower Arenigian, yields P. cf. parallelus (Pander) and P. cf. flabellum (Lindström). In a northwestward direction the red-colored member is replaced by the 10-m-thick phthanite bed that contains at Point 1 (Figures 1, 2, 3, and 4) Walliserodus ethingtoni (Fahraeus), Scalpellodus gracilis (Sergeeva), Dapsilodus viruensis (Fahraeus), and Panderodus cf. gracilis (Branson et Mehl). This association is peculiar to the Paroistodus horridus, Periodon zgierzensis, and Ansella jemtlandica Beds corresponding to the Lower Llanvirnian analogs (Figures 2 and 3). Upward from the base grey-green aphyric basalts are exposed. [47] South of the Polyakovka-Voznesenka road, 1.5 km south-southeast of the Starobalbukovo Village, on the hillock (Point 22, Figure 1) the following units are recorded upward from the base (Figure 4): [48] 1. Siliceous, red with hematite crystals siltstones, at Point 22b bearing Oepikodus evae Lindström (see Appendix, Plate 1, figs. 2, 3, 5), Periodon flabellum (Lindström), Oistodus lanceolatus Pander, and Drepanoistodus forceps (Lindström), referred to the Oepikodus evae Beds (Figure 3) correlative with the "Middle'' Arenigian. Thickness 30 m. [49] 2. Grey siliceous siltstones, at Point 22c yielding Periodon flabellum (Lindström), Oepikodus cf. intermedius Serpagli, and Protopanderodus cf. rectus (Lindström), assigned to the O. intermedius Beds (Figure 3) that correspond to the "Middle'' Arenigian. Thickness 10 m. [50] 3. Grey aphyric basalts with grey-green chert beds, at Point 22d (Figure 4) containing Pygodus serra (Hadding) (see Appendix, Plate 2, figs. 6-8), Periodon aculeatus Hadding, Spinodus spinatus (Hadding), and Drepanoistodus suberectus (Branson et Mehl), referred to the Pygodus serra and Periodon aculeatus Beds (Figure 3) correlative with the Upper Llanvirnian. Thickness 20 m. [51] Thus the fourth nappe is composed of rocks ranging in age from the Lower Arenigian to Upper Llanvirnian except for the uppermost Llanvirnian. [52] The available conodont records indicate that the Polyakovka siliceous basaltic complex embraces the interval from Arenigian to Ashgillian inclusive. The stratigraphic range of the Polyakovka Formation can be extended owing to a finding of redeposited Late Tremadocian Loxodus cf. latibasis Ji et Barnes (see above). The state of preservation and color index, common to all conodont elements in the assemblage, suggest that the Late Tremadocian Loxodus cf. latibasis Ji et Barnes was redeposited just from the Polyakovka Formation. [53] The Polyakovka complex grades into the sequence of interbedded basalts and carbonaceous siliceous shales that can be correlated with the Dergaish Formation of the Orenburg region [Tishchenko and Cherkasov, 1985]. It is in turn conformably overlain by the carbonaceous shale sequence bearing Middle Llandoverian-Wenlockian conodonts and graptolites, which is the analog of the Sakmara Formation of the stratotype region. [54] The recognition of the Polyakovka Formation only in the range of the Middle Ordovician [Maslov and Artyushkova, 2000] seems improper. The composite section of the Polyakovka Formation,[55] which combines the sections of the four nappes, includes the succession of eleven faunal beds (Figure 3) correlated with the conodont zones of Balto-Scandia [Cooper and Sadler, 2004] and traditional stages of the British Ordovician, and referred to the Upper Tremadocian-Ashgillian stratigraphic interval.[56] The stratigraphic succession of the Pygodus, Periodon, Protopanderodus, and Ansella species recognized in the Polyakovka Formation, does not contradict the present notion of morphophylogeny of these genera.
[57] The following peculiarities of the discussed section should be noted: (1) the
successive chronological alteration of the species Periodon flabellum
Paleoenvironments during the formation of the siliceous basaltic complex.[58] The Ordovician Polyakovka siliceous basaltic complex of the Western Magnitogorsk zone represents an upper unit of the ophiolitic suite [Ruzhentsev, 1976; Ryazantsev et al., 1999]. The fragment of the ophiolitic section we observed (Figure 4) within the first nappe (see above). According to A. V. Ryazantsev, the siliceous basaltic complex was formed at different stages of evolution of the oceanic basin located east of the Ordovician Guberlya ensimatic island arc [Ryazantsev et al., 2005]. To date, the complex is studied in the set of deformed nappes which represent a fragment of the accretionary prism formed in front of the Devonian Magnitogorsk arc. The nappes are characterized by close occurrence of the fragments of sections from different paleooceanic zones, which vary, as shown above, in stratigraphic completeness and in composition of its certain parts.[59] The siliceous tuffaceous and compound volcanogenic section types of the Kuragan, Guberlya, and Baulus formations are connected by gradual facies changes and, partly, by stratigraphic transitions and are widespread in the Sakmara and southern Sakmara-Voznesenka zones. They are represented in the nappes separated by serpentinite melange and olistostromes. Siliceous Tuffaceous Type of Section
[62] Nearby the Blyava station (Figure 5a), on the slopes of the Kolnabuk gully, almost vertical occurrence of rocks is observed. The following units are recorded in westward direction upward from the base (Figure 6): [63] 1. Aphyric basalts decoupled by 0.5-1-m-thick lenses of red tuffaceous siltstones, at Point 19K2 bearing Tripodus sp. A Tolmach. and Drepanodus arcuatus Pander; and 30 m westwards at Point PK1A with Acodus delicatus Branson et Mehl. and D. arcuatus Pander, referred to the Bergstroemognathus extensus and Drepanoistodus forceps Beds corresponding to the upper Lower-"Middle'' Arenigian (Figures 2 and 3). Thickness 100 m. [64] 2. Red tuffaceous siltstones and siliceous tuffites decoupled by 2-10-m-thick beds and lenses of aphyric basalts and at Point 19K1 yielding Bergstroemognathus extensus (Graves et Ellison), Acodus delicatus Branson et Mehl, Drepanodus arcuatus Pander, and Drepanoistodus forceps (Lind.); at Point K66 bearing A. delicatus Branson et Mehl, Drepanodus conulatus Lindström, Scandodus furnishi Lindström, and numerous molds of inarticulate brachiopods and ostracodes. They are assigned to the same Bergstroemognathus extensus and Drepanoistodus forceps Beds, i.e. to the upper Lower-"Middle'' Arenigian analogs (Figure 3). Thickness 150 m. [65] 3. Red tuffaceous siltstones with rare lenses of grey-green tuffaceous siltstones. Thickness 300 m. [66] 4. Back of the fault, red siltstones similar to that of Bed 3, bearing gabbroid sills. Thickness 150 m. [67] 5. Massive alternated pink, violet, and pistachio-colored tuffites. Thickness 50 m. [68] In the stratotype area, 1.4 km northeast of the Kidryasovo Village (Figure 5), on the right bank of the Pis'menka River, the cherry-colored tuffaceous argillites of the Kuragan Formation yield Arenigian-Llanvirnian conodonts identified by S. V. Dubinina [Korinevskii, 1988]. [69] The younger units of the section were studied in the Shaitantau Mountains northwest of the Novokurskii Village (Figure 5b). The Shaitantau Mountains are mainly composed of the Silurian Sakmara Formation making up the core of the synform, on which flanks the Ordovician Kuragan rocks are exposed. The Ordovician and Silurian sediments are tectonically overlain by the Ordovician Guberlya Formation and by the Devonian siliceous basaltic complex. [70] The basal part of the section on the western synform flank is represented by poorly exposed red-colored tuffaceous siltstones bearing grey lenses. The rock is enriched with inarticulate brachiopod and ostracode detritus. The red siltstones at Point 7 (Figure 6) contain Drepanodus arcuatus Pander, Scalpellodus cf. S. viruensis (Löfgren), and Ansella jemtlandica (Löfgren) and are referred to the Ansella jemtlandica Beds that correspond to the Lower Llanvirnian (Figure 3). [71] On the eastern flank of the synform (Figures 5b and 6) the red tuffaceous siltstone sequence is overlain by the 50- to 150-m thick member of red and grey-green siliceous tephroids bearing scarce lenses of variegated tuffaceous sandstones. The tephroids at Point 1 yield Hamarodus brevirameus (Walliser), Protopanderodus liripipus Kennedy et al., Panderodus gracilis (Branson et Mehl), Scabbardella altipes (Henningsmoen), and redeposited Periodon aculeatus Hadding and are referred to the Hamarodus brevirameus, Dapsilodus mutatus, and Scabbardella altipes Beds corresponding to the Upper Caradocian-Ashgillian (Figure 3).
[72] Crowns of the eastern ridges in the Shaitantau Mountains are made up of the thrust
sheet, in the lower part of which grey siliceous tephroids at Point 2
[73] It is conformably overlain by carbonaceous, clayey siliceous and dolomitic shales of the Sakmara Formation, 250-300 m thick (Figures 5b and 6). At Point 5 they contain Llandoverian conodonts Ozarkodina aldridgei Uyeno et Barnes and Dapsilodus obliquicostatus (Branson et Mehl) and at Point 6, Llandoverian graptolites Lagarograptus acinaces (Tornquist). [74] According to composition, the red-colored tuffaceous siltstone sequence at the base of the Shaitantau section is correlated with the Kuragan sediments from the Blyava Station area (Figure 6). The overlying siliceous tephroids are recognized as the Novokursk [Khvorova et al., 1978] or Pis'menka [Puchkov, 2000; Puchkov et al., 1990] sequence in the upper part of the Kuragan Formation. In this paper we accept the first variant. [75] Both sequences of the Kuragan Formation, i.e. the lower (red-colored tuffaceous siltstone with basalt flows) and the upper (siliceous tephroid) Novokursk sequence, make up the northeastern flank of the Utyagulovo synform on the left bank of the Sakmara River, east of the Churaevo and Yumaguzino 1 Villages (Figure 5). [76] Nearby the Yumaguzino 1 Village, on the right bank of the Tereklya River (left tributary of the Sakmara River) (Figure 6), the red tuffaceous sandstones and siltstones are exposed at the ridge foot. The slope and top of the ridge are composed of mainly grey and grey-green siliceous tephroids. The siliceous rocks are decoupled by variegated tuffaceous sandstones and gravelstones. The beds steeply (60o-70o) dip westward (overturned occurrence). [77] The grey tephroids (enriched in sand-sized tephra) at Point K8a (Figure 6), 40 m above the top of the red sequence, yield Hamarodus brevirameus (Walliser), Scabbardella cf. altipes (Henningsmoen), Periodon grandis (Ethington), Protopanderodus cf. liripipus Kennedy et al., Amorphognathus cf. superbus (Rhodes), Prioniodus cf. gerdae Bergstrom, and Panderodus gracilis (Branson et Mehl), and are referred to the Hamarodus brevirameus, Dapsilodus mutatus, and Scabbardella altipes Beds (Figure 3) corresponding to the Upper Caradocian-Ashgillian. The similar rocks exposed 50 m upward from the top, at Point K8 (Figures 2 and 6), and bearing H. brevirameus (Walliser), P. liripipus Kennedy et al., P. grandis (Ethington), Drepanodus robustus (Hadding), Istorinus sp., Baltoniodus sp., Amorphognathus sp., and Panderodus sp., are assigned to the same faunal beds. The similar conodont assemblage was recorded 70 m above, in the member of manganese-bearing siliceous rocks. This association at Point K6 (Figures 2, 5, and 6) includes H. brevirameus (Walliser), Dapsilodus mutatus (Branson et Mehl), P. liripipus Kennedy et al., P. grandis (Ethington), Drepanoistodus suberectus (Branson et Mehl), Panderodus gracilis (Branson et Mehl), and Spinodus sp. and is peculiar to the same faunal beds corresponding to the Upper Caradocian-Ashgillian analogs (Figure 3). The siliceous tephroid (i.e. Novokursk) sequence is overlain there by the carbonaceous shales of the Sakmara Formation (Figure 6). [78] North of the Yumaguzino 1 Village and 1.3 km northwest of Point K8a, the Novokursk sequence becomes up to 230 m thick [Ruzhentsev, 2005]. The thickness of the overlying carbonaceous shales of the Sakmara Formation increases as well. [79] Twenty meters above the top of the red-colored sequence, at Point NK-2 (Figure 6), the grey siliceous tephroids of the Novokursk unit contain Pygodus anserinus (Lamont et Lindström), Periodon aculeatus Hadding, Panderodus gracilis (Branson et Mehl), and Protopanderodus sp. and are referred to the Pygodus anserinus and Periodon aculeatus Beds correlative with the uppermost Llanvirnian-Lower Caradocian (Figure 3). [80] The light grey tuffosilicites 100 m above the top of the red-colored sequence, at Points 264 and 264a (Figures 2 and 6) bear Hamarodus brevirameus (Walliser), Protopanderodus liripipus Kennedy et al., Scabbardella altipes (Henningsmoen), Periodon cf. grandis (Ethington), Belodina compressa (Branson et Mehl), Panderodus gracilis (Branson et Mehl), and Drepanoistodus suberectus (Branson et Mehl) and are assigned to the Hamarodus brevirameus, Dapsilodus mutatus, and Scabbardella altipes Beds (Figure 3) corresponding to the Upper Caradocian-Ashgillian. [81] The fragment of the section that includes the contact between the red-colored siltstone and siliceous tephroid Novokursk sequences is recorded further southward, nearby the crossing of the roads to Churaevo and Kuvandyk [Ryazantsev et al., 2005, Fig. 9]. The grey siliceous tephroids at Point B-38 yield Protopanderodus liripipus Kennedy et al., Periodon grandis (Ethington), Panderodus gracilis (Branson et Mehl), and Plectodina sp. The association is characteristic of the Hamarodus brevirameus, Dapsilodus mutatus, and Scabbardella altipes Beds (Figure 3) corresponding to the Upper Caradocian-Ashgillian analogs. [82] Thus the siliceous tuffaceous type of section (Kuragan Formation) was accumulated from the upper Lower Arenigian to Ashgillian inclusive. The composite section of the Kuragan Formation.[83] It includes the succession of five faunal beds (Figure 3) that are correlated with the conodont zones of Balto-Scandia [Cooper and Sadler, 2004] and traditional Ordovician stages of Britain, and embrace the interval from the upper Lower Arenigian to Ashgillian. In two points of the Kuragan section the continuity of faunal beds is broken (Figure 3).
[84] The stratigraphic succession of the species Periodon aculeatus
Paleoenvironments during the formation of the siliceous tuffaceous complex.[85] As shown above, in the Ordovician siliceous tuffaceous complex (Kuragan Formation) the lower and upper parts are clearly distinguished. The lower part represented by red-colored tuffaceous sandstones and siltstones with basalt flows, was likely formed in a distal part of the marginal basin (Figure 6) that was located west of the Ordovician Guberlya ensimatic island arc [Ryazantsev et al., 2005]. Formation of the upper part of the complex, which is composed of red- and grey-colored siliceous tephroids (Novokursk sequence of the Kuragan Formation), was associated with environments of an island arc foot (Figure 6).Compound Volcanogenic Type of Section[86] Sections of this type are characteristic of the island arc Guberlya and Baulus formations; they are succeeding and are connected by gradual facies changes. The Guberlya Formation.[87] The Guberlya Formation was distinguished on the eastern slope of the South Urals by K. E. Razumovskaya in 1941 and is recorded in the discussed region as well [Ruzhentsev, 1976]. According to the above authors, the Guberlya Formation overlies with a gradual transition the Kuragan Formation. Other geologists [Kheraskov and Milanovskii, 1953; Sidorenko et al., 1964] considered them as synchronous units.
[89] In the stratotype area (Figure 7) the Ordovician Kidryasovo and Guberlya formations were distinguished [Sharfman and Tsetlin, 1968]. The Tremadocian age of the Kidryasovo Formation was defined from the trilobite and brachiopod evidence, whereas the Guberlya Formation was estimated as Middle or Early-Middle Ordovician based on its correlation with the fauna-bearing Kuragan Formation. [90] Subsequently, in the distribution area of the Guberlya Formation the Devonian organic remains were found in limestones [Sharfman and Tsetlin, 1968] and cherts [Puchkov and Ivanov, 1985]. As a result of finding in cherts of Early-Middle Devonian conodonts made by K. S. Ivanov and V. N. Puchkov, the Guberlya Formation was considered invalid and was eliminated from stratigraphic scales [Antsygin et al., 1993]. The volcanogenic sedimentary, mixtite-bearing rocks distributed in the area were suggested to consider the analogs of the Devonian Kosistek Formation. [91] Our research revealed that the discussed area is mainly occupied by the Ordovician volcanogenic sedimentary rocks (Guberlya Formation) and by the olistostrome yielding cherty olistoliths with conodonts ranging in age from the Lochkovian to Lower Famennian inclusive [Borisenok and Ryazantsev, 2005; Ryazantsev et al., 2005]. The Tremadocian terrigenous sediments of the Kidryasovo Formation are locally distributed there. [92] The Guberlya Formation is variable in composition. It is largely made up of tuffites and pillow basalts with lesser participation of rhyolites. The basalts contain limestone lenslike beds, small lenses, and interpillow bodies and are decoupled by horizons, members, and lenses of siliceous tuffites that commonly look like smears on pillow surface. [93] The relatively undisturbed section occurs on the left bank of the Guberlya River, west of the Guberlya station (Figures 6 and 7). It begins with the pillow basalt member, less than 100 m thick, decoupled by 0.5-2-m-thick horizons of red tuffites, and is further built up by the 180-m-thick red tuffite member. The overlying sequence of over 500 m thick is represented by alternated lenslike light pistachio, rarer red, tuffites bearing beds of quartz rhyolite and basalt tuffs. The tuffites of the lower member at Point 424 (Figures 6 and 7) yield Periodon zgierzensis Dzik, Walliserodus ethingtoni (Fahraeus), Ansella jemtlandica (Löfgren), Oistodus? tablepointensis Stouge, Dapsilodus cf. mutatus (Branson et Mehl), Drepanodus robustus Hadding, and Protopanderodus cf. liripipus Kennedy et al. sensu Stouge [1984]. The association is characteristic of the Periodon zgierzensis and Ansella jemtlandica Beds corresponding to the mid-Llanvirnian (Figure 3). [94] On the right bank of the Guberlya River, at Point B-87 (Figures 6 and 7), the 10-cm-thick red tuffite bed among basalts contains Ansella nevadensis (Ethington et Schumacher), Periodon aculeatus Hadding, Dapsilodus similaris (Rhodes), and Walliserodus sp. This assemblage is specific for the Pygodus anserinus, Periodon aculeatus, and Ansella nevadensis Beds (Figure 3) correlative with the uppermost Llanvirnian-Lower Caradocian analogs. [95] Further northward the section is mainly composed of monotonous, red and pistachio-colored tuffites bearing rare members of basalts and basalt tuffs with carbonate cement. The following conodonts were found on the left bank of the Malaya Kayala River, in the 10-20-m-thick red tuffite bed, at Point B-102 (Figures 6 and 7): Pygodus serra (Hadding), Periodon aculeatus Hadding, Eoplacognathus cf. robustus Bergstrom, Dapsilodus viruensis (Fahraeus), Protopanderodus varicostatus (Sweet et Bergstrom), Drepanoistodus suberectus (Branson et Mehl), Plectodina sp., and Ansella sp. (Figure 2). The association is characteristic of the Eoplacognathus robustus and Pygodus serra Beds (Figure 3) and provides their correlation with the robustus Subzone of the serra Zone in Balto-Scandia, i.e. with the uppermost Llanvirnian. [96] Sections of the Guberlya Formation were studied beyond the stratotype area as well. [97] On the northeastern flank of the Utyagulovo synform (Figure 5) tuffs, tuffites, and basalts are more common in the section. The following units are recorded on the left bank of the Sakmara River, on the watershed of the Almash and Kyzyl Yar gullies, nearby their junction (Figure 6): [98] 1. Frequently alternating fine-clastic tuffs of quartz and basic rhyolites and grey-green, turquoise-colored and, rarer, red ashstones and tuffites. Aphyric basalt lenses occur in the upper part. Total thickness is 300 m. [99] 2. Red siltstones and tuffaceous siltstones, bearing grey-green tuffite lenses, numerous molds of ostracodes and inarticulate brachiopods, and at Point K-32 including Baltoniodus medius (Dzik), Strachanognathus parvus Rhodes, Periodon aculeatus Hadding, Drepanodus arcuatus Pander, and Protopanderodus sp. referred to the Baltoniodus medius and Strachanognathus parvus Beds (Figure 3), i.e. to the Lower Llanvirnian analogs. Thickness 20 m. [100] 3. Grey-green, tinged with violet, coarse-bladed aphyric basalts. Thickness 200 m. [101] Nearby the base of the section this structure is complicated with fault wedges composed of ultrabasites and siliciclastic rocks bearing Early Devonian (Emsian) conodonts. North of the structure the section is represented by tuffaceous rocks of the Kuragan Formation. [102] The section of the Guberlya Formation was also studied at the eastern margin of the Churaevo Village (Figure 5). Here, at the northern foot of the mountain with a fire-prevention post at the top, above the poorly exposed red tuffaceous siltstones of the lowermost Kuragan Formation, the following units are recorded in the westward direction (Figure 6): [103] 1. Grey-yellow tuffaceous sandstones and gravelstones with a peculiar pseudopillow shelly parting. Thickness 15 m. [104] 2. Interbedded red and pink tuffaceous sandstones and siltstones, yellowish and greenish tuffites, and red radiolarites. The red-brown tuffaceous siltstones with 0.5-cm-thick tephra layers of mixed composition at Point E-26 (Figures 5 and 6) contain numerous large Spinodus spinatus (Hadding), Walliserodus ethingtoni (Fahraeus), Protopanderodus liripipus Kennedy et al., and Drepanodus robustus Hadding (Figure 2) and are referred to the upper part of the Pygodus anserinus, Periodon aculeatus, and Ansella nevadensis Beds (Figure 3) corresponding to the uppermost Llanvirnian-Lower Caradocian. Thickness 30 m. [105] 3. Tuffaceous siltstones and sandstones decoupled by tuffaceous conglomerate lenses. Thickness 60 m. [106] In the Shaitantau Mountains sections of the Guberlya and Kuragan formations are tectonically juxtaposed. The Guberlya Formation fringes the Shaitantau Mountains with a narrow stripe marking the common antiform structure (Figure 5b). [107] South and east of the mountains the section is composed of rhythmically alternated, variegated, sand- and silt-sized tuffites. The rhythms commonly begin with a gravel-sized sediments. Southeast of the Shaitantau Mountains, 500 m southwest of the western margin of the Novokurskii Village, the tuffites terminate with a 5-mm-thick layer of grey cherts yielding at Point 3-15 (Figure 5b) Spinodus spinatus (Hadding), Periodon aculeatus Hadding, Protopanderodus cooperi (Sweet et Bergstrom), Drepanodus cf. robustus Hadding, Panderodus gracilis (Branson et Mehl), Drepanoistodus suberectus (Branson et Mehl), and Ansella sp. (Figure 2). These sediments correspond to the Protopanderodus cooperi Beds, i.e. to the Upper Llanvirnian analogs (Figure 3). [108] On the flanks of the Blyava synform (Figure 5) the tuffaceous sediments typical for the Guberlya Formation are associated with tuffaceous sandstones and siltstones bearing basalt flows, which are characteristic of the lower part of the Kuragan Formation, and occupy a higher structural and, most likely, stratigraphic position. Eastern part of the Sakmara-Voznesenka zone.[109] The volcanogenic and tuffaceous rocks recorded in the area represent the analogs of the Guberlya Formation. They make up small-sized tectonic lenses in the structure of serpentinite melange between the Nurali and Tatlambet ophiolitic massifs (Figure 1).[110] At 250 m southeast of the Absalyamovo Village (Figure 1) the 40-m-thick block of grey-green aphyric basalts bears the 15-m-thick, greenish, siliceous tuffite bed. The tuffites at Point 669 (Figures 1 and 6) contain Pygodus protoanserinus Zhang, P. serra (Hadding), Periodon aculeatus Hadding, Protopanderodus varicostatus (Sweet et Bergstrom), Drepanodus arcuatus Pander, Panderodus sp., and Ansella sp. (Figure 2) and are assigned to the Pygodus protoanserinus and P. serra Beds (Figure 3) correlative with the uppermost Llanvirnian. [111] The serpentinite melange located 2.2 km northwest of the Yal'chigulovo Village (Figure 1) is overlain by the 20-m-thick mixtite horizon bearing ophiolite rock fragments. The mixtites are replaced upwards by the 200-m-thick grey-green aphyric basalts and basaltic andesites. In the upper part of the section, at Point 6033 (Figures 1 and 6), the 6-10-m-thick lenses of grey-green, brown and dark grey cherts yield Periodon aculeatus Hadding, Pygodus cf. anserinus Lamont et Lindström, Drepanodus robustus Hadding, and Ansella sp., referred to the Pygodus anserinus, Periodon aculeatus, and Ansella nevadensis Beds (Figure 3), i.e. to the analogs of the uppermost Llanvirnian-Lower Caradocian. [112] Thus, the research of sections of the Guberlya Formation located within and beyond the stratotype area provided reason enough to re-establish the formation in the Ordovician stratigraphic scheme, with more refined composition and age. According to composition, the Guberlya Formation is intermediate between the Ordovician volcanogenic Baulus Formation (see below) and the siliceous tuffaceous Kuragan Formation. The Guberlya Formation is of Llanvirnian-Early Caradocian age. The composite section of the Guberlya Formation.[113] The section includes the succession of six faunal beds (Figure 3) that are correlated with the conodont zones of Balto-Scandia [Cooper and Sadler, 2004] and traditional Ordovician stages of Britain, and are referred to the Lower Llanvirnian-Lower Caradocian interval.
[114] The stratigraphic successions of the Periodon, Ansella, and Pygodus species
revealed in the section of the formation does not contradict the present notion on
morphophylogeny of these genera. In the composite section one can record the
chronological successions Periodon zgierzensis
[115] The Baulus Formation distinguished by V.T. Tishchenko in 1988 is represented by acidic, basic, and, to a lesser degree, intermediate extrusive rocks. Its sediments make up cores of the large Blyava and Utyagulovo synforms (Figure 5), between which the stratotype section was described in a small synform at the Blyava and Kuragan Rivers confluence [Artyushkova et al., 1991; Maslov et al., 1993].
[116] In the Blyavtamak Village area, on the southern slope of the Baulus Mountain
(Figures 5 and 6) upward from the foot, the basalts and basaltic tuffs grade into rhyolites
and rhyolite tuffs. The beds dip gently at
[117] On the left bank of the Blyava River, southwest of the Blyavtamak Village, an aphyric basalt block with grey chert beds is recorded. It is likely a fragment of section of the Baulus Formation. At Point 11 (Figure 2) the cherts contain Hamarodus brevirameus (Walliser), Periodon grandis (Ethington), Dapsilodus mutatus (Branson et Mehl), Plectodina furcata (Hinde), Panderodus gracilis (Branson et Mehl), and Belodina sp. and are also referred to the Hamarodus brevirameus, Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds that correspond to the Upper Caradocian-Ashgillian (Figure 3). The Blyava synform
[119] In the western wall of the Blyava quarry (Figure 8), above the ore stratum, the member of interbedded rhyolites, their tuffs, and basalts is crowned by the 0.5-2-m-thick bed of red siliceous siltstones and hematite-siliceous shales. At Point 38 (Figure 6) the red siliceous siltstones bearing Dapsilodus mutatus (Branson et Mehl), the form transitional from Periodon cf. aculeatus Hadding to P. cf. grandis (Ethington), Drepanodus robustus Hadding, Protopanderodus cf. liripipus Kennedy et al., and Panderodus sp. (Figure 2), are referred to the Hamarodus brevirameus, Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds of the Baulus Formation section (Figure 3), which are correlative with the Upper Caradocian-Ashgillian. Previously the findings in this horizon of inarticulate brachiopods [Korinevskii, 1992] and, evidently in the same bed, of Ordovician conodonts [Ivanov et al., 1989; Maslov et al., 1993] were reported. The siliceous rocks terminating the Baulus Formation are overlain by the 0.5-m-thick bed that represents the base of the Blyava Formation. It is composed (upward from the base) of siliciclastic sandstones, basaltic tuffites, and carbonaceous siliceous shales. The latter are recorded above, among the pillow basalts as well. The Llandoverian, including Late Llandoverian, graptolites were encountered in the shales at different levels [Borodaev et al., 1963]. [120] At the Komsomol'skoe deposit (Figure 8), on the left bank of the Khersonka River, the member overlying the ore-bearing sequence is composed of interbedded basalts, quartz rhyolite tuffs and tuffites, red flaggy siltstones, cherts, jaspers, and hematite shales. This level is analogous to the supraore bed recovered in the Blyava quarry. The Middle and Lower-Middle Ordovician conodonts were found there previously [Ivanov et al., 1989]. At Point 3-37 (Figure 8) we collected in red jaspers the Late Caradocian-Ashgillian Hamarodus brevirameus (Walliser), Panderodus sp., and Protopanderodus sp., characteristic of the above-mentioned beds of the Baulus Formation (Figure 3). This confirms the validity of correlation of the supraore horizons in the Blyava and Komsomol'skoe deposits. [121] At the northern closure of the Blyava synform (Figure 8) the section of the Baulus Formation is overturned. With the normal succession of rocks of the formation the quartz and basic rhyolites are overlain by basalts (100-m-wide outcrop). In the mid-section the basalt member is decoupled by a 2-10-m-thick, red, brown, and grey siliceous tuffaceous siltstone bed. At Point 599 (Figures 2 and 8) the siltstones contain Hamarodus cf. brevirameus (Walliser), Scabbardella cf. altipes (Henningsmoen), and Periodon sp. of the Late Caradocian-Ashgillian age. The basalts are further replaced by siliceous and siliciclastic rocks of the Lower-Middle Devonian Akchura Formation. The same siliceous tuffaceous siltstone bed, 200 m to the northeast, at Point 5-184 (Figures 2 and 8) yields H. cf. brevirameus (Walliser), Protopanderodus cf. liripipus Kennedy et al., Drepanodus robustus (Hadding), and Panderodus sp. In both points the conodont associations are characteristic of the Hamarodus brevirameus, Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds of the Baulus Formation (Figure 3), which correspond to the Upper Caradocian-Ashgillian. The overturned occurrence previously resulted in the mistaken inference that the volcanites belong to the Devonian section, and they were distinguished as the Utyagulovo Formation [Maslov et al., 1993]. [122] The question of the age of ore-bearing sequences and the massive sulfide ore deposits, was debated over a long period [Artyushkova et al., 1991; Ivanov, 1996; Ivanov et al., 1989; Korinevskii, 1992]. From the available records we infer the Ordovician and Silurian age of the ore-bearing volcanogenic complex and the Late Ordovician age of the massive sulfide ore deposits (Figure 6). The schematic section of the ore-bearing volcanogenic complex is described in the other paper [Ryazantsev et al., 2005]. [123] The data on composition and age of the ore-bearing volcanogenic complex of the Sakmara zone permit its comparison with that of the Tagil zone [Karetin, 2000] (see inset map Figure 1). Their similarity is in association of the ore deposits with the Ordovician-Silurian boundary and in the occurrence of siliceous rocks at this level. Utyagulovo synform.[124] The Baulus Formation is there of similar composition and along with the Silurian and Devonian siliceous basaltic sequences makes up the synform core (Figure 5). The sequence of interbedded aphyric and amygdaloidal basalts, quartz rhyolites and tuffs, is recorded from the western flank to the core of the synform. The sequence is replaced by the 40-m-thick member of red siliceous tuffaceous siltstones bearing lenses of grey cherts and turquoise-colored tuffites. At Point E-4 (Figure 5) the red tuffaceous siltstones yield Hamarodus cf. brevirameus (Walliser), Protopanderodus liripipus Kennedy et al., Scabbardella altipes (Henningsmoen), Periodon cf. grandis (Ethington), Belodina confluens Sweet, Drepanodus cf. robustus Hadding, and Istorinus sp. (Figure 2) and are referred to the Hamarodus brevirameus, Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds (Figure 3) correlative with the Upper Caradocian-Ashgillian. The member is further built up by the aphyric and vesicular basalt sequence with lenses of acidic extrusive rocks.Khmelevka synform.[125] The similar in composition sediments located nearby the Khmelevka Village (Figure 5) are assigned to the Baulus Formation as well. Here, the extrusive sequences of different age occur on the synform flanks structurally above the serpentinite melange. Three kilometers west of the Khmelevka Village, on the left bank of the Kyzymbadka River (18 m west of the felsitic stack), the up to 0.2-m-thick red chert lenses among the basalts contain conodonts. The previous data available from this point are contradictory (K. S. Ivanov and V. N. Puchkov, preprint, 1984), [Korinevskii, 1988]. Subsequent to Korinevskii, we collected at Point B-115 (Figure 6) H. brevirameus (Walliser), P. liripipus Kennedey et al. (see Appendix, Plate 5, fig. 15), S. altipes (Henningsmoen), Istorinus cf. erectus Knupfer, Periodon sp., Histiodella sp., and Plectodina sp. (Figure 2). This assemblage is typical for the Hamarodus brevirameus, Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds that correspond to the Upper Caradocian-Ashgillian (Figure 3). Thickness of the basalts bearing lenses of cherts of the Baulus Formation does not exceed 100 m.Southern part of the Sakmara-Voznesenka zone.[126] In this area the Baulus Formation analogs are recorded. In the Baiguskarovo Village region (see inset map in Figure 1), on the left bank of the Dergamysh River, the section is dominated by pillow basalts with rare chert, phthanite, and jasper lenses. The Caradocian conodonts were encountered in the jasper beds [Artyushkova and Maslov, 1998]. The faunal characteristic of the section was supplemented by Borisenok [Borisenok and Ryazantsev, 2005].[127] In the 2-4-m-thick and about 200-m-long bed of jaspers and red siliceous siltstones, at 600 m from the Baiguskarovo Village at Point 037 (Figure 2) we collected Periodon grandis (Ethington), Panderodus sp. A Dzik, and Dapsilodus sp. The phthanite lens among basalts, at 1.5 km from the Baiguskarovo Village margin, at Point 1-184 (Figure 2), yields Scabbardalla altipes subsp. B Orchard, Panderodus gracilis (Branson et Mehl), and Plectodina sp. Five meters upward from the base the bluish-green cherts alternating with phthanites, at Point 029 (Figure 2) bear S. altipes (Henningsmoen) and D. mutatus (Branson et Mehl). [128] According to composition and age, these sediments are correlative with the Baulus Formation of the Sakmara zone, namely, with the Hamarodus brevirameus, Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds (Figure 3) corresponding to the Upper Caradocian-Ashgillian. The composite section of the Baulus Formation[129] includes the succession of two faunal beds (Figure 3) that are correlated with the conodont zones of Balto-Scandia [Cooper and Sadler, 2004] and traditional Ordovician stages of Britain, and are referred to the Upper Caradocian-Ashgillian interval.
[130] The stratigraphic succession Periodon aculeatus
Paleoenvironments during the formation of the compound volcanogenic complex.[131] As shown above, the Guberlya Formation rocks are dominated by tuffites, acidic and intermediate tuffs, and basalt and rhyolite flows. The Baulus Formation is represented by acidic, basic, and, to a lesser extent, intermediate extrusive rocks. The Guberlya and Baulus formations are successive in time (Figures 2, 3, and 6). They form an island arc complex. Its formation and, correspondingly, the generation of the volcanic (Guberlya) arc occurred from the mid-Ordovician (Early Llanvirnian) to the terminal Ordovician (Late Ashgillian). The sediments of the Guberlya Formation (Llanvirnian-Lower Caradocian) record the initiation and development of the Guberlya ensimatic island arc. Subsequently, in the Late Caradocian-Ashgillian the Guberlya arc likely experienced splitting (riftogenesis). During this period the sediments of the Baulus Formation were accumulated. Black smokers that were associated with the ore formation in the Baulus (Late Caradocian-Ashgillian) time, were formed in the island-arc structure, under its splitting.[132] Volcanites of both formations are strongly heterogeneous in chemical composition. Basalts of the Guberlya Formation are correlative with intraplate oceanic and island-arc tholeiites [Borisenok and Ryazantsev, 2005]; basaltoids of the Baulus Formation, with island-arc tholeiites, MORB, and intraplate oceanic basalts. All the available data indicate that the volcanogenic complex reflects the evolution of the ensimatic island arc which experienced splitting, riftogenesis, and died off by the beginning of the Silurian. 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
[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).
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