RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES5001, doi:10.2205/2008ES000302, 2008
[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.
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Figure 4 |
[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
P. protoanserinus
P. anserinus was traced in Sweden
[Zhang, 1998]
and in the South Urals (this paper). These records permit the distinction of narrow
stratigraphic units in the uppermost Llanvirnian and its analogs.
For instance, the Pygodus protoanserinus and P. serra Beds in the South Uralian sections
correspond to the upper part of the serra Zone of Balto-Scandia, whereas the Uralian
Pygodus protoanserinus and P. anserinus Beds, to the lower part of the anserinus Zone of
Balto-Scandia (Figure 3).
[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.
[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.
[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
P. grandis
Beds (Figure 3) corresponding to the analogs of the mid-Caradocian.
[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.
[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.
[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
P. zgierzensis
P. aculeatus
P. grandis bellus (Figure 2); (2) the distinct succession in evolution of
Protopanderodus rectus
P. varicostatus
P. liripipus
P. insculptus;
(3) the lineage Ansella jemtlandica
A. nevadensis
A. erecta; and finally
(4) the succession Pygodus serra
P. protoanserinus
P. anserinus, in which
P. protoanserinus is the most important link that permitted the distinction of the
Pygodus protoanserinus and P. anserinus Beds of a narrow stratigraphic range (Figure 3).
Apparently, it will be possible in the future to recognize the Pygodus protoanserinus and
P. serra Beds within the Polyakovka rocks, as is done in the Guberlya Formation (see below).
[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.
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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