Cyclostratigraphy and sequence boundaries of inner platform mixed carbonate–siliciclastic successions (Barremian–Aptian) (Zonguldak, NW Turkey)

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Journal of Asian Earth Sciences 30 (2007) 253–270

Cyclostratigraphy and sequence boundaries of inner platformmixed carbonate–siliciclastic successions

(Barremian–Aptian) (Zonguldak, NW Turkey)

_Ismail Omer Yılmaz *, Demir Altıner

Department of Geological Engineering, Middle East Technical University, 06531 Ankara, Turkey

Received 12 December 2005; received in revised form 14 July 2006; accepted 14 August 2006

Abstract

Barremian–Aptian shallow marine mixed carbonate–siliciclastic successions of the Zonguldak area (NW Turkey) have been studied indetail along three measured stratigraphic sections. Field and laboratory analysis of microfacies and sedimentary structures have demon-strated that sections are composed of meter-scale shallowing-upward cycles. Cyclicity of shallow water mixed carbonate–siliciclastic suc-cessions are generally observed in the form of alternating sandstone/siltstone or sandy limestone and limestone. A prominent level ofcharophyte packstone within the successions of Zonguldak and Kozlu is interpreted as a sequence boundary. In the Cengellidere section,cyclicity is generally represented by an alternation of sandstone/conglomerate and bioclastic limestone. The first level of conglomeratewithin the section is interpreted as a sequence boundary. Smaller-scale cyclic arrangements are also recorded within meter-scale cycles.4th-, 5th- and 6th-order cycles are superimposed and create a hierarchical stacking. However, 6th-order cycles present a relatively ran-dom distribution and termed as episodic. Thickness variation of cycles along the section and frequency of sand and/or silt rich cycle typesare used to interpret systems tracts within the sequences.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Cyclostratigraphy; Sequence stratigraphy; Carbonate platform; Mixed carbonate–siliciclastic successions; Barremian; Aptian; Zonguldak;Turkey

1. Introduction

Peritidal carbonate-dominated or mixed carbonate–silic-iclastic successions frequently display shallowing-upwardcycles at meter-scale. Vertically stacked shallowing-upwarddeposits are either bounded by marine flooding surfacesthrough transgressive events or by subaerial exposure sur-faces which represent sea-level fall. In shallow-water car-bonate depositional settings, shallowing-upward meter-scale cycles usually correspond to parasequences whichare bounded by marine flooding surfaces as originallydefined by Van Wagoner et al. (1988), and are interpretedas building blocks of larger sequences (Wright, 1986;

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doi:10.1016/j.jseaes.2006.08.008

* Corresponding author.E-mail address: [email protected] (_I.O. Yılmaz).

Grotzinger, 1986; Hardie et al., 1986; Osleger and Read,1992; Tucker et al., 1993). However, shallowing-upwardmeter-scale cycles are also interpreted as small-scalesequences (Strasser et al., 1999). These cycles can be con-sidered as the reflection of climatic changes (Milankovitchcycles) controlled by the Earth’s orbital parameters (eg.,Einsele et al., 1991).

The cyclic nature of shallow-water carbonate-dominatedsuccessions can be revealed by examination of sedimentarystructures, microfacies and textures at centimetre- (micro-stratigraphy), decimetre- or metre-scale. Careful examina-tion of these microstratigraphic data helps to betterunderstand the nature of the platform carbonate sequenc-es. Strasser (1994), Raspini (1998), D’Argenio et al.(1997) and Strasser and Hillgartner (1998) have publishedgood examples of well-documented studies about the hier-archy of high-frequency cyclic carbonate successions.

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254 _I.O. Yılmaz, D. Altıner / Journal of Asian Earth Sciences 30 (2007) 253–270

This study mainly focuses on the cyclicity and sequenceboundaries within the Barremian–Aptian peritidal succes-sions of the Pontide carbonate platform (_Istanbul Zone,Okay, 1989; Tuysuz, 1999) located at the northern side ofthe Neotethyan Ocean in the Zonguldak region, NW Tur-key. Although dense vegetation and soil cover haveobscured the geometrical structures of sequence boundariesand systems tracts in the study area, possible systems tractsand sequence boundaries can be identified by interpretingthe cyclicity of the successions.

2. Methods of the study

Three stratigraphic sections covering the Barremian andLower Aptian have been measured in detail in the Zongul-dak, Kozlu, and Cengellidere areas of the Zonguldakregion (Figs. 1 and 2). The biostratigraphy and chronostra-tigraphy have mainly been established by the zonations ofbenthic foraminifera.

A total of 95.84 m stratigraphic thickness was measured.Two hundred and sixteen beds were examined vertically

Fig. 1. Geographic location of section

and laterally in the field and 119 oriented and unorientedsamples, 46 for the Zonguldak section, 48 for the Cengelli-dere section, and 25 for the Kozlu section were recovered.For microfacies studies, point-counting of components hasbeen applied. Both microfacies analysis and field examina-tions of sedimentary structures are used in the reconstruc-tion of the cyclicity. The point counting results displayingthe changes in ratio of the components in each bed havebeen applied for identifying the smaller-scale cycles. Thecalcitic matrix, bioclasts, algae, benthic foraminifera,quartz, intraclasts and pellets are counted for each bedand their relative ratio changes are used to detect thechanges in the selected cycles. The comparison of the stableisotope data of Yılmaz et al. (2004) with the ratio changesof constituents of the facies from the same selected cyclesare used to check the cyclic hierarchy in the sections.

3. Geologic setting

The study areas comprise the centre of the Zonguldakcity and localities near the town of Kozlu and of Cengelli-

s measured in the Zonguldak area.

Fig. 2. Generalised geological map of the Zonguldak and surrounding areas (simplified from Derman, 1990). Locations of the measured sections areshown with numbers.

_I.O. Yılmaz, D. Altıner / Journal of Asian Earth Sciences 30 (2007) 253–270 255

dere village in the Black Sea region (NW of Turkey)(Fig. 1). They are geologically located on the _Istanbul Zone(Okay, 1989; Okay and Tuysuz, 1999) of the Pontides,which was situated at the northern passive margin of theNeotethys Ocean during the Late Jurassic–Early Creta-ceous (S�engor and Yılmaz, 1981; Ozgul, 1984; Altıneret al., 1999; Tuysuz, 1999). The _Istanbul Zone is partlycharacterised by Cretaceous outcrops along the southwest-ern Black Sea coast (Tokay, 1952, 1954/55; Kaya et al.,1983; Derman, 1990; Orhan, 1995; Yalcın, 1996; Gorur,1997). According to Tuysuz (1999), the studied Cretaceousrocks belong to the Zonguldak-Ulus Basin, which wasopened during the Late Barremian by the rifting of theUpper Jurassic and older basement rocks and survived asa single basin during the Late Barremian-Maastrichtian.

In this region, Upper Paleozoic rocks of the westernPontides, composed of continental and shallow-water car-bonates and clastics, are unconformably overlain by Upper

Jurassic–lowermost Cretaceous shallow-water carbonates,the _Inaltı Formation, with a basal conglomerate (the Bur-nuk Formation), which is Late Jurassic in age. A Creta-ceous sequence unconformably rests on the Paleozoic andUpper Jurassic–Lowermost Cretaceous units. The base ofthis sequence is represented by the _Incigez, Kapuz andCengellidere Formations. The Kapuz Formation of EarlyCretaceous age is characterised by peritidal carbonatesintercalated with thin siliciclastic layers (Tuysuz, 1999;Tuysuz et al., 2004) (Figs. 2 and 3). The Cengellidere For-mation is partly lateral equivalent of the Kapuz Forma-tion, and consists of alternating bioclastic limestones andthick siliciclastics. The Lower Cretaceous platform carbon-ates (Kapuz and Cengellidere Formations) transgressivelyoverlie a red continental clastic unit (_Incigez Formation),which itself unconformably lies over the _Inaltı Formation.They are unconformably or partly conformably overlain bythe Kilimli Formation composed of an alternation of

Fig. 3. (A) Generalised columnar section of the study area (modified from Tuysuz et al., 2004) displaying the studied interval and positions of themeasured sections (indicated by numbered solid black lines). (B) Biostratigraphic framework including biozones determined in the Zonguldak, sectionsand their correlation (Biozones are exactly equal to biozones determined in Yılmaz (2002) and Yılmaz et al. (2004)). (C) Correlation of sequenceboundaries and biozones.


sandstones and marls of Aptian-Albian age, followed bythe Velibey Formation characterised by quartz arenites ofAptian-Albian age (Tuysuz, 1999; Tuysuz et al., 2004).The Sapca Formation, composed of an alternation of sand-stones with glauconite and marls, conformably overlies theVelibey Formation. The Cenomanian Tasmaca Formationis represented by flysch-type successions and it is uncon-formably overlain by Upper Cretaceous – Eocene volca-nics, volcanoclastics and turbiditic siliciclastics.

The studied sections are measured in the lower and mid-dle-upper parts of the Kapuz and Cengellidere Formations(Fig. 3).

4. Biostratigraphy

Biostratigraphy and chronostratigraphy of the sectionshave been already studied by Yılmaz et al. (2004) andYılmaz (2002), and will therefore not be repeated here.The two biozones (A and B) were established based on ben-thic foraminifers in the measured sections (Fig. 3). Zone Aencompasses the interval between the uppermost Barremi-an and the Lower Aptian and is characterised by the totalrange of Palorbitolina lenticularis. Zone B is an assemblagezone corresponding to the Barremian and is characterisedby Rectodictyoconus giganteus, Orbitolinopsis debelmasi,


Arenobulimina cochleata, Choffatella tingitana, and Orbito-

linopsis flandrini (Yılmaz et al., 2004).Among the measured sections, Section 1 and an impor-

tant part of the Section 2 correspond to the Zone A, how-ever, Section 3 entirely belongs to the Zone B (Fig. 3C).According to De Graciansky et al. (1998), the first occur-rences of two important orbitolinid index foraminifera P.

lenticularis and R. giganteus are within the Late Barremianand contemporaneous, corresponding to time line of 123.5my. These two forms are present in our zones, however,their first occurrences are not contemporaneous. Recto-

dictyoconus giganteus occurs for the first time in the upperpart of the zone B and P. lenticularis is recorded in the suc-ceeding Zone A. This observation suggest that the bound-ary between these two zones is diachronous in the region,however the resolution is still within the Barremian stage(more precisely within the Upper Barremian substage)encompassing the interval characterised by the apparentfirst occurrences of the taxa.

Within the frame of the biostratigraphic data, whenphysically identified sequence boundaries in the study arecorrelated with the Spanish Pyrenees (Bernaus et al.,2003) and the global picture of De Graciansky et al.(1998), the best fit in the correlation occurs as illustratedin Fig. 3C. Our three main sequence boundaries corre-spond to 124 my (Bar 5), 122 my (Bar 6) and 121 my(Ap1) of De Graciansky et al. (1998).

5. Cyclostratigraphy

Meter-scale cycles are recorded throughout the mea-sured sections. They are interpreted to represent Milankov-itch cyclicity and are ranked as the 4th-order. These type ofcycles can also resemble to the small-scale sequences ofStrasser et al. (1999). Smaller-scale cycles, the 5th-ordercycles, are also recorded and constitute parts of meter-scalecycles where they are visible in the measured sections andconsidered as rhythmic cycles (Einsele, 2001). They canalso resemble to the elementary sequences of Strasseret al. (1999). The 6th-order episodic cycles are randomlypreserved and are rarely recorded within the 4th- and5th-order cycles. The 6th-order cycles are also consideredas episodic event deposits therefore may not be stated ascyclic.

5.1. Zonguldak section

The Zonguldak section has a thickness of 18.93 m.(Fig. 1, Section 1, and Fig. 4). The succession is composedof medium- to thick-bedded carbonates alternating withthin- to medium-bedded calcareous sandstones, siltstones,sandy limestones and limestones. Calcareous clastic beds,generally rich in quartz grains (Fig. 5C), contain plantremains (Fig. 5D) and intraclasts derived from underlyinglimestones (Fig. 5B). They contain less marine biota thanthe overlying limestones and are transitional with lime-stones. Some levels are characterised by flat pebbles with

imbricate structure and parallel laminations. Limestonesdevoid of clastic material are represented by mudstone,wackestone, packstone and grainstone facies includingforaminifera, dasyclad algae, pelloids, intraclasts and frag-ments of rudist shells.

In general, cycles start with thinner siltstones or calcar-eous sandstones as transgressive deposits at the bottom andcontinue upward with alternating sandy limestones andsiltstones. Thicker limestones capping the cycles representthe sea-level highstand as high carbonate production andaccumulation outpacing the relative sea level rise (Einsele,2001; Strasser et al., 1999). This type of cyclicity is inter-preted as the 4th-order (Einsele et al., 1991).

These cycles display the following microfacies associa-tions (Figs. 5 and 6):

Cycle type – A: The type starts with parallel laminatedcalcareous sandstone or intraclastic sandy limestone(Fig. 5A) or siltstones and ends with intraclastic, bioclasticforaminiferal packstones/grainstones reflecting the high-stand condition of the cycle.

Cycle type – B: This type comprises parallel laminatedand foraminiferal calcareous sandstones at the bottomand continues upward with an alternation of silty marlsand sandy limestones. It is capped by bioclastic pelloidalpackstones/grainstones (Fig. 5H) containing foraminiferaand dasyclad algae at the top of the cycle.

Cycle type – C: In this type, foraminiferal sandy lime-stones displaying parallel lamination or calcareous sand-stones/siltstones occur at the bottom of the cycle. Thesuccession continues with an alternation of sandy/siltylimestone and sandstone/siltstone and is overlain by limemudstones (Fig. 5E) and then by bioclastic pelloidal fora-miniferal packstones/grainstones at the top.

Cycle type – D: Pelloidal sandy limestones with bivalveshells and foraminifera or thin-bedded calcareous sand-stones or siltstones are at the bottom whereas thicker-bed-ded pelloidal foraminiferal packstones or grainstones(Fig. 5F) with gastropoda or rudist shells or in some levels,wackestones with foraminifera and dasyclad algae are atthe top of the cycle.

Cycle type – E: This type of cycle starts with thin-beddedcalcareous sandstones or siltstones containing ostracodsand intraclasts at the bottom which are followed by thincalcareous siltstones and sandstones. It ends with thickcharophyte packstones including ostracods possibly depos-ited in a brackish/fresh water environment (Fig. 5G) indi-cating a clear shallowing trend in the section.

In a general sense, 4th-order meter-scale cycles do notactually exhibit perfect shallowing-upward propertiesincluding subaerial exposure structures such as mudcracks,karst breccias, or dissolution vugs etc., except charophytepackstones facies at the top. Therefore they can be inter-preted as submerged cycles (Elrick, 1995; Altıner et al.,1999; Yılmaz, 2002; Yılmaz et al., 2004).

The 4th-order cycles also contain smaller-scale 5th-ordercycles (Fig. 6). These cycles are facies repetitions within4th-order cycles (Fig. 6). Generally, 2–5 5th-order cycles

Fig. 4. Zonguldak measured stratigraphic section. Numbers on right side of the column represent samples numbers (for the symbols, see Appendix A).


build up one 4th-order cycle (Fig. 4). However, throughoutthe measured section, a definite and homogeneous arrange-ment of 5th-order cycles within 4th-order cycles is notclearly observed. This may be due to error in detection ofthe cycles along the section (hidden cycles), loss of sedi-ments by differential dissolution or compaction and styloli-

zation causing loss of some of the cycles or as a result of theeffect of all above parameters acting at the same time in aparticular location on the shelf. Within the 5th-order cyclessome 6th-order episodic cycles are recorded as well. 6th-or-der episodic cycles are characterised by fining-upwardstructures confined to one bed (Figs. 6 and 4). For

Fig. 5. Photomicrographs of the selected samples in Zonguldak section, (A) intraclastic sandy limestone (Q: Quartz) (OZ-4) (white bar is the scale: 0.5 mmand valid for all photographs), (B) sandy/silty limestone including large caliche clast (F: Foraminifera) (OZ-18b), (C) silty limestone (F: Foraminifera)(OZ-1), (D) sandy/silty limestone including large plant fragments (P: Plant fragment, Q: Quartz, F: Foraminifera) (OZ-2), (E) lime mudstone (OZ-23), (F)foraminiferal, intraclastic, pelloidal grainstone (F: Foraminifera, I: Intraclast, M: Micritized clast, O: Orbitolinid Foraminifera, P: Pelloid) (OZ-14). (G)Charophyta packstone (OZ-25), (H) pelloidal, foraminiferal, bioclastic moderate-well sorted grainstone (F: Foraminifera, P: Pelloid) (OZ-10).


example, within the intraclastic foraminiferal packstone/grainstone facies, intraclasts are coarser at the bottomand abundant. Towards the top of the same bed, intraclastsdecrease in abundance and size, and may be totallyreplaced with pelloids and shell accumulations (Figs. 4and 6). This type of cycle is not frequently observedthroughout the section. Their occurrence is random andrelated with episodic storm events.

According to ages of sequence boundaries within thechart of De Graciansky et al. (1998) and Haq et al.(1988), the average time of duration per 5th order-cycle isestimated to range between 32 and 75 ka., and per 4thorder-cycles between 115 and 200 ka. This duration periodalso fit to the values proposed from the Tauride platform,southern Turkey (Altıner et al., 1999; Yılmaz and Altıner,2001) showing this cyclicity is at least of regional-scale.

Fig. 6. Representative cycle types determined in the Zonguldak section.


5.2. Zonguldak–Kozlu section

The Kozlu section consists of 28 cyclic beds andmeasures 17.73 m in thickness (Fig. 1, Section 2 and

Fig. 7. Zonguldak–Kozlu measured stratigraphic section. Numbers on the righ

Fig. 7). This section and the Zonguldak section displaynearly the same microfacies, micropaleontological con-tent and sedimentary structures. The cycle types andthe hierarchy are similar except for some cycles in the

t side of the column are sample numbers (for the symbols, see Appendix A).

Fig. 8. Representative cycle types determined in the Zonguldak–Kozlu section.


Kozlu section which displays dissolution vugs at the topof the cycles.

Meter-scale cycles are basically the 4th-order cycles anddisplay the following microfacies associations (Figs. 7 and8):

Cycle type – A: Transgressive sandy, silty, bioclastic andforaminiferal limestones at the bottom and packestones withabundant charophytes and ostracods representing high-stand conditions at the top are the fundamental facies varia-tions within this type of cycle. Limestones with charophytescontain some dissolution vugs evidencing subaerialexposure.

Cycle type – B: Sandy, silty, pelloidal and bioclastic thick-bedded limestones at the bottom and algal and foraminiferalwackestones with some dissolution vugs representing expo-sure conditions at the top are the fundamental faciesvariations.

Cycle type – C: This type of cycle contains thicker-bed-ded carbonate facies and includes less siliciclastic material.It starts with foraminiferal and algal limestones with somepelloids and quartz or thin-bedded and laminated, sand-stones and siltstones containing lime clasts and continuesupward with bioclastic and pelloidal, foraminiferal wacke-stones including rudist fragments.

Point counting analysis of some of selected samples of theKozlu section suggests that facies associations in cyclicchanges are conformable with variations in siliciclastic andcarbonate content. The samples are especially selected fromwhere facies changes are not clear or very similar to eachother. Even the bed-scale facies variations are observed inpoint counting results confirming the presence of the small-er-scale cycles in this study. The point counting dataobtained from the calcitic matrix, bioclasts, algae, benthicforaminifera, quartz, intraclasts and pelloids for each bedhas resulted in relative ratio changes along the selected cycles(Fig. 9): the matrix increases towards the top of the cycleswhereas intraclasts, quartz content and benthic foraminiferadecrease in both smaller- and larger-scale cycles. The

comparison of the stable isotope data of Yılmaz (2002)and Yılmaz et al. (2004) with the ratio changes of constitu-ents of the facies from the same selected cycles has displayeda parallel cyclic fluctuation along the cycles in both smaller-and larger-scale (Fig. 7). The facies rich in siliciclastic con-tent at the bottom of the cycles interpreted as the transgres-sive part display enrichment in intraclast, quartz contentand benthic foraminifera. In contrast, the facies rich in car-bonate at the top of the cycles interpreted as the regressivepart display enrichment in calcitic matrix and decrease inintraclast, quartz content and benthic foraminifera. Thisrelationship is also consistent with the stable isotope excur-sions indicating warmer climatic conditions in the transgres-sive part causing the accumulation of more siliciclastic andclastic materials appeared in the point counting results andcooler climatic conditions in the regressive part causing adecrease in the accumulation of siliciclastic and clastic mate-rials and an increase in the accumulation of carbonatematrix appeared in the point counting results. This may alsoindicate that during the warmer periods more sediment hasbeen involved in the transportation due to increase in sedi-ment influx by the running water (Einsele, 2001).

5.3. Cengellidere section

This section is measured around Cengellidere village ofthe Zonguldak city (Fig. 1, Section 3). The total thicknessof the section is 59.18 m (Fig. 10). It covers the Upper Bar-remian–Lower Aptian interval and is completely composedof meter-scale cycles. Unlike Zonguldak–Kozlu and Zon-guldak sections, the amount of siliciclastic intercalationsincluding cross-bedded quartz arenites, calcareous silt-stones, sandstones and conglomerates and the limestoneswith rudists increases upward in this section. This indicatesthat the sedimentation has taken place in a more proximalposition relative to the shoreline. According to verticalfacies variations and differences with other sections, thedepositional setting of this section can be explained by

Fig. 9. The results of point counting method indicating number of counts on each component and their pie-graphs displaying changes in relativeabundance of components of the facies within the cyclic alternation in Zonguldak–Kozlu section.


sedimentation close to wave/tide dominated shorelines(Doyle and Roberts, 1988; Bosellini, 1998; Emery andMyers, 1996) in but also associated with reefal carbonates.

Calcareous sandstones and siltstones contain limestoneclasts and quartz grains and can be classified as sublithar-enite to litharenite (Folk, 1980). Quartz arenites alternatingwith sandstones are calcite cemented and include limestoneclasts. Conglomerates are calcite cemented and containabundant sedimentary rock clasts such as limestones, silt-stones, mudrocks and cherts, and can be classified aspolymict paraconglomerates (Tucker, 2001).

Limestone microfacies consist of packstones, wacke-stones, grainstones including foraminifera, dasyclad algae,pelloids, intraclasts and fragments of rudist shells. Baffle-stone facies includes coral branches and micritic ormicrosparitic matrix. The coral branches are partly micri-tized and replaced by sparry calcite.

In general, meter-scale cyclicity is represented by polym-ict conglomerates, calcareous sandstones/siltstones andmudstones as transgressive deposits at the bottom; alter-nating sandy limestones and siltstones at the middle, andlimestones representing the sea-level highstand at the top.

Fig. 10. Cengellidere measured stratigraphic section. Numbers on right side of the section represent samples (for the symbols, see Appendix A).


This type of cyclicity is interpreted as 4th-order (Einseleet al., 1991).

The increase in siliciclastic input starts with calcareous,polygenetic shallow marine conglomerates including

Fig. 11. Representative cycle types dete

foraminifera, rudist fragments and sandstones. Towardsthe top of the section, siliciclastic content decreases.

Microfacies variations within recorded cycle types are asfollows (Figs. 10 and 11);

rmined in the Cengellidere section.


Cycle type – A: This type of cycle is completely com-posed of siliciclastic material. It starts with thick-beddedcalcareous polymict conglomerates with scoured basesand is capped by an alternation of cross-bedded calcareoussandstones (Fig. 12B) and quartz arenites (Fig. 12A) at thetop (Fig. 11). In some places, quartz arenites may not beobserved and the cycle ends up with cross-bedded calcare-ous sandstones.

Fig. 12. Photomicrographs of the selected samples in Cengellidere section, (A)clast, Q: Quartz, C: Calcite cement) (white bar is the scale: 0.5 mm and valid fcement (OCG-29), (C) foraminiferal pelloidal packstone (P: Pellet, F: Foramin(OCG-35) (Q: Quartz, F: Foraminifera, P: Pellet), (E) intraclastic sandy limforaminiferal silty wackestone (I: Intraclast, Q: Quartz, F: Foraminifera) (OC10C), (H) Bufflestone with micritized and replaced coral branches (OCG-36) (

Cycle type – B: In this type of cycle, the invasion of thin-bedded black mudstones is observed between the cross-laminated calcareous sandstones or bioclastic packstones/wackestones. Within the cycle, cross-bedded sandstonesor conglomerates take place at the bottom, the alternationof mudstone and sandstone in the middle, and bioclasticforaminiferal packstones/wackestones (Fig. 12C) or calcar-eous sandstones at the top.

Quartzarenite with calcite cement and micritic clast (OCG-28) (M: Micriticor all photographs), (B) calcarenite with quartz, micritic clasts and calciteifera) (OCG-14), (D) pelloidal, intraclastic, foraminiferal sandy limestoneestone (I: Intraclast, Q: Quartz) (OCG-10A), (F) intraclastic, pelloidal,G-10B), (G) bioclastic pelloidal wackestone (R: Rudist fragment) (OCG-MC : Micritized coral branch, MSM: Microspar matrix).


Cycle type – C: This type of cycle starts with thin-bed-ded sandstones at the bottom and ends up with thick-bed-ded limestones represented by foraminiferal and pelloidalwackestone–packstone facies with abundant rudist frag-ments or with medium-bedded pebbly sandstones at thebottom and thick-bedded bioclastic and pelloidal, sandylimestones (Fig. 12D) at the top. The cyclicity can alsobe expressed by having sandy limestones at the base ofcycles and limestones in bafflestone (Fig. 12H) or pack-stone facies with corals, red algae and rudist fragmentsat the top. This type of cyclicity is generally observed inhighstand position of the sea level towards the top ofthe section.

We generally consider that the described cyclicity corre-sponds to 4th-order fitting to eccentricity band of Milan-kovitch cycles. Each 4th-order cycle is found to becomposed of 2–6 number of smaller-scale cycles whichare formed by the alternation of genetically related twofacies. Average number of these smaller-scale cycles isabout three per 4th-order cycles and they are termed as5th-order cycles. Polygenetic calcareous conglomerates dis-play thinning and fining upward structures and passupward into calcareous, cross-bedded, laminated marinesandstones including foraminifera. Alternation of thesetwo facies completes a cycle just before the next conglom-erate bed. Some cross-bedded quartz arenites intercalatewithin this cycle type (Fig. 11).

Within the cyclic nature of the succession, as in othermeasured sections in this study, fining-upward or coarsen-ing-upward structures are recorded within single beds(Fig. 11) and termed as 6th-order episodic cycles. Thesebeds occur rather randomly and actually are not cyclic(Einsele, 2001). They are interpreted as episodic eventdeposits.

Oxygen and carbon stable isotope variations on someof the selected cycles along the measured section dis-played parallel variations with facies (Yılmaz, 2002;Yılmaz et al., 2004) and even smallest-scale cycles, 6th-order episodic cycles, are isotopically detected within asingle bed (Fig. 10 samples 10A–C and Figs. 12E–Gand 7).

6. Sequence stratigraphy

Tracing of large-scale geometrical relationships ofsequence boundaries and systems tracts with overlyingand underlying strata in the field were not possible due todense vegetation, soil cover and populated settlements.Therefore, in this study, sequence boundaries have beendetermined by the correlation of facies and microfaciesanalysis in the field and in the laboratory along andbetween the measured sections and correlated with bound-aries identified by De Graciansky et al. (1998) and Haqet al. (1988).

In the Zonguldak section, sequence boundaries dated as122 and 124 My in the global sea-level curve of De Gra-ciansky et al. (1998) are represented by a prominent charo-

phyte packstone level (Figs. 13, 5G and 14C) and caliche/calcrete fragments embedded within sandy/silty limestonerespectively (Fig. 5B). In the Zonguldak–Kozlu and Ceng-ellidere sections, the sequence boundary at 113.5 My in thecurve of Haq et al. (1988) corresponds to 121 My in thecurve of De Graciansky et al. (1998) (Fig. 13). In the Zon-guldak–Kozlu section, this boundary is represented by aprominent charophyte packstone level as in the Zonguldaksection (Fig. 7). However, in the Cengellidere section, itcorresponds to the first appearance of a calcareous con-glomerate over the bioclastic limestones. The sequenceboundary at 112 My of (Haq et al., 1988) is probably veryclose to the upper end of the Cengellidere section (Fig. 13)according to the thickness variations of cycles and the bio-stratigraphic framework.

When the position of the cycle types and sequenceboundaries within the biostratigraphic framework are ana-lysed, it can be concluded that global sea-level changes ofHaq et al. (1988) and De Graciansky et al. (1998) wererecorded in the Zonguldak region (Fig. 13). The prominentcharophyte packstone levels in the section evidencing shal-lowing-up conditions are interpreted as Type 2 sequenceboundaries. Additionally, a very similar charophyte pack-stone level recorded close to the Barremian–Aptian bound-ary in the Kozlu section (Fig. 7) displays that the globalsea-level fall can be laterally traced over the platform(Fig. 13). When the position of the section is consideredwithin the platform, subaerial exposure structures such askarstic breccia, caliche, mud cracks (Fischer, 1965; Demic-co and Hardie, 1994; Wright and Tucker, 1991; Estebanand Klappa, 1983; Scholle et al., 1983) must have beenrecorded in the innermost parts of the platform towardsland. Another sequence boundary far below the charo-phyte rich level is characterised by the presence of peb-ble-sized caliche clasts (Fig. 5B). It is interpreted to be aravinement surface. Eroded materials from the exposedsurfaces are deposited in silty/sandy facies. Therefore, theType-2 sequence boundary which was recorded on anexposure structure landward is hidden in this bed. Thenumber of cycles determined within the same time periodin the Tauride carbonate platform in the study of Altıneret al. (1999) and Yılmaz (2002) is comparable to the num-ber of cycles in this study. In the Tauride platform, the low-er Aptian comprises 15 cycles in the Fele 1 section, 10cycles in the Fele 2 section, and 10 cycles in the Uzumlusection. In this study, 13 cycles in the Cengellidere sectionare detected within the same interval. As for the upper Bar-remian of Taurides, it comprises 18 cycles in the Fele 1 sec-tion, 15 cycles in the Fele 2 section, 19 cycles in the Uzumlusection, and 14 cycles in the Seydis�ehir-2 section. In thisstudy, the corresponding number of cycles is 16 in the Zon-guldak section. The number of compacted cycles on bothplatforms separated by thousands of kilometres is correlat-able (Kaufmann et al., 1991). Moreover, the two charo-phyte rich level interpreted as two sequence boundarieswithin the Late Barremian–Early Aptian interval in theUrgonian Carbonate platform by Bernaus et al. (2003) in

Fig. 13. Correlation of sequences and meter-scale cycles between the sections measured in the Zonguldak area and with global sea-level chart (Haq et al.,1988 and De Graciansky et al., 1998).


Spanish Pyrenees can also be used as an indication of valid-ity of these levels for correlation (Fig. 3C).

It could be interpreted that another sequence bound-ary might also be present just at the bottom of the Zon-guldak measured section (Fig. 14B), at least in thisparticular study area, because of the presence of a conti-nental succession including thick paleosoils alternatingwith coal-bearing sandstones and mudstones within the_Incigez Formation, marine sandy limestones and calcare-ous sandstones including marine fauna in the overlyingbeds of the Kapuz Formation. Few onlapping patternsdue to limited exposures of outcrops on the beds ofKapuz Formation just over the contact with _Incigez For-

mation is recognised (Fig. 14B). However, it is indicatedthat the age of the _Incigez Formation obtained from pre-vious studies is consistent with that of the Kapuz Forma-tion and a transitional boundary exists between them insome areas (Tuysuz et al., 2004). This indicates that theKapuz Formation transgressively overlies the _IncigezFormation without a significant time gap and onlappingpatterns may be observed in some areas. Nevertheless, itis thought that the _Incigez Formation constitutes a depo-sition over a tectonically enhanced sequence boundary(Vail et al., 1991) because of the long lasting continentaldeposits above the _Inaltı Formation (Tuysuz et al.,2004).

Fig. 14. Field views of some particular segments of Zonguldak and Zonguldak–Kozlu and Cengellidere sections. Geological hammer is circled in thephotographs for scale; (A) the lower to middle part of the Zonguldak section. Beds are relatively thinner compared to upper part of the section, (B) lowerpart of the Zonguldak section. Black line draws the unconformable boundary between the Kapuz and _Incigez Formations, (C) shallowing-upward meter-scale cycles and sequence boundary in the Zonguldak section. SBS: sequence boundary surface. Black line draws the sequence boundary just overCharophyta packestone. Black triangles: Shallowing-upward meter-scale cycles, (D) cycles composed of thicker beds in Zonguldak–Kozlu section. Whitetriangles are shallowing-upward meter-scale cycles, (E) cross-beddings in thicker sandstones (Cengellidere section), (F) close-up view of the cross-beddings(Cengellidere section), (G) close-up view of calcareous conglomerates (Cengellidere section), (H) outcrop of meter-scale cycles composed of sandstones,sandy limestones and limestones in the lower part of the section. White triangles represent meter-scale cycles, (Cengellidere section).


Determination of the boundaries of the systems tractswas not possible along the measured sections. However,within sequence packages, observable thickness changesin cycles (Fig. 13) and increase in the frequency of occur-rence of certain type of cycles in certain position are usedto approach the probable systems tract boundaries. Forexample, cycle with thicker limestones at their tops aredominantly observed towards the upper portion of thesequence packages (Figs. 14A and D and 15) or cycles withthicker sandstones at their bottoms are dominantlyobserved towards the lower portion of the sequences (Figs.14H and 15).

7. Conclusions

In the Zonguldak area, within the inner platform car-bonate-dominated successions, all measured stratigraphicsections are completely composed of meter-scale cycles.

Sections measured in the Barremian–Aptian intervalshow differences in cyclicity and lithofacies. In the Zongul-dak-Cengellidere section, the alternation of limestones withrudist fragments and sandstones is followed by the invasionof conglomerates (Fig. 14G) and cross-bedded sandstones(Fig. 14E and F) towards the top. However, in Zongul-dak–Kozlu and Zonguldak sections, contributions of

Fig. 15. Schematic correlation to illustrate the similarities between meter-scale cycles and sequences of the Zonguldak and Zonguldak–Cengellideresections. This figure also shows the similarity between small-scale and large-scale sea-level changes.


sandstones are less and thinner and limestones with rudistfragments are not frequently recorded.

At the bottom of the Zonguldak and Zonguldak–Kozlusections, limestones exhibit thinner beds and bed thicknessincrease towards the top (Figs. 14A, B and D). This mayindicate a large-scale increase in bed thickness from bottomto top.

In the Cengellidere section, sandstones and conglomer-ates decrease and the thickness of limestones increasestowards the top of the section, and in turn, thicknesses ofcycles increase in this section.

Dominant shallowing-upward cycles correspond to theMilankovitch cycles (Eccentricity band, E-2 signal) andare termed as 4th-order cycles in this study. Fourth ordercycles superimpose on top of each other and form 3rd-or-der sequence packages (Goldhammer et al., 1990). Howev-er, higher frequency smaller-scale cycles are also recordedwithin the 4th-order cycles. The alternation of two geneti-cally related facies without disturbing the shallowingupward structure within the 4th-order cycles is interpretedas smaller-scale cycles and hierarchically ranked as 5th-or-der (D’Argenio et al., 1997) and termed as rhythmic alter-nations (Einsele et al., 1991). One 4th-order cycle cancontain 2–6 5th-order cycles.

Some individual beds within 5th-order cycles exhibit fin-ing-upward or coarsening-upward structures and aretermed as 6th-order episodic cycles. However they are rare-ly observed as compared to 5th- and 4th-order cycles.Because of their random distribution, these cycles can beinterpreted as episodic deposits and may be related toshort-term storm events (Aigner, 1985; Walliser, 1996).

Consequently, high-frequency studies on small-scalecyclic arrangements of facies can be taken as a key to ana-lyse the arrangement of facies in large-scale successions onspot outcrops, especially where geometrical structures ofstrata to observe the features of systems tracts or to followthe sequence boundaries in long distance are not possible(Fig. 15).

The arrangement of cycle types shows a well-definedpattern according to large-scale sea-level changes. Abun-dance of cycles defined as having thin-bedded sandy lime-stones/sandstones at the bottom, thick-bedded limestonesat the top at certain position within sequence packagesand cycle thickness variations are used to determine prob-able boundaries of systems tracts. Generally, increase insand-dominated cycles concentrate in transgressive systemstracts and limestone-dominated cycles with thicker lime-stone beds concentrate in highstand systems tracts(Fig. 15).

Even this relationship is observed in smaller-scale cyclesand is detected by constituent changes within the faciesalong the cycles obtained by point counting method. It dis-plays a close relationship with the results of stable isotopeanalysis applied on the same cycles (Yılmaz et al., 2004)such that the stable isotope excursions indicating warmerclimatic conditions at the transgressive part causing theaccumulation of more siliciclastic and clastic materialsdue to relatively more erosive conditions and cooler climat-ic conditions at the regressive part causing the decrease inaccumulation of silciclastic and clastic materials andincrease in accumulation of carbonate matrix due to rela-tively more stagnant conditions.

Acknowledgements

This work was supported by the Turkish Scientific andTechnical Research Council (TUB_ITAK, Project No:YDABCAG-198Y040, Ankara, Turkey) and the MiddleEast Technical University (Ankara, Turkey). We are grate-ful to Yakup Ozcelik (Turkish Petroleum Co., Ankara) forhis help in characterising the well-exposed sections in theZonguldak region. We are thankful to reviewers Prof.Dr. Andre Strasser (University of Fribourg, Switzerland)and Prof. Dr. Okan Tuysuz (_Istanbul Technical University,Turkey) for their constructive comments.


Appendix A

Calcareous sandstone/siltstone

Silty marl

SYMBOLS USED IN DETAILED SECTIONS

Silty/Sandy Limestone

Calcareousconglomerate

Quartz arenites

Bafflestone facies withcorals

Pelloidal packstone - grainstone containingforaminifera, bioclasts and intraclasts

Lime mudstone

Bioturbation structures Pelloids

Large intraclasts

Oncoids

Pelloids

Micrite coated bioclastsOncoids

Ooids

PelecypodaOstracoda

Dasyclad algae

Intraclast

Limestone tongue withpellets and intraclasts

Rudists

Charophyta

Keystone vugs

Bivalves

Plant particles

Orbitolinids

Gastrapoda

Miliolids

Foraminifera

Cross laminae

Parallel laminae

Sandstone tongue

Limestone clast

Birdseye structures

Dissolution structures

SandstoneForaminiferal pelloidalwackestone

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Documents

Cyclostratigraphy and sequence boundaries of inner platform mixed carbonate–siliciclastic successions (Barremian–Aptian) (Zonguldak, NW Turkey)