Conodont stratigraphy and correlation of the Ordovician volcanogenic and volcanogenic sedimentary sequences in the South Urals Текст научной статьи по специальности «Науки о Земле и смежные экологические науки»

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 Urals

S. V. Dubinina1 and A. V. Ryazantsev1

Received 19 January 2008; accepted 27 April 2008; published 1 June 2008.

[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. Index

TERMS: 0410 Biogeosciences: Biodiversity; 0459 Biogeosciences: Macro- and micropaleontology; 3030 Marine Geology and Geophysics: Micropaleontology; 4950 Paleoceanography: Paleoecology; KEYWORDS: Stratigraphy,

Ordovician, conodonts, formations, “faunal beds”, biofacies, volcanogenic sedimentary deposits, South Urals.

Citation: Dubinina, S. V. and A. V. Ryazantsev (2008), Conodont stratigraphy and correlation of the Ordovician volcanogenic and volcanogenic sedimentary sequences in the South Urals, Russ. J. Earth. Sci., 10, ES5001, doi:10.2205/2008ES000302.

_____________________ Introduction

1 Geological Institute, Russian Academy of Sciences, Moscow,

Russia

[2] In the South Urals the Paleozoic complexes of various Copyright 2008 by the Russian Journal of Earth Sciences. structural-and-formational paleozones are tectonically jux-

ISSN: 1681-1208 (online) taposed. They make up a system of nappes with facially dif-

ferent 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, ph-thanites, 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., 1995] in the historically type area, Anglo-Wales. It primarily concerns the expanded range of the Llanvirnian and the reduced Llan-deilian 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 Llan-deilian. 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 Trema-docian 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; Figure 1].

[6] The first occurrence of conodont Iapetognathus flucti-

vagus 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 (Lindstrom) 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 Nema-graptus 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 out-

Figure 1. Scheme of the geological structure in the north of the Western Magnitogorsk and Sakmara-Voznesenka zones (compiled using records by V. I. Kozlov, I. S. Anisimov, V. V. Babkin, A. V. Zhdanov and others). 1 - terrigenous carbonate sediments (C1); 2 - Zilair Formation (D3-C1) - flyschoid clayey terrigenous sediments with tuffaceous admixture; 3 - Bugodak Formation (D3fm) - tuffs, tuffites, andesites, basaltic andesites; 4-5 - Mukasovo Formation (D3f) and its analogs: 4 - cherts, siliceous siltstones, 5 -grey- and red-colored sandstones and siltstones; 6 - limestones (D2zv); 7 - Ulutau Formation and analogs of the Yarlykapovo Horizon, nonsubdivided (D2zv) - basalts, rhyolite tuffs, cherts, jaspers, manganese ores; 8 - Irendyk and Karamalytash formations, pooled and their analogs (D1-2) - intermediate and basic volcanites; 9 - Mansurovo sequence (D1e) - tuffaceous terrigenous sediments bearing mixtite and chert beds; 10 - terrigenous siliceous and basaltic siliceous sequences (S-D1); 11 - limestones (S2-D1);

12 - Sakmara Formation (S1-2) - carbonaceous siliceous shales and siltstones; 13 - Guberlya Formation (O2-3) - basalts, tuffites, cherts, tuffs of mixed composition; 14-17 - Polyakovka Formation (O1-3) in the nappes’ section: 14 - first (lower) nappe - basalts, cherts (O2l-O3k1), 15 - second nappe - basalts and cherts (O2l-O3k1), 16 - third nappe - basalts and cherts (O1a-O3k-as), 17 - fourth nappe - basalts, red cherts, specular schists (O1a-O212); 18 - meta-arkoses R3?; 19 - Balbukovka complex of alkalic granitoids (PZ3); 20 - gabbro-diorites (D1); 21 - dunite-wehrlite-clinopyroxenite complexes; 22 - peridotites (lherzolites) of the Nurali (on the north) and Tatlambet (on the south) massifs; 23 - serpentinite melange; 24 - fault deformations; 25 - findings of paleontological remains, their numbers and age.

The inset map shows rock complexes in the South Urals (after Savel’ev et al. [1998], simplified) and location of the studied areas (Figures 1, 5, 7, and 8, Baiguskarovo area): 1 - East European platform margin; 2 - Late Devonian-Early Carboniferous graywackes (Zilair series): 3 - ophiolitic massifs and serpentinite melanges; 4 - Ordovician and Devonian complexes of accretionary prisms and island arcs (SV - Sakmara-Voznesenka, M - Magnitogorsk, S - Sakmara, T - Tagil zones); 5 - Late Devonian and Carboniferous suprasubduction and riftogenic volcanogenic and carbonate complexes; 6-7 - East Uralian zone: 6 - pre-Paleozoic and Early Paleozoic metamorphic complexes, 7 - terrigenous covers, riftogenic volcanic complexes; 8 - Late Paleozoic granitoids of the Dzhabyk massif; 9 - Major Uralian Fault.

4 of 31

~P

o

<£ <L)

Ötsu

eg

CJ

Conodont scale of Balto-Scandia Cooper et Sadler, 2004

ordovicicus

superbus

tvaerensis

ansennus

lindstromi

robustus

reclinatus

foliaceus

suecicus

variabilis

parva

originalis

triangularis

evae

elegans

proteus

deltifer

angulatus

Iapetognathus

fluctivagus

Siliceous basaltic type of section

48

44/1

58/10

55 '

6180/1 ' 2c, 15/2, ' 24 !

58/4, 25, 532, , 51/1 129,511. 6176/1

(

12, 51/2, 14| 22d, 2b, 2a,, 127, 128 '

15/3 1 1 1 59 '

6175/1 1

15/1

56/4

22c, 32 22b

1/2, 2 2e

25*

Levels (

intervals

assemblages

) and stratigraphie of conodont

Hamarodus brevirameus, P. grandis bellus, D.suberectus, P.liripipus B. borealis, B.confluens, P. aculeatoides

Belodina

compressa,

Periodon aculeatus P. grandis

Scabbardella altipes Periodon grandis bellus,

Yaoxinognathus ani, Ansella erecta, Panderodus gracilis

Pygodus anserinus, Periodon aculeatus, Protopanderodus varicostatus, Ansella neva-

densis, Dapsilodus viruensis_________

^M Pygodus protoanserinus

Pygodus serra, 'P. anserinus, P. serra, Periodon aculeatus, iP- aculeatus, A. neva-Ansella jemtlandica, [dcn_sis,_P. gracilis Baltoniodus sp.

Paroistodus horridus, Periodon zgierzensis, Ansella jemtlandica, Walliserodus ethingtoni, Drepanoistodus forceps,

Scalpcllodus gracilis,

Dapsilodus viruensis

Periodon flabellum, Baltoniodus sp

O.evae, P.flabellum O. lanceolatus

( ) intermedius.

P. flabellum, i P. flabellum Paroistodus ! P. rectus parallelus

c

o

•o

o

Loxodus cf. latibasis ( redeposited )

CO ¿3

§

als«

/

1,3,4' |B-38 264 -264a' K6 -K8 |K8a

K9 -

NK2

K66 . 19K1 -RK1A' 19K2 1

Levels (* and stratigraphic intervals i= of conodont assemblages

Hamarodus brevirameus Scabbardella altipes, Dapsilodus mutatus, Periodon grandis, Protopanderodus liripipus Plectodina furcata, Amorphognathus cf. superbus, P.cf.gerdae Panderodus gracilis,D.sub pq |erectus,D.robustus, ¡^.compressa

Pygodus anserinus, Periodon aculeatus, Panderodus gracilis

Drepanodus arcuatus, Scalpellodus viruensis, Ansella jemtlandica

Bergstroemognathus extensus, A.delicatus,

D.arcuatus, D.conulatus,

1 S.fumishi, Tripodus sp.A. i Drepanoistodus forceps

> g ° M

u 2 o>

^3 &•«

J -

V £

3-37 -E-4 . B-115 5-184,' 599 1 1-184, 037,029' 38 ■

11 -10

10a .

a

6033

E26

B 87 669

B-102

3-15

424

K-32

Levels (•-----► )

and stratigraphic intervals i= of conodont assemblages

Hamarodus brevirameu: Scabbardella altipes, Dapsilodus mutatus, Periodon grandis, Protopanderodus liripipus, D.robustus, Panderodus gracilis, Plectodina furcata, Istorinus erectus

P.liripipus,S.spinatus Periodon aculeatus

A.nevadensis P.aculeatus.

S.spinatus, P.liripipus,

D.robustus ,JS£

—— »[J g-1

_Eo|)lacognathus

___. ! rnhnctnc ^

|—I robustus, 1-5 I Pygod w>j| i serra,

!£ £ I Periodon

•S'!?' aculeatus,

:> < in .

i D.viruensis,

: a o

8* 8 3)

GO CU 0_‘

! P. varie i

ostatug

i D.suberectus

.2 3

o £ ¿ r3

£ tx

CQ C/2

Figure 2. Successions of Ordovician conodont assemblages with the indicated stratigraphic ranges in sections of the Polyakovka, Kuragan, Guberlya, and Baulus formations of the South Urals.

ES5001 DUBININA AND RYAZANTSEV: ORDOVICIAN CONODONT STRATIGRAPHY OF THE SOUTH URALS ES5001

lined (International Subcommission..., 2006 (http://www. ordovician.cn)).

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

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

[10] On the same basis we refined correlations and analyzed the conodont biofacies in the region. We illustrated some diagnostic species of Arenigian (see Appendix, Plate 1), Llanvirnian (see Appendix, Plate 2), Uppermost Llanvir-nian (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 Section

[13] Polyakovka Formation. 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.

[14] In the stratotype area, the Polyakovka Village region (Figure 1), the siliceous basaltic complex forms a stripe of exposures up to 5 km wide and over 30 km long. Here, on the flanks of antiform complicated by strike-slip faults and reversed faults, in the set of four nappes, the sections of siliceous basaltic complex differing in composition and thickness of synchronous units, are juxtaposed (Figure 4). This structure steeply dips northwestward (at 60° to 80°), i.e. the antiform southern flank is overturned.

[15] The first nappe. 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 1-4), contains numerous Pygodus protoanserinus Zhang, P. anserinus Lamont et Lindstrom, 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 Lindstroom, Periodon aculea-tus 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

6 of 31

Compound volcanogenic type of section

O

Conodont scale of Balto-Scandia

Cooper et Sadler, 2004

ordovicicus

superbus

tvaerensis

ansennus

lindstromi

robustus

reclinatus

foliaceus

suecicus

variabilis

parva

originalis

triangularis

evae

elegans

proteus

deltifer

angulatus

Iapetognathus

fluctivagus

Siliceous basaltic type of section

Points with conodonts and their numbers

58/4, 25, 532 151/1.129,51 IT I 6176/1 I----\

12, 51/2, 14' 22d, 2b, 2a ' 127,128 ;

48

44/1

58/10

55

6180/1 2c, 15/2, 24

15/3

1

59

6175/1

15/1

56/4

22c, 32

22b

1/2, 2 2e

25*

"Faunal beds"

Hamarodus brevirameus, Scabbardella altipes, and Periodon grandis bellus Beds

Belodina compressa, Periodon aculeatus -► P grandis Beds

Pygodus anserinus, Periodon aculeatus, and Ansella nevadensis Beds

Pygodus protoanserinus and P anserinus Beds

Pygodus serra and Periodon aculeatus Beds

Paroistodus horridus, Periodon zgierzensis, and Ansella jemtlandica Beds

Periodon flabellum Beds

Oepikodus intermedius Beds

Oepikodus evae Beds

Paroistodus parallelus and Periodon flabellum Beds

Loxodus cf. latibasis ( redeposited )

Siliceous tuffaceous type of section

Points with conodonts and their ni mhers

«

1,3,4. B-3 8 < 264 ' 264a' K6. K8, K8a ' K 9 .

N IvjJ

K66

19K1

RK1A

19K2

"Faunal beds"

Hamarodus brevirameus. Dapsilodus mutatus, and Scabbardella altipes Beds

Pygodus anserinus and Periodon aculeatus Beds

Periodon aculeatus Beds

Ansella jemtlandica

Beds

Bergstroemognathus extensus and Drepanoistodus forceps Beds

Points with conodonts and their numbers

CQ

O

3-37

E-4

B-115

5-184,

599

1-184,

037,029

38

11

10

10a

6033

E-26

B-87

669

B-102

3-15

424

K32

"Faunal beds"

Hamarodus brevirameus Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds

Spinodus spinatus and Periodon aculeatus Beds

Pygodus anserinus, Periodon aculeatus, and Ansella nevadensis Beds

Pygodus protoanserinus, P. serra Beds

E.robustus, P.serra Beds

Protopanderodus cooperi Beds

’ Periodon zgierzensis and Ansella jemtlandica Beds

Baltoniodus medius and Strachanognathus parvus Beds

Figure 3. Correlation of the Ordovician faunal beds’ successions in the sections of the Polyakovka, Kuragan, Guberlya, and Baulus formations in the South Urals with the Balto-Scandia conodont scale and British Ordovician stages.

ES5001 DUBININA AND RYAZANTSEV: ORDOVICIAN CONODONT STRATIGRAPHY OF THE SOUTH URALS ES5001

Figure 4. Correlation of sections of the Polyakovka siliceous basaltic complex (Western Magnitogorsk zone): 1 - grey-green, rarely red-brown cherts and siliceous siltstones, phthanites, clayey siliceous shales; 2 - red-brown and red cherts, siltstones, specular schists; 3 - grey-green aphyric basalts, hyaloclastites; 4 - dolerites of the parallel dike (sills) complex; 5 - serpentinite melange; 6 - points of conodont localities and their numbers.

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 stratigraphie 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 Lindstrom, 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 З), which correspond to the lowermost anserinus Zone of Balto-Scandia, i.e. to the uppermost Llanvirnian. We can say about a part of the anser-inus 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 З).

[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 ІЗ-m-thick horizon of massive grey-green and red-brown bedded cherts, is recorded. The horizon extends for a distance of l 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.

[21] The second nappe (Figures 1 and 4). The faults separating the second nappe are associated with ser-pentinite 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 Llanvir-nian 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 (Lofgren), Drepanoistodus forceps (Lindstrom), and Pro-topanderodus 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 З) that correspond to the uppermost Llanvirnian. At Point 129 located 2.З km southwest of Point 59 (Figures 1 and 4) the cherts yield Pygodus cf. pro-toanserinus Zhang, Periodon aculeatus Hadding, Drepanois-todus suberectus (Branson et Mehl), and Drepanodus sp.

[25] The third nappe (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.

[2S] 2. Grey-green, compact cherts, at Point 15/1 bearing Periodon flabellum (Lindströom) (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/З bearing Periodon zgierzensis Dzik, Ansella cf. jemt-landica (Lofgren), Paroistodus cf. horridus (Barnes et Popla-wski), and Pygodus sp., and representing the Paroistodus horridus, Periodon zgierzensis, and Ansella jemtlandica Beds (Figure З) referred to the Lower Llanvirnian analogs. Thickness 12 m.

[31] 5. Grey-green aphyric basalts. Thickness 15 m.

[32] б. Pale-yellow, flaggy siltstones, at Point 15/2 yielding Pygodus anserinus Lamont et Lindstroöm, Periodon ac-uleatus 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 б m.

[33] At 1.2 km northeast of Point 15/2, at Point 2c (Figures 1 and 4) the similar rocks bearing Pygodus cf. anser-inus Lamont et Lindströom, 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 5б/4 (Figure 1) yield Periodon flabellum (Lindstrom) 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 З2 (Figure 1) the rocks bear Periodon flabellum (Lindströom) 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 pro-toanserinus Zhang, the combined occurrence of the two other species, namely, P. anserinus Lamont et Lindstrom 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 З- 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. gran-

dis (Ethington) form (see Appendix, Plate 4, figs. б, Т, 912), 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 Lindstroöm, P. serra (Hadding), Periodon cf. aculeatus Hadding, Wallis-erodus ethingtoni (Fahraeus), Drepanodus cf. arcuatus Pander, and Panderodus sp. are referred to the Pygodus pro-toanserinus and P. anserinus Beds (Figure З) that are correlative to the lowermost anserinus Zone of Balto-Scandia, i.e. to the uppermost Llanvirnian.

[3S] The grey cherts occurring among basalts at the southwestern extension of the nappe on the left bank of the Kras-nokhty River (Points 12Т and 128) (Figures 1 and 4) correspond to the Upper Llanvirnian Pygodus serra and Peri-odon aculeatus Beds (Figure З). At Point 12Т 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 (Hanning-smoen), 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 З) 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 Kras-nokhty 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 б1Т5/1 Parois-todus 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 З) 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 б1Тб/1 (Figures 1 and 4) the rocks contain the predominant Periodon ac-uleatus Hadding, along with Pygodus protoanserinus Zhang, P. anserinus Lamont et Lindströom, Drepanoistodus suberec-tus (Branson et Mehl), and Panderodus gracilis (Branson et Mehl) that are peculiar for the Pygodus protoanserinus and P. anserinus Beds (Figure З) 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 Lindstrom, 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 б1Тб/1, in a 70-m-wide stripe, the ser-pentinite melange with lenses of basalts decoupled by grey-green cherts is exposed. The conodont association derived at Point б180/1 (Figures 1 and 4) includes the predominant in the assemblage, transitional from Periodon aculeatus Hadding to P. grandis (Ethington) forms, as well as Pygo-dus cf. anserinus Lamont et Lindstroöm, 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 anser-inus, 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 З).

[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 1-4), 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 (Hen-ningsmoen), Protopanderodus liripipus Kennedy et al., Pe-riodon grandis bellus (Moskalenko), Plectodina aculeatoides Sweet, Besselodus borealis Nowlan et McCracken, Belod-ina cf. confluens Sweet, Drepanoistodus suberectus (Branson et Mehl), and Ozarkodina sp. At 4 km southeast of the Abzakovo Village, at Point 48 (Figures 1-4) Hamaro-dus brevirameus (Walliser), Periodon cf. grandis (Ethington), and Belodina sp. were encountered. The conodont assemblages at both points are peculiar to the Hamaro-dus brevirameus, Scabbardella altipes, and Periodon grandis bellus Beds (Figure З) 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.

[44] The fourth nappe is represented on the northern flank of the antiform (Figure 1) and includes the Areni-gian 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 1-4) the middle part of the red siltstone member yields Periodon flabellum (Lindstroöm) and Paroistodus parallelus (Pander) characteristic of the Parois-todus parallelus and Periodon flabellum Beds that correspond to the Lower Arenigian (Figure 3). At Point 2a, б0 m to the west, the red with grey-green spots siltstones contain rare Pygodus serra (Hadding) and Ansella jemtlandica (Löofgren) 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, З, 4) the red-colored member recovered on the hill, on the Balbukovka River left bank and referred to the Paroistodus parallelus and Periodon fla-bellum Beds corresponding to the Lower Arenigian, yields P. cf. parallelus (Pander) and P. cf. flabellum (Lindstrom). In a northwestward direction the red-colored member is replaced by the 10-m-thick phthanite bed that contains at Point 1 (Figures 1-4) Walliserodus ethingtoni (Fahraeus), Scalpel-lodus 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):

[4S] 1. Siliceous, red with hematite crystals siltstones, at Point 22b bearing Oepikodus evae Lindströom (see Appendix, Plate 1, figs. 2, 3, 5), Periodon flabellum (Lindström), Ois-todus lanceolatus Pander, and Drepanoistodus forceps (Lindström), referred to the Oepikodus evae Beds (Figure 3) correlative with the “Middle” Arenigian. Thickness З0 m.

[49] 2. Grey siliceous siltstones, at Point 22c yielding Peri-odon flabellum (Lindström), Oepikodus cf. intermedius Ser-pagli, and Protopanderodus cf. rectus (Lindström), assigned to the O. intermedius Beds (Figure З) that correspond to the “Middle” Arenigian. Thickness 10 m.

[50] З. Grey aphyric basalts with grey-green chert beds, at Point 22d (Figure 4) containing Pygodus serra (Hadding) (see Appendix, Plate 2, figs. б-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 Loxo-dus 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.

[55] The composite section of the Polyakovka Formation, 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 ^ P. zgierzensis ^ P. aculeatus ^ P. grandis bellus (Figure 2); (2) the distinct succession in evolution of Protopanderodus rectus ^ P. vari-costatus ^ 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 strati-graphic 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).

[58] Paleoenvironments during the formation of the siliceous basaltic complex. 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

[60] The Kuragan Formation characterized by the siliceous tuffaceous type of section is widespread on the flanks of the Utyagulovo and Blyava synforms (Figure 5). The formation was distinguished in the 1930s [Lermontova and Razumovskii, 1933]. Its section was described in the range of all Ordovician series [Ruzhentsev, 1976] and dated by means of conodonts [Artyushkova et al., 1991; Maslov et al., 1993; Ryazantsev et al., 2000, 2005], (K. S. Ivanov and V. N. Puchkov, preprint, 1984).

[61] The Kuragan Formation is represented in the lower part by red or grey-green tuffaceous siltstones decoupled by basalt flows and sills. The grey and, rarer, red siliceous tephroids (Novokursk sequence) dominate in the upper part of the section. The close spatial association of the Kuragan Formation with the Tremadocian terrigenous Kidryasovo series is recorded (Figures 5 and 6). In places the formation is conformably overlain by carbonaceous shales of the Silurian Sakmara Formation (Figures 5 and 6).

[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 Bergstroe-mognathus 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 Lindstrom, Scandodus furnishi Lindström, and numerous molds of inarticulate brachiopods and ostracodes. They are assigned to the same Bergstroemog-nathus 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 Kidrya-sovo 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 com-

posed of the Silurian Sakmara Formation making up the core of the synform, on which flanks the Ordovician Kura-gan 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 tuffa-ceous siltstones bearing grey lenses. The rock is enriched with inarticulate brachiopod and ostracode detritus. The red siltstones at Point Т (Figure б) contain Drepanodus arcuatus Pander, Scalpellodus cf. S. viruensis (Lofgren), and Ansella jemtlandica (Löofgren) 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 б) 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 bre-virameus (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* (Figures 5b and б) contain redeposited conodont elements of Periodon aculeatus Hadding that serve as a marker of the Periodon aculeatus Beds of the Kuragan Formation, corresponding to the Upper Llanvirnian analogs (Figure 3). Further upwards the grey siliceous tephroids at Point З (Figures 5b and б) yield Hamarodus brevirameus (Walliser), Scabbardella altipes (Henningsmoen), Dapsilodus mutatus (Branson et Mehl), Periodon grandis (Ethington), Protopanderodus liripipus Kennedy et al., Panderodus sp., Ansella sp., and Plectodina furcata (Hinde); at Point 4, Hamarodus brevi-rameus (Walliser), Protopanderodus liripipus Kennedy et al., Periodon grandis (Ethington), and Spinodus spinatus (Hadding) (see Appendix, Plate 5, fig. 18), and are assigned to the Hamarodus brevirameus, Dapsilodus mutatus, and Scabbardella altipes Beds corresponding to the Upper Caradocian-Ashgillian (Figure 3). The grey-colored member is 50- to 150-m-thick.

[73] It is conformably overlain by carbonaceous, clayey siliceous and dolomitic shales of the Sakmara Formation, 250-З00 m thick (Figures 5b and б). At Point 5 they contain Llandoverian conodonts Ozarkodina aldridgei Uyeno et Barnes and Dapsilodus obliquicostatus (Branson et Mehl) and at Point б, Llandoverian graptolites Lagarograptus aci-naces (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 б). The overlying siliceous tephroids are recognized as the Novokursk [Khvorova et al., 19Т8] 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.

Churaevo

:19KlU9K ZjW > ' n L //l

Kuvandvk

Mcdnoiiorsk

Kidrvasovo

•3-15 Novokurskii

Khmelevka

p

V

’c' . / 0 ' \ c 11

n ■VmVmiV 13 14 15 + + + + 16 17 / / 18 2 •

Figure 5. Scheme of the geological structure of the Sakmara zone in the Mednogorsk massive sulphide ore deposit area (compiled using record of geological survey by B. F. Khromykh, V. T. Tishchenko, N. T. Vidyukov and the authors’ data): 1 - soft Mesozoic-Cenozoic sediments; 2 - flyschoid terrigenous and terrigenous carbonate sediments (D3-Ci); 3 - siliceous condensed section (D1-3); 4 - Devonian basaltoid sequences with mixtite beds (D1-2); 5 - siliceous-black-shale sediments of the Silurian Sakmara Formation (S1-2); 6 - siliceous tuffaceous sediments of the Kuragan Formation (O1-3) and volcanogenic tuffaceous deposits of the Guberlya Formation (O2-3); 7-10 - elements of the section of the Kuragan Formation (in insets): 7 - tuffaceous sandstones and siltstones with basalt lenses and sills, 8 - basalts with siliceous tuffite lenses, 9 - grey siliceous tephroids, 10 - red siliceous tephroids; 11 - Guberlya Formation (O2-3), tuffs and tuffites of mixed composition (in insets); 12 - compound volcanogenic complex, Baulus

(O3) and Blyava (S1) formations; 13 - terrigenous arkosic sediments of the Kidryasovo Formation (O11);

Figure б. Correlation of sections of the Kuragan, Guberlya, and Baulus formations in the Sakmara and eastern Sakmara-Voznesenka zones: 1 - carbonaceous shales and cherts; 2 - arkoses; 3 - siltstones, argillites, and arkoses; 4 - tuffaceous sandstones and siltstones; 5 - tuffites of mixed composition; б -tuffaceous sandstones and gravelstones; Т - variegated siliceous tephroids; 8 - grey siliceous tephroids; 9 - basalts; 10 - andesites; 11 - rhyolites and dacites; 12 - rhyolite and dacite tuffs; 13 - massive sulfide ores; 14 - points of conodont and other organic remains localities and their numbers. Redeposited conodonts are recorded at Point 2*.

[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 (60°-70°) 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. al-tipes (Henningsmoen), Periodon grandis (Ethington), Pro-topanderodus 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), Is-torinus sp., Baltoniodus sp., Amorphognathus sp., and Pan-

14 - serpentinite melanges, ophiolitoclastic mixtites, olistostromes; 15 - metamorphic complexes of the Uraltau zone; 16 - diorites (Оз); 17 - faults; 18 - normal and overturned elements of occurrence; 19 -conodont localities, their numbers and age. Letters indicate synforms: B - Blyava, U - Utyagulovo, Kh - Khmelevka. The insets demonstrate structure of the Blyava Station area (A) and Shaitantau Mountains (B).

derodus sp., are assigned to the same faunal beds. The similar conodont assemblage was recorded Т0 m above, in the member of manganese-bearing siliceous rocks. This association at Point ^ (Figures 2, 5, б) 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 fau-nal 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 б).

[7S] 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 б), the grey siliceous tephroids of the Novokursk unit contain Pygodus anserinus (Lamont et Lindstrom), 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 Carado-cian (Figure 3).

[50] The light grey tuffosilicites 100 m above the top of the red-colored sequence, at Points 2б4 and 264a (Figures 2 and б) bear Hamarodus brevirameus (Walliser), Protopanderodus liripipus Kennedy et al., Scabbardella altipes (Hen-ningsmoen), 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.

[51] The fragment of the section that includes the contact between the red-colored siltstone and siliceous tephroid No-vokursk sequences is recorded further southward, nearby the crossing of the roads to Churaevo and Kuvandyk [Ryazant-sev 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 Scab-bardella altipes Beds (Figure 3) corresponding to the Upper Caradocian-Ashgillian analogs.

[52] Thus the siliceous tuffaceous type of section (Kuragan Formation) was accumulated from the upper Lower Areni-gian to Ashgillian inclusive.

[53] The composite section of the Kuragan Formation includes the succession of five faunal beds (Figure З) 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).

[54] The stratigraphic succession of the species Periodon aculeatus ^ P. grandis does not contradict the present notion of morphophylogeny of the genus.

[S5] Paleoenvironments during the formation of the siliceous tuffaceous complex. 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 б) 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 б).

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.

[87] The Guberlya Formation was distinguished on the eastern slope of the South Urals by K. E. Razu-movskaya 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.

[88] The Guberlya Formation is mainly composed of tuf-fites, acidic and intermediate tuffs, and basalt and rhyolite flows. It is located in the stratotype area in the Guberlya, Malaya and Bol’shaya Kayala River basins (Figure 7), and on the flanks of the Blyava and Utyagulovo synforms (Figure 5). The analogs of the Guberlya Formation are first recorded in the eastern Sakmara-Voznesenka zone (Figures 1 and 6) (see below).

[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 cher-ty olistoliths with conodonts ranging in age from the Lochko-vian to Lower Famennian inclusive [Borisenok and Ryazant-

Figure T. Scheme of the geological structure of the Guberlya, Bol’shaya and Malaya Kayala Rivers’ interfluve: 1 - polymictic olistostrome (C1?); 2-5 - Guberlya Formation: 2 - tuffs, tuffites, siltstones, 3 - basalts, 4 - quartz rhyolites, 5 - red tuffaceous siltstones; б - Kidryasovo Formation (O11) - sandstones and siltstones; Т - granitoids (PZ1); 8 - ophiolites and serpentinite melanges; 9 - metabasalts -amphibolites and green schists; 10 - faults; 11-12 - findings of organic remains, their numbers and age: 11 - shelly fauna, 12 - conodonts.

sev, 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 ething-toni (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 sim-ilaris (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), Pro-topanderodus varicostatus (Sweet et Bergstrom), Drepanois-todus 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 Bal-toniodus 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 sili-ciclastic rocks bearing Early Devonian (Emsian) conodonts. North of the structure the section is represented by tuffa-ceous 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 б):

[103] 1. Grey-yellow tuffaceous sandstones and gravel-stones 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 ra-diolarites. The red-brown tuffaceous siltstones with 0.5-cm-thick tephra layers of mixed composition at Point E-26 (Figures 5 and б) contain numerous large Spinodus spinatus (Hadding), Walliserodus ethingtoni (Fahraeus), Protopan-derodus liripipus Kennedy et al., and Drepanodus robus-tus 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 б0 m.

[106] In the Shaitantau Mountains sections of the Gu-berlya 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 No-vokurskii Village, the tuffites terminate with a 5-mm-thick layer of grey cherts yielding at Point 3-15 (Figure 5b) Spin-odus spinatus (Hadding), Periodon aculeatus Hadding, Pro-topanderodus 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 Pro-topanderodus cooperi Beds, i.e. to the Upper Llanvirnian analogs (Figure 3).

[10S] 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.

[109] Eastern part of the Sakmara—Voznesenka zone. 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 бб9 (Figures 1 and б) contain Pygodus protoanserinus Zhang, P. serra (Hadding), Periodon ac-uleatus 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 б033 (Figures 1 and б), the б-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.

[113] The composite section of the Guberlya Formation 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, Ansel-la, and Pygodus species revealed in the section of the formation does not contradict the present notion on morpho-phylogeny of these genera. In the composite section one can record the chronological successions Periodon zgierzen-sis ^ P. aculeatus (Figure 2); Ansella jemtlandica ^ A. nevadensis; and Pygodus serra ^ P. protoanserinus ^ P. anserinus. The concurrent occurrence of P. protoanserinus Zhang and P. serra (Hadding) permitted the distinction of the Pygodus protoanserinus and P. serra Beds of a narrow stratigraphic range for the Upper Llanvirnian analogs (Figure 3), which is of fundamental importance for development

of high-resolution Ordovician conodont scale in the South Urals.

[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 б) upward from the foot, the basalts and basaltic tuffs grade into rhyolites and rhyolite tuffs. The beds dip gently at <20o. Nearby the contact between acidic and basic rocks, in the 20-m-thick member, extrusive rocks are decoupled by beds of fine-clastic tuffs, tuffites, and red cherts. The lower chert stratum, at Point 10a (Figures 2 and б) bearing Spinodus spina-tus (Hadding), Periodon aculeatus Hadding, Protopandero-dus liripipus Kennedy et al., and Ansella sp. identified by Dubinina [Artyushkova et al., 1991], is referred to the Spin-odus spinatus and Periodon aculeatus Beds (Figure 3) corresponding to the mid-Caradocian. The upper chert bed with burrow molds yields at Point 10 (Figure б) Protopandero-dus insculptus (Branson et Mehl) (see Appendix, Plate 5, fig. 19), Scabbardella altipes (Henningsmoen), Dapsilodus mutatus (Branson et Mehl), Walliserodus cf. nakholmensis (Hamar), Panderodus gracilis (Branson et Mehl), Cornuodus longibasis (Lindstrom), Icriodella sp., and Istorinus erectus Knupfer (Figure 2) and is assigned to the Hamarodus brevi-rameus, Dapsilodus mutatus, Scabbardella altipes, and Istor-inus erectus Beds correlative with the Upper Caradocian-Ashgillian (Figure 3).

[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 Hamar-odus brevirameus, Dapsilodus mutatus, Scabbardella altipes, and Istorinus erectus Beds that correspond to the Upper Caradocian-Ashgillian (Figure 3).

[11S] The Blyava synform (Figures 5 and 8) is characterized by the most complete sections of the formation. In the Mednogorsk region the section of the Baulus Formation is associated with a set of massive sulfide ore deposits among which the Yamankasy deposit (Figure 8) is recognized as typical, associated with black smokers [Herrington et al., 2002; Maslennikov, 1999; Zaikov et al., 2001]. The ores yield vesti-mentifers, brachiopods, and pelecypods of wide stratigraphic range and occur in transitional part of the section from the rhyolite-dacite (with andesites) to basaltic sequence. The transition between the sequences is gradual in the Blyava and Komsomol’skoe deposits.

[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

Figure 8. Structural elements of the Blyava synform: 1 - soft Cenozoic sediments; 2 - polymictic olistostromes; 3 - ophiolitoclastic mixtites; 4 - siliciclastic olistostromes; 5 - siliceous sediments (Di_2); 6 - basalts with chert and siliciclastic rock lenses - Ishmuratovo Formation (Di_2); 7 - siliceous black-shale Sakmara Formation (Si_2); 8 - tuffaceous sediments of the upper part of the Kuragan Formation (Guberlya Formation analogs O2_3); 9 - tuffaceous sediments and basalts of the lower part of the Kuragan Formation (Oi_2); 10-12 - extrusive complex of mixed composition (Baulus (O3) and Blyava (Si) formations): 10 - nonsubdivided basaltoids of the Blyava and upper Baulus formations, 11 - acidic extrusions of the upper Baulus Formation, 12 - acidic and basic extrusions of the lower Baulus Formation; 13 - arkosic sandstones and siltstones of the Kidryasovo Series (O1t); 14 - diorites (O3); 15 - massive sulfide ores; 16 - nappe boundaries; 17 - other faults; 18 - points of conodont localities, their numbers and age. Figures indicate massive sulfide ore deposits: 1 - Blyava, 2 - Komsomol’skoe, 3 - Yamankasy, 4 - Razumovskoe.

Point 38 (Figure 6) the red siliceous siltstones bearing Dap-silodus mutatus (Branson et Mehl), the form transitional from Periodon cf. aculeatus Hadding to P. cf. grandis (Ethington), Drepanodus robustus Hadding, Protopandero-

dus cf. liripipus Kennedy et al., and Panderodus sp. (Figure 2), are referred to the Hamarodus brevirameus, Dapsilo-dus mutatus, Scabbardella altipes, and Istorinus erectus Beds of the Baulus Formation section (Figure 3), which are cor-

relative with the Upper Caradocian-Ashgillian. Previously the findings in this horizon of inarticulate brachiopods [Ko-rinevskii, 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 Pro-topanderodus 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.

[124] Utyagulovo synform. The Baulus Formation is there of similar composition and along with the Silurian and Devonian siliceous basaltic sequences makes up the syn-form 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. brevi-rameus (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, Scab-bardella 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.

[125] Khmelevka synform. 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 Hamaro-dus 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.

[126] Southern part of the Sakmara—Voznesenka zone. 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, ph-thanite, 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 Bai-

guskarovo Village at Point 0ЗТ (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, Pan-derodus 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).

[12S] According to composition and age, these sediments are correlative with the Baulus Formation of the Sakmara zone, namely, with the Hamarodus brevirameus, Dapsilo-dus mutatus, Scabbardella altipes, and Istorinus erectus Beds (Figure 3) corresponding to the Upper Caradocian-Ashgillian.

[129] The composite section of the Baulus Formation 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 ^ P. grandis established in the section of the Baulus Formation does not contradict the present notion on evolution of this genus.

[131] Paleoenvironments during the formation of the compound volcanogenic complex. As shown above, the Guberlya Formation rocks are dominated by tuffites, acidic and intermediate tuffs, and basalt and rhyo-lite 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, 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, riftoge-nesis, 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.

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

[135] The fauna of this biofacies includes dominating Oepikodus evae Lindstrom and Periodon flabellum (Lindstrom) and less common Protopanderodus rectus (Lindstrom) 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 fau-nal 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 Lindstrom and P. flabellum (Lindstrom) constituting about 80% of the total assemblage. Less numerous are Drepanodus arcuatus Pander, P. rectus (Lindstrom), 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 exten-sus (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.

[139] The Periodon biofacies. Six corresponding biofacies differing in composition and ratio of faunal components are recorded in the Upper Arenigian-Middle Carado-cian 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.

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

[141] 2. Periodon biofacies of the Lower Llanvir-nian (= Periodon zgierzensis biofacies). The predominant component of this fauna includes Periodon zgierzensis Dzik, Ansella jemtlandica (Lofgren), Paroistodus hor-ridus (Barnes et Poplawski), and Walliserodus ethingtoni (Fahraeus). The attendant species are Drepanoistodus forceps (Lindstrom), Drepanodus arcuatus Pander, Strachanog-nathus parvus Rhodes, Protopanderodus sp. Scalpellodus viruensis (Lofgren), 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.

[142] 3. Periodon biofacies of the Upper Llan-virnian (= Periodon aculeatus-Pygodus serra biofacies). This fauna is dominated by Periodon aculeatus Hadding and Pygodus serra (Hadding). Significantly numerous faunal component is represented by Protopandero-dus varicostatus (Sweet et Bergstrom) and Drepanoistodus suberectus (Branson et Mehl). Among attendant species are Ansella jemtlandica (Lofgren), 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.

[143] 4. Periodon biofacies of the uppermost Llanvirnian (= Periodon aculeatus-Pygodus pro-toanserinus biofacies). The predominant component of the fauna involves Periodon aculeatus Hadding and Pygodus protoanserinus Zhang. Relatively numerous are Pygodus serra (Hadding), Pygodus anserinus Lamont et Lindstrom, Drepanoistodus suberectus (Branson et Mehl), and Spinodus spinatus (Hadding), whereas Ansella nevadensis (Ething-ton 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).

[144] 5. Periodon biofacies of the Lower Carado-cian (= Periodon aculeatus-Pygodus anserinus biofacies). The assemblage is dominated by Periodon ac-uleatus Hadding and Pygodus anserinus Lamont et Lind-stroom. Common occurrence is characteristic of Protopan-derodus varicostatus (Sweet et Bergstrom), Ansella nevaden-sis (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, Protopan-derodus 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 (Ku-ruktag) was referred [Wang and Qi, 2001] to relatively deep and cold-water environments of the North Atlantic Realm.

[146] 6. Periodon biofacies of the “Middle”

Caradocian (= Periodon aculeatus ^ Periodon grandis biofacies). The predominant component of this fauna includes the form transitional from Periodon aculeatus Hadding to Periodon grandis (Ethington). The following species are less common: Belodina compressa (Branson

et Mehl), Spinodus spinatus (Hadding), Protopanderodus liripipus Kennedy et al., Ansella sp., and Baltoniodus sp. (Figure 2). The fauna inhabited oceanic basin (upper part of the Polyakovka Formation) and environment associated with the splitting of the island arc during riftogenesis (lower part of the Baulus Formation).

[147] The Periodon biofacies of the South Urals belongs to that of the North Atlantic Realm, or more precisely to a part of this realm, still imperfectly studied [Zhen and Percival,

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

[148] The Hamarodus brevirameus-Dapsilodus mutatus-Scabbardella altipes or HDS biofacies of the Upper Caradocian—Ashgillian. The fauna of the

HDS biofacies includes (Figure 2): (1) the predominant

species Hamarodus brevirameus (Walliser), Periodon grandis (Ethington), Protopanderodus liripipus Kennedey et al., Drepanodus robustus Hadding, and Drepanoistodus suberec-tus (Branson et Mehl) that are widespread in the North Atlantic Realm; (2) significant in number of specimens fau-nal component represented by Dapsilodus mutatus (Branson et Mehl), Scabbardella altipes (Henningsmoen), and Istori-nus erectus Knupfer that are characteristic of the cold-water Mediterranean province of the North Atlantic Realm; (3) important though low in number component including Belodina confluens Sweet, Panderodus gracilis (Branson et Mehl), and Plectodina furcata (Hinde), i.e. species typical for the North American Midcontinent Realm; (4) few Yaoxianognathus ani Zhen et al. known from Australia and China.

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

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

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

Conclusions

[152] The findings of Ordovician conodonts in volcanogenic and volcanogenic sedimentary sequences of different tectonic zones in the South Urals first provided a significant advancement of stratigraphy in the region. To do this, we studied three types of Ordovician sections, namely, siliceous basaltic, siliceous tuffaceous, and volcanogenic differing in stratigraphic completeness and being associated with various elements of the Uralian paleoocean active margin.

[153] The siliceous basaltic type of section (Polyakovka Formation) in the Western Magnitogorsk zone was formed at different stages of the ocean basin evolution. It makes up a system of nappes which represent a fragment of the ac-cretionary prism in front of the Devonian Magnitogorsk arc. The formation embraces the Upper Tremadocian-Ashgillian stratigraphic interval. The other types of section are connected by gradual facies changes and, partly, by strati-graphic transitions and are recorded in structures of the Sak-mara and Sakmara-Voznesenka zones.

[154] The siliceous tuffaceous type of section (Kuragan Formation) has been formed from the Arenigian to Ashgillian inclusive. The Arenigian-Llanvirnian interval, i.e. the lower part of the Kuragan Formation, is mainly composed of tuffa-ceous sandstones and siltstones, which in the Caradocian-Ashgillian were replaced by siliceous tephroids of the Novokursk sequence (upper part of the Kuragan Formation). The former were accumulated in the distal part of a marginal basin; the latter were associated with environments of the Ordovician Guberlya ensimatic island-arc foot area.

[155] The island-arc complex corresponding to the vol-canogenic type of section is represented by the successive Guberlya and Baulus formations.

[156] The conodont studies provided the age estimate of the Guberlya Formation as Llanvirnian-lowermost Carado-cian; its status as the Ordovician formation is regained; and its conformable contact with the Upper Caradocian-Ashgillian Baulus Formation is recognized.

[157] The section of the Baulus Formation includes a set of massive sulfide ore deposits associated with the black smokers activity. Directly in the supraore cherts of the Blyava and Komsomol’skoe deposits and in many localities along the supraore cherts strike the similar in composition Late Caradocian-Ashgillian conodont associations are recorded.

[158] From the conodont evidence we distinguished additional biostratigraphic units or faunal beds in the sections of the studied formations. The Ordovician sections of the Polyakovka and Kuragan formations include successions of eleven and five faunal beds, respectively. The section of the Guberlya Formation is characterized by six successive faunal beds embracing the upper Middle to lower Upper Ordovician. The Upper Ordovician Baulus Formation includes two faunal beds.

[159] The Ordovician conodont scale of the South Urals comprises eleven (11) units and is based on successive alteration of conodont associations with the use of stratigraphic successions of the Periodon, Pygodus, Ansella, and Protopanderodus species, which does not contradict the present notion of morphophylogeny of these genera.

[160] The stratigraphic succession of the Pygodus species is of particular importance; its middle member P. protoanser-inus provided the distinction of fine-resolution units. In the uppermost Llanvirnian analogs the P. protoanserinus and P. anserinus Beds replace the P. protoanserinus and P. serra Beds (Figure 3). The former are distinguished in the Polyakovka Formation; the latter, in the Guberlya Formation. There are strong grounds to believe that both units will be recognized in the single siliceous basaltic (oceanic) type of section. Then the Ordovician stratigraphic scale of the South Urals will include twelve (12) conodont units.

[161] The Ordovician conodont scale of the South Urals represents one of still few variants of scales related to the biogeographic Open-Ocean (= Open-Sea) Realm (OSR) of the Tropical Domain [Zhen and Percival, 2003]. The realm was dominated by cosmopolitan and widespread taxa, whereas endemics were of minor importance. This fact prevents from distinction of a province, i.e. smaller biogeographic unit within the Realm. Recall that among the scales of this Realm are the Upper Cambrian-Lower Ordovician Sarykum scale [Dubinina, 1991, 1998, 2000] and the Upper Cambrian-Middle OrdovicianBurubaitalscale [Tolmacheva et al., 2001,

2004] of Central Kazakhstan.

[162] We identified eight deep-water and/or relatively deep-water biofacies, i.e. (1) “Middle” Arenigian Oepikodus evae; (2) Upper Arenigian Periodon flabellum; (3) Lower Llanvir-nian Periodon zgierzensis; (4) Upper Llanvirnian Periodon aculeatus-Pygodus serra; (5) uppermost Llanvirnian Periodon aculeatus-Pygodus protoanserinus; (6) Lower Carado-cian Periodon aculeatus-Pygodus anserinus; (7) “Middle” Caradocian Periodon aculeatus ^ Periodon grandis; and

(8) Upper Caradocian-Ashgillian Hamarodus brevirameus-Dapsilodus mutatus-Scabbardella altipes or HDS biofacies. The listed Ordovician conodont biofacies of the South Urals refer to those of still imperfectly studied part of the North Atlantic Realm that was recently considered [Zhen and Per-cival, 2003] as a distinct paleobiogeographic Open-Ocean Realm of the Tropical Domain.

[163] The South Uralian conodont biofacies were compared within the studied region to an extent that the fossil material permitted. For the most part, they were compared with biofacies of the North Atlantic Realm or of the Open-Ocean Realm of the Tropical Domain using the literature records.

[164] Acknowledgments. We are grateful to Yu. Gladenkov, O. Korchagin, V. Chernykh, and V. Aristov for critical remarks that contributed to improvement of the manuscript. This work was supported by the Program of Basic Research, Earth Sciences Department, Russian Academy of Sciences, no. 10, and by the Russian Foundation for Basic Research, projects no. 06-05-64717 and no. 07-05-01158.

References

Antsygin, N. Ya., et al. (1993), Stratigraphic Schemes

of the Urals (Precambrian, Paleozoic), 151 pp., UrNTs RAN, Yekaterinburg.

Armstrong, H. A., and A. W. Owen (2002), Euconodont diversity changes in a cooling and closing Iapetus Ocean, in Palaeo-biogeography and Biodiversity Change: the Ordovician and

Mesozoic-Cenozoic Radiations, no. 194, edited by J. A. Crame and A. W. Owen, p. 85, Geol. Soc., London.

Artyushkova, O. V., and V. A. Maslov (1998), Paleonto-

logical Basis for Stratigraphic Subdivision of Pre-Famennian Complexes in the Verkhneural’sk and Magnitogorsk Regions, 156 pp., Inst. Geol. UNTs RAS, Ufa.

Artyushkova, O. V., V. A. Maslov, andV. T. Tishchenko(1991), New Records on Paleozoic Stratigraphy of the Northern Sak-mara Zone and Adjacent Area in the South Urals, 29 pp., BNTs UrO AN SSSR, Ufa.

Borisenok, D. V., and A. V. Ryazantsev (2005), Lower Paleozoic volcanogenic complexes at the junction of the Sakmara and Sakmara-Voznesenka zones in the South Urals, in Essays on regional tectonics, vol. 1. South Urals, p. 135, Nauka, Moscow.

Borisenok, V. I., L. Kurkovskaya, and A. V. Ryazantsev (1998), Ordovician conodonts from the siliceous basaltic complex of the South Urals (results of scientific research at the Uralian training ground), Vestn. Mosk. Univ. Ser. Geol., no. 3, 52.

Borodaev, Yu. S., T. N. Koren’, and A. D. Petrovskii (1963), On graptolite finding in the quarry of the Blyava copper-sulfide deposit in the South Urals, Dokl. Akad. Nauk SSSR, 150(5), 1107.

Cooper, R. A., and P. M. Sadler (2004), The Ordovician Period, in A Geologic Time Scale, edited by F. M. Gradstein, J. G. Ogg, A. G. Smith, p. 165, Cambridge University, Cambridge.

Dubinina, S. V. (1991), Upper Cambrian and Lower Ordovician conodont associations from open-ocean paleoenvironments, illustrated by Batyrbay and Sarykum sections in Kazakhstan, in Advances in Ordovician Geology, Paper 90-9, edited by C. R. Barnes and S. H. Williams, p. 107, Geol. Surv. of Can., Ottawa.

Dubinina, S. V. (1998), Conodonts from the Early Ordovician (mid-Arenig) deep water deposits of Central Asian pale-obasins, Proceedings of the Sixth European Conodont Symposium (ECOS VI), edited by H. Szaniawski, Palaeontologica Polonica, no. 58, 79.

Dubinina, S. V. (2000), Conodonts and Zonal Stratigraphy of the Cambrian-Ordovician Boundary Deposits, 240 pp., Nauka, Moscow.

Ferretti, A., and C. R. Barnes (1997), Upper Ordovician con-odonts from the Kalkbank limestone of Thuringia, Germany, Palaeontology, 40(1), 15.

Ferretti, A., and H. P. Schonlaub (2001), New conodont faunas from the Late Ordovician of the Central Carnic Alps, Austria, Soc. Pal. Italiana. Boll., 40(1), 3.

Ferretti, A., and E. Serpagli (1999), Late Ordovician conodont faunas from southern Sardinia, Italy: biostratigraphic and pa-leogeographic implications, Soc. Pal. Italiana. Boll., 37(2—3), 215.

Fortey, R. A. (1995), The Ordovician Series of the historical type area: revision as a contribution to their utility in international correlation, in Ordovician Odyssey: Short Papers for the Seventh International Symposium on the Ordovician System. Las Vegas, Nevada, USA, June 1995, edited by J. D. Cooper, M. L. Groser, and S. C. Finney, p. 1-11, SEPM, California.

Fortey, R. A., et al. (1995), A revision of Ordovician series and stages from the historical type area, Geol. Mag., 132, 15.

Herrington, R. J., et al. (2002), Massive sulfide deposits in the South Urals: Geological setting within the framework of the Uralide Orogene, in Mountain Building in the Uralides, p. 155, AGU, Washington, D.C..

Ivanov, K. S. (1996), Problems of stratigraphy and tectonics of volcanogenic sequences in the Sakmara zone and Major Uralian Fault stripe, in Materials on Stratigraphy and Paleontology of the Urals, p. 5, Ural Depart. RAS, Yekaterinburg.

Ivanov, K. S., et al. (1989), First results of revision

of the Polyakovka Formation stratigraphy using conodonts, Ezhegodnik-1988, p. 12, Inst. Geol. Geophys. UrO AN SSSR, Sverdlovsk.

Johnston, D. I. (1987), Early Ordovician (Arenig) conodonts from St. Pauls Inlet and Martin Point, Cow Head Group, western Newfoundland, M. Sc. Thesis, p. 226, Memorial Univ. of Newfoundland, St. John’s, Newfoundland.

Karetin, Yu. S. (2000), Geology and Volcanic Formations of the Uralian Superdeep Core SD-4 region, 275 pp., Inst. Geol. Geophys. Ural Depart. RAS, Yekaterinburg.

Kheraskov, N. P., and E. V. Milanovskii (1953), Cambrian and Lower Ordovician of the Orsk Urals, In memory of Professor A. N. Mazarovich, p. 106, MOIP, Moscow.

Khvorova, I. V., et al. (1978), Formations of the Sakmara Allochthon (South Urals), 232 pp., Nauka, Moscow.

Koptev-Dvornikov, V. S., E. S. Dobrokhotov, I. S. Rozhkov, and G. A. Mirlin (1940), Geologic Section of the Urals From Zlatoust to Chelyabinsk, 67 pp., Acad. Nauk SSSR, Moscow.

Korinevskii, V. G. (1988), Ordovician conodont findings in the South Urals, Sov. Geol., no. 2, 66.

Korinevskii, V. G. (1992), Age of the ore-bearing sequence of the Blyava copper-sulfide deposit in the Urals, Sov. Geol., no. 7, 24.

Lermontova, E. V., and N. K. Razumovskii (1933), On the oldest sediments of the Urals (Lower Silurian and Cambrian in the Kidryasovo area, South Urals), Zap. Ross. Mineral. Ob-va. Second Ser. (in Russian), 62(1), 185.

Maletz, J., A. Lofgren, and S. M. Bergström (1996), The base of the Tetragraptus approximatus Zone at Mt. Hunneberg, S.W. Sweden: A proposed Global Stratotype for the Base of the Second Series of the Ordovician System, Newsl. Stratigr., 34(3), 129.

Maslennikov, V. V. (1999), Sedimentogenesis, Halmyrolysis, and Ecology of Sulfide-Bearing Paleohydrothermal Fields (as Exemplified by the South Urals), 348 pp., Geotur, Miass.

Maslov, V. A., and O. V. Artyushkova (2000), Stratigraphy of Paleozoic Sediments in the Uchaly Region, Bashkiria, 140 pp., Inst. Geol. UNTs RAS, Ufa.

Maslov, V. A., et al. (1993), Stratigraphy and Correlation of Middle Paleozoic Volcanogenic Complexes in Major CopperSulfide Areas of the South Urals, 217 pp., UNTs RAS, Ufa.

Mitchell, C. E., et al. (1997), Definition of a global boundary stratotype for the Darriwilian Stage of the Ordovician System, Episodes, 20(3), 158.

Pohler, S. M. L. (1994), Conodont Biofacies of Lower to

Lower Middle Ordovician Megaconglomerates, Cow Head Group, Western Newfoundland, vol. 459, Bull, 1—70 pp., Geol. Surv. Canada, Ottawa.

Puchkov, V. N. (2000), Paleogeodynamics of the South and Central Urals, 145 pp., Dauriya, Ufa.

Puchkov, V. N., and K. S. Ivanov (1985), On the age of the Guberlya Formation, South Urals, Ezhegodnik 1984, p. 20, Inst. Geol. Geophys. UNTs Acad. Nauk SSSR, Sverdlovsk.

Puchkov, V. N., K. S. Ivanov, andV. A. Nasedkina(1990), First evidence on Ordovician siliceous sequences on the Urals western slope, in New Records on Geology of the Urals, Western Siberia, and Kazakhstan, p. 16, UrO AN SSSR, Sverdlovsk.

Rasmussen, J. A. (1998), A reinterpretation of the conodont Atlantic Realm in the Late Early Ordovician (Early Llanvirn), Palaeontologica Polonica, 58, 67.

Ruzhentsev, S. V. (1976), Marginal Ophiolitic Allochthons

(Tectonic Origin and Structural Position), 170 pp., Nauka, Moscow.

Ruzhentsev, S. V. (2005), “Facies” nappes in the structure of the South Uralian western slope, in Essays on Regional Tectonics, vol. 1, South Urals, p. 56, Nauka, Moscow.

Ryazantsev, A. V., et al. (2005), General structure of the South Uralian Sakmara zone in the Mednogorsk sulfide deposits area, in Essays on Regional Tectonics, vol. 1, South Urals, p. 84, Nauka, Moscow.

Ryazantsev, A. V., S. V. Dubinina, and D. V. Borisenok (2000), Tectonic convergence of Paleozoic complexes in the Devonian

accretionary prism, in General and Regional Problems of Geology, vol. 2, p. 5, GEOS, Moscow.

Ryazantsev, A. V., S. V. Dubinina, and L. Kurkovskaya (1999), Ordovician siliceous basaltic complex of the South Urals and its association with ophiolites, in General and Regional Problems of Geology, vol. 1, p. 5, GEOS, Moscow.

Savel’ev, A. A., et al. (1998), Structure and deformation stages of the northern margin of the Uralian Magnitogorsk zone, Geotectonics, no. 3, 38.

Sharfman, V. S., and V. P. Tsetlin (1968), On the age ofthe Gu-berlya Formation in the South Urals, Bul. MOIP. Otd. Geol., 43(2), 53.

Sidorenko, V. V., V. S. Baikova, and I. V. Stepanov (1964), Group of Volcanogenic Siliceous Formations in the Sakmara Zone on the South Uralian Western Slope, 67 pp., Nauka, Moscow-Leningrad.

Stouge, S. (1984), Conodonts of the Middle Ordovician Table Head Formation, western Newfoundland, in Fossils and Strata, no. 16, p. 1-145, Blackwell Publ., Oxford.

Stouge, S., and J. A. Rasmussen (1996), Upper Opdovician conodonts from Bornholm and possible migration routes in the Palaeotethys Ocean, Geol. Soc. Denmark. Bull., 43, 54.

Sweet, W. C., and S. M. Bergstrom (1984), Conodont provinces and biofacies of the Late Ordovician, Geol. Soc. America, 196, 69.

Tishchenko, V. T., and V. L. Cherkasov (1985), New records on stratigraphy of Silurian and Lower Devonian formations in the southern (Orenburg) part of the Magnitogorsk fore-deep western flank, in Paleozoic Biostratigraphy and Lithology of the South and Central Urals, p. 3, BF Inst. Geol. AN SSSR, Ufa.

Tolmacheva, T., T. Danelian, and L. Popov (2001), Evidence for 15 m.y. of continuous deep-sea biogenic siliceous sedimentation in early Paleozoic oceans, Geology, no. 29, 755, doi:10.1130/0091-7613(2001)029<0755:EFMYOC>2.0.CO;2.

Tolmacheva, T., L. Holmer, L. Popov, and I. Gogin (2004), Conodont biostratigraphy and faunal assemblages in radi-olarian ribbon-banded cherts of the Burubaital Formation, West Balkhash Region, Kazakhstan, Geol. Mag., 141(6), 699, doi:10.1017/S0016756804009902.

Wang, Zhi-hao, and Yu-ping Qi (2001), Ordovician conodonts from drillings in the Taklimakan Desert, Xinjiang, NW China, Acta Micropalaeontologica Sinica, 18(2), 133.

Zaikov, V. V., et al. (2001), Ore-Formational and

Ore-Facies Analysis of Sulphide Deposites of the Uralian Pa-leoocean, 315 pp., Imin UrO RAN, Miass.

Zhang, J. (1998), The Ordovician conodont genus Pygodus, Palaeontologica Polonica, 58, 87.

Zhen, Yong-Yi, and I. G. Percival (2003), Ordovician

conodont biogeography - reconsidered, Lethaia, 36, 357, doi:10.1080/00241160310006402.

Zhen, Yong-Yi, B. D. Webby, and C. R. Barnes (1999), Upper Ordovician conodonts from the Bowan Park succession, central New South Wales, Australia, GEOBIOS, 32(1), 73, doi:10.1016/S0016-6995(99)80084-9.

Zonenshain, L. P., et al. (1984), Structure and evolution of the South Urals in the light of lithosphere plate tectonics, in History of Evolution of the Uralian Paleoocean, p. 6, Nauka, Moscow.

S. V. Dubinina and A. V. Ryazantsev, Geological Institute, Russian Academy of Sciences, Moscow, Russia (sbubinina7@gmail.com)

Appendix: Diagnostic species of various Ordovician conodont associations of the South Urals

Plate 1. Arenigian conodonts from the Polyakovka Formation.

1, 4, 6, 7, 9, 11-13 - Periodon flabellum (Lindstrom, 1954). 1 - Sc element, specimen no. 4876/4, x 53.4,1 lateral view, Sample 15/1; 4 - Sd element, specimen no. 4876/5, x 46, postero-lateral view, Sample 15/1; 6 - Pa element, specimen no. 4876/6, x 62.7, posterior view, Sample 2; 7 - Sc element, specimen no. 4876/7, x 47, lateral view, Sample 2; 9 - Pb element, specimen no. 4876/8, x 91, posterior view, Sample 2; 11 - Sd element, specimen no. 4876/9, x 108, lateral view, Sample 22c; 12 - Pa element, specimen no. 4876/11, x 36.3, anterior view, Sample 22c; 13 - Pa element, specimen no. 4876/10, x 74.5, anterior view, Sample 22c; 2, 3, 5 - Oepikodus cf. O. evae (Lindstrom, 1954). 2 -Pa element (mold), specimen no. 4876/3, x 50, lateral view, Sample 22b; 3 - Pa element (mold), specimen no. 4876/2, x 44, lateral view, Sample 22b; 5 - Sd element, specimen no. 4876/1, x 85, lateral view, Sample 22b; 8 - Paroistodus parallelus (Pander, 1856). M element, specimen no. 4876/12, x 73.5, lateral view, Sample 2; 10 - Drepanoistodus forceps (Lindstrom, 1954). M element, specimen no. 4876/13, x 56, lateral view, Sample 22b.

Plate 2. Llanvirnian conodonts from the Polyakovka and Kuragan formations.

1 - Drepanodus arcuatus Pander, 1S66. Sd element, specimen no. 4S76/24, x 4S, lateral view, Sample 7, Kuragan Formation; 2, 3 - Scalpellodus cf. S. viruensis (Löofgren, 197S). 2 - specimen no. 4S76/26, x 119.б, lateral view,

Sample 7, Kuragan Formation; 3 - specimen no. 4S76/26, x S2.3, lateral view, Sample 7, Kuragan Formation; 4 -Drepanoistodus suberectus (Branson et Mehl, 1933). Sc element, specimen no. 4S76/23, x 10S.S, lateral view, Sample 22d, Polyakovka Formation; б, 9, 11-13 - Periodon aculea-tus Hadding, 1913. б - Pa element, specimen no. 4S76/20, x 7б.б, posterior view, Sample 22d, Polyakovka Formation; 9 - Pa element, specimen no. 4S76/21, x бб, posterior view, Sample 2, Kuragan Formation; 11 - M element, specimen no. 4S76/1S, x 72.б, lateral view, Sample 22d, Polyakovka Formation; 12 - Pb element, specimen no. 4S76/22, x 104, posterior view, Sample 2, Kuragan Formation; 13 - Sd element, specimen no. 4S76/19, x S1.3, lateral view, Sample 22d, Polyakovka Formation; б-S - Pygodus serra (Hadding, 1913). б - Pa element, specimen no. 4S76/14, x 100, upper view, Sample 22d, Polyakovka Formation; 7 - Pb element, specimen no. 4S76/16, x 130, lateral view, Sample 22d, Polyakovka Formation; S - Pa element, specimen no. 4S76/16, x 117.б, upper view, Sample 22d, Polyakovka

1 Here and below scale coefficients are set for the plate images (pages 29—33) printed without magnification (41pc width).

Formation; 10 - Spinodus spinatus (Hadding, 1913). Sb element, specimen no. 4876/17, x 112, lateral view, Sample 22d, Polyakovka Formation.

Plate 3. Uppermost Llanvirnian conodonts (Pygodus protoanserinus and P. anserinus Beds) from the Polyakovka Formation.

1-4, 6, 10, 13, 17 - Periodon aculeatus Hadding, 1913. 1 - Sc element, specimen no. 4876/27, x 64.7, lateral view, Sample 58/4; 2 - Sd element, specimen no. 4876/28, x 48.6, lateral view, Sample 58/4; 3 - Sc element, specimen no. 4876/29, x 60.8, lateral view, Sample 58/4; 4 - M element, specimen no. 4876/31, x 56.8, lateral view, Sample 58/4; 6 - M element, specimen no. 4876/30, x 57.8, lateral view, Sample 58/4; 10 - Pa element, specimen no. 4876/32, x 39.2, posterior view, Sample 58/4; 13 - Sc element, specimen no. 4876/33, x 70.6, lateral view, Sample 58/4; 17 - Sc element, specimen no. 4876/34, x 47, lateral view, Sample 25; 5, 12, 16 - Pygodus anserinus Lamont et Lindstrom, 1957. 5 - Pa element, specimen no. 4876/35, x 51, upper view, Sample 58/4; 12 - Pa element, specimen no. 4876/36, x 56, upper view, Sample 58/4; 16 - Pa element, specimen no. 4876/37, x 62.7, upper view, Sample 25; 7-9, 11, 15 -Pygodus protoanserinus Zhang, 1998. 7 - Pa element, specimen no. 4876/40, x 49, upper view, Sample 58/4; 8 - Pa element, specimen no. 4876/41, x 82.3, upper view, Sample 58/4; 9 - Pa element, specimen no. 4876/38, x 71.5, upper view, Sample 58/4; 11 - Pa element, specimen no. 4876/39, x 86.2, upper view, Sample 58/4; 15 - Pa element, specimen no. 4876/42, x 68.6, upper view, Sample 25; 14 - Loxodus cf. latibasis Ji et Barnes, 1994. Specimen no. 4876/43 (redeposited), x 110, inner lateral view, Sample 25; 18 - Pygodus cf. P. serra (Hadding, 1913). Pa element, specimen no. 4876/44, x 71.5, upper view, Sample 25.

Plate 4. Uppermost Llanvirnian—Caradocian conodonts from the Polyakovka Formation.

1,5- Pygodus anserinus Lamont et Lindström, 1957. 1 -Pb element, specimen no. 4876/46, x 74.5, lateral view, Sample 51/1; 5 - Pa element, specimen no. 4876/45, x 102, upper view, Sample 51/1; 4 - Plectodina sp. Specimen no. 4876/47, x 117.6, anterior view, Sample 51/1; 2, 3 - Periodon aculeatus Hadding, 1913. 2 - Sd element, specimen no. 4876/48, x 98, inner lateral view, Sample 51/1; 3 - M element, specimen no. 4876/49, x 78.4, lateral view, Sample 51/1; 8 - Drepanodus robustus Hadding, 1913. Sd element, specimen no. 4876/50, x 47, lateral view, Sample 2c;

6, 7, 9-12 - Periodon aculeatus Hadding, 1913, transitional to Periodon grandis (Ethington, 1959). 6 - M element, specimen no. 4876/51, x 74.3, lateral view, Sample 55; 7 - Pb element, specimen no. 4876/52, x 79.4, postero-lateral view, Sample 55; 9 - Pa element, specimen no. 4876/53, x 100, antero-lateral view, Sample 55; 10 - Sa element, specimen no. 4876/10, x 81.3, lateral view, Sample 55; 11 - Pb element, specimen no. 4876/55, x 66.6, anterior view, Sample

бб; 12 - Sc element, specimen no. 4S76/66, x 110.3, lateral view, Sample бб.

Plate б. Upper Caradocian-Ashgillian conodonts from the Polyakovka, Kuragan (Novokursk sequence), and Baulus formations.

1 - Panderodus sp. Specimen no. 4S76/67, x б9.б, lateral view, Sample 3, Kuragan Formation (Novokursk sequence);

2-4, S - Hamarodus brevirameus (Walliser, 19б4). 2 - Pb element, specimen no. 4S76/6S, x 137.2, posterior view, Sample 44/1, Polyakovka Formation; 3 - Pb element, specimen no. 4S76/69, x 71.б, posterior view, Sample B-115, Baulus Formation; 4 - Sc element, specimen no. 4S76/60, x б2.7, lateral view, Sample 3, Kuragan Formation (Novokursk sequence); S - M element, specimen no. 4S76/61, x 50, lateral view, Sample 44/1, Polyakovka Formation; б - Pan-derodus gracilis (Branson et Mehl, 1933). S element, specimen no. 4S76/62, x 71.5, antero-lateral view, Sample 5S/10, Polyakovka Formation; б, 9 - Scabbardella altipes (Hen-ningsmoen, 1947). б - specimen no. 4S76/63 (redeposited), x 72.5, lateral view, Sample 134, Blyava Formation; 9 -specimen no. 4S76/64, x 7S.4, lateral view, Sample 44/1,

Polyakovka Formation; 7, 14 - Dapsilodus mutatus (Branson et Mehl, 1933). 7 - specimen no. 4S76/65 (redeposited), x б7.б, lateral view, Sample 134, Blyava Formation; 14 -specimen no. 4S76/66 (redeposited), x 70.5, lateral view, Sample 134, Blyava Formation; 10, 11 - Periodon grandis (Ethington, 1959). 10 - Sd element, specimen no. 4S76/67, x б1.3, lateral view, Sample 4, Kuragan Formation (No-vokursk sequence); 11 - Pb element, specimen no. 4S76/6S, x б2.7, postero-lateral view, Sample 3, Kuragan Formation (Novokursk sequence); 12, 13 - Periodon grandis bellus (Moskalenko, 19SS). 12 - Sc element, specimen no. 4S76/70, x S0.4, lateral view, Sample 44/1, Polyakovka Formation;

13 - M element, specimen no. 4S76/69, x 44, lateral view, Sample 5S/10, Polyakovka Formation; 15 - Protopandero-dus liripipus Kennedy, Barnes et Uyeno, 1979. Specimen no. 4S76/71, x бб, lateral view, Sample B-115, Baulus Formation; 1б - Walliserodus cf. W. nakholmensis (Hamar, 19бб). Specimen no. 4S76/72 (mold), x 32.3, lateral view, Sample 10, Baulus Formation; 17 - Ansella sp. Specimen no. 4S76/74, x 34.3, outer lateral view, Sample 3, Kuragan Formation (Novokursk sequence); 1S - Spinodus spinatus (Hadding, 1913). Sc element, specimen no. 4S76/75, x 4б, lateral view, Sample 4, Kuragan Formation (Novokursk sequence); 19 - Protopanderodus insculptus (Branson et Mehl, 1933). Specimen no. 4S76/73, x бб, lateral view, Sample 10, Baulus Formation.

Plate З. Uppermost Llanvirnian conodonts (Pygodus protoanserinus and P. anserinus Beds) from the Polyakovka Formation.

Plate б. Upper Caradocian-Ashgillian conodonts from the Polyakovka, Kuragan (Novokursk sequence), and Baulus formations.