Main phytostratigraphic boundaries in the Jurassic deposits of Western Siberia

ISSN 0869�5938, Stratigraphy and Geological Correlation, 2014, Vol. 22, No. 3, pp. 231–238. © Pleiades Publishing, Ltd., 2014.Original Russian Text © N.K. Mogutcheva, 2014, published in Stratigrafiya. Geologicheskaya Korrelyatsiya, 2014, Vol. 22, No. 3, pp. 6–13.

231

INTRODUCTION

A significant role in the remarkable legacy left byV.A. Vakhrameev belongs to the substantiation ofboundaries between Jurassic epochs in the continentaldeposits on the basis of paleobotanic data and the pos�sibility of their stage subdivision. Certain success inthis direction was achieved by studying the Jurassicfauna of the Indo�European paleoflorisitic region(Vakhrameev, 1989). As Vakhrameev noted, the Juras�sic flora in the Siberian region developed slowly andgradually, without sharp reorganization under warmand wet climatic conditions, and differed from theflora of the southern regions in the poorer taxonomiccomposition and stages of evolution. Therefore, it wasdifficult to distinguish phytostratigraphic units in theJurassic continental sediments of southern Siberia.Presently accumulated paleobotanic data are suffi�cient to substantiate a detailed phytostratigraphicscheme.

Within the Siberian paleofloristic region, the Juras�sic plant�bearing continental sediments are wide�spread in the Kuznetsk, Irkutsk, Chulym–Yenisei,and Kansk–Achinsk basins. The study of these sedi�ments and plant remains in them in natural exposureswas started as early as the end of 19th century. The firstbrief description of the Jurassic flora of the Irkutskcoal basin was conducted by Heer (1876) and Prynada(1962). The results of study of the stratigraphy and

flora of the Jurassic deposits of Southern and WesternSiberia were generalized by Teslenko (1970) and laterby Kiritchkova et al. (1992). Teslenko (1970) recog�nized two floral assemblages in the Lower and MiddleJurassic. Kiritchkova et al. (1992) distinguished andcorrelated the main turnovers in the evolution of theJurassic flora of the Kuznetsk Basin, Krasnoyarsk krai,and Tomsk oblast. They distinguished two phytohori�zons in the Kuznetsk Basin and Krasnoyarsk krai andalso determined the sequence of phytostratigraphiccomplexes and plant�bearing beds, which are fairlyconditionally correlated with a general stratigraphicscale. As was correctly noted by Vakhrameev (1989),paleofloras can be reliably dated only by study ofsequences represented by alternating marine and con�tinental sediments. Precisely these materials have beenobtained in recent decades during study of the Jurassicflora of Western Siberia, which can be presently con�sidered as the reference flora for Jurassic sediments ofthe Siberian phytochoria.

MATERIALS

Owing to prospecting works carried out in recentdecades for discovery of the petroleum potential of theLower–Middle Jurassic polyfacies terrigenous sedi�ments of Western Siberia, a large collection of plantremains was bed�by�bed sampled from borehole coresby researchers from SNIIGGiMS. All finds have an

Main Phytostratigraphic Boundaries in the Jurassic Deposits of Western Siberia

N. K. MogutchevaSiberian Research Institute of Geology, Geophysics, and Mineral Resources (SNIIGGiMS), Ministry of Natural Resources,

pr. Krasnyi 67, Novosibirsk, 630014 Russiae�mail: [email protected]

Received April 12, 2012; in final form, January 23, 2013

Abstract—The study of the large collections of plant remains gained from cores of numerous boreholesdrilled in Western Siberia made it possible to determine the taxonomic composition of the Jurassic flora ofthis region, the stages of its evolution, and the sequence of floral assemblages, which characterize the regionalstratigraphic horizons indirectly correlated via series of parallel faunal, microfaunal, spore and pollen zonalscales with a general stratigraphic scale. The compositions of floral assemblages was established in the Het�tangian–lower part of the upper Pliensbachian, upper part of the upper Pliensbachian, lower Toarcian, upperToarcian, Aalenian, Bajocian, Bathonian, and Callovian–Oxfordian sediments. Criteria were elaborated tosubstantiate the Triassic–Jurassic and Lower–Middle Jurassic boundaries. Lithologically and biostrati�graphically, the Middle–Upper Jurassic boundary is poorly expressed.

Keywords: Western Siberia, Jurassic, flora, phytostratigraphy, boundaries, floral assemblages

DOI: 10.1134/S0869593814030083

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STRATIGRAPHY AND GEOLOGICAL CORRELATION Vol. 22 No. 3 2014

MOGUTCHEVA

accurate assignment to sections, formations, andregional stratigraphic horizons. The stratigraphicposition and stage dating of these horizons have beenreliably substantiated by macro� and microfauna anddynocyst parallel zonal scales and reference palyns�patic scale (Mogutcheva, 2000; Stratigrafiya…, 2000;Kiritchkova et al., 2005).

The study of collections sampled in sections of over170 boreholes allowed us to determine the taxonomiccomposition of the Jurassic flora of Western Siberia,including over 100 plant species from more than40 genera. The succession and stages in the floral evo�lution during the Jurassic have been investigated andfloral complexes typical of ten regional stratigraphicJurassic horizons have been recognized. The age offloral assemblages is determined by their assignmentto regional horizons. This made it possible for the firsttime to compile a detailed phytostratigraphic scheme(figure) for the southern West Siberian plate. In thisscheme, peculiar floral assemblages are demonstratedpractically for all stratigraphic regional horizons andseven floral beds are distinguished: five beds in theLower Jurassic, one phytostraton in the upper MiddleJurassic (Malyshevka Horizon), and one in the Call�ovian–Oxfordian (Mogutcheva, 2000; Geolog�icheskoe…, 2005). Floral beds have not been recog�nized yet in the Laida, Vymskoe, and Leont’evskiyhorizons, but floral beds characterized by the Verkh�nepeshkovsky, Azharminsky, Malyshevsky, andNaunaksky floral assemblages were distinguished inthe Middle Jurassic by Kiritchkova et al. (2005).Established phytostratigraphic units play an importantrole in substantiating wide intra� and interregionalcorrelations and determining criteria for the recogni�tion of the Jurassic phytostratigraphic boundaries inthe West Siberian continental sediments and in speci�fying the age of the Jurassic continental plant�bearingsediments in the Siberian coal basins (Mogutcheva,2009).

Similar studies were conducted at the same time byA.I. Kiritchkova at the All�Russia Petroleum ResearchExploration Institute and by L.I. Bystritskaya atTomsk University. Their results were published in themonograph Phytostratigraphy and Flora of the WestSiberian Jurassic Sediments (Kiritchkova et al., 2005),which describes more than 100 species of plantremains, including first described epidermally studiednumerous Ginkgoales, Czekanowskiales, and coni�fers. This significantly expands our concepts concern�ing the taxonomic composition of the Jurassic flora ofSiberia. This work also presents materials of E.I. Kos�tina on the Jurassic phytostratigraphy of the KanskBasin. According to Kiritchkova et al. (2005), theJurassic succession is subdivided into three large phy�tohorizons: Lower Jurassic Urengoi, Middle JurassicTomsk, and Callovian–Oxfordian Naunaksky hori�zons. The former two horizons were subdivided into

six floral beds correlated with regional horizons:(1) Levinskiy and Sharapovo horizons, (2) Kiterbyutand Nadoyakh horizons, (3) Laida Horizon, (4) Vym�skoe, Leont’evskiy and lower Malyshevka horizons;(5) Malyshevka Horizon, and (6) Vasyugan Horizon.Their correlation is complicated by different strati�graphic ranges determined by us and A.I. Kiritchkovaet al. for phytostratons.

RESULTS

Over the greater part of the territory of WesternSiberia, Jurassic sediments with erosion overlie differ�ent horizons of the Triassic, Paleozoic, and Precam�brian. Only in the north of the region is the visible hia�tus between Jurassic and Upper Triassic continentalsediments missing. Such uninterrupted successions ofthe boundary Triassic–Jurassic sediments were recov�ered in the Urengoi distinct, the north of WesternSiberia, and in the North Sos’va district in the Uralianarea (Fradkina et al., 2003; Mogutcheva, 2011). Thefloral assemblages of the studied successions havesomewhat different composition. In the Urengoi dis�trict, the plant remains in the Upper Triassic Varen�gayakh (Carnian–Norian) and Vityuta (Rhaetian)formations of the Tampei Group are mainly repre�sented by large horsetails, mainly species Neocalamitescarrerei (Zeil.) Halle, sometimes with subordinateferns. The overlying Lower Jurassic rocks (BeregovayaFormation, Zimnyaya Horizon, Hettangian–initialupper Pliensbachian) also contain horsetails, but ofdifferent composition: Neocalamites pinitoides(Chachl.) Chachl. and Equisetites turgaicus (Vlad.)Kiritch., as well as Czekanowskiales, Ginkgoales, andconifers typical of the Siberian Jurassic flora. The Tri�assic–Jurassic boundary in this area is identified by achange in species composition of horsetails and fernsand by the first appearance of Jurassic gymnosperms.

In the North Sos’va district of the Uralian area ofWestern Siberia, the Triassic and Jurassic floral assem�blages have richer taxonomic composition. The floralassemblage of the Raethian deposits (Yatrin Forma�tion) bears 20 species, half of which (fern Cladophle�bis, Czekanowskiales, and conifers) later receivedextremely wide development among Jurassic flora ofSiberia. Typical taxa of this assemblage (Cladophlebisstenolopha Brick, Scytophyllum, Tmematostrobus, Yuc�cites, Podozamites gutiformis (Migatch.) Stanisl., andothers) do not leave the Triassic and Jurassic bound�aries. Jurassic deposits (Zimnyaya Horizon, Hettan�gian–initial upper Pliensbachian) in the studied suc�cessions yielded Equisetites turgaicus (Vlad.) Kiritch.,Osmundopsis plectrophora Harris, Podozamites sp., andElatocladus aff. manchuricus (Yok.) Yabe. In this partof Western Siberia, the Triassic–Jurassic phytostrati�graphic boundary is identified by the complete disap�pearance of Triassic plants, decrease in horsetails


MAIN PHYTOSTRATIGRAPHIC BOUNDARIES IN THE JURASSIC DEPOSITS 233

System

Series

Stage

Substage

Ho

rizo

nB

eds

wit

h f

lora

Typ

ical

flo

ral a

ssem

blag

esJURASSIC

Upper

Oxfor�dian

L.С. U. L. С. U.

Geo

rgie

vski

y

Vas

yuga

n

Co

nio

pte

ris

lati

lobu

s,

C. d

epen

sis,

L

obi

foli

a aj

aken

sis,

N

ilss

on

ia m

ajsk

aja

Equ

iset

ites

late

ralis

(P

hill

.) P

hill

., C

onio

pter

is la

tilob

us B

istr

., C

. bur

ejen

sis

(Zal

.) S

ew.,

C. v

sevo

lodi

i E. L

eb.,

C. d

epen

sis

E.L

eb.,

Lob

ifolia

aja

kens

is E

. Leb

., N

ilsso

nia

maj

skaj

a B

istr

., N

. vitt

aefo

rmis

Pry

n.,

Pag

ioph

yllu

m s

etos

um (

Ph

ill.)

Sew

.

Middle

Callo�vian

Bajo�cian

Bathonian

U. L. Middle U.

Mal

ysh

evka

Co

nio

pte

ris

sim

ple

x,

C. f

urs

sen

koi,

C

. via

lovi

i,

Nil

sso

nia

sp

p.

Equ

iset

ites

late

ralis

(P

hill

.) P

hill

., E

. bea

nii (

Bun

b.)

Sew

., C

onio

pter

is s

impl

ex (

L. e

t H.)

Har

r., C

. fur

ssen

koi P

ryn

.,

C. v

ialo

vii T

ur.�

Kel

., C

. em

bens

is P

ryn

., C

. lat

ilobu

s B

istr

., C

. ner

ifolia

Gen

., C

. sni

gire

vska

e Te

sl.,

Cla

doph

lebi

s w

illia

mso

nii

(Bro

ngn

.) B

ron

gn.,

C. d

entic

ulat

a (B

ron

gn.)

Fon

t., R

apha

elia

dia

men

sis S

ew.,

Nils

soni

a po

lym

orph

a S

chen

k, N

. vitt

aefo

rmis

P

ryn

., N

. maj

skaj

a B

istr

., N

. gra

cilli

ma

Pry

n.,

Hei

lung

ia c

f. a

mur

ensi

s (N

ovop

.) P

ryn

., P

agio

phyl

lum

set

osum

(P

hill

.) S

ew.,

T

axoc

ladu

s si

biri

cus

(Ch

ach

.) T

esl.,

Ela

tocl

adus

man

chur

ica

(Yok

.) Y

abe,

Bra

chyp

hyllu

m s

p.

Leo

nt’

evsk

iyE

quis

etite

s as

iatic

us P

ryn

., C

onio

pter

is h

ymen

ophy

lloid

es (

Bro

ngn

.) S

ew.,

C. b

urej

ensi

s (Z

al.)

Sew

., N

ilsso

nia

maj

skaj

a B

ist.

Aalenian

Upper

Vym

sko

e

Hep

atic

ites

won

naco

tti H

arr.,

Equ

iset

ites

late

ralis

(P

hill

.) P

hill

., E

. bea

nii (

Bun

b.)

Sew

., C

onio

pter

is h

ymen

ophy

lloid

es

(Bro

ngn

.) S

ew.,

C. m

aaki

ana

Hee

r, C

lado

phle

bis w

illia

mso

nii (

Bro

ngn

.) B

ron

gn.,

C. n

ebbe

nsis

(Bro

ngn

.) N

ath

., C

. kam

enk�

ensi

s T

hor

n.,

C. b

iden

tata

Tur

.�K

et.,

Cze

kano

wsk

ia r

igid

a H

eer,

Pho

enic

opsi

s an

gust

ifolia

Hee

r, P

ityop

hyllu

m n

orde

nski

oldi

i (H

eer)

Nat

h.

Lower

Lai

da

Equ

iset

ites l

ater

alis

(P

hill

.) P

hill

., E

. asi

atic

us P

ryn

., C

onio

pter

is p

orci

na B

rick

, C. s

pect

abili

s B

rick

, Cla

doph

lebi

s kam

enke

n�si

s T

hom

., G

inkg

o si

biri

ca H

eer,

Pse

udot

orel

lia s

p., P

odoz

amite

s sp

., P

ityop

hyllu

m s

p., C

zeka

now

skia

, P

hoen

icop

sis

Lower

Toarcian

Upper

Nad

oya

kh

Phlebopteris polypodioides

NeokoretrophyllitesClathropteris obovata

Neo

kore

trop

hylli

tes

linea

rifo

rmis

Tes

l., N

eoca

lam

ites

sp.,

Ann

ular

iops

is s

p., E

quis

etite

s la

tera

lis (

Phi

ll.)

Phi

ll., C

onio

pter

is la

ti�fo

lia B

rick

, C. s

pect

abili

s B

rick

, C. k

irgi

sica

Bri

ck, C

lath

ropt

eris

obo

vata

Ois

hi,

Gin

kgo

sibi

rica

Hee

r, G

. dig

itata

(B

ron

gn.)

H

eer,

Pte

roph

yllu

m te

slen

koi B

at.,

Cze

kano

wsk

ia, P

hoen

icop

sis,

Kov

alia

gra

ndifo

lia T

esl.

Lower

Kit

erby

ut

Equisetites turgaicus

Oto

zam

ites

Neo

kore

trop

hylli

tes

linea

rifo

rmis

Tes

l., N

eoca

lam

ites

sp.,

Equ

iset

ites

turg

aicu

s (V

lad.

) K

irit

ch.,

Con

iopt

eris

maa

kian

a H

eer,

C

. ner

ifolia

Gen

k., C

lath

ropt

eris

sp.

, Phl

ebop

teri

s po

lypo

dioi

des

(Bro

ngn

.) B

ron

gn.,

Tod

ites

prin

ceps

Pre

sl, C

lado

phle

bis

den�

ticul

ata

(Bro

ngn

.) S

ew.,

C. d

elic

atul

a Ya

be e

t Ois

hi,

Oto

zam

ites

cf. l

atio

r S

apor

ta, C

zeka

now

skia

, Pho

enic

opsi

s

Pliens�bachian

Upper

Sh

arap

ovo

Co

nio

pte

ris

zin

dan

ensi

s

Neo

cala

mite

s sp

., E

quis

etite

s tu

rgai

cus

(Vla

d.)

Kir

itch

., C

onio

pter

is m

aaki

ana

Hee

r, C

. zin

dane

nsis

Bri

ck, C

. por

cina

Bri

ck,

C. s

pect

abili

s B

rick

, Phl

ebop

teri

s po

lypo

dioi

des

(Bro

ngn

.) B

rong

n., C

lado

phle

bis

will

iam

soni

i (B

ron

gn.)

Bro

ngn

., G

inkg

o si

biri

ca H

eer,

Sph

enob

aier

a cz

ekan

owsk

iana

(P

om.)

Flo

r., C

zeka

now

skia

, Pho

enic

opsi

s

Lev

insk

iy

Ric

ciop

sis

tom

iens

is B

at.,

Neo

cala

mit

es p

init

oide

s (C

hac

hl.

) C

hac

hl.

, E

quis

etit

es tu

rgai

cus

(Vla

d.)

Kir

itch

.,

E. g

raci

llis

(Nat

h.)

Hal

le,

Sph

enob

aier

a sp

ecta

bili

s (N

ath

.) F

lor.

, C

zeka

now

skia

, P

ityo

phyl

lum

, S

amar

opsi

s pl

icat

ifor

mis

, C

arpo

lith

es iv

anov

skyi

Tes

l.,

C. t

rico

stat

us T

ur.

�Ket

., P

ityo

sper

mum

par

vum

Tu

r.�K

et.

L.

Zim

nya

ya

Hettangian–Sinemurian

L.

Jur

assi

c ph

ytos

trat

igra

phy

of W

este

rn S

iber

ia.

234


MOGUTCHEVA

(especially Neocalamites), change in species composi�tion, a partial change in species composition of fernsCladophlebis, and impoverishment of flora at the ini�tial Early Jurassic.

The Late Triassic Siberian flora is dominated byferns, mainly Cladophlebis gen., and conifers (Yuc�cites, Podozamites), with some taxa in common withJurassic flora. Since the Jurassic flora of Siberia inher�ited many taxa from the Triassic, the Triassic–Jurassicboundary can be defined only by the presence of char�acteristic plant taxa.

The flora of the very beginning of the Early Jurassic(Zimnyaya and Levinskiy horizons) has a poorer taxo�nomic composition than the Late Triassic flora. Itcontains a few mosses and horsetails, rare Cze�kanowskiales, Ginkgoales, and conifers, and singleCladophlebis (Mogutcheva, 2000). Since the floralassemblages of the Zimnyaya and Levinskiy horizonsare similar in composition, they were united into a sin�gle assemblage of arbitrarily Hettangian–initial latePliensbachian age. The assemblage comprises Ricciop�sis tomiensis Bat., Thallites sp., Neocalamites pinitoides(Chachl.) Chachl., Equisetites cf. gracilis (Nath.)Halle, E. turgaicus (Vlad.) Kiritch., Annulariopsis sp.,Osmundopsis plectrophora Harris, Cladophlebis sp.,Sphenopteris sp., Czekanowskia rigida Heer, Sphe�nobaiera spectabilis (Nath.) Flor., Ixostrobus heeriPryn., Podozamites sp., Elatocladus aff. manchuricus(Yok.) Yabe, Pityophyllum nordenskioldii (Heer)Nath., Samaropsis plicatiformis Tesl., S. rotundataHeer, S. tersiensis Bat., Pityospermum parvum Tur.�Ket., Carpolithes cinctus Heer, C. minor Pryn., C. tri�costatus Tur.�Ket., and C. ivanovskyi Tesl.

During the Early Jurassic, the taxonomic diversityof flora gradually increased and reached its peak by theterminal Toarcian. Significant changes in flora havealready been noted in the terminal Upper Pliensba�chian (Sharapovo Horizon). That time was marked bythe significant reorganization the plant cover. Unlikethe Zimnyaya–Levinskiy assemblage, the Sharapovoassemblage (terminal Pliensbachian) is dominated byferns at a significant decrease in gymnosperms. Thislevel is marked by the first occurrence of Todites andPhlebopteris, in addition to cladophlebis, and, first ofall, different species of Coniopteris gen. This is animportant feature of the Siberian flora, unlike the floraof the Indo�European phytochoria, where Coniopterisappeared in the lower Middle Jurassic, thus definingthe Lower–Middle Jurassic boundary (Vakhrameev,1989).

The late Pliensbachian (Sharapovo) assemblageyielded Hepaticites haiburnensis Harris, Neocalamites sp.,Equisetites turgaicus (Vlad.) Kiritch., Todites sp., Phle�bopteris polypodioides Brongn., Coniopteris maakiana(Heer) Pryn., C. zindanensis Brick, C. spectabilisBrick, C. porcina Brick, C. cf. hymenophylloides

(Brongn.) Sew., Cladophlebis haiburnensis (L. et H.)Brongn., C. williamsoni (Brongn.) Brongn., C. nebben�sis (Brongn.) Nath., Sphenobaiera czekanowskiana(Heer) Flor., Czekanowskia rigida Heer, Phoenicopsisangustifolia Heer, and Carpolithes minor Pryn.

During the Toarcian stage, the flora experiencedfurther transformation. In Western Siberia, the Toar�cian starts from the lower Toarcian Kiterbyut Horizonlying on the Sharapovo Horizon and having mainlyclayey composition. It is regarded as the interregionalstratigraphic marker over the huge territory of WesternSiberia and farther beyond its limits and correlateswith the early Toarcian climatic optimum (Strati�grafiya…, 2000). A change in the climatic conditionscaused reorganization of the plant cover. The earlyToarcian (Kiterbyut Horizon) flora, as the terminallate Pliensbachian (Sharapovo Horizon) flora, con�sists mainly of sporiferous plants sharply dominated byferns (around 80%): three species of Phlebopteris(P. polypodioides (Brongn.) Brongn., P. angustiloba(Presl) Hirm. et Hoerh., P. braunii (Goep.) Hirm. etHoerh.), six species of Coniopteris (C. maakiana(Heer) Pryn., C. spectabilis Brick, C. murrayana(Brongn.) Brongn., C. zindanensis Brick, C. nerifoliaGenk., and C. pulcherrima Brick), and six species ofCladophlebis (C. denticulata (Brongn.) Font., C. deli�catula Yabe et Oishi, C. nebbensis (Brongn.) Nath.,C. haiburnensis (L. et H.) Brongn., C. crispata Racib.,C. concinna (Presl) Du Toit). There are also Toditesprinceps (Presl) Goth., Klukia, Clathropteris, and sin�gle cycadophytes Otozamites latior Saporta. HorsetailsNeocalamites sp., Equisetites turgaicus (Vlad.) Kiritch.,and Neokoretrophyllites lineariformis Tesl. are few innumber. The latter species was found only in the lowerand upper Toarcian deposits (Kiterbyut and Nadoyakhhorizons), i.e., within the Early Jurassic climatic opti�mum range of Siberia. Czekanowskiales, Ginkgoales,and conifers in the early Toarcian flora occur in subor�dinate amounts (around 5%). These are Ginkgo sp.,Baiera setacea (Heer) Pryn., Czekanowskia rigidaHeer, Phoenicopsis angustifolia Heer, Ixostrobus heeriPryn., Pityophyllum sp., and Carpolithes minor Pryn.

Noteworthy is the presence of significant amountsof thermophillous floras from the southern paleoflo�ristic regions such as ferns Phlebopteris, Klukia, andClathropteris, Middle Asian Coniopteris, and cycado�phytes Otozamites. These features make the early Toar�cian flora different from the late Pliensbachian. Thepresence of thermophillous elements—migrants fromthe southern paleofloristic regions—is the result of theearly Toarcian climatic warming.

The late Toarcian flora (Nadoyakh Horizon) dif�fers from the early Toarcian in the wider taxonomiccomposition. It is represented by almost the entiregroup of Mesozoic flora (in total, around 50 species):mosses Hepaticites and Thallites; horsetails Annulari�



opsis, Neokoretrophyllites lineariformis Tesl., Neoca�lamites pinitoides (Chachl.) Chachl., Eqisetites lateralis(Phill.) Phill., E. asiaticus Pryn.; ferns Phlebopteris sp.,Todites princeps (Presl) Goth., Clathropteris obovataOishi, and Klukia; two Cladophlebis species; numerousConiopteris—C. latifolia Brick, C. nerifolia Genk.,C. porcina Brick, C. kirgisica Brick, C. spectabilisBrick, C. zindanensis Brick, and others; cycadophytesHeilungia sp., Pterophyllum teslenkoi Bat.; GinkgoalesGinkgo, Baiera, Pseudotorellia, Czekanowskia, Phoeni�copsis, Leptostrobus, and Ixostrobus; and confers Kov�alia, Pityophyllum, Podozamites, Pityospermum, Pityo�lepis, Schizolepis, Samaropsis, and Carpolithes. Apeculiar feature of the late Toarcian assemblage is thegreat abundance of Ginkgo remains, which is not typi�cal of other intervals of the studied succession, and thepresence of Heilungia, Pterophyllum cycadophytes,and Middle Asian Coniopteris. Coniopteris(ten species) continues to be the predominant genusamong ferns, while the role of other ferns decreases.The occurrence of some plants in the late Toarcian flo�ral assemblage is limited to the Lower Jurassic (Neoca�lamites pinitoides (Chachl.) Chachl., Neokoretrophyl�lites, Annulariopsis, Phlebopteris, Clathropteris obovataOishi, and others). These plants are missing from theMiddle Jurassic of Siberia, but occur among the Mid�dle Jurassic flora in the Indo�European region. In theAalenian and younger deposits of Western Siberia,these species were not found. The disappearance ofthese plants serves as a reliable biostratigraphic markerfor the Lower–Middle Jurassic boundary. This bound�ary is also peculiar in the decrease in amount anddiversity of Coniopteris and in general impoverishmentof the Allenian flora as compared to the Toarcian one.Presicely these changes in flora allowed Teslenko(1970) to position the boundary between the Lowerand Middle Jurassic of Siberia. New data confirm thisassumption. Vakhrameev (1989) at first disagreed withthis idea, because this boundary in the floras of theIndo�European phytochoria was defined by other cri�teria: the first appearance and presence of at least twoto three species of Coniopteris in the Middle Jurassic atpreserved occurrence of thermophillous ferns andhorsetails, which disappeared in Siberia by the end ofthe Early Jurassic. Vakhrameev (1988) later agreedwith arguments of Teslenko (1970) and Il’ina (1985)concerning rapid reorganization of Siberian flora inthe Early Jurassic in response to the repeated climaticchanges, which resulted in the first occurrence ofConiopteris as early as the late Pliensbachian. It wasalso established that ferns Klukia appeared in Siberiain the early Toarcian, not in the Aalenian.

Some cooling at the beginning of the Middle Juras�sic caused definite changes in the Aalenian flora(Laida Horizon): extinction of thermophillous ele�ments typical of the Toarcian flora, decrease inConiopteris, and the absence of Cycadophyta. This led

to the impoverishment of the Aalenian flora, whichwas also established from palynological data (Il’ina,1985). This flora, unlike the late Toarcian flora,showed a decrease in amount and diversity of horse�tails and ferns: Equisetites lateralis (Phill.) Phill.,E. asiaticus Pryn., E. aff. gracilis (Nath.) Halle,Coniopteris porcina Brick, C. spectabilis Brick, C. lati�folia Brick, C. hymenophylloides (Brongn.) Sew.,C. nerifolia Gen., Cladophlebis williamsonii (Brongn.)Brongn., C. argutula (Heer) Font., C. whitbiensis(Brongn.) Brongn. More abundant plant remains ofgymnosperms are represented by Ginkgo sibirica Heer,Sphenobaiera sp., Czekanowskia rigida Heer, Phoeni�copsis angustifolia Heer, Pseudotorellia longifoliaDolud., P. angustifolia Dolud., Leptostrobus sp., Soro�saccus sp., Podozamites lanceolatus (L. et H.) Schimp.,Pityophyllum nordenskioldii (Heer) Nath., Schizolepis sp.,Samaropsis rotundata Heer, Carpolithes heeri Tur.�Ket., and Carpolithes minor Pryn. In general, the plantremains are rather scarce in the Laida Horizon.

The Bajocian and Bathonian revealed a gradualflourishing of the Middle Jurassic flora, which reachedits maximum in the Bathonian. Already at the termi�nal Aalenian–initial Bajocian (Vymskoe Horizon),the composition and amount of ferns changed:Coniopteris burejensis (Zal.) Sew., C. porcina Brick,C. hymenophylloides (Brongn.) Sew., C. maakiana(Heer) Pryn., C. margaretae Harris, C. simplex (L. etH.) Harris, C. murrayana (Brongn.) Brongn., C. cf.embensis Pryn., C. cf. vsevolodii E. Leb., C. cf. sapor�tana (Heer) Vachr., Lobifolia lobifolia (Phill.) Rass. etE. Leb., Raphaelia diamensis Sew., R. ex gr. tapkensis(Heer) Pryn., Cladophlebis williamsonii (Brongn.)Brongn., C. denticulata (Brongn.) Font., C. delicatulaYabe et Oishi, C. bidentata Tur.�Ket., C. haiburnensis(L. et H.) Brongn., C. nebbensis (Brongn.) Nath.,C. kamenkensis Thom., C. auriculus Tesl., C. aktashen�sis Tur.�Ket., and C. suluktensis Brick. The flora of theVymskoe Horizon is characterized by the presence ofsingle moss, Lycopodiaceae, and horsetails: Hepati�cites wonnacottii Harris, Lycopodites sp., Equisetiteslateralis (Phill.) Phill., Cycadophyta Heilungia sp.,Pterophyllum sp., Williamsoniella sp.

Other gymnosperms are as follows: Ginkgo sibiricaHeer, G. tapkensis Dolud. et Rass., Baiera ahnerti Krysht.,Sphenobaiera angustiloba (Heer) Flor., Czekanowskiarigida Heer, Phoenicopsis angustifolia Heer, Pseudo�torellia sp., Leptostrobus laxiflora Heer, Ixostrobusheeri Pryn., Podozamites sp., Pityophyllum nordenski�oldii (Heer) Nath., Sorosaccus sp., Antholithes sp.,Stenorhachis sp., Elatides curvifolia (Dunk.) Nath.,Pityospermum parvum Tur.�Ket., Schizolepis antiqua(Heer) Pryn., Samaropsis rotundata Heer, Carpolithesheeri Tur.�Ket., and C. minor Pryn.

This proportion of different plant groups was pre�served during the entire Bajocian, although the Bajo�

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cian assemblage of the Leont’evskiy Horizon includesslightly more than 20 species. This is presumablyrelated to the unequal sampling of plant remains: veryfew samples from these deposits were available for ourstudy. The Leont’evskiy assemblage showed someincrease in mosses and horsetails: Hepaticites wonna�cottii Harris, Thallites sp., Equisetites lateralis (Phill.)Phill., E. beanii (Bunb.) Sew., E. asiaticus Pryn. At thesame time, this assemblage became lower in ferns,especially in Cladophlebis gen. (only two species—C. denticulata (Brongn.) Font. and C. nebbensis(Brongn.) Nath.), and contains as few as six species ofConiopteris (C. simplex (L. et H.) Harris, C. burejensis(Zal.) Sew., C. hymenophylloides ((Brongn.) Sew.,C. maakiana (Heer) Pryn., C. murrayana (Brongn.)Brongn., C. margaretae Harris). Cycadophytes belongto Nilssonia majskaja Bistr., N. acuminata (Presl)Goep., Doratophyllum ninae Kiritch., and Williamso�niella sp. Czekanowskiales and conifers are repre�sented by typical Jurassic species of genera Pseudo�torellia, Czekanowskia, Phoenicopsis, Podozamites,Pityophyllum, and Carpolithes.

The Bathonian flora (Malyshevka Horizon) has therichest taxonomic composition in the Jurassic. Thisflora includes more than 60 plant species from allgroups of the Mesozoic flora. The most numerousplants are ferns (23 species), in particular, Conio�pteris: C. burejensis (Zal.) Sew., C. porcina Brick,C. hymenophylloides (Brongn.) Sew., C. maakiana(Heer) Pryn., C. margaretae Harris, C. simplex(L. et H.) Harris, C. murrayana (Brongn.) Brongn.,C. embensis Pryn., C. vsevolodii E. Leb., C. furssenkoiPryn., C. vialovii Tur.�Ket., C. snigirevskae Tesl.,C. latilobus Bistr., C. latifolia Brick, and others. Thecharacteristic feature of fern associations is the fre�quent occurrence and great amount of pinnulate spe�cies Coniopteris simplex (L. et H.) Harris, C. furssenkoiPryn., C. vialovii Tur.�Ket., and C. embensis Pryn.Among frequent species are also Raphaelia, Lobifolialobifolia (Phill.) Rass., and five species of commonJurassic Cladophlebis: Cladophlebis denticulata(Brongn.) Font., C. williamsonii (Brongn.) Brongn.,C. delicatula Yabe et Oishi, C. argutula (Heer) Font.,and C. embensis Pryn.

The Malyshevka floral assemblage also differs fromother complexes in the greater amount and frequencyof cycadophytes of genus Nilssonia (N. majskaja Bistr.,N. compta (Phill.) Bronn., N. acuminata (Presl)Goep., N. gracillima Pryn., N. rara Sixtel, N. iniquaKiritch., N. lobatidentata Kiritch., N. lacinulata Bat.)and in the presence of Heilungia amurensis (Novop.)Pryn., H. iczetujensis Vachr. et Srebr., and Jacutiella sp.This assemblage also yields diverse conifers. In addi�tion to common Pityophyllum and Podozamites, thereare also frequent Pagiophyllum setosum (Phill.) Sew.,Brachyphyllum sp., Taxocladus sibiricus (Chachl.)Tesl., Elatocladus manchurica (Yok.) Yabe, and Pityo�

cladus aff. ferganensis Brick. This horizon is abundantin Ginkgoales and Czekanowskiales and richer inmoss and lepidophytes (Hepaticites sp., Thallites sp.,Selaginella sp., Lycopodites tenerrimus Heer, L. trichi�atus Pryn., L. falcatus L. et H.), as well as horsetails(Equisetits lateralis (Phill.) Phill., E. beanii (Bunb.)Sew., and E. asiaticus Pryn.). Among numerous anddiverse reproductive organs and seeds are Sorosaccus,Schizolepis, Pityospermum, Burejospermum, Samarop�sis, and Carpolithes. The aforementioned featuresmake the Bathonian assemblage (Malyshevka Hori�zon) recognizable.

The Malyshevka Horizon in the West SiberianJurassic successions is overlain by the Vasyugan Hori�zon, the age of which is presently determined as thelate Bathonian–Callovian–Oxfordian. The remainsof plants in it, with rare exceptions, were collectedmainly in the upper part of the horizon. The floralassociations of the Vasyugan Horizon in compositionare similar to the Bathonian one, but have moredepleted composition, which may be related to theuneven sampling. In spite of similarity, there are somedifferences between these floral associations, whichare expressed in the renewal of species composition offerns in the Vasyugan Horizon assemblage. CommonBathonian Coniopteris latilobus Bistr., C. burejensis(Zal.) Sew., C. simplex (L. et H.) Harris, C. furssenkoiPryn., C. depensis E. Leb., C. vsevolodii E. Leb., andothers are supplemented by Lobifolia ajakensis(E. Leb.) Rass. et E. Leb., Osmundiella vachrameeviVass. et Pavl., Gleichenia aff. lobata Vachr., Cladophle�bis vasilevskae Vachr., C. dunkeri (Schimp.) Sew., andothers. Horsetails are represented by single Annulari�opsis simpsonii (Phill.) Harris, Equisetites lindensisKiritch., E. ishinae Mark., E. dissimilis Kiritch., andE. tschetschumensis Vass. The frequency of occurrenceof cycadophytes, especially, nilssonia, decreases sig�nificantly, with renewal of their composition: Nilssoniamajskaja Bistr., N. serrulata Oishi, N. vittaeformisPryn., N. kendalii Harris, N. baranovae Kiritch., Wil�liamsoniella aff. elegans Tur.�Ket., Heilungia sp.,?Beania gracilis Carr. In addition to the common Cze�kanowskiales, there are also rare Ginkgoales: Ginkgosibirica Heer, Sphenobaiera uninervis Sam., S. cze�kanowskiana (Heer) Flor., Pseudotorellia sp., andEretmophyllum sp. Conifers are few in number andcomprise Podozamites reinii Geyl., P. gracilis Vassil.,P. lanceolatus (L. et H.) Schimp., P. eichwaldiiSchimp., Pityophyllum, Pagiophyllum setosum (Phill.)Sew., Elatocladus manchuricus (Yok.) Yabe and E. sub�manchuricus Yabe et Oishi, and Brachyphyllum sp.(Geologicheskoe…, 2005).

Since the Vasyugan Horizon has a wide age range(late Bathonian–Callovian–Oxfordian), it is as yetdifficult to determine to which stratigraphic level thenoted changes belong. Taking into account its con�finement mainly to the upper part of the horizon and



the occurrence of Late Jurassic plants that are absentin the Middle Jurassic, we may conditionally suggestthat these changes in flora probably mark the Middle–Upper Jurassic boundary. The upper horizons of theUpper Jurassic of Western Siberia sporadically containsingle plant remains.

The established sequence in changes of floralassemblages allowed us to compile a phytostrati�graphic scheme of the Jurassic deposits of WesternSiberia (figure). This scheme demonstrates seven flo�ral beds and nine floral assemblages typical of tenregional stratigraphic horizons (Mogutcheva, 2000;Geologicheskoe…, 2005). Owing to this, the revealedcomplexes have reliable stage dating and could serve asthe reference for the wide intra� and interregional cor�relations and substantiation of phytostratigraphicboundaries. This scheme is also of great significancefor specification of the age of continental floral depos�its of the Siberian coal basins (Mogutcheva, 2009).

CONCLUSIONS

Multiyear study of the Jurassic flora from the coreof numerous boreholes from different districts of West�ern Siberia made it possible to determine its taxo�nomic composition, character and stages of evolution,and sequence of floral assemblages, which are con�fined to the regional stratigraphic horizons. Dating ofthe latter and their typical floral assemblages is justi�fied by their correlation with parallel bivalve, foramin�ifer, ostracode, dinoflagellate cyst, and palynologicalzonal scales (Stratigrafiya…, 2000). This makes it pos�sible to consider the West Siberian Jurassic flora as thereference for Siberia.

Owing to accurate assignment of all finds of plantremains to the studied borehole successions anddetailed correlation of the latter, we established com�positions of floral assemblages typical of ten regionalhorizons. Therefore, the assemblages have exact stagedating. We identified the reference assemblages for theHettangian–lower Upper Pliensbachian (Zimnyaya +Levinskiy horizons), upper portions of the UpperPliensbachian (Sharapovo Horizon), lower Toarcian(Kiterbyut Horizon), upper Toarcian (NadoyakhHorizon), Aalenian (Laida Horizon), Lower Bajocian(Vymskoe Horizon), Bajocian (Leont’evskiy Hori�zon), Bajocian–Bathonian (Malyshevka Horizon),and upper Bathonian–Callovian–Oxfordian (Vasyu�gan Horizon). On the basis of these data, a detailedphytostratigraphic scheme of West Siberian Jurassicdeposits was compiled for the first time. This schemeis of great importance for wide correlations and agespecification of sediments of different facies.

The data obtained were used to reveal criteria forsubstantiation of phytostratigraphic boundaries. TheTriassic–Jurassic boundary is identified by the disap�pearance of typical Triassic generic and species taxa

(Neocalamites carrerei (Zeil.) Halle, Cladophlebisstenolopha Brick, Scytophyllum, Tmematostrobus, Yuc�cites, Podozamites guttiformis (Migatch.) Stanisl., andothers), by the change in species composition ofhorsetails and ferns, and by floral impoverishment atthe beginning of Early Jurassic (Hettangian–begin�ning of late Pliensbachian).

The boundary between the Lower and MiddleJurassic is defined by the disappearance of typicalLower Jurassic thermophillous horsetails and ferns(Neocalamites pinitoides (Chachl.) Chachl., Neokoret�rophyllites, Annulariopsis, Phlebopteris, Clathropterisobovata Oishi, and others), by the decrease in the roleof Coniopteris, and by general impoverishment of theAalenian flora, which is related to some cooling inSiberia at the Early–Middle Jurassic boundary(Teslenko, 1970; Il’ina, 1985).

The phytostratigraphic boundary between theMiddle and Upper Jurassic in Western Siberia is indis�tinct. The finds of plant remains in the Upper Jurassicsediments are lower in abundance as compared to theLower and Middle Jurassic ones. The Late Jurassicfloral assemblages of Western Siberia are similar to theMiddle Jurassic ones, but differ in the appearance ofplants (Lobifolia ajakensis (E. Leb.) Rass. et E. Leb.,Osmundiella vachrameevi Vass. et Pavl., Cladophlebisvasilevskae Vachr., and others) typical of the SiberianUpper Jurassic deposits.

Reviewer E.I. Kostina

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Translated by M. Bogina

Documents

Main phytostratigraphic boundaries in the Jurassic deposits of Western Siberia