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Cretaceous Research 32 (2011) 806e822

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Cretaceous Research

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Larger foraminifera distribution and strontium isotope stratigraphy of the La Covalimestones (ConiacianeSantonian, “Serra del Montsec”, Pyrenees, NE Spain)

Carme Boix a, Gianluca Frijia b, Vicent Vicedo c, Josep M. Bernaus d, Matteo Di Lucia b, Mariano Parente e,Esmeralda Caus c,*

aBadley Ashton & Associates Ltd, Horcastle, United Kingdomb Institut für Erd- und Umweltwissenschaften, Universität Potsdam, GermanycDepartament de Geologia (Paleontologia), Universitat Autònoma de Barcelona, Spaind StatoilHydro ASA, Stavanger, NorwayeDipartimento di Scienze della Terra, Università di Napoli “Federico II”, Italy

a r t i c l e i n f o

Article history:Received 3 December 2010Accepted in revised form 18 May 2011Available online 27 May 2011

Keywords:Larger ForaminiferaBiostratigraphyStrontium isotope stratigraphyConiacian-Santonian boundaryShallow-water carbonatesPyreneesSpain

* Corresponding author. Tel.: þ34 935812031; fax:E-mail address: esmeralda.caus@uab.es (E. Caus).

0195-6671/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.cretres.2011.05.009

a b s t r a c t

The Upper Cretaceous La Cova limestones (southern Pyrenees, Spain) host a rich and diverse largerforaminiferal fauna, which represents the first diversification of K-strategists after the mass extinction atthe CenomanianeTuronian boundary.

The stratigraphic distribution of the main taxa of larger foraminifera defines two assemblages. The firstassemblage is characterised by the first appearance of lacazinids (Pseudolacazina loeblichi) and mean-dropsinids (Eofallotia simplex), by the large agglutinated Montsechiana montsechiensis, and by severalspecies of complex rotalids (Rotorbinella campaniola, Iberorotalia reicheli, Orbitokhatina wondersmitti andCalcarinella schaubi). The second assemblage is defined by the appearance of Lacazina pyrenaica, Pal-androsina taxyae and Martiguesia cyclamminiformis.

A late Coniacian-early Santonian age was so far accepted for the La Cova limestones, based on indirectcorrelation with deep-water facies bearing planktic foraminifers of the Dicarinella concavata zone.Strontium isotope stratigraphy, based on many samples of pristine biotic calcite of rudists and ostreids,indicates that the La Cova limestones span from the early Coniacian to the early-middle Santonianboundary. The first assemblage of larger foraminifera appears very close to the early-middle Coniacianboundary and reaches its full diversity by the middle Coniacian. The originations defining the secondassemblage are dated as earliest Santonian: they represent important bioevents to define the Coniacian-Santonian boundary in the shallow-water facies of the South Pyrenean province.

By means of the calibration of strontium isotope stratigraphy to the Geological Time Scale, the largerforaminiferal assemblages of the La Cova limestones can be correlated to the standard biozonal schemeof ammonites, planktonic foraminifers and calcareous nannoplankton. This correlation is a first steptoward a larger foraminifera standard biozonation for Upper Cretaceous carbonate platform facies.

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1. Introduction

The “Serra del Montsec” is a continuous mountain ridgeextending east-west for more than 50 km south of the Pyrenees,between the Tremp and Ager basins (Fig. 1). It is mainly made ofUpper Cretaceous shallow-water carbonates covering with nointerruption the Cenomanian to Maastrichtian time interval. Theseshallow-water carbonates contain a very rich and diverse fauna oflarger foraminifera that has long attracted the attention of

þ34 935811263.

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micropaleontologists (Aubert et al., 1963; Hottinger, 1966; Caus andCornella, 1981a,b; Hottinger and Caus, 1982, 1993; Hottinger et al.,1989; Cherchi and Schroeder, 1999 among them). The larger fora-minifera of the La Cova limestones represent in the Pyrenean Basinthe first great diversification of the K-strategists in the Late Creta-ceous Global Community Maturation cycle (Hottinger, 2001) afterthe CenomanianeTuronian mass extinction (Caus et al., 1993, 1997;Parente et al., 2008). During this interval of time, the Pyreneanfossil record documents the “explosive” development of twogroups of porcelaneous larger foraminifera, the lacazinids (largermilioliform foraminifera with a tremathoporate aperture) and themeandropsinids (a group of Late Cretaceous foraminifera withOphtalmidiid origin), and of a group of lamellar-perforate, the

Fig. 1. Location of the “Serra del Montsec” and of the measured Sections (1e12).

C. Boix et al. / Cretaceous Research 32 (2011) 806e822 807

rotaliids. Moreover, new genera of agglutinated foraminifera, suchas Ramirezella and Martiguesia, appear, while a few survivors fromthe Cenomanian (such as Cuneolina, Dicyclina or Cyclolina) are stillpresent.

Recently, several taxonomic studies have been carried out on thelarger foraminifera of the La Cova limestones (Boix, 2004; Boix,2009; Albrich, 2008; Villalonga, 2009; Boix et al., 2009; Hottingerand Caus, 2009). However, the stratigraphic ranges of many taxaare still not known in detail and their chronostratigraphic age is notwell constrained.

The La Cova limestones have been classically attributed to the lateConiacianeearly Santonian time interval (Hottinger andRosell,1973;Pons, 1977) and their larger foraminiferal assemblages have beencorrelatedwith theDicarinella concavata planktic foraminiferal zone(Caus and Gómez-Garrido, 1989). However, this correlation is basedexclusively on indirect lithostratigraphic correlation with deep-water facies exposed more to the north (Bòixols Thrust Sheet). Nodirect evidence is available as ammonites, calcareous plankton andnannoplankton have never been found in the La Cova limestones.

The lack of precise correlation with deep-water facies and ofaccurate chronostratigraphic dating is often a severe drawback inthe study of Cretaceous carbonate platforms which has in somecases generated harsh controversies (Föllmi, 2008). During the lastdecades, chemostratigraphy has been successfully applied to tacklethese problems. Carbon isotope stratigraphy proved particularlysuccessful for high resolution dating and correlation of middleCretaceous carbonate platforms (Parente et al., 2007 and referencestherein) while strontium isotope stratigraphy (SIS) has provenparticularly suitable for Upper Cretaceous rudist limestones(Steuber, 2003, 2003a; Steuber et al., 2005; Frijia and Parente,2008a; Schlüter et al., 2008).

The potential of SIS as a high resolution tool of chronostrati-graphic dating and correlation of marine sedimentary rocks hasbeen widely demonstrated (see Veizer et al., 1997 and McArthurand Howarth, 2004 for recent reviews). The curve describing thesecular variation of marine Sr isotope ratio during the Cretaceous isknown in considerable detail. In particular, the late Turonian-Maastrichtian segment of the reference curve is characterised bya narrow error band and by a fairly high gradient. As a result,a maximum resolution of�0.5e1 My can be attained over this timeinterval with SIS (McArthur and Howarth, 2004).

The aim of this work is to supply new data on the stratigraphicdistribution of the main taxa of larger foraminifera in the La Covalimestones and to constrain their chronostratigraphic age by SIS.We propose two larger foraminiferal assemblage zones and corre-late them to standard ammonite, planktic foraminiferal andcalcareous nannoplankton zones. Finally, we use SIS to define theposition of the Coniacian-Santonian boundary within the La Covalimestones and propose a biostratigraphic criterion, based on largerforaminifera, for the definition of the boundary in shallow-watercarbonate facies of the Pyrenean basin.

2. Geological setting

During the Late Cretaceous the “Serra del Montsec” was part ofthe southern margin of the Pyrenean Basin, which was a narrowand deep gulf open to the Atlantic Ocean and extending approxi-mately east-west between the Iberian and European plates, fromthe Galician offshore to the “Bassin of Beausset” near Marseille.

Nowadays, the “Serra del Montsec” forms part of the MontsecThrust Sheet that, together with the Bòixols Thrust Sheet to thenorth and the Serres Marginals Thrust Sheet to the south,

Fig. 2. Upper Cretaceous stratigraphic units of the “Serra del Montsec”.

C. Boix et al. / Cretaceous Research 32 (2011) 806e822808

constitutes the southern verging thrusts of the Upper Thrust Sheetsunit (Berastegui et al., 1993; Muñoz et al., 1984). The MontsecThrust Sheet has a simple internal structure, comprising a frontalanticline to the south, formed by Mesozoic rocks, and a largesyncline to the north (Tremp syncline), which includes earlyPalaeogene deposits.

The Upper Cretaceous stratigraphic units exposed in the “Serradel Montsec” are, from bottom to top (Figs. 2 and 3):

1. Santa Fe Formation pro parte. Well-bedded shallow-wateralveolinid limestones, resting unconformably on Lower Creta-ceous deposits. The age of this unit is middle to upper Cen-omanian (Hottinger and Rosell, 1973; Caus et al., 1997; Calongeet al., 2002).

Fig. 3. The western edge of the “Serra del Montsec” (Noguera-Ribagorçana valley) seen frosouthern slope of the “Congost de Montrebei”.

2. Pardina Formation. Massive to poorly-bedded, deep-ramp lightgrey to white micritic limestones, dominated by calcispheres.Planktonic foraminifera of the Helvetoglobotruncana helveticaandMarginotruncana schneegansi zones point to a Turonian age(Caus et al., 1993).

3. La Cova limestones. Shallow-water limestones and nodularmarly limestones with a very rich and diverse fauna of rudists,gastropods, corals and larger foraminifera. The larger forami-niferal content and the chronostratigraphic age of this unit arethe objects of the present work. Previous works support anupper Coniacianelower Santonian age (Hottinger and Rosell,1973; Pons, 1977).

4. Font de les Bagasses marls. Yellow and/or grey marls alternatingwith thin-bedded marly limestones. They yield a very richfauna of larger foraminifera, echinoids, brachiopods and soli-tary corals. Some nautiloids and sponge spicules are alsopresent locally. This unit was attributed to the upper Santonian(Hottinger, 1966; Caus and Gómez-Garrido, 1989).

5. Terradets limestones. Grey to beige bioclastic limestones ofCampanian age (Caus and Cornella, 1983). The fossil assem-blages of this unit are dominated by larger foraminifera (mainlyorbitoids and siderolitids), rudists and bryozoa, with minorcalcareous algae.

6. Montsec sandy limestones and sandstones. Calcareous sand-stones, sandy limestones and sandstones with siderolitids,orbitoids, rudists and some brachiopods. The upper part of thisunit includes some levels of limestones with charophytes andcontinental gastropods. The age given in the literature is upperCampanian-lowermostMaastrichtian (Caus and Cornella,1983;Pons, 1977).

7. Tremp Group. Continental red-beds representing the end of themarine sedimentation in this area.Dinosaur remainsare frequentin its lower part. The Cretaceous-Tertiary boundary is placedwithin this unit (López-Martínez et al., 2006).

3. Stratigraphy and sedimentology

The stratigraphic and sedimentologic study of the La Covalimestones is based on detailed field observations on 12 strati-graphic sections distributed along an east-west 42 km transept

m the south. The Upper Cretaceous stratigraphic units are beautifully exposed on the

C. Boix et al. / Cretaceous Research 32 (2011) 806e822 809

(Fig. 1) Field observations were complemented by microfaciesanalysis of over 300 thin sections.

The shallow-water carbonate deposits of the La Cova limestonesrepresent a complete transgressiveeregressive cycle bounded byerosional surfaces (Caus et al., 1999). From the base to the top, it canbe subdivided into the following subunits (Fig. 4):

3.1. Subunit 1

Limestones with charophytes and smaller benthic foraminifera(wackestones, packstones and more rarely fine-grained grain-stones). This subunit overlies the Pardina limestone by means of anerosive contact. Some beds show abundant root bioturbations(paleosols). Charophyte oogons and stems occur at the base of thisunit, but they are gradually replaced by small benthic foraminiferamoving upward. The presence of charophyte remains, ostracodsand small and poorly diversified foraminifera indicates that theselimestones were deposited in shallow-water coastal pools.

3.2. Subunit 2

Grey nodular marly limestones (mainly packstones with rarewackestones) with rudists and rare stony corals and brachiopods.Larger foraminifera are very abundant and highly diversified. Three

Fig. 4. Lithostratigraphy of the La Cova limestones. This formation has been subdivided intisotope stratigraphy are drawn in this figure. The position of the samples discussed in the

discontinuous and temporally successive rudist build-ups and somelens-shaped bodies of finely cross-stratified dark yellow bioclasticlimestones are intercalated in this subunit (Figs. 5 and 6). The oldestbuild-up is very thinand ismadeof rudists (assemblage Tu inPascualet al., 1989) and chaetetids. The middle one consists of a thin lens-shaped body dominated by shells of Hippuritella praetoucasi (Tou-cas) (assemblage Co in Pascual et al., 1989). The youngest one is verywell developed and comprises several successive bioconstructionswith abundant hippuritids (assemblage Co-Sa in Pascual et al.,1989).

The marly limestones of this unit are interpreted as deposited ina wide lagoon, separated from the open sea by a system of discon-tinuous, poorly developed bars. The moderate hydrodynamic condi-tions and theabundanceof seagrasses favored thedevelopmentof richand diverse assemblages of larger foraminifera. Occasionally, rudistcommunities flourished in the lagoon. The bioclastic sediments of thecross-laminated intercalations are interpreted as tidal delta deposits.Their composition is the result of themixing of grains produced in situwith grains which originated in the open-sea and were transportedinto the lagoon through tidal channels (Caus et al., 1999).

3.3. Subunit 3

Massive limestones (mainly packstone-grainstones; more rarelywackestones at the base of the unit) that form a vertical cliff on the

o four subunits (see text for further details). Only the sections sampled for strontiumtext and their numerical ages are marked.

Fig. 5. Sharp contact between marly limestones (lagoon deposits) and bioclasticlimestones (tidal delta deposits) of the subunit 2. Clot d’Olsí gully (Section 3).

C. Boix et al. / Cretaceous Research 32 (2011) 806e822810

mountain slopes (Fig. 7). The fossil content is abundant (mainlyrudists, larger foraminifera and crustose red algae), but isfrequently fragmented. Rudist shells are locally silicified. Overall,these massive limestones formed a bar, locally subdivided intoseveral superposed bars, which separated the protected lagoonfrom the open sea.

Fig. 6. Close-up of the youngest rudist build-up of subunit 2. Barranquils area(Section 4).

3.4. Subunit 4

Grey bioturbated nodular marls alternating with well-beddedlimestones (packstones and wackestones) (Fig. 7). The fossilcontent is abundant and dominated by larger foraminifera andrudists. Some distinctive facies are intercalated within this subunit:a) rudist build-ups with a short lateral extension, constitutedmainly by radiolitids and hippuritids, with some requienids(assemblage Sa in Pascual et al., 1989); b) lens-shaped bodies ofcross-stratified bioclastic limestones, similar to those found insubunit 2; c) nodular marls and marly limestones with abundantfragments of irregularly disposed nerineids (Wieczorek andLlompart, 1994) and echinoids.

The marls, the bedded limestones and the rudist build-upsformed in the interior of a lagoon. The cross-stratified limestonesare interpreted as tidal delta deposits which originated in moreexternal areas and were transported into the lagoon throughinterbar tidal channels. The nodular marls with nerineids representwashover deposits transported into the lagoon by storm waves.

4. Strontium isotope stratigraphy

4.1. Materials and methods

The SIS database for this paper consists of 42 samples of bioticlow Mg-calcite (mainly rudists, a few ostreids and some unidenti-fied bivalve fragments) coming from fourteen stratigraphic levels infive sections.

Samples were selected in the field using as first guidance colourpreservation (yellowish to dark brown or dark grey) and prelimi-nary analysis of shell microstructure with the hand lens. Wheneverpossible, multiple samples were collected from each stratigraphiclevel in order to test the internal consistency of data. When wholeshells and fragments could not be isolated in the field, samples ofthe encasing rockwere collected. In the lab, rock samples and largershells were cut to produce 0.5e2 cm-thick slabs. The slabs wereground and polished on all sides in order to eliminate surficialcoatings (mainly clay material and oxides) and visible calciteovergrowths.

Smaller shells and fragments were carefully washed in anultrasound bath (5 min filled with deionised water at 50 �C) andthen dried at room temperature. The shells collected from clay-richlevels were passed through repeated cycles of ultrasonic washing ina solution of water andmethanol to remove adhering clayminerals.The shells were then bathed for 20e45 s in HCL 1M, to eliminatecalcite overgrowths, and then rinsed carefully with deionizedwater. As a final step all the samples were washed ultrasonically ina bath of ultrapurewater (milli-Qwater) for 3min and then dried ina clean environment.

A careful examination of the samples before and after thecleaning steps confirmed that this procedure effectively removedsurficial coatings and thin calcite overgrowths.

All the samples (rock slabs and isolated shell fragments) werethen passed through a complete petrographic screening (opticalmicroscope, cathodoluminescence, SEM) to assess the preservationof the original shell microstructure.

The elemental (Mg, Sr, Mn and Fe) composition of the shells wasanalysed as a further screening step. Themicritic matrix and calciticcements of some samples were also analysed in order to get deeperinsight into the diagenetic evolution. Samples for geochemicalanalyses were obtained by microsampling selected areas of pol-ished slabs and shell fragments with a hand-operated microdrillequipped with 0.3e0.5 mm B tungsten drill bits. All thegeochemical analyses were performed at the Institute for Geology,Mineralogy and Geophysics of the Ruhr-University (Bochum,

Fig. 7. Massive limestones (vertical cliff) of the subunit 3, sandwiched between the marly limestones of subunit 2 and subunit 4. The latter is overlain by the Font de les Bagassesmarls. Barranquils area (Section 4).

Fig. 8. Thin section (AeB) and SEM (CeD) photomicrographs of rudist and ostreid shells. (A) compact portion of the outer shell layer of a rudist shell, showing a well preservedprismatic structure with growth lines. Sample LC1B, thin section, crossed nicols. (B) well preserved crossed-lamellar structure of an ostreid shell. Sample LC1C, thin section, crossednicols. (CeD) different portions of the same rudist shell showing different degrees of alteration. The individual prisms are partially fused in (C) and almost completely obliterated bymicritization in (D). Sample MB12.1 C-A, fresh fracture, SEM.

C. Boix et al. / Cretaceous Research 32 (2011) 806e822 811

C. Boix et al. / Cretaceous Research 32 (2011) 806e822812

Germany). The elemental concentration of Ca, Mg, Sr, Fe and Mnwas established by inductively coupled plasma-atomic emissionspectrometry using a Thermo Fisher Scientific iCAP6500 Dual ViewICP-OES. The external reproducibility, expressed as relative stan-dard deviation, is �1% of the measured concentrations for Ca, Mgand Sr, �2% for Mn and �5.6% for Fe.

After strontium separation by standard ion-exchange methods,strontium-isotope ratios were analysed on a Finnigan MAT 262thermal-ionization mass spectrometer and normalized to an86Sr/88Sr value of 0.1194. The long term mean of modern seawater(USGS EN-1) measured at the Bochum isotope laboratory is0.709158 � 0.000004 (2 s.e., n ¼ 180) and the mean value of theUSGS EN-1 standards run together with the samples analysed forthis study is 0.709166 � 0.000005 (2 s.e., n ¼ 10).

The 87Sr/86Sr ratios of the samples were adjusted to the value of0.709175 of USGS EN-1 standard, to be consistent with the normal-ization used in the compilation of the “look-up’’ table of McArthuret al. (2001; version 4: 08/04), which was used to derive thenumerical ages. When more than one sample was available for onestratigraphic level, the mean value was calculated. Whenmore thanone level was sampled in a thin interval (less than 20m, i.e. samplesMB12.1eMB13; tab.), with no evidence of intervening stratigraphicgaps, we derived the numerical age from the mean value of all the

Table 1Elemental and isotopic composition of biotic calcite, micrite and cement from the La Cov

Sample Section Component Mg (ppm) Sr (ppm)

CCCI-A(MC6) 6 rudist 2247 1354MB12-1C-A 4 rudist 2442 1391MB12.1C-B 4 rudist 6959 1467MB12.1G-A 4 rudist 944 1126MB12.1F-A 4 rudist 926 1462MB12.1E-C 4 rudist 2657 1339MB12.2-C 4 rudist 3294 1344MB12.2B 4 rudist 4177 1372MB 12.2D-A 4 rudist 5070 769MB 12.3C-A 4 rudist 2685 1423MB 12.3C-B 4 rudist 1903 1264MB12.3B-A 4 rudist 2329 1357MB13.1A 4 rudist 5342 847MB 13.1 C-A 4 rudist 6032 1785MB13.1B-A 4 rudist 3833 700MB17-18A 4 rudist 8633 1591MB-17/18-4 4 rudist na naMB17/18-B 4 rudist 3271 1157MEC 12.1-3 2 rudist na naMEC12.1-5 2 rudist 2958 1061MEC18-1-C 2 rudist 1832 1277MEC18-1 BA 2 rudist na naMEC18-1 AA 2 rudist na naMEC18.1-E 2 rudist 1876 1532MEC18-1-F 2 rudist 2304 1493SL-CO4 3 rudist 3573 1355SL-CO2 3 rudist 5754 654SL-CO5 3 rudist 3763 1510SM 1 (NC) 3 rudist 2426 546SM 3 3 rudist 1114 1357SOP-A 6 rudist na naMC8eSC 6 indet. bivalve 4064 1433MC8-SCM 6 matrix 4097 296LC 1 (C) (2) 9 ostreid 2628 963LC 1 B (A) 9 rudist 3120 825LC 1 B (B) (2) 9 rudist 2031 1204LC 2 (B) 9 ostreid 1213 959LC 2 (D) 9 ostreid 1163 990LC 2 (E) 9 ostreid 869 1069LC 2 (F) 9 ostreid 665 986LC 2 (M) 9 matrix 4861 568LC 2 (T) 9 cement 3310 160

P ¼ pristine; PA ¼ probably altered; A ¼ altered. na ¼ not available.

samples. This procedure is based: a) on the assumption that signif-icant changes of Sr isotope ratios are not expected to occur over timeintervals shorter than the Sr residence time in modern seawater(2My), b) on an estimated long-termaccumulation rate of a few tensof m/My, typical of “cool-water” carbonates (Schlager, 2000). A“cool-water” affinity is inferred for the La Cova Limestones, based onthe dominant carbonate producing biota.

The precision of the 87Sr/86Sr mean value for each stratigraphiclevel is given as 2 s.e. of the mean when the number of samples (n)is �4. When n < 4, the precision is considered to be not better thanthe average precision of single measurements (2 s.e. ¼ 0.000007)and is calculated from the standard deviation of the mean value ofthe standards run with the samples.

The numerical ages of the samples analysed in this study werederived from the look-up table of McArthur et al. (2001, version 4:08/04), which is tied to the Geological Time Scale of Gradstein et al.(2004; hereinafter GTS2004). Minimum and maximum ages wereobtained by combining the statistical uncertainty (2 s. e.) of themean values of the Sr-isotope ratios of the samples with theuncertainty of the seawater curve. The numerical ages were thentranslated into chronostratigraphic ages and corresponding stan-dard biozones (ammonites, planktic foraminifera and calcareousnannoplankton) by reference to the GTS2004.

a limestones.

Fe (ppm) Mn (ppm) 87Sr/86Sr 2 s.e. (*10�6) Preservation

45 4 0.707409 7 P38 3 0.707417 5 A29 1 na na P11 1 na na P44 4 na na P52 1 0.707387 6 P157 5 0.707422 6 P199 5 0.707430 5 P56 2 na na P133 4 na na P55 2 0.707407 6 P100 7 0.707418 6 P110 5 0.707424 5 P56 < 5 0.707417 7 P88 3 na na PA266 6.6 0.707448 6 PAna na 0.707454 6 PA68 5 0.707434 6 Pna na 0.707341 5 P56 4 0.707405 6 P635 14 na na Ana na 0.707395 6 Pna na 0.707402 6 P50 6 0.707415 6 P71 2 0.707398 6 P179 14 0.707355 5 P199 47 0.707363 5 A62 8.5 0.707343 7 P822 29 na na A26 2 0.707403 6 Pna na 0.707662 6 A39 <5 0.707435 6 P1453 33 0.707511 5159 6 0.707431 7 P166 11 0.71 5 P60 1 0.707437 6 P984 72 0.71 6 A728 87 0.707450 5 A352 49 0.707446 5 A475 55 0.71 6 A8004 309 na na7992 421 na na

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4.2. Diagenetic screening

SIS is based on the fact that the Sr isotope ratio of the Sr dis-solved in the ocean varied through time and on the assumption thatat any time the ocean was well mixed with respect to 87Sr/86Sr.Based on the latter assumption and on the empirical knowledge ofthe marine Sr isotope ratio through time, accurate age informationcan be obtained from samples of marine precipitates. However, thisis only true for those samples that retained the original Sr isotoperatio of the seawater from which they precipitated. Therefore,selection of the best preserved samples through an accurateprocedure of diagenetic screening is a prerequisite for successfulapplication of SIS (McArthur, 1994). The low-Mg biotic calcite ofcompact-shelled bivalves and other marine invertebrates is

Fig. 9. Sr, Fe, Mn, Mg concentration and 87Sr/86Sr in samples analysed for this study. The fieldThe arrows indicate the diagenetic trend from pristine shells through altered shells to diagestratigraphy even if they are in the field of pristine samples (see text for further details).

considered a particularly suitable material because it is resistant todiagenesis and because the preservation of the original micro-structure can be used to assess diagenetic alteration (McArthur,1994; Veizer et al., 1997).

Petrographic observation, with the optical microscope (underpolarized light and cathodoluminescence) and the SEM, revealedan excellent preservation of the original microstructure for most ofthe shells collected from the La Cova limestones (Fig. 8A and B).Only a few samples showed evidence of a more or less severealteration (Fig. 8C and D).

Diagenesis of biogenic low-Mg calcite usually results ina pattern of decreasing Sr concentration and increasing Mn and Feconcentration (Brand and Veizer, 1980; Al-Aasm and Veizer, 1986).However, depending on the diagenetic environment and on the

s delimited by the dashed lines include the samples that have been deemed as pristine.netic calcite (micrite and cement). Circled samples were not used for strontium isotope

C. Boix et al. / Cretaceous Research 32 (2011) 806e822814

composition of the diagenetic fluids, lowMn and Fe content can befound also in diagenetic calcite (Steuber et al., 2005; Frijia andParente, 2008b; Vicedo et al., 2011).

Integrating the results of the petrographic screening and ofgeochemical analyses, we considered as diagenetically altered allthe samples withMn concentration> 20 ppm and Fe concentration> 250ppm (Table 1). Beyond these thresholds our data depict a cleardiagenetic trend of increasing Fe and Mn and decreasing Srconcentration for moderately to poorly preserved shells. Micriticmatrix and blockycement are the endmembers of this trend (Fig. 9).

Since Sr is easily lost even in the early stages of diagenesis(Brand and Veizer, 1980), and taking into account the caveats dis-cussed for Mn and Fe concentrations, we relied mainly on theconcentration of Sr for diagenetic screening. In our dataset, speci-mens with Sr concentration < 700 ppm have at least one indicatorof diagenetic alteration (either high Mn or Fe content or moderateto poor preservation of the original shell microstructure). There-fore, a concentration of Sr >700 ppm was taken as the limit forsamples to be used for SIS. It is worth mentioning that some shellswith Sr concentration well above this threshold were not used forSIS because SEM observations showed evidence of recrystallization(i.e. samples MB12-1C-A and MB17-18A, Fig. 8ced).

Rudist shells ranked as pristine by the petrographic screeninghave Sr and Mg concentrations in the range of well preservedhippuritids (Steuber, 1999). All the ostreid shells have Sr and Mgconcentrations in the range of well preserved ostreids as compiledby Schneider et al. (2009).

In many diagenetic environments diagenetic fluids are enrichedin radiogenic Sr derived by interaction with crustal rocks, anddiagenesis proceeds with decreasing Sr concentration andincreasing Sr isotope ratio (McArthur, 1994). A few exceptions havebeen documented, with diagenetic materials characterised by Srisotope ratios lower than pristine marine precipitates (Steuber,2003; Steuber et al., 2005; Vicedo et al., 2011). In our samples themicritic matrix and poorly preserved shells invariably show lowerSr concentration and more radiogenic Sr isotope ratios than asso-ciated pristine shells. For this reasonwhen different shells from thesame level showed significantly different Sr isotope ratios, wediscarded the subsamples showing the more radiogenic values

Table 2Strontium isotope stratigraphy of the La Cova limestones.

Sample Section 87Sr/86Sr 2 s.e. (*10�6) 0\ 87Sr/86Sr mean 2

LC 1 (C) (2) 9 0.707431 7LC 1 B (B) (2) 9 0.707437 6LC 1 B (A) 9 0.707440 5 0.707436SM 3 3 0.707403 6 0.707403 1CCCI-A(MC6) 6 0.707409 7 0.707409 1MB12.2-C 4 0.707422 6MB12.2B 4 0.707430 0MB12.1E-C 4 0.707387 6MB 12.3C-B 4 0.707407 6MB12.3B-A 4 0.707418 6MB13.1A 4 0.707424 5MB 13.1 C-A 4 0.707417 7MB (12e13) 0.707415 1MB17/18-B 4 0.707434 6 0.707434 1MC8eSC 6 0.707435 6 0.707435 1MEC 12.1e3 2 0.707341 5 0.707341 1MEC18.1-E 2 0.707415 6MEC18-1-F 2 0.707398 6MEC18-1 AA 2 0.707402 6MEC18-1 BA 2 0.707395 6 0.707402SL-CO4 3 0.707355 5SL-CO5 3 0.707343 7 0.707349 1

Numerical ages from McArthur et al. (2001; look-up table version 4: 08/04). Chronostranumerical ages and on the estimates of the precision.

(e.g. MB17/18A and MB17/18-4; see Table 1), even in absence ofother indicators of diagenetic alteration.

With the few exceptions cited above, well preserved shells fromthe same level have values of 87Sr/86Sr within analytical precision(2 s.e. ¼ �7 � 10�6) (Table 2; Fig. 9). In accordance with previousstudies (McArthur, 1994; McArthur et al., 2004; Frijia and Parente,2008b), the internal consistency of data is taken as furtherevidence that the samples that we selected as pristine retainedtheir original marine isotopic signature.

4.3. Numerical ages and chronostratigraphic dating

Samples for SIS were collected close to the base, in the middleand in the upper part of the La Cova limestones. This allowed high-precision chronostratigraphic dating of this formation and of itssubunits.

The age of the lower part of the La Cova limestones is con-strained in thewestern part of the studied area (sections 2 and 3) bythe samples obtained from two rudist levels (SL-CO and MEC12.1)occurring close to the base of subunit 2 (Fig. 4). Their 87Sr/86Srmeanvalues are almost identical (Table 2) and translate into numericalages of 88.48 Ma and 88.68 Ma, respectively. These ages are veryclose to the early-middle Coniacian boundary.

The upper part of subunit 2 was sampled in different levels ofSections 2, 3, 4 and 6. The SIS ages obtained for these levels go from86.73 to 86.31 Ma (Fig. 4; Table 2), which correspond to the lateConiacian.

The next levels analysed for SIS are from the upper part of the LaCova limestones, a few meters above the base of subunit 4. Thesamples collected from these levels in sections 4, 6 and 9 givenumerical ages ranging from 85.46 to 85.31 Ma (Fig. 4; Table 2),corresponding to the early Santonian.

5. The larger foraminifera of the La Cova limestones

The micropaleontological and biostratigraphical study is basedon over 400 samples. Isolated specimens were obtained from thewashing residues of unconsolidated lithotypes (120 samples) andmore than 650 thin sections were prepared from limestones. The

s.e. mean (*10�6) Age (Ma) Chronostratigraphy

min preferred max

9 83.65 85.31 86.06 early Santonian5 86.02 86.7 87.27 late Coniacian5 85.75 86.51 87.10 late Coniacian

0 85.7 86.31 86.78 late Coniacian5 83.39 85.46 86.34 early Santonian5 83.33 85.39 86.31 early Santonian5 88.02 88.68 89.14 early Coniacian

9 86.28 86.73 87.13 late Coniacian

2 87.76 88.48 88.92 early-middle Coniacian

tigraphy from Gradstein et al. (2004). See the text for details on the calculations of

Fig. 10. aed Cyclolina aff. cretacea d’Orbigny, 1846. a: nearly centred axial section of a microspheric specimen. b: axial section of a megalospheric specimen. c: low-angle obliquesection of a probable microspheric form. d: half a part of an axial section showing the single foramina in the median part of the chambers. eeh Cyclopsinella steinmanni (Munier-Chalmas, 1887). e: fragment of a subaxial section close to the axial plane of a microspheric specimen. f: fragment of a section slightly oblique with respect to the equatorial plane. g,

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excellent exposures of the “Serra del Montsec” allowed bed by bedcorrelation across the whole mountain ridge. In this way thechanges in faunal composition due to shifts of environmentalconditions could be discriminated from changes through time,which are the basis of biostratigraphy.

Agglutinated, porcelaneous and lamellar-perforate larger fora-minifera are equally abundant in the La Cova limestones. Thestratigraphic distribution of the main taxa is given in Fig. 13. Thetaxa included in the range-chart are figured at the standardmagnifications in order to facilitate their comparison and identifi-cation in thin-sections (Figs. 10e12). The terminology of thearchitectural and structural elements refers to Hottinger (2006). Inthis paper a detailed description is given only for some agglutinatedforaminifera. The shell architecture and the systematics of the mainporcelaneous larger foraminifera of the La Cova limestones (laca-zines and meandropsines) have been studied previously (Hottingeret al., 1989; Hottinger and Caus, 2009), while the lamellar-perforaterotaliforms have been studied recently by Boix (2009) and Boixet al. (2009). Some comments about the regional distribution oflacazinids are also added.

5.1. Architecture of selected agglutinated foraminifera

5.1.1. Cyclolina aff. cretacea D’Orbigny, 1846 (Fig. 10aed)Finely agglutinated, disc-shaped test, slightly undulated and

sometimes with a small protuberance in its central part. Thethickness of the disc increases slightly towards the periphery. Thechambers consist of several concentric annuli with slightlydepressed sutures. The early chambers in the B forms are planis-pirally coiled. The multiple foramina are uniformly distributed inone row on the periphery of the disc. The chamber lumen is free ofexo- or endoskeletal elements. The specimens found in the La Covalimestones are larger than C. cretacea from the type-locality (Cen-omanian from Île Madame, southwest of France), and they may beconsidered to be a new (younger) species, but the new taxonwill benot formally described here.

5.1.2. Cyclopsinella steinmanni (Munier-Chalmas, 1887) (Fig. 10eeh)

The shape of the shell is similar to Cyclolina, but the periphery ofthe disc is pierced by two vertically superimposed rows of roundedopenings. In contrast with the simple genus Cyclolina, the chamberlumen is occupied by endoskeletal elements consisting of pillarsdisposed radially from one septum to the next. The pillars can befused, giving an irregular appearance in axial or tangential section.The communication between two successive chambers is assuredby stolons with radially disposed axes. Cyclopsinella, as Cyclolina,does not present exoskeletal elements.

5.1.3. Ramirezella montsechiensis (Caus and Cornella, 1981a)(Fig. 10ieq)

Axially compressed shell with flabelliform to discoid shape. Theperiphery of the disc is slightly undulated with multiple small androunded apertures arranged in alternating rows. The chambersdisplay a planispiral evolute growth model in the early stages,which becomes peneropliform to annular in the adult stages. Theexternal part of the chamber lumen is occupied by an exoskeleton

h: subaxial sections showing the disposition of the pillars in the central part of the shells. iethe progressive increase of the thickness of the disc. j: oblique section showing the thick waelements. k, l: axial or slightly subaxial sections of the first chambers, showing the apparitionplane, showing the alternating position of the pillars in successive stolon planes. n: fragmetransverse sections. Note the appearance of endoskeletal elements only in the equatorialAbbreviations: c: chamber; E: embryo; exo: exoskeletal elements; f: foramen; pi: pillar; s:

consisting of simple beams, while the inner part is occupied by anendoskeleton consisting of radial pillars aligned from one chamberto the next, but alternating in position in successive stolon planes.The early ontogenetic stages do not have endoskeletal elements;their appearance seems linked to the kidney-shape of the cham-bers. The stolon axes are radial but, if pillars are cut obliquely, thisarrangement could be erroneously interpreted as a crosswise-oblique system due to the high degree of curvature of the septa.

5.1.4. Pseudochoffatella? aff. gigantica Kaever, 1967 (Fig. 11aec)Large discoidal foraminifera (their diameter is up to 1 cm) with

a coarse agglutinated texture. The test could be free or attached,and the adult chambers are arranged in several wide annuli, whosethickness increases progressively towards the periphery. Modifi-cations of the annular chambers may occur in the attached forms(see Fig. 11b). The intercameral foramina show one, two or threerows of large openings depending on the ontogenetic stage. Theshell structure consists of thick beams and rafters distributedregularly in a coarse subepidermic network. Most of the specimensseem to be A-forms. Owing to the absence of centred B-forms, theattribution of the Pyrenean specimens to the Lower Cretaceousgenus Pseudochoffatella Deloffre, 1961 (type species: Pseudochoffa-tella cuvillieri) is not certain.

5.1.5. Pseudocyclammina massiliensis Maync, 1959(Fig. 11deh)Lobulate, subcylindric agglutinated shells characterised by pla-

nispiral arrangement in the early chambers and becoming uncoiledin the later stages. The large openings are irregularly distributed onthe whole apertural face. The exoskeleton is represented bya coarse, deep and well developed subepidermic network. The LaCova specimens are identical to those illustrated by Maync (1959)from Martigues (south-east of France).

5.1.6. Martiguesia cyclamminiformis Maync, 1959 (Fig. 11ieq)Nautiloid agglutinated larger foraminifera with biumbilicate

shells with rounded periphery. In the early growing stage thechambers are planispiral and involute, becoming uncoiled in thefinal stages as in Pseudocyclammina. The septa are gently curved.The apertural face is large and pierced by rounded openings as inthe genus Pseudocyclammina. The exoskeleton consists of at leasttwo orders of beams and rafters which are extraordinarily deep.The tangential section show the typical polygonal network, butwith very large lateral cavities. The endoskeleton is made of pillars,which alternate from one chamber to the next.

5.1.7. Dicyclina schlumbergeri Munier-Chalmas, 1887Finely agglutinated, discoidal shell with a biserial arrangement

of chambers. The chambers are annular and disposed parallel to theplane of the biseriality. The openings are positioned in a unique rowat the base of the apertural face. The exoskeleton comprises beamsand rafters organized in a subepidermic network. It lacks endo-skeletal structures. The beams penetrate deep into the chamberlumen leaving only a narrow passage.

5.1.8. Cuneolina spp.The species of the conical to flabelliform biserial genus Cuneo-

lina are not included into the distribution chart. This genus has

q Ramirezella montsechiensis (Caus and Cornella, 1981a,b). i: subaxial section showingll, the endoskeleton and the free space between the endoskeleton and the exoskeletalof the first pillars. m: detail of a fragment of an oblique section, close to the equatorial

nt of a section slightly oblique with respect to the equatorial plane. o, p: fragments ofplane. q: subaxial-tangential section showing the thick external wall and the pillars.septum.

Fig. 11. aec Pseudochoffatella? aff. gigantica Kaever, 1967. a: fragment of an axial section showing the coarse subepidermic network. b: specimen partially attached. c: fragment ofa section oblique with respect to the equatorial plane, showing the large foramina and the typical polygonal network produced by the tangential sections of the exoskeletalelements (beams and rafters). deh Pseudocyclammina massiliensis Maync, 1959. deg: sections parallel to the axial section or slightly oblique, showing the uncoiling later stages andthe multiple foramina. Note the lack of endoskeleton and the exoskeletal elements similar to those showed in Pseudochoffatella? aff. gigantica. h: centred oblique section. ieqMartiguesia cyclamminiformis Maync, 1959. i, l: slightly oblique subaxial sections showing the pillars. j, n: tangential sections showing the typical coarse subepidermic reticularnetwork. k: non-centred axial section showing the deep exoskeletal elements. m,o,p slightly oblique or subequatorial sections showing the deep exoskeletal elements and thepillars. q: subequatorial section showing the exo- and endoskeletal elements. Abbreviations: c: chamber; E: embryo; exo: exoskeletal elements; f: foramen; pi: pillar; s: septum.

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a record spanning the whole Cretaceous. A revision of its species isneeded before they can be used for biostratigraphy.

5.1.9. Dictyopsella spp.The genusDictyopsella has a typical low conical shell shape, with

a finely agglutinated wall. The chambers are trochospirallyarranged and show a very characteristic half-moon shape ina dorsal view. The ventral side presents an open umbilicus. Themain aperture is a low, interiomarginal arch situated at the base ofthe apertural face. Small supplementary apertures are openeddirectly into the umbilicus. The exoskeleton forms a fine sub-epidermic network that penetrates very deep into the chamber’slumen. At least two orders of beams have been observed. It does notpresent endoskeletal structures. Two species have been describedfrom the late Cretaceous sediments: D. kiliani Munier-Chalmas inSchlumberger (1899), and D. muretae Hottinger, 1967, but theircharacteristics are not clear enough and difficult to identify in thin-section. For this reason, we avoided including the species in therange-chart of Fig. 13.

5.2. Comments on lacazinid porcelaneous foraminifera

The lacazinid genera Pseudolacazina loeblichi (Fig. 12aei) andLacazina pyrenaica (Fig. 12jen) are the dominating porcelaneouslarger foraminifera in the ConicacianeSantonian shallow-watersediments of the Pyrenean basin. In many regional studiesP. loeblichiiHottinger et al., 1989, (type-locality Collades de Busturs)was erroneously assigned to the younger taxon Lacazina elongataSchlumberger 1900, which appears in the Font de les Bagassesmarls. L. pyrenaicaHottinger et al., 1989, previously cited in regionalstudies as Lacazina compressa Munier-Chalmas and Schlumberger,1895, was described from the La Cova limestones (in the middlepart of the subunit 4, section 9 in this work, see Fig. 4). Thesewrongdeterminations led frequently to wrong age assignments. Fordetails about the architecture of lacazinid species see Hottingeret al. (1989).

6. Larger foraminifera biostratigraphy andchronostratigraphic correlations

The stratigraphic distribution of the larger foraminifera permitsto separate two assemblages (Fig. 13). Assemblage 1 occurs insubunit 2 and is characterised by the rise of many new genera andspecies of agglutinated, porcelaneous and lamellar-perforate largerforaminifera after the extinction related to the Cen-omanianeTuronian boundary events. The most significant taxa are:Cyclopsinella aff. steinmanni, Montsechiana montsechiensis, Pseudo-lacazina loeblichi, Rotorbinella campaniola Boix et al., 2009, Iberor-otalia reicheli (Hottinger, 1966), Orbitokhatina wondersmittiHottinger, 1966, and Calcarinella schaubi (Hottinger, 1966).

The full diversity of this assemblage is reached in at least threesuccessive steps (see Fig. 13). All the species of the Assemblage 1persist in the Assemblage 2, which is defined by a new burst oforiginations occurring at the base of subunit 4. The most importantnewcomers are the agglutinated M. cyclamminiformis and theporcelaneous L. pyrenaica, Hellenalveolina tappanae Hottinger et al.,1989, and Palandroxina taxyae Fleury and Tronchetti, 1994. It isworth noting that the bioclastic grainstones/packstones of subunit 3are rich in fragments of larger foraminifera. However, no specimenof the taxa defining assemblage 2 has been identified in subunit 3.

The larger foraminiferal assemblages of the La Cova Limestoneshave been so far correlated to the D. concavata zone and assigned tothe late Coniacianeearly Santonian time interval (Hottinger andCaus, 2009). This age assignment was based on indirect lithos-tratigraphic correlation of the shallow-water facies of the Montsec

ridge with the deep-water facies with planktic foraminiferaoccurring more to the north, in the Pallaresa valley (Caus andGómez-Garrido, 1989). The SIS of the La Cova limestones allowsfor the first time to test this correlation and to better constrain thechronostratigraphic age of the larger foraminiferal assemblages.

The numerical age of 88.5e88.7 Ma, obtained for the rudistlimestones at the base of subunit 2, indicates that Assemblage 1appears very close to the early-middle Coniacian boundary. Thisnumerical age corresponds to the boundary between the Forresteriapetrocoriensis and Peroniceras tridorsatum ammonite zones, to themiddle part of the D. concavata planktic foraminiferal zone and tothe middle part of the CC13 calcareous nannoplankton zone(Fig. 13).

The chronostratigraphic age of the larger foraminiferal Assem-blage 2 is constrained by the numerical age of 85.3e85.5 Ma,obtained from levels a few meters above the base of subunit 4. Thisnumerical age corresponds to the early Santonian and is very closeto the Coniacian-Santonian boundary, which is placed at 85.8 Ma inthe GTS2004. The FOs of the larger foraminifera defining Assem-blage 2 could therefore offer suitable biostratigraphic events for theConiacian-Santonian boundary in the shallow-water carbonatefacies of the Pyrenean paleobiogeographic province (Caus andHottinger, 1986).

In terms of standard biochronologic scales, the base of theAssemblage zone 2 can be correlated with the middle part ofthe Texanites gallicus ammonite zone, with the uppermost part ofthe D. concavata planktic foraminiferal zone and with the baseof the calcareous nannoplankton biozone CC15 (Fig. 13).

The age of the base and of the top of the La Cova Limestones canbe estimated by means of age-thickness plots, making theassumption of constant accumulation rate between successivelevels dated by SIS. The age-thickness plot for the upper part ofSection 4 is built by extrapolating to the top of the section theaverage accumulation rate calculated between the two successivelevels dated by SIS (Fig. 14). The plot produces an age of 85.5 Ma forthe top of subunit 3: this level is taken as a datum because itrepresents a maximum flooding surface. For Section 2, we extrap-olated downward to the base of the section the average accumu-lation rate calculated between the two successive levels dated bySIS. The age-thickness plot for this section is then completedupward by using as a calibration point the age of 85.5Ma for the topof subunit 3, and by using for subunit 4 the accumulation ratecalculated for the lower part of the section (which has a very similarfacies). Finally, the lower part of the age-thickness plot for Section 4is completed by using the accumulation rate calculated for thelower part of Section 2.

The age-thickness plots produce an age of 89.3e88.7 Ma for thebase of the La Cova limestones in Sections 2 and 4 respectively andof 84.9e85.1 Ma for the top (Fig. 14). Therefore, based on our SISdata and extrapolation of accumulation rates, we estimate that theLa Cova limestones represents a time interval of about 4 My,spanning from the early Coniacian to the early-middle Santonianboundary. Under the same assumptions, the Coniacian-Santonianboundary (85.8 Ma in GTS2004) should be placed in the subunit3 (Fig. 14).

7. Conclusions and perspectives

Precise chronostratigraphic dating of shallow-water carbonatefacies and correlation with coeval deep-water successions oftenpose serious problems. In this paper we applied strontium isotopestratigraphy (SIS) to constrain more precisely the age of the UpperCretaceous La Cova limestones of the southern margin of the Pyr-enean basin, which were previously dated only by indirect(sequence stratigraphic) correlation to deep-water units exposed

Fig. 12. aei Pseudolacazina loeblichi Hottinger et al., 1989. a: slightly oblique-longitudinal section in the apertural axis, showing the trematophore (aperture) of a microsphericspecimen. b: section perpendicular to the apertural axis of a microspheric specimen. c: oblique-centred section almost perpendicular to the apertural axis. Megalospheric specimen.d: section perpendicular to the apertural axis. Megalospheric specimen. e: oblique-centred section almost in the apertural axis, associated to an oblique section showing the

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Fig. 13. Larger foraminiferal biostratigraphy and chronostratigraphy of the La Cova limestones. The time scale and the calcareous nannoplankton, planktonic foraminifera andammonite zones are from the GTS2004 (Gradstein et al., 2004). The calibration of the lithostratigraphic units and of the larger foraminiferal assemblages to the time scale is basedon strontium isotope stratigraphy. The most recent version (GTS2008; Ogg et al., 2008) could not be used because a calibration of the marine strontium isotope curve to this scale isnot yet available.

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farther to the north. The base of this unit can be referred to the earlyConiacian and is therefore considerably older thanwhat previouslyaccepted. The top is close to the early-middle Santonian boundary.Precise numerical and chronostratigraphic ages can be assigned tothe 4 subunits recognized in the La Cova limestones.

The La Cova limestones host a rich and diverse fauna ofagglutinated, porcelaneous and lamellar-perforate larger forami-nifera. Two assemblages can be defined on the basis of thestratigraphic distribution of the main taxa. Assemblage 1 appearsat the base of subunit 2 and reaches its full diversity in the upperpart of the same subunit. This assemblage is characterised bycomplex miliolids (lacazinids and meandropsinids), large discoidalagglutinated (Ramirezella) and large complex lamellar-perforateforaminifera (Iberorotalia, Orbithokathina and Calcarinella). It

flexostyle. Megalospheric specimens. f: centred longitudinal-oblique section. Megalosphericentre of the upper half of the picture (microspheric specimen) associated to a transversspecimen. i: longitudinal-oblique section together with a tangential section. Megalosphericthe apertural axis. Microspheric specimen. k: slightly oblique section of a microspheric spapertural axis in the upper part of the megalospheric specimen. m: longitudinal-obliquespecimen showing a large embryo. oeq Hellenalveolina tappanae Hottinger et al., 1989. o:septula. q: oblique section. ret Palandrosina taxyae Fleury and Tronchetti, 1994. r: almost cesection of a megalospheric specimen. t: detail of the structural elements in an oblique sectionE: embryo; end: endoskeletal elements; f: foramen; fl: flexostyle, pi: pillar; prp: preseptal

represents the first diversification of K-strategists within the LateCretaceous Global Community Maturation cycle after the massextinction at the CenomanianeTuronian boundary. Assemblage 2starts at the base of subunit 4 and is defined by the appearance ofL. pyrenaica, Hellenalveolina tappanae, Palandrosina taxyae andM. cyclamminiformis.

SIS allows dating the appearance of assemblage 1 as very closeto the early-middle Coniacian boundary. The new burst of origi-nations defining assemblage 2 occurs very close to the Coniacian-Santonian boundary, offering a precious biostratigraphic event todefine this boundary in the shallow-water carbonate facies of thePyrenean paleobioprovince (Caus and Hottinger, 1986). By refer-ence to the geological time scale of Gradstein et al. (2004), to whichSIS is calibrated, precise correlations can be established between

c specimen. g: longitudinal-oblique section showing the pillared trematophore in thee section (megalospheric specimen). h: almost centred oblique section. Microsphericspecimens. jen Lacazina pyrenaica Hottinger et al., 1989. j: oblique section not far fromecimen showing the disposition of pillars. l: oblique-longitudinal section showing thesection of a megalospheric specimen. n: Oblique-centred section of a megalosphericalmost axial section of a megalospheric specimen. p: tangential section showing thentred axial section of a microspheric specimen showing the annular stages. s: obliqueof the annular stage. Compare to picture r. Abbreviations: c: chamber; chl: chamberlet;passage; s: septum; sl: septulum.

Fig. 14. Age-thickness plot for sections 2 and 4. The average accumulation rate iscalculated for segments bracketed by two successive levels dated by strontium isotopestratigraphy and then extrapolated to other intervals. The top of subunit 3 is taken asa datum because it represents a maximum flooding surface. See text for furtherexplanations.

C. Boix et al. / Cretaceous Research 32 (2011) 806e822 821

the larger foraminiferal assemblage zones of the La Cova limestonesand the standard zones of ammonites, planktic foraminifera andcalcareous nannoplankton.

The chronostratigraphic dating of the La Cova limestones, and ofits larger foraminiferal assemblages, opens up some interestingresearch avenues. It represents a first step toward a larger forami-nifera standard biozonation for Upper Cretaceous carbonate plat-form facies. Precise correlations could be established between thebiostratigraphic events defined by the endemic larger foraminiferaof the Pyrenean basin and the bioevents recognized in thecarbonate platforms of the southern Tethyan margin (Chiocchiniet al., 2008; Velic, 2007), when SIS ages will be available also forthose platforms.

The rich and diverse rudist assemblages of the La Cova lime-stones (Pons, 1977; Pascual et al., 1989) have been so far dated onlyindirectly. Precise ages could now be given to these rudist assem-blages by SIS and an integrated zonation based on rudists and largerforaminifera could be established.

The La Cova limestones have been so far correlated with the SantCorneli (Aramunt vell and Montagut members) and Carreu (Clot deMoreu, El Grau and Prats de Carreu members) formations (sensuGallemí et al., 1982) exposed in the northern flank of the SantCorneli-Bòixols anticline. The lithostratigraphy of the Sant Cornelianticline has been recently a matter of considerable debate (Vicenset al., 1998). A middle Coniacianeupper Santonian age is docu-mented by Vicens et al. (1998) for the lithostratigraphic units origi-nally described as the Sant Corneli and Carreu formations by Gallemíet al. (1982). The first occurrence of the inoceramid Platyceramusundulatoplicatus pinpoints the Coniacian-Santonian boundary in theuppermost part of the Montagut limestones (Vicens et al., 1998;Gallemí et al., 2004). The chronostratigraphic ages defined by SISfor the La Cova limestones offer new constrains for the correlationbetween the shallow-water units of the Serra del Montsec and theunits exposed in the Sant Corneli-Bòixols anticline, with importantimplications for the sequence-stratigraphic interpretation of thesouthern margin of the Pyrenean basin.

Acknowledgments

We wish to express our sincere gratitude to Prof. Joan Rosell(Universitat Autònoma de Barcelona) and Gonzalo Rivas (InstitutGeologic de Catalunya) for their help in the field work. Dr. DieterBuhl (Ruhr-Universität, Bochum) is warmly thanked for taking careof the geochemical analyses. The editor of Cretaceous Research andan anonymous reviewer are thanked for their useful suggestions.This research was funded by the Spanish Ministry of Science andInnovation, projects: CGL2006-02899/BTE and CGL2009-08371.

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