sedimentology, geochemistry, and biota of volgian ...volgian stage (oil shales) are developed over a...

ISSN 0024-4902, Lithology and Mineral Resources, 2008, Vol. 43, No. 4, pp. 354–379. © Pleiades Publishing, Inc., 2008.Original Russian Text © Yu.O. Gavrilov, E.V. Shchepetova, M.A. Rogov, E.A. Shcherbinina, 2008, published in Litologiya i Poleznye Iskopaemye, 2008, No. 4, pp. 396–424.

354

The second half of the Late Jurassic Period wascharacterized by the development of conditions favor-able for the accumulation of high-carbonaceous (fre-quently, oil-generating) sediments in different regionsof the Northern Hemisphere (West Europe, RussianPlate, Barents Sea, West Siberia, and others). In theEast European Platform, organic-rich sediments of theVolgian Stage (oil shales) are developed over a spaciousarea. Researchers have proposed different concepts forthe accumulation of high-carbonaceous sediments.However, formation mechanisms of such sedimentsremain unclear so far. Development of their geneticmodels requires data on sections from different parts ofthe spacious Volgian Basin, which would make it pos-sible to understand general regularities in its evolutionand determine the influence of local factors on sedi-mentation processes. Data on the typical shale-bearingsections with the most complete record of their sedi-mentation history are also essential.

The best studied Gorodishche and Kashpir strato-type sections of the Volgian Stage in the Middle Volgaregion represent the relatively condensed successionswith numerous indications of syn- and postsedimentaryerosion. This process promoted the concentration ofcoarse fractions at some levels, i.e., in the formation ofcondensed units (Baraboshkin et al., 2002). In case ofexhumation of the relatively compact sediments to thebasin bottom, they were subjected frequently (oftenintensely) reworked by burrowing organisms, resultingin the formation of softground-type surface (Bromley,

1996). These and some other phenomena were respon-sible for the fragmentary record of the shale-bearingsequence formation in sections of the Volga region.

Such disadvantages of the sedimentation record aresignificantly less manifested in the examined sectionnear the Ivkino Settlement located on the right bank ofthe Unzha River (left tributary of the Volga River, Kos-troma region) in the area known as the Manturov groupof Volgian oil shale deposits (Dobryanskii, 1947;Yavkhuta, 1973; and others) (Fig. 1).

In addition to the relative sedimentary complete-ness, the Ivkino shale-bearing section is also interestingfrom the standpoint of sedimentation settings in thenorthern (least studied) part of the Volgian Basin. Inthis work, we present a detailed characteristic of theshale-bearing sequence with the consideration of dif-ferent sedimentological and geochemical features,which are crucial for understanding sedimentation set-tings of the Volgian carbonaceous sediments.

When studying the shale-bearing sequence and theunderlying and overlying sediments, we paid particularattention to the initial structure of sediments, degree ofbiogenic reworking, character of bioturbation, grain-size distribution, mineral composition, petrographicand pyrolytic parameters of organic matter (OM), anddistribution of different chemical elements in the heter-ogeneous laminated sequence.

It should be noted that the shale-bearing sequence isoverlain by Cretaceous and Quaternary sediments with

Sedimentology, Geochemistry, and Biotaof Volgian Carbonaceous Sequences in the Northern Part

of the Central Russian Sea (Kostroma Region)

Yu. O. Gavrilov, E. V. Shchepetova, M. A. Rogov, and E. A. Shcherbinina

Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017 Russiae-mail: [email protected], [email protected]

Received January 28, 2008

Abstract

—Lithological, geochemical, stratigraphic, and paleoecological features of carbonaceous sedimentsin the Late Jurassic Volgian Basin of the East European Platform (Kostroma region) are considered. The shale-bearing sequence studied is characterized by greater sedimentological completeness as compared with its stra-totype sections in the Middle Volga region (Gorodishche, Kashpir). Stratigraphic position and stratigraphy ofthe shale-bearing sequence, as well as the distribution of biota in different sedimentation settings, are specified.It is shown that Volgian sediments show a distinct cyclic structure. The lower and upper elements of cyclitesconsist of high-carbonaceous shales and clayey–calcareous sediments, respectively, separated by transitionalvarieties. Bioturbation structures in different rocks are discussed. Microcomponent composition and pyrolyticparameters of organic matter, as well as distribution of chemical elements in the lithologically variable sedi-ments are analyzed. Possible reasons responsible for the appearance of cyclicity and accumulation of organic-rich sediments are discussed.

DOI:

10.1134/S002449020804007X

LITHOLOGY AND MINERAL RESOURCES

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SEDIMENTOLOGY, GEOCHEMISTRY, AND BIOTA 355

the total thickness of 150 m. Intensity of postsedimen-tary transformations of the shale-bearing sedimentscorresponds to early catagenesis. Consequently, thepresent-day geochemical parameters reflect largelytheir state at sedimentary and diagenetic stages.

The mineral composition of rocks was determinedby the X-ray diffraction method in the Laboratory ofPhysical Methods for the Study of Rock-Forming Min-erals (GIN). Pyrolytic parameters were measured usinga Rock-Eval II equipment at the Institute of Geologyand Development of Fuel Minerals. Concentrations of

C

org

, CO

2

, Fe, Mn, Ti, P, and S were determined bychemical methods, while concentrations of other ele-ments were measured by the emission spectral method(spectrometer PGS-1) in the Laboratory of PhysicalMethods for the Study of Rock-Forming Minerals(GIN).

Particular attention was paid to specification of thestratigraphic position of the examined sequence andresponse of the biota to sharp fluctuations of paleoeco-logical settings during the accumulation of alternatinghigh- and low-carbonaceous sediments. The obtained

Fig. 1.

Schematic paleogeographic map of the East European Platform for the Volgian Age (sea and land distribution, after (Thierry,2000); areas of shale-bearing sequence development, after (

Prognoz goryuchikh…

, 1974; Lyyurov, 1996; and others). (1) Land;(2) epicontinental seas; (3) Tethys Ocean; (4) areas of shale-bearing sequence development. Asterisk in the inset shows location ofthe Ivkino section.

KostromaKostromaKostroma

Ul’yanovskUl’yanovskUl’yanovsk

SaratovSaratovSaratov

MoscowMoscowMoscow

CCCeee nnn

ttt rrraaa lll

RRR uuusss sss

iii aaannn

SSS eeeaaa

TethysTethysTethys

NeyaNeyaNeyaManturovoManturovoManturovo

Makar’evMakar’evMakar’ev

UUUnnnzzzhhh

aaa RRR...

NNNeeeyyy

aaaRRR

...

UUUnnnzzzhhhaaa

RRR...

VVVooolllgggaaa RRR...

KostromaKostromaKostroma

KKK ooo sss ttt rrr ooo mmm aaa

rrr eee ggg iii ooo nnn

Gor'kovskoeReservoir

0 16 32km

60

50

40

30

1 2 3 4

356


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GAVRILOV et al.

data are important for the more reliable correlation ofsections in the spacious East European Platform and theelucidation of relationships between the Volgian shale-bearing sequence and carbonaceous sediments in otherregions of the Northern Hemisphere. Ammonites andnannofossils were the main objects in these studies,although other fossil remains were also taken into con-sideration.

GENERAL CHARACTERISTIC AND STRATIGRAPHIC POSITION

OF THE SHALE-BEARING SEQUENCE

The present-day spacious fields of Volgian high-car-bonaceous sediments make up the Volga–Pechorashale-bearing province (Fig. 1) in the eastern RussianPlate (Yavkhuta, 1973). They constitute a sequencewith well-developed bedding related to the alternationof beds (thickness from

n

dm to ~1 m or more) of darkbrown high-carbonaceous shales and notably lighter(mainly gray) clayey–calcareous sediments. The thick-ness of sequence varies from a few meters in the Rus-sian Plate to several tens of meters in the southern andnortheastern parts. The shales extend laterally over sig-nificant distances (more than 10

n

km) only in rare casesand their amount in the sequence is variable. The shale-bearing sequence is frequently replaced laterally bycondensed sections composed of phosphorites, carbon-ate and siliceous coquinas, and glauconitic sandstones.

Although shale-bearing sediments are known in theKostroma and northern regions for more than a century(Krotov, 1879; Ivanov, 1909; Rozanov, 1913; Dobryan-skii, 1947, Gerasimov et al., 1962; Yavkhuta, 1973),data on the structure of their particular sections arelacking. Territory characterized by the preservation ofthese sediments form erosion is bounded by the sublat-itudinal line extending from Manturovo to Neya in thenorth and to the Unzha Settlement in the south. Theyoccur at depths of 0 to 50 m and their thicknessincreases gradually from 0.9 m in the Unzha Settlementarea to ~8 m near Manturovo; i.e., one can see transitionfrom the condensed section (Gerasimov et al., 1962) tothe more or less complete section.

Olfer’ev (1986) identified Volgian sediments (oilshales) of the Kostroma region into the Kostroma For-mation and proposed to consider sections outcroppingat the right bank of the Unzha River as its stratotypes.The Ivkino section (58

°

14

′

36

′′

N, 44

°

39

′

29

′′

E) is oneof them.

Based on lithology, the Ivkino section is divided intothree sequences: the lower Kimmeridgian sequence ofcalcareous clays (apparent thickness approximately1.2 m), the middle Volgian shale-bearing sequence(~6 m thick) subdivided into three members, and theoverlying sandstone–siltstone sequences (Fig. 2).

Results of the Study of Ammonitesand Other Macrofaunal Fossils

Already at the beginning of stratigraphic studies inthe early 20th century, it was established that the shale-bearing sequence correlates with the

Dorsoplanitespanderi

ammonite zone of the middle Volgian Substage(initially,

Perisphinctes panderi

Zone of the VolgianStage). Identification of this zone was followed byattempts to subdivide it into smaller units (Rozanov,1919; Ilovaiskii and Florenskii, 1941). Mikhailov(1962): the lower

Pavlovia pavlovi

Subzone and theupper

Dorsoplanites panderi

Subzone, which was laterrenamed as

Zaraiskites zarajskensis

Subzone (Gerasi-mov and Mikhailov, 1966).

Based on the

Zaraiskites

phyletic lineage (from thebase to top:

quenstedti, scythicus, regularis, zarajsken-sis

), Kutek (1994) subdivided the

Scythicus

Zone, ananalogue of the

Panderi

Zone (Poland), into two sub-zones and four faunal horizons. In the Russian Plate,this stratigraphic interval can also be subdivided intotwo subzones (based on the

Zaraiskites

ammonite spe-cies):

Scythicus

and

Zarajskensis.

Recently,

Zaraiskitesregularis

was found in the Russian Plate (Rogov,2004). Subsequently, all the four faunal horizonsdefined by Kutek in Poland were established in the Gor-odishche section of the Middle Volga region (Rogov,2005). The species

Zaraiskites regularis

is subdividedin two chronological subspecies that characterize suc-cessive faunal horizons (Pimenov et al., 2005).

It was noted long ago that the boreal (cold-resistant)ammonite species of the genera

Pavlovia

and

Dor-soplanites

prevail in northern areas of the RussianPlate, while assemblages from its central areas aredominated by the Subboreal (relatively more thermo-philic)

Zaraiskites

forms (Rozanov, 1913).Until recently, stratigraphy of the Volgian sediments

was not virtually studied in the Kostroma region.Among recent publications, one can note a find of theammonite species

Pavlovia pavlovi

in the Ivkino sec-tion (Mitta and Starodubtseva, 1998), a species charac-teristic of the

Panderi

Zone in the Russian Plate. Thisfind confirmed the previously established age of theshale-bearing sequence.

The lower part of the Ivkino section contains thelower Kimmeridgian ammonite species

Amoebites

sp.and

Rasenia

sp. characteristic of the

Cymodoce

Zone(Fig. 2), which occur as both deformed clayey casts ofshells and phosphatized body chambers.

The upper Kimmeridgian and lower Volgian sedi-ments are absent in the section. The boundary betweenstages is distinctly indicated by a regional thin interbedof phosphoritic conglomerate with reworked Kim-meridgian ammonites. The next phosphorite interbed,which occurs approximately 0.17 m above the previousone, is likely of the middle Volgian age because it con-tains rare

Dorsoplanites

cf.

dorsoplanus

(Fig. 2).Based on numerous finds of ammonite genera

Pav-lovia, Dorsoplanites

, and

Zaraiskites

, the shale-bearing


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sequence is reliably correlated with the

Panderi

Zoneof the middle Volgian Substage.

Ammonite shells in the Ivkino section are stronglydeformed and unidentifiable in some cases. Neverthe-less, it is evident that finds of the boreal ammonites

Pavlovia pavlovi

and

Dorsoplanites

are most common(>98%) and the former species is particularly abundant.

In the lower part of the section,

P. pavlovi

(Figs. 3b, 3c)specimens are usually small, which is generally charac-teristic of the lower part of the

Panderi

Zone in the Rus-sian Plate (Fig. 3a). Only Bed 24 contains the relativelylarge specimens (Fig. 3e). In small

Dorsoplanites

spec-imens, their generic features are often poorly devel-oped, which makes them almost indistinguishable from

Stag

e

Subs

tage

Am

mon

itezo

nes

Am

mon

itesu

bzon

es

Faun

al h

oriz

on(b

ased

on

31

amm

onite

s)

Mem

bers

Lith

olog

y

302928

272625

24

21-23

20

14

1312

11

10

9

8

7

6

5

4

3

2

1

Sam

plin

gle

vels

and

beds

Levels

taxa finds

First occurrence

Nan

nopl

ankt

onzo

natio

n

Subs

tage

Stag

e

?

Vo

lgia

n

Mid

dle

Pand

eri

Scyt

hicu

sZ

araj

sken

sis

Z. r

egul

aris

subs

p. n

ov.

8

7

6

5

3

2

1

0

m

4

?

Kim

mer

idgi

an

Low

er

Cym

odoc

e

a

b

a

b

ab

ba

ca

b

ababca

b

ababcdab

ababc

a

b

Aa

b12

abc123

1 21

2

Ras

enia

sp.

Am

oebo

cera

s

sp.

Dor

sopl

anite

s

cf

. dor

sopl

anus

Zara

iski

tes

sp. j

uv.

Dor

sopl

anite

s

pand

eri

Pav

lovi

a

cf.

pavl

ovi

Zara

iski

tes

regu

lari

s

subs

p. n

ov.

Hex

apod

orha

bdus

cuvi

llier

iC

onus

phae

ra m

exic

ana

Pol

icos

tela

bec

kman

niM

icro

stau

rus

chia

stus

NJ-

20A

-B(?

)N

JK-A

?

Mid

dle

Tit

ho

nia

n

Kim

mer

idgi

an

123

456

789

101112

1314

of ammonite levels of zonalnannoplanktontaxa

Fig. 2.

Schematic stratigraphy of the Ivkino section and distribution of guide ammonite and nannoplankton species. (1) Carbon-aceous shales; (2) laminated calcareous–clayey rocks; (3) massive calcareous–clayey rocks; (4) clayey–sandy siltstones; (5) phos-phorite pebbles and nodules; (6) phosphate concretions; (7) calcareous concretions; (8) pyrite nodules; (9) erosion and sedimentcondensation surfaces; (10) fossils; (11) bioturbation; (12) “softground” with fucoids confined to levels of decelerated sedimenta-tion or nondeposition, after (Bromley, 1996); (13) sampling sites; (14) ammonite finds.

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GAVRILOV et al.

1 cm

(a)

(b)

(c)

(d)

(e)

(f)

(h)

(i)

(g)

Fig. 3.

Ammonites from the

Panderi

Zone of the middle Volgian Substage. (a–c, e)

Pavlovia pavlovi

(Mikhalski): (a) SpecimenMK2341, Khanskaya Gora section, Berdyanka River,

Scythites

Subzone, Bed C7, (b) Specimen MK2321, Ivkino section,

Scyth-ites

(?) Subzone, Bed 7, (c) Specimen MK2320, Ivkino section,

Scythites

(?) Subzone, Bed 8, (e) Specimen MK2466,

Zarajskensis

Subzone,

regularis

subsp. nov. faunal horizon, Ivkino section, Bed 24 (collection by E.K. Iosifova); (d)

Zaraiskites

sp. juv.(cf.

scythicus

(Vischn.)), Specimen MK2307-2,

Scythicus

Subzone, Ivkino section, Bed 7; (f, h)

Zaraiskites regularis

subsp. nov.,

Zarajskensis

Subzone,

regularis

subsp. nov. faunal horizon: (f) Specimen MK1250, Kashpir section, Bed 2/10, (h) SpecimenMK2467, Ivkino section, Bed 24 (collection by E.K. Iosifova); (g)

Zaraiskites

sp., Specimen 66J1,

Zarajskensis

Subzone,

regularis

subsp. nov. (?) faunal horizon, Sysola River, Bed Yb; (i)

Dorsoplanites

cf.

panderi

(Eichw.), Specimen MK2464,

Zarajskensis

Sub-zone,

regularis

subsp. nov. faunal horizon, Ivkino section, Bed 24 (collection by E.K. Iosifova). Depository of the Geological Insti-tute (specimens from the collection by M.A. Rogov unless otherwise stated. Scale bar 1 cm (magn. 1.5 in 3d).


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Pavlovia representatives of the similar size. Therefore,their share in the ammonite assemblage is unclear sofar. Dorsoplanites cf. dorsoplanus is found at the baseof Volgian sediments (Fig. 4). D. cf. panderi (Fig. 3i)occurs slightly higher in the section.

In northern areas of the Russian Plate, Zaraiskitessuitable for the subdivision of the Panderi Zone intofaunal horizons are rare. They are unknown in the lowerpart of this zone because of paleobiogeographic rea-sons (Rogov, 2005). Sections of the Pechora (Repinet al., 2006) and Sysola river basins contain only Z. reg-ularis. In the Ivkino section, specimens of Zaraiskitesare found at two stratigraphic levels: the lower part ofthe section (Bed 7) includes a small specimen ofZaraiskites sp. juv. (cf. scythicus) (Fig. 3d), which mayindicate the presence of the Scythicus Subzone,whereas the upper Bed 24 includes a well-preservedrepresentative of the early Z. regularis chronologicalsubspecies with the stage of biplicate ribs preserved inthe ontogenesis (Fig. 3h). Similar ammonites occur inthe Gorodishche stratotype section beginning approxi-mately from the middle part of the shaly sequence(Rogov, 2005), in the Kashpir section (Fig. 3f), and innorthern sections, e.g., the Sysola River basin (Fig. 3g).

Finds of early Zaraiskites regularis indicate that thePanderi Zone in the Ivkino section also includes the upperPanderi Subzone represented by the Zaraiskites regularis

subsp. nov. faunal horizon. It is conceivable that upper lay-ers of the shale-bearing sequence, which virtually lackammonites, represent analogues of the Z. regularis regu-laris faunal horizon, because precisely the younger chro-nological subspecies is relatively abundant in northernareas (Khudyaev, 1927; Repin et al., 2006).

In general, the molluscan assemblage in shales of thePanderi Zone is less diverse. It is dominated by smallbivalve Buchia species, gastropods Scurria maeotis, andammonites largely belonging to the genus Pavlovia.Coleoids are rare and represented only by Acanthoteuthis.Like coeval sediments of the Gorodishche section (Vish-nevskaya et al., 1999), shales are characterized by theprevalence of juvenile molluscan specimens, probably,owing to the unfavorable influence of anoxic environ-ments. At the same time, intercalating beds (for example,Bed 24) include a slightly more diverse assemblage: juve-nile forms of ammonites are accompanied by large shellsof adult specimens belonging to genera Pavlovia, Dor-soplanites (dominant), and Zaraiskites (rare).

The palynological analysis of OM from some car-bonaceous beds (samples 4,7, 11) revealed that the OMis largely represented by disintegrated algal remainswith rare specimens of relatively well-preserved Chor-ata dinocysts (private communication by G.N. Aleksan-drova). Scarcity of dinocysts in the high-carbonaceoussediments is probably explained by some unfavorable

Fig. 4. Fragment of the prepared surface of phosphorite bed (roof of Bed 3) with inclusions of phosphorite nodules (black)and ammonite casts Dorsoplanites cf. dorsoplanus (Vischn.), Panderi Zone of the middle Volgian Substage. Photograph byM.A. Rogov. Scale bar 5 cm.

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conditions. However, it is rather difficult to identify theunfavorable factor at present.

Results of the Study of Calcareous Nannofossils

Calcareous nannofossils, one of the main compo-nents of the Mesozoic–Cenozoic biogenic sedimenta-tion, represents a taxonomically uniform group of uni-cellular phytoplankton organisms with calcite skeletonsof variable morphology. Under favorable conditions, itproduces a huge volume of biomass, which promoteshigh sedimentation rates particularly in epicontinentalsea basins. The Jurassic–Cretaceous transition wascharacterized by high evolutionary rates of nanno-plankton, which made it possible to develop the rela-tively detailed zonations for the Tethyan (Bralower etal., 1989) and boreal (Bown et al., 1988) realms.

The distribution of nannofossils is very irregular inthe Ivkino section. The lower part of the section (upper-most Kimmeridgian and Member I of the shale-bearingsequence) is characterized by the high abundance ofnannofossils. Their content decreases gradually upwardthe section (Member II) up to the point of complete dis-appearance in the upper part (Member III and overlyingsandy–silty sediments). Therefore, the results of thestatistical analysis are given only for the lower half ofthe section (Fig. 5). The nannofossil assemblage islargely composed of representatives of the highly toler-ant genus Watznaueria: W. britannica (Figs. 6a–6c),

W. barnesae (Fig. 6d), W. fossacincta (Fig. 6e),W. manivitae (Fig. 6f), W. ovata (Fig. 6g), and others,which constitute 70–99% of the assemblage in thelower and upper parts of the sequence, respectively.Such an oligotaxonic composition of the assemblagecan be related not only to initially high concentrationsof these species, which is typical of Jurassic–Creta-ceous sediments worldwide, but also to the fact that andWatznaueria spp. are most resistant to dissolution dur-ing diagenesis. Therefore, the dominant role of thesespecies can be related to secondary processes, althoughdistinct dissolution signs in nannofossils are noted onlyin organic-rich beds.

The nannofossil assemblage from the Ivkino sectiondiffers substantially from its counterparts in the southernGorodishche and Kashpir sections of the Ul’yanovskregion (Lord et al., 1987; Kessels et al., 2003) containingfrequent the boreal species Stephanolithion atmetros,Crucibiscutum salebrosum, and eutrophic forms (Bis-cutum constans, Zeugrhabdotus erectus). In the exam-ined section, the first two species are practically missing.The two last forms (Figs. 6k, 6l) are relatively abundantin the Kimmeridgian sediments and are represented bysingle specimens in the shale-bearing sequence.

In sediments underlying the shaly sequence, the nan-nofossils are relatively diverse (up to 10 species) butless abundant: average ~15 species, maximum 25 speci-mens in the microscopic observation field at a magnifica-

10c

10b

10a9b9a

8b

8a7c

7a7b

6b6a5b

5a

4c

4b

4a3b3a

2b

2a

1b

1a

W. barnesae group W. britannicaB. constans

Zeugrh. spp.

Average numberof nannofossils

in 50 microscopicfields of view

0 20 40 60 80 0 20 40 60 0 20 0 20 0 20 40 60 %

Fig. 5. Distribution of main nannofossil taxa and its total abundance (%) in Kimmeridgian–Volgian sediments of the Ivkino section.For lithological legend, see Fig. 2.



(a) (b) (c) (d)

(i)

(f) (g) (h)

(p)

(j)

(k)

(l)

(m)

(n)

(o)

(q) (r) (s)

(e)

Fig. 6. Photomicrographs of nannofossils from Kimmeridgian–Volgian sediments of the Ivkino section. (a–o) Polarized light,×6000; (p–s) scanning electron microscope, magn. 12000. (a) typical Watznaueria britannica (Stradner, 1963) Reinhardt, 1964,Sample 1a; (b) small W. britannica sp. specimen with a thick massive placolith and narrow central opening entirely covered by sep-tum, Sample 3a; (c) thin large W. britannica sp. specimen with a wide central opening and thin septum, Sample 9b; (d) W. barnesae(Black, 1959) Perch-Nielsen, 1968, Sample 1a; (e) W. fossacincta Wind and Cepek, 1979, Sample 1b; (f) W. manivitae Bukry,1979b, Sample 4b; (g) W. ovata Bukry, 1969b, Sample 6a; (h) Cyclagelosphaera margerelii Noël, 1965, Sample 2a; (i) Conus-phaera mexicana Trejo, 1969, Sample 3a; (j) Polycostella beckmannii Thierstein, 1971, Sample 6a; (k) Zeugrhabdotus erectus(Deflandre in Deflandre and Fert, 1954) Reinhardt, 1965, Sample 9b; (l) Biscutum constans (Gorka, 1957) Black in Black and Bar-nes, 1959, Sample 4a; (m) Microstaurus chiastius (Worsley, 1971) Grün in Grün and Allermann,1975, Sample 20b; (n) M. chiastius,Sample 10b; (o) coccosphaera of Watznaueria sp., Sample 4b; (p) W. ovata, Sample 2a; (q) W. fossacincta, Sample 2a; (r) W. bri-tannica, proximal disc, (s) W. britannica, distal disc.

362


GAVRILOV et al.

tion of 1250. The assemblage is largely composed ofnumerous Watznaueria barnesae, W. britannica, Cycla-gelosphaera margerellii (up to 25%), and rare Biscutumconstans (up to 11%) (Fig. 6). Such an assemblage ischaracteristic of normal marine (probably, mesotrophic)environments. Of stratigraphic significance is only thefind of single Hexapodorhabdus cuvillieri, which disap-pears at the upper Kimmeridgian boundary.

Conusphaera mexicana (Fig. 6i), which marks thelower boundary of Zone NJ20A (middle Tithonian orupper part of the lower Volgian Substage), appears atthe base of the shale-bearing sequence above the lowerphosphorite horizon (Sample 3a). The slightly higherhorizon (Sample 6) includes Polycostella beckmanni(Fig. 6j), which marks the base of Subzone NJ20-B(Fig. 2), and Microstaurus chiastius (Figs. 6m, 6n),which marks the lower boundary of Zone NJK-A(uppermost middle–basal upper Tithonian, uppermostlower–basal middle Volgian). Thus, it can be assumedthat the base of the third black shale bed coincides withhiatus corresponding to Subzone NJ20B. It should betaken into consideration, however, that the above-men-tioned species are abundant in the Tethyan realm andscarce in the examined section. Therefore, their appear-ance levels in the study region may slightly differ from thetrue evolutionary appearance; i.e., the real age of sedi-ments may be slightly older than that indicated by nanno-plankton. It is remarkable that the high-latitude Ivkino sec-tion contains (although scarce) warm-water forms, whichdefine zonal units of the Tethyan scale corresponding tothe middle Tithonian Substage. Their occurrence in thisboreal section is most likely related to a complex systemof currents in the shallow Central Russian Basin.

The nannofossil assemblage from the lower part ofthe shale-bearing sequence, which is compositionallysimilar to that in the underlying Kimmeridgian sedi-ments, shows a notably lower abundance of eutrophictaxa and a prevalence of Watznaueria spp. The basallayers contain Cyclagelosphaera margerellii (Fig. 6h)and Biscutum constans, which are practically missingabove the second shale interbed. Beginning from thislevel, the content of all nannofossil taxa, exceptWatznaueria representatives, does not exceed 1%. Incarbonaceous shales, abundance of nannofossilsdecreases sharply, while they are abundant and play therock-forming role in the adjacent carbonate interbeds(Fig. 5). The population of different representatives ofWatznaueria species demonstrates notable changes. Inthe Kimmeridgian assemblage, taxa lacking the centralopening or with a small opening without central bridge,e.g., W. barnesae, W. fossacincta, W. ovalis, W. manivi-tae (Figs. 6d–6g), which are conditionally united intothe W. barnesae group, and forms with a more or lesswide central opening divided by the central bridge(W. britannica; Fig. 6) occur in equal proportions. Incontrast, the assemblage from the lower part of theshale-bearing sequence is dominated by taxa of the firstgroup, the population of W. britannica is drastically

decreased, and its content slightly increases only insome carbonaceous layers. In the basal part of Volgiansediments, the W. britannica group is divided distinctlyinto two morhotypes. Morphotype 1 (W. britannica sp. 1,Figs. 6b, 6o) is represented by small massive formswith a small central opening almost closed by centralbridge. Morphotype 2 (W. britannica sp. 2, Fig. 6c) ischaracterized by large thin coccoliths with a wide cen-tral opening; i.e., they are less calcified. No transitionalforms are observed between these morphotypes, sug-gesting that the taxon likely attempted to adapt to sub-stantially different ecological niches. It is conceivablethat they dwelt at different depths of the basin and thesignificant differentiation of planktonic forms is relatedto the strong stratification of its water column. Higherin the section, the share of thin large W. britannica sp. 2with the wide central opening is notably higher. This issometimes interpreted as indicator of some warming(Giraud et al., 2006) and deficiency of Ca in the basin. Inthe overlying sample 10 (Figs. 2, 5), the amount of nan-nofossils decrease sharply and disappears completelyabove sample 21. Only sample 24 contains an abundantnannofossil assemblage composed largely of W. fos-sacincta, which is less calcified than W. barnesae.

The results of the statistical analysis of nannofossilassemblages from the Ivkino section differ from thedata obtained for coeval assemblages from the lowerlatitudes of the Atlantic (Bornemann et al., 2003; Trem-olada et al., 2006). In the Atlantic region, the upper partof Subzone NJK-A demonstrates a sharp increase inspecies diversity of nannofossils and an abundance ofstrongly calcified massive forms Cornusphaera mexi-cana, Polycostella beckmannii, and Nannoconus spp.In the Ivkino section, these taxa occur as single speci-mens in the lower part of the shale-bearing sequence.These data indicate different concentrations of dissolvedCa in waters of oceanic and epicontinental basins in theterminal Jurassic. In oceans, its content was sufficient toprovide high productivity of strongly calcified taxa,while epicontiental seas were populated by forms thatconsumed minimal concentrations of dissolved calciumcarbonate for the formation of skeletons.

Enhanced eutrophication of the basin affected nega-tively the nannoflora and provoked the extinction ofmost taxa, including the eutrophic forms that could tol-erate only an insignificant eutrophication. The releasedecological niches were occupied by the most toleranttaxa that could tolerate significant changes in environ-mental parameters.

Glauconite-bearing sandy–silty sediments overly-ing the shaly sequence in the examined section (Fig. 2)lack ammonites and nannofossils. Therefore, the reli-able dating of such sediments is rather difficult. Itshould be noted that lithologically similar sedimentsoverlying the shale-bearing sequence with an erosionalsurface in other areas of the Moscow Basin can be cor-related with either the Virgatites virgatus Zone of theVolgian Stage, which is defined as the Egor’evsk For-



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wer

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e

J 3 V

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eri Z

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7

6

8

5

4

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458

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87

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Fig. 7. Distribution of Corg, CaCO3, and hydrogen index (HI) in sediments of the Ivkino section (based on OM pyrolysis data). Inthe HI column, solid and open circles designate oil shales and calcareous clays, respectively.

mation according to (Olfer’ev, 1986), or the lower Val-anginian (Gerasimov et al., 1982).

LITHOLOGICAL CHARACTERISTICS OF SEDIMENTS

Figure 7 illustrates lithological column of the Ivkinosection with the distribution of Corg and CaCO3 andvariations of the hydrogen index (HI) based on thepyrolytic analysis of OM.

Lower Kimmeridgian Sediments

Lower Kimmeridgian sediments (apparent thick-ness ~1.2 m) are represented by alternation of the rela-tively dark gray and notably lighter beds (a few tens ofcentimeters thick) of calcareous clays (CaCO3 11–33%). The clays contain an admixture of silty material(10–20%) (Figs. 8a, 8b).

Couplets of differently colored beds make upcyclites. Their dark and light sediments should be con-

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0.50 mm 0.10 mm

0.20 mm

0.20 mm

(a) (b)

(c)

(d)

Fig. 8. Microtexture of sediments in the Ivkino section. (a, b) Lower Kimmeridgian calcareous clays: (a) rocks with bioturbationsigns from the lower part of a cyclite, Sample 1a (white and black correspond to fucoids filled with sediments and pyrite, respec-tively), (b) rocks from the upper part of a cyclite, Sample 2b (OM is practically missing); (c–f) rocks from different parts of a cyclitein the shale-bearing sequence: (c) carbonaceous shale from the lower part of a cyclite, Sample 4a from Member I (dark zones aresmall colloalginite lenses), (d) light calcareous clays from the upper part of the same cyclite, Sample 4c (black zone is a small plantdetritus), (e) high-carbonaceous oil shales from the lower part of a cyclite, Sample 28a from Member III (dark and gray zones arecolloalginite lenses and laminae, respectively), (f) low-carbonate clays from the upper part of the same cyclite, Sample 28b (darkzone is finely dispersed OM; (g, h) sandy–silty sediments overlying the shale-bearing sequence: (g) bioturbation structure of sandy–silty rocks (dark zone is clayey matrix, while light zones are transverse sections of fucoids filled with silty glauconite–quartz mate-rial), (h) glauconite in sandy–silty rocks.



sidered the lower and upper elements, respectively,because transition from the dark sediments to the lightvariety is gradual, whereas transition from the light layerto the dark layer is relatively sharp. This part of the sectionincludes two such cyclites (0.75 and 0.45 m). Each ofthem demonstrates upsection increase in the carbonatecontent and decrease in the Corg content (Fig. 7).

The lower Kimmeridgian sediments are character-ized by platy jointing and negligible sedimentary lami-nation. Fucoids of burrowing organisms occur as sub-horizontal thin slightly curved tubes up to 1–2 cm long.Thin sections show that the fucoids are largely filledwith clayey material that is similar to the groundmassbut more compact. The sediments contain mainly small

molluscan shells. Morphological uniformity of fucoidsand low content of benthic fossils indicate unfavorableconditions for dwelling of the benthos at the bottom ofthe early Kimmeridgian basin. Development of reduc-ing environments in the bottom sediments is evidentfrom numerous sulfide aggregates, which fill cavities ofshells and fucoids (Fig. 8a) and make up a regular dis-semination of microframboids.

The lower Kimmeridgian section is crowned by aphosphoritic conglomerate bed (0.08 m) consisting ofdark phosphate nodules (up to ~5 cm across) and theirsmall angular fragments of the gravel–sand size. It alsoencloses rostra of small belemnites and fragments ofsmall ammonite shells.

0.20 mm

0.20 mm

0.50 mm 0.10 mm

(e)

(f)

(g) (h)

Fig. 8. (Contd.)

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GAVRILOV et al.

Table 1. Concentrations of chemical elements in sediments of the Ivkino section

Age Sample Corg CaCO3 Fe Mn Ti P S Cr V Ni Co

J3, lower Kim-meridgian

1a 0.98 22.7 4.03 0.053 0.44 0.05 0.75 120 132 151 861b 1.00 21.00 4.4 0.038 0.46 0.05 1.02 137 142 115 922a 1.59 11.58 4.85 0.036 0.53 0.08 – 135 157 239 942b 1.09 18.05 4.76 0.046 0.48 0.06 1.79 136 247 470 170

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3a 1.09 25.54 3.71 0.044 0.39 0.08 0.77 99 170 530 1753b 1.03 33.03 3.07 0.357 0.37 – 78 125 292 924a 9.40 22.36 3.18 0.034 0.34 0.12 – 111 182 112 294b 3.00 16.34 3.49 0.032 0.46 0.07 – 106 132 109 304c 2.46 24.52 3.17 0.032 0.42 0.06 – 103 115 101 315a 5.80 23.84 2.78 0.032 0.35 0.05 – 123 160 94 265b 1.34 30.08 2.97 0.044 0.37 0.04 – 92 101 242 1206a 9.90 32.12 2.67 0.039 0.33 0.05 – 98 107 135 686b 1.30 36.77 2.94 0.044 0.39 92 78 230 1007a 7.95 27.13 2.53 0.036 0.28 0.12 – 104 182 84 237b 7.80 26.11 3.16 0.046 0.28 0.09 – 102 137 103 447c 0.51 34.50 2.69 0.034 0.35 0.06 – 102 150 98 318a 8.50 30.65 2.76 0.035 0.26 0.11 _ 88 155 107 238b 0.52 43.24 2.4 0.075 0.3 0.05 – 84 88 257 1009a 5.20 37.46 2.64 0.054 0.29 0.05 – 88 90 110 339b 0.57 46.54 2.27 0.096 0.28 0.05 – 71 61 203 7610a 0.62 34.39 2.85 0.048 0.37 0.05 – 82 85 196 6110b 0.68 44.27 2.28 0.056 0.27 0.03 – 73 78 155 4310c 0.40 55.62 2.1 0.106 0.22 0.06 – 61 54 170 3511 8.70 36.32 2.53 0.048 0.18 0.12 – 67 127 81 1612 20.10 7.49 3.79 0.03 0.27 0.18 – 114 450 94 2813a 14.80 3.75 4.9 0.037 0.32 0.16 – 162 760 110 2914b 9.40 21.68 3.47 0.027 0.28 0.15 – 105 440 107 2214c 0.82 19.07 3.99 0.034 0.38 0.08 – 107 145 114 3520a 0.68 37.68 2.61 0.055 0.29 0.04 – 82 88 137 3620b 1.36 42.34 2.88 0.039 0.31 0.07 – 76 75 74 2521 12.20 0.91 5.96 0.032 0.38 0.23 2.64 145 760 135 3222 15.60 3.41 6.29 0.036 0.31 0.32 1.89 136 1000 132 3523 19.30 1.93 5.27 0.033 0.32 0.34 2.56 132 1070 135 3524a 1.8 35.19 3.55 0.054 0.34 0.12 – 103 152 125 5824b 2.16 4.77 4.9 0.037 0.32 0.16 0.69 127 240 212 9125-2 20.5 0.03 3.89 0.018 0.32 0.19 4.32 142 580 160 3726 21.5 0.45 3.84 0.019 0.3 0.25 4.30 143 640 151 3227a 6.5 5.90 5.15 0.026 0.44 0.23 2.91 157 350 155 5127b 7.9 7.60 4.39 0.027 0.39 0.27 2.52 130 310 145 4827c 2.13 2.04 4.97 0.051 0.47 0.43 0.87 109 282 140 5628-1 13 0.03 4.65 0.023 0.36 0.05 4.64 144 600 172 7628-2 20.4 2.38 3.26 0.02 0.29 0.04 – 130 1200 252 7328-3 27 2.61 3.32 0.016 0.31 0.11 – 124 1100 210 5529 1.79 19.52 13.45 0.339 0.37 0.16 0.78 100 205 219 7630 26.2 2.72 3.03 0.0068 0.18 0.19 – 120 1000 315 125

?, overlying sediments

31-1 0.98 None 7.44 0.013 0.5 0.13 – 130 290 53 2831-2 1.01 None 6.91 0.014 0.47 0.14 0.24 145 325 64 35

Note: Contents of Corg, CaCO3, Fe, Mn, Ti, P, S, SiO2, and Al2O3 are given in %; others, in ppm. (–) Not determined.



Cu Pb Zn Mo Ag As Hg Sn Ga Ge B SiO2 Al2O3

37 22 147 3.8 0.08 7.4 0.10 3.7 11 1.5 54 40.51 13.8731 14 92 2.8

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GAVRILOV et al.

The conglomerate is overlain by a dark gray claybed (0.17 m) with inclusions of black angular phos-phate and glauconite grains of the gravel–sand size.The clay bed is overlain by another thin (0.02–0.03 m)bed of phosphoritic conglomerate (Fig. 4). As was men-tioned, the clayey rock and conglomerate bed is attrib-uted to the basal layer of the middle Volgian Substage.

Middle Volgian Sediments of the Panderi Zone

Structure of the shale-bearing sequence. Thesequence is characterized by distinct cyclic pattern withat least 17 cyclites. The complete elementary cyclite con-sists of three elements (Fig. 7, samples 7a–7c, 8a, 8b,21−24b, and others). The lowest element is representedby high-carbonaceous rocks (shales), while the middleand upper elements are composed of the dark and lightcarbonate clays, respectively (Figs. 8a–8f). However,rocks of the middle (darker) element are usually enrichedin Corg and depleted in CaCO3 as compared with theupper (lighter) element. Some cyclites are lacking theupper (lightest) layer. The lower shaly layer is absent inrare cases (Fig. 7, samples 10a–10c, 20a–20c). Boundarybetween the cyclites is sharp and distinct at the base ofthe shaly layer, while others transitions are more gradualand considered as internal boundaries.

The elementary cyclites in the shale-bearingsequence are grouped into three members.

The lower Member I (2.8 m thick) unites sevencyclites (Fig. 7, samples 4–10) with the thickness ofindividual cyclites up to 0.5 m or more in some cases.Table 1 shows that the Corg content in shales does notexceed 12% and is minimal for the shale-bearingsequence (samples 4–10). In contrast, the CaCO3 con-tent in these rocks (particularly, in the upper elementsof cyclites) is highest for the sequence and increasesupsection from 22 to 55%.

The middle Member II (1.7 m) comprises sixcyclites with an irregular distribution of shale beds(Figs. 2, 7). The lower part of the member includesthree cyclites, in which the middle element is reducedand the upper element is absent. The thickness ofcyclites does not exceed 10–15 cm. In contrast, theoverlying two cyclites are substantially thicker (0.7 mor more). Moreover, the lower cyclite can be subdividedinto two smaller cyclites. One of them includes only themiddle and upper elements. The second member withirregular cyclic patterns encloses several erosional sur-faces (condensation horizons). Relative to the lowermember, the Corg content in shales from the middle mem-ber is notably higher (almost 27%). The CaCO3 concen-tration remains relatively high and shows a slightdecrease in the uppermost part of the member.

Boundaries between the lower and middle membersare drawn at the roof of the relatively thick light graycalcareous clays.

The upper Member III (0.7 m) unites four cycliteseach 0.2 m thick. The uppermost cyclite is reducedlikely due to erosion of its upper part. All cyclites of this

member are lacking the upper element (light calcareousclay). The Corg content in shales from this member ismaximal (20–26%), while the CaCO3 concentration issignificantly lower as compared with the remaining partof the section.

No distinct cyclicity is observed in sandy siltstones(apparent thickness ~5 m), which overlie the shale-bearing sequence with a sharp boundary.

Lithology of the shale-bearing sequence. Despitethe apparent similarity of different parts of the shale-bearing sequence, its lithology shows notable varia-tions through the section.

In the lower part of Member I (~1.2 m), carbon-aceous shales (Fig. 8c) are dark gray with brownish tint.Their bedding surfaces contain rare small thin-walledbivalve shells and abundant small carbonate and phos-phate biogenic detritus.

Both shales and clayey sediments demonstrate thinhorizontal lamination related to the alternation of lam-inae enriched in OM or carbonate material (CM).Lamination is irregularly bioturbated (locally up to50 vol %). Bioturbation patterns indicate that the sed-iments were intermittently reworked by small burrow-ing organisms in the uppermost (a few centimetersthick and substantially water-saturated) layer. There-fore, sediments of this member are lacking morpho-logically distinct ichnofossils.

Clayey sediments from the upper part of Member Iare characterized by a notably higher carbonate admix-ture, lighter color, and more contrasting differencesbetween the lower and upper parts of cyclites. Faunalremains are more abundant and diverse. They are fre-quently represented by intact shells of bivalves, gastro-pods, and small (several centimeters across) ammonites.The fossils are accumulated along bedding surfaces ofshales. One can also see numerous small trails of Chon-drites and solitary subhorizontal ribbon-shaped mean-dering trails of Planolites (up to 10 cm long and ~0.5 cmacross). Both trails are filled with lighter material.

Calcareous clays in the upper parts of cyclites arealmost entirely homogenized by bioturbation (Fig. 8d).Therefore, they have a massive homogenous texture.

Dark gray clay interbeds separating the carbon-aceous shales and light clayey–calcareous sedimentswithin the elementary cyclites show transitional char-acteristics. They are usually enriched in Corg and CaCO3(Table 1). These interbeds, particularly their upperparts, are saturated with small fucoids (Chondrites,Planolites, and others are most abundant). They arewell distinguished owing to their sedimentary infill rep-resented by light material similar to that in the overly-ing sediments. Fucoids show distinct external bound-aries and preserve their fine morphological features.According to (Bromley, 1996; Mikulash and Dronov,2006), such ichnofossils are formed in plastic sedi-ments that can retain fine morphological features oftraces of life activity. They could be left either byorganisms that populated slightly lithified sediments



and were exhumed owing to the erosion of the overly-ing beds or by infauna that dwelt in the relatively deephorizons of sediments and crosscut lithological bound-aries (the lower bioturbation level).

Member I is crowned by a bed of large lenslike con-cretions (~1 m across, ~0.3 m thick) composed ofhomogenous microcrystalline calcite.

In Member II, carbonaceous shales are character-ized by thin platy uniform jointing and slight bioturba-tion. Traces of biogenic reworking are usually rare.They are usually distinguished in thin sections owing topeculiar small (a few millimeters long) subvertical fun-nel-shaped holes, cracks, and bends in the laminae. It isconceivable that accumulation of carbonaceous sedi-ments was accompanied by the episodic dwelling ofmobile (mostly small) organisms or meiofauna accord-ing to (Bromley, 1996). Bedding surfaces of shales inthis member contain accumulations of deformed shellsof bivalves, gastropods, and small ammonites, as wellas shelly detritus, rare small Chondrites fucoids andsingle ribbon-like meandering trails of crustacean (?).

Upper parts of cyclites in this member are composedof light gray homogenous bioturbated clayey–calcare-ous sediments that are lithologically similar to theircounterparts from the underlying member.

In contrast to Member I, the section of Member IIoften contains cyclites with significantly reduced thick-ness or without the upper clayey parts. Intermittent ero-sion of sediments is evident from the presence of inter-beds with numerous fragments of belemnite rostra andshelly detritus. It should be noted that belemniteremains are generally rare in the sequence and aremainly confined to these interbeds.

Dark clays in the middle element of the uppercyclite of Member II host a bed of lenslike calcareousconcretions (up to 0.3 m across and 0.15 m thick). Theconcretions of microcrystalline calcite are character-ized by fine lamination, which is almost destroyed inthe host sediments due to bioturbation. The concretionswere formed rather rapidly at the earliest diageneticstage: in contrast to host sediments, they are lackingnotable traces of distortion by the infaunal organisms.

Member III encloses shale varieties with the maxi-mal Corg and minimal CaCO3 contents. In terms ofproperties, the sediments correspond to the term “oilshale.” The associated clayey sediments are character-ized by the lowest CaCO3 contents in the entiresequence (Table 1, Figs. 8e, 8f).

Shales in this part of the sequence differ from theunderlying varieties even by their appearance—rocksconsisting of frequently alternating laminae (a few cen-timeters thick) of laminated and organic-rich oil shales(Corg up to 20% or more) and more massive beds ofclayey–silty material (Corg 6.5–13.0%). In the oil shalelaminae, clay particles, OM aggregates, and other com-ponents are oriented parallel to bedding surfaces andsigns of bioturbation are absent. In contrast, clayey–silty laminae demonstrate numerous small biogenic

distortions of bedding as funnel-shaped holes filledwith silty material, which are readily distinguishable inthin sections.

The uppermost part of the shaly sequence encloses abed of dark brown uniform thin foliated oil shales(0.10−0.12 m thick) with the Corg content up to 26%. Theshales are readily separated into thin (a few millimeters)laminae with bedding surfaces lacking any faunalremains. Thin sections show that OM in the rock is con-centrated in thin frequent laminae and is represented bybrownish yellow amorphous colloalginite (Fig. 8e).

The upper parts of reduced cyclites of Member IIIare composed of greenish gray low-carbonate massiveclays with admixture of silty material (up to 10%).These sediments contain small tubular fucoids (up to0.5 mm in diameter) frequently filled with pyrite anduniform dissemination of brown microspherulites com-posed of iron hydroxides (oxidized pyrite framboids).One can also see rare pyrite aggregates (up to 2 cmacross) with shapes indicating their formation withinmolluscan shells.

Fossils in carbonaceous sediments of Member IIIare represented by single finds of ammonite and largebivalve (inocerams and buchias) conchs. The bivalvesare dissolved to a variable extent and sometimes onlytheir impressions are preserved. Clays contain a smallamount of faunal remains and rare fragments of largecarbonate shells.

The upper part of the finely foliated shales crowningthe shale-bearing sequence is cemented by phosphatematerial and traced within the outcrop as a thin (up to5 cm) continuous light brown bed. Thin sections demon-strate a distinct sedimentary texture of the carbonaceousmaterial cemented by a colloform phosphate matter(francolite and hydroxylapatite). The surface of the phos-phate bed is uneven hummocky, and, probably, biotur-bated. Depressions are filled with an incoherent greenishgray sandy–silty material, which is compositionally sim-ilar to the sediments overlying with an erosion.

As follows from this description, the main rock-forming components of the shale-bearing sequence arerepresented by terrigenous (carbonate) and biogenic(organic) material with the subordinate admixture ofauthigenic sulfide, silicate, and phosphate aggregates.

The terrigenous material consists of clay mineralswith silty admixture (quartz with the subordinate feld-spar, mica, and glauconite), which constitutes an insig-nificant share of sediments and rarely exceeds 10%.

We studied a series of samples taken from differentparts of the section to study the mineral composition ofclays. The X-ray diffraction analysis revealed that frac-tion

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GAVRILOV et al.

tion. The pattern is slightly different in coeval sedimentsof the middle Volga region. For example, the upper partof the shale-bearing sequence in the Gorodishche sectiondemonstrates a drastic decrease in the content of kaolin-ite (up to the point of its complete disappearance) accom-panied by the increase in the smectite content and theappearance of clinoptilolite as one of the rock-formingcomponents (Shimkyavichus, 1986; Ruffell et al., 2002;Riboulleau et al., 2001; original data).

Like in the underlying lower Kimmeridgian strata, car-bonate material in the middle Volgian sediments is repre-sented by the finely dispersed calcite (based on micro-scopic and X-ray spectroscopic studies). The biogenic CMconsists of irregularly distributed calcareous nannoplank-ton skeletons and small fragments of molluscan shells.The CaCO3 content in clays of Member I increasesupward the section and reaches 55% in its uppermost part(Table 1, Fig. 7). The carbonaceous shales also show ahigh content of the calcareous admixture.

Member II is characterized by the permanently highcontent of CaCO3 in the clayey sediments and its signif-icant variation in different cycles within the carbon-aceous shales. In Member III, CM is insignificant in thealternating carbonaceous shales (with variable Corg con-centrations) and clays. Hence, this sedimentation stagewas marked by significant ecological changes in thepaleobasin and the consequent sharp reduction of theproductivity of calcareous organisms.

Comparison of the distribution of CaCO3 and Corg inthe shale-bearing sequence revealed a negative correla-tion between these components: increase in the contentof one component is accompanied by decrease in theconcentration of another component (Table 1, Fig. 7),although this correlation is expressed in variable extent.

Organic matter occurs in different forms in carbon-aceous shales and clays.

Carbonaceous shales are dominated by the amorphousOM (up to 90–99%) corresponding to colloalginiteaccording to terminology in (Ginsburg, 1991). Colloalg-inite has different tints of yellow, orange and brown colors.It is concentrated in the clayey matrix as thin laminae orflattened lenses (from 0.0n to n mm long) arranged paral-lel to bedding surfaces (Figs. 8c, 8e). The Corg content inthe shales shows a positive correlation with the contentand dimension of colloalginite aggregates.

One can observe in thin sections under great magni-fications that the clayey matrix in carbonaceous shalescontains a uniformly dissemination of the amorphouscolloalginite-type OM. According to (Ginsburg, 1991),such aggregates of the clayey material (dominant com-ponent) and OM belong to “sorbomixtinite.” The originof sorbomixtinite in shales is evidently similar to that ofcolloalginite except for a slight compositional differ-ence. Increase in the share of sorbomixtinite (relative tocolloalginite) is particularly notable in the relativelylow-carbonaceous (calcareous and clayey) shale variet-ies. For example, the relatively Corg-rich calcareousshales in members I and, partly, II are characterized by

small colloalginite aggregates in the calcareous–clayeymatrix. In such shales, the content of sorbomixtinite iscomparable with that of colloalginite.

Shale varieties with the highest Corg contents frommembers II and, particularly, III include numerouslarge colloalginite aggregates. In contrast, they areinsignificant in the OM of sorbomixtinite. Such OM isprimarily confined to the foliated oil shales in theuppermost part of the shale-bearing sequence.

Carbonaceous shales always contain an admixture(5–10%) of small detritus of terrestrial OM representedby black to dark brown opaque angular fragments ofplant tissues (inertinite). The size and abundance ofplant remains increase notably in the upper part of theshale-bearing sequence and reach maximum values inshale varieties with the maximal silt contents.

High-carbonaceous shales contain an insignificantadmixture (up to 1%) of sporomorph remains repre-sented by cysts of dinoflagellates and spores and pollenof terrestrial plants.

In clayey–calcareous sediments, OM occurs usuallyin finely dispersed form (Fig. 8). The OM contentshows negative correlation with CM. It is difficult todetermine the origin (terrestrial or marine) of finely dis-persed OM in these sediments. Colloalginite and sorbo-mixtinite occur as an admixture mainly in relicts of theprimarily carbonaceous laminae destroyed by bioturba-tion. An admixture of finely dispersed plant detritus ispresent in clayey sediments. However, since other plantremains were not preserved or accumulated, preciselythis OM becomes dominant and governs pyrolyticparameters to a significant extent.

Pyrolytic parameters of OM. Table 2 presents theresults of pyrolytic studies of OM in samples of themost representative carbonaceous shales and clayeysediments from different parts of the section.

One can see that OM in sediments is generally char-acterized by low catagenetic maturity, which is evidentfrom temperature values corresponding to the maximalextraction of hydrocarbon compounds during the heatingof samples in an inert atmosphere (ímax < 435°ë) (Tissotand Welte, 1978; Lopatin and Emets, 1987).

In some samples of clayey sediments with low Corgcontents (upper half of the section, samples 20b and 29)ímax values are higher (up to 444 and 458°C, respec-tively), which may be explained by the occurrence ofinertinite, i.e., detritus of substantially transformed OMredeposited from older sediments. The presence ofinertinite increases usually ímax of OM in sedimentaryrocks (Lopatin and Emets, 1987).

The OM generation potential, which is determined bythe (S1 + S2) value, corresponds to the total quantity ofHC compounds released from OM during the heating ofsamples up to S and 300–600°ë (S2) S1 characterizes thebitumen content in the rock, while S2 characterizes thepetroleum potential of kerogen at the moment of analysis(Tissot and Welte, 1978; Lopatin and Emets, 1987).



Tab

le 2

. C

hara

cter

istic

of

OM

fro

m V

olgi

an s

edim

ents

of

the

Ivki

no s

ectio

n ba

sed

on p

etro

grap

hic

and

pyro

lytic

dat

a

Mem

-be

rSa

m-

ple

Roc

kO

M d

omin

ant t

ype

Tm

ax, °

CS 1

,m

g H

C/g

roc

kS 2

,m

g H

C/g

roc

kT

OC

,%

of

rock

HI,

mg

HC

/g T

OC

I

4aC

arbo

nace

ous

shal

e, c

al-

care

ous

Col

loal

gini

te, s

orbo

mix

tinite

409

0.99

53.7

811

.74

458

4bD

ark

calc

areo

us–c

laye

y ro

ckFi

nely

dis

pers

ed O

M, s

mal

l ph

ytod

etri

tus

425

0.04

2.88

2.45

117

7aC

arbo

nace

ous

shal

e, c

al-

care

ous

Col

loal

gini

te, s

orbo

mix

tinite

409

1.77

69.1

314

.04

402

7cL

ight

cal

care

ous–

clay

ey

rock

Fine

ly d

ispe

rsed

OM

424

0.03

1.76

1.83

96

9aD

ark

calc

areo

us–c

laye

y ro

ckSo

rbom

ixtin

ite, c

ollo

algi

nite

, sm

all p

hyto

detr

itus

419

0.08

8.63

3.68

234

10c

Lig

ht c

alca

reou

s–cl

ayey

ro

ckFi

nely

dis

pers

ed41

90.

010.

080.

4617

II

12a

Hig

h-ca

rbon

aceo

us s

hale

Col

loal

gini

te, s

orbo

mix

tinite

, sp

orom

orph

com

plex

es39

93.

5518

4.6

26.7

469

0

14c

Lig

ht c

alca

reou

s–cl

ayey

ro

ckFi

nely

dis

pers

ed O

M, s

mal

l ph

ytod

etri

tus

426

0.04

2.82

2.82

100

20b

Dar

k ca

lcar

eous

–cla

yey

rock

Fine

ly d

ispe

rsed

OM

, sm

all

phyt

odet

ritu

s44

40.

041.

671.

9187

21C

arbo

nace

ous

shal

e,

clay

eyC

ollo

algi

nite

, sor

bom

ixtin

ite41

44.

2674

.29

16.5

450

22H

igh-

carb

onac

eous

sha

leC

ollo

algi

nite

412

2.35

110.

220

.553

7

III

27a

Car

bona

ceou

s sh

ale,

silt

ySo

rbom

ixtin

ite, s

mal

l phy

tode

-tr

itus,

col

loal

gini

te40

61.

3232

.86

10.8

730

2

28-1

Hig

h-ca

rbon

aceo

us s

hale

Col

loal

gini

te41

03.

2714

6.1

26.1

755

8

29D

ark

calc

areo

us–c

laye

y ro

ckSm

all p

hyto

detr

itus

458

0.07

0.4

1.78

22

30-1

Hig

h-ca

rbon

aceo

us s

hale

Col

loal

gini

te40

75.

0915

9.6

24.0

766

3

372


GAVRILOV et al.

The generation potential of Volgian carbonaceousshales in the study area is high (32.86–159.6 HC/g),while the potential of clayey–calcareous rocks is low(0.40–2.82 HC/g).

The total organic carbon (TOC) parameter corre-sponds to the sum of pyrolyzed and residual organiccarbon (%). The TOC and Corg values determined bydifferent methods in samples taken from the same lay-ers are similar for the OM-depleted clays and carbon-aceous shales with the highest Corg concentrations. Atthe same time, the TOC and Corg values are sometimesdifferent in shales with the relatively low OM contents,which may be explained by different potentialities ofanalytical methods and extremely irregular distributionof colloalginite in layers of these rocks.

The value of hydrogen index (HI = S2/TOC) obtainedfor kerogen from carbonaceous shales varies from 402 to690 mg HC/g TOC, which indicates its largely marine ori-gin (Tissot and Welte, 1981). The highest HI values (537–690) are characteristic of carbonaceous shales with themaximal Corg concentrations in the upper half of thesequence (members II and III) enriched in colloalginiteand subjected to minimal bioturbation. The maximal HIvalues are established for the high-carbonaceous foliatedshale interbed from Member III (HI = 653, sample 30),which lacks fossils and bioturbation signs, and for sample12 from Member II (HI = 690), where OM contains sporo-morph components, in addition to the dominant colloalg-inite and the subordinate sorbomixtinite. The notablylower HI values are characteristic of calcareous and vari-ably bioturbated clayey–silty shales, in which sorbomix-tinite plays a notable role along with colloalginite.

Kerogen in Volgian shales belongs to types I and IIoriginating largely from marine microplankton. Thehydrocarbon part of primary OM is well preserveddespite its oxidizing biochemical destruction.

Clayey–calcareous sediments are characterized bysubstantially lower HI values (17–302). Its minimal val-ues (17–100) are typical for the colloalginite-free rocks(Corg



Fig

. 9. A

l-no

rmal

ized

dis

trib

utio

n of

che

mic

al e

lem

ents

in

the

Ivki

no s

ectio

n. V

ertic

al d

ashe

d lin

e sh

ows

el/A

l va

lues

cal

cula

ted

as r

atio

bet

wee

n av

erag

e co

nten

ts o

f a

spec

ific

elem

ent a

nd A

l (8.

0%)

in c

laye

y ro

cks,

aft

er (

Tur

ekia

n an

d W

edep

ohl,

1961

; Wed

epoh

l, 19

91).

For

lith

olog

ical

lege

nd, s

ee F

ig. 2

.

III II I

02

13

45 0

100

200

010

20

00.

020.

0100.

040.

08 0

10 20

2030

100

20400

1020

30 010

020

0

01

23

45

6

00.

020.

0401.

02.

0

00.

020.

04

I

Ti/A

lFe

/Al

Mn/

Al

Pb/A

lZ

n/A

lC

o/A

lN

i/Al

Cr/

Al

Cu/

Al

Ag/

Al

P/A

lM

o/A

lV

/Al

Cor

g/A

l

?31 30 29 28 27 25 24 23 20 14 13 12 11 10 9 8 7 6 5 4 3 2 1

2 1 1 3 c a 2 b a b ca b a da c b a b a b a c a b a b a c a ba

b a b a

J3 Kimmeridgian Stagelower substage

Cymodoce Zone

J3 Volgian Stagemiddle substage

Panderi Zone

374


GAVRILOV et al.

imentation stage, but no reliable evidence for the exist-ence of such provenances is available.

Increase in contents of these elements in the upper-most lower Kimmeridgian sediments is most likelyrelated to their redistribution during diagenesis. Termi-nation of the accumulation of Kimmeridgian sedimentsdue to regression could promote the exhumation ofrecently accumulated sediments, in which diagenetictransformations were in progress. Development ofreducing conditions in bottom sediments is evidentfrom the abundance of small pyrite aggregates. Due todrying of bottom sediments, interstitial waters with dis-solved compounds of different elements began tomigrate intensely toward the surface and evaporate,resulting in the precipitation of elements.

High concentrations of approximately the same ele-ments are also observed in a thin (no more than 0.2 m)bed of calcareous clays at the base of the shale-bearingsequence. They were likely accumulated at the initialstage of the middle Volgian transgression due to the par-tial erosion of underlying sediments with high concen-trations of these elements; i.e., enrichment with somechemical elements was most likely an inherited process.

In the shale-bearing sequence, the distribution ofelements was controlled by factors that were active atboth sedimentation and diagenesis stages accompaniedby the redistribution of elements within sediments. It ismost probable, however, that these factors acted simul-taneously and their contribution to the final distributionpattern of elements was different.

Some elements correlate distinctly with OM, highconcentrations of which in the carbonaceous shalesappeared at the sedimentation stage. Correlationbetween OM and V is traced well from the parallelincrease in contents of these components upward thesection and within separate cycles: high V contents arenoted in the carbonaceous shales at the base of cycles,while relatively low V concentrations are characteristicof clayey sediments. As follows from Table 1 and Fig. 9,concentrations of V in shales from the upper part of thesection are very high (1000–1200 ppm), which exceeds8–10 times its background values. Mn shows a similarbehavior: its concentration in shales from some parts ofmembers II and III is 1.0–1.5 orders of magnitudehigher than the background value. Similar or closebehavior is also noted for P, As, and, locally, Cu and Cr.

The main process responsible for high concentra-tions of elements in the shales was likely to be theirsorption on the surface of OM particles at the sedimen-tation stage. Diffusion of elements from bottom watersinto carbonaceous sediments could also contribute tothe concentration of elements.

The distribution of Ni and geochemically relatedelements across the sequence is characterized by differ-ent patterns. Table 1 and Figure 9 show that the Ni con-tent in clayey sediments in sedimentary cycles of mem-bers I and II is locally higher than in the underlying car-bonaceous shales. However, high Ni concentrations are

observed in carbonaceous shales rather than clays inrocks of Member III.

Such distribution pattern of Ni could result from itsdiagenetic redistribution in sediments. Lithologicallycontrasting cyclites with high and extremely low OMcontents in carbonaceous and clayey–calcareous sedi-ments, respectively, were accumulated in substantiallydifferent redox environments: sharply reducing condi-tions in the carbonaceous sediments and slightly reduc-ing (sometimes, probably oxidizing) environment inthe clayey-calcareous sediments. Correspondingly, Niin carbonaceous sediments was transformed into labileforms, which diffused into the adjacent clayey–calcar-eous sediments and precipitated under different Eh con-ditions. Moreover, Ni could migrate into both overlyingand underlying strata. At the sedimentation stage, car-bonaceous sediments were evidently characterized byhigher Ni contents, and the present-day distribution ofNi is related to its redistribution between the adjacentlayers during diagenesis. This inference is supported bythe Ni distribution in sediments of Member III charac-terized by the maximal OM concentration. At the sametime, notable Corg concentrations are also recorded in theclayey sediments alternating with the shales. Hence, bothtypes of sediments were characterized by well-devel-oped reducing conditions during diagenesis. Corre-spondingly, no significant Eh gradient existed betweenthese sediments. Hence, the environment did not fosterintense redistribution of elements and their diffusionfrom the carbonaceous shales into the clayey sediments.

Based on the behavior in geochemically contrastinglayers, Co, Zn, Mn, and, to a lesser extent, Pb belong tothe Ni group. The low-contrast distribution of elementsacross the sequence, as well as slightly increased con-tents of B, Ga, and Ge in the clayey sediments, are mostlikely related to their association with the clayey matter.

Distribution of Fe lacks any correlation with partic-ular parts of sedimentary cyclites. Its general distribu-tion across the section demonstrates a decreasing trendin sediments of Member I and slight increasing trend inmembers II and III. This statement is also valid for Crand Sn.

It should be noted that behavior of chemical ele-ments in the Kimmeridgian and Volgian sediments wasalso certainly affected by bioturbation that mixed thesediments, changed their permeability, and influenceddiagenetic processes. However, issue of the influence ofbioturbation on the geochemistry of sediments requiresspecial consideration.

The shale formation stage was followed by the accu-mulation of sandy–silty sediments that differ substan-tially from the underlying strata in terms of geochemis-try: moderate concentrations of OM (Corg ~1%) andclarke-level contents of most elements. High Fe con-tents (6.9–7.4%) in these sediments are rather typical ofbasal layers of the sequence accumulated in the courseof transgression. Since iron hydroxides are good sor-bents, it seems quite natural that V concentrations in



these sediments are also high: geochemical behavior ofthese elements during sedimentation is often similar(Kholodov, 1973 and others). A significant share of Fein these sediments was incorporated into glauconiteduring diagenetic transformations.

DISCUSSION

The study of the Ivkino section reveals that its shale-bearing sequence is more complete than other coevalsuccessions in the Middle Volga region: rarer traces oferosion, condensation, and softgrounds. At the sametime, the Ivkino section lacks the sandstone horizonand “phosphorite plate” developed at the top of the Vol-gian sequence in the Gorodishche section. This can berelated to the following reasons: (1) subsequent erosionof the upper part of the section; (2) greater distancefrom the bank; and (3) existence of geomorphologicalbarriers for the progradation of sandy fractions. TheMiddle Volga region could also accumulate the painportion of sandy material transported by rivers from thewest. The greater completeness of the shale-bearingsequence could be related to its accumulation either indeeper settings (there is no direct evidence for thisassumption except the lesser content of littoral benthicfauna in sediments) or in hydrodynamically calm set-tings of the paleobasin probably surrounded by mor-phological barriers.

Analysis of fossils in lithologically different sedi-ments of the Ivkino section shows that ecology in thepaleobasin was extremely unstable during accumula-tion of the shale-bearing sequence. Clayey–calcareoussediments with low OM contents were accumulated inenvironments favorable for different organisms typicalof normally aerated basins. In contrast, their habitatconditions was sharply deteriorated during the deposi-tion of organic-rich sediments, resulting in the abun-dance of juvenile molluscan shells in carbonaceousshales and the drastic reduction of nannoplankton andother benthic and planktonic remains.

Significant outburst in bioproductivity of organic-walled plankton (planktonic bacteria, various species ofmicroalgae, and others) could foster the development ofdifferent factors unfavorable for the biota. For example,accumulation of high-carbonaceous sediments couldstimulate the development of anoxic conditions in thewater column. This is evident from finds in other sec-tions (Bushnev, 2007) of isorenierathene derivatives—biomarkers of the photosynthesizing alga of the genusChlorobium that dwelt in the photic zone at the bound-ary with the H2S-contaminated watermass (Damste etal., 1993, 1995; Repeta and Simpson, 1991; Repeta etal., 1989; and others). At the same time, anoxic condi-tions in the shallow basin could hardly be stable andshould include a relatively thick bottom water layer.Another negative factor could be represented byextremely high intensity of organic-walled planktonbloom, which consumed the main portion of biophileelements and restricted the development of other micr-

biotic forms, carbonate-producing organisms included.As was shown above, sharp increase in the OM contentwas accompanied by a significant decrease in the con-tent of the carbonate component in sediments (Fig. 7).Finally, we should not also rule out an additional aspectof this bloom (red tides). Observations in present-dayseas have revealed that the red tides can often be toxicand they can affect negatively most macro- and microbi-otic organisms. Thus, intense bloom of organic-walledplankton could provoke sharp deterioration of paleoeco-logical conditions in both bottom and surface waters ofthe Volgian sea (Gavrilov and Kopaevich, 1996). More-over, ecological conditions could fluctuate during theaccumulation of carbonaceous sediments, which is evi-dent from finds of benthic organisms (although distinctlysuppressed) at bedding surfaces.

It should also be kept in mind that abundance ofboth macro- and microfossils decreases upward the sec-tion up to their complete disappearance, which is evi-dent from the distribution of ammonites and nannofos-sils. We should note a brief episode of ecological set-ting normalization at the level of Bed 24, as suggestedby the episodic appearance of these fossils.

What is the factor responsible for significant fluctu-ations in paleoecological settings and their generaldeterioration trend toward the late accumulation stageof Volgian sediments? The answer to this question maybe obtained after considering dynamics of the shale-bearing sequence formation.

Like in sections of the Middle Volga region, thestructure of the Upper Jurassic sequence in the Ivkinosection implies that the general (mainly regressive)development trend of the late Kimmeridgian basin wascomplicated by regression–transgression cycles of atleast three orders. Among them, two regression epi-sodes (prior to and after the accumulation of the shale-bearing sequence) are most distinct. Both these epi-sodes are reflected in the section as sharp changes insedimentation settings, which produced lithologicallyand geochemically different sediments.

Two other regression episodes occurred during theformation of the shale-bearing sequence. They dividedthe single stage of its deposition into three substagescorresponding to accumulation of members I, II, andIII. Member I is crowned by a bed of carbonate concre-tions. Their appearance is evidently related to a rela-tively slight erosion of the condensed bed with the highcontent of reworked biogenic CM, which was redistrib-uted during diagenesis and transformed into concre-tions. Such beds frequently mark the roofs of sedimen-tary cycles at the end of regressive episodes.

The smallest elements of the section related to sea-level fluctuations are represented by sedimentarycyclites mentioned above that are identified in bothKimmeridgian and Volgian sediments with specific fea-tures at different stratigraphic levels. They are mostlikely governed by Milankovitch cycles. Sharp contactsof shales with the underlying sediments and, in con-

376


GAVRILOV et al.

trast, their gradual transitions to the overlying elementsof cyclites suggest that their formation began withtransgressions and terminated with sealevel fall andinsignificant erosion of accumulated sediments (at theroof of cyclites). The amplitude of sealevel fluctuationsprobably did not exceed a few meters.

Based on finds of chlorophyll in the OM of shales,Strakhov (1934, 1962) explained episodes of intenseaccumulations of organic-rich sediments by the forma-tion of “underwater meadows” in the basin with thedevelopment of phytobenthos. Cyclicity of these eventswas attributed to presumably tectonic movements of sea-floor and its intermittent exhumation to the photic zone.

In our opinion, the formation mechanism of theshale-bearing sequence was slightly different. First, weshould elucidate why the initial stage of cyclite forma-tion was marked by the accumulation of carbonaceoussediments that gave way to the clayey–calcareous sedi-ments. The pulsating character of carbonaceous sedi-ment accumulation can best be explained by the modelpreviously proposed for the formation of organic-richsediments during rapid and relatively brief transgres-sions (Gavrilov, 1994; Gavrilov et al., 1997, 2002,2003, 2004). Its essence consists in the following.Areas released from seawater during the humid climateat the regression stage were marked by the rapid forma-tion of soils and lacustrine–boggy landscapes that werefavorable for the accumulation of both solid and dis-solved forms of OM and compounds of biophile ele-ments, such as P, N, Fe, and others. When the regres-sion gave way to the next transgressive stage, seawatersinteracted with the landscapes and released biophileelements, resulting in the outburst in bioproductivity ofplanktonic bacteria, organic-walled dinoflagellates, andother low-organized biotic forms (correspondingly, inthe accumulation of organic-rich sediments). At thesame time, solid plant OM released from the landscapesalso participated in the formation of carbonaceous sedi-ments. This assumption is supported by the pyrolyticanalysis of OM indicating its mixed composition: domi-nant aqueous and subordinate continental materials. Atthe initial transgression stages, biophile elements weremainly provided by terrestrial coastal landscapes. Begin-ning of the accumulation of organic-rich sediments trig-gered the recycling of biophile elements in the sedimen-tation basin itself, i.e., return of some elements (prima-rily, phosphorus) from the reduced sediments. Recyclingcould maintain high bioproductivity in the basin evenunder conditions of decelerated transgression character-ized by decrease in the influx of biophile elements. Infer-ence on recycling is confirmed by the data on the distri-bution of phosphorus in the shaly sequence. As was men-tioned above, we can see a positive correlation betweenCorg and P. However, if the entire phosphorus transportedwith OM had been buried in the sediments, the present-day phosphorus concentrations in shales should be sig-nificantly higher. When the transgression ceased and therecycling attenuated, the formation of carbonaceousshales was replaced by the accumulation of background

clayey–calcareous sediments. Fluctuation of bioproduc-tivity could also be affected by an additional factor. Thecyclic sealevel fall in the basin and its shoaling at thisstage could provide conditions for greater heating of thewater column phytoplankton development.

Thus, different microplankton groups dominated atdifferent formation stages of sedimentary cycles:organic-walled organisms dominated at the initialstage, whereas the carbonate-producing organisms(nannoplankton, foraminifers, and others) prevailed atthe later stages. Multiple changes in biogenic sedimen-tation produced the cyclic shale-bearing sequence.

It should be noted that accumulation of upper ele-ments of cyclites (clayey–calcareous sediments) wasaccompanied by the influx of OM from differentsources. However, OM buried together with the calcar-eous microplankton was unstable and almost com-pletely decomposed in the course of diagenetic pro-cesses. In contrast, the relatively small amount of ter-restrial OM transported to the basin at this stage wasalready oxidized to a significant extent. Hence, the OMwas inert and buried in sediments without substantiallosses. Therefore, hydrogen index in clayey–calcareoussediments was very low (Table 2).

Appearance of the Volgian high-carbonaceousrocks, which differ substantially in terms of lithologi-cal–geochemical parameters from other Jurassic sedi-ments of the East European Platform, poses the ques-tion concerning factors responsible for the sharpincrease in OM accumulation rate at the particular stageof basin development. The formation mechanism ofsedimentary cycles (OM concentration in the lowerparts) also functioned before the accumulation ofshales: cyclicity is observable in the entire Upper Juras-sic section of the East European Platform. During thehigh stable stand of sea level, however, influence of itslow-amplitude fluctuations influenced on the influx ofbiophile elements from coastal landscapes was insig-nificant, resulting in a weak compositional differencebetween the lower and upper elements of the cycle.

The paleogeographic situation changed notablyafter the hiatus related to the sealevel fall in the terminalKimmeridgian time. Although the Volgian transgres-sion was rapid and powerful, amplitude of the sealevelrise was substantially lower than that in the Kimmerid-gian time. This feature promoted the following pro-cesses. First, a wide coastal zone with the developmentof soils and lacustrine–boggy landscapes was formed.Second, intrabasin uplifts turned into islands (archipel-agoes) with landscapes similar to those on the mainland were exposed. The relief in these landscapes wasflat, because it was previously smoothed away bymarine erosion and sedimentation. Correspondingly,even low-amplitude sealevel fluctuations (in particular,rises) resulted in the flooding of spacious lowlands,intense influx of biophile elements, rapid growth of bio-productivity of the organic-walled microbiota, andaccumulation of high-carbonaceous sediments.



Such processes were in progress during the Panderiphase of the middle Volgian Age. The accumulationperiod of shale-bearing sequence was marked by twohigh sealevel falls, in addition to its low-amplitude fluc-tuations responsible for the formation of elementarycycles. The subsequent sealevel fluctuations were char-acterized by the attenuating patterns: each subsequentsealevel rise was always lower than the previous one.Consequently, sediments of each of the three successivemembers were accumulated in progressively shallowerenvironments. It should also be noted that the closespacing of carbonaceous beds at the base of Member II(as well as in the reduced Member III) is related to theiraccumulation in relatively shallow-water settings in thecourse of transgression. The transgression peak andsealevel stabilization were followed by the accumula-tion of thicker cyclites with well-developed lower, mid-dle, and upper elements.

Such a sedimentation scenario in the Late Jurassicalso explains some geochemical features of the shale-bearing sequence. In particular, the upsection increasein the OM content becomes clear, because bioproduc-tivity of the basin was maximal near sources of biophileelements (land areas) and the basin was graduallyshoaling. Since land provided the majority of chemicalelements, their maximal concentrations are also con-fined to the coastal zone of maximal bioproductivity.The coastal zone served as a peculiar biofilter thatentrapped a wide range of elements. Since the elementswere delivered from both land and newly formedislands, the basin hosted many zones of elevated bio-productivity at this stage. Around relatively high upliftsturned into islands prior to other areas, carbonaceoussediments could also begin to accumulate earlier thanin other areas of the basin where islands appeared later.It is rather difficult to correlate the sections because ofthe patchy distribution of facies related to the intricaterelationships described above. Thu

sedimentology, geochemistry, and biota of volgian ...volgian stage (oil shales) are developed over a...

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