The Carboniferous carbonates of the Dombar Hills (western Kazakhstan) and the problem of the Viséan–Serpukhovian boundary

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Palaeoworld 18 (2009) 80–93

The Carboniferous carbonates of the Dombar Hills (western Kazakhstan)and the problem of the Viséan–Serpukhovian boundary

Svetlana V. Nikolaeva a,∗, Lemuza Z. Akhmetshina b, Vera A. Konovalova a,Vladimir F. Korobkov b, Gulnara F. Zainakaeva c

a Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya 123, Moscow 117997, Russiab AktyubNIGRI, ul. Mirzoyana 17, Aktobe, Kazakhstan

c Institute of Geology, Ufa Research Center, Russian Academy of Sciences, ul. Karla Marksa 16/2, Ufa 450000, Russia

Received 14 August 2008; received in revised form 2 January 2009; accepted 16 April 2009Available online 24 April 2009

Abstract

The Upper Viséan–Serpukhovian ammonoid-rich carbonates in the Dombar Hills (Aktobe Region, western Kazakhstan) provide an excellentopportunity to calibrate the ammonoid and conodont zonations around the base of the Serpukhovian Stage, and are important for interregionalcorrelation. A section in the Dombar Hills spanning the Viséan–Serpukhovian boundary is measured and its fossil content is analyzed. Twoammonoid genozones (Hypergoniatites-Ferganoceras and Uralopronorites-Cravenoceras) and two conodont zones (Lochriea nodosa and Lochrieaziegleri) are recognized. The section displays a contact between the Hypergoniatites-Ferganoceras and Uralopronorites-Cravenoceras ammonoidgenozones in a continuous succession and an evolutionarily early appearance of the genus Cravenoceras. The base of the Serpukhovian Stageis drawn at the level of the first appearance datum (FAD) of the conodont Lochriea ziegleri, which, as in the Verkhnyaya Kardailovka section(potential GSSP candidate, South Urals, Russia) enters within the Hypergoniatites-Ferganoceras ammonoid Genozone.© 2009 Nanjing Institute of Geology and Palaeontology, CAS. Published by Elsevier Ltd. All rights reserved.

Keywords: Carboniferous; Stratigraphy; Viséan; Serpukhovian; Dombar; Kazakhstan

1. Introduction

The Mississippian successions in the Dombar Hills inwestern Kazakhstan (southern fringes of the South Urals)(Fig. 1) are renowned for their rich ammonoid occurrences. Thesections in this area have been known for decades (Khvorova,1961; Ruzhencev and Bogoslovskaya, 1971) but their geology,lithology and biostratigraphy are still insufficiently studied.However, their importance has recently increased because of theongoing search for the Viséan–Serpukhovian boundary marker,one of the high priority tasks announced by the Subcommissionon Carboniferous stratigraphy (Richards, 2007, p. 99). Richards,speaking for the ICS Task Group to establish a GSSP for theViséan–Serpukhovian boundary, stated that “the first evolution-ary appearance of the conodont Lochriea ziegleri in the lineage

∗ Corresponding author.E-mail addresses: [email protected] (S.V. Nikolaeva), [email protected]

(L.Z. Akhmetshina), [email protected] (V.A. Konovalova),[email protected] (G.F. Zainakaeva).

Lochriea nodosa to Lochriea ziegleri presented the best potentialmarker for defining the boundary”. It has been identified in sev-eral successions including the type Serpukhovian area (MoscowBasin) (Table 1), Western and Eastern Europe, the Urals, Kaza-khstan, and China (Nemirovskaya et al., 1994; Skompski et al.,1995; Nikolaeva et al., 2001, 2005; Wang and Qi, 2003; Qi andWang, 2005; Nemyrovska with an appendix by Samankassou,2006). A major shortcoming of this choice of boundary markeris that it has not been found in North America. A reliablecorrelation of the boundary level into North America andGondwana may be achieved by the comparison of the detailedconodont zonation with zonations based on other fossil groups.After several years of intense research in the type Serpukhovianarea in the Moscow Basin, it has become clear that the boundarycould not be reliably identified in the type Serpukhovian sectionin the Zaborie Quarry near Moscow. A section in the DombarHills (western Kazakhstan) has been measured and sampled toconfirm the correlation between the conodont and ammonoidzonations around the Viséan–Serpukhovian boundary. TheDombar Hills area is very important for Carboniferous stratig-raphy in the countries of the former Soviet Union, because it

1871-174X/$ – see front matter © 2009 Nanjing Institute of Geology and Palaeontology, CAS. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.palwor.2009.04.004

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Fig. 1. (A and B) Location of the Viséan–Serpukhovian sections on the geological map of the South Urals. (1) Pre-Paleozoic and metamorphic rocks; (2) pre-Carboniferous Paleozoic rocks; (3) Carboniferous and younger rocks. The western slope of the Dombar Hills (section Dombar 1) in the photograph (C) is almostentirely built of the Upper Viséan carbonates. View looking SE. The Viséan–Serpukhovian boundary itself is in a small gully on the left-hand side of the hill.

was the place where the Lower Carboniferous (Mississippian)ammonoid genozones were first established (Ruzhencev andBogoslovskaya, 1971). Since then, these genozones havebecome the backbone of Carboniferous stratigraphic schemesin Russia, Kazakhstan, and Uzbekistan. They are included in allofficial stratigraphic schemes and are broadly cited. However,the conodont biostratigraphy of the Viséan and Serpukhovianbeds in the Dombar Hills has not previously been studied, and nocomplete section that would include the Viséan–Serpukhovianboundary has been previously described from this area. Thesection at Verkhnyaya Kardailovka (South Urals) containingthe conodont lineage L. nodosa–L. ziegleri has been proposed

as a possible GSSP candidate for the Viséan–Serpukhovianboundary, and the Dombar section, which is not very farfrom Verkhnyaya Kardailovka (ca. 200 km to the NE, Fig. 1),provides important information on the ranges of ammonoidsand conodonts near the boundary (Kulagina et al., 2006).

2. Geographical and palaeogeographical details

Several sections spanning the Viséan–Serpukhovian bound-ary ammonoid genozones (Hypergoniatites-Ferganoceras andUralopronorites-Cravenoceras) were studied in the DombarHills, the type region for these genozones. The Dombar area

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Table 1Regional stratigraphic subdivisions of the Upper Viséan and Ser-pukhovian in the Russian Platform and the South Urals. Theshaded area shows the geochronological range of the Dombar 2section.

is a series of carbonate hills extended as a band 4 km longand 500 m wide, in the Aktyubinsk region of Kazakhstan tothe north of the city of Aktobe. Geologically, the sections aresituated on the southern fringes of the South Urals within theband of crinoidal carbonates of Upper Viséan–Serpukhovian agewith numerous ammonoids, conodonts, and ostracodes. Occa-sionally, there are accumulations of gastropods, bivalves andtrilobites, but these are clearly subdominant. The rocks containvery few brachiopods, foraminifers, or bryozoans.

Tectonically, this area belongs to the Kiya-Shandy regionof the Prizilair Subzone (Central Uralian Tectonic Zone) andborders other tectonic zones to the east and west. V.N. Puchkov(pers. comm.) interpreted this zone as a continuation of the ZilairZone, and considered the Viséan deposits here to be similar tothe dark-grey carbonates of the Bukharcha Formation of theZilair Zone. The Upper Viséan and Serpukhovian carbonatesin this region have been studied in detail by Khvorova (1961).Ruzhencev and Bogoslovskaya (1971) referred to these carbon-ates as the “Dombar Limestone”, a name that has since becomegenerally attributed to the exposures of carbonates extended in anorth-south direction along the rivers Kiya, Dombar, Alabaital,Zhaksy-Kargaly, Shandy, and Sholak-Sai. The carbonates in thisarea everywhere overlie the Viséan Chanchar Shale (Fig. 2) andare strongly dislocated. Ruzhencev and Bogoslovskaya (1971,pp. 54–55, text-figs. 2–3) showed that the area of the hillsrepresents a complex combination of anticline–syncline struc-tures with multiple faults. According to Khvorova (1961) theDombar area was strongly dislocated at the end of the Serpukho-vian, and then over a long period of time remained above sea

Fig. 2. Correlation of the ammonoid genozones and local stratigraphic units inthe Kiya-Shandy Zone. The Dombar Limestone corresponds to the range fromthe upper part of the Beyrichoceras-Goniatites Genozone to the Fayettevillea-Delepinoceras Genozone.

level, until the Sakmarian (Permian). In the section studied, theViséan–Serpukhovian carbonates are not covered by youngerrocks, but elsewhere they are unconformably overlain by theSakmarian sandstones, which are mostly found in the west ofthe area, and only occasionally on the Dombar Hills.

The palaeogeographical situation is similar to that in theSouth Urals, but apparently there was a slightly greater dif-ferentiation between areas of local uplifts with smaller basinssemi-isolated by zones of deeper oceans and currents. The Dom-bar Sea was a relatively deep (probably below the photic zone)tropical or subtropical basin on the margin of a local submergeduplift, heavily populated by crinoids and cephalopods and semi-isolated from the rest of the basin by deeper zones and strongcurrents.

3. Geology and lithology

Ruzhencev and Bogoslovskaya (1971) recorded about 30ammonoid occurrences in the eastern and western slopes ofthe Dombar Hills. All occurrences (including the Dombar1 and Dombar 2 sections) are either uppermost Viséan or

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Fig. 3. Eastern slope of the Dombar Hills, with the section Dombar 2. Thesuccession is upturned, with the stratigraphically older rocks exposed higher upon the slope.

lowermost Serpukhovian. We measured the Dombar 2 sec-tion between Sample 8 and Sample 14 of Ruzhencev andBogoslovskaya (1971), as these two occurrences are locatedon the same eastern slope of the Dombar Hills slope, closeto each other (Fig. 3). The crinoidal–cephalopod beds of the

Dombar Hills overlie the Viséan Chanchar Shale. The sectionbegins almost at the top of the hill (5 m down the slope fromthe watershed) in a small trench (Sample 8 of Ruzhencev andBogoslovskaya (1971) = our Sample 108). Another trench (Sam-ple 14 of Ruzhencev and Bogoslovskaya (1971) = our Sample114) is 10–12 m down the slope. The boundary beds are com-posed of an alternation of light-grey, thickly bedded crinoidallimestone with frequent trilobite remains and light-grey thick-bedded limestone with numerous ammonoid shells, conodonts,ostracodes, gastropods, trilobites, bivalves, and rare colonialcorals (Figs. 4 and 5). In places the rock is almost completelybuilt of unsorted and tightly packed ammonoid shells. The com-pressed shells are orientated along the bedding planes. The bedswith ammonoid shells also contain numerous uncoiled nau-tiloids, and stalks and cups of crinoids. Foraminifers are veryrare.

4. Distribution of fossils in the Dombar section

4.1. Conodonts

Conodonts are found throughout the Dombar Hills section.It is the first time that conodonts have been studied from theViséan–Serpukhovian boundary beds in the Dombar Hills. Twoconodont zones are recognized in the Dombar 2 section (the

Fig. 4. Distribution of ammonoids, conodonts and ostracodes in the Dombar 2.

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Fig. 5. Rock lithology in Dombar 2. (A) Mudstone-wackestone, with spheres; (B) bioclastic, detrital wackestone with trilobites; (C) wackestone–floatstone; (D)bioclastic, detrital floatstone, wackestone; (E) detrital–crinoidal wackestone with ammonoids; (F) ammonoid–bioclastic floatstone–wackestone. Scale bar = 0.5 mm.

characteristic species from the Dombar Hills are shown inFig. 6).

4.1.1. Lochriea nodosa ZoneThis zone is recognized in Bed 1. Gnathodus bilineatus bilin-

eatus (Roundy), Gn. girtyi Hass, Lochriea commutata (Bransonet Mehl), L. mononodosa (Rhodes, Austin et Druce), Pseudog-nathodus homopunctatus (Ziegler), Ps. symmutatus (Rhodes,Austin et Druce) continue from the underlying Viséan beds,whereas L. monocostata Pazukhin et Nemirovskaya and L.costata Pazukhin et Nemirovskaya appear at level 108. Thisassemblage is found in the upper part of the Hypergoniatites-Ferganoceras Genozone (Nm1a2 Zone) (Samples 8 and 9 ofRuzhencev and Bogoslovskaya (1971), and in our Samples

108 and 200/1). In the sections of the Peri-Caspian Depres-sion, a similar assemblage is found in the Eostaffella tenebrosaforaminiferal Subzone. A similar zone is recognized in the sec-tion at Verkhnyaya Kardailovka (eastern slope of the Urals),in the interval between Bed 21.5 (upper part) to Bed 21.10(inclusive) (Pazukhin et al., 2002). In the South Urals, this zonecorresponds to upper part of the Venevian Horizon (Kulagina etal., 1992). A similar assemblage is found in the upper part ofthe L. nodosa Zone in the South Tien-Shan (Neevin, 2005). Inall these regions, the L. nodosa Zone defines the topmost part ofthe Viséan. In the Russian Platform, this assemblage is found inthe Venevian (Skompski et al., 1995; Alekseev et al., 2004), andthe uppermost Mikhailovian. In South China, a similar assem-blage was recognized in the basal part of the Nashui section

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Fig. 6. Conodonts from the Dombar Hills. All conodonts are housed in AktyubNIGRI (coll. no. 76). Scale bar = 0.125 mm. (A and B) Lochriea ziegleri Nemirovskaya,Perret et Meischner, 1994; (A) specimen no. 76/418, Sample 24 (29); (B) specimen no. 76/148, Sample 14 (= 114); Serpukhovian, L. ziegleri Zone. (C and D) Gnathodusbilineatus bilineatus (Roundy), 1926; (C) specimen no. 76/419, Sample 15 (= 101); (D) specimen no. 76/420, Sample16 (= 110); Serpukhovian, L. ziegleri Zone.(E and M) Lochriea monocostata Pazukhin et Nemirovskaya, 1992; (E) specimen no. 76/421, Sample 8 (108), Viséan, L. nodosa Zone; (M) specimen no. 76/429,Sample 13 (= 113), Serpukhovian, L. ziegleri Zone. (F) Pseudognathodus homopunctatus (Ziegler, 1960), specimen no. 76/422, Sample 12 (= 112a), Serpukhovian,L. ziegleri Zone. (G) Lochriea senckenbergica Nemirovskaya, Perret et Meischner, 1994, specimen no. 76/423, Sample 13 (= 113), Serpukhovian, L. ziegleri Zone.(H) Lochriea mononodosa (Rhodes, Austin et Druce, 1965), specimen no. 76/124, Sample 24 (= 29), Serpukhovian, L. ziegleri Zone. (I and J) Lochriea nodosa(Bischoff, 1957); (I) specimen no. 76/425, Sample 8 (= 108), Viséan, L. nodosa Zone; (J) specimen no. 76/426, Sample 24 (= 29), Serpukhovian, L. ziegleri Zone.(K) Lochriea commutata (Branson et Mehl, 1941), specimen No. 76/427, Sample 16 (= 110), Serpukhovian, L. ziegleri Zone. (L) Lochriea cruciformis (Clarke,1968), specimen no. 76/428, Sample 13 (= 113), Serpukhovian, L. ziegleri Zone. (N) Lochriea multinodosa (Wirth, 1967), specimen no. 76/430, Sample 15 (= 101),Serpukhovian, L. ziegleri Zone. (O) Gnathodus girtyi girtyi Hass, 1953, specimen No. 76/122, Sample 8 (= 108), Viséan, L. nodosa Zone.

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of Luodian, Guizhou, South China (Qi and Wang, 2005). Theindex species is also recorded at the base of the ammonoid Neo-glyphioceras spirale (III � spi) in the Rhenish Massif (Skompskiet al., 1995).

The correlation of this zone with the Gn.bilineatus–Cavusgnathus altus Zone in the USA is appar-ent. The presence of the forms transitional to Lochriea ziegleriis yet to be confirmed.

4.1.2. Lochriea ziegleri ZoneThe zone is recognized in beds 2–4. The species Gnathodus

girtyi girtyi Hass, G. girtyi collinsoni Rhodes, Austin et Druce,Mestognathus beckmanni Bischoff, M. bipluti Higgins, Lochrieamononodosa (Rhodes, Austin et Druce), L. nodosa (Bischoff), L.costata Pazukhin et Nemirovskaya, Pseudognathodus symmu-tatus (Rhodes, Austin et Druce), Ps. homopunctatus (Ziegler)continue from the underlying L. nodosa Zone, while L.ziegleri Nemirovskaya, Perret et Meischner (frequent), L. cruci-formis (Clarke), L. multinodosa (Wirth), and L. senckenbergicaNemirovskaya, Perret et Meischner (rare) appear for the firsttime. This assemblage is found in the beds with ammonoidsof the Uralopronorites-Cravenoceras Genozone (Nm1b1 Zone)(Samples 12, 13, 14, 15, 16, 23, 24, 28 of Ruzhencev andBogoslovskaya (1971) and in our Samples 200/3, 201, 203,and 114). The species Lochriea ziegleri shows high consis-tency in appearance in many sections in the basal layers of theSerpukhovian and is therefore presently considered as the bestcandidate for the definition of the Viséan–Serpukhovian bound-ary (Richards, 2006, 2007). Its first appearance is recorded inthe basal Namurian and Serpukhovian in the sections of theDonets Basin and Cantabrian Mountains (Nemirovskaya et al.,1994; Nemyrovska with an appendix by Samankassou, 2006). InGermany this species appears within the Emstites schaelkensisammonoid Zone (Skompski et al., 1995, fig. 3).

4.2. Ammonoids

In the section Dombar 2 (total thickness 6.8 m), ammonoidsand conodonts were collected from 11 levels. Ruzhencev andBogoslovskaya (1971) described this section and subdividedit, based on ammonoids, into two genozones (Hypergoniatites-Ferganoceras and Uralopronorites-Cravenoceras) (Fig. 4) andfour species zones to which they proposed the following codes:Nm1a1, Nm1a2, Nm1b1, and Nm1b2). The species zones arerecognized in most of the Uralian sections and in Tien-Shan.The abbreviation “Nm” stands for “Namurian”, which wasused in the global Carboniferous stratigraphy at the time whenRuzhencev and Bogoslovskaya (1971) proposed their zonation.Because the “Namurian” is not at present part of the globalCarboniferous scale (see Heckel and Clayton, 2006), the abbre-viation has lost its original meaning, but it is still in use in thelocal schemes as a means of subdividing the ammonoid geno-zones into smaller units. Ten of the levels that we sampled in theDombar 2 section represent the Hypergoniatites-Ferganocerasammonoid Genozone (Nm1a) (in part, approximately equivalentto the Upper Brigantian and in part, to the Serpukho-vian) (Samples 108–205), and one, the Lower Serpukhovian

Uralopronorites-Cravenoceras Genozone (Nm1b) (approxi-mately equivalent to the Pendleian) (Sample 114 = Sample14 of Ruzhencev and Bogoslovskaya (1971)). Occurrence108 is uppermost Viséan in age and contains ammonoidsof the Hypergoniatites-Ferganoceras Genozone (Sample108 = Sample 8 of Ruzhencev and Bogoslovskaya (1971)).Occurrence 114 is lowermost Serpukhovian (Nm1b1 = lowerpart of the Uralopronorites-Cravenoceras Genozone). Theammonoids of the Hypergoniatites-Ferganoceras Genozonein Samples 108–205 represent the same levels as inthe Hypergoniatites-Ferganoceras Genozone in VerkhnyayaKardailovka, while the ammonoids of the Uralopronorites-Cravenoceras Genozone (Nm1b1 Zone in Sample 114)are clearly older than the earliest ammonoids of theUralopronorites-Cravenoceras Genozone in Verkhnyaya Kar-dailovka (Nm1b2 Zone in Sample 011), as they contain elementscontinuing from the Hypergoniatites-Ferganoceras Zone (e.g.,Lyrogoniatites mutabilis Ruzh. et Bogosl., and Lusitanites con-cavus Ruzh. et Bogosl.), which are absent in VerkhnyayaKardailovka. The Hypergoniatites-Ferganoceras Genozone ispartly Viséan and partly Serpukhovian, as the conodont speciesLochriea ziegleri enters within it, more precisely within thespecies zone Nm1a2.

Most ammonoids in these beds are endemic to theUrals–Kazakhstan and Tien-Shan regions (Figs. 7 and 8).

4.2.1. Hypergoniatites-Ferganoceras GenozoneThe sections in the Dombar Hills are the type sections for this

genozone. The Hypergoniatites-Ferganoceras Genozone con-tains an assemblage dominated by Dombarites, Platygoniatites,Dombarigloria, Alaoceras, and Megapronorites.

In Dombar 2 the beds are in an upturned position. The strati-graphically older beds are exposed on the top of the hill, andthe younger beds occur down the slope. The Hypergoniatites-Ferganoceras Genozone is here defined by the presenceof the genera Neogoniatites, Dombarocanites, Dombarites,Ferganoceras, etc. The lower boundary of the genozone is notdetermined in Dombar 2 (as the older beds are weathered awayand no contact with the underlying rocks is possible to observein this section. However, it will be possible in the future to trenchthe contact elsewhere in the hills). The assemblage includesDombarocanites chancharensis Ruzh., Epicanites aktubensisRuzh., Megapronorites sakmarensis Ruzh., Irinoceras arcua-tum Ruzh., Sudeticeras grande Ruzh. et Bogosl., Girtyoceraslatum Ruzh. et Bogosl., Sulcogirtyoceras artumbilicatum Ruzh.et Bogosl., Trizonoceras medioximum Ruzh. et Bogosl., Paradi-morphoceras orientale Ruzh. et Bogosl., Arcanoceras burmai(Miller et Downs), Neogoniatites milleri Ruzh. et Bogosl., Platy-goniatites omniliratus Ruzh. et Bogosl., Dombarites falcatoidesRuzh. et Bogosl., D. linteroides Ruzh. et Bogosl., Alaocerasbajtalense Ruzh. et Bogosl., Lyrogoniatites mediator Ruzh. etBogosl., Neoglyphioceras gradatum Ruzh. et Bogosl., Lusitan-ites subcircularis (Mill.), Ferganoceras elegans Libr., F. gracileRuzh. et Bogosl., and Ophilyroceras tersum Ruzh. et Bogosl.Most of these species are found throughout the genozone, andmany are also found in the overlying beds. Ruzhencev andBogoslovskaya (1971) recognized two species zones within the

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Fig. 7. Ammonoids from the Dombar 2 Section. Sample 8 of Ruzhencev and Bogoslovskaya (1971) (= Sample 108 in this paper); Hypergoniatites-FerganocerasGenozone (Viséan part). All ammonoids are housed in the Paleontological Institute, Russian Academy of Sciences, coll. no. 455 (collected by Ruzhencev andBogoslovskaya). Scale bar = 1 cm. (A and B) Ophilyroceras tersum Ruzhencev et Bogoslovskaya, 1971, specimen no. 455/33404. (C, D, P, and Q) Dombaritesfalcatoides Ruzhencev et Bogoslovskaya, 1971; (C and D) specimen no., 455/10520; (P and Q) specimen no. 455/10524. (E and F) Prolecanites librovitchi(Ruzhencev, 1949), specimen no. 455/10520. (G) Platygoniatites omniliratus Ruzhencev et Bogoslovskaya, 1971, specimen no. 455/8230. (H and J) Alaocerasbajtalense Ruzhencev et Bogoslovskaya, 1971, specimen no. 455/29758. (K and L) Neoglyphioceras gradatum Ruzhencev et Bogoslovskaya, 1971, holotype,specimen no. 455/34100. (M) Lyrogoniatites mediator Ruzhencev et Bogoslovskaya, 1971, holotype, specimen no. 455/31658. (N and O) Ferganoceras gracileRuzhencev et Bogoslovskaya, 1971, holotype, specimen no. 455/5902. (R and S) Ferganoceras elegans Ruzhencev et Bogoslovskaya, 1971, specimen no. 455/5998.(T and U) Trizonoceras medioximum Ruzhencev et Bogoslovskaya, 1971, specimen no. 455/6810.

Hypergoniatites-Ferganoceras Genozone, different in speciescomposition. The species Ophilyroceras tersum and Alaocerasbajtalense, according to Ruzhencev and Bogoslovskaya (1971),indicate the Nm1a2 Zone, the upper of two zones within theHypergoniatites-Ferganoceras Genozone. The lower Nm1a1Zone is not observed in this section but it is present in manyother sections in the Dombar Hills.

4.2.2. Uralopronorites-Cravenoceras GenozoneThe lower boundary of the genozone is defined by the

first appearance of species of the genus Cravenoceras, par-ticularly C. leionoides. The assemblage includes Prolecaniteslibrovitchi (Ruzh.), Dombarocanites chancharensis Ruzh., D.catillus Ruzh. et Bogosl., Epicanites aktubensis (Ruzh.),Megapronorites sakmarensis Ruzh., Uralopronorites mirus

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Fig. 8. Ammonoids from the Dombar 2 section. Sample 14 of Ruzhencev and Bogoslovskaya (1971) (= Sample 114 in this paper). Hypergoniatites-FerganocerasGenozone (Serpukhovian part). All ammonoids are housed in the Paleontological Institute, Russian Academy of Sciences, coll. no. 455 (collected by Ruzhencev andBogoslovskaya). Scale bar = 1 cm. (A and B) Megapronorites sakmarensis Ruzhencev, 1949, specimen no. 455/2370. (C and D) Dombarites liratus Ruzhencev etBogoslovskaya, 1971, specimen no. 455/21699. (E, F, J, and K). Cravenoceras shimanskyi Ruzhencev et Bogoslovskaya, 1971; (E and F) specimen no. 455/30568; (Jand K) holotype, specimen no. 455/30567. (G and H) Cravenoceras leionoides Ruzhencev et Bogoslovskaya, 1971, specimen no. 455/30607. (L and M) Dombaritescarinatus Ruzhencev et Bogoslovskaya, 1971, specimen no. 455/11624. (N and O) Cravenoceras crassum Ruzhencev et Bogoslovskaya, 1971, specimen no.455/30522.

Libr., Irinoceras arcuatum Ruzh., Kazakhoceras hawkinsi(Moore), Neogoniatites milleri Ruzh. et Bogosl., Platygoni-atites molaris Ruzh., Dombarites carinatus Ruzh. et Bogosl.,D. liratus Ruzh. et Bogosl., Alaoceras bajtalense Ruzh.et Bogosl., Cravenoceras shimanskyi Ruzh. et Bogosl., C.leionoides Ruzh. et Bogosl., C. crassum Ruzh. et Bogosl., Tym-panoceras trisulcatum Ruzh., Lyrogoniatites mutabilis Ruzh.et Bogosl., and Lusitanites concavus Ruzh. et Bogosl. Amongthese species, Lyrogoniatites mutabilis and Neogoniatites milleri

continue from the underlying Hypergoniatites-FerganocerasGenozone along with the newly appeared Cravenoceras shi-manskyi and Lusitanites concavus. This transitional assemblagedefines the Nm1b1 Zone, the lower of the two zones withinthe Uralopronorites-Cravenoceras Genozone (according toRuzhencev and Bogoslovskaya, 1971). A similar assemblage isfound in Kyzyl-Shin, on the Kiya, Alimbet, and Zhaksy-Kargalyrivers (all these localities are nearby in the Aktyubinsk Region ofKazakhstan and the Orenburg Region of Russia) (Ruzhencev and

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Fig. 9. Ostracodes from the Dombar 2 section. (A) Bolbozoella nodosa Robinson, 1959, right valve view; (B) the same as A, dorsal view; (C) Bolbozoella sp. 1,right valve view; (D) the same as D, dorsal view; (E) Rectoplacera sp. 1, left valve view. Scale bar = 0.2 mm.

Bogoslovskaya, 1971). The assemblage is older than the earli-est Serpukhovian ammonoid fauna in Verkhnyaya Kardailovka,because the latter lacks any elements of the Viséan fauna andhence may be determined as belonging to the Nm1b2 Zone(Nikolaeva et al., 2005).

4.3. Ostracodes

The Viséan–Serpukhovian boundary beds contain ostracodesthat become abundant in the Serpukhovian: Bairdiocypris?ambigens, Bolbozoella nodosa Robinson, 1959, Polycope per-minuta (Kellett), Rectoplacera sp., etc. A similar assemblageis found in the Urals (Verkhnyaya Kardailovka, Kiya, andSholak-Sai) (Kulagina et al., 1992; Pazukhin et al., 2002)(Fig. 9).

5. Discussion and comparison with other successions

5.1. Definition of the Viséan–Serpukhovian boundary

In the Dombar section, the Viséan–Serpukhovian bound-ary defined by the first appearance of Lochriea ziegleri isdrawn at the level of Sample 200/3, in the upper part of theHypergoniatites-Ferganoceras Genozone, below the bed withthe assemblage of the Uralopronorites-Cravenoceras Geno-zone. A similar situation is observed in the Rhenish Massif,where Lochriea ziegleri first appears in the Emstites schaelkensisZone (Skompski et al., 1995), and in England, where Lochrieaziegleri first appears in the Upper Brigantian (Skompski etal., 1995). In the type Serpukhovian section Zaborie, the con-odonts L. ziegleri and L. cruciformis are found near the baseof the Serpukhovian (Tarusian Horizon) (Barskov et al., 1984;Skompski et al., 1995; Nikolaeva et al., 2002), whereas inthe Lanshino Section in the Moscow Basin Lochriea zieg-leri appears in the upper part of the Venevian (Skompskiet al., 1995). In the Donets Basin the first occurrence ofLochriea ziegleri is recorded much higher in the sectioncompared to other regions—in the Protvian through to theZapaltyubinian and at the base of the Voznesenskian (lime-stones D5

1–D105 ) (Skompski et al., 1995; Nemyrovska, 1999).

The reasons for such discrepancies are not entirely clear, butmay result from facial constraints, or more probably from dif-

ferent approaches to species identification. However, this speciesis currently the best candidate for the Viséan–Serpukhovianboundary definition. The chronostratigraphy of other con-odont species that appear near the base of the Serpukhovian(L. cruciformis and L. multinodosa), which could potentiallybe proposed as boundary markers, is much more debatable.For instance, in the Dombar Hills L. cruciformis appearsslightly above L. ziegleri, while Skompski et al. (1995)recorded that in Germany L. cruciformis appears below L. zieg-leri.

In the Kardailovka section, the Viséan–Serpukhovian bound-ary is defined by the first appearance of the conodont Lochrieaziegleri in the lineage L. nodosa–L. ziegleri in Bed 22/1 whereLochriea ziegleri appears along with the appearance of L. cruci-formis. Lochriea ziegleri Nemyrovska, Perret et Meischner isalso a convenient global marker for the identification of theViséan–Serpukhovian boundary in Eurasia (Skompski et al.,1995; Nemyrovska, 1999; Nemyrovska with an appendix bySamankassou, 2006), because it appears near the levels of thefirst appearance of the ammonoid species Cravenoceras leionand Edmooroceras pseudocoronula, which are commonly usedfor marking the Viséan–Namurian boundary in Western Europe(Bisat, 1950; Horn, 1960; Korn and Horn, 1997a,b; Nikolaevaand Kullmann, 2001, 2003; Korn and Tilsley, 2002). However,the usefulness of Cravenoceras leion is mainly restricted tothe British Isles, whereas in other regions of Western Europe(for instance, in Germany) the occurrences of this species arequestionable (see Korn, 1988, 1996). In the Rhenish Massif(Germany) the transitional Viséan–Serpukhovian beds containa range of girtyoceratids that have been used in biostratigra-phy (Horn, 1960; Korn, 1988, 1997; Korn and Horn, 1997a,b),and Horn (1960) suggested using Tumulites pseudobilinguis(an ammonoid species appearing above Cravenoceras leion) asa boundary marker. However, Ruzhencev and Bogoslovskaya(1971) argued that the use of this species to mark the endof the Viséan would cause complications elsewhere outsidethe Rhenish Massif because this species appears above thefirst entry of Cravenoceras leion, considerably above the baseof the Pendleian in Europe. Considering these difficultiesRuzhencev and Bogoslovskaya (1971) proposed placing theboundary at the base of their Hypergoniatites-FerganocerasGenozone, the boundary readily recognized in the Urals and

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in Tien-Shan. However, this level is practically unrecognizablein most sections in Western Europe because of the absenceof the reliable occurrences of the genus Dombarites, the keyelement of the Hypergoniatites-Ferganoceras ammonoid fauna(Nikolaeva and Kullmann, 2001). The lowermost Serpukhovianbeds in Kardailovka do not contain ammonoids, but 0.7–0.8 mabove the boundary (Sample 012/3) the first species of theammonoid genus Dombarites appear suggesting the presence ofthe Uralopronorites-Cravenoceras Genozone (Nm1b). The firstand the most typical species in this genozone is Dombarites tec-tus Librovitch, which enters in Bed 22a (Sample 011) (Nikolaevaet al., 2001; Pazukhin et al., 2002). This is by no means the evo-lutionary earliest appearance of Dombarites, since the earliestDombarites species to appear in Kardailovka (Dombarites tec-tus Librovitch) lacks falcatoid ornamentation, which indicatesthe upper part (Nm1b2) of the Uralopronorites-CravenocerasGenozone. In the Dombar 2 section, we observed a succession ofDombarites species from Dombarites falcatoides (with falcatoidornamentation) in the Hypergoniatites-Ferganoceras Genozone(layers that Ruzhencev and Bogoslovskaya, 1971 referred toas Nm1a2) to Dombarites carinatus and D. liratus (withoutfalcatoid ornamentation) in the Uralopronorites-CravenocerasGenozone (layers that Ruzhencev and Bogoslovskaya, 1971referred to as Nm1b1).

5.2. Correlation of the conodont and ammonoid zones

The Lochriea nodosa conodont Zone correlates with theupper part (Nm1a2) of the Hypergoniatites-Ferganoceras Geno-zone (in the Dombar Hills levels 108, 200/1, 200/2). In thePeri-Caspian Basin, the Lochriea nodosa Zone correlates withthe Eostaffella tenebrosa foraminiferal Subzone, and in theVerkhnyaya Kardailovka section it is recognized from Bed 21/5(upper part) to Bed 21/10 (Pazukhin et al., 2002), in the upperpart of the Venevian. This zone is recorded in the middle partof the Mashat Horizon of Tien-Shan (Neevin, 2005), and inthe Upper Viséan of the Urals (Pazukhin et al., 2002). In theMoscow Basin, a similar assemblage is recorded in the upperpart of the Mikhailovian and in the Venevian (Alekseev et al.,2004).

The Lochriea ziegleri Zone in the Dombar Hills comprises theupper part of the Hypergoniatites-Ferganoceras Genozone andthe Uralopronorites-Cravenoceras Genozone (samples 200/3,201, 203, 114). Lochriea ziegleri is widespread in the Serpukho-vian of the Per-Caspian Basin beginning from the Tarusian. InKyrgyzstan, the Lochriea ziegleri Zone is recorded in the LowerSerpukhovian, and in Uzbekistan, it correlates with the L. cru-ciformis Zone (Neevin, 2005). In the Urals, it is recorded fromthe Tarusian and Steshevian.

Thus, as the Viséan–Serpukhovian boundary is defined bythe appearance of L. ziegleri, the Hypergoniatites-FerganocerasGenozone is partly placed in the Serpukhovian, although it waspreviously completely placed in the Upper Viséan (Nikolaeva etal., 2001). The lower part of the Hypergoniatites-FerganocerasGenozone section (below the first appearance of Lochriea zieg-leri) approximately correlates with the upper Brigantian inWestern Europe and Morocco (for records see Korn et al., 1999;

Korn and Feist, 2007) and provisionally with the Sulcogirty-oceras ornatissimum Zone of Texas (Titus, 1999).

5.3. Correlation with the Zaborie section (Moscow Basin)and Kardailovka section (South Urals)

5.3.1. ZaborieThe Dombar section correlates with the Venevian and part

of the Tarusian horizons of Zaborie. The Serpukhovian succes-sion in Zaborie is composed of shallow water carbonates andshale and is incomplete in its upper part (the uppermost bedsof the Serpukhovian Stage – the Pestovo Series – are missing).In addition, in the Zaborie section, the Viséan–Serpukhovian(V–S) boundary interval normally lies below the water level ofa lake at the bottom of this disused quarry, and is only vis-ible in dry years. Nikolaeva et al. (2001), Gibshman (2001,2003), and Kabanov (2003, 2004) were able to observe andexamine the boundary beds, which since then have not beenexposed. The V–S boundary level was observed at the base of theTarusian of the Zaborie section, where the conodont L. ziegleriwas found near the occurrence of the foraminifer “Millerella”tortula along with the foraminifer Neoarchaediscus postrugo-sus (Gibshman, 2001, 2003). No ammonoids were found nearthe Viséan–Serpukhovian boundary, although a few shells ofCravenoceras shimanskyi were recorded from the upper Taru-sian and Lower Steshevian (A.V. Shkolin, pers. comm.).

Thus, the correlation between the Dombar section andZaborie has to be based on conodonts, because the Dombar sec-tion contains very few foraminifers, while only a few ammonoidsare found in Zaborie. The conodont-based boundary level canbe defined in both cases by the first appearance of the conodontLochriea ziegleri.

5.3.2. Kardailovka section (South Urals)In the South Urals and western Kazakhstan, the Viséan–

Serpukhovian successions are mostly represented by carbonatesyielding many fossils, including conodonts, ammonoids, andforaminifers. A section opposite the village of Verkhnyaya Kar-dailovka in the South Urals has been discussed in the literatureas a potential GSSP candidate for the base of the Serpukho-vian (Kulagina et al., 1992; Nikolaeva et al., 2001, 2002, 2005;Pazukhin et al., 2002; Richards, 2007). This section is locatedon the right bank of the Ural River, 2 km east-north-east of thevillage of Prigorodnyi, in the southeast of the Baimak Regionof the Republic of Bashkortostan (Russian Federation). A com-prehensive study of this section resulted in a series of papersdepicting the geography, lithology, foraminiferal, conodont andammonoid zones. Uninterrupted outcrops of Lower Viséan car-bonates are observed from east-northeast to west-southwest,3 km along the river from the Lower Viséan to Moscovian.The mainly bioclastic Lower Viséan limestone is underlainby the Upper Tournaisian–Lower Viséan calcareous limestone,calcareous tuff and tuffaceous limestone of the BerezovskayaFormation. The Lower Viséan limestone formation is more than300 m thick (Eoparastaffella simplex and Uralodiscus rotundusforaminiferal zones). These carbonates are overlain by crinoidallimestone with the foraminifer Paraarchaediscus koktjubensis

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(equivalent of the Upper Viséan lower Tula Horizon), whichis followed by 6 m of previously covered interval and 37 m ofcondensed Upper Viséan–Serpukhovian succession (Pazukhinet al., 2002, in press). The previously covered interval is exposedby a series of overlapping trenches displaying an uninterruptedsuccession of two ammonoid genozones (Beyrichoceras-Goniatites and Hypergoniatites-Ferganoceras). The upperpart is exposed and represents two Serpukhovian ammonoidgenozones (Uralopronorites-Cravenoceras and Fayettevillea-Delepinoceras). The first appearance of the conodont L. ziegleriis observed within the uppermost Viséan–lower SerpukhovianHypergoniatites-Ferganoceras ammonoid Genozone, 0.35 mbelow the base of the Uralopronorites-Cravenoceras Genozone.This level is shown to be near the base of the Eolasiodis-cus donbassicus foraminiferal Zone (Pazukhin et al., 2002).The uppermost Viséan beds (Hypergoniatites-FerganocerasGenozone) contain very few ammonoids (Neogoniatites mil-leri Ruzhencev et Bogoslovskaya, Prolecanites librovitchiRuzhencev, ?Lyrogoniatites sp.). These ammonoids allow thereliable recognition of the Hypergoniatites-Ferganoceras Geno-zone, but are insufficient to determine whether they belong tothe upper or the lower parts of the zone.

The first appearance of the Serpukhovian (Uralopronorites-Cravenoceras) ammonoids in Sample 011 in VerkhnyayaKardailovka shows the presence of the Nm1b2 Zone, whereasthe lowermost Serpukhovian Nm1b1 Zone could not be recog-nized. The interval between Sample 015 in Trench 4 and 011 inthe outcrop contained no ammonoids.

The conodont succession in the Dombar Hills is more or lesssimilar to those found in the Urals, particularly in VerkhnyayaKardailovka (Pazukhin et al., 2002). The conodont lineageLochriea nodosa–Lochriea ziegleri is identified in the sec-tion. Lochriea ziegleri appears earlier (Sample 200/3) thanthe first Cravenoceras and Tympanoceras (Sample 114). Thefirst entry of Lochriea ziegleri is recorded at the level ofSample 200/3 that is certainly within the upper part of theHypergoniatites-Ferganoceras Genozone, which is consistentwith Verkhnyaya Kardailovka. However, because several bedsabove this entry contain ammonoids of the Hypergoniatites-Ferganoceras Genozone, it becomes clear that part of thisgenozone lies in the Serpukhovian. Similarly, in VerkhnyayaKardailovka, the first specimens of L. ziegleri are recordedbelow the first Cravenoceras, even though the first appear-ance of Cravenoceras in the Dombar 2 section (in associationwith Dombarites carinatus, Dombarites liratus, and Lusitan-ites concavus) is evolutionarily earlier than that in VerkhnyayaKardailovka.

Compared to the section in the Dombar Hills, the sec-tion of Verkhnyaya Kardailovka has a better potential forthe V–S GSSP, because of the exceedingly complex tecton-ics in the area where the Dombar section is located, andthe lack of clarity concerning the relationship of the Dom-bar Limestone with other formations. However, the Dombar 2section can be used as an important auxiliary section, espe-cially to clarify the correlation between the ammonoid andconodont distributions in the Hypergoniatites-FerganocerasGenozone.

6. Conclusions

1. In the Dombar 2 section, the zonal subdivision of the upper-most Viséan and lowermost Serpukhovian corresponds tothose in the Moscow Basin and in the Urals, and in all theseareas the base of the Serpukhovian is defined by the firstappearance of the conodont Lochriea ziegleri.

2. The Viséan–Serpukhovian boundary in the Dombar 2 sectiondefined by the entry of the conodonts Lochriea ziegleri liesbelow the first appearance of ammonoids of the SerpukhovianUralopronorites-Cravenoceras ammonoid Genozone. Thislevel is close to those at which Lochriea ziegleri enters inVerkhnyaya Kardailovka, and in other sections in the SouthUrals, Moscow Basin, Germany, and England.

3. The ammonoid Hypergoniatites-Ferganoceras Genozone ispartly Upper Viséan and partly Serpukhovian, if the boundaryis defined by the entry of Lochriea ziegleri. It is establishedthat Lochriea ziegleri first appears within the upper part ofthe Hypergoniatites-Ferganoceras Genozone.

4. Conodont geochronology suggests that the Hypergoniatites-Ferganoceras Genozone in the Urals partly correlates withthe upper part of the P2c Zone in Western Europe.

Acknowledgements

Fieldwork was organized by AktyubNIGRI (Kazakhstan).We thank B.K. Baimagambetov for help and support. The partic-ipation of S.V. Nikolaeva and V.A. Konovalova was supportedby the program of the Presidium of the Russian Academy ofSciences No. 18. G.F. Zainakaeva was supported by the RussianFoundation for Basic Research (project 05-65022). The paper isbenefited from the thoughtful reviews by Markus Aretz (Köln),Barry Richards (Calgary), and Ian Somerville (Dublin).

Appendix A. Description of the Dombar 2 section (seeFig. 4)

Member 1(a) Light-grey, thickly bedded and unbedded, nodu-

lar mudstone–wackestone, with numerous unsortedammonoid shells, crinoid debris and complete smallcrinoidal cups, and many trilobites. Occasionally the rockbecomes an ammonoid shellstone. Ammonoid shells aremostly complete, sometimes in fragments up to 10 cm indiameter. The average size of the shells is 2.5–3 cm, withmany juvenile shells present. The rock contains manyshells of uncoiled nautiloids. Sample 108.

(b) Light-grey, fractured, thickly bedded brachiopod–crinoidal floatstone with ostracode debris limestone withnumerous ammonoids. Sample 200.

(c) Grey, more compact, darker wackestone. Sample 200/1.(d) Grey, crinoidal, loosely cemented bioclastic wackestone

with pelmatozoan and trilobite debris and small ammonoidshells and ostracodes. Small spherical structures are repre-sented by single-chambered foraminiferans Bisphaera sp.,and Archaesphaera sp. There are also spherical structurescompletely filled with sparite, possibly re-crystallized radi-

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olarians. In places the rock is composed almost completelyof large, irregularly orientated fragments of crinoids. Sam-ples 200/2, 200/3.

(e) Grey floatstone with remains of tabulate corals, with frag-ments of ammonoid shells, echinoderms, and spheres. Inplaces blocky sparite was observed. Only a few ammonoidshells and rare uncoiled nautiloids were observed. Sample200/4. Thickness 0.8 m.

Member 2(a) Pinkish-grey, compact, crinoidal–ammonoid floatstone–

wackestone with encrustations of sparite. Crevices andcavities in place of leached ammonoid shells are filled inby blocky sparite. The basal layers of the member (10 cm)contain many crinoid remains; upward in the section accu-mulations of weakly ferruginous ammonoid shells wereobserved. Fossils—crinoids, ostracodes, and tubular algae.Sample 201.

(b) Grey, compact wackestone, with ammonoids, rare bra-chiopods and trilobites. Sample 202. Thickness 0.8 m.

Member 3(a) Grey wackestone with large ammonoid shells. Sample 203.(b) Crinoidal wackestone. The spheres are re-crystallized

with no distinct boundaries. In places, the limestone ismicritic; with a terrigenous admixture of quartz and occa-sional fragments of siliceous rocks. Clastic grains are0.15–0.25 mm in diameter (fine-grained sandstone), semi-angular, semirounded, subisometric and constitute about6–8% of the rock. Sample 204. Thickness 4.2 m.

Member 4(a) Grey micritic limestone (wackestone) with ammonoid

shells. Sample 205.(b) Grey micritic, compact limestone (wackestone) with

ammonoid shells. Sample 114. Thickness 0.8 m.

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