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
Entreprise Tunisienne d’Activités Pétroli
res, ETAP-CRDP 4 Rue des Entrepreneurs, 2035 la Charguia II, Tunisia; firstname.lastname@example.org
(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.
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
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.
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
Fig. 1. Location of the studied sections. (A) Bahloul isopach map and C/T paleogeography in Central Tunisia. (B) Stratigraphy and bios-
tratigraphy of Jerissa (CES), Guern Halfaya (GH), Hammem Mellegue (HM) and Bargou (COK) sections.
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.
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.
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-
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,
CENOMANIAN—TURONIAN TIME SERIES ANALYSIS USING FORAMINIFERA
Fig. 3. A—D – Time series analyses performed on the two opportunistic C/T foraminiferal species, the OMZ dweller Heterohelix spp. and
the eutrophic surface dweller Guembelitria spp. showing the power spectrum of their relative abundance logs (%) in the studied interval of
the Bahloul in each section. The values on the vertical axis indicate the power while the horizontal axis refers to frequencies in cycles/
meter, from low-frequency (left) to high-frequency (right) periodicities. Only the statistically significant periodicities are labelled; numbers
above the significant peaks are frequencies in cycle meter. Dashed lines indicate filtered signals.
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.
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.
These latter sequences represent variable
indurate levels that also show % TOC, %
C fluctuations (Fig. 5).
Influence of orbital forcing during
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.
CENOMANIAN—TURONIAN TIME SERIES ANALYSIS USING FORAMINIFERA
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
Fig. 5A – Geochemical (% CaCO
; % TOC and %
C) and biostratigraphic (foraminifera, radiolarians and ammonites) data of the four
studied sections (modified from Soua & Tribovillard 2007). I, II and III indicate the three worldwide isotopic spikes of the C/T boundary.
Black stars indicate the Heterohelix shift (within each section).
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
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-
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