Upper Campanian–Maastrichtian nannofossil biostratigraphy and high-resolution carbon-isotope stratigraphy of the Danish Basin: Towards a standard δ 13C curve for the Boreal Realm

Upper Campanian–Maastrichtian nannofossil biostratigraphy and high-resolution carbon-isotope stratigraphy of the Danish Basin: Towards a standard δ 13C curve for the Boreal Realm

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  Upper Campanian e Maastrichtian nannofossil biostratigraphy and high-resolutioncarbon-isotope stratigraphy of the Danish Basin: Towards a standard  d 13 C curvefor the Boreal Realm Nicolas Thibault a , * , Rikke Harlou a , Niels Schovsbo b , Poul Schiøler c , Fabrice Minoletti d , Bruno Galbrun d ,Bodil W. Lauridsen a , Emma Sheldon b , Lars Stemmerik a , Finn Surlyk a a Department of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark b Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark c GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand d Université Pierre et Marie Curie Paris 06, UMR CNRS 7193, ISTeP, F-75005 Paris, France a r t i c l e i n f o  Article history: Received 30 September 2010Accepted in revised form 6 September 2011Available online 10 September 2011 Keywords: CampanianMaastrichtianDanish Basin d 13 C stratigraphyCalcareous nannofossil biostratigraphyDino 󿬂 agellate biostratigraphy a b s t r a c t High-resolution carbon isotope stratigraphy of the upper Campanian e Maastrichtian is recorded in theBoreal Realm from a total of 1968 bulk chalk samples of the Stevns-1 core, eastern Denmark. Isotopictrends are calibrated by calcareous nannofossil bio-events and are correlated with a lower-resolution d 13 C pro 󿬁 le from Rørdal, northwestern Denmark. A quantitative approach is used to test the reliabilityof Upper Cretaceous nannofossil bio-events and provides accurate biohorizons for the correlation of   d 13 Cpro 󿬁 les. The Campanian e Maastrichtian boundary (CMB) is identi 󿬁 ed through the correlation of dino- 󿬂 agellate biostratigraphy and  d 13 C stratigraphy between Stevns-1 and the Global boundary StandardStratotype-section and Point at Tercis les Bains (SW France), allowing the identi 󿬁 cation of new chemicaland biostratigraphic markers that provide a precise placement of the stage boundary on a regional scale.The boundary interval corresponds to the third phase of a stepwise 0.8 & negative  d 13 C excursion, lies incalcareous nannofossil subzone UC16d BP , and encompasses the last occurrence of nannofossil  Tranolithusstemmerikii  and  󿬁 rst occurrence of nannofossil  Prediscosphaera mgayae . Fifteen  d 13 C events are de 󿬁 nedand correlated to sixteen reliable nannofossil biohorizons, thus providing a well-calibrated standardhigh-resolution  d 13 C curve for the Boreal Realm.   2011 Elsevier Ltd. All rights reserved. 1. Introduction The latest Cretaceous (late Campanian e Maastrichtian) repre-sents a major transition in oceanic circulation and climate. Anoverall 8-million-year widespread long-term cooling (Douglas andSavin, 1973; Arthur et al., 1985; Barrera et al., 1997) led to a pronounced provincialism of microfossil groups throughout thelate Campanian e Maastrichtian interval (Huber and Watkins,1992; Lees, 2002). Superimposed on this long-term trend, two pronounced cooling episodes are documented between 71 and 69Ma, close to the Campanian e Maastrichtian boundary (CMB), andin the late Maastrichtian between 67.7 and 66 Ma (Barrera andSavin, 1999; Li and Keller, 1999). These episodes are marked indeep and intermediate water masses through the record of benthic foraminifer stable isotopes and reveal prominent changesin the sources of deep-water formation and in the thermohalinecirculation which started to behave on a mode more similar totoday ’ socean(BarreraandSavin,1999).Understandingthecausesof these changes has been hampered by dif  󿬁 culties in the applicationand correlation of biostratigraphic schemes for the Tethyan  e IntermediateandBorealRealms.Severalnannofossilandplanktonicforaminiferal bio-events are only observed in low- or high-latitudeprovinces, respectively. Others have proved to be time-transgressive across latitudes as a result of climatic changes(PospichalandWise,1990;HuberandWatkins,1992;Thibaultetal., 2010). For calcareous nannofossil assemblages, this has led to theestablishment of a Boreal Upper Cretaceous biozonation (UC BP forthe Upper Cretaceous Boreal Province, Burnett, 1998). In addition, d 13 C stratigraphy has proved to be a powerful tool for correlatingUpper Cretaceous strata on a global scale (Scholle and Arthur,1980; Arthur et al., 1985, 1987; Gale et al., 1993; Jenkyns *  Corresponding author. Tel.:  þ 45 29 87 60 64. E-mail address:  nt@geo.ku.dk (N. Thibault). Contents lists available at SciVerse ScienceDirect Cretaceous Research journal homepage: www.elsevier.com/locate/CretRes 0195-6671/$  e  see front matter    2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.cretres.2011.09.001 Cretaceous Research 33 (2012) 72 e 90  et al., 1994; Mitchell et al., 1996; Voigt and Hilbrecht, 1997; Voigt, 2000; Jarvis et al., 2002, 2006). The  d 13 C stratigraphy must be cali-brated with biostratigraphic events, but it can then be used to testthe synchroneity of bio-events and reconcile inter-regionalbiostratigraphic schemes.In this study, a new carbon stable-isotope pro 󿬁 le calibrated bynannofossil bio-events is presented for two continuously coredboreholes through the upper Campanian e Maastrichtian chalk of the Danish Basin and is correlated to a new carbon-isotope recordestablished on the Campanian e Maastrichtian Global boundaryStandardStratotype-sectionandPoint(GSSP)atTercislesBains,SWFrance. The precise correlation of these sections is achieved bycombined dino 󿬂 agellate biostratigraphy and  d 13 C stratigraphy. Thiscorrelation allows proposal of new nannofossil markers for theCampanian e Maastrichtian boundary in the Boreal Realm, andadditional stratigraphically important nannofossil bio-events forthe studied interval. Some discrepancies relative to publishedbiozonation schemes of the Boreal Realm are highlighted. Regionaldiachroneity of several bio-events and the usefulness of additionalbio-eventsbasedonabsoluteabundancepatternsofindividualtaxaare discussed. 2. Material The Upper Cretaceous of the Danish Basin consists of whitechalk with several intercalated marly intervals. Three sections arestudied.TheStevns-1core,drilledimmediatelyadjacenttothecoastalcliff of Stevns Klint, eastern Denmark, where the Cretaceous  e Paleogene boundary (K-Pg) section is well exposed. A compositesectionisbuiltfromtheRørdal-1coreandtheRørdalquarrysectionin the town of Aalborg in northern Jylland (Fig.1). The GSSP of theCampanian e Maastrichtian boundary, is located in the main quarryof Tercis les Bains, near Dax, SW France.  2.1. Stevns-1 The Stevns-1 core was drilled in 2005 at Stevns Klint (Fig. 1).Seismic pro 󿬁 les immediatelyoffshore Stevns Klint indicatethat theChalk Group, as de 󿬁 ned by Deegan and Scull (1977), was stronglyin 󿬂 uenced by powerful, long-lasting bottom currents during itsdeposition. The LateCretaceous sea 󿬂 oor was continuously sculptedby dynamic contour-parallel bottom currents into systems of ridges, valleys, drifts, and moats with amplitudes up to 150 m andwidth of several kilometres (Lykke-Andersen and Surlyk, 2004;Esmerode et al., 2007; Surlyk and Lykke-Andersen, 2007). Stevns-1 penetrated the upper part of a ridge succession and recovered456 m of upper Campanian to basal Danian chalk and bryozoanlimestone with a core recovery of nearly 100%. Comparison of calcareous nannofossil biostratigraphy of Stevns-1 (Sheldon, 2008)with global schemes allows a broad estimation of a duration of   c.  8myr for this 456 m succession and suggests a mean sedimentationrate of   c.  5.7 cm/kyr. Stevns-1 is the  󿬁 rst complete section throughthe upper Campanian e Maastrichtian chalk of NW Europe and is todate among the most expanded Maastrichtian sections worldwide(Thibault, 2010a). The lowermost 13 m of the core (  456 to  443 m) consist of upper Campanian bioturbated chalk. The upper Campanian  e lowermost Maastrichtian interval (  443 to   307 m) is char-acterised by chalk-marl cycles (Fig. 2). The marly beds are lightgrey and commonly about 50 cm thick, and the chalk beds are Fig.1.  Geological setting and isopach contours of the Upper Cretaceous e Danian Chalk Group in the Danish Basin showing the locations of the Stevns-1 (S) and Rørdal-1 boreholesand Rørdal quarry section (R). The map in the upper right corner shows other important European geological sites of the Campanian and Maastrichtian stages. R: Rørdal compositesection, S: Stevns-1, KHL: Kronsmoor, Lägerdorf, Hemmoor, M: Maastricht, N: Norfolk, T: Tercis les Bains, G: Gubbio. N. Thibault et al. / Cretaceous Research 33 (2012) 72 e 90  73  generally thicker. Distinctive grey to dark-grey centimetre thicklayers occur in the middle of the marl beds, and highlight thecyclical pattern in this interval. The   307 to   104 minterval consists of alternating white bioturbated chalk andmarls. Chalk-marl cycles are irregular, less distinctive thanin the underlying interval, and the thickness of the marlsvaries from 50 cm to 4 m thick. This interval is overlain by theRørdal Member (  104 to   76 m) which is characterised by apronounced chalk-marl cyclicity and higher gamma-raylog values, and can be correlated in outcrop and boreholessouth of Aalborg and south of Copenhagen (Surlyk et al., 2010). The Rørdal Member is overlain by the Sigerslev Member (Surlyket al., 2006) between   76 and   15 m, which consists of pure chalk with a few marl beds. The grey chalk of theHøjerup Member (Surlyk et al., 2006) is between   15and   12.8 m. The upper 12.8 m of Stevns-1 consists of lowerDanian bryozoan limestone of the Stevns Klint Formation(Surlyk et al., 2006), topped by 1 m of Quaternary deposits (S. Rasmussen, pers. comm., 2011). The whole succession bearscelestite nodules and chert nodules  󿬁 rst appear around   200 mand become common in the upper Maastrichtian  e  Daniansuccession (Madsen and Stemmerik, 2009). A calcareous nan- nofossil biostratigraphic study of the core, based on presence/absence data was presented by Sheldon (2008) and quantita-tive data are added here. The Stevns-1 carbon-isotope curvedescribed in this study was presented in a preliminary study bySchovsbo and Stemmerik (2007).  2.2. Rørdal quarry and Rørdal-1 The Rørdal quarry is situated near Aalborg in northern Jyl-land (Fig. 1). It is located in the inverted Aalborg Graben whichforms a part of the NW e SE orientated Sorgenfrei e TornquistZone (Fig. 1). The quarry exposes up to 30 m of upper Maas-trichtian chalk (Surlyk, 1970, 1972; Surlyk et al., 2010) and comprises a w 10 m-thick cyclic chalk-marl unit, de 󿬁 ned as theRørdal Member (Surlyk et al., 2010) (Fig. 3). This unit has been analysed for the palaeoecology of benthic faunas and tracefossil assemblages (Lauridsen and Surlyk, 2008; Lauridsenet al., 2011). The chalk and marl beds are highly bioturbatedwith well-de 󿬁 ned trace fossils (Lauridsen et al., 2011). Rørdal-1was drilled in 1967 at a location adjacent to the quarry wallwhich corresponds today to the NE of the central lake of thequarry (57  03 0 06.81 00 N, 9  59 0 17.61 00 E). A 100 m-long successionof chalk was recovered from the upper Campanian e lowermostupper Maastrichtian. Occasional clay laminae and thin marllayers occur in the lowermost and uppermost parts of the core,respectively, but no cyclic sedimentation is expressed (Fig. 3).Apart from the few marl layers, the core consists of a rathermonotonous bioturbated white chalk. Rørdal-1 was studied forforaminifer biostratigraphy (Stenestad, 2005) and forstable isotopes (Harlou et al., 2011). The obtained compositesection spans 130 m in total and is now on referred to asRørdal. The correlation with Stevns-1 presented below showsthat Rørdal corresponds to about 375 m of Stevns-1 and coversabout 6.6 myr, resulting in a mean sedimentation rate of  c.  2 cm/kyr.  2.3. Tercis les Bains The outcrop of Tercis is situated 8 km SW of Dax, SW France,between the Basque region to the south and the Charente prov-ince to the north (Fig. 1). Sediments were deposited in the smallAturian basin, north of the Pyrenees, which belongs to theTethyan Realm and is an appendix of the North Atlantic Ocean.This palaeogeographic situation was favourable to faunal and 󿬂 oral exchanges between the Boreal and Tethyan Realms. Thesediments consist of metre-thick indurated limestone beds witha lower unit (d ’ Azevac unit) of glauconitic limestones and anoverlying unit (Les Vignes unit) of   󿬂 int-bearing limestones (Fig. 4).A nearly constant mean sedimentation rate of 2.5 cm/kyr has beenestimated for this section (Odin and Amorosi, 2001). More detailed information about the section of Tercis les Bains can befound in Odin (2001). Fig. 2.  Absolute abundances of selected calcareous nannofossil taxa in Stevns-1. R   ¼  reworked specimens. FO  ¼  First Occurrence. LO  ¼  Last Occurrence. LCO  ¼  Last ConsistentOccurrence. N. Thibault et al. / Cretaceous Research 33 (2012) 72 e 90 74  3. Analytical methods  3.1. Stable isotopes Oxygen and carbon isotopic composition of bulk carbonateswere measured on a total of 1968 chalk samples from Stevns-1,with a mean sampling resolution of   c.  20 cm (3.5 kyr witha mean sedimentation rate of 5.7 cm/kyr). A total of 100 chalksamples from Rørdal-1 and 65 chalk samples from the Rørdalquarry have been analysed. The analyses were carried out atthe Department of Geography and Geology, University of Copenhagen, Denmark, using a micromass isoprime spectrom-eter. The extraction of CO 2  was executed by reaction withanhydrous orthophosphoric acid at 70   C. In addition, 102 bulksamples have been analysed along the 164 m-long successionof the Tercis section with a mean sampling resolution of 1.6 m,corresponding to about 64 kyr, assuming an obliquity-drivenmetre-scale rhythm of the sedimentation (40 kyr/m; Odinand Amorosi, 2001). These analysis were done at the Labo-ratoire Biominéralisations et Environnements Sédimentaires(Université Pierre et Marie Curie Paris 06, France) using a massspectrometer Finnigan Delta E. The extraction of CO 2  was madeby reaction with anhydrous orthophosphoric acid at 50   C. Theoxygen and carbon isotope values are expressed in per milrelative to the V-PDB standard reference. The analytical preci-sion is estimated at 0.1 & for oxygen and 0.05 & for carbon forboth laboratories.  3.2. Calcareous nannofossil assemblages The preservation of nannofossil assemblages varies in chalksamples of the two localities and is generally poor in the marllayers. A total of 448 samples were processed and samples withpoor preservation and very low species richness were discardedfor a more accurate record of nannofossil abundances. Forcalcareous nannofossil analysis, 88 samples from Stevns-1, 42samples from Rørdal-1, and 20 samples from the Rørdal quarrywere selected, with a sampling resolution of ca. 5 m ( w 85 kyr) forStevns-1, and ca. 2 m ( w 100 kyr) for Rørdal. The sediments weregently disaggregated in a mortar and 50 mg (  0.5) of driedsediment were dispersed in 50 ml of distilled water. Thesuspension was treated in an ultrasonic bath for 15 s and furtherhomogenised with a magnetic stirrer. An aliquot of 0.75 ml of thissuspension was dropped on a microscopic slide using a micropi-pette, ensuring an even distribution of the particles over theentire slide. This method is similar to that described by Koch andYoung (2007), who proved it to provide high reproducibility witha minimal margin of error when calculating nannofossil absoluteabundances.Species abundance counts were performed on at least 500specimens, randomly counted at a magni 󿬁 cation of    1600. Addi-tional 󿬁 eldsofview(FOV)wereexaminedandtakenintoaccounttodocument rare species. The absolute abundance for one species iscalculated as the ratio between the number of counted specimensand the number of FOV examined. After measuring the area of oneFOV (0.0184 mm 2 ), these abundances were converted into numberof specimens per mm 2 . The number of additional FOV were takeninto account for the abundances of rare species. Similar methodswere used by Backman and Shackleton (1983), Henriksson (1993)and Koch and Young (2007).In addition to the record of   󿬁 rst occurrences (FOs) and lastoccurrences (LOs), absolute abundances of nannofossil markers areused to determine acme events of taxa and last consistent occur-rences (LCOs). Occurrences are here considered inconsistent whena taxon is absent in at least two consecutive samples (Fig. 2). Thebiozonations of  Burnett (1998) and Fritsen (1999) are applied (Fig. 5) and the taxonomic concepts of  Perch-Nielsen (1985) and Young and Bown (1997) are considered here.  3.3. Dino  󿬂 agellate cysts Eight samples were collected from Stevns-1 in the intervalbetween   353 m and   250 m, covering the upper Campanian tolowermost Maastrichtian with an average sample spacing of  Fig. 3.  Absolute abundances of selected calcareous nannofossil taxa in the Rørdal-1 borehole and Rørdal quarry section. R  ¼ reworked specimens. FO ¼ First Occurrence. LO ¼ LastOccurrence. LCO  ¼  Last Consistent Occurrence. N. Thibault et al. / Cretaceous Research 33 (2012) 72 e 90  75  approximately 12 m. The samples were processed following stan-dard palynological processing techniques, including 2 min of oxidation in concentrated nitric acid and heavy liquid separation(Batten, 1999). The sample residue was  󿬁 ltered through a 11  m m 󿬁 lter cloth and mounted in glycerine jelly. The samples werestudied qualitatively for dino 󿬂 agellates and acritarchs and quanti-tatively for palynofacies. The biostratigraphic results from thisinterval are presented in an attempt to better identify the CMB inStevns-1 by comparison with previous results obtained from theTercis les Bains section (Antonescu et al., 2001a, 2001b; Schiølerand Wilson, 2001). 4. Results 4.1. Nannofossil analysis Three modes of preservation of calcareous nannofossils (poor,moderate and good) have been considered using the visualcriteria of  Roth (1983) on etching and overgrowth. Samples of poor preservation showing major etching and/or overgrowth of the nannofossil assemblage were discarded. All investigatedsamples considered in this study exhibit moderate preservation.Species richness in individual samples is generally higher inStevns-1 (50 e 80 species) than at Rørdal (36 e 64 species).Enrichment of dissolution-resistant taxa was not observed in theassemblages. 4.1.1. Precision and accuracy of nannofossil bio-events Important innovations in nannofossil biostratigraphy wereintroduced by the use of a quantitative approach (Hay, 1972;Hay and Steinmetz, 1977; Hay and Southam, 1978; Southamand Hay, 1978) and applied in the Cenozoic (Backman andShackleton, 1983; Backman, 1986). Abundance patterns of  nannofossil species may sometimes reveal a tail of stronglyreduced and inconsistent occurrences above the last consistentabundance (Backman, 1986). These tails may point to Fig. 4.  Dino 󿬂 agellate and nannofossil bio-events, nannofossil biozonation for Tethyan and Intermediate Provinces (TP),  d 13 C pro 󿬁 le, magnetostratigraphy and age model for theTercis les Bains section, GSSP of the Campanian e Maastrichtian boundary (CMB). (1) Lithology after Barchi et al. (1997). (a) Tectonised limestone, (b) scree, (c)  󿬂 int, (d) clay. (2)Antonescu et al. (2001a). (3) Antonescu et al. (2001b). (4) Schiøler and Wilson (2001). (5) Gardin et al. (2001b). (6) Odin and Lamaurelle (2001). The CMB level is highlighted by the dotted grey line. Key bio-events quoted and discussed in the text are highlighted in bold. N. Thibault et al. / Cretaceous Research 33 (2012) 72 e 90 76
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