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Stratal, sedimentary and faunal relationships in the Coniacian 3rd-order sequenceof the Iberian Basin, Spain

José F. García-Hidalgo a, Fernando Barroso-Barcenilla a,b, Javier Gil-Gil a,*, Ricardo Martínez c,Jose Maria Pons c, Manuel Segura a

a IBERCRETA UAH Research Team CCTE 2007/R23, Universidad de Alcalá de Henares, 28871 Alcalá de Henares, SpainbDepartamento de Paleontología, Universidad Complutense de Madrid, 28040 Madrid, SpaincDepartament de Geologia, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain

a r t i c l e i n f o

Article history:Received 3 August 2011Accepted in revised form 12 November 2011Available online 25 November 2011

Keywords:Depositional sequencesAmmonitesRudistsConiacianIberian BasinSpain

a b s t r a c t

The Coniacian 3rd-order sequence in the Iberian Basin is represented by a carbonate ramp-like openplatform. The biofacies is mainly dominated by nekto-benthic (such as ammonites) and benthicorganisms (such as bivalves, mainly rudists) with scarce solitary corals (hermatypics are absent),showing major differences among the Transgressive System Tract (TST) and Highstand Normal Regres-sion (HNR). During the TST, platform environments were dominated by Pycnodonte, other oysters andmolluscs (with only subordinate rudists) and ammonites, which were represented by ornamentedplatycones (Tissotioides and Prionocycloceras), and by smooth oxycones (Tissotia and Hemitissotia). Duringthe HNR, shallow water depositional areas were occupied by rudist-dominated associations. Storm- andwind-induced currents and waves acting on these associations produced large amounts of loosebioclastic debris that covered outer platform areas. This facies belt graded landwards into protected,lower-energy settings (inner platform, lagoon and littoral environments). Rudist biostromes werepreserved in seaward areas of these protected shallow environments of overall moderate to lowhydrodynamic gradient, which was punctuated by storms. In this environment and landwards, largeareas of marly substrate favoured the presence of gastropods, other bivalves, echinoderms, benthicforaminifera and solitary corals. Because of the input of siliciclastics and, probably, the lack of nutrients insuspension, the establishment of rudist communities was difficult in more landward areas of the lagoonand in tidal environments. This heterozoan carbonate factory was thus controlled by warm-waterconditions and high energy levels, which were responsible for high-nutrient contents in suspension.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The Upper Cretaceous carbonate platforms in the Iberian Basinare represented by successive transgressiveeregressive 3rd-ordersequences, suggesting the presence of extremely dynamic sedi-mentary systems over time in this basin. The stacking of thesesequences generally shows a similar palaeogeographic patternwitha siliciclastic facies belt at the coastal margin and an innercarbonate platform facies in central areas of the basin (Gil et al.,2006a, 2008; García-Hidalgo et al., 2007). In these facies, bio-stratigraphically useful fossils are relatively poor because earlydiagenetic processes (mainly dolomitization) obliterated primarysedimentary structures and fossil content. Nevertheless, there aretwo particular sequences displaying deep and open platform faciesin most of the basin, which flooded wide areas of the coastal

margins with the development of thick carbonate platforms(Segura et al., 1989, 2001; Floquet, 1998; Gräfe, 1999; García et al.,2004), in which dolomitization processes during early diagenesisdid not significantly affect carbonate facies; consequently, fossilsare common and relevant in both deep and open facies (Barroso-Barcenilla, 2006) and shallow platform facies (Gil et al., 2002,2009). These sequences are related to the globally recognisedCenomanian/Turonian boundary and late Coniacian eustaticmaxima (Haq et al., 1988; Hardenbol et al., 1998).

Studies of the older sequence (Late CenomanianeEarly Turo-nian) have been undertaken by Segura et al. (1989, 1993a, b),García-Hidalgo et al. (2003, 2007), Barroso-Barcenilla (2006) andBarroso-Barcenilla et al. (2009, 2011) among others, and haveprovided: (1) a deep understanding of its stratal and depositionalframework, with a superimposed high-frequency depositionalstacking pattern; (2) a detailed biostratigraphy based on ammonitefaunas, allowing the revision of several ammonite families; and (3)an understanding of the geological history of this interval.

* Corresponding author. Tel.: þ34 918854997; fax: þ34 918855090.E-mail address: [email protected] (J. Gil-Gil).

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Cretaceous Research

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Cretaceous Research 34 (2012) 268e283


In contrast, the palaeoenvironments along the entire basin ofthe second episode (Coniacian) are less well understood andprevious interest has mainly focused on rudist associations andtheir vertical successions, fabrics and facies of shallow carbonate-platform settings (Gil et al., 2002, 2009, among others). In thepast, the detailed sedimentology and fossil content of the succes-sion were typically overlooked, although biotic communities areparticularly important for sedimentological analysis, because theyrecord ecological and environmental conditions with manydifferent features, determining accumulation rates and faciesdistribution, thus controlling platform geometry (Pomar, 2001).

The Coniacian sequence (named here DS-2), which constitutesthe main objective of this paper, is a superb example of a symmet-rical depositional event, with faunas and facies of both deep andshallow platform environments that retrogradate and progradatedepending on the eustatic signal (system tract). Thus, themain aimsof this paper are to: (1) describe the stratal architecture of thesequence from distal platform environments to coastal marginareas; (2) identify themain stratigraphic reference surfaces in orderto define the system tracts, describing the vertical succession offacies in different areas of the basin; (3) describe the faunalsuccession of ammonites, rudists and other bivalves, and theircorrelation from carbonate platform to coastal margin areas; (4)relate their occurrence with the depositional stacking pattern; and(5) discuss their biostratigraphic and palaeoecological implications.

2. Geological setting

The deposition of Cretaceous strata on the Iberian Microplatetook place in an intracratonic basin, withmaximum subsidence andsediment accumulation rates in the areas between the HesperianMassif and the Ebro Massif: the Iberian Basin (Fig. 1A). Reducedsubsidence in the west and southeast, and siliciclastic sedimentinput, mainly derived from the Hesperian Massif, resulted indeposition of the Coniacian sequence with an overall wedge-shaped, westwards- and southeastwards-thinning geometry. In

northeastern Iberia, the emerged Ebro Massif and different highsseparated the Iberian Basin from the Pyrenean Trough. Connectionwith the Tethyan Realm occurred discontinuously along south-eastern Iberia (García et al., 2004).

During these times, the Iberian Microplate was located in thetropical belt, south of a latitude of 30�N (Dercourt et al., 2000), andwas exposed to the warm, circum-global Tethyan current and awayfrom cold Boreal influences. This palaeogeographic location fav-oured a warm, humid climate and the proliferation of benthiccommunities in the Tethyan peri-continental areas (Philip, 2003).As a consequence, significant carbonate production together witha remarkable widespread development of platforms took place inthe basin.

The significant global eustatic sea-level rise during the LateCretaceous was the main factor controlling the depositionalepisodes in the Iberian Basin (Rat, 1982; García et al., 1996, 2004;Segura et al., 2001; Gil et al., 2004). Owing to the shallow char-acter of the Iberian Basin, it was particularly sensitive to any sea-level oscillation, registering even those of smaller amplitude(high frequency) (Gil et al., 2006a, b; García-Hidalgo et al., 2007).Four 2nd order eustatic sea-level cycles have been described for theLate Cretaceous in the Iberian Basin (MS-1 toMS-4megasequences;Segura et al., 2006). For the interval discussed in this paper, theDS-2 sequence represents the transgressive peak and the onset ofthe regressive phase of the upper Turonianelower Campanianmegasequence (MS-2; Segura et al., 2006).

Several of the stratigraphic sections discussed in this paper(Fig. 1B) have been previously studied by others. The ammonites ofthe Cervera section were first studied by Wiedmann (1975), whoerected Hemitissotia celtiberica Wiedmann, 1975 here (for authornames and dates of all species mentioned in the text, see Table 1);later the stratigraphy and sedimentology of this outcrop wasdescribed by Floquet (1991). The Castrojimeno section was firstdescribed by Alonso (1981), who reported ammonite and rudistassemblages; more recently it has also been studied by Gil et al.(2009), with a description and interpretation of the evolution of

Fig. 1. Location of the study area. A, palaeogeographical scheme of the Iberian Basin within the Tethyan Domain during the Coniacian, indicating main depositional environmentsand the cross-section of Fig. 7 (red line). B, geographical and geological scheme showing the cross-section of Fig. 7 (red line) and the location of the following reference key sections:1, Cuevas de San Clemente; 2, Contreras; 3, Hoz de Silos; 4, Hortezuelos; 5, Casuar-Linares; 6, Castrojimeno-Castroserracín; 7, Barranco de las Cuevas; 8, Embalse de Entrepeñas; 9,Estrecho de Paredes. Modified from Gil et al. (2009). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

J.F. García-Hidalgo et al. / Cretaceous Research 34 (2012) 268e283 269


different rudist assemblages. Finally, the depositional, biostrati-graphic and chronostratigraphic framework of different coastalmargin sequences at the Barranco de las Cuevas section has beenextensively investigated (Gil et al., 2002; Gil and García, 1996,among others).

3. Stratigraphic successions, facies and stratal stackingpattern

Sequence DS-2 is a southeastward-thinning, 90e20 m thick,marly and calcareous wedge. Eight reference sections have beenstudied in detail to create a composite NNWeSE cross-section(Fig. 1B). From NNW to SSE, DS-2 consists of the HortezuelosFormation (nodular limestones, bioclastic limestones and fossilif-erous marls) and the upper half of the Alarcón Formation (greenmarls and thin-bedded dolostones) (Gil et al., 2004). Three distinctreference areas (NNW, central and SE), with slightly differentstratigraphic successions can be distinguished, owing to the inter-digitation of both formations and the presence of several membersin the Hortezuelos Formation, showing a transition from outerplatform to littoral environments. Facies and subfacies describedbelow, as well as their environmental interpretation, are alsosummarised in Table 2.

The Hortezuelos Formation is lithologically subdivided in theNNWreference area (sections 1e4 in Fig.1B) into lower, middle andupper members (Fig. 2). The lower member consists of thin-beddedbioclastic limestones (subfacies 22) grading upwards to nodular,fossiliferous and intensely bioturbated, micritic limestones (subf-acies 11), alternating with thin marly beds and clayey joints,showing a general thickening-upwards trend. The middle memberconsists of fossiliferous grey marls and calcareous mudstones(subfacies 12); the thickness of this member progressively increases

northwards where it has been recognised as the NidáguilaFormation (Floquet et al., 1982; Floquet, 1998). The upper memberconsists of nodular limestones (subfacies 11; Table 2) grading tovery thick- and poorly-bedded bioclastic and/or oolitic limestones(subfacies 22 and 23, respectively; Table 2) with thin marly inter-calations (subfacies 12 or 31; Table 2), having a clear and distinctivemorphological expression in outcrops; the thickness of this uppermember progressively increases southwards, reaching over 50 m,compensating for the loss of thickness of the middle marlymember.

At the central reference area (sections 5e7 in Fig. 1B), the lowermember of the Hortezuelos Formation is almost absent (Fig. 3),being occasionally represented by a thin set of bioclastic and ooliticthin-bedded limestones (subfacies 22 and 23; Table 2). The thick-ness of the middle marly member has decreased in favour of theupper one, which is composed of an alternation of bioclastic,micritic limestones with rudist buildups (subfacies 21e25; Table 2)and yellowmarls (subfacies 31; Table 2). The lower boundary of theHortezuelos Formation here is the top of the calcareous CaballarFormation, marked by a single hardground and a distinct litho-logical change (Fig. 3 in Gil et al., 2009) from red tidal dolostonesand stromatolites to inner platform deposits (subfacies 21e25;Table 2). The overall trend of DS-2 in this area shows severalmarlstone-limestone bundles (sedimentary cycles), grading fromouter platform facies (subfacies 11 and 12; Table 2) to inner platformfacies (subfacies 21e25) or even restricted littoral deposits (subfa-cies 31; Table 2). In the basal cycles, outer platform faciespredominate; whereas in the upper cycles, inner platform faciesfollowed by restricted littoral facies are predominant (Fig. 4).

In the SE reference area (sections 8 and 9 in Fig.1B) DS-2 pinchesout to a 20-m-thick alternation of marls and dolostones, belongingto the upper half of Alarcón Formation (Fig. 5). DS-2 entirelyconsists here of restricted littoral sediments grading upwards fromgreen marls rich in organic matter (subfacies 43; Table 2) and thin-bedded dolostones with ripples, wavy and flaser bedding, algallaminations and ferruginous surfaces (subfacies 41; Table 2), toseveral bed-sets of green marls (subfacies 43; Table 2) and thin-bedded red dolostones (subfacies 41; Table 2) or intensively bio-turbated dolomitised breccias (subfacies 42).

All of these sedimentary areas clearly show a trans-gressiveeregressive depositional trend with a southeastward basalonlap. DS-2 is bounded by two major sequence boundaries (SB-2and SB-3, Figs. 2e5, 7), based on: (1) the presence of major sedi-mentary discontinuities and diagenetic overprints; (2) stratalrelationships (onlap, offlap, toplap) with underlying and overlying3rd-order sequences; and (3) the presence of the most importantbreaks in the vertical succession of facies, reflecting major changesin the sedimentary trends below and above DS-2. All of theseaspects suggest the existence of significant falls in the relativesea-level in both cases.

The lower sequence boundary (SB-2) is characterised bya widespread hardground with a well-developed lateritic crust andboring structures (Fig. 6A, B), affecting the underlying sediments(DS-1; Gil et al., 2006a). Occasionally, it is also characterised bycollapse dolomitic breccias related to early dissolution of evaporites(Hoz de Silos section; Floquet, 1991; Fig. 4C in Gil et al., 2006b).From NNW to SE, also a progressively more intense ferruginizationand early dolomitization of the underlying sediments occurred(Gil et al., 2009). Moreover, toplap relationships with at least threeunderlying 5th order parasequences (Gil et al., 2006a) and the basalonlap of DS-2 also suggest the existence of a sequence boundary(SB-2). Finally, a significant landward displacement of thecarbonate platform facies belts occurred at this boundary, and mid-outer shelf carbonate facies (subfacies 23 and 12; Table 2) at thebase of DS-2 rest on tidal carbonate facies (subfacies 41; Table 2) of

Table 1Taxonomic list of species mentioned in text.

AmmonoideaEulophoceras Hyatt, 1903Forresteria petrocoriensis (Coquand, 1859)Hemitissotia Peron, 1897Hemitissotia celtiberica Wiedmann, 1975Hemitissotia turzoi Karrenberg, 1935Placenticeras Meek, 1876Plesiotissotia cantabria Karrenberg, 1935 (¼ H. turzoi)Plesiotissotia dullai var. plana Karrenberg, 1935 (¼ H. turzoi)Prionocycloceras Spath, 1926Prionocycloceras iberiense (Basse, 1947)Pseudoschloenbachia Spath, 1921Texanites gallicus Collignon, 1948Tissotia Douvillé, 1890Tissotioides Reyment, 1958Tissotiodes hispanicus Wiedmann, 1960

HippuritoidaBournonia gardonica (Toucas, 1907)Bournonia fascicularis (Pirona, 1869)Praeradiolites requieni (d’Hombres-Firmas, 1838)Biradiolites canaliculatus d’Orbigny, 1850[Biradiolites angulosus (d’Orbigny, 1842)]Radiolites sauvagesi (d’Hombres-Firmas, 1838)Hippurites incisus Douvillé, 1895[Hippurites resectus Defrance, 1821][Hippurites vasseuri Douvillé, 1894]Vaccinites giganteus (d’Hombres-Firmas, 1838)Vaccinites moulinsi (d’Hombres-Firmas, 1838)Apricardia sp.

GryphaeidaePycnodonte Fischer von Waldheim (1835)

InoceramidaeCladoceramus undulatoplicatus (Römer, 1852)

J.F. García-Hidalgo et al. / Cretaceous Research 34 (2012) 268e283270


Table 2Summary of main facies associations and subfacies of the DS-2 sequence (Coniacian) in the Iberian Basin, and their environmental interpretation.

Facies association Subfacies Lithofacies Biofacies and bioturbation Environmental interpretation

Outer carbonateplatform

11 Nodularlimestones

Nodular clayey limestones(mudstone to wackestone).Poorly-bedded. Ferruginoussurfaces.

Gastropods, bivalves, bryozoans,discorbids, miliolids and green algae.Bioturbation common

Low-energy, open-marine settingwith low sedimentation rate. Outercarbonate platform environmentbelow storm wave base

12 Fossiliferousmarls

Massive, grey marls andcalcareous mudstones

Oysters, ammonites, irregular echinoids,isocardiids and other bivalves,gastropods, brachiopods. Locally, oystersand ammonites are bioeroded,ferruginised and colonised by annelids.

Inner carbonateplatform

21 Micriticlimestones

Thin- to medium-bedded limestones(mudstone to wackestone).Grading-upwards to 22 and 23subfacies. Locally, hard grounds at top

Benthic foraminifera, gastropods, bivalves,solitary corals, thin-shelled bivalvefragments. Bioturbation and bioerosionat top of beds is locally common

Open-marine settings ranging fromdistal to proximal carbonate platform.Alternation of low-energy and shoalsettings between storm wave baseand fair-weather wave base.22 Bioclastic

limestonesCoarsening- and thickening-upwardslimestones (floatstones to rudstones);angular to well rounded, andimbricated clasts. Reworkedmicritic intraclasts are common

Oyster fragments in the lower part ofplatform succession and rudist fragmentsin the upper part. Bioeroded shellscommon. Rare bioturbation

23 Ooliticlimestones

Oolitic and bioclastic limestones(packstone-grainstone). Well-sortedintraclasts and oolitised bioclasts.Large through and planarcross-bedding or massive.Rarely quartz extraclasts

Radiolitids, gastropods, echinoderms,bryozoans and red algae fragments,benthic foraminifera. Bioturbationabsent

24 Rudistboundstromes(build-ups)

Rudist biostromes, open anddensely packed, autochthonousand parautochthonous fabricsalternating with floatstones torudstones.

Mono- and paucispecific associationsof radiolitids, biradiolitids, hippuritidsand vacinitids. Poorly-sorted rudistfragments

Lagoon 31 Yellow marls Yellow and ochre marls and nodularclayey limestones. Silt layers.

Radiolitids and hippuritids, isolatedbenthic foraminifera. Rudist and otherbivalve bioclasts. Organic matter-rich.Abundant plants fragments andcarbonaceous remains. Bioturbationcommon

Lagoonal settings and low-energysubtidal ponds in back-barrierramp-coastal environments

Littoral 41 Thin-beddeddolostones

Thin-bedded to fine laminateddolostones, red dolostones and sandydolostones. Ripples and wavy bedding.Fenestral structures. Pseudocolumnar,laterally-linked stromatolites.Ferruginous surfaces.

Moldic porosity. Plant fragments. Rarebioturbation at top of beds

Intertidal to supratidal settings(including swamp and mangroveenvironments) with commonpresence of acids meteoric watersand siliciclastic inputs

42 Dolomitisedbreccias

Nodular to brecciated yellow andbrown dolostones; chicken wire

Intensive bioturbation

43 Green marls Slaty green marls (yellow when arealterated)

Organic matter-rich

Fig. 2. Field view of the upper Turonian 3rd-order sequence below (DS-1) and Coniacian 3rd-order sequence above (DS-2) in the Hoz de Silos section (3 in Fig. 1; Burgos). DS-2 isrepresented by the Hortezuelos Formation showing three clear lithosomes: a, lower calcareous member; b, middle marly and fossiliferous member; c, upper poorly-beddedcalcareous member. *, collapse dolomitic breccias level associated with SB-2 and cited by Floquet (1991) and Gil et al. (2006a, b). Bar for scale is 20 m long.



the underlying sequence (Gil et al., 2009). Thus, the basal onlap anddisplacement of facies belts suggest a major sea-level fall at thesequence boundary followed by a rapid rise during the subsequenttransgressive stage of the next depositional episode (DS-2). Theseobservations suggest a stratigraphic gap at SB-2.

The upper sequence boundary (SB-3) also represents an intervalof interruption of sedimentation with intermittent, minor episodesof subaerial exposure. The presence of the boundary is also shownby a sudden, although less pronounced, change in the vertical faciestrend (first described by Floquet, 1991), and by the presence ofa landward early dolomitization processes, also less intense thanthose related to SB-2.

The Maximum Flooding Surface (MFS) of DS-2 is recognisedwithin the middle marly member of Hortezuelos Formation(Figs. 2e4). Deeper water sediments of this member even reachedareas of the Iberian Basin that were previously coastal environ-ments in underlying sequences. As a consequence, a stratal stackingpattern of two system tracts can be recognised (Fig. 7): (1) ATransgressive Systems Tract (TST) between SB-2 and MFS, showingboth a deepening-upwards and a retrogradational trend of faciesbelts, with a pronounced coastal onlap. This onlap geometry causesthe MFS to be contained within SB-2 southeastwards, therebyincreasing the hiatus contained in that boundary. (2) A HighstandNormal Regression (HNR) (sensu Catuneanu et al., 2009) betweenMFS and SB-3, showing an aggradational and progradational trend.

4. Faunal succession

The lower and middle members of the Hortezuelos Formationare barren of rudists and contain ammonites, Pycnodonte and otheroysters, inoceramids, gastropods and echinoderms. The uppermember of the Hortezuelos Formation contains, however, anassociation of ammonites and rudists along with other bivalves,gastropods and benthic foraminifera. Thus, two different fossilassemblages can be distinguished in DS-2; the first is dominated byammonites and non-rudistid bivalves found in the TST and external(deeper) facies of the HNR; the second is dominated by rudists andcoincides with the internal (shallower) facies of the HNR.

4.1. Ammonoidea

Three different ammonite associations can be distinguishedwithin DS-2. Ammonites in the lower association are scarce andrepresented by ornamented platycones (Tissotioides and Prionocy-cloceras); ammonites in the second association, however, arecommon and characterised by smooth oxycones (Tissotia andHemitissotia).

Tissotioides hispanicus and Prionocycloceras iberiense have beenidentified in the first ammonite association. The former is a small tointermediate-sized, involute species with a suboval or sub-pentagonal compressed section, prominent and bullate umbilical

Fig. 3. Field view of the Coniacian 3rd-order sequence (DS-2) in Castroserracín section (6 in Fig. 1B), showing characteristic members of the Hortezuelos Formation (a, b and c,respectively) and the presence of a yellowish marly lithosome (d) of restricted littoral environments (subfacies 31 in Table 2) at top.

Fig. 4. Field view of the Coniacian 3rd-order sequence (DS-2) in Barranco de las Cuevas section (7 in Fig. 1B), showing the alternation between the upper calcareous member(c) of the Hortezuelos Formation and yellowish marly lithosomes of restricted littoral environments (d). Lower calcareous member (a in Figs. 2, 3) and middle marly member (b inFigs. 2, 3) are absent because of basal onlap.



tubercles, feeble, straight ribs, clavate ventrolateral tubercles, andrelatively simple sutures with narrow, dentate lobes and wide,entire saddles (Fig. 8AeC). Specimens of T. hispanicus were previ-ously described by Wiedmann (1960, 1964, 1979), Wiedmann andKauffman (1978) and Santamaría-Zabala (1991, 1995) and,possibly, by Carretero-Moreno (1982); in the study area, thisspecies has been found in Cuevas de San Clemente section. Prio-nocycloceras iberiense is an intermediate-sized and involute toevolute species, with a characteristic subrectangular compressedsection, rounded umbilical tubercles and simplified sutures(Fig. 8DeF). Specimens of P. iberiense were previously identifiedand figured by Basse (1947), Santamaría-Zabala (1991, 1995) andGallemí et al. (2007); in the study area, this species has beencollected from the Contreras section.

The taxa Tissotia sp., Hemitissotia celtiberica and Hemitissotiaturzoi have been identified in the second ammonite association.Tissotia sp. is an intermediate-sized and very involute taxon,with aninflated suboval compressed section, low tubercles and ribs duringearly ontogeny, and relatively complex sutures, with numerouselements. Among them, first lateral saddles are divided into twoequal intervals with few indentations, and entire saddles remain(Fig. 8GeI). Specimens similar to those identified here in the Cuevasde San Clemente, Contreras and Hortezuelos sections, have beendescribed by Santamaría-Zabala (1991, 1995). H. celtiberica isa discoidal and involute species with a subogival to subovalcompressed section that reaches its maximum width close to themiddle part of the flanks. During juvenile stages it shows an acuteventer and, occasionally, some weak ornamentation that becomesrounded and disappears throughout ontogeny. It has pseudocer-atitic sutures, with about three indented lateral lobes and threerounded saddles per flank (Fig. 8JeL). Specimens of H. celtibericawere previously described and figured by Wiedmann (1975, 1979)and Wiedmann and Kauffman (1978). This species has beenidentified in the Contreras, Hortezuelos, Casuar-Linares andCastrojimeno-Castroserracín sections. Hemitissotia turzoi is verysimilar to H. celtiberica. The only differences are that the former hasa completely smooth surface, a slightly more compressed andinvolute section with a maximumwidth close to the inner third ofthe flanks, an acute adult venter, and about two additional laterallobes and saddles per flank (Fig. 8MeO). Although these two formscould be considered as mere synonyms, on the basis of these slightdifferences (although overlapping in certain cases), themorphologyand their temporal and geographical distribution suggest that it ispreferable to maintain the specific division. In Spain, specimens of

H. turzoi were previously described and illustrated by Karrenberg(1935), Bataller (1950), Wiedmann and Kauffman (1978),Wiedmann (1979), Martínez (1982) and Santamaría-Zabala (1991,1995). The forms Plesiotissotia dullai var. plana and Pleiotissotiacantabria, only established on the basis of small differences in thesuture line, can be considered as synonyms of this species, asindicated by Santamaría-Zabala (1991, 1995). During the researchpresented here, this species was identified in the Cuevas deSan Clemente, Contreras, Hortezuelos, Casuar-Linares andCastrojimeno-Castroserracín sections.

Stratigraphically above these two ammonite associations, thethird association is composed of some poorly preserved specimenswith characteristics close to those of the genera Placenticeras,Eulophoceras and Pseudoschloenbachia. They have been collectedfromtheCuevas de SanClemente, Casuar-Linares andCastrojimeno-Castroserracín sections.

4.2. Hippuritoidea

The second assemblage consists of rudist lithosomes that cropout in the Castrojimeno-Castroserracín and Barranco de las Cuevassections, which developed mainly in the upper part of the HNR ofthe DS-2.

Bournonia gardonica (Fig. 9A) has only been identified inBarranco de las Cuevas section where it forms small, monospecificcluster reefs in fine micritic limestones, intercalated with bioclasticlimestones. The matrix was affected by early dolomitization, butthe outer shell layer remained well preserved, showing bothexternal morphological characters and shell structure. Knowledgeof the shell structure of B. gardonica has been improved based onthe study of these specimens (Gil et al., 2002). The species was firstdescribed and figured by Toucas (1909) as Agria gardonica, from theConiacian of Gatigues (Gard), together with other specimens fromPiolenc (Vaucluse) and Beausset (Var), and reported also fromRochefort (Landes), all in France. It has also been reported fromIstria in Croatia (Parona,1926) and Cilento in southern Italy (Cestariand Pons, 2004) corresponding to the Coniacian event K in Cestariand Sartorio (1995).

Bournonia fascicularis (Fig. 9B) has been identified in Cas-troserracín outcrops. The species was first described from theConiacian of Colle di Medea (Friuli) and reported subsequently frommany other localities in Italy (see Steuber, 2002). Toucas (1909)figured specimens from the Coniacian of Gatigues (Gard), France.

Fig. 5. Field view of the Coniacian 3rd-order sequence (DS-2) in Embalse de Entrepeñas section (8 in Fig. 1B), showing the alternation between green marls (d) and thin-beddedtidal dolostones (e) belonging to Alarcón Formation. Sequence-boundary surfaces (SB-2 and SB-3) are out of the photograph.



Praeradiolites requieni (Fig. 9C) has been identified in the Cas-trojimeno and Castroserracín outcrops. Some specimens are conical(120mmhigh, 70mmwide), but others are remarkably flat (40mmhigh, 80 mm wide), lying reclined on their flat dorsal margin,particularly those from Castroserracín. The species has beenreported from the Coniacian of Gatigues and Bagnols (Gard),Martigues (Bouches du Rhône), Noyères (Vaucluse), Nyons (Drôme)and Le Beausset (Var) in France (Toucas, 1907), and also fromMontsec (Catalonia) in Spain (Pascual et al., 1989).

Biradiolites canaliculatus (Fig. 9D,E) is the typespeciesof thegenusBiradiolites. It was first reported from the Coniacian of Martigues(Bouches du Rhône), Beausset (Var), Gatigues and Bagnols (Gard), allin southeast France, and has been reported sporadically from otherlocalities including the southern Pyrenees, such as Montsec (Pascualet al., 1989). The species has traditionally been described as higherthan wide. The specimens identified in the close cluster/frame reefsof Castrojimeno outcrop are extremely long and thin and developacute ribs (Fig. 9E), similar to those described for the Turonianspecies B. angulosus, while those in the open cluster reefs of theCastrocerracín outcrop, lying on their dorsal or anterior side, areremarkablyflat andexpandedand, in someof them, the characteristicinter-band fold develops downwards below the apex of the rightvalve (Fig. 9D). This extreme intra-specific diversity is probably thecause of some misidentifications in literature.

Radiolites sauvagesi (Fig. 9F) occurs in nearly all bioconstructiontypes at Castrojimeno, and as isolated specimens at the Cas-troserracín outcrops. Specimens represent a wide spectrum of shellgrowth conditions. Radial ribbing ranges from acute and narrow torounded and broad. Spacing of growth lamellae is highly variable.The width of radial sinuses (always up-folds of growth lamellae) isalso variable. Inter-band down-fold is either simple, subdivided, orformed by three folds when radial sinuses are deep and theirmargins well defined. The undivided inter-band fold was consid-ered a primitive feature by Toucas (1908) justifying the erection ofthe species R. praesauvagesi, but it appears to be related to growthconstraints and ecological factors. This species was first describedfrom the Coniacian of Gattigues (Gard) in southeast France and hasbeen frequently reported from deposits of the same age all alongthe Mediterranean Tethys margins.

AHippurites species occurs as aminor component, togetherwithradiolitids, in paucispecific close-cluster and segment reefs or inmonospecific close-cluster/frame reefs at Castrojimeno (Fig. 9G). Ithas been identified asHippurites incisus, whichwasfirst described asa variety of H. resectus from the Coniacian of Espluga de Serra(southern Pyrenees), subsequently identified in other Coniacianlocalities in the Pyrenees (Pons, 1982) and reported later from otherdistant areas. It is considered here to be the Coniacian species ofToucas’ (1904) Hippurites canaliculatus Group; however, it is not

Fig. 7. Dip cross-section of the Coniacian 3rd-order sequence (DS-2) in the Iberian Basin (see Fig. 1 for location), showing the depositional architecture of (1) the facies associations;(2) the faunal distribution; and (3) the system tracts and stratigraphic reference surfaces (sequence boundaries and Maximum Flooding Surface). SB-2, lower sequence boundary;SB-3, upper sequence boundary; TST, Transgressive Systems Tract; MFS, Maximum Flooding Surface; HNR, Highstand Normal Regression.

Fig. 6. Stratigraphic and biosedimentary features of the Coniacian 3rd-order sequence (DS-2). A, lower 3rd-order sequence boundary (SB-2) of DS-2 in Villaverde de MontejoOutcrop (near the Casuar-Linares section), showing a hardground surface and the boundary between the Muñecas Formation (below) and Hortezuelos Formation (above). B, detailof the same hardground surface in Casuar-Linares section (5 in Fig. 1B). C, large cross-bedding surfaces in thick-bedded sets of micritic to bioclastic limestones (subfacies 21 to 23 inTable 2) in the upper half of the HNR near the Contreras section (2 in Fig. 1B). D, densely packed autochthonous fabric of rudist boundstromes (subfacies 24 in Table 2), composed ofspecimens of R. sauvagesi, B. angulosus and scarce H. incisus; Castrojimeno-Castroserracín section (6 in Fig. 1B). E, alternation of micritic limestones (Subfacies 21) with benthicforaminifera and fenestral lamination (above) and bioclastic rudstones (Subfacies 22), composed of fine, well-sorted rudist fragments (above); note the irregular stratigraphicsurface between both beds, pointing to intense bioeroding processes at the top of the previously lithified micritic bed; Barranco de las Cuevas section (7 in Fig. 1B). F, fine planaralgal lamination within a set of thin-bedded yellowish dolostones; Embalse de Entrepeñas section (8 in Fig. 1B). G, detail of nodular to brecciated dolostones cropping out at the topof DS-2 at the southeastwards end of the transect; the nodular and brecciated aspect is a consequence of the intense bioturbation; Embalse de Entrepeñas section (8 in Fig. 1B).



definitively established whether this specific name should bereserved for the specimens with deeply folded growth lamellae,producing acute ribs on the right valve surface, and having pustuleson the left valve, and thosewithout these characteristicsmaintainedas H. resectus, which is a Turonian cosmopolitan species but withyounger representatives that range up to the Maastrichtian. Byaccepting the last alternative, the stratigraphical distribution ofH. resectuswould also reach the Coniacian, co-occurring in this stagewith H. incisus and H. vasseuri; the latter species is currently relatedto the former but has characters that differentiate it. During thecourse of our research the co-occurrence of different combinationsof two of the three species in different Coniacian fossil localitiesfrom southern France and northern Spain has been revealed.

Vaccinites moulinsi (Fig. 9H) occurs in the Castroserracín expo-sures. It was first reported from the Coniacian of Gatigues (Gard).Until now it has only been reported from southern France andnorthern Spain. Some misinterpretation of the tip of the ligamentridge lead Douvillé (1895) to propose a new species,H. praemoulinsi,which was later considered invalid by Toucas (1904).

Vaccinites giganteus (Fig. 9I) occurs in the outcrops of both Cas-trojimeno and Castroserracín. It has been reported previously fromthe Coniacian of Gatigues (Gard), Noyères (Vaucluse), Nyons(Drôme), Martigues (Bouches du Rhône), Le Beausset (Var), andBugarach (Aude) in France (Toucas, 1904), Espluga de Serra andother Pyrenean localities in Catalonia, Spain (Pons, 1982), andmanyother localities in the Adriatic area (see references in Steuber, 2002).

The specimens of Apricardia sp. collected at Castroserracín areisolated individuals in which both valves are very similar in size,coiling, and transverse section; the section is triangular, higherthan wide and carinate ventrally. Inner moulds show the impres-sion of the posterior myophore plate in both valves. Theyundoubtedly correspond to the genus Apricardia but its specificdetermination is doubtful. Late Cretaceous requieniids havereceived less attention than Early (or mid) Cretaceous ones andlittle progress has been achieved since the nineteenth century.

Overall, this rudist assemblage is characteristic of the Coniacianof southern France and north-eastern Spain, although a few specieshave also been reported from other Mediterranean areas.

5. Relationships between stratigraphic patterns, sedimentaryenvironments and biotic associations

During the Coniacian, the Iberian Basin experienced a majortransgressive event, resulting in the inundation of shallow areas,a phenomenon that usually causes changes in facies and bioticassociations on carbonate platforms (e.g. Pomar and Kendall, 2008).This transgression was also coincident with the beginning of themaximum spreading and diversity of rudists in the Late Cretaceous(Cestari and Sartorio, 1995; Pomar and Hallock, 2008). It resulted inthe deposition of the bioclastic-rich limestones of the HortezuelosFormation resting upon the shallower Caballar and Muñecasformations; siliciclastic influx was scarce and mainly derived froma crystalline Hercynian basement (Hesperian Massif), located to thewest of the study area (Fig. 1A). The sedimentary sequence reflectsthose of a carbonate ramp passing distally into a deep pelagic basin.Along this sedimentary sequence, a change in biotic associationsfrom transgressive to highstand deposits within a single deposi-tional sequence (DS-2) occurred.

The transgressive deposits reflect the flooding of the underlyingshallower sediments at the onset of the TST (Fig. 6A). The trans-gressive biotic associations are dominated by bivalves (mainlyPycnodonte and other oysters, with the presence of banks of thesemolluscs being common), and gastropods; ammonites and echino-derms are less common.

The rise in sea-level during transgressive periods leads to theinflux of terrigenous sediments and nutrients (mainly silt, clay andorganic matter) from flooded coastal plains, allowing increasedproductivity and suggesting fluctuating environmental conditions(including possible short term salinity changes; Wilmsem andVoigt, 2006). The broad presence in these sediments of oysterswell-adapted to resist prolonged environmental stress is indicativeof both the transgressive nature of the sedimentation and the newenvironmental niches produced by the incursion of the sea onto thecoast (Bauer et al., 2003; Pufahl and James, 2006). Oysters arepresently marine shallow-water inhabitants, and because they arefilter feeders, they can tolerate a wide range of environmentalconditions, ranging from tidal brackish settings (Stenzel, 1971;Pufahl and James, 2006) to subtidal, lagoonal environments(Mahboubi et al., 2006; El-Azabi and El-Araby, 2007). The substratefor oyster shells needs to be firm because they are sessile benthic.Fossil forms usually appear embedded in predominantly calcareoussediments (Stenzel, 1971; Pufahl and James, 2006) whereas,Pycnodonte, a typical free-lying oyster, obtained stability on a softsubstrate with its large, convex and thickened valve; theirappearance is usually related to marly substrates (Wilmsem andVoigt, 2006).

The tops of the carbonate beds are usually highly bioturbatedand red-stained, suggesting early lithification of substrates. Theselithified sediments were probably unfavourable to the rudists,which are considered to have been partially buried in soft sedi-ments (Cestari and Sartorio, 1995; Cestari and Pons, 2007); they arealmost completely absent from transgressive sediments. Coralswould have been able to grow on these firm substrates, but theirabsence implies that unstable environmental conditions probablyinhibited their growth. In the Pyrenean Trough, however, theywerecommonly associatedwith rudists during the Coniacian (Booler andTucker, 2002).

Regarding the ammonite distribution in the TST, the key indi-cators T. hispanicus and P. iberiense are restricted to the lowermember of Hortezuelos Formation (lower set of DS-2 sequence;Fig. 2) in the northern outcrops (Cuevas de San Clemente andContreras sections (1 and 2 in Fig. 1B). To date, these species havenot been found in southern outcrops, suggesting that their absenceis owing to the onlap at the base of the formation related to the3rd-order sea-level rise of the entire sequence (stratigraphic gap).

Duringmaximum transgression, around the Maximum FloodingSurface (MFS), the basin was completely flooded and marly deep-water sedimentation occurred along the northern and centralparts of the Iberian Basin. During this late TST, the replacement ofTissotioides and Prionocycloceras by Tissotia and Hemitissotia tookplace (Fig. 7); mostly, Tissotia sp. and H. celtiberica below the MFS,followed by H. turzoi. Comparing the morphology of these ammo-nites, a clear change from platycones (Tissotioides and Prionocyclo-ceras), close to the morphogroup 9 of Batt (1989) and Westermann(1996), to oxycones (Tissotia and Hemitissotia), close to the mor-phogroup 11 of these authors, can easily be observed. This gradual

Fig. 8. Ammonite assemblage of the Coniacian 3rd-order sequence in the Iberian Basin. AeC, Tissotioides hispanicus, SC-S-985, Tissotioides hispanicus and Prionocycloceras iberiensezone of Cuevas de San Clemente, in A, ventral, B, lateral and C, apertural views. DeF, Prionocycloceras iberiense, CT-R-950, Tissotioides hispanicus and Prionocycloceras iberiense zone ofContreras, in D, apertural, E, lateral and F, ventral views. GeI, Tissotia sp., CT-R-980, Tissotia sp. zone of Contreras, in G, ventral, H, lateral and I, apertural views. JeL, Hemitissotiaceltiberica, CT-S-954, Hemitissotia spp. zone of Contreras, in J, apertural, K, lateral and L, ventral views. MeO, Hemitissotia turzoi, CJ-S-924, Hemitissotia spp. zone of Castrojimeno, inM, ventral, N, lateral and O, apertural views. Scale bar represents 5 cm. Figured specimens to date are housed in the Coniacian Palaeontological Collection of the Universidad deAlcalá, Spain.



replacement bymorehydrodynamic and less ornamented forms canalso be perceived in the evolution of the more abundant genus(Hemitissotia): its earlier representatives have low, roundedumbilical tubercles and a moderately compressed section, and areprogressively replaced by completely smooth and tightlycompressed later specimens.

Similar morphological transitions have been reported andrelated to changes in the marine environment of the depositionalbasins (deepening-upwards cycles), among others, in the MiddleJurassic of Germany (Bayer and McGhee, 1984), and the UpperCretaceous of the Western Interior of the USA (Jacobs et al., 1994),Germany and Iran (Wilmsem and Mosavinia, 2011).

The morphologic transformations observed in these ammonitefaunas appear to be hydrodynamic adaptive responses to sea-levelchanges (ecophenotypic variations), because in terms of hydrody-namics, a compressed, smooth form usually has a lower dragcoefficient and faster, more efficient locomotion than a robust one.Consequently, platycones (more abundant in the TST) reflectcomparatively shallower near-shore (proximal) environments,with higher energy, whereas oxycones (more numerous in theMFS) correspond to relatively deeper open marine (distal) waters(Wilmsem and Mosavinia, 2011), being well-adapted to a nekto-benthic lifestyle (Chamberlain, 1980).

Two lines of evidence suggest that Hemitissotia oxycones wererelated to these open marine and lower energy environments.Firstly, they are present in deeper, quiet-water marly and nodularlimestone facies (subfacies 11 and 12), with inoceramids (Gallemíet al., 2007) and echinoderms rather than in high energy facies.Secondly, in many of the intervals studied, Hemitissotia is the onlyammonoid present, occurring as scattered, isolated specimens inbenthos-poor strata. The fact that Hemitissotia so commonly occursin strata devoid of both benthic fauna and other ammonoidssuggests a pelagic habit in the upper levels of the water column(as proposed for other oxycone ammonites by Tsujita andWestermann, 1998), where water oxygenation was probablyhigher and more or less constant. Both pieces of evidence supportthe idea that most oxyconic ammonoids were well-adapted toa predatory lifestyle (Westermann, 1996), mainly by short chaseanywhere in the water column (Tsujita and Westermann, 1998). Infact, the incursions of predatory oxycones, such as Hemitissotia, to

very shallow and coastal waters have been reported previously(Hewitt and Westermann, 1989; Kauffman, 1990) The presence ofoxycones in these near-coast environments could be increased byonshore transport of shells as a result of post-mortem drift(Wilmsem and Mosavinia, 2011).

During the early HNR, platform facies and their biotic commu-nities started to recover, but they were different from those of theTST. In this case a complete transect from deeper marls to tidalfacies can be observed (Fig. 10).

Deeper environments are similar to those of the MFS, beingcharacterised by marls (subfacies 12) and nodular limestones(subfacies 11); inoceramids and ammonites were common in thisfacies in northward sections (Fig. 10; Gallemí et al., 2007). The innerramp facies are characterised by skeletal packstones very rich inrudist fragments and oolitic grainstones (subfacies 22 and 23). Theyare interpreted as high-energy facies, reflecting the development ofsandy (bioclastic or oolitic) shoal sediments (Fig. 10). No rudistbuild-ups or shells have been observed in these facies exceptlocally; all fragments are worn and rounded, indicating abrasionand transport before deposition. Thus, this bioclastic fraction wasswept off to the external platform areas, where the skeletal debriscommonly accumulated forming the bulk of the skeletal compo-nent. The basal beds of the early HNR contain the smallest amountsof rudists; however, as long as low-energy, unconsolidatedsubstrates developed, these substrates allowed rudist communitiesto recover and substitute previous biotic associations.

Large cross-bedding (up to 10m thick) has been observed locally(Fig. 6C), but this is not indicative of the flanks of steep-sided, high-relief build-ups, because no evidence of reef-flat to reef-slopesediments has been found. These beds were thus deposited fromshoals with a relatively gentle relief (Fig. 10).

Although rudists could have been spread over all inner shelfsectors, they have only been preserved in growth position overdifferent substrata on the leeward sides of the shoals and in lagoonalareas (Fig. 10). Scarce, poorly preserved ammonites (possiblyPlacenticeras, Eulophoceras, Pseudoschloenbachia) have also beenfound,which unlike openmarineHemitissotia, havemoredepressedshells, probably reflecting shallower but quiet environments.

Rudists grew in soft sediments giving rise to laterally limited andscattered rudist-rich lithosomes (see below), which were of low

Fig. 10. Sedimentological model for the HNR of the DS-2 sequence in the Iberian Basin (for explanation of subfacies, see Table 2).

Fig. 9. Rudists of the Coniacian 3rd-order sequence in the Iberian Basin. A, Bournonia gardonica, transverse section of right valve, PUAB-43933, Barranco de las Cuevas. B, Bournoniafascicularis, ventral posterior view of right valve, PUAB-75899, Castroserracín. C, Praeradiolites requieni, ventral posterior view of both valves, PUAB-74442, Castroserracín. D, E,Biradiolites canaliculatus. D, ventral posterior view of both valves, PUAB-74455, Castroserracín. E, transverse section of right valve, PUAB-43735, Castrojimeno. F, Radiolites sauvagesi,ventral views of two right valves from the same bouquet, PUAB-43748, Castrojimeno. G, Hippurites incisus, transverse section of several right valves from a thicket, PUAB-43745,Castrojimeno. H, Vaccinites moulinsi, transverse section of right valve, PUAB-74426, Castroserracín. I, Vaccinites giganteus, transverse section of right valve, PUAB-74419,Castroserracín. J, Apricardia sp., inner mould of both valves, posterior view, PUAB-74463, Castroserracín. PB, posterior radial band; PML, posterior myophore lamina; VB, ventralradial band. Scale bar represents 10 mm. Figured specimens are housed in the Palaeontological Collections of the Universitat Autònoma de Barcelona (PUAB).



relief on the surrounding sea bed and limited to the last generationof specimens (Fig.10). Storm-andwind-induced currents andwavesrepeatedly mobilised the rudist-supporting loose sediments; asa consequence, a well-sorted and rounded bioclastic fraction wasreworked to the external platform areas; meanwhile toppled shellswere very abundant landwards. Only in deposits reflecting themoreinternal and protected shelf sectors can the growth-relatedarrangement of the rudist shells be easily observed.

Inner platform areas (Casuar-Linares and Castrojimeno-Castroserracín sections; Fig. 7; 5 and 6 in Fig. 1B) are characterisedby an alternation of mudstones-wackestones with a highly diversebenthic fauna (solitary corals, chaetetids, echinoderms, brachio-pods and benthic foraminifera), alternating within rudist elevatorbiostromes (Fig. 10). These biostromes show both open and denselypacked autochthonous fabrics (Fig. 6D), parautochthonous fabrics,and bioclastic levels of reworked rudist fragments with floatstoneto rudstone textures (Gil et al., 2002, 2009). These fabrics andtextures correspond to matrix-supported cluster and segmentreefs, and even skeleton-supported frame reefs, according to thestructural categories of organic reefs of Riding (2002).

In the innermost platform areas (Barranco de las Cuevas section,Fig. 7; 7 in Fig. 1B), rudists and benthic foraminifera inhabited low tomedium energy muddy bottoms, where the rudists formed limitedbuild-ups in restricted lagoon areas (Fig. 10). These shallow waterlimestones rich in rudists contain a preponderance of skeletalcomponents (molluscs and benthic foraminifera) and lack non-skeletal grains. The most prominent sediments are rudist-dominatedfine- to coarse-grained rudstones (Fig. 6E). The sediments weregenerated in situ on protected areas where rudists were the primarysediment producers. These sediments were subsequently moved bystorms, waves and currents. The finer fractions were probablywinnowed out and deposited in deeper waters.

Finally, an alternation of green marls with plant debris, massiveor stromatolitic dolostones (Fig. 6F), well-bedded dolomitisedmudstones/wackestones locally with benthic foraminifera, anddolomitised breccias (Fig. 6G), characterise the more landwardsoutcrops (Embalse de Entrepeñas and Estrecho de Paredes sections,Fig. 7; 8 and 9 in Fig. 1B), where they rhythmically alternate andtestify to deposition in tidal-flat environments (Fig. 10).

DS-2 contains several ammonite and rudist assemblages. Thefirst assemblage is dominated by Middle Coniacian ammonites(Tissotioides hispanicus and Prionocycloceras iberiense); the second,characteristic of the Upper Coniacian, is mainly composed ofTissotia sp., Hemitissotia celtiberica and H. turzoi. On the other hand,the DS-2 sequence can be clearly correlated with (1) UC9/10 andUC10/11 sequences of Gräfe (1994) and Gräfe and Wiedmann(1998); (2) DS Co II and DS Co III of Wiese and Wilmsen (1999),and (3) DC8 depositional cycle of Floquet (1998), suggesting that itsbase is probably upper Lower Coniacian in the northern outcropswhere a hiatus at the base of DS-2 existed, as shown by the pres-ence of collapse breccias (Fig. 2) and hardground development(Fig. 6A, B). This hiatus in deposition was longer in the southernoutcrops, where sediments of the entire TST are missing (Fig. 7).

A third assemblage, which can be identified in the southernsections, is dominated by rudists that are usually attributed to theUpper Coniacian (Biradiolites, Praeradiolites, Radiolites, Apricardia,Hippurites, Vaccinites). The upper part of DS-2 has a poorlypreserved ammonite assemblage, some of whose representatives(Placenticeras, Eulophoceras, Pseudoschloenbachia) are commonlyattributed to the Santonian, coexisting with Coniacian rudists,which raises a problem concerning the precise age of the top of thissequence. On this point, it should be mentioned here that severalpossible representatives of this upper assemblage have beenlocated below the FAD (first appearance datum) of the inoceramidCladoceramus undulatoplicatus (index species for the base of the

Santonian) in Riu de Carreu and Prat de Carreu (Gallemí et al.,2004), and Villamartín (Gallemí et al., 2007) sections in northernSpain. This low stratigraphic position for these genera could not beconfirmed by the present work, because C. undulatoplicatus has notbeen identified in the sections studies. Nevertheless, this researchseems to confirm the presence of some ammonite generacommonly attributed to the Santonian in lithosomes strati-graphically below or coincident with those containing the charac-teristic Coniacian rudist assemblage.

The stratigraphic and sedimentologic analysis shows that theentire sequence DS-2 in the Iberian Basin was deposited ona carbonate ramp (Fig. 10). The palaeogeographic reconstructions ofthe basin during this interval suggest that sedimentation took placein tropical waters (Fig. 1A), a typical environment for photozoanassemblages. Biotic assemblages, however, show the presence ofa mollusc-dominated diverse association during the TST, but a low-diversity association of rudists, which were heterotrophic suspen-sion feeders (Scott, 1995), during the HNR. These can be consideredas heterozoan skeletal assemblages, which tend to be related tocooler waters of temperate to polar latitudes (Lees and Buller, 1972;James, 1997). The upper Coniacian heterozoan assemblages cannot,however, be strictly interpreted as foramol facies (sensu Lees andBuller, 1972) because firstly, organisms such as rudists, solitarycorals, gastropods, chaetetids and benthic foraminifera are gener-ally considered to have flourished in warm seawater environmentsand secondly, cool-water (pectinid bivalves, among others) andinfaunal molluscs were extremely scarce and even totally absentwithin these carbonates. These data altogether with the absence ofevaporites suggest that changes in temperature (or too hightemperatures) and salinity can be excluded as major factorscontrolling sedimentation; the presence, however, of monospecificoyster assemblages indicates that sedimentation in lagoonal areaswas punctuated by salinity crises (Wilmsem and Voigt, 2006).

Similar heterozoan assemblages inwarm-water conditions havebeen recognised in several Coniacian carbonate platforms of theTethys (Simone et al., 2003; Philip and Gari, 2005); meanwhile, inother Tethyan areas, rudists occurred in close association withcorals (Pyrenees, Booler and Tucker, 2002; Alps, Sanders and Pons,1999). Some of these occurrences were interpreted to imply thatthe sediments formed under deeper, darker, and more eutrophicconditions than typical, present-day tropical, coral-bearingcarbonates (Philip and Gari, 2005). The close association withcorals, however, might be interpreted as reflecting deposition inlighter, shallower environments, although Cretaceous corals prob-ably flourished in deeper water than their modern equivalents(Pomar et al., 2005). This supports the former interpretation, butadds a further complication to the interpretation of carbonatesdominated by this skeletal association.

An alternative interpretation, which is preferred here, points tothe existence of a strong sedimentary and trophic control on thebiotic assemblages, as suggested for other Coniacian platforms(Carannante et al., 1995). Nutrients, high hydrodynamic gradients,mobile substrates, and the presence of siliciclastics were probablythe factors that controlled the development of these biotic associ-ations in the Iberian Basin. Rudists in life position occupied a narrowfringemainly located on the seaward side of lagoonal areas. Facies inseaward areas of this fringe were composed of high-energy bars;although rudists were able to occupy mobile substrates (Pomaret al., 2005), they have not been found in life position in these facies.

Thehigh ratesof erosion related to theseenvironmentswithhighlymobile substrates suggests theexistence of large amountsof nutrientsin suspension, allowing the predominance of nutrient-tolerant,suspension feeder groups of calcareous organisms, such as rudists.The presence of high-nutrient levels might not be the cause of rudistdevelopment, but a consequence of the high energy environment. In



quieter, shallower, landward areas rudists are absent (Embalse deEntrepeñas andEstrechodeParedes sections; Fig. 7; 8 and9 in Fig.1B);probably, because of both the absence of abundant nutrients insuspension and higher siliciclastic input. In fact nutrients may havebeen abundant, but the presence of siliciclastics would have stronglydiluted (and thus reduced) the nutritional values of the suspendedparticulate matter (Witbaard et al., 2001); thus, today in suchcircumstances nearshore communities may be impoverished and/orcharacterised by the presence of smaller individuals. As on otherplatforms, the relationships between rudists and siliciclastics suggestthat moderate siliciclastic influxes controlled neither the presenceand absence of rudists, nor the composition of rudist associations(Sanders and Pons, 1999). However, substrate colonization by rudistswas probably more difficult, or even impossible in the shallowestareas with low nutrients and higher siliciclastic input (landwards),and in outer platform areas under frequent shifting of the bioclasticsubstrates.

6. Conclusions

The Coniacian sequence DS-2 in the Iberian Basin representsdeposition on a low angle, carbonate ramp-like, open platform. Thebiofacies is mainly dominated by a nekto-benthic (such as ammo-nites) and benthic (such as bivalves, mainly rudists) organismswithscarce solitary corals (hermatypics are absent), with major differ-ences apparent between the Transgressive Systems Tract (TST) andHighstand Normal Regression (HNR) tract.

In the TST, the molluscs are dominated by Pycnodonte, otheroysters, ammonoids and other molluscs (with only subordinaterudists). Initially ammonites were scarce and represented byornamented platycones (Tissotioides and Prionocycloceras),becoming more abundant and characterised by smooth oxycones(Tissotia and Hemitissotia) in the later part of the TST.

In the HNR, shallow-water depositional areas were occupied bya rudist-dominated association. Erosive processes (storm- andwind-induced currents and waves) acting on this associationproduced large amounts of loose, bioclastic fragments. These bio-clastic sediments covered external platform areas, characterised byopen circulation, and formed large, coalescing beds of winnowedskeletal sands, suggesting the presence of significant offshoretransport of these carbonatematerials. Thisouter, high-energy faciesbelt passed landwards into inner ramp sediments, which accumu-lated in protected, lower-energy settings. Rudist biostromes devel-oped in seawards areas of these protected shallow environments ofoverall moderate to low hydrodynamic gradient, which was punc-tuated by storms. In this environment and landwards, large areas ofmarly substrate persisted (and became re-established after inter-mittent smothering of rudists), which favoured the development ofother bivalves, gastropods, echinoderms, benthic foraminifera andsolitary corals. Because of the higher input of siliciclastic and,probably, the lack or dilution of nutrients in suspension by silici-clastics, the establishment of rudist communities was difficult inmore landwards areas of the lagoon and in tidal environments.

This heterozoan carbonate factory was thus controlled bywarm-water conditions and high energy levels, which wereresponsible for high-nutrient levels in suspension.

The reconstruction of the depositional architecture of a 3rd-ordersequence from platform to coastal areas allows us to recognise notonly the spatial-temporal distribution of the facies belts (therebydefining systems tract distribution in detail) but also to examine therelationships between the standard zones of different biomarkers inplatform environments, whose different boundaries are not alwayscoincident (i.e., outer shelf ammonites vs. inner shelf rudists)between them and/or with the internationally recognised markers(i.e., C. undulatoplicatus for the base of the Santonian).

Acknowledgements

This study has been carried out within the projects CGL2007-60054, CGL2008-03112/BTE, and CGL2009-12008/BTE of theDirección General de Investigación y Gestión del Plan NacionalI+D+i Spanish Ministerio de Ciencia e Innovación, and PEII11-0237-7926 of the Junta de Comunidades de Castilla-LaMancha, Spain.Wegratefully acknowledge Markus Wilmsen, Christopher J. Wood andDavid J. Batten for their valuable comments, constructive reviewand assistance in revising the English text, which helped to improvethe initial version of the manuscript.

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