Lower Silurian Stratigraphy and Brachiopods of the ... - Springer Link

ISSN 08695938, Stratigraphy and Geological Correlation, 2015, Vol. 23, No. 3, pp. 262–280. © Pleiades Publishing, Ltd., 2015. Original Russian Text © O.I. Nikitina, I.F. Nikitin, M.A. Olenicheva, L.M. Palets, 2015, published in Stratigrafiya. Geologicheskaya Korrelyatsiya, 2015, Vol. 23, No. 3, pp. 30–49.

Lower Silurian Stratigraphy and Brachiopods of the Chingiz Range, Eastern Kazakhstan O. I. Nikitinaa, I. F. Nikitinb, †, M. A. Olenichevac, L. M. Paletsb, † a

Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russia email: [email protected] b Satpaev Institute of Geological Sciences, Kabanbai batyr street 69a, Almaty, 050010 Kazakhstan c Orel State University, Komsomol’skaya ul. 95, Orel, 302026 Russia Received May 21, 2013; in final form, November 5, 2013

Abstract—New data on the stratigraphy and faunal assemblages of the Lower Silurian of the Chingiz region are presented. Owing to the discovery of Ruddanian brachiopods in the basal Alpeis Formation, the position of the Ordovician–Silurian boundary has been revised. The stratigraphic range of the Alpeis Formation has been revised to correspond to the range of the Alpeis Horizon in the stratotype and is limited to the beds with the brachiopod Eospirifer cinghizicus and the beds with the graptolites of the Coronograptus gregarius Zone. Beds with Pentamerus longiseptatus of the Donenzhal Horizon are assigned to the Zhumak Formation. A new Ruddanian brachiopod assemblage (ten species) is recognized in the lower part of the beds with E. cinghizicus. Keywords: Lower Silurian, boundaries, Alpeis Formation, Donenzhal Horizon, beds, brachiopods DOI: 10.1134/S0869593815030065 †

INTRODUCTION

STRATIGRAPHY

The basis for the Silurian stratigraphy of the Chin giz Range region was founded in the 1940s–1960s by M.A. Borissiak, O.P. Kovalevsky, G.A. Stukalina, I.M. Kolobova, and others (Russian Geological Research Institute); C.M. Bandaletov, J.S. Sultan bekova, and others (Satpaev Institute of Geological Sciences, Kazakhstan); T.B. Rukavishnikova, N.V. Poltavtseva, N.F. Mikhailova, L.N. Klenina, and others (Territorial Geological Survey). After the pub lication in 1969 of a wellknown work by S.M. Ban daletov The Silurian of Kazakhstan, paleontological and stratigraphic studies, on the basis of a detailed study of reference sections, were carried out in the 1970s–1980s by L.M. Palets, M.A. Olenicheva, and S.V. Kim with I.F. Nikitin, M.K. Apollonov, and D.T. Tsai. (Satpaev Institute of Geological Sciences, Kazakhstan). In the early 1990s, this work was sus pended, and the results remained unpublished. It was recommenced only in 2002–2005 by O.I. Nikitina and colleagues (Nikitina, 2004; Nikitina et al., 2006). The following are the main results of the studies important for the modernization of regional and local strati graphic schemes, as well as for correlation with the schemes of other regions and the International Strati graphic Scale.

Silurian deposits in the region studied belong to the Chingiz Subzone of the ChingizTarbagatai Zone and Prechinghizian Subzone of the ShidertyPrechinghi zian Zone. These zones represent geological structures of the Early Paleozoic BozschakolChinghizTarba gatai island arc of the western segment of the Central Asian Folded Belt (Bandaletov, 1969; Zhautikov et al., 1977; Resheniya…, 1991; Degtyarev, 1999; Seitmura tova and Nikitina, 2012). Faunistically, the Lower Silu rian stratigraphic succession studied includes two regional horizons: Alpeis (Ruddanian Stage–lower part of the Aeronian Stage) and Donenzhal (upper part of the Aeronian Stage–lower part of the Shein woodian Stage). They are represented by the Alpeis and Zhumak formations. The Alpeis Formation con formably overlies the Upper Ordovician Akdombak Formation. The succession is dominated by shallow water marine siliciclasticcarbonate shelf facies with abundant and diverse fossil assemblages, mainly benthic. The upper part of the Alpeis Formation is represented by marginalshelf and slope siliciclastic facies with graptolites (Fig. 1). The upper subforma tion of the Zhumak Formation is composed of redbed ashclastic rocks and middlebasic volcanics. In the 1960s, M.A. Borissiak and her colleagues recognized in the Alpeis Horizon, apart from the beds with Holo rhynchus giganteus later assigned to the Ordovician, the following beds with brachiopods (from bottom to top): Eospirifer cinghizicus, Holorhynchus cinghizicus, Pentamerus longiseptatus (Bandaletov, 1969; Borissiak

† Deceased.

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LOWER SILURIAN STRATIGRAPHY AND BRACHIOPODS OF THE CHINGIZ RANGE

et al., 1974). Later, the beds with E. cinghizicus and H. cinghizicus were united because they were shown to be synchronous, whereas beds with P. longiseptatus were assigned to the Donenzhal Horizon; hence, the range of the Alpeis Horizon became smaller than the range of the Alpeis Formation (Sapel’nikov and Rukavishni kova, 1975; Resheniya…, 1991; Koren’ et al., 2003; Nikitina, 2004). One of the main problems in stratigraphy of the Chingiz Region was the position of the Ordovician– Silurian boundary. The Kazakhstan limitotype of this boundary is located in the ChuIli region, in an unin terrupted succession of the graptolite facies, where Late Ordovician graptolites are found in association with trilobites of the Dalmanitina Beds and beds with brachiopod Hirnantia. This section includes the Chokpar and Durben horizons of the terminal Ordov ician (Duplexograptus inuiti, Paraorthograptus pacifi cus zones of the Lower Hirnantian and Normalograp tus extraordinarius, N. persculptus zones of the Upper Hirnantian) and lower part of the Llandovery Alpeis Horizon with the succession of zones from Akidograp tus ascensus–Parakidograptus acuminatus to Coro nograptus gregarius (Granitsa…, 1980). In the Chingiz region, the position of the Ordovician–Silurian boundary for a long time remained debatable, because here the shallow water sections of the boundary inter val lack representative orthostratigraphic fossils. The system boundary was traditionally correlated with the boundaries between the Akdombak and Alpeis forma tions, but was placed by different authors at different levels inside the sedimentary cycles. The Ordovician– Silurian boundary was thought to correlate with the base of a regressive element in the upper part of the Ordovician transgressiveregressive succession (Ban daletov, 1969, p. 33) or with a considerably strati graphically higher level of mass appearance of the bra chiopods E. cinghizicus or H. cinghizicus in the contin uous transgressive sequence of the next similar cycle (Silurian) (Resheniya…, 1991). This was partly caused by the resemblance of the composition of the transi tional beds, but to a large extent by an uncertainty of the geological age and stratigraphic position of the so called “small fauna beds” (Figs. 2, 3), assigned either to the Alpeis Formation and the Lower Silurian (Ban daletov, 1969) or to the Akdombak Formation and Upper Ordovician (Nikitin, 1972, p. 91). The Ordovi cian age was supported by the occurrence of the trilo bite species Dalmanitina mucronata Brongniart (Apol lonov, 1974; Granitsa…, 1980). However, according to a subsequent personal communication by M.K. Apol lonov, this identification had been erroneous, and this specimen was in fact a representative of the Silurian genus Chattiaspis. Confirmed D. mucronata have been found only near the Mizek Deposit in the Prechinghi zian Subzone (Apollonov, 1974). Therefore, it was considered that there was no gap in sedimentation in the Chingiz Subzone at the Ordovician–Silurian boundary, in contrast to the Prechinghizian Subzone neighboring to the southwest, where the base of the STRATIGRAPHY AND GEOLOGICAL CORRELATION

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Silurian was drawn at the base of conglomerate that was thought to represent the basal conglomerate (Ban daletov, 1969; Resheniya…, 1991). New data allow reinterpretation of these correlations. REFERENCE SECTIONS The main reference sections, including the strato types of the Alpeis Horizons and Alpeis and Zhumak formations, are located on the southwest slopes of the Chingiz Range to the southwest of Semipalatinsk (now Semei) and the southeast of Karaganda. Bandaletov (1969) described a section containing beds with E. cin ghizicus and P. longiseptatus on the right bank of the Zhanybek River near Akdombak Mountain (Fig. 4, inset) as the stratotype of the Alpeis Horizon and Alpeis Formation. The stratotype of the Donenzhal Horizon is in the Donenzhal Mountains southeast of the Alkamer gen’Tarbagatai Subzone (Resheniya…, 1991). The Ordovician–Silurian boundary beds have been most thoroughly studied in the generalized Akdomak reference section. The base of the section contains bioherm limestone—beds with corals Agetolites mira bilis of the Dulankara Horizon assigned to the lower subformation of the Akdombak Formation (Katian Stage; Upper Ordovician) (Borissiak et al., 1974; Nikitin, 1972). The upper siliciclastic subformation begins with a member of rhythmic twoelement alter nation of graygreen siltstone and finegrained sand stone with coral and brachiopods (beds with Holo rhynchus giganteus and graptolites of the Paraor thograptus pacificus Zone of the Chokpar Horizon of the lower half of the Hirnantian (Outcrops 594, 1037, 1037/1). The upper subformation is gradually overlain by a variegated member (200 m) of brown, crimson, and graygreen siltstone, rhythmically alternating with some gray, green quartzfeldspar and polymictic sand stone, with lensshaped and rounded claycarbonate nodules. Previously, this member was considered as barren. Bandaletov (1969) assigned it to the Alpeis Formation because of the variegated facies typical of that formation. This interpretation did not leave any possible space in the section for the Durben Horizon of the upper half of the Hirnantian, and this was inter preted as evidence of a gap and concealed stratigraphic unconformity. Although no evidence of erosion and unconformity at this level was observed, these are present 200 m above where the thin clastic facies are replaced by basal conglomerates of the stratigraphi cally higher sedimentary sequence. The variegated member contains brachiopods (obolids) with phos phate shells, not suitable for dating, but typical of the shallow water lingulid facies, in this case completing the transgressiveregressive series. The unconformity surface is preferable as a stratigraphic boundary, and therefore we accepted it as the base of the Alpeis For mation, whereas the variegated member is assigned to the Akdombak Formation and is provisionally corre lated with the Durben Horizon. Vol. 23

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NIKITINA et al. Beds with fossil → Fossils ↓ Brachiopods Leangella cf. scissa (Davidson) Anisopleurella micula Nikitin et Nikitina, sp. n. Saughina? sp. Leptostrophia tenuis Williams L. compressa Sowerby Eopholidostrophia sefmensis (Williams) Dolerorthis sowerbyana (Dalman) Isorthis (Protocortezorthis) prima Walmsley et Boucot Holorhynchus cinghizicus M. Borissiak Pentamerus oblongus Sowerby P. longiseptatus M. Borissiak P. longiseptatus tuberosa Sapelnikov et Rukavishnikova Striklandia lens progressa Williams Clorinda undata Sowerby C. undata vassilevskii M. Borissiak Antirhynchonella linguifera (Sowerby) Alispira cf. putilla Savage Zygatrypa (Ramazania) separata Nikitin et Nikitina, subgen. et sp. nov. Zygospiraella duboisi (Verneuil) Z. planoconvexa Hall Eospirigerina gaspeensis Cooper E. porkuniana (Jaanusson in Rubel, 1970) Meifodia tulkulensis Modzalevskaya et Popov Tuvaella? sp. Pronalivkinia numerosa Rukavishnikova P. nasuta Rukavishnikova Lissatrypa atheroidea Twenhofel Atrypopsis reclinans Rubel Atrypa hedei Struve A. reticularis (Linnaeus) Eospirifer cinghizicus M. Borissiak E. radiatus (Sowerby) E. olgae M. Borissiak Colonial corals Mesofavosites fleximurinus Sokolov M. aff. dualis Sokolov Paleofavosites minutus Kovalevsky P. poulsoni Teichert P. simplex Tchernyshev P. cf. alveolaris (Goldfuss) P. forbesiformis septatus Barskaja P. maximus Tchernyshev

E. cinghizicus A

B

H. cinghi C. gre zicus (C) garius

P. longi septatus

x x x x x x x x x

x

x x x x x x x

x x x x

x x x

x

x x x x x x x x x x

x x

x

x x x x x x

x

x x

x x x

Fig. 1. Brachiopod and associated fauna occurrences in the Lower Silurian of the Chingiz Range. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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LOWER SILURIAN STRATIGRAPHY AND BRACHIOPODS OF THE CHINGIZ RANGE Beds with fossil → Fossils ↓ Propora conferta MilneEdwards et Haime P. subdecipiens Kovalevsky Catenipora copulata Klaaman C. micropora (Whitfield) C. panga Klaaman C. algabasensis Kovalevsky Syringoporinus aff. raritabulatus Kovalevsky Custipalipfyllum kimi Kovalevsky Halysites catenularius (Linnaeus) H. espensis Kovalevsky H. fixus Kovalevsky Heliolites humilis Kovalevsky H. interstinctus (Linnaeus) H. cf. lebedevae Tchernyshev H. aff. yavorskyi Tchernyshev Rugoses Crassilasma crassiseptatum (Smith) C. ramasanicum Sultanbekova Tryplasma medius Sultanbekova Schlotheimophyllum patellatum (Schlotheim) Holacantia flexuosa (Linnaeus) Calostylis gregaris Sultanbekova Strombodes schrenki (Dubatolova) S. socialis (Soshkina) Cystiphyllum siluriensis Lonsdale Hedstroemophyllum obscurum (Smell.) Crinoids Cretalocrinus borealis (Yeltyscheva) Medinecrinus lenitus Stukalina Chinghizocrinus kokajgirensis Stukalina Trilobites Dalmanitina (Chattiaspis) sp. Sphaeroexochus mirus Beyrich Encrinurus mullochensis Reed E. punctatus Wahlen Graptolites Climacograptus ex gr. normalis Nicholson Cl. cf angustus Perner Glyptograptus maderneii Koren et Mikhailova Pseudoclimacograptus (Metaclimacograptus) cf. hugesi (Nicholson) Rectograptus ex gr. truncatus (Lapworth) Monograptus ex gr. revolutus (Kurck) M. denticulatus Tornquist M. triangulatus (Harkness) M. cf. tenuipraecursor Obut et Sobolevskaya Pribylograptus cf. argutus (Lapworth)

E. cinghizicus H. cinghi C. gre P. longi zicus (C) garius septatus A B x x x x x x x x x x x x x

x x x x x x x x x x x x x x x x x x x x

x

x x x x x x x

Fig. 1. (Contd.) STRATIGRAPHY AND GEOLOGICAL CORRELATION

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NIKITINA et al.

Variegated member Holorhynchus giganteus

Chokpar Durbenian Akdombak

Akdombak

Chokpar Ulkuntas

Akdombak

Chokpar Ulkuntas

Upper

Ordovician

Holorhynchus giganteus

Variegated member Holorhynchus giganteus

Beds with brachiopods, subformation (marker members) Upper subformation: barren volcanics lower subformation: beds with Pentamerus longiseptatus

Holorhynchus chinghizicus

Formation Zhumak

Horizon Donenzhal

Holorhynchus chinghizicus

Variegated member Sandstone, siltstone with Dalmantina “small fauna beds,”

Eospirifer chinghizicus

Alpeis

Holorhynchus chinghizicus– Eospirifer chinghizicus

Pentamerus longiseptatus

Akdombak

Alpeis

Holorhynchus chinghizicus and Eospirifer chinghizicus

Alpeis

Alpeis

Alpeis

Llandovery


not recognized (barren volcanics)

Alpeis


Beds with brachiopods (marker members)

Proposed scheme

Chokpar Durbenian

not recognized (corals of the beds with Propora obrutchevi?)

Horizon Formation


Resheniya…, 1991

Donenzhal Zhumak

Formation

Zhumak

Horizon

Formation Zhumak

Horizon Zhumak

Series Wenlock

System

not recognized (corals of the beds with Propora obrutchevi?)

Silurian


Zhumak

Nikitin, 1972; Sapel’nikov and Rukavishnikova, 1975

Bandaletov, 1969

Alpeis Alpeis

266

Eospirifer chinghizicus upper part B Eospirifer chinghizicus lower part A: “small fauna beds,”

Variegated member (lingulid facies) Holorhynchus giganteus

Fig. 2. Correlation of the stratigraphic schemes of the Ordovician–Silurian boundary beds in the Chingiz Range.

The Akdombak reference section in its Silurian part includes the Zhanybek and Kazbala sections, the holostratotype and hypostratotype of the Alpeis For mation and Alpeis Horizon, correspondingly, and the stratotype of the lower part of the Zhumak Formation of the Donenzhal Horizon, together corresponding to the beds with the brachiopods E. cinghizicus and P. longiseptatus and graptolites of the Coronograptus gregarius Zone. The sections are included into the area stratotype located to the northwest of Akdombak Mountain on the northern wing of the Akdombak Anticline, where all the beds are clearly traceable lat erally from the Zhanybek River to the west along the Kazbala River Valley, up to its confluence with the Baikashkar River, and further westnorthwest to Kur bakanas River (Figs. 3, 4). Zhanybek reference section (holostratotype; traverse I in Fig. 4). The section is recorded in a north western direction from locality 593 on the right bank of Zhanybek Creek, 500 m west of its submeridional riverbed, along several traverses with shifts along the marker levels. The transverse and oblique faults do not distort the bedding. The section is subdivided into lithological units and beds with fossils (E. cinghizicus,

C. gregarius). The variegated member of the top part of the Akdombak Formation with remains of obolids is structurally conformably overlain by (from bottom to top): Alpeis Formation Beds with Eospirifer cinghizicus, lower part (A) 1. Basal member (1.5–23 m). At the base with bed 1a (1.5–2 m): compact green and greenish gray finegrained quartzpolymictic sandstone with mass accumulations of obolod remains. The previous bed is overlain by bed 1b (0–4 m): conglomerates greenish, unsorted, with sandy or sandy calcareous matrix, with abundant pebbles of light gray and pink sandstones, finegrained sandstone, siltstone, acidic volcanics, dioritic porphyrites, less commonly granites. The middle part contains bed 1c (10 m): beds and lenses of conglomerate (0.1–1 m), alternating with deep red and green mediumgrained sandstone. The distribu tion of clasts in conglomerates is irregular: along with saturated beds 0.1–0.2 m thick with large pebbles (5– 10 cm), small and mediumsize unsaturated pebble conglomerates, pudding sandstone, and gravelite lenses alternate. The sandstone pebbles contain colo nial corals from the beds with Agetolites mirabilis of the

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Zhanybek section 15

C. gregarius Eospirifer cinghizicus–Holorhynchus cinghizicus

20 19 18

894/1 894

17

893

300 200 100

16

13

Ramazan section

0

15b 10, 33

15а

11

19

10 9 8 7 6

889, 890 891, 892

17, 18

14 13

5

4 913, 1042 3 2 912, 1041 1

12 11 10 892 9 8 7 A 593, 593а, 5–6 8831, 884 1–4

3 2 1

915, 1051

B

B

4

942 918 916, 917, 1052

10 8 7 6 5

896, 897

? 650v A 650b 1023 = 938 650а, 1031

? B

8991, 1050 899, 1049 C 20 595а, B 1048

1040

A

?

Tatuadam section (Elgunov’s Grave)

8

930 928, 929

7 6 5 4 3

927 926

2 138b, 925

1 ?

Holorhynchus giganteus

Alpeis Alpeis

400

21

12

Katian Hirtantian Chokparian Durbenian Akdombak

Upper Ordovician

267

m 500

Kazbala section

14

Ruddanian

Lower Silurian–Llandovery Aeronian

System, series Stage Horizon Donenzhal Zhumak (lower subformation) Formation Pentamerus longiseptatus Beds


? 146 539, 539/1

594

523

10010 10009 10371

1

2

8

9

3 10

524

4 11

5

6

7

12

Fig. 3. Correlation of the reference sections in the Lower Silurian of Chingiz Range. (1) Green siltstone and argillite; (2) varie gated (brown, crimson, green) argillite and siltstone; (3) green sandstone; (4) calcareous sandstone; (5) red sandstone; (6) con glomerate and gravelite; (7) limestone; (8) sandstone nodules and lenses; (9) andesite; (10–12) fossils: (10) brachiopods, corals, trilobites; (11) lingulates (inarticulate brachiopods); (12) graptolites. Numbers of beds and members are shown on the left of the lithological logs, and sample numbers on the right. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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10

18

a

b

19

8 7

17 16 15

4

5

250 500 750 1000 m

14 13 12

23

2 1

78°48′

3

250 75°

913 75° 9121 912 914 50° 650v 652 650а

0

1037(564)

O3ak2

898

1037/1

1039 1040 1038

1041

S1al 1042

6

18 19

78°50′ E

O3ak1

892 65°

17 1030 75°

8961 . R la a deserted village 895 zb Ka

894 897S zm 1 1 35° 894а 8952 1043 8951

60°

893

III

S1zm2 80° E

II Balkhash

0

120 km

20

9 593а

884

593

I Karaganda

Akdombak Mountain Koryk (Algabas) Ramazan Ayaguz

50° N

Semipalatinsk

21

11

48° 31′ 594

891 886 883 882 885 60° 33

10

60°

887

R.

890

Zha nybe k

50°

22

NIKITINA et al. 48° 32′ N

268

Fig. 4. Geological scheme of the outcrops of the Ordovician and Silurian depists near Akdombak Mountain. (1–5) Akdombak Formation: (1) limestone; (2) green siltstone, argillite; (3) variegated siltstone; (4) green sandstone, siltstone; (5) calcareous sand stone, siltstone; (6–13) Alpeis Formation: (6) basal member (green sandstone, conglomerate, gravelite, limestone); (7) variegated siltstone; (8) variegated calcareous sandstone, gravelite; (9) arenaceous limestone; (10) green calcareous sandstone; (11) calcar eous argillite with carbonate nodules; (12) green, variegated sandstone, siltstone; (13) gray, red sandstone, interbed of green silt stone; (14–17) Zhumak Formation, lower subformation: (14) red conglomerate; (15) gray calcareous sandstone, crinoid lime stone; (16) green sandstone, siltstone with carbonate nodules; (17) red and green sandstone with interbeds of conglomerate; (18) Zhumak Formation, the upper subformation: tuffites, tuffs, andesite lavas; (19) acidic dikes (a) and basic dikes (b); (20, 21) fossil occurrence sites: (20) benthic, (21) graptolites; (22) faults; (23) section traverses: (I) Zhanybek, (II, III) Kazbala, Kazbala1. Localities are indicated by numbers. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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Akdombak Formation (Sample 593a), and the con glomerate matrix and sandstone beds contain early Silurian brachiopods Isorthis (Protocortezorthis) prima and Eospirifer cinghizicus (Sample 8831). The top of the member contains bed 1d (7 m): light green silt stone and finegrained sandstone with beds of com pact, less commonly nodular sandstones (0.3–0.5 m), containing brachiopods Eospirigerina porkuniana, Zygospiraella sp., Eoplectodonta sp., Alispira sp., Lep taena sp., and I. (Protocortezorthis) prima (Samples 593, 883, 884). The basal member, of variable thickness, is readily traceable laterally. 2. Siltstone, green, lathlike, with sandstone nod ules. Thickness up to 80 m. 3. Siltstone and aleuropelite, crimson, and dark crimson and grayish crimson, lathlike, with pancake shaped nodules of marly sandstone and calcareous sandstone, at the top interbedded with greencolored varieties. Member is pierced by numerous sills, less commonly by cross dikes of diabases. Thickness 130– 180 m. Beds with Eospirifer cinghizicus, upper part (B) 4. Sandstone with varying grain size, predomi nantly graygreen, calcareous, at the base coarse grained, up to gravelite. Weakly calcareous beds (10– 20 cm) contain accumulations of Zygospiraella sp. (Sample 882). Thickness 35 m. 5. Sandstone yellowish green and greenish beige, mediumgrained, calcareous, with gravelite layers (0.25–0.5 m). Abundant remains of brachiopods Eospirifer radiatus, E. cinghizicus, Pronalivkinia cf. numerosa, and Spirigerina sp. (Samples 886, 887) often forming 20 to 30cm layers of coquina. Thick ness 180 m. 6. Mediumgrained sandstone and aleurosand stone crimson, thinly bedded. Thickness 8 m. 7. Limestone lightcolored pinkish gray, bioclastic, massive, and thicklybedded, with brachiopods Lep tostrophia sp., Dolerorthis sp., Resserella sp., Isorthis sp., Eospirigerina gaspeensis Cooper, Zygospiraella duboisi, Z. planoconvexa Hall, Pronalivkinia numerosa, P. nasuta, Whitfieldella? sp., Eospirifer cinghizicus; colonial corals (identifications by N.V. Poltavtseva) Mesofavosites fleximurinus, M. aff. dualis, Paleofavosites minutus, P. poulsoni, P. cf. simplex, Propora conferta, Catenipora copulata, Heliolites sp. (Samples 891, 892). Thickness 7 m. 8. Sandstone mediumgrained, equivalent to Bed 5, and aleurosandstone dark olivegreen, calcareous, breached by several diabase sills. In the middle part of the bed, there are accumulations of brachiopods (Sam ple 0333), mainly large flat shells of Saughina? sp. Thickness up to 17 m. 9. Limestone light gray, unevenly bedded, nodular, becoming calcareous siltstone, argillite yellowish gray with pancake nodules of lightgray micritic sandstones with brachiopods Resserella sp., Isorthis sp., Antirhyn chonella linguifera, Eoplectodonta sp., Spirigerina sp., Lissatrypa atheroidea, Atrypopsis reclinans, Eospirifer STRATIGRAPHY AND GEOLOGICAL CORRELATION

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cinghizicus, E. radiatus; colonial corals (identifica tions by N.B. Poltavtseva) Heliolites humilis, H. inter stinctus, Halysites catenularius, Paleofavosites cf. alveo laris, Propora subdecipiens, Catenipora micropora, Syringoporinus aff. raritabulatus; trilobites (identifica tions by I.M. Kolobova) Encrinurus mullochensis (Samples 889, 890, 892/1). Thickness 18–20 m. 10. Siltstone and argillite yellowish gray, beige, and olive green, with moniliform layers and small lenses of gray micritic sandstone. Numerous fossils as in the previous bed (Sample 885). Thickness 80 m. 11. Siltstone light and dark green, interbedded with finegrained sandstone with nodules of compact gray sandstones and beds of dark lilac gray siltstone. Thick ness 60 m. 12. Beds with graptolites of Coronograptus grega rius Zone: sandstone greenish gray, finegrained, indistinctly bedded, with large pencil jointing, inter bedded with siltstones of a similar color. The bed of orangegray siltstone contained graptolites (identifi cations by T.N. Koren’): Climacograptus ex gr. norma lis, Pseudoclimacograptus (Metaclimacograptus) cf. hugesi, Monograptus cf. tenuipraecursor, M. ex gr. rev olutus, M. denticulatus, M. triangulatus, Pribylograptus cf. argutus (Samples 10, 33). Thickness 110 m. The visible thickness of the formation is about 740 m. Upward in the section, the succession contains a barren series of mainly redbedded siliciclastics, the upper part of which is cut out by a fault (members 13– 15 in Fig. 3). It is well represented in the upper reaches of Kazbala Creek, 1.1 km southwest of Outcrop 33 along the strike of Bed 12, where it is described by Bandaletov (1969) as the upper part of the Alpeis For mation (see the description of the Kazbala section and traverse III in Fig. 4). Kazbala reference section (hypostratotype; Fig. 3; traverses II, III in Fig. 4) apart from the Alpeis Forma tion includes the lower part pf the Zhumak Forma tion—beds with Pentamerus longiseptatus (stratotype). The basal part of the Alpeis Formation contains so called “small fauna beds” with abundant small brachi opods, trilobites, and fragments of crinoid stems. The section continues toward the north from point 650a located 2600 m west of Akdombak Mountain and crosses the sublatitudinal valley of Kazbala Creek. The Alpeis Formation, as in the holostratotype, overlies the variegated member of the upper part of the Akdombak Formation with numerous obolids. It is subdivided into lithological members and beds with fauna. The description of the basal member is given bedtobed (Beds 1–10); subsequent numeration cor responds to the members: Alpeis Formation Basal member: beds with Eospirifer cinghizicus, lower part (A) 1. Greenish gray, dark green coarsegrained sand stone, gravelite, and smallpebble conglomerate with clasts of red and green jasper, sandstone, siltstone, Vol. 23

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Akdombak sandstones, basic and middle volcanics. Thickness 0.2–0.3 m. 2. Greenish gray, crimson siltstone and fine grained sandstone, breached by diabase sills. Thick ness 10 m. 3. Greenish siltstone with lathlike jointing, with beds (up 5 cm) of gray micritic limestone. Thickness 7 m. 4. Greenish gray siltstone with beds of bluish gray, compact, bedded quartzcalcareous sandstone, con taining accumulations of isolated valves and shells of E. cinghizicus (Samples 650a, 1038). Thickness 12 m. Overlying beds 5–7 are so called “small fauna beds”: 5. Limestone gray, nodular, bioclastic, with beds of calcareous siltstone, with brachiopods I. (Protocoster orthis) prima, Alispira cf. putilla, Eospirigerina porkuni ana, Leptaena sp. (Samples 650, 1023, 938, 1036, 1039). The same brachiopods are found near the Baikashkar River (Sample 1031). Thickness 1–15 m. 6. Alternation of green siltstone and finegrained sandstone. Beds contain small brachiopods Lean gella cf. scissa, Anisopleurella micula; unidentifiable brachiopods of the families Orthidae, Strophomenidae, Atrypidae (Sample 650b); and trilobites (identifications of M.K. Apollonov) Dalmanitina (Chattiaspis) sp., Remopleuridae, Lichadidae, Odontopleuridae, Otari onidae. Thickness 30 m. 7. Green argillite, siltstone, and aleurosandstone, forming loose talus on the northern slope of the hill and in the nearest sublatitudinal valley. The upper part of the bed contains brachiopods A. micula, Leangella cf. scissa, Plectambonitidae gen. et sp. indet., Tuvaella? sp., Eospirifer sp.; graptolites (iden tifications by T.N. Koren’) Climacograptus ex gr. nor malis, Cl. cf. angustus, Glyptograptus maderneii; and gastropods, crinoids, and others (Samples 650v, 652). Thickness 30 m. 8. Siltstone crimson, with subdominant beds of greenish varieties, breached by diabase sills covered in the upper part. Thickness 45–60 m. Beds with Eospirifer cinghizicus, upper part (B): 9. Alternation of brownish lilac and green siltstone and greenish gray finegrained sandstone with rare E. cinghizicus (Sample 1040). Thickness 40 m. 10. Sandstone grayish green, mediumgrained and finegrained, calcareous, with beds of brachiopod coquina and sandy bioclastic sandstones (10–15 cm). An inconsistent bed of gravelite is observed at the mem ber base. In the upper third, there is a bed (0.3–0.4 m) of gray dense sandstones and calcareous gravelite. Bra chiopods E. cinghizicus, E. radiatus, Pronalivkinia cf. numerosa, and Spirigerina sp. are found across the member (Samples 912, 9121, 914, 898, 1041). Thick ness 48 m. 11. Sandstone crimson, mediumgrained, banded. Thickness 10 m. 12. Limestone gray bedded and nodular, light gray massive, bioclastic, with numerous brachiopods Dole rorthis sp., Resserella sp., Isorthis sp., Leptostrophia sp., Eospirigerina sp., Pronalivkinia numerosa, P. nasuta,

Zygospiraella duboisi, Z. planoconvexa, E. cinghizicus and numerous rugose corals and colonial corals (Sam ples 913, 1042). Thickness 10–50 m. 13. Siltstone olivegreen, calcareous, with pancake like nodules of micritic marly sandstones. Rare bra chiopods and rugoses. Thickness 90 m. 14. Beds with graptolites of Coronograptus gregarius Zone: sandstone member dirty green, finegrained, interbedded with green, less commonly brownish silt stone. The member includes a series of subconformal sills and branching dikes, mainly of diabase. In some sites eastward along the strike, beds of platy orangey gray siltstone contain graptolites, equivalent to those in Bed 12 of the Zhanybek section (identifications by T.N. Koren’): Climacograptus ex gr. normalis, Pseudocli macograptus (Metaclimacograptus) cf. hugesi, Mono graptus cf. tenuipraecursor, M. ex gr. revolutus, M. den ticulatus, M. triangulatus, Pribylograptus cf. argutus, and others (Samples 17–19). Thickness up to 70 m. 15. Barren member (280–500 m): sandstone medium and coarsegrained gray and graycrimson, interbedded with finegrained varieties and bright green siltstone. Along the section, this member is interrupted by the floodplain of Kazbala Creek and is better exposed more to the east, in the upper reaches of the creek, near traverse III, where it is distinctly subdivided into two parts: 15a—sandstone gray, crim son, from coarse up to finegrained, alternating with bright green siltstone (200–400 m); 15b—red and green siltstone, with lathlike jointing, sandstone beds (7–15 cm) with spheroidal jointing (over 80 m). The thickness of the Alpeis Formation in the Kazbala reaches 970 m. Zhumak Formation (Donenzhal Horizon). Lower Subformation 16. Basal member: conglomerate and gravelites reddish, largepebbled to bullion, with volcanic frag ments, sealing jasper, clastic rocks, in the upper part gradually becoming coarsegrained sandstone. In the eastern region of the structure, where it is crossed by traverse III, this level is dominated by red and reddish gray coarsegrained sandstone with inconsistent beds of gravelite (0.4–1.0 m) and smallpebble conglomer ate. Thickness 50–55 m. Beds with Pentamerus longiseptatus: 17. Greenish gray, mediumgrained calcareous sandstone with coquina beds (0.2–0.8 m), usually of accumulations of small crinoid fragments (identifica tions by G.A. Stukalina) Cretalocrinus borealis, Medi necrinus lenitus, and others (Samples 895, 896). These are referred to the literature as the “Crinoidea beds,” marking the base of the beds with P. longiseptatus (Bandaletov, 1969; Borissiak et al., 1974; Sultanbe kova, 1986). Brachiopods of these beds are consider ably different from those of beds with E. cinghizicus. Apart from the continuing Dolerorthis sp., Isorthis sp., and Zygospiraella duboisi, here Leptostrophia tenuis, L. compressa, Eospirifer olgae and diverse pentamerids appear, especially numerous in member 19 (according


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to T.B. Rukavishnikova (Sapel’nikov and Rukavishni kova, 1975) and identifications by M.A. Olenicheva). Rugoses (identifications by Zh.S. Sultanbekova): Crassilasma crassiseptatum, Tryplasma medius, Schlo theimophyllum patellatum, Holacantia flexuosa, Calos tilis sp., Kungeiophyllum sp. (Samples 893, 8961). Thickness 80 m. 18. Sandstone, aleurosandstone, and siltstone yel lowish green, with abundant pancakelike nodules of compact gray sandstones. Thickness 80 m. 19. Sandstone finegrained, light green, calcare ous, thinly plated, with coquina beds, composed of brachiopods (identifications of T.B. Rukavishnikova and M.A. Olenicheva) Leptostrophia tenuis, Eoplect odonta sp., Eopholidostrophia sefinensis, Dolerorthis sp., Resserella sp., Isorthis sp., Pentamerus longiseptatus tuberosa, P. oblongus, Stricklandia lens progressa, Clorinda undata, C. undata vassilevskii, Zygospiraella duboisi, Atrypa hedei, A. reticularis, Lissatrypa athe roidea, Eospirifer radiatus, E. olgae, Nucleospira sp., Striispirifer sp. (Samples 894a, 8951, 1043). Rugoses (identifications of Zh.S. Sultanbekova): Crassilasma ramasanicum, T. medius, Ptychophyllum sp., Strom bodes schrenki, Calostilis sp., Kungeiophyllum sp. Thickness 40 m. 20. Sandstone dark greenish gray, conchoidal, with spheroidal jointing, with beds of siltstone and tuff sandstone (80 m). Numerous fossils (Samples 894, 8952, 897, 1030), including brachiopods (identifica tions by M.A. Olenicheva) Resserella sp., C. undata vassilevskii, L. atheroidea, Atrypa sp., E. radiatus; crinoids (identifications by G.A. Stukalina) Medine crinus lenitus, Bystrovicrinus sp.; colonial corals (iden tifications by O.P. Kovalevsky) Halysites cf. catenu laris, Heliolites aff. interstinctus; rugoses (identifications by Zh.S. Sultanbekova) C. ramazanicum, T. medius, Strombodes socialis, Cystiphyllum siluriensis, Hedstroe mophyllum obscurum, Calostylis gregaris; trilobites (identifications by I.M. Kolobova) Sphaeroexochus mirus, Encrinurus sp. Thickness 40 m. Apparently, this level is the same as a level with the occurrence (characteristic, according to O.P. Kova levsky) of beds with Propora obrutschevi in the Donen zhal Horizon: Palaeofavosites forbesiformis septatus, Catenipora panga, Halysites espensis, H. fixus, Helio lites cf. lebedevae, and also brachiopods and trilobites (Bandaletov, 1969, p. 34, Outcrop 870). The thickness of the lower subformation of the Zhumak Formation is about 340 m. Zhumak Formation (Donenzhal Horizon). The Upper Subformation 21. Transitional member: sandstone greenish and red, finegrained up to coarsegrained, tuff sandstone, less commonly gravelite and smallpebble conglomer ate. At the base, a bed of medium and largepebble conglomerate (3 m) with volcanic pebbles and less com monly those of sedimentary rocks. Thickness 420 m. 22. Red tuffs and andesibasalt lavas with interbeds of sandstone. Thickness up to 3000 m. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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The total thickness of the continuous Lower Sil urian sedimentary series in the Akdombak section is 1400–1800 m. The Alpeis Formation was traditionally subdivided into two subformations, i.e., the lower variegated green series, with brachiopods and corals including the beds with E. cinghizicus, H. cinghizicus, and the graptolite C. gregarius Zone of the Alpeis Horizon, and the upper, mainly red series, including beds with P. longi septatus Donenzhal Horizons (Bandaletov, 1969; Koren’ et al., 2003; Resheniya…, 1991). Thus recog nized, the lower subformation represents a transgres siveregressive cycle, limited below and above by unconformities. The upper subformation is very differ ent from the lower and resembles the Zhumak Forma tion. The boundary of the upper subformation (mem ber 16 of the Kazbala section, Fig. 3) coincides with the beginning of the next transgressiveregressive cycle and is a distinct lithological marker, optimal for the defini tion of a boundary, similar to the boundary of the basal member of the preceding Alpeis cycle (see above). We exclude the upper subformation from the Alpeis For mation and assign it to the lower subformation of the Zhumak Formation. Accordingly, the lower boundary is lowered to the base of member 16 (Figs. 2, 3). Apart from beds with E. cinghizicus, beds with the brachiopod Holorhynchus cinghizicus have a less dis tinct stratigraphic position and are found both below and at the level of the beds with E. cinghizicus, some times completely replacing them in many sections in the Alpeis Formation and its equivalents beyond the stratotype region. The Ramazan reference section is 9 km southeast of Akdombak Mountain, near the mouth of the Bakanas River, 400 m north of the village of Ramazan (Figs. 3, 5). The lower part of the Alpeis Formation crops out on the left bank of the Bakanas River, where it is complicated with small folds and faults. The barren basal series (about 50 m) is com posed of coarsegrained polymictic sandstone with beds of brownish, greenish gray siltstone and two beds of smallpebble polymictic conglomerate. These are overlain by a variegated member, corresponding to Members 3–6 of the Zhanybek section. The member continues to the right bank of the Bakanas River, where its upper part crops out on the slope of the floodplain ledge. An almost uninterrupted succession of the Alpeis and Zhumak formations is exposed above the ledge base in the northwestern direction (from bot tom to top): Alpeis Formation Beds with Eospirifer cinghizicus, upper part (B): 1. Alternation of brownish red, greenish siltstone and argillites with lathlike jointing, numerous pan cakelike nodules 5–6 cm in diameter and beds of gray nodular micritic sandstones (0.5–2 cm). Several strati graphic levels contain brachiopods Isorthis sp., Zyga trypa (Ramazania) separata, Zygospiraella duboisi, E. cinghizicus, Tuvaella? sp., and small rugoses and colonial corals (Samples 595a, 1048). Thickness 50 m. Vol. 23

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Fig. 5. Geological scheme of the outcrops of the Lower Silurian deposits near the village of Ramazan. (1) Upper Ordovician silt stone and sandstone; (2–7) Alpeis Formation: (2) polymictic conglomerate; (3) green, variegated siltstone; (4) limestone, cal careous sandstone; (5) sandstone, aleurosandstone green calcareous; (6) sandstone, gravelite graygreen (a) and red (b); (7) yel lowish argillite, nodular limestone; (8, 9) Zhumak Formation, lower subformation: (8) sandstone, gravelite, conglomerate; (9) calcareous siltstone, sandstone, crinoid limestone; (10) Zhumak Formation, upper subformation: andesite tuffs, volcanomic tic sandstone; (11) loose Cenozoic deposits; (12) acidic dikes; (13) faults; (14) traverse of the Ramazan section; (15, 16) fossil occurrences: (15) benthic, (16) graptolites. Localities are indicated by numbers. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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2. Sandstone calcareous, dark green, finegrained (up to aleurosandstone). Graptolites (identifications by N.F. Mikhailova): Pseudoclimacograptus hughesi, Rec tograptus ex gr. truncatus, Akidograptus sp. (Sample 20). Thickness up to 20 m. 3. Beds with Holorhynchus cinghizicus (C): limestone yellowish gray, nodular, clayey, unevenly bedded, with subdominant beds of calcareous sandstone and silt stone. Numerous colonies of colonial corals, rugoses, and brachiopods Holorhynchus cinghizicus, Z. duboisi, and others. (Samples 899, 1049). Thickness 5–6 m. 4. Sandstone dark graygreen, brownish, with large (up to 15 cm) pancakelike nodules and lenses of argillaceous sandstones, with occasional beds of coquina (10–15 cm thick) with the brachiopods H. cinghizicus, Z. duboisi, Spirigerina sp., Eopholi dostrophia sp., Strophomenidae, in places with abun dant crinoid fragments Chinghizocrinus kokajgirensis (Samples 8991, 1050). Thickness 40 m. 5. Continuation of beds with E. cinghizicus (B): sandstone light green mediumgrained calcareous and coquinae with brachiopods Resserella sp., Isorthis sp., Zygospiraella sp., and E. cinghizicus (Samples 915, 1051, and others). Thickness 120 m. 6. Sandstone lightcolored greenish gray, stained, medium and coarsegrained, in the upper part including crimsongray varieties. Thickness 120 m. Zhumak Formation, Lower Subformation (Donenzhal Horizon) 7. Sandstone red, crossbedded, with beds of grav elite. Thickness 70 m. Beds with Pentamerus longiseptatus: 8. Argillite and siltstone yellowish beige, greenish gray, with thin moniliform layers and nodules of gray bioclastic sandstones, with abundant rugoses, colonial corals, and brachiopods Pentamerus longiseptatus, Orthidae, Strophomenidae (Samples 916, 917). In the upper part with coquinae with Dolerorthis sp., Resse rella sp., P. longiseptatus, and others. (Samples 1052, 1056). Thickness 12–15 m. 9. Conglomerate red, small to large rubbly, with pebbles of acidic volcanics, sealing jasper, with beds of gravelite and sandstone. Thickness 15–20 m. 10. Siltstone greenish, calcareous, with beds of gray crinoidal sandstones of the marker “Crinoidea beds,” with abundant crinoid fragments (Sample 918), upward in the section replaced by whitish calcareous finegrained, thinbedded sandstone. The lower part contains brachiopods of the families Pentameridae, Atrypidae, Strophomenidae, etc., rugoses, trilobites (Samples 919, 942, 1057), and higher in the section numerous large Atrypidae, corals (identifications by N.V. Poltavtseva): Paleofavosites sp., Halysites ex gr. catenularis, Heliolites cf. lebedevae, H. aff. yavorskyi Tchernyshev (Samples 9421, 1058). The upper layers are dominated by brachiopods Clorinda sp., Penta merus sp., Atrypa sp., Strophomenidae (Sample 9422). Thickness 50 m. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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The upper subformation of the Zhumak Formation overlies the lower, with a gradual transition and is composed of coarseclastic andesite tuffs and basalts with beds of red sandstone and gravelite. The total (visible) thickness of Lower Silurian sedimentary series with fossils in the Ramazan section is 560 m. A similar succession is observed 11 km westnorth west, on the right bank of the Baikoshkar River, near its mouth. Further northwest, a section in the upper reaches of the Kurbakanas River, in sections described by S.M. Bandaletov and I.F. Nikitin 1–1.5 km north east of Elgunov’s Grave (currently village of Tatuadam), exposes beds with H. cinghizicus (Bandaletov, 1969, p. 6, textfig. 10; Nikitin, 1972, p. 92, textfig. 25). The Alpeis Formation rests here, apparently with a hidden unconformity, on beds with H. giganteus of the Akdom bak Formation. The basal member of green sandstone, siltstone, and sandstones with conglomerate beds con tains brachiopods of most likely the lower part of the beds with E. cinghizicus (A): Leangella ex gr. scissa, Eoplectodonta sp., Leptaena sp., Eostropheodonta sp., Eospirigerina ex gr. porkuniana, Coolinia sp., Holorhyn chus sp., Virgianella? sp., and Stricklandia? sp. (Sam ples 138b, 925/1). Pronalivkinia numerosa, Zygospi raella sp., Eospirifer sp. and corals (identifications by N.V. Poltavtseva), including Mesofavosites sp., and Pro pora conferta, are found higher up in the section (Sam ples 926, 927). Approximately at the level of the middle part of beds with E. cinghizicus, we recognized beds with H. cinghizicus (C) composed of calcareous siltstone and sandstone with beds of sandstones and coquina, con sisting of large valves of H. cinghizicus, and also Eopholi dostrophia sp., Zygospiraella sp., Leptostrophia sp., and Eospirifer sp. (Samples 927, 928, 929). They are overlain by the upper part of the beds with E. cinghizicus (B) rep resented by green and crimson sandstone, with coquina beds with E. cinghizicus (Sample 930). The upper part of the formation is cut off by a fault, and its visible thick ness is about 200 m (Fig. 3). In the northwestern continuation of the Chingiz Subzone, to the south of the Tokai Mountains, the lower part of beds with E. cinghizicus (A) is not recog nized but is replaced by nodular, impure limestone with H. cinghizicus. This limestone lies at the base of the Alpeis Formation below a thick series of variegated sandstone with brachiopods of the upper part of the beds with E. cinghizicus (B). In the southeast termina tion of the subzone, in western Tarbagatai, both of these taxa are found together in carbonates (Poltav tseva et al., 1974; Sapel’nikov and Rukavishnikova, 1975; Nikitina, 2004). In the Prechinghizian Subzone (Southwestern Prechinghizian Region), in the Akbastau district, the beds with H. cinghizicus are recognized at the base of the Silurian section (Bandaletov, 1969). The Ordovi cian–Silurian boundary beds are represented here by the siliciclasticvolcanic series with large organic mounds, up to 900 thick, composed of thickbedded and massive bioherm limestones. The Ordovician part of the section is dominated by bryozoanalgal variet Vol. 23

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ies, and the Silurian beds, by coralstromatopore vari eties, with beds and lenses of carbonatesiliciclastic and tuffsiliciclastic rocks. The buildups usually rest on Upper Ordovician island arc volcanics. In the val ley of the Koryk River, to the north of the village of Algabas (Fig. 4, inset), the carbonate series, like the region near Akdombak Mountain, is dated by the fauna of the Dulankara Horizon in the lower part, while in the middle part it contains trilobites and bra chiopods of beds with H. giganteus of the Chokpar Horizon. The upper part of the series yielded fossils indicative of the Silurian Alpeis Horizon: corals Paleo favosites maximus, P. simplex, Mesofavosites fleximuri nus, Heliolitidae, and others (identifications by N.I. Ivanova); trilobites Encrinurus punctatus and oth ers; and also brachiopod banks, composed of shells and valves of H. cinghizicus. The top of the mounds contains beds of argillaceous nodular coralstromato pore sandstones of varying thickness. They contain abundant corals P. maximus, M. fleximurinus, Cateni pora algabasensis, Custipalipfyllum kimi, and others (identifications by N.I. Ivanova) and brachiopods Dolerorthis sowerbyana, Isorthis sp., H. cinghizicus, Virgianella? sp., Zygospiraella ex gr. duboisi, Alispira sp., Eospirigerina porkuniana, and Meifodia tulkulensis in association with crinoids, nautiloids, gastropods, and other fossils. The limestones laterally alternate with lenses of unsorted (up to bullion) polymictic or sand stone conglomerates (reef facies), which were previ ously interpreted as basal facies, and also with tuffites and agglomerate middle tuffs. Thus, in contrast to pre vious interpretations (Bandaletov, 1969; Resheniya…, 1991), the base of the Silurian here lies within a con tinuous carbonate series, whereas the overlying con glomerates are not basal. The bioherm limestones are overlain by a thick (up to 2000 m) series of marginal shelf and slope gray green flysch facies, i.e., sandstonesiltstone rhythms, often of tuff origin, with flows, sills, and intralayer dikes of middle volcanics. These flysch facies contain successive graptolite assemblages from the Coro nograptus cyphus Zone of the Ruddanian Stage up to the Lituigraptus convolutus Zone of the Aeronian Stage inclusive (identifications by T.N. Koren’). The basal part contains lenses of polymictic and sandy

impure conglomerates with brachiopod banks, con sisting of valves of H. cinghizicus. The beds with E. cin ghizicus are not established here, whereas accumula tions of brachiopods of the Nucleospira associations are found at the level of the beds with P. longiseptatus composed of the same mainly red clastic facies as the Zhumak Formation of the Chingiz Subzone. Beds in the northeastern Prechingizian region (Alkamergen’ Tarbagatai Subzone) are dominated at the same level by brachiopods of the Nalivkinia association (Koren’ et al., 2003; Nikitina et al., 2006). BRACHIOPODS The brachiopod collection comes from seven locali ties in the lower part of the Alpeis Formation on the southwestern slopes of the Chingiz Range on the Kazbala and Zhanybek creeks, Baikoshkar River, and Bakanas River. It contains ten taxa (eight from the lower part and two from the upper part of beds with E. cing hizicus), including two new species, of which one belongs to a new subgenus. The Ruddanian brachiopod assemblage of the lower part of the beds with E. cing hizicus is presented for the first time. It dates the base of the Alpeis Formation of the Alpeis Horizon and allows the refinement of the Ordovician–Silurian boundary in this region. The only similar assemblage in Kazakhstan has been described by Modzalevskaya and Popov (1995) from the Lower Silurian of the Akkerme Peninsula on the southeastern coast of Lake Balkhash. Brachiopods of the lower part of the beds with E. cinghizicus belong to two benthic associations. The first, with low taxonomic diversity or monotax onic, is found in the shallowest water facies in the basal part of the transgressive succession. It corresponds to benthic assemblage 2 (according to A. Boucot) and is mainly composed of numerous shells of E. cing hizicus. These are dominated by specimens rounded in outline, with a shortened hingeline, a shallow bowlshaped sulcus with a flattened base and flat tened fold, possessing an anterior longitudinal groove (Plate II, figs. 10–14). These characters of the shell are within the species variability limits, the wide range of which was note by Borissiak (1955). I. (Protocortezor this) prima (Plate I, figs. 12–15, 23), identical to the

Plate I. Brachiopods of subfamilies Sowerbyellinae, Isorthinae, Tuvaellinae, Atrypinae, Clintonellinae and family Leptestiidae from the lower part of the beds with Eospirifer cinghizicus. Abbreviations: v.v.– ventral valve, d.v.–dorsal valve. (1–5) Anisopleurella micula Nikitin et Nikitina, sp. nov.: (1) open shell no. 5298/1650v (holotype), ×11.6; (2) v.v. no. 5298/2650v, imprint, ×12; (3) d.v. no. 5298/3650v, external mould, ×12; (4) v.v. mold and fragment of d.v. mold no. 5298/4650v, ×12; (5) v.v. no. 5298/5650v, external mould cast, ×12; Kazbala section, locality 650v; (6–10) Leangella cf. scissa (Davidson): (6) v.v. internal mold no. 5298/9a650b, ×8.3, (7) v.v. internal mold no. 5298/13650v, ×7.5; (8, 9) v.v. no. 5298/15650v, (8) external mould, (9) external mould cast, ×9; (10) d.v. external mould no. 5298/11650v, ×10; Kazbala section, localities 650b, 650v; (11) Plectambonitacea gen. et sp. indet., v.v. internal mold no. 5298/043650v, ×7.5, Kazbala section, locality 650v; (12–15, 23) Isorthis (Protocorte zorthis) prima Walmsley et Boucot: (12⎯14, 23) complete shell no. 5298/018831, ×5: (12) ventral view, (13) dorsal view, (14) anterior view, (23) lateral view; Zhanybek section, locality 8831; (15) d.v. no. 5298/031044, external ornamentation, ×7, Zhanybek section, locality 1044; (16–18) Tuvaella? sp., complete shell no. 5298/02650v: (16) ventral view, (17) lateral view, (18) anterior view, ×5.6; Kazbala section, locality 650v; (19, 20, 26) Zygospiraella duboisi Verneuil, complete shell no. 5298/31048: (19) ventral view, (20) dorsal view, (26) anterior view, ×6; Ramazan section, locality 1048; (21, 22, 24, 25) Alispira cf. putilla (Hall et Clarke), complete shell no. 5298/10311: (21) ventral view, (22) dorsal view, (24) anterior view, (25) posterior view, ×7.5; Kazbala section, Outcrop 1031. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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species described from the Akkerme Peninsula (Modzalevskaya and Popov, 1995), is less common. A stratigraphically higher association is the socalled “small fauna beds,” composed of numerous and diverse brachiopods, most likely of benthic assemblage 3 (according to A. Bucout). This association typically STRATIGRAPHY AND GEOLOGICAL CORRELATION

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has tiny thinwalled Anisopleurella micula Nikitin et Nikitina sp. nov. and also Leangella cf. scissa (David son), which are distinguished from this species only by the smaller (average) size (Plate I, figs. 6–10). Small Alispira cf. putilla (Hall et Clark) (Plate I, figs. 21, 22, 24, 25) closely related to A. putilla from the Lower Sil Vol. 23

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urian of Illinois and Missouri in the United States (Amsden, 1974, pl. 17, fig. 7; pl. 18, figs. 1–3, 5–7) are less common. Similarly small Plectambonitacea gen. et sp. indet. most likely belong to the subfamily Aegi romeninae (Plate I, fig. 11). Eospirigerina porkuniana (Plate II, figs. 6–9) dominate among larger speci mens and are similar to specimens described from the Juuru Horizon of Estonia (Rubel, 1970) and Rudda nian of the Akkerme Peninsula (Modzalevskaya and Popov, 1995). Tuvaella? sp. (Plate I, figs. 16–18) externally appear to belong to this species, but their interior is not studied because of poor preservation. Descriptions and lists of brachiopods from the upper part of the beds with E. cinghizicus are cited in many pub lications (Borissiak, 1955; Sapel’nikov and Rukavish nikova, 1975; Resheniya…, 1991; etc.). Near the village of Ramazan (Figs. 3, 5) this level contains, together with E. cinghizicus and Zygospiraella duboisi widely dis tributed in the Lower and Middle Llandovery, also described from the Akkerme Peninsula (Modzalevskaya and Popov, 1995), small atripids Zygatrypa (Ramazania) separata Nikitin et Nikitina subgen. et sp. nov. SYSTEMATIC PALEONTOLOGY The following abbreviations were used for the main parameters of brachiopod shells and isolated valves: vs—ventral valve, vp—dorsal valve, l—tongue. Maximum parameters: L, H—length, height of the shell or the valve; W, T—shell width, shell thickness. S, MF—sulcus width near the anterior margin, median fold width near the anterior margin. A—area length, TO—tongue length, HL—hingeline length. All dimensions are in millimeters. The collection is housed under no. 5298 in the Orlov Paleontological Museum, Russian Academy of Sciences (PIN), Moscow, Russia. O R D E R STROPHOMENIDA ÖPIK, 1934 S U P E R F A M I L Y PLECTAMBONITOIDEA JONES, 1928 FA M I L Y SOWERBYELLIDAE ÖPIK, 1930 S U B F A M I L Y SOWERBYELLINAE ÖPIK, 1930

Genus Anisopleurella Cooper, 1956 Anisopleurella micula Nikitin et Nikitina, sp. nov. Plate I, figs. 1–5

E t y m o l o g y. From the Latin micula (mica, a lit tle crumb, little grain, little bit).

H o l o t y p e. PIN no. 5298/1650v, open shell: L—1.6, W—2.5; Chingiz Range, Kazbala Creek 9.5 km northwest of the village of Ramazan, locality 650v; lower part of the Alpeis Formation, Alpeis Formation, Ruddanian Stage. M a t e r i a l. Locality 650b—one ventral and one dorsal valve. Locality 650v—imprints of the inner sur face of two open shells and outer surface of ten ventral and five dorsal valves. Diagnosis. Tiny thinshelled Anisopleurella with three major radially ornamented costae, slightly visible muscle field on ventral valve, and almost indiscernible bema on dorsal valve. D e s c r i p t i o n. The shell is thinwalled, very small, transverse, weakly concavoconvex semicircu lar in outline with the cardinal extremities at a near right angle. The shell width corresponds to the hinge line width. The ventral valve is weakly and evenly con vex; cardinal extremities are flattened. The beak is not pronounced. Area low, almost linear, apsacline, with small pseudodeltidium. The dorsal valve is weakly concave and almost flattened. The area is linear. The protegulum is shaped like a tiny tubercle. The radial ornamentation consists of three threadlike main cos tae on the ventral valve and four on the dorsal valve, dividing the surfaces into four or five sectors, respec tively. Inner sectors between the primary costae have up to 7–8 costellae of the second order, and outer sec tors have 12–14; about six costellae in 1 mm at the anterolateral shell margin. Shell interior. The ventral interior does not show the muscle field on the molds. The dental plates are absent; the hingeline has no denticles. The dental inte rior has an undercut cardinal process; the dental sock ets are long, conical, almost parallel to the hingeline, anteriorly bound by the socket ridges. Three short septa appear at some distance from the beak and diverge at 30°–32° and in adult shells finish at the midvalve. The bema is hardly noticeable. M e a n s i z e s: (eight ventral and six dorsal valves). Ventral valve: L—1.4 (min—1.0, max—1.6), W—2.2 (min—1.3, max—2.5), L/W—0.6 (min— 0.5, max—0.8). Dorsal valve: L—1.2 (min—1.1, max—1.6), W—1.8 (min—1.0, max—2.5), L/W— 0.7 (min—0.6, max—0.7). C o m p a r i s o n a n d r e m a r k s. A. micula sp. nov. is half or onethird of the size of all other known species of the genus and differs in the thin shell, and hence, the indistinct muscle field in the ventral valve,

Plate II. Brachiopods of subfamilies Anazyginae, Spiriferininae, and Eospiriferinae from the lower and upper parts of the beds with Eospirifer cinghizicus. (1–5) Zygatrypa (Ramazania) separata Nikitin et Nikitina, subgen. et sp. nov., complete shell no. 5298/10481, holotype: (1) ventral view, (2) dorsal view, (3) lateral view, (4) posterior view, (5) anterior view, ×5; Ramazan section, locality 1048; (6–9) Eospirigerina porkuniana (Jaanusson in Rubel, 1970): (6–8) complete shell no. 5298/10312: (6) ventral view, (7) dorsal view, (8) Posterior view, ×2.7; Kazbala section, locality 1031; (9) complete shell no. 5298/8831/1, lateral view, ×4, Zhanybek section, locality 8831; (10–22) Eospirifer cinghizicus M. Borissiak: (10–12) incomplete shell no. 5298/650a1: (10) ventral view, (11) dorsal view, (12) lateral view, ×4.6); Kazbala section, locality 650a; (13, 15–17) complete shell no. 5298/9382: (13) anterior view, (15) dorsal view, (16) posterior view, (17) lateral view, ×5.5; Kazbala section, locality 938; (14) fragment of the ventral beak area internal mold no. 5298/650v25, ×10, Kazbala section, locality 650v; (18–22) complete shell no. 5298/10481: (18) ventral view, (19) dorsal view, (20) anterior view, (21) posterior view, (22) lateral view, ×4.2; Ramazan section, locality 1048. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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and in the hardly discernible bema in the dorsal valve (Jones, 1928, p. 472, pl. 24, figs. 21–25; Cocks, 1970, p. 189, pl. 16, figs. 1–9; Temple, 1970, p. 38, pl. 8, figs. 4–7, 10–13; Temple, 1968, p. 58, pl. 5, figs. 9– STRATIGRAPHY AND GEOLOGICAL CORRELATION

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22

17). Most known species of the genus Anisopleurella are found in the Middle and Upper Ordovician and only A. gracilis (Jones) occurs from the Upper Ordov ician (Hirnantian) to the Lower Silurian (Cocks and Vol. 23

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Rong, 1989, p. 133). A. micula sp. nov. is distinguished from this closely related species, apart from the above characters, by the weakly and evenly convex ventral and concave dorsal valves, and also by the coarser radial ribbing: the number of costellae of the second order per 1 mm of shell surface does not exceed six, whereas A. gracilis (Jones) has eight or nine. O c c u r r e n c e. Lower Silurian, Llandovery, Rud danian Stage, lower part of the Alpeis Horizon, lower part of the beds with Eospirifer cinghizicus. L o c a l i t y. Lower part of the Alpeis Formation, Chingiz Range, localities 650b, 650v, left bank of the Kazbala Creek. O R D E R ATRYPIDA RZHONSNITSKAYA, 1980 S U B O R D E R ANAZYGIDINA COPPER, 1996 S U P E R F A M I L Y ANAZYGOIDEA DAVIDSON, 1883 FA M I L Y ANAZYGIDAE DAVIDSON, 1883 S U B F A M I L Y ANAZYGINAE DAVIDSON, 1883

Genus Zygatrypa Copper, 1977 Subgenus Zygatrypa (Ramazania) Nikitin et Nikitina, subgen. nov.

E t y m o l o g y. In honor of Academician of the Kazakh Academy of Sciences Ramazan Aslambekov ich Borukaev—researcher of the Lower Paleozoic of Kazakhstan. Ty p e s p e c i e s. Zygatrypa (Ramazania) sepa rata Nikitin et Nikitina, subgen. et sp. nov.—lower part of the Alpeis Formation of the Chingiz Range, Kazakhstan. D i a g n o s i s. Coarse costate Zygatrypa with cos tae covering entire shell surface, including its postero lateral areas, with sulcus on ventral valve and median fold, additional sulcus on dorsal valve. C o m p a r i s o n. The new subgenus is distin guished from the subgenus Z. (Zygatrypa) Copper, containing the North American species Z. mica (Bill ings), Z. plicata (Hall), Z. paupera (Hall), and Z. min ima (Hall) (Copper, 1977, pp. 307–310), by charac ters listed in its diagnosis. Zygatrypa (Ramazania) separata Nikitin et Nikitina subgen. et sp. nov. Plate II, figs. 1–5

E t y m o l o g y. From the Latin separatus (sepa rate). H o l o t y p e. PIN, no. 5298/11048, complete shell: Lv—5.9, Ld—5.6, W—6.5, Hv—2.2, Hd—1.5, T—3.7, v.v.—2.5, d.v.—3.2; Chingiz Range, right bank of the Bakanas River opposite the village of Ramazan, locality 1048; Alpeis Formation, Alpeis Horizon, Ruddanian Stage. M a t e r i a l. Locality 1048: 13 shells of various state of preservation and two ventral valves. D i a g n o s i s. Small ventribiconvex shell rounded pentagonal up to suboval in outline, transverse or slightly elongated, with narrow sulcus on the ventral

valve and with wider dorsal sulcus, with a median fold in its middle. Radial ornamentation composed of 16– 17 coarse costae. D e s c r i p t i o n. The shell is small ventribiconvex, transverse or slightly longitudinally elongated, roundedpentagonal or suboval in outline. The maxi mum width in transverse elongated specimens is between the hingeline and the middle of the shell; in elongated and rounded specimens, it is near the mid dle of the shell. The hingeline is about half of the shell width. The cardinal extremities are rounded, and the anterior margin is intraplicate. The ventral valve is more strongly convex than the dorsal (Hd/Hv—0.4), with flattened or slightly concave lateral surfaces and an evenly convex profile. The beak is acute, directed backward or curved upward. The area is short, ana cline, weakly concave. The deltirium is wide, with convex deltidial plate. The narrow sulcus is gently arched in cross section and begins immediately pos terior to the midvalve. It is limited by sharp shoulders on the sides. Its width near the anterior margin is about half the shell width (Sv/W—0.4). The dorsal valve is weakly convex, sometimes almost flat, shorter than the ventral (Ld/Lv—0.9). The lateral surface is weakly convex; the profile is evenly convex or sharply flexed near the midvalve. The beak is small, not sepa rated. Median sulcus is wide, with sloping margins, beginning from the beak and near the anterior margin, occupying 0.4 of the valve width. Median fold also begins near the beak, but becomes prominent near the midvalve and visible up to the anterior margin, like the sulcus on the ventral valve. Near the anterior margin, the height of this fold does not exceed the depth of the sulcus, giving an impression of a median fold demar cated laterally by wide and shallow grooves. The radial ornamentation consists of ctenoform, mainly simple costae running from the beak up to anterior margin along the entire shell surface. Bifurcating costae are observed on the lateral sides of some specimens. The largest costae are on the sides of the median sulcus of the ventral valve and one is in the middle of the fold within the sulcus of the dorsal valve. The lateral surfaces have six to seven costae. In the sulcus of the ventral valve, there are two costae; on the fold of the dorsal valve, three costae. Apart from the radial ribbing, the entire surface is covered by very thin concentric lines forming zigzag ornamentation on the costae. Shell interior. The ventral interior with massive oblique teeth. The dorsal interior with massive cardi nal ridgelike process. The dental plates are fused. The conical spiralia of the spirals are composed of three to four volutions, and their apices are directed toward the middle of the dorsal valve. M e a n s i z e s: 11 complete shells: Lv—6.99 (min—5.3, max—9.6); Ld—6.6 (min—4.6, max— 9.0); W—6.6 (min—4.4, max—7.8); T—4.1 (min— 4.1, max—3.0); Hv—2.9 (min—1.4, max—4.0); Hd—1.2 (min—0.5, max—2.1); with v—2.4 (min— 4.1, max—3.0); with d—3.7 (min—2.5, max—4.5).


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Variability. The species is highly variable. The shell outline varies from transverse roundedpentagonal to elongatedoval. The shell thickness also varies. In spec imens with weakly convex valves, the median sulcus on the ventral valve and the fold of the dorsal valve are usu ally weakly developed, but are always discernible. A g e v a r i a b i l i t y. In young shells less than 4 mm long, the valves are most commonly weakly con vex, whereas the sulcus of the ventral valve and the median fold in the sulcus on the dorsal valve are absent or hardly noticeable near the anterior margin. O c c u r r e n c e. Lower Silurian, Llandovery, Rud danian Stage, Alpeis Formation, upper part of the beds with Eospirifer cinghizicus. L o c a l i t y. Localities 1048, 595a (a1–a3), Alpeis Formation, right bank of the Bakanas River, opposite the village of Ramazan, Chingiz Range. CONCLUSIONS 1. Owing to the occurrences of Ruddanian brachio pods in the lower part of the Alpeis Horizons, the OrdovicianSilurian boundary is revised and its position in the Chingiz Region is refined. In the Chingiz Sub zone, the boundary coincides with the unconformity separating the transgressiveregressive successions and corresponds to a gap in sedimentation. However, there is no evidence that the gap was strong: many characters, including the structural conformity of the boundary beds, small thickness, and frequent wedging out of the Silurian basal conglomerates, show that the gap was local, with no structural change or change in the sedi mentary settings. In the adjacent deeper Prechinghizian Subzone, at the contact of the island arc ChingizTar bagatai region and ZhungarBalkhash backarc basin, sedimentation at the Ordovician–Silurian boundary was not interrupted. A sharp change in the sedimentary settings from a shallow shelf with local volcanism and reef carbonate sedimentation to an outer shelf and slope settings with thick siliciclastic sediment gravity flow deposits only happened in the Ruddanian. 2. The beds with Eospirifer cinghizicus increase in thickness and are subdivided into two parts (Figs. 1–3). The lower part (A) is dominated by Isorthis (Protocorte zorthis) prima, Eospirigerina porkuniana, small plectambonitids, atripids, trilobites, and crinoids typ ical of the Ruddanian Stage against a background of relatively rare E. cinghizicus. In the upper part of the beds (B), where they were originally established, they are sharply dominated by E. cinghizicus in association with E. radiatus, Zygospiraella duboisi, and large shells of atripids (Pronalivkinia, Lissatrypa), strophomenids, and orthids (Leptostrophia, Resserella). 3. Certain patterns are observed in the spatial distri bution of brachiopods of the Alpeis Formation. The central part of the Chingiz Subzone (Fig. 3, Zhanybek, Kazbala sections) are dominated by eospiriferids, on the basis of which the beds with E. cinghizicus were rec ognized. The pentamerids of genus Holorhynchus are much less common and occur mainly in the periphery STRATIGRAPHY AND GEOLOGICAL CORRELATION

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of the Chingiz Subzone (Ramazan and Tatuadam sec tions) and in the Prechinghizian Subzone, where they often form pentamerid banks in the carbonate reef facies. It is in these regions that beds with H. cinghizicus are recognized. These beds show low taxonomic diver sity and are assigned to shallow water shelf biofacies of benthic assemblages 2–3 of the upper and medium sublittoral. These are synchronous with the deeper eospiriferid facies of benthic assemblages 3–4 in medium sublittoral with high taxonomic diversity. Beds with E. cinghizicus and with P. longiseptatus are the most consistent laterally, widely distributed, and reli able correlative markers in the Lower Silurian beds of the Chingiz Range, also known in other regions of Kazakhstan. The beds with H. cinghizicus occur patch ily and represent pentamerid biofacies, which usually occur in small thickness at various levels within the beds with E. cinghizicus. Occasional finds of H. cing hizicus in South Zhungary are the only records of this species beyond the Chingiz Subzone. 4. The boundary between the Alpeis and Zhumak formations is revised and correlated with the bound aries of biostratigraphic horizons; the lower position of the Zhumak Formation (Fig. 3) is revised. The range and composition of the formations corresponding to the sedimentary sequences are detailed. The revised Zhumak Formation of the Chingiz and Prechinghi zian subzones much better correlates with the Donen zhal and Ajaguz formations of the Alkamergen’Tar bagatai Subzone adjacent in the northeast and also with the Tiulkuli Formation of western Tarbagatai (southeastern margin of the Chingiz Subzone). All this allows simplification and refinement of stratigraphic schemes and correlation with other section types in the Chingiz Subzone and beyond its boundaries. In addition, all units and boundaries are readily recog nizable, easily mapped, and only need for their status to be fixed in the new generation stratigraphic schemes (the latest schemes for the Paleozoic of Kazakhstan accepted in 1986; Resheniya…, 1991). 5. The Upper Ordovician and Lower Silurian sedi ments in the Chingiz Subzone contain two gaps corre sponding to the base and top of the Alpeis Formation. The former corresponds to the end of the Ordovician and was likely caused by a glacialeustatic event of the Late Ordovician glaciation (Brenchley, 1988). The sec ond gap, preceding the Zhumak Formation, was related to the intensified regression leading to the shallowing of the marine basins and expanding of the landmass in Kazakhstan at the end of the Early Silurian. The general regression in the Late Ordovician–Early Silurian time was complicated by two transgressiveregressive cycles reflected in the deposits of the Akdombak and Alpeis formations. They include two shortterm high stands, marked by the appearance of graptolite facies of the deep shelf and slope against a background of the domi nant shallow shelf and coastal facies. Vol. 23

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ACKNOWLEDGMENTS We are sincerely grateful to T.N. Koren’, the late O.P. Kovalevsky, T.L. Modzalevskaya, L.E. Popov, T.Yu. Tolmacheva, and L.M. Melnikova for valuable consultations and useful comments on the paper. Reviewed by T.L. Modzalevskaya, T.Yu. Tolmacheva, and A.S. Alekseev. REFERENCES Amsden, T.W., Late Ordovician and Early Silurian articu late brachiopods from Oklahoma, SouthWestern Illinois, and Eastern Missouri, Bull. Oklah. Geol. Surv., 1974, no. 119, pp. 1–154. Apollonov, M.K., Ashgill’skie trilobity Kazakhstana (Ashgil lian Trilobites of Kazakhstan), AlmaAta: Nauka, 1974 [in Russian]. Bandaletov, S.M., Silur Kazakhstana (The Silurian of Kaza khstan), AlmaAta: Nauka, 1969 [in Russian]. Borissiak, M.A., Stratigraphy and brachiopods of Silurian deposits of the Chingiz Ridge, Mat. VSEGEI. Nov. Ser., 1955, no. 5. Borissiak, M.A., Kovalevskii, O.P., Kolobova, I.M., and Stukalina, G.A., Biostratigraphical scheme for the Upper Ordovician and Lower Silurian of Chingiz and Tarbagatai (Eastern Kazakhstan), in Dopaleozoi i paleozoi Kazakh stana. T. 1 (The PrePaleozoic and Paleozoic of Kazakh stan), AlmaAta: Nauka, 1974, pp. 228–238. Brenchley, R.J., Environmental changes close to the Ordovician–Silurian boundary, Bull. Brit. Mus. (Nat. Hist.). Geol., 1988, vol. 43, pp. 376–385. Cocks, L.R.M., Silurian brachiopods of the superfamily Plectambonitacea, Bull. Brit. Mus. (Nat. Hist.). Geol., 1970, vol. 19, pp. 139–203. Cocks, L.R.M. and Rong, JiaYu., Classification and review of the brachiopod superfamily Plectambonitacea, Bull. Brit. Mus. (Nat. Hist.). Geol., 1989, vol. 45, pp. 77–163. Copper, P., Zygospira and some related Ordovician and Sil urian atrypoid brachiopods, Palaeontol., 1977, vol. 20, no. 2, pp. 295–335. Degtyarev, K.E., Tektonicheskaya evolyutsiya rannepaleozo iskoi aktivnoi okrainy v Kazakhstane (Tectonic Evolution of the Early Paleozoic Active Margin in Kazakhstan), Mos cow: Nauka, 1999 [in Russian]. Granitsa ordovika i silura v Kazakhstane (The Ordovician— Silurian Boundary in Kazakhstan), AlmaAta: Nauka, 1980 [in Russian]. Jones, O., Plectambonites and some allied genera, Geol. Surv. Great Brit. Mem., 1928, vol. 1, pp. 367–527. Koren’, T., Popov, L.E. Degtjarev, K.E., et al., Kazakhstan in the Silurian, in Silurian Lands and Seas. Paleogeography Out side of Laurentia, New York State Mus. Bull., 2003, vol. 493, pp. 323–343.

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Translated by S.V. Nikolaeva


Vol. 23

No. 3

2015