High resolution stratigraphy of the Lower Jurassic and ...of Western Siberia (Fig. 2) (Nikitenko 1992, 1994, 2008; Nikitenko et al. 2000; Shurygin et al. 2000). Several marker-hrozons

267

High resolution stratigraphy of the Lower Jurassic and Aalenian of Arctic regions as the basis for detailed palaeobiogeographic reconstructions

NORWEGIAN JOURNAL OF GEOLOGY Stratigraphy of the Lower Jurassic and Aalenian stratigraphy of Arctic regions

Boris Nikitenko, Boris Shurygin & Michael Mickey

Nikitenko, B., Shurygin, B. & Mickey, M.B.: High resolution stratigraphy of the Lower Jurassic and Aalenian of Arctic regions as the basis for detailed palaeobiogeographic reconstructions. Norwegian Journal of Geology, Vol. 88, pp. 267-278. Trondheim 2008. ISSN 029-196X..

Lower Jurassic and Aalenian deposits are widespread in the Arctic Basin and in northwestern Europe. The succession of Lower Jurassic lithologic units is very similar throughout the Arctic regions. The Lower Toarcian is represented by dark-gray, organic-rich mudstones and often bituminous mudstones developed uniformly over the whole area. All deposits contain rich assemblages of foraminifers, ostracods and palynomorphs, as well as less abundant ammonites and bivalves, providing a mean for correlation of the lithologic units. The established Jurassic zonal subdivisions have been studied in all areas of the Arctic Basin using original material as well as published data. Thus, the Jurassic zonal subdivision of northern Siberia, based on ammonites, bivalves, foraminifers and ostracods, may be considered as the Boreal Zonal Standard. The analysis of biotic and abiotic events as well as the geographic distribution of microbenthos result in the definition of biogeographic realms and provinces based on foraminifer and ostracode data for the Early Jurassic and Aalenian. The differentiation of microbenthos associations is based on the multivariate analysis of characteristic taxa. The configurations of the boundaries between provinces and realms, based on the geographic distribution of the different groups of benthos, changed during geological time.

Boris Nikitenko & Boris Shurygin, Institute of Petroleum Geology and Geophysics SB of RAS, Koptyug av. 3, Novosibirsk, 630090, Russia. E-mail: [email protected]; [email protected].; Michael Mickey, Micropaleo Consultants, 329 Chapalita Drive, Encinitas, CA 92024, USA. E-mail: [email protected]

IntroductionLower Jurassic and Aalenian deposits are widespread in both the Arctic Basin and in northwestern Europe (Fig. 1). Numerous Jurassic sections as well as ammonites, bivalves, foraminifers and ostracodes from the Western and Eastern Siberia, NE Russia, Barents Sea and northern Alaska have been investigated by the authors (Nikitenko 1994, 2008; Shurygin et al. 2000; Nikitenko & Mickey 2004). For comparative analysis, published data on lithostratigraphy, biostratigraphy and palaeontology of Lower Jurassic and Aalenian sections from the northwestern portion of western Europe, the North Sea, the Barents Sea and Arctic Canada were included in this study (Norling 1972; Bate & Coleman 1975; Souaya 1976; Løfaldli & Nagy 1980; Copestake & Johnson 1984, 1989; Wall 1983; Riegraf 1985; Ainsworth 1986, 1987; Gramberg 1988; Malz & Nagy 1989; Basov et al. 1989; Nagy & Johansen 1991; Dibner 1998; Leith et al. 1992; Embry 1993; Arias 2000).

A series of remarkable events characterized the Early Jurassic–Aalenian period of the Arctic Basin. Sedimentation in Arctic seas was to a considerable extent under control of transgressive-regressive events largely interrelated with eustatic sea-level changes. Accumulation of sandy and silty material prevailed

in many regions during the Hettangian–initial Late Pliensbachian, when the Jurassic Arctic basin was under formation, whereas deposition of mud and clay was in progress only far away from the basin shoreline. A vast transgression in the mid-Pliensbachian resulted in deposition of predominantly clayey deposits widespread in the study region. The Lower Toarcian dark-gray clays enriched in organic matter (bituminous in places) are recognizable practically everywhere within the Arctic Basin area (Nikitenko & Mickey 2004). The structural-tectonic reorganization of the late Aalenian changed the sedimentation regime at most sites in Siberia. In Arctic Alaska a considerable break in sedimentation is recorded during the terminal Late Aalenian.

StratigraphyThe Lower Jurassic and the Aalenian deposits in northeastern Siberia and northeastern Russia are represented by marine and shallow-water marine clays, silts and sandstones, sometimes with interlayers of continental and sub-continental sandstones at the base, and are characterized by ammonites and bivalves, and also by rich and diverse foraminiferal and ostracod assemblages (Shurygin et al. 2000) (Fig. 2). Nevertheless, ammonite occurrences in the Lower and Middle Jurassic of the Arctic region are rather rare, so there is often a

268

disjunction of biostratigraphic units in sections, reducing their correlation potential. In these cases, the ammonite occurrences serve to date the marker levels allowing the correlation of biostratigraphic units based on different fossil groups against Standard Stratigraphic Scale (Hardenbol et al. 1998).

At the same time, benthic fossils (bivalves, foraminifers and ostracods) are the most effective instruments for subdivision and correlation of the Jurassic (Fig. 2). The occurrences of these fossils are well known in the Jurassic both from typical marine sediments and marine interlayers in “transitional” (from marine to continental) deposits. Biostratigraphic studies allow definition of the most complete successions of zones based on ammonites, bivalves, foraminifers and ostracods from Lower Jurassic to Aalenian deposits for northern regions of Siberia and northeastern Russia (Fig. 2).

In northwestern Siberia, Lower and Middle Jurassic sediments have only been described from boreholes. Conventional core samples from these subsurface strata contain relatively rich microfossil assemblages and rare bivalves. The deposits were formed within “transitional” environments. There are numerous continental interlayers in the shallow marine deposits. As a result, stenohaline organisms such as ammonites are very rare. Autonomous Jurassic zonations based on bivalves, foraminifers and ostracodes, which have been developed from the ammonite-dated coastal outcrops of northeastern Siberia, are valid for the northern regions of Western Siberia (Fig. 2) (Nikitenko 1992, 1994, 2008;

Nikitenko et al. 2000; Shurygin et al. 2000). Several marker-hrozons are well developed in central and even southern regions of Western Siberia, where continental deposits with rare shallow-marine interlayers are present.

There is much less information about Early and Middle Jurassic fossils from the Barents Sea region. The oldest foraminiferal assemblages of Late Pliensbachian age have been described from the middle of the Wilhelmøya Formation in Svalbard, Vasilievka Formation in Franz Josef Land and in wells in the Barents Sea (Dibner 1998; Klubov 1965; Nikitenko & Mickey 2004) (Fig. 3). These deposits in Svalbard are overlain by sandy sediments, often beginning with a layer of phosphoritic concretions, which contain Toarcian and Aalenian ammonites and bivalves. Studies of microfauna from the upper part of the Tegetthoff Formation allow determination of the foraminifers which are characteristic of the uppermost Pliensbachian – lowermost Toarcian Recurvoides taimyrensis JF9 foraminiferal Zone (Fig. 3). There are only rare ostracodes which are typical for the assemblage of Ogmoconcha longula JO2 ostracod Zone (Basov et al. 2007). No typical Lower Toarcian microfauna have been found in outcrops on Svalbard and Franz Josef Land. The only evidence are rare Toarcian ostracods (Camptocythere mandelstami JO4 and Camptocythere occalata JO6 Zones) in ditch-cutting samples from central areas of the Barents Sea shelf (Nikitenko & Mickey 2004; Basov et al. 2007). Impoverished foraminiferal assemblages from the uppermost Lower Toarcian (Astacolus praefoliaceus JF12 Zone) are known

1500 km

Atlantic Ocean

PacificOcean

BarentsSea

Laptev Sea

OkhotskSea

2

1

3

4

5

6

7

8

1-Northwest of western Europe 2-North Sea 3-Barents Sea area: South Barents depression,Franz Josef Land, Svalbard 4-Northwestern Siberia 5-Northeastern Siberia 6-NortheasternRussia 7-Northern Alaska 8-Canadian Arctic Archipelago and Northwest territories of Canada

; ;; ; ;

; ;

Figure 1. The Arctic Basin and northwestern Europe and the Lower Jurassic - Aalenian study areas.

B. Nikitenko, B. Shurygin & M. Mickey NORWEGIAN JOURNAL OF GEOLOGY

269

from a well section in South-Barents depression. A foraminiferal assemblage from the lower part of Lower Aalenian (Verneuilinoides syndascoensis JF14 Zone) has been found upwards in the section (Basov et al. 1989) (Fig. 3). It is more typical for sections of the Barents-Kara region. Foraminiferal assemblages are dominated by agglutinated forms, while calcareous foraminifers are rare. Younger Aalenian horizons containing assemblages of macro- and microfaunas have been observed in Franz Josef Land (Dibner 1998) (Fig. 3).

The Hettangian and Pliensbachian of Arctic Canada are mainly represented by sandy deposits (Fig. 4). Hettangian and Sinemurian foraminiferal assemblages together with ammonites have been found only in central regions of the Canadian Arctic (Poulton et al. 1982; Harrison et al. 1999). The sediments lack macro- and microfossils, and

Lai

da

Fm

.

Verneuilinoidessyndascoensis

JF14

pra

esq

uam

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15

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aco

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zwet

kovi

,L

enti

culin

an

ord

vike

nsi

s

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per

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r.

JO8

Recurvoidestaimyrensis

Trochammina lapidosa

Ammod.siliceus

Ammobacul.lobus,

Saccamminainanis

Ammodiscusglumaceus

JF11-JF12

Trochammina inusitata

JF2

JF3

JF4

JF9

JF11

?

Anmarginulinaarctica,

Anmarginulinagerkei

JF7-JF8

Ogmoconchalongula

JO2

Camptocytheremandelstami

JO4

?

?

?

Camptocytherefoveolata

Camptocytherenordvikensis

JO9

Dactylioc.commune

?

?

Arc

toti

s le

nae

nsi

s,A

rcti

ca h

um

ilicu

lmin

ata

Arctotismarchaensis

Meleagrinellafaminaestriata

Dacryomyainflata,

Tancredia bicarinata

Tancrediakuznetsovi

Har

pax

laev

igat

us,

An

rad

ulo

nec

tite

s

Arctotislenaensis,Unionidae

ex gr.

R.elegans,

A. lenaensis

ex gr.

D. gigantea,Sowerbya sp.

?

Foraminiferazones

(F-zones)

Ostracodazones

(O-zones)

Ammonitezones

Bivalvezones

(B-zones)

Yag

eln

oe

Fm

.C

her

nic

hn

oe

Tog

ur

Fm

.N

ovo

go

gn

yay

Fm

.,L

ow

erM

br.

Lev

insk

iyF

m.

Zim

nya

y F

m.

Sh

arap

ovo

Fm

.K

iterb

yut

Fm

.N

ado

yakh

Fm

.

Lithostratigraphy(Shurygin et al. 2000)

Ber

ego

voe

Fm

.

Northwestern Siberia

falcodiscus

wuerttembergeri

compactile

stokesi

margaritatus

viligaensis

falciferum

antiquum

commune

monestieri

spinatum

Polymorphites

?

libratus

siverti

colymicum

angulata

liasicus

planorbis

Astacolus

praefoliaceus,

Lenticulina multa

Ammobaculiteslobus,

Trochammina kisselmani

Co

no

rbo

ides

bu

limin

oid

es

Anmarginulinagerkei

Anmarginulinaarctica

Tr. lapidosa, Fr. dubiella

Ammodiscus siliceus


Tro

cham

. kis

selm

ani

Tro

cham

min

a la

pid

osa

Trochammina inusitata,Turritellella volubilis

Trochamminasublapidosa

JF1

JF2

JF3

JF4

JF5

JF6 JF7

JF8

JF9JF

10

JF11

JF12

T.ta

imyr

ensi

s

bra

un

ian

us

JF1

3

whiteavesi

beyrichi

maclintocki

JF14

Lenticulinanordvikensis

Astacolus zwetkovi


T. p

raes

qu

amat

aJF

15

JF16

JF17

Camptocytherepraespinulosa


JO8

JO10JO9

Prima s primulump il.

Psiloceras planorbis

Alsatites liasicus

Schlotheimiaangulata

Arietites libratus

Coronicerassiverti

Angulaticerascolymicum

Polymorphites

Dactyliocerascommune

Amaltheusstokesi

Amaltheusmargaritatus

Amaltheusviligaensis

Tiltoniceras antiquum

Harpoceras falciferum

Eleganticeraselegantulum

Porpocerasspinatum

Pseudoliocerasfalcodiscus

Pseudolioceraswu rtte bergerie m

Pseudoliocerascompactile

Pseudoliocerasmaclintocki

Pseudoliocerasbeyrichi

Zugodactylitesmonestieri Z

ug

od

act.

bra

un

ian

us

Pseudolioceras(Tugurites)whiteavesi,

P.(T.) tugurensis

?

Retroceramusjurensis

Retroceramuselegans

Mclearniakelymiarensis

Arc

toti

s le

nae

nsi

sD

acry

om

yag

igan

tea

Arctotismarchaensis

Pseudomytiloidesmarchaensis


Dacryomya inflata,Tancredia bicarinata

Tancredia kuznetsovi

Har

pax

laev

igat

us

Anradulonectitesincertus

Velata viligaensis

Harpax ex gr. spinosus

Pseudomytiloidessinuosus

Meleagrinellasubolifex,

Pseudomytiloidessinuosus

Otapiria limaeformis

Corbulomina sp.

Foraminiferazones

(F-zones)

Ostracodazones

(O-zones)

Ammonitezones

(Shurygin et al. 2000;Knyazev et al. 2003)

Bivalvezones

(O-zones)

Su

bst

age

Sta

ge

PL

IEN

SB

AC

HIA

NT

OA

RC

IAN

UP

PE

RL

OW

ER

SIN

EM

UR

IAN

HE

TTA

NG

IAN

AA

LE

NIA

N

LO

WE

RU

PP

ER

LO

WE

RU

PP

ER

LO

WE

RU

PP

ER

Ammonite BorealStandard

(7)

Zakharov et al.199

Ogmoconchalongula

Campt. mandelstami

Camptocythereoccalata

Campt occalata. aff.

JO2

JO4

JO6

JO7

Ogmoconchabuurensis

Nanacytherecostata

Trachycythereverrucosa

JO3

JO5

JO1

Lev

insk

iyF

m.

Sh

arap

ovo

Fm

.K

iterb

yut

Nad

oya

khF

m.

Zim

nya

y F

m.

??

Kyr

a F

m.

Kel

imya

r F

m.

Air

kat

Fm

.K

iterb

yut

Ere

nF

m.

Norteastern Siberia(Shurygin et al. 2000)

NortheasternRussia(Knyazev

)et

al. 2003

Nal

edn

aya

Fm

.

?

Ast

ron

om

ich

eska

yaM

rach

nay

a F

m.

Ko

rotk

iyF

m.

Eks

a F

m.

Lai

da

Fm

.

Ap

rele

vka

Fm

.A

ran

g.

Kh

or.

Lithostratigraphy

Northeastern Siberia and northeastern Russia

Sandstone SiltstoneSilty clay and claystone

Organic-richshale

Bituminousshale

Legend

Figure 2. Lithostratigraphic summary and zonal subdivision of Lower Jurassic and Aalenian of Northeastern Siberia, Northeastern Russia and Northwestern Siberia.

Ogmoconchalongula

JO4

JO6


JF12

Len

ticu

lina

no

rdvi

ken

sis

T. p

raes

qu

amat

a

falcodiscus

wuerttembergeri

compactile

stokesi

margaritatus

viligaensis

falciferum

antiquum

commune

monestieri

spinatum

Polymorphites

?

libratus

siverti

colymicum

angulata

liasicus

planorbis

Trochammina lapidosa

?

?

Verneuilino-ides syndas-coensis

JF14

JF4

JF9


Camptocytheremandelstami

?

?

JO4

bra

un

ian

us

elegantulum

beyrichi

maclintocki

whiteavesi

Astacoluspraefoliaceus

JF16-17

Harpoceras spp.


Zugodactylitesbraunianusex gr.

P. p

ola

rePs.rosenkrantzi

?

?

Pseudoliocerasmaclintocki,

Liocerasopalinum

Pseudolioceras(Tugurites)whiteavesi

Rem

aine

dep

osits

in th

e co

nden

sed

Bre

ntsk

ardh

auge

n B

ed

?

?

?

Har

pax

laev

igat

us

Maclerniakelimyarensis

Arc

toti

s le

nae

nsi

s

Su

bst

age

Sta

ge

PL

IEN

SB

AC

HIA

NT

OA

RC

IAN

UP

PE

RL

OW

ER

SIN

EM

UR

IAN

HE

TTA

NG

IAN

AA

LE

NIA

N

LO

WE

RU

PP

ER

LO

WE

RU

PP

ER

LO

WE

RU

PP

ER


()

Zakharov et al.1997

Foraminiferazones (F-zones)

(Gramberg1988, Basov et al.

1989 withmodifications)

Ostracodazones

(O-zones)

Ammonitezones

(Ershova & Repin 1983with modification)

Bivalvezones

(B-zones)

Pseudomytilmarchaensis

.

Tancredia kuznetsovi

Anradulonectitesincertus

-

Velata viligaensis

?

?

Lithostratigraphy(Gramberg 1988;Leith et al. 1992;

Dibner 1998)

Wilh

elm

oya

Fm

.

Teg

etth

off

Fm

.

Vas

ilevk

a F

m.

Barents Sea area

Figure 3. Lithostratigraphic summary and zonal subdivision of the Lower Jurassic and Aalenian of the Barents Sea area.


270

are regarded to be related to delta complex, which are widespread in eastern regions of Arctic Canada (Embry 1993). Several discoveries of macro- and microfossils have been observed in northern parts of the Yukon and Richardson Mountains in the upper part of the Murrey Formation, where diverse foraminiferal assemblages have been revealed together with Sinemurian ammonites (Poulton et al. 1982) (Fig. 4). Rare foraminifers and Late Pliensbachian ammonites have been described from sandy silts of the Almstrom Creek Formation. In the overlying silty clays and mudstones of the Manuel Creek Formation a richer foraminiferal assemblage from the lower part of Lower Toarcian has been recovered. This assemblage is most typical for the Ammobaculites lobus, Trochammina kisselmani JF11 Zone. The stratigraphic position of this part of the Manuel Creek Formation is based on the co-occurrences of these foraminifers with the ammonite Dactylioceras sp. (Poulton et al. 1982). Similar assemblages (Flabellammina sp. assemblage) are recovered from sections in the Canadian Arctic Archipelago, i.e. in the clays of the Jameson Bay Formation (Wall 1983) (Fig. 4). Similar foraminiferal

assemblages have also been recovered in the Emerald K-33 well on Melville Island. Foraminiferal assemblages of the lowermost Aalenian (Verneuilinoides syndascoensis JF14 Zone) have been described in the upper part of the Jameson Bay Formation as well as in sands and silts of the Sandy Point Formation (Wall 1983).

In the northern regions of Arctic Alaska, the Lower Jurassic deposits are recognized in many well sections. Macro- and micropalaeontological subdivision of these sediments is based on the analysis of conventional core and ditch-cutting samples. The deposits in these sections are often represented by an alternation of sandstones, mudstones and siltstones in the lower member of the Kingak Formation (Hettangian-Aalenian) (Mickey & Haga 1987; Nikitenko & Mickey 2004) (Fig. 4). The underlying Triassic sediments are overlain by Jurassic beds of varying ages. Some sections, however, are characterized only by mudstones with rare interbedded siltstones. The Lower Toarcian deposits of the Kingak Formation are represented everywhere by bituminous mudstones changing to

Gro

sven

or

Isla

nd

Fm

.

?

Lihostratigraphy(Poulton et al. 1982; Embry 1993)

Mu

rray

Rig

de

Fm

.

Fo

shei

m M

br.

, Hei

ber

g F

m.

Mac

lean

St.

Fm

.

Lo

ug

hee

d Is

lan

d F

m

Kin

g C

hri

stia

n F

m.

Alm

stro

m C

reek

Fm

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mes

on

Bay

Fm

.M

anu

el C

reek

Fm

.

Jam

eso

n B

ay F

m.

Rem

us

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r., H

eib

erg

Fm

.

San

dy

Pt

Fm

.

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on

nel

l Isl

.

Flabellamminasp.1

?

?

Glomospiraperplexa

?

JF11

JF2

JF1-JF2

Trochammina inusitata

Ammodiscus aspercf.

JF14

Badouxia (?)

Cornioceras, Arnioceras (?), Charmasseiceras

Echioceras arcticum, E. aklavikenseOxynoticeras oxynotum, Gleviceras plauc-huti, Microderoceras (?), Aegoceras jeletzkiy

?

?

?

Amaltheus stokesi,A. bifurcus

Erycitoides howelli

Leioceras opalinum,Pseudolioceras maclintocki

?

ZugodactylitesZ. braunianus

sp. cf.

Dactylioceras commune

Hildaites, Harpoceras H. exaratum

sp. cf.

Protogrammoceras paltum

?

Peronoceras polare, P.spinatum, Pseudolio-ceras spitsbergense, P. P. compactilesp. cf.

?

Ammonitezones

(Harrison et al., 1999)

Foraminiferazones (F-zones)

(Poulton et al.1982; Wall 1983;

Harrison et al.1999 with

modifications)

Pseudoliocerasmaclintocki

Erycitoideshowelli,

Pseudolioceras(Tugurites)whiteavesi

Astacolus

praefoliaceus,

Lenticulina multa

Ammobaculiteslobus,

Trochammina kisselmani


Anmarginulinaarctica,

Anmarginulinagerkei

Tro

cham

min

a la

pid

osa

Tro

cham

. kis

selm

ani

Trochamminainusitata,

Turritellella volubilis

Tro

cham

.tai

myr

ensi

s

Trochamminasublapidosa

JF1

JF2

JF4

JF7-JF8

JF9

JF11 JF10

JF12

falcodiscus

wuerttembergeri

compactile

stokesi

margaritatus

viligaensis

falciferum

antiquum

commune

monestieri

spinatum

Polymorphites

?

libratus

siverti

colymicum

angulata

liasicus

planorbis

whiteavesi

beyrichi

maclintocki

Lenticulinanordvikensis JF17


JF14

Trochammina praesquamata,Astacolus zwetkovi

JF16

JF13

Foraminiferazones

(F-zones)

Psiloceras(Franziceras)

Arietites, Coroniceras,Charmass

-eiceras

Amaltheusstokesi

Amaltheusmargaritatus


Tiltoniceras antiquum

Harpocerasfalciferum

E elegantulum.

Waehneroceras ?

Amaltheusengelhardti

Ps compactile. cf.

Pseudolioceras lythensecf.

Pseudolioceras

?

?

Ogmoconchalongula

Camptocythere mandelstami


?

Camptocythere occalataaff.

JO2

JO4

JO6

JO7


JO8

Camptocytherenordvikensis JO9

Ostracodazones

(O-zones)

Otapirialimaeformis

ex gr.

Myophorialingonensis,

Harpax laevigatus

Chlamystapensis

ex gr.

Corbulomina

Dacryomya inflata


Pseudomytiloidesmarchaensis

Luciniola,Dacryomya

Arctotis

Ammonitezones

(Imlay & Detterman 1973with modification)

Bivalvezones

(B-zones)

Su

bst

age

Sta

ge

PL

IEN

SB

AC

HIA

NT

OA

RC

IAN

UP

PE

RL

OW

ER

SIN

EM

UR

IAN

HE

TTA

NG

IAN

AA

LE

NIA

N

LO

WE

RU

PP

ER

LO

WE

RU

PP

ER

LO

WE

RU

PP

ER


()

Zakharov et al.1997

Lithostratigraphy(Mickey & Haga 1987

with modifications)

Kin

gak

Fo

rmat

ion

, Lo

wer

Mem

ber

N Alaskaorthern Arctic Canada

Figure 4. Lithostratigraphic summary and zonal subdivision of the Lower Jurassic and Aalenian of Northern Alaska and the Canadian Arctic.


271

banks of non-bituminous mudstones and siltstones. The most complete assemblages of ammonites, bivalves, foraminifers and ostracodes, which are very similar to the assemblages of eastern Siberia, are present in the sections in northern Alaska (Nikitenko & Mickey 2004). The Pliensbachian, Toarcian and Aalenian deposits are overlain with stratigraphic unconformity by greenish-gray siltstones and mudstones of the upper member of the Kingak Formation (?Upper Callovian, Oxfordian-Kimmeridgian).

Thus, Lower Jurassic zonal scales developed in northeastern Siberia on the basis of ammonites, bivalves, ostracodes, and foraminifers allow the correlation of biostratigraphic subdivisions based on these fossil groups over the whole Arctic Basin (Figs. 2-4). Moreover, several marker-horizons based on bivalves, foraminifers and ostracodes allow calibration with the Jurassic macro- and microbenthic zonal subdivisions of the Arctic regions and of Western Europe (Shurygin et al. 2000; Nikitenko & Mickey 2004; Nikitenko 2008). Therefore, these zonal scales may be considered as a Boreal Zonal Standard.

Combined use of these scales provides the basis for a very detailed subdivision (at the intra-zonal level) of the Arctic Jurassic. The universality of this system of scales, allows one to choose those which are most effective in resolving concrete stratigraphic problems, taking into consideration specific geological situations. The use of all sets of Jurassic zonal scales permits detailed and consistent correlation of deposits in the Arctic Basin (Nikitenko & Shurygin 1994a).

Palaeobiogeography, main biotic and abiotic eventsThe Lower Jurassic and Aalenian of the Arctic region are characterized by a succession of sedimentological cycles caused by eustatic events (Fig. 5). The widespread development of Lower Toarcian organic-rich shales allows us to divide the Lower and Middle Jurassic deposits into two parts: the Hettangian-Pliensbachian and the uppermost Lower Toarcian-Aalenian (Fig. 2-4). During the Hettangian to Sinemurian, sandy and

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Figure 5. Phases in the evolution of Siberian Jurassic benthos and nekton and the main abiotic events (the data on the phases evolution of ammonites, belemnites and bivalves adapted from Meledina et al. 2005).


272

silty sediments were deposited in many regions during the formation of the Jurassic Arctic Basin. Mudstones and clays accumulated only in offshore areas. An extensive middle Late Pliensbachian transgression led to deposition of primarily clayey sediments over the greater part of the Arctic. Lower Toarcian dark-gray clays enriched by organic matter (sometimes bituminous) have been observed almost everywhere in the Arctic Basin. The upper parts of the Lower and Upper Toarcian are dominated by sandy and silty deposits (Figs. 2-4).

Comparative analysis of taxonomic diversity of Jurassic benthic associations in different facies areas of the Boreal basins allows the definition of development phases of macro- and microfaunal associations and certain marker levels with the sharpest structural and taxonomic changes as well as their relation to the main abiotic factors (Nikitenko & Mickey 2004; Meledina et al. 2005; Nikitenko 2008) (Fig. 5). In the Early Jurassic and Aalenian there was a stable connection between the biota of the Arctic and Palaeopacific, while the connection with the Palaeoatlantic was only periodically intensified during the major transgressions, which were accompanied by climate warming.

The first migrant taxa from northwestern regions of the European seas appeared at the beginning of the Late Pliensbachian. These European taxa almost always occur in microbenthic communities in the middle of the Late Pliensbachian, in the beginning of the Early Toarcian and in the Early Aalenian (Fig. 5). Connections between the Arctic and the Palaeoatlantic cease at the end of the Late Aalenian, and caused a substantial difference in microbenthic communities of the Arctic and northwestern regions of Europe (Nikitenko 2008).

The comprehensive analysis of biotic and abiotic events, as well of the biostratigraphic data, permits reliable palaeobiogeographic reconstructions to be made. The traditional principles of palaeobiogeographic zonation based on molluscs (ammonites, belemnites, bivalves), such as the definition of realms based on endemic families, as well as the definition of provinces based on the distribution of endemic genera (Saks 1972; Westermann 2000), cannot be used for the microbenthos. In the Arctic palaeobasin, Jurassic microbenthos is generally characterized only by foraminiferal and ostracode families and genera of rather wide (cosmopolitan) geographic distribution. In this study, we performed cluster analysis of foraminifer and ostracode genera for biogeographic zonation of different regions of the northern hemisphere for several time intervals (Norling 1972; Bate & Coleman 1975; Souaya 1976; Klubov 1965; Løfaldli & Nagy 1980; Poulton et al. 1982; Wall 1983; Riegraf, 1985; Copestake & Johnson 1984, 1989; Morris & Coleman, 1989; Ainsworth 1986, 1987; Basov et al. 1989; Malz & Nagy 1989; Nagy & Johansen 1991; Arias 2000; Harrison et al. 1999; Dibner 1998; Nagy & Seidenkrantz 2003).

The BioDiversity Professional Program has been used to calculate the matrix and construct the dendrograms (McAleece et al. 1997). Cluster analysis was carried out with the group average link method and based upon the Jaccard coefficient. Taxa of high rank from tropical basins are almost totally lacking in Jurassic microbenthic associations in Arctic sea areas. This reflecs the influence of factors such as climatic zoning and geographic barriers, which resulted in the partial isolation of Arctic biota. Areas providing links between the Arctic Basin and Boreal seas occurred in the north of the Palaeoatlantic and the Palaeopacific basins. Taking into consideration the specific features of Mesozoic Arctic biota, we may assume the presence of a deep-water basin, possibly with oceanic depths, in the Arctic during the Mesozoic (Fig. 6). Such a basin was necessary to support the considerable diversity of specific Arctic marine biota (Zakharov et al. 2002; Nikitenko 2008). Lower Jurassic and Aalenian microbiotas of Arctic regions were dominated by panboreal (cosmopolitan) taxa. Nevertheless some migrant taxa from southern areas reached the Arctic seas (Nikitenko & Mickey 2004). On the other hand, some typically Arctic forms occurred in the Boreal basins.

Boreal transgression took place in the beginning of the Hettangian (Fig. 5). Foraminiferal associations in northwestern European seas are characterized by specific features inherited from Late Triassic communities. The assemblages were dominated by diverse calcareous forms. Judging from the considerable number of specific microfauna taxa (Copestake & Johnson 1989; Ainsworth 1989), northwestern European seas may be reconstructed as shallow-water basins periodically exposed to desalination. Hettangian foraminifers from Arctic seas have been recovered in northeastern Siberia, northern Alaska, and possibly in Arctic Canada. Taxonomic compositions of European and Arctic associations of Hettangian foraminifers are very different. The Siberia – Northern Alaska Province may be established within the Arctic Realm, based on the detailed taxonomic composition of both foraminifers and ostracodes (Fig. 6). The environments were unfavorable for the development of microbenthos during the Hettangian and Early Pliensbachian. In contrast to Western Europe, the taxonomic diversity of Arctic microbenthos was almost unchanged, and varied from 2 to 6 genera (Nikitenko 1992).

In the northwest of Europe (Northwestern Europe Province), a considerable increase in microfaunal diversity occurred in the Sinemurian and Early Pliensbachian. Foraminiferal associations of northeastern Siberia and northern Alaska were of “primitive” character as in the Hettangian (Siberia – Northern Alaska Province). Calcareous forms, which are predominant in the associations of Western Europe seas, are very rare. Foraminiferal associations of the Canadian Arctic were even more impoverished.


273

?

?

on the Foraminifera

on the Ostracoda

Depth, m

14% Similarity43 100

11% Similarity 44.5 100

Northern Alaska

England

Canadian Arctic

Eastern Siberia

Dendrograms of Jaccard claster analysis for Early HettangianForaminifera genera from the Arctic and Boreal-Atlantic Realms

Provinces:

Fastnet Basin

Dendrograms of Jaccard cluster analysis for Early HettangianOstracoda genera from the Arctic and Boreal-Atlantic Realms

Provinces:Northern Alaska

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Depth, m

Dendrograms of Jaccard cluster analysis for mid Late PliensbachianOstracoda genera from the Arctic and Boreal-Atlantic Realms

Provinces:

ArcticRealm


Dendrograms of Jaccard cluster analysis for mid Late PliensbachianForaminifera genera from the Arctic and Boreal-Atlantic Realms

Provinces:

ArcticRealm


Northeastern Russia

Eastern Siberia


NorthwesternEurope

Figure 6. Palaeobiogeographic zonation of the Arctic Realm in the Early Hettangian and mid Late Pliensbachian on the basis of foraminifers and on ostracods with dendrograms showing results of cluster analysis (Base map after Golonka & Scotese (1995); palaeogeographic recon-structions after Bogolepov (1983) with modifications).


274

In the Arctic Basin and northwestern European seas, several simultaneous biotic and abiotic events have been recognized during the Late Pliensbachian - Early Toarcian. One of the most striking biotic events is the Early Toarcian biotic crisis (Nikitenko & Shurygin 1994b; Little & Benton 1995; Nikitenko & Mickey 2004). The beginning of the Late Pliensbachian is characterized by a transgression in the Arctic Basin (Fig. 5). The climate was warm and wet. The taxonomic diversity of microbenthic associations gradually increased in the Arctic palaeobasin, and in palaeoseas of Western Europe. The transgression and climatic warming caused the invasion of a number of thermophilic migrant taxa (at both the species and generic levels) in the microbenthic communities of the Arctic palaeoseas. The regressive stage of the Arctic palaeobasin began at the end of the Late Pliensbachian (Fig. 5). In association with an eustatic sea-level fall, a rather sharp cooling has been observed, as indicated by the presence of glendonites in the northern regions of eastern Siberia. At the same time, the climate gradually changed to arid. The palaeobasins became shallower. There was possibly a series of geographical barriers. The profile of the seabed and the current system also changed. These events apparently resulted in biotic connections with western European seas being partially broken. Simultaneous eustatic sea-level fall accompanied by climatic cooling, and change of the seabed profile (and consequently changes in the current system), caused microbenthos depletion at the specific and generic levels, as well as dominance shifts in the Arctic microbenthos communities. A climatic warming in association with a major eustatic sea-level rise took place in the earliest Toarcian (Fig. 5). The seabed also profile changed resulting in changes to the current system and a wider distribution of stagnant environments. As a consequence of a sharp change in primary abiotic factors, the taxonomical diversity of microbiota was strongly reduced over a rather short period of time, initiating a new stage in the development of microbenthic communities. This microbiotic crisis was more striking in Arctic seas than in the western European palaeobasins (Nikitenko & Mickey 2004).

During the post-crisis period, there were reliable links between the microbenthic communities of the Arctic and western European seas caused by transgression and climatic warming. In the Arctic palaeobasin, this stage is characterized by periodic invasions of migrant taxa of both foraminifers and ostracods which are widespread in the Toarcian palaeoseas of Western Europe (Fig. 5). At the same time, some specific Arctic forms migrated into western European seas giving rise to new taxa there. Judging from the analysis of the geographic distribution of microbenthos in the first half of the Late Pliensbachian, two biogeographic provinces based upon foraminifers within the Boreal-Atlantic Realm were identified: the Northwestern Europe Province and the North Sea Province. In the Arctic Realm, two provinces were also established: the Western Siberia

- Canadian Arctic and Eastern Siberia - Northern Alaska (Fig. 6). The Northwestern Europe Province, based upon ostracodes, has been established in the north of the Boreal-Atlantic Realm and three provinces have been identified in the Arctic Realm: North Sea, Western Siberia and Eastern Siberia – Northern Alaska Provinces. At the end of the Late Pliensbachian, foraminiferal associations of the North Sea Province are the closest to associations of the Arctic Realm, but not to the Boreal-Atlantic associations. There were only two ostracode provinces: the North Sea Province and the Eastern Siberia – Northern Alaska Province (Nikitenko & Mickey 2004).

In the Early Toarcian, only one province has been identified in the northern part of the Boreal-Atlantic Realm, while the Arctic Realm may be subdivided into three provinces based on foraminifers: North Sea, Western Siberia–Canadian Arctic and Eastern Siberia – Northern Alaska (Fig. 7). The Northwestern Europe Province, based on ostracods, remained in the northern part of the Boreal-Atlantic Realm. Its boundaries were not changed. Two ostracod provinces have been identified in the Arctic Realm: Western Siberia – Barents Sea and Eastern Siberia – Northern Alaska (Nikitenko & Mickey 2004).

In Arctic basins, the regression of the end of the Early Toarcian was replaced by a transgression in the Early Aalenian (Fig. 5). In this period, the climate became cooler and more humid in comparison to the Early Toarcian (Nikitenko 2008). Sandy silts accumulated in the Arctic basins (Fig. 2-4). A characteristic feature of the benthic communities of this period is the presence of migrant taxa from the seas of Western Europe. In the latest Toarcian and Early Aalenian, the richest foraminiferal associations occurred in the eastern part of the Arctic region (Eastern Siberia – Northern Alaska Province) and North Sea. Impoverished foraminiferal associations existed in the western part of the Arctic region (Western Siberia – Canadian Arctic Province). At the end of the Toarcian and in the beginning of the Early Aalenian, the differences between ostracode associations of the Boreal-Atlantic and Arctic Realms are even more significant than the difference between the foraminiferal associations.

At the beginning of the Middle Jurassic, a number of geographic barriers were formed in the region of the North Sea. These separated the Arctic and Boreal-Atlantic Basins making reciprocal migrations of the benthos difficult. In the latest Aalenian, similarity coefficients of microbenthos associations reduced almost in two times pointing indicating isolation of the basins. In the Late Aalenian, the geographic situation established in the Early Aalenian remained in the Arctic Basin. Foraminiferal and ostracod associations of the Arctic and Boreal-Atlantic Realms were somewhat similar in having several common genera, but they


275


NorthwesternEurope

on the Ostracoda

on the Foraminifera

Depth, m

Dendrograms of Jaccard cluster analysis for Early ToarcianOstracoda genera from the Arctic and Boreal-Atlantic Realms

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?

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11 % Similarity 44.5 100

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Dendrograms of Jaccard cluster analysis for Early ToarcianForaminifera genera from the Arctic and Boreal-Atlantic Realms

Provinces:

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Land

Upper sublittoral zone (~0-50 m)

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Deep Sea:Pseudoabyssal,Slope (more than 200 m)

5% Similarity 47.5 100

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England

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Depth, m

Dendrograms of Jaccard cluster analysis for Late AalenianOstracoda genera from the Arctic and Boreal-Atlantic Realms

Provinces:

ArcticRealm


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Dendrograms of Jaccard cluster analysis for Late AalenianForaminifera genera from the Arctic and Boreal-Atlantic Realms

Provinces:

ArcticRealm


Northern Alaska

Eastern Siberia

NorthwesternEurope


Western Siberia-Barents Sea

Figure 7. Palaeobiogeographic zonation of the Arctic Realm in the Early Toarcian and Late Aalenian on the basis of foraminifers and on ostra-cods with dendrograms showing results of cluster analysis (Base map after Golonka & Scotese (1995); palaeogeographic reconstructions after Bogolepov 1983, with modifications).


276

lacked almost any common species. The specific features of biogeographic differentiation of the foraminifers and ostracodes allow the definition of several provinces within both the Boreal-Atlantic and Arctic Realms (Fig. 7).

ConclusionsThe results of these investigations allow us to trace Lower Jurassic ammonite, bivalve, ostracode and foraminiferal zonations developed in northern regions of eastern Siberia sections over the whole Arctic Basin. Therefore, Lower Jurassic zonations based on these groups established for northern Siberia can be considered as a Boreal Zonal Standard (Zakharov et al. 1997; Shurygin et al. 2000; Nikitenko & Mickey 2004; Nikitenko 2008) (Figs. 2-4). The analysis of the dynamics of taxonomic diversity and community structure of the benthos allows us to establish several phases in the development of macro- and microfossil associations. The Hettangian-Sinemurian and the second half of the Late Aalenian – first half of the Bathonian were periods of maximum isolation of the Arctic Basin and northern seas of Western Europe (Fig. 5). Early Jurassic and Aalenian biogeographic provinces of the Arctic basins have been defined on the basis of cluster analysis of foraminiferal and ostracod genera (Figs. 6, 7). A number of biochores with the rank of Realms and Provinces have been identified. It has been determined that the boundaries of Provinces and Realms based on the palaeogeographic distribution of different microfossil groups were not the same and changed their positions during geological time. Ecotone zones (North Sea) between the realms changed their locations in different periods belonging sometimes to the Arctic Realm, and at other times to the Boreal-Atlantic Realm (Figs. 6, 7).

Acknowledgements: The authors are grateful to Dr R. Blodgett, Dr J. Clough, Dr Kai-Uwe Gräfe and an anonymous reviewer for reading the manuscript and offering useful comments. This investigation has been carried out with the financial support of the Russian Fund for Basic Research N 06-05-54291.

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Documents

High resolution stratigraphy of the Lower Jurassic and ...of Western Siberia (Fig. 2) (Nikitenko 1992, 1994, 2008; Nikitenko et al. 2000; Shurygin et al. 2000). Several marker-hrozons