The Carbonate Massif of Voskresenka Mount in the ... - Springer Link

ISSN 0869-5938, Stratigraphy and Geological Correlation, 2018, Vol. 26, No. 2, pp. 139–156. © Pleiades Publishing, Ltd., 2018. Original Russian Text © E.N. Gorozhanina, V.M. Gorozhanin, T.N. Isakova, 2018, published in Stratigrafiya, Geologicheskaya Korrelyatsiya, 2018, Vol. 26, No. 2, pp. 21–37.

The Carbonate Massif of Voskresenka Mount in the Southern Pre-Urals: Age and Development of the Submerged Carbonate Platform E. N. Gorozhaninaa, V. M. Gorozhanina, *, and T. N. Isakovab aInstitute

of Geology, Ufa Science Center, Russian Academy of Sciences, ul. Karla Marksa 16/2, Ufa, 450077 Russia b Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russia *e-mail: [email protected] Received September 26, 2016; in final form, December 20, 2016

Abstract—In the conjunction zone of the East European Platform and the Uralian foredeep, involved in structures of the Southern Urals (Bashkiria), sediments deposited at the shelf zone edge in the Late Carboniferous–Early Permian crop out. The Upper Carboniferous bioherm and Lower Permian deep marine–shelf boundary limestones, composing Voskresenka Mount near Tabynsk township, were studied. Results of the complex analysis of lithofacies, paleontological, structural, and also geological and geophysical data show that the Voskresenka carbonate massif, previously attributed to a single reef structure, represents the SW-dipping tectonic horst block, composed of Upper Carboniferous shelf–bioherm limestones, which is uplifted in a near break zone. As a result of tectonic processes, the edge of the late Carboniferous carbonate platform, overlain by Asselian deep-water sediments, was exhumed. The sedimentary succession shows that the paleogeographic setting at the margin of the East European Craton changed at the Carboniferous–Permian boundary during the formation of the Ural collisional orogen. Keywords: Late Carboniferous, Early Permian, biostratigraphy, zoning, Southern Urals, Uralian foredeep, shelf, reef, carbonate platform, tectonic block DOI: 10.1134/S0869593818010045

INTRODUCTION The carbonate massif of Voskresenka Mount is located within the Belskaya Depression of the Uralian foredeep on the territory of Bashkiria (Southern Urals) (Fig. 1). Along the western side of the foredeep, in the conjunction zone with the Eastern European Platform, lies the chain of early Permian Ishimbaytype reef structures (Kulagina et al., 2015), extending along the entire margin of the platform, including its southern periphery within the Caspian Basin (Geologicheskoe…, 1997). At the base of the Asselian structures usually lie buried Upper Carboniferous sediments (Chuvashov, 1985, 1998; Chuvashov et al., 1990; Antoshkina, 2003). Outcrops of Upper Carboniferous carbonate formations are known in the northern part of the basin: in the Sylva (Orel Cliff in the village of Kyn) and Solikamsk depressions (Chuvashov, 1985), and also in the Pechora Urals (Antoshkina, 2003). The Voskresenka carbonate massif, exposed at the latitude of the Tabynsk township, north of the Krasnousolsky township, is a unique object of study in the Southern Pre-Urals. This is due to its accessibility and picturesque views over the small Belaya Mount, which towers above the Voskresenka River. Carbonate rocks

form a 70 m ridge extending in sublatitudinal direction for a distance of about 1 km with a low-angle northeastern slope and a steeper southwestern slope (Fig. 2). The eastern part represents a rock ledge. It is believed that light massive limestones are of a reef origin and they compose a chain of separate outcrops (Putevoditel’…, 1975, 1984; Alksne, 1999; Alekseev et al., 2010). In the late 1960s, the Tabynsk oil field (1967–1968) was discovered near Voskresenka Mount. This oil field is still in operation. The field is confined to the Upper Devonian–Lower Carboniferous (Tournaisian Stage) carbonates, which form an asymmetric anticline structure with gently sloping eastern and steep western wings (Baimukhametov et al., 1997). The Voskresenka carbonate massif is located in the central elevated part of the Tabynsk structure. The fundamentals of biostratigraphy of the Voskresenka massif were developed by D.L. Stepanov (1951), O.L. Einor, and V.A. Aleksandrov on brachiopods, A.E. Alksne and M.V. Shcherbakova on foraminifers, and R.S. Furdui on conodonts, taking into account data on other groups of macrofauna (Putevoditel’…, 1975, Atlas…, 1979). When describing the section along the southern slope in the central part (the western block) of the mountain, V.A. Aleksan-

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drov and R.S. Furdui identified fusulinid zones belonging to the upper part of the Kasimovian and Gzhelian stages (Aleksandrov in Putevoditel’…, 1975). However, the data obtained later as a result of further appraisal of fusulinids (Alksne, 1999, Isakova in Alekseev et al., 2010) showed the possibility of establishing the fusulinid zones of the Kasimovian Stage and only the lower part of the Gzhelian Stage. At the same time, the study of conodonts revealed the occurrence of late Gzhelian and early Asselian species in crinoid limestones that overlie bioherm deposits (Alekseev et al., 2010). The Voskresenka massif as a Late Carboniferous reef complex, located in the Uralian foredeep, was repeatedly demonstrated to the participants of a number of field excursions, in particular, during the VIII International Congress on Carboniferous Stratigraphy and Geology (Moscow, 1975) and the International Geological Congress (Moscow, 1984). The tasks of this work are to specify the age of bioherm limestones, to clarify their relationships with the overlying Asselian sediments, to determine the character of variations in the paleogeographic conditions at the Carboniferous–Permian boundary at the marginal part of the carbonate shelf paleozone cropping out in the southern part of the Uralian foredeep, and to develop the model of formation of the Voskresenka carbonate massif. This work is based on studying the

original collection of samples from the eastern and western blocks of the massif, as well as archival data (core description and well-log interpretation) from a number of boreholes drilled in the Tabynsk field. Dunham’s classification (Dunham, 1962) was used for the description of carbonate rocks. The taxonomy of fusulinids is given in accordance with the Reference Book on the Systematics of the Paleozoic Foraminifera (Spravochnik…, 1996). STRUCTURE OF VOSKRESENKA CARBONATE MASSIF Previous researchers have repeatedly noted the block structure of Voskresenka Mount (Aleksandrov in Putevoditel’…, 1975; Alekseev et al., 2010). According to these data, the Voskresenka River valley follows the sublatitudinal fault zone, the branches of which detach the Voskresenka massif into blocks (Fig. 2c). It seems that this fault is feathering relative to a large submeridional fault zone, which runs parallel to the Belaya River valley and controls the structural position of the Tabynsk anticline (Baimukhametov et al., 1997). A series of faults and feathering fractures divides the territory into diamond-shaped blocks, which are deciphered by a satellite image (Fig. 2c). Upper Carboniferous limestones of Voskresenka Mount compose the

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100 m Fig. 2. Geological structure of Voskresenka mount. (a) According to Alekseev et al., 2010; (b) with the additions of authors; legend: 1, slope–depression crinoid limestones of late Gzhelian–Asselian age (С3g2–P1a); 2, bioherm limestones of Kasimovian– Gzhelian age (С3k–g1); 3, proposed faults; (c) block structure of Voskresenka Mount according to the data of satellite image interpretation; legend: (Á) dip and strike of rock layers, (dashed lines) proposed faults, (circles) the position of boreholes. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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block, elevated in the near break zone and segmented into smaller blocks. The dip directions of limestones on Voskresenka Mount and overlying clayey–siltstone sediments of the Artinskian Stage that crop out in quarries and excavations at the northern foot of the mountain do not coincide. Artinskian beds dip to the northwest (Az. 330°–350°) at an angle varying from 10° to 30°. The beds on Voskresenka Mount dip mainly to the southwest with dip angles varying from 20° to 40°. This indicates that we are dealing with the anticlinal structure of sublatitudinal strike with the subvertical displacement of blocks along the faults. The western closure of the Voskresenka massif is the most studied and visited part. The western block is usually demonstrated to participants of geological excursions. The eastern part of Voskresenka Mount represents a block where white bioherm limestones form a high rocky ledge (Fig. 3 (1)). The central part of Voskresenka Mount is poorly exposed. On the southern slope, the dip angle of beds is close to that of mountain slope, which gives the impression of massiveness of limestones. The northern slope of the mount is gently sloping and grass-covered. At the base of the northern slope of Voskresenka Mount, overlying carbonate–terrigenous sediments of the Artinskian Stage are exposed in a road ditch (Fig. 2c). Eastern Block Under a detailed examination of a rocky ledge on the east side, one can observe the layered-stratified structure of limestone beds. It seems that these are folded into a small fold of sublatitudinal strike, the hinge of which occurs as a rocky ledge in the apical part (Fig. 3 (2)). From the top down, several layers can be distinguished. The upper layer 10–15 m thick is composed of massive light limestones with calcite incrustation. It is underlain by darker distinctly layered crinoid limestones represented by alternation of crinoid grainstones and dark gray pelitomorphic wackestones 3–5 cm thick. The layers are dipping southwest (Az. 230°, dip angle 35°, according to V.A. Aleksandrov in Putevoditel’…, 1975). Below, there are separate outcrops of massive bryozoan and brachiopod limestones (baundstones and rudstones) and layered crinoid limestones (grainstones and wackestones). In the eastern block, several lithotypes of rocks are distinguished: bioherm bryozoan limestone with

incrustation cement (baundstones and rudstones), brachiopod limestones (bafflestone), recrystallized limestones with ammonoids, crinoids, and rare orthoceratids (grainstones and rudstones), and layered crinoid limestones with gray micrite matrix (wackestones). The rocks are light, intensely recrystallized, and fractured. In part, they are transformed into carbonate milonites. Sometimes, it is difficult to distinguish the recrystallization structures and cleavage cracks incrusted with calcite druses in these rocks from the stromataxis structures, associated with the filling of primary voids between bryozoan and shell fragments in the matrix with cement. In the upper part of Voskresenka Mount, bryozoan limestones of bioherm type (bryozoan baundstones and rudstones) crop out. Owing to a large amount of bryozoan skeletons, they form the basic structure of the rock (Fig. 3 (3, 4)). The apparent thickness of bryozoan limestones is about 10 m. They overlie a unit of crinoid limestones– grainstones (Fig. 3 (5)). Light brachiopod-containing limestones are observed in rare outcrops and in talus at the foot of the mountain. Similar brachiopod limestones occur in the talus on the slope of a forested low hill (Lokhmataya Mount), which is apparently another exposure (southeastern block) of bioherm limestones (Fig. 2c). The studied samples from the eastern peak of Voskresenka Mount (Belaya Mountain) are represented by detrital and finely fragmented limestones, mainly bryozoan; algae are rare; there are single fragments of cyanobacterial fouling Tubiphytes sp. The following foraminiferal assemblage was established (Plate I): in the upper part of the exposure—Eolasiodiscus sp., Eotuberitina maljavkini (Mikhailov), Tetrataxis sp., Diplosphaerina grandis (Reitlinger), Palaeonubecularia rustica Reitlinger; 20 m down slope—Triticites ex gr. simplex (Schellwien), Rauserites cf. tabinicus (Alksne), R. atelicus (Rauser), R. dictyophorus (Rosovskaya), Usvaella usvae (Dutkevich), Quasifusulina sp., Schubertella sp., Tetrataxis sp., Eolasiodiscus sp., Pseudoglomospira vulgaris (Lipina), Palaeonubecularia rustica Reitlinger, Ammovertella sp., Tolypammina sp. The foraminiferal assemblage is unrepresentative, evidently of late Kasimovian–early Gzhelian. The sample from the lower part of the outcrop yielded only one median section of the fusulinid shell, which indicates the probable occurrence of the genus Rauserites. However, according to A.V. Aleksandrov (Putevoditel’…, 1975), interlayers containing

Fig. 3. Typical facies of limestones of the Voskresenka carbonate massif. (1–5) Eastern block (eastern peak of Voskresenka Mount): (1) view from the south; (2) outcrops of limestones in the top part, view from the east; (3–5) bioherm limestones (С3k–g1): (3, 4) bryozoan baundstone and rudstone; (3) image of outcrop, image length of 10 cm; (4) micrograph of the thin section, parallel nicols, image length of 2 mm; (5) layered crinoid limestones, image of outcrop, image length of 30 cm; (6‒8) western block, layered limestones (С3g2–P1a): (6) layered crinoid limestones from a cave on the southern slope of Voskresenka Mount; (7) large-crinoid limestones (grainstone) on the cave wall, image length of 12 cm; (8) patterned silicified and recrystallized crinoid–bioclastic limestones (wackestone) from the quarry wall on the southwestern slope, micrograph of the thin section (parallel nicols), image length of 2 mm. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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Jigulites longus longus (Rosovskaya), J. longus mucronatus (Rosovskaya), Schwageriniformes schwageriniformis nanus (Rosovskaya), Rauserites dictyophorus (Rosovskaya), and some other taxa were distinguished in the eastern block (brecciated limestones at the lower part of the slope), and this assemblage indicates the Gzhelian age of these sediments. Thus, taking into account the previous results of studying fusulinids in the eastern block (Putevoditel’…, 1975), as well as our results, one can suggest that the bioherm massif is of Kasimovian–early Gzhelian age. The character of the dip of layers and an analysis of bedding elements indicate that bioherm light limestones of the eastern block lie below the layered sequence of the western block (Fig. 2), that is, below point 29 or at the same level, where the lower Gzhelian conodonts were previously determined (Sample BC-8 according to Alekseev et al., 2010). This gives grounds for assuming that a successive sequence of carbonate sediments can be traced from the east to the west. It is possible that the layers are folded into the near break folds, which cause variations in the dip and strike of rocks. Western Block The rocks on the western side of Voskresenka Mount are represented mainly by layered brownish gray crinoid limestones (Fig. 3 (6–8)). These rocks are underlain by light gray bryozoan–brachiopod limestones that is observable on the northern wall of a cave located in the middle of the southern slope of the mountain (Putevoditel’…, 1975, 1984). The dip and strike of rocks (Az. 240°, dip angle 40°) are nearly the same as in the eastern block. In some separate interlayers of foraminiferal algal limestones cropping out below the cave at the lowermost part of the succession of the western block, the late Kasimovian fusulinid assemblage, including Triticites secalicus (Say), T. shikhanensis Rosovskaya, Rauserites dictyophorus (Rosovskaya), R. exilis (Rosovskaya), Schwageriniformes schwageriniformis (Rauser), etc., was previously identified. At 4 m above, the early Gzhelian fusulinid assemblage containing Rugosofusulina cf. uralensis Rosovskaya, R. ex gr. praevia Shlykova, Rauserites variabilis

(Rosovskaya), and some others was identified in an interlayer containing large foraminifers (Putevoditel’…, 1975). There occur cyanobacteria (Cyanophyta), as well as Girvanella permica, green algae (Chlorophyta: Globuliferoporella symmetrica, Atractyliopsis carnica), and purple algae (Rhodophyta: Donezella intertexta) (Atlas…, 1979). In addition, trilobite Griffithides uralicus Konstantinenko (Atlas…, 1979) was found in the lower part of the Gzhelian Stage, which is attributed to the Rauserites stuckenbergi (C3C) Zone. In layered limestones of the upper part of the western block, there are no fusulinides. The recent studies of foraminifers and conodonts (Alekseev et al., 2010) made it possible to obtain a microfaunistic characteristic of the western block of the Voskresenka carbonate massif (Fig. 2a). At the lower part of the slope (below the cave), the organogenic limestones, including the microfauna of fusulinides and conodonts, incorporate the lower Gzhelian fusulinid assemblage. This complex is characterized by the predominance of the species of the genus Rauserites: R. postarcticus (Rauser), R. voskresenicus (Alksne), R. cf. modificatus Rosovskaya, R. aff. lucidus (Rauser), R. tabinicus Alksne; also, there are Triticites umbus Rosovskaya, T. parvulus ishimbaji Rosovskaya, and T. cf. shikhanensis Rosovskaya. The taxonomic composition of the assemblage is supplemented by Rugosofusulina flexuosa Rosovskaya, Quasifusulina aff. tenuissima (Schellwien), and Quasifusulinoides fusulinoides brevis (Putrja) (Plate II), characteristic of the lower zone of the Gzhelian Stage. A conodont assemblage with the dominant species Streptognathodus pawhuskaensis Harris et Hollingsworth, typical of the upper part of the Kasimovian Stage and the lower part of the Gzhelian Stage, was identified at the same level. Upward (at the level of the cave), bioherm limestones are overlapped by layered crinoidal limestones (Fig. 2a), not containing fusulinids, but with conodonts attributed to the upper Gzhelian Stage and the lower parts of the Asselian Stage. Conodonts (Streptognathodus aff. Lanceatus Chernykh, S. constrictus Chernykh et Reshetkova) belonging to the middle and upper parts of the Asselian Stage were identified in dark gray coarse-crinoidal layered limestones opened in a quarry within the western block (Alekseev et al.,

Plate I. Fusulinids from the eastern block of the Voskresenka carbonate massif. All specimens were collected in the late Kasimovian–early Gzhelian sediments. Figs. 1–5, ×15 (scale bar 0.5 mm); figs. 6, 7, 9–11, ×100 (scale bar 0.1 mm); figs. 8, 12–14, ×50. (1, 2) Rauserites dictyophorus (Rosovskaya, 1950): (1) axial section, spec. no. 4915/1 GIN RAS, Sample Vos-06; (2) median section, spec. no. 4915/2 GIN RAS, Sample Vos-05; (3) Rauserites atelicus (Rauser, 1958), oblique section, spec. no. 4915/3 GIN RAS, Sample Vos-06; (4) Usvaella usvae (Dutkevich, 1932), paraxial oblique section, spec. no. 4915/4 GIN RAS, Sample Vos-06; (5) Rauserites tabinicus (Alksne, 1979), paraxial oblique section, spec. no. 4915/5 GIN RAS, Sample Vos-06; (6, 7) Eolasiodiscus sp.: (6) spec. no. 4915/6 GIN RAS, Sample Vos-06; (7) spec.. no. 4915/7 GIN RAS, Sample Vos-06; (8) Syzrania confusa Reitlinger, 1950, spec. no. 4915/8 GIN RAS, Sample Vos-05; (9) Eotuberitina maljavkini (Mikhailov, 1939), spec. no. 4915/9 GIN RAS, Sample Vos-05; (10, 11) Pseudoglomospira vulgaris (Lipina, 1949): (10) spec. no. 4915/10 GIN RAS, Sample Vos-05; (11) spec. no. 4915/11 GIN RAS, Sample Vos-05; (12) Ammovertella sp., spec. no. 4915/12 GIN RAS, Sample Vos-05; (13) Tolypammina sp., spec. no. 4915/13 GIN RAS, Sample Vos-05; (14) Palaeonubecularia rustica Reitlinger, 1950, spec. no. 4915/14 GIN RAS, Sample Vos-05. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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2010). In lower Permian deposits of the Voskresenka massif, trilobites Neoproetus bashkiricus Konstantinenko were found (Atlas…, 1979). The occurrence of conodonts belonging to the Kasimovian, Gzhelian, and, possibly, Asselian stages in sediments of the Voskresenka carbonate massif (without accurate location) was previously recorded. R.S. Furdui (Atlas…, 1979) described Streptognathus gracilis Stauffer et Plummer from deposits of the Kasimovian Stage, S. elegulus Staufer et Plummer and S. simulator Ellison from sediments of the Gzhelian Stage, and S. elongatus Gunnell possibly from deposits of the upper part of the Gzhelian Stage. Thus, on the basis of the study of conodonts, layered limestones compose a continuous condensed section in the range from the upper Gzhelian Stage of the Upper Carboniferous to the Asselian Stage represented by deeper water facies of the shelf slope. Besides crinoids, limestones also contain solitary corals and brachiopod and ammonoid shells. Rocks are intensely recrystallized, leached, and silicified (Fig. 3 (8)). ANALYSIS OF CORE AND SEISMIC DATA According to the core data, the shelf type of the Upper Carboniferous sequence is widely developed on the platform westward, where its thickness reaches 200–300 m. Deposits are represented by dolomites and limestones with an abundance of faunistic remains. In some areas along the margin of the Uralian foredeep, there are small reef structures (Syundyukov, 1975). Within the Uralian foredeep, the facies zoning in Upper Carboniferous sediments is traced from west to east. Four types of sections are distinguished: shelf section, composed of layered carbonates (Ishimbay); reef (Voskresenka) section; clayey–carbonate (depression) section; and flyschoid (Sakmara-Ik) section. The Voskresenka (reef) type of the section is exposed on Voskresenka Mount and is opened by some boreholes in the Tabynsk field near this mountain and to the south in the Karly field and in the area of Tra-Tau Mount near the town of Sterlitamak. The deposits are represented by layered limestones with interlayers of reef (bioherm) limestones, including those composed of algae and bryozoans (Alksne and Shamov, 1975). An analysis of core and seismic data shows that the distribution in area of different

types of Upper Carboniferous sediments in the Uralian foredeep is mainly controlled by a structure factor. Boreholes of the Tabynsk Field The Tabynsk oil-bearing field is located in the structural and tectonic zone of the near-fault anticline structures stretching from the village of Bakrak in the north to the Krasnousolsky township in the south. In 1966– 1972, PJSC Bashneft drilled a number of boreholes with core sampling in the environs of Voskresenka Mount within the Tabynsk oil-bearing field (Figs. 2c, 4). Later, two boreholes, 772 and 773, were drilled in the Voskresenka massif. In Borehole 773 drilled at the foot of the northern slope of Voskresenka Mount and Borehole 772 drilled at the top of the massif, Upper Carboniferous bioherm limestones with a thickness varying from 100 to 140 m were opened. An analysis of the distribution of fusulinids through the section (Alksne, 1999) showed that the Upper Carboniferous sequence of Voskresenka Mount includes all three fusulinid zones of the Kasimovian Stage. The lower zone Protriticites pseudomontiparus, Obsoletes obsoletus on the basis of the occurrence of fusulinid assemblage, which includes index species with predominance of Pulchrella pulchra Rauser and Usvaella usvae Dutkevich, was established in Borehole 773 (int. 65–133 m) and in Borehole 772 (int. 131–150 m). The microfaunistic characteristic of the lower part of the Kasimovian Stage is complemented by the find of conodonts Idiognathodus magnificus Stauffer et Plummer in Borehole 773 (int. 68.7–70.7 m and 102.2– 105.6 m) (Atlas…, 1979). The next zone Montiparus montiparus, the lower boundary of which is established by the first occurrence of the genus Montiparus, was distinguished in Borehole 773 (int. 38–65 m) and in Borehole 772 (int. 117–130 m). The upper zone of the Kasimovian Stage (Triticites acutus, T. quasiarcticus, according to A.E. Alksne (1999)) distinguished in Borehole 772 (int. 77–80 m) is characterized by the occurrence of Triticites subacutus Z. Mikhailova, Rauserites pseudoarcticus Rauser, Montiparus rhombiformis Rosovskaya, and M. umbonoplicatus (Rauser et Beljaev) (Atlas…, 1979). The conodonts Gondolella clarki Koike were found in sediments opened in int. 109.0– 115.5 m of Borehole 772, which is evidently attributed to the Zone Triticites acutus, T. quasiarcticus (Atlas…, 1979). It should be noted that the determinations of

Plate II. Fusulinids from the western block of the Voskresenka carbonate massif. All specimens were collected in the lower zone of the Gzhelian Stage Rauserites stuckenbergi, Rauserites rossicus (Sample VS-1); ×15 magnification in all cases. (1) Quasifusulina aff. tenuissima (Schellwien, 1898), spec. no. 4915/15 GIN RAS; (2, 3) Quasifusulinoides fusulinoides brevis (Putrja, 1947): (2) spec. no. 4915/16 GIN RAS, (3) no. 4915/17 GIN RAS; (4, 5) Rauserites voskresenicus (Alksne, 1979): (4) spec. no. 4915/18 GIN RAS, (5) no. 4915/19 GIN RAS; (6, 7) Rauserites postarcticus (Rauser, 1958): (6) spec. no. 4915/20 GIN RAS, (7) spec. no. 4915/21 GIN RAS; (8, 9) Rauserites tabinicus Alksne, 1979: (8) spec. no. 4915/22 GIN RAS, (9) spec. no. 4915/23 GIN RAS; (10–12) Triticites parvulus ishimbaji Rosovskaya, 1950: (10) spec. no. 4915/24 GIN RAS, (11) spec. no. 4915/25 GIN RAS, (12) spec. no. 4915/26 GIN RAS; (13) Rauserites aff. lucidus (Rauser, 1958), spec. no. 4915/27 GIN RAS; (14, 15) Rauserites cf. modificatus Rosovskaya, 1958: (14) spec. no. 4915/28 GIN RAS, (15) spec. no. 4915/29 GIN RAS; (16) Triticites umbus Rosovskaya, 1958, spec. no. 4915/30 GIN RAS; (17, 18) Triticites cf. shikhanensis Rosovskaya, 1950: (17) spec. no. 4915/31 GIN RAS, (18) spec. no. 4915/32 GIN RAS; (19) Rugosofusulina flexuosa Rosovskaya, 1958, spec. no. 4915/33 GIN RAS. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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conodonts made by R.S. Furdui (Putevoditel’…, 1975, Atlas…, 1979) in some cases do not correspond to present state of knowledge (Alekseev and Goreva, 2012). So, Gondolella clarki Koike is typical of the Middle Carboniferous sediments. In this regard, it is obviously necessary to revise determinations of the conodont collection of R.S. Furdui, in particular, to reexamine conodonts from sediments of Voskresenka Mount. The Gzhelian Stage is represented only by the lower fusulinid zone Rauserites stuckenbergi, Rauserites rossicus, opened by Borehole 772 in the int. 20– 50 m. According to (Alksne, 1999), limestones of this zone are covered by carbonate sediments that do not contain fusulinides and are attributed to the Asselian Stage in terms of lithology.

The sediments of the Sakmarian Stage are not reliably established. The flysch-type terrigenous sediments are attributed to the Artinskian Stage. In the lower part of the sequence, they are represented by dark gray limestones, marls, mudstones, and siltstones; in the upper part, by gray siltstones and calcareous sandstones (thickness of 70–170 m). As seen in core sections of most boreholes, they are overlapped by anhydrites and clayey rocks of the Kungurian Stage of the Lower Permian (thickness of 50–350 m). In the core section of the South Tabynsk Borehole 8, salt beds up to 80 m thick were described. The Paleozoic sequence is crowned by red deposits of the Ufimian Stage of the Middle Permian, represented by mudstones, siltstones, and sandstones (up to 360 m thick).

According to drilling data within the Tabynsk field (the core description and interpretation of well logs from archives of PJSC Bashneft and OJSC Bashneftegeofizika), the upper open part of the sequence is composed of Permian and Upper–Middle Carboniferous sediments. The Middle Carboniferous sediments (Myachkovskian and Podolskian horizons) are represented by gray fine-grained limestones with crinoids, brachiopods, ostracods, and foraminifera Kamaina kamensis (Safonova), Fusulina cf. mjachkovensis Rauser, F. aff. chernovi Rauser, Schubertella sp., Glomospira sp., Tuberitina sp., etc., as well as with the algae Ungdarella (Borehole 7, determinations by V.V. Arkhipova, materials of PJSC Bashneft, 1967), with chert inclusions and dolomite interlayers. The thickness of the Myachkovskian–Podolskian sediments is 150–220 m (about 200 m on average).

The sequence of the Upper Paleozoic sediments is well subdivided and correlated in the character of the logs. A comparison of log data obtained from a number of boreholes in the Tabynsk field (Fig. 5) made it possible to establish that the Upper Carboniferous shelf limestones with an average thickness of about 100 m are common in this zone. They are underlain by Middle Carboniferous limestones of the Moscovian Stage, which differ in log characteristics. The Middle–Upper Carboniferous transition is determined on the basis of small variations of the log curves. The Upper Carboniferous carbonates are covered by a 20– 30 m unit of bituminous–argillaceous limestones of the Asselian Stage, which are clearly distinguished on the logs by peaks of increased gamma activity. Similar sediments can be traced to the north of the Zilim field and to the south of the South Tabynsk field. In our opinion, in the primary description of the core sections of South Tabynsk Boreholes 8 and 9 (materials of PJSC Bashneft, 1971), the upper Carboniferous shelf carbonate sequence was attributed to the Middle Carboniferous without confirmation of its age by paleontological data. The thickness of limestones of the upper part of the Moscovian Stage (Podolskian and Myachkovskian horizons) was incorrectly increased in a sense by approximately 100 m. At the same time, thin deep-water shelf and depression sediments of the Asselian Stage (Early Permian), whose outcrops can be observed in a quarry at the northwestern outskirts of Voskresenka Mount (western block), were assigned to the Upper Carboniferous. An analysis of log data made it possible to specify the distribution boundaries of Upper Carboniferous bioherm–shelf (reef) sedi-

They are overlain conformably by the thick (70– 150 m) rock sequence represented by light gray to gray limestones, organogenic, crinoid-bryozoan, porous, oil-impregnated, with interlayers of fine-grained dolomites. The core section of Borehole 5 is characterized by fusulinids of the Triticites acutus–Triticites quasiarcticus Zone (Alksne and Shamov, 1975). In other boreholes, the section is attributed to the Upper Carboniferous in terms of its stratigraphic position, since an overlying thin unit (10–30 m) of dark gray limestone with crinoids, ostracods, rare bryozoans, and foraminifera Nodosinelloides sp. and Tuberitina sp., as well as with interlayers of greenish gray mudstones, is attributed to the Asselian Stage on the basis of lithological features and the first occurrence of lagenids.

Fig. 4. Geological structure of the Bashkir Pre-Urals in the area of Tabynsk and Krasnousolsky townships (fragments of 1 : 200000 geological maps N–40–XV and N–40–ХХI, I.I. Sinitsyn, 1962, with amendments). (1) Quaternary (Q) and Neogene (N) sediments; (2) Triassic sediments (T1); (3) Upper Permian, red deposits of the Ufimian Stage (P2u); (4–7) Lower Permian: (4) Kungurian Stage (P1k), gypsiferous sediments; (5, 6) Artinskian Stage (P1ar), carbonate–terrigenous and flysch sediments, respectively; (7) Sakmarian and Asselian stages (P1а–s), carbonate–terrigenous depression and flysch sediments; Asselian Stage (Р1а), clayey– carbonate depression sediments; (8–12) limestones: (8) Upper Carboniferous (С3), (9) Middle Carboniferous (С2), (10, 11) Lower Carboniferous (С1), (12) Upper and Middle Devonian; (13) Lower Devonian sandstones of the Takata Horizon (D1e1tk); (14) boreholes and their numbers; (15) Boreholes 772 and 773, drilled on slope of mount Voskresenka; (16) distribution area of the Late Carboniferous carbonate platform based on the drilling data; (17) rivers; (18) Tournaisian Stage top outlines of the Tabynsk oil field (at a depth of about 1400 m); (19) tectonic contact; (20) position of the regional seismic profile no. 3. STRATIGRAPHY AND GEOLOGICAL CORRELATION

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480 m

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Fig. 5. Correlation scheme of the core sections of Tabynsk, South Tabynsk, Zilim, and Saitbaba fields based on well-logging records: GRN—gamma-ray neutron log, GR— gamma-ray log. (1) Artinskian Stage (P1ar), terrigenous flysch sediments; (2) Asselian Stage (Р1а), clayey-carbonate slope–depression sediments; (3) Upper Carboniferous bioherm and shelf limestones (С3); (4, 5) Upper Carboniferous shelf limestones of the Moscovian Stage: (4) Myachkovskian and Podolskian horizons (C2m2), (5) Kashirskian and Vereiskian horizons (C2m1); (6) numbers below line—depths of the upper and lower contacts of Upper Carboniferous limestones (without elevation) in different boreholes (for Borehole 8 based on reinterpreted data). The position of boreholes is aligned with the upper contact of the Upper Carboniferous. The lithological core log is given for Tabynsk Borehole 1. Locations of boreholes are shown in Figs. 2 and 4.

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ments. To the east, the strata of the Upper Carboniferous bioherm limestones are sharply replaced by thin depression sediments (Kazantsev, 1984, p. 48, Fig. 30). The Upper Carboniferous depression facies crop out in a section along the Usolka River at the Krasnousolsky sanatorium (Chuvashov et al., 1990; Chernykh, 2005). To the east of the Voskresenka structure, the sediments belonging to the Upper Carboniferous and the Asselian Stage are covered by Artinskian flysch; there are no boreholes drilled here. To the northeast, within the Zilim field (Zilim Boreholes 16, 18, and 19), there is a sharp transition from shelf carbonates (Borehole 16) to depression clayey–carbonate sediments of much lower thickness, exposed in Boreholes 18 and 19, which is fixed by change of character of the logs (Fig. 5). The transition zone is confined to a fault zone, interpreted as the Zilim–Krasnousolsky overthrust (Kazantsev, 1984, p. 51, fig. 31), which should rather be regarded as an overthrust fault dipping to the east at an angle of 30°–35° (Baimukhametov et al., 1997). To the east, in the Saitbaba structure, Boreholes 1, 2, and 4 opened the Upper Carboniferous deposits represented by clayey–carbonate depression deposits with a thickness of about 50 m, similar to those described in the section along the Usolka River. These data indicate that the distribution areas of the Upper Carboniferous carbonate shelf and depression clayey facies are separated by the submeridional fault zone. In boreholes drilled to the north (22, 14, 12) and to the south (7, 21, 28, 31, 8, 9) from the Voskresenka massif (Fig. 4), the roof of the Upper Carboniferous limestones is submerged following a series of sublatitudinal faults to a depth of 300 m in Borehole 22, to a depth of 590 m in Borehole 31, and to a depth of 860 m in South Tabynsk Borehole 8 (Fig. 5). Thus, the drilling data show a step-block structure of the Tabynsk field. Analysis of Seismic Data The Tabynsk structure is crossed by the regional seismic profile no. 3, which passes through Boreholes 22, 14, and 12 and to the northeast crosses the Saitbaba structure along the profile of Boreholes 1 and 2 (Fig. 4). The seismic profile clearly demonstrates the modern trough-like structure of this part of the Uralian foredeep (Fig. 6). The Tabynsk structure is controlled from the west by a high-angle fault, along which the uplifting amplitude of sediments is about 1 km. Most geologists attribute these structures to uplifts (Yusupov et al., 1975) and thrusts (Kazantsev, 1984). The nature of faults indicates their formation under the influence of strike-slip movements (Sylvester, 1988; Gorozhanin and Gorozhanina, 2016). More to the east, the layers dip to the east at an angle of up to 45°. Subvertical and inclined faults separating blocks can be traced to a depth of more than 5 km. As a result of the branching of faults, wedge-shaped oblique-slip (Fig. 6) and STRATIGRAPHY AND GEOLOGICAL CORRELATION

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overthrust structures, such as the Saitbaba structure (Kazantsev, 1984; Baimukhametov et al., 1997), are developed. According to the geological and geophysical data, the Tabynsk structure is a narrow (2–5 km) anticline structure (Figs. 4, 6) or a fold broken by longitudinal and transverse fault zones (Baimukhametov et al., 1997). In terms of structure, there is a noncoincidence between the arches composed of the Devonian–Carboniferous sediments and those of the overlying Upper Carboniferous–Permian strata. This was previously associated with an increase in the thickness of the Upper Carboniferous on the western wing of the structure, represented by bioherm limestones (up to 120 m) (Yusupov et al., 1975). The Voskresenka massif crops out in the central, most elevated part of this structure. An analysis of seismic materials and their correlation with the core data have shown that the Voskresenka carbonate massif represents the upper part of the horst-shaped uplift formed in the large fault zone of the strike-slip type. To the east, the distribution area of the Upper Carboniferous bioherm–shelf (“reef”) sediments is constrained by the submeridional fault separating the facies of the deep-water zone of the Uralian foredeep. To the west, there is a large tectonic fault, marked by the Belaya River valley. The sublatitudinal feathering faults reflect the block structure of the studied area (Fig. 2c). The Usolka River valley is traced along the largest one. DISCUSSION OF RESULTS The Voskresenka carbonate massif, located in the central part of the Tabynsk structure (Fig. 4), is confined to the axial zone of the Uralian foredeep (Alksne and Shamov, 1975, Baimukhametov et al., 1997), the lithological boundaries of which are considered to be the boundaries of the distribution of the Lower Permian sediments. The eastern boundary of the foredeep is drawn on geological maps along the boundary of the Early Permian flysch deposits; the western boundary is marked by the Early Permian reef structures; the depression facies are widespread in the axial part. The lithological boundaries of the Uralian foredeep were located more to the east in the Late Carboniferous, since the facial boundary, marked by the distribution area of flysch deposits, shifted gradually to the west during the Early Permian. This was associated with the advance of the flysch accumulation zone toward the platform during the Uralian orogenesis (Chuvashov, 1985, 1998; Puchkov 2010). This means that the shelf area corresponding to the Voskresenka massif was in the Late Carboniferous in the western platform part of the foredeep. As a result of the displacement of facies zones in the Early Permian, this site found itself in the center of the Early Permian basin and was overlain by deep-sea sediments. It should be noted that the lithofacies and structural boundaries of the Uralian foreVol. 26

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Tabynsk structure Platform

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0

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Abs. mark, m

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–4500 –5000 –5500 –6000 –7000

Fig. 6. Fragment of regional seismic profile no. 3 across the Tabynsk structure (based on materials of OJSC Bashneftegeofizika, 2004). P1—Lower Permian; С3—Upper Carboniferous; C2m—Middle Carboniferous, Moscovian Stage; C1–2—Lower–Middle Carboniferous; C1—Lower Carboniferous; D—Devonian; V—Vendian. Boundaries of the Upper Carboniferous (С3) carbonate platform are shown by light lines. Main fault zones are shown by dashed lines. The zone of the Uralian foredeep yields a chaotic seismic record, which indicates the rock faulting.

deep do not coincide (Puchkov and Timonin, 1970). Seismic data demonstrate that the modern structural position of the Voskresenka massif corresponds to the marginal part of the folding zone (the eastern side of the Uralian foredeep). According to B.I. Chuvashov (Chuvashov, 1985; Chuvashov et al., 1990), five facies zones with different types of Upper Carboniferous sections are distinguished in the Uralian foredeep from the west to the east: (1) zone of layered limestones and shelf dolomites, (2) zone of reef structures, (3) zone of carbonate–clayey sediments (two-member section type), (4) zone of clayey sediments (depression type of section), and (5) zone of carbonate–terrigenous flysch deposits. A two-member type of section occupies the transitional position, represented at the base by the shelf carbonates, covered by deeper clayey and carbon-

ate-clayey units. In the massif of Voskresenka Mount there is a two-member (or transitional) type of sequence formed as a result of the subsidence (flooding) of the carbonate platform margin with brachiopod–bryozoan bioherms at the end of the Late Carboniferous. Later, this part of the platform was overlapped by a unit of Asselian layered crinoid limestones. The term “carbonate platform” is applied to sedimentary sequences which deposited under conditions of open carbonate shelf with a relatively flat level surface with increased wave action in different geodynamic settings (Wilson, 1980; Tucker and Wright, 1990). The shelf zone edge or the edge of the carbonate platform is often marked by occurrence of organogenic reef structures. The subsidence and flooding of the carbonate shelf zone is recorded in a sharp change of shallow-water sediments by deeper ones. For this


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sequence, the term “drowned carbonate platform” is used since the platform surface is below the photic zone. The reason for a sharp change in a depth of the basin could be a sea level rise that advanced the development of carbonate structures for a number of reasons, both eustatic and tectonic or geodynamic (Schlager, 1981, Proust et al., 1998, Kim et al., 2012). This carbonate platform was formed along the periphery of the East European craton at the western frame of the Uralian foredeep and in the Northern Caspian region during the Carboniferous–Early Permian (Gorozhanina et al., 2007). According to the geodynamic position, this platform is synorogenic (Dorobek, 1995) and it was formed at the foreland basin border off the orogen (Proust et al., 1998; Gorozhanina, 2010). The carbonate deposition in this marginal part of the shelf of the East European Platform occurred during the late Visean, Bashkirian, and Moscovian ages within a gently sloping carbonate ramp under active wave activity. The carbonate platform, bounded by reef hills, existed from the Kasimovian time of the Late Carboniferous to the late Artinskian–early Kungurian time of the Permian (Proust et al., 1998). The sedimentary succession in the section of Voskresenka Mount and neighboring Tabynsk boreholes records the evolution of sedimentation during the Carboniferous and Early Permian at advancement of the foreland basin toward the platform. In the section, one can observe a change of the Middle Carboniferous shelf carbonates by the Upper Carboniferous shallow marine–shelf and bioherm formations. Bioherm facies are represented by bryozoan limestones (baundstones and rudstones) with brachiopods, algae, and ammonoids. They are underlain by layered crinoid limestones (wackestones and grainstones). The rocks contain a foraminiferal assemblage, indicating the Kasimovian–early Gzhelian interval (Alekseev et al., 2010). According to our data and those of Alksne (1999), the upper part of the Gzhelian Stage (Jigulites jigulensis–Daixina sokensis Zone and Daixina robusta– Daixina bosbytauensis Zone) is absent in the section of the eastern block of Voskresenka Mount. The lower Gzhelian light bioherm (reef) limestones of the Rauserites stuckenbergi Zone in the western block are conformably overlain by gray pelitomorphic finegrained dolomitized limestones that were previously conditionally assigned to the Asselian Stage (Putevoditel’…, 1975, 1984; Alksne, 1999). According to these data, there is a hiatus at the boundary between the Gzhelian and Asselian stages, at the contact of the bioherm facies with overlying deeper water layered limestones. Finds of conodonts Streptognathodus simplex Gunnell and Streptognathodus aff. paraisolatus Chernykh and a juvenile specimen of Solkognathus velivolus Chernykh, described by Chernykh (2005) from the bellus Zone of the Gzhelian Stage, in overlapping layered crinoid limestones (Alekseev et al., 2010) allow us to attribute these limestones to the STRATIGRAPHY AND GEOLOGICAL CORRELATION

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upper Gzhelian–lower Asselian. Consequently, the transition from the bioherm to deepwater-shelf facies can be considered almost continuous. The Upper Carboniferous depression sediments are distributed more to the east and they crop out in a section along the Usolka River at the Krasnousolsky sanatorium. In this area, the Kasimovian–Gzhelian– Asselian sediments form a continuous succession in depression clayey–carbonate facies (Chuvashov et al., 1990, Chernykh, 2005), marking the submerged zone of the Uralian foredeep, which are overlapped by Sakmarian–Artinskian carbonate–silty–clayey sediments of the remote facies (“pre-flysch” according to B.I. Chuvashov). In the Usolka section, the Kasimovian Stage of the Upper Carboniferous is confirmed by the occurrence of fusulinid assemblage, including Quasifusulina longissima (Moeller), Montiparus umbonoplicatus (Rauser et Beljaev), Schwageriniformes schwageriniformis (Rauser), and others (Chuvashov et al., 1991). Owing to study of conodonts, the Gzhelian Stage in this section was characterized in detail. The distribution of conodonts allows us to establish the sequence of zones of the conodont scale in its entirety: simulator– vitali–virgilicus–simplex–bellus–wabaunsensis (Chernykh, 2012). Fusulinids of the Gzhelian Stage were defined at several levels. The taxonomic composition of fusulinid assemblages is diverse and also indicates the Gzhelian age of the deposits. In this part of the section, there are common Gzhelian species: Rauserites stuckenbergi (Rauser), R. mogutovensis Rosovskaya, R. triangulus (Rosovskaya), etc.; Schwageriniformes fusiformis (Bensh), Sch. perstabilis (Scherbovich), etc.; and Daixina azantashiensis Davydov, D. enormis Scherbovich, and some others. According to the definitions of V.I. Davydov (Davydov et al., 2008; Chernykh et al., 2009), the Gzhelian fusulinid assemblage of the Usolka section includes 29 species, belonging to eight genera. The thickness of the Upper Carboniferous in the Usolka section is 28 m. The Asselian sediments in this section are represented by the Kholodnilozhian and Shikhanian horizons, composed of clayey–carbonate sediments (aphanitic and clayey limestones, marls with interlayers of dolomites and mudstones), “enriched in conodonts” (Chernykh, 2005, p. 59). These sediments are relatively weakly characterized by fusulinids. In the Kholodnilozhian Horizon, fusulinids were not found. The fusulinid assemblage belonging to the upper fusulinid zone of the Asselian Stage was recorded only in the upper part of the Shikhanian Horizon (Chuvashov et al., 1991). Thus, in the Usolka River section, as well as in the section of the carbonate massif of Voskresenka Mount, fusulinids are absent in the layers containing Asselian conodonts. The thickness of the Asselian Stage is about 17 m (Chuvashov et al., 1990, 1991). The distribution of conodonts in the section of the Usolka River served as the basis for the development Vol. 26

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of the conodont zonal scale of the Asselian Stage. In this section, a continuous succession of conodont zones was established for the total volume of the Asselian Stage: isolatus–glenisteri–cristellaris–sigmoidalis–constrictus-fusus–postfusus (Chernykh, 2005, 2006). Conodont species Streptognathodus constrictus Chernykh et Reshetkova and Streptognathodus aff. lanceatus Chernykh found in sediments on Voskresenka Mount are common for the Asselian Stage. The same species were found in the Usolka section, respectively, in layer 17 of the Kholodnolozhian Horizon and layer 21 of the Shikhanian Horizon (Chernykh, 2005). According to B.I. Chuvashov, the Usolka section belongs to the depression type, whereas the Voskresenka section situated westward belongs to the binomial type. During the formation of the Uralian foredeep in the Carboniferous and Early Permian, the depression facies overlapped consistently the shelf carbonates (Chuvashov et al., 1990; Proust et al., 1998; Puchkov, 2000, 2010; Gorozhanina and Gorozhanin, 2015). Formation of flooded marginal parts of carbonate platforms is typical of the foreland basins developed at the boundary between the platform and collision orogens. In the post-Paleozoic time, the structure of the Uralian foredeep underwent a number of changes under the influence of the Alpine tectonic events (Puchkov, 2010) mainly of a strike-slip type (Kopp, 2005). There is a discrepancy between the facies and structural boundaries of the Uralian foredeep, which indicates the superimposition of later tectonic structures on the facial profile of the foreland basin. The discrepancy between the lithological boundaries of the foredeep and its modern structure is a characteristic feature of the northern part of the Uralian foredeep (Puchkov and Timonin, 1970). The main stage of tectonic deformations was in the Neogene (Kopp, 2005; Puchkov, 2010). The Voskresenka block, as well as the Ishimbay Shikhans (Kulagina et al., 2015), was probably exhumed at the end of the Neogene. CONCLUSIONS The obtained complex lithofacies, paleontological, structural, and geological-geophysical data show that the Voskresenka massif is not a single reef (as is commonly believed) in the axial zone of the Uralian foredeep, but a tectonic horst-like block composed of the Late Carboniferous bioherm–shelf limestones located in the center of the Tabynsk anticline structure. The marginal part of the carbonate platform with bioherms, which are overlain by the upper Gzhelian– Asselian slope depressions, crops out. The age range of the biogerm facies corresponds to the Kasimovian Stage (foraminiferal zones Protriticites pseudomontiparus–Obsolete obsoletus, Montiparus montiparus, Triticites acutus–T. Quasiarcticus) and the lower Gzhelian substage (Zone Rauserites stuckenbergi, Rauserites rossicus). The overlying layered crinoidal

limestones are attributed to the late Gzhelian–Asselian, which is confirmed by the unified biostratigraphic sequence of zonal conodont assemblages from the simulator Zone to the constrictus Zone. In the Voskresenka carbonate massif, a two-member or transitional type of the carbonate–clayey sediments is recorded. These sediments deposited as a result of subsidence of the carbonate platform margin with brachiopod-bryozoan bioherms at the end of Late Carboniferous, which was overlain by a condensed unit of the Lower Permian layered crinoidal limestones. The succession of the Gzhelian–Asselian sediments in the carbonate massif of Voskresenka Mount corresponds to the setting of a flooded carbonate platform formed at the Late Carboniferous–Early Permian boundary within the Uralian foredeep during the collision process. The Upper Carboniferous shelf zone is bounded from the east by a submeridional tectonic fault, along which chains of oil fields associated with anticline structures are traced. The sequences of the shelf (Voskresenka) and depression (Krasnousolsky) types of sediments are contiguous as a result of syncollisional strike-slip and post-collisional block (neotectonic) movements. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (project no. 15-05-06393) and within the framework of State Tasks (project nos. 02522014-0002 IG USC RAS, 0135-2014-0070 IG RAS). Reviewers E.I. Kulagina, M.G. Leonov, and V.N. Puchkov REFERENCES Alekseev, A.S. and Goreva, N.V., Conodonts of the Moscovian–Kasimovian Boundary interval (Carboniferous) in the Southern Urals, in Tr. XV Vseross. mikropaleontol. soveshch. “Sovremennaya mikropaleontologiya”, 12-16 sentyabrya 2012 g., Gelendzhik (Proc. XV All-Russ. Micropaleontol. Conf. “Modern Micropaleontology”, September 12– 16, 2012), Moscow, 2012, pp. 189–193. Alekseev, A.S., Goreva, N.V., Kossovaya, O.L., and Isakova, T.N., About the age of the reef complex of Mount Voskresenka (Southern Bashkiria), in Mater. VIII Mezhreg. nauchno-prakt. konf. (Proc. VIII Interreg. Res. Pract. Conf.), Ufa: DizainPoligrafServis, 2010, pp. 35–38. Alksne, A.E. and Shamov, D.F., Upper Carboniferous deposits of Bashkiria, in Stratigrafiya i geologiya karbona Yuzhnogo Urala i vostochnoi okrainy Russkoi platformy (Carboniferous Stratigraphy and Geology of the Southern Urals and the Eastern Margin of the Russian Platform), Ufa: Bashkir. Fil. Akad. Nauk SSSR, 1975, pp. 102–112. Alksne, A.E., Fusulinids of the Upper Carboniferous Voskresenka reef in the Southern Urals, Izv. Otd. Nauk Zemle Ekol. Akad. Nauk Bashkirii, 1999, no. 4, pp. 52–58.


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Translated by D. Voroshchuk


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2018