Paleomagnetism of boundary oligocene-miocene deposits in the Kompasskii Bor tract on the Tym River (Western Siberia)

70

ISSN 1069-3513, Izvestiya, Physics of the Solid Earth, 2009, Vol. 45, No. 1, pp. 70–79. © Pleiades Publishing, Ltd., 2009.Original Russian Text © Z.N. Gnibidenko, N.N. Semakov, 2009, published in Fizika Zemli, 2009, No. 1, pp. 74–84.

INTRODUCTION

As was noted previously, determination of the mag-netic polarity scale in greater detail is an importantproblem of the Cenozoic paleomagnetism of the west-ern Siberian plate [Gnibidenko, 2006a, 2006b, 2007].There are informational gaps in the paleomagneticscale of the boundary region between the Oligoceneand Miocene in the Late Oligocene–Early Miocenetime interval, which is obvious from comparison of thisscale with the Berggren time scale [Berggren et al.,1995]. The absence of data is either caused by an inter-ruption in sedimentation or by the fact that some sec-tions of these deposits are inaccessible for study.Recently, we had the opportunity to comprehensivelystudy one such difficult to access section of Oligocene–Miocene deposits, which is located in the KompasskiiBor tract on the Tym River in western Siberia. The goalwas to study the character of distribution of magneticpolarity zones at the Oligocene–Mioccene boundary inthe reference sections (Belyi Yar and Dunaevskii Yar)of the Kompasskii Bor tract. All these investigationswere complex and were based on paleomagnetic, geo-logical–stratigraphic, mineralogical, and paleontologi-cal (faunal, paleocarpological, and palynological) data.The Kompasskii Bor deposits were paleontologicallystudied by G.A. Pospelova [Gorbunov and Pospelova,1966]. The authors of the cited work carried out recon-noitering investigations of a lens of clays in the BelyiYar outcrop with rare sampling. They showed that thepaleolatitude determined from paleomagnetic data

agrees well with the data obtained as a result of a studyof fossil plant remains by paleobotanists.

GEOLOGICAL AND STRATIGRAPHIC CHARACTERIZATION

The Kompasskii Bor tract is located in the middleflow of the Tym River (right-hand tributary of the ObRiver) on its right-hand bank at a distance of 470 kmfrom its mouth (Fig. 1). In the tract area, the Tym Riverforms two extended ravines: Belyi Yar and DunaevskiiYar, with well-outcropped Oligocene–Miocene depos-its abundant in excellently preserved organic remains[Gorbunov, 1962]. These outcrops, owing to the diver-sity of rocks and fossil remains, are important referencesections for geological and paleomagnetic studies andhelp in understanding the history of the formation ofcontinental Paleogene–Neogene deposits in the vastterritory of the Tym River basin and adjacent regions ofwestern Siberia.

In the Kompasskii Bor tract, on the Tym River, theBelyi Yar outcrop is located upstream of the DunaevskiiYar outcrop (see the inset in Fig. 1). In this region, thegeological section is composed of Paleogene–Neogeneand Quaternary deposits. Rocks outcropping in the notnumerous bluffs are represented by continental facies.

The geological structure of the region is describedbelow after M.G. Gorbunov [1962]. A layer of clayeyconglomerate occurs in the lowermost part of theKompasskii Bor tract. Outcrops of this conglomerate

Paleomagnetism of Boundary Oligocene–Miocene Depositsin the Kompasskii Bor Tract on the Tym River

(Western Siberia)

Z. N. Gnibidenko

a

and N. N. Semakov

b

a

Institute of Oil and Gas Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia

b

Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia

Received July 1, 2008

Abstract

—We present the results of a complex (paleomagnetic, geological–stratigraphic, and paleontological)study of reference sections of Oligocene–Miocene deposits in the Kompasskii Bor tract on the Tym River in theeastern part of the western Siberian plate. Three sections of the Kompasskii Bor tract are studied: Belyi Yar(200-m mark), Belyi Yar (300-m mark), and Dunaevskii Yar (200-m mark). A composite magnetostratigraphicsection of deposits, including the Oligocene–Miocene boundary dated at 23.8 Ma, is compiled with allowancefor the complex data. Two magnetozones (of normal and reversed polarities) are identified in this section. Thescale of magnetic polarity of the Cenozoic of the western Siberian plate at the Oligocene–Miocene boundary isdetermined in greater detail. The time interval of the upper part of the Zhuravka series (Chattian) and theLyamin layers of the Abrosimov series (Aquitanian) is 24.6–22.9 Ma.

PACS numbers: 91.25.Ph

DOI:

10.1134/S1069351309010108

IZVESTIYA, PHYSICS OF THE SOLID EARTH

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2009

PALEOMAGNETISM OF BOUNDARY OLIGOCENE–MIOCENE DEPOSITS 71

are fixed at the bases of both ravines: Belyi Yar andDunaevskii Yar. For convenience of orientation withinthe outcrops (ravines), they were marked at 100-mintervals. For the Belyi Yar outcrop, the marking beganat its downstream end; for the Dunaevskii Yar outcrop,at its upstream end.

Clayey conglomerates in the Tym River basin areknown only in the Kompasskii Bor tract. The presenceof lumps of brown coal and fragments of lignified woodin these conglomerates indicates that a coal-bearing(brown-coal) sequence, which existed in this areabefore the beginning of the Miocene, was destroyed inthe process of washout. Apparently, the interruption insedimentation during the Oligocene was caused by aconsiderable tectonic uplift of the region.

A sequence of sands with vegetable detritus overliesdeposits in the Kompasskii Bor tract. This sequence isdivided into two layers; lower and upper. The lowerlayer outcrops in both of the ravines; the upper layer,only in the Dunaevskii Yar ravine. According to Gor-bunov’s observations, sands of the upper layer overliethe washed surface of sands of the lower layer. Thesesands are clayey medium- and fine-grained horizontallylayered formations of dark gray, yellowish, and brown-ish color. The abundance of vegetable remains—trunksof lignified wood, roots, and small plant detritus (fruitsand seeds)—is a characteristic indicator of sands of thelower layer. The upper layer of the sandy sequence, rep-resented by clayey fine- and medium-grained light graysands, has been detected, as mentioned above, only inthe Dunaevskii Yar outcrop. In contrast to the upperQuaternary sands, these sands are more consolidated

Taz R.

Vakh R.

Tym

R.

Enisei R

Ket’ R.

Irtysh

R.

Tomsk

Kolpashevo

Surgut

K A R A S E A

Kompasskii

Bor tract

Gul

f of O

b

Turukhansk

Nadym R.

Krasnoyarsk

Angara R.

Lake Babushkino

0 300 km

66° 72° 78° 84°

N

Tym

Riv

er

K

64°

60°

56°

1

2

Fig. 1.

General scheme of the eastern part of the western Siberian plate and the position of the studied section: K is the KompasskiiBor tract. The positions of the Belyi Yar (

1

) and Dunaevskii Yar (

2

) are shown in the inset.

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GNIBIDENKO, SEMAKOV

due to an insignificant admixture of clayey material.The thickness of this layer varies from 5 to 11 m.

A lens of lacustrine clay with multiple remains ofplants—leaf prints and remains of seeds, fruits, pollen,and diatoms (more than 130 forms)—outcrops in thelower half of the Belyi Yar ravine in the sequence oflower-layer sands between marks 200 and 375 m.Imprints of insects and fish are also met here. Gorbunov[1962] refers to the complex of plants presented by leafprints and remains of seeds, fruits, cones, woods, anddiatoms, which is contained in the lens of lacustrineclays and the sequence of clays in both outcrops (BelyiYar and Dunaevskii Yar), as “Kompasskii Bor flora.” Herelates these clays and the sands underlying them to thebrown-coal formation of western Siberia and datesthem at the Early Miocene; he believes that theKompasskii Bor flora belongs to the Tara type of floras.The mentioned paleontological remains were studiednot only by Gorbunov, but also by many other scientists[Dorofeev, 1960, 1963; . Nikitin and Gorbunov, 1962;Nikitin, 1967, 1999]. The majority of scientists believethat the sediments are of Early Miocene age. However,P.I. Dorofeev [1960, 1963] relates the layers withKompasskii Bor flora to the lower (pre-brown-coal)part of the Novomikhailovka series and believes thattheir age is Upper Oligocene. Thus, it should be notedthat at present, researchers have no unified opinion onthe age of these deposits.

OBJECTS AND METHODS OF INVESTIGATIONS

The Belyi Yar (

ϕ

= 60°06.3

′

N,

λ

= 83°14.4

′

E) andDunaevskii Yar (

ϕ

= 60°06

′

N,

λ

= 83°12.6

′

E) referencenatural sections (outcrops) represented by lacustrineclays and fine-grained sands were the objects of paleo-magnetic studies. We took 137 oriented cubic samplesfrom 49 stratigraphic levels in two strippings (marks200 and 300 m) of the Belyi Yar outcrop and 120 ori-ented cubic samples from 40 stratigraphic levels in theDunaevskii Bor outcrop (mark 200 m). The orientedsamples were taken uniformly at intervals of 0.15–0.20 m.Sandy varieties of rocks were taken with the use of aBishaev sampler. Laboratory investigations and pro-cessing of the data were performed in accordance withstandard methods [

Paleomagnetology…

, 1982; Molos-

tovskii and Khramov, 1997]. The components of thenatural remanent magnetization (NRM)

J

n

were deter-mined by the methods of stepwise thermal demagneti-zation and demagnetization by an alternating magneticfield. In interpretating the results, the NRM componentcomposition was established on the basis of orthogonalprojections [Zijderveld, 1967]. The

J

n

componentswere determined by the Kirschvink [1980] method. TheEnkin [1994] computer programs were used for all con-structions and graphical presentations. Laboratoryexperiments were performed in a room screened fromthe external magnetic field. Domestic and foreigninstruments and setups (KT-5, KLY-2, ION-1, JR-4, JR-6, LDA-3A setup for alternating-field demagnetization,Aparin setup for thermal demagnetization) were usedin field and laboratory experiments. Normal magnetiza-tion curves of rock samples were measured with a 5-Pconstant-field electromagnet, whose maximum magne-tizing field was 1088 kA/m. Under field conditions, thesections under study were correlated with the use of theGPS system.

RESULTS OF INVESTIGATIONS

Magnetic Properties of Rocks

On the whole, the sedimentary rocks (clays and fine-grained sands) under investigation belong to the classof weakly magnetic rocks. The minimum, maximum,and average values of

J

n

and

χ

(magnetic susceptibility)for the Belyi Yar and Dunaevskii Yar sections are pre-sented in the table, and the along-section (from the topdownward) changes are shown in Figs. 3–5. As followsfrom these data, the magnetic susceptibility varieswithin the range (1.3–12.0)

×

10

–5

SI units, and theNRM varies from fractions of unity to 6.1 mA/m. TheKoenigsberger factor (

Q

) has the values from 0.02 to1.2. The minimum

J

n

values are observed in clays of theBelyi Yar section (200-m mark) and the minimum

χ

values, in fine-grained sands of the Dunaevskii Yarsection. The maximum values of

J

n

and

χ

are fixed indeposits of the Belyi Yar section (300-m mark).

Magnetic characteristics of Oligocene–Miocene deposits of the Kompasskii Bor tract

Outcrop (section)

N

(

n

)

χ

, 10

–5

SI units

J

n

, mA/m

Q

Belyi Yar (200-m mark) 27 (75)

Belyi Yar (300-m mark) 24 (62)

Dunaevskii Yar (200-m mark) 40 (120)

4.97–9.067.66

------------------------ 0.10–0.830.57

------------------------ 0.02–0.180.13

------------------------

0.10–11.605.34

--------------------------- 0.21–6.111.04

------------------------ 0.17–1.20.77

---------------------

1.33–4.932.65

------------------------ 0.23–0.910.58

------------------------ 0.17–0.870.47

------------------------


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Magnetic Minerals and the NRM Component Composition of Rocks

According to the results of granulometric and min-eralogical analyses (data presented by M.M Tel’tsovain [Gorbunov, 1962] and our data), the sandy fractionwith a size of more than 0.1 mm prevails in fine-grainedsands of the lower layer in the Belyi Yar and DunaevskiiYar sections. The main rock-forming minerals of fine-grained sands are represented by terrigenous grains ofquartz (35–63%) and, to a lesser extent, by feldspargrains (14–32%), muscovite grains (4–8%), and clayeyparticles (12–38%). The heavy-fraction minerals are

represented by terrigenous grains of ore minerals (50–72%), epidote, hornblende, and accessory minerals(zircon and granite). According to granulometric anal-ysis, the clayey fraction with a size smaller than0.01 mm prevails in lacustrine clays of the Belyi Yarsection. The content of this fraction varies from 45 to80%. According to mineralogical analysis, the mainrock-forming minerals of the light fraction are pre-sented by terrigenous grains of quartz and (to a muchlesser extent) grains of feldspars, fragments of micro-quartzites, and other rocks. The heavy fraction containsgrains of magnetite, hornblende, and pyroxenite. Frag-ments of quartz grains (46–67%), feldspar grains (20–

Fig. 2.

Characteristic plots of the

J

n

demagnetization by an alternating field and temperature and the Zijderveld curves for rocks ofthe Belyi Yar (I) and Dunaevskii Yar (II): (a) normalized curve of thermal and alternating-field demagnetizations of

J

n

; (b) Zijdervelddiagram: projections of

J

n

onto the horizontal (filled symbols) and vertical (open symbols) planes; and (c) stereograms of

J

n

direc-tions onto the lower (filled symbols) and upper (open symbols) hemispheres.

1.0

0

0.5

10 20 30 40 50

H

, mT

M

/

M

0

28-1

(‡)

1.0

0

0.5

10 20 30 40 50

H

, mT

M

/

M

0

60 70

11-2

x

–x

–

z

z

–y–y

yy

M – b =

0.3 mA/m

M – b =

0.2 mA/m

x

–x

–

z

z

–y–y

yy

(b)

+––––––––

––––––––– – – – – – – – – – – – – – – –

–x

x

–y y

+––––––––

––––––––– – – – – – – – – – – – – – – –

–x

x

–y y

(c)

(I)

74


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GNIBIDENKO, SEMAKOV

35%), muscovite (4%), and microquartzites (2.5–5%)are the main rock-forming minerals of fine-grainedsands of the upper sandy layer in the Dunaevskii Yarsection. The heavy fraction contains magnetite (65%);the remaining 35% of this fraction are represented byepidote and accessories (zircon and granite).

In order to establish the fundamental composition ofmagnetic minerals-carriers of magnetization, we mea-sured the curves of normal magnetization for somesamples of the collection. The values of the remanentsaturation magnetization

J

rs

(

H

) for some samples of theBelyi Yar and Dunaevskii Yar sections are 700–750 mA/m at a saturation field of

H

s

= 110–120 kA/m.For other samples of these sections,

J

rs

varies from 40to 50 mA/m at a saturation field of 200 kA/m or higher,

which possibly indicates that minerals of magnetite–titanomagnetite and hematite–hemoilmenite groups arepresent in the rock samples under investigation.

All samples were subjected to demagnetization byan alternating magnetic field and to stepwise thermaldemagnetization. Characteristic examples of the

J

n

vec-tor behavior in the process of demagnetization by analternating field and temperature are presented in Fig. 2.Our experiments showed that one to two, And some-times three, components of the

J

n

vector, both unstable(low-temperature) and stable (high-temperature), arepresent in the rock samples under investigation. Thefirst unstable components are usually eliminated inalternating fields of up to 20 mT and at a temperature of

100–300°ë

; they are associated with the destruction of

Fig. 2.

(Contd.)

1.0

0

0.5

10 20 30 40 50

H

, mT

M

/

M

0

96-3

(‡)

1.0

0

0.5

100 300 500

T

, °C

M

/

M

0

700

97-1

x

–x

–

z

z

–y–y

yy

M – b =

0.2 mA/m

M – b =

0.2 mA/m

x

–x

–

z

z

–y–y

yy

(b)

+––––––––

––––––––– – – – – – – – – – – – – – – –

–x

x

–y y +––––––––

––––––––– – – – – – – – – – – – – – – –

–x

x

–y y

(c)

60 70 80

(II)

IZVESTIYA, PHYSICS OF THE SOLID EARTH Vol. 45 No. 1 2009


107.5

8.0

8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

12.5

50 1.00.50 1.5 3200 180

280 240 20004080 8040

+–Paleo-

sectionI0

D0Jn, mA/mχ, 10–5 SI unit0

Q

m

Zhu

ravk

a se

ries

1 2 3 4 5 6 7 8

Belyi Yar (200-m mark)40 80 120 160

magnetic

Fig. 3. Paleomagnetic section of deposits of the Belyi Yar outcrop (200-m mark): (1) clay; (2) sand; (3) aleurite clay; (4) vegetabledetritus; (5) seed complexes; (6), (7) magnetizations: (6) normal, (7) reversed; (8) levels of taking oriented samples.

105

6

7

8

9

10

50 1.00.50 1.5 3200 180

280 240 200

04080 8040+–

Paleo-I0

D0Jn, mA/mχ, 10–5 SI units

0Q

m

Abr

osim

ov s

ectio

n

Belyi Yar (300-m mark)40 80 120 160

6.11

magneticsection

Fig. 4. Paleomagnetic section of deposits of the Belyi Yar outcrop (300-m mark). Notation as in Fig. 3.

laboratory and viscous magnetizations. After the elimi-nation of these components, Jn contains only the stablehigh-temperature component, which is associated with

hematite and magnetite. We assumed that this compo-nent was the characteristic, possibly, primary magneti-zation.

76


GNIBIDENKO, SEMAKOV

Paleomagnetic Sections of the Oligocene–Lower Miocene Deposits in the Kompasskii Bor Tract

Taking the conditionally primary (characteristic)magnetization as the basis, we constructed paleomag-netic sections of the Belyi Yar (marks 200 and 300 m)and Dunaevskii Yar (mark 200 m) outcrops, as well asthe correlation scheme of these deposits in theKompasskii Bor tract. In order to increase the reliabilityof the data, microintervals covered by samples of onlyone stratigraphic level were disregarded in determiningthe subzones and microzones of magnetic polarity,when the paleomagnetic sections of these outcropswere constructed.

Below, we present the comprehensive geological–geostratigraphic and paleomagnetic characteristics ofthese sections.

Belyi Yar outcrop (200-m mark). This section isrepresented by a layer of plastic homogeneous clays 5.0 mthick bedding in the sandy sequence of the lower layer(Fig. 3). A layer of homogeneous dense light gray clay0.4 m thick, called the “plate,” occurs at the base of thelens of lacustrine clays. This plate underlies the layer oflacustrine clays proper containing remains of leaves,and it is 4.6 m thick. According to Gorbunov [1962],this layer consists of three members. The lower mem-ber, 2–1.5 m thick, is composed of dense, homoge-neous, dark gray clay with characteristic platy partingand horizontal layering. This member abounds in leaf

prints. The middle member, 1.9–1.6 m thick, is repre-sented by dense, weakly sandy, dark gray clay. Imprintsof leaves (Popular Pterocarya) and nuts of the Trapagenus are met in the lower part of this member. Theupper member, 1.8–1.5 m thick, is composed of brown-ish clay and contains almost no leaf prints. Vertical frostcracks (wedges) up to 4 cm wide are observed in thismember. Genetically, this sequence represents sedi-ments of small bodies of water, such as lakes of the ox-bow type. As noted above, the lens of lacustrine clayscontains a great amount of imprints of leaves, fruits,and seeds, as well as pollen and spores.

The paleomagnetic studies allowed us to constructthe paleomagnetic section of the Belyi Yar outcrop(200-m mark). Changes in the direction of the identi-fied high-temperature components and the magneticzoning corresponding to these changes are shown inFig. 3. One zone of reversed polarity with two thin unitsof normal magnetization in the upper and middle partsof the section (0.3 and 0.6 m thick, respectively) havebeen identified in the paleomagnetic section of thesedeposits; it is 5.0 m thick.

The declination D of reversely magnetized rocksvaries within 240°–180°; their inclination I, from –8° to–78°.

A palynocomplex, whose pollen composition iscomparable with that of the palynocomplex of theFagus grandifoliiformis–Pterocara stenopteroides

103

5

7

9

11

13

50 1.00.50 1.5 0 180

280 240 200

04080 8040+–

Paleo-I0

D0Jn, mA/mχ, 10–5 SI units0

Q

m

Dunaevskii Yar (200-m mark)40 80 120 160

320

15

Zhu

ravk

a se

ries

,L

ower

laye

r of

san

Abr

osim

ov s

erie

s,U

pper

laye

r of

san

dsmagneticsection

Fig. 5. Paleomagnetic section of deposits of the Dunaevskii Yar outcrop (200-m mark). Notation as in Fig. 3.



regional palynozone, was identified by O.B. Kuz’minafrom samples taken by us at depths of 10.5, 9.5, and8.3 m and which were subjected to palynological anal-ysis. This palynozone is characteristic of the Zhuravkaunit of western Siberia [Unified…, 2001], which allowsus to date these deposits at the Late Oligocene. Vegeta-ble remains characteristic of the entire clayey sequencewere detected by V.P. Nikitin in vegetable detritus sam-pled from interlayers in the sequence of lacustrine clayswith leaf prints at a depth of 9.7 m. These remains allowus to compare the established flora with complexes ofthe Lyamin floristic level [Nikitin, 1999]. According toV.P. Nikitin, seed floras of this time are known in themajor parts of the West Siberian plain: from the LyaminRiver basin and the lower part of the Irtysh River to thecentral part of the Kulunda steppe and the Kamenka–Barnaul part of the Ob region. In accordance with thestratigraphic scheme [Unified…, 2001], layers withLyamin floras at the base of the Abrosimov unit arerelated to the Lower Miocene. However, in Nikitin’sopinion, this flora remains close to Lagernosad Oli-gocene flora. Hence, he believes that at the present levelof study of Tertiary Siberian floras, it would be morelogical to regard the Lyamin floras as intermediatebetween the Oligocene and Miocene. However, P.I.Dorofeev [1963], who studied the Kompasskii Borflora, thinks that layers with this flora belong to the

sandy facies of the lower (pre-brown-coal) part of theNovomikhailovka series and must be related to theUpper Oligocene.

Belyi Yar outcrop (300-m mark). The 300-m marksection, 5.5 m thick, is represented by clays and sands(Fig. 4). Paleontological investigations showed that theidentified characteristic component has one polarity.Thus, the entire investigated sequence of the 300-m-mark section, represented by clays in its upper part andfine-grained sands in its lower part, has normal polarityand forms one magnetozone (see Fig. 4). Within thiszone, the magnetic declination varies in the range320°–60°, and the magnetic inclination, from 44° to82°. The scalar magnetic parameters are given in thetable.

Palynospectra supporting the existence of onepalynocomplex were established by Kuz’mina fromsamples taken at depths of 8.85, 7.65, 6.95, and 5.95 mfor palynological analysis. Analysis of the percentageof small- and broad-leaved plants in the complexallowed her to compare the identified palynocomplexwith the palynocomplex of the Quercus sibirica–Ulmuss crassa regional palynozone established for theLower Miocene (Aquitanian stage) of western Siberia[Unified…, 2001]. On this basis, the age of these depos-its is dated at the Early Miocene. We did not sample

Low

er la

yer

of s

ands

Zhu

ravk

a se

ries

CH

AT

TIA

N

Upp

er la

yer

of s

ands

Abr

osim

ov s

erie

s

AQ

UIT

AN

IAN

Stag

e

Uni

t

laye

r Dunaevskii Yar(200-m mark)

Belyi Yar (200-m mark)

Belyi Yar (300-m mark)

Paleo-magneticsection

Complexes oforganic remains

(Nikitin, Kuz'mina)

Lyaminfloristic

level

Lyaminfloristic

level

Quercus-Sibirica-

Ulmus crassapalynozonein the BelyiYar section

Fagusgrandi-

foliiformis–Pterocarya

stenopteroidespalynozonein the Belyi

Rzr

Nab

Nab 22

23

24

25

26

27

28

Chr

ons

Pola

rity

Stag

e

Subd

ivis

ion

Div

isio

n

Syst

em

Ma

Aqu

itani

an

Low

er

Mio

cene

NE

OG

EN

EPA

LE

OG

EN

E

Olig

ocen

e

Upp

er

Cha

ttian

C6ArC6AAn

C6AAr

C6B

C6C

C7

C8

C9

C10

n

r

n

r

n

r

n

r

n

r

n

Berggren et al., 1995

(300-m mark)

Yar section(200-m mark)

Fig. 6. Composite magnetostratigraphic section and correlation scheme of the Oligocene–Miocene sections of the Kompasskii Bortract. Notation as in Fig. 3.

78


GNIBIDENKO, SEMAKOV

vegetable detritus for paleocarpological analysis; how-ever, previously, V.P. Nikitin [1967] and Gorbunov andP.A. Nikitin [1962] performed a paleocarpologicalanalysis of these deposits. V.P. Nikitin believes that theestablished flora can be compared with complexes ofthe Lyamin floristic level.

Dunaevskii Yar outcrop (200-m mark). TheDunaevskii Yar section, 12.5 m thick, is represented bygray and light gray fine-grained sands with clayeyinterlayers (Fig. 5). Sands of the lower and upper layersform the structure of this section. The upper sands over-lie, with washout, the lower sands (see Fig. 5). Twopolar magnetozones—the magnetozone of reversedpolarity (the lower 4.5 m of the section) encompassingthe layer of lower sands and the magnetozone of normalpolarity (upper 7.5 m of the section) encompassing thelayer of upper sands—are recognizable in the paleo-magnetic pattern of the Dunaevskii Yar section. A unitof normally magnetized rocks 0.3 m thick representedby two stratigraphic levels is fixed in the reversed-polarity magnetozone. The scalar magnetic parametersof the Dunaevskii Yar section are given in the table andin Fig. 5. The NRM values for the rocks of this sectionvary within 1.33–4.93 mA/m, and the average value is2.65 mA/m.

Composite Section and Correlation Schemeof Boundary Oligocene–Miocene Deposits

of the Kompasskii Bor TractThe magnetostratigraphic section of boundary Oli-

gocene–Miocene deposits of the Kompasskii Bor tractis constructed on the basis of the comparison and cor-relation of the Belyi Yar (marks 200 and 300 m) andDunaevskii Yar (mark 200 m) paleomagnetic sectionswith the palynological and paleocarpological dataobtained by P.A. Nikitin, V.P. Nikitin, P.I. Dorofeev,and O.B. Kuz’mina. We also used the geological dataobtained by M.G. Gorbunov. Thus, the positions andsuccession of magnetozones in the compiled magneto-stratigraphic section are controlled by geological–stratigraphic and paleontological data.

The lowermost part of the Oligocene–Miocenedeposits in the Kompasskii Bor tract is represented inthe Belyi Yar outcrop (200-m mark) by the lens oflacustrine clays related to the Zhuravka series andforming the reversed-polarity magnetozone Rzr 5.0 mthick (see Fig. 6). The palynocomplex established inthis zone, which is related to the Fagus grandifoliifor-mis–Pterocarya stenopteroides regional palynozone,makes it possible to date these deposits at the Chattianstage of the Late Oligocene. In its stratigraphic positionand sign of polarity, this magnetozone correlates withthe reversed-polarity magnetozone 4.5 m thick identi-fied in the Dunaevskii Yar outcrop (200-m mark) andpresented by fine-grained sands of the lower layer (itslower part). In the composite section, this magnetozoneis overlain by normally magnetized sands of the lowerlayer (its upper part), which outcrop in the Belyi Yar

section (300-m mark) and have a thickness of 4.5 m.According to paleontological data, these sands arerelated to the Abrosimov series and form the normal-polarity magnetozone Nab in the composite section.Flora of the Lyamin level (data of V.P. Nikitin) and thepalynocomplex characteristic of the Quercus sibirika–Ulmus crassa regional palynozone allow us to date theNab magnetozone at the Early Miocene (Aquitanian).V.P. Nikitin believes that the Lyamin floras are interme-diate between the Oligocene and Miocene. The magne-tostratigraphic section is crowned by normally magne-tized sands of the upper layer (normal-polarity zoneNab) overlying with washout sands of the lower layerand reaching the surface within the Kompasskii Bortract only in the Dunaevskii Yar outcrop (200-m mark).The thickness of this magnetozone is 7.5 m, and thedeposits composing it are related, according to paleo-carpological data, also to the Abrosimov series. At thisstage of investigations, one cannot state with certaintywhether normally magnetized sands of the lower andupper layers form one polarity zone or they are relatedto different normal-polarity magnetozones. In our con-structions, we assume that normally magnetized sandsof the lower and upper layers form one magnetozone ofnormal polarity Nab 14 m thick.

DISCUSSION

The regional magnetostratigraphic section of Oli-gocene–Miocene deposits in the Kompasskii Bor tractwas compared with the scale of magnetic polarity of thewestern Siberian plate during the Cenozoic [Gni-bidenko, 2006a, 2006b, 2007]. In this scale, the magne-tozone R1N1aq identified in the Abrosimov series andcharacterized by flora of the Lyamin level has reversedpolarity and is dated at the Aquitanian. The polarityzone Nab encompassing deposits of the Abrosimovseries with Lyamin flora has normal polarity in themagnetostratigraphic section of Oligocene–Miocenedeposits in the Kompasskii Bor tract. Thus, taking intoaccount this investigation, the Miocene deposits withthe Lyamin-layer flora in the territory of the westernSiberian plate are represented not by one polarity zone,but by two zones of normal and reversed polarity. Sincethe normal-polarity magnetozone Nab (Abrosimovseries, Lower Miocene, Aquitanian) in the magneto-stratigraphic section of Oligocene–Miocene deposits ofthe Kompasskii Bor tract is underlain without interrup-tion by the reversed-polarity magnetozone Rzr of theZhuravka serias of the Upper Oligocene (Chattian), wecan state that this reversed-polarity magnetozonecrowns the paleomagnetic section of the Zhuravkaseries and the Oligocene as a whole in the paleomag-netic section of the Cenozoic of the western Siberianplate. In the previously compiled Cenozoic scale of thewestern Siberian plate [Gnibidenko, 2006a], the paleo-magnetic section of the Zhuravka series and the Oli-gocene as a whole is crowned by the normal-polaritymagnetozone Nzr. With consideration for newly



obtained data, the reversed-polarity magnetozoneR1N1aq previously identified in the Abrosimov serieswith the Lyamin-level flora and the newly identifiedmagnetozones can be arranged in the following way(from the bottom upward): Rzr, Nab, (R1N1aq = R1ab).Then, taking into account the reversed-polarity magne-tozone R1N1aq, the following correlations with theBerggren scale [Berggren et al., 1995] are possible: themagnetozones Rzr, Nab, (R1N1aq = R1ab), and the Cen-ozoic polarity scales of the western Siberian plate cor-relate with the chrons C6Cr–C6Cn–C5Br of the Berg-gren scale in the interval 24.6–22.9 Ma.

Thus, not one magnetozone but two magnetozones:the normal-polarity magnetozone N1N1aq (N1ab) andthe reversed-polarity magnetozone R1N1aq (R1ab), arefixed in the Abrosimov series characterized by theLyamin-level flora. In addition to four orthozones(R5E3h, N4E3h, R6E3h, N5E3h) previously identified inthe Zhuravka series, the reversed-polarity magnetozoneRzr(let us designate it as R7E3h), which crowns thepaleomagnetic section of the Zuravka series belongingto the Upper Oligocene (Chattian), is established in theCenozoic polarity scale of the western Siberian plate.The investigations of the paleomagnetic record ofUpper Oligocene–Lower Miocene deposits in theKompasskii Bor tract allowed us to fix the Oligocene–Miocene boundary and the magnetozones adjoining itin the range 24.6–22.9 Ma, which made it possible torefine the magnetic polarity scale of the Cenozoic of thewestern Siberian plate in this time interval.

CONCLUSIONS

Detailed reference paleomagnetic sections, a com-posite paleomagnetic section, and a correlation schemeof Oligocene–Miocene deposits of the Kompasskii Bortract located on the Tym River of the Ob region of thewestern Siberian plate were compiled for the first timeon the basis of complex (paleomagnetic, geological–stratigraphic, paleontological) data. A stable high-tem-perature component associated with magnetite andhematite is possibly the primary NRM in the investi-gated Oligocene–Miocene rocks. The geomagneticfield of the Cenozoic at the Oligocene–Miocene bound-ary reconstructed from the NRM vectors of rocks fixedtwo regimes of polarity (normal and reversed).

The investigations performed made it possible toobtain a detailed paleomagnetic record of the Oli-gocene–Miocene boundary and refine the magneticpolarity scale of the Cenozoic of the western Siberianplate, which was compiled previously [Gnibidenko,2006a, 2006b, 2007], in the time interval 24.6–22.9Ma, having identified the normal-polarity magnetozonecharacterized by the Lyamin-level flora in the LowerMiocene (Aquitanian) and the reversed-polarity mag-netozone crowning the section of the Zhuravka seriesbelonging to the Upper Oligocene (Chattian).

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research, project no. 02-05-64787.

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Paleomagnetism of boundary oligocene-miocene deposits in the Kompasskii Bor tract on the Tym River (Western Siberia)