Coniacian–Maastrichtian calcareous nannofossil biostratigraphy and carbon-isotope stratigraphy in...

Newsletters on Stratigraphy, Vol. 47/2 (2014), 183–209 ArticleStuttgart, June 2014

Coniacian–Maastrichtian calcareous nannofossilbiostratigraphy and carbon-isotope stratigraphy in the Zagros Basin (Iran): consequences for thecorrelation of Late Cretaceous Stage Boundariesbetween the Tethyan and Boreal realms

Mohammad Javad Razmjooei1, Nicolas Thibault2, Anoshiravan Kani1, Azam Mahanipour3, Myriam Boussaha2, and Christoph Korte2

With 7 figures, 2 plates and 1 table

Abstract. Calcareous nannofossil biostratigraphy and stable isotope stratigraphy have been investigated inthe Shahneshin section of the Gurpi Formation from the Zagros Basin (Iran). The results show that the GurpiFormation spans the late early Coniacian to late Thanetian. The age-model shows that the Shahneshin sectionspans the Coniacian to mid-Campanian with a good continuity whereas condensation is highlighted in thelate Campanian, across the Campanian/Maastrichtian boundary and in the late Maastrichtian. Extreme con-densation is recorded after the Cretaceous-Paleogene boundary with the complete absence of the Danian, andthe Selandian and lower Thanetian being comprised in only 6 m at the top of the Gurpi Formation. Correla-tion of the carbon-isotope profile with other reference curves allows the recognition of several Late Creta-ceous excursions at the Shahneshin section such as the Beeding, White Fall, Kingsdown, Michel Dean, HavenBrow, Horseshoe Bay, Buckle, Hawks Brow, Santonian/Campanian boundary (SCBE) and Campanian/Maas-trichtian boundary (CMBE) events. Correlation to a recently proposed global δ13C stack for the Late Creta-ceous points to a major mismatch of this compilation with magnetostratigraphy in the Santonian–early Cam-panian interval. The δ13C correlation, supported by calcareous nannofossil biostratigraphy, brings insightsinto: (1) the position of the Coniacian/Santonian, Santonian/Campanian and Campanian/Maastrichtianboundaries with respect to carbon-isotope stratigraphy and calcareous nannofossil bio-horizons, and (2) theircorrelation between the Tethyan and Boreal realms.

Key words. Late Cretaceous, calcareous nannofossil biostratigraphy, carbon-isotope stratigraphy, Zagros

© 2014 Gebrüder Borntraeger, Stuttgart, GermanyDOI: 10.1127/0078-0421/2014/0045

www.borntraeger-cramer.de0078-0421/2014/0045 $ 6.75

Authors’ addresses:1 Department of Geology, Faculty of Earth Science, Shahid Beheshti University, Tehran, Iran. M. J. Razmjooei: [email protected] Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, DK-1350Copenhagen K, Denmark. N. Thibault (corresp. author, present address): [email protected] Department of Geology, Faculty of Science, Shahid Bahonar University, 22 Bahman Bolvar, Kerman, Iran.

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1. Introduction

The Cretaceous of Iran is characterised by a large di-versity of rocks and facies (Nabavi 1976). There are 27 different formations in the Mesozoic of the Zagrosarea, 17 of those belong to the Cretaceous (Motiei1994). Sediments from the Cretaceous are well ex-posed in the Zagros basin, southwest of Iran, with anoverall good continuity and high sedimentation rates.For a variety of reasons, such as oil and gas reservoirsin Mesozoic and Cenozoic deposits, numerous studieshave been focused on the formation of the basin (Mol-nar 2006, Talbot and Alavi 1996, Sepehr and Cosgrove2005). The Gurpi Formation, which spans the LateCretaceous (Santonian–Maastrichtian) to Paleocenein the west and southwestern part of the Zagros basin,predominantly consists of shales and marls (Stocklinand Setudehnia 1970). The Gurpi Formation is highlyfossiliferous, and for this reason, has been extensivelystudied for different biostratigraphical aspects. Nu-

merous studies in the Zagros folded zone reveal sig-nificant lateral changes in the thickness, age and litho-logical composition of the Gurpi Formation (Ghasemi-Nejad 2006, Hosseini 2006, Nabavi 2008, Asleshirin2011). The present study focuses on the Shahneshinsection of the Gurpi Formation in the west of the Za-gros Basin (Fig. 1). The aim of the study is to establisha solid stratigraphic framework of the Shahneshin sec-tion based on calcareous nannofossils and bulk carbonstable isotopes, and to provide correlations to other areas and oceanic basins spanning the same interval(Fig. 2). The Late Cretaceous of this area remains apoorly known part of the Tethyan Realm and can im-prove the knowledge on calcareous nannofossil andstable isotope stratigraphy for the considered time in-terval which is mostly well-documented from Euro-pean and North American sections (Jenkyns et al.1994, Jarvis et al. 2002, 2006, Sprovieri et al. 2013,Thibault et al. 2012a, Voigt et al. 2010, 2012). Strati-graphic correlations to other reference sections for this

M. J. Razmjooei et al.184

Fig. 1. Location and geological map of the studied area. After the Geological map of Kazerun at 1:100000.

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time interval are shown and address a number of issuesregarding global δ13C stratigraphy and the identifica-tion of Late Cretaceous stage boundaries.

2. Geological setting

The Zagros basin is located in the western and south-western parts of Iran, between the Arabian andEurasian lithospheric plates. This structural zone iswidespread in nearby countries like Kuwait and Iraq(Takin 1972, Agard et al. 2005). Numerous northwest/southeast trending parallel folds were formed as a re-sult of the collision of Eurasia and Arabia during theCenozoic (Takin 1972, Agard et al. 2005).

The Cretaceous deposits of the Zagros Basin gener-ally cover the Berriasian to Maastrichtian interval(Motiei 1994). Among these deposits, the Gurpi For-mation investigated here is characterized by grey to blue marls and shales with, occasionally, intercalationsof thin beds of argillaceous limestones. The thickness ofthe Gurpi Formation is changing significantly in differ-ent parts of the Zagros Basin (Motiei 1994) and this isprobably the result of the major Kazerum Fault and other north-south trending faults in the central part ofZagros (Sepehr and Cosgrove 2005). Based on fora -

minifera and calcareous nannofossil data, the age of theGurpi formation has been shown to extend from lateSantonian to early Paleocene in some places (Asleshirin2011, in the Kuh-Sephid section, Hadavi and Rasa Ezadi 2008, in the Dare-Shahr section) whereas onlyCampanian to Maastrichtian deposits are present in other areas of the Zagros Basin (Etemad et al. 2008, inthe Kuh-Ghach section; Sina et al. 2010, in the Kuh-Soltan section). The Gurpi Formation was deposited ina deep shelf to basin margin setting during a majortransgressive phase (Bahrami and Parvanehnezhad Shi-razi 2010). Data from planktonic foraminifers (Abrari et al. 2011 in the southwest of Firozabad, Etemad et al.2008 in Lar Area, Hemmati-Nasab et al. 2008 in theKaaver section, Moradi 2010 in Farhad Abad section),benthic foraminifers (Hemmati-Nasab et al. 2008,Moradi 2010), palynology and sequence stratigraphicstudies (Rabani et al. 2009 in the Dare Shahr section) indicate relatively deep basin conditions for the GurpiFormation in the studied area. Ghasemi-Nejad et al.(2006) suggest an open marine, upper bathyal environ-ment for the deposition of the Gurpi Formation in theShahneshin section on the basis of palynofacies. Hem-mati-Nasab et al. (2008) suggested a rough estimationof 800 to 1200 m paleo-water depth for the Gurpi For-mation based on foraminifer planktic/benthic ratios.

Coniacian–Maastrichtian calcareous nannofossil biostratigraphy 185

Fig. 2. Maastrichtian paleogeography with positions of several sections mentioned in the text. (1) Shahneshin section, Iran,(2) Gubbio, Italy, (3) German chalk sections, (4) English chalk sections, (5) Olazagutia, Spain, (6) Waxahachie Dam Spill-way section, Texas, USA.

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The presence of various calcareous nannofossil and dinoflagellate cysts species with low latitude affinitiespoint toward deposition under subtropical latitudes forthe Gurpi Formation during the Late Cretaceous (Asle -shirin et al. 2011, Hadavi et al. 2007, Beiranvand et al.2013). The studied section is situated on the southwest-ern flank of the Shahneshin anticline, in the west of theFars province and in the north east of Kazerun city, withthe first sample collected at the following coordinatesN 29° 44� 47�; E 51° 46� 31� (Fig. 1). The Coniacian-Maastrichtian interval of this section is ca. 243 m thickwhich would point to an average sedimentation rate ofca. 1 cm/kyr but as shown further, several condensed in-tervals can be identified so this value is under-estimated.

3. Methods

3.1 Calcareous nannofossil assemblages

A total of 135 samples with a sampling resolution ofnearly 2 m were collected from the Shahneshin sectionand were processed using the gravity settling techniqueof Bown and Young (1998). This technique allows toconcentrate calcareous nannofossils in the slides by get-ting rid of much bigger and much finer particles. Resultsobtained on absolute abundances (specimens per fieldof view) are not directly comparable to studies using asimple smear-slide technique. However, this techniqueis useful in biostratigraphy for the observation of rarespecies due to the absence of fine and big sedimentaryparticles that can potentially shroud the calcareous nan-nofossil assemblage. The slides were prepared at thesedimentology laboratory of Shahid Beheshti Univer-sity and studied with a binocular microscope (EclipseE-600 pol) at 1000 � magnification. Calcareous nanno-fossil biostratigraphy and species richness were estab-lished based on presence/absence of biostratigraphicmarkers and by examining an average of 100 fields ofview. In addition, the abundance of the different specieswas evaluated in 62 samples from the Cretaceous inter-val over a total of 300 specimens. Key species are illus-trated in Plates 1 and 2. Bibliographic references for thecalcareous nannofossils are provided by Perch-Nielsen(1985) and Bown (1998). The CC biozonation of Sis -singh (1977) modified by Perch-Nielsen (1985) and theUCTP (Tethyan Province) of Burnett (1998) are appliedfor the Cretaceous of the investigated section (Fig. 3).For the Paleogene, the biozonation of Martini (1971)was applied. The taxonomic concepts follow Perch-Nielsen (1985) and Bown (1998).


Fig. 3. Calcareous nannofossil bio-horizons and biozona-tions used for the Cretaceous stratigraphy of the Gurpi For-mation in Shahneshin section based on Sissingh (1977)modified by Perch-Nielsen (1985) and Burnett (1998). Thenannofossil subzonation of Burnett (1998) corresponds tothe scheme for the Tethyan realm (TP).

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3.2 Stable isotopes

Oxygen and carbon isotope composition of bulk rockswere measured on the same 135 samples analyzed forcalcareous nannofossils. The analyses were carried outat the Department of Geosciences and Natural Re-source Management, University of Copenhagen, Den-mark, using the Micromass Isoprime mass spectrome-ter. The extraction of CO2 was executed by reactionwith anhydrous orthophosphoric acid at 70°C. The analytical precision is measured at 0.15‰ for oxygenand 0.08‰ for carbon.

3.3 Calcium carbonate content

135 samples were analysed for their CaCO3 content.The CaCO3 content (%) was measured using the fol-lowing method: bulk rock samples (0.2 g) were driedand homogenised, and then dissolved with HCl (4 cc,1 M) in a Bernard apparatus conical flask. The CaCO3

content (%) was calculated using calibration to the relative pressure of carbon dioxide.

4. Results

4.1 Sedimentological description of the Gurpi Formation

The type section of Gurpi Formation is located at thenorthwest of Zagros Basin on the southeastern plungeof Tang-e-Pabdeh, north of the Lali Oil field inKhuzestan. The succession consists of 320 meters ofgrey to blue marl and shale beds with occasional in-tercalations of thin beds of argillaceous limestone(James and Wynd 1965, Setudehnia 1972, Motiei1994). At the type section, the Gurpi Formation over-lays the Ilam Formation disconformably as indicatedby paleontological data that suggest a 10 Myr hiatusbetween the two Formations (Wynd 1965). The GurpiFormation is overlain by the Pabdeh Formation con-formably at the type section (Motiei 1994).

In the Shahneshin section, the Gurpi Formation con-sists of 255.4 m of marl and marly limestone cycleswhich overlay limestones of the Ilam Formation withan apparent lithological discontinuity. The basal con-tact of the Gurpi formation is marked in the studiedarea by an erosional surface, which is associated withiron oxide nodules (Fig. 4). The marly beds are grey toblack green and the marly limestone beds are lightgray to yellow and their thickness vary from a few centimeters to several meters. The Gurpi Formation is

overlain by purple shales of the Pabdeh Formationwith an apparent concordant contact. Subtle differ-ences in marls and marly limestones can be observedin the field, and from the results from the CaCO3 con-tent (Fig. 4). The definition of marls, limy marls, marl-limestone, marly limestone and limestone relies on the Pettijohn et al. (1975) classification based on theCaCO3 content as shown in Figure 4. The section canbe subdivided into 8 lithological units from the base to the top, as defined on Figure 4.

4.2 Calcareous nannofossil bio-horizonsand biozonations

The abundance of calcareous nannofossils varies be-tween 4 and 25 specimens per fields of view with anaverage of 12. The overall assemblage is dominated by Watznaueria barnesiae and W. biporta. Other abun-dant species are Cribrosphaerella ehrenbergii, Rete-capsa angustiforata, Micula staurophora and Predis-cosphaera cretacea. Eiffelithus eximius and Trano-lithus orionatus are also quite abundant in the intervalbetween the base of the studied section and 164 m (topof zone CC21/UC15c), reaching relative abundancesup to 17 and 8%, respectively. The preservation of thecalcareous nannofossils varies in the studied sectionfrom moderate to poor in the studied section, based onvisual inspection of etching and overgrowth as de-scribed by Roth (1978). The species richness of Creta-ceous samples is quite low (varying between 18 and39) compared to typical Tethyan assemblages and thissuggests an important impact of diagenesis on the cal-careous nannofossil assemblage. However, most bio -stratigraphic markers of the studied interval have beenfound and show a consistent record with few sporadicoccurrences (Table 1; Plate 1 and 2). According to thedistribution of calcareous nannofossil biostratigraphicmarkers, the Cretaceous part of the section spans zonesCC15 to CC26 of Sissingh (1977) and UC10 to UC20of Burnett (1998). The Paleocene part of the GurpiFormation includes zones NP5, NP7 and NP9 of Mar-tini (1971) and the base of the Pabdeh Formation iswithin NP9 (Fig. 4, Table 1).

Results of the biostratigraphy suggest several strati-graphical gaps in the Late Cretaceous part of the GurpiFormation. Also, it was not always possible to retrieveall the different zones and subzones of Perch-Nielsen(1985) and Burnett (1998) schemes because of the absence of given stratigraphic markers or reversals in the supposed order of those markers (Fig. 5). The po-sition of the Coniacian/Santonian, Santonian/Campan-


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Fig. 4. Stratigraphy, log and descriptionof the Shahneshin section. The calcare-ous nannofossil UC subzonation of Bur-nett (1998) corresponds to the scheme forthe Tethyan realm (TP).

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ian and Campanian/Maastrichtian boundaries is notdetailed here and is rather discussed along with iso-topic results in chapter 5.3. The main bio-horizonsused for biozonation are highlighted in bold on Figs. 5,6 and 7 to distinguish them from other bio-horizonsthat might have a regional significance. These bio -stratigraphic results are based here on moderately topoorly preserved assemblages and the reliability of thestratigraphic position of bio-horizons in the Shah-neshin section should thus be considered with caution.

The presence of Reinhardtites anthophorus in thefirst 19 m of the section attests of zone CC15. For theBurnett scheme, the absence of Lithastrinus grillii inthe first 0.86 m of the section suggests the transitionfrom the top of UC10 to the base of UC11. The first occurrence (FO) of Lucianorhabdus cayeuxii recordedat 19 m marks the top of CC15 and UC11a-bTP. Thesebiostratigraphic results thus suggest a Coniacian agefor the very base of the Gurpi Formation in the Shah-neshin section. The FO of Hexalithus hexalithus, is not-


Fig. 5. Calcareous nannofossil biostratigraphy, main calcareous nannofossil bio-horizons, calcium carbonate content andbulk carbon and oxygen isotopic records of the Shahneshin section. (1) The Cretaceous CC biozonation is that of Sissingh(1977) modified by Perch-Nielsen (1985). The UC biozonation is from Burnett (1998) and the subzonation corresponds tothe scheme for the Tethyan realm (TP). The low-pass filter of the δ13C curve shows 3 well-defined cycles in the Coniacianto lower Campanian interval. Nannofossil bio-horizons used for the biozonations are in bold. Secondary bio-horizons arealso shown for their potential use at the regional scale.

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Table 1 Distribution chart of calcareous nannofossil taxa in the Shahneshin section and inferred biozonations (upper part).

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Table 1 Distribution chart of calcareous nannofossil taxa in the Shahneshin section and inferred biozonations (lower part).

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ed at 11.5 m within CC15/UC11a-b. This species is asynonym of H. gardetae whose FO is recorded withinUC12 in shelf sections of South England (Burnett1998). The FO of Calculites obscurus is recorded at20.8 m, immediately after the FO of Lucianorhabduscayeuxii, and marks the base of CC17. The resultingCC16 Zone is thus quite dense (only about 1.8 m). Thisbiozone is equivalent to part of UC11c (Fig. 5). The FOof Broinsonia parca is recorded at 65.8 m and marksthe base of CC18 and UC14. Zone CC17 thus corre-sponds to zones UC11cTP to UC13 of the Burnettscheme. The FO of A. cymbiformis (143.1 m) whichnormally defines the base of UC13 is recorded muchlater here (within CC21 and UC15cTP) than as proposedin the Burnett UC scheme. Therefore, the base of UC13cannot be defined here. The FO of Bukryaster hayi co-incides with that of B. parca constricta at 71.8 m andmarks the base of CC18b and UC14cTP. Marthasteritesfurcatus has been observed in many samples, oftenwith broken arms, but shows sporadic occurrences.However, it is continuously recorded toward the end ofits range and a last occurrence (LO) of this species canbe confidently placed at 77.4 m, marking the base ofCC19 (Table 1). The FO of Ceratolithoides verbeekii at82.3 m marks the base of UC14dTP. Misceomarginatuspleniporus was not found in any samples and for thisreason the top of UC14 is not clear. The FO of Cera-tolithoides aculeus was found at 92.6 m and marks thebase of CC20 and UC15bTP. The LO of Bukryaster hayiwhich normally marks the base of CC19b is hererecorded at 109 m, above the FO of C. aculeus, thuswithin CC20. Uniplanarius sissinghii is rare and spo-radic but a FO can be recorded at 136 m and marks thebase of CC21 and UC15cTP.

In the upper part of CC21/UC15cTP lies the first occurrence of a characteristic “curved spine” nannolithwhich was already documented by Lees (2002) andshows a narrow range in the late Campanian of ODPHole 762C (Thibault et al. 2012a). A similar narrowrange of this form is also documented here in the sameinterval and the FO of this curved spine form lies with-

in the uppermost part of CC21/UC15cTP (Fig. 5). Al-though this species has not been formally described, it may prove useful as a potential new biostratigraphicmarker in the future. The LO of Lithastrinus grillii isplaced at 163.9 m but this species shows a very spo-radic signal before this level and thus the reliability ofthis bio-horizon is not very good here (Table 1). TheFO of Uniplanarius trifidus at 167 m marks the base ofCC22 and UC15d-eTP. The LOs of Eiffellithus angus-tus (181.1 m), Zeugrhabdotus diplogrammus (176.5 m)and the curved spine (178.4 m) are recorded withinCC22/UC15d. The LO of Reinhardtites anthophorus isrecorded here at 147.4 m marking the end of a contin-uous consistent presence whereas the specimen record-ed at 157.4 m is probably reworked (Table 1). The LOof Reinhardtites anthophorus is before the FO of U. tri-fidus at 167 m, in contradiction with Perch-Nielsen’sscheme. Therefore, it is not a reliable event for the baseof CC23 in this section. The LO of Eiffellithus eximiusis difficult to place with certainty because of its incon-sistent occurrence toward the end of its range (Table 1).However, this taxon is quite common up to 163.9 m and shows a neat drop in abundance above this level.The presence of this species then remains consistent upto 170.9 m, after which it is either absent or very rare(Table 1). This pattern is typical of the potential re-working of common species (Backman 1986, Raffi1999). Therefore the LO of E. eximius is rather chosenhere at 170.9 m. According to Perch-Nielsen (1979),the LO of E. eximius can be used as a secondary mark-er for the base of CC23. In addition, this bio-horizonmarks the base of UC16. The bases of Zones CC23b/UC17, CC24/UC18 and CC25a/UC19 are respectivelymarked by the LOs of Broinsonia parca constricta(204.1 m), Tranolithus orionatus (206.6 m) and Rein-hardtites levis (212.15 m). The LO of Zeugrhabdotusbicrescenticus (202.8 m) is observed within CC23/UC16. One single specimen of Uniplanarius trifiduslong-rayed was found at 208.4 m but it is probably re-worked, given its absence in the preceding 5 samplesand the coincident LO of U. trifidus short and medium-


Plate 1. Calcareous nannofossils of the Gurpi Formation in the Shahneshin section. A – Eiffellithus eximius, XPL;B – Watznaueria barnesae, XPL; C – Tranolithus orionatus, XPL; D – Arkhangelskiella cymbiformis, XPL; E – Micula stau-rophora, XPL; F – Micula murus, XPL; G – Eiffellithus turriseiffelii, XPL; H – Broinsonia parca constricta, XPL; I – Cer-atolithoides aculeus, XPL; J – Eiffellithus angustus, XPL, as amended by Shamrock and Watkins (2009); K – Lucianorhab-dus cayeuxii, XPL; L – Ceratolithoides kamptneri, XPL; M – Reinhardtites levis, XPL; N – Reinhardtites anthophorus, XPL;O – Uniplanarius sissinghii, XPL; P – Uniplanarius trifidus, XPL; Q – Cruciplacolithus tenuis, XPL; R – Heliolithus klein-pellii, XPL; S – Micula prinsii, XPL; T – Curved spine, XPL; U – Discoaster multiradiatus, PPL; V – Discoaster mohleri,PPL; W – Fasciculithus tympaniformis, XPL; X – Biantholithus sparsus, XPL.

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rayed at 195.1 m (Table 1). The concomitant FOs ofLithraphidites quadratus, Micula murus and Cerato -lithoides kamptneri that respectively mark the bases ofUC20aTP, UC20bTP and UC20cTP at 222.2 m suggest ahiatus in the late Maastrichtian of the Gurpi Formation.In the Perch-Nielsen scheme, these subzones corre-spond to the interval CC25b to CC26a. The FO of Mi -cula prinsii at 242.8 m marks the base of UC20dTP andCC26b. This last subzone is only 1.5 m thick, suggest-ing that this interval is also condensed. In the next sam-ple at 244.3 m, the concomitant presence of Crucipla-colithus tenuis, Ellipsolithus macellus, Biantholithussparsus and Fasciculithus tympaniformis mark zoneNP5 of Martini (1971) and thus highlight an importantgap across the Cretaceous/Paleogene transition of theGurpi Formation (Fig. 5). These findings suggest thatthe Danian is completely absent here. Moreover, thetop of the section is also very condensed. The FO ofDiscoaster mohleri marks the base of NP7 at 247.3 mand the FO of Discoaster multiradiatus at 249 m attestsof NP9. These two zones belong to the Thanetian. TheSelandian stage is thus solely represented by NP5 andcovers an interval of 3 m only. From 249 m to the topof the section at 256.9 m, samples belong to zone NP9.

4.3 Carbonate stable isotopes

4.3.1 Carbon isotopesThe long-term trend of the bulk carbonate δ13C of theShahneshin section shows a relative increase from val-ues ca. 1.2 to ca. 1.8‰ from the base of the section inthe Coniacian to the Santonian/Campanian boundary(Fig. 5). In addition to this long-term trend, the latterinterval is characterized by three main negative excur-sions, each immediately followed by positive excur-sions (Fig. 4). A number of identifiable minor positiveand negative excursions can also be observed in thisinterval of the Coniacian to late Campanian (Fig. 4).Values remain relatively stable around 1.8‰ up to themiddle of the late Campanian. A stepwise negativeshift is observed at the transition between nannofossilzones UC15cTP and UC15d-eTP in the late Campanian

and values remain lower around 1.3‰ up to zoneUC18 in the lower Maastrichtian. The δ13C increasesagain sharply within zone UC18 and values remainstable upward at around 1.6‰ up to the topmost partof zones UC20a-c. A sharp decrease takes place fromthe upper part of UC20a-c and across the K-Pg bound-ary, reaching the lightest values of –0.2‰ in the firstsample of the Paleogene. A very fast recovery of theδ13C is observed in the Paleogene, reaching values of1.3‰ at the top of the section within nannofossil zoneNP9 (Thanetian).

4.3.2 Oxygen isotopesThe bulk carbonate δ18O curve shows values fluctuat-ing around a stable average of ca. –5‰ from the baseof the section to the middle of zone UC15bTP in thelower Campanian. A sharp increase is observed withinthe upper part of UC15bTP to reach values fluctuatingaround –4.5‰ within UC15cTP. Values increase againslightly up to ca. –4.2‰ within UC15d-eTP and reachminima around –4.1‰ within zones UC16 to UC18 inthe early Maastrichtian. The δ18O values fluctuatearound –4.4‰ in zone UC20a-cTP and sharply de-crease within UC20dTP. This latter decrease continuesacross the K-Pg boundary and reaches the lightest val-ues of –4.8‰ in the Selandian and Thanetian (zonesNP5 and NP7). A sharp increase follows within NP9(Tha netian) reaching a value of –3.5‰ in the last sam-ple of the section (Fig. 5).

5. Discussion

5.1 Diagenetic overprint

The moderate to poor preservation of the calcareousnannofossil assemblage and the very low species rich-ness point to a potential diagenetic impact on primarygeochemical signatures. However, the range of vari-ability recorded in the measured carbon-isotopes cor-responds well to biogenic calcite precipitated in openand shelf oceans of the Late Cretaceous (Stoll and


Plate 2. Calcareous nannofossils of Gurpi Formation in the Shahneshin section. A – Lithastrinus grillii, XPL; B – Zeug -rhabdotus embergeri, XPL; C – Marthasterites furcatus, PPL; D – Calculites ovalis, XPL; E – Rhagodiscus angustus, XPL;F – Lucianorhabdus maleformis, XPL; G – Tranolithus gabalus, XPL; H – Zeugrhabdotus bicrescenticus, XPL;I – Lithraphidites carniolensis, XPL; J – Cribrosphaerella ehrenbergii, XPL; K – Bukryaster hayi, XPL; L – Ceratolithoidesverbeekii, XPL; M – Micula praemurus, XPL; N – Ceratolithoides indiensis, XPL; O – Lithraphidites quadratus, PPL;P – Eiffelithus angustus, rotated, XPL, as amended by Shamrock and Watkins (2009); Q – Watznaueria barnesae, XPL;R – Thoracosphaera operculata, XPL; S – Thoracosphaera sp. 2, XPL; T – Ellipsolithus macellus, XPL.

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Schrag 2000, Jarvis et al. 2006, Voigt et al. 2010,2012). The general trends observed here match wellthose delineated in previous studies whereas absolutevalues are somewhat ca. 1‰ lower than coeval valuesin these studies (Jenkyns et al. 1994, Jarvis et al. 2006,Voigt et al. 2012, Sprovieri et al. 2013, Wendler 2013)(Figs. 6–7). This suggests that long-term trends in car-bonate δ13C are not profoundly affected by diagenesishere and that this geochemical proxy can be used forstratigraphy. The lighter δ13C values at Shahneshinmay reflect either a slight diagenetic overprint or, al-ternatively, local conditions in the nannofossil speciescomposition and marine productivity. Few diageneticoverprint of carbon isotope values is also supported bythe fact that the δ13C–δ18O cross-plot does not displayany correlation (R2 = 0.0008, Mitchell et al. 1997).

Conversely, the range of δ18O values (–3.5 to–5.5‰) is relatively low compared to contemporane-ous diagenetically unaltered marine calcite of low latitude planktic foraminifers (Grossman 2012). Thismay indicate that the primary δ18O signal was over-printed by diagenetic fluids. Therefore, trends in bulkδ18O are not discussed further.

5.2 Correlations to other Late Cretaceoussections

5.2.1 Correlation of carbon-isotopesDespite the influence of many potential local factors on the isotopic composition of carbon in carbonates ofepeiric shallow seas and pelagic oceanic basins, greatsimilarities have been observed in the secular trends ofδ13C records. This chemostratigraphic proxy is widelyused today for correlation at the global scale, even between shelf and open ocean areas (Voigt et al. 2010,Wendler 2013 and references therein). Carbon isotopesrecords are thus used here for their application in strati -graphy and global correlations. For further explanationson potential causes for δ13C fluctuations in the Late Cretaceous, the reader is referred to Wendler (2013) andreferences therein. Several studies have focused recent-ly on the global correlation of δ13C trends in numeroussections and deep-sea sites of the late Campanian–Maastrichtian (Voigt et al. 2010, 2012, Thibault et al.2012a, 2012b, Wendler 2013). Few studies have cov-ered the Coniacian to Campanian interval but a refer-ence curve was proposed for this interval, based on theEnglish chalk (Jarvis et al. 2006). These authors identi-fied and defined a number of isotopic events within thisinterval, some of which can also be correlated to therecord of Seaford Head (Sussex, Jenkyns et al. 1994)

(Fig. 6). From the Boreal Turonian/Coniacian boundarymarked by a pronounced δ13C minimum defined as theNavigation event (Fig. 6), many characteristic positiveand negative excursions with different amplitudes andshapes have been defined by Jarvis et al. (2006)throughout the Turonian to early Maastrichtian. Most ofthese defined excursions from the Coniacian to earlyCampanian can actually be identified at Shahneshin andcorrelated to the English chalk (Fig. 6). These excur-sions have also been previously identified and similarlycorrelated to the English chalk in the Lägerdorf recordof North Germany and in the Gubbio record which areused here for correlation (Fig. 7) (Voigt et al. 2010,Sprovieri et al. 2013). All together, the long-term trendsof the Coniacian to early Campanian δ13C record is systematically characterized by three main cycles whichhave proved to constitute the expression of a 2.4 Myrlong-term eccentricity oscillation mode in the Earth’sclimate system (Sprovieri et al. 2013). Following the pioneering work of Jarvis et al. (2006), these δ13C ex-cursions, as well as the Late Campanian negative event(LCE) and the Campanian/Maastrichtian boundaryevent (CMBE) have been successfully used for the cor-relation of the English chalk with the German Chalk andthe Gubbio reference section in Italy (Voigt et al. 2010,Sprovieri et al. 2013). The two latter studies present byfar the highest resolution δ13C records for the completeConiacian–Maastrichtian interval and have thus beenchosen in the present study for correlation with theShahneshin section (Fig. 7). Moreover, the Gubbio sec-tions bear the reference magnetostratigraphic record forthe Late Cretaceous, allowing a direct correlation to theGeologic Time Scale 2012 whereas the German andEnglish Chalk have an excellent macrofossil biozona-tion allowing a good tie to Boreal stage and substagedefinitions.

Although the resolution of the δ13C curve of Shah-neshin is lower than the previous studies at Gubbio and in the German Chalk, the comparison of the δ13Crecords shows a good correspondence in shapes andlong-term trends (Fig. 7). In particular, within the Coniacian-Santonian interval, it is possible to identifyand correlate the Beeding, White Fall, Kingsdown,Michell Dean, Haven Brow, Horseshoe Bay, Buckle,Hawks Brow and the SCBE events between Shah-neshin, the English chalk, the German chalk and Gub-bio (Figs. 6–7). Moreover, the extraction of long-termtrends of the Shahneshin record within this intervalthrough a low-pass filter delineates the 2.4 Myr cyclesin δ13C as also shown in Gubbio (Fig. 7) (Sprovieri etal. 2013). Finally, the correlation of these isotopic


eschweizerbart_xxx


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eschweizerbart_xxx

curves is further supported by comparable Tethyan cal-careous nannofossil bio-horizons between Shahneshinand the Gubbio area (Fig. 7). The similar biostratigra-phies at Gubbio and Shaneshin make the tie of carbonisotopes very solid (Fig. 7). Carbon isotopes were alsopreviously correlated between Gubbio and the Germanchalk by Sprovieri et al. (2013) while Voigt et al. (2010)correlated the German chalk to the English chalk. Thecorrelation we present in Figure 5 between Shahneshinand the English chalk is thus a direct consequence ofthese past studies and of the tie between Shahneshinand Gubbio (Fig. 7). This correlation suggests that it ishowever not possible to correlate calcareous nannofos-sil events between Iran and the English chalk (Fig. 6).Therefore, it is here suggested that a few common cal-careous nannofossil bio-horizons are time-transgres-sive between the Tethyan and Boreal realms in the Santonian-Campanian interval (Fig. 6). The LCE couldnot be identified in the Shahneshin section. In addition,immediately after the FO of U. trifidus which can becorrelated between Shahneshin and Gubbio, the δ13Cpatterns at Shahneshin are characteristic of the largenegative CMBE. Therefore, our correlation suggestsanother important hiatus in the Shahneshin section at165 m within the top of UC15cTP, corresponding to theoverall interval between the base of the LCE and thebase of CMBE expressed at Gubbio and in the Germanchalk (Fig. 7). This means that a large part of the lateCampanian is missing at Shahneshin, as also supportedby the condensation of zone CC22/UC15d-eTP (Fig. 7).The CMBE can be correlated between the three recordsand several similar calcareous nannofossil bio-hori-zons are recorded in the three sections within the upperhalf of the excursion, immediately before or within therecovery to more positive values (Fig. 7). Condensationof the Maastrichtian interval at Shahneshin preventsany further correlation of the δ13C curve with other ref-erence records in the upper part of the Maastrichtian.Finally, the negative excursion observed across theK-Pg boundary and the very rapid positive recovery isin accordance with the timing of global δ13C recordsand with the condensation identified from the bio -stratigraphy in the Paleogene of the Shahneshin section.Global δ13C values of the Danian and early Selandianremain relatively low after the major K-Pg negative excursion and the full recovery toward more positivevalues is only observed in the interval between the lateSelandian and early Thanetian (Cramer et al. 2009).Therefore, the extremely sharp positive peak observedafter the K-Pg boundary in the δ13C of the Shahneshinsection (Fig. 7) is in agreement with a Thanetian age

and reflects the extreme condensation of the Paleogenein the Gurpi and Pabdeh Formations.

5.3 Implications for the definition and correlation of Late Cretaceousstage boundaries between the Boreal and Tethyan Realms

Following the first international symposium on Creta-ceous stage boundaries in Copenhagen in 1983, LateCretaceous stage boundaries have been quite well de-fined in the Boreal Realm, based on the correlation ofmany European sections and proposals of several bio-horizons have been made to correlate to the Tropicalrealm (Birkelund et al. 1984). However, the provin-ciality observed in many fossil groups makes the cor-relation between the Boreal Realm, the Western Inte-rior, the Tethys and deep-sea sites of the South At-lantic, Pacific and Indian Oceans a difficult exercise.The results of the Shahneshin section and correlationto Gubbio (Italy) and to the German and English Chalkhave implications on the definition and correlation ofLate Cretaceous stage boundaries with respect to cal-careous nannofossil biostratigraphy and carbon-iso-tope stratigraphy.

5.3.1 Age-model for the Shahneshin sectionThe calcareous nannofossil biostratigraphy and thepreceding isotopic correlations suggest that the GurpiFormation presents a number of gaps and condensedintervals throughout the Coniacian–Paleogene at theShahneshin section. The base of the Shahneshin sec-tion corresponds to the lower Coniacian UC10 biozone(Fig. 5). Underneath, the base of the Coniacian is miss-ing and an unconformity between the Turonian and the upper lower Coniacian is marked by the transitionbetween the Ilam and Gurpi Formations. Three otherhiati have been characterized in the late CampanianCC22/UC15d-eTP zone, in the Maastrichtian at theUC19/UC20 transition and at the K-Pg boundary,above which the whole Danian and most of the Se-landian and Thanetian are missing. This extreme con-densation is supported by the results on stable isotopesand it demonstrates that the first 8.5 Myr of the Paleo-gene are actually comprised in ca. 6 m in the top partof the Gurpi Formation. In addition, the upper Maas-trichtian is very condensed as attested by the absenceof the characteristic δ13C excursions of this intervaland the limited thickness of UC20dTP. However, theupper lower Coniacian to lower Campanian intervalseems rather continuous and the correlation of the


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calcareous nannofossil bio stratigraphy and isotopictrends with Gubbio and the German chalk has impor-tant implications for the Coniacian/Santonian and San-tonian/Campanian boundaries which have not yet beenratified by the International Commission on Stratigra-phy (ICS).

5.3.2 Definition and correlation of the Coniacian/Santonian boundary of the Tethyan and Boreal Realms

In the Tethyan realm, the first appearance of the am-monite subgenus Texanites was originally consideredas a good indicator for the base of the Santonian(Kennedy 1984). This bio-horizon has been rejected bythe Santonian Working Group and the favored primarymarker is currently the first appearance of Cladocera-mus undulatoplicatus (Inoceramid) which is easily rec-ognizable and widespread (It is known from N. Amer-ica, Europe, Africa, Madagascar, and Central Asia,Lamolda et al. 1996). The first appearance of thisspecies defines the base of the pachti/undulatoplicatusmacrofossil zone in the German Chalk (pu on Fig. 7)and is recorded at the level of the Michel Dean Flint inthe English chalk, immediately below the Michel Deanevent, within the Micraster coranguinum zone (Fig. 6)(Mortimore et al. 2001, Paul and Lamolda 2009). Sofar, the coincidence between the FO of C. undulatopli-catus and the Michel Dean δ13C event has been shownat Olazagutia (Spain), Ten Mile Creek (Texas), CulverCliff, Trunch and Dover (England), and Lägerdorf(Germany) (Paul and Lamolda 2009, Voigt et al. 2010).The Michel Dean event is thus chosen for placement ofthe Coniacian/Santonian boundary in the Shahneshinsection (Figs. 6–7). The FOs of calcareous nannofossilsC. obscurus and L. cayeuxii conformably lie slightlybelow the Kingsdown event at Gubbio and Shahneshinand hence seem to constitute reliable markers for thebase of the Santonian in the Tethyan province (Fig. 7).The FOs of the two latter taxa are also recorded in thesame stratigraphic order slightly below the Coniacian/Santonian boundary in the Olazagutia section, Spain(Lamolda and Paul 2007) and in the RomanianCarpathians (Melinte and Lamolda 2007). The FO ofC. obscurus is recorded within the Micraster coran-guinum zone, ca. 18 m below the Michel Dean Flint atSeaford Head (Sussex, English chalk, Hampton et al.2007). Assuming similar sedimentation rates along thesouthwestern coast of England, this level projects ap-proximately between the Kingsdown and White Fallevents on the Trunch/Dover carbon isotope profile(Fig. 6). This projection is also supported by a good

correspondence of facies between the two records, theHope Point Marls and Otty Bottom marls of Dover(Jarvis et al. 2006) representing the equivalent of theBelle Tout Marls at Seaford Head (Hampton et al.2007) whereas the prominent East Cliff SemitabularFlint of Dover likely represents the equivalent of theSeven Sisters Flint at Seaford Head. Therefore, the level of the FO of C. obscurus at Seaford Head wouldlie slightly below the Kingsdown event as defined inDover (Fig. 6), conformably to the Tethyan record. Inthe Trunch borehole, the FO of C. obscurus was record-ed much further up in the stratigraphy, in coincidencewith the SCBE (Jarvis et al. 2002). Interestingly, thislevel corresponds well to the first occurrence of an in-flux in C. obscurus at Seaford Head (Fig. 6, Hampton etal. 2007). It is possible that the apparent FO of C. ob-scurus in the Trunch borehole actually marks the sameinflux event as at Seaford Head and is caused by the extreme scarcity of this marker below this level. Theuse of the FO of C. obscurus as a calcareous nannofos-sil marker for the Coniacian/Santonian boundary maythus be somewhat reliable between the Tethyan and Boreal realms but it must be used with caution in theBoreal realm due to its sporadic occurrence close to the base of its range (Hampton et al. 2007). The posi-tion of the Coniacian/Santonian at Shahneshin usingthe Michel Dean event for correlation with the Borealrealm lies in the lower half of calcareous nannofossilzone CC17 (UC11cTP–UC13) (Figs. 6–7).

5.3.3 Definition and correlation of the Santonian/Campanian boundary of the Tethyan and Boreal Realms

Currently, the favored boundary criterion chosen bythe Campanian Working Group for the base of theCampanian is the extinction level of the crinoid Mar-supites testudinarius which also marks the LO of thewhole Marsupites genus (Gale et al. 2008). Amongother criteria proposed for the base of the Campanianare the FO of calcareous nannofossil Broinsonia par-ca parca which also marks the FO of the whole Broin-sonia parca lineage, the LO of planktonic foraminiferDicarinella asymetrica and the C33R/C34N paleo-magnetic reversal (Montgomery et al. 1998, Gale et al. 2008). The level of extinction of M. testudinariusdefines the top of the testudinarius/granulata zone inthe German chalk (ts/gr on Fig. 7) and is thus directlycorrelatable to the English chalk, coinciding in bothrecords to the base of δ13C event SCBE (Voigt et al.2010) (Figs. 6–7). The FO of B. parca parca lies abovethat level within zone Offaster pilula in the English


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chalk (Fig. 6). The C33R/C34N reversal as interpretedby Montgomery et al. (1998) in the English chalk cor-responds to the Buckle Marls at Seaford Head and iswithin the Uintacrinus socialis zone (Us on Fig. 6),well below the extinction of M. testudinarius. Actual-ly, the C33R/C34N reversal is just above the Horse-shoe Bay event and nearly in coincidence with theBuckle event (Fig. 6). The C33R/C34N reversal atGubbio lies in-between the Buckle and Horseshoe Bayevents which is reasonably similar to the English chalkinterpretation (Fig. 7). This level at Gubbio is also co-incident with the LO of planktonic foraminifer D. asy-metrica and with the FO of calcareous nannofossilB. parca parca (Fig. 7). The good match in the corre-lation of carbon isotopes and main nannofossil bio-horizons in the Coniacian to lower Campanian intervalbetween Gubbio and Shahneshin and the correlation to the English chalk (Figs. 6–7) allows us to draw thefollowing conclusions with respect to the position ofthe Santonian/Campanian boundary:

(1) The base of the Campanian as defined by the ex-tinction of Marsupites testudinarius corresponds to theso-called SCBE carbon isotope event and lies withinChron C33R in the English chalk (Boreal realm). Thislevel corresponds to the very top of Chron C33R atGubbio (Tethyan realm), close to the C33N/C33R re-versal. Based on the comparison with Gubbio and onthe occurrence of many additional zones of normal po-larity within macrofossil zones O. pilula and G. qua d-rata (Fig. 6), a possible revised interpretation of themagnetostratigraphic Montgomery record could pointto a large part of the lower Campanian of the Englishchalk actually corresponding to chron C33N (Fig. 6).

(2) The base of the Campanian as defined by the coin-cident C34N/C33R reversal and by the LO of plankton-ic foraminifer D. asymetrica lies in-between the Horse-shoe Bay and Buckle carbon isotope events at Gubbio(Fig. 7) and thus corresponds in the English chalk to thebase of the Uintacrinus socialis zone (Fig. 6).

(3) The FO of B. parca parca appears to be time-transgressive between the Tethyan (Shahneshin andGubbio) and the Boreal realm (English and Germanchalk). In the Boreal realm, it lies well above theSCBE (Fig. 6) whereas in the Tethyan realm, it lies in-between the Horseshoe Bay and Buckle carbon isotopeevents, in coincidence with the FO of D. asymetricaand the C34N/C33R reversal (Fig. 7).

(4) Consequently, some of the criteria chosen by theCampanian Working Group for the definition of the

base of the Campanian appear to be diachronous. Sincethe favored marker is the LO of M. testudinarius, theBoreal definition should be adopted, corresponding tothe SCBE. However, this implies that, at least in someparts of the Tethyan Realm, this level is actually closeto the C33N/C33R reversal as observed in Gubbio(Fig. 7). This stratigraphic level would also be muchbetter defined by the FO of the calcareous nannofossilCeratolithoides aculeus as suggested by our correlationbetween Gubbio and Shahneshin (Fig. 7).

To confirm these results and resolve the problematiccorrelation of the Santonian/Campanian boundary be-tween North America, Europe and southern Tethyansites, more work is needed on potential boundary stra-totype sections such as the currently favored Waxa-hachie Dam Spillway section (Texas) (Gale et al.2008) but also on other potential sections such as in the Hateg area, Romania (Melinte-Dobrinescu and Bojar 2010). To resolve this issue, it may be necessaryto study the boundary in a much broader context, toperform very high-resolution carbon isotope analysisin addition to the biostratigraphy in order to retrievewith precision the numerous events of Jarvis et al.(2006). It appears necessary to find sections wherelarge parts of the Coniacian and lower Campanian arealso exposed in order to document the 2.4 Myr cyclesof Sprovieri et al. (2013). In particular, the two2.4 Myr δ13C cycles with maxima at the HorseshoeBay event and SCBE have quite similar amplitudes atShahneshin, in the German chalk and in the Englishchalk and can be easily misidentified if not studied ina broad context (Figs. 6–7).

According to Burnett (1998), the Santonian/Cam-panian boundary would lie somewhere within nanno-fossil zone CC17/UC11cTP–UC13, below the FO ofB. parca parca. Our results and correlations suggestthat this boundary level, as defined by the LO ofM. testudinarius in the English and German chalk,would actually correspond in the Tethyan realm to thebase of CC20/UC15bTP.

5.3.4 Definition and correlation of the Campanian/Maastrichtian boundaryof the Tethyan and Boreal Realms

The Campanian/Maastrichtian boundary (CMB) isnow very well defined globally and tied to the firstminimum in δ13C values of the CMBE, preceding theso-called event M1+ (Thibault et al. 2012a, 2012b,Voigt et al. 2012). This level projects approximately at195 m at Shahneshin within zone CC23 (UC16), with-


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in CC22/23 (UC16) at Gubbio and in the top of UC16in the German chalk (Fig. 7). The last occurrence ofUniplanarius trifidus is the first bio-horizon lying im-mediately above the Campanian/Maastrichtian bound-ary at Shahneshin and Gubbio. In the German chalk,the LO of a form defined as U. cf. U. trifidus is alsofound above this boundary at the base of δ13C eventM1+, similarly to the Gubbio record (Fig. 7). Unipla-narius trifidus is a very characteristic tropical nanno-lith unlike any other calcareous nannofossil species ofthis time interval. Although this species is quite rareand/or generally absent in the Boreal realm, it is verylikely that U. cf. U. trifidus identified in Germany byBurnett in Schönfeld et al. (1996) is actually a trueU. trifidus. The LO of U. trifidus is one of the rare re-liable Maastrichtian calcareous nannofossil strati-graphic markers in the Indian Ocean ODP Site 762C,and it was also found there immediately above theCMB (200 kyr after the identified CMB level, Thi-bault et al. 2012b). Consequently, the LO of U. trifidusappears to be an excellent nannofossil marker for theCampanian/Maastrichtian boundary and should beconsidered in future revised nannofossil biozonations.

5.3.5 Position of substage boundaries at Shahneshin

The lower/upper subdivision of the Campanian used inthe Tethyan scheme of Burnett (1998) projects corre-sponds to the FO of Uniplanarius sissinghii. Unipla-narius sissinghii is rare and sporadic at Shahneshin but its FO can be placed at 136 m, marking the base ofCC21 and UC15cTP. According to the Burnett scheme,this level corresponds to the base of the belemniteBelemnitella mucronata zone in Northwest Europe andthis level projects slightly below the “lower/middle”Campanian boundary in North American usage, as defined by the first occurrence of ammonite Baculitesobtusus (Ogg et al. 2012). In the German chalk, thislevel corresponds to the base of the conica/senior zone(Voigt et al. 2010) (Fig. 7). The lower/upper Maas-trichtian boundary may be placed here at the FO ofLithraphidites quadratus (Fig. 7), following Thibault etal. (2012a, 2012b).

5.4 Implications for the global Late Cretaceous δ13C stack and the Geologic Time Scale

Wendler (2013) recently proposed a global averageδ13C stack for the Turonian through Maastrichtianbased on a very large literature compilation. This stack

was presented against the magnetostratigraphy, the for-mer Geologic Time Scale (GTS 2004, Gradstein et al.2004) and the most recent one (GTS 2012, Ogg et al.2012). This stack was redrawn here based on ages fromthe GTS 2012 and compared to δ13C records and strati-graphic data of Shahneshin, Gubbio and the Germanchalk (Fig. 7). The comparison of Coniacian to lowerCampanian stage and substage boundaries as defined inthe Boreal realm and of δ13C correlations of the eventsdefined by Jarvis et al. (2006) between Gubbio, NorthGermany and the Wendler compilation delineates a ma-jor mismatch of this stacked curve with the magne-tostratigraphy of this interval (Fig. 7). In particular, asshown at Gubbio, the negative excursion following theHorseshoe Bay event marks the C33r/C34n reversaland the SCBE occurs exactly at the C33n/C33r rever-sal. In contrast, the Wendler compilation places theSCBE at the C33r/C34n reversal, i. e. ca. 4 Myr earlierthan suggested by the Gubbio record. This mismatchhas a very strong impact on the respective duration es-timations of the Santonian and Campanian stages andtherefore needs to be addressed thoroughly for the next generation of Cretaceous Geologic Time Scales.Sprovieri et al. (2013) showed that the long-term δ13C trends across the Coniacian to Santonian intervalactually correspond to three well-pronounced 2.4 Myrlong-term eccentricity cycles (Fig. 6). The expressionof these three 2.4 Myr cycles is a remarkable commonfeature of Shahneshin, Gubbio and the German chalk(Fig. 7). Therefore, using either the age of the C33n/C33r reversal or that of C33r/C34n as potential an-chors, as well as the cyclostratigraphic frame of Spro -vieri et al. (2013) for the high-resolution carbon isotopecurve of Gubbio, it should be possible to correct theLate Cretaceous Wendler δ13C stack which will provevery useful for future geologic time scales.

6. Conclusions

The calcareous nannofossil biostratigraphy and stableisotope stratigraphy of the Shahneshin section alloweddrawing the following conclusions:

(1) The Gurpi Formation in the western part of the Zagros Basin spans the Coniacian to Thanetian.

(2) Calcareous nannofossil bio-horizons and carbon-isotope stratigraphy at Shahneshin suggest: condensa-tion in the late Campanian and in the lower part of thelate Maastrichtian; a gap corresponding to the entireDanian stage; and a major condensation of the Paleo -


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cene with ca. 8.5 Myr comprised in only 6 m at the topof the Gurpi Formation.

(3) Correlation of the carbon-isotope profile with other reference records has allowed the identificationof the following Late Cretaceous excursions at Shah-neshin: the Beeding, White Fall, Kingsdown, MichellDean, Haven Brow, Horseshoe Bay, Buckle, andHawks Brow events as well as the SCBE and CMBE.

(4) The FO of Calculites obscurus, which marks the base of zone CC17 of the Perch-Nielsen (1985)scheme, occurs nearly in coincidence with the positiveKingsdown δ13C event at Shahneshin and Gubbio.This bio-horizon is situated below the Coniacian/San-tonian boundary as defined in the English chalk by theFO of Inoceramid Cladoceramus undulatoplicatusand coincident to the Michel Dean δ13C event.

(5) The FO of Broinsonia parca parca is recorded in-between the Horseshoe bay and Buckle δ13C eventsat Shahneshin which correlates well with the Gubbiorecord. Correlation of the Tethyan Shahneshin andGubbio sections to the German and English chalk (Bo-real realm) suggests that this bio-horizon is time-trans-gressive between the two provinces and is thus not re-liable as a global marker for the Santonian/Campanianboundary.

(6) The Santonian/Campanian boundary as definedby the LO of Marsupites testudinarius corresponds tothe δ13C SCBE. In the English chalk (Boreal realm),this stratigraphic level lies below the FO of B. parcaparca in subzone UC13iiiBP. At Shahneshin and Gub-bio (Tethyan realm), this level actually corresponds tothe FO of Ceratolithoides aculeus which defines thebase of CC20/UC15bTP. UC Boreal and Tethyan nan-nofossil zones are not analogous in this interval.

(7) Correlation of δ13C records with the global stackof Wendler (2013) highlights a mismatch of this com-piled record to magnetostratigraphy and the GeologicTime Scale in the Santonian and lower Campanian.This is mainly due to the position of the SCBE that isnot coincident with the C33R/C34N reversal. As sug-gested by the Gubbio record and a proposed reinter-pretation of the English chalk magnetostratigraphy, the SCBE rather coincides with the top of C33R, andpossibly with the C33N/C33R reversal.

Acknowledgements. We are grateful to B. Salehipour(GeoPardazesh Petroleum Exploration Company, Iran) forhis help during the sampling in the field. Geochemicalanalyses received support from the Carlsberg foundation,

Denmark. We thank Bo Petersen (Univ. Copenhagen) forsupport in the MS lab. We thank Silke Voigt and an anony-mous reviewer for their constructive and helpful commentsand suggestions.

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Samples Height δ13C δ18O CaCO3

(m) ‰ ‰ Bernard (PDB) (PDB) method

[%]

MB_Pa2 257.90 –1.33 –3.53 64.6MB_Pa1 256.91 –1.28 –3.78 76.8MB_G133 255.39 –0.54 –4.22 73.7MB_G132 254.97 –0.50 –4.22 80MB_G131 252.90 –0.32 –4.89 64.5MB_G130 250.81 –0.21 –4.52 56.6MB_G129 248.97 –0.40 –4.51 68.8MB_G128 247.30 –0.06 –4.97 60.6MB_G127 244.27 –0.21 –4.73 65.7MB_G126 242.79 –1.12 –4.63 84.3MB_G125 240.70 –1.59 –4.12 82.6MB_G124 238.44 –1.58 –4.21 86MB_G123 236.43 –1.61 –4.33 84.6MB_G122 234.00 –1.41 –4.52 79.4MB_G121 231.06 –1.58 –4.33 82MB_G120 228.70 –1.51 –4.38 85.7MB_G119 225.75 –1.65 –4.45 67.1MB_G118 223.60 –1.51 –4.64 78.8MB_G117 222.18 –1.63 –4.41 85.7MB_G116 219.80 –1.72 –4.30 89.4MB_G115 217.48 –1.64 –4.36 91.4MB_G114 214.68 –1.53 –4.26 89.4MB_G113 212.15 –1.55 –4.40 89.1MB_G112 210.30 –1.55 –3.97 82.8MB_G111 208.40 –1.38 –4.53 87.4MB_G110 206.61 –1.18 –4.38 86.8MB_G109 204.10 –1.39 –3.94 96.8MB_G108 202.75 –1.19 –4.34 94.6



[%]

MB_G107 200.30 1.42 –4.23 90.8MB_G106 197.62 1.28 –4.00 82.8MB_G105 195.14 1.39 –4.12 80.8MB_G104 192.03 1.35 –4.48 85.1MB_G103 188.90 1.36 –4.37 78.3MB_G102 186.20 1.23 –4.39 92.8MB_G101 183.36 1.31 –4.10 69.1MB_G100 181.09 1.29 –4.20 89.7MB_G099 178.40 1.28 –4.42 86.8MB_G098 176.46 1.48 –4.53 89.7MB_G097 173.75 1.66 –4.44 89.7MB_G096 170.87 1.31 –4.47 74.3MB_G095 168.90 1.60 –4.48 79.4MB_G094 166.99 1.79 –4.36 79.1MB_G093 165.14 1.93 –4.24 77.4MB_G092 163.90 1.82 –4.14 81.1MB_G091 161.93 1.75 –4.14 80MB_G090 159.92 1.60 –4.22 84.6MB_G089 157.40 1.65 –4.49 85.1MB_G088 155.61 1.61 –4.60 88.3MB_G087 153.51 1.37 –4.14 93.8MB_G086 151.12 1.68 –4.42 77.1MB_G085 149.30 1.74 –4.29 81.4MB_G084 147.44 1.45 –4.54 84.6MB_G083 145.24 1.50 –4.48 77.7MB_G082 143.14 1.70 –4.50 84.3MB_G081 140.63 1.56 –4.41 82.8MB_G080 138.42 1.61 –4.80 87.4

Appendix 1. CaCO3 content, Bulk carbon and oxygen stable isotopes at Shahneshin section.

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[%]

MB_G079 136.13 1.49 –4.81 76MB_G078 135.20 1.78 –4.47 83.7MB_G077 133.41 1.69 –4.73 74.6MB_G076 131.38 1.82 –4.05 82.8MB_G075 129.45 1.62 –4.44 87.7MB_G074 128.29 1.72 –4.67 76.8MB_G073 126.44 1.66 –4.55 89.7MB_G072 125.13 1.71 –4.72 82MB_G071 122.94 1.77 –4.67 69.4MB_G070 120.28 1.64 –5.00 85.4MB_G069 118.57 1.74 –5.01 78.6MB_G068 116.30 1.67 –5.05 95.4MB_G067 113.97 1.69 –4.93 82.6MB_G066 111.67 1.94 –4.97 82.8MB_G065 108.97 1.80 –4.98 84.3MB_G064 106.88 1.77 –5.03 90.8MB_G063 105.31 1.66 –4.90 87.4MB_G062 103.22 1.68 –4.78 88.6MB_G061 100.80 1.77 –4.78 75.4MB_G060 98.77 1.60 –4.87 80.3MB_G059 94.68 1.57 –4.97 81.1MB_G058 94.61 1.78 –5.07 79.4MB_G057 92.63 1.89 –4.45 82MB_G056 90.88 1.77 –4.77 71.7MB_G055 88.83 1.52 –4.93 68MB_G054 86.90 1.49 –5.03 71.1MB_G053 84.79 1.56 –4.92 78.8MB_G052 82.33 1.46 –4.93 77.1MB_G051 80.99 1.67 –5.06 69.7MB_G050 79.12 1.49 –4.76 78MB_G049 77.43 1.49 –5.07 76.6MB_G048 75.90 1.66 –5.21 77.1MB_G047 74.14 1.37 –4.82 79.4MB_G046 71.80 1.46 –4.12 70.8MB_G045 69.76 1.49 –4.93 69.4MB_G044 67.78 1.11 –4.95 77.1MB_G043 65.80 1.16 –4.49 78.8MB_G042 63.50 1.63 –5.00 78.6MB_G041 61.10 1.63 –4.93 76MB_G040 59.77 1.47 –4.68 76



[%]

MB_G039 58.60 1.48 –5.11 77.7MB_G038 57.44 1.45 –4.95 72.8MB_G037 55.00 1.46 –5.04 81.7MB_G036 52.72 1.35 –5.44 82.3MB_G035 50.15 1.35 –4.92 76.3MB_G034 48.00 1.45 –5.01 72.6MB_G033 44.92 1.29 –4.84 72.8MB_G032 43.08 1.32 –4.83 76.3MB_G031 41.58 1.20 –4.52 79.1MB_G030 38.90 1.27 –4.94 64.8MB_G029 37.38 1.04 –5.18 82.3MB_G028 34.50 1.51 –4.89 77.1MB_G027 33.46 1.26 –4.86 84.8MB_G026 31.90 1.05 –4.65 74.8MB_G025 29.76 1.12 –5.19 70MB_G024 27.70 1.39 –4.99 80.3MB_G023 25.17 1.33 –5.05 82.6MB_G022 24.38 1.55 –4.80 86.8MB_G021 22.41 1.47 –4.94 83.1MB_G020 20.79 1.62 –4.91 74.8MB_G019 19.03 1.70 –5.59 79.1MB_G018 17.20 1.45 –4.70 90.6MB_G017 15.73 1.40 –4.88 95.4MB_G016 14.60 1.03 –4.88 90.5MB_G015 12.74 1.05 –4.98 86.3MB_G014 11.54 1.04 –5.18 87.1MB_G013 9.51 1.24 –5.38 72.3MB_G012 8.15 1.31 –5.06 78.6MB_G011 6.80 1.17 –5.21 86.8MB_G010 5.84 1.33 –4.97 79.7MB_G009 4.70 1.49 –4.88 65.7MB_G008 3.67 1.46 –5.14 76MB_G007 2.90 1.36 –5.06 75.1MB_G006 2.07 1.47 –4.99 74.8MB_G005 1.56 1.47 –5.15 79.4MB_G004 0.86 1.41 –5.14 90.6MB_G003 0.58 1.47 –4.94 81.7MB_G002 0.30 1.47 –4.91 93.4MB_G001 0.00 1.44 –4.98 83.7


Appendix 1. Continued.

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