Depositional sequences and palaeoceanographic change in the ...

Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 285–296

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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o

Depositional sequences and palaeoceanographic change in the Ordovician of the Siberian craton Alexander Kanygin a, Andrei Dronov b,⁎, Alexander Timokhin a, Taras Gonta a a b

Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of Russian Academy of Sciences, Acad. Koptyug 3, 630090, Novosibirsk, Russia Geological Institute, Russian Academy of Sciences, Pyzhevsky per.7, 119017, Moscow, Russia

a r t i c l e

i n f o

Article history: Received 25 May 2009 Received in revised form 26 January 2010 Accepted 16 February 2010 Available online 24 February 2010 Keywords: Depositional sequences Sea-level change Upwelling Ordovician Siberia

a b s t r a c t Nine major depositional sequences separated by regional unconformities are recognized in the Ordovician of the Siberian Craton. These sequences are typically characterized by an absence of lowstand systems tracts, relatively thin transgressive systems tracts and somewhat thicker shallowing upward highstand systems tracts. The most prominent unconformities, marking forced regressions are associated with the base of the Baykit depositional sequence (Early Darriwilian) and the top of the Ordovician. Across the former, the tropical-type carbonate factory was terminated and siliciclastic sediments (Baykit Sandstone) were deposited. These changes in lithology inferably correspond to tectonic events along the western margin of the Siberian continent. With major flooding events during the Volgino, Kirensk–Kudrino and Mangazeya depositional sequences phosphatization became much more widespread and carbonate rock compositions shift from tropical-type to temperate-type. This lithological shift is due to a combination of upwelling of cool water, increased nutrient influx and increased water turbidity. Major transgressions marked by widespread black shales and/or deep-water marine red bed facies occurred during the Volgino, Kirensk–Kudrino (Late Darriwilian), Mangazeya (Early Sandbian) and Kety (Middle Katian) depositional sequences. The Ordovician depositional sequences of the Siberian Craton can be tentatively correlated with those developed in the Baltica palaeocontinent, which suggests that most of them were controlled mainly by eustasy. The pattern of long-term lithological changes in the Ordovician of the Siberian Craton and our established sea-level curve demonstrate more similarities with North America than with Baltica. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The Russian territory includes the Siberian and Baltic palaeocontinents, which were located far away from each other throughout the Early Palaeozoic. In the Cambrian and Early Ordovician Baltica was located in the high latitudes of the Southern Hemisphere. During the Middle–Late Ordovician it moved at a relatively high velocity into the tropical zone of this hemisphere (Cocks and Torsvik, 2005). In contrast, Siberia was located in the equatorial zone throughout the Cambrian and Ordovician shifting slowly from the southern half to the northern half of the equatorial zone (Cocks and Torsvik, 2007). The Siberian palaeocontinent, which is represented today in Eurasia by the Siberian Platform (Siberian Craton), embraced several separate Early Palaeozoic sedimentary basins. The largest ones were the Irkutsk and Tungus epicontinental basins, which are also known as the Irkutsk Amphitheatre and the Tungus Syneclise, respectively (Kanygin et al., 1984, 2007), (Fig. 1). In order to construct a sequence stratigraphic chart for the Ordovician of the Siberian Platform we have

⁎ Corresponding author. Tel.: +7 495 959 30 17; fax: +7 495 953 07 60. E-mail address: [email protected] (A. Dronov). 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.02.014

studied the key Ordovician sections of both basins. In the Tungus basin the Kulumbe River section in the vicinity of Turukhan Land and the main outcrops in the Podkamennaya Tunguska River valley and its tributaries have been investigated. In the Irkutsk basin the main natural outcrops and quarries near the town of Bratsk, as well as in the Angara River valley, and in the Lena River valley between the towns of Ust'-Kut and Kirensk have been studied (Fig. 1). Particular attention was paid to the recognition, interpretation and correlation of the sequence boundaries. Identification of the systems tracts was not always possible. Amplitudes of regressions and transgressions were estimated based on the depth of erosion of the beds underlying unconformities and on the distribution of relatively deep-water facies such as black shale and marine red bed facies. Drastic changes in fauna and facies allow us to recognize five biotic events in the Ordovician of the Siberian Platform. These are associated with forced regression sequence boundaries and/or transgressive surfaces (Dronov et al., 2008). The term “forced regression” is applied to seaward translation of facies and shoreline regression in response to relative sea-level lowering. Forced regression is in contrast with “normal” regression where the seaward migration of the shoreline occurs because the rate of sediment supply exceeds the rate of new accommodation produced by

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sea-level rise. Forced regressions commonly are associated with a zone of sedimentary bypass, subaerial exposure and possible fluvial erosion between newly formed and preceding shorelines (Posamentier et al., 1992). Most of the discussed in the paper regional unconformities in the Siberian Ordovician were produced by forced regressions. The uppermost part of the Ordovician succession on the Siberian Platform is eroded in exposed sections (Kanygin et al., 2007), but we could not exclude existence of one or two additional sequences in the Hirnantian. Precise biostratigraphic correlation of the sequence boundaries between Baltica and Siberia also remains problematic due to the absence of common species in faunal assemblages but the number of sequences in broader stratigraphic intervals is comparable. Comparative facies analysis of the Ordovician succession on both palaeocontinents indicates an opposite trend in facies evolution. This long-term lithologic change in the case of the Siberian Craton, is probably due to palaeoceanographic change caused by tectonic events along the southwestern margin (present day orientation) in the Middle Ordovician. 2. Depositional sequences Based mainly on outcrop data from the western margin of the Tungus basin and the northern and northeastern margin of the Irkutsk basin, nine depositional sequences bounded by regional unconformities or their correlative conformities have been distinguished. The sequences correspond to sea-level fluctuations of the third order (according to Vail et al., 1977) and have an average duration between 1 and 10 Myr. For the purpose of convenience, individual names derived from the names of regional stages, series and formations have been given to them. From the base upwards these are: (1) Nya; (2) Ugor; (3) Kimai; (4) Baykit; (5) Volgino; (6) Kirensk–Kudrino; (7) Mangazea; (8) Dolbor; and (9) Kety sequence (Fig. 2). 2.1. The Nya Sequence There are no apparent signs of significant erosion near the Cambrian–Ordovician boundary on the Siberian Platform in contrast to the Russian Platform (main part of the Baltica palaeocontinent) where an erosional unconformity is well displayed. In the Kulumbe River valley the Upper Cambrian–Lower Ordovician interval is represented by a 50–70 m thick unit of limestone and dolomite with abundant stromatolites, oolitic grainstones, and flat-pebbled conglomerates formed on a tropical shallow-water carbonate platform (Il'tyk Formation). Some transgressive–regressive cycles can be recognized within this unit but no significant gaps have been recorded. The position of the Cambrian–Ordovician boundary on the Siberian Craton has long been a matter of discussion. Based on the first appearance of dendroid graptolites in the Loparian Regional Stage, the boundary has been drawn at the base of the underlying Mansian Regional Stage where distinct lithological changes and traces of erosion occur (Kanygin et al., 1982, 2006). However, recent data on the distribution of conodonts of the Cordylodus lineage indicate a considerably higher position for the boundary, approximately near the base of the Nyaian Regional Stage (Abaimova, 2006; Abaimova et al., 2008; Tolmacheva and Abaimova, 2009). The Nyaian Regional Stage has always been interpreted as a separate phase in the evolution of biota on the Siberian Platform (Kanygin et al., 2006). Most likely it also represents a separate depositional sequence. Knowledge of the internal structure of this sequence, however, requires further investigations. Based on conodonts distribution in the Kulumbe River section the Nya Sequence correlates approximately with the Pakerort Sequence of the Russian Platform (Tolmacheva and Abaimova, 2009).

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2.2. The Ugor Sequence The Ugor depositional sequence corresponds to the Ugorian Regional Stage. Its lower boundary is best exposed in the Irkutsk basin, where it is represented by a regional unconformity separating Ust'kut and Iya formations. The surface is well exposed at the Angara River valley, 25 km upstream of the town of Kodinsk (Fig. 3A). It is interpreted as a Type I sequence boundary and marks a shift from predominantly carbonate to siliciclastic sedimentation. The shift was probably caused by a rapid sealevel fall, which resulted in subaerial exposure and karstification of a tropical carbonate platform. A subsequent rapid sea-level rise was followed by deposition of coarse-grained and cross-bedded quartz sandstone of the Iya Formation. In the northwestern part of the Tungus basin (the Kulumbe River section) the Ugor Sequence has indistinct boundaries. The Ugor Regional Stage deposits correspond to the middle part of the Il'tyk Formation and lithologically they are very similar to the underlying Nyaian Regional Stage and the overlying Kimaian Regional Stage deposits of the same formation. The Ugorian deposits are represented here by yellowish-grey, locally clay-rich dolomites and marls with rare intercalations of grey oolitic grainstones and marls. The Ugor Sequence correlates approximately with the Latorp Sequence of the Russian Platform. 2.3. The Kimai Sequence The Kimai depositional sequence coincides with the Kimai Regional Stage. In the Angara River valley the base of the stage corresponds to the boundary between the Iya and Badaranovo formations. The sequence boundary coincides with a transgressive erosional surface at the base of a 0.6–1.5 m thick unit of glauconite-enriched sandstone with interbeds of bioclastic, glauconitic limestone and pebbly limestone conglomerate. The underlying Iya Formation and overlying Badaranovo Formation are represented by similar pure quartz sandstones with well developed unidirectional cross stratification. The glauconite-rich unit is interpreted as a condensed section representing the upper part of the transgressive systems tracts (Van Wagoner et al., 1988; Loutit et al., 1988; Schutter, 1996). The glauconite-rich deposits were formed during rapid sea-level rise, when siliciclastic supply from the adjacent land area ceased and sedimentation rate was very slow. The overlying cross-bedded quartz sandstones of the Badaranovo Formation are considered to be deposits of the subsequent highstand systems tract. In most regions of the Siberian Platform the Kimaian deposits marks a transgressive episode (Kanygin et al., 2006), which was favorable for a wide distribution of relatively uniform benthic and pelagic assemblages. The Kimai Sequence can be correlated to the Volkhov Sequence of the Russian platform. It should be noted that both sequences exhibits Type II sequence boundary (Dronov and Holmer, 1999) i.e. with no considerable erosion of underlying beds and no signs of subaerial exposure. 2.4. The Baykit Sequence The Baykit depositional sequence corresponds to the Baykit formation which includes deposits of the Vikhorevian and Mukteian regional stages. The best outcrops of the formation are located along the Podkamennaya Tunguska River valley, where monotonous light grey and yellowish quartz sandstones, 5 to 80 m thick are exposed on both sides of the river. The sandstones are coarsely bedded and frequently massive. At certain levels a well developed crossstratification and locally, especially at the base, conglomerates are typical (Markov, 1970).

Fig. 1. General distribution of the Ordovician rocks on the Siberian Platform with the position of the Russian and Siberian platforms in the structure of Eurasian continent. Legend: 1. Supposed continuation of the Siberian Craton under the Mesozoic cover of the West Siberian basin; 2. Territories covered by waters of seas, lakes and estuaries; 3. Land areas without Ordovician deposits; 4. Ordovician deposits in the subsurface areas; 5. Ordovician outcrop areas; 6. Boundary of the Siberian Craton; 7. Provisional boundaries of the Siberian craton; 8. Boundaries of the Land areas and sedimentary basins; 9. Position of the studied sections.

288 A. Kanygin et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 296 (2010) 285–296 Fig. 2. Stratigraphic chart showing selected correlations of the studied Ordovician successions of the Irkutsk and Tungus basins. It also shows depositional sequences, sea-level fluctuations, long-term lithological changes and main biotic events. Legend: 1. Warm-water carbonates; 2. Cool-water carbonates; 3. Deep-water carbonates; 4. Quartz sandstones; 5. Siltstones; 6. Red siltstones; 7. Gaps.


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Fig. 3. Regional unconformities and sequence boundaries in the Ordovician of Siberian Platform. A. Regional unconformity at the base of the Ugor depositional sequence in the Irkutsk Basin (left (southern) bank of the Angara River 25 km upstream of the town of Kodinsk). Sequence boundary separates oolitic grainstones with stromatolites of the Ust'kut Formation (below) and coarse-grained quartz sandstone and gravelstone with cross stratification of the Iya Formation (above). B. Transgressive surface coinciding with the sequence boundary at the base of the Volgino depositional sequence in northwestern part of the Tungus Basin (left (southern) bank of the Kulumbe river section). C. Sequence boundary at the base of the Mangazea depositional sequence in the Irkutsk Basin (left (western) bank of the Lena River near the village of Makarovo). Black shales of the Chertovskaya Formation overlie quartz sandstones of the Krivaya Luka Formation with phosphate conglomerate at the base (transgressive lag deposits). D. Sequence boundary at the base of the Dolbor depositional sequence in the Tunguska Basin (right (northern) bank of the Podkamennaya Tunguska River 5 km upstream of the mouth of the Stolbovaya River). Siliciclastic tempestites of the Dolbor Formation (above) overlie bioclastic calcareous tempestites of the Mangazea Formation (below).

The Baykit Sandstone constitutes a distinctive sedimentary body extending for over 600 km along the Podkamennaya Tunguska River valley. The lower boundary of the sequence is poorly exposed in the natural outcrops. Near the village of Sulomai in the vicinity of the Yenisei Land, however, the Baykit Sandstone overlies different Lower Ordovician strata and even Cambrian units with an angular unconformity (Markov, 1970). The monotonous composition of the sporadically exposed Baykit Sandstone prevents identification of systems tracts. The Baykit Sandstone represents a regressive stage in the evolution of the Tunguska basin. It should also be noted that the underlying Kimai depositional sequence is partly eroded almost over the entire Siberian Platform. The Kimaian deposits were completely removed by erosion in the eastern and northeastern parts of the platform (Kanygin et al., 2006). The unconformity at the base of the Baykit depositional sequence reflects probably one of the most significant forced regressions in the Siberian Platform during the Ordovician. The Baykit Sequence roughly correlates with the Kunda Sequence of the Russian Platform. 2.5. The Volgino Sequence The Volgino depositional sequence coincides with the Volginian Regional Stage, which deposits constitute a distinct and characteristic transgressive–regressive cycle traceable over the entire Siberian

Platform. An erosional unconformity at the base of the sequence is well pronounced in the Podkamennaya Tunguska River valley where it is marked by conglomerates with pebbles of the underlying Baykit Sandstone (Kazarinov et al., 1969; Markov et al., 1971). An erosional surface at the base of the Volgino depositional sequence is very distinct also in the Kulumbe River section on the northwestern margin of the Tungus basin (Fig. 3B). The Volginian deposits of the section belong to the Angir Formation. The upper beds of the Angir Formation show abundant signs of shallowing, including quartz sandstone and multidirectional cross bedding. In the Podkamennaya Tunguska River valley the uppermost Volginian deposits include a thick bed of medium-grained quartz sandstone. The bed may correspond to a highstand systems tract of the Volgino depositional sequence based on its shallow-water depositional environment and stratigraphical position directly below transgressive surface at the base of the overlying Kirensk–Kudrino depositional sequence. The sequence marks the beginning of one of the most extensive transgressions in the Siberian Platform. The faunal assemblage of the Volginian Stage is easily recognizable and markedly different from the assemblage of underlying strata. The unconformity at the top is not as distinct as that at the base of the sequence although a shallowing upward trend is clearly seen in all sections. The Volgino sequence correlates approximately with the Aseri Regional Stage of the Russian Platform.

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2.6. The Kirensk–Kudrino Sequence The Kirensk–Kudrino depositional sequence coincides with the Kirensko-Kudrinian Regional Stage. Like the underlying Volgino Sequence it forms a full cycle of deposition. In the outcrops along the Podkamennaya Tunguska River the sequence represents a distinct regressive–transgressive–regressive cycle with three components corresponding to lowstand systems tract, transgressive systems tract, and highstand systems tract. The lowstand systems tract is a uniform unit of greenish-grey and bluish-grey siltstones with rare interbeds of quartz sandstone and/or bioclastic limestone, which directly overlie coarse-grained quartz sandstone of the uppermost Volgino Sequence. The transgressive systems tract is mainly composed of red siltstones with abundant nautiloid shells. It was formed in an open marine environment and represents a deep-water setting, as evident from the abundant cephalopod fauna as well as finegrained sediments and their red colour. Phosphatization is also associated with these sediments. The highstand systems tract is represented by greenish-grey and bluish-grey siltstones similar to those of the lowstand systems tract but coarser grained in composition. In the section along the Kulumbe River valley the sequence is represented by the Amarkan Formation. Similar to the situation observed in Podkamennaya Tunguska River, its lower boundary is a transgressive surface, along which greenish-grey siltstones of the Amarkan Formation overlie nodular limestone and cross-bedded quartz sandstone of the Angir Formation. The middle and upper parts of the Amarkan Formation are composed mostly by red siltstone. In the Irkutsk basin the boundary between the Volgino and Kirensk– Kudrino sequences is represented by the regional unconformity in the middle of the Mamyr Series (Kanygin et al., 1984). Lack of natural outcrops and adequate drill-core data prevent the distinction of any systems tracts. The Volgino and Kirensk–Kudrino sequences are similar in structure and sediment composition. The best exposed unconformities are observed at the base of the Volginian Regional Stage and at the top of the Kirensko-Kudrinian Regional Stage. Thus, the Volginian and Kirensko-Kudrinian deposits can be united into one composite depositional sequence of a higher order. The composite sequence is a correlative with the Tallinn Sequence of the Russian Platform, with Volgino sequence correlating with the Aseri Regional Stage and the Kirensk–Kudrino sequence with the, Lasnamägi, Uhaku and Kukruze stages of the Russian Platform. 2.7. The Mangazea Sequence The Mangazea depositional sequence includes the Chertovskian and Baksian regional stages. Its lower boundary coincides with that of the Chertovskian Regional Stage and is represented by a transgressive surface over the entire Siberian Platform. In the Ordovician succession at the Kulumbe and Lena Rivers this level demonstrates clear signs of erosion and a conglomerate which can be interpreted as a transgressive lag deposit (Fig. 3C). In the Podkamennaya Tunguska sections the Mangazea Sequence includes the upper part (corresponding to the Chertovskian Stage) of the Ust'–Stolbovaya Formation and the Mangazea Formation. The upper part of the Ust'–Stolbovaya Formation is interpreted as a transgressive systems tract and the Mangazea Formation as a highstand systems tract. The transgressive systems tract consists of two members: (1) greenish-grey siltstones interbedded with fine-grained yellowish-grey sandstones and black shales containing carbonate concretions with Chertovskian fauna; and (2) red siltstones with scattered rounded phosphate pebbles 0.5 to 2 cm across and with beds of red conglomerates of phosphate pebbles. Both the black shales and the red phosphate conglomerates are interpreted as relatively deep-water deposits. At the Podkamennaya Tunguska region the highstand systems tract of Mangazea sequence is a unit of greenish-grey siltstones

alternating with bioclastic limestones. The bioclasts are predominantly fragments of brachiopods and trilobites as well as echinoderms, ostracods, and bryozoans. The limestone interbeds sometime show ripple marks on its upper bedding plane. At some levels bioclastic limestones contain glauconite grains. The intercalations of siltstones and bioclastic limestones of the Mangazea formation are interpreted as cool-water calcareous tempestites. The Mangazea Sequence correlates with the Kegel Sequence of the Russian Platform.

2.8. The Dolbor Sequence The Dolbor depositional sequence corresponds to the Dolbor Formation and approximately to the Dolborian Regional Stage. The deposits are yellowish-grey fine-grained sandstone and siltstone, locally with carbonate cement. In the outcrops along the right bank (northern) of the Podkamennaya Tunguska river, between the Stolbovaya and Listvennichnaya rivers, the base of the sequence is rather sharp and easily recognizable (Fig. 3D). The alternating bioclastic limestones and greenish-grey siltstones of the Mangazea Formation are replaced by predominantly yellowish fine-grained sandstones and siltstones. According to earlier observations based on studies of the outcrops higher upstream along the Podkamennaya Tunguska River, the uppermost beds of the Baksian Regional Stage were eroded and a sequence boundary is indicated by erosional pockets up to 0.4 m deep filled with fragments of the underlying rocks (Bgatov, 1973). The Dolbor Sequence correlates approximately with the Wesenberg Sequence of the Russian Platform.

2.9. The Kety Sequence The Kety depositional sequence comprises the Nirundinian and Burian regional stages. They were referred to as substages of Kety Regional Stage in earlier publications (e.g. Tesakov, 1975). The sequence is best exposed in the outcrops along the Bolshaya Nirunda and Nizhnyaya Chunku River valleys, the tributaries of Podkamennaya Tunguska. The lower boundary is defined by a sharp change of yellowish-grey and greenish-grey sandstone and siltstone of the Dolbor Formation to the cherry-red mudstone of the Nirunda Formation. By analogy with the cherry-red mudstone facies of the Kirensk–Kudrino sequence, the Nirundian deposits seem to be related to a transgressive systems tract. The overlying deposits of the Bur Regional Stage are interpreted as the highstand systems tract of the Kety depositional sequence. The Silurian deposits rest unconformably on the deeply eroded Ordovician strata. This does not exclude the presence of younger Ordovician deposits, belonging to the one or two Hirnantian depositional sequences in the deeper parts of the Tungus basin. The Kety sequence is interpreted to correlate with the Fjäcka Sequence of the Russian Platform.

3. The main Ordovician transgressions and regressions on the Siberian Platform Each of the described depositional sequences represents a transgressive–regressive cycle deposited in response to sea-level fluctuations. The base of every sequence is a level of marked facies and biotic changes. But the most prominent changes are associated with the following five stratigraphic levels: (1) the base of the Baykit Sequence; (2) the base of the Volgino Sequence; (3) the base of the Mangazea Sequence; (4) the base of the Kety Sequence; and (5) the Ordovician/Silurian boundary. These levels represent remarkable faunal turnovers that can be traced all over the Siberian Craton (Fig. 2).


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3.1. The base of the Baykit Sequence

3.4. The base of the Kety Sequence

One of the most pronounced faunal turnovers is associated with the regional unconformity at the base of the Baykit Sandstone. In the southwestern part of the Siberian Platform (near the Yenisei Land) the Baykit Sandstones rest with a slight angular unconformity on various Ordovician and even Cambrian deposits (Markov, 1970). Input of great volumes of siliciclastic material into the basin points to enlargement of the source area (Bgatov, 1973) and probably to regional tectonic uplift, accompanied by a forced regression. The regression resulted in the final destruction of the tropical carbonate platform that existed on the Siberian palaeocontinent throughout the Riphean, Vendian, Cambrian and Early Ordovician. The carbonate platform in the Irkutsk basin disappeared even earlier, during the regression marked by the base of the Ugor Sequence. The Baykit regression is manifest in the northwestern part of the Tungus basin in the vicinity of the Turukhan Land by the quartz sandstones and siltstones of the Guragir Formation. In the eastern part of the basin near the Anabar Land, it is marked by a considerable hiatus (Kanygin et al., 2007). The underlying Kimaian deposits have been entirely or partly eroded in many areas of the Siberian Platform (Kanygin et al., 2006, 2007). The regression seems to have had a major amplitude (N75 m) according to the semi-quantitative classification of Haq and Schutter (2008). It was one of the largest Ordovician regressions on the Siberian Craton and resulted in significant changes in the development of sedimentation and biotic evolution.

The cherry-red siltstone of the Nirunda Formation seems to represent a facies analogue of the relatively deep-water marine red bed facies of the Kirensk–Kudrinian and Chertovskian stages. This implies that the sharp Dolbor/Kety sequence boundary is also a transgressive surface that reflects a rapid sea-level rise. This sequence boundary has not been adequately studied because this stratigraphic interval has a limited number of natural exposures. The best of them are located in the inner part of the Siberian Platform and are not easily accessible.

3.2. The base of the Volgino Sequence The transgressive surface at the base of the Volgino depositional sequence marks the beginning of an impressive transgressive event that succeeded the lowstand phase displayed by the deposition of the Baykit Sandstone. The transgression is associated with abrupt changes in faunal assemblages and sedimentation. It is not surprising therefore that the base of the Volginian Regional Stage has been suggested as the boundary between the Lower and the Upper Ordovician series on the Siberian Platform (Tesakov, 1975). These series differ both in lithological composition, facies and thickness of deposits, and in composition and structural organization of biota (Kanygin et al., 2006). Although the transgression that produced the Volgino Sequence was not the largest in amplitude (probably it was in range of 25–75 m) according to the classification by Haq and Schutter (2008), the transgressive surface at the base of the sequence represents one of the most significant biotic events in the Ordovician of the Siberian Platform. 3.3. The base of the Mangazea Sequence The transgressive surface at the base of the Mangazea Sequence is well pronounced all over the platform. In many places it is associated with erosion of underlying deposits and a basal conglomerate. This surface was probably formed during a forced regression and was reworked afterwards into a transgressive surface of erosion. The phosphate pebble conglomerate at the base of the Chertovskian Regional Stage has a widespread distribution and is interpreted as a transgressive lag deposits. It is usually overlain by a relatively deepwater black shale or red bed facies. The transgressions of the Volginian and the Chertovskian stages are the most prominent in the Ordovician of the Siberian Platform (Kanygin et al., 2006). But the amplitude of the Mangazea sea-level rise seems to exceed the Volginian one reaching the amplitude of N75 m. The minimal facies differentiation provided distribution of the uniform benthic and pelagic assemblages all over the entire Siberian platform. This transgression corresponds well to the global eustatic sea-level rise reported at the base of the Nemagraptus gracilis Zone in many regions of the World (Fortey, 1984; Barnes et al., 1996).

3.5. The Ordovician–Silurian boundary The Ordovician–Silurian boundary is marked by a distinct gap generated by a high amplitude forced regression. The sea-level fall was of eustatic origin, supposedly connected with continental glaciation of Gondwana. The uppermost beds of the Ordovician (Hirnantian) are absent almost everywhere on the Siberian Platform. The beginning of the Silurian Period was characterized by a new transgression and an essential renovation of marine ecosystems. 4. Long-term lithologic change The described transgressions and regressions overlap with longterm lithologic changes, which probably reflect changes in climate and oceanography. In the Ordovician of the Siberian Platform the most significant changes include shifts from tropical-type to temperatetype carbonates, pulses of considerable siliciclastic supply and intensive phosphate formation. Based on lithological composition, associated minerals, and a set of primary sedimentary structures the carbonate sediments can be differentiated into so called warm-water, or tropical-type (photozoan), and cool-water, or temperate-type (heterozoan) carbonates (Lindström, 1984; James, 1997; Dronov, 2001; Dronov and Rozhnov, 2007). Tropical-type carbonates are characterized by a wide range of carbonate grain types including skeletal grains, intraclasts, peloids, and ooids. Different types of reefs are abundant in tropical-type carbonates including stromatolites and cyanobacterial mats. Carbonate mud occurs in abundance in tropical-type carbonates. Sediment textures vary widely from mud to sands to gravels and flat-pebble conglomerates. Temperate-type carbonates demonstrate a limited spectrum of skeletal grains, which are dominated by fragments of bryozoans, brachiopods, trilobites, ostracods and echinoderms. Ooids and peloids are absent or scarce. Reefs, stromatolites and cyanobacterial mats are extremely rare or absent. In the shallow shelf areas insignificant amounts of carbonate mud could be accumulated owing to bioerosion below the storm wave base. Sediment textures are mainly sands and gravels. In addition to water temperature, carbonate sediments are affected by water transparency and content of nutrients. Increased amounts of nutrients and terrigenous input can cause a temperatetype appearance even to tropical carbonates (James, 1997). As noted above, the lithological and palaeontological differences between the lower and upper parts of the Ordovician of the Siberian Platform served as a basis for dividing the system into two series (Tesakov, 1975). The boundary between the two coincides with the transgressive surface at the base of the Volgino Sequence. The lower series (from the Nyaian to the Mukteian stages) is composed of predominantly tropical-type carbonates and near shore quartz sandstone. The carbonate lithotypes include oolitic grainstones, widespread stromatolites, and cyanobacterial mats. Flat-pebble conglomerates are also common. They are products of the destruction and redeposition of carbonate crusts by tropical storms in the tidal flats of the carbonate platforms. The lower series vary from 120 m to 700 m in thickness, being about 440 m on average.

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The upper series (from the Volginian to the Burian stages) is formed by fine-grained terrigenous sediments with subordinate carbonates represented mostly by temperate-type lithologies. Bioclastic wackestone and packstone dominate. Stromatolites, dolomites, and organic buildups are absent. Ooids and peloids are scarce. At some levels glauconite grains and films occur. The upper series has a relatively reduced thickness of 90 to 300 m and is about 170 m on average. The thickness decrease reflects reduced rates of sedimentation and, correspondingly, diminished productivity of the shallowwater “carbonate factory”. The shift from tropical-type carbonate sedimentation to temperate-type was preceded by the destruction of the warm-water “carbonate factory” and a large input of siliciclastic material. According to palaeomagnetic data, the Siberian palaeocontinent was located in the tropical zone during the Cambrian, Ordovician and Silurian (Cocks and Torsvik, 2007). Therefore the shift from tropicaltype to temperate-type carbonates in the Middle and Late Ordovician (Fig. 4) can only be explained by the upwelling of cold oceanic waters along the margin of the continent. The upwelling may have caused the suppression of the tropical-type carbonate accumulation, absence of carbonate mud, peloids and ooids, suppression of growth of organogenic buildups, stromatolites, and cyanobacterial mats, occurrence of glauconite and cool-water skeletal carbonates. The upwelling of cold oxygen-depleted oceanic waters may also provide a nutrient supply which was reflected in the widespread phosphatization (phosphate rocks) and phosphatic conglomerates in the Volgino, Kirensk–Kudrino and Mangazea sequences. The significant nutrient supply enhanced the cold-water effect and also suppressed tropicaltype carbonate sedimentation. In the case of such condition the water temperature need not be necessarily be very much lower to produce a “cool-water” carbonate effect (James, 1997). 5. Comparison with the Russian Platform The main difference between the Russian and Siberian platforms in the Ordovician is an opposing trend in long-term lithologic changes. On the Russian Platform the average thickness of the regional stages persistently increases upwards through the Ordovician succession, reflecting a rise in the average rate of sedimentation. In addition, predominantly terrigenous sediments are successively replaced, first by cool-water and then by tropical carbonates formed in shallow-water environments (Dronov and Rozhnov, 2007). These climate-controlled changes in sedimentation reflect a drift of the Baltic palaeocontinent

during the Ordovician from the near-polar latitudes of the Southern Hemisphere into the Equatorial zone (Cocks and Torsvik, 2007). On the Siberian Platform the regional stages of the Lower and lower Middle Ordovician have a considerably greater thickness than those of the upper Middle and Upper Ordovician. In addition, upwards through the succession the predominantly tropical-type warm-water carbonates changed to terrigenous and temperate-type carbonate deposits. The shift from tropical-type to temperate-type sedimentation was rather sharp and recorded at the base of the Volgino Sequence. The Ordovician successions of the Russian and Siberian platforms show a certain similarity in the number of depositional sequences and the stratigraphic position of their boundaries. The Baltic and Siberian palaeocontinents belong to different palaeobiogeographic provinces, however and share no common species in the assemblages of trilobites, ostracods and brachiopods. In the Early Ordovician no common genus between the two palaeocontinents has been reported. Some similarity of trilobites and brachiopods at generic level occurred in the Middle Ordovician. Six genera of trilobites and five genera of brachiopods were common for both continents but their stratigraphic distribution was quite different. This fact prevents precise biostratigraphic correlation. To date there has been no direct correlations between the Ordovician successions of the Siberian and Russian platforms. The palaeontological background for the regional biostratigraphic scheme of the Siberian Ordovician was summarized for the first time in the classical monograph of Nikiforova and Andreeva (1961). In this book the stratigraphical distribution of all the main groups of fossils (brachiopods, trilobites, corals, bryozoans, stromatoporoids, nautiloids, gastropods, bivalves, crinoids and ostracods) was described. It was also stressed that all benthic groups in the Siberian Ordovician fauna were represented by endemic taxa, but some genera were typical of the Mid-Continent province of North America and much rarer in the Baltoscandian basin of the Russian Platform. Further investigations of ostracods and co-occurring groups of fauna (mainly brachiopods and trilobites) in Verkhoyansk–Chukotka Folded Belt demonstrated a close similarity with the fauna of the Siberian Platform. There are many species in common for both regions. In many Ordovician successions in the Verkhoyansk– Chukotka Folded Belt containing a benthic fauna of the Volginian and Chertovskian regional stages of the Siberian Platform graptolite assemblages typical for the Hustedograptus teretiusculus and Nemagraptus gracilis zones of the British standard co-occur (Kanygin, 1967, 1971; Oradovskaya, 1988).

Fig. 4. Palaeogeographical position of the Siberian (SP) and Russian (RP) Platforms for the Upper Ordovician (450 Myr). Based on Torsvik and Cocks (2009).


At the beginning of the 1960s studies of the Ordovician conodonts on the Siberian Platform started. These studies also clearly demonstrated some similarity at generic level with the Mid-Continent province of North America and a large difference from Baltoscandia (Moskalenko, 1973; Abaimova, 1990). All these data were summarized in a comprehensive monograph on the Ordovician stratigraphy of the Siberian Platform (Sokolov and Tesakov, 1975). A revised version of the regional biostratigraphic scheme for the Siberian Ordovician was later published in English (Kanygin et al., 1988). (For most recent additional data see Kanygin et al. (2007), Bergström et al. (2009) and references herein.).For intercontinental biostratigraphic correlation of the Ordovician of Siberian Platform we have so far only four reliable levels: 1. Base of the Nyaian Regional Stage that correlates with the base of the Ordovician based on most recent conodont investigations (Abaimova et al., 2008; Tolmacheva and Abaimova, 2009); 2. Base of the Volginian Regional Stage that correlates with the base of the Hustedograptus teretiusculus graptolite zone; 3. Base of the Chertovskian regional Stage that correlates with the base of the Nemagraptus gracilis graptolite zone and 4. Base of the Silurian (Bergström et al., 2009). These four stratigraphic levels constitute three major stratigraphic intervals that can be correlated globally. It is noteworthy that these major stratigraphic intervals include an equal number of sedimentary sequences in the two different regions. This suggests the synchronous formation of the sequences on both palaeocontinents (Fig. 5). As for the main turnover events in the Ordovician biota and sedimentation patterns, they were different on the Russian and Siberian Platforms. The global Hirnantian regression is well displayed on both platforms. The turnover event in the mid-Darriwilian time is also very distinct. The Siberian Platform is marked by a transgressive surface at the base of the Volgino depositional sequence, which indicates an extended transgression, upwelling, and a significant change in sedimentation. The Russian Platform it marked by a transgressive surface at the base of the Aseri Stage, which coincides with an important change in shelly fossils across the basin. The change allowed definition of the regional Tallinn Stage with the lower boundary coinciding with the base of the Aseri Stage (Männil, 1966). Also distinct is the base of the Sandbian Stage (the base of the Caradoc Series of Great Britain). Probably, this level marks a global eustatic transgression reflected in the basal beds of the Nemagraptus gracilis Zone (Fortey, 1984; Barnes et al., 1996; Haq and Schutter, 2008). By contrast, the important level at the base of the Baykit Sequence of the Siberian Platform, which was related to the sedimentation change, destruction of the carbonate platform and input of enormous siliciclastic masses into the basin, was less pronounced on the Russian Platform. The very distinct unconformity at the Volkhov–Kunda sequence boundary and the important faunal change (Männil, 1966) were not combined with major changes in sedimentation. Probably, the global eustasy was reinforced by tectonic elevation of the region adjacent to the Yenisei Land area on the Siberian Platform. A transgressive event corresponding to the base of Nirunda Formation on the Siberian Platform is represented by deep-water red bed facies contrary to the black Fjäcka Shale on the Russian Platform but probably these contrast lithologies reflects the same eustatic sea-level rise. In general, eustasy seems to have been the main controlling factor for depositional sequence formation on both platforms, but it was combined with regional tectonic effects. The trend of increasing water depth is well displayed in the upper Middle and Upper Ordovician deposits (until the Hirnantian regression) on both palaeocontinents. 6. Discussion The wide distribution of the Middle–Upper Ordovician temperatetype carbonates over the Siberian Platform, located in the low latitudes near the Equator, can be adequately explained only by the

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upwelling of deep cold oceanic waters and their penetration into shallow-water epicontinental seas. The fact that the underlying Upper Cambrian–Lower Ordovician and overlying Silurian deposits of the Siberian Platform are represented by typical warm-water tropicaltype carbonates indicates that in the absence of upwelling the tropical-type carbonates should have formed and did in these palaeolatitudes. The beginning and end of the upwelling are important events in the Ordovician history of the Siberian Platform. The upwelling began during an extensive transgression, the start of which is reflected in the transgressive surface at the base of the Volgino Sequence. The relative sea-level highstands occurred successively during formation of the Kirensk–Kudrino, Mangazea and Kety sequences. So, the sea level reached its maximum in the Middle–Late Ordovician. The upwelling coincided temporally with the highest sea-level stand on the Siberian Platform. Sea-level rise itself, however significant, cannot necessarily create upwelling. On the Siberian Platform the upwelling occurred during the transgression which occurred just after the greatest regression marked by the formation of the Baykit Sandstone. The regression was caused by or, at least, reinforced by the tectonic elevation of the Yenisei land area along the western margin of the Siberian Platform. The tectonic activation of the platform margin was probably a result of the attachment of a terrain and/or island arc. The tectonic reorganization probably altered the directions of large oceanic currents, which generated the upwelling. The end of upwelling in the latest Ordovician was likely related to the eustatic sea-level fall resulting from the Hirnantian glaciation. The following sea-level rise in the Silurian did not regenerate an identical system of oceanic currents and, correspondingly, the upwelling. In the absence of upwelling the warm-water tropical-type carbonate accumulation typical of the low palaeolatitudes was restored in the shallow-water epicontinental seas. The same situation was recorded for the North American platform. In the Ordovician this platform was also located in the tropical zone, and similarly, the warm-water carbonate sedimentation of the early Middle Ordovician was replaced by a cool-water environment in the terminal Middle–Late Ordovician (Holland and Patzkowsky, 1996). Similarly, tropical carbonates accumulated again in the Latest Ordovician–Silurian. This situation in the American platform was explained by an upwelling that also resulted from an extended transgression reflected in the M-5 sequence (Holland and Patzkowsky, 1996). The transgression took place after a major regression and tectonic rearrangement related to the beginning of the Taconian orogeny, i.e., the initial attachment of the Taconian island arc to the North American continent. In North America the Taconian orogeny and the subsequent coldwater upwelling started later, in the terminal Middle–early Late Ordovician, while on the Siberian Platform these events occurred slightly earlier. However the general succession of events and their character are very similar. Thus, the geological history and evolution of sedimentation in the Siberian Platform is much more similar to that of the North American platform than to the Russian. 7. Conclusions 1. Nine depositional sequences have been recognized in the Ordovician of the Siberian Platform The sequences correspond to 3rd order sealevel fluctuations with an average duration from 1 to 10 my. The sequence boundaries are represented by erosional unconformities and transgressive surfaces. The major regressive events were recorded at the base of the Baykit Sequence (lowermost Darriwilian) and near the Ordovician/Silurian boundary (in the Hirnantian). The most extensive transgressions coincided with the formation of the Volgino, Kirensk–Kudrino (upper Darriwilian), Mangazea (Sandbian) and Kety (upper Katian) sequences.

Fig. 5. Comparison between the Ordovician sequence stratigraphic charts of Baltica and Siberia.

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2. Comparative analysis of the Ordovician sequences of the Russian and Siberian platforms shows that they are similar in the number and approximate stratigraphic position of their boundaries. Such close similarity suggests a eustatic nature of the sequences. The differences in amplitude of the transgressive and regressive events on the Russian and Siberian platforms are possibly due to regional tectonic factors which have not been completely suppressed by the eustatic component. 3. Occurrence of the cool-water non-tropical carbonates in the epicontinental seas of the Siberian Platform, which was located in the equatorial zone in the Ordovician, is explained by the upwelling of cool oceanic water, caused by the rearrangement of oceanic currents induced by a Middle Ordovician tectonic event. Invasion of cold water into the epicontinental sea was encouraged by the highstand of the sea level in the late Middle–Late Ordovician. Acknowledgments We thank Robin Cocks (London), Thomas Servais (Lille), Mikael Calner (Lund) and David Harper (Copenhagen) for their useful comments and linguistic improvements. This work was supported by the Russian Fund for Basic Research grant 07-05-01035 and represents a contribution to the IGCP Project-503 “The Ordovician Palaeogeography and Palaeoclimate”. References Abaimova, G.P., 1990. Konodontovye zony verkhnego kembrija i nizhnego ordovika Sibirskoi platformy. V: Stratigrafija i paleontologija dokembrija i fanerozoja Sibiri (Conodont zones of the Upper Cambrian and Lower Ordovician of the Siberian Platform). Stratigraphy and Palaeontology of the Precambrian and Phanerozoic of Siberia. Nedra, Novosibirsk, pp. 57–65. in Russian. Abaimova, G.P., 2006. 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