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United Kingdom - Russia scientific workshop ""T
«What does the Devonian of the Arctic tell us
УДК 551.734.470.13 DOI: 10.19110/2221-1381-2019-10-28-38
CONODONTS AND OSTRACODES FROM THE GIVETIAN-FRASNIAN SHALLOW-WATER DEPOSITS
OF THE SOUTHERN TIMAN
M. A. Soboleva, D. B. Sobolev
Institute of Geology FRC Komi SC UB RAS, Syktyvkar [email protected], [email protected]
Conodont and ostracode distribution in the Timan (Middle — Upper Devonian, Upper Givetian — Lower Frasnian), Ust'-Yarega (Upper Devonian, Lower Frasnian) and Yba (Middle? — Upper Devonian, Upper Givetian? — Lower Frasnian) formations of Southern Timan was analyzed. The regional and interregional correlation of the established associations was conducted. The significant changes in conodont and ostracode associations are observed almost at the base of the Ust'-Yarega Formation, and they are associated with a gradual increase in the depth of the paleobasin. The conodonts of Ancyrodella rugosa and Mesotaxis asymmetricus appearing at this level characterize the Ancyrodella rugosa conodont Zone. The ostracodes of the Ust'-Yarega Formation are represented by the assemblage of the Cavellina chvorostanensis — Entomozoe (R.) scabrosa Zone. The significant changes in the conodont associations of the Yba Formation reflect an important sequence of appearance of the index-species: Mesotaxis falsiovalis, Ancyrodella rotundiloba soluta, Ancyrodella rotundiloba rotundiloba. The Ancyrodella soluta conodont Zone provide a reliable correlation of the sections of the Timan-North Urals Region and are essential for resolving the issue of the lower boundary of the Frasnian stage of the Upper Devonian in the East European Platform.
Keywords: conodonts, ostracodes, biostratigraphy, correlation, Southern Timan, Givetian-Frasnian boundary interval.
КОНОДОНТЫ И ОСТРАКОДЫ ИЗ МЕЛКОВОДНЫХ ЖИВЕТСКО-ФРАНСКИХ ОТЛОЖЕНИЙ ЮЖНОГО ТИМАНА
М. А. Соболева, Д. Б. Соболев
Институт геологии ФИЦ Коми НЦ УрО РАН, Сыктывкар
Проведен анализ распространения конодонтов и остракод в разрезах тиманской (средний — верхний девон, верхний живет — нижний фран), усть-ярегской (верхний девон, нижний фран) и ыбской (средний? — верхний девон, верхний живет? — нижний фран) свит Южного Тимана. Осуществлены региональная и межрегиональная корреляции установленных комплексов. Заметные изменения в конодонтовых и остракодовых комплексах отмечаются практически с основания усть-ярегской свиты и связаны с постепенным увеличением глубины морского бассейна. На этом уровне появляются конодонты Ancyrodella rugosa и Mesotaxis asymmetricus, которые характеризуют зону Ancyrodella rugosa. Остра-коды усть-ярегской свиты представлены комплексом зоны Cavellina chvorostanensis — Entomozoe (R.) scabrosa. В то же время заметные изменения в конодонтовых комплексах ыбской свиты отражают важную последовательность появления зональных видов: Mesotaxis falsiovalis, Ancyrodella rotundiloba soluta, Ancyrodella rotundiloba rotundiloba. Конодонто-вая зона Ancyrodella soluta обеспечивает надежную корреляцию разрезов Тимано-Североуральского региона и имеет существенное значение для разрешения вопроса о нижней границе франского яруса верхнего девона на территории Восточно-Европейской платформы.
Ключевые слова: конодонты, остракоды, биостратиграфия, корреляция, Южный Тиман, живетско-франский пограничный интервал.
The problem of biostratigraphic subdivision and correlation of the Givetian and Frasnian stages (i. e. the Middle and Upper Devonian) in the north-east of European Russia has been considered in many publications. Despite repeated discussion of this problem [7, 9, 17, 25], it remains unresolved and relevant. There are different points of view regarding the position of the lower boundary of the Frasnian Stage in various regions. Several authors proposed to draw this boundary in the TimanNorth Urals Region [8—9, 11, 17, 25]. The lowest position of the boundary roughly coincides with the base of the Pashy For-
mation, and the highest position coincides with the base of the Ust'-Yarega Formation or the upper part of the Kedzydshor Formation [2, 25], characterized by the late forms of the conodonts Ancyrodella rotundiloba (Bryant). In addition, in sections of the Timan-Pechora Province (TPP), it is proposed to draw the boundary at the base or inside the Timan Formation [8—9, 11, 17], which is approximately correlated with the Kedzyd-shor Formation of the western slope of the Urals and the Chernyshev Ridge. The options proposed by different authors for the regional level of the base of the Frasnian Stage do not
meet the recommendations of the International Subcommission on the Devonian Stratigraphy System (SDS) [27].
In accordance with the decision of SDS and the International Stratigraphic Commission, the position of the boundary between the Middle and Upper Devonian is assumed to be at the base of the lower subzone of the Polygnathus asymmetricus conodont Zone by the first appearance of the late forms of An-cyrodella rotundiloba [27]. This is very important level in the evolution of conodonts. Here, for the first time, most species of the genus Ancyrodella and Mesotaxis, which are widespread in the Lower Frasnian of Upper Devonian, appear.
In the modern conodont zonation (Fig. 1), the boundary between the Givetian and Frasnian stages is drawn inside the lower subzone of the Mesotaxis falsiovalis conodont Zone marked by the first appearance of Ancyrodella rotundiloba [20, 28]. This early form stands out as a separate subspecies — Ancyrodella rotundiloba pristina Khalymbadzha et Chernysheva, which is zonal tax-on in the Tafilalt (Southern Morocco) conodont zonation [1] and in the new scheme of the conodont zonation [3].
In addition to biostratigraphic markers of the boundary between the Givetian and Frasnian stages, there are lithologi-cal markers, reflecting eustatic fluctuations. As a result of a significant sea level rise (the Frasne Event), fundamental changes in the composition of the marine biota occurred. Perhaps the Frasne Event occurred in several stages [26]. O. Wallizer [26] considered that the main biological event coincides with the geological event, i.e. the level of the lower subzone of Mesotax-
is falsiovalis conodont Zone (below the base of the Frasnian stage) is correlated with the beginning of transgression of the marine basin.
Conodont zonation is widely used for biostratigraphic subdivision and correlation of the Givetian and Frasnian deposits. However, the absence of zonal taxa of conodonts in many areas of the Timan-North Urals Region because of their facies dependence complicates biostratigraphy. Therefore, we analyzed the distribution of conodonts and ostracodes in the Givetian-Frasnian shallow-water deposits of the Southern Timan. Here are the stratotype sections of the Timan and Ust'-Yarega formations, where the lower boundary of the Frasnian stage of the Upper Devonian was traditionally considered. The section of the Yba Formation is lithologically the closest to the predominantly carbonate stratotype Col du Puech de la Suque section in Montagne Noir (Southern France) [27]. This section, in our opinion, is the most promising to confirm the level of the poorly defined boundary between the Givetian and Frasnian stages in the East European Platform.
Material and methods
We analyze the distribution of conodonts and ostracodes in the Givetian-Frasnian shallow-water deposits of the Southern Timan (Fig. 2, A), as in this region there are stratotypes of a number of local and regional subdivisions of the Middle and Upper Devonian. A significant part of the sections is located in the Ukhta Region of the Komi Republic. This region is related to the north-
Fig. 1. Middle to Upper Devonian correlation of conodont and ostracode zones compared to the distribution of the ostracodes, which are widespread in the Timan-North Urals Region, according to our data and literature data
Рис. 1. Корреляция конодонтовых и остракодовых зон среднего и верхнего девона с распределением остракод, широко распространенных в Тимано-Североуральском регионе (наши и литературные данные)
Fig. 2. A — location of the studied area on the tectonic scheme of the Timan-Pechora Province (after [4] with simplifications); B — locality map of the Givetian-Frasnian Shera Creek section; C — locality map of the Givetian-Frasnian Ukhta River section (outcrops 18, 13, 14, 16)
and 1-Balneological Borehole (1-B)
Рис. 2. A — место района исследований на тектонической схеме Тимано-Печорской провинции (по [4] с упрощениями); B — расположение живетско-франских отложений на руч. Шера; С — расположение живетско-франских отложений на р. Ухте
(обнажения 18, 13, 14, 16) и скв. 1-Бальнеологической (1-B)
eastern limb of the Ukhta anticline located in the north of the Ukhta-Izhma Uplift. The Ukhta River (the tributary of the Izh-ma River) section and 1-Balneological Borehole demonstrate fairly complete stratigraphic sequence. They are represented by shallow-water clayey and clay-carbonate deposits of the Timan and Ust'-Yarega formations. The 1-Balneological Borehole was drilled in south-western outskirts of the Ukhta City (Fig. 2, C). The predominantly carbonate Yba Formation of the Shera Creek Section is located to the south of the Ukhta Region sections. This section was cropper out on the left bank of the Shera Creek, upstream of the Sherael' Quarry located in the Ust-Kulom Region of the Komi Republic (Fig. 2, B). The research area is related to the Djejimparma Uplift located in the south-south-western part of the Timan Ridge, practically in the zone of junction of the Mezen Syneclise and the Timan Ridge. The level of knowledge on the biostratigraphy of these regions varies significantly. While the scheme of conodont and ostracode zonations has been de-
veloped for the Ukhta Region sections of the Southern Timan, it remains to be done for sections in the Ust'-Kulom Region.
Biostratigraphic data on conodonts and ostracodes were used to subdivide the Timan and Ust'-Yarega formations [6, 8—9, 13—15]. The most detailed materials on the biostratigraphy of the 1-Balneological Borehole are contained in the geological report [6].
Biostratigraphic subdivision of the Yba Formation was carried out by the authors based on the appearance of zonal taxa of conodonts and ostracodes. Microfauna was isolated from carbonate rocks by dissolving in 10 % acetic acid with the addition of a buffer solution. Clay rocks were soaked in water and washed through a 0.05—0.1 mm sieve. Some clayey and porous limestones were dissolved in glacial acetic acid for obtaining the ostracodes. We processed 95 samples of 0.2 to 5 kg in weight. The conodonts were photographed using a Tescan Vega 3 LMH scanning electron microscope.
Results and discussion
The territory of Southern Timan was the object of close study by geologists of various kinds. The stratotypes of many local units (formations) of the Middle and Upper Devonian are here. Most of the formations were established by N. N. Tikhonovich in 1930, A. I. Lyashenko in 1956, Z. V. Larionova in 1987, and others. The Givetian-Frasnian boundary interval comprises sequence of the Timan and Ust'-Yarega formations in the Ukhta Region. In the Ust'-Kulom Region, this interval is represented by the Yba Formation.
The Timan Formation was established by A. I. Lyashenko in 1956 in the volume of the variegated horizon of the Oil-bearing Formation identified by N. N. Tikhonovich. This formation is divided into two subformations. The stratotype of the Lower Timan Subformation is a section of 1-Balneological
Borehole (depth interval of 239.5—307 m). The section in the borehole is represented by sandy-clay rocks and tuffites, alternating above with variegated clays and siltstones with interlay-ers of marls and limestones. The stratotype of the Upper Timan Subformation is the Ukhta River section. This section mainly consists of three outcrops 18, 13, 14 (Fig. 2, C). The section consists of chocolate-brown and greenish-gray clays with layers of siltstones, sandstones and limestones. The boundary between the subformations is placed at top of a sandy limestone bed (Bed A) in the 1-Balneological Borehole (Fig. 3).
The Ust'-Yarega Formation was established by A. I. Lyashenko in 1956 to include the cuboid horizon of the Oil-bearing Formation identified by N. N. Tikhonovich. The stratotype is the Yarega River section (Outcrop 16). The lower part of the formation is represented by sandstones, siltstones, and clays with arenaceous limestone interbeds. The upper part of the Forma-
Fig. 3. Log of the 1-Balneological Borehole with conodont and ostracode distribution after [6, 13]
Рис. 3. Литостратиграфическая колонка разреза скв. 1-Бальнеологической с распределением конодонтов и остракод
по данным [6, 13]
tion is represented by bluish-gray clays with beds of marl and clay limestone.
The boundary between the Timan and Ust'-Yarega formations is drawn in different ways by different researchers. A. I. Lyashenko [10] did not indicate the exact position of the boundary between the formations. He noted only the gradual nature of the transition and insufficient palaeontological information. Currently, there are two main options for interpreting the base of the Ust'-Yarega Formation in the Ukhta Region. Yu. A. Yudi-na and M. N. Moskalenko [13] draw it in the upper part of the Outcrop 14 on the Ukhta River (base of Bed 7) based on a change in the brachiopod associations. In some papers [9, 17—18], the boundary of the formations is accepted in the lower part of Outcrop 14 (base of Bed 2). A change in the fish assemblage is indicated here [9]. However, there are no convincing litholog-ical criteria for placing the base of the Ust'-Yarega Formation here. This interpretation inevitably leads to errors in the correlation of the already adopted unified biostratigraphic subdivisions [19] with local stratigraphic units.
In our opinion, the boundary between the Timan and Ust'-Yarega formations should be drawn in the upper part of Outcrop 14 by the appearance of the first layer of pure mudstones,
without admixture of quartz material and mica (inside of Bed 7, Fig. 4). This bed fixes the first transgression maximum and is clearly recognizable in the field.
Above, there are two more clay layers separated by bio-clastic detrital limestone, overlain by a 40 cm bed of bioclastic micritic limestone. Above, there are two more clay interbeds separated by bioclastic detrital limestone, overlain by a 40 cm bed of bioclastic detrital micritic limestone. This transgression seems to be a regional manifestation of the global Frasne Event. A synchronous level of the onset of such a transgressive cycle is reported by S. V. Tikhomirov [24] as the Novospassky Horizon, allocated in the central regions of the East European Platform (P. 90, after [24]). In the lithological characteristic of these deposits, substantial reduction of coarse terrigenous material, or its absence, is particularly emphasized. Areas of paleo-up-lifts are an exception, where the synchronous deposits lie over an erosional surface, and contain reworked material (Djejim-parma and Ochpharma uplifts).
The Givetian-Frasnian boundary interval in the Ust'-Ku-lom Region is represented by deposits of the Yba Formation. The stratotype is a section of a borehole located in the headwaters of the Yba River [16]. The Yba Formation is represented by
Fig. 4. Log of the Ukhta River section with conodont and ostracode distribution after [8—9, 13] (see Fig. 3 for legend)
Рис. 4. Литостратиграфическая колонка разреза р. Ухты с распределением конодонтов и остракод по данным [8—9, 13]
(обозначения см. на рис. 3)
limestone and clay. Together with the underlying Izyayol' Formation, it composes the northern limbs of the Djejim and Asyv-vozh structures.
Biostratigraphy
of the Givetian-Frasnian deposits
Biostratigraphic studies of the Devonian deposits of Southern Timan have a history of more than 50 years. The conodonts of the Upper Timan and Ust'-Yarega formations in the territory under consideration were studied by V. G. Khalymbadzha, Yu. V. Deulin, L. I. Melnikova, A. V. Kuzmin, L. I. Kononova, N. S. Ovnatanova, et al. The ostracodes were studied by G. P. Martynova, E. N. Polenova, V. G. Egorov, N. A. Fokin, M. N. Moskalenko, A. N. Orlov, etc. This paper [13] provides a biostratigraphic scheme of the Frasnian deposits of the Ukhta Region. A conodont zonation was developed by V. G. Khalym-badzha and A. V. Kuzmin. An ostracode zonation was proposed by M. N. Moskalenko. Later, a local conodont zonation was established based on a sequence of conodont associations (TP associations) using the sections of boreholes and outcrops of Southern Timan [17—18]. The local ostrocode zonation of Southern Timan is traced in the Chernyshev Ridge sections [15], despite the significant differentiation of the shallow paleobasin during the Givetian-Frasnian time. In the Fig. 1 shows the distribution of the ostracodes that are widespread in the Givetian-Frasnian deposits of the Timan-North Urals Region according to our data and data from literature.
We analyze the distribution of conodonts and ostracodes in the sections of the Timan, Ust'-Yarega and Yba formations of Southern Timan. The standard [28] and new [3] conodont zonations, the Tafilalt (Southern Morocco) conodont zonation [1], and the TPP ostracode zonation [6, 12, 15, 19] were used as a biostratigraphic framework.
1-Balneological Borehole (Fig. 3). Here, the Lower Timan Subformation (depth of 307—239.5 m), the Upper Timan Subformation (depth of 239.5—147.5 m) and the Ust'-Yarega Formation (depth of 147.5—121 m) stand out [12].
We analyzed the conodont and ostracode associations in unpublished geological report [6], and in some publications [13]. The unpublished materials show the distribution of conodonts and ostracodes and their correlation with ostracode zonation TPP and regional stratigraphic subdivisions (horizons). Con-odont zones are not recognized. The conodonts were identified by V. G. Khalymbadzha. The ostracodes were identified by N. A. Fokin and M. N. Moskalenko. The distribution of zonal and characteristic conodonts and ostracodes is shown in Fig. 3.
Conodonts. The Lower Timan Subformation is characterized by a conodont association with the following taxonomic composition: Icriodus nodosus Branson et Mehl, Polygnathus normalis Mehl et Youngquist, Polygnathus decorosus Stauffer, Polygnathus webbi Stauffer, Polygnathus varcus Stauffer, Polygnathus cf. brevilaminus Branson et Mehl [6, 13]. The species Polygnathus varcus characterizes the Givetian varcus — hermani-cristatus conodont zones. In the section of 1-Balneological Borehole it continues up to the base of the Domanik Formation, which causes great doubts. In the Upper Timan Subformation, in addition to the above-mentioned species, the appearance of Polygnathus foliatus Bryant is recorded, which is also traced higher in the deposits of the Frasnian stage. It should be noted that the conodont association presented is of an unremarkable composition and is widely distributed in the Upper Givetian and Lower Frasnian deposits. A more substantial renewal of conodonts is noted almost from the base of the Ust'-
Yarega Formation. First, Ancyrodella rotundiloba rotundiloba Bryant, probably characterizing the Ancyrodella rotundiloba conodont Zone [3], then Mesotaxis asymmetricus Bischoff et Ziegler, Klapperina ovalis Ziegler et Klapper and Icriodus sym-metricus Branson et Mehl appear. A conodont association with the index-species Mesotaxis asymmetricus characterizes the upper subzone of the Mesotaxis falsiovalis conodont Zone of the Frasnian [28]. This part of the zone is equivalent to the Ancy-rodella rugosa Zone [3]. Ancyrodella rotundiloba alata Glenister et Klapper appears at the top of the formation. Palmatolepis tran-sitans Muller noted above, characterizing the transitans con-odont Zone of the Lower Frasnian.
Ostracodes. The Lower Timan Subformation in the 1-Bal-neological Borehole contains an ostracode association of the following taxonomic composition: Pseudonodellina strelniensis Polenova, Cavellina devoniana Egorov, Nodella faceta Rozh-destvenskaya, Buregia krestovnikovi Polenova, Uchtovia polenovae Egorov, Bairdia kynovensis Rozhdestvenskaya et al [6, 13]. Since the index-species Ornatella multiplex Rozhdestvenskaya is absent here, we have identified the Beds with Pseudonodellina strelniensis, conventionally compared with the Ornatella multiplex ostracode Zone.
In the regional stratigraphic scheme [19], the Upper Timan Subformation corresponds to the Cavellina devoniana os-tracode Zone. However, the index-species of this zone appears much earlier and is included in the association characterizing of the underlying Ornatella multiplex ostracode Zone. Therefore, in our opinion, one should choose a zonal species of wide geographic distribution, which would appear at this stratigraphic level. As a zonal species, we propose the most common Acrati-na pestrozvetica Egorov and recognize the Beds with Acratina pestrozvetica by its first appearance. The Beds with Acratina pestrozvetica contain, in addition to the taxa Cytherellina cuneate (Rozhdestvenskaya), Healdianella inclinata Polenova, Schneideria schigrovskiensis (Polenova), and Rectella elata Zaspelova. Almost from the base of the Ust'-Yarega Formation there is a significant changes of the ostracode association. Acratia gassano-vae Egorov, Entomozoe (R.) scabrosa Polenova, Ungerella (F.) jaregae (Martynova) and others appear here. The ostracode association with the index-species Entomozoe (R.) scabrosa characterizes the Cavellina chvorostanensis - Entomozoe (R.) scabrosa ostracode Zone [13, 19].
Ukhta River section (Fig. 4). Here, the Upper Timan Subformation (outcrops 18, 13, 14) and the Ust'-Yarega Formation (upper part of outcrops 14 and 16) stand out [12—13]. The boundary between the formations was adopted by the authors in the upper part of Outcrop 14 by the appearance of the first layer of pure mudstones (inside of Bed 7, Fig. 4), fixing a transgression maximum.
The biostratigraphy of this section based on conodonts is covered in a number of publications [8—9, 12—13, 17—18, etc.]. We analyzed the conodont and ostracode associations given in [8—9, 13]. The definitions of conodonts made by L. I. Melnikova, A. V. Kuzmin and V. G. Khalymbadzha are given in [13]. The definitions of ostracodes were made M. N. Moskalenko. The distribution of zonal and characteristic conodonts and os-tracodes is shown in Fig. 4.
Conodonts. The Upper Timan Subformation (Outcrop 18) is characterized by a conodont association of the following tax-onomic composition: Polygnathus normalis, Polygnathus decorosus, Polygnathus incompletes Uyeno, Polygnathus brevilaminus, Polygnathus pennatus Hinde, Polygnathus foliatus, Icriodus expansus Branson et Mehl (definitions of L. I. Melnikova). A
Fig. 5. Detailed log of the Shera Creek section with conodont and ostracode distribution at the level of the supposed transgressive Frasne Event
(see Fig. 3 for legend)
Рис. 5. Детальная литостратиграфическая колонка разреза руч. Шера с распределением конодонтов и остракод на уровне предполагаемого трансгрессивного события Frasne (обозначения см. на рис. 3)
somewhat different complex was determined by A. V. Kuzmin [9]. He established an association of the following taxonomic composition: Icriodus expansus, Polygnathus alatus Huddle, Polygnathus pennatus, Polygnathus webbi, Polygnathus lanei Kuzmin. According to A. V. Kuzmin, the appearance of the species Polygnathus lanei fixes the Givetian-Frasnian boundary within the Timan Formation. The paper [9] also gives the corrected results of the definitions of L. I. Melnikova, where Polygnathus brevilaminus (= Polygnathus lanei), Polygnathus normalis (= Polygnathus webbi).
V. G. Khalymbadzha [8] established in Outcrop 18 a con-odont association of the following taxonomic composition: An-cyrodella binodosa Uyeno, Icriodus expansus, Icriodus nodosus, Polygnathus brevilaminus, Polygnathus varcus, Polygnathus fo-liatus. Unfortunately, in the paper [8] summarized lists of con-odont associations are indicated, which makes biostratigraphic reconstructions difficult. Also important is the lower limit of distribution of Ancyrodella binodosa. Most of these species in Outcrop 18 are of undistinctive composition and of wide strati-graphic distribution. However, the conodonts Ancyrodella bin-odosa, Polygnathus lanei and Polygnathus incompletus established by L. I. Melnikova, A. V. Kuzmin and V. G. Khalymbadzha suggest Frasnian age of these deposits. These species are additional markers of the lower boundary of the Frasnian stage of the Upper Devonian [ 1 ] and characterize the lower subzone of the Mesotaxis falsiovalis conodont Zone [28]. This lower subzone is equivalent to the Moroccan Ancyrodella rotundiloba pristina Zone [1, 3]. An index-species for the lower boundary of the Frasnian stage — Ancyrodella rotundiloba pristina in the Ukhta River section has not established.
The Upper Timan Subformation in outcrops 18 and 14 is characterized by a conodont association of similar taxonomic composition. The difference is in the appearance of the species Polygnathus posterus Kuzmin and Polygnathus ljaschenkoi Kuzmin, which are widely distributed in the Lower Frasnian of the Upper Devonian. The interval of the Upper Timan Subformation considered above characterizes the local Ancyrodella binodosa Zone [8, 13].
A more significant renewal of conodonts is noted in the upper part of the Outcrop 14 almost from the base of the Ust'-Yarega Formation. A. V. Kuzmin [9, 13] established a conodont association of the following taxonomic composition: Ancyrodella rotundiloba, Ancyrodella alata, Ancyrodella rugosa Branson et Mehl, Ancyrodella mouraviffi Garcia-Lopez, Ancyrodella soluta Sandberg, Ziegler et Bultynck, Ancyrodella binodosa, Mesotaxis falsiovalis Sandberg, Ziegler et Bultynck, Mesotaxis asymmetri-cus, Polygnathus posterus, Polygnathus ljaschenkoi, Polygnathus pennatus, Polygnathus webbi, Polygnathus lanei, Polygnathus spat-ulatus Youngquist et al. Of these, the conodonts Ancyrodella rugosa and Mesotaxis asymmetricus are index-species of the upper subzone of the Mesotaxis falsiovalis conodont Zone [28] and Ancyrodella rugosa conodont Zone [3] of the Frasnian stage. It is necessary to note the absence of the Ancyrodella ro-tundiloba soluta and Ancyrodella rotundiloba rotundiloba con-odont zones, which form the basis of the conodont zonation just above the boundary of the Givetian and Frasnian stages [3]. Apparently, part of the Upper Timan deposits was eroded in the Early Frasnian time. This is indirectly indicated by the presence of poorly rounded fragments (gravel) of quartz sandstones with carbonate cement in the upper part of the Upper Timan Subformation (Fig. 4).
The main part of the Ust'-Yarega Formation is presented in Outcrop 16. In addition to the above conodont species, Mesotax-is bogoslovskyi Ovnatanova et Kuzmin and Palmatolepis transitans
appear. The interval of the Ust'-Yarega Formation according to [8, 13] characterizes the local Ancyrodella rotundiloba Zone.
Ostracodes. The Upper Timan Subformation in the Ukhta River section contains an ostracode association widespread in the underlying deposits: Cavellina devoniana, Nodella faceta, Buregia krestovnikovi, Uchtovia polenovae. The appearance of the species Cavellina uchtensis Egorov, Rectella elata, Graviafabra Zaspelova, Acratina pestrozvetica is noted in the Outcrop 18. The Upper Timan Subformation is characterized by an ostracode association of similar taxonomic composition in outcrops 13 and 14.
Here we have identified Beds with Acratina pestrozvetica. Significant but gradual changes of the ostracode association are observed in the upper part of the Upper Timan Subformation (Outcrop 14, down 3.5 m from the top of Bed 7 after [13]). Cytherellina cunaeta and Nodella sp. (sensu Moskalenko, 1997) appear here. Similar ostracodes Nodella sp. with a characteristic posterior-abdominal spike were also noted by I. O. Evdoki-mova in the Ust'-Yarega Formation of Middle Timan [5].
Almost from the base of the Ust'-Yarega Formation, the renewal of the ostracode association continues. First, Cavellina chvorostanensis Polenova appear, then Entomozoe (R.) scabrosa and Ungerella (F.) jaregae appear, characterizing of the Cavelli-na chvorostanensis - Entomozoe (R.) scabrosa ostracode Zone [13, 19]. The last species are widely represented in Outcrop 16 of the Ust'-Yarega Formation.
Shera Creek section (Fig. 5). The Yba Formation stands out here [16]. Previously, the authors made a preliminary biostratigraphic subdivision of the Shera Creek section [21]. However, additional material has now appeared, clarifying the zo-nation of this section. The distribution of zonal and characteristic conodonts and ostracodes is shown in Figs. 5 and 6.
Conodonts. In the lower part of the section (samples 1a— 24) we have established a conodont association of the following taxonomic composition: Polygnathus pennatus, Polygnathus re-imersi Kuzmin, Polygnathus ljaschenkoi, Mehlina gradata Youngquist, Polygnathus aff. foliatus. Most species have a wide strati-graphic range. The conodont association of with Polygnathus ljaschenkoi compares well with the complex of Beds with Polygnathus pollocki — Polygnathus ljaschenkoi, identified in the Ko-syu River section on the western slope of the Subpolar Urals [22]. The joint appearance of the conodonts Polygnathus pollocki M1 Druce and Polygnathus ljaschenkoi is noted significantly below the first appearance of Ancyrodella soluta [22]. The Polygnathus pollocki is also defined in the section of the Yba Formation, but above the first appearance of Polygnathus ljaschenkoi. Since the identification of conodont zones [1, 3, 28] is not possible in the Yba Formation, we have identified Beds with Polygnathus ljaschenkoi.
In the middle part of the section (samples 24a—36c), the following conodont association is established: Mesotaxis falsiova-lis, Mesotaxis falsiovalis —> Mesotaxis asymmetricus, Polygnathus webbi, Polygnathus pollockiM1, Polygnathus pennatus, Polygnathus reimersi, Polygnathus decorosus, Polygnathus ljaschenkoi, Polygnathus dengleri dengleri Bischoff et Ziegler, Polygnathus cf. alatus, Polygnathus dubius Hinde, Klapperina ovalis et al. Most of the species are widespread in the Upper Givetian and Lower Fras-nian deposits. Mesotaxis falsiovalis is an index-species of the lower subzone of the Mesotaxis falsiovalis conodont Zone [28]. Its appearance is recorded in sample 27b. This part of subzone is equivalent to the Skeletognathus norrisi conodont Zone characterizing the upper part of the Givetian stage [3]. However, the species Mesotaxis falsiovalis can be traced and higher in the de-
Fig. 6. Detailed log of the Shera Creek section with zonal and characteristic conodonts (see Fig. 3 for legend). a — Polygnathus pennatus Hinde, upper view of 492/17-1, sample 1b; b — Polygnathus Ijaschenkoi Kuzmin, upper view of 492/17-4, sample 1b; c, d — Polygnathus webbi Stauf-fer, lower and upper view of 492/18-3, sample 41; e — Polygnathus webbi Stauffer, upper view of492/17-9, sample 24a; f — Polygnathuspollocki Morphotype 1 Druce, upper view of492/17-38, sample 24a; g — Mesotaxisfalsiovalis Sandberg, Ziegler et Bultynck, upper view of 492/17-12, sample 27b; h, k — Mesotaxis falsiovalis Sandberg, Ziegler et Bultynck, upper and lower view of 492/18-2, sample 38; l, m — Ancyrodella rotundiloba soluta Sandberg, Ziegler et Bultynck, upper and lower view of 492/18-4, sample 37; n, o — Ancyrodella rotundiloba rotundiloba
Bryant, upper and lower view of 492/18-1, sample 39b. Scale bar is 200 |m
Рис. 6. Детальная литостратиграфическая колонка разреза руч. Шера с зональными и характерными конодонтами (обозначения см. на рис. 3). FAD — уровень первого появления. a — Polygnathus pennatus Hinde, вид сверху, № 492/17-1, обр. 1b; b — Polygnathus ljaschenkoi Kuzmin, вид сверху, № 492/17-4, обр. 1b; c, d — Polygnathus webbi Stauffer, вид снизу и сверху, № 492/18-3, обр. 41; e — Polygnathus webbi Stauffer, вид сверху, № 492/17-9, обр. 24a; f — Polygnathus pollocki Ml Druce, вид сверху, № 492/17-38, обр. 24a; g — Mesotaxis falsiovalis Sandberg, Ziegler et Bultynck, вид сверху, № 492/17-12, обр. 27b; h, k — Mesotaxis falsiovalis Sandberg, Ziegler et Bultynck, вид сверху и снизу, № 492/18-2, обр. 38; l, m — Ancyrodella rotundiloba soluta Sandberg, Ziegler et Bultynck, вид сверху и снизу, № 492/18-4, обр. 37; n, o — Ancyrodella rotundiloba rotundiloba Bryant, вид сверху и снизу, № 492/18-1, обр. 39b.
Масштабная линейка 200 мкм
posits of the Frasnian stage, therefore it is more usful to distinguish the Beds with Polygnathus webbi. The conodonts Polygnathus webbi are the most abundant in this interval.
The overlying interval of the Yba Formation (samples 37— 41) characterizes the Ancyrodella rotundiloba soluta and An-cyrodella rotundiloba rotundiloba zones of the Frasnian stage [3]. Here is established a conodont association of the following taxonomic composition: Mesotaxis falsiovalis, Ancyrodella rotundiloba soluta, Ancyrodella rotundiloba rotundiloba, Polygnathus webbi, Polygnathus cf. reimersi. The first Ancyrodella rotundiloba soluta appears in sample 37, and then Ancyrodella rotundiloba rotundiloba appears in sample 39b. Such a sequence reflects their appearance in the new scheme of the conodont zonation [1, 3]. The first appearance of the conodonts Ancyrodella rotundiloba soluta established by the authors in the sections of the western slope of the Subpolar Urals (Kozhym River and Kosyu River sections) in the interval of the upper subformation of the Kedzydshor Formation [22—23].
Future research will focus on detailed biostratigraphic study of the sections, especially below the level of the first appearance of the conodonts Ancyrodella rotundiloba soluta.
Ostracodes. In the lower part of the section (sample 7) we have established an ostracode association with the following taxonomic composition: Bairdia resima Schevtsov, Gravia fab-ra, Cytherellina cuneata, Aparchitellina incognita T. Soz. and Bairdia timanica Moskalenko. The last two species appear in Early Timan time. Ostracodes Gravia fabra and Cytherellina cu-naeta are predominantly distributed in Upper Timan Subformation. So, this interval of the Yba Formation we compare with the Upper Timan Subformation. At the same time, we do not exclude its comparison with the deposits of the Lower Timan Subformation.
In the overlying interval of the Yba Formation (samples 20—31b), a significant renewal of the ostracode association is observed. Acratina pestrozvetica, Rectella elata, Bairdia aff. nau-movae Egorov and many other species appear. Based on the first
appearance of Acratina pestrozvetica, we have established ostra-code beds that correspond to the Cavellina devoniana ostra-code Zone. The Beds with Acratina pestrozvetica are widespread and provide a reliable correlation of the sections of the TimanNorth Urals Region.
In the middle part of the section (samples 31c—37a), the following significant renewal occurs in the ostracode associations. Bairdia sokolovi Egorov, Bairdia aff. uchtaensis Egorov, Polenovia nurlatica Schevtsov, Bairdia (Rectobairdia) aff. cha-lonensis Casier, Nodella sp. (sensu Moskalenko, 1997), Nodella faceta and Schneideria schigrovskiensis appears. All of these species, except for the last two, are characteristic of the Denisov Beds of the northeast of the East European Platform and the Ust'-Yarega Formation of Southern Timan. At this interval, we identified Beds with Bairdia sokolovi, which probably correspond to the beginning of changes in the ostracode association before the appearance of the Cavellina chvorostanensis - Entomozoe (R.) scabrosa ostracode Zone.
Conclusions
According to the conodont zonation, the boundary between the Givetian and Frasnian stages is drawn inside the lower subzone of the Mesotaxis falsiovalis conodont Zone by the appearance of Ancyrodella rotundiloba pristina. The index-species of the lower boundary of the Frasnian stage is not established in the Southern Timan. Additional markers of the boundary are conodonts Ancyrodella binodosa, Polygnathus lanei and Polygnathus incompletus, which are found in the Upper Timan Subformation of the Southern Timan sequences. However, the lower limit of their distribution in the Timan Formation is currently unknown.
The significant changes in conodont and ostracode associations are observed almost from the base of the Ust'-Yarega Formation and are associated with a gradual increase in the depth of the sea basin. Conodonts Ancyrodella rugosa and Me-sotaxis asymmetricus appear on this level, which characterize the Ancyrodella rugosa conodont Zone. The ostracodes of the Ust'-Yarega Formation are represented by the assemblage of the Cavellina chvorostanensis — Entomozoe (R.) scabrosa Zone.
The significant changes in the ostracode associations of the Yba Formation make it possible to compare its middle part with the Upper Timan Subformation. The ostracode Beds with Acratina pestrozvetiva identified at this level correspond to the Cavellina devoniana ostracode Zone and provide a reliable correlation of the sections of the Timan- North Ural Region. Changes in the conodont associations of the Yba Formation reflect an important sequence of appearance of the index-species Mesotaxis falsiovalis, Ancyrodella rotundiloba soluta, Ancy-rodella rotundiloba rotundiloba. Based on conodonts, the Ancy-rodella rotundiloba soluta and Ancyrodella rotundiloba rotundiloba section intervals can be correlated with the Ancyrodella ro-tundiloba soluta and Ancyrodella rotundiloba rotundiloba zones established in the Tafilalt section and in the new scheme of the conodont zonation. The Ancyrodella rotundiloba soluta con-odont Zone is well traced in the sections of the Yba and Kedzyd-shor formations. This level is certainly more reliable with respect to the validity of conodonts, although it characterizes the position of the old boundary in the Conodont Scale. The established conodont and ostracode sequence, as well as the expression of the global Frasne Event in the Yba Formation, are essential for resolving the issue of the lower boundary of the Fras-nian stage of the Upper Devonian in the East European Platform.
Acknowledgements
The authors are appreciated to A. V. Zhuravlev and L. I. Kononova for assistance in determining the conodonts of the Yba Formation and to S. V. Nikolaeva for assistance in writing the English text of the article. Also the authors would like to thank the referees for constructive remarks that substantially improved this article.
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