Correlation of Eustatic and Biotic Events in the Ordovician Paleobasins of the Siberian and Russian...

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

INTRODUCTION

There are two large paleocontinents with a long his-tory of evolution (Siberian and Russian Platforms),which in the Early Paleozoic were located at a consid-erable distance apart (based on the paleomagnetic data)and in different biogeographic zones (based on paleon-tological and sedimentological data). Based on the lat-est palinspastic reconstructions, in the Cambrian andEarly Ordovician the Russian Platform was located inthe subpolar latitudes of the southern hemisphere nearGondwana and was rapidly moving northward towardthe tropics within the same hemisphere (Cocks andTorsvik, 2005). At the same time, the Siberian Platformmigrated more slowly, within the equatorial belt fromthe southern to the Northern Hemisphere (Cocks andTorsvik, 2007). Thus, despite the epicontinental basinsof these platform were at a distance from each other atthat time, the climates in which their biotas existedbecame more similar, which enables their comparisonusing the same paleontological criteria (taxonomiccomposition of the biotas and biodiversity dynamics).

The Ordovician sedimentary complexes on theseplatforms also appear promising for revelation andcomparison of important evolutionary events, becausethey contain an almost complete record of sedimenta-tion, and the existence and composition of fossil com-munities. Like the Devonian, the Ordovician was a timewhen epicontinental seas became most widespread(Ronov, 1993; Morrow et al., 1996). Therefore the sed-imentary successions on both platforms contain the

most complete stratigraphic record of the Ordovician,apart from the topmost Ordovician on the Siberian plat-form, where this part of the section is represented bybarren lagoon facies or is absent. On the Russian Plat-form, most Ordovician beds are overlain by youngersedimentary complexes, but they are relatively wellexposed in Baltoscandia and thoroughly studied pale-ontologically and sedimentologically by several gener-ations of workers. Based on this and using a method ofrecognition large transgressive-regressive cyclites (orsedimentary sequences) Dronov and Holmer (1999;2002) constructed a eustatic curve showing sea levelfluctuations in the Ordovician on the Russian Platform.However it is known that transgressive-regressiveevents in paleobasins result from two factors, i.e., glo-bal sea level fluctuations and regional tectonics. Moreadequate global sea level fluctuations may be obtainedby construction of integral eustatic curves based on thecomparison of isochronic sequences of remote paleoba-sins, which allow the recognition of the effects of theregional tectonics.

To solve this task, in 2006–2008 we conducted addi-tional examination of the most complete sections torecognize sedimentary sequences and compare themwith a eustatic curve for Baltoscandia with a note ofpaleontological data from various tectonic and facialregions of the Siberian Platform. These classic sectionspreviously studied by Nikiforova and Andreeva (1961)and Sokolov and Tesakov (1975) are used as a basis forpaleontological substantiation of three generations of

Correlation of Eustatic and Biotic Events in the Ordovician Paleobasins of the Siberian and Russian Platforms

A. V. Dronov

a

, A. V. Kanygin

b

, A. V. Timokhin

b

, T. Yu. Tolmacheva

c

, and T. V. Gonta

b

a

Geological Institute, Russian Academy of Sciences, Pyzhevsky per. 7, Moscow, 119017 Russiae-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. Thesesequences correspond to sea level fluctuations of the 3rd order, from 1 to 6 My. Correlation with the sequencesrecognized in the Ordovician of the Russian Platform suggest their possible eustatic nature. Cold water non-tropical carbonates are suggested in the Ordovician of the Tungus Syneclise, which may be explained by theupwelling of cold oceanic waters. The upwelling was caused by re-distribution of oceanic currents due to large-scale tectonic events in the mid-Ordovician. The Ordovician evolution of the Siberian Platform was much moresimilar to that of the North American Platform than of the Russian Platform.

DOI:

10.1134/S0031030109110124

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stratigraphic schemes (Kanygin et al., 2007). A detaileddescription of these sections with bed-by-bed paleonto-logical characterization is included in several collectivevolumes (Myagkova et al., 1977; Moskalenko et al.,1978; Kanygin et al., 1982, 1984a, 1984b; 1989; Tesa-kov et al., 2003). These data were used for correlationof stratigraphic subdivisions on the Siberian Platformand Baltoscandia. A preliminary version of sequencestratigraphic scheme of the Ordovician of the SiberianPlatform was published previously (Dronov, 2008;Dronov et al., 2008). These papers give a more detailedcharacterization of this scheme and its correlation withthe scheme for Baltoscandia.

SEDIMENTARY SEQUENCES IN THE ORDOVICIAN

OF THE SIBERIAN PLATFORM

Based on a summary of all existing paleontologicaland lithological data, including data from drilling,schemes of facies zonation for all regional stages wereproposed by Kanygin et al. (2007). A generalizedscheme of the Siberian Platform showing two sedimen-tary basins (Tungus Syneclise and Irkutsk Amphithe-ater) strongly differing in their sedimentary fillings isshown in Fig. 1. Within the Tungus Syneclise a normalmarine environment with mainly carbonate sedimenta-tion prevailed. In the marginal zones of this depression,in the northwest of the area (Kulyumbe River) andsouthwest (Podkamennaya Tunguska River) thick ter-rigenous series formed during the pre-Volginian regres-sion indicating a nearby source of this clastic material.In the basin of the Kulyumbe River they are representedby the variegated Guragir Formation with traces ofhalite crystals, desiccation cracks, and wave ripples. Inthe basin of the Podkamennaya Tunguska River, at thattime there accumulated the Baikit Formation, com-posed of coarse-grained quartz sandstones. The clasticmaterial for that formation was most likely suppliedfrom the land in the place of the modern Yenisei Range,where pre-Cambrian rocks are exposed. In the north-east of the Tungus Syneclise, in places where it is adja-cent to the Viljuy Syneclise beginning from the Volgin-ian Regional Stage, the amount of red-colored finelyterrigenous material with carbonate cement increases,while in the Late Ordovician these beds are interbeddedwith gypsum and anhydrite, with the thickness of evap-orate members increasing toward the upper part of thesection.

In the Irkutsk Amphitheater, Middle Ordovicianbeds are mainly represented by terrigenous shorefacies, at some levels containing lingulids, large crusta-ceans, endemic conodontophorids, nautiloids, andoccasionally ostracodes, i.e., groups typical of suchmarginal environments. To substantiate a sequence-stratigraphic scheme, we also studied Ordovician refer-ence sections in both sedimentary basins. In the Tun-guska Syneclise we examined a section on the Kuly-umbe River and main Ordovician outcrops in the Pod-

kamennaya Tunguska River. In the IrkutskAmphitheater, we studied sections in the vicinity of thecity of Bratsk, in the middle reaches of the AngaraRiver and in the valley of the Lena River, between thetowns of Ust’-Kut and Kirensk.

A total of nine large sedimentary sequences are rec-ognized in the Ordovician of the Siberian Platform,separated by surfaces of regional unconformities andtheir correlative conformities (Dronov et al., 2008). Therank of the sequence corresponds to sea level fluctua-tions of the third order (in the sense of Vail et al. 1977)and they have an average duration of between 1 and10 Ma long. For the convenience of further discussion,we gave names to these sedimentary sequences, whichare chosen after the names of corresponding strata(regional stages, formations, or superstages). From bot-tom to top these are: (1) Nya; (2) Ugry; (3) Kimai;(4) Baikit; (5) Volgina; (6) Kirensk-Kudrino; (7) Man-gazeya; (8) Dolbor, and (9) Keto sequences.

Below the above sequences are described and com-pared with synchronous sequences on the Russian Plat-form.

1. Nya Sequence

In contrast to the Russian Platform, where an ero-sional unconformity is clearly observed at the base ofthe Ordovician, no visible erosional unconformity hasbeen observed on the Siberian platform at the Cam-brian–Ordovician boundary. In the section on the Kuly-umbe River, the Upper Cambrian–Lower Ordovicianbeds are represented by a thick series of limestones anddolomites with numerous stromatolites, oolitic grain-stones and flat-pebble conglomerates. The series wasdeposited in a shallow sea on a tropical carbonate plat-form. It shows some cyclicity with transgressive-regressive cyclites, often with traces of slight erosion atthe base. However, no large erosional gaps are recordedin this stratigraphic interval.

The correlation of the base of the Ordovician in theregional stratigraphic scale of the Siberian Platformwith the GSSP of the base of the Ordovician remainsdebatable. In the previous regional stratigraphicschemes, the Cambrian–Ordovician boundary wasdrawn based on the occurrence of dendroid graptolitesin the Loparian Regional Stage at the base of the under-lying Mansian Regional Stage based on the clearlydefined lithological boundary, which is possibly asequence boundary (Kanygin et al., 2006). However,recent data on the distribution of

Cordilodus

conodontsin the sections, suggest that the Cambrian-Ordovicianboundary lies much higher, and correlates approxi-mately with the base of the Nyaian Regional Stage(Abaimova, 2006; Abaimova et al., 2008). The NyaianRegional Stage has always been considered as a sepa-rate stage in the evolution of fauna on the Siberian plat-form (Kanygin et al., 2006) and, apparently, it corre-sponds to one sedimentary sequence. However, the


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CORRELATION OF EUSTATIC AND BIOTIC EVENTS 1479

structure of this sequence needs clarification and,hence, additional study.

On the Russian Platform, the Nya sequence corre-lates with the Pakerort sequence (Fig. 3). There is littlein common between the two. The Nya sequence is rep-resented by tropical carbonates, whereas the Pakerortsequence consists of cross-bedded quartz sands withfragments of shells of phosphatic brachiopods and

black graptolite shales. The Pakerort sequence showswell-developed lower and upper boundaries repre-sented by regional erosional unconformities. It is alsoclearly subdivided into the lower, shallow-water, andupper, deep-water parts. The lower portion is inter-preted as a lowstand systems tract, while the upper por-tion is interpreted as a highstand systems tract. Thetransgressive systems tract in the Pakerort sequences iscut off by the erosion at the base of the overlying Latorp

1

Anabar Land

TunguskaSyncline

KatangaLand

Irkutsk

Amphitheater

Enis

ei L

and

2

3

4

5

6

7

Turu

khan

sk L

and

Baical Lake

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 bedscovered 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.

Upper Lena Facial Zone

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System

Stage

Series

Trema- Floian Dapin- Darriwiliangian Sandby Katian Hirnan-tian

UpperMiddleLower

Bur

Kim

ai

Nyai

Ugorsk

Muktei

Vik

horev

Volg

in

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san

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or

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nda

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or

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ai

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it

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a

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a

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1 2 3 4 5 6 7 8 9

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Len

a Riv

erA

ngara R

iver

Irkutsk

Basin

(form

ations)

Bratsk

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arovo

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Ust-K

ut

Ust-K

ut

Iya

Bad

aran

Kriv

olu

tsk

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orn

insk

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arkan

9

87

65432

1 5432

Iltyk

Gurag

ir

Angir

1

Tungusk

a Basin

(form

ations)

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ennay

a

Cold-water carbonatesWarm-water carbonates

Critical L

evels

(even

ts)

Baik

it

Ust-R

ybnin

sk

Kuly

um

be R

iver

Sed

imen

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Sea lev

elflu

ctuatio

ns.

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gr. - R

egr.

(low

er reaches)

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a Riv

er

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Coastal

line

doc

Ordovician

Intern

ational

Stratig

raphic

Scale

Cherto

vo

Sib

erianP

latform

regio

nal

horizo

ns

Fig. 2.

Ordovician Sedimentary Sequences of the Siberian Platform.


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Keg

el

Tallin

n

Kunda

Vesen

berg

Fjack

a

Pek

erort

Varan

gu

Hunneb

erg

Billin

gen

Volk

hov

Kunda

Aseri

Lasn

am

ä

agi

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u

Kukru

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Idav

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Keila

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du

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vere

Nab

alaV

orm

si

Pirg

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Baltica

HirnantianKatianSandbyDarriwilianDapin-

Floian

Middle Upper

Junsto

rp

Tom

marp

Seq

uen

ces

Lower

Ordovician

Series

Stage

Seq

uen

ce units

Lato

rp

Volk

hov

Pek

erort

Sib

eria

Seq

uen

ces

Seq

uen

ce units

Reg

ional

stratigrap

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units

Not reco

gnized

Kety

Dolb

or

Man

gazey

a

Kiren

-Kudri

Volg

in

Kim

ai

Baik

it

Ugorsk

Nyai

Ugorsk

Nyai

Kim

ai

Kiren

-Kudri

Volg

in

Dolb

or

Tremadoc

Cherto

vo

SB

SB

SB

SB

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Boundaries

Boundaries

987653 421

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ional

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Stratig

raphic

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10987654321

SB

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Bur

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nda

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AshgillCaradocLlanvirnArenigTremadoc

gianStage

British series

Fig. 3.

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

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sequence (Dronov and Holmer, 1999). In the Nyasequence, the unconformity at the base is almost invis-ible and despite the rhythmic cycles observed in thesections, no contrast between the facies allowing theidentification of systems tracts is observed.

2. Ugry Sequence

The Ugry sequence corresponds stratigraphically tothe Ugorian Regional Stage. The base of the sequenceis particularly clearly observed in the Ordovician basinof the Irkutsk Amphitheater, where it is represented byregional erosional surface at the boundary between theUst’-Kut and Iya formations. In the latitudinal portionof the Angara River, 25 km upstream of the town ofKodinsk, this surface is clearly visible in outcrops(Fig. 4). It is interpreted as a 1st-type sequence bound-ary and is marked by a sharp shift from carbonate tosiliciclastic sedimentation. This apparently happenedas a result of a large-scale sea level drop causing thetropical carbonate platform to dry up and develop karstin the Ust’-Kut time, which led to the destruction of theshallow-water “carbonate factory.” Despite the follow-ing rapid sea level rise, carbonate sedimentation did notresume, and coarse-grained and cross-bedded sandsbegan accumulating in the shallow-water areas.

In the outcrops of the Lena River, the base of theUgry Sequence is represented by a transgressive flood-ing surface, which reflected an abrupt change in sedi-mentation. This surface separates shallow-water marine

tropical carbonates with stromatolitic bioherms of theUst’-Kut Formation from relatively deep-water fly-schoid series of the overlying Krivaya Luka Formation.These deposits are represented by interbeds of quartztempestites alternating with interbeds of dark-gray silt-stones. In the Tungus Syneclise (section on the Kuly-umbe River), the boundaries of the Ugry sequence areless discernible. Deposits of the Ugorian RegionalStage compose the middle part of the Iltyk Formationand are not fundamentally different from the beds of theunderlying Nyaian Regional Stage and from the rocksof the overlying Kimai Regional Stage. They are repre-sented by yellowish-gray, in places clayey dolomitesand marls with rare interbeds of gray oolitic limestones(grainstones) and marls.

The Ugry sequence of the Siberian Platform approx-imately corresponds to the Latorp Sequence of the Rus-sian Platform (Fig. 3). A large regional unconformityand all three sedimentary systems tracts are relativelyeasy to recognize. In the south of the Siberian Platform,a regional unconformity (Fig. 4) in the Irkutsk Basin, atthe base of the Ugry sequence, is distinguished,although parasequences and identification of sedimen-tary systems tracts require more detailed examination.

3. Kimai Sequence

The Kimai Sequence corresponds to the KimaiRegional Stage. In the sections of the Angara River val-ley, the base of the Kimai Regional Stage corresponds

Iya Formation

Yst-Kut Formation

SB

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 andgravelites of the Iya Formation (above).


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to the boundary between the Iya and Badaranovo for-mations. The boundary between sequences coincideswith an erosional surface that is overlain by a 0.6-m-thick unit of quartz-glauconite sandstones with inter-beds of bioclastic glauconite limestones and gravelconglomerates. The underlying beds of the Iya Forma-tion and the overlying beds of the Badaranovo Forma-tion are represented by uniform quartz sands and sand-stones with cross-bedding inclined in the same direc-tion. A unit with glauconite is interpreted as acondensed section, developing in the upper part of thetransgressive systems tracts (Van Wagoner et al., 1988).The boundary of the sedimentary sequence, in this casecoincides with the transgressive surface. Beds withglauconite were formed during a rapid rise of the sealevel, when the input of siliciclastic material from theadjacent land was essentially suppressed, and thereforethe sedimentation rates were considerably lower.

Glauconites are very characteristic of condensedsections of transgressive systems tracts (Loutit et al.,1988; Schutter, 1996). The overlying cross-beddedcoastal quartz sands of the Badaranovo Formation areinterpreted as a highstand sedimentary systems tract.

In the Tungus Syneclise (section on the KulyumbeRiver), the deposits of the Kimai sequence are mainlyrepresented by gray fine-grained oolitic grainstoneswith subdominant beds of dolomites and siltstones.They are represented by more marine and more purevarieties of limestone, compared to the underlyingdolomites of the Ugry Sequence. However, no clearerosional surface similar to that between the Ugry andKimai sequences in the Irkutsk Amphitheater have sofar been observed. In most regions of the Siberian Plat-form, Kimaian beds represent transgression (Kanyginet al., 2006). That time is characterized by the unifica-tion of sedimentary environments, which facilitateswide dissemination of similar benthic and pelagic bio-cenoses.

The Kimai Sequence approximately corresponds tothe Volkhov sequence of the Russian Platform (Fig. 3).Deposits of both sequences are represented mainly bycarbonates, although in the Volkhov sequence, these arecold-water carbonates with many glauconite grains inthe rock matrix, and in the Kimai sequence these aremainly tropical oolitic grainstone in the Tungus Synec-lise and quartz sands in the Irkutsk Amphitheater. It isnoteworthy that the base of sequences, both in the Rus-sian and Siberian platforms, is represented by the2nd-type sequence boundary (Van Wagoner et al.,1988), i.e., by a boundary, on which no deep erosion ofunderlying beds, or traces of subaerial exposition maybe observed. The Volkhov Sequence forms a completesedimentary cycle, within which all three systems tractsare recognized. Inside the Kimai Sequence it is stillimpossible to identify sedimentary systems tracts.

4. Baikit Sequence

The Baikit Sequence is represented by Baikit For-mation, including beds of the Vikhorevian and Mukte-ian regional stages. Sections of this formation are themost complete in the basin of the Podkamennaya Tun-guska River, where it is represented by a uniform seriesof light-gray and yellowish quartz sands and sandstones5 to 80 m thick. The sands are composed of angular,less commonly well-rounded grains of quartz (up to80% of the total rock weight) with rare grains of feld-spar and leaves of muscovite. The sands are coarsely-bedded often massive. Cross-bedding resulted from thealternation of beds differing in the size of the grains.The sandstones of the Baikit Formation are verymature, the grains are well sorted. Several horizonsshow cross-bedded series and sometimes layers of con-glomerate, especially at the base (Markov, 1970).

The Baikit sandstones constitute a sedimentarybody continuing laterally and traceable in the basin ofthe Podkamennaya Tunguska River at a distance ofover 600 km. The base of the Baikit sequence can rarelybe observed in outcrops because of the generally poorexposition. However, Markov (1970) noted that nearthe village of Sulomai, the Baikit sandstones overlievarious lithostratigraphic units of the Ordovician andeven the Lower Cambrian. Markov also noted an angu-lar unconformity between the rocks of the Evenk For-mation (Upper Cambrian) and Baikit sandstones nearGlinyanyi Creek in the basin of the PodkamennayaTunguska River.

The inner structure of the Baikit sequence is poorlyknown because of its relatively monotonous composi-tion and poor exposure. However, there is no doubtabout the unconformities at the base and at the top ofthe Baikit sandstones. Generally, the Baikit Sequencerepresents a regressive stage in the development of theTungus Basin. It is noteworthy that the underlyingKimai Sequence is partly eroded almost over the entireterritory of the Siberian Platform. In the east and north-east of the platform, the deposits of Kimai age are com-pletely eroded (Kanygin et al., 2006). Unconformity atthe base of the Baikit sequence may be a result of oneof the strongest regressions in the Ordovician on theSiberian Platform.

The Baikit sequence on the Siberian Platform corre-lates with the Kunda Sequence on the Russian Platform(Fig. 3). These sequences are considerably different inthe composition of their sediments. The KundaSequence, similar to the underlying Volkhov Sequence,is represented mainly by cold-water carbonates. TheBaikit Sequence mainly consists of terrigenous depos-its, often represented by quartz sandstones. At the baseof the Baikit sandstones there observed a gap and aregional unconformity. At the base of the KundaSequence, there is also a large unconformity suggestingthe erosion of underlying beds. Thus, both sequences inthe base have a 1st-type sequence boundary (Van Wag-oner et al., 1988). Within the Kunda Sequence there are

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three sedimentary systems tracts approximately corre-sponding to three substages of the Kunda RegionalStage. Within the Baikit Sequence sedimentary systemstracts have not yet been recognized.

5. Volgina Sequence

The Volgina Sequence corresponds to the entireVolginian Regional Stage, with deposits forming a dis-tinct transgressive-regressive cyclite, clearly recogniz-able over the Siberian Platform. The erosional uncon-formity at the base of the Volgina Sequence is clearlyobserved in the basin of the Podkamennaya TunguskaRiver, where in the outcrops between the Listvennich-naya and Stolbovaya rivers, there is a conglomerate,composed of fragments of the underlying Baikit sand-stones. In addition, at the top of the Baikit sandstones,there was observed a subaerial weathering crust, con-nected with a large gap, whereas the redeposited prod-ucts of this crust are recorded in the basal beds of theVolgina Sequence (Kazarinov et al., 1969; Markov etal., 1971).

A clear erosional surface at the base of the VolginaSequence is also recognized in the northwestern partsof the Tungus Syneclise, in the section on the Kuly-umbe River (Fig. 5). Here, it is also expressed as atransgressive surface, with traces of considerable deep-ening of the basin toward the upper part of the section.In the section on the Kulyumbe River, the deposits ofthe Volginian Regional Stage correspond to the AngirFormation. The topmost part of the Angir Formation

shows many features suggesting that the basin becameshallow, the process accompanied by the influx ofquartz sands and characteristic textures with variouslyorientated cross-bedding structures.

In the basin of the Podkamennaya Tunguska River,at the top of the Volginian Regional Stage there is a bedof the massive coarse-grained sandstones, apparentlycorresponding to the highstand systems tract of the Vol-gina Sequence (Fig. 6). The Volgina Sequence marksthe beginning of one of the most prominent transgres-sions on the Siberian platform. Faunal assemblages ofVolginian age are clearly recognizable and are readilydistinguished from those of the underlying beds. Theunconformity at the top of the Volgina Sequence is notwell developed compared to its base, although the shal-lowing is registered in all sections. This was the reasonto unite the Volgina Sequence and the overlyingKirensk-Kudrino Sequence into a joint Krivaya Lukasedimentary sequence (Dronov et al., 2008). However,it is apparently more logical to recognize them as sepa-rate sedimentary sequences, as it is done in this study.

6. Kirensk-Kudrino Sequence

The Kirensk-Kudrino Sequence stratigraphicallyequals the Kirensk-Kudrinian Regional Stage. Like theunderlying Volgina Sequence, it forms a complete sed-imentary cycle. In sections in the lower reaches of thePodkamennaya Tunguska River, this sequence forms adistinct regressive-transgressive-regressive cyclite,comprising three elements, corresponding to a low-

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 KulyumbeRiver, contact of the Guragir and Angir Formations.


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stand systems tract, transgressive systems tract, andhighstand systems tract. The base of the Kirensk-Kudrino Sequence is represented by a transgressivesurface (Fig. 6). In the section on the Kulyumbe River,at the base of the Kirensk-Kudrino Sequence there isalso a transgressive surface, at which the thick-beddedlimestones and cross-bedded quartz sandstones withcarbonate cement, developed at the top of the AngirFormation, are overlain by greenish-gray siltstones ofthe Amarkan Formation. The middle and upper parts ofthe Formation are mainly composed of reddish silt-stones. No certain highstand tract deposits have beenidentified. It is possible that they are cut off by the ero-sion at the base of the overlying Mangazeya Sequence.

In the Irkutsk Amphitheater, the boundary betweenthe Volgina and Kirensk-Kudrino sequences is appar-ently represented by the so-called “Middle MamyrUnconformity.” Individual sedimentary systems tractscannot be presently recognized in the Irkutsk Amphi-theater because of the poor expose of this stratigraphicinterval and the absence of adequate drilling data.

The Volgina and Kirensk-Kudrino sequences on theSiberian Platform are very similar in the nature andcomposition of their sediments. The upper, Kirensk-Kudrino Sequence, has a greater thickness. It overliesthe Volgina Sequence and represents its natural contin-uation. The most prominent unconformities arerecorded at the base of the Volginian Regional Stageand at the top of the Kirensk-Kudrinian Regional Stage.At the boundary between the Volginian and Kirensk-

Kudrinian regional stages, the unconformity is not thatclear, although Volginian beds form a well recognizedsedimentation cycle. In fact, the Volginian and Kirensk-Kudrinian beds can be united in one sequence (Dronov,et al., 2008). In that case the Volginian sedimentarycyclite could be considered as a lowstand systems tractof this combined sequence, or as a separate parase-quences set with a displayed retrogradational stackingpatterns within the transgressive sedimentary systemstract.

This combined sequences corresponds to the Tallinnsequence on the Russian Platform, which includes theAseri, Lasnamägi, Uhaku, and Kukruse regional stages(Fig. 3). The Volgian sedimentary cyclite correspondsto the Aseri Regional Stage on the Russian Platform.These have a similar strata architecture and can beinterpreted either as a lowstand tract, or as an initialportion of a transgressive systems tract. The deposits ofthe Lasnamägi and, partly, the Uhaku regional stageconstitute a transgressive sedimentary systems tracts,whereas the uppermost part of the Uhaku RegionalStage and the Kukruse Regional Stage constitute ahighstand systems tract.

7. Mangazeya Sequence

The Mangazeya Sequence corresponds to the Cher-tovskian and Baksanian regional stages. Its lower bound-ary coincides with the base of the Chertovskian RegionalStage, which is represented by a transgressive surface

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.

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over almost the entire territory of the Siberian Platform.In the Ordovician sections on the Kulyumbe River andalong the Lena River, traces of erosion and conglomerateare documented at this level, which can be interpreted asa transgressive lag (Fig. 7). In sections in the valley of thePodkamennaya Tunguska River, the MangazeyaSequence corresponds to the upper portion of the Ust’-Stolbovaya Formation (referred to the ChertovskianRegional Stage) and the Mangazeya Formation. Therespective portion of the Ust’-Stolbovaya Formation isinterpreted as a transgressive systems tract, whereas theMangazeya Formation is interpreted as a highstand sys-tems tract. The transgressive systems tract in this sectionconsists of two subdivisions: (1) a member of intrelay-ered greenish-gray siltstones, fine-grained yellowish-gray sandstones and black shale with large carbonatenodules containing faunal remains of the ChertovskianRegional Stage; and (2) a member of red siltstones withscattered rounded phosphate pebbles from 0.5 to 2 cm indiameter, alternating with beds of red gravel phosphaticconglomerates composed of these pebbles. Both blackshales and red phosphatic conglomerates are interpretedas relatively deep-water deposits.

The highstand systems tracts in the section on thePodkamennaya Tunguska River is represented by a fly-schoid series of greenish-gray siltstones interbeddedwith micritic (in the lower part) and bioclastic lime-stones. Bioclasts are mainly represented by fragmentsof brachiopod shells and trilobites carapaces. Frag-

ments of crinoids, ostracodes and bryozoans are alsopresent. At some levels at the top of limestones layers,wave ripples are observed. Several layers contain glau-conite. Deposits of the Mangazeya Formation are inter-preted as cold-water carbonate tempestites.

The Mangazeya Sequence on the Siberian Platformcorresponds to the Kegel Sequence on the Russian Plat-form (Fig. 3). Both sequences have at the base a cleartransgressive surface and in both cases, transgressivetract and a highstand tract can be clearly recognized.The erosion of the underlain beds is best seen on theSiberian Platform, which may be related to a greaterhydrodynamic energy in the basin, or a stronger sealevel drop before the transgression. The character ofsedimentological content of these sequences is con-trasting. The Keila beds on the Russian Platform arerepresented by tropical carbonates, whereas theBasksan beds on the Siberian Platforms are cold-watercarbonates.

8. Dolbor Sequence

The Dolbor Sequence corresponds to the DolborFormation, which approximately corresponds to theDolborian Regional Stage. It is noteworthy that the fau-nal assemblage typical of the Dolborian Regional Stageis actually found a few meters below the boundarybetween the Mangazeya and Dolbor Sequence i.e., inthe Mangazeya Sequence. The Dolbor beds are repre-

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 Man-gazeya sedimentary sequence. Left bank of the Lena River near the village of Makarovo. The boundary between the Krivaya Lukaand Chertovskaya formations.


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sented by yellowish-gray fine-grained sandstones andsiltstones, in places with carbonate cement. In sectionsin the lower reaches of the Podkamennaya TunguskaRiver between the mouth of the Stolbovaya River andListvennichnaya River the base of the sequence isprominent, easily discernible in the outcrop and readilydiagnosed (Fig. 8). This boundary is marked by achange in sedimentation. A series of bioclastic lime-stones interbedded with greenish-gray siltstones char-acteristic of the Mangazeya Formation is overlain by aseries of mainly sandy and siltstone beds. In the outcropnear the mouth of the Stolbovaya River, the boundary isdistinct but appears conformable. No traces of largeerosion of underlying beds are noticeable here.

At the same time, V.I. Bgatov indicates that in thesections in the middle reaches of the PodkamennayaTunguska River, the upper horizons of the Baksanianbeds are eroded, and sharp-angled fragments of rocksfrom the underlying beds are found in the depressionson the ancient relief. Erosional pockets can be up to0.4 m deep. The same author noted the presence of thepre-Dolbor weathering crust in these sections (Bgatov,1973) and, hence suggested a large gap and unconfor-mities. The boundary between the Mangazeya and Dol-bor sedimentary sequences is drawn at this unconfor-mity. Individual rhythms are recognized within the Dol-bor Sequence, but the recognition of sedimentarysystems tracts is as yet impossible. The DolborSequence corresponds to the Wesenberg Sequence onthe Russian Platform (Fig. 3). The latter contains a

transgressive tract corresponding to the OanduRegional Stage Horizon and a highstand tract corre-sponding to the Rakvere Regional Stage. The base ofthe sequences in both cases coincides with a transgres-sive surface.

9. Keto Sequence

The Keto Sequence corresponds to the Nirundianand Burian regional stages, referred to the KetoRegional Supestage. The best sections of this sequenceare in the basin of the Podkamennaya Tunguska River,along the Bolshaya Nirunda and Nizhnyaya ChunkuRivers. We did not observe this sequence in natural out-crops, and the conclusions presented in this paper arebased on previous publications. Judging from thesedata, the base of the Keto sequence is quite distinct andis drawn at a level where cherry-red shales of theNurunda Formation overlie the yellowish-gray andgreenish-gray sandstones and siltstones of the DolborFormation. Like the cherry-red shale of the Kirensk-Kudrino Sequence, these beds are interpreted as rela-tively deep-water open sea sediments of a transgressivesystems tract. The overlying beds of the BurianRegional Stage can be interpreted as a highstand sys-tems tract of the Keto sedimentary sequence, althoughit is possible that these beds may be recognized as aseparate sequence. The boundary with the Silurian isunconformable and is marked by strong erosion ofunderlying Ordovician beds. It is possible, that the cen-

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 kmabove the mouth of the Stolbovaya River. A boundary between the Mangazeya and Dolbor Formations.

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tral zones of the basin may contain stratigraphicallyhigher Ordovician beds, which will be referred to a dif-ferent sequence.

The Keto sequence on the Siberian platform, appar-ently, corresponds to the Fjäcka Sequence on the Rus-sian Platform (Fig. 3). However, the relatively deep-water deposits of the Fjäcka sequence is characterizedby black-shale sedimentation, whereas for the Ketosequence marine red bed sedimentation is typical. Inthis respect, the Keto Sequence is the most similar tothe Jonstorp Sequence of the Russian Platform, whichoverlies the Fjäcka Sequence. The degree of knowledgeon the sequence stratigraphy of the Upper Ordovicianon the Siberian Platform does not yet allow identifica-tion of the number and features of sedimentarysequences in this stratigraphic interval. The correlationsuggested is just preliminary.

In the uppermost Ordovician of the Russian Plat-form there are at least two more sequences—Jonstorpand Tommarp (Dronov and Holmer, 1999), but on theSiberian platform these beds are in most cases cut byerosion, and in places of their possible presence, e.g., inthe central areas of the Tungus Syneclise, they areinsufficiently studied. Therefore positive correlation ofsequences 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 rel-atively large transgressive-regressive cyclite, connectedwith fluctuations of the relative sea-level and accompa-nied by noticeable lithological changes in sedimentaryenvironments. In fact, the base of each sequence is alevel marked by noticeable lithological changes. How-ever, depending on the magnitude of transgressions andregressions, the rate of sea level changes, relationshipsbetween transgressive and regressive components andaccompanying paleogeographic and paleoclimaticchanges it is possible to recognize certain levels markedby even stronger changes. These levels have the largestcorrelative potential. In the Ordovician of the SiberianPlatform five such levels may be recognized: (1) base ofthe Baikit Sequence; (2) base of the Volgina Sequence;(3) base of the Mangazeya Sequence; (4) base of theKeto Sequence, and (5) Ordovician–Silurian boundary(Fig. 2).

I. Base of the Baikit Sequence

Regional unconformity at the base of the Baikitsandstones is one of the most prominent levels in theOrdovician of the Siberian Platform. In the west of theplatform, in the region of the Yenisei Range, Baikitsandstones overlie various horizons including those ofLate Cambrian age, with a small angular unconformity(Markov, 1970). The influx of large amount of silici-clastic material into the basin indicates an expansion ofthe source area on the Yenisei Land (Bgatov, 1973), an

apparent tectonic elevation of this area, and a forcedregression accompanying this elevation. This forcedregression led to the destruction of the tropical carbon-ate platform, which had existed on the Siberian Plat-form in the Riphean, Vendian, and early Cambrian. Thisregression is revealed in the Igarka–Norilsk Zone bythe deposition of quartz sandstones and siltstones of theGuragir Formation, and in the Berezovo and Markha-Markoka zones (Kanygin et

al., 2007) as a large ero-sional gap. The underlying Kimai deposits were partlyor completely eroded in many places on the Siberianplatform (Kanygin et al., 2006; 2007). In the IrkutskBasin, the tropical carbonate platform had beendestroyed even earlier, during the regression at the baseof the Ugry Sequence. However the final disappearanceof the tropical carbonate factory on the Platform wasnot until the forced regression at the base of the Baikitsequence. The amplitude of this regression may be esti-mated as “large”, i.e., >75 m (Haq and Schutter, 2008).This was one of the largest regressions in the Ordovi-cian of the Siberian Platform and one of the mostimportant turning points in the evolution of sedimenta-tion and the biota.

II. Base of the Volgina Sequence

The transgressive surface at the base of the VolginaSequence marks the beginning of a large transgressiveevent after a lowstand, marked by the Baikit sand-stones. This transgression led to a complete change offaunal assemblages and prevailing type of sedimenta-tion. The base of the Volgina Sequence (VolginianRegional Stage) was a chosen level of the boundarybetween two large stages in the geological history ofthe Ordovician of the Siberian Platform, distinct inlithology, facies and thickness of rocks, and in the com-position of biotas (Kanygin et al., 2006). Contrastingdifferences in the lithological and paleontological char-acterization of the Ordovician above and below thisboundary served as a basis for subdivision of theOrdovician on the Siberian Platform into two series(lower and upper) (Sokolov and Tesakov, 1975).Although the transgression that led to the formation ofthe Volgina Sequence was not the largest, the transgres-sive surface at the base of the Volgina Sequence is oneof the most significant turning points in the Ordovicianof the Siberian Platform.

III. Base of the Mangazeya Sequence

The transgressive surface at the base of the Man-gazeya Sequence is clearly discernible over the entireplatform and is in many places accompanied by fea-tures of erosion of the underlying rocks and presence ofbasal conglomerates at its base. The top of the Kirensk-Kudrino Sequence appears to be marked by a forcedregression, whereas the subsequent transgressionresulted in the formation of the transgressive surface oferosion with conglomerates that constitute an overlying


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transgressive lag. Apparently, the transgressions in thebasal beds of the Volginian and Chertovskian regionalstages were the largest in the Ordovician of the SiberianPlatform (Kanygin et al., 2006). In rate and magnitudethe transgression at the base of the MangazeyaSequence possibly exceeded the Volginian transgres-sion. The facial differentiation at the time of this trans-gression was minimal, which facilitated wide distribu-tion of the similar benthic and pelagic biocoenoses inmost regions of the Siberian Platform. The transgres-sion at the base of the Mangazeya Sequence apparentlycorresponds to a global eustatic maximum at the base ofthe graptolite

Nemagraptus gracilis

Zone (Fortey,1984; Barnes et al., 1996).

IV. Base of the Keto Sequence

Judging from published descriptions, cherry-redsiltstones of the Nirunda Horizon are facial equivalentsof relatively deep-water facies of the Kirensk-Kudrin-ian and Chertovskian regional stages. Hence the bound-ary between the Dolbor and Keto sequences representsa transgressive surface and indicates a level of a rapidsea-level rise. This level is insufficiently studiedbecause the deposits at this stratigraphic interval arepoorly exposed, while the best outcrops of the Dolbo-rian, Kirensk-Kudrinian, and Burian regional stages arelocated in poorly accessible regions of the SiberianPlatform. Thus, this level is at present drawn only pro-visionally.

V. Ordovician–Silurian Boundary

The Ordovician–Silurian Boundary is marked by adistinct discontinuity in sedimentation, which resultedfrom a large-scale forced regression. It is possible thatthis drop in the sea level was connected to the continen-tal glaciation of Gondwana and is hence eustatic. Theuppermost beds of the Ordovician on the Siberian Plat-form are always eroded. The beginning of the Silurianmarks a new transgression and essential renewal of theecosystems. A regressive event at the Ordovician–Sil-urian boundary is one of the most significant turningpoints, reflected in changes in fauna and sedimentationin the Ordovician and even in the entire Phanerozoic.

Thus, the history of the evolution of the Ordovicianepicontinental basins of the Siberian paleocontinentcontained five most important levels of turnover(Fig. 2). Two of these levels (base of the BaikitSequence and the Ordovician–Silurian boundary) arerelated to forced regressions. The other levels (bases ofthe Volgina, Mangazeya, and Keto sequences) arerelated to transgressive surfaces marking a rapid rise insea level followed by large-scale transgressions.

LONG-TERM CHANGES IN SEDIMENTATION

The recorded transgressions and regression areoften superimposed on the long-term changes in the

character of sedimentation reflecting climatic, oceano-graphic and paleogeographic changes. Among suchlithological changes in the Ordovician of the SiberianPlatform it is possible to recognize a sharp change incarbonate sedimentation from the typically tropical car-bonates to temperate carbonates, and large impulses ofinflux of terrigenous sediments in the sedimentarybasin and impulses of deposition of phosphates.

Carbonate sediments are clearly distinguished bytheir lithology, associated minerals and set of primarysedimentary textures between so-called warm-water ortropical (photozoan) and cold-water or nontropical(heterozoan) carbonates (Lindström, 1984; James,1997; Dronov, 2001; Dronov and Rozhnov, 2007). Wewill not describe here details and characteristics ofeither type, but indicate that the temperature boundarybetween these two types of carbonates was an annualtemperature of water around 22

°

C (James, 1997). Trop-ical carbonates are characterized by a wide spectrum ofvarious types of grains in the sediment including skele-tal remains (detritus), intraclasts, pellets, and oolites.These carbonates typically show the development ofbarrier carbonate reef buildups, including numerousstromatolites and cyanobacterial mats. The presence offine carbonate mud in the sediment is typical.

Nontropical carbonates are characterized by muchmore restricted spectrum of allochems, dominated bybioclasts. Pellets and ooids are usually absent, or veryrare. Barrier reef buildups and cyanobacterial mats arevirtually absent. In the shallow-water shelf sedimentscarbonate mud is virtually absent, although it mayaccumulate due to bioerosion in small quantities belowthe storm wave base. Apart from temperature, the char-acter of carbonate sediments is influenced by watertransparency and content of nutrients. An increase inthe amount of nutrient and terrigenous suspension inthe water (increased concentration of suspended mate-rial) shifts the spectrum of carbonates to the cold-waterside and can make carbonates look nontropical evenwhen the water temperature is high.

As noted above, contrasting differences in the litho-logical and paleontological characterization of theOrdovician of the Siberian Platform served as a basisfor subdivision of this system into two series (lower andupper) (Sokolov and Tesakov, 1975). These series areseparated by a transgressive surface at the base of theVolgina Sequence. The lower series (Nyaian–Mukteianregional stages) are characterized by predominance oftropical carbonate sedimentation. The dominatinglithotypes include oolitic grainstones, stromatolites andcyanobacterial mats are widespread. The beds charac-teristically contain flat-pebble conglomerates formedby breaking and redeposition by tropical storms of car-bonated crusts on the tidal flats of carbonate platforms.The thickness of the deposits of the lower series rangesfrom 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 amountof carbonates which are mainly represented by non-tropical varieties (Fig. 2). The beds are dominated bylimestones (bioclastic packstones). Stromatolites, dolo-mites, and large organic buildups are virtually absent.Pellets and ooids are very rare, and in places wherefound they are subdominant. Some levels show accu-mulations of glauconitic films and grains. The thick-ness of deposits of the upper series is sharply reducedranging from 90 to 300 m and on average about 170 m.Reduced thickness suggests reduced sedimentationrate, and hence sharp decrease in productivity of theshallow-water “carbonate factory.” The replacement ofthe tropical carbonate sedimentation by non-tropicalsedimentations was preceded by the destruction of thewarm-water carbonate factory and large influx of silici-clastic material into the basin.

Based on the paleomagnetic data, it is establishedthat in the Cambrian, Ordovician, and Silurian, theSiberian paleocontinent was located in the tropics(Cocks and Torsvik, 2007). Therefore the replacementof tropical carbonates by cold-water carbonated in theMiddle and Upper Ordovician can only be explained byupwelling of cold water from the depth to the continent.The upwelling of the colder oceanic water can explainthe suppressed tropical carbonate sedimentation, theabsence of the carbonate mud, pellets, and ooids, sup-pressed growth of organic buildups, stromatolites, andcyanobacterial mats, presence of glauconite and skele-tal cold-water carbonates. The upwelling of cold, oxy-gen-deprived water from the depths of the ocean, couldalso facilitate the influx of nutrients, which wasreflected in the wide distribution of phosphatization andphosphate conglomerates in the Volgina, Kirensk-Kudrino, and Mangazeya sequences. It should be notedthat the large influx of nutrients increases the effect ofcold water and also suppresses tropical carbonate sedi-mentation in favor of temperate carbonates. In the caseof large supply of nutrients, the water temperature maybe higher than necessary to create the effect of “cold-water carbonates.” The increased opacity of sea waterresulting from increased input of the terrigenous mate-rial from the nearby land could also contribute to thiseffect.

CORRELATION OF THE ORDOVICIAN BIOTAS IN THE SIBERIAN AND RUSSIAN PLATFORMS

The states of knowledge on fossil record from theSiberian and Russian platforms are compatible. Datafrom these regions may be considered sufficiently rep-resentative for comparison of the biotas in two criteria:(1) taxonomic composition of dominant fossil groupsand (2) dynamics of changes in their biodiversity at sig-nificant levels in the evolution of these paleobasins.These data are the integral indicator of the optimal con-dition of the development of biotas in normal marineenvironment and reflect major evolutionary trends, withthe minimum of error, and the latter may be due not to

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

In the Ordovician, a global ecosystem reorganiza-tion of the marine sector of the biosphere, the largest inthe Phanerozoic, took place, and its scale and evolu-tionary consequences is comparable with the explosiveCambrian biodiversification of the organic world onEarth, when all major phyla of invertebrate organismsappeared. In the Ordovician, the biodiversity of marineanimals increased almost twofold due to the appearanceand rapid divergence of pioneering groups or diversifi-cation of previously small benthic groups with newecological specializations and more diverse adaptiveopportunities (corals, bryozoans, echinoderms, articu-late brachiopods, stromatoporoids, and ostracodes). Inthe Ordovician, the biodiversity and population densityof trilobites previously monopolizing the benthal,became to decrease, because of the increased competi-tion for food resources, although they still retained theirrole as major components of benthic biocoenoses. Atthe same time, a stable zoopelagial was formed for thefirst time (to replace the previous facultative one) tocontain chitinozoans, a new highly productive group ofmicrophytoplankton, which apparently became themain initial food resource for specialized groups ofzooplankton and nekton (graptolites, radiolarians, con-odontophorids, nautiloids, and agnostids) (Kanygin,2001; Kanygin, 2008).

Of the above fossil groups, trilobites, ostracodes,brachiopod, and conodonts are the best indicators ofevolutionary changes in the biotas, due to their highpopulation density and taxonomic diversity, similarlevel of knowledge, and reliable identification based ona variety of morphological criteria. The taxonomy oflarge groups (colonial corals, stromatoporoids, and bry-ozoans) is complicated by broad intraspecific variationsof live forms and difficulties of identification based onthin sections. Data on echinoderms (although thisgroup is large) are still not representative because theyare unequally studied, especially on the Siberian Plat-form. The data on naulitoids, graptolites, radiolarians,chitinozoans, and other pioneer Ordovician groups arealso important for characterization of the ecosystems ofthe epicontinental basins of the Siberian and Russianplatforms, but these are even less complete.

The comparison of the taxonomic composition andbiodiversity dynamics of four major dominant groupsallow the recognition of general evolutionary trendsand features of biotas of the Siberian and Russian plat-forms. The comparison of the taxonomic compositionof these two major components of biotas shows thatbenthic communities differ considerably throughoutthe Ordovician. No species in common are documentedin the trilobite, ostracode, and brachiopod assemblages.In the Early Ordovician (Tremadocian, Arenig), thesegroups show no genera in common between the twoplatforms. Some similarities in the generic compositionof the trilobite and brachiopod assemblages appear


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from the Middle Ordovician (Llanvirn). Six trilobitegenera and five brachiopod genera in common arerecorded from these regions, but their stratigraphicranges are essentially different: in the Siberian Platformthey are found in the Volginian Regional Stage only(Llanvirn), which corresponds to the beginning of alarge transgression, whereas in Baltoscandia these gen-era continued to exist to the Mid-Ashgill. Even strongerdifferences are recorded in the taxonomic compositionof the benthic assemblages of ostracodes, which con-tain no genera or even families in common (exceptsome of doubtful identification). At the same time manyreadily identified ostracode taxa (e.g., species withlobe-like dissected shells) show relatively synchronousphenotypic changes in parallel phylogenetic lineages.Such uniform phenotypic changes in isolated popula-tions of species with similar morphological architec-tonics in genetics are referred to as “mutation fashion”,which can certainly can be revealed under the influenceof global environmental factors. This “fashion” is taxo-nomically expressed by the presence of the parallelphylogenetic lineages of twin species, which belong,for instance, on the Siberian Platform to the familyCherskiellidae, and in Baltoscandia to the family Tet-radellidae.

The autochthonous nature of benthic communitieson the Siberian and Russian platforms suggests theirpaleogeographic separation, especially strongly devel-oped in the Early Ordovician. The appearance of thesame genera of trilobites and brachiopods in the Early–Middle Ordovician boundary interval can be used asevidence of the convergence of these paleobasins,which agrees with modern palinspastic reconstructions.The life cycle of trilobites and brachiopods has ameroplanktonic stage, during which larvae are capableof moving within relatively short distances in theocean, and therefore provided entries in similarbiotopes of other epicontinental basins. At this timemany genera and even species in common of pelagicostracodes (

Coelochilina, Eurychilina, Laccochilina,Sigmobolbina, Oepikella

n. sp.) appear in both paleoba-sins, which indicates the migration links between thebasins in the pelagic zone without significant climaticbarriers.

The most adequate representation of general ten-dencies, their connections with global ecosystem re-organizations and eustatic events can be obtained fromgraphs of changes in biodiversity (Fig. 9), despite someconventions in their synchronization. The typificationof data in two regions was based on regional strati-graphic scales, and their correlation using the previousBritish standard of stage subdivision of the Ordovicianand its subdivision into series by the decisions of theInterdepartmental Stratigraphic Committee of theUSSR/Russia.

Unexpectedly, the comparison of the graphs ofbiodiversity showed a much lower biodiversification ofdominant faunal groups on the Siberian Platform than

in Baltoscandia. Beginning from classical reconstruc-tions of climatic zonation in the Phanerozoic in theframework of the “fixism” paradigm (Strakhov, 1968),and in all subsequent paleogeographic reconstructions(now using the plate tectonics paradigm) the Siberiancontinent in the Ordovician was located in a warmerzone than Baltoscandia. Therefore, following generalpatterns of chorological differentiation of biodiversityin climatic belts one could expect the opposite situa-tion, i.e., a higher number of taxa on the Siberian Plat-form. This paradox can be explained by the existenceduring the entire Ordovician in the Baltoscandian partof the Russian Platform of a more stable normal marineenvironment, whereas the paleobasin of the SiberianPlatform was generally shallower with contrastingchanging environments from normal marine to evapor-ite, brackish-water, and subaerial. This conclusion issupported by the fact that in the relatively deeper car-bonates of the Verkhoyansk–Chukotka Folded Regionand Taimyr, which at that time along with the Siberianplatform were part of the large Kolyma-Siberian Prov-ince, the number of ostracodes species (the best studiedgroup) increased two- or threefold (Kanygin, 1967,1971), which is comparable with that in Baltoscandia.Another explanation could be in the cooling effect onthe Siberian epicontinental basin of an upwelling,which supposedly emerged on its western margin (inmodern geographical coordinates) due to the openingin the Ordovician of the northern branch of the Paleoa-siatic Ocean (often also called Paleouralian Ocean).

Graphs showing changes in biodiversity both inindividual groups and in the integral representationsagree well and can be correlated with global eustaticevents. Some incongruity in synchronization of graphsof biodiversity of these regions could result from:(1) insufficient precision of biostratigraphic correlationmarkers, (2) effect of regional paleogeographic and tec-tonic features in the development of paleobasins, and(3) differences in the methods of interval-based countsof species. On the Siberian Platform, the typification ofspecies composition is based on regional stages; on thegraphs by Hammer on Baltoscandia (Hammer, 2003b),which are here used for comparison, data on the num-ber of species were summed in conventional one-dimensional chronological intervals. However, possibleerrors resulting from the above factors do not rule out theconclusion that the major trends and events in the devel-opment of biotas of the paleobasins of the Siberian andRussian Platforms were more or less synchronous.

Two maxima of biodiversity of the benthic fauna inthe Middle Ordovician particularly clearly correlate.The first of these on the Siberian Platform correspondsto the Volginian Regional Stage, and in Baltoscandia tothree regional stages of the Estonian scale—Aseri, Las-namägi, and Uhaku. The second peak on the Siberianplatform corresponds to the Chertovskian and Baksa-nian regional stages, and in Baltoscandia to the Kuk-ruse and Haljala regional stages. Both maximum peaksof biodiversity (like other less dramatic such peaks),

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coincide with transgressive phases in the developmentof paleobasins, and minima of biodiversity with regres-sive phases. A relative synchronicity of transgressive-regressive events and changes in biodiversity of domi-nating faunal groups in epicontinental paleobasins sep-arated by ocean proves a global nature of such signifi-cant levels and can be used as an additional criterion forintercontinental stratigraphic correlations.

To evaluate the dynamics of biodiversity of con-odonts in the Siberian and Baltoscandian basins, anadditional comment is necessary because this fossilgroup has become a primary paleontological indicatorfor global correlations. However, conodonts in differentregions are studied to a varying extent, whereas data ontheir true taxonomic composition are difficult to sys-tematize because at different times they were classifiedbased on two alternative criteria—multi-element appa-ratuses and isolated elements. Even in Baltoscandia,where conodonts are justifiably considered as the beststudied group compared to other regions of the world,curves based on statistical data using mathematicalmethods (Hammer, 2003a, 2003b), are insufficientlyprecise in interpretation of true diversity of this group.

In this regions, the most reliable is the dynamics of thediversity of conodonts from the Lower to lower part ofthe Middle Ordovician, showing a gradual increase innumber of species from 25–30 in the Upper Tremado-cian to 45–50 in the Lower Llanvirn (Fig. 9). A negativeanomaly of the graph of the medium-weight diversity inthe Middle Llanvirn–Lower Caradoc (Hammer, 2003b)is largely related to the widespread terrigenous facies,from where conodonts have virtually never been stud-ied. The information on the Upper Llanvirn and LowerCaradoc conodonts have become more complete onlyrecently because of the substantiation of the limitotypeof the Sandbian Stage in southern Sweden (Bergström,2007; Pålsson et al., 2002; Viira, 2008).

The existing data show that compared to benthiccommunities, changes in sea level only slightly affectedthe diversity of conodonts of the Lower and beginningof the Middle Ordovician in Baltoscandia. Apparently,despite the changes in facial conditions in the regions ofcarbonate sedimentation related to sea level fluctua-tions, optimal environment for this pelagic groupremained in this district. Lower diversity of conodonts(15–25 species) in the upper part of the Caradoc–begin-

15202530

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Conodonts

Ostracodes

Tiolobites

Brachiopods

of dominant groups of fauna of Baltoscandia

Hirn.KatianSandbianDarriwillianDapin-FloianTremadoc


gian

Porkun.Pirgu

Vorm

.

Nab.Rak

Oan

du

Kei

la

Hal.KukruseUhaku

Las

n.

Ase

ri

KundaVolkhovVaran.Pake-rort BillingenHunneberg

Baltoscandia (Hammer, 2003)

Benthic species

Pelagic species

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

PALEONTOLOGICAL JOURNAL Vol. 43 No. 11 2009


ning of the Ashgill may be explained by displacementof the basin in the region of lower latitudes and warmtemperatures. The distorting effect of the taphonomy onthe contents of samples should also be taken intoaccount because in the warm-water carbonates theamount of conodont elements is usually small, whichmakes the examination more difficult, collectionssmaller, and the number of species identified lower.

On the Siberian Platform, in contrast to Baltoscan-dia, the Ordovician conodonts are studied more com-pletely across different facial zones (Moskalenko,1973; Kanygin et al., 1984b; 1989; Tesakov et al., 2003and others.). Almost all regional subdivisions containthis group, conodonts are found in various facies, andare represented by numerous specimens. However,when calculating the diversity, there is an immediateproblem of counting species, because in the majority ofpapers, conodonts are described using formal taxon-omy, which considerably increases the number of taxa.The transition from the formal to multi-element taxon-omy required detailed revision of faunal composition.Many Siberian conodonts are endemics and possiblyhave apparatuses not characteristic of most known taxa.To reduce the distorting effect of using different meth-ods of calculations and classifications, the approximate

number of multi-element taxa may be accepted as halfof known formal species, because in most cases P andS-elements, or M and S-elements of real species havebeen described as separate species. Thus, it is mostlikely that no more than 15–20 conodont speciesexisted at the same time in the Paleozoic basin of Sibe-ria, and their diversity at the period of time consideredremained relatively constant (Fig. 9). The small numberof species in the Muktei Regional Stage is possiblyrelated to the very low thickness of this unit. Low diver-sity of conodonts in the Volginian time, in contrast tohigh diversity of other fossil groups, is possibly connectedwith significant dominance of Phragmodus flexousus inthe assemblages.

Different stages of the evolution of the Siberianbasin were characterized by different level of ende-mism of the conodont fauna. Assemblages of the Nya-ian and Ugorian Regional stages contain up to 20% cos-mopolitan species (Eoconodontus notchpeakensis,Cordylodus angulatus, C. proavus), and species char-acteristic of Laurentia and other regions of the Mid-Continent paleobiogeographic conodont province. Inthe Late Ugorian, Kimaian, and Vikhorevian regionalstages, the endemism of conodonts was at its highest,with no species recorded from other continents found

15Pelagic species

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An integrated graph showing the diversity

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of dominant groups of fauna of Baltoscandia

Hirn.KatianSandbianDarriwillianDapin-FloianTremadoc


gian

Not recognizedKimai

Kir

.-

Kudri

VolginNyai Ugorsk DolborVikhorev

MukteiBaks.Chert. Nir. Bur

Siberian Platform

Benthic species

Fig. 9. (Contnd.)

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on the Siberian Platform. Some similarity was main-tained only with the warm basins of Laurentia at thegeneric level (genera Loxodus, Coelodus, and Erismo-dus). However, beginning from the Volginian RegionalStage, Phragmodus flexuosus, Ph. inflexus and otherspecies typical of the warm-water Mid-Continent Prov-ince became to appear. From that time the assemblagesbecame more similar on the generic level with Bal-toscandia, and this similarity gradually increases by theBaksanian and Dolborian epochs. These results agreewell with the notion that the Baltoscandian basin bythen was shifted to the tropic, and now there existedwarm-water environment, similar to that of the Sibe-rian. In the Late Ordovician (Dolborian time) conodontassemblages of the Siberian Platform, which was at thattime dominated by shallow marine environment, expe-rienced an influx of pelagic species typical of openmarine basins (e.g., Periodon grandis).

Thus, the data on the taxonomic composition andbiodiversity dynamics of four dominant faunal groups,despite their considerable chorological and ecologicaldifferences, confirm similar trends in the evolution ofthe biotas and gradual change in the paleogeographicpositions of paleobasins in the Ordovician–from beinglargely spatially and climatically separated to be con-vergent near the equator.

CORRELATION WITH THE RUSSIAN PLATFORM

The analysis of the geological history of the Russianand Siberian platforms in the Ordovician shows moststrikingly in the contrasting sedimentation, thickness,and facies in these two paleocontinents. On the RussianPlatform, the average thickness of regional horizonsgradually increases upward in the sections, whichreflects an increase in the mean rate of sedimentation.In addition, a gradual change from mainly terrigenoussedimentation to first cold-water, and then to tropicalcarbonates upward in the succession is observed inshallow-water environments (Dronov and Rozhnov,2007). This climate-related successive change in sedi-mentation reflects the drift of the Baltica Paleoconti-nent in the Ordovician from the Subpolar regions of thesouthern hemisphere to the subequatorial regions,which agrees well with the paleomagnetic data (Cocksand Torsvik, 2005). On the Siberian platform the thick-ness of regional horizons in the lower and lower middlepart of the Middle Ordovician is considerably higherthan in the Upper Ordovician and the upper part of theMiddle Ordovician. In addition, upward in the section,the predominantly carbonate tropical warm-water sedi-mentation changes by mainly terrigenous and cold-water terrigenous-carbonate. In other words, the trendwas strictly the opposite of the trend observed on theRussian Platform. In addition, the replacement of onetype of sedimentation to another on the Siberian Plat-form was not gradual, but quite abrupt and occurs at thebase of the Volgina Sequence.

There is some similarity between the Ordovician onthe Russian and Siberian platforms in the number oflarge sedimentary rhythms and stratigraphic position oftheir boundaries. Unfortunately, the Baltica and Sibe-rian paleocontinents belonged to different paleobiogeo-graphic provinces, and their fauna in the Ordoviciancontains almost no species in common. This prohibitsdetailed biostratigraphic correlation. However, forsome stratigraphic intervals, which can be correlated,the number of sedimentary basins recognized withinthem is the same. This suggests synchronous develop-ment of the sedimentary sequences on both platformsand their eustatic nature.

However, speaking about the major turning points inthe history of the biota and sedimentation on the Rus-sian and Siberian platforms in the Ordovician, theirexpression on both platforms is not always the same.For instance, a global regression at the Ordovician-Sil-urian boundary is clearly observed both on the Russian,and the Siberian platforms. It is apparently connectedwith the global drop in sea level caused by the Hirnan-tian Glaciation. In both platforms levels of re-organiza-tion in the Darriwilian time are clearly discernible. Onthe Siberian platform this is a transgressive surface atthe base of the Volginian Regional Stage, associatedwith transgression, upwelling, and significant change insedimentation. On the Russian Platform, this is a trans-gressive surface at the base of the Aseri Regional Stage,marked by a prominent renewal of the shelly fauna inall parts of the basin. This renewal was the basis for rec-ognition of the Tallinn regional stage with a lowerboundary coinciding with the base of the AseriRegional Stage (Männil, 1966). A prominent turningpoint at the base of the Sandbian Stage (base of theCaradoc in Great Britain) is also distinct on both plat-forms. This level apparently coincides with a globaleustatic transgression at the base of the Nemagraptusgracilis graptolite zone (Fortey, 1984; Barnes et al.,1996; Haq and Schutter, 2008).

At the same time, a very important turning point forthe Siberian Platform, at the base of the BaikitSequence, related to a marked change in sedimentation,destruction of a tropical carbonate platform and influxof a vast quantity of siliciclastic material in the sedi-mentary basin is less pronounced on the Russian Plat-form. The unconformity at this level (boundarybetween the Volkhov and Kunda sequences) is well pro-nounced and shows a considerable renewal of faunalcontent (Männil, 1966), although no fundamentalchange in sedimentation took place at that time. Appar-ently, on the Siberian Platform, a global eustatic eventwas reinforced by a regional tectonic uplift of the area,adjacent to the Yenisei Land. The boundary correspond-ing to the Keto Sequence on the Siberian Platformapparently correlates with the transgressive FjäckaShale on the Russian Platform. On both platforms thislevel is poorly exposed and insufficiently studied.



In general, despite complications superimposed byregional tectonics, eustatic events were the main factorinfluencing the evolution of sedimentary basins on bothplatforms. A general trend toward increased depth ofthe basin in the upper part of the Middle and in theUpper Ordovician (before the Hirnantian Regression)is clearly traced on both paleocontinents.

DISCUSSION

Taking into account that the Siberian Platform in theMiddle and Upper Ordovician was situated in the lowlatitudes, near the equator, the wide distribution ofcold-water carbonates on this platform can be onlyexplained if the presence of a powerful upwelling ofcold oceanic water coming from the depths whichreached shallow-water epicontinental seas is assumed.The presence of typical warm-water carbonates inunderlying (Upper Cambrian–Lower Ordovician) andoverlying beds (Silurian) on the Siberian Platformshows that in normal environments (i.e., without anupwelling), tropical carbonates developed in this pale-olatitudes (as it should be).

The beginning and the end of the upwelling were thelargest geohistorical events in the Ordovician of theSiberian Platform. The beginning of the upwelling isconnected with the largest transgression, the beginningof which is marked by a transgressive surface at thebase of the Volgina sedimentary sequence. At the timeof the following transgressions (Kirensk-Kudrino,Mangazeya, and Keto sequences) the relative sea levelrose even more. Thus, in the upper part of the Middleand in the Upper Ordovician it apparently reached itsmaximum in the Ordovician. The time of the existenceof the upwelling coincides with the time of the maxi-mum sea level highstand on the Siberian Platform.

However, even a significant rise of sea level does notnecessarily result in an upwelling. On the Siberian Plat-form, the upwelling developed at the time of the trans-gression, which immediately followed the largestregression (the latter resulted in the deposition of Baikitsandstones). This regression was either caused orgreatly amplified by tectonic processes (elevation of theYenisei Land on the western margin of the SiberianPlatform). The uplift of the margin of the Siberian Pale-ocontinent at that time could be connected to its tec-tonic activization as a result of the adherence of a ter-rain or an island arc. This reorganization of tectonic ele-ments could cause a redistribution of directions of largeoceanic currents, which could result in an upwelling atthe time of the subsequent transgression.

The termination of the upwelling at the Ordovician-Silurian boundary was apparently caused by theeustatic sea level drop resulted from the HirnantianGlaciation and subaerial exposure of all continents. There-distribution of the areas occupied by sea and land ledto the re-organization of the system of global oceaniccurrents and cessation of the upwelling. Later, when in

the Silurian, the sea level rose again, this did not lead tothe restoration of the system of oceanic currents, whichexisted in the Ordovician and, hence, the upwelling wasnot restored either. Because of its absence, a warm-water marine carbonate sedimentation was restored inthe Silurian shallow-water epicontinental seas, whichwas normal for the paleolatitudes where it occurred.

It is noteworthy that a situation similar to that in theOrdovician on the Siberian Platform, is recorded on theNorth American Platform. This platform throughoutthe Ordovician was in the tropical zone, but in theLower and Middle Ordovician the succession is repre-sented by warm-water carbonates, which was replacedin the middle part of the Upper Ordovician by cold-water carbonates (Holland and Patzkowsky, 1996).Tropical carbonate sedimentation was restored at thevery end of the Ordovician and in the Silurian. On theAmerican Platform, this situation is explained by anupwelling, corresponding to a strong transgression atthe base of the sequence M-5 (Holland and Patzkowsky,1996). This transgression, similarly, appeared after alarge regression and tectonic-reorganization connectedwith the beginning of the Takonic Orogeny, i.e., thebeginning of the accretion of the Takonic Island Arc tothe North American continent.

In the case with the North America, the beginning ofthe Takonic Orogeny and of the upwelling of cold oce-anic waters onto the continent occurred somewhat later,in the middle of the Upper Ordovician. On the SiberianPlatform, the orogeny and subsequent upwelling hap-pened somewhat earlier, in the upper part of the MiddleOrdovician. However, a general trend of events andtheir nature are very similar. Thus, the history of devel-opment and evolution of sedimentation in the Ordovi-cian period of the Siberian Platform are far more simi-lar to those on the North American than on the EastEuropean Platform.

CONCLUSIONS

(1) In the Ordovician of the Siberian Platform werecognized and traced nine sedimentary sequences,corresponding to sea level fluctuations of the third orderwith an average duration from 1 to 6 Myr. The bound-aries of the sequences are represented by erosionalunconformities and transgressive surfaces. Sedimen-tary systems tracts were identified for some sequences.The largest regressive events occurred on the Siberianplatform at the base of the Baikit sequence and at theOrdovician–Silurian boundary. The largest transgres-sions are observed in the Volgina, Mangazeya, and Ketosequences.

(2) The comparison with the sequences, recognizedin the Ordovician of the Russian Platform, shows thattheir number and stratigraphic position of boundarieson both platforms almost coincide, which suggests thatthe sea-level changes were possibly eustatic. Differ-ences 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-levelchanges.

(3) Presence of cold-water nontropical carbonates inepicontinental seas of the Siberian paleocontinent,which in the Ordovician was situated in the equatorialzone may be explained by the effect of an upwelling ofcold oceanic water, caused by redistribution of oceaniccurrents due to the tectonic re-organization in the Mid-dle Ordovician. The penetration of cold oceanic waterinto epicontinental seas resulted from a sea-level high-stand in the upper part of the Middle Ordovician andupper Ordovician.

(4) Pathways in the development of the Siberian Plat-form in the Ordovician are much more similar to theNorth-American, rather than to the Russian Platform.

ACKNOWLEDGMENTS

The study is supported by the Russian Foundationfor Basic Research, project no. 07-05-01035a and is acontribution to the International project IGCP no. 503“Ordovician Paleogeography and Paleoclimate.”

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