ISSN 08695938, Stratigraphy and Geological Correlation, 2010, Vol. 18, No. 6, pp. 635659. Pleiades Publishing, Ltd., 2010.Original Russian Text M.A. Akhmetev, N.I. Zaporozhets, A.I. Iakovleva, G.N. Aleksandrova, V.N. Beniamovsky, T.V. Oreshkina, Z.N. Gnibidenko, Zh.A. Dolya, 2010, published in Stratigrafiya. Geologicheskaya Korrelyatsiya, 2010, Vol. 18, No. 6, pp. 78103.

635

INTRODUCTION

Drilling carried out on the Lomonosov Ridge (Backman and Moran, 2004) revealed several peculiar featuresin the marine Paleogene section and its biota, which areinconsistent with traditional views on natural settings inthe Arctic basin. This is primarily true of anomalousevents established by the study of the drill core.

The first of such anomalous events is an increase inthe surface water temperatures in the polar segment upto 2023, which is thought to be associated with theglobal warming episode at the PaleoceneEocenetransition (PETM). The second anomalous event isthe mass development of the aquatic fern Azolla,which indicates desalination of surface waters at theearlymiddle Eocene transition. Desalination of thephotic layer in polar waters is also evident from otherlayers, in addition to the Azolla Beds. It is also reflectedin the appearance of Chrysophyta algae (Stickleyet al., 2008) at least at fivesix levels of the middleEocene section above the Azolla Beds, althoughremains of this aquatic fern are missing in the latter.There are also data on desalination of surface watersover the Lomonosov Ridge observed in the late Thanetian, in addition to Eocene (Brinkhuis et al., 2006).

Until recently, searches for factors responsible theabovementioned phenomena were limited to comparison of the data obtained during drilling on theLomonosov Ridge and in the NorwegianGreenlandsegment of the Arctic basin. At the same time, thesephenomena are readily explainable owing to the comparative analysis of peculiar features of Early Paleogene marine sedimentation in the Arctic and WestSiberian basins. Until recently, such analysis was neverused by specialists, who studied the core from holesdrilled on the Lomonosov Ridge, although both basinswere characterized by paleogeographic and paleobiogeographic connections in the Early Paleogene.

The study of dinocysts, i.e., palynomorphs ofhigher plants, from marine Paleogene sediments of theWest Siberian Plate that were deposited under conditions similar to settings in the Arctic basin allows theseevents to be at least partly interpreted. In West Siberia,these sediments were studied both in boreholes andnatural outcrops over large areas (Fig. 1).

The approval of the recent Unified Paleogenestratigraphic scale for the West Siberian Plate (Unified, 2001) was preceded by the study of new material from Lower Paleogene sediments of theplate,based on the reviously available data and their

Comparative Analysis of Marine Paleogene Sections and Biota from West Siberia and the Arctic Region

M. A. Akhmeteva, N. I. Zaporozhetsa, A. I. Iakovlevaa, G. N. Aleksandrovaa, V. N. Beniamovskya, T. V. Oreshkinaa, Z. N. Gnibidenkob, and Zh. A. DolyacaGeological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russia

email: akhmetiev@ginras.rubInstitute of Petroleum Geology and Geophysics, Siberian Division, Russian Academy of Sciences,

pr. akademika Koptyuga 3, Novosibirsk, 630090 RussiacOpen JointStock Company Omsk GeologicalProspecting Expedition

Received May 25, 2010; in final form June 20, 2010

AbstractThe analysis of the main biospheric events that took place in West Siberia and the Arctic regionduring the Early Paleogene revealed the paleogeographic and paleobiogeographic unity of marine sedimentation basins and close biogeographic relations between their separate parts. Most biotic and abiotic events ofthe first half of the Paleogene in the Arctic region and West Siberia were synchronous, unidirectional, andinterrelated. Shelf settings, sedimentation breaks, and microfaunal assemblages characteristic of these basinsduring the Paleogene are compared. The comparative analysis primarily concerned events of the PaleoceneEocene thermal maximum (PETM) and beds with Azolla (aquatic fern). The formation of the Eocene AzollaBeds in the Arctic region and West Siberia was asynchronous, although it proceeded in line with a commonscenario related to the development of a system of estuarinetype currents in a sea basin partly isolated fromthe World Ocean.

Keywords: stratigraphy, Paleogene, Eocene, Oligocene, dinocysts, diatom algae, radiolarians, foraminifers,Azolla Beds.

DOI: 10.1134/S0869593810060043

636

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

AKHMETEV et al.

revision and on the combined analysis of biostratigraphic problems (Unified., 2001; Akhmetev et al.,2001, 2004a, 2004b; Iakovleva and Kulkova, 2003; andothers). Previously, it had been considered that sedimentation was relatively continuous through the Paleocene and Eocene. At the same time, recent studiesrevealed several hiatuses usually of a eustatic nature(Akhmetev et al., 2004a, 2004b). Some of these hiatuses are probably related to block movements of thebasement of the plate and partial erosion of previouslyaccumulated sediments during reorganizations of thesedimentation regime. The latter resulted in a weakened or, on the contrary, strengthened influence of theopen seas located to the north and to the south of theWest Siberian basin on the hydrological regime andsedimentation of the inner basin (Akhmetev et al.,2004b, p. 83).

Until 2004, there was almost no information on theage of sediments constituting the sedimentary cover ofthe Arctic Ocean in its polar segment. Barron (1985) wasthe first to report on the Campanian diatom algae fromclays obtained from the Alpha Ridge. Subsequently, in hisreport at the international meeting in London dedicatedto problems of the Palearctic region, J. Thiede noted thepresence of Lower Paleogene clayey and clayeysiliceousrocks with siliceous microplankton on the same ridge.These and other materials, including data obtained bydeepsea drilling in highlatitude ODP Leg 151, weresummarized in his papers (Thiede, 1991; Thiede et al.,

1996). In the 1990s, international teams on the icebreakers Polarstern and Oden sampled sediments from gravitycores in the junction zone of the Lomonosov Ridge withthe continental margin of the East Siberian Sea(Rachold, 1997).

In 2004, the Arctic Core Expedition (ACEX) wasconducted in the framework of Leg 302 of the Integrated Ocean Drilling Program (IODP). During thisleg, several holes were drilled on the Lomonosov Ridge(Moran and Backman, 2006). Of five holes drilled atdepths of 12001300 m along a single seismic profile,two sections (M0002A and M0004A) were subjected tocomplex study. The second of these holes, that wasabandoned at a depth of 430 m, recovered a most complete section of the uppermost Cretaceous and LowerPaleogene sediments, including the PETM interval.

According to the interpretations proposed by theresearch team, the Lomonosov Ridge represents afragment of the continental crust separated during rifting from the shelf margin of Eurasia and displaced tohigh latitudes. This opinion is reflected in recent publications dedicated to ACEX materials: During thespreading, Gakkel Ridge had separated from the Siberian shelf (Sluijs et al., 2008, p. 10). In this connection, the comparison of the Paleogene sedimentarysection of the Arctic region with coeval sequences ofWest Siberia became particularly crucial. Informationon sediments recovered by the first holes (Backmanet al., 2006) was continuously supplemented by newmaterial.

The composite Upper CretaceousHolocene section of the Lomonosov Ridge consists of four units.The thickness of the first complex (Unit I) composedof middle EoceneHolocene sediments ranges from220 m in Hole M0002A to 265 m in Hole M0004A. Itis subdivided into six subcomplexes (subunits), theconsideration of which is outside the scope of thiswork. Unit II is dated back to the middle Eocene. It iscomposed of horizontally and crossbedded biosiliceous mud with rare pebbles and contains lenses andnodules of pyrite. The unit is recovered in the intervalsof 220.2267.7 and 263313 m in Holes M0002A andM0004A, respectively. The attribution of this unitentirely to the middle Eocene gives rise to doubts,which are substantiated below. The third complex(Unit III) entirely recovered only by Hole M0004A inthe interval of 313.6404.8 m is composed of dark graycompact clay, locally silty, with rare intercalations andlenses of pyrite. The clays contain abundant phytoplankton remains, while siliceous microplanktonand biogenic opal are practically absent. The unit iscorrelated with the upper Paleocenelower Eocene.The fourth complex (Unit IV) is penetrated by HoleM004A in the interval of 424.5427.6 m, where it isrepresented by dark gray locally sandy clay and siltysand with sandstone fragments and pyrite nodules. Thecomplex is presumably Campanian in age (Fig. 2).

In this work, special attention is paid to the thirdand, partly, second complexes, which are character

M0004A,M0002A

Well 228

Well 4Vasyugan R.

Well011BP

Well 9

Well8, 10Sokolovskii

Quarry

PershinskiiQuarry

Well 19 UstManya

140E

120

100

8060

50

70N

Fig. 1. Schematic location of marine Paleogene sectionsmentioned in the text.

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

COMPARATIVE ANALYSIS OF MARINE PALEOGENE SECTIONS AND BIOTA 637

ized by similar, in our opinion, composition and formation conditions in the West Siberian Plate and onthe Lomonosov Ridge.

Of particular interest are beds with remains of theaquatic fern Azolla recently discovered in the sectiondrilled on the Lomonosov Ridge at depths of approximately 300 m (Brinkhuis et al., 2006). This is a freshwater fern with an optimal salinity of 11.6. Experiments have shown that Azolla cannot tolerate salinityexceeding 5.5. Repeated desalination of surfacewaters in the Arctic Ocean and subsequent restorationof normal salinity during the Eocene was confirmed byabundant finds of Azolla mega and microspores aswell as by intermittent increase in concentrations ofChrysophyta algae in siliceous sediments. The alternation of beds formed under desalination and layerssaturated with marine siliceous and organicwalledphytoplankton of open seas in the sedimentary sectionof the Lomonosov Ridge confidently demonstrates thepulsating mode of the hydrological regime in the basinunder consideration. At the same time, the limiteddata on Paleogene sediments from the polar segmentof the Arctic region prevents unambiguous interpretation of factors responsible for desalination of surfacewaters. They are most frequently thought to be relatedto intense atmospheric precipitation and its prevalentinfluence over evaporation or cicular contour of seacurrents (Onodera et al., 2008).

Comparing formation conditions of the AzollaBeds on the Lomonosov Ridge and in the West Siberian basin, we aim to demonstrate (1) that these bedswere forming in line with a common scenario in theinner basin partly isolated from the open ocean and(2) that this phenomenon was determined by processes that took place on the West Siberian Plate.

Recently, information on the composition ofmarine Paleogene sediments and biota, that of theAzolla Beds included, was substantially supplementedby a comprehensive biostratigraphic study of coevalsections recovered by wells 8 and 10 in the RusskayaPolyana area in the marginal part of the Omsk Trough.These sections were analyzed by V.S. Volkova andO.B. Kuzmina. Their inferences were included intothe Explanatory Notes to the geological map of theRusskaya Polyana District (scale 1 : 200000) (Dolyaand Marineev, 2008).

In this work, we largely use the lithostratigraphicunits of the West Siberian marine Paleogene section,which are presented in the Unified Stratigraphic Scaleapproved by the Interdepartmental Stratigraphic Committee of Russia in 2002 (Unified, 2001). The lowerpart of the section is paleontologically best characterized in Well 10 and its upper part, in Well 8. Z.N. Gnibidenko provided the magnetostratigraphic scale for themarine Paleogene section of Well 8 (Fig. 4).

MARINE PALEOGENE SECTION OF THE OMSK TROUGH

In the Well 10 section, Paleogene sediments restwith the erosional surface upon the MaastrichtianGankino Formation composed of alternating greenquartzglauconite sandstones and clayey siltstones.The contact between the Gankino and PaleogeneLyulinvor formations is marked by a bed of dark graysiltstone with phosphorite nodules. The GankinoFormation contains the early Maastrichtian dinocystassemblage.

The Upper Paleocene (Thanetian Stage)Lower Eocene (Ypresian Stage). The Lyulinvor Formation

The lower part of the Lyulinvor Formation recoveredin the interval of 259.7237.0 m (Fig. 3) is characterized by species Alisocysta margarita, Alisocysta sp. 2, andDeflandrea denticulata, which allow these sediments tobe attributed to the Thanetian (~57.455.8 Ma; Luterbacher et al., 2004). Thus, hiatus between the Gankinoand Lyulinvor formations in Well 10 corresponds to theperiod of the late Maastrrichtian to Thanetian (~69.359.5 Ma).

The upper Thanetian Apectodinium hyperacanthumZone in the Omsk Trough is established in Well Chistoozernaya9. The same stratigraphic interval is correlated with the Glomospira gordialiformis andSpiroplectammina (Bolivinopsis) spectabilis beds basedon benthic foraminifers (Akhmetev et al., 2004a,2004b).

The dinocyst assemblage defined in the interval of237.0233.7 m in Well 10 (Fig. 4) includes stratigraphically significant species Apectodinium angustum,A. hyperacanthum, A. paniculatum, A. parvum, A. summissum, and Wilsonidium pechoricum, which correlatethese sediments with the PETM event (PaleoceneEocene Thermal Maximum, ~55.855.6 Ma; Crouchet al., 2001). The beds recovered in the interval of233.7229.0 m contain a dinocyst assemblage characterized by mass abundance of species from the Areoligera/Glaphirocysta group, and lack typical Ypresiantaxa. This part of the Well 10 section is most likely correlative with the basal Ypresian (~55.655.3 Ma). Thedepth of 229 m is marked by the first appearance of theWetzeliella astralobisca group, which corresponds tothe onset of the Ypresian s. str. (~55.354.0 Ma; Luterbacher et al., 2004). Higher through the section of theLyulinvor Formation, the succession of dinocystevents (first appearances of Wetzeliella meckelfeldensis, Dracodinium simile, D. varielongitudum, Charlesdowniea coleothrypta, Dracodinium politum, and Charlesdowniea columna) indicates earlymiddle Ypresianage of host sediments (~5451.5 Ma; Luterbacheret al., 2004). In the Well 011BP section, the uppermostLyulinvor Formation demonstrates the successiveappearance of the stratigraphically significant speciesAreosphaeridium diktyoplokum, A. michoudii, Wetze

638

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

AKHMETEV et al.

Kim

an

d G

leze

r,

2007

Bac

kman

et

al.,

2006

Pri

nci

pal b

ioti

c an

d ab

ioti

c ev

ents

(Bac

kman

et

al.,

200

6; O

no

der

a et

al.

, 20

08;

Slu

jis e

t al

., 2

008;

Sti

ckey

et

al.,

200

8, 2

009)

Oli

goce

ne

4 4B 5

200

220

240

260

280

300

320

340

360

380

400

14

45X

46X

47X

48X

49X

50X

51X

52X

53X

54X

55X

56X

57X

58X

59X

60X

61X

62X

5X 6X 7X 8X 9X 10X

11X

12X

13X

14X

15X

16X

18X

19X

20X

22X

23X

24X

25X

26X

27X

28X

29X

30X

31X

32X

33X

34X

35X

Mid

. Mio

c. 1

2 31/5

1/6

44.6

Ma

45.4

Ma

49.7

Ma

Dia

tom

s an

d si

licof

lage

llate

sD

inoc

ysts

Ben

thic

fora

min

ifers

Ver

neul

inoi

des

Ass

embl

age

Reo

phax

P

sam

mos

phae

ra A

ssem

blag

e

Ret

icul

ophr

agm

ium

Ass

embl

age

12

3

45

b

46.9

Ma

47.5

Ma 48

.5M

a

ET

M2

(X e

vent

)

PE

TM

Paleo

cene

Selan

dian

Lower Eocene

Middleupper Eocene

Lutetian, Bartonian,

Serie ,

subseries

Stage

Assemblage,

subassemblage

Thickness, m

Depth, m

Core

Core

Lithology

Age

Late Paleoceneearly EoceneMiddle Eocene

Middle Eocene

Middle Miocene

Ebridian algae

Diatomian algae

Synedropsis spp.

Silicoflagellates

Endemic silicoflagellate species ~50%

Chrysophyta > 50%

Riedelia claviger

Ps. radiosoreticulatum

Pyxilla oligocaenica

Anaulus arcticus sp. n.

Huttonia norwegica

Brightwellia hyperborea

Macrora barbadiensis

Cavitatus jouseanus

Kozloviella subrotundata

Hemiaulus incisus

Monobranchia unicornuta

Distephanus antiquus

Ps. inconspicuus var. trilobata

Monobranchia simplex

Navicula udintsevii

Naviculopsis punctilia

Dictyocha hexacantha

Dictyocha deflandrei

Dictyocha frenguellii

Dictyocha rotundata

Corbisema toxeuma

Brackishwater settings

Dominant role of peridinoid dinocysts

Oxygen deficiency or/and anoxia

Cerodinium striatum Deflandrea denticulata

Apectodinium augustum

Cerodinium wardense

Azolla spp.

Phthanoperidinium echinatum complex

Phthanoperidinium clithridium

Deflandrea oebisfeldensis

Hystrichosphaeridium tubiferum

Lentinia cf. wetzelii

Diphyes colligerum

Glaphyrocysta ordinata

Cerodinium depressum

Wetzeliella articulata

Wetzeliella hampdenensis

Deflandrea phosphoritica

Thalassiphora delicata

acme Azolla

Seasonal glaciations

Ypresian

138

52

recovery

Litho

assemblage

Priabonian

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

COMPARATIVE ANALYSIS OF MARINE PALEOGENE SECTIONS AND BIOTA 639

liella articulata subsp. brevicornuta, Wetzeliellla eocaenica, Hystrichosphaeropsis costae, and Duosphaeridiumnudum, which date the corresponding sediments backto the late Ypresian, its terminal part included (~51.548.6 Ma) (Kulkova and Zaporozhets in Akhmetevet al., 2004a, 2004b; Iakovleva, 2008).

The Well 10 section yielded benthic foraminiferalassemblages of the Saccammina grzybowskiNothiarobusta (interval of 259.7257,7 m) and Reophax subfusiformisNothia excelsa (interval of 224.1189.0 m)zones (Table 1). The interval of 195.0192.5 m in thesame section is characterized by abundant siliceousmicrofossils. These are diverse sponge spicules, diatoms, silicoflagellates, radiolarian fragments, andebridians. The taxonomic composition of diatoms andsilicoflagellates in the two samples is similar, althoughthere are some differences. The assemblage from thesample taken at depth of 195 m reflects the onset of thetransgressive stage, which is evident from prevalenceof sponge spicules, benthic (Arachnodiscus) andtychopelagic (Paralia, Anuloplicata, Hyalodiscus) dia

toms. In the higher sample (depth of 192.5 m), the fossil assemblage is different on account of the reducedcontent of spicules and more diverse taxonomic composition and higher abundance of diatoms, which arerepresented by Pyxilla gracilis, Pyxilla sp. 1, Coscinodiscus payeri, Golovenkinia polyactis (=Coscinodiscuspolyactis), Grunowiella gemmata, Pyxidicula moelleri,Stephanopyxis marginata, and Coscinodiscus descrescens. The silicoflagelalte assemblage includes stratigraphically important Naviculopsis robusta andN. punctilia. Both these samples from the interval of195.0192.5 m characterize the lower Eocene Pyxillagracilis Zone of the extratropical Paleogene scale(Strelnikova, 1992).

In the Well 8 section, siliceous plankton characterizes the wider interval (224.5203.5 m), although representative silicoflagellate and diatom assemblages areconfined to the interval of 219.4211.3 m (maximaldiversity is recorded at depth of 217.3 m), which alsocorresponds to the Pyxilla gracilis Zone. Their taxonomic compositions in these sediments are slightly

NP9

NP8

NP7

NP6

NP5

NP4

55

56

57

58

59

60

61

C24

C25

C26

m237.9

240

244.7

247

248.7249.9

252.4

254.3

257.7

259.7259.9

Deflandrea oebisfeldensis

Alterbidinium rugulum,Senegalinium? dylwinense

A. margarita, Defl. denticulataAlisocysta sp. 2 Heilm.Cl.

Dinocysts

Benthicforaminifers

Saccamina grzybowskiNothia robusta Zone

1

2

3

4

5

6

7

8

9

10

11

12

Sel

andi

anT

han

etia

nYp

resi

ans.

l.S

tage

Nan

no

zon

atio

n

Ma

Pol

arit

y

b

b

b

Fig. 3. The stratigraphic distribution of dinocysts in the lower part of the Lyulonvor Formation in the Well 10 section and correlation with the General stratigraphic scale.(1) siltstone; (2) sandstone; (3) sand; (4) siliceous clay; (5) opoka; (6) clay; (7a) molluscan remains; (7b) fish remains; (8a) pyrite;(8b) gravel, pebble; (9a) marcasite inclusions; (9b) silty inclusions, films; (10a) Azolla macroremains, (10b) siderite inclusions,concretions; (11) first appearance levels of taxa and bases of zones; (12) last appearance levels of taxa and tops of zones.

plan

kton

Fig. 2. The composite section of sedimentary complexes 31 recovered by Holes M0002A and M0004A on the Lomonosov Ridgeand main biotic events.(1) silty mud, silty clay, clayey silt, sandy beds and lenses, isolated pebbles; (2) biosiliceous (diatom) ooze with occasional pebblesand pyrite lenses; (3) clay and silty clay with pyrite beds and nodules; (4) Azolla Beds; (5) stratigraphic levels of taxa distribution:(5a) first appearance, (5b) last appearance.

640

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

AKHMETEV et al.

NP

15

NP

14

NP

13

NP

12

NP

11

NP

10

NP

9

43 44 45 46 47 48 49 50 51 52 53 54 55 56

C20

C21

C22

C23

C24

Wel

l 10

Rus

skay

a P

olya

na B

eds

m

Din

ocys

ts

Abu

nda

nt

Cor

rudi

nium

inco

mpo

situ

m,

Cos

tacy

sta

buci

na,

Cor

d. c

anth

arel

lus

Lyu

linvo

r Fo

rmat

ion

Ch.

col

umna

gro

up

,O

ch. r

oman

um

Dra

codi

nium

pol

itum

Ch.

col

eoth

rypt

a,W

il. o

rnat

um

Dr.

var

ielo

ngit

udum

Dra

codi

nium

sim

ile

W. m

ecke

lfel

dens

is,

W. l

unar

isW

etze

liel

la a

stra

lob

isca

A. a

ugus

tum

, W

il. p

echo

ricu

m

Ape

ctod

iniu

m a

ugus

tum

Wil

. pec

hori

cum

, A

. hyp

erac

anth

um,

A. p

anic

ulat

um,

A. p

arvu

m,

A. s

umm

issu

m

Din

ocys

ts

184.

2

188.

1 189 19

2.5

195

198

200.

5

203.

5

205

208

211.

121

2.5

215.

3

217.

8

219.

5

222

224.

122

4.7

224.

922

6.7

227.

722

8.7

228.

822

9.1

230.

523

1.7

233.

723

5.5

236

236.

423

6.7

237.

8523

6.9

237

NP

21C

13

NP

18

NP

17

NP

16

NP

15

C15

C16

C17

C18

C19

C20

33 34 35 36 37 38 39 40 41 42 43

Mag

net

ic c

hro

ns

N4

R3

m

m

R2

N3

N2

R1

N1

R3

106

107

109

110.

311

1.6

113

115

117

119

121

123.

4

125

127

129

131.

313

2.5

134.

713

6

138.

5

141

143.

714

5.2

147

149

150.

615

0.8

153

155.

5

158.

3

160.

6

163.

6

165.

4

168.

5

170.

9

173.

2

176

179

182

185

188.

5

191.

7

195.

2

198

200

201.

5

203.

5

205.

5

207.

520

921

1.3

214.

3

Ch.

cla

thra

ta a

ngul

osa

Tha

lass

ipho

ra r

etic

ulat

a

Pht

hano

peri

dini

um a

moe

num

Th.

fene

stra

ta,

G. s

emit

ecta

acm

e H

ydro

pter

is in

dutu

s

acm

e H

ydro

pter

is in

dutu

s

Wet

zeli

ella

irty

sche

nsis

Mem

bran

opho

ridi

umas

pina

tum

, W

. ech

inul

ata

Kis

sele

via

orna

ta,

Th.

elo

ngat

a

Rho

mbo

dini

um d

raco

,W

etze

liel

la ir

tysc

hens

is

R. d

enta

lini

form

is

S. e

x gr

. car

inat

ifor

mis

G

. ex

gr. s

uper

turk

esta

nica

Zon

e

Dia

tom

s

Ben

thic

fora

min

ifers

Top

of t

he

P. g

raci

lis Z

one

nn

Than

YpresianLutetianStage

Nannoplankton

zonation

Ma

Polarity

Lower subformationUpper subformation

P. gracilis

Zone

H. lentis Zone

Reophax subfusiformisNothia exscelsa Zone

Diatoms

Radiolarians

Benthic foraminifers

PETM

LutetianBartonianPriabonianRupelian

NP1920

Lu

Russkaya Polyana BedsLower Tavda SubformationUpper Tavda Subformation

Azolla Beds

Polarity

Ma

Nannoplankton

zonation

Stage

Wel

l 8D

inoc

ysts

etian

tet.

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

COMPARATIVE ANALYSIS OF MARINE PALEOGENE SECTIONS AND BIOTA 641

Fig. 4. The stratigraphic distribution of dinocysts and diatom, radiolarian, and benthic foraminifer zones in Well 10 (LyulinvorFormation and Russkaya Polyana Beds) and Well 8 (Russkaya Polyana Beds and Tavda Formation) sections. Correlation of wells 8and 10 sections and their lithostratigraphic units with units of the General stratigraphic scale. For the legend, see Fig. 3.

more diverse as compared with their counterparts inthe Well 10 section. They include Brightwelia hyperborea and Pseudotriceratum radiosoreticulatum, inaddition to the abovementioned stratigraphicallyimportant diatom species and silicoflagellates Naviculopsis constricta and N. foliacea. It should be noted thatthe appearance of Golovenkinia polyactis together withBrightwellia hyperborea represents an important stratigraphic event in the West Siberian basin. This level isreadily traceable through West Siberia, which previously provided grounds for defining the synonymoussubassemblage in the upper part of the Pyxilla gracilisZone (Rubina, 1973) in the regional scale of WestSiberia. As a whole, this interval of biogenic siliceoussediments reflects increased bioproductivity of surfacewaters determined by a transgressive episode and, probably, a warm climatic trend. The overlying sediments demonstrate the opposite trend: reduction of the speciesdiversity of the assemblage and gradual disappearanceof diatoms and silicoflagellates. Based on correlation ofthis zone with zonations by dinocysts, radiolarians, andcarbonate groups of organisms, its age is estimated ascorresponding to that of the NP11NP12 Zone(Kozlova et al., 1998; Iakovleva et al., 2000; Akhmetevet al., 2001; Oreshkina et al., 2004, 2008).

Radiolarians from the Well 10 section were identified by G.E. Kozlova. The sample from a depth of237 m (basal PETM) contains several specimens ofS. irinae and the sample from a depth of 219.5 yieldsAxoprunum sp. cf. A. inclarum, a type species from bedswith the zonal assemblage of lower Eocene planktonicforaminifers (Morosovella subbotinae Zone). Theinterval of 212.5 to 189.0 m (the top of the LyulinvorFormation s. str.) corresponds to the lower EoceneHeliosdiscus lentis radiolarian zone. The latter is established in the lower Eocene interval, where the Pyxillagracilis diatom zone is defined.

The Russkaya Polyana (intermediate) Beds of the Terminal Lyulinvor Cyclothem

The sections of wells 9, 011BP, 8, and 10 in theOmsk Trough include an interval (bed) that occupiesan intermediate position between the Lyulinvor s. str.and Tavda s. str. formations (Fig. 5). From the Lyulinvor Formation, these beds differ by the sharplyreduced SiO2 content of the rocks, the higher proportion of sandy fractions, the appearance of glauconiteand smallsized gravel at the base. The features incommon with the Tavda Formation are abundant fishremains and marcasite concretions indicating H2Scontamination of the basin. Unlike the Tavda Formation s. str., the Russkaya Polyana Beds lack siderite

concretions and endemic dinocysts characteristic ofthe lower Tavda Subformation.

The intervals of 188.2182.0 m in Well 10 and259.0256.2 m in Well 011BP represented by greengray or green clays contain the composite middleLutetian dinocyst assemblage with Costacysta bucina,Cordosphaeridium cantharellus, Wilsonidiun echinosuturatum, and abundant Corrudinium incompositum.These sediments are attributed to the upper transgressive member of the Russkaya Polyana Beds. Based onfinds of Wetzeliella articulata, the underlying memberin the Well 8 section is conditionally correlated withthe lower Lutetian Wetzeliella articulataSystematophora placacantha Zone (Akhmetev et al., 2004a).Thus, the sections of wells 9, 10, 011BP are characterized by a stratigraphically important hiatus corresponding to the terminal Ypresian and YpresianLutetian boundary interval (~50.045.4 Ma) (Luterbacher et al., 2004). It should be noted that theRusskaya Polyana Beds in the Well 8 section (interval of208.9185.2 m) still contain Charlesdowniea coleothrypta and Ch. coleothrypta subsp. rotundata, which areparticularly characteristic of the two lower members.

In our opinion, the Russkaya Polyana Beds accumulated in transforming hydrological settings inresponse to the first (after significant regression at theYpresianLutetian transition) northward transgression in the middle Lutetian. The estuarine circulationin the basin closed in the north and opening in thesouth to the Turan basin was discussed elsewhere(Akhmetev et al., 2004a, 2004b). The status of thistransitional lithostratigraphic unit has drawn theattention ofWest Siberian stratigraphers and paleontologists for a long time. Some of them (Martynov,Nikitin) attribute these strata to the Tavda Formation,while others (Gurari, Shatskii) consider them tobelong to the Nyurolka Formation, which occupiescentral and northern areas of Siberia and in the TransUrals region an intermediate position between theIrbit Formation saturated with biogenic silica and theTavda Formation s. str. together with its continentalanalogs in the marginal part of the plate. The attribution of the intermediate sequence to the Lutetian isconsistent with the unstable regime in the Arctic basinat that time. It is, in fact, not essential to which lithostratigraphic unit the intermediate sequence belongs. Itis of importance that the greenish gray lowsiliceousclays with marcasite inclusions accumulated duringthe initial phase of West Siberian basins isolation fromthe Arctic Ocean. The late Ypresianearly Lutetiansedimentation break proper is synchronous with theaccumulation of Azolla Beds on the LomonosovRidge, which were formed, in our opinion, owing tothis isolation.

642

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

AKHMETEV et al.

The Middle and Upper Eocene (Bartonian and Priabonian stages). The Tavda Formation

In the Omsk Trough, the Tavda Formation is from150 to 200 m thick in its axial part and 7080 m thickin the southern limb. The unit is subdivided into the

lower and upper Tavda subformations with the boundary between them placed at the base of coarsegrainedsandstone bed or at the base of the Azolla Beds, wherethe sandstone member is missing. The sandstonemember is traceable along geological and seismic pro

Dinocyst zonation

Diatom zonation

Pechora Depression,

Well 228

Northernregion, TransUrals, Well 19(UstManya)

Vasyugan

Well 4

Omsk Trough

Well 9Well

Well 10 Well 8

West Siberian Arctic basin

NP21 34 C13

basin011BP

NP19/20

NP18

NP17

NP16

NP15

NP14

NP13

NP12

NP11

NP10

NP9

NP8

NP7

NP6

NP5

NP4

NP3

NP2

NP1

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

C15

C16

C17

C18

C19

C20

C21

C22

C23

C24

C25

C26

C27

C28

C29

Charlesdowniea

clatharata

angulosa

Kisselevia

ornata

Rhombodiniumdraco

Beds with Paucilobimorpha,Micrhystridium

Beds with C. bucina,W. echinosuturatum,

C. cantharellus

W. articulata (acme) S. placacantha

?Charlesdowniea

coleothrypta

rotundata

Charlesdowniea

coleothrypta

Dracodinium

varielongitudum

D. simile

W. meckelfeldensisW. astralobiscaD. oebisfeldensis

A. augustum

Apectodiniumhyperacanthum

Alisocysta

margarita

Cerodinium

speciosum

Senoniasphaera

inornata

P. oligocaenica

var. tenue

P. gracilis

C. payeri

M. uralensisH. proteus

T. ventriculosa

T. heibergiana

Global glacioeustatic regression

Transgression

Termination of Azolla Beds formation under intermittent

surface water desalination

Beginning of Azolla Bedsformation. Horizon B

Alternation of surface water desalination episodes and periods of strengthened water exchange with the PeriTethys

Isolation from the Arctic basin.Development of the estuarine

circulation regime. Horizon T

Formation of intermediate layers under partiall isolation

from the Arctic basin and permanent water

exchange with marginal seas of the PeriTethys

Shortterm isolation from the Arctic basin.

Replacement of siliceousclayey

sediments. Highenergy hydrologic regime

in marginal parts of the basin

Reflection of the PETM event in theorganicwalled and siliceous biotas

Accumulation of terrigenoussiliceous sediments

Regional hiatus

Formation of thinbedded biosiliceous oozes.

Seasonal stratification and episodical surface water

desalination. Appearance of icerafted material

Formation of Azolla Beds.Development of

the estuarine circulation regime

Accumulation of clayeysiliceous sediments under intermittent surface water

desalination and anoxia in bottom waters

Reflection of the PETM event in the organicwalled biota

Accumulation of shallowwaterterrigenous sediments

sedimentation by the clayey one

Maximal expansion of the basin. Accumulation of siliceousclayey

?

? ?

Dan

ian

Sel

andi

anT

han

etia

nYp

resi

ans.

l.Yp

resi

anL

utet

ian

Bar

ton

ian

Pri

abon

ian

Sta

geN

anno

plan

kton

zo

nat

ion

Ma

Pol

arit

y

Kir

shor

For

mat

ion

Vora

vozh

For

mat

ion

Mar

syat

For

mat

ion

Ser

ov F

orm

atio

nIr

bit

For

mat

ion

Low

er L

yuli

nvo

r S

ubfo

rmat

ion

Upp

er L

yuli

nvo

r

Sub

form

atio

nT

avda

For

mat

ion

Rus

skay

a P

olya

na

Bed

s

Lyu

lin

vor

For

mat

ion

Low

er L

yuli

nvo

rS

ubfo

rmat

ion

Upp

er L

yuli

nvo

rS

ubfo

rmat

ion

Upp

er L

yuli

nvo

rS

ubfo

rmat

ion

Tav

da F

orm

atio

n

Tav

da F

orm

atio

n

Tav

da F

orm

atio

n

Tav

da F

orm

atio

n

Rus

skay

aP

olya

na

Bed

s

Rus

skay

aP

olya

na

Bed

s

Rus

skay

aP

olya

na

Bed

sL

yuli

nvo

r F

orm

atio

n

?

Fig. 5. Correlation of principal Paleogene geological events in the West Siberian and Arctic basins.

River,

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

COMPARATIVE ANALYSIS OF MARINE PALEOGENE SECTIONS AND BIOTA 643

files across the West Siberian Plate, particularly alongits southeastern margin as a long horizon B is replacedtoward the Siberian Platform by shallowwater sandswith lenses of lignites, which were deposited in theperiod of maximal basin regression (Zaltsman, 1968).The same is true of the Azolla Beds of West Siberia.

In the Well 8 section, the lower and upper boundaries of the Tavda Formation are located at depths of185.2 and 106.0 m, respectively. We place the lowerboundary of the lower Tavda Subformation conditionally above the Russkaya Polyana Beds based on thereplacement of greenish gray pelitic foliated clays withmarcasite inclusions by their massive varieties withlarge marcasite and siderite concretions. The lowerboundary of the upper Tavda Subformation is drawn ata depth of 158.5 m, although macroremains of thesubaquatic fern shoots appear at a depth of 150.8 mand the underlying member is characterized by massfind of its mega and microspores beginning from adepth of 155.5 m. The lower and upper Tavda subformations are 26.7 and 52.5 m thick, respectively. In thelatter, 29.5 m correspond to the Azolla beds and 23.0 mare represented by marine clays.

Lower Tavda Subformation. The lower subformation begins with the Rhombodinium draco Zone widespread in West Siberia and the Transuralian region(Vasileva, 1990; Kulkova, 1994; Akhmetev et al.,2001; Iakovleva and Kulkova, 2003). The zone is correlative with the basal Bartonian Stage. In the Well8 section, the base of the subformation is placed at theappearance level of the type species and endemic Wetzeliella irtyshensis in the basal part of the RusskayaPolyana Beds. The Rh. draco assemblage gives way tothe assemblage characterizing the Kisselovia ornataZone. The Bartonian age of the latter is confirmed byfinds of the index species and characteristic taxa of theKuma Horizon of the North Caucasus: Quercus ex gr.gracilisQ. graciliformis, Castanea crenataeformis,Castanopsis pseusdocingulum, Rhiopites porrectus, andothers). The composition of higher plant palynomorphs is correlative with that of the Castanopsis pseusdocingulumRhiopites pseusdocingulumQuercus graciliformis Zone (Quercus gracilis Zone, afterI.A. Kulkova) (Practical, 1990). The dinocystassemblages include abundant endemic West Siberianspecies: Wetzeliella irtyshensis, Thalassiphora elongata,Kisselevia ornata. The appearance of endemic speciesindicates the complete cessation of the water exchangebetween the Tethyan and Arctic basins. The waterexchange was still in progress only between the WestSiberian sea and southerly located Turan basin, whichrepresented one of the marginal seas of the NorthernPeriTethys. This is confirmed by finds of the abovementioned endemic dinocyst species in the TurgaiTrough. The reduced water exchange and strengthened isolation of the West Siberian sea, which was subsequently responsible for the formation of Azolla Beds,are also evident from the growing share of prasino

phytes in the palynological complexes toward the topof the lower Tavda Subformation.

The upper Tavda Subformation and Azolla beds.Recently, the data on the composition of sedimentsand biota in the Omsk Trough (Akhmetev et al., 2001,2004a, 2004b, Beniamovsky et al., 2002) have substantially increased owing to the thorough biostratigraphicand geochemical analyses of the Well 8 section(Figs. 4, 5, Plate II).

Dinocysts are a major microplankton group, whichserves as a basis for zonal subdivision of marine Paleogene sediments of the West Siberian Plate. In total,over 40 samples were taken from the Tavda Formation.The maximal sampling density (every 2 m on average)was applied to the Azolla Beds.

The dinocyst assemblage from the basal layers ofthe upper Tavda Subformation is dated to the Bartonian, since it contains abundant Araneosphaera araneosa, one of the characteristic taxa of the Bartonianstratotype in southern England (Eaton, 1969). Thisspecies occurs together with other Bartonian taxa(Rhombodinium draco, Membranophoridium aspinatum, Phthanoperidinium stockmansii).

As has been mentioned, Azolla microspores andconjugates appear in the Well 8 section beginning froma depth of 155.5 m, although their abundance suddenly increases from a depth of 153 m (up to 95% inthe palynomorph spectrum). The dynamics in the proportions of dinocysts, prasinophites, palynomorphs ofhigher plants, and Azolla microspores in the lower partof the upper Tavda Subformation (Akhmetev andZaporozhets, 2010) is noteworthy. The dinocyst diversity successively decreases from 43 taxa at a depth of155.5 to 24 at depths of 153 m, and to 10 species atdepths of 150.8 m. At a depth of 149.7 m, it againincreases to 19 species, reducing once more to 13 taxaat a depth of 145 m. In the samples from a depth of143 m, their assemblage consists of six taxa and at adepth of 141 m, a single species of the genus Phthanoperidinum was found. The share of acritarchs and prasinophytes in the 5mthick basal layer, where they arerepresented by 1012 taxa, is as high as 20% of thetotal content of organicwalled phytoplankton. Thebasal part of the Azolla Beds still contains Paucilobimorpha trirdiata, a species characteristic of the middleEocene constituting up to 50% of total prasinophyteand arcitarch abundance; higher in the section, thistaxon is absent. The overlying strata contain Tyttodiscus spp., Pterospermella spp., Crassosphaera spp.Cymatiosphaera spp. Tetraporina spp., Micrhystridiumspp., Veryachyum spp., and Pediastrum spp. As forAzolla microspores, they are abundant up to a depth of130 m; in the interval of 129125 m, their abundancerapidly decreases to zero in overlying sediments.Therefore, the upper boundary of the Azolla Beds isplaced at depth of 128 m.

Dinocysts appear again in notable abundance only2 m below the top of the Azolla Beds. At a depth of131 m, their assemblage consists of seven species

644

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

AKHMETEV et al.

Plate I

200 m 1200 m

200 m 200 m200 m

200 m100 m200 m200 m100 m

100 m 200 m 200 m 200 m 200 m 100 m 100 m

2

200 m 200 m200 m

200 m 200 m 200 m

3 4

5

6 7 89

1011

12 13 1415 16

17

18 1920 21

22

23

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

COMPARATIVE ANALYSIS OF MARINE PALEOGENE SECTIONS AND BIOTA 645

Plate I. Agglutinated benthic foraminifers from the Lyulinvor Formation.(13) Nothia robusta: Well 9: (1, 2) Sample 444 m, (3) Sample 445.4 m; (4, 5) Nothia excelsa: Well 9: (4) Sample 468.5 m,(5) Sample 444.2 m; (69) Reophax subfusiformis: (6, 7) Well 9 (Sample 468.5 m, (8) Well 10 (Sample 211.1 m), (9) Well 9 (Sample 454 m); (10, 11) R. dentaliniformis: Well 9: (10) Sample 446.6 m, (11) Sample 454 m; (12) Psammosphaera irregularis, Well 9(Sample 512 m); (13, 14) Saccammina grzybovski: Well 9: (13) Sample 464 m, (14) Sample 447.5 m; (15) Glomospira gordialis,Well 9, Sample 518 m; (16) G. serpens, Well 9, Sample 512 m; (17) Ammodiscus planus, Well 9, Sample 446.5 m; (18, 19) Verneuilinoides sp.: Well 9: (18) Sample 446.5 m, (19) Sample 454 m; (20) Gaudryinoides ex gr. superturkestanica, Well 9, Sample 454 m;(21) Spiroplectammina cf. carinatiformis, Well 11, Sample 459; (22, 23) Spiroplectammina spectablis: Well 9: (22) Sample 518 m,(23) Sample 481 m.

increasing up to 25 taxa at 129 m. Upward almost tothe top of the Tavda Formation, their diversity variesfrom 25 to 30 taxa and then suddenly increases to50 species immediately at its top (depth of 106.6 m).This level corresponds to the peak of the late Priabonian transgression.

The upper part of the upper Tavda Subformation,which accumulated in deeper settings, is characterizedby Charlesdowniea clathrata subsp. angulosa, a typePriabonian species, which appears at depth of 115 mand occurs in all samples up to the formation top,where it prevails over other taxa.

Based on the dinocyst distribution, the most part ofthe Azolla Beds and overlying marine clays of the upperTavda Subformation are dated back to the Priabonian.The middleupper Eocene boundary is located insidethe lower part of this subformation.

The interval of 127113 m yields Phthanoperidiniun amoenum. In West Siberia, this zonal species of thefirst Oligocene (base of the Rupelian) dinocyst zone ofthe southern Russia scale likely appeared in the Priabonian and subsequently migrated south and westward following the retreating sea.

Three morphotypes of conjugates are recognizablein the upper Tavda Subformation. Two of them differfrom species of the Pseudokomewuia genus describedfrom the Zhuravka (Turtas) Formation of South andCentral Siberia, where they dwelt in the freshwater orslightly brackishwater lacustrine basin (Kuzmina andVolkova, 2008). At the same time, they are morpohologically similar to species reported by Chinese specialists from marine and brackishwater upper Eocene andlower Oligocene sediments of the Bohai Bay of the EastChina Sea (He, 1984). In the Well 8 section, the conjugates were found in the interval of 155.5143.7 m, i.e.,in the lower part of the Azolla Beds. Their first finds areconfined to a depth of 155.5 m, where the dinocystassemblage consists of 43 species. Thus, it may be concluded that dwelling first in relatively open sea settings,the conjugates could subsequently adapt to desalinatedsurface waters.

Macroremains of the subaquatic fern Azolla veraare represented by shoots with attached sori (Plate II).Vegetative parts of ferns intermittently formed mats atthe water surface. In the sandyclayey matrix, they areaccompanied by mega and microspores and massuleswith glochidians or without them. All the microremains are organically isolated from the shoots.Microspores are dominated by Hydropteris indutus

(=Azolla) with banner, owing to which this speciesdiffers from another morphotype of Azolla spores lacking such banner (Plate II). Megaspores also belongto two morphotypes attributed to sections Rhizophoraand Prisca (Antiqua). Megaspores and massules formmass accumulations in layers with impressions of vegetative shoots, which are particularly abundant in theintervals of 150145 and 132130 m and less commonin the interval of 143145 m, where diversity ofdinocysts increases up to 68 species, while abundance of Azolla microspores and prasinophytes isreduced. This fact indicates that salinity in the basinduring accumulation of the middle part of the AzollaBeds increased somewhat intermittently, althoughAzolla microspores still dominated among palynomorphs. The distribution of Azolla microspores isexplained by eustatic sea level fluctuations and, probably, seasonal reproduction of these ferns, which wasasynchronous with that of dinoflagellates.

Kulkova (1994) was first to define two palynocomplexes in the Tavda Formation. The lower complex withCastanopsis pseudocingulumRhoipites pseudocingulumQuercus graciliformis (Q. gracilis, after I.A. Kulkova)characterizes the lower Tavda Subformation and basalpart of the upper Tavda Subformation. The uppercomplex Quercus gracilisQuercus graciliformisappears at depth of 145 m. It is characteristic of theremaining part of the upper Tavda Subformation. Inthe interval of 153149 m, the change of the complexis preceded by a sharp increase in abundance of smallpollen of xeromorphic oaks (Quercus gracilis, Q. graciliformis, Q. conferta) against the background of notably reduced coniferous pollen abundance. Theseevents correspond to the stage of maximal reduction inannual precipitation at that time. Beginning from adepth of 145 m, the share of thermophilic elements inthe Quercus gracilisQuercus graciliformis complexbecomes lower, although the dominant role ofFagaceae is retained against a background of subordinate development of Juglandaceae and Hamamelidaceae. The spectrum of gymnosperm pollen, theshare of which becomes notably higher in the middleand upper parts of the Azolla Beds includes first grainsof dark coniferous taxa (Picea, Tsuga). This indicatesa probable decrease in annual temperatures. At leastthree addition intervals with the notable prevalence ofFagaceae pollen are defined above the Azolla Beds: at127 and 125 m and in the uppermost part of the formation, where coniferous pollen constitutes up to 30% of

646

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

AKHMETEV et al.

Plate II

50 m

1a 1b2

3a

4a

3b

4b

50 m

7 8

65

Plate II. Megasori, macro and microspores of Azolla and cell envelopes of conjugates (Order Saccodendrophyceae) from theupper Tavda Subformation (Well 8). Magnifications: 600 for Figs 15, 400 for Fig 6, and 4 for Figs. 7, 8.(1, 2) conjugates, Order Saccodendrophyceae, cell envelopes: (1a, 1b) Depth 150 m, (2) Depth 145 m; (3, 4) Hydropteris spp.,microspores, Depth 149 m; (5) fragments of the massula with glochidias, Depth 150 m; (6) accumulation of Hydropteris indutusmicrospores, Depth 149 m; (7, 8) vegetative shoots of Azolla vera Krysht., Depth 131.5 m: (7) with sori, (8) sterile.

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

COMPARATIVE ANALYSIS OF MARINE PALEOGENE SECTIONS AND BIOTA 647

arborealfruticulose pollen. This is also confirmed byevidence for the sea level rise and warming in the terminal Priabonian derived from the Well Chistoozernaya 9section (Fig. 6) and eustatic curves (Haq et al.,1987; Popov et al., 2009).

The section of the upper Tavda Subformationencloses two thin coquina intercalations composed ofCultellus orientalis and stenohaline Nucula spp. at

depths of 134 m and supplemented by shells of Arcticaaff. alexeevi at depth 131.5 m. Both intercalations areconfined to the upper part of the Azolla Beds. Similarto the wells Chistozernaya9 and 011BP sections, themain finds of ostracods and small suppressed dwarfedbenthic foraminifers are confined to this interval.According to Ovechkin (1954) and S.V. Popov, whostudied our collections, the mollusks identified dwelt

Aph

CCCGan

277

326330

380392

408412

435437

444444.6

449.6451

466468

478.8480

492.5496502

518522.5525.5

Gymnosperms

Angiosperms

Dinocysts

10 30 50 70 90100 300 500 More than 2 1120 610 up to 570 90 503010

Cre

ta

Pal

eoce

ne

Eoc

ene

Ser

ies

Sta

ge

For

mat

ion

, sub

form

atio

n

Din

ocys

t as

sem

blag

es,

zon

es

Inte

rval

, m

BIT

inde

x, %

Abu

nda

nce

of

taxa

fo

un

d

Pal

ynom

orph

ab

unda

nce

K2m

Th

anet

ian

Ypre

sian

Lut

etia

nB

arto

nia

nP

riab

onia

nL

yuli

nvo

rT

avda

Lo

wer

Up

per

DV

A

HA

MD

V s

.s.

Cc

Wa

Sp

P

MA

d

Rd

Rp

Hi

PC

CA

Up

per

Lo

wer

Fro

mea

chy

tra

Fro

mea

laev

igat

aF

rom

ea fr

agil

isM

icrh

ystr

idiu

m s

p.

Nor

emia

sp

.P

tero

sper

mel

la s

p.

Tyt

thod

iscu

s sp

.C

rass

osph

aera

sp

.F

rom

ea s

p.

Pau

cilo

bim

orph

a sp

inig

era

Cym

atio

spha

era

sp.

Dia

croc

anti

dium

sp

.P

auci

lobi

mor

pha

gran

ulif

era

Pau

cilo

bim

orph

a tr

irad

iata

Pau

cilo

bim

orph

a tr

ipus

Pau

cilo

bim

orph

a sp

.T

etra

pori

na s

p.

Azo

lla s

p.

Sch

izoc

ysta

sp

.O

void

ites

sp

.H

ydro

pter

is in

dutu

sP

tero

sper

mel

la a

ustr

alis

Ped

iast

rum

sp

.

Fig. 6. The distribution of acritarchs, prasinophytes, and reproductive organs of Azolla in the Well 9 section (Akhmetev et al.,2004a).Dinocyst zones and beds: (CCC) Cerodinium diebeliiChatangiella spp., (AM) Alisocysta margarita, (Aph) Apectodinium homomorphum, (DV) Dracodinium varielongitudum, (AH) Alterbidinum sp. 1Hystrichosphaeridium tubiferum, (DV s.s.) Dracodiniumvarielongitudum s. str., (Cc) Charlesdowniea coleothrypta, (WaSP) Wetzeliella articulateSystematophora placacantha, (PM) Paucilobimorpha triradiataMicrhystridium, (AdRd) Areosphaeridium diktyoplokumRhombodinium draco, (Rp) Rhombodinium porosum,(CCA) Charlesdowniea clathrata angulosa, (HiP) Hydropteris indutusPediastrum.

ceou

s

kino

648

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

AKHMETEV et al.

on coastal shoals beyond the wave zone and characterize the boreal coldresistant biota. These species werefirst described from the middle part of the CheganFormation of the Aral region, where they inhabited abasin with disoxic bottom waters.

Ostracods occur in abundance in the interval of130135 m; they are so far unstudied. Previously, itwas established that the Azolla Beds in the Well Chistoozernaya9 section contain a diverse assemblage ofbrackishwater ostracods (Nikolaeva in Akhmetevet al., 2004a, 2004b). Similar to mollusks from theWell 8 section, their assemblage is coldresistant andcharacterizes the stratified sea basin with desalinatedsurface waters. The analogous assemblage was firstdescribed from the Chegan Formation of the Aralregion (Nikolaeva, 1985).

In most lithotectonic zones of the West SiberianPlate, the formation of the Azolla Beds was closelyrelated to transformation of the inner sea into a basinisolated from the Arctic Ocean. This unit traceableover a spacious area is an element of the Tavda Formation and its formation was determined by the globalsea level fall in the second half of the Bartonian. Thelatter resulted in desalination of surface waters, differentiation of the water column, and development ofanoxic conditions in the bottom waters of both theWest Siberian and several other marginal basins of theNorthern PeriTethys: Kuma (CrimeanCaucasusregion) and SaksaulChegan (Turan Plate).

The samples from the Lyulinvor and Tavda formations and also from the Russkaya Polyana Beds weresubjected to the Xray fluorescence analysis, whichmade it possible to discriminate geochemically sediments deposited under conditions of the normal salinity and desalinated setting during the Tavda time. Intotal, 23 samples taken from the interval of 214113 mwere analyzed by this method. The samples were takenfrom the Lyulinvor Formation (at depths of 214.2,201, and 188 m) and Tavda (at depths of 179, 163, and155.5 m) formations, which were forming under conditions of normal and elevated salinity, respectively.The Azolla Beds were sampled in an interval of 151128 m (12 samples with an average step of 1.8 m).Three samples were taken at depths of 119, 115, and113 m from the upper clay member of the upper TavdaSubformation that accumulated after restoration ofthe saline regime in the sea basin.

The geochemical background of sedimentsthrough the section with respect of the S, Sc, Cr, As,Ru, Y, Zr, Nb, Ta, Mo, Zn, and Pb composition as wellas that of rockforming oxides remains relatively stable. Moreover, most samples from the Azolla Bedsexhibit systemic, although insignificant deviation inconcentrations of some elements, which is probablyrelated to peculiar features of the hydrological regimeand more intense influx of organic matter into thebasin at that time. This is primarily true of higher concentrations of U, Th (some samples demonstrate doubled concentrations of these elements as compared

with their background values), Ce, Ga, Zn, Cu, Ni,and Co (in all the samples, their concentrations are1.31.5 times (on average) higher as compared withbackground values). At the same time, V, Ba, and Srconcentrations in the Azolla Beds are 25% lower onaverage than in beds formed under normal salinity.Among rockforming oxides, the lowest contents inthe Azolla Beds are characteristic of SiO2 particularlyrelative to that in the Lyulinvor Formation and MgO.In all the samples, the FeO, Fe2O3, TiO2, and P2O5concentrations appeared to be higher than in sediments deposited under conditions of normal salinewaters.

COMPARISON BETWEEN MAIN PALEOGENE ABIOTIC AND BIOTIC EVENTS IN THE WEST

SIBERIAN AND ARCTIC BASINS

PreAzolla Events in the Arctic and West Siberian Basins (Late PaleoceneEarly Eocene)

Paleocene. The base of the Paleogene section inWest Siberia, the Pechora basin, and Transuralianregion is diachronous, which reflects different levels oftransgressions during different Paleogene epochs. Themarginal parts of the West Siberian basin and PechoraDepression are characterized by development ofThanetian sediments, which rest with a hiatus uponCretaceous strata (Iakovleva et al., 2000). In thenorthern Transuralian region (Lozva Pier outcrop,Well UstManya19), many researchers (Strelnikova,1992; Kozlova and Strelnikova, 1984; Vasileva, 1990;Glezer and Grundan, 2005; Oreshkina et al., 2008)reported on the presence of Danian marine sedimentswith siliceous plankton and dinocysts. The lower Paleocene diatom assemblages are correlated with the Trinacria herbergiana Zone corresponding to the NP2NP3 nannoplankton zones.

In most sections examined of West Siberia and theTransuralian region, the dinocyst assemblages characterize higher Paleogene strata: partly Selandian (interval of the West European Cerodinium speciosum Zone)and the entire Thanetian (interval of the EuropeanAlisocysta margarita and Apectodinium hyperacanthumzones). By its stratigraphic position, the base of thesedimentary section on the Lomonosov Ridge is closeto that in the Well 228 Section in the Pechora Depression (Iakovleva et al., 2000) and wells 9 and 10 sectionsin the Omsk Trough (Akhmetev et al., 2004a, 2004b),where upper Paleocenelower Eocene sediments alsorest upon Cretaceous rocks (reflection of a largescalePaleocene transgressive cycle) (Fig. 4).

In the Lomonosov Ridge section, presumableupper Paleocene sediments overlie Campanian (?)rocks. Their late Paleocene age is deduced from presence of Deflandrea denticulata and Membranosphaeraspp. in the assemblage; these taxa are described fromsediments of the Danian basin (HeilmanClausen,1985).

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

COMPARATIVE ANALYSIS OF MARINE PALEOGENE SECTIONS AND BIOTA 649

As was noted, the third sedimentary complex onthe Lomonosov Ridge is represented by sandyclayeysediments that reflect the regressive stage of the latePaleocene and transgressive stage beginning from theEocene. The participants of the Arctic expedition(APEX) arrived at the conclusion that the HoleM0004A area was located near the shoreline in the terminal Paleocene where there were fluvial flows, as evidenced by the high share of higher plant spores andpollen (up to 90%) with prevalence of angiosperms inpalynospectra and dinocyst taxa tolerant to low salinity of waters, although preferring nutrientrich environments (Shuijs, 2006). The upper Thanetian sediments contain abundant organic matter of terrestrialorigin.

Unlike the Lomonosov Ridge section, Paleocenesections in the northern and central parts of West Siberia and the Pechora Depression (wells 228,UstManya19, Vasyugan4, wells in the lowerreaches of the Pur River, wells 9 and 10 in the marginalpart of the Omsk Trough) are more complete and theirdinocyst assemblages are more diverse. As on theLomonosov Ridge, the upper Thanetian sedimentsusually up to 10 m thick belong to the Apectodiniumhyperacanthum dinocyst zone.

The upper Paleocene part of the Well M0004A section on the Lomonosov Ridge contains the benthicforaminiferal assemblage in the interval of 404.8381.0 m (Cores 35X32X). Its most characteristic feature is the presence of three species belonging to thegenus Reticulophragmium (R. ministicoogmium,R. arcticum, and R. boreale). In addition to Reticulophragmium representatives, the assemblage includestubular, rodshaped, straight, and curved forms ofprimitive genera Nothia and Rhabdammina (Backman et al., 2006, p. 18). This assemblage is called asthe Reticulophragmium Assemblage. It is remarkablethat one of the characteristic Paleocene species of theWest Siberian Plate is Reticulophragmium (Cyclammina) coksuvorovae, which plays the role of the zonaltaxon in this region (Foraminifers, 1964).E.M. Bugrova identified this species in the samplefrom the interval of 390391 m of Well M00004A(Gusev et al., 2006). She considers the host sedimentsto be Selandian in age. The published and original dataindicate, however, the Thanetian age of the Reticulophragmium assemblage. This is confirmed by the factthat the Kyrshor Formation in Well 228 drilled on thewestern slope of the Polar Urals demonstrates thestandard succession of three Thanetian dinocyst zonesof the upper Paleocene in addition to benthic foraminifers: Alisocsta margarita, Apectodinium hyperacanthum, and Apectodinium angustum (Oreshkina et al.,1998; Iakovleva et al., 2000). It should be noted thatthe Reticulophragmium event is traceable globally inThanetian sediments (Kaminski and Gradstein, 2005,p. 102, fig. 46). When comparing the lower assemblagefrom the upper Paleocene sediments of the polar HoleM0004A section with the upper Paleocene assemblage

from the West Siberian Reophax subfusiformisNothiarobusta Beds (Zone), we should mention anothercharacteristic feature in common for them: in bothshallowwater areas, they are accompanied by abundant tubular (tubularrodshaped) representatives ofthe genus Nothia. The studies in the Russkaya Polyanaarea revealed that dark gray upper Paleocene (based ondinocysts) carbonatefree sandysilty rocks from thebasal part of the Lyulinvor Formation represented bythe 2mthick member of dark gray sandyclayey rockscontain species of the genus Nothia (N. robusta andN. excelsa) occurring through the entire section(Fig. 7, Plate I).

In deeper facies represented by the 20mthickmember of silicified clays of the lower Lyulinvor Subformation, late Paleocene dinocysts are accompaniedby a benthic foraminiferal assemblage with GlomospiragordialisSpiroplectammina spectabilis (Beniamovskyet al., 2002). Unlike shallower assemblages from Arctic wells and marginal parts of the Omsk Trough, itincludes abundant representatives of the genus Glomospira (G. gordialis, G. cf. charoides, G. serpens) andSpiroplectammina spectabilis (Plate I). It should benoted that the Glomospira acme event is globallyrecorded in the terminal Paleocene and transitionalP/E Glomospira acme interval (Kaminski and Gradstein, 2005, p. 102, fig. 46) and is related to globalanoxia at that time. Another zonal speciesSpiroplectammina spectabilis is a typical form in upperPaleocene assemblages of the deepwater Torsk Formation from the Barents Sea (Nagy et al., 2000), whichwas deposited at bathyal depths. Thus, based onbenthic foraminifers, the Thanetian stage in development of the epicontinentaloceanic (West SiberianArctic) basin is readily distinguishable in evolution ofthe latter. Moreover, the shallow shelf and shallowneritic settings were occupied by the ReticulophragmiumNothia coenoses, while its deeper part was populated by the cosmopolitan ArcticBoreal disoxicGlomospira paleocoenosis, which is traced through theOmsk Trough of the West Siberian Plate, Torsk Basinof the Barents Sea (northeastern periphery of theNorth Atlantic), eastern North Caspian Depression(Kamsaktykol Formation), and southern TurgaiTrough (relatively deepwater Shelkarteniz Formation)(Beniamovsky, 1994; Beniamovsky et al., 1993).

The most distinct in the Lomonosov Ridge sectionis the PETM level. In addition to isotopic data indicating CIE, the dinocyst assemblage is characterized bypresence of Apectodinium angustum, A. hyperacanthum, A. parvum, and A. homomorphum (the interval of~388.61378.4 m, according to CIE). With respect ofthe taxonomic composition, the assemblage is considered to be impoverished (Brinkhuis et al., 2006).

The characteristic feature of the terminal Paleocene in the Arctic region, which corresponds to theonset of PETM phenomenon, is the decrease BITindex (proportions of terrestrial and marine organicmatter). This indicates the reduced influx of organic

650

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

AKHMETEV et al.

matter by rivers with the simultaneous notableincrease of dinocyst abundance in palynospectra(up to 60%) and aquatic plants among spores and pollen, which was determined by the global sea level riseat the PaleoceneEocene transition. In the Eocene,the percentage of higher plant palynomorphs inpalynospectra again increases in the North Sea andArctic regions (Schouten et al., 2006).

In the Pechora Depression and West Siberia, thePETM level and presence of Apectodinium angustumare established in wells 228 (Kyrshor Formation,Pechora Depression), 19 (Irbit Formation,UsManya), 4 (Irbit Formation, Vasyugan Riverbasin), 10 (Lyulinvor Formation, southern Omskoblast), and the Pershinskii Quarry (Irbit Formation,southern Transuralian region). The assemblageincludes Apectodinium angustum, A. hyperacanthum,A. parvum, A. summissum, A. quinquelatum, A. homomorphum, and Wilsonidium pechoricum.

Despite the similar taxonomic composition, quantitative proportions of dinocyst groups vary in differentsections. For example, in the Pechora Depression,approximately 50% of the assemblage are representedby Apectodinium spp. and abundant representatives ofthe Deflandrea group (Deflandrea spp., Cerodiniumspp., and others), which makes this assemblage closeto the assemblage from the Lomonosov Ridge. At thesame time, the Pechora dinocyst assemblage of thePETM level includes approximately 60 taxa, which is

substantially more than the assemblage from theLomonosov Ridge. In the northern Transuralianregion (Well UstManya19), representatives ofApectodinium spp. at the PETM level constitute only7% of the assemblage. In the Vasyugan River basin(Well 4), the assemblage from the PETM level alsoincludes many peridinoid forms (Apectodinium,Deflandrea, and others) accompanied by acritarchsand abundant pollen. In the southern Omsk Trough,Apectodinium representatives (10 species) constitutefrom 0.5 to 10.0% (up to 25% in a single sample) of thedinocyst assemblage. The important feature, whichmakes this assemblage different from the Arctic one, isthe dominant role of exclusively gonyoaulacoiddinocysts, i.e., cysts of authotrophic dinoflagellates.

Based on diatom assemblages, the PaleocenetoEocene transition (interval of the Trinacria ventriculosaHemiaulus proteus diatom zones) was analyzedin detail in sections of Well 19 (UstManya) andKamyshlov, Korkino, Chumlak, and Sokolovskiiquarries (Radionova et al,, 2001; Oreshkina et al.,2004, 2008; Aleksandrova et al., 2020). In the Kamyshlov (Sverdlovsk region), Korkino, and Chumlyak(Chelyabinsk region) quarries, where the Paleocene Eocene boundary interval is represented by thesequence of diatomaceous clays and clayey diatomitesup to 45 m thick, diatom assemblages allow the entirestratigraphic succession of corresponding zonal unitsto be traced. Like the sections in the Middle Volga

Levels of changes in foraminiferal assemblages

West Siberian sea

Assemblages with rare foraminifers of Central Asia origin

Isolation from the Arctic basin

Disappearance of BorealArcticforaminiferal taxa

Reophax dentiliniformisSpiroplectammina ex. gr. carinatiformisGaudryinopsis ex gr.superturkestanica Zone

Assemblage of the ReophaxsubfusiformisNothia excelsa Zone

Assemblages of the Glomospira gordialisand Saccammina grzybowskiNothia robusta zones1

2

3

4

5 Polar Arctic basin in the Hole M0004A areaon the Lomonosov Ridge

Formation of the Azolla Beds.Development of the estuarinecirculation regime

Disappearanceof foraminifers

Verneuilinoides Assemblage

ReophaxPsammosphaeraAssemblage

Reticulophragmium Assemblage1

2

3

4

Lat

eP

aleo

cen

eE

arly

Eoc

ene

Mid

dle

Eoc

ene

Fig. 7. Levels of changes in Paleocene and Eocene benthic foraminiferal assemblages in the West Siberian and Arctic basins.

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

COMPARATIVE ANALYSIS OF MARINE PALEOGENE SECTIONS AND BIOTA 651

region, the upper part of the Trinacria ventriculosaZone demonstrates the sharp increase in taxonomicdiversity on account the first appearance of severalgenera (Craspedodiscus, Fenestrella, Moisseevia,Soleum, Pseudotriceratum) and the radiation of thegenera Grunowiella, Hemiaulus, Stephanopyxis, andTrinacria. The silicoflagellate assemblage is characterized by the stable presence of a new taxon with atypicalmorphological features. The Hemiaulus proteus Zonecorresponding to culmination of the climatic optimum (interval of the negative carbon isotope excursion) is marked by the appearance of several shortlived taxa, the index species included. The next, Coscinodiscus uralensis Zone reflects termination of the climatic optimum evident from the reduced taxonomicdiversity, acme of the index species, and sporadicappearance of species typical of the overlying Coscinodiscus payeri Zone.

Early Eocene. The lower Eocene marine sedimentsabove the PETM level are represented by the Irbit Formation (part) in the western areas of the West SiberianPlate and Lyulinvor Formation in its eastern part. Theearly Eocene was universally marked by accumulation ofbiogenic siliceous sediments (diatomites), which aregradually replaced laterally (from the west eastward) andupward the section by clayeysiliceous rocks. The thickness of the Ypresian sediments through the plate rangesfrom 80 to 120 m. They include the complete successionof lower Eocene zones: Hemiaulus proteus, Moisseeviauralensis, Coscinodiscus payeri, Pyxilla gracilis, andPyxilla oligocaenica var. tenue.

The similar 70mthick lower Eocene section ofthe Lomonosov Ridge is composed of uniform blackclays, slightly bioturbated and barren of siliceousmicrofossils. These clays accumulated in the period,when the West Siberian sea was of maximum size.

Based on the defined carbon and oxygen isotopeshifts, the depth of 368 m in the Hole M0004A sectionis conditionally correlated with the ETM2 event (or,probably, X event). As for the dinocyst composition,this depth is marked by presence of Cerodinium wardenense and abundant Hysrichosphaeridium tubiferum.Based on these finds and analogy with the North Sea(Bujak and Mudge, 1994), it was assumed that the hostsediments are early Eocene in age (Zone E1 of theNorth Sea, NP10basal NP11; the last appearance ofC. wardenense marks the boundary between zonesNP10 and NP11, slightly below the C24rC24n.3ntransition). The interval with C. wardenense is overlainby sediments with the Deflandrea oebisfeldensis acme.

It remains unclear what part of the lower Eocenesection is preserved in the Arctic region/ The sediments overlying the Deflandrea oebisfeldensis acmeyield Wetzeliella articulata and W. hampdenenesis(Brinkhuis et al., 2006), which correlate them to theinterval of the NP12NP13 zones (most likely,NP13).

In addition to these taxa, the lower Eocene clayeysiliceous sediments located above the PETM level on

the Lomonosov Ridge contain many dinocists speciesin common with the assemblage from the LyulinvorFormation recovered by Well 9 in the axial part of theOmsk Trough (Backman et al., 2006; Akhmetev et al.,2004a, 2004b). Despite the fact that many of thesespecies are cosmopolitan and characterized by thewide stratigraphic range, their presence in differentparts of the single Early Eocene basin indicates theirpaleogeographic connections. The group of these taxaincludes Hystrichosphaeridium tubiferum, Diphyes colligerum, Glaphyrocysta ordinata, and Deflandrea phosphoritica. They are accompanied by at least two dozencosmopolitan species in common, which are characteristic of medium and high latitudes (Spiniferitesramosus, Thalassiphora delicata, Palaeocystodiniumgolzowense, Homotryblium tenuispinosum, and others),and at least ten forms identified in both sections in theopen nomenclature.

In the Pechora Depression, the section corresponding to the PaleoceneEocene transition isalmost complete: the PETM level is overlain by basalEocene sediments with the taxonomically impoverished assemblage of dinocysts from the EuropeanDeflandrea oebisfeldensis Zone; the hiatus correspondsto the basal Ypresian s. str. (i.e., the first appearancelevel of Wetzeliella astralobisca).

The dinocyst assemblages from the lower half of thelower Eocene section in West Siberia and PechoraDepression demonstrate the complete succession ofkey taxa (Fig. 4), some of which are missing from theLomonosov Ridge. The Ypresian section is most complete in the southeastern part of West Siberia: wells 9,10, and 011BP, Omsk Trough (Kulkova and Zaporozhets in Akhmetev 2004a, 2004b, Iakovleva, 2008;Iakovleva and HeilmanClausen, in press). Accordingto dinocyst data, Well 10 recovered the most completesection of the lower half of the Ypresian, while wells011BP and 9 drilled in the Omsk Trough also penetrated the upper Ypresian sediments, the uppermostlayers included (first appearance levels of Wetzeliellaeocaenica and Duosphaeridium nudum). The sectionsdrilled in the southern marginal part of the OmskTrough (wells 8 and 10) exhibit a single developmentstage of siliceous plankton corresponding in both wellsto the second half of the Pyxilla gracilis and Helicodiscus lentis Zone.

It is assumed that the early Eocene basin was characterized by intermittently decreasing surface watersalinity, which is supported by the dominant role ofperidinoid dinocysts (Senegalinium, Cerodinium), highabundance of spores, and general taxonomic diversityof dinocysts (Sluijs et al., 2008). The late Paleoceneearly Eocene dinocyst assemblages from the PechoraDepression and West Siberia are also characterized bythe presence of abundant Deflandrea, Cerodinium, andAlterbidinium representatives. The dominant role ofperidinoid dinocysts may also be explained, in additionto the lowered salinity, by their specific food source taking into consideration their heterotrophic nature.

652

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

AKHMETEV et al.

The foraminiferal assemblage recorded above thePETM level in the Lomonosov Ridge section (HoleM0004A) in the interval of 378380 m (Core 29XCC)is characterized by the dominant role of primitiveagglutinated species from the genus Reophax andspherical forms from the genus Psammosphaera(Backman et al., 2006, p. 17). The similar successionis also noted in well sections from the southeasternpart of the West Siberian Plate. In these areas, sectionsof wells 9, 10, and 011BP in the Omsk Trough yieldedthe foraminiferal assemblage of the Reophax subfusiformisNothia excelsa Zone. It is recorded at the baseof the upper Lyulinvor Subformation represented bycompact carbonatefree clays, locally slightly opokalike or sandy. The characteristic feature of this assemblage, which makes it close to the polar assemblageunder consideration, is permanent presence of abundant agglutinated Reophax subfusiformis, Psammosphaera irregularis, and Saccammina grzybowski tests(Fig. 7, Plate I). The mass abundance of the genusReophax in the Lyulinvor Formation was established inthe 1960s1970s (Freiman, 1970). The figures illustrating the variability of Reophax subfusiformis presented by the last author were subsequently used for thecharacteristic of this cosmopolitan species (Kaminskiand Gradstein, 2005). Statistical calculations ofReophax abundance in the Lyulinvor Formation forWell 011BP are noteworthy. In this unit, Reophax isdominant, with abundances up to 600 specimens. TheReophax species typical of the ArcticBoreal segmentare absent from the CarpathianAlpine realm of theTethyan segment (Kaminski and Gradstein, 2005). Inaddition to spherical forms, this assemblage containsabundant tubular rodshaped Nothia species:N. robusta and N. excelsa (Plate I). The Nothiaexcelsa Acme event is considered as characteristic ofthe early Eocene (at the level of middleupper Ypresian NP11NP13 nannoplankton zones), which happened after the PETM Glomospira Acme (Kaminskiand Gradstein, 2005).

It should be noted that in polar Hole M0004A, thesediments with the assemblage under considerationare preceded by the 5m interval of the Eocene section(Cores 31X and 32X) lacking foraminifers. In WestSiberia, foraminifers are also missing from the upperpart of the lower Lyulinvor Subformation composed ofhighsiliceous facies (opokas, diatomites, opokalikeand diatomaceous clays) in the lower part and varioussandysilty sediments with the opokasiliceouscement in the upper part. It is remarkable that theupper part of the lower Lyulinvor Subformation in theWell 10 section (interval of 237229 m) represented byfrequently alternating sands, sandstones cemented byopoka material, and siliceousopoka clays containsabundant spicules of siliceous sponges (beds with siliceous sponge spicules). These sediments reflectdynamically unstable sedimentation in very shallowpart of the basin with the formation of a peculiar latitudinal belt of siliceous sponges.

The uppermost Verneuilinoides foraminiferalassemblage in the polar Hole M0004A section is established in the interval of 372374 m (Section 28X). Itdiffers distinctly from the preceding one by therenewed taxonomic composition on account of species from the planispiral genus Ammodiscus, tubularcoiled Haplophragmoides, and trochoid Verneuilinoides: A. planus, H. excavatus, H. perecilis, V. subtilis,V. macintyrei. The last two species are most abundant.They are recorded from the PaleoceneEocene AklakFormation in the Beaufort Sea (McNeil, 1997). Inaddition, the Verneuilinoides species represent a frequent component of the lower Eocene foraminiferalassemblage from the western (Norwegian) part of theBarents Sea (Nagy et al., 2000). In West Siberia, similar taxonomic transformations are observable in theassemblage of the Reophax dentaliniformisSpiroplectammina ex. gr. carinatiformisGaudryinopsisex gr. superturkestanica Zone, which is established inthe upper YpresianLutetian part of the LyulinvorFormation represented by a 2535mthick memberof variably sandy clays and clayey silts. Like the polarassemblage, it is marked by the appearance of Verneuilinoides representatives and Ammodiscus planus(Plate I).

It should be emphasized that final stages in development of Paleogene foraminifers of the connectedArctic basin and West Siberian epicontinental sea aresimilar as well. For example, in the Lomonosov Ridgesection the sediments above the Azolla Beds are barrenof foraminifers and the Lyulinvor stage in developmentof foraminifers in West Siberia is characterized by thedisappearance by BorealArctic agglutinated taxa,which may be explained only by cessation of troughconnections between the Arctic and Tethys oceans viathe West Siberian seaway. In the newly formed Tavdabasin, foraminifers became very rare and similar toTethyan taxa of Central Asian origin (Fig. 7).

The similarity of Early Paleogene benthic foraminiferal assemblages from the West Siberian and Arcticregions indicates their belonging to a single biochoreand confinement to shallow and intermediate depthsof the neritic zone. The decisive role of the meridionalcommunication system in the dispersion of benthicforaminifers during the Early Paleogene north of central regions of extratropical Europe, where they weretaxonomically substantially more diverse than in theArctic region, is undoubted.

As was mentioned, the early Eocene in West Siberiais a period characterized by intense accumulation ofbiogenic siliceous sediments of the upper LyulinvorSubformation and Irbit Formation. The most complete succession of lower Eocene diatom zones (Moisseevia uralensisCoscinodiscus payeriPyxilla gracilis)is established in sections of wells 228 and 19(UstManya) in the Pechora Depression and NorthUrals, respectively (Kozlova and Strelnikova, 1984;Strelnikova, 1992; Oreshkina et al., 2008). The Pyxilla gracilis Zone that reflects the maximum of the

STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 18 No. 6 2010

COMPARATIVE ANALYSIS OF MARINE PALEOGENE SECTIONS AND BIOTA 653

Ypresian transgression is characterized by the widestgeographic distribution. The stratigraphic positionand range of the overlying Russkaya Polyana Beds separating the Lyulinvor (s. str.) and Tavda formationswith the diatom assemblage of the Pyxilla oligocaenicavar. tenue Zone. This zone is characterized by the limited distribution in marginal parts of the West SiberianPlate (Rubina, 1973). Glezer and Stepanova (1994)established the type zonal assemblage in Well 157drilled on the Kara Sea shelf (Leningradskaya field).The data on the complex zonal subdivision of the wells9 and 011BP sections in the Omsk Trough (Akhmetevet al., 2004a, 2004b) as well as earlier materials byN.I. Strelnikova from wells SP1 and 148 drilled inthe southeastern Urals region (Kozlova et al., 1998)indicate that the Pyxilla oligocaenica var. tenue Zone iscorrelated with the Buriella longa and Lichnocaniumseparatum radiolarian zones, Charlesdowniea coleothrypa s. l. organicwalled plankton zone, NP13NP14 nannoplankton zones and dated back to the terminal Ypresianinitial Lutetian.

Azolla and Subsequent Events in the Arctic and West Siberian Sea Basins (Middle and Late Eocene)

The participants of the ACEX expedition found,for the first time, abundant massules with glochidiansand microspores as well as megaspores of the aquaticfern Azolla in the Hole M0004A section on theLomonosov Ridge (Brinkhuis et al., 2006). Unfortunately, different researchers report on different depthsintervals for the Azolla remains. Some of them(Brinkhuis et al., 2006) mention four Azola peaks atdepths of 303.0, 302.0, 301.0, and 295.5 m. Others(Backman et al., 2006) indicate two peaks: at depth of313.35 and in the interval of 302.7301.5 m. The lastauthors place the Azolla Beds at the base of the secondsedimentary complex instead of attributing them tothe third one. These beds form the member of thinlaminated mudstones (laminae up to 1 mm thick)intercalated with dark and lightcolored varieties.Moreover, Azolla remains occur only in the dark varieties, while light mudstones contain diverse siliceousmicroplankton. It is assumed that such an alternationis of seasonal origin determined by the replacement ofthe Azolla early spring bloom by its summer reproductive cycle. It is conceivable that Azolla remains belongto a new species or, probably, to the early EoceneCanadian species Azolla areolata (section Crematospora), which was described from the EurekaSound Formation (Banks Island of the Canadian Arctic Archipelago) (Sweet and Hills, 1976). The participants of the ACEX expedition believe that the Azollaremains are of local origin and they inhabited a basinthat was located near the North Pole, characterized bya salinity of a few parts pro mille, and that the Azollacould have been dispersed by currents from the polarregion to peripheral parts of the Arctic basin and evento North Atlantic basins. Based on finds of abundant

Azolla in basal layers of the middle Eocene sectiondrilled in the North Sea (Eldrett et al., 2004) and byODP Hole 913 in the northern part of the GreenlandSea (Thiede, 1991; Thiede et al., 1996), the AzollaBeds in the M0004A section are attributed to the initial Lutetian (Sluijs, 2006; Brinkhuis et al., 2006). Theduration of the existence of the desalinated basin nearthe pole is estimated to be 800 kyr. At the same time, itis conceivable that desalination of surface water was inprogress in the terminal early Eocene, not in the initialmiddle Eocene, which is supported by two facts. First,the Azolla Beds in the Eureka Sound Formation weredeposited prior to the middle Eocene and, second, thelower boundary of the middle Eocene drawn in theODP 213 section based on presence of Eatonicystaursulae seems doubtful. It is known that the type species of this taxon was described from the lower Eocenesediments of Canada.

The replacement of dominant siliceous sedimentation by the clayeysiliceous one at the base of theRusskaya Polyana Beds in the southern West SiberianPlate was accompanied by a break in sedimentation.The next sedimentation cycle started at the end of theearly Eocene and terminated in the Lutetian. Judgingfrom the change in zonal diatom assemblages (Pyxillagracilis/P. oligocaenica var. tenue) in west Siberian andArctic section, the onset of the cycle corresponded tothe period of Azolla Bed accumulation on theLomonosov Ridge. The difference consisted in thefact that in the Arctic basin largely siliceous andclayeysiliceous (with abiogenic silica) sedimentswere replaced after the Azolla Beds formation by biogenic Sibearing mud, while in West Siberia lowsiliceous terrigenousclayey sediments started accumulating at that time. The changes in sedimentation werealso accompanied by quantitative changes in the composition of siliceous and organicwalled microplankton in response to weakened sea communica