SEA LEVEL CHANGES IN PLATFORM AND PELAGIC CARBONATE SUCCESSIONS 19
GEOLOGICA CARPATHICA, 55, 1, BRATISLAVA, FEBRUARY 2004
STABLE ISOTOPE EVIDENCE FOR METER-SCALE SEA LEVEL
CHANGES IN LOWER CRETACEOUS INNER PLATFORM
AND PELAGIC CARBONATE SUCCESSIONS OF TURKEY
ùSMAùL ÖMER YILMAZ
, TORSTEN VENNEMANN
and MUHARREM SATIR
Department of Geological Engineering, Middle East Technical University, 06531 Ankara, Turkey; email@example.com
Lehrstuhl für Geochemie, Institut für Mineralogie, Petrologie, und Geochemie, University of Tübingen, Wilhelmstraße 56,
72074 Tübingen, Germany
(Manuscript received November 26, 2002; accepted in revised form June 23, 2003)
Abstract: In this study, stable C and O isotope compositions of bulk rock samples from Barremian-Aptian platform
carbonates of the Taurides, the Pontides, and of their pelagic counterparts of the Sakarya continent in Turkey were
measured for the first time. Platform carbonates and pelagic successions including a prominent black shale interval are
composed of distinct sedimentary cycles in the form of meter-scale shallowing- upward sequences and limestone-marl/
shale couplets, respectively. The biostratigraphy and the
C values allow this black shale interval to be correlated with
the Selli level. Variations in lithofacies and
O values with the magnitudes of about 2 within the cycles
indicate high frequency fluctuations in sea level, probably in response to alternate periods of cooling and warming
occurring on Milankovitch-cycle scales even during the greenhouse-type Cretaceous. A good overall correlation be-
tween sedimentary cyclicity and variations in
O values suggests preservation of the primary isotopic signal.
Key words: Barremian, Aptian, Taurides, Pontides, Sakarya, stable isotopes, cyclicity.
Over the past decade, many studies have documented changes
in sea level during the Early Cretaceous (e.g. Haq et al. 1988;
De Boer & Smith 1994; Graciansky et al. 1998; Altôner et al.
1999; Raspini 2001; Pittet et al. 2002). Studies on the global
climate indicate that greenhouse-type conditions existed dur-
ing the Early Cretaceous, but that even within this type of cli-
mate, periods of prolonged cooling occurred (Weissert & Lini
1991; Sellwood et al. 1994; Pirrie et al. 1995; Barrera &
Johnson 1999; Jenkyns & Wilson 1999; Price 1999; Stoll &
Schrag 2000). While discussions on the ultimate cause of the
climatic variation continue, correlations of both
O values from different carbonate sequences of the Creta-
ceous justify a global interpretation and elucidate the control-
ling factors and scales of paleoclimatic and paleoceanograph-
ic changes (e.g. Menegatti & Weissert 1998; Larson & Erba
1999; Strasser et al. 2001).
The main purpose of this study is to document the relation-
ship between stable isotope variations and meter- to larger-
scale changes in sea level as documented by sedimentologi-
cal, micropaleontological and petrographical features of both
platform carbonates and pelagic successions of the eastern
Tethyan margins. This study is part of a larger project dealing
with sequence- and cyclostratigraphy of Barremian-Aptian
sediments in the NW, WNW and SW of Turkey. First, an in-
terpretation of the depositional environments of the observed
facies and their cyclic patterns is presented; second, analysis
O data obtained from selected cycles and their
interpretation in relation with sea level changes are given.
This second part forms the core of this study. Detailed docu-
mentation of the high-resolution sequence-stratigraphic and
cyclostratigraphic correlations of the sections are the topic of a
continuing parallel study and will be published later.
This study covers three different regions: Taurus, Sakarya,
and Zonguldak. Each region has its own specific geological
A The Taurus region in the southern part of Turkey is
characterized by platform carbonates of the southern Neot-
ethys Ocean. Of particular interest to this study is the
Seydiºehir area, where platform carbonates dominate
(Figs. 1a, 1b, 2).
Fig. 1a. Geographic location of study regions (A, B, C). A Taurus
region (locality: Seydiþehir area). B Sakarya region (localities:
Mudurnu and Nallôhan area). C Zonguldak region (localities: Zon-
guldak, Kozlu and Çengellidere areas) (Yôlmaz et al. 1997, modified).
20 YILMAZ et al.
The geology of this area has been studied extensively
(Monod 1977; Özgül 1983, 1997; Altôner et al. 1999; Yôlmaz
1999; Yôlmaz & Altôner 2001). In the Seydiþehir area (Fig. 2)
typical autochthonous/para-autochthonous platform carbon-
ates of the Taurides rest unconformably on sedimentary Cam-
brian-Ordovician and Triassic basement rocks. Shallow water
platform carbonates ranging from Dogger to Paleocene in age
form thick, relatively homogeneous successions over the
basement rocks. Nappe movements during the Eocene result-
ed in prominent Eocene flysch successions covering much of
the Taurides. Allochthonous units, which are generally com-
posed of ophiolitic successions and underlying basement
rocks, are unconformably covered by Neogene/Quarternary
successions. Two stratigraphic sections have been studied,
covering the BarremianAptian of the Polat Formation
(Özgül 1997), which crops out extensively in the Western
B The Sakarya region is part of the so-called Sakarya
continent (ýengör & Yôlmaz 1981). Metamorphic basement
and overlying Mesozoic and Cenozoic sedimentary and vol-
canic successions characterize the stratigraphy of the region.
Paleozoic metamorphic basement rocks were overthrusted by
Triassic sedimentary and metamorphic rocks during the Kar-
akaya Orogeny. Liassic successions including Ammonitico
Rosso facies unconformably overlie all these metamorphic
suites and deformed sedimentary units. The rest of the Juras-
sic and Cretaceous successions start with alternations of shelf
carbonates, pelagics, cherts, volcanics, and volcaniclastics,
and continue with slope and basin deposits of the Upper Creta-
ceous. In the Mudurnu and Nallôhan areas studied, slope/basin
pelagic carbonates dominate the successions (Figs. 1a, 3).
These pelagics have been interpreted as deposits filling the in-
tra-continental Mudurnu Trough of the Sakarya continent
(Önal et al. 1988; Altôner et al. 1991; Altôner 1991; Altôner &
The two stratigraphic sections in the Nallôhan and Mudurnu
areas include Lower Cretaceous pelagic carbonates of the
Soûukçam Limestone (Figs. 1a, 1c, 3, 4). In the studied area,
the Soûukçam Limestone is distinguished from other forma-
tions by the presence of limestone-shale/marl couplets.
C The Zonguldak region is characterized by Cretaceous
outcrops along the Western Black Sea Coast of Northwestern
Turkey (Tokay 1954, 1955; Kaya et al. 1983; Derman 1990;
Orhan 1995; Görür 1997). They represent a continental shelf
facing towards an ocean in its south. In this region, Paleozoic
rocks of the Western Pontides, composed of continental and
shallow-water carbonates, are unconformably overlain by Ju-
rassic-Cretaceous shallow-water carbonate-dominated succes-
sions, and followed by Upper Cretaceous flysch-type succes-
sions. The Zonguldak, Kozlu, and Çengellidere areas studied
cover these Lower Cretaceous platform carbonates. Three
stratigraphic sections have been investigated within the Öküs-
medere and Çengellidere Formations (Figs. 1a, 1c, 5). The
Öküsmedere Formation is composed of peritidal carbonates in-
tercalated with thin siliciclastics. The Çengellidere Formation
is the lateral extension of the Öküsmedere Formation, but also
Fig. 1bc. Geographic location of sections measured in the Seydiþehir area in the Western Taurides (SW Turkey) (b). Geographic loca-
tion of sections measured in the MudurnuNallôhan (Sakarya) and Zonguldak (Pontides) areas (c).
SEA LEVEL CHANGES IN PLATFORM AND PELAGIC CARBONATE SUCCESSIONS 21
includes alternating reefal carbonates and thick siliciclastics.
These two formations are separated from the Upper Jurassic to
Lower Cretaceous Inaltô Formation by an unconformity,
which is represented by red continental clastics of the ùncigez
Formation (Fig. 5).
Biostratigraphy and chronostratigraphy
Within the sections and on a regional basis the chronos-
tratigraphy was established using benthic foraminiferal bio-
zones in platform carbonates and planktonic foraminiferal
biozones in pelagic successions (Altôner & Decrouez 1982;
Derman 1990; Altôner 1991; Altôner et al. 1991, 1999; Altôner
& Özkan 1991; Yôlmaz 1999, 2002; Altôner & Yôlmaz 2000;
Yôlmaz & Altôner 2001).
In the sections measured in the Zonguldak area (Pontides),
two biozones (A and B) were established based on benthic
foraminifers (Fig. 6). Zone A encompasses the interval be-
tween the uppermost Barremian and the Lower Aptian and is
characterized by the total range of Palorbitolina lenticularis.
Zone B is an assemblage zone corresponding to the Barremian
and is characterized by Orbitolinopsis debelmasi, Arenobu-
limina cochleata, Choffatella tingitana, and Orbitolinopsis
flandrini. In the sections measured in the Mudurnu and
Nallôhan areas (Sakarya), the Barremian-Aptian boundary is
defined between the Hedbergella sigali and Hedbergella si-
milis Interval Zones. The Aptian is divided into 7 zones by the
successive appearances of Hedbergella similis, Globigerinel-
loides blowi, Leopoldina cabri, Globigerinelloides ferroelen-
sis, Globigerinelloides algerianus and Planomalina cheriiou-
rensis (Fig. 6). In the Nallôhan section, the Planomalina
cheriiourensis Zone is not recorded due to truncated upper-
most part of the section by a thrust fault in the area. In the sec-
tions measured in the Seydiþehir area (Taurides), the studied
successions correspond to the K2b and K3 Zones of Altôner et
al. (1999). Zone K2 is characterized by the Vercorsella
scarsellaiSalpingoporella dinarica Assemblage Zone and
covers the Upper HauterivianLower Aptian interval. K2b is
the Voloshinoides murgensis Subzone defined within the K2
Zone and corresponds to the Lower Aptian (Fig. 6). Zone K3
is characterized by Cuneolina gr. pavonia Miliolidae 1 as-
semblage and encompasses the interval between the Upper
Aptian to Cenomanian. All the biozones are correlated with
Fig. 2. Simplified and modified geological map of Seydiþehir area
(Monod 1977). The locations of measured sections are shown with
numbers: 1 Seydiþehir-1; 2 Seydiþehir-Madenli.
Fig. 3. Geological map of the Mudurnu (Sakarya) area (Altôner et
al. 1991). The location of the measured section is shown with a
22 YILMAZ et al.
the biozones in the biostratigraphic chart of Graciansky et al.
(1998). The approximate durations of zones will be used for a
rough calculation of the duration per cycle in the studied sec-
tions. The upper and lower boundaries of the studied sections
do not coincide with stage boundaries because of limited ex-
posure. Therefore, the dates attributed to the base and the top
of the sections are approximated.
Cyclic stratigraphy of the sections
A total of five outcrop sections were measured in the
Seydiºehir (Taurides), Sakarya (Sakarya), and Zonguldak
(Pontides) regions of Turkey. All the studied sections have
been measured bed-by-bed in the field and sampled at a meter
or sub-meter-scale. Semi-quantitative analysis of 448 thin sec-
tions and hand samples was performed in the laboratory to de-
fine microfacies types. Bedding surfaces, sedimentary struc-
tures, weathering profiles, and facies compositions were
identified in the field. Vertical and lateral facies changes, as-
sociations of micro- and macro-sedimentary structures, and
microfacies types are used in the reconstruction of cyclicity.
Sedimentary cyclicity is recorded as shallowing-upward
meter-scale cycles in inner platform settings, and as lime-
stone-marl/shale couplets in pelagic settings. The Seydiþehir
and Zonguldak sections represent inner platform carbonates,
whereas pelagic successions are represented by the Nallôhan
and Mudurnu sections.
Inner platform carbonates
Taurides Seydiºehir sections
In the Seydiºehir area, two stratigraphic sections,
Seydiºehir-1 and Seydiºehir-Madenli have been studied in de-
tail. They are separated by a distance of 2 km (Figs. 1b, 2).
The Seydiºehir-1 section has a thickness of 36.03 m and cov-
ers the Early AptianLate Aptian/Albian time interval (Fig.
7). The Seydiþehir-Madenli section covers the Aptian period
and is 15.37 m thick (Fig. 8). The sections are composed of a
continuous succession of shallowing-upward meter-scale cy-
cles (Altôner et al. 1999; Yôlmaz & Altôner 2001) (Figs. 7, 8).
Cyclicity is mainly defined by the vertical arrangement of
subtidal, intertidal, and supratidal facies. Cycles generally
start with intraclastic, peloidal, foraminiferal, dasyclad algal
pack- to wackestones of a shallow subtidal environment and
continue vertically with algal, foraminiferal lime mud- to
wackestone facies. They are capped by cryptalgal laminites/
stromatolites, or fenestral limestones of intertidal/supratidal
environments. The top of each cycle is generally characterized
Fig. 4. Geological map of the Nallôhan (Sakarya) area (Altôner et al.
1991). The locations of measured sections are shown with numbers.
Fig. 5. Generalized geological map of the Zonguldak and sur-
rounding areas (simplified from Derman 1990). Locations of the
measured sections are shown with numbers: 1 Zonguldak; 2
Kozlu; 3 Çengellidere.
SEA LEVEL CHANGES IN PLATFORM AND PELAGIC CARBONATE SUCCESSIONS 23
Fig. 6. Biostratigraphic framework including biozones determined in the Zonguldak, Mudurnu and Nallôhan, and Seydiþehir sections and
their correlation. (Biozones are exactly equal to biozones determined in Altôner et al. (1999), Yôlmaz (1999, 2002).)
by subaerial exposure features such as karstic breccias and
dissolution vugs (Demicco & Hardie 1994; Altôner & Yôlmaz
2000; Yôlmaz et al. 2000; Yôlmaz & Altôner 2001). Therefore,
each cycle is represented by an asymmetrical transgressive-re-
gressive sequence (Strasser 1991). Transgressive and regres-
sive portions of the cycles reflect transgressive and highstand
conditions of small scale sea level changes. Records of low-
stand condition may be reworked in transgressive phase or
hidden in the subaerial exposure structures. A predominantly
shallowing-upward nature is consistently recorded in both
sections. The shallowing-upward cycles resemble the small-
scale sequences of Strasser et al. (1999). The thicknesses of
cycles range between 1 m and 1.5 m. The durations can be
calculated simply by dividing the time interval represented in
the section by the number of cycles detected, assuming that
each cycle has the same duration and that there are no gaps.
The estimated approximate durations of the parts of the stages
covered in the sections are about 12 Ma for the Seydiºehir-
Madenli section and 24 Ma for the Seydiºehir-1 section, ac-
cording to the correlation of the biostratigraphic framework in
the sections with the chronostratigraphic charts of Haq et al.
(1988) and Graciansky et al. (1998). Thus, the duration per
cycle in the Seydiºehir-1 section ranges between 74 Ka and
148 Ka, and between 62.5 Ka and 125 Ka in the Seydiºehir-
Madenli section (Table 1). These durations are close to the
100 Ka of the first eccentricity cycle (Fischer 1991), suggest-
ing that sea level changes were related to changes in the
Earths orbital parameters. Similar patterns of cyclicity and
durations of cycles are described by Pittet et al. (2002) from
Barremian-Aptian successions of northern Oman.
Name of the section
of the section
Between 74148 Ka
Eccentricity signals (E1 and E2)
Between 62.5125 Ka
Obliquity and eccentricity signals (E1 and E2)
Between 7093 Ka
Eccentricity signal (E1)
Between 125250 Ka
Eccentricity signal of E2
Between 4895 Ka
Obliquity and eccentricity signal of E1
Between 4957 Ka
Between 3443 Ka
Table 1: Table showing the calculated durations per cycles in measured sections and comparison with the frequencies in Milankovitch band.
24 YILMAZ et al.
Pontides Zonguldak sections
The Zonguldak sections studied (Zonguldak, Zonguldak-
Kozlu, Zonguldak-Çengellidere) are well-exposed outcrops of
inner platform carbonates and their siliciclastic intercalations.
The Zonguldak and Zonguldak-Kozlu sections represent parts
of the Öküsmedere Formation, whereas the Zonguldak-Çen-
gellidere section is part of the Çengellidere Formation (Fig. 5)
(Derman 1990; Orhan 1995; Görür 1997). Distances between
the sections are about 3 to 5 km (Fig. 5).
The Zonguldak and Zonguldak-Kozlu sections have very
similar features and complement each other. The Zonguldak
section has a thickness of 18.93 m and covers the Late Hau-
terivianBarremian period. The Zonguldak-Kozlu section is
17.73 m thick and covers the Late BarremianAptian time in-
terval (Figs. 9, 10).
In both sections, cyclicity is defined by vertical facies ar-
rangements. Cyclicity is recorded as an alternation of thinner
limey sandstones or sandy limestones/siltstones and thicker
limestones. Sandstones and siltstones occur at the bottom of
each cycle and are interpreted to represent lowstand and/or
transgressive conditions. They are overlain by limestones de-
posited during highstand condition of cycles (Osleger 1991;
Strasser 1999). Sandstones and siltstones are generally com-
posed of quartz, benthic foraminifers, dasyclad algae, plant
particles, and carbonate matrix. Within the siliciclastics, faint
laminations are also observed. Limestones constitute bioclas-
tic, peloidal, foraminiferal, dasyclad algal pack- and wacke-
Fig. 7. Shallowing-upward meter-scale cycles and stable isotope variations within the cycles of the Seydiþehir-1 section (for symbols see
Legend to Fig. 7).
Legend to Fig. 7.
SEA LEVEL CHANGES IN PLATFORM AND PELAGIC CARBONATE SUCCESSIONS 25
Fig. 8. Shallowing-upward meter-scale cycles and stable isotope variations within the cycles of the Seydiþehir-Madenli section (for sym-
bols see Legend to Fig. 7).
Fig. 9. Shallowing-upward meter-scale cycles (sandstone-limestone couplets) and stable isotope variations within the cycles of the Zongul-
dak section (for symbols see Legend to Fig. 7).
26 YILMAZ et al.
stones. Lime mudstones and charophyte packstones are infre-
quently intercalated (Fig. 9).
In both sections, cycles do not exhibit subaerial exposure
structures but instead are more like submerged cycles (Al-
tôner et al. 1999) or subtidal cycles of Osleger (1991). In a
few cycles, dissolution vugs filled with vadose silts are re-
corded and overlain by sandy beds including clasts derived
from the lower units. Some caliche clasts are also recorded in
these sandstones. Consequently, the bases of the sandstones
are interpreted as ravinment surfaces. In addition, the lime-
stones contain fragments of rudists and other bivalves increas-
ing in proportion relative to the sandstones.
In contrast to the Zonguldak section, the Zonguldak-Kozlu
section represents a less sandy succession. The cycles start
with sandy limestones and/or peloidal wackestones/pack-
stones and end with thicker pure limestones having bioclastic,
peloidal, foraminiferal, dasyclad algal packstones/wacke-
stones or grainstones at the top. In both sections, a prominent
charophyte packstone level composed of successive beds is
recorded. This level is interpreted as a record of a sea level
still-stand and/or fall in the sea level. The charophyte pack-
stone occurs at the top of the cycles and overlies siltstones/
sandstones at the bottom of the cycles. This also supports the
view that limestone-sandstone couplets are formed by shal-
lowing-upward conditions (Fig. 10).
The ZonguldakÇengellidere section covers the Early Ap-
tian period and has a thickness of 59.18 m (Fig. 11). This sec-
tion has more siliciclastic intercalations within its succession,
compared to the Zonguldak and Zonguldak-Kozlu sections.
Thick cross-bedded calcareous sandstones, quartzarenites,
calcareous conglomerates, and pebbly sandstones occur in the
lower to middle parts of the section. Some black mudstones
and siltstones alternating with sandstones are also observed.
This section is composed of shallowing-upward meter-scale
cycles. In this section two types of cycles are observed: con-
glomerate-sandstone couplets and sandstones/sandy lime-
stonelimestone couplets. Conglomerates occur at the bottom
and sandstones at the top of these siliciclastic cycles, repre-
senting fining-upward cycles. Sandstones and sandy lime-
stones form the bottom part and limestones composed of bio-
clastic packstone, peloidal, foraminiferal wackestone/
packstone or even bafflestone facies form the upper part of the
sandstone-limestone cycles (Fig. 11). In all cycles, subaerial
exposure structures are not recorded. In all conglomerates and
sandstones, except in the quartzarenites alternating with con-
glomerates and sandstones, in situ benthic foraminifers are
found. Therefore, the cycles in Zonguldak sections are inter-
preted as submerged cycles (Altôner et al. 1999) in this
study. Because of the thick sandstones and conglomerates, av-
erage cycle thickness rises up to 3 m in this section.
As in the Tauride sections, the duration of each cycle is cal-
culated by dividing the time interval represented in the section
by the number of cycles detected. (The estimated rough ap-
proximate durations of part of the stages covered in the sec-
Fig. 10. Shallowing-upward meter-scale cycles (sandstone-limestone couplets) and stable isotope variations within the cycles of the
Zonguldak-Kozlu section (for symbols see Legend to Fig. 7).
SEA LEVEL CHANGES IN PLATFORM AND PELAGIC CARBONATE SUCCESSIONS 27
Fig. 11. Shallowing-upward meter-scale cycles (sandstone-limestone couplets) and stable isotope variations within the cycles of the
Zonguldak-Çengellidere section (for symbols see Legend to Fig. 7).
28 YILMAZ et al.
tions are about 34 Ma for Zonguldak section, and 12 Ma
for Zonguldak-Kozlu section, and 12 Ma for Zonguldak-
Çengellidere section after the correlation of the biostratigraph-
ic framework in the sections with the global sea level chart
(Haq et al. 1988 and Graciansky et al. 1998).) The duration of
each cycle on the sections fits to the orbital eccentricity of the
Milankovitch band, E1 (98 Ka) and E2 (126 Ka) signals of
Fischer (1991) (Table 1) indicating that sea level changes
were related to changes in Earths orbital parameters.
Additionally, smaller-scale cycles have been observed
within meter-scale cycles. These cycles are fining-upward
within a particular bed or form repetitions of thin sandy/
clayey beds with limestone beds (Fig. 11). Although grouping
and ordering of the smaller scale cycles are still in progress, it
is found that 2 or 3 of them form a bundle within one meter-
scale cycle in the Zonguldak-Çengellidere section (Fig. 11
lower detailed section). However, the distribution of these cy-
cles is rather random compared to the meter-scale cycles. Fur-
ther studies are required to interpret these cycles.
Sakarya Nallôhan and Mudurnu sections
Both sections comprise the pelagic counterparts of the stud-
ied Barremian-Aptian shallow water carbonates of Seydiþehir
and Zonguldak regions. The Nallôhan section has a thickness
of 68.81 m, whereas the Mudurnu section is 45.01 m thick
(Figs. 12, 13). The distance between the two sections is about
50 km. Both sections occur within pelagic carbonates overly-
ing successions of the Mudurnu Trough with a large trans-
gression (Altôner 1991; Altôner & Özkan 1991; Altôner et al.
1991). They are composed of cm to m scale cycles represent-
ed by limestone-marl/shale couplets. The regular alternation
of couplets throughout the sections is impressive on the out-
crop scale and indicates a relatively homogeneous cyclicity.
Limestones composed of planktonic foraminiferal, radiolar-
ian wackestone/packstone facies form the top of the cycles,
whereas marls and shales with some radiolarians and plank-
tonic foraminifers form the bottom of the cycles. A prominent
black shale interval is recorded in both sections. It occurs
within the G. blowi Zone, and is interpreted as equivalent of
the Selli level (Wezel 1985) (Figs. 12, 13). It is 11 m thick
in the Nallôhan section and 2 m in the Mudurnu section. These
differences in thickness might be related to paleotopographic
conditions. In the Nallôhan section, the black shales are well
exposed. The black shale interval displays cm- to m-scale cy-
cles, which are generally composed of marl-shale couplets
(Yôlmaz et al. 2000) (Figs. 12, 13). Within the black shale,
presence of thin glauconitic sandstone beds rich in quartz and
feldspars, relatively ammonite-rich marls and iron enrichment
suggest that this interval is a condensed section deposited dur-
ing a maximum transgression of sea level (Yôlmaz et al. 2000;
Yôlmaz 2002). Similar records are also described in Martire
(1992), Peybernes et al. (2000), Prokoph & Thurow (2001),
Vennin & Aurell (2001), Hesselbo & Huggett (2001) and in
many others. Boundaries between limestones and marls/shales
are transitional. Bioturbations in the limestones are followed
by shales and marls. Decreasing clay contents in limestones,
Fig. 12. Limestone-marl/shale cycles, black shale interval and stable isotope variations within the cycles of the Mudurnu section (for
symbols see Legend to Fig. 7).
SEA LEVEL CHANGES IN PLATFORM AND PELAGIC CARBONATE SUCCESSIONS 29
and comparison of the position of the black shale within lime-
stone-marl/shale couplets with black shale intervals overlain
by the pelagic limestones in both sections, which are already
interpreted as a transgressive record in the large-scale, lead to
the interpretation that the thin marls and black shales were de-
posited due to small-scale transgressions (Yôlmaz et al. 2000
and Yôlmaz 2002). The thin transggressive shales and marls
are also described by Bralower et al. (1994, 1999), Claps &
Masetti (1994), Claps et al. (1995), Bellonca et al. (1996).
Similarly, Mutterlose & Ruffell (1999) indicated the position
of dark marls deposited in transgressive and pale marls depos-
ited in high stand conditions of sea level within the Hauterivi-
an-Barremian deposits of Eastern England and Northern Ger-
many. The limestone beds are commonly bioturbated and
filled with black, grey clayey material derived from overlying
black shales/marls. Therefore, limestones are set at the tops of
cycles and interpreted as records of sea level high stand. The
limestone-marl/shale couplets are interpreted as the records of
small-scale sea level changes induced by climatic effects (as
in Claps & Masetti 1994; Claps et al. 1995; Yôlmaz et al.
2000; Pittet et al. 2002; Yôlmaz 2002 and many others).
As in the Seydiþehir and Zonguldak sections, the duration
of each cycle is similarly calculated by dividing the time inter-
val of the whole section by the number of cycles detected.
(The approximate durations of part of the stages covered in
the sections are about 12 Ma for Barremian and 5 Ma for
Aptian in Mudurnu section and 45 Ma for Aptian in the
Nallôhan section after the correlation of biostratigraphic
framework in the sections with the global sea level chart
(based on time-scales of Haq et al. 1988).) The duration of
each cycle in the sections fits the axial obliquity of the Mi-
lankovitch band, with a model period of 41 Ka of Fischer
(1991) (Table 1) indicating that sea level changes were related
to changes in the Earths orbital parameters.
However, on the basis of the time-scale of Graciansky et al.
(1998) and Premoli Silva & Sliter (1999), the approximate du-
ration for the Aptian represented in the Mudurnu section is
about 88.8 Ma, and 78 Ma in the Nallôhan section.
Therefore, the duration per cycle in pelagic sections gives a
range between 45 Ka and 76 Ka, depending on the different
Stable isotope analysis
Because of the lithified texture, and fine-grain size of the
samples, all stable isotope analyses were obtained from bulk
rock samples. Individual foraminiferal species or other micro-
Fig. 13. Limestone-marl/shale cycles, black shale interval and stable isotope variations within the cycles of the Nallôhan section (for
symbols see Legend to Fig. 7).
30 YILMAZ et al.
Table 2: Carbon and oxygen stable isotope values of bulk carbonate rock samples, veins, infillings, bioturbations, shells and corals ob-
tained from measured sections in this study.
fossils could not be separated. During the sampling, the fine-
grained matrix and homogeneous micritic parts of limestones
were used and, unless specifically sampled for, care was taken
to avoid larger clasts and veins. Powders were obtained from
well-polished surfaces of hand specimens by means of a hand-
held drill. Two milligrams of powdered sample collected from
several spots of the sample slab were homogenized, washed with
2.5% NaOCl for 24 hours and rinsed several times with distilled
water. Samples were subsequently dried at 70 °C overnight.
Isotopic compositions were measured at the University of
Tübingen using an automated GasBench II online to a Finni-
gan MAT 252 mass spectrometer, and He carrier gas. Samples
of about 100 µg size were dissolved by several drops of ortho-
phosphoric acid (100%) at 70 °C in individual borosilicate
glass vials. Isotopic compositions are expressed as conven-
-values in permil () relative to V-PDB. Precision is
about 0.08 and accuracy 0.1 for both oxygen and car-
bon. The isotopic ratios were normalized relative to the in-
house standard (Laaser Marble, with
C = +1.5 and
O = 5.2 , calibrated against NBS-19
C = +1.95 ,
O = 2.20 ). A total of 126 bulk rock samples, 15
vein samples, 5 shell samples, 8 samples from visibly biotur-
bated parts, 4 infilling samples, and 6 clast samples have been
SEA LEVEL CHANGES IN PLATFORM AND PELAGIC CARBONATE SUCCESSIONS 31
Table 2: Continued.
Host rock for Infillings
Host rock for clasts
Host rock for bioturbations
Host rock for corals and shells
Corals and shells
Stable isotope data
The carbon isotope values for bulk rock samples in pelagic
successions range from 0 to +4.2 , oxygen isotope val-
ues in pelagic successions range from 5.3 to 1.9 , with
most samples having values between 2 and 4 . For sam-
ples from platform successions, carbon isotope values for
bulk rock samples vary between 2.26 and +3.05 . Oxy-
gen isotope values for inner platform successions range from
9.7 to 2.6 , with most samples having values between
3 and 6 . The carbon isotope values for vein samples
range from +1.2 to 5.2 , and oxygen isotope values
range between 10.7 and 4.2 (Table 2).
Effects of diagenesis
The stable isotope compositions of carbonates can be sig-
nificantly modified through diagenesis. Whether or not alter-
ation of the isotopic composition occurs during recrystalliza-
tion is a function of the original mineralogical constitution of
the carbonate (e.g. metastable aragonite and high-Mg calcite
versus low-Mg calcite; Patterson & Walter 1994), the isotopic
composition of the fluid, and the temperature during alteration
and recrystallization. Recrystallization, in the presence of me-
teoric water, tends to lower both the
O values of
carbonates (e.g. Lohmann 1988). The diagenetic history of the
sections shows two different general patterns between the
platform and pelagic sections in this study. The platform car-
bonates mainly composed of lime mudstone, wackestone,
packstone display development of marine diagenesis (includ-
ing cement A and B of Flügel 1982 in some parts) at the bot-
tom and mid-parts of the cycle and meteoric diagenesis at the
top of the cycle where subaerial exposure structures are devel-
oped. However, pelagic carbonates do not present any meteor-
ic diagenesis along the sections and are mainly dominated by
primary micrites. Joachimski (1994) states that micrites have
a good potential for preservation of their primary carbon iso-
topic composition if the duration of subaerial exposure is rath-
By comparison to other carbonates analysed from Creta-
ceous sequences and considered to represent primary isotopic
compositions (e.g. Weissert & Lini 1991; Menegatti & Weis-
sert 1998; Stoll & Schrag 2000; Pittet et al. 2002) the
values of the sampled profiles are generally lower and hence
are interpreted as having been affected by diagenesis. Support
for a diagenetic influence on the bulk rock isotopic composi-
tions is also given by a comparison of the isotopic composi-
tions of the bulk rock with those measured from individual
shells, corals, bivalves, and small veins within the same sec-
tions (Fig. 15 and Table 2). Shells, corals, and bivalves are on
average, somewhat enriched in
O compared to their parent
bulk rocks (Fig. 15 and Table 2). In contrast, late genetic
veins in platform and pelagic carbonate samples are always
significantly depleted in
O and C
compared to the bulk
rock (Fig. 15), indicating the influence of meteoric water in
veins. The average carbon isotope compositions of clasts,
shells, and corals are similar to average values of their host
rocks. Furthermore, the carbon isotope compositions have a
range that is very similar to that of other Cretaceous carbon-
32 YILMAZ et al.
ates and as illustrated in Figure 14a and discussed further be-
low, the variations in
C values of the Nallôhan section also
correlate with global Aptian
C curves (Weissert & Lini
1991; Weissert & Bréhéret 1991; Menegatti & Weissert 1998,
and many others). These features, in addition to the observa-
tion that there is little co-variation in
compositions (Fig. 15), and that the pelagic carbonates still
retain higher average
O values (3.55 ) compared to the
inner platform shallow water carbonates (5.03 ) (Fig. 15
and Table 2), but with a parallel stratigraphic variation, indi-
cate that the diagenetic influence was limited. Thus, while a
diagenetic influence may somewhat shift oxygen isotope val-
ues, the original variations are still preserved.
Other possible effects causing variations of
values can be changes in ocean water circulation, changes in
amount of terrestrial input, rate of carbonate productivity,
and/or salinity changes (De Boer 1982).
Similar cyclic isotope curves are obtained from peritidal
carbonates of the isolated platform (Taurides), from shallow
marine deposits of the continental shelf (Zonguldak) and from
pelagic successions (Sakarya). This is explained by common
mechanisms for all studied settings. Eustatic sea level changes
in relation to climatic variations appear to be the most likely
factors. The marl/limestone couplets showing similar cyclic
isotope curves with their shallow water counterparts
(Fig. 14b) indicate the presence of small-scale sea level
changes induced by short-term climatic fluctuations and the
positions of limestone-marl/shale facies within a cycle be-
The relationship between stable isotope compositions and
sea level changes
Stable isotope compositions were measured with the aim of
determining the mechanisms responsible for the meter-scale
shallowing-upward cycles in platform carbonates and the
limestone-marl couplets in pelagic successions. Many studies
have concentrated on the causes of cyclicity within Creta-
ceous sediments deposited under what is often considered a
relatively equable greenhouse-type climate (e.g. Einsele et al.
1991; De Boer & Smith 1994; Miall 1997; Einsele 2000; Fiet
et al. 2001; Strasser et al. 2001). Some of the studies suggest-
Fig. 14. a Comparison of the
C curves in G. blowi Zone. The dotted line represents the global curve (Wiessert & Lini 1991; Menegatti
& Weissert 1998 and many others), the solid line represents the carbon isotope curve in this study. Curves are smoothed and simplified.
b Representative diagram illustrating small-scale and large-scale facies and stable isotope synchronicity in pelagic and inner platform set-
tings in this study.
SEA LEVEL CHANGES IN PLATFORM AND PELAGIC CARBONATE SUCCESSIONS 33
ed fluctuating glacial activities within a greenhouse Creta-
ceous (Weissert & Bréhéret 1991; Weissert & Lini 1991; Sel-
wood et al. 1994; Pirrie et al. 1995; Jenkyns & Wilson 1999;
Price 1999; Stoll & Schrag 2000). Therefore, waxing or wan-
ing ice volumes even during Cretaceous greenhouse condi-
tions may be the most apparent cause for fluctuating eustatic
sea levels. Strasser et al. (1999) stated that thermal expansion
and retraction of the uppermost layer of ocean water by ther-
mally induced volume changes in deep water circulation
might have contributed to high-frequency sea level variations,
and also noticed that a cool mode in paleoclimate from
Middle Jurassic to Early Cretaceous with a pronounced sea-
sonality was present, and although ice-volumes were not suf-
ficient to introduce important glacio-eustatic fluctuations,
they could make small contributions.
Changes towards more positive
O values can be inter-
preted as a result of lowering of eustatic sea level caused by
cooling episodes and build up of ice-sheets, while changes to-
wards more negative
O values may be correlated with a
rise in sea level, corresponding to warming episodes. In addi-
tion to changes in
O values, changes in the
C values of
carbonates may also be linked to changes in climate as the
carbon isotope compositions are ultimately linked to the car-
bon cycle and the bioproductivity in the water column of the
Cretaceous ocean (e.g. Weissert & Lini 1991). Hence, chang-
es towards lower
C values may be associated with lower
biological productivity, which in turn may be related to re-
duced nutrient flux and water cycle and overall cooling.
For the sedimentary cycles of the Taurides (Seydiºehir), the
Pontides (Zonguldak), as well as the pelagic Mudurnu Trough
carbonates, the variations in
O values are all similar in that
they generally increase in parallel with the shallowing-upward
cycles (Figs. 7 to 13, 14a,b). The overlying cycle returns to
lower, more negative
O values at the bottom and again
ends up with higher values towards the top. This is the case
even though the cycles are composed of different lithofacies
in different settings. This pattern is recorded in most of the cy-
cles and, despite the diagenetic overprint, can be interpreted
as a record of changing sea level and temperature as a result of
changes in the ice-volume. Hence, transgressive portions of
the cycles with lower
O values indicate warmer periods,
whereas regressive portions of cycles with more positive
values indicate cooler periods. This relationship is also ob-
served on the scales of the smallest cycles recognized, with
O being on the order of about 1 for smaller
cycles to about 2 for larger cycles.
While changes in the
C values are not as constant with
regard to the direction of change within a cycle compared to
O values, there is nonetheless also a general ten-
dency to have more positive values in the transgressive por-
tions and more negative values in the regressive portions of
the cycles. This also supports the idea of warming during the
transgressive phase and cooling during the regressive phase.
However, there are also a number of cycles where the changes
C values with respect to the transgressive-regressive cy-
cles are not as clear. This is likely to be related to other factors
besides climate affecting the bioproductivity in the water col-
umn as well as complex changes in the types of organisms
and their relative abundances as a function of time and sedi-
mentary facies, and pedogenetic alterations at the top of the
subaerially exposed cycles.
The curve depicting the
C values of the pelagic Nallôhan
section, the most complete section sampled in this study
(Fig. 13), is quite comparable to other Aptian
both in terms of the magnitude of change in
C values (0
to +4 ) and in terms of the absolute
C values (Fig. 14a)
(Weissert & Bréhéret 1991; Weissert & Lini 1991; Ferreri et
al. 1997; Grötsch et al. 1998; Kuhnt et al. 1998; Jenkyns &
Wilson 1999; Erba et al. 1999; Strasser et al. 2001). The
Nallôhan section includes a prominent black shale interval
within the G. blowi Zone. This position of this black shale is
directly comparable to the Selli level, which marks a promi-
nent large-scale transgressive event in sections studied else-
where (Menegatti & Weissert 1998). The correlation to other
sections where this event has been interpreted as being related
to a large-scale transgression also supports the conclusion that
the marl-shale sequences within the cycles are related to small-
scale rises in sea level (Yôlmaz et al. 2000; and Yôlmaz 2002).
The variations in
O values observed for the small cycles
as well as the larger cycles (Figs. 7 to 13, 14) imply that
small-scale variations of climate and sea level were superim-
Bulk shallow water carbonates vs. pelagics
Bulk rocks vs. veins, shells and corals
Shells and Corals
Fig. 15. Graphic expressions of relationships between carbon and
oxygen stable isotope compositions of bulk rock samples, veins,
shells and corals (data from Table 2).
34 YILMAZ et al.
posed over larger-scale variations. Hence, even during the
Cretaceous greenhouse-type climate, significant short-term
variations were possible, and the climate during the Creta-
ceous was not as equable as previously thought.
The Tauride and Pontide platforms are interpreted to have
formed in tropical-subtropical belts during the Early Creta-
ceous. The shallow water carbonates of these platforms dis-
play a warm-water facies. No cool-water carbonate facies has
been recorded in the studied sections. However, cooling peri-
ods are interpreted from the stable isotope variations mea-
sured in these sections. This implies that temperature changes
were not sufficiently large to change the character of the fa-
cies in the tropical-subtropical regions, but large enough to
cause sufficient melting in the polar regions in order to affect
sea level and the oxygen isotope composition of the seawater.
In this study, changes in the C and O isotope composition
of Barremian-Aptian platform carbonates of the Taurides and
Pontides and their pelagic counterparts are documented.
values generally change towards more positive values at the
top of the cycle, but again more negative at the base of the
next cycle. This cyclicity in
O values is interpreted as a pri-
mary variation reflecting changes in temperature and sea level
on the order of the Milankovitch frequency band. Meter-scale
cycles of inner platform carbonates and of pelagic successions
are thus likely to be related to small-scale changes in glacial
activity and/or thermal expansion of the ocean water.
values are more difficult to interpret for the small-scale cycles,
but on a larger scale the
C curve compares well with the
ones measured for the same time interval. This is particularly
the case for the pelagic Nallôhan section, which can be corre-
lated on a global scale and for which the changes in values are
of similar magnitude.
The C and O isotope compositions measured in the sections
of this study can thus be interpreted to reflect primary varia-
tions, even though the primary compositions have been slight-
ly modified by diagenetic changes. Such changes appear to
have had a more pronounced effect on the oxygen rather than
the carbon isotope compositions.
Acknowledgments: This work was supported by the Turkish
Scientific and Technical Research Council (TÜBITAK,
(Project No: YDABÇAG-198Y040) Ankara, Turkey) and the
Middle East Technical University (Ankara, Turkey), and by
the Institute of Mineralogy, Petrology, and Geochemistry of
the University of Tübingen (Germany). We thank Prof. Dr.
André Strasser (Fribourg, Switzerland), Dr. Otília Lintnerová
(Bratislava, Slovak Republic) and Prof. Dr. Okan Tüysüz
(Avrasya Yerbilimleri Enstitüsü, ùTÜ, Turkey) for the review
of the manuscript. We are grateful to Necdet Özgül (Geomar,
Istanbul) for sharing his experiences during fieldwork and to
Yakup Özcelik (TPAO, Ankara) for his help in characterizing
the well-exposed sections in the Zonguldak region. We also
want to thank Bernd Steinhilber (University of Tübingen) for
his help with the laboratory work.
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