1. Introduction
The Muradymovo section is the best known of the several measured sections spanning the Mid-Carboniferous Boundary (MCB) in the basin of the Bolshoi Ik River, Zilair Megasynclinorium (ZM), western slope of the South Urals, Bashkortostan (Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, Reference Kulagina, Pazukhin, Nikolaeva and Kochetova2000, Reference Kulagina, Pazukhin, Kochetova, Nikolaeva and Chuvashov2002). Here we characterize the zonal biostratigraphy in this section, which is important both for regional stratigraphy and for global correlation because of the paucity of sections containing all three biostratigraphically significant groups (conodonts, foraminifers and ammonoids) in a single succession across the MCB. The presence of the above three fossil groups in the Muradymovo section allows the correlation between the first appearance datum (FAD) of D. noduliferus, the first appearance of the ammonoid family Homoceratidae and the first Bashkirian foraminifers.
The section was first discovered in 1984 in the course of regional geological research work, and has been visited many times by geologists since then. However, its true value only became apparent when the search for the MCB stratotype revealed a number of sections with similar conodont faunas in the critical interval. This deep-water section was first described bed-by-bed by Kulagina & Pazukhin (Reference Kulagina and Pazukhin1986), who recognized the presence of the MCB beds and sampled the section for conodonts and foraminifers. They recognized the equivalents of the basal (Bogdanovkian) beds of the regional Syuranian Substage, representing the lowermost Bashkirian (MCB beds). Over several consecutive years they resampled the critical MCB portion of the section to increase the stratigraphic resolution and identify zonal boundaries. The base of the Bashkirian was originally placed at the base of the foraminiferal Plectostaffella bogdanovkensis Zone and the conodont Declinognathodus noduliferus-D. lateralis Zone in Unit 11 (Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992). After the acceptance of the MCB at the base of the D. noduliferus s.l. Zone, the base of the Bashkirian was placed at the base of Unit 9 (Kulagina et al. Reference Kulagina, Pazukhin, Kochetova, Nikolaeva and Chuvashov2002).
The Muradymovo section was not proposed as a Russian candidate section for the Mid-Carboniferous Boundary Global Boundary Stratotype Section and Point (GSSP) for a number of reasons, mainly because of the suspected redeposition of conodonts at some levels and because of the presumed absence of ammonoid occurrences. After ammonoids were discovered in this section in 1997, it was re-excavated and re-sampled during several field seasons. This enabled its comparison with the previously known candidate sections for the MCB GSSP, including the chosen GSSP section in the Arrow Canyon.
Previous search for the MCB stratotype resulted in the choice of a section in Arrow Canyon, Nevada, USA where the Mississippian–Pennsylvanian boundary is marked by the first appearance of the conodont Declinognathodus noduliferus (Ellison & Graves, Reference Ellison and Graves1941) in Unit G (sample 61b) in the Bird Spring Formation (Lane et al. Reference Lane, Baesemann, Brenckle and West1985, Reference Lane, Brenckle, Baesemann and Richards1999; Lane & Manger, Reference Lane and Manger1985; Brenckle et al. Reference Brenckle, Baesemann, Lane, West, Webster, Langenheim, Brand and Richards1997; Richards et al. Reference Richards, Lane, Brenckle, Hills, Henderson and Bamber2002). This boundary closely coincides in time with the first appearance of the ammonoid family Homoceratidae (Bisat, Reference Bisat1924, Reference Bisat1928; Ruzhencev & Bogoslovskaya, Reference Ruzhencev and Bogoslovskaya1978; Manger & Saunders, Reference Manger, Saunders, Ramsbottom, Saunders and Owens1982; Pareyn et al. Reference Pareyn, Saunders, Manger, Lemosquet, Sutherland and Manger1984; Manger et al. Reference Manger, Weyant and Pareyn1985; Ramsbottom & Saunders, Reference Ramsbottom and Saunders1985; Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992; Nikolaeva, Reference Nikolaeva1994, Reference Nikolaeva1995a ,Reference Nikolaeva b ; Riley, Reference Riley1998; Titus et al. Reference Titus, Webster, Manger and Dewey1997; Titus & Manger, Reference Titus and Manger2001). Other sections containing the boundary level and proposed as possible candidates included successions in Britain (Riley, Reference Riley1987; Riley et al. Reference Riley, Varker, Owens, Higgins and Ramsbottom1987), China (Li et al. Reference Li, Shen, Wu, Tong and Wang1987), the Donets Basin, Ukraine (Aizenverg et al. Reference Aizenverg, Astakhova, Berchenko, Brazhnikova, Vdovenko, Dunaeva, Zernetzkaja, Poletaev and Sergeeva1983; Skipp et al. Reference Skipp, Brenckle, Poletaev, Nemirovskaya, Lane and Manger1989) and Uzbekistan (Nigmadganov & Nemirovskaya, Reference Nigmadganov and Nemirovskaya1992; Nikolaeva & Nigmadganov, Reference Nikolaeva and Nigmadganov1992; Nikolaeva, Reference Nikolaeva1994, Reference Nikolaeva1995a ,Reference Nikolaeva b ). Each of the above sections, including the approved GSSP section, had some shortcomings, and the vote was restricted to only three sections (Arrow Canyon in Nevada, Stonehead Beck in England and Aksu in Uzbekistan). The GSSP section in Nevada consists of shallow-water carbonates, with many discontinuities immediately below and above the boundary level and has no ammonoids, although the MCB ammonoids have been found in the Scotty Wash Formation in the Test Site locality in Nevada at a considerable distance from Arrow Canyon (Titus et al. Reference Titus, Webster, Manger and Dewey1997; Titus & Manger, Reference Titus and Manger2001). Because of the many disconformities and the absence of ammonoids in the GSSP section, the correlation between the FAD of Declinognathodus noduliferus and the first Homoceratidae has remained unclear (Kullmann & Nikolaeva, Reference Kullmann, Nikolaeva, Hills, Henderson and Bamber2002). In some areas the first homoceratids (Isohomoceras) are recorded below the entry of D. noduliferus (Stonehead Beck and Aksu; Riley et al. Reference Riley, Varker, Owens, Higgins and Ramsbottom1987; Nigmadganov & Nemirovskaya, Reference Nigmadganov and Nemirovskaya1992; Nikolaeva & Nigmadganov, Reference Nikolaeva and Nigmadganov1992; Nemirovskaya & Nigmadganov, Reference Nemirovskaya and Nigmadganov1993, Reference Nemirovskaya and Nigmadganov1994), in other places D. noduliferus precedes Isohomoceras (Test Site).
The Muradymovo section is one of the best MCB sections in the southern Ural Mountains and probably worldwide in terms of the continuity of the strata (lack of unconformities) and the number of biostratigraphically significant fossil groups present. This deep-water succession is very useful for testing the MCB definition (the first evolutionary appearance of the conodont Declinognathodus noduliferus s.l.) and evaluating the correlative potential of associated fossil groups.
2. Geological background
According to the modern tectonic interpretation of the geological structures in the Urals (Kondiajn et al. Reference Kondiajn, Belyaev, Melgunov and Rumayntseva2011), the section is located in the west of the Zilair Megasynclinorium (ZM), which belongs to the southern West Uralian Megazone. The predominant deposition in the Carboniferous in this area was the accumulation of limestones and siliceous-carbonate members. The deposition was influenced by the active growth of the Ural Mountains, which provided an influx of siliciclastics. The limestone turbidites and debris-flow deposits were formed on the slope of a carbonate ramp during phases of intense tectonism followed by increased hinterland erosion, supplying a large amount of siliciclastics (Gorozhanina et al. Reference Gorozhanina, Kulagina, Brown, Alvarez-Marron and Pazukhin2001). Siliciclastics and carbonates were deposited in a relatively stable tectonic environment. The sedimentary series of the Zilair Megasynclinorium is thought to be the product of syntectonic sedimentation on the subsiding shelf of Baltica with platform sediments involved in the process of folding in the frontal area of the collision orogen (Alvarez-Marron et al. Reference Alvarez-marron, Brown, Perez-Estaun, Puchkov and Gorozhanina2000; Puchkov, Reference Puchkov2002, Reference Puchkov, Brown, Juhlin and Puchkov2009). The beds are folded in N–S-trending isoclinal folds, torn by large thrusts in places (Kamaletdinov, Reference Kamaletdinov1974). The deposits are subdivided in several formations of similar lithology (Khvorova, Reference Khvorova1961; Keller, Reference Keller1949) in the belt of the Palaeozoic slope formation in the basin of the Sakmara River, traceable throughout the ZM.
The MCB is fixed within the Bukharcha Formation established by Khvorova (Reference Khvorova1961) and exposed as a narrow N–S-directed belt along the west slope of the ZM. The Bukharcha formation conformably overlies the Verkhneitkulovo Subformation of the Itkulovo Formation, represented by an alternation of siltstone, mudstone, silicite and limestone beds. The Verkhneitkulovo Subformation is c. 350–450 m thick and largely unexposed in the area, having been examined mostly in quarries, road-cuts and in a few outcrops. The section of the Verkhneitkulovo Subformation in the road-cut 3 km north of the village of Kugarchi, near the village of Suleimanovo on the Bolshaya Suren River, contains all Upper Viséan foraminiferal zones (Sinitsyna et al. Reference Sinitsyna, Kulagina, Klimenko and Lozin1997). The Bukharcha Formation is overlain by the Unbetovo Formation (Pazukhin & Gorozhanina, Reference Pazukhin and Gorozhanina2003).
The Bukharcha formation is composed of limestones with interbeds of spongolite, argillite, argillaceous-chert shale and cherty nodules. The formation is mainly composed of limestone in the northern ZM (Muradymovo and Kurair), whereas in the southern ZM (Kugarchi and Bogdanovka) the basal part of the formation contains beds of argillaceous to cherty shale. The limestones are dominated by argillaceous, micritic and fine-grained varieties. The lower part of the formation contains lithoclast-bioclast-lime grainstone and limestone breccia (Kugarchi, Barangul Creek) and conglomeratic breccia (Azakla River) with semi-rounded fragments (1–3 cm) of fine-grained limestone. Limestones often show fluorite mineralization. The upper part of the formation contains a traceable bed of grainstone, becoming a lime rudstone in places. The Bukharcha Formation has a thickness of 250–320 m. The deposits of the upper portion of the Bukharcha Formation show the deepening trend indicated by spiculite and radiolarite mudstones and wackestones appearining and becoming more abundant in the Yuldybaevian. In other sections in the area (Bogdanovka and Bolshoi Uskalyk) we observed a transition from the deep shelf facies to more basinal depression facies as bioclastic and, in places, gradational-bedded wackestones of the Bukharcha Formation become replaced by laminated siliceous carbonates including spiculite, radiolarite and argillaceous basinal mudstone accumulated in deep-water conditions (Pazukhin & Gorozhanina, Reference Pazukhin and Gorozhanina2003).
The Bukharcha Formation contains diverse well-preserved fossils which include crinoids, foraminifers and conodonts and, in the upper part, ammonoids. Bryozoans and sponge spicules are also present. The best sections are in the basins of the Bolshoi Ik River (Muradymovo section) and Bolshaya Suren River (Bogdanovka and Kugarchi sections) (Krestovnikov, Reference Krestovnikov1935; Librovitch, Reference Librovitch1947; Einor et al. Reference Einor, Furdui and Aleksandrov1973; Ruzhencev & Bogoslovskaya, Reference Ruzhencev and Bogoslovskaya1971, Reference Ruzhencev and Bogoslovskaya1978; Nikolaeva, Reference Nikolaeva1999; Kulagina et al. Reference Kulagina, Pazukhin, Nikolaeva and Kochetova2000, Reference Kulagina, Pazukhin, Kotschetkova, Sinitsyna and Kochetova2001, Reference Kulagina, Pazukhin, Kochetova, Nikolaeva and Chuvashov2002). Stratigraphically, the Bukharcha Formation corresponds to the interval from the Serpukhovian Stage to the Syuranian Substage of the Bashkirian Stage. The Serpukhovian includes the Protvian and Yuldybaevian substages, while the Syuranian Substage includes the Bogdanovkian and Kamennogorian regional substages (Kulagina et al. Reference Kulagina, Pazukhin, Kotschetkova, Sinitsyna and Kochetova2001).
3. Geographical location and lithology
The Muradymovo section is located in the Kugarchi District of the Republic of Bashkortostan, 20 km to the SE of the town of Mrakovo, 3 km to the west of the village of Muradymovo (52° 35ʹ 10″ N, 56° 48ʹ 02″ E; Fig. 1). The outcrops are observed in the middle part of the steep slope of a hill on the left bank of the Abai–Elga Creek, left tributary of the Bolshoi Ik (Fig. 2). A large portion of the section is exposed in trenches. The beds are overturned, dipping at azimuth 80–120° at an angle of 45–80°. The sampled levels are marked on the section. The Bukharcha Formation in this section is composed mainly of dark-grey medium-bedded limestones, often siliceous, with interbeds, lenses and nodules of black chert. The limestones are dominated by micritic varieties with interbeds of bioclastic, less commonly lithoclastic, lime grainstone. The azimuth of the dip varies over the range 80–120° and the dip angle is 55–45°. The rock units in the section are marked by paint from 55 to 90, and are described in the following sections (Fig. 3).
Figure 1. (a) Position of the Muradymovo section. 1 – Pre-Palaeozoic deposits and metamorphic rocks, 2 – Pre-Carboniferous Palaeozoic deposits, 3 – Carboniferous deposits. I–IV – Structural zones of the Urals: I – Cisuralian Foredeep, II – West Uralian, III – Central Uralian, IV – Magnitogorsk. (b) Geological map of the Muradymovo section (modified after Kamaletdinov, Reference Kamaletdinov1974). Upper Devonian: D3fm1–2 – Famennian (Zilair Formation): siltstone, sandstone, cherts in the lower part; D3fm3 – Famennian, Kushelgian and Lytvinian substages (Yamashla Formation): limestone, shale. Lower Carboniferous: C1t1 – Lower Tournaisian (Mazitovo Formation): limestone, shale, siltstone, sandstone; C1t2 –Upper Tournaisian (Kuruil Formation): limestone, in places cherty, shale, siltstone; C1v – Viséan (Itkulovo Formation): limestone, shale, siltstone; C1s – Serpukhovian, C2b1– Bashkirian, C1s–C2b1– Serpukhovian and Bashkirian (Syuranian Substage), Bukharcha Formation, limestone; Upper Carboniferous: C2b2–4 – Bashkirian (Akavassian, Askynbashian and Arkhangelskian substages): limestone, lime breccia (Unbetovo Formation) and shale, siltstone, sandstone (lower part of the Kugarchi Formation); C2m – Moscovian (upper part of the Kugarchi and Zolotogorsk formations): shale, siltstone, sandstone, limestone; C3 – Kasimovian and Gzhelian (Abzanovo and Zianchurino Regional substages): sandstone, shale, marl and limestone, clastic in the upper part; Q – Quaternary. Dashed lines indicate faults covered by younger deposits.
Figure 2. An outcrop of the Serpukhovian and Bashkirian deposits 3 km west of the village of Muradymovo. Drawing (b) from the photograph (a) is modified from Kulagina et al. (Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992). Marker beds in the trench: (c) sample 77; (d) sample 75; and (e) sample 74. С2b1 ?kmg – ?Kamennogorian Substage of the Bashkirian Stage; C2b1bg – Bogdanovkian Substage of the Bashkirian Stage; C1s jul – Serpukhovian Stage, Yuldybaevian Substage; C1s pr – Serpukhovian Stage, Protvian Substage.
Figure 3.
The distribution of fossils (ammonoids by S.V. Nikolaeva, selected foraminifers by E.I. Kulagina, conodonts by V.N. Pazukhin and ostracods by N.N. Kochetova) in the MCB beds of the Muradymovo section. The stratigraphic log was composed by E.I. Kulagina and V.N. Pazukhin. Limestone: 1 – mudstone, packstone; 2 – grainstone; 3 – limestone with lenses of chert. Fossils: 4 – crinoids; 5 – ammonoids; 6 – brachiopods; 7 – bryozoans.
3.a. Serpukhovian
3.a.1. Kosogorian
The total thickness of the Kosogorian is 34.5 m.
3.a.1.a. Unit 1 (55, 56)
The lowest unit described here is numbered 55 and exposed at the base of the slope of a small gully. It is composed of medium-bedded, in places cherty, mudstone (bed thickness 0.1–0.15 m). This mudstone is overlain by a bed of micrograined grainstone (56), microscopically weakly recrystallized with infrequent foraminifers Archaesphaera sp., Earlandia minima (Birina), E. elegans (Birina), Pseudoglomospira spp. and Mediocris breviscula Vissarionova. Conodonts from this unit (samples 23, 55, 56) are Lochriea commutata (Branson & Mehl), L. cruciformis (Clarke) and L. mononodosa (Rhodes, Austin & Druce). The thickness is 1 m. This interval is overlain by a covered interval 5 m thick. The overlying beds are measured up the slope along azimuth 270 at a slope angle of 1–20º.
3.a.1.b. Unit 2 (57, 58)
The covered interval terminating Unit 1 is overlain by a member of skeletal crinoid-foraminiferal packstone, with lenses of chert, with a bed of peloid mudstone in the middle that contains only single-chambered spherical foraminifers. The thickness is 4 m. Samples 24, 57, 57a, 58, 58a contain foraminifers and conodonts, the distribution of which is shown in Figure 3. This unit is terminated by a covered interval of thickness 2.5 m.
3.a.1.c. Unit 3 (59)
The covered interval terminating Unit 2 is overlain by a unit composed of fine bioclastic grainstone/packstone and packstone with frequent calcisphaeras, crinoids, bryozoans, algae Koninckopora, foraminifers, conodonts Gnathodus, Lochriea, Neoprioniodus and others (samples 59, 59a; see Fig. 3) and also redeposited Famennian taxa Palmatolepis perlobata schindewolfi Müller (sample 59). The thickness is 2.5 m. Further on along azimuth 270 there is a covered interval of thickness 2.5 m.
3.a.1.d. Unit 4 (60–64)
The covered interval terminating Unit 3 is overlain by a unit composed of medium- to thin-bedded micritic, in the lower part predominantly fine-grained wackestones/packstone, with peloids and foraminifers Archaesphaera and Eotuberitina (samples 59b, 25), occasionally recrystallized (sample 61). Limestones contain a small admixture of argillaceous material. In the upper part of the unit, the limestone is argillaceous and microlaminated in places. The rock is weakly dolomitized in places. The limestone contains rare foraminifers and conodonts (Fig. 3). The thickness of the unit is 17 m. The marked bed 60 is 2 m above the base of the interval. Sample 61 was collected 9.5 m away from the marked level of sample 60 along azimuth 240. The bed is traced along-strike up the slope for 10 m, from where the description resumes along azimuth 260.
3.a.2. Protvian
The total thickness of the Protvian is 49.5 m.
3.a.2.a. Unit 5 (65–67)
This unit directly overlies dark-grey mudstone with cherty nodules and is composed of a lighter-coloured bioclastic lime packstone to grainstone consisting of foraminifers, skeletal fragments of crinoids, bryozoans, algae and unidentifiable fine bioclastic grains, frequent conodonts and foraminifers from underlying beds. Monotaxinoides subplanus Brazhnikova & Jarzeva and Gnathodus bollandensis Higgins & Bouckaert appear here for the first time. The basal part of the bed contains infrequent redeposited Upper Devonian conodonts and foraminifers. The thickness of the bed (sample 65) is 0.5 m. This level is overlain by dark-grey mudstone 5 m thick. A small ridge of dark-grey silicified mudstone can be easily traced upwards in the section. Upwards and along-strike, this mudstone becomes bioclastic grainstone (samples 66 and 66a). This bed is overlain by a bed of dark-grey fine-grained limestone (sample 67). The thickness of Unit 5 is 6.5 m, and it is overlain by a covered interval of 3.5 m.
3.a.2.b. Unit 6 (68, 69, 70)
The base of the unit is composed of a thin interrupted bed of thinly laminated mudstone, overlain by a bed of bioclastic wackestone (sample 68) with Calcisphaera and rare fragments of bryozoans and foraminifers. The bed is of thickness 0.5 m and can be easily traced as a small ridge up the slope. These beds are overlain by medium-bedded bioclastic packstone (samples 69, 70, 70/1, 70/2) with foraminifers, fragments of algae, crinoids and bryozoans and single oolites. The conodont assemblage contains a redeposited specimen of Palmatolepis cf. glabra lepta Ziegler & Huddle. Marked bed 70 contains rare imprints of small brachiopods. The entire unit contains lenses and laminae of chert, and is of thickness 5.5 m.
3.a.2.c. Unit 6a (70/2–72)
Unit 6a (thickness 11.9 m) is predominantly composed of medium- and thin-bedded mudstones and wackestones with radiolarians and less-commonly packstones. The assemblage contains ostracods, fish teeth and an impoverished conodont assemblage (see Fig. 3) that includes a redeposited specimen of Siphonodella sp. The beds are exposed in discontinuous outcrops which were sampled (samples 70/2, 71, 72). The bed with sample 72 at the top of the unit is 0.2 m thick and contains lenses of chert.
3.a.2.d. Unit 7a (72/1–73/6)
Unit 7a (thickness 8 m) was exposed in trenches 1–3. Trench 1 was described along azimuth 260. A thin bed (0.13 m) of bioclastic and lithoclastic grey loosely cemented limestone with crinoids lies on bed 72. A thin-section of this limestone (sample 72/1) showed clastic limestone containing a rounded fragment of mudstone from the underlying unit. These limestones are likely to represent a debris flow. This bed (thickness 1.7 m) is overlain by a unit of thin- to medium-bedded (predominating thickness 0.10–0.15 m) loosely cemented, crinoid wackestone and dense, interbedded with thin-bedded compact silicified mudstone, wackestones with foraminifers, crinoid remains, rare imprints of small brachiopods and beds of lithoclastic limestones (samples 72/2, 72/4, 72/5, 72/7). The overlying member consists of dark-grey thinly bedded, compact laminated mudstone with subdominant laminae of chert. In trench 1, from where samples 72/1–72/11 were collected, the visible thickness is 3 m. Trench 2 contains mudstone and packstone with infrequent foraminifers and spongolite in the upper part (samples 73/1–73/4, thickness 3.5 m). Further up the slope, the succession continues in trench 3 exposing bioclastic wackestone/packstone with sponge spicules and foraminifers (samples 73/5–73/6).
3.a.3. Yuldybaevian
The total thickness of the Yuldybaevian is 13.5 m.
3.a.3.a. Unit 7b (73/7–73A)
The thinly bedded limestone of Unit 7a is overlain by fine-grained foraminiferal packstone, in places silicified, 0.55 m thick (sample 73/7). The packstone is overlain by a bed of thinly laminated limestone (0.1 m), underlying a layer (0.1 m) of bioclastic crinoidal loosely cemented limestone. This interval is overlain by thickly and medium-bedded wackestone and packstone with cherty lenses and cherty limestone, of thickness 1.8 m. This limestone is immediately overlain by a bed of fine-grained, bioclastic, crinoidal, weathered limestone, with rare solitary corals, 0.5 m thick. The overlying bed of thick-bedded fine-grained limestone 4.25 m thick contains bioclastic wackestones and packstones with lenses of cherty limestone, in places strongly weathered, loosely cemented. These are overlain by argillaceous micritic limestone, with a lens of ammonoid shells (sample 73A), 0.45 m thick. The thickness of Unit 7b is 7.5 m.
3.a.3.b. Unit 7c (73/9–73/14, 74)
This unit (thickness 2.1 m) continues the section in trench 3 and consists of thin-bedded micritic argillaceous limestone (beds are 0.10–0.15 m thick) and includes the beds of fine-grained packstone-grainstone with sparite cement, with terrigenous admixture (up to 10%) with conodonts, foraminifers, ostracods and fragments of algae, bryozoans and brachiopods. Sample 73/14 contains a rich assemblage of conodonts with numerous Gnathodus bilineatus (Roundy) (42 specimens), Gn. bollandensis (49 specimens) and others (see Fig. 3), and also occasional redeposited Famennian and Tournaisian species Palmatolepis marginifera marginifera Helms, Bispathodus aculeatus aculeatus (Branson & Mehl) and Scaliognathus anchoralis Branson & Mehl (samples 73/9–73/14 and 74).
3.a.3.c. Unit 8 (74/1–8)
The base of the unit immediately above Unit 7c is composed of a bed of packstone 0.4 m thick. The unit is composed of medium- and thinly bedded limestone, micritic, argillaceous, spiculite, microlaminated, in places finely detrital, peloidal, with radiolarians, foraminiferans, bryozoans, algae and conodonts. The thickness is 3.9 m. The conodont assemblage from sample 74/1 contained 50 specimens of Gnathodus bollandensis and infrequent redeposited Famennian conodonts Palmatolepis cf. marginifera Helms and Polygnathus spp. (samples 74/1–74/8).
3.b. Bashkirian
The Syuranian, a substage of the Bashkirian, includes the Bogdanovkian and Kamennogorian regional substages. The total thickness of the Syuranian is c. 60–63 m.
3.b.1. Bogdanovkian
The total thickness of the Bogdanovkian is c. 40 m.
3.b.1.a. Unit 9a (74/9–10, 75–76)
Unit 9a is a continuation of Unit 8 and consists of medium- and thinly-bedded limestone (beds are 0.1–0.2 m thick). The unit is composed of wackestone and packstone, foraminiferal fine-bioclastic, slightly recrystallized, sometimes with carbonate debris, pellets, in places peloidal micritic, spiculite and microlaminated. The foraminiferal assemblage is almost the same as in the underlying beds; however, this unit shows first appearance of the conodonts Declinognathodus inaequalis (Higgins) at the base (sample 74/9). There are three specimens along with numerous Gnathodus bollandensis (104 specimens). In the next sample (sample 74/10) from the top of the overlying bed of similar lithology (0.55 m thick) we found three specimens of Declinognathodus inaequalis and a single Declinognathodus noduliferus. The rocks are complicated by syndepositional slump folds; the thickness (c. 6 m) is therefore difficult to estimate. Marked bed 75 is 0.2 m thick and is located 0.68 m above the base of the unit. Samples found in this unit are 74/9, 74/10, 75, 75/1–75/4, 76/1–76/7; sample 76/6 contained one redeposited specimen of Pseudopolygnathus triangulus (Voges).
3.b.1.b. Unit 9b (77)
Unit 9b begins from marked bed 77. The basal bed of limestone, 0.35 m thick, is overlain by a unit of thick- and medium-bedded limestone. These are bioclastic mudstone and wackestone with layers of bio-lithoclastic packstone grading to limestone breccia, usually slightly recrystallized, with calcite veins. Bio-lithoclastic packstones consist of bioclastic and micritic grains of various sizes (0.5–5 mm) and variously rounded, with micritic cement (sample 77b). Samples 77/1, 77 b, 77/2, 77/3 contained several redeposited conodont species ranging in age from Famennian to the Viséan, including Polygnathus inornatus Branson & Mehl, Palmatolepis sp., Hindeodella sp., Siphonodella cf. obsoleta Hass, Scaliognathus anchoralis Branson & Mehl. Lime breccia (sample 77/4) is composed of bioclasts and lithoclasts (micritic, algal, etc.) and contained a redeposited specimen of the early Viséan foraminifer Eoendothyranopsis sp. Sample 77/v is represented by micritic finely grained limestone with infrequent quartz grains and clay minerals, with scarce small-sized bioclasts. The bed contains foraminifers, fragments of crinoids, bryozoans and brachiopods and ammonoids. Sample 77 contains Plectostaffella varvariensis (Brazhnikova & Potievskaja) and numerous Gnathodus bilineatus and Gn. bollandensis (more than 50 specimens). The beds are complicated by syndepositional slump folds. Samples 77/1–77/6, 77a, 77b and 77v were collected from this unit, the thickness of which is estimated to be c. 6 m.
3.b.1.c. Unit 9c (78, 79)
The basal layers of Unit 9c (thickness 1.7 m) are composed of bioclastic packstone 0.8 m thick. The top of the unit is marked by a layer of limestone 0.7 m thick. The unit consists of bioclastic wackestones and packstone, in places cherty, with foraminifers, crinoids, bryozoans, conodonts and Calcifolium algae. This unit shows the first appearance of the zonal species Plectostaffella bogdanovkensis Reitlinger (sample 78). The conodont assemblage includes a single specimen of Declinognathodus noduliferus, numerous Gnathodus bollandensis and Gn. bilineatus and redeposited specimens of Palmatolepis glabra pectinata Ziegler, Neopolygnathus communis (Branson & Mehl) and Pseudopolygnathus marburgensis Bischoff & Ziegler (samples 78 and 79).
3.b.1.d. Unit 10 (79a)
Unit 10 includes a covered interval above marked bed 79 (1.5 m thick) and marked bed 79a (0.7 m thick), composed of large-grain bio-lithoclastic grainstone with foraminifers, crinoids, conodonts and the algal species Fasciella kizilia Ivanova. The basal portion of the bed is silicified. The unit is 2.2 m thick and is overlain by two beds of limestone of Unit 11, each 0.5 m thick. Upwards in the section, the succession is interrupted by a gully. Bed 79a can be traced towards the top of the hill.
3.b.1.e. Unit 11 (80)
This unit (thickness 3.0 m) is described in the upper portion of the slope. It is exposed 2 m above the covered interval overlying the marked bed 79a and is composed of medium-bedded limestone, mudstone and foraminiferal fine-bioclastic packstone, with terrigenous admixture of infrequent quartz grains, plagioclase, glauconite and fragments of hematized microlaminated rocks (Gorozhanina, pers. comm., 2012). The bed contains abundant conodonts, typical of the Bashkirian and also redeposited late Famennian, Tournaisian, Viséan (rare) and Serpukhovian (frequent) conodonts. Tournaisian conodonts include Pseudopolygnathus triangulus (Voges), Siphonodella duplicata Branson & Mehl, Polygnathus inornatus Branson & Mehl, Polygnathus symmetricus Branson, Siphonodella isosticha (Cooper) and S. aff. duplicata Branson & Mehl (samples 79v, 80, 80/1, 80/2).
3.b.1.f. Unit 12 (81a)
The covered interval, which is c. 2 m thick, is overlain by a medium-bedded, foraminiferal grainstone with rare fine-bioclastic materials, small quartz grains and in places micritic. The thickness of the unit is 3.5 m and yielded samples 81 and 81a. It is overlain by 2 m of covered interval.
3.b.1.g. Unit13 (81b, v, g)
The covered interval is overlain by interrupted exposures of grey, micritic and fine-grained, fine-bioclastic limestone with foraminifers, with a layer of foraminiferal grainstone in the upper part (sample 81g) that consists of rounded grains of limestone, bioclasts, oolites and terrigenous grains of quartz and other minerals (up to 15%). The thickness of this unit is 4.5 m.
3.b.1.h. Unit 14 (81d–81z)
The grainstone bed is followed by outcrops of isolated beds of lime mudstone and lime wackestones, with rare bioclastic grains and foraminifers, from which we collected samples 81g, 81e and 81zh. The isolated outcrops are exposed in a shallow gully. The thickness of the unit is 9 m.
3.b.1.i. Unit 15a (82, 83)
Outcrops of lime mudstone, with rare bioclastic grains, with lime packstone and grainstone in the upper part with microsparite cement, oolites, intraclasts and large bioclasts, grains of plagioclase, are observed on the right bank of a gully. Foraminifers, ammonoids and conodonts are present in samples 82, 83 and 83a. The thickness of this unit is 2.5 m.
3.b.2. Kamennogorian
After the covered interval (4 m) the section continues with units 15b–16 (18.6 m), tentatively assigned to the Kamennogorian.
3.b.2.a. Unit 15b (84–86)
The unit comprises limestone dark-grey medium-bedded fine-grained bioclastic, in places micritic with fine-bioclastic grains. In the upper part, litho-bioclastic packstone-grainstone with foraminifers, algae and other organic remains can be found. The thickness of this unit is 5.6 m, which yielded samples 84–86. Unit 15 is overlain by a covered interval 3 m thick.
3.b.2.b. Unit 16 (87–89)
This unit is dominated by medium-bedded limestone, mudstone and peloid packstone, argillaceous in the lower part, with rare fine-bioclastic grains. Samples 87, 87a, 88, 88a and 89 were collected from this interval and total unit thickness is 10 m.
The slope above the covered interval (c. 18–24 m thick) shows exposures of lime grainstone, becoming carbonate breccia with micritic, partly dolomitized cement in places, contains oolites, numerous foraminifers, crinoids and algae (0.2 m). Upwards in the section, these beds are replaced by micritic and lithoclastic bioclastic packstone with foraminifers, bryozoans and conodonts. The total thickness of the outcrop is 7 m. The foraminifers suggest the upper part of the Pseudostaffella antiqua and Pseudostaffella praegorski zones and conodonts of the Idiognathodus sinuosus Zone. The lithology and fossils correspond to those from the overlying Unbetovo Formation. There is no visible contact between the Bukharcha and Unbetovo formations.
4. Zonal subdivision
In the Muradymovo section, the MCB beds contain four major fossil groups. According to the international standard, the base of the Bashkirian is defined by the appearance of the conodont Declinognathodus noduliferus in Unit 9a. This level shows changes in other fossil groups (Fig. 4).
Figure 4. Correlation of the Mid-Carboniferous Boundary beds of the South Urals and the GSSP at Arrow Canyon, Nevada.
4.a. Conodonts
In the Muradymovo section, the Serpukhovian and Bashkirian zonations were first established by Pazukhin in Kulagina et al. (Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992) and Pazukhin (Reference Pazukhin1995). Since then the zonation has been changed considerably to incorporate changes in the knowledge of the conodont distribution and taxonomy. Conodonts are found here at almost all levels examined in the Serpukhovian and lower and middle Bashkirian (except in the Akavassian). More than 90 samples were processed (the average weight of a sample was 2 kg); 87 samples contained over 4500 conodont elements. The conodont collection is mainly represented by platform elements of the genera Gnathodus, Lochriea, Declinognathodus and Idiognathoides. Some samples contained redeposited conodonts, including some Famennian and Tournaisian and more commonly Serpukhovian (in the Syuranian). The section contains an uninterrupted succession of zones from Lochriea ziegleri to Idiognathodus sinuatus (Fig. 5).
Figure 5. Conodonts from the Muradymovo section. The scale bars are 0.2 mm. 1–4, Declinognathodus noduliferus (Ellison & Graves, Reference Ellison and Graves1941). 1, specimen no. 104/518, Unit 9a, sample no. 74/10. 2, Specimen no. 104/517, Unit 9a, sample no. 75/1. 3, Specimen no. 104/523, Unit 9a, sample no. 74/9. 4. Specimen no. 104/519, Unit 9a, sample no. 74/9. 5, 6, Declinognathodus praenoduliferus Nigmadganov & Nemirovskaya (Reference Nigmadganov and Nemirovskaya1992). 5, Specimen no. 104/530, Unit 9b, sample no. 77/6. 6, Specimen no. 104/532, Unit 9b, sample no. 77/6. 7, 8, Gnathodus bilineatus (Roundy, Reference Roundy, Roundy, Girty and Goldman1926). 7, Specimen no. 104/383, Unit 9a, sample no. 75. 8, Specimen no. 104/493, Unit 9b, sample no. 77. 9, Gnathodus bollandensis Higgins & Bouckaert (Reference Higgins and Bouckaert1968), specimen no. 104/493, Unit 9a, sample no. 76/3. 10, 11, Lochriea commutata (Branson & Mehl, Reference Branson and Mehl1941). 10, Specimen no. 104/627, Unit 9c, sample no. 79. 11, Specimen no. 104/501, Unit 9a, sample no. 74/9. 12, Lochriea ziegleri Nemirovskaya, Perret & Meischner (1994) specimen no. 104/628, Unit 9c, sample no. 79. 13, Lochriea costata (Pazukhin & Nemirovskaya in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992), specimen no. 104/493, Unit 9b, sample no. 7. 14, Lochriea monocostata (Pazukhin & Nemirovskaya in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992), specimen no. 104/505, Unit 9c, sample no. 79. 15, 16, Cavusgnathus unicornis Youngquist & Miller (Reference Youngquist and Miller1949). 15, Specimen no. 104/553: 15a, Upper view and 15b, Lateral view, Unit 9a, sample no. 74/10. 16, Specimen no. 104/554: 16a, upper view and 16b, lateral view, Unit 9b, sample no. 77/6.
4.a.1. Lochriea ziegleri Zone (Units 1–4; Serpukhovian)
The base of the Lochriea ziegleri Zone is defined based on the FAD of the index species. The zone shows the first appearance of Lochriea cruciformis (Clarke) and Lochriea multinodosa (Wirth). The assemblage also contains Gnathodus bilineatus (Roundy), Gn. girtyi girtyi Hass, Gn. girtyi collinsoni Rhodes, Austin & Druce (in the upper horizons), Lochriea commutata (Branson & Mehl), L. costata (Pazukhin & Nemirovskaya), L. monocostata (Pazukhin & Nemirovskaya), L. mononodosa (Rhodes, Austin & Druce), L. nodosa (Bischoff) and others. The species Lochriea ziegleri is widespread in Eurasia and has been considered as a potential marker for the base of the Serpukhovian (Skompski et al. Reference Skompski, Alekseev, Meischner, Nemirovskaya, Perret and Varker1995; Nemyrovska, Reference Nemyrovska2005; Somerville, Reference Somerville2008) and is currently considered to be the best marker by the Subcommission on the Carboniferous Stratigraphy (Richards, Reference Richards2011). It appears 3 m above the base of the Venevian in the Moscow Basin (A.S. Alekseev, pers. comm., 2012) and in the P1d Zone of the Brigantian of the British Isles (G. Sevastopulo, pers. comm., 2012).
4.a.2. Gnathodus bollandensis Zone (Units 5–8; Serpukhovian)
The base of the Gnathodus bollandensis Zone is based on the FAD of the index species. The zone shows the appearance of Gnathodus postbilineatus Nigmadganov & Nemirovskaya. The assemblage contains Gnathodus bilineatus, Lochriea commutata, L. costata, L. cruciformis, L. monocostata, L. mononodosa, L. multinodosa, L. nodosa and infrequent Cavusgnathus unicornis Youngquist & Miller. The species Gnathodus postbilineatus appearing in the upper part of the zone is used in some sections elsewhere to recognize the Gnathodus postbilineatus Zone (Nemyrovska et al. Reference Nemyrovska, Wagner, Winkler Prins and Montanez2011), but no such zone has been established so far in the Muradymovo section. The base of the Gnathodus bollandensis Zone coincides with the base of the Protvian.
4.a.3. Declinognathodus noduliferus Zone (Units 9a–11; Bashkirian)
The Declinognathodus noduliferus Zone in the South Urals is subdivided into two subzones: early Declinognathodus noduliferus Subzone showing the appearance of the first members of Declinognathodus against the background of the Viséan–Serpukhovian fauna and the late Declinognathodus noduliferus Subzone recognized based on the predominant occurrence of the Declinognathodus species, marked increase in the diversity of the Declinognathodus species and almost complete disappearance of Gnathodus and Lochriea species. In the Muradymovo section the boundary between the subzones is difficult to fix because of the redeposition of the Serpukhovian conodonts recorded up to the top of the Declinognathodus noduliferus Zone. The base of the zone is marked by the appearance of the infrequent Declinognathodus inaequalis, D. noduliferus against the background of the typical Serpukhovian conodont fauna. The middle part of the zone (sample 77/6) contains the first Declinognathodus praenoduliferus, closer to the top showing the appearance of Declinognathodus lateralis (Higgins & Bouckaert). The species continuing from the Serpukhovian include Cavusgnathus unicornis Youngquist & Miller, Gnathodus bilineatus, Gn. bollandensis, Gn. postbilineatus, Lochriea commutata, L. costata, L. cruciformis, L. monocostata, L. mononodosa, L. multinodosa, L. nodosa, L. ziegleri Nemirovskaya, Perret & Meischner and Mestognathus bipluti Higgins.
4.a.4. Idiognathoides sinuatus Zone (Units 12–16; Bashkirian)
The Idiognathoides sinuatus Zone is recognized in the upper Bogdanovkian and Kamennogorian. The base is drawn based on the appearance of the conodont Idiognathoides sinuatus Harris & Hollingsworth. The species Idiognathoides corrugatus Harris & Hollingsworth, Id. asiaticus Nigmadganov & Nemirovskaya, Id. sulcatus Higgins & Bouckaert and Neognathodus symmetricus Lane appear in this zone. D. inaequalis, D. lateralis, D. japonicus (Igo & Koike), D. noduliferus and D. praenoduliferus continue from the underlying beds. The assemblage also contains redeposited Serpukhovian and less commonly Famennian conodonts. This zone and its equivalents are established in many regions of the world, but their positions relative to other fossil groups are not always the same. In the Pennines (British Isles), the boundary between the zones D. noduliferus and Id. corrugatus – Id. sulcatus is near the base of the R1 Zone (Higgins, Reference Higgins1975). Nemyrovska (Reference Nemyrovska1999) reported similar results. In the Gissar Range (South Tien Shan), the base of the Id. corrugatus Zone is placed in the upper part of the Homoceras – Hudsonoceras Genozone (Nemirovskaya & Nigmadganov, Reference Nemirovskaya and Nigmadganov1994) and approximately coincides with the Id. sinuatus Uralian Zone.
4.b. Foraminifers
In the Serpukhovian and Bashkirian stages of the Muradymovo section six foraminiferal assemblages and zonal subdivisions were established (Kulagina & Pazukhin, Reference Kulagina and Pazukhin1986). After the subsequent study of the previously covered intervals however, the foraminiferal zonation of the section was modified (Kulagina et al. Reference Kulagina, Pazukhin, Kotschetkova, Sinitsyna and Kochetova2001, Reference Kulagina, Pazukhin, Kochetova, Nikolaeva and Chuvashov2002). Some specimens from the Serpukhovian and lower Bashkirian of the Muradymovo section were figured previously (Kulagina, Reference Kulagina1988; Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, Reference Kulagina, Pazukhin, Kotschetkova, Sinitsyna and Kochetova2001). The foraminiferal taxonomy in this paper is accepted according to the revisions of Rauser-Chernousova et al. (Reference Rauser-Chernousova, Bensh, Vdovenko, Gibshman, Leven, Lipina, Reitlinger, Solovjeva and Chediya1996), Brenckle & Grelecki (Reference Brenckle and Grelecki1993), Brenckle (Reference Brenckle2005) and Ginkel (Reference Ginkel2010). Six general and local zones are recognized from bed 2 of the Muradymovo section. The foraminifers are illustrated in Figures 6–9.
Figure 6. Foraminifers of the Muradymovo section. Asteroarchaediscus parvus and Eostaffellina actuosa zones. The scale bars are 0.2 mm. 1, Endothyra bowmani Phillips, Reference Phillips1846, specimen no. 121/50, sample 59, Kosogorian. 2, Omphalotis sp., specimen no. 121/260a, sample 69, Protvian. 3–6, Pojarkovella nibelis (Durkina, Reference Durkina1959). 3, Specimen no. 121/300. 4, Specimen no. 121/301. 5, Specimen no. 121/241. 6, Specimen no. 121/240, all from sample 69, Protvian. 7, Endothyranopsis sphaerica (Rauser-Chernousova & Reitlinger in Rauser-Chernousova et al. Reference Rauser-Chernousova, Belyaev and Reitlinger1936), specimen no. 121/233, sample 58. 8, Janischewskina typica Mikhailov, Reference Mikhailov1935, specimen no. 121/245, sample 66a, Protvian. 9, Ikensieformis ex gr. ikensis (Vissarionova, Reference Vissarionova1948), specimen no. 121/261, sample 69, Protvian. 10, Paramillerella (Acutella) aff. grozdilovae (Maslo & Vachard, Reference Maslo and Vachard1997), specimen no. 121/192, sample 69, Protvian. 11, 12, Eostaffella postovoidea Orlova in Orlova & Bensh, Reference Orlova and Bensh2004. 11, Specimen no. 121/256, sample 69. 12, Specimen no. 121/257, both from sample 69, Protvian. 13, Eostaffella ovoidea Rauser-Chernousova, Reference Rauser-Chernousova1948 a, specimen no. 121/199, sample 69, Protvian. 14, 15, Eostaffellina actuosa Reitlinger, Reference Reitlinger1963. 14, Specimen no. 121/198, sample 69, Protvian. 15, specimen no. 121/253, sample 72/3, Protvian. 16, Globivalvulina bulloides (Brady, Reference Brady1876), specimen no. 121/247, sample 70/1, Protvian. 17, Archaediscus karreri Brady, Reference Brady1873, specimen no. 121/74, sample 59, Kosogorian. 18, Asteroarchaediscus parvus (Rauser-Chernousova, Reference Rauser-Chernousova1948 b), specimen no. 121/1190, sample 59, Kosogorian. 19, Archaediscus gigas Rauser-Chernousova, Reference Rauser-Chernousova1948 b, specimen no. 121/238, sample 58, Kosogorian. 20, Planospirodiscus sp., specimen no. 121/270, sample 70, Protvian. 21, Neoarchaediscus probatus (Reitlinger, Reference Reitlinger1949), specimen no. 121/237, sample 57a, Kosogorian. 22, Neoarchaediscus tumefactus Ivanova, Reference Ivanova, Breiel and Papulov1970, specimen no. 121/269, sample 72/3, Protvian. 23, Archaediscus timanicus Reitlinger, Reference Reitlinger1949, specimen no. 121/236, sample 65, Protvian. 24, Monotaxinoides subplanus (Brazhnikova & Jarzeva, Reference Brazhnikova and Jarzeva1956), specimen no. 121/97, sample 65. 25, Bioclastic grainstone with Valvulinella sp., sample 59, Kosogorian.
Figure 7. Foraminifers of the Muradymovo section. Monotaxinoides transitorius Zone. The scale bars are 0.2 mm. 1, Endothyra sp., specimen no. 121/331, sample 77/1, Yuldybaevian. 2, Endostaffella pauperis (Durkina, Reference Durkina1959), specimen no. 121/281, sample 73/7, Yuldybaevian. 3, Endothyranopsis sp., specimen no. 121/281, sample 73/13, Yuldybaevian. 4, Bradyina ex gr. minima Reitlinger, Reference Reitlinger1950, specimen no. 121/348, sample 75, Bogdanovkian. 5, Janischewskina delicata (Malakhova, Reference Malakhova1956), specimen no. 121/278, sample 73/13, Yuldybaevian. 6, Planoendothyra sp. A. Axial section, specimen no. 121/335, sample 74/10, Bogdanovkian. 7, Mediocris ovalis Vissarionova, Reference Vissarionova1948, specimen no. 121/334, sample 74/10, Bogdanovkian. 8–10, Plectomillerella prisca (Rauser-Chernousova, Reference Rauser-Chernousova1948 a). 8, Specimen no. 121/1196, sample 73/13. 9, Specimen no. 121/279, sample, 73/7. 10, specimen no. 121/282, sample 73/7, all from the Yuldybaevian. 11, Pseudoendothyra sp., specimen no. 121/347, sample 74/10, Bogdanovkian. 12, 13, Eostaffella mosquensis Vissarionova, Reference Vissarionova1948. 12, Specimen no. 121/286, sample 73/13. 13, Specimen no. 121/1191, sample 73/13, Yuldybaevian. 14, Ikensieformis cf. ikensis (Vissarionova, Reference Vissarionova1948), specimen no. 121/292, sample 73/13, Yuldybaevian. 15, Neoarchaediscus tumefactus Ivanova, Reference Ivanova, Breiel and Papulov1970, specimen no. 121/314, sample 73/7, Yuldybaevian. 16, Paraarchaediscus vischerensis (Grozdilova & Lebedeva, Reference Grozdilova and Lebedeva1954), specimen no. 121/309, sample 73/7, Yuldybaevian. 17, 18, Archaediscus ex gr. gigas Rauser- Chernousova, Reference Rauser-Chernousova1948. 17, N121/315, sample 73/10. 18, N121/1192, sample 73/7, Yuldybaevian. 19, Archaediscus moelleri Rauser-Chernousova, Reference Rauser-Chernousova1948 b, N121/317, sample 73/11, Yuldybaevian. 20, Paraarchaediscus grandiculus (Schlykova, 1951), specimen no. 121/308, sample 73/7, Yuldybaevian. 21, Asteroarchaediscus baschkiricus (Krestovnikov & Theodorovich, Reference Krestovnikov and Theodorovich1936), specimen no. 121/311, sample 73/11, Yuldybaevian. 22, Monotaxinoides ex gr. transitorius Brazhnikova & Jarzeva, Reference Brazhnikova and Jarzeva1956, specimen no. 121/305, sample 73/7, Yuldybaevian. 23, Monotaxinoides transitorius Brazhnikova & Jarzeva, Reference Brazhnikova and Jarzeva1956, specimen no. 121/171, sample 75, Bogdanovkian. 24, 26, Monotaxinoides gracilis (Dain in Reitlinger, Reference Reitlinger1956): 24, 121/739, sample 74/10. 26, Specimen no. 121/365, sample 74/9, Bogdanovkian. 25, Monotaxinoides ex. gr. subplanus (Brazhnikova & Jarzeva, Reference Brazhnikova and Jarzeva1956), specimen no. 121/302, sample 73/10, Yuldybaevian. 27, Palaeotextularia longiseptata Lipina, Reference Lipina1948, specimen no. 121/312, sample 73/7, Yuldybaevian. 28, Palaeotextularia gibbosa minima Lipina, Reference Lipina1948, specimen no. 121/313, sample 73/13, Yuldybaevian. 29, Eolasiodiscus muradymicus Kulagina in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, specimen no. 121/363, sample 74/10 Bogdanovkian. 30–32, Monotaxinoides subplanus (Brazhnikova & Jarzeva, Reference Brazhnikova and Jarzeva1956): 30, Specimen no. 121/303, sample 73/13, Yuldybaevian. 31, Specimen no. 121/361, sample 74/10, Bogdanovkian, x 100. 32, 121/206, sample 73/13, Yuldybaevian.
Figure 8. Foraminifers of the Muradymovo section. Plectostaffella varvariensis Zone. Bogdanovkian. The scale bar is 0.2 mm. 1, Tolypammina sp., specimen no. 121/327, sample 77. 2–4, Pseudoglomospira spp.: 2, Specimen no. 121/319, sample 77/1. 3, specimen no. 121/321, sample 77. 4, Specimen no. 121/114, sample 77. 5, Glomospiroides sp., specimen no. 121/323, sample 77/6. 6, Syzygial cysts, specimen no. 121/116, sample 77. 7, Tolypammina sp., specimen no. 121/325, sample 74/10. 8, Janischewskina cf. delicata (Malakhova, Reference Malakhova1956), 121/119, sample 77. 9, Endothyra bowmani Phillips, Reference Phillips1846, specimen no. 121/313, sample 77. 10, Pseudoammodiscus multivolutus Reitlinger, Reference Reitlinger1949, specimen no. 121/1193, sample 77. 11, Eostaffella postovoidea Orlova in Orlova & Bensh, Reference Orlova and Bensh2004, specimen no. 121/122, sample 77. 12, 13, Endothyranopsis sp.: 12, Specimen no. 121/1194, sample 79a. 13, Specimen no. 121/329, sample 776. 14, Eolasiodiscus grandis R. Ivanova, Reference Ivanova1973, specimen no. 121/134, sample 77. 15, 16, Ikensieformis ex gr. ikensis (Vissarionova, Reference Vissarionova1948). 15, Specimen no. 121/342. 16, Specimen no. 121/341, both from sample 77/3. 17, Bradyina aff. pauciseptata Reitlinger, Reference Reitlinger1950, specimen no. 121/347, sample 77. 18–20, Plectostaffella varvariensis (Brazhnikova & Potievskaja, Reference Brazhnikova and Potievskaja1948). 18, Specimen no. 121/336, sample 77/6. 19, Specimen no. 121/169, sample 77. 20, Specimen no. 121/125, sample 77. 21–24, Biseriella minima (Reitlinger, Reference Reitlinger1950). 21, Specimen no. 121/354, sample 77. 22, Specimen no. 121/120, sample 77. 23, Specimen no. 121/121, sample 77. 24, Specimen no. 121/355, sample 77. 25, Wackestone/packstone, recrystallized with encrusting foraminifers, sample 77. 26, Climacammina ex gr. prisca Lipina, Reference Lipina1948, specimen no. 121/353, sample 77b. 27, Paraarchaediscus koktjubensis (Rauser-Chernousova, Reference Rauser-Chernousova1948 a), specimen no. 121/172, sample 77.
Figure 9. Foraminifers of the Muradymovo section. Plectostaffella bogdanovkensis and Semistaffella minuscilaria zones, Bogdanovkian. The scale bars are 0.2 mm. 1, Bradyina concinna Reitlinger, Reference Reitlinger1950, specimen no. 121/384, sample 81. 2, Bradyina ex gr. minima Reitlinger, Reference Reitlinger1950, specimen no. 121/382, sample 81. 3, Planoendothyra aljutovica (Reitlinger, Reference Reitlinger1950), specimen no. 121/381, sample 81. 4, Endostaffella pauperis (Durkina, Reference Durkina1959), specimen no. 121/388, sample 81. 5, Millerella umbilicata Kireeva in Rauser-Chernousova et al. Reference Rauser-Chernousova, Gryzlova, Kireeva, Leontovich, Safonova and Chernova1951, specimen no. 121/216, sample 80. 6, Eostaffella cf. paraprisca Durkina, Reference Durkina1959, specimen no. 121/339, sample 74/9. 7, Paramillerella advena (Thompson, Reference Thompson1944), specimen no. 121/144, sample 81. 8, Plectostaffella sp., specimen no. 121/407, sample 83a. 9, Eostaffella sp., specimen no. 121/389, sample 81. 10, Plectostaffella akkujlukia Rumjanzeva in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, specimen no. 121/1195, sample 83a. 11, Plectostaffella jachakia Rumjanzeva in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, specimen no. 121/150, sample 80. 12, Plectostaffella sp. 1, specimen no. 121/209, sample 81. 13, Plectostaffella ex gr. bogdanovkensis Reitlinger, Reference Reitlinger1980, specimen no. 121/208, sample 81. 14, Plectostaffella sp., specimen no. 121/414, sample 81z. 15, Plectostaffella sp., specimen no. 121/211, sample 81. 16, Plectostaffella bogdanovkensis Reitlinger, Reference Reitlinger1980, specimen no. 121/1175, sample 82/1. 17–19, Plectostaffella varvariensis (Brazhnikova & Potievskaja, Reference Brazhnikova and Potievskaja1948). 17, Specimen no. 121/203. 18, Specimen no. 121/152. 19, Specimen no. 121/210, all from sample 81. 20, Plectomediocris asymmetrica Brazhnikova & Vdovenko, Reference Brazhnikova, Vdovenko, Aisenverg, Astakhova, Berchenko, Brazhnikova, Vdovenko, Dunaeva, Zernetzkaya, Poletaev and Sergeeva1983, specimen no. 121/222, sample 79v. 21–23, Semistaffella minuscilaria Reitlinger, Reference Reitlinger1971. 21, Specimen no. 121/205. 22, Specimen no. 121/206. 23, Specimen no. 121/207, all from sample 81. 24, 25, Plectomillerella ex gr. angusta (Kireeva in Rauser-Chernousova et al. Reference Rauser-Chernousova, Gryzlova, Kireeva, Leontovich, Safonova and Chernova1951) ( = Eostaffella pseudostruvei Rauser-Chernousova and Belyaev, in Rauser-Chernousova et al. Reference Rauser-Chernousova, Belyaev and Reitlinger1936 in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, pl. 2, fig. 3 and Kulagina et al. Reference Kulagina, Pazukhin, Kotschetkova, Sinitsyna and Kochetova2001, pl. 1, fig. 13). 24, Specimen no. 121/386, sample 81. 25, Specimen no. 121/370, sample 79. 26, 27, Plectostaffella cf. evolutica (Rumjanzeva, Reference Rumjanzeva and Kuzichkina1970). 26, Specimen no. 121/147, sample 81b. 27, Specimen no. 121/221, sample 80. 28, 29, Eostaffellina paraprotvae (Rauser-Chernousova, Reference Rauser-Chernousova1948 b). 28, Specimen no. 121/146. 29, Specimen no. 121/204, both from sample 81. 30, Eostaffella postovoidea Orlova in Orlova & Bensh, Reference Orlova and Bensh2004, specimen no. 121/219, sample 81. 31, Biseriella minima (Reitlinger, Reference Reitlinger1950), specimen no. 121/166, sample 81. 32, 33, Globivalvulina moderata Reitlinger, Reference Reitlinger1950. 32, 121/160. 33, 121/202, both from sample 81. 34–36, Globivalvulina kamensis Reitlinger, Reference Reitlinger1950. 34, 121/162, sample 81. 35, Specimen no. 121/215, sample 80. 36, Specimen no. 121/393, sample 81.
4.b.1. Asteroarchaediscus parvus Zone (Units 2–4; Serpukhovian)
This zone correlates with the N. postrugosus Zone of the General Stratigraphic Scale of Russia (GSSR). The most diverse assemblages are found in the 2–3 beds of section (samples 57–59, Fig. 6, photographs 1, 7, 17–19, 21, 25). Foraminifers are here represented by diverse archaediscids: Archaediscus ex gr. gigas Rauser-Chernousova, A. krestovnikovi Rauser-Chernousova, A. grandiculus Schlykova, A. angulatus Sosnina, A. timanicus Reitlinger, Paraarchaediscus koktjubensis (Rauser-Chernousova), P. stilus (Grozdilova & Lebedeva), Asteroarchaediscus parvus (Rauser-Chernousova), and A. baschkiricus (Krestovnikov & Theodorovich). There are also frequent Omphalotis omphalota (Rauser-Chernousova & Reitlinger in Rauser-Chernousova et al.), Endothyranopsis crassa (Brady), E. sphaerica (Rauser-Chernousova & Reitlinger in Rauser-Chernousova et al.), Ikensieformis ex. gr. ikensis (Vissarionova) and species of Earlandia, Endostaffella, Palaeotextularia, Mediocris and others. The beds contain rare Biseriella parva (N. Tchernysheva). This assemblage corresponds to that of the type Kosogorian of the western slope of the Urals (Ponomareva et al. Reference Ponomareva, Lyadova, Shcherbakova, Pazukhin, Nasedkina, Kitaev, Chuvashov and Amon2002; Ponomareva, Reference Ponomareva2010), the Sunturian of the Khudolaz section (Stepanova & Kucheva, Reference Stepanova and Kucheva2006) and the Neoarchaediscus regularis – Biseriella parva Zone of the Central Tien Shan (Orlov-Labkovsky et al. Reference Orlov-Labkovsky, Bensh and Mikhno2003).
4.b.2. Monotaxinoides subplanus – Eostaffellina actuosa Zone (Units 5–6, 7a; Serpukhovian)
This zone correlates with the Eostaffellina paraprotvae Zone of the GSSR and E. paraprotvae – Ikensieformis mirifica Zone of the east slope of the Urals (Stepanova & Kucheva, Reference Stepanova and Kucheva2006). The base of the Monotaxinoides subplanus – Eostaffellina actuosa Zone is defined by the appearance of Monotaxinoides subplanus (Brazhnikova & Jarzeva) (Fig. 6, photograph 24). Eostaffellina actuosa Reitlinger (Fig. 6, photographs 14, 15), Ikensieformis mirifica (Brazhnikova), Globivalvulina bulloides (Brady) (Fig. 6, photograph 16) appear upwards in the section (Unit 6). Monotaxinoides subplanus is typical of the equivalents of the Protvian in the Donets Basin (Aizenverg et al. Reference Aizenverg, Astakhova, Berchenko, Brazhnikova, Vdovenko, Dunaeva, Zernetzkaja, Poletaev and Sergeeva1983).
4.b.3. Monotaxinoides transitorius Zone of the GSSR (Units 7b, 7c, 8, 9a; Serpukhovian and Bashkirian)
The Monotaxinoides transitorius Zone is based on the appearance of the zonal species Monotaxinoides ex gr. transitorius Brazhnikova & Jarzeva (Fig. 7, photographs 22, 23) and other species of Monotaxinoides and Eolasiodiscus (Fig. 7, photographs 24–26, 29–31). This zone shows the appearance of Plectostaffella orbiculata Ivanova and contains infrequent representatives of species continuing from the underlying beds, including Endothyranopsis sp., Ikensieformis ex gr. ikensis, I. mirifica, Omphalotis sp., Howchinia gibba (Moeller), H. bradyana (Howchin) and Monotaxinoides subplanus. Bradyina ex gr. minima Reitlinger enters above the base of the zone (sample 75, Fig. 7, photograph 4).
The foraminiferal assemblage is similar to that of the Monotaxinoides transitorius Zone of the Peri-Caspian (Zaitseva & Klenina, Reference Zaitseva and Klenina2008). However, based on the data from the Tengiz borehole it is possible to correlate the M. transitorius Zone of the Muradymovo section with the lower part of the M. transitorius Zone of the Peri-Caspian up to the level of the appearance of Pl. varvariensis. Based on the presence of Monotaxinoides transitorius, this zone correlates with the Monotaxinoides transitorius Zone of the Donets Basin (Vdovenko, Reference Vdovenko1988). Ponomareva (Reference Ponomareva2004) correlated this zone with the local Plectostaffella reitlingeri Zone of the Gostinskyi Section (Vishera Region) of the western slope of the Middle Urals. Monotaxinoides transitorius allows correlation with the upper portion of the Arnsbergian Cf7 Zone of the Dinant Basin (Laloux, Reference Laloux1988). In North America, this zone apparently correlates with the interval Eosigmoilina explicata – Brenckleina rugosa (Brenckle, Reference Brenckle, Brenckle and Manger1991).
4.b.4. Plectostaffella varvariensis Zone (Unit 9b; Bashkirian)
The Plectostaffella varvariensis Zone is defined by the appearance of Pl. varvariensis (Brazhnikova & Potievskaja) (samples 77, Fig. 8, photographs 18–20). This zone shows the first appearance of Bradyina aff. pauciseptata Reitlinger (Fig. 8, photograph 17) and Eolasiodiscus grandis Ivanova (Fig. 8, photograph 14). The zone typically contains Ikensieformis mirifica, Eostaffella postovoidea Orlova, Eolasiodiscus donbassicus and Globivalvulina bulloides which continue from the Serpukhovian. Of species originating from the Viséan, this zone contains infrequent Janischewskina (Fig. 8, photograph 8), Endothyranopsis ex gr. crassa, Ikensieformis ex gr. ikensis (Fig. 8, photographs 15, 16), Howchinia gibba, H. bradyana and long-ranging species Mediocris breviscula, acquiring species of Tolypammina, diverse Pseudoglomospira (Fig. 8, photographs 2–4), some Eostaffellina, Parastaffella and Neoarchaediscus. The base of the zone coincides with the extinction of the large species of Ikensieformis, Omphalotis, Globoendothyra and most Viséan archaediscids. This zone is characterized in the Muradymovo section by the occurrence of large specimens of Endothyranopsis and Howchinia bradyiana, which are traditionally considered Lower Carboniferous. It could be assumed that these specimens are redeposited since there are occurrences of redeposition in this section, and the foraminifers are found in litho-bioclastic limestone. However, it is worth mentioning that in the algal limestones with no traces of redeposition in the Bolshoi Kizil section, Endothyranopsis and Howchinia bradyiana are also found along with D. noduliferus. In addition, Endothyranopsis differs from the Serpukhovian species of this genus in the rapid increase of the height of the last whorl (more than twice) (Fig. 8, photographs 12, 13). A similar assemblage was recognized by Ponomareva (Reference Ponomareva2004) in the Staroutkinsk Regional Substage of the Gostinskii section of the Western Urals. In that section the species Pl. varvariensis is recorded in association with the first representatives of Declinognathodus below the last brachiopod bank with Striatifera. As in Muradymovo, the assemblage of the Gostinskii section is similar to that of the underlying zone and possibly corresponds to the M. transitorius of the Peri-Caspian with the earliest Pl. varvariensis (Zaitseva & Klenina, Reference Zaitseva and Klenina2008). In the Donets Basin the Plectostaffella varvariensis Zone correlates with the upper portion of the Loeblichia minima – Monotaxinoides transitorius – Eosigmoilina explicata Zone (Vdovenko et al. Reference Vdovenko, Aizenverg, Brazhnikova and Poletaev1989). In the Central Tien Shan Mountains it correlates with the Plectostaffella posohovae Zone, recognized by Z.S. Rumjanzeva (Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992) at the base of the Bashkirian below the Pl. bogdanovkensis Zone.
4.b.5. Plectostaffella bogdanovkensis Zone (Unit 9c–11; Bashkirian)
The base of this zone is defined by the appearance of Plectostaffella bogdanovkensis Reitlinger, Plectomillerella ex gr. angusta (Kireeva) (Fig. 9, photograph 25) and Millerella umbilicata Kireeva (Fig. 9, photograph 5). This zone shows an increase in the species diversity and abundance of representatives of Plectostaffella (Fig. 9, photographs 13–15) and the appearance of Plectostaffella species with a strongly displaced axis, which are difficult to identify to species (Fig. 9, photograph 12). The assemblage is identical to that described by Reitlinger (Reference Reitlinger1980) from the stratotype of Bogdanovkian (Bogdanovka section) and to that established by us previously in the same section (Kulagina et al. Reference Kulagina, Pazukhin, Nikolaeva and Kochetova2000). The most significant change in the foraminiferal assemblages of the Muradymovo section is recorded at the base of this zone. The assemblage of this zone is widespread in Eurasia (Gibshman & Akhmetshina, Reference Gibshman and Akhmetshina1991; Ivanova & Chuvashov, Reference Ivanova and Chuvashov1993; Mizuno & Ueno, Reference Mizuno and Ueno1997; Ponomareva, Reference Ponomareva2004; Zaitseva & Klenina, Reference Zaitseva and Klenina2008). The zone corresponds to the lower portion of the Voznesenskian and the Plectostaffella bogdanovkensis Zone of the Donets Basin (Vachard & Maslo, Reference Vachard and Maslo1996) and Koikebiltau Substage of Central Asia (Rumjanzeva, Reference Rumjanzeva1989). In the GSSP section in Arrow Canyon, North America this zone can apparently be correlated with the earliest Millerella marblensis (Brenckle et al. Reference Brenckle, Baesemann, Lane, West, Webster, Langenheim, Brand and Richards1997).
4.b.6. Semistaffella minuscilaria Zone (Units 12–15a; Bashkirian)
The assemblage of the zone is dominated by species continuing from the underlying beds. This interval shows the first appearance of Semiendothyra surenica Reitlinger, Bradyina concinna Reitlinger and Semistaffella minuscilaria Reitlinger. The zone correlates with the upper part of the Voznesenskian and the Millerella marblensis – M. angusta Zone of the Donets Basin (Vachard & Maslo, Reference Vachard and Maslo1996) and the Seslavino Substage of the Middle and South Tien Shan Mountains, containing the Plectostaffella seslavica (Rumjanzeva, Reference Rumjanzeva1989; Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992) and Pl. longiscula – Pl. varvariensis Zone (Orlov-Labkovsky et al. Reference Orlov-Labkovsky, Bensh and Mikhno2003).
4.c. Ammonoids
Two ammonoid genozones (Fayettevillea–Delepinoceras and Homoceras–Hudsonoceras) of the Russian Carboniferous Scale are recognized at Muradymovo (Fig. 10).
Figure 10. Ammonoids from the Muradymovo section. The scale bars are 2 mm. 1, Fayettevillea occidentalis Ruzhencev & Bogoslovskaya, specimen no. 4715/200, sample 74. 1a, Lateral view; 1b, ventral view. 2, Proshumardites delepinei Schindewolf, specimen no. 4715/207, 3 m below sample 74. 2a, Lateral view; 2b, ventral view. 3, Stenoglaphyrites sp., specimen no. 4715/208, sample 74. 3a, Ventral view; 3b, lateral view; 3c, apertural view. 4, Ramosites sp., specimen no. 4715/201, sample 73. 5, Juveniles of Homoceras haugi astrictum Ruzhencev & Bogoslovskaya and Isohomoceras sp., specimen no. 4715/210, sample 83. 6, Juvenile of Homoceras sp. Ruzhencev & Bogoslovskaya, specimen no. 4715/201, sample 83. 7, 8, Homoceras haugi astrictum Ruzhencev & Bogoslovskaya, sample 83. 7, Specimen no. 4715/203. 7a, Ventral view; 7b, lateral view. 8, Specimen no. 4715/205. 8a, Lateral view; 8b, ventral view. 9, Isohomoceras sp., specimen no. 4715/206, sample 83.
4.c.1. Fayettevillea–Delepinoceras Genozone (Units 7b–9; Serpukhovian)
The Fayettevillea-Delepinoceras Genozone is established beginning from the base of Unit 7b. The lowermost ammonoid-containing sample (73A) contains numerous shells of Proshumardites delepinei Schindewolf. These ammonoids are also found in sample 75 (Unit 9a). Sample 77 (Unit 9b) contains the same species in association with Glaphyrites sp. These ammonoids indicate the upper part of the Fayettevillea-Delepinoceras Genozone (Nm1c2 Zone) of Ruzhencev & Bogoslovskaya (Reference Ruzhencev and Bogoslovskaya1971, Reference Ruzhencev and Bogoslovskaya1978). Both these species appear at this level in many sections of the South Urals, Central Asia and North Africa near the Mid-Carboniferous Boundary (Manger et al. Reference Manger, Weyant and Pareyn1985). The Nm1c2 Zone corresponds to the upper part of the E2 Zone of Western Europe (Bisat, Reference Bisat1924, Reference Bisat1928; Ramsbottom & Saunders, Reference Ramsbottom and Saunders1985; Nikolaeva & Kullmann, Reference Nikolaeva and Kullmann1998, etc.).
4.c.2. Homoceras-Hudsonoceras Genozone (from Unit 12; Bashkirian)
The ammonoid geochronology of the interval between samples 77 and 81 is difficult to assess. The absence of the association of the index Upper Carboniferous species and Proshumardites delepinei, which was recorded in Sholak-Sai (Ruzhencev & Bogoslovskaya, Reference Ruzhencev and Bogoslovskaya1971, Reference Ruzhencev and Bogoslovskaya1978) and Aksu (Nikolaeva, Reference Nikolaeva1995 a,b) suggests that the basal Pennsylvanian Homoceras–Hudsonoceras Genozone begins slightly upwards in the section, approximately at the level of sample 81 (Unit 12) where Ramosites sp. enters. This is indirectly supported by the presence of Ramosites without accompanying Isohomoceras or Homoceras, which was recorded in the type Bogdanovkian section by Nikolaeva (Reference Nikolaeva1999) and Kulagina et al. (Reference Kulagina, Pazukhin, Nikolaeva and Kochetova2000).
Upward in the section, sample 83 (Unit 15a) contains a typical assemblage of the upper portion of the Homoceras–Hudsonoceras Genozone ((Isohomoceras sp., Ramosites ramosus Ruzhencev & Bogoslovskaya, Homoceras haugi astrictum Ruzhencev & Bogoslovskaya and Anthracoceratidae gen. & sp. indet.), indicating the upper part of this genozone: the Nm2a2 Zone. This zone is usually correlated with the H2 Zone of Western Europe (Kullmann & Nikolaeva, Reference Nikolaeva1999).
4.d. Ostracods
Ostracods were obtained from samples collected from the MCB boundary beds in the Muradymovo section and treated with acetic acid to obtain conodonts (Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992; Figs 11, 12). Ostracods are therefore represented by isolated and fragile valves and less commonly by complete shells with preserved structures and ornamentation. The assemblage contains mainly benthic taxa, including paraparchitids (Javatius, Coeloenellina, Libumella, Fellerites), kirkbyellids (Kirkbyella), kirkbyoids (Kirkbya, Amphissites, Ectodemites, Polytylites, Amphizona, Editia, Kelletina), kloedenelloids (Limnoprimitia, Kirkbyina), glyptopleurids (Glyptopleura), paraparchitoids (Shishaella, Shivaella, Chamishaella, Microcoeloenella, Dorsoobliquella), healdiids (Healdia, Bolbozoella, Carbonita), bairdioids (Bairdia, Bairdiocypris, Bairdianella, Basslerella, Acratia, Bohlenatia, Bairdiocypris, Acanthoscapha, Macrocypris), roundyellids (Roundyella), microcheilinids (Microcheilinella) and polycopids (Polycope) from the orders Palaeocopida, Platycopida, Metacopida, Podocopida and Cladocopida. The assemblage characteristically contains rectonarids (Rectoplacera, Rectonaria, Triplacera). The ostracods were studied mainly by Nataliya Kochetova (= N. Kotchetova). Three successive assemblages are recognized in the MCB portion of the Muradymovo section, described in the following.
Figure 11. Ostracods from the Muradymovo section. The scale bars are 0.2 mm. 1, Microcheilinella ? shiloi Bless in Simakov et al. Reference Simakov, Bless, Bouckaert, Conil, Gagiev, Kolesov, Onoprienko, Roty, Razina, Shilo, Smirnova, Streel and Swennen1984, specimen no. 66–325, sample 76/3, right valve view, Bogdanovkian. 2, Libumella sp., specimen no. 66–12, sample 73/7, left valve view, Yuldybaevian. 3, Healdia sp. 1, specimen no. 66–316, sample 74/3, right valve view, Yuldybaevian. 4, Kirkbyina tenella N. Kotchetova in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, specimen no. 66–311, sample 74/3, right valve view, Yuldybaevian. 5, Bolbozoella inflata Gründel, Reference Gründel1975, specimen no. 66–320, sample 74/3, right valve view, Yuldybaevian. 6, Ectodemites tumidus Cooper, Reference Cooper1941, specimen no. 66–207, sample 74/3, left valve view, Yuldybaevian. 7, Bohlenatia aff. inornata (Cordell, Reference Cordell1952), specimen no. 66–203, sample 74/3, right valve view, Yuldybaevian. 8, Editia sp. 2, specimen no. 66–150, sample 76/3, left valve view, Bogdanovkian. 9, Editia sp. 2, specimen no. 66–309, sample 74/9, left valve view, Yuldybaevian. 10, Pseudoparaparchites celsus N. Kotchetova, 1991, sample 74/3, left valve view, Yuldybaevian. 11, Healdia ikensis N. Kotchetova in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, holotype no. 66–66, sample 74/3, right valve view, Yuldybaevian. 12, Rectonaria aff. accepta N. Kotchetova in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, specimen no. 66–321, sample 74/3, left valve view, Yuldybaevian. 13, Editia sp. 3, specimen no. 66–310, sample 74/9, left valve view, Yuldybaevian. 14a, 14b, Microcheilinella extuberata, specimen no. 66–326, sample 76/3, right valve view and dorsal view, Bogdanovkian. 15, Healdia ikensis N. Kotchetova in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, specimen no. 66–68, sample 76/3, left valve view, Bogdanovkian. 16, Kirkbya aff. punctata Kellett, Reference Kellett1933, specimen no. 66–238, sample 76/3, right valve view, Bogdanovkian. 17, Rectoplacera sp. 1, specimen no. 66–99, sample 74/3, right valve view, Yuldybaevian. 18, Healdia sp. 2, specimen no. 66–318, sample 76/3, right valve view, Bogdanovkian. 19, Acratia sp., specimen no. 66–323, sample 74/3, right valve view, Yuldybaevian. 20, Ectodemites sp., specimen no. 66–208, sample 74/3, right valve view, Yuldybaevian. 21a, 21b, Healdia uralica N. Kotchetova in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, specimen no. 66–60, holotype, sample 76/3, right valve view and dorsal view, Bogdanovkian. 22, Healdia sp. 2, specimen no. 66–319, sample 74/3, left valve view, Yuldybaevian. 23, Kirkbyella sp., specimen no. 66–239, sample 76/3, left valve view, Bogdanovkian. 24, Amphissites aff. centronotus (Ulrich & Bassler, Reference Ulrich and Bassler1906), specimen no. 66–24, sample 76/3, left valve view, Bogdanovkian. 25, Chamishaella uniformis Kotschetkova, Reference Kotschetkova1983, specimen no. 66–312, sample 74/3, right valve view, Yuldybaevian.
Figure 12. Ostracods from the Muradymovo section. The scale bars are 0.2 mm. All except photographs 22a,b from sample 76/3. Bogdanovkian: 1a, 1b, Basslerella simonovae Kotschetkova, Reference Kotschetkova1983, specimen no. 66–141, right valve view and dorsal view. 2, Dorsoobliquella cf. ovalis Kotschetkova, Reference Kotschetkova1983, specimen no. 66–314, right valve view. 3, Editia sp. 1, specimen no. 66–152, right valve view. 4, Bairdiocypris subalia N. Kotchetova in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, specimen no. 66–79, right valve view. 5, Microcoeloenella orbiculata Kotschetkova, Reference Kotschetkova1983, specimen no. 66–313, right valve view. 6, Kellettina sp., specimen no. 66–32, right valve view. 7, Rectonaria sp. 1, specimen no. 66–104, right valve view. 8, Kirkbya aff. elongata Cooper, Reference Cooper1941, specimen no. 66–213, right valve view. 9a, 9b, Rectonaria sp. 2, specimen no. 66–95, right valve view and dorsal view. 10a, 10b, Coeloenellina serotina Kotschetkova, Reference Kotschetkova1983, specimen no. 66–4, right valve view and dorsal view. 11, Healdia sp. 1, specimen no. 66–317, left valve view. 12, Kirkbya sp. C, specimen no. 66–23, right valve view. 13, Amphisona sp., specimen no. 66–30, left valve view. 14, Bairdianella sp., specimen no. 66–136, right valve view. 15, Libumella aff. reticulata Robinson, Reference Robinson1978, specimen no. 66–13, left valve view. 16, Bairdia alula Kotschetkova, Reference Kotschetkova1983, specimen no. 66–322, right valve view. 17, Bairdia chudolasensis Kotschetkova, Reference Kotschetkova1983, specimen no. 66–129, right valve view. 18. Ectodemites tumidus Cooper, Reference Cooper1941, specimen no. 66–206, left valve view. 19, Acanthoscapha aff. limata N. Kotchetova in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, specimen no. 66–324, right valve view. 20, Kirkbya sp. B, specimen no. 66- 214, right valve view. 21, Bairdiocypris indiges Kotschetkova, Reference Kotschetkova1983, specimen no. 66–215, right valve view. 22a, 22b, Chamishaella opima Kotschetkova, Reference Kotschetkova1983, specimen no. 66–223, right valve view and dorsal view, sample 80a. 23, Fellerites gratus N. Kotchetova & Vakula in Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992, specimen no. 66–21, left valve view. 24, Kirkbya sp. A, specimen no. 66–308, right valve view.
4.d.1. Ostracod assemblage I
This assemblage is found in limestones of the upper part of the Yuldybaevian in Unit 8 (Fig. 3). This assemblage includes species characteristic of the Pseudoparaparchites celsus regional zone, which are usually associated with the Protvian and lower part of the Yuldybaevian (Kulagina et al. Reference Kulagina, Rumjanzeva, Pazukhin and Kotchetova1992) and Pseudoparaparchites celsus N. Kotchetova in association with kirkbyids, Healdia ikensis N. Kotchetova, H. uralica N. Kotchetova, Bolbozoella inflata Grundel, Bohlenatia aff. inornata (Green), Acanthoscapha cf. limata N. Kotchetova, Microcheilinella ? shiloi Bless, singular rectonarids and Editia spp., Acratia sp. However, in this section we observe the occurrence of Pseudoparaparchites celsus unusually high in the section of several valves, which could possibly be redeposited.
4.d.2. Ostracod assemblage II
The second ostracod assemblage (II) is found in the limestone of the upper part of Unit 9a and in Units 10 and 11 (Fig. 3), typical of the lower Bogdanovkian containing the transitional assemblage with surviving Serpukhovian species. This association is characteristic of the Fellerites gratus Zone, the base of which is based on the appearance of the index species alongside Javatius kisilensis (Kotschetkova). The assemblage contains species continuing from the underlying beds, diverse kirkbyaceans, Kirkbyella sp. and Chamishaella opima Kotschetkova, Discoidella perspicua Kotschetkova, Bairdia chudolasensis Kotschetkova, Bairdiocypris indiges Kotschetkova and other taxa commonly found in the Bashkirian of the South Urals (Kotschetkova, Reference Kotschetkova1983).
4.d.3. Ostracod assemblage III
The basal portion of Unit 12 (sample 81) contains several taxa of Assemblage III, which is typical of the middle and upper Bogdanovkian. This assemblage includes Limnoprimitia cf. arcuata (Bean), allowing the tentative recognition of the Limnoprimitia arcuata Zone (Kulagina et al. Reference Kulagina, Pazukhin, Kotschetkova, Sinitsyna and Kochetova2001; Kochetova, Reference Kochetova2008). The assemblage also contains Dorsoobliquella ovalis Kotschetkova, Microcoeloenella orbiculata Kotschetkova, Bairdia cestriensis var. granulosa Girty, Bairdia cf. seminalis (Knight) and Acratia grandis Kotschetkova.
5. Correlation of the zones
In the Muradymovo section, there are successions of conodont, foraminiferal, ammonoid and ostracod zones (Fig. 4). The succession of the conodont zones is the most complete and representative and allows the reliable correlation with zones based on other fossil groups. The lower Serpukhovian Substage contains the assemblage of the conodont Lochriea ziegleri Zone, while its foraminiferal assemblage contains species continuing from the upper Viséan in association with the species Archaediscus timanicus Reitlinger (Reference Reitlinger1950) that was originally described from the Bashkirian.
The upper Serpukhovian Substage (Protvian and Yuldybaevian) includes the conodont Gn. bollandensis Zone. This zone correlates with foraminiferal Monotaxinoides subplanus – Eostaffellina actuosa Zone and the lower, largest portion of the Monotaxinoides transitorius Zone that corresponds to the beds with the ammonoids assemblage of the Fayettevillea–Delepinoceras Zone. The association of the foraminifers, conodonts and ammonoids is recorded in the interval c. 4 m thick (upper part of Unit 7b to Unit 7c).
The Declinognathodus noduliferus Zone identified by the first appearance of the conodont Declinognathodus inaequalis and D. noduliferus at the base of Unit 9a (samples 74/9, 74/10, 75) corresponds to the upper part of the foraminiferal Monotaxinoides transitorius Zone, Pl. varvariensis and Pl. bogdanovkensis Zones. The deposits at 5 m above the base of the conodont D. noduliferus Zone and up to its top contain the ostracod assemblage II with Fellerites gratus.
The Declinognathodus noduliferus Zone in all studied sections of this interval lies near the boundary between the Fayettevillea–Delepinoceras and Homoceras–Hudsonoceras genozones and their equivalents (zones E and H in Western Europe and North America). The association of Proshumardites delepinei and Isohomoceras established in the lower part of the Homoceras–Hudsonoceras Genozone in Sholak-Sai (South Urals; Ruzhencev & Bogoslovskaya, Reference Ruzhencev and Bogoslovskaya1978) and in the Aksu section (Tien Shan Mountains; Nikolaeva, Reference Nikolaeva1994), containing the first occurrence of D. noduliferus (Nikolaeva & Nigmadganov, Reference Nikolaeva and Nigmadganov1992), is not recorded at Muradymovo. The base of the Homoceras–Hudsonoceras Genozone should possibly be around the base of the Pl. varvariensis foraminiferal Zone (as in other sections in the area; Kulagina et al. Reference Kulagina, Pazukhin, Nikolaeva and Kochetova2000), but the lowest ammonoid occurrences of the Homoceras–Hudsonoceras Genozone is in sample 81 (Unit 12), containing Ramosites sp. Sample 83 (Unit 15a) contains a typical assemblage of the Homoceras–Hudsonoceras Genozone of the Uralian scale usually correlating with the Alportian. This level corresponds to the base of the foraminiferal Semistaffella minuscilaria Zone and conodont Idiognathoides sinuatus Zone.
The boundary level defined by conodonts in different regions may occur both above and below the first appearance of the ammonoid genus Isohomoceras, which indicates the base of the Homoceras–Hudsonoceras Genozone. In the Muradymovo section, the boundary between the genozones can be at any level between samples 73 and 83 because this interval contains ammonoids that, in other regions, occur both in the Fayettevillea–Delepinoceras and Homoceras–Hudsonoceras genozones. However, it is most likely that the base of the Homoceras–Hudsonoceras Genozone lies between samples 77 and 81 because the genus Ramosites was not found below this interval, whereas no characteristic Mississippian ammonoid species is found above it.
The middle part of the conodont Idiognathoides sinuatus Zone, 18 m above the base, contains an ammonoid assemblage of the upper part of the Homoceras–Hudsonoceras genozone. In this part of the zone, Id. sulcatus has been recorded. Ostracod assemblage III is found in association with foraminifers of the Semistaffella minuscilaria Zone and conodonts of the lower part of the Idiognathoides sinuatus Zone.
6. Summary
In the Muradymovo section the position of the MCB is presently identified by the first appearance of the conodonts Declinognathodus inaequalis and D. noduliferus. It is placed within the foraminiferal Monotaxinoides transitorius Zone. The base of the Pl. varvariensis Zone is a foraminiferal level nearest to the base of the Pennsylvanian (6 m above the base of D. noduliferus Zone), but the most significant change in the foraminiferal assemblages of the Muradymovo section is recorded higher, at the base of Plectostaffella bogdanovkensis Zone, which is widespread in Eurasia. Notably, the assemblage of the boundary beds in Muradymovo does not contain Rhachistognathus primus Dunn, so the Rhachistognathus primus – D. noduliferus conodont zone fixed near the MCB at Arrow Canyon (Lane et al. Reference Lane, Brenckle, Baesemann and Richards1999; Richards et al. Reference Richards, Lane, Brenckle, Hills, Henderson and Bamber2002) cannot be recognized. Instead, the assemblage contains D. praenoduliferus and D. postbilineatus; the situation is somewhat similar to that of the Aksu Section, Tien Shan Mountains and La Lastra section in Spain (Nemirovskaya & Nigmadganov, Reference Nemirovskaya and Nigmadganov1994; Nemyrovska et al. Reference Nemyrovska, Wagner, Winkler Prins and Montanez2011). Ammonoids indicate the presence of the Fayettevillea–Delepinoceras and Homoceras–Hudsonoceras zones (E2 to H), facilitating broad correlations across the South Urals, Western Europe, Central Asia and Nevada in the USA. In summary, the Muradymovo section is a very good auxiliary section for the MCB interval allowing reliable correlations with many other successions worldwide.
Acknowledgements
The authors are grateful to A.S. Alekseev and B.C. Richards for critically reviewing the manuscript and providing valuable comment. The study was supported by the Program of Presidium of the Russian Academy of Sciences ‘Origin of the Biosphere and Evolution of GeoBiological Systems’, the project ‘Geobiological Events in the Evolution of the Biota on an Example of Cephalopods and Radiolarians’ and Russian Foundation for Basic Research (project no. 10-05-01076).
