GeolCarp_Vol61_No2_111

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA, APRIL 2010, 61, 2, 111—120 doi: 10.2478/v10096-010-0004-5

Time series analysis (orbital cycles) of the uppermost

Cenomanian—Lower Turonian sequence on the southern

Tethyan margin using foraminifera

MOHAMED SOUA

Entreprise Tunisienne d’Activités Pétroli

res, ETAP-CRDP 4 Rue des Entrepreneurs, 2035 la Charguia II, Tunisia; mohamed.soua@etap.com.tn

(Manuscript received April 30, 2009; accepted in revised form December 11, 2009)

Abstract: Time series analysis has been performed for the first time on the Cenomanian-Turonian sequence in Central
Tunisia in order to shed light on its Milankovitch-like cyclicity. This analysis was applied to two foraminiferal genera:
the biserial Heterohelix, an oxygen-minimum zone (OMZ) dweller, and the triserial Guembelitria, a eutrophic surface
dweller. Average sedimentary rates and the duration of the oceanic anoxic event (OAE2) in each studied section were
estimated. The fluctuations in abundance of these two opportunistic species can be related mainly to both precessional
(ca. 20 kyr) and eccentricity (100 and 400 kyr) cyclicity suggesting that changes in surface water fertility were linked to
climate changes in the Milankovitch frequency band.

Key words: Cenomanian-Turonian boundary, time series analysis, cyclostratigraphy, Milankovitch cyclicity, Heterohelix
spp., Guembelitria spp.

Introduction

Understanding  of  the  biotic  and  sedimentary  events  that
marked  Earth  history  is  of  great  interest  in  the  geoscience
community (e.g. Haq et al. 1987; Sepkoski 1993; Kauffeman
1995; Sageman et al. 1998; O’Dogherty & Guex 2001; Soua
&  Tribovillard  2007).  The  Cenomanian-Turonian  (C/T)
boundary  lies  in  a  time  interval  characterized  by  a  cluster  of
significant  events,  some  of  which  are  biotic  (mass,  step-by-
step  or  even  pseudo-extinction  (O’Dogherty,  personal  com-
munication, progressive recolonization of new habitats, etc.),
others are sedimentary (forced regression following an intense
transgression) (e.g. Robaszynski et al. 1990; Hardenbol et al.
1998;  De  Wever  et  al.  2003;  Zhao  et  al.  2004).  The  chrono-
metric scale across the C/T boundary has been defined by (1)
radiometric ages, such as 40

/39

determined from bentonite

layers at the Pueblo section (e.g. Obradovich 1993; Kowallis
et al. 1995), (2) ammonite biostratigraphy (e.g. Kennedy &
Cobban 1991), and (3) linear interpolation (where the sedi-
mentation rate is assumed to be constant) inside the magnetic
anomaly C34 (Cretaceous Normal Super-Chron).

It has been suggested that the marl-limestone alternations

characterizing the C/T transition in Central Tunisia may have
originated from orbital forcing (Vonhof et al. 1998; Neder-
bragt & Fiorentino 1999; Caron et al. 1999; Soua & Tribovil-
lard 2007). The thus induced climatic changes produced
alternations between terrigenous input and pelagic-hemipelag-
ic carbonate sedimentation.

Geological framework

According to the tectonic setting, this area is regarded as be-

ing a transitional zone between the Tunisian Northern Atlas,

dominated by diapiric salt structures, and the Tunisian Central
Atlas, dominated by a NE-SW trending fold axis, which is in-
tersected  by  many  trough  faults  bordering  on  Neogene-filled
half-graben  systems  (Soua  et  al.  2009).  The  C/T  interval  is
represented  by  the    black  shales  of  the  Bahloul  Formation
(Robaszynski et al. 1990; Maamouri et al. 1994; Caron et al.
1999;  Nederbrag  &  Fiorentino  1999;  Soua  2005;  Soua  et  al.
2006, 2009; Soua & Tribovillard 2007) in some localities, the
sedimentation  was  controlled  by  basin  morphology  created
either  by  Triassic  salt  halokinesis  (Fig. 1A)  affecting  the
Cretaceous  series  or  by  other  syn-sedimentary  tectonic  activi-
ties. The eastern part belongs to the Bargou area that is connect-
ed paleogeographically to central Tunisia. It is characterized by
(1)  emerged  paleohighs  displaying  gaps  and  discontinuities,
and by (2) subsiding zones affected by deep-water sedimenta-
tion.  This  area  is  dominated  by  N140°  and  N70°  trending
faults limiting several blocks.

Uniquely, in the Dir Ouled Yahia (COK) situated in Bargou

area and Guern Halfaya (GH) sections, the top of the Bahloul
Formation represents many Cenomanian olistolith levels
marking syndepositional tectonic activities. Elsewhere, these
syn-sedimentary features are represented by local slumping as
in the Jerissa (CES) and Hammem Mellegue (HM) sections.

Material and methods

A total of 219 samples were collected throughout the Bahloul

Formation, which is well exposed in these four sections
(Fig. 1A,B). Calcium carbonate (CaCO

) analysis was con-

ducted in the Geochemistry Laboratory at the Faculty of Sci-
ence of Tunis University, the total organic carbon (TOC) was
determined on crushed samples with a Rock-Eval II type ma-
chine in ETAP. Thin section analysis was conducted in the

113

CENOMANIAN—TURONIAN TIME SERIES ANALYSIS USING FORAMINIFERA

geological  laboratory  of  the  Office  National  des  Mines
(ONM).  High-resolution  biostratigraphical  and  geochemical
studies have previously been undertaken (Soua 2005; Soua &
Tribovillard  2007)  and  the  essential  foraminiferal  zone  and
subzone divisions have been established there. In the present
paper, only the abundances of Heterohelix spp. (total Hetero-
helix species) and Guembelitria spp. (total Guembelitria spe-
cies)  in  each  section  were  used  for  time  series  analysis.
Spectral analysis was carried out using an integrated software
package (PAST, Hammer et al. 2001). Spectral estimation was
applied  to  the  Heterohelix  spp.  and  Guembelitria  spp.  fre-
quency (%) data series. The use of the spectral estimation en-
sures a better confidence factor when spectral analysis is used
as a cyclostratigraphic tool.

Results

The Bahloul Formation in the studied sections (Fig. 1B)

consists  of  alternating  limestone,  argillaceous  limestone  and
rhythmically  bedded  marls.  Uniquely,  in  the  Bargou  and
Guern Halfaya areas (COK and GH sections) it is represented
by  an  original  organic-rich  and  siliceous  facies  (Soua  et  al.
2006; Soua & Tribovillard 2007; Fig. 2.3). The Bahloul lime-
stone  consists  of  wackstone/packstone  clay-poor  carbonates
rich in microfossils (filaments, Fig. 2.1; radiolarians, Fig. 2.2;
foraminifera, Fig. 2.4; and calcispheres, Fig. 2.5) and macro-
fossils  (ammonites),  whereas  marls  consist  of  clay-rich  car-
bonates  (Robaszynski  et  al.  1990;  Maamouri  et  al.  1994;
Nederbragt & Fiorentino 1999; Caron et al. 1999; Soua 2005;

Fig. 2.  1 – Micrograph of a filament-rich layer from the upper
Bahloul Formation showing elongated and thin forms with imbri-
cated  and  tangled  arrangement  enveloping  sparitic  elements.
They also display several features such as tangling and overlap-
ping.  Moreover,  they  display  heterogeneous  sizes.  2  –  Micro-
graph of a radiolarian-rich layer from the Bahloul in COK section
showing both nassellarian and spumellarian species. 3 – A view
of  the  Bahloul  outcrop  in  GH  section  showing  silica-rich  layers
characterizing  organic-rich  and  siliceous  facies  of  the  GH  and
COK  section.  4  –  Micrograph  of  a  foraminiferal-rich  level
showing  whiteinellids  and  hedbergellids.  5  –  Micrograph  of  a
calcispheres-rich layer from the lower Bahloul Formation.

114

SOUA

Soua & Tribovillard 2007). According to Caron et al. (1999),
periods of low sea levels generally coincide with high detrital
influx (marls) and increased erosion and are predominantly as-
sociated  with  arid  and/or  cooler  climatic  conditions.  Periods
of high sea levels are generally characterized by increased car-
bonate  production  (limestone)  and  low  detrital  influx  under
warm  and  humid  climatic  conditions.  The  Bahloul  black
shales  are  on  average  about  20 m  thick.  A  prominent  micro-
conglomeratic  limestone,  50 cm  thick  on  average,  marks  the
base of the Bahloul. It contains a late Cenomanian ammonite
assemblage  of  the  Metoicoceras  geslinianum  Zone  in  the
Jerissa  (CES)  section  (Accarie  et  al.  1999),  and  a  planktonic
foraminiferal  assemblage  indicative  of  the  uppermost  Rotali-
pora  cushmani  Zone.  It  is  composed  of  grey  limestone  beds
overlain by thinly bedded marls with a few laminated darker
limestone  beds.  These  layers  are  overlain  by  increasingly
clayey limestone beds. In the middle part, the microfauna dis-
play  a  more  dissolved  aspect  and  dwarfism.  Ammonite-rich
and ichnofossil-rich beds are present in the Jerissa (CES), Guern
Halfaya  (GH)  and  Bargou  (COK)  sections  (Fig. 1B)  and  they
are  used  as  a  correlation  tool.  Two  opportunistic  species  (the
biserial Heterohelix and the triserial Guembelitria spp.) were se-
lected for this study to carry out spectral analysis. Heterohelix
is considered to be a global biomarker that reflects the expan-
sion of the oxygen-minimum zone (OMZ) (Soua & Tribovillard
2007).  This  low  oxygen  tolerant  species  dominates  the  faunal
assemblages, averaging up to 80 % in the Bahloul black shales
(Fig. 3), and especially in the lower W. archaeocretacea Zone
(Soua  &  Tribovillard  2007;  Fig. 1B;  Fig. 3).  The  Guembeli-
tria

species thrived in eutrophic surface waters with variable sa-

linities at times of severe ecological stress (Soua & Zaghbib-
Turki 2007; Soua & Tribovillard 2007).

Time series analysis

In the stratigraphic record, sedimentary rhythmicity has

been  well  discussed  and  debated  (see  mainly  Berger  et  al.
1989; House 1995; Weedon 2003). Such records are often in-
terpreted as orbitally controlled especially with the hierarchi-
cal  stacking  of  limestone-marl  couplets  (Berger  1978;  House
1995; Negri et al. 2003; Scopelliti et al. 2006). It is believed that
periodicities  of  precession  and  obliquity  were  not  the  same  in
the past as today (e.g. Berger et al. 1989; Strasser et al. 2006).

Well established correlations between rhythmic sequences

and  geochemical  and  biotic  parameters  have  been  described
by many authors (e.g. Ditchfield & Marshall 1989; Weedon &
Jenkyns  1990),  confirming  temperature  dependence  and  cor-
relations  with  %  CaCO

, nannofossils and foraminifera fre-

quencies  (%),  which  might  be  expected  to  be  controlled  by
temperature  or  other  parameters  related  to  Milankovitch  cy-
clicity (House 1995; Negri et al. 2003). The frequency distri-
bution of limestone-marl alternations and their relation to the
Milankovitch  parameters  is  commonly  tested  by  time  series
analysis  (e.g.  House  1986,  1995;  Weedon  2003).  The  power
spectrum  shows  generally  squared  amplitudes  and  wave-
lengths that represent the periods of regular components in the
time  series  (Weedon  2003).  Conventionally,  the  horizontal
axis  of  the  power  spectrum  represents  Frequency  (Frequen-
cy = 1/period) with the highest frequencies (i.e. shortest oscil-

lations) appearing on the right and the vertical axis represents
Power spectra (Fig. 3). Zero frequency usually corresponds to
oscillations  that  have  periods  exceeding  the  length  of  the
whole  data  set  (Weedon  2003).  The  Heterohelix  spp.  and
Guembelitria spp. relative abundance curves in the four locali-
ties  clearly  exhibit  strong  cyclic  signals  (Fig. 3),  which  al-
lowed us to perform a spectral analysis study, computed using
the PAST software (Hammer et al. 2001), to test for the exist-
ence of periodicities through the oceanic anoxic event (OAE2)
deposits in Central Tunisia. The results of the spectral analysis
are illustrated in Fig. 3. The strongest peaks point to a period of
25 m (Heterohelix spp.) in the HM section (Fig. 3A) and a peri-
od of 17.54 m (Guembelitria spp.) in the CES section (Fig. 3D).

Hammem Mellegue (HM section)

Nederbragt & Fiorentino (1999) argued that in the nearby

Mellegue section the Bahloul Formation is suggestive of Mi-
lankovitch  cycles  due  to  its  regular  strata  (Fig. 1B).  Average
accumulation  rates  of  about  10 cm/kyr  for  the  same  Bahloul
interval  was  previously  determined  by  Vonhof  et  al.  (1998)
suggesting  that  each  2-m-thick  pair  of  laminated  limestone
and bioturbated marly limestone represents a precession cycle
(20 kyr) rather than an obliquity cycle (40 kyr) as seen in other
C/T intervals (e.g. Arthur et al. 1987; Kuhnt et al. 1997; Sage-
man et al. 1998). Time series analysis performed on Heterohe-
lix  spp.  shows  that  the  strongest  peak  points  to  a  period  of
25 m (Fig. 3A). Other statistically significant peaks are found
at  11.9  m  and  5.26 m.  This  latter  peak,  although  statistically
significant, falls in the low-energy field of the spectrum. The
spectral  curve  of  the  Guembelitria  spp.  shows  significant
peaks  at  about  15.62 m  and  7.14 m.  The  4.54 m  and  3.12 m
peaks (Fig. 3A) recorded in the low energy field of the spec-
trum are very close, and they might be considered as compo-
nents of the same periodicity.

Dir Ouled Yahia (COK section, Bargou area) and Guern

Halfaya (GH section)

The COK section, about 24 m thick (Fig. 1B), is exposed in

Oued  El  Kharroub  (Bargou  area)  (Fig. 1A),  whereas  the  GH
section, about 17 m thick, is located between Hammem Melle-
gue and Tajerouine and is one of the most expanded CT tran-
sitions in Tunisia (Soua & Tribovillard 2007). The results of
the spectral analysis are also shown in Fig. 3 (B and C). At the
COK and GH sections, the Bahloul Formation is characterized
by  several  radiolarian-rich  siliceous  beds  (Soua  et  al.  2006;
Fig. 2.3).  The  time  series  analysis  performed  on  Heterohelix
spp.  shows  that  the  strongest  peak  points  to  a  period  of
17.85 m (Fig. 3B). Other significant values are represented at
9.1 m, 5 m and 3.84 m.

Dealing with Guembelitria time series analysis, we observe

strong peaks at 13.16 m and 12.82 m respectively in the GH
and COK sections (Fig. 3B; Fig. 3C).

Jerissa (CES section)

The Heterohelix signal was filtered and three significant

peaks of different wavelengths were detected (i.e. 19.6 m,

116

SOUA

7.69 m, and 4.1 m; Fig. 3D). They were selected for this study
because they represent accurate values for time series analysis.
These  values  are  normalized  to  the  shortest  periodicity  (i.e.
4.1)  and  then  compared  to  the  relative  precession  ratios  of
classical  Milankovitch  periodicities  (see  Table 1).  Table 1
shows the relationship between the ratios obtained by the nor-
malization both of the eccentricity (100—400 kyr) and obliqui-
ty (40 kyr) to the precession (20 kyr). All wavelengths (related
respectively to Heterohelix and Guembelitria in the four sec-
tions) are normalized to the related shortest periodicity.

Cyclostratigraphy

According to the Park & Oglesby (1991) model, which re-

produces  Cretaceous  Milankovitch  cyclicity,  the  Bahloul  se-
quence  may  display  a  Milankovitch  style  climatic  cyclicity
corresponding  to  a  20 kyr  precession  cycle  that  prevailed  at

Fig. 4. Comparison between the filtered signals (precession and eccentricity) of the
Heterohelix spp. of the four studied sections.

low  latitudes  during  the  C/T  transition.  In
this study we have tried to calculate the pa-
leolatitudes  of  the  Kef  and  Bargou  areas
(where  the  four  studied  sections  are  situat-
ed) during the C/T transition, namely about
93.5 Ma.  The  calculated  paleolatitudes  are
about 15 to 17° N according to Philip et al.
(1993),  or  16—19°  N  according  to  Camoin
et  al.  (1993).  In  contrast  they  are  37°  to
37.5° N according to the ODSN Plate Tec-
tonic  Reconstruction  Service  (Hay  et  al.
1999).

These latter sequences represent variable

indurate levels that also show % TOC, %
CaCO

and ‰

C fluctuations (Fig. 5).

Influence of orbital forcing during

Bahloul deposition

Negri et al. (2003) discussed the duration

of  the  orbital  cycles  in  a  time-equivalent
Bonarelli black shale level, and have shown
that  they  can  be  normalized  and  tuned  to
precession  cycles.  The  Heterohelix  and
Guembelitria signals in the studied sections
were filtered, and show significant fluctua-
tions  in  the  OMZ  and  eutrophic  surface
dweller signals (see Fig. 4).

This time series analysis allowed us to

develop a cyclostratigraphic approach in or-
der to estimate the duration of the organic-
rich  Bahloul  Formation  in  the  four  studied
sections. The evaluation of the OAE2 dura-
tion  was  the  subject  of  several  studies  and
was  originally  estimated  based  mainly  on
biostratigraphic  (ammonites  and  foramin-
iferal)  ranges  (e.g.  Hardenbol  et  al.  1998)
and/or  orbital  cyclicity.  Estimates  are:
720 kyr in the Pueblo section (Meyers et al.
2004),  about  400 kyr  in  Tarfaya  and  Wadi
Bahloul  (Kuhnt  et  al.  1997;  Caron  et  al.
1999),  320 kyr  in  western  Canada  and  the

Bottaccione  section  in  Central  Italy  (Prokoph  et  al.  2001;
Scopelliti et al. 2006), 450 kyr for the Douvres section (Gale
1991), 500 kyr to 600 kyr for the Thomel level (Morel 1998).
Wavelength ratios were tuned to the associated orbital cyclici-
ty of the C/T transition strata (see Table 1). Alternatively we
may estimate an average sedimentary rate for the Bahloul For-
mation  (OAE2)  by  tuning  each  calculated  wavelength  to  the
corresponding orbital cyclicity (e.g. 19.6/100; 7.69/40; 4.1/20;
Heterohelix data of the CES section). Figure 4 shows that for
the  CES  section,  the  Heterohelix  precession  curve  (filter 20)
exhibits 18 to 19 cycles. This means that a duration of about
400 kyr  can  be  attributed  to  the  Bahloul  Formation,  which
represents  the  anoxic  event.  The  eccentricity  curve  exhibits
about 4  to  4.5  cycles,  which  also  corresponds  to  about  (i.e.
~400 kyr).  The  average  duration  of  the  Bahloul  Formation
(OAE2)  may  thus  be  evaluated  as  ~ 400 kyr.  The  results  are
displayed in Fig. 4, and Table 1.

117

CENOMANIAN—TURONIAN TIME SERIES ANALYSIS USING FORAMINIFERA

Table 1:

Wavelengths

evaluated

from

the

power

spectra

Fig. 3;

averag

sedimentary

rates

(cm/kyr)

and

estimated

duration

the

OAE2

each

section.

Note

that

the

Bahloul

Formation

de-

posited

within

interval

350—410 kyr

with

sedimentation

rate

encompassed

between

and

20 cm/kyr.

Discussion

The solar influx which penetrates the atmosphere is con-

trolled by the orbital parameters of the Earth: namely eccen-
tricity,  obliquity  and  precession  (Berger  1978;  House  1995).
These orbital variations lead to climatic variations, which then
influence oceanic circulation and the sedimentary systems. A
dominance  of  precession  cycles  on  climate  change  and  sedi-
mentation  can  be  discerned  during  the  C/T  interval  (Berger
1978).  In  addition,  ignoring  the  obliquity  signal  component
will  greatly  simplify  data  interpretation  (Vonhof  et  al.  1998;
Nederbragt & Fiorentino 1999; Caron et al. 1999; Negri et al.
2003; Scopelliti et al. 2004). Eccentricity is known to be very
stable through the Mesozoic time and did not vary significant-
ly through the last 100 Ma (Laskar 1989, 1999; Berger et al.
1992).  Figure 3  and  Figure 4  show  the  cyclostratigraphic  in-
terpretation of the four studied sections, leading to the recog-
nition  of  precession  cycles  and  the  identification  of  100 kyr
cycles (short eccentricity). The spectral analysis applied to the
Heterohelix/Guembelitria  spp.  fluctuations  (Fig. 3)  points  to
the presence of a meter-scale periodicity, for example at 25 m
in the HM section (Fig. 3A), and other major relevant peaks at
11.9 m and 5.26 m using  Heterohelix spp. counts. Regarding
Guembelitria  ssp.,  there  is  a  meter-scale  periodicity  at
17.54 m  in  the  CES  section  and  other  important  peaks  at
8.33 and 3.84 m (Fig. 3D). The relationship between the ra-
tios  obtained  by  the  normalization  both  of  the  eccentricity
(100—400 kyr)  and  obliquity  (40 kyr)  to  the  precession
(20 kyr)  are  displayed  in  Table 1.  The  high  correlation  be-
tween  the  spectral  peak  ratios  (of  both  the  Heterohelix  and
Guembelitria in the four sections) and those of the orbital com-
ponents is very interesting (Fig. 3A, Table 1). Calculated aver-
age  sedimentary  rates  (Table 1)  are  in  perfect  agreement  with
the  sedimentation  rate  estimated  from  Hardenbol  et  al.  (1998)
and the relatively close value (10 cm/kyr) obtained by Vanhof
et al. (1998) for the Mellegue section. This interpretation clearly
highlights the existence of cyclicity within the Bahloul Forma-
tion that is more complex than that which can be recognized by
visual inspection. It implies that orbital forcing has caused the
Heterohelix and Guembelitria abundance fluctuations.

Paleoecology of the two opportunistic species

The biserial heterohelicids (Heterohelix spp.), are believed

to be ecological opportunists and  low-oxygen tolerant fauna,
which  thrive  in  well-stratified  open  marine  settings  with  a
well-developed  oxygen  minimum  zone  (OMZ)  (Boersma  &
Premoli Silva 1988; Barrera & Keller 1994). Thus, high abun-
dance of Heterohelix indicates an expanded oxygen minimum
zone (Soua & Tribovillard 2007).

The triserial heterohelicacea Guembelitria species thrived in

eutrophic surface waters with variable salinities at times of se-
vere ecological stress (Leckie 1987; Kroon & Nederbragt 1990;
Soua & Tribovillard 2007; Soua & Zaghbib-Turki 2007).

Relationship between paleoecology and the orbital cycles

We have shown that in the four studied sections strong cy-

clicity is reflected not only in the lithological pattern but also

119

CENOMANIAN—TURONIAN TIME SERIES ANALYSIS USING FORAMINIFERA

in the biotic signal. In particular, our data recognize a strong
precessional signal that is comparable to others identified all
over  the  world  (Kuhnt  et  al.  1997;  Morel  1998;  Caron  et  al.
1999;  Prokoph  et  al.  2001;  Negri  et  al.  2003;  Meyers  et  al.
2004; Scopelliti et al. 2006). The paleoecological preferences
of both Heterohelix spp. and Guembelitria spp. seem to be re-
lated to areas of enhanced surface water fertility and oxygene-
minimum zone (e.g. Nederbragt & Fierontino 1999; Luning
et  al.  2004;  Soua  &  Tribovillard  2007).  Thus,  the  long  term
fluctuations related to both precession and eccentricity suggest
that  changes  in  surface  water  fertility  were  linked  to  the
Milankovitch parameters.

Conclusion

Four C/T transition sections were sampled in Central Tuni-

sia  displaying  biotic  and  lithological  variations.  Time  series
analysis  was  applied  to  the  abundances  of  two  opportunistic
foraminiferal genera (Heterohelix  and  Guembelitria spp.) for
all the four sections. This analysis allowed us to calculate and
estimate both the average sedimentary rate and the duration of
the OAE2 in each section. In addition, the biotic signal repre-
sented  by  the  Heterohelix  and  Guembelitria  spp.  relative

abundance  fluctuations  in  the  studied  sections  is  probably
driven  by  Milankovitch  frequencies  through  a  climate-con-
trolled productivity model. We can deduce that the fluctua-
tions  of  these  two  species  are  related  mainly  to  both
precessional and eccentricity cyclicity, suggesting that chang-
es  in  surface  water  fertility  were  linked  to  the  Milankovitch
parameters.  Thus  the  Bahloul  Formation  in  the  four  studied
sections exhibits a duration interval between 350—400 kyr and
a sedimentation rate interval between 12 and 20 cm/kyr.

Acknowledgment: Prof. André Strasser provided a thorough
review  of  this  article  and  has  made  numerous  suggestions  to
improve the text. The author is also grateful to the reviewers,
Dr.  Jozef  Michalík  (Responsible  Editor)  and  Prof.  Adam
Gasiński  (Jagiellonian  University)  for  providing  constructive
comments and Nigel Collins (Terrasciences Ltd.) for improv-
ing the English.

References

Accarie M., Robaszynski F., Amedro F., Caron M. & Zagrarni M.F.

1999: Stratigraphie événementielle au passage Cénomanien—Tur-
onien dans le secteur occidental de la plateforme de Tunisie cen-
trale (Formation Bahloul, région Kalaat Senen). Ann. Min. Géol.
40, 63—80.

Arthur M.A., Schlanger S.O. & Jenkyns H.C. 1987: The Cenomanian—

Turonian oceanic anoxia event: II. Paleoceanographic controls on
organic matter production and preservation. In: Brooks J. & Fleet
A.J. (Eds.): Marine and petroleum source rocks. Geol. Soc. Lon-
don, Spec. Publ. 26, 401—420.

Barrera E. & Keller G. 1994: Productivity across the Cretaceous-Ter-

tiary boundary in high latitudes. Geol. Soc. Amer. Bull. 106,
1254—1266.

Berger A.L. 1976: Obliquity and precession for the last 5,000,000

years. Astromony and Astrophysics 51, 127—135.

Berger A.L. 1978: Long-term variations of caloric insolation resulting

from the Earth’s orbital elements. Quat. Res. 9, 139—167.

Boersma A. & Premoli Silva I. 1988: Atlantic Palaeogene biserial het-

erohelicid foraminifers and oxygen minima. Palaeoceanography
4, 271—286.

Camoin G., Bellion Y., Dercourt J., Guiraud R., Lucas J., Poisson A.,

Ricou L.E. & Vrielynck B. 1993: Late Maastrichtian (69.5—
65 Ma). In: Dercourt J., Ricou L.E. & Vrielinck B. (Eds.): Atlas
Tethys palaeoenvironmental maps. Explanatory notes. Gauthier-
Villars, Paris, 179—196.

Caron M., Robaszynski F., Amedro F., Baudin F., Deckonink J.F., Ho-

chuli P., Salis Perch Nielsen K. (von) & Tribovillard N. 1999: Es-
timation de la durée de l’événement anoxique global au passage
Cénomanien Turonien. Approche cyclostratigraphique dans la
Formation Bahloul en Tunisie centrale. Bull. Soc. Géol. France,
170, 2, 145—160.

Ditchfield P. & Marshall J.O. 1989: Isotopic variation in rhythmically

bedded chalks: paleotemperature variation in the Upper Creta-
ceous. Geology 17, 842—845.

Fischer A.G., Herbert T.D., Napoleone G., Premoli Silva I. & Ripepe

M. 1991: Albian pelagic rhythms (Piobbico Core). J. Sed. Pe-
trology 61, 1164—1172.

Gale A.S., Jenkyns H.C., Kennedy W.J. & Corfield R.M. 1993:

Chemostratigraphy versus biostratigraphy: data from around the
Cenomanian Turonian boundary. J. Geol. Soc. London 150,
1993, 29—32.

Hammer

., Harper D.A.T. & Ryan P.D. 2001: PAST: Paleontological

Statistics software package for education and data analysis.
Palaeontologia Electronica 4, 1, 4, 1—9.

Fig. 5B – Distribution and correlation of foraminiferal and radi-
olarians profiles in the same graphic at Bargou section.

120

SOUA

Haq B.U., Hardenbol J. & Vail P.R. 1987: Chronology of fluctuating

sea levels since the Triassic. Science 235, 1156—1167.

Hardenbol J., Thierry J., Farley M.B., Jacquin T., De Graciansky P.-C. &

Vail P.-R. 1998: Cretaceous sequence chronostratigraphy. In: De
Graciansky P.-C., Hardenbol J., Jacquin T. & Vail P.R. (Eds.): Me-
sozoic and Cenozoic sequence stratigraphy of European Basins.
Soc. Econ. Paleontol. Mineral. Spec. Publ., Vol. 60, Chart 4.

Hay W., DeConto R., Wold C.N., Wilson K.M., Voigt S., Schulz M.,

Wold-Rossby A., Dullo W.-C., Ronov A.B., Balukhovsky A.N. &
Soeding E. 1999: Alternative global Cretaceous paleogeography.
In: Barrera E. & Johnson C. (Eds.): The evolution of Cretaceous
ocean/climate systems. Geol. Soc. Amer., Spec. Pap. 332, 1—47.

Hinnov L.A. & Goldhammer R.K. 1991: Spectral analysis of the Middle

Triassic Latemar Limestone. J. Sed. Petrology 61, 1173—1193.

House M.R. 1995: Orbital forcing timescales: an introduction. Geol.

Soc. London, Spec. Publ. 1995, 85, 1—18.

Kauffman E.G. 1995: Global change leading to biodiversity crisis in a

Greenhouse World: The Cenomanian—Turonian (Cretaceous)
mass extinction – effects of past global change on life. National
Academy Press, 48—71.

Kroon D. & Nederbragt A.J. 1990: Ecology and palaeoecology of tris-

erial planktic foraminifers. Mar. Micropaleontology 16, 25—38.

Kuhnt W., Nederbragt A. & Leine L. 1997: Cyclicity of Cenomanian—

Turonian organic-carbon-rich sediments in the Tarfaya Atlantic
coastal basin (Morocco). Cretaceous Research 18, 587—601.

Laskar J. 1989: A numerical experiment on the chaotic behaviour of

the solar system. Nature 338, 237—238.

Leckie R.M. 1987: Paleoecology of mid-Cretaceous planktic foramin-

ifera: a comparison of open ocean and epicontinental sea assem-
blages. Micropaleontology 33, 164—176.

Luning S., Kolonic S., Belhaj E.M., Belhaj Z., Cota L., Baric G. &

Wagner T. 2004: An integrated depositional model for the Ceno-
manian-Turonian organic-rich strata in North Africa. Earth Sci.
Rev. 64, 1—2, 51—117.

Maâmouri A.L., Zaghbib-Turki D., Matmati M.F., Chikhaoui M. &

Salaj J. 1994: La formation Bahloul en Tunisie centro-septentrio-
nale: variation latérales nouvelle datation et nouvelle interpréta-
tion en terme de stratigraphie séquentielle. J. African Earth Sci.
18, 1, 37—50, 1994.

Morel L. 1998: Stratigraphie

haute résolution du passage Cén-

omanien—Turonien. Th

se de l’Université Pierre et Marie Curie,

Paris, VI, 1—224 (Unpublished).

Nederbragt A.J. & Fiorentino A. 1999: Stratigraphy and paleoceanogra-

phy of the Cenomanian-Turonian Boundary Event in Oued Melle-
gue, north-western Tunisia. Cretaceous Research 20, 47—62.

Negri A., Cobianchi M., Luciani V., Fraboni R. & Milani A.M. Claps

2003: Tethyan Cenomanian pelagic rhythmic sedimentation and
Pleistocene Mediterranean sapropels: is the biotic signal compa-
rable? Palaeogeogr. Palaeoclimatol. Palaeoecol. 190, 373—397.

O’Dogherty L. & Guex J. 2001: Rates and pattern of evolution

among Cretaceous radiolarians: Relations with Global Paleocean-
ographic Events. Micropaleontology Vol. 48, Supplement 1: Mi-
cropaleontology of radiolarians: Proceedings of INTERRAD 9
(2001), 1—22.

Park J. & Oglesby R.J. 1991: Milankovitch rhythms in the Creta-

ceous: a GCM modelling study. Glob. Planet. Change 1991, 4,
4, 329—355 (2 p.)

Paul C.R.C., Lamolda M.A., Mitchel S.F., Vaziri M.R., Gorostidi A. &

Marshall J.D. 1999: The Cenomanian-Turonian boundary at East-
bourne (Sussex, UK): a proposed European reference section.
Palaeogeogr. Palaeoclimatol. Palaeoecol. 150, 83—121.

Philip J. et al. 1993 : Late Cenomanian (94—92 Ma). In: Dercourt J. et

al. (Eds.): Atlas Tethys, palaeoenvironmental maps. Explanatory
notes. Gauthier-Villars, Paris, 153—178.

Prokoph A., Villeneuve M., Agterberg F.P. & Rachold V. 2001: Geo-

chronology and calibration of global Milankovitch cyclicity at the
Cenomanian-Turonian boundary. Geology 29, 523—526.

Robaszynski F., Caron M., Dupuis C., Amedro F., Gonzalez-Donso

J.M., Linares D., Hardenbol J., Gartner J., Calandra F. & Deloffre
R. 1990: A tentative integrated stratigraphy in the Turonian of
Central Tunisia: Formations, zones and sequential stratigraphy in
the Kalaat Senan area. Bull. Centres Rech. Explor. Prod. Elf-
Aquitaine 14, 1, 213—384.

Sageman B., Rich J., Savrda C.E., Bralower T., Arthur M.A. & Dean

W.E. 1998: Multiple Milankovitch cycles in the Bridge Creek
Limestone (Cenomanian—Turonian), Western Interior basin. In:
Arthur M.A. & Dean W.E. (Eds.): Stratigraphy and paleoenviron-
ments of the Cretaceous Western Interior seaway, USA. Soc. Sed.
Geol., Concepts in Sedimentology and Paleontology 6, 153—171.

Scopelliti G., Bellanca A., Coccioni R., Luciani V., Neri R., Baudin F.,

Chiari M. & Marcucci M. 2004: High-resolution geochemical
and biotic records of the Tethyan ‘Bonarelli Level’ (OAE2, latest
Cenomanian) from the Calabianca—Guidaloca composite section,
northwestern Sicily, Italy. Palaeogeogr. Palaeoclimatol. Palaeo-
ecol. 208, 293—317.

Sepkoski J., Jr. 1993: Ten years in the library: New data confirm pale-

ontological patterns. Paleobiology 19, 43—51.

Soua M. 2005: Biostratigraphie de haute résolution des foraminif

res

planctoniques du passage Cénomanien Turonien et impact de
l’événement anoxique EAO-2 sur ce groupe dans la marge sud de
la Téthys, exemple régions de Jerissa et Bargou. Mém. Mast

re,

Univ. Tunis, El Manar, 1—169.

Soua M. & Tribovillard N. 2007: Depositional model at the Cenoma-

nian/Turonian boundary for the Bahloul Formation, Tunisia.
Comptes rendus — Geoscience 339, 10, 692—701.

Soua M. & Zaghbib-Turki D. 2007: Biotic response during times of se-

vere ecological stress, how paleoceanographic changes in latest
Cenomanian—earliest Turonian Ocean induced step-by-step ex-
tinction and dwarfism. Geol. Soc. Amer., Northeastern Section –
42nd Annual Meeting (12—14 March 2007), Paper No. 29—4, Ses-
sion No. 29.

Soua M., Zaghbib-Turki D. & O’Dogherty L. 2006: Radiolarian biotic

responses to the Latest Cenomanian global event across the
southern Tethyan margin (Tunisia). Proceeding of the tenth Ex-
ploration and Production Conference. Memoir 26, 195—216.

Soua M., Echihi O., Herkat M., Zaghbib-Turki D., Smaoui J., Fakhfakh-

Ben Jemia H. & Belghaji H. 2009: Structural context of the paleo-
geography of the Cenomanian-Turonian anoxic event in the
eastern Atlas basins of the Maghreb. Comptes rendus Geosci.
341, 12, 1029—1037.

Tsikos H., Jenkyns H.C., Walsworth-Bell B., Petrizzo M.R., Forster

A., Kolonic S., Erba E., Premoli Silva I., Baas M., Wagner T. &
Sinninghe Damsté J.S. 2004a: Carbon-isotope stratigraphy re-
corded by the Cenomanian—Turonian oceanic anoxic event: cor-
relation and implications based on three keylocalities. J. Geol.
Soc. London 161, 711—720.

Vonhof H.B., Nederbragt A.J., Kuypers M., Van Hinte J.E., Ganssen

G.M. & Smit J. 1998: Synchronous strontium and carbon isotope
excursions across the Cenomanian-Turonian Oceanic Anoxic
Event. In: Vonhof H.B. (Ed.): The strontium isotope stratigraphic
record of selected geologic events. PhD Thesis, Free University,
Amsterdam, 93—109.

Weedon G.P. 2003: Time-series analysis and cyclostratigraphy.

Cambridge University Press, 1—274.

Weedon G.P. & Jenkyns H.C. 1990: Regular and irregular climatic cy-

cles and the Belemnite Marls (Pliensbachian, Lower Jurassic,
Wessex Basin). J. Geol. Soc. London 147, 915—918.

Zhao X., Riisager P., Riisager J., Draeger U., Coe R.S. & Zheng Z. 2004:

New palaeointensity results from Cretaceous basalt of Inner
Mongolia, China. Phys. Earth Planet. Int. 141, 2, 16, 131—140.