Correlation of Eustatic and Biotic Events in the Ordovician ...

ISSN 0031-0301, Paleontological Journal, 2009, Vol. 43, No. 11, pp. 1477–1497. © Pleiades Publishing, Ltd., 2009.

Correlation of Eustatic and Biotic Events in the Ordovician Paleobasins of the Siberian and Russian Platforms A. V. Dronova, A. V. Kanyginb, A. V. Timokhinb, T. Yu. Tolmachevac, and T. V. Gontab a Geological

Institute, Russian Academy of Sciences, Pyzhevsky per. 7, Moscow, 119017 Russia e-mail: [email protected] b Trofimuk Institute of Oil-and-Gas Geology and Geophysics, Siberian Division, Russian Academy of Sciences, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090 Russia c All-Russia Geological Research Institute, 74 Sredny Pr., St.-Petersburg, 199106, Russia Received March 4, 2009

Abstract—Nine sedimentary sequences are recognized in the Ordovician of the Siberian Platform. These sequences correspond to sea level fluctuations of the 3rd order, from 1 to 6 My. Correlation with the sequences recognized in the Ordovician of the Russian Platform suggest their possible eustatic nature. Cold water nontropical carbonates are suggested in the Ordovician of the Tungus Syneclise, which may be explained by the upwelling of cold oceanic waters. The upwelling was caused by re-distribution of oceanic currents due to largescale tectonic events in the mid-Ordovician. The Ordovician evolution of the Siberian Platform was much more similar to that of the North American Platform than of the Russian Platform. DOI: 10.1134/S0031030109110124

INTRODUCTION There are two large paleocontinents with a long history of evolution (Siberian and Russian Platforms), which in the Early Paleozoic were located at a considerable distance apart (based on the paleomagnetic data) and in different biogeographic zones (based on paleontological and sedimentological data). Based on the latest palinspastic reconstructions, in the Cambrian and Early Ordovician the Russian Platform was located in the subpolar latitudes of the southern hemisphere near Gondwana and was rapidly moving northward toward the tropics within the same hemisphere (Cocks and Torsvik, 2005). At the same time, the Siberian Platform migrated more slowly, within the equatorial belt from the southern to the Northern Hemisphere (Cocks and Torsvik, 2007). Thus, despite the epicontinental basins of these platform were at a distance from each other at that time, the climates in which their biotas existed became more similar, which enables their comparison using the same paleontological criteria (taxonomic composition of the biotas and biodiversity dynamics). The Ordovician sedimentary complexes on these platforms also appear promising for revelation and comparison of important evolutionary events, because they contain an almost complete record of sedimentation, and the existence and composition of fossil communities. Like the Devonian, the Ordovician was a time when epicontinental seas became most widespread (Ronov, 1993; Morrow et al., 1996). Therefore the sedimentary successions on both platforms contain the

most complete stratigraphic record of the Ordovician, apart from the topmost Ordovician on the Siberian platform, where this part of the section is represented by barren lagoon facies or is absent. On the Russian Platform, most Ordovician beds are overlain by younger sedimentary complexes, but they are relatively well exposed in Baltoscandia and thoroughly studied paleontologically and sedimentologically by several generations of workers. Based on this and using a method of recognition large transgressive-regressive cyclites (or sedimentary sequences) Dronov and Holmer (1999; 2002) constructed a eustatic curve showing sea level fluctuations in the Ordovician on the Russian Platform. However it is known that transgressive-regressive events in paleobasins result from two factors, i.e., global sea level fluctuations and regional tectonics. More adequate global sea level fluctuations may be obtained by construction of integral eustatic curves based on the comparison of isochronic sequences of remote paleobasins, which allow the recognition of the effects of the regional tectonics. To solve this task, in 2006–2008 we conducted additional examination of the most complete sections to recognize sedimentary sequences and compare them with a eustatic curve for Baltoscandia with a note of paleontological data from various tectonic and facial regions of the Siberian Platform. These classic sections previously studied by Nikiforova and Andreeva (1961) and Sokolov and Tesakov (1975) are used as a basis for paleontological substantiation of three generations of

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stratigraphic schemes (Kanygin et al., 2007). A detailed description of these sections with bed-by-bed paleontological characterization is included in several collective volumes (Myagkova et al., 1977; Moskalenko et al., 1978; Kanygin et al., 1982, 1984a, 1984b; 1989; Tesakov et al., 2003). These data were used for correlation of stratigraphic subdivisions on the Siberian Platform and Baltoscandia. A preliminary version of sequence stratigraphic scheme of the Ordovician of the Siberian Platform was published previously (Dronov, 2008; Dronov et al., 2008). These papers give a more detailed characterization of this scheme and its correlation with the scheme for Baltoscandia. SEDIMENTARY SEQUENCES IN THE ORDOVICIAN OF THE SIBERIAN PLATFORM Based on a summary of all existing paleontological and lithological data, including data from drilling, schemes of facies zonation for all regional stages were proposed by Kanygin et al. (2007). A generalized scheme of the Siberian Platform showing two sedimentary basins (Tungus Syneclise and Irkutsk Amphitheater) strongly differing in their sedimentary fillings is shown in Fig. 1. Within the Tungus Syneclise a normal marine environment with mainly carbonate sedimentation prevailed. In the marginal zones of this depression, in the northwest of the area (Kulyumbe River) and southwest (Podkamennaya Tunguska River) thick terrigenous series formed during the pre-Volginian regression indicating a nearby source of this clastic material. In the basin of the Kulyumbe River they are represented by the variegated Guragir Formation with traces of halite crystals, desiccation cracks, and wave ripples. In the basin of the Podkamennaya Tunguska River, at that time there accumulated the Baikit Formation, composed of coarse-grained quartz sandstones. The clastic material for that formation was most likely supplied from the land in the place of the modern Yenisei Range, where pre-Cambrian rocks are exposed. In the northeast of the Tungus Syneclise, in places where it is adjacent to the Viljuy Syneclise beginning from the Volginian Regional Stage, the amount of red-colored finely terrigenous material with carbonate cement increases, while in the Late Ordovician these beds are interbedded with gypsum and anhydrite, with the thickness of evaporate members increasing toward the upper part of the section. In the Irkutsk Amphitheater, Middle Ordovician beds are mainly represented by terrigenous shore facies, at some levels containing lingulids, large crustaceans, endemic conodontophorids, nautiloids, and occasionally ostracodes, i.e., groups typical of such marginal environments. To substantiate a sequencestratigraphic scheme, we also studied Ordovician reference sections in both sedimentary basins. In the Tunguska Syneclise we examined a section on the Kulyumbe River and main Ordovician outcrops in the Pod-

kamennaya Tunguska River. In the Irkutsk Amphitheater, we studied sections in the vicinity of the city of Bratsk, in the middle reaches of the Angara River and in the valley of the Lena River, between the towns of Ust’-Kut and Kirensk. A total of nine large sedimentary sequences are recognized in the Ordovician of the Siberian Platform, separated by surfaces of regional unconformities and their correlative conformities (Dronov et al., 2008). The rank of the sequence corresponds to sea level fluctuations of the third order (in the sense of Vail et al. 1977) and they have an average duration of between 1 and 10 Ma long. For the convenience of further discussion, we gave names to these sedimentary sequences, which are chosen after the names of corresponding strata (regional stages, formations, or superstages). From bottom to top these are: (1) Nya; (2) Ugry; (3) Kimai; (4) Baikit; (5) Volgina; (6) Kirensk-Kudrino; (7) Mangazeya; (8) Dolbor, and (9) Keto sequences. Below the above sequences are described and compared with synchronous sequences on the Russian Platform. 1. Nya Sequence In contrast to the Russian Platform, where an erosional unconformity is clearly observed at the base of the Ordovician, no visible erosional unconformity has been observed on the Siberian platform at the Cambrian–Ordovician boundary. In the section on the Kulyumbe River, the Upper Cambrian–Lower Ordovician beds are represented by a thick series of limestones and dolomites with numerous stromatolites, oolitic grainstones and flat-pebble conglomerates. The series was deposited in a shallow sea on a tropical carbonate platform. It shows some cyclicity with transgressiveregressive cyclites, often with traces of slight erosion at the base. However, no large erosional gaps are recorded in this stratigraphic interval. The correlation of the base of the Ordovician in the regional stratigraphic scale of the Siberian Platform with the GSSP of the base of the Ordovician remains debatable. In the previous regional stratigraphic schemes, the Cambrian–Ordovician boundary was drawn based on the occurrence of dendroid graptolites in the Loparian Regional Stage at the base of the underlying Mansian Regional Stage based on the clearly defined lithological boundary, which is possibly a sequence boundary (Kanygin et al., 2006). However, recent data on the distribution of Cordilodus conodonts in the sections, suggest that the Cambrian-Ordovician boundary lies much higher, and correlates approximately with the base of the Nyaian Regional Stage (Abaimova, 2006; Abaimova et al., 2008). The Nyaian Regional Stage has always been considered as a separate stage in the evolution of fauna on the Siberian platform (Kanygin et al., 2006) and, apparently, it corresponds to one sedimentary sequence. However, the

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Turukhansk Land

Anabar Land

Tu

ng

usk

a

Syncli

ne

Enisei Land

Katanga Land

Upper Lena Facial Zone Irkutsk Amphitheater Baical Lake

1 2

4

6

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5

7

Fig. 1. Distribution of the Ordovician beds on the Siberian Platform. Explanations: (1) Submerged regions of the Siberian Platform; (2) Regions lacking Ordovician deposits; (3) Boundaries of the Siberian Platform; (4) Zones of distribution of the Ordovician beds covered by younger beds; (5) Provisional boundaries of the Siberian Platform and lands; (6) Outcrops of the Ordovician deposits; (7) Boundary between the Tungus Syneclise and Irkutsk Amphitheater.

structure of this sequence needs clarification and, hence, additional study. On the Russian Platform, the Nya sequence correlates with the Pakerort sequence (Fig. 3). There is little in common between the two. The Nya sequence is represented by tropical carbonates, whereas the Pakerort sequence consists of cross-bedded quartz sands with fragments of shells of phosphatic brachiopods and PALEONTOLOGICAL JOURNAL

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black graptolite shales. The Pakerort sequence shows well-developed lower and upper boundaries represented by regional erosional unconformities. It is also clearly subdivided into the lower, shallow-water, and upper, deep-water parts. The lower portion is interpreted as a lowstand systems tract, while the upper portion is interpreted as a highstand systems tract. The transgressive systems tract in the Pakerort sequences is cut off by the erosion at the base of the overlying Latorp

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International Stratigraphic Scale

Siberian Platform regional horizons

Kety

Sedimentary sequences

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Dolbor

Makarovo

Lena River

Irkutsk Basin (formations) Angara River

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Chertovo

Krivolutsk

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Amarkan

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Sea level fluctuations.

Trangr. - Regr.

Critical Levels (events)

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? Bur Nirunda 8

7 Mangazeya Ancherik

Tareev

Ust-Rybninsk

Baikit

Ust-Stolbovaya

Dolbor Baksan Chertovo

Volgin

6 Kiren-Kudri 5

Ust-Kut

Cold-water carbonates

Warm-water carbonates

Kiren-Kudri Volgin Muktei

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Ugorsk

Ust-Kut

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Kimai

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Nyai

Badaran

Ugorsk

1

Vikhorev

Nyai

Mamyr

Stage Katian Sandby Darriwilian

Series Upper Middle Lower

Hirnantian Trema- Floian Dapindoc gian

System Ordovician

Fig. 2. Ordovician Sedimentary Sequences of the Siberian Platform.

International Stratigraphic Scale

Tommarp

Hunneberg Varangu

SB

SB

SB SB

SB

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SB SB SB

Boundaries

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SB SB SB

SB SB SB SB

SB

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Sequences

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Bur Nirunda

Not recognized

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Kiren-Kudri Volgin Muktei Vikhorev

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Ugorsk

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6 5

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Dolbor

Mangazeya

Kiren-Kudri Volgin Baikit Kimai

Ugorsk

Nyai

Fig. 3. Correlation of the Ordovician sedimentary sequences of the Baltica and Siberian paleocontinents.

Baltica Sequence units

10 Junstorp Fjacka

Sequences

Porku 9 8 Vesenberg

Regional stratigraphic units

Pirgu 7 Kegel

Vormsi Nabala Rakvere Oandu Keila

6

J’o~hvi Idavere Kukruse

Kunda

Tallinn

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Volkhov

5

Kunda

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Latorp

Uhaku Lasnamäagi Aseri

Volkhov

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Pekerort

Billingen

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British series Ashgill Caradoc Llanvirn Arenig Tremadoc

Stage Hirnantian Katian Sandby Darriwilian Dapingian Floian Tremadoc

Series Upper Middle Lower

Stage Ordovician

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Iya Formation

SB

Yst-Kut Formation

Fig. 4. Erosional unconformity at the base of the Ugry Sequence. Left bank of the Angara River near the former village of Pashino. Contact of oolitic grainstones and pillar stromatolites of the Ust’-Kut Formation (below) and coarse-grained quartz sandstones and gravelites of the Iya Formation (above).

sequence (Dronov and Holmer, 1999). In the Nya sequence, the unconformity at the base is almost invisible and despite the rhythmic cycles observed in the sections, no contrast between the facies allowing the identification of systems tracts is observed. 2. Ugry Sequence The Ugry sequence corresponds stratigraphically to the Ugorian Regional Stage. The base of the sequence is particularly clearly observed in the Ordovician basin of the Irkutsk Amphitheater, where it is represented by regional erosional surface at the boundary between the Ust’-Kut and Iya formations. In the latitudinal portion of the Angara River, 25 km upstream of the town of Kodinsk, this surface is clearly visible in outcrops (Fig. 4). It is interpreted as a 1st-type sequence boundary and is marked by a sharp shift from carbonate to siliciclastic sedimentation. This apparently happened as a result of a large-scale sea level drop causing the tropical carbonate platform to dry up and develop karst in the Ust’-Kut time, which led to the destruction of the shallow-water “carbonate factory.” Despite the following rapid sea level rise, carbonate sedimentation did not resume, and coarse-grained and cross-bedded sands began accumulating in the shallow-water areas. In the outcrops of the Lena River, the base of the Ugry Sequence is represented by a transgressive flooding surface, which reflected an abrupt change in sedimentation. This surface separates shallow-water marine

tropical carbonates with stromatolitic bioherms of the Ust’-Kut Formation from relatively deep-water flyschoid series of the overlying Krivaya Luka Formation. These deposits are represented by interbeds of quartz tempestites alternating with interbeds of dark-gray siltstones. In the Tungus Syneclise (section on the Kulyumbe River), the boundaries of the Ugry sequence are less discernible. Deposits of the Ugorian Regional Stage compose the middle part of the Iltyk Formation and are not fundamentally different from the beds of the underlying Nyaian Regional Stage and from the rocks of the overlying Kimai Regional Stage. They are represented by yellowish-gray, in places clayey dolomites and marls with rare interbeds of gray oolitic limestones (grainstones) and marls. The Ugry sequence of the Siberian Platform approximately corresponds to the Latorp Sequence of the Russian Platform (Fig. 3). A large regional unconformity and all three sedimentary systems tracts are relatively easy to recognize. In the south of the Siberian Platform, a regional unconformity (Fig. 4) in the Irkutsk Basin, at the base of the Ugry sequence, is distinguished, although parasequences and identification of sedimentary systems tracts require more detailed examination. 3. Kimai Sequence The Kimai Sequence corresponds to the Kimai Regional Stage. In the sections of the Angara River valley, the base of the Kimai Regional Stage corresponds


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to the boundary between the Iya and Badaranovo formations. The boundary between sequences coincides with an erosional surface that is overlain by a 0.6-mthick unit of quartz-glauconite sandstones with interbeds of bioclastic glauconite limestones and gravel conglomerates. The underlying beds of the Iya Formation and the overlying beds of the Badaranovo Formation are represented by uniform quartz sands and sandstones with cross-bedding inclined in the same direction. A unit with glauconite is interpreted as a condensed section, developing in the upper part of the transgressive systems tracts (Van Wagoner et al., 1988). The boundary of the sedimentary sequence, in this case coincides with the transgressive surface. Beds with glauconite were formed during a rapid rise of the sea level, when the input of siliciclastic material from the adjacent land was essentially suppressed, and therefore the sedimentation rates were considerably lower. Glauconites are very characteristic of condensed sections of transgressive systems tracts (Loutit et al., 1988; Schutter, 1996). The overlying cross-bedded coastal quartz sands of the Badaranovo Formation are interpreted as a highstand sedimentary systems tract. In the Tungus Syneclise (section on the Kulyumbe River), the deposits of the Kimai sequence are mainly represented by gray fine-grained oolitic grainstones with subdominant beds of dolomites and siltstones. They are represented by more marine and more pure varieties of limestone, compared to the underlying dolomites of the Ugry Sequence. However, no clear erosional surface similar to that between the Ugry and Kimai sequences in the Irkutsk Amphitheater have so far been observed. In most regions of the Siberian Platform, Kimaian beds represent transgression (Kanygin et al., 2006). That time is characterized by the unification of sedimentary environments, which facilitates wide dissemination of similar benthic and pelagic biocenoses. The Kimai Sequence approximately corresponds to the Volkhov sequence of the Russian Platform (Fig. 3). Deposits of both sequences are represented mainly by carbonates, although in the Volkhov sequence, these are cold-water carbonates with many glauconite grains in the rock matrix, and in the Kimai sequence these are mainly tropical oolitic grainstone in the Tungus Syneclise and quartz sands in the Irkutsk Amphitheater. It is noteworthy that the base of sequences, both in the Russian and Siberian platforms, is represented by the 2nd-type sequence boundary (Van Wagoner et al., 1988), i.e., by a boundary, on which no deep erosion of underlying beds, or traces of subaerial exposition may be observed. The Volkhov Sequence forms a complete sedimentary cycle, within which all three systems tracts are recognized. Inside the Kimai Sequence it is still impossible to identify sedimentary systems tracts. PALEONTOLOGICAL JOURNAL

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4. Baikit Sequence The Baikit Sequence is represented by Baikit Formation, including beds of the Vikhorevian and Mukteian regional stages. Sections of this formation are the most complete in the basin of the Podkamennaya Tunguska River, where it is represented by a uniform series of light-gray and yellowish quartz sands and sandstones 5 to 80 m thick. The sands are composed of angular, less commonly well-rounded grains of quartz (up to 80% of the total rock weight) with rare grains of feldspar and leaves of muscovite. The sands are coarselybedded often massive. Cross-bedding resulted from the alternation of beds differing in the size of the grains. The sandstones of the Baikit Formation are very mature, the grains are well sorted. Several horizons show cross-bedded series and sometimes layers of conglomerate, especially at the base (Markov, 1970). The Baikit sandstones constitute a sedimentary body continuing laterally and traceable in the basin of the Podkamennaya Tunguska River at a distance of over 600 km. The base of the Baikit sequence can rarely be observed in outcrops because of the generally poor exposition. However, Markov (1970) noted that near the village of Sulomai, the Baikit sandstones overlie various lithostratigraphic units of the Ordovician and even the Lower Cambrian. Markov also noted an angular unconformity between the rocks of the Evenk Formation (Upper Cambrian) and Baikit sandstones near Glinyanyi Creek in the basin of the Podkamennaya Tunguska River. The inner structure of the Baikit sequence is poorly known because of its relatively monotonous composition and poor exposure. However, there is no doubt about the unconformities at the base and at the top of the Baikit sandstones. Generally, the Baikit Sequence represents a regressive stage in the development of the Tungus Basin. It is noteworthy that the underlying Kimai Sequence is partly eroded almost over the entire territory of the Siberian Platform. In the east and northeast of the platform, the deposits of Kimai age are completely eroded (Kanygin et al., 2006). Unconformity at the base of the Baikit sequence may be a result of one of the strongest regressions in the Ordovician on the Siberian Platform. The Baikit sequence on the Siberian Platform correlates with the Kunda Sequence on the Russian Platform (Fig. 3). These sequences are considerably different in the composition of their sediments. The Kunda Sequence, similar to the underlying Volkhov Sequence, is represented mainly by cold-water carbonates. The Baikit Sequence mainly consists of terrigenous deposits, often represented by quartz sandstones. At the base of the Baikit sandstones there observed a gap and a regional unconformity. At the base of the Kunda Sequence, there is also a large unconformity suggesting the erosion of underlying beds. Thus, both sequences in the base have a 1st-type sequence boundary (Van Wagoner et al., 1988). Within the Kunda Sequence there are

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Angir Formation SB

Guragir Formation

Fig. 5. Transgressive surface at the base of the Volgina Sequence (base of the member of black shales). Left bank of the Kulyumbe River, contact of the Guragir and Angir Formations.

three sedimentary systems tracts approximately corresponding to three substages of the Kunda Regional Stage. Within the Baikit Sequence sedimentary systems tracts have not yet been recognized. 5. Volgina Sequence The Volgina Sequence corresponds to the entire Volginian Regional Stage, with deposits forming a distinct transgressive-regressive cyclite, clearly recognizable over the Siberian Platform. The erosional unconformity at the base of the Volgina Sequence is clearly observed in the basin of the Podkamennaya Tunguska River, where in the outcrops between the Listvennichnaya and Stolbovaya rivers, there is a conglomerate, composed of fragments of the underlying Baikit sandstones. In addition, at the top of the Baikit sandstones, there was observed a subaerial weathering crust, connected with a large gap, whereas the redeposited products of this crust are recorded in the basal beds of the Volgina Sequence (Kazarinov et al., 1969; Markov et al., 1971). A clear erosional surface at the base of the Volgina Sequence is also recognized in the northwestern parts of the Tungus Syneclise, in the section on the Kulyumbe River (Fig. 5). Here, it is also expressed as a transgressive surface, with traces of considerable deepening of the basin toward the upper part of the section. In the section on the Kulyumbe River, the deposits of the Volginian Regional Stage correspond to the Angir Formation. The topmost part of the Angir Formation

shows many features suggesting that the basin became shallow, the process accompanied by the influx of quartz sands and characteristic textures with variously orientated cross-bedding structures. In the basin of the Podkamennaya Tunguska River, at the top of the Volginian Regional Stage there is a bed of the massive coarse-grained sandstones, apparently corresponding to the highstand systems tract of the Volgina Sequence (Fig. 6). The Volgina Sequence marks the beginning of one of the most prominent transgressions on the Siberian platform. Faunal assemblages of Volginian age are clearly recognizable and are readily distinguished from those of the underlying beds. The unconformity at the top of the Volgina Sequence is not well developed compared to its base, although the shallowing is registered in all sections. This was the reason to unite the Volgina Sequence and the overlying Kirensk-Kudrino Sequence into a joint Krivaya Luka sedimentary sequence (Dronov et al., 2008). However, it is apparently more logical to recognize them as separate sedimentary sequences, as it is done in this study. 6. Kirensk-Kudrino Sequence The Kirensk-Kudrino Sequence stratigraphically equals the Kirensk-Kudrinian Regional Stage. Like the underlying Volgina Sequence, it forms a complete sedimentary cycle. In sections in the lower reaches of the Podkamennaya Tunguska River, this sequence forms a distinct regressive-transgressive-regressive cyclite, comprising three elements, corresponding to a low-


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Kiren-Kudri Horizon SB Volgin Horizon

Fig. 6. A bed of coarse-grained quartz sandstones in the upper part of the Volgina Sequence. Left bank of the Stolbovaya River. Ust’-Stolbovaya Formation.

stand systems tract, transgressive systems tract, and highstand systems tract. The base of the KirenskKudrino Sequence is represented by a transgressive surface (Fig. 6). In the section on the Kulyumbe River, at the base of the Kirensk-Kudrino Sequence there is also a transgressive surface, at which the thick-bedded limestones and cross-bedded quartz sandstones with carbonate cement, developed at the top of the Angir Formation, are overlain by greenish-gray siltstones of the Amarkan Formation. The middle and upper parts of the Formation are mainly composed of reddish siltstones. No certain highstand tract deposits have been identified. It is possible that they are cut off by the erosion at the base of the overlying Mangazeya Sequence. In the Irkutsk Amphitheater, the boundary between the Volgina and Kirensk-Kudrino sequences is apparently represented by the so-called “Middle Mamyr Unconformity.” Individual sedimentary systems tracts cannot be presently recognized in the Irkutsk Amphitheater because of the poor expose of this stratigraphic interval and the absence of adequate drilling data. The Volgina and Kirensk-Kudrino sequences on the Siberian Platform are very similar in the nature and composition of their sediments. The upper, KirenskKudrino Sequence, has a greater thickness. It overlies the Volgina Sequence and represents its natural continuation. The most prominent unconformities are recorded at the base of the Volginian Regional Stage and at the top of the Kirensk-Kudrinian Regional Stage. At the boundary between the Volginian and KirenskPALEONTOLOGICAL JOURNAL

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Kudrinian regional stages, the unconformity is not that clear, although Volginian beds form a well recognized sedimentation cycle. In fact, the Volginian and KirenskKudrinian beds can be united in one sequence (Dronov, et al., 2008). In that case the Volginian sedimentary cyclite could be considered as a lowstand systems tract of this combined sequence, or as a separate parasequences set with a displayed retrogradational stacking patterns within the transgressive sedimentary systems tract. This combined sequences corresponds to the Tallinn sequence on the Russian Platform, which includes the Aseri, Lasnamägi, Uhaku, and Kukruse regional stages (Fig. 3). The Volgian sedimentary cyclite corresponds to the Aseri Regional Stage on the Russian Platform. These have a similar strata architecture and can be interpreted either as a lowstand tract, or as an initial portion of a transgressive systems tract. The deposits of the Lasnamägi and, partly, the Uhaku regional stage constitute a transgressive sedimentary systems tracts, whereas the uppermost part of the Uhaku Regional Stage and the Kukruse Regional Stage constitute a highstand systems tract. 7. Mangazeya Sequence The Mangazeya Sequence corresponds to the Chertovskian and Baksanian regional stages. Its lower boundary coincides with the base of the Chertovskian Regional Stage, which is represented by a transgressive surface

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Chertovo Horizon SB Kiren-Kudri Horizon

Fig. 7. A bed of phosphatic conglomerates, forming a transgressive lag, overlying the transgressive surface at the base of the Mangazeya sedimentary sequence. Left bank of the Lena River near the village of Makarovo. The boundary between the Krivaya Luka and Chertovskaya formations.

over almost the entire territory of the Siberian Platform. In the Ordovician sections on the Kulyumbe River and along the Lena River, traces of erosion and conglomerate are documented at this level, which can be interpreted as a transgressive lag (Fig. 7). In sections in the valley of the Podkamennaya Tunguska River, the Mangazeya Sequence corresponds to the upper portion of the Ust’Stolbovaya Formation (referred to the Chertovskian Regional Stage) and the Mangazeya Formation. The respective portion of the Ust’-Stolbovaya Formation is interpreted as a transgressive systems tract, whereas the Mangazeya Formation is interpreted as a highstand systems tract. The transgressive systems tract in this section consists of two subdivisions: (1) a member of intrelayered greenish-gray siltstones, fine-grained yellowishgray sandstones and black shale with large carbonate nodules containing faunal remains of the Chertovskian Regional Stage; and (2) a member of red siltstones with scattered rounded phosphate pebbles from 0.5 to 2 cm in diameter, alternating with beds of red gravel phosphatic conglomerates composed of these pebbles. Both black shales and red phosphatic conglomerates are interpreted as relatively deep-water deposits. The highstand systems tracts in the section on the Podkamennaya Tunguska River is represented by a flyschoid series of greenish-gray siltstones interbedded with micritic (in the lower part) and bioclastic limestones. Bioclasts are mainly represented by fragments of brachiopod shells and trilobites carapaces. Frag-

ments of crinoids, ostracodes and bryozoans are also present. At some levels at the top of limestones layers, wave ripples are observed. Several layers contain glauconite. Deposits of the Mangazeya Formation are interpreted as cold-water carbonate tempestites. The Mangazeya Sequence on the Siberian Platform corresponds to the Kegel Sequence on the Russian Platform (Fig. 3). Both sequences have at the base a clear transgressive surface and in both cases, transgressive tract and a highstand tract can be clearly recognized. The erosion of the underlain beds is best seen on the Siberian Platform, which may be related to a greater hydrodynamic energy in the basin, or a stronger sea level drop before the transgression. The character of sedimentological content of these sequences is contrasting. The Keila beds on the Russian Platform are represented by tropical carbonates, whereas the Basksan beds on the Siberian Platforms are cold-water carbonates. 8. Dolbor Sequence The Dolbor Sequence corresponds to the Dolbor Formation, which approximately corresponds to the Dolborian Regional Stage. It is noteworthy that the faunal assemblage typical of the Dolborian Regional Stage is actually found a few meters below the boundary between the Mangazeya and Dolbor Sequence i.e., in the Mangazeya Sequence. The Dolbor beds are repre-


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Dolbor Horizon SB Baksan Horizon

Fig. 8. An abrupt change in lithology at the base of the Dolbor Sequence. Right bank of the Podkamennaya Tunguska River, 1 km above the mouth of the Stolbovaya River. A boundary between the Mangazeya and Dolbor Formations.

sented by yellowish-gray fine-grained sandstones and siltstones, in places with carbonate cement. In sections in the lower reaches of the Podkamennaya Tunguska River between the mouth of the Stolbovaya River and Listvennichnaya River the base of the sequence is prominent, easily discernible in the outcrop and readily diagnosed (Fig. 8). This boundary is marked by a change in sedimentation. A series of bioclastic limestones interbedded with greenish-gray siltstones characteristic of the Mangazeya Formation is overlain by a series of mainly sandy and siltstone beds. In the outcrop near the mouth of the Stolbovaya River, the boundary is distinct but appears conformable. No traces of large erosion of underlying beds are noticeable here. At the same time, V.I. Bgatov indicates that in the sections in the middle reaches of the Podkamennaya Tunguska River, the upper horizons of the Baksanian beds are eroded, and sharp-angled fragments of rocks from the underlying beds are found in the depressions on the ancient relief. Erosional pockets can be up to 0.4 m deep. The same author noted the presence of the pre-Dolbor weathering crust in these sections (Bgatov, 1973) and, hence suggested a large gap and unconformities. The boundary between the Mangazeya and Dolbor sedimentary sequences is drawn at this unconformity. Individual rhythms are recognized within the Dolbor Sequence, but the recognition of sedimentary systems tracts is as yet impossible. The Dolbor Sequence corresponds to the Wesenberg Sequence on the Russian Platform (Fig. 3). The latter contains a PALEONTOLOGICAL JOURNAL

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transgressive tract corresponding to the Oandu Regional Stage Horizon and a highstand tract corresponding to the Rakvere Regional Stage. The base of the sequences in both cases coincides with a transgressive surface. 9. Keto Sequence The Keto Sequence corresponds to the Nirundian and Burian regional stages, referred to the Keto Regional Supestage. The best sections of this sequence are in the basin of the Podkamennaya Tunguska River, along the Bolshaya Nirunda and Nizhnyaya Chunku Rivers. We did not observe this sequence in natural outcrops, and the conclusions presented in this paper are based on previous publications. Judging from these data, the base of the Keto sequence is quite distinct and is drawn at a level where cherry-red shales of the Nurunda Formation overlie the yellowish-gray and greenish-gray sandstones and siltstones of the Dolbor Formation. Like the cherry-red shale of the KirenskKudrino Sequence, these beds are interpreted as relatively deep-water open sea sediments of a transgressive systems tract. The overlying beds of the Burian Regional Stage can be interpreted as a highstand systems tract of the Keto sedimentary sequence, although it is possible that these beds may be recognized as a separate sequence. The boundary with the Silurian is unconformable and is marked by strong erosion of underlying Ordovician beds. It is possible, that the cen-

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tral zones of the basin may contain stratigraphically higher Ordovician beds, which will be referred to a different sequence. The Keto sequence on the Siberian platform, apparently, corresponds to the Fjäcka Sequence on the Russian Platform (Fig. 3). However, the relatively deepwater deposits of the Fjäcka sequence is characterized by black-shale sedimentation, whereas for the Keto sequence marine red bed sedimentation is typical. In this respect, the Keto Sequence is the most similar to the Jonstorp Sequence of the Russian Platform, which overlies the Fjäcka Sequence. The degree of knowledge on the sequence stratigraphy of the Upper Ordovician on the Siberian Platform does not yet allow identification of the number and features of sedimentary sequences in this stratigraphic interval. The correlation suggested is just preliminary. In the uppermost Ordovician of the Russian Platform there are at least two more sequences—Jonstorp and Tommarp (Dronov and Holmer, 1999), but on the Siberian platform these beds are in most cases cut by erosion, and in places of their possible presence, e.g., in the central areas of the Tungus Syneclise, they are insufficiently studied. Therefore positive correlation of sequences in this part of the section is still impossible. MAJOR TURNING POINTS IN THE ORDOVICIAN OF THE SIBERIAN PLATFORM Each of sedimentary sequences recognized is a relatively large transgressive-regressive cyclite, connected with fluctuations of the relative sea-level and accompanied by noticeable lithological changes in sedimentary environments. In fact, the base of each sequence is a level marked by noticeable lithological changes. However, depending on the magnitude of transgressions and regressions, the rate of sea level changes, relationships between transgressive and regressive components and accompanying paleogeographic and paleoclimatic changes it is possible to recognize certain levels marked by even stronger changes. These levels have the largest correlative potential. In the Ordovician of the Siberian Platform five such levels may be recognized: (1) base of the Baikit Sequence; (2) base of the Volgina Sequence; (3) base of the Mangazeya Sequence; (4) base of the Keto Sequence, and (5) Ordovician–Silurian boundary (Fig. 2). I. Base of the Baikit Sequence Regional unconformity at the base of the Baikit sandstones is one of the most prominent levels in the Ordovician of the Siberian Platform. In the west of the platform, in the region of the Yenisei Range, Baikit sandstones overlie various horizons including those of Late Cambrian age, with a small angular unconformity (Markov, 1970). The influx of large amount of siliciclastic material into the basin indicates an expansion of the source area on the Yenisei Land (Bgatov, 1973), an

apparent tectonic elevation of this area, and a forced regression accompanying this elevation. This forced regression led to the destruction of the tropical carbonate platform, which had existed on the Siberian Platform in the Riphean, Vendian, and early Cambrian. This regression is revealed in the Igarka–Norilsk Zone by the deposition of quartz sandstones and siltstones of the Guragir Formation, and in the Berezovo and MarkhaMarkoka zones (Kanygin et al., 2007) as a large erosional gap. The underlying Kimai deposits were partly or completely eroded in many places on the Siberian platform (Kanygin et al., 2006; 2007). In the Irkutsk Basin, the tropical carbonate platform had been destroyed even earlier, during the regression at the base of the Ugry Sequence. However the final disappearance of the tropical carbonate factory on the Platform was not until the forced regression at the base of the Baikit sequence. The amplitude of this regression may be estimated as “large”, i.e., >75 m (Haq and Schutter, 2008). This was one of the largest regressions in the Ordovician of the Siberian Platform and one of the most important turning points in the evolution of sedimentation and the biota. II. Base of the Volgina Sequence The transgressive surface at the base of the Volgina Sequence marks the beginning of a large transgressive event after a lowstand, marked by the Baikit sandstones. This transgression led to a complete change of faunal assemblages and prevailing type of sedimentation. The base of the Volgina Sequence (Volginian Regional Stage) was a chosen level of the boundary between two large stages in the geological history of the Ordovician of the Siberian Platform, distinct in lithology, facies and thickness of rocks, and in the composition of biotas (Kanygin et al., 2006). Contrasting differences in the lithological and paleontological characterization of the Ordovician above and below this boundary served as a basis for subdivision of the Ordovician on the Siberian Platform into two series (lower and upper) (Sokolov and Tesakov, 1975). Although the transgression that led to the formation of the Volgina Sequence was not the largest, the transgressive surface at the base of the Volgina Sequence is one of the most significant turning points in the Ordovician of the Siberian Platform. III. Base of the Mangazeya Sequence The transgressive surface at the base of the Mangazeya Sequence is clearly discernible over the entire platform and is in many places accompanied by features of erosion of the underlying rocks and presence of basal conglomerates at its base. The top of the KirenskKudrino Sequence appears to be marked by a forced regression, whereas the subsequent transgression resulted in the formation of the transgressive surface of erosion with conglomerates that constitute an overlying


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transgressive lag. Apparently, the transgressions in the basal beds of the Volginian and Chertovskian regional stages were the largest in the Ordovician of the Siberian Platform (Kanygin et al., 2006). In rate and magnitude the transgression at the base of the Mangazeya Sequence possibly exceeded the Volginian transgression. The facial differentiation at the time of this transgression was minimal, which facilitated wide distribution of the similar benthic and pelagic biocoenoses in most regions of the Siberian Platform. The transgression at the base of the Mangazeya Sequence apparently corresponds to a global eustatic maximum at the base of the graptolite Nemagraptus gracilis Zone (Fortey, 1984; Barnes et al., 1996). IV. Base of the Keto Sequence Judging from published descriptions, cherry-red siltstones of the Nirunda Horizon are facial equivalents of relatively deep-water facies of the Kirensk-Kudrinian and Chertovskian regional stages. Hence the boundary between the Dolbor and Keto sequences represents a transgressive surface and indicates a level of a rapid sea-level rise. This level is insufficiently studied because the deposits at this stratigraphic interval are poorly exposed, while the best outcrops of the Dolborian, Kirensk-Kudrinian, and Burian regional stages are located in poorly accessible regions of the Siberian Platform. Thus, this level is at present drawn only provisionally. V. Ordovician–Silurian Boundary The Ordovician–Silurian Boundary is marked by a distinct discontinuity in sedimentation, which resulted from a large-scale forced regression. It is possible that this drop in the sea level was connected to the continental glaciation of Gondwana and is hence eustatic. The uppermost beds of the Ordovician on the Siberian Platform are always eroded. The beginning of the Silurian marks a new transgression and essential renewal of the ecosystems. A regressive event at the Ordovician–Silurian boundary is one of the most significant turning points, reflected in changes in fauna and sedimentation in the Ordovician and even in the entire Phanerozoic. Thus, the history of the evolution of the Ordovician epicontinental basins of the Siberian paleocontinent contained five most important levels of turnover (Fig. 2). Two of these levels (base of the Baikit Sequence and the Ordovician–Silurian boundary) are related to forced regressions. The other levels (bases of the Volgina, Mangazeya, and Keto sequences) are related to transgressive surfaces marking a rapid rise in sea level followed by large-scale transgressions. LONG-TERM CHANGES IN SEDIMENTATION The recorded transgressions and regression are often superimposed on the long-term changes in the PALEONTOLOGICAL JOURNAL

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character of sedimentation reflecting climatic, oceanographic and paleogeographic changes. Among such lithological changes in the Ordovician of the Siberian Platform it is possible to recognize a sharp change in carbonate sedimentation from the typically tropical carbonates to temperate carbonates, and large impulses of influx of terrigenous sediments in the sedimentary basin and impulses of deposition of phosphates. Carbonate sediments are clearly distinguished by their lithology, associated minerals and set of primary sedimentary textures between so-called warm-water or tropical (photozoan) and cold-water or nontropical (heterozoan) carbonates (Lindström, 1984; James, 1997; Dronov, 2001; Dronov and Rozhnov, 2007). We will not describe here details and characteristics of either type, but indicate that the temperature boundary between these two types of carbonates was an annual temperature of water around 22°C (James, 1997). Tropical carbonates are characterized by a wide spectrum of various types of grains in the sediment including skeletal remains (detritus), intraclasts, pellets, and oolites. These carbonates typically show the development of barrier carbonate reef buildups, including numerous stromatolites and cyanobacterial mats. The presence of fine carbonate mud in the sediment is typical. Nontropical carbonates are characterized by much more restricted spectrum of allochems, dominated by bioclasts. Pellets and ooids are usually absent, or very rare. Barrier reef buildups and cyanobacterial mats are virtually absent. In the shallow-water shelf sediments carbonate mud is virtually absent, although it may accumulate due to bioerosion in small quantities below the storm wave base. Apart from temperature, the character of carbonate sediments is influenced by water transparency and content of nutrients. An increase in the amount of nutrient and terrigenous suspension in the water (increased concentration of suspended material) shifts the spectrum of carbonates to the cold-water side and can make carbonates look nontropical even when the water temperature is high. As noted above, contrasting differences in the lithological and paleontological characterization of the Ordovician of the Siberian Platform served as a basis for subdivision of this system into two series (lower and upper) (Sokolov and Tesakov, 1975). These series are separated by a transgressive surface at the base of the Volgina Sequence. The lower series (Nyaian–Mukteian regional stages) are characterized by predominance of tropical carbonate sedimentation. The dominating lithotypes include oolitic grainstones, stromatolites and cyanobacterial mats are widespread. The beds characteristically contain flat-pebble conglomerates formed by breaking and redeposition by tropical storms of carbonated crusts on the tidal flats of carbonate platforms. The thickness of the deposits of the lower series ranges from 120 to 700 m (440 in average). The upper series (Volginian–Burian regional stages) is characterized by accumulation of mainly fine-

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grained terrigenous deposits with subdominant amount of carbonates which are mainly represented by nontropical varieties (Fig. 2). The beds are dominated by limestones (bioclastic packstones). Stromatolites, dolomites, and large organic buildups are virtually absent. Pellets and ooids are very rare, and in places where found they are subdominant. Some levels show accumulations of glauconitic films and grains. The thickness of deposits of the upper series is sharply reduced ranging from 90 to 300 m and on average about 170 m. Reduced thickness suggests reduced sedimentation rate, and hence sharp decrease in productivity of the shallow-water “carbonate factory.” The replacement of the tropical carbonate sedimentation by non-tropical sedimentations was preceded by the destruction of the warm-water carbonate factory and large influx of siliciclastic material into the basin. Based on the paleomagnetic data, it is established that in the Cambrian, Ordovician, and Silurian, the Siberian paleocontinent was located in the tropics (Cocks and Torsvik, 2007). Therefore the replacement of tropical carbonates by cold-water carbonated in the Middle and Upper Ordovician can only be explained by upwelling of cold water from the depth to the continent. The upwelling of the colder oceanic water can explain the suppressed tropical carbonate sedimentation, the absence of the carbonate mud, pellets, and ooids, suppressed growth of organic buildups, stromatolites, and cyanobacterial mats, presence of glauconite and skeletal cold-water carbonates. The upwelling of cold, oxygen-deprived water from the depths of the ocean, could also facilitate the influx of nutrients, which was reflected in the wide distribution of phosphatization and phosphate conglomerates in the Volgina, KirenskKudrino, and Mangazeya sequences. It should be noted that the large influx of nutrients increases the effect of cold water and also suppresses tropical carbonate sedimentation in favor of temperate carbonates. In the case of large supply of nutrients, the water temperature may be higher than necessary to create the effect of “coldwater carbonates.” The increased opacity of sea water resulting from increased input of the terrigenous material from the nearby land could also contribute to this effect.

global, but to local factors (facies, taphonomy, state of knowledge).

CORRELATION OF THE ORDOVICIAN BIOTAS IN THE SIBERIAN AND RUSSIAN PLATFORMS

The comparison of the taxonomic composition and biodiversity dynamics of four major dominant groups allow the recognition of general evolutionary trends and features of biotas of the Siberian and Russian platforms. The comparison of the taxonomic composition of these two major components of biotas shows that benthic communities differ considerably throughout the Ordovician. No species in common are documented in the trilobite, ostracode, and brachiopod assemblages. In the Early Ordovician (Tremadocian, Arenig), these groups show no genera in common between the two platforms. Some similarities in the generic composition of the trilobite and brachiopod assemblages appear

The states of knowledge on fossil record from the Siberian and Russian platforms are compatible. Data from these regions may be considered sufficiently representative for comparison of the biotas in two criteria: (1) taxonomic composition of dominant fossil groups and (2) dynamics of changes in their biodiversity at significant levels in the evolution of these paleobasins. These data are the integral indicator of the optimal condition of the development of biotas in normal marine environment and reflect major evolutionary trends, with the minimum of error, and the latter may be due not to

In the Ordovician, a global ecosystem reorganization of the marine sector of the biosphere, the largest in the Phanerozoic, took place, and its scale and evolutionary consequences is comparable with the explosive Cambrian biodiversification of the organic world on Earth, when all major phyla of invertebrate organisms appeared. In the Ordovician, the biodiversity of marine animals increased almost twofold due to the appearance and rapid divergence of pioneering groups or diversification of previously small benthic groups with new ecological specializations and more diverse adaptive opportunities (corals, bryozoans, echinoderms, articulate brachiopods, stromatoporoids, and ostracodes). In the Ordovician, the biodiversity and population density of trilobites previously monopolizing the benthal, became to decrease, because of the increased competition for food resources, although they still retained their role as major components of benthic biocoenoses. At the same time, a stable zoopelagial was formed for the first time (to replace the previous facultative one) to contain chitinozoans, a new highly productive group of microphytoplankton, which apparently became the main initial food resource for specialized groups of zooplankton and nekton (graptolites, radiolarians, conodontophorids, nautiloids, and agnostids) (Kanygin, 2001; Kanygin, 2008). Of the above fossil groups, trilobites, ostracodes, brachiopod, and conodonts are the best indicators of evolutionary changes in the biotas, due to their high population density and taxonomic diversity, similar level of knowledge, and reliable identification based on a variety of morphological criteria. The taxonomy of large groups (colonial corals, stromatoporoids, and bryozoans) is complicated by broad intraspecific variations of live forms and difficulties of identification based on thin sections. Data on echinoderms (although this group is large) are still not representative because they are unequally studied, especially on the Siberian Platform. The data on naulitoids, graptolites, radiolarians, chitinozoans, and other pioneer Ordovician groups are also important for characterization of the ecosystems of the epicontinental basins of the Siberian and Russian platforms, but these are even less complete.


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from the Middle Ordovician (Llanvirn). Six trilobite genera and five brachiopod genera in common are recorded from these regions, but their stratigraphic ranges are essentially different: in the Siberian Platform they are found in the Volginian Regional Stage only (Llanvirn), which corresponds to the beginning of a large transgression, whereas in Baltoscandia these genera continued to exist to the Mid-Ashgill. Even stronger differences are recorded in the taxonomic composition of the benthic assemblages of ostracodes, which contain no genera or even families in common (except some of doubtful identification). At the same time many readily identified ostracode taxa (e.g., species with lobe-like dissected shells) show relatively synchronous phenotypic changes in parallel phylogenetic lineages. Such uniform phenotypic changes in isolated populations of species with similar morphological architectonics in genetics are referred to as “mutation fashion”, which can certainly can be revealed under the influence of global environmental factors. This “fashion” is taxonomically expressed by the presence of the parallel phylogenetic lineages of twin species, which belong, for instance, on the Siberian Platform to the family Cherskiellidae, and in Baltoscandia to the family Tetradellidae. The autochthonous nature of benthic communities on the Siberian and Russian platforms suggests their paleogeographic separation, especially strongly developed in the Early Ordovician. The appearance of the same genera of trilobites and brachiopods in the Early– Middle Ordovician boundary interval can be used as evidence of the convergence of these paleobasins, which agrees with modern palinspastic reconstructions. The life cycle of trilobites and brachiopods has a meroplanktonic stage, during which larvae are capable of moving within relatively short distances in the ocean, and therefore provided entries in similar biotopes of other epicontinental basins. At this time many genera and even species in common of pelagic ostracodes (Coelochilina, Eurychilina, Laccochilina, Sigmobolbina, Oepikella n. sp.) appear in both paleobasins, which indicates the migration links between the basins in the pelagic zone without significant climatic barriers. The most adequate representation of general tendencies, their connections with global ecosystem reorganizations and eustatic events can be obtained from graphs of changes in biodiversity (Fig. 9), despite some conventions in their synchronization. The typification of data in two regions was based on regional stratigraphic scales, and their correlation using the previous British standard of stage subdivision of the Ordovician and its subdivision into series by the decisions of the Interdepartmental Stratigraphic Committee of the USSR/Russia. Unexpectedly, the comparison of the graphs of biodiversity showed a much lower biodiversification of dominant faunal groups on the Siberian Platform than PALEONTOLOGICAL JOURNAL

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in Baltoscandia. Beginning from classical reconstructions of climatic zonation in the Phanerozoic in the framework of the “fixism” paradigm (Strakhov, 1968), and in all subsequent paleogeographic reconstructions (now using the plate tectonics paradigm) the Siberian continent in the Ordovician was located in a warmer zone than Baltoscandia. Therefore, following general patterns of chorological differentiation of biodiversity in climatic belts one could expect the opposite situation, i.e., a higher number of taxa on the Siberian Platform. This paradox can be explained by the existence during the entire Ordovician in the Baltoscandian part of the Russian Platform of a more stable normal marine environment, whereas the paleobasin of the Siberian Platform was generally shallower with contrasting changing environments from normal marine to evaporite, brackish-water, and subaerial. This conclusion is supported by the fact that in the relatively deeper carbonates of the Verkhoyansk–Chukotka Folded Region and Taimyr, which at that time along with the Siberian platform were part of the large Kolyma-Siberian Province, the number of ostracodes species (the best studied group) increased two- or threefold (Kanygin, 1967, 1971), which is comparable with that in Baltoscandia. Another explanation could be in the cooling effect on the Siberian epicontinental basin of an upwelling, which supposedly emerged on its western margin (in modern geographical coordinates) due to the opening in the Ordovician of the northern branch of the Paleoasiatic Ocean (often also called Paleouralian Ocean). Graphs showing changes in biodiversity both in individual groups and in the integral representations agree well and can be correlated with global eustatic events. Some incongruity in synchronization of graphs of biodiversity of these regions could result from: (1) insufficient precision of biostratigraphic correlation markers, (2) effect of regional paleogeographic and tectonic features in the development of paleobasins, and (3) differences in the methods of interval-based counts of species. On the Siberian Platform, the typification of species composition is based on regional stages; on the graphs by Hammer on Baltoscandia (Hammer, 2003b), which are here used for comparison, data on the number of species were summed in conventional onedimensional chronological intervals. However, possible errors resulting from the above factors do not rule out the conclusion that the major trends and events in the development of biotas of the paleobasins of the Siberian and Russian Platforms were more or less synchronous. Two maxima of biodiversity of the benthic fauna in the Middle Ordovician particularly clearly correlate. The first of these on the Siberian Platform corresponds to the Volginian Regional Stage, and in Baltoscandia to three regional stages of the Estonian scale—Aseri, Lasnamägi, and Uhaku. The second peak on the Siberian platform corresponds to the Chertovskian and Baksanian regional stages, and in Baltoscandia to the Kukruse and Haljala regional stages. Both maximum peaks of biodiversity (like other less dramatic such peaks),

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Baltoscandia (Hammer, 2003) 280 240 An integrated graph showing the diversity 200 of dominant groups of fauna of Baltoscandia 160 120 80 40 50 Conodonts 40 30 20 10 120 Brachiopods 100 80 60 40 20 100 Tiolobites 80 60 40 20 30 Ostracodes 25 20 15 10 5 Billingen

Tremadoc

Arenig

Tremadoc

Floian Dapingian

Volkhov

Kunda Llanvirn

Darriwillian

Uhaku Kukruse Hal.

Vorm.

Hunneberg

Keila Oandu Rak

Pelagic species Aseri Lasn.

Pakerort Varan.

Benthic species

Nab.

Caradoc Sandbian

Pirgu

Porkun.

Ashgill Katian

Hirn.

Fig. 9. Comparison of graphs of diversity of dominant groups of fauna of the Siberia and Baltica paleocontinents.

coincide with transgressive phases in the development of paleobasins, and minima of biodiversity with regressive phases. A relative synchronicity of transgressiveregressive events and changes in biodiversity of dominating faunal groups in epicontinental paleobasins separated by ocean proves a global nature of such significant levels and can be used as an additional criterion for intercontinental stratigraphic correlations. To evaluate the dynamics of biodiversity of conodonts in the Siberian and Baltoscandian basins, an additional comment is necessary because this fossil group has become a primary paleontological indicator for global correlations. However, conodonts in different regions are studied to a varying extent, whereas data on their true taxonomic composition are difficult to systematize because at different times they were classified based on two alternative criteria—multi-element apparatuses and isolated elements. Even in Baltoscandia, where conodonts are justifiably considered as the best studied group compared to other regions of the world, curves based on statistical data using mathematical methods (Hammer, 2003a, 2003b), are insufficiently precise in interpretation of true diversity of this group.

In this regions, the most reliable is the dynamics of the diversity of conodonts from the Lower to lower part of the Middle Ordovician, showing a gradual increase in number of species from 25–30 in the Upper Tremadocian to 45–50 in the Lower Llanvirn (Fig. 9). A negative anomaly of the graph of the medium-weight diversity in the Middle Llanvirn–Lower Caradoc (Hammer, 2003b) is largely related to the widespread terrigenous facies, from where conodonts have virtually never been studied. The information on the Upper Llanvirn and Lower Caradoc conodonts have become more complete only recently because of the substantiation of the limitotype of the Sandbian Stage in southern Sweden (Bergström, 2007; Pålsson et al., 2002; Viira, 2008). The existing data show that compared to benthic communities, changes in sea level only slightly affected the diversity of conodonts of the Lower and beginning of the Middle Ordovician in Baltoscandia. Apparently, despite the changes in facial conditions in the regions of carbonate sedimentation related to sea level fluctuations, optimal environment for this pelagic group remained in this district. Lower diversity of conodonts (15–25 species) in the upper part of the Caradoc–begin-


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25 20 15 10 5 40 30 20 10 30 20 10 30 25 20 15 10 5

An integrated graph showing the diversity Siberian Platform of dominant groups of fauna of Baltoscandia

Conodonts

Brachiopods

Tiolobites

Ostracodes Benthic species Pelagic species Nyai

Ugorsk

Vikhorev Muktei

Kimai

Tremadoc

Arenig

Tremadoc

Floian Dapingian

Llanvirn Darriwillian

Volgin

Kudri Kir.-

80 60 40 20

1493

Chert. Baks. Dolbor

Nir. Bur

Caradoc Sandbian

Not recognized Ashgill

Katian

Hirn.

Fig. 9. (Contnd.)

ning of the Ashgill may be explained by displacement of the basin in the region of lower latitudes and warm temperatures. The distorting effect of the taphonomy on the contents of samples should also be taken into account because in the warm-water carbonates the amount of conodont elements is usually small, which makes the examination more difficult, collections smaller, and the number of species identified lower. On the Siberian Platform, in contrast to Baltoscandia, the Ordovician conodonts are studied more completely across different facial zones (Moskalenko, 1973; Kanygin et al., 1984b; 1989; Tesakov et al., 2003 and others.). Almost all regional subdivisions contain this group, conodonts are found in various facies, and are represented by numerous specimens. However, when calculating the diversity, there is an immediate problem of counting species, because in the majority of papers, conodonts are described using formal taxonomy, which considerably increases the number of taxa. The transition from the formal to multi-element taxonomy required detailed revision of faunal composition. Many Siberian conodonts are endemics and possibly have apparatuses not characteristic of most known taxa. To reduce the distorting effect of using different methods of calculations and classifications, the approximate PALEONTOLOGICAL JOURNAL

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number of multi-element taxa may be accepted as half of known formal species, because in most cases P and S-elements, or M and S-elements of real species have been described as separate species. Thus, it is most likely that no more than 15–20 conodont species existed at the same time in the Paleozoic basin of Siberia, and their diversity at the period of time considered remained relatively constant (Fig. 9). The small number of species in the Muktei Regional Stage is possibly related to the very low thickness of this unit. Low diversity of conodonts in the Volginian time, in contrast to high diversity of other fossil groups, is possibly connected with significant dominance of Phragmodus flexousus in the assemblages. Different stages of the evolution of the Siberian basin were characterized by different level of endemism of the conodont fauna. Assemblages of the Nyaian and Ugorian Regional stages contain up to 20% cosmopolitan species (Eoconodontus notchpeakensis, Cordylodus angulatus, C. proavus), and species characteristic of Laurentia and other regions of the MidContinent paleobiogeographic conodont province. In the Late Ugorian, Kimaian, and Vikhorevian regional stages, the endemism of conodonts was at its highest, with no species recorded from other continents found

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on the Siberian Platform. Some similarity was maintained only with the warm basins of Laurentia at the generic level (genera Loxodus, Coelodus, and Erismodus). However, beginning from the Volginian Regional Stage, Phragmodus flexuosus, Ph. inflexus and other species typical of the warm-water Mid-Continent Province became to appear. From that time the assemblages became more similar on the generic level with Baltoscandia, and this similarity gradually increases by the Baksanian and Dolborian epochs. These results agree well with the notion that the Baltoscandian basin by then was shifted to the tropic, and now there existed warm-water environment, similar to that of the Siberian. In the Late Ordovician (Dolborian time) conodont assemblages of the Siberian Platform, which was at that time dominated by shallow marine environment, experienced an influx of pelagic species typical of open marine basins (e.g., Periodon grandis). Thus, the data on the taxonomic composition and biodiversity dynamics of four dominant faunal groups, despite their considerable chorological and ecological differences, confirm similar trends in the evolution of the biotas and gradual change in the paleogeographic positions of paleobasins in the Ordovician–from being largely spatially and climatically separated to be convergent near the equator. CORRELATION WITH THE RUSSIAN PLATFORM The analysis of the geological history of the Russian and Siberian platforms in the Ordovician shows most strikingly in the contrasting sedimentation, thickness, and facies in these two paleocontinents. On the Russian Platform, the average thickness of regional horizons gradually increases upward in the sections, which reflects an increase in the mean rate of sedimentation. In addition, a gradual change from mainly terrigenous sedimentation to first cold-water, and then to tropical carbonates upward in the succession is observed in shallow-water environments (Dronov and Rozhnov, 2007). This climate-related successive change in sedimentation reflects the drift of the Baltica Paleocontinent in the Ordovician from the Subpolar regions of the southern hemisphere to the subequatorial regions, which agrees well with the paleomagnetic data (Cocks and Torsvik, 2005). On the Siberian platform the thickness of regional horizons in the lower and lower middle part of the Middle Ordovician is considerably higher than in the Upper Ordovician and the upper part of the Middle Ordovician. In addition, upward in the section, the predominantly carbonate tropical warm-water sedimentation changes by mainly terrigenous and coldwater terrigenous-carbonate. In other words, the trend was strictly the opposite of the trend observed on the Russian Platform. In addition, the replacement of one type of sedimentation to another on the Siberian Platform was not gradual, but quite abrupt and occurs at the base of the Volgina Sequence.

There is some similarity between the Ordovician on the Russian and Siberian platforms in the number of large sedimentary rhythms and stratigraphic position of their boundaries. Unfortunately, the Baltica and Siberian paleocontinents belonged to different paleobiogeographic provinces, and their fauna in the Ordovician contains almost no species in common. This prohibits detailed biostratigraphic correlation. However, for some stratigraphic intervals, which can be correlated, the number of sedimentary basins recognized within them is the same. This suggests synchronous development of the sedimentary sequences on both platforms and their eustatic nature. However, speaking about the major turning points in the history of the biota and sedimentation on the Russian and Siberian platforms in the Ordovician, their expression on both platforms is not always the same. For instance, a global regression at the Ordovician-Silurian boundary is clearly observed both on the Russian, and the Siberian platforms. It is apparently connected with the global drop in sea level caused by the Hirnantian Glaciation. In both platforms levels of re-organization in the Darriwilian time are clearly discernible. On the Siberian platform this is a transgressive surface at the base of the Volginian Regional Stage, associated with transgression, upwelling, and significant change in sedimentation. On the Russian Platform, this is a transgressive surface at the base of the Aseri Regional Stage, marked by a prominent renewal of the shelly fauna in all parts of the basin. This renewal was the basis for recognition of the Tallinn regional stage with a lower boundary coinciding with the base of the Aseri Regional Stage (Männil, 1966). A prominent turning point at the base of the Sandbian Stage (base of the Caradoc in Great Britain) is also distinct on both platforms. This level apparently coincides with a global eustatic transgression at the base of the Nemagraptus gracilis graptolite zone (Fortey, 1984; Barnes et al., 1996; Haq and Schutter, 2008). At the same time, a very important turning point for the Siberian Platform, at the base of the Baikit Sequence, related to a marked change in sedimentation, destruction of a tropical carbonate platform and influx of a vast quantity of siliciclastic material in the sedimentary basin is less pronounced on the Russian Platform. The unconformity at this level (boundary between the Volkhov and Kunda sequences) is well pronounced and shows a considerable renewal of faunal content (Männil, 1966), although no fundamental change in sedimentation took place at that time. Apparently, on the Siberian Platform, a global eustatic event was reinforced by a regional tectonic uplift of the area, adjacent to the Yenisei Land. The boundary corresponding to the Keto Sequence on the Siberian Platform apparently correlates with the transgressive Fjäcka Shale on the Russian Platform. On both platforms this level is poorly exposed and insufficiently studied.


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In general, despite complications superimposed by regional tectonics, eustatic events were the main factor influencing the evolution of sedimentary basins on both platforms. A general trend toward increased depth of the basin in the upper part of the Middle and in the Upper Ordovician (before the Hirnantian Regression) is clearly traced on both paleocontinents. DISCUSSION Taking into account that the Siberian Platform in the Middle and Upper Ordovician was situated in the low latitudes, near the equator, the wide distribution of cold-water carbonates on this platform can be only explained if the presence of a powerful upwelling of cold oceanic water coming from the depths which reached shallow-water epicontinental seas is assumed. The presence of typical warm-water carbonates in underlying (Upper Cambrian–Lower Ordovician) and overlying beds (Silurian) on the Siberian Platform shows that in normal environments (i.e., without an upwelling), tropical carbonates developed in this paleolatitudes (as it should be). The beginning and the end of the upwelling were the largest geohistorical events in the Ordovician of the Siberian Platform. The beginning of the upwelling is connected with the largest transgression, the beginning of which is marked by a transgressive surface at the base of the Volgina sedimentary sequence. At the time of the following transgressions (Kirensk-Kudrino, Mangazeya, and Keto sequences) the relative sea level rose even more. Thus, in the upper part of the Middle and in the Upper Ordovician it apparently reached its maximum in the Ordovician. The time of the existence of the upwelling coincides with the time of the maximum sea level highstand on the Siberian Platform. However, even a significant rise of sea level does not necessarily result in an upwelling. On the Siberian Platform, the upwelling developed at the time of the transgression, which immediately followed the largest regression (the latter resulted in the deposition of Baikit sandstones). This regression was either caused or greatly amplified by tectonic processes (elevation of the Yenisei Land on the western margin of the Siberian Platform). The uplift of the margin of the Siberian Paleocontinent at that time could be connected to its tectonic activization as a result of the adherence of a terrain or an island arc. This reorganization of tectonic elements could cause a redistribution of directions of large oceanic currents, which could result in an upwelling at the time of the subsequent transgression. The termination of the upwelling at the OrdovicianSilurian boundary was apparently caused by the eustatic sea level drop resulted from the Hirnantian Glaciation and subaerial exposure of all continents. The re-distribution of the areas occupied by sea and land led to the re-organization of the system of global oceanic currents and cessation of the upwelling. Later, when in PALEONTOLOGICAL JOURNAL

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the Silurian, the sea level rose again, this did not lead to the restoration of the system of oceanic currents, which existed in the Ordovician and, hence, the upwelling was not restored either. Because of its absence, a warmwater marine carbonate sedimentation was restored in the Silurian shallow-water epicontinental seas, which was normal for the paleolatitudes where it occurred. It is noteworthy that a situation similar to that in the Ordovician on the Siberian Platform, is recorded on the North American Platform. This platform throughout the Ordovician was in the tropical zone, but in the Lower and Middle Ordovician the succession is represented by warm-water carbonates, which was replaced in the middle part of the Upper Ordovician by coldwater carbonates (Holland and Patzkowsky, 1996). Tropical carbonate sedimentation was restored at the very end of the Ordovician and in the Silurian. On the American Platform, this situation is explained by an upwelling, corresponding to a strong transgression at the base of the sequence M-5 (Holland and Patzkowsky, 1996). This transgression, similarly, appeared after a large regression and tectonic-reorganization connected with the beginning of the Takonic Orogeny, i.e., the beginning of the accretion of the Takonic Island Arc to the North American continent. In the case with the North America, the beginning of the Takonic Orogeny and of the upwelling of cold oceanic waters onto the continent occurred somewhat later, in the middle of the Upper Ordovician. On the Siberian Platform, the orogeny and subsequent upwelling happened somewhat earlier, in the upper part of the Middle Ordovician. However, a general trend of events and their nature are very similar. Thus, the history of development and evolution of sedimentation in the Ordovician period of the Siberian Platform are far more similar to those on the North American than on the East European Platform. CONCLUSIONS (1) In the Ordovician of the Siberian Platform we recognized and traced nine sedimentary sequences, corresponding to sea level fluctuations of the third order with an average duration from 1 to 6 Myr. The boundaries of the sequences are represented by erosional unconformities and transgressive surfaces. Sedimentary systems tracts were identified for some sequences. The largest regressive events occurred on the Siberian platform at the base of the Baikit sequence and at the Ordovician–Silurian boundary. The largest transgressions are observed in the Volgina, Mangazeya, and Keto sequences. (2) The comparison with the sequences, recognized in the Ordovician of the Russian Platform, shows that their number and stratigraphic position of boundaries on both platforms almost coincide, which suggests that the sea-level changes were possibly eustatic. Differences in the magnitude of transgressions and regres-

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sions emphasize the effect of regional tectonic factors, which are not completely obscured by eustatic sea-level changes. (3) Presence of cold-water nontropical carbonates in epicontinental seas of the Siberian paleocontinent, which in the Ordovician was situated in the equatorial zone may be explained by the effect of an upwelling of cold oceanic water, caused by redistribution of oceanic currents due to the tectonic re-organization in the Middle Ordovician. The penetration of cold oceanic water into epicontinental seas resulted from a sea-level highstand in the upper part of the Middle Ordovician and upper Ordovician. (4) Pathways in the development of the Siberian Platform in the Ordovician are much more similar to the North-American, rather than to the Russian Platform. ACKNOWLEDGMENTS The study is supported by the Russian Foundation for Basic Research, project no. 07-05-01035a and is a contribution to the International project IGCP no. 503 “Ordovician Paleogeography and Paleoclimate.” REFERENCES 1. G. P. Abaimova, “Conodonts of the Cambrian–Ordovician Boundary Beds of the Siberian Platform in the Context of the Problem of Global Correlation of the Lower Boundary of the Ordovician System,” in Paleogeography and Global Correlation of the Ordovician Events (Project 503 MPGK “Ordovician Paleogeography and Paleoclimate”): Materials of the International Symposium, Novosibirsk, August 5–7, 2006 (Geo, Novosibirsk, 2006), pp. 6–8 [in Russian]. 2. G. P. Abaimova, T. Tolmacheva, and D. Komlev, New Data on Upper Cambrian and Lower Ordovician Conodonts from the Loparian and Nyaian Regional Stages on the Kulumbe River Section, North-West of Siberian Platform," in International Conference “Development of Early Paleozoic Biodiversity: Role of Biotic and Abiotic Factors, and Event Correlation" (KMK Scientific Press, Moscow, 2008), pp. 3–5. 3. C. R. Barnes, R. A. Fortey, and S. H. Williams, “The Pattern of Global Bio-Events during the Ordovician Period,” in Global Events and Event Stratigraphy in the Phanerozoic (Springer-Verlag, 1996), pp. 139–172. 4. S. M. Bergström, Middle and Upper Ordovician Conodonts from the Fågelsång GSSP, Scania, Southern Sweden," Geol. Fören. i Stockholm Förhandlingar, 77–82 (2007). 5. V. I. Bgatov, Lithologic-Geochemical Regularities in Sedimentogenesis in the Ordovician and Silurian of the Siberian Platform: Proceedings of the Siberian Research Institute of Geology, Geophysics, and Mineral Resources, Issue 147 (Krasnoyarskoe Knizhn. Izd., Krasnoyarsk, 1973) [in Russian]. 6. L. R. M. Cocks and T. H. Torsvik, “Baltica from the Late Precambrian to Mid-Paleozoic Times: The Gain and Loss of Terrane’s Identity,” Earth-Science Rev., 39–66 (2005).

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