Late Jurassic -earliest Cretaceous prolonged shelf dysoxic-anoxic event and its possible causes

Abstract

The Late Jurassic – earliest Cretaceous time interval was characterized by a widespread distribution of dysoxiс–anoxiс environments in temperate- and high-latitude epicontinental seas, which could be defined as a shelf dysoxic–anoxic event (SDAE). In contrast to black shales related to oceanic anoxic events, deposits generated by the SDAE were especially common in shelf sites in the Northern Hemisphere. The onset and termination of the SDAE was strongly diachronous across different regions. The SDAE was not associated with significant disturbances of the carbon cycle. Deposition of organic-carbon-rich sediment and the existence of dysoxic–anoxic conditions during the SDAE lasted up to c. 20 Ma, but this event did not cause any remarkable biotic extinction. Temperate- and high-latitude black shale occurrences across the Jurassic–Cretaceous boundary have been reviewed. Two patterns of black shale deposition during the SDAE are recognized: (1) Subboreal type, with numerous thin black shale beds, bounded by sediments with very low total organic carbon (TOC) values; and (2) Boreal type, distinguished by predominantly thick black shale successions showing high TOC values and prolonged anoxic–dysoxic conditions. These types appear to be unrelated to differences in accommodation space, and can be clearly recognized irrespective of the thickness of shale-bearing units. Black shales in high-latitude areas in the Southern Hemisphere strongly resemble Boreal types of black shale by their mode of occurrence. The causes of this SDAE are linked to long-term warming and changes in oceanic circulation. Additionally, the long-term disturbance of planktonic communities may have triggered overall increased productivity in anoxia-prone environments.

Full text

Geological Magazine
www.cambridge.org/geo
Original Article
Cite this article: Rogov MA, Shchepetova EV,
and Zakharov VA (2020) Late Jurassic –earliest
Cretaceous prolonged shelf dysoxic–anoxic
event and its possible causes. Geological
Magazine 157: 1622–1642. https://doi.org/
10.1017/S001675682000076X
Received: 13 February 2019
Revised: 25 June 2020
Accepted: 29 June 2020
First published online: 19 August 2020
Keywords:
black shales; oceanic anoxic events (OAEs);
Jurassic; Cretaceous; shelf dysoxic–anoxic
event (SDAE); nannoplankton; radiolarians
Author for correspondence: MA Rogov,
Email: russianjurassic@gmail.com
© The Author(s), 2020. Published by Cambridge
University Press.
Late Jurassic –earliest Cretaceous prolonged
shelf dysoxic–anoxic event and its possible
causes
MA Rogov1
,
2, EV Shchepetova1and VA Zakharov1
1Geological Institute of RAS, Pyzhevski lane 7/1, Moscow 119017, Russia and 2Saint-Petersburg State University,
Universitetskaya nab. 7/9, Saint Petersburg 199034, Russia
Abstract
The Late Jurassic –earliest Cretaceous time interval was characterized by a widespread
distribution of dysoxiс–anoxiсenvironments in temperate- and high-latitude epicontinental
seas, which could be defined as a shelf dysoxic–anoxic event (SDAE). In contrast to black shales
related to oceanic anoxic events, deposits generated by the SDAE were especially common in
shelf sites in the Northern Hemisphere. The onset and termination of the SDAE was strongly
diachronous across different regions. The SDAE was not associated with significant disturb-
ances of the carbon cycle. Deposition of organic-carbon-rich sediment and the existence of
dysoxic–anoxic conditions during the SDAE lasted up to c. 20 Ma, but this event did not cause
any remarkable biotic extinction. Temperate- and high-latitude black shale occurrences
across the Jurassic–Cretaceous boundary have been reviewed. Two patterns of black shale
deposition during the SDAE are recognized: (1) Subboreal type, with numerous thin black shale
beds, bounded by sediments with very low total organic carbon (TOC) values; and (2) Boreal
type, distinguished by predominantly thick black shale successions showing high TOC values
and prolonged anoxic–dysoxic conditions. These types appear to be unrelated to differences in
accommodation space, and can be clearly recognized irrespective of the thickness of shale-
bearing units. Black shales in high-latitude areas in the Southern Hemisphere strongly resemble
Boreal types of black shale by their mode of occurrence. The causes of this SDAE are linked to
long-term warming and changes in oceanic circulation. Additionally, the long-term disturbance
of planktonic communities may have triggered overall increased productivity in anoxia-prone
environments.
1. Introduction
Oceanic anoxic events (OAEs) are attracting much attention due to their impact on the develop-
ment of life, and the burial of organic carbon in marine sediments leading to the formation of
black to organic-rich shales of high economical value. During the last decades, numerous studies
of OAEs across the world have revealed some of their distinctive features. OAEs were widely
distributed across both oceanic and shelf basins, and were associated with perturbations of
the global carbon cycle (Jenkyns, 2010). They are generally of short duration, lasting some tens
to hundreds of thousand years. The onset of typical OAEs is nearly synchronous across different
basins, and associated excursions in carbon isotope values can also be traced in non-marine
successions. OAEs are also associated with elevated global temperatures, as well as significant
faunal turnovers and sometimes extinctions (Jenkyns, 1999).
In addition to typical OAEs, other intervals characterized by widespread black shale distri-
bution are known of. In particular, the Upper Jurassic –lowermost Cretaceous interval stands
out as one of the most important. It can be traced across multiple middle- to high-latitude basins
and sub-basins from NW Europe to the Polish Lowlands and the European part of Russia, and
from the North Sea and East Greenland to Western and Northern Siberia, Alaska and Arctic
Canada. Significant differences from typical OAEs preclude this period from being identified
as a Late Jurassic anoxic event (Nozaki et al. 2013; Arora et al. 2015; Carmeille et al.2020)
or an Oxfordian–Kimmeridgian OAE (Trabucho-Alexandre et al. 2012; Martinez & Dera,
2015). Here we propose to recognize the Late Jurassic –earliest Cretaceous shelf dysoxic–anoxic
event (SDAE), named in such a manner because it influenced shelf environments, mainly in
high latitudes. It is characterized by the following set of key features.
(1) Latest Jurassic –earliest Cretaceous black shales are very rare in low-latitude areas and
oceanic sites, but very widely distributed in Boreal shelves (and also known from the
high-latitude sites of the Southern Hemisphere).
(2) The onset and termination of this SDAE were strongly diachronous within laterally differ-
ent palaeobasins and sometimes inside the same basin.

(3) The deposition of finely laminated organic-carbon-rich
sediments and, accordingly, the existence of associated
dysoxic–anoxic environments lasted for several million years
(up to c. 20 Ma; see Georgiev et al. 2017).
(4) There is no evidence for any significant perturbations in the
global carbon cycle during the SDAE.
(5) The long-term development of dysoxic–anoxic conditions
near the sediment–water interface strongly affected benthic
faunas, but did not lead to any remarkable extinction events.
This paper is focused on an analysis of the distribution of black
shales near the Jurassic–Cretaceous (J/K) boundary. The term
‘black shales’follows the definition by Tyson (1987): “dark-
coloured, fine grained mudrocks having the sedimentological,
palaeoecological and geochemical characteristics associated with
deposition under oxygen-deficient or oxygen-free bottom waters”.
Additionally, Tyson (1987) emphasized the high content of total
organic carbon (TOC usually more than 1%; see also Arthur &
Sageman, 1994), and a predominantly marine origin of the organic
matter (OM). Black shales are usually well-laminated due to a
general lack of bioturbation (Savrda & Bottjer, 1986).
An additional comment should be made concerning the stage
names used in this paper and Boreal–Tethyan correlation (Fig. 1).
Significant faunal provincialism near to the J/K boundary (Enay,
1972; Rawson, 1973; Remane, 1991; Cecca, 1999; Zakharov &
Rogov, 2003; Wimbledon, 2008) has led to continuous use of inde-
pendent stages for Tethyan (internationally accepted Tithonian
and Berriasian stages) and Boreal (Volgian and Ryazanian stages)
regions. Although the base of the Volgian stage is coinciding with
the base of the Tithonian stage (Rogov, 2004,2010) and top of the
Ryazanian stage lies close to the top of the Berriasian stage
(Baraboshkin, 2004), the Volgian–Ryazanian boundary corre-
sponds to a level somewhere within the lower Berriasian stage
(Houšaet al. 2007; Bragin et al.2013). It should also be noted that
although the Volgian and Tithonian ammonite zones can be cor-
related (Rogov, 2014), the use of the Volgian and Ryazanian stages
is preferred for the Boreal regions in this paper as all boundaries of
these stages are clearly traced across the Boreal areas (Baraboshkin,
2004; Rogov & Zakharov, 2009), while substages of the Tithonian
and Berriasian stages cannot be recognized here.
2. Late Jurassic –earliest Cretaceous black shales in
space and time
The wide distribution of Late Jurassic –earliest Cretaceous black
shales in temperate- and high-latitude areas of the Northern
Hemisphere is well-known, especially due to the high source-rock
potential of these rocks. However, there exist relatively few papers
summarizing the occurrences of these black shales (cf. Braduchan
et al. 1989; Wignall, 1990; Leith et al. 1992); we therefore provide a
brief review of black shale occurrences. We also consider low-
latitude Subboreal black shales (such as those from England and
northern France), as both the mode of occurrence and faunal con-
tents of these black shales show similarities with those of northern
high-latitudes.
Two different patterns of black shale deposition during the Late
Jurassic –earliest Cretaceous SDAE can be recognized. (a) Type 1
are Subboreal (Kimmeridge Clay Formation) and are characterized
by intercalations of black shales and typical shallow-water
mudstones, marlstones, sandstones, etc; the sands, marls and muds
were deposited in well-oxygenated environments. This type of
black shale is mainly restricted to relatively low latitudes
(35–50° N). (b) Type 2 are Boreal (Bazhenovo Formation of the
Western Siberia) and characterized by thick, monotonous black
shale units of variable thickness that were deposited in anoxic–
dysoxic environments of long duration. Type 2 black shales are more
typical of high palaeolatitudes (Fig. 2;onlineSupplementary
Table S1,availableathttp://journals.cambridge.org/geo).
Type 1 black shales of the Subboreal type are especially
well-studied in the type area of the famous Kimmeridge Clay
Formation (Figs 2,3a). These black shales span the upper
Kimmeridgian –lowermost middle Volgian interval (Cope,
1967,1978; Callomon & Cope, 1971; Morgans-Bell et al. 2001;
Gallois, 2004,2011). Numerous black shale bands are exposed
along the Dorset and Yorkshire coasts, penetrated by many explo-
ration or scientific boreholes, and further expanded offshore to the
North Sea (Gallois, 2004). These black shales, characterized by
specific dysoxic–anoxic benthic assemblages (Wignall, 1990;
Oschmann, 1994), are intercalated with mudstone–marlstone beds
indicating well-oxygenated near-bottom conditions. It should be
noted, however, that not all black shale bands are clearly associated
with prominent anoxia, but additional factors controlling black
shale deposition (enhanced bioproductivity, high sediment accu-
mulation rates and rapid burial) are also important (Tribovillard
et al. 2005). Fossil assemblages of the Kimmeridge Clay
Formation (including black shale bands) are especially diverse
and include some unique records of both invertebrates and verte-
brates (Etches & Clarke, 1999; Gallois, 2004). The oxygen-depleted
environments clearly favoured preservation of organic material.
Total pyrolysed and residual organic carbon (TOC, %) values in
the black shales vary from 2–4% to 10–12%, sometimes reaching
13–15% or 28–32% (Scotchman, 1991; Sælen et al. 2000; Morgans-
Bell et al. 2001). RockEval Pyrolysis data and the distribution of the
hydrogen index of kerogen (HI = S2/TOC, where S2: the amount of
hydrocarbons from the thermal cracking of insoluble OM, mea-
sured in mg HC g–1) with the temperature of the maximum rate
of hydrocarbon generation occurring in a kerogen sample during
pyrolysis (measured at the top of the S2 peak; T
max
) (Fig. 4a) deter-
mine mainly ‘immature’kerogen of Type II and III, which indicates
a mixture of marine and terrestrial OM present in variable propor-
tions (Scotchman, 1991; Sælen et al. 2000). Increased TOC values
in the black shales typically coincide with elevated HI (Fig. 4a),
indicating an increase in the marine OM contribution to the ker-
ogen composition. In northern Scotland (Isle of Skye) the deposi-
tion of black shales began earlier, and here black shales of the
Subboreal type are known from the Oxfordian –lower
Kimmeridgian Staffin Shale Formation (Nunn et al. 2009). TOC
values here range from 0.2 to 9.2 wt% with values increasing up
the sequence, although highest values were reported from the lower
Oxfordian substage. RockEval pyrolysis indicated mainly Type III
kerogen, while c. 10% of samples indicated Type II kerogen (Fig. 2).
Similar occurrences and stratigraphic ranges of black shales are
known from the opposite coast of the English Channel, that is, in
northern France (Herbin et al. 1995; Proust et al. 1995; Samson
et al. 1996; Gallois, 2005). However, the duration of black shale
deposition is reduced and the number of elementary black shale
bands here are fewer when compared with coeval strata in
Dorset (Fig. 2). Black shale bands in this region, although bounded
by TOC-depleted beds, are sometimes relatively thick (Geyssant
et al. 1993). According to Tribovillard et al.(2001) and Hatem
et al. (2018), the TOC values in the black shales falls in the range
2–7% and sometimes 9%. Type II–III kerogen is represented
mainly by amorphous OM interpreted as marine biomass
degraded as a result of selective oxidation of metabolizable
Shelf dysoxic–anoxic event 1623

components, and partly by incorporation of reduced inorganic sul-
phur into lipids (Hatem et al.2018).
Black shales belonging to the Subboreal type can also be found
in the Polish Lowlands. Both lithologies and fossil contents of the
Kimmeridgian and lower Volgian deposits here are very close to
those of the Volga Basin (Rogov, 2010), and the measured TOC
concentrations (0.2–9.2 wt%) from the central–eastern part of
the Ł´odźSynclinorium are comparable to those of the
LOWER KIMMERIDGIAN UPPER KIMMERIDGIAN
Rasenia
cymodoce
Pictonia
baylei
Suboxydiscites
taimyrensis
Aulacostephanus
autissiodorensis
Sub-
stage
Amoeboceras
rosenkrantzi
Amoeboceras
regulare
Amoeboceras
serratum
Amoeboceras
alternoides
Vertebriceras
densiplicatum
Miticardioceras
tenuiserratum
MIDDLE OXF. UPPER OXFORDIAN
Regional ammonite
zones
Aulacostephanus
eudoxus
Euprionoceras
sokolovi
Hoplocard.
decipiens
Aulacostephanus
mutabilis
Amoebites kitchini
Plasmatites
bauhini
12 3
LOWER KIMMERIDGIAN UPPER KIMMERIDGIAN
MIDDLE OXFORDIAN UPPER OXFORD.
Sub-
stage
Hybonoticeras
beckeri
Aulacostephanus
pseudomutabilis
Aspidoceras
acanthicum
Crussoliceras
divisum
Ataxioceras
hypselocyclum
Sutneria
platynota
Idoceras
planula
Epipeltoceras
bimammatum
Euaspidoceras
hypselum
Dichotomoceras
bifurcatus
Gregoryceras
transversarium
Perisphinctes
plicatilis
Submediterranean
succession
Sub-
stage
LOWER VOLGIAN MIDDLE VOLGIAN UPPER VOLGIAN RYAZ ANIAN
Ilowaisk.
klimovi
Ilowaiskya
sokolovi Ilowaiskya
pseudosc.
“Pseudovirg.”
puschi Virgatit.
virgatus
Epivirgatites
nikitini
Kachpurites
fulgens
Garn.
caten.
Crasp.
nodig.
Volgid.
singul.
Rias.
rjasan.
Bojark.
tzikw.
Dorsoplanites panderi Pereg.
albid.
Sphinctoceras
subcrassum
Eosp.
magn.
Pect.
eleg.
Pect.
scitulus
Pect.
wheatl.
Pectinatites
hudlestoni
Pectinatites
pectinatus
Paravirg.
lideri
Pect.
fedor.
Pavlovia
iatriens.
Strajev.
strajev.
Dorsoplanit.
ilovaiskii
Pavlovia
pallasioides
Pavlovia
rotumda
Virgpv.
fittoni
P.albani
S. prim.
Volgid.
lampl.
Subcraspedites
preplicomphalus
Glauc.
glauc.
G. okus.
P.opres.
T. ang.
Kerb.
kerber.
Pereg.
albid.
Bojark.
sten.
P.runc.
H.kochiH.kochi
Ch.sib.
S.icenii
S.analog.
Bojark.
mesezh.
Tollia
tolli
Regional ammonite
zones
Crasp.
taim.
Chet.
chetae
Craspedites
okensis
Dorsoplanit.
maximus
T.exc.
Epivirgatites
variabilis
P. exot.
12 3
UPPER TITHONIAN
LOWER TITHONIAN
Sub-
stage
Tirnovella
occitanica
Protacanthodiscus
andreai
Micracanthoceras
microcanthum
Danubisphinctes
palmatus
“Lemencia”
ciliata
Neochetoceras
mucronatum
Hybonoticeras
beckeri
Submediterranean
succession
Franconites
vimineus
BERRIASIAN
Berriasella
jacobi
Subthurmannia
boissieri
Fig. 1. Correlation of the regional ammonite biostratigraphic scales for the Oxfordian–Berriasian period. Boreal zonal successions are provided for 3 regions: (1) the Russian
Platform; (2) England and (3) Northern Siberia. Correlation of the Volgian part of the succession is after Rogov & Zakharov (2009), with minor corrections.
1624 MA Rogov et al.

Kimmeridge Clay facies of NW Europe (Wierzbowski &
Wierzbowski, 2019). The geochemical studies of the Upper
Jurassic deposits in Central Poland (Socha & Makos, 2016;
Więcław, 2016; Wierzbowski & Wierzbowski, 2019) indicate high
potential for hydrocarbon generation in the dark-grey shaly calca-
reous claystones, interlayered with light-grey mudstones, marls or
marly limestones of the upper Kimeridgian –middle Volgian
Pałuki Formation (100–125 m in thickness). Dark shaly claystones
with TOC values of c. 3–6% on average, sometimes reaching
9–11%, contain mainly Type II kerogen, sometimes with HI reach-
ing 500–700 (Socha & Makos, 2016;Więcław, 2016; Wierzbowski
& Wierzbowski, 2019). However, the T
max
values (423–439°C)
indicate the low thermal maturity of the rocks in the Pałuki
Formation (Wierzbowski & Wierzbowski, 2019).
Black shales are especially widely distributed on the Russian
Platform (Braduchan et al. 1989; Zakharov et al. 2017) but can
be subdivided into three parts based on their stratigraphic ranges
(Figs 2,5a, b).
(1) The lower interval corresponds to the uppermost middle
Oxfordian –uppermost lower Volgian deposits and is character-
ized by very few black shale bands, with TOC values varying from
4–6% to 16–17%. These black shales can be traced over distances
from a few kilometres to c. 1000 km. Black shales are highly
enriched in amorphous OM and commonly contain fine plant
detritus (Bushnev et al. 2006; Gavrilov et al.2014). RockEval
parameters HI and T
max
(Hantzpergue et al.1998; Shchepetova
& Rogov, 2013,2016; Gavrilov et al.2014; Ilyasov et al. 2018) indi-
cate that the Type II and III kerogen is of low thermal maturity
(Fig. 4b), originating largely from marine microplankton with
an admixture of terrestrial plant components. Fossil contents of
these black shales are very different from one band to another.
For example, black shale bands in the basal part of the upper
Oxfordian substage (i.e. Głowniak et al.2010) are characterized
by relatively diverse ammonites, coleoids and benthonic fossils,
which are usually overcrowding bedding planes (Fig. 6f–g). On
the other hand, black shale bands in the upper Kimmeridgian
Mutabilis Zone typically contain very few ammonites and
sometimes bivalve (Aulacomyella) and gastropod accumula-
tions (Fig. 6h).
(2) The middle interval belongs to the middle Volgian
Dorsoplanites panderi Zone (Rogov, 2013). It is characterized by
a succession with well-defined decimetre- to metre-scale cyclicity,
formed by alternations of black shales (ТОС up to 25%) and cal-
careous clays or mudstones. Volgian black shale (so-called Kashpir
Oil Shale, according to Riboulleau et al.2001) developed in a wide
area extending from the Caspian to Pechora seas. Although the
thickness of this sequence varies from a few metres on the mid-
lands of Russia to a hundred metres in the northern slope of the
Oxfordian Ryazanian
middle
Kim Claymeridge
> 4-5 wt % ТОС (higher gamma-ray readings)> 1-2 wt % ТОС (lower gamma-ray readings)
upper lower upper
Kimmeridgian Volgian
lower uppermiddle
Formation
Mandal Formation
Draupne Formation
Tau Formation
N.Sea
south
part
N.Sea
central
graben
N.Sea
Norw.-
Greenl.
N.Sea
Viking
graben
Spekk Formation
Norweg.
Sea East
Greenl.
Svalbard
Hareelv Formation
Bernbjerg Formation
Bernbjerg
Formation
Agardhfjellet Formation
Hekkingen Formation
Alge Member
Barents
Sea
Bazhenovo Formation
Yanov Stan Formation
Urdyuk-Khaya Formation Paksa Formation
Western
Siberia North.
Siberia
England
Scotland
Kim Clay
meridge
Formation
Staffin Shale Formation
Argiles de
Châtillon
Formation
France
Boulonn.
Pa uki Formłation
Podm.
Form.
Mikhalenino
Formation
Novik.
Form.
Promzin.
Form.
Trazovs.
Form.
Boreal black shales Subboreal black shales
Lopsiya ationForm
Central
Poland
European
Russia Subpolar
Ural
Farsund Formation
Fig. 2. Black shale distribution across the J/K boundary in the Boreal areas of the Northern Hemisphere. N –North; Norw –Norway; Norweg –Norwegian; Greenl –Greenland;
North –Northern; Boulonn –Boulonnais; Form –Formation; Novik –Novikovka; Podm –Podmoskovie; Trazovs –Trazovskaya; Promzin –Promzinskaya; for data source see text.
Shelf dysoxic–anoxic event 1625

Caspian depression (Fig. 5a, b), and the thickness of individual
black shale bands changes from one site to other, the general cyclic
structure of this black shale unit remains nearly constant through-
out the whole Russian Platform area (Strachoff, 1934; Shchepetova,
2009; see Figs 5a, b, 7a). Kerogen in the Volgian black shales shows
a low degree of thermal evolution, as determined by the range of
T
max
<435°C derived from RockEval Pyrolysis (Fig. 4b). Most of
these shales are characterized by TOC values >10%; a high hydro-
gen index (HI) corresponds mainly to a Type II kerogen, indicating
predominance of marine OM, and sometimes to a Type I kerogen,
resulting from the accumulation of the most resistant organic com-
ponents. The middle Volgian black shales are especially rich in
ammonites, and can be easily observed in borehole sections, as well
as a diverse bivalve fauna dominated by common Buchia or
Inoceramus, gastropods, brachiopods, echinoderms and diverse
marine vertebrates. In contrast to many other examples of the
SDAE black shales, these beds are also characterized by relatively
diverse and abundant benthic fossils (i.e. Strachoff, 1934;
Vischnevskaya et al.1999; Fig. 6i). Benthic taxa are usually repre-
sented by numerous juveniles in death assemblages related to
anoxic events (Vischnevskaya et al. 1999; Turov, 2000).
Northwards from the middle Volga basin benthic faunal diversity
in these black shales drastically decrease, and only Buchia and
Inoceramus usually occur within black shale beds. Remains of
coleoid molluscs (i.e. Rogov & Bizikov, 2006) and marine reptiles,
especially ichthyosaurs (Zverkov & Efimov, 2019), are also very
typical here.
(3) The upper interval is represented by two occurrences of
black shale beds corresponding to the upper middle Volgian
and lower Ryazanian substages, each known from only a single
locality in Central Russia. Very thin (decimetre-scale) black shale
horizons, highly enriched in marine organic carbon (TOC up to
16–40%, HI up to 278–444; Fig. 4b) are present within the sandy
shallow-water upper middle Volgian –Ryazanian succession
Fig. 3. Typical lithological logs, TOC values,
thickness and stratigraphic distribution of black
shales belonging to SDAE (western Europe).
(a) Swanworth Quarry 1 borehole, England
(Morgans-Bell et al.2001); (b) Blokelv-1
borehole, Jameson Land, East Greenland
(Bojesen-Koefoed et al.2018; age after Alsen
& Piasecki, 2018); and (c) DH2 and DH5R bore-
holes, Spitsbergen (Koevoets et al.2016,2019;
age after Rogov, unpubl. data). Abbreviations:
ba. –bayi; Bath. –Bathonian; Call. –
Callovian; cymod. –cymodoce; dc. –decipiens;
eleg. –elegans; exot –exoticus; I –iatriensis;
Km., Kimmer. –Kimmeridgian; L., Low. –
Lower; lamb. –lambecki; L.Volg. –Lower
Volgian; max. –maximus; Mid. –Middle; mt. –
mutabilis; okens. –okensis; Oxford. –
Oxfordian; ru. –Rugosa; U. –Upper; U. V. –
Upper Volgian; Volg. –Volgian; whea. –
wheatleyensis.
1626 MA Rogov et al.

(6–7 m) (Rogov et al.2015). The upper middle Volgian black shale
is characterized by ammonites, indicating the Epivirgatites nikitini
Zone. In contrast, the Ryazanian shales are nearly barren of
macrofossils and their age assignments are based on their relative
stratigraphic position and a single ammonite record (Rogov
et al. 2015). Black shales belonging to the third interval are
characterized by being bounded by sandstone units, not by mud-
stones (Fig. 7b).
Further eastwards, black shales of the Subboreal type are known
from the eastern slope of the Subpolar Urals (Fig. 2). Here, a single
band of marine black shale with TOC 12–13%, HI 278–446 and
T
max
408–420°C is recorded in the upper Kimmeridgian substage
(Zakharov et al. 2005). It is characterized by a low-diversity mol-
luscan assemblage consisting of cardioceratid ammonites and
numerous Meleagrinella bivalves (Zakharov et al. 2005). This black
shale band was previously ascribed to the Mutabilis Zone
(Zakharov et al. 2005) but, because of the presence of
Aulacostephanus species (indicative of the Eudoxus Zone in the
underlying bed), these black shales were ascribed here to the lower
Eudoxus Zone.
Fig. 4. (a–d) Hydrogen index (HI) versus pyrolysis T
max
diagrams, showing kerogen type and thermal maturity (after Delvaux et al.1990; Tyson, 1995) of Subboreal and Boreal
black shales of Europe. Dashed curve distinguishes kerogen of Type II (marine) and mixed Type II–III (marine and terrestrial). Dotted vertical line (T
max
=430°C)
subdivides immature and mature kerogen. R
0
, vitrinite reflectance.
Shelf dysoxic–anoxic event 1627

All aforementioned black shale occurrences of the Subboreal
type are characterized by variable vertical TOC profiles, caused
by alternation of the black shales with clayey or sandy beds char-
acterized by very low TOC values (Figs 2, 3a, 5a, c). All basins and
sub-basins show strongly diachronous onset and termination of
the black shale deposition. The occurrences of individual black
shale beds or members have significantly changed in space and
time: some of the black shale bands are known from a single local-
ity only, while others cover millions of square kilometres.
Irrespective of such patchy distribution, individual black shale beds
are sometimes characterized by very high TOC values, up to
c. 30–40% (Fig. 2). Taking into account strongly irregular black
shale records, it is not surprising that their occurrences are not
associated with any carbon isotope excursions or faunal turnovers
(except local turnovers influenced by local environmental pertur-
bations). Cyclicity in the Subboreal black shale could be caused by
short-term climate oscillations associated with Milankovich
cycles. A very similar type of cyclicity was described in the
Lower Cretaceous strata of England and northern Germany
(Mutterlose & Ruffell, 1999). Here, the numerous thin pale beds
of mudstone with Tethyan fauna, likely indicating surface waters
depleted in nutrients, were formed during periods with warm arid
Fig. 5. Typical lithological logs, TOC values, thickness and strati-
graphic distribution of black shales belonging to SDAE (eastern
Europe and Siberia). (a) Gorodischi, Memei and Kashpir, composite,
Volga area, Central Russia (Hantzpergue et al.1998; Rogov, 2010,
2013; Gavrilov et al. 2014); (b) borehole 559, Samara region,
Central Russia (Kulyova et al. 2004); (c) Cape Urdyuk-Khaya,
Nordvik peninsula, Northern Siberia (Kashirtsev et al.2018); and
(d) borehole no. 6, Western Siberia (Panchenko et al. 2015,2016).
autissiod. –autissiodorensis; Kimm. –Kimmeridgian; L., Low. –
Lower; LV, L.V. –Lower Volgian; Mid. –Middle; mt. –mutabilis;
Oxford. –Oxfordian; U. –Upper; UV –Upper Volgian. For legend
see Figure 3.
1628 MA Rogov et al.

Fig. 6. Typical fossils encountered in Upper Jurassic black shales in Russia. (a, c) Megaonychites, coleoid hooks from the Volgian Stage of Western Siberia; (b, e) Ammonites
associated with Buchia bivalves and oyster; (e) in the ammonite umbilical region, Volgian Stage of Western Siberia; (d) Inoceramus from the Bazhenovo Formation of Western
Siberia; (f) Upper Oxfordian black shales showing typical accumulation of juvenile and mature ammonite shells, occasional bivalves and shell debris; Mikhalenino section,
Kostroma region, Russia; (g) Fossilized soft tissue remains of the fin and part of the body of coleoid mollusc (age and locality as for (f)); (h) Numerous complete and disarticulated
shells of planktonic bivalve Aulacomyella, Upper Kimmeridgian, Memei section, Middle Volga area, Russia; (i) Coleoid Acanthoteuthis (in the central part of the figure), piece of
big-sized inoceramid bivalve (in the top), shell debris and limped gastropod(below coleoi d), Middle Volgian of well 559, Samara region, Russia. Scale bar for (f, g) 5 cm;scale barfor
other specimens is 1 cm (see lower right corner of the figure).
Shelf dysoxic–anoxic event 1629

climate while the dark mudstone layers, with a dominantly Boreal
fauna, were deposited under cooler-water conditions, rich in
nutrients. Although calcareous nannofossils are poorly preserved
in black shales due to diagenetic overprint (e.g. in the middle
Volgian black shales of the Volga area; see Ruffell et al. 2002),
ammonite data at least partially support cooler environments dur-
ing deposition of the dark shale beds. The well-traced black-
shale-bearing interval near the top of the Kimmeridgian
Eudoxus Zone is commonly overcrowded by Boreal cardioceratids
Nannocardioceras, while the overlying grey mudstone beds of the
base of the Autissiodorensis Zone are usually characterized by
numerous Tethyan aspidoceratids both on the Russian Platform
and in England (Rogov, 2010). However, the changes in ammonite
assemblages of the middle Volgian Panderi zone rather show a
gradual warming through time, with increasing Subboreal
and/or Boreal ammonite ratios within both the black shales and
grey clays (Rogov, 2013).
High-latitude black shale deposits, referred to here as Boreal
type (Fig. 2), have been the focus of numerous integrated studies
over the past decades. Black shales of the Boreal type are very
common in the Arctic, ranging from the North Sea (Vollset &
Doré, 1984; Cornford, 1998), East Greenland (Stemmerik et al.
1998; Alsgaard et al. 2003; Bojesen-Koefoed et al. 2018) through
Norwegian and Barents sea basins (Dalland et al.1988;
Mutterlose et al. 2003; Langrock & Stein, 2004; Georgiev et al.
2017) to Spitsbergen (Dypvik, 1985; Koevoets et al. 2018a,b),
and further E-wards via the Russian part of the Barents Sea shelf,
South Kara basin and the Western Siberian depression (Braduchan
et al. 1986; Ryzhkova et al. 2018) to the Yenissei–Khatanga depres-
sion (Zakharov et al. 2014; Kashirtsev et al. 2018) and the Lena
River lower reaches in the north (Rogov et al. 2011). Eastwards
from the Lena River basin, Upper Jurassic successions (sometimes
extremely thick, up to 5–6 km thickness) are characterized by the
common presence of volcanic rocks, including tuffs and lava flows.
Upper Jurassic –lowermost Cretaceous black shales appear in
Alaska and the Arctic Canada (Leith et al.1992).
Black shales are inherent constituents of the Late Jurassic –
Early Cretaceous formations of the North Sea Graben Province.
Thick accumulations of moderately organic carbon-rich shale
commonly include extended intervals (from tens to several hun-
dred metres) represented by dark-olive-grey fissile shales, enriched
in marine organic carbon up to 5–6% in average, in some cases to 8
to 12–15% (Miller, 1990; Cornford, 1994,1998). They are also
known as ‘hot shale’due to the strong natural gamma-ray response
(Miller, 1990; Clark et al. 1993; Cornford, 1994,1998; Underhill,
1998; Gautier, 2005). The stratigraphic range of the ‘hot shales’
in the Central Graben (unit B of the Upper Kimmeridge Clay
Formation, Mandal and Farsund Formations) and in the Viking
Graben (Draupne Formation) is Volgian–Ryazanian (Vollset &
Doré, 1984; Clark et al.1993, Cornford, 1994; Ineson et al.
2003; Badics et al.2015; Ziegs et al. 2017), comparable to the
Bazhenovo Formation in Western Siberia. In some cases the
‘hot shale’is overlain by upper Ryazanian black shales, which
are less rich in organic carbon (Lott et al.1989). The lithostratig-
raphy and gamma-ray log patterns of the core sections (Vollset &
Doré, 1984; Clark et al.1993; Ineson et al.2003) when using cali-
brated relationships between gamma-log response and TOC values
(Cornford, 1998) suggest an almost uniform distribution of the
elevated TOC values within the ‘hot shale’units, thus also revealing
its resemblance to the Bazhenovo Formation. Kerogen in the ‘hot
shales’is of Type II, originating from a mixture of degraded terres-
trial and planktonic marine OM, resulting in the high hydrogen
index values of 350–650 (Cornford, 1994,1998; Gautier, 2005;
Ponsaing et al.2020; see Fig. 4c). The lower interval (upper
Kimmeridgian –lower Volgian) of the black shale (TOC, 2–6 wt%)
with lower gamma-ray values is present in the Moray Firth, Viking
Graben and Norwegian–Danish Basin (Miller, 1990).
In Greenland, the Upper Jurassic –lowermost Cretaceous black
shales are well-known but have only been studied in detail in
Jameson Land. Here, upper Oxfordian –lower Volgian black shales
of the Hareelv Formation, with TOC values of 5–10% and kerogen
Type III or Type II–III, seem to be degraded as a result of their pre-
oil-window maturity (Bojesen-Koefoed et al.2018; Figs 2, 3b, 4d).
Macrofossils recovered from black shale of the Blokelv-1 borehole
are represented by ammonites, belemnites, onychites, bivalves and
vertebrates, belonging to typical Boreal and Subboreal taxa (Alsen
& Piasecki, 2018). Coeval strata of the Kuhn Ø belonging to the
upper Oxfordian –lowermost Ryazanian Bernbjerg Formation,
are dominated by dark-grey to black mudstones with TOC values
of 2.8–5.4% (TOC data for the Volgian part of the succession
remain unpublished); these rocks are also rich in ammonites,
belemnites and buchiid bivalves (Alsgaard et al. 2003; Pauly
et al. 2013; Kelly et al. 2015). The presence of black shale facies
was also reported from North Greenland, where black shales of
the Ladegardsaen Formation (Kimmeridgian–Volgian) are known
Dorsoplanites panderi zone
Ryazanian
(a)
(b)
Volgian
Fig. 7. Black shales of the Subboreal type. (a) Middle Volgian black shales in the
Gorodischi section (middle Volga area, Russia). Thickness of black shale member is
c. 6 m (photograph by MA Rogov). (b) Ryazanian black shales of the Subboreal type
embedded between two sandy units in the Kashpir section (middle Volga are a, Russia).
Diameter of a coin c. 2 cm (photograph by SV Maleonkina).
1630 MA Rogov et al.

from Peary Land (Håkansson et al. 1981), while in Kilen black
shales of the Dromledome Formation are Ryazanian –?
Hauterivian in age (Hovikoski et al.2018). No TOC data have been
published from north Greenland.
Black shales of the Late Jurassic –earliest Cretaceous age are
well-represented in Svalbard (Figs 2,3c, 8a). In contrast to afore-
mentioned examples, in some cases the oldest black shale occur-
rences (with TOC contents up to 12%) are of Callovian age
(Dypvik, 1985). However, mass deposition of black shales in this
area generally began during late Oxfordian time and ended during
late Volgian or early Ryazanian time (Koevoets et al.2016,2019).
Two peaks of TOC values are recorded in the black paper shale,
traced in both near-shore and offshore areas (Nagy et al.1988;
Koevoets et al.2018a): upper Oxfordian –lower Kimmeridgian
(ТOC up to 6–11%); and uppermost middle –upper Volgian
(ТОС up to c. 12–14%) (Fig. 2), with background values oscillating
around 1–3%. According to Koevoets et al.(2018a), the low HI val-
ues (50–200) in the black shales result from extensive thermal deg-
radation, as indicated by T
max
values of 448–476°C (Fig. 4d). These
suggest a higher initial quality of kerogen (Type II), which is most
likely marine in origin. Black shales belong to the upper part of the
Agardhfjellet Formation (Fig. 8a), which is well-exposed in
Spitsbergen, and its fossil assemblages have attracted much atten-
tion during the last decade (Hammer et al. 2013; Delsett et al. 2016;
Koevoets et al.2016,2018a,b,2019). Ammonites and Buchia
bivalves are the most typical fossils, but sometimes other bivalves
and gastropods as well as belemnites, onychites and echinoderms
are also abundant (Hammer et al.2013; Koevoets et al.2018a).
Vertebrate remains, including skeletons of diverse marine reptiles
(Delsett et al.2016) as well as fish bones and scales, are also very
typical components of the black shale fossil assemblage (Koevoets
et al. 2018b).
Black shales from the southern part of the Norwegian Sea
belong to the Spekk Formation, ranging in age from Oxfordian
to Ryazanian (Dalland et al. 1988). Sometimes the Spekk
Formation is referred to the Volgian–Ryazanian interval only,
underlain by the Oxfordian–Kimmeridgian siltstones of the
Rogn Formation. The high TOC values (from 4–5% to 10–13%)
and mixed Type II and III kerogens were identified for the
Spekk Formation of the Haltenbanken area (Cornford, 1998). In
the near-shore part of the basin (core 6307/07-U-02) highest
TOC values (up to 7%) are reported from the Volgian stage (espe-
cially the lower Volgian substage), and show a gradual decline
towards the Ryazanian stage (with TOC values up to 4%)
(Langrock & Stein, 2004). Black shales in the northern part of
the Norwegian Sea shelf (Hekkingen Formation) are of late
Oxfordian –Ryazanian age. These rocks are represented by dark
silty claystone, which is characterized by the uniformly elevated
(2–7%) level of total organic carbon, with a peak in the approxi-
mate upper Volgian substage (Smelror et al.2001; Mutterlose
et al.2003; Fig. 2). As at other Boreal sites, macrofossils here are
dominantly represented by ammonites and Buchia bivalves, as
revealed from the borehole 6814/04-U-02 (Smelror et al.2001).
Very similar patterns of black shale occurrences are reported from
the Norwegian sector of the Barents Sea shelf (Langrock et al.2003;
Langrock & Stein, 2004; Georgiev et al.2017). Here, these rocks are
also of late Oxfordian –late Ryazanian age (Fig. 2). In the
southwestern part of the Barents Sea, the maximum TOC
(15.4%) and HI (300–430) values and the highest hydrocarbon
generation potential (Fig. 4b) have been estimated within the upper
Oxfordian –Kimmeridgian Alge Member of the Hekkingen
Formation (Helleren, 2019). However, macrofaunas of these
black shales are insufficiently known. Only Oxfordian –
Kimmeridgian ammonites are well-studied (Wierzbowski et al.
2002; Wierzbowski & Smelror, 2020), and very few Ryazanian
Buchia bivalves and ammonites were mentioned and/or figured
from this region (Wierzbowski et al.2011). Information concern-
ing the black shales of the Russian sector of the Barents Sea is very
limited, as all boreholes drilled here are poorly sampled, and char-
acteristics of rocks were mainly based on an analysis of geophysical
data and cuttings. The range of the black shale facies here can be
roughly estimated as Kimmeridgian–Ryazanian. TOC values of
these black shales lie mainly between 2 and 23%, but fluctuations
of TOC distribution through the succession remains unclear
(Basov et al.2009). Fossils recovered from these black shales
mainly belong to ammonites and Buchia; in addition to these
groups, lingulid brachiopods, coleoid hooks and onychites
(including big-sized megaonychites) were reported from the
Volgian interval of black shales.
Black shales enriched by organic carbon were also reported
from the upper Oxfordian –Ryazanian Hofer Formation of
Franz-Josef Land (Kosteva, 2005), but information concerning
TOC remains unpublished. Leith et al.(1992) indicated that
Upper Jurassic samples from this area have organic carbon con-
tents of only 1–3%.
Upper Jurassic –Lower Cretaceous black shales are especially
well-known in Western Siberia. Although the black shale lithofa-
cies ranges from the upper Oxfordian to the Hauterivian
Ryazanian
(a)
(b)
Volgian
Fig. 8. Black shales of the Boreal type. (a) Kimmeridgian black shales at
Myklegardfjellet, Spitsbergen (photograph by DS Zykov). (b) Volgian–Ryazanian
boundary beds at Nordvik, northern Siberia (photograph by M Mazuch).
Shelf dysoxic–anoxic event 1631

(Braduchan et al.1989), the Volgian–Ryazanian black shales
of the famous Bazhenovo Formation (Braduchan et al.1986;
Panchenko et al.2015; Ryzhkova et al. 2018) and its time
equivalents are more widely distributed across Siberia. Upper
Oxfordian –Kimmeridgian black shale facies here are restricted
by the NE part of Western Siberia. These black shales belong to
the Yanov Stan Formation (upper Oxfordian –Ryazanian, up to
300 m), and show relatively low TOC values (2.8% average) with
a general trend to gradual TOC rise upsection, with maximum val-
ues up to c. 8%. As follows from the RockEval data, Type II kerogen
and Type III kerogen occurred here in nearly equal quantities
(Afanasenkov et al.2018). Several members of the Bazhenovo
Formation characterized by different dominant micro- and macro-
fossil groups can be recognized within the Bazhenovo Formation
(Panchenko et al.2015,2016). In the central regions of Western
Siberia (Khanty-Mansiysk–Tyumen), the Bazhenovo Formation
as well as its time analogue in the west of this area (lower
Tutleim Member) can be subdivided into the lower (lower–middle
Volgian) and upper (upper Volgian –Valanginian) members. The
lower member (15–20 m) is composed mainly of dark-brown lami-
nated siliceous shales rich in radiolarians, with low (30–40%) clay
content and high (5–10%) organic carbon enrichment. Almost
pure, devoid of clay, structureless dark chalcedonite with thin
radiolarite intercalations prevail near the top (2–5 m) of the lower
member. The upper member of Bazhenovo Formation (15–25 m)
starts with very dark-brown clayey silicite (2–6 m) abundant in
bivalve shells. This thin, but laterally traceable clayey unit corre-
sponds to the upper Volgian substage. It is overlain by thinly lami-
nated black shale with abundant calcareous nannoplankton of
Ryazanian age, which is highly enriched in organic carbon
(up to 20% and more). Within the upper Valanginian interval,
the black shales are gradually followed by terrigenous clays with
decreasing TOC, suggesting an increase in sedimentation rates.
In the eastern part of Western Siberia (Tomsk region) the lower
member, rich in radiolaria, is reduced, and the overlying black
shale unit within the upper member, which is highly enriched in
organic carbon and calcareous nannoplankton remnants, declines
or disappears, while the clayey unit is much thicker. Such a dilution
results in lower ТOC values (5–7%) on average and a displacement
of TOC maxima in the lower member of the Bazhenovo
Formation. It is noteworthy that in spite of non-uniformities in
the organic carbon distribution in core sections from two different
parts of Western Siberia, the numerous RockEval data show mainly
Type II kerogen of marine origin (Lopatin & Yemets, 1987; Peters
et al.1993; Ulmishek, 1993; Kozlova et al. 2015; see Fig. 9c), and the
general trend is characterized by a gradual increase in TOC values
during Volgian time. This is followed by a gradual decrease during
Ryazanian time, although some levels are characterized by
extremely high TOC values (Panchenko et al. 2016; Fig. 2;
Fig. 5d). Highest average values of TOC are reached in the black
shales of the central and southwestern regions of Western
Siberia (Ponomareva et al.2018). Ammonites (Fig. 6b, e), oysters,
Inoceramus and Buchia bivalves (Fig. 6b, d), fish remains and ony-
chites (Fig. 6a, c) are among the most common fossils of the
Bazhenovo Formation (Braduchan et al.1986; Panchenko et al.
2015). Surprisingly, large-sized fossil reptiles are extremely
rare in this area: only one occurrence of ichthyosaur bones is
known to date from the Bazhenovo Formation. This is consistent
with the lithofacies characteristics and usually interpreted as
resulting from deposition in restricted deep-marine environment
persistently prone to anoxic conditions (Zakharov, 2006;
Grishkevich, 2018).
Eastwards from Western Siberia, in the Yenissei–Khatanga
depression, Upper Jurassic and Lower Cretaceous rocks are mainly
represented by shallow-water deposits with low organic carbon
contents (cf. Shurygin & Dzyuba, 2015). However, in the north
of Central Siberia, from the Khatanga Bay in the west to the
Lena River lower reaches in the east, Upper Oxfordian –
Ryazanian black shales become the dominant rock type. Only
the well-known Nordvik section (Fig. 8b), which is the most thor-
oughly studied Boreal section across the J/K boundary (Zakharov
et al.1983,2014; Houšaet al. 2007; Nikitenko et al. 2008,2015;
Kashirtsev et al. 2018), provides information concerning the
organic geochemistry of these black shales. As has been revealed
by Zakharov & Yudovny (1974) and recently approved by
Kashirtsev et al.(2018), typical black shales with elevated TOC val-
ues here are mainly restricted to the upper Oxfordian and middle
Volgian –Ryazanian substages (Figs 2,5c). However, their hydro-
gen index values indicate only kerogen of Type III, with a low
contribution of marine OM, probably due to high sedimentation
rates and dilution with terrigenous components (Fig. 9d). Fossil
assemblages of these black shales are dominated by ammonites,
belemnites, onychites and Buchia bivalves (Zakharov et al.
1983,2014).
Black shales in Arctic Canada still remain insufficiently studied
in terms of high-resolution biostratigraphy and geochemistry.
Generally they are restricted to the Kimmeridgian–Ryazanian
interval, although locally a first occurrence of black shales could
be dated as Oxfordian (Leith et al.1992). Highest TOC values
are reported from the Kimmeridgian part of the succession
(Gentzis et al.1996), while the Volgian–Ryazanian interval is char-
acterized by less than 5% of TOC. Recent data by Galloway et al.
(2019) have revealed that the Deer Bay Formation of the Axel
Heiberg island is generally characterized by low TOC values
throughout the succession. Their two measured sections show
median values of TOC 1.6%, with range 0.9–4.6% (Buchanan
Lake section) and TOC 1.8%, with range 0.8–5.7% (Geodetic
Hills section). Highest TOC values in both sections are recorded
near the top of the formation in the upper Valanginian substage.
Organic matter is represented either by Type III kerogen
(at Buchanan Lake) or by a mixture of Type II and III kerogen
(at Geodetic Hills). As well as in other Boreal shaly facies, ammon-
ites and Buchia strongly dominate in faunal assemblages of these
rocks of Arctic Canada (Jeletzky, 1984).
Rocks that resemble black shales in Northern Alaska belong to
the Oxfordian–Valanginian Kingak Shale Formation (Bird &
Molenaar, 1987). TOC values reported from these shales are rela-
tively few (mainly less than 2%, see Magoon et al.1987; Bayliss &
Magoon, 1988). Imlay (1981) reported ammonites and Buchia
from Kingak Shale, indicating an early Oxfordian –earliest
Ryazanian age of this formation.
3. High-latitude Late Jurassic –earliest Cretaceous black
shale occurrences in the Southern Hemisphere
Black shale facies, mainly represented by ‘massive’black shale
members or formations that resemble black shales of the Boreal
type, are common in the high-latitude areas of the Southern
Hemisphere (Figs 10,11). These are the Tithonian –lower
Valanginian Vaca Muerta Formation of the Neuquen Basin
(Argentina, see Kietzmann et al.2016); upper Callovian –
Berriasian Spiti Shale Formation of Nepal (Cariou et al.
1994; Enay, 2009); Tithonian black shales of the Suowa
Formation, Tibet (Chen et al.2012; Yang et al. 2017);
1632 MA Rogov et al.

Kimmeridgian–Tithonian black shales of the Jhuran Formation
in Kutch (Arora et al. 2015); and Kimmeridgian–Berriasian black
shales of the Nordenskjöld Formation, Graham Land, Antarctic
(Doyle & Whitham, 1991). Oxfordian–Tithonian black shales of
the Falkland Plateau (Deroo et al.1983) are the only Upper
Jurassic black shales recorded during the Deep Sea Drilling
Program. Macrofossil assemblages of these black shales mainly
includes ammonites, vertebrates and, in some cases, Buchia
homoeomorphs ascribed to Australobuchia (Doyle &
Whitham, 1991).
All aforementioned high-latitude black shales of the Southern
Hemisphere shared many common features with Boreal black
shales discussed in the previous section. These are relatively thick
black shale units characterized by elevated TOC values. They were
deposited in anoxic–dysoxic environments, and characterized by
low-diversity benthonic faunas represented by taxa tolerant to
low-oxygen contents.
4. Low-latitude Late Jurassic –earliest Cretaceous black
shales
The ‘Late Jurassic ocean anoxic event’(Nozaki et al.2013; Arora
et al.2015)or‘Oxfordian–Kimmeridgian anoxic event’earlier sug-
gested by (Trabucho-Alexandre et al.2012) is not global in the
strict sense because low-latitude occurrences of Upper Jurassic –
lowermost Cretaceous black shales are scarce and known from
only a few areas (Figs 10,11). These are the upper Oxfordian –
Kimmeridgian Haynesville Shale Formation in east Texas and
Louisiana (Hammes et al. 2011) and the upper Oxfordian –
Tithonian black shales of Mexico (La Casita Formation, La Caja
Fig. 9. (a–d) Hydrogen index (HI) versus pyrolysis T
max
diagram, showing kerogen type and thermal maturity (after Delvaux et al.1990; Tyson, 1995) of Subboreal and Boreal black
shales of the Barents Sea region and Siberia. Dashed curve distinguishes kerogen of Type II (marine) and mixed Type II –III (marine and terrestrial). Dotted vertical line (T
max
=430°
C) subdivides immature and mature kerogen; R
0
, vitrinite reflectance.
Shelf dysoxic–anoxic event 1633

Formation, Pimienta Shale Formation; see Goldhammer &
Johnson, 2002 for details). Middle Oxfordian black shales
showing elevated TOC values (up to 6%) are known in the
Khodjaipak Formation, Uzbekistan (Carmeille et al. 2020), while
Kimmeridgian black marlstones of the Akkuyu Formation (SW
Turkey) show very high TOC values (up to 30%, see Baudin
et al. 1999), comparable with those of Boreal regions.
Oxfordian–Kimmeridgian black shales intercalated with lime-
stones were also reported from the Antalo Limestone Formation
in Ethiopia (Mohammedyasin et al.2019) and Oxfordian
Fig. 10. Oxfordian and Kimmeridgian palaeogeography
(after Rees et al. 2000, with minor changes) and
worldwide distribution of black shales.
1634 MA Rogov et al.

black shales are known of in Morocco (Davison, 2005). Upper
Jurassic black shale occurrences are also known from Yemen
(Hakimi & Ahmed, 2016, restricted to Kimmeridgian Madbi
Formation) and northern Iraq (Tithonian–Berriasian Chia
Gara Formation; see Tobia et al.2019). In all aforementioned
examples, low-latitude black shales occur in restricted basins
(Figs 10,11). In contrast to high-latitude black shales, these beds
are frequently intercalated with limestones and marls and their
deposition seems to be influenced mainly by local environmen-
tal factors.
Fig. 11. Volgian and Ryazanian palaeogeography and
worldwide black shale distribution.
Shelf dysoxic–anoxic event 1635

5. Key features of the Late Jurassic –earliest Cretaceous
high-latitude shelf dysoxic–anoxic event
As emphasized in Section 1, in contrast to OAEs, the onset and
offset of the SDAE are strongly diachronous in different basins
and sub-basins. In all cases, however, the duration of black shale
deposition was very long compared with those of OAE-related
black shales, indicating nearly constant presence of anoxic–dysoxic
near-bottom depositional conditions over periods of 10–20 Ma.
The very widespread geographic distribution of these black shale
facies in high latitudes excludes any significant influence of local
factors on their deposition, while the traceability of individual
black shale beds and their relationship with overlying and under-
lying units were controlled by local environmental conditions.
Although different regions are characterized by different patterns
of TOC values through time, at least for black shales of the Boreal
type, maximum TOC contents were reported for the upper
Volgian, that is, for the Jurassic–Cretaceous transitional strata.
Although some attempts at chemostratigraphic correlation of these
rocks were undertaken recently (Turner et al.2019), deposition of
high-latitude Upper Jurassic –lowermost Cretaceous black shales
are not associated with any significant carbon isotope excursions.
The only carbon isotope excursion during middle Volgian time,
the so-called VOICE (Volgian Isotopic Carbon Excursion), can
be recognized in some Boreal areas (Hammer et al. 2012;
Koevoets et al. 2016; Galloway et al. 2019); in contrast to excur-
sions related to the typical OAEs it is relatively long, spanning
nearly the whole middle Volgian age, and showing some diachro-
neity between the basins. The onset of black shale deposition was
strongly asynchronous in the different basins and even different
sub-basins, and the same is also true for the termination of black
shale deposition with the end of the SDAE.
By comparison with coeval units deposited in the well-
oxygenated environments, these black shales are characterized
by a very low diversity of benthic organisms represented by a
few dominant genera tolerant of low oxygen contents, especially
by burrowing suspension-feeding bivalves (Oschmann, 1988,
1994) as well as by nektonic and planktonic animals. In black
shales of the Boreal type, benthic fossils usually only occur in thin
intervals, which indicate short-time increases in oxygen contents
near the sea floor, while other parts of the successions are typically
barren of benthic macrofossils. Nevertheless, these long-lasting
anoxic conditions did not cause significant faunal turnover or
extinction.
6. Possible causes of the Late Jurassic –earliest
Cretaceous shelf dysoxic–anoxic events
Rising temperatures accompanied by increased productivity,
together with slow ocean circulation, are considered among the
factors responsible for prolonged black shale deposition in the
Arctic (Georgiev et al. 2017). Indeed, gradual warming during
the Late Jurassic epoch in the Boreal areas is supported by palynol-
ogy (Dzyuba et al.2018), clay mineralogy (Ruffell et al.2002) and
oxygen stable isotope data (Price & Rogov, 2009; Zakharov et al.
2014; Dzyuba et al.2018). Cold-water glendonite pseudomorphs
are common in the Middle Jurassic deposits of Siberia (Morales
et al.2017), but they rarely occur in the Upper Jurassic deposits.
Glendonites are absent from the upper Kimmeridgian –lower
Ryazanian interval of Siberia (Fig. 12). However, additional factors
can be invoked to explain such unique SDAE, because other
Mesozoic warming events are not associated with such long-time
black shale deposition. It is very possible that, in tandem with
warming and changes in oceanic circulation caused by Pangaea
break-up, significant changes in the plankton ecosystems could
be responsible for long-lasting high productivity. Relatively higher
humidity in the Subboreal and Boreal realms compared with low-
latitude territories caused more precipitation and more intensive
land drainage; the chemical weathering therefore prevailed, at least
periodically (or seasonally). Hypopycnal plumes of muddy fresh-
water could have supplied high amounts of dissolved nutrients to
the basins. This led to unfavourable conditions for accustomed
Fig. 12. Climate indicators and black shale distribution in Western and north of Eastern Siberia. Blue bars showing amount of glendonite-bearing sites (Rogov et al.2019, with
minor changes), while orange and red lines showing Classopollis pollen abundance (based on Vakhrameyev, 1982, with some additions from Ilyina, 1985; Nikitenko et al.2015).
1636 MA Rogov et al.

marine plankton in the surface waters because of their opacity,
abnormal salinity or ‘nutrient oversaturation’; their high buoyancy
prevented mixing and contributed to stratification. Such condi-
tions in the upper photic zone could cause rapid growth of new,
more tolerant, planktonic communities. Periodical (or seasonal)
restoration of such conditions was important for long-lasting
changes of planktonic communities, resulting in black shale
deposition directly or indirectly. In our opinion, immigration of
calcareous nannoplankton to high latitudes is an indicator of such
a new but very widespread ecological condition in the Subboreal
and Boreal seas.
Enhanced productivity of high-latitude basins may have had a
significant influence on black shale deposition. Some intervals of
the Bazhenovo Formation are so enriched by radiolarians
(Khotylev et al.2019) that they are considered here as rock-
building, while other intervals consist mainly of calcareous nanno-
plankton (Zanin et al. 2012). However, silicites, typical of the
Bazhenovo Formation, rarely occur outside of Western Siberia.
In our opinion, immigration of calcareous nannoplankton to
high latitudes is especially important for long-lasting changes of
plankton communities and the resulting black shale deposition.
It should be noted that coccolithophores, which originated during
the Triassic Period (Mutterlose et al.2005), are now responsible for
much of the primary oceanic productivity (Malone, 1971; Rost &
Riebesell, 2004). During the Late Jurassic epoch, calcareous nanno-
plankton shows a significant growth in diversity (Bown et al.2004;
Suchéras-Marx et al.2019), but in high-latitude Arctic it is only
recognized near the Jurassic–Cretaceous transition (Smelror
et al.1998; Mutterlose et al.2003; Zanin et al.2012;Pauly et al.
2013; Rogov & Ustinova, 2018). Calcareous nannoplankton taxa
have therefore controlled an increase in primary productivity,
and an ecosystem disturbance seems to be one of the causes of
Late Jurassic –earliest Cretaceous high-latitude SDAE. This
hypothesis should be investigated in future studies. It should be
noted that the onset of the studied interval of black-shale deposi-
tion across the Jurassic–Cretaceous boundary coincides with a shift
from abiotic to biotic control on evolution of calcareous plankton
(Eichenseer et al. 2019). Among the other biotic factors, radiolar-
ian blooms should also be considered as regionally responsible for
black-shale deposition, that is, for the Bazhenov Formation of the
Western Siberia (Khotylev et al. 2019). Moreover, mass occur-
rences of radiolarians were also considered as very important
for deposition of the Late Devonian black shales in European
Russia (Afanasieva & Mikhailova, 2001) However, among the
studied Upper Jurassic –lowermost Cretaceous successions with
black shale abundance, there are very few showing mass occur-
rence of radiolarians. In these cases, radiolarians are easily visible
in thin-sections, and these microfossils became rock-forming.
Reduced salinity can also be favourable for black shale deposi-
tion. For example, recent studies of the Toarcian strata of the
Cleveland Basin have revealed a significant salinity drop during
the black shale interval associated with the Toarcian OAE, in spite
of the common occurrence of ammonites that sharing a low toler-
ance to salinity decrease with other cephalopods (Remírez & Algeo,
2020). However, the gradual decrease of salinity in the Middle
Russian Sea during Oxfordian–Kimmeridgian time, recognized
through clumped isotope studies (Wierzbowski et al. 2018), shows
no correlation with the prominent black shale horizons that
occurred mainly in two very short intervals (at the beginning of
the late Oxfordian and middle part of the late Kimmeridgian
Mutabilis Chron). Although reduced salinity of the Arctic area
was suggested as the one of the factors controlling regional distri-
bution of faunas by Hallam (1969), any evidence of a long-term
drop in salinity in high-latitude areas during Late Jurassic –
earliest Cretaceous time is missing. During the period considered
here, both planktonic and nektonic faunas in high latitudes were
nearly of the same type as before or after the SDAE, and were
represented by the same taxa (at least at the family level).
7. Conclusions
The Late Jurassic –earliest Cretaceous episode of prolonged
(10–20 Ma) black shale deposition in extra-tropical latitude areas
is identified as a shelf dysoxic–anoxic event (SDAE). This specific
type of anoxic event differs from typical OAEs. During the Late
Jurassic –earliest Cretaceous time interval, OM-rich sediment
accumulated mainly in the high latitudes. The onset and termina-
tion of SDAE was diachronous in separate palaeobasins. Subboreal
and Boreal patterns of black shale deposition, caused by variations
in the stability of oxygen-depleted environments, has been
recognized. Among the main drivers of the SDAE, global climatic
warming led to significant palaeo-oceanographic changes favour-
ing a sluggish stratification-prone circulation pattern, driving the
advancement of widespread dysoxia–anoxia-prone environments
onto shallow shelves. These could have led to a disturbance in
planktonic ecosystems and to an invasion of calcareous, siliceous
and organic-walled nannoplankton to high-latitude palaeobasins,
providing huge reserves for warmer and more transparent water
masses during the considered period.
Acknowledgements. This study was supported by RSF grant 17-17-01171.
The authors thank IV Panchenko (JSC MiMGO, Moscow) and YuA
Gatovsky (Moscow State University, Moscow) for providing fossil specimens
from western Siberia figured in this article. Two anonymous reviewers are
kindly acknowledged for their valuable comments. We are especially grateful
to Bas van de Schootbrugge for his work on improvement of the English
language.
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