GEOLOGIJA 49/2, 205–217, Ljubljana 2006 Usolka section (southern Urals, Russia): a potential candidate for GSSP to define the base of the Gzhelian Stage in the global chronostratigraphic scale Valery V. CHERNYKH1, Boris I. CHUVASHOV1, Vladimir I. DAVYDOV2, Mark SCHMITZ2 & Walter S. SNYDER2 1 Laboratory of Stratigraphy and Paleontology, Institute of Geology and Geoshemistry Uralian Scientific Ctr of Russian Academy, Pochtovy Per. 7, Ekaterinburg, 620151, Russia, chernykh@igg.uran.ru, chuvashov@igg.uran.ru 2 Permian Research Institute, Dept. Geosciences, Boise State University, 1910 University Drive, Boise, ID, 83725, USA, vdavydov@boisestate.edu, wsnyder@boisestate.edu Key words: Carboniferous, Gzhelian, chronostratigraphy, type-section, conodonts, fusulinids, Russia, Urals Abstract Conodont species Streptognathodus simulator Ellison, 1941 has been proposed recently to define the Kasimovian-Gzhelian boundary in the global chronostratigraphic scale. The species distributed globally and traditionally has been used as a marker of the base of the Gzhelian Stage in the type sections in Moscow Basin and Urals. Recent studies of conodont taxonomy and biostratigraphy in southern Urals have established the chronoc-line with ascendant and descendant to Streptognathodus simulator species. Usolka section proposed here as a potential candidate for the GSSP (Global Stratotype Section and Point) to define the global Gzhelian Stage at the FAD of the Streptognathodus simulator within the chronocline Streptognathodus praenuntius Chernykh, 2005 - St. simulator Ellison, 1941 - St. auritus Chernykh, 2005. The chronocline recovered within 2.7 m of beds 4 and 5 at the Usolka section, with all three species described and properly figured. No obvious interruptions in sedimentation are recorded within the Kasimovian-Gzhelian transition there. Several volcanic ash beds are present below and above the proposed boundary, making radiometric calibration highly possible in the near future. Mode of preservation of conodonts with a CAI of around 1.0-1.5 provides excellent basis for the geochemical studies. Accessibility presently is adequate, and this exposure will be improved and maintained permanently for interested scientists. Future access will be guaranteed by means of legislative action to create a scientific preserve. Introduction In the recent report of the Task Group to establish a Global Stratotype Section and Point (GSSP) at the Kasimovian–Gzhe-lian boundary (Villa and Task Group, 2005) and in a series of recent publications (Heck-el et al., 2005; Chernykh, 2005; Menning et al., 2006; etc.), the conodont species Strep-tognathodus simulator Ellison, 1941 (regarded as Idiognathodus by some authors), has been proposed as the best index-fossil for the definition of the base of the global Gzhelian Stage. This species was originally described from the Heebner Shale Member of the Oread Limestone (Ellison, 1941) in Midcontinent North America, and has been traditionally used as a marker for the boundary in the Moscow Basin (Barskov & Alek-seev, 1979) and in the Urals (Chernykh & Reshetkova, 1987; Davydov & Popov, 1991; Chernykh, 2002). Therefore it easily can be adopted in the stratotype region as the event marker for the base of the global Gzhelian Stage. Heckel et al. (2005) have shown that this species is definitely global in 206 Valery V. Chernykh, Boris I. Chuvashov, Vladimir I. Davydov, Mark Schmity & Walter S. Snyder distribution. Moreover, the taxonomy of the species has been updated recently by Bar-rick et al. (2004), who recognize two separate species: St. simulator [sensu stricto], the concept of which is based on the holo-type from Midcontinent Heebner Shale of the Oread cyclothem, and its potential ancestor St. aff. simulator, which occurs in the older Midcontinent Eudora Shale of the Stanton cyclothem and the Merriman-Up-per Winchell cyclothem of Texas (Heckel et al., 2005). Current studies of upper Paleozoic stratigraphy and biostratigraphy in the Urals during the last few years (Chuvashov et al., 1990, 1993, 2002; Davydov & Popov, 1986; Leven & Davydov, 2001; Davydov & Leven, 2003) have established a refined biostratigraphic framework for this time interval and described several new conodont species (Chernykh, 2002, 2005). The latter publication establishes a chronocline of Streptognathodus praenuntius Chernykh, 2005 - St. simulator Ellison, 1941 - St. au-ritus Chernykh, 2005, which is recovered within 2.7 m of beds 4 and 5 at the Usolka section, with all three species described and properly figured. Besides biostratigraphy and sedimentology, comprehensive geo-chemical study has been done at Usolka section. Numerous and frequent volcanic ashes occur throught succession in Usolka section (Davydov et al., 2002). One volcanic ash layer very close to the boundary has been dated just recently (Schmitz et al., 2006). There are several more ashes that potentially will precisely constrain proposed boundary in term of radiometric calibration. Strontium study from conodonts also has been performed in Usolka section (Needham et al., 2006). We are envisioning studying 818O isotopes from conodonts as well. We propose here the Usolka section as a potential candidate for establishing a GSSP for the base of the global Gzhelian Stage. Material The Usolka section is located approximately 120 km southeast of Ufa and about 60 km northeast of Sterlitamak on the northeastern margin of the city of Krasnousolsk, just north of the Usolka River across from the hot-springs resort, in the Bashkortostan Republic of Russia (Fig. 1). This section is exposed along the roadcut on the right bank of the Usolka River in the core of the meridi-onally striking Usolkian brachianticline. The section occurs in the axial part of the Belsk depression in the relatively deeper– water portion of the Preuralian Foredeep, and therefore the sedimentary succession there is likely to be relatively undisturbed. The section starts with dolomitic limestone with chert nodules and rare volcanic ash beds of the Zilim Formation, approximately 10–12 m thick, which is overlain across a covered interval by the predominantly mixed carbonate-siliciclastic succession of the Kurkin (or Kurortnaya) Formation (Fig. 2). During a 2001 field trip, we collected samples and recovered conodonts from volcanic ash near the top of the Zilim Formation. Conodonts there include typical Moscovian Neognathodus and are under current study. The measured thickness between the exposed top of the Zilim Formation and bed 1 of the Kurkin Formation in the section is approximately 5–6 meters. The Kurkin Formation contains numerous micritic limestone beds and up to 50 volcanic ash layers (Fig. 2) more or less evenly distributed throughout the section (Davy-dov et al., 2002, 2003). More general information and details on the section can be obtained from several sources (Chuvashov et al., 1991, 1993; Chuvashov & Chernykh, 2002). Chernykh (2005) provided the most comprehensive record of conodont distribution in the section. Because of the undisturbed sedimentary record and abundance of conodonts in the section, higher parts of the Usolka section were proposed as an auxiliary section for the Carboniferous-Permian boundary (Chuvashov et al., 2002), and as a candidate for the GSSP for the base of the Sakmarian Stage (Wardlaw et al., 1999). The lithology of the Kasimovian-Gzhe-lian transition is not yet described in great detail, but we plan to re-measure and re-study it in the near future. Our plan is to collect additional conodont samples at a centimeter scale along with samples for fu-sulinids, smaller foraminifers, and volcanic ash beds where the lithology is appropriate. Below is the latest available description of the Kasimovian-Gzhelian transition (Chu-vashov & Chernykh, 2002), in ascending order of numbered beds (with thicknesses based on Fig. 2): 1. Thin-bedded, slightly silicified dark– grey to black siltstone with bioclastic debris in the uppermost part. acri-tarchs and palinospores are found in siltstone .......... 1.1 m Usolka section (southern Urals, Russia): a potential candidate for GSSP to define the base of the... 207 Figure 1. Location map and Google Earth image of Usolka section, Southern Urals, Bashkortostan, Russia. 2. Bluish-grey micritic limestone, strongly silicified in the lower 10–12 centimeters. One orange-yellow volcanic ash layer has been found in this bed. In the middle of the bed, a lens of packstone to grainstone limestone contains smaller foraminifers, rugose corals, brachiopods, crinoids and conodonts Idiognathodus delicatus Gunnell, Id. sagittalis Kozitskaya, Id. tersus Ellison, Streptognathodus cancellosus (Gunnell), Gondolella sinuata Gun-nell, G. merrilli Gunnell .......... 0.4 m 3. Bluish-grey, strongly silicified, medium to thin-bedded (5–20 cm), slightly silty micritic limestone interbedded with grayish foliated, silicified siltstone. Two very thin (2–3 cm) orange-yellow volcanic ash layers occur within the lower third of this bed. In the uppermost part of the bed, a few very thin (1–2 cm) layers of packstone to micrit-ic limestone occur. Packstone contains smaller foraminifers, brachiopods, crinoids and other bioclastic debris. Radiolaria and sponge spicules are found in micritic layers. Conodonts Idiognathodus delicatus Gunnell, and Id. tersus Ellison are found in this bed .......... 4.0 m 4. Alternations of dolomitic micrite and wackstone and siltstones. Wackstone layers are 0.18–0.3 m thick, dark-grey, with microgranular matrix. They contain smaller foraminifers, brachiopods, crinoids and conodonts. At the top of bed 4 relatively thick (0.7 m) layer of grayish packstone with small lenses of fine grainstone at its base (bed 4–1) occurs. Smaller foraminifers, fusulinids, brachiopods and rugose corals are found in this grainstone. The following species are identified among fusu-linids: Quasifusulina ex gr. longissima (Moeller), Pseudofusulinella minuta (Grozdilova), Ps. pulchra (Rauser & Belyaev), Schwageriniformis petchori-cus brevis (Rauser & Belyaev), Sch. petchoricus varsanofievi (Z. Mikhailo-va), Sch. schwageriniformis mosquen-sis (Rosovskaya), Schwageriniformis baisunensis (Bensh), Schwagerinifor- 208 Valery V. Chernykh, Boris I. Chuvashov, Vladimir I. Davydov, Mark Schmity & Walter S. Snyder Figure 2. Distribution of conodonts and fusulinids within Kasimovian-Gzhelian transition in Usolka section. Bed boundaries are shown by thick marks across from bed numbers on line to left of metric scale. mis (Tumefactus) sp., Rauserites bash-kiricus (Rosovskaya), R. shikhanensis compactus (Rosovskaya), Rauserites dictiophorus (Rosovskaya). Conodonts in the lower part of this 0.7 m limestone layer are: Streptognathodus fir-mus Kozitskaya, St. gracilis Staffer & Plummer, St. zethus Chernykh & Reshetkova, St. pawhuskaensis Harris & Hollingsworth, St. praenuntius Chernykh, Gondolella merrilli Gun-nell, and some other conodonts that are undergoing study. 20 centimeters above, in bed 4–2 (Fig. 2), the assemblage of conodonts is more diverse and includes: Streptognathodus elegantu-lus (Stauffer & Plummer), St. firmus Kozitskaya 1978, St. gracilis Stauffer & Plummer, St. makhlinae Alekseev et Goreva. St. simulator Ellison, St. dolioliformis Chernykh 2005, St. pic- tus Chernykh 2005, and Id. undatus Chernykh, 2005 .......... 1.1 m 5. Alteration of predominantly micritic limestone (0.05 to 0.2 m thick layers), with a few thin (0.1–0.2 m) siltstone layers. Micritic limestone contains con-odonts Idiognathodus lobulatus Koz-itskaya, Id. brevisulcatus Chernykh 2005, Id. pictus Chernykh 2005, Strep-tognathodus luganicus Kozitskaya, St. pawhuskaensis (Harris & Holling-sworth), St. simulator Ellison, St. au-ritus Chernykh 2005, and Gondolella sublanceolata Gunnell .......... 2.1 m 6. Grey silicified siltstone and thin layers of micritic limestone. Three volcanic ash layers are found in the upper third of this bed .......... 1.2 m 7. A band of silty brownish-grey, finegrained dolomite with conchoidal cleavage (0.3 m thick) forms the bot- Usolka section (southern Urals, Russia): a potential candidate for GSSP to define the base of the... 209 tom of this bed; overlain by brownish-grey siltstone with numerous calcareous and phosphatic small concretions (3–5 cm) and one orange-yellow 3 cm thick ash layer. The upper part of the bed consists of grey dolomitized packstone with one ash layer near the top. The packstone contains smaller fora-minifers, poorly preserved fusulinids, brachiopods, crinoids and conodonts. The latter are Streptognathodus au-ritus Chernykh, 2005, St. elegantulus (Stauffer & Plummer), St. luganicus Kozitskaya, St. pawhuskaensis (Harris & Hollingsworth), St. simulator Ellison, Idiognathodus pictus Chernykh, 2005 and Id. sagittalis Kozitskaya. The following species are found among fu-sulinids: Pseudofusulinella eopulchra (Rauser), Ps. usvae (Dutkevich), Qua-sifusulina cf. elegantula Schlykova, Rauserites dictiophorus (Rosovskaya), R. shikhanensis (Rosovskaya), R. cf. cybea (Putrja) .......... 2.6 m 8. Alternation of dolomitic marl and grey foliated siltstone. A lens of graded grainstone occurs 10 cm above the base of the bed. Five volcanic ash layers are recognized within bed 8. The lower part of the grainstone contains fish remains, brachiopods, and diverse fusulinid and conodont assemblages. Conodonts: Idiognathodus lobulatus Kozitskaya, Id. tersus Ellison, Strep-tognathodus simulator Ellison, St. ec-centricus Ellison, St. pawhuskaensis (Harris & Hollingsworth), St. elegan-tulus (Stauffer & Plummer), St. in-signitus Akhmetshina. Fusulinids: Quasifusulina eleganta Schlykova, Rauserites stuckenbergi (Rauser), R. petchoricus (Rauser & Belyaev), R. triangulus (Rosovskaya), R. elongatis-simus (Rosovskaya), R. mogutovensis Rosovskaya, R. noinskyi (Rauser), R. tjanshanensis (Bensh), R. samaricus (Rauser), R. sphaericus Rosovskaya, R. variabilis (Rosovskaya), Schwagerini-formis kurshabensis (Bensh), Sch. per-stabilis (Scherbovich), Sch. baisunensis (Bensh), Sch. fusiformis (Bensh), Daix-ina rugosa Rosovskaya .......... 2.2 m. Analysis In the described succession (Fig. 2), bed 1 was not sampled for either conodonts or foraminifers and therefore its age cannot be determined. However, because conodonts recovered 6 meters below the top of the Zilim Formation are Moscovian Neogna-thodus, bed 1 is in a transitional position between the Moscovian and Kasimovian stages. Beds 2 and 3 have yielded the typical Kasimovian conodonts Id. sagittalis Koz-itskaya and Streptognathodus cancellosus (Gunnell), and thus belong to the Kasimov-ian Stage. Bed 3 was not properly sampled in the past, so we will re-sample it in greater detail. In the lower part of bed 4 (4–1) at 4.7 meters above the base of the section (mab), the conodont species are more advanced than in bed 3, with St. zethus Chernykh & Reshetkova and St. pawhuskaensis Harris & Hollingsworth, of which the former marks the base of the regional Virgilian Stage in North America (Heckel, 2004). The newly described species St. praenuntius Chernykh, 2005, which closely resembles St. simulator, also occurs in this level (Fig. 2). The fusulinid assemblage that is found at the same level is usually characteristic of the late Kasimo-vian, although some species range up into the early Gzhelian. In the upper part of bed 4 (4–2), starting from 4.9 mab, conodonts St. elegantulus and St. simulator occur. The latter species, as mentioned above, is the traditional index for determining the base of the Gzhelian Stage in the Moscow Basin and the Urals and is recently proposed index of the base of the global Gzhelian Stage (Heckel et al., 2005). Upwards, in bed 5 at 6.4 mab, the conodont assemblage is very similar to that from bed 4–2, except that new and more advanced forms that resemble St. simulator occur. These forms were recently described as a new species St. auritus Chernykh, 2005 (Figs. 2–3). No fusulinids were found in beds 5 or 6, and the fusulinids that were recovered from bed 7 (8.2 mab) are poorly preserved. In bed 8 (10.8 mab), the typical Gzhelian species Rauserites stuckenbergi and Daixina rugosa were recovered among other fusulinid species (Fig. 2). Discussion The traditional base of the Gzhelian in the Moscow Basin was proposed by Nikitin (1890) at the base of the Rusavkino unit in a limestone near Gzhel village that discon-formably overlies the Troshkovo unit of the Kasimovian (Ivanova & Khvorova, 1955). Makhlina et al. (1979) recognized four sed- 210 Valery V. Chernykh, Boris I. Chuvashov, Vladimir I. Davydov, Mark Schmity & Walter S. Snyder Transitional from St. simulator to S. auritus St. simulator St. praenuntius K as i mo vi an Idiognathodus Figure 3. Evolutionary trend within the chronocline of St. simulator and related species. imentary cycles within the Rusavkino. However, more recently the Rusavkino has been divided into three cycles: Lower, Middle, and Upper Rusavkino (Heckel et al., 2005). The conodonts Streptognathodus zethus, St. firmus and St. pawhuskaensis are reported in the Lower and Middle Rusavkino, and Streptognathodus simulator is added to this assemblage in the Upper Rusavkino. The younger Amerevo unit contains the con-odonts St. ruzhenzevi and St. vitali (Heckel et al., 2005). No fusulinids were shown for the Lower and Middle Rusavkino (although the presence of fusulinids in the lowermost Rusavkino was noted by Makhklina et al., 1979). Rauserites rossicus is recorded in the Upper Rusavkino, and Rauserites stuck-enbergi in the Amerevo unit. Based on this observation, the FAD of St. simulator in the Usolka section at 4.9 mab corresponds to the Upper Rusavkino and the newly redefined base of the Gzhelian Stage. In the Usolka section (Fig. 2) the FAD of St. simulator is recorded within the evolutionary morphocline (Fig. 3): Streptogna-thodus praenuntius – St. simulator – St. auritus within 2.7 m of beds 4 and 5. The undisturbed character of sedimentation in Plate 1 (scale bar 300 microns) Conodonts from uppermost part of the Kasimovian Stage, bed 4/1. IGG – Institute Geology and Geochemistry Uralian Branch of Russian Academy of Sciences, Ekaterinburg, Russia. I, 2 Idiognathodus sagittalis Kozitskaya, 1978. 1 – IGG U12-6; 2 – IGG U12–21 3, 4 Swadelina sp. 3 – IGG U12–3; 4 – IGG U12–5, specimen with weakly developed median trough 5 Streptognathodus ?ancellosus Gunnell, 1933. IGG U12–27, juvenile form (Note that scale bar for this specimen slightly enlarged) 6. Idiognathodus delicatus Gunnell, 1931. IGG U12–18 7, 8 Gondolella merrilli Gunnell, 1933. 7 – IGG U12–12; 8 – IGG U12–20 9 Streptognathodus gracilis Stauffer et Plummer, 1933. IGG U12–16 10 Streptognathodus pictus Chernykh, 2005. IGG U15–35 II, 12 Streptognathodus zethus Chernykh et Reshetkova, 1987. 11 – IGG U15–20; 12 – IGG U15–30 13-15 Streptognathodus praenuntius Chernykh, 2005. 13 – U13–23; 14 – IGG U13–11; 15 – IGG U13–11; (note that scale bar for this specimen slightly reduced) Usolka section (southern Urals, Russia): a potential candidate for GSSP to define the base of the... 211 212 Valery V. Chernykh, Boris I. Chuvashov, Vladimir I. Davydov, Mark Schmity & Walter S. Snyder this interval and the data on conodont evolution suggest that there are no documented breaks in sedimentation during this transition, and allow the Usolka section to be considered as a candidate for the Global Stratotype Section and Point (GSSP) for the base of the Gzhelian Stage. St. praenuntius in the established chronocline possess strong similarity to St. simulator, but differs from the latter in a less eccentric and less well developed trough along the medial line of the platform element (Fig. 2). The phylogenetic evolutionary relation between Streptogna-thodus praenuntius and St. simulator is proposed because: (1) these two species appear in successive order, and (2) there are numerous transitional forms from an almost undeveloped and nearly symmetric trough (assigned to St. praenuntius) to those with the clear and well-developed trough that is significantly shifted toward one side of the platform (assigned to St. simulator). This trend indicates that Streptognathodus prae-nuntius is the most probable ancestor of St. simulator. A very similar trend in conodont evolution is observed in Midcontinent North America. “Idiognathodus” n. sp. aff. Id. simulator (Barrick et al., 2004, pl. 5, fig. 11) from the Eudora Shale (middle Stanton cy-clothem) most probably belongs to St. prae-nuntius Chernykh. St. simulator there occurs in Heebner Shale of Oread Limestone (Oread-Heebner cyclothem). Streptognatho-dus praenuntius and St. simulator in North America occur in the same successive order as in the southern Urals. The next step in the evolutionary development of Strepto-gnathodus simulator resulted in the appearance of St. auritus Chernykh, 2005 (Fig. 3). The latter species has a slightly eccentric trough similar to St. simulator, but also has small nodes on one (inner) or both sides of the platform. Conclusions 1. Although the proposed definition of the base of the global Gzhelian Stage, the FAD of St. simulator is slightly above its traditional position in the Moscow region, this does not affect regional and interregional stratigraphy and correlation, and therefore would be supported and accepted by the geologic communities. 2. The Usolka section is one of the best candidates so far known for the GSSP that will define the global Gzhelian, although additional study is still required. No obvious interruptions in sedimentation are recorded within the Kasimovian-Gzhelian transition there. The chronocline that defines the FAD of St. simulator is established within 20 cm of undisturbed sedimentary sequence. Several volcanic ash beds are present below and above the proposed boundary, making radiometric calibration highly possible in the near future (Schmitz et al., 2005). 3. The relatively deeper water facies and the mode of preservation of conodonts with a CAI of around 1.0–1.5 suggest that chemostratigraphic and paleo-magnetic studies would be highly possible. The first steps in this direction, Sr isotope studies, are already in progress (Needham et al., 2006). 4. Accessibility presently is adequate, and we have an agreement with the Bashkirian Academy of Sciences via the Institute of Geology in Ufa, Bashkortostan, Russia, that this exposure will be improved and maintained permanently for interested scientists. Future access will be guaranteed by means of legislative action to create a scientific preserve. Plate 2 (scale bar 200 microns) Conodonts from lowermost part of the Gzhelian Stage, bed 4/2. 1–5 Streptognathodus simulator, Ellison, 1941. 1 – IGG U1–5, right element; 2 – IGG U1–7, left element; 3 – IGG U1–8, left element; 4 – IGG U1–28, left element; 5 – IGG U1–6, juvenile right element. 6, 7 Streptognathodus auritus Chernykh, 2005. 6 – IGG U1–29, form transition from S. simulator Ellison to S. auritus Chernykh; 7 – holotype IGG 1–18. 8, 9 Streptognathodus pictus Chernykh, 2005. 8 – holotype IGG U14–32, right element; 9 – IGG U14– 22, left element. 10 Swadelina sp., IGG U14–1. 11–12 Streptognathodus sinistrum Chernykh, 2005. 11 – IGG U1–11; 12 – holotype IGG U1–9. Usolka section (southern Urals, Russia): a potential candidate for GSSP to define the base of the... 213 214 Valery V. Chernykh, Boris I. Chuvashov, Vladimir I. Davydov, Mark Schmity & Walter S. Snyder 5. A better relationship among conodont, fusulinid, and ammonoid biozona-tions and other fossil groups must still be worked out. Acknowledgements This study has been supported from NSF EAR–0418703 and EAR–0510876, EAR–0545247 Help of our colleagues and students from Boise State University greatly appreciated. References Barrick, J. E., Lambert, L. L., Heckel, P. H. and Boardman, D. R. 2004: Pennsylvanian con-odont zonation for Midcontinent North America. – Revista Española de Micropaleontología, 36/2, 231–250. Barskov, I. S., & Alekseev, A. S. 1979: Con-odonts of the middle and upper Carboniferous of the Moscow Basin. – News of the Academy of Sciences of the USSR, series geological, 6, 84–99, (in Russian). Chernykh, V. V. , & Reshetkova, N. P. 1987: Biostratigraphy and conodonts of Carboniferous-Permian boundary beds of western slope of southern and central Urals. – Academy of Sciences of the U.S.S.R., Ural Scientific Center, Institute of Geology and Geochemistry Preprint, 1–50, Sverdlovsk, (in Russian). Chernykh, V. V. 2002: Zonal scale of the Gzhe-lian and Kasimovian Stages based on conodonts of genus Streptognathodus. In: B. I. Chuvashov & E. A. Amon (eds.), Stratigraphy and paleogeogra-phy of Carboniferous of Eurasia. 302–306, Ekaterinburg, (in Russian). Chernykh, V. V. 2005: Zonal method in bio-stratigraphy. Zonal Conodont Scale of the lower Permian in the Urals. – Institute of Geology and Geochemistry of RAN, 217 p., Ekaterinburg, (in Russian). Chuvashov, B. I., & Chernykh, V. V. 2002: Krasnousol’sky Section. In: B.I. Chuvashov (ed.), Guidebook of the geological excursions on the Carboniferous of Urals. Part 1. Southern Urals excursion. – Institute of Geology and Geochemistry, 18–33, Ekaterinburg. Chuvashov, B. I., Djupina, G. V., Mizens, G. A., & Chernykh, V. V. 1990: Key-sections of the Upper Carboniferous and Lower Permian of the western slope of the Urals and Preurals. – Uralian Branch of Rassian Academy of Sciences, 367 p., 41 pl., Sverdlovsk, (in Russian). Chuvashov, B. I., Djupina, G. V., Mizens, G. A., & Chernykh, V. V. 1993: Krasnouso-lsk section. In: B. I. Chuvashov, V. A. Chernykh, V. J. Kipnin, V. A. Molin, V. P. Ozhgibesov & P. A. Sofronitsky (eds.), Permian System: Guides to Geological Excursions in the Uralian Type Localities – Jointly published by Uralian Branch Russian Academy of Sciences, Ekaterinburg, Russia and ESRI, Occasional Publications ESRI, University of South Carolina, New Series 10, 45–70. Chuvashov, B. I., Davydov, V. I., Chernykh, V. V., Northrup, C. J., Snyder, W. S., & Ward-law, B. R. 2002: Cisuralian (Lower Permian) Time-Scale in Southern Urals. – Geological Society of America, Denver, Colorado, Abstracts with Program, Paper 61–2, 134, Denver, USA. Davydov, V. I. 1996: Fusulinid biostratigraphy and correlation of Moscovian-Guadalupian North American, Tethyan and Boreal (Russian Platform/ Uralian) standards. – Permophiles, 29, 47–52. Davydov, V. I., Chernykh, V. V. , Chuvash-ov, B. I., Northrup, C. J., & Snyder, W. S. 2002: Volcanic ashes in the upper Paleozoic of the southern Urals: new perspectives in the Pennsyl- Plate 3 (scale bar 10 mm; magnification for all figures x20, except 4, 15 and 17, that is x 15) Figures 1-9: bed 4-1 (4.7 meters above the base of the section [mab]) I Schwageriniformis (Tumefactus)? sp.. USO-4-6BCh. 2, 7 Schwageriniformis schwageriniformis mosquensis (Rosovskaya). 2 - USO-4–11BCh; 7 - USO-4-4BCh. 3 Pseudofusulinella pulchra (Rauser-Chernousova). USO-4-5BCh. 4 Quasifusulina brevis Brazhnikova. USO–4–10BCh. 6 Schwageriniformis baisunensis (Bensh). USO–4–2BCh. 8 Rauserites shikhanensis compactus (Rosovskaya). USO-4-12BCh. 9 Rauserites bashkiricus (Rosovskaya). USO-4–1BCh. Figures 10-13: bed 7 (8.2 mab) 10 Rauserites shikhanensis (Rosovskaya). USO-7-6aVD. II Rauserites dictiophorus (Rosovskaya). USO-7-6bVD. 12 Rauserites cf compactus (Rosovskaya). USO-7-3VD. 13 Pseudofusulinella usvae (Dutkevitch). USO-7-2VD. Figures 14-17: bed 8 (10.8 mab) 14 Schwageriniformis sp. 1. USO-8-17VD. 15, 17 Schwageriniformis schwageriniformis mosquensis (Rosovskaya). 15 - USO-8-12BCh; 17 - USO-8- 16 29VD. Schwageriniformis kurshabensis (Bensh). USO-8-3VD. Usolka section (southern Urals, Russia): a potential candidate for GSSP to define the base of the... 215 216 Valery V. Chernykh, Boris I. Chuvashov, Vladimir I. Davydov, Mark Schmity & Walter S. Snyder vanian time scale calibration. In: B. I. Chuvashov & E. A. Amon (eds.), Stratigraphy and paleoge-ography of Carboniferous of Eurasia. - 112-122, Ekaterinburg, (in Russian), extended abstract in English, 123. Davydov, V. I., & Leven, E. Ya. 2003: Correlation of Upper Carboniferous (Pennsylvanian) and Lower Permian (Cisuralian) Marine Deposits of the Peri-Tethys. In: M. Gaetani (ed.), Peri-Te-thys Program. - Palaeogeography Palaeoclima-tology, Palaeoecology 196, 1-2, 39-57. Davydov, V. I., & Popov, A. V. 1986: Upper Carboniferous and Lower Permian sections of the Southern Urals. Carboniferous/Permian Boundary beds of the Urals, Pre-Urals area and Central Asia. - Nauka, 29-33. Davydov, V. I., & Popov, A. V. 1991: The Carboniferous system. In: T. N. Koren (ed.), Zonal stratigraphy of the Phanerozoic of the USSR. - Nedra, 64-76, (in Russian). Ellison, S. P. Jr. 1941: Revisions of Pennsylvanian conodonts. - Journal of Paleontology, 15, 107-143. Heckel, P. H. 2004: Updated Cyclothem grouping chart and observations on the grouping of Upper Pennsylvanian cyclothems in Midconti-nent North America. - Newsletter on Carboniferous stratigraphy, 22, 18-22. Heckel, P. H., Alekseev, A. S. & Nemy-rovska, T. I. 1998: Preliminary conodont correlations of late Middle to early Upper Pennsylvanian rocks between North America and eastern Europe. - Newsletters on Carboniferous Stratigraphy, 16, 8-12. Heckel, P. H., Alekseev, A. S., Barrick, J. E., Boardman, D. R., Chernykh, V. V, Davydov, V. I., Forke, H. C, Goreva, N. V., Lup-pold, F. W., Mendez, C. A., Nemyrovska, T. I., Ueno, K., Villa, E., & Work, D. M. 2005: Cyclothem [sequence-stratigraphic] correlation and biostratigraphy across the Moscovian-Kasimo-vian and Kasimovian-Gzhelian Stages (Upper Pennsylvanian Series) in North America and Eurasia. - Newsletter on Carboniferous stratigraphy, 23, 37-44. Ivanova, E. A. & Khvorova, I. V. 1955: Stratigraphy of the Middle and Upper Carboniferous of the western part of the Moscow syneclise. - Transactions of Paleontological Institute, 53, p. 279, (in Russian). Leven, E. Ya., & Davydov, V. I. 2001: Stratigraphy and fusulinids of the Kasimovian and Lower Gzhelian (Upper Carboniferous) in the Southwestern Darvas (Pamir). – Rivista Italiana di Paleontologia e Stratigrafia, 107, 1, 3–46. Makhlina, M. Kh., Kulikova, A. M., & Nikitina, T. A. 1979: Stratigraphy, biostratigraphy and paleogeography of Upper Carboniferous of the Moscow Syneclise. In: M. Kh. Makhlina & C. M. Shik (eds.), Stratigraphy, paleontology and paleogeography of Carboniferous of the Moscow Syneclise. – Transactions of Geological foundation of Ministry of Geology of Russian Federation, 25–69, (in Russian). Menning, M., Alekseev, A. S., Chuvashov, B. I., Davydov, V. I., Devuyst, F.–X., Forke, H. C., Grunt, T. A., Hance, L., Heckel, P. H., Izo-kh, N. G., Jin, Y.–G., Jones, P. J., Kotlyar, G. V. , Kozur, H. W., Nemyrovska, T. I., Schneider, J. W., Wang, X.–D., Weddige, K., Weyer, D. & Work, D. M. 2006: Global time scale and regional stratigraphic reference scales of Central and West Europe, East Europe, Tethys, South China, and North America as used in the Devonian-Carboniferous-Permian Correlation Chart 2003 (DCP 2003). – Palaeogeography, Palaeoclimatology, Pa-laeoecology, 240, 1–2, 318–372. Needham, L., Schmitz, M. D., & Davydov, V. I. 2006: A Precise and Accurate Seawater Sr Curve From Late Carboniferous-Early Permian Conodonts. – Geological Society of America, Abstracts with Programs, 38, 7, 184. Nikitin, S. N. 1890: The Carboniferous of the Moscow Basin and artesian water in the region of the Moscow Basin. – Transactions of Geological Committee, 5, 5, p. 138 (in Russian), 139-182 (in French). Schmitz, M. D., Davydov, V. I., & Snyder, W. S. 2005: New ID–TIMS U–Pb Zircon Ages Bracketing The Asselian-Sakmarian Stage Boundary (Lower Permian), Southern Urals – Russia, Ka-zaskhstan. – Geological Society of America, Abstracts with Programs, 37, 7, p. 483. Villa, E., and Task Group. 2005: Report of the Task Group to establish GSSPs at the Moscovian-Kasimovian and Kasimovian-Gzhelian boundaries. – Newsletter on Carboniferous stratigraphy, 23, 9–10. Wardlaw, B. R., Leven, E. Ya., Davydov, V. I., Schiappa, T. A. & Snyder, W. S. 1999: The base of the Sakmarian Stage: call for discussion (possible GSSP in the Kondurovsky section, southern Urals, Russia). – Permophiles, 34, 19–26. Plate 4 (scale bars 10mm, magnification for all figures x 15, except 2 and 3 that are x 10) All figures are from bed 8 (10.8 meters above the base of the section) I Pseudofusulinella pulchra (Rauser–Chernousova). USO–8–34BCh. 2, 3 Quasifusulina eleganta (Schlykova). 2 – USO–8–4BCh; 3 – USO–8–9VD. 4–5, 7 Rauiserites elongatissimus (Rosovskaya). 4 – USO–8–22VD; 5 – USO–8–12VD; 7 – USO–8–4VD. 6 Rauiserites triangulus (Rosovskaya). USO–8–2VD. 8 Rauiserites sphaericus (Rosovskaya). USO–8–11VD. 9 Rauiserites samaricus (Rauser–Chernousova). USO–8–10VD. 10 Rauiserites mogutovensis Rosovskaya. USO–8–5VD. II Rauiserites tjanshanensis (Besnh). USO–8–7VD. 12–14 Rauiserites stuckenbergi (Rauser–Chernousova). 12 – USO–8–21BCh; 13 – USO–8–19BCh; 14 – USO–8–17BCh. 15 Daixina rugosa (Rosovskaya). USO–8–13VD. Usolka section (southern Urals, Russia): a potential candidate for GSSP to define the base of the... 217