Depositional sequences and ammonoid assemblages in the upper Cenomanian-lower Santonian of the Iberian Peninsula ( Spain and Portugal )

A clear relationship exists between eustatic sea-level rises and falls recorded as cyclical depositional sequences and ammonite faunas during the Cenomanian-Santonian in the Iberian and West Portuguese basins. Most of the faunal turnovers correlate with stratigraphic intervals related to marine transgressions, maximum flooding of the shelf (locally associated to anoxic events), and marine regressions. Specifically, within each depositional sequence, three distinct and identical events of morphological change occur, involving ammonoids belonging to different groups. Transgressive sediments are characterized by moderately ornamented, inflated and evolute morphologies, which are replaced by smooth, involute and compressed oxycones (the most hydrodynamic shells) during maximum flooding (and to a lesser extent at the early highstand) of the sequences. The latter morphologies in turn are replaced by coarsely ornamented and evolute shells during late highstands. We conclude that ammonoid faunal analysis can be used to trace sea-level changes and provides an additional tool for sequence stratigraphy. Depositional sequences. Ammonoid assemblages. Upper Cenomanian-lower Santonian. Spain. Portugal. KEYWORDS


INTRODUCTION
During the Late Cretaceous, the Iberian Peninsula was a relatively independent tectonic block.Its privileged palaeogeographical location favoured the arrival of boreal faunas from the Protoatlantic and temperate-warm faunas from the Tethys to its epicontinental flooded regions, such as the Iberian and the West Portuguese basins.As the arrangement of these regions enabled the development of rather confined marine environments, endemic species arose in its waters as well.The combined incidence of eustatic changes and local tectonics generated several depositional sequences with characteristic faunal assemblages (particularly well-recorded during major sea-level rises) from the upper Cenomanian to the lower Santonian in the Iberian and the West Portuguese basins.
These sequences and their ammonoid assemblages, after a first approximation limited to the upper Cenomanian to lower Turonian interval (Barroso-Barcenilla et al., 2011a), have been related and interpreted in detail for the first time in this paper, considering the narrow relationship that can be established in the epicontinental platforms between palaeoenvironmental changes (e.g.sea-level oscillations, anoxic events) and palaeontological successions (Fernández-López, 1999;Hirano et al., 2000).

GEOGRAPHICAL AND GEOLOGICAL SETTING
The Iberian Peninsula contains Upper Cretaceous sedimentary sequences, mainly composed of marls and limestones with several terrigenous and dolomitic intervals.These sequences yield fossils of the Iberian and West Portuguese basins (Fig. 1A-C).The Iberian Basin was a relatively stable intracratonic platform and comprised certain parts of the northern, central and south-eastern regions the Iberian Peninsula that were temporally or permanently flooded by the Protoatlantic ocean, the Tethys sea or both (Fig. 1B-C).During the late Cenomanian-early Santonian the Iberian basin had high subsidence and sedimentation rates, mainly controlled by eustatic changes (Floquet et al., 1982;Segura et al., 2001Segura et al., , 2002;;Barroso-Barcenilla et al., 2011a).The sedimentary and palaeontological successions of the northern part of the basin (southern Cantabrian Range), show a nearly continuous record in marly materials of relatively deep and open inner platform with ammonites, inoceramids and other benthic, non-rudist bivalves.In the southern part of the Iberian Basin (northern Central System and western Iberian Range) nodular carbonates (limestones and dolostones) of relatively shallow and restricted inner platform and siliciclastic rocks corresponding to a narrow coastal belt predominate (Gil et al., 2006;García-Hidalgo et al., 2007) (Fig. 1C).The nodular carbonates contain thinner and less abundant levels with ammonites, rudists (mainly Coniacian) and other molluscs, whereas the siliciclastic rocks have scarce palaeontological content.
The West Portuguese Basin included the western-central regions of the Iberian Peninsula temporally or permanently flooded by the Protoatlantic (Fig. 1B-C).It was part of an active continental margin controlled by reactivated Late Hercynian faults and halokinetic structures with Triassic-Lower Jurassic evaporites.Nevertheless, as in the Iberian Basin, the main depositional episodes and faunal assemblages were related to global sea-level changes, but the sedimentary infill was less continuous and influenced by local tectonics and continental influx episodes (Ferreira Soares, 1980;Callapez, 1998Callapez, , 2008;;Rey et al., 2006;Dinis et al., 2008;Barroso-Barcenilla et al., 2011a).During the Cenomanian a carbonate platform with different domains developed.A nodular carbonate shelf with ammonites, and a related micaceous littoral plain occurred at the northern part of the West Portuguese Basin (Beira Littoral Range).The southern part of the basin (Estremadura Range) was occupied by a rimmed carbonate platform with coral and rudist fringes, and by a lagoonal system with other molluscs and echinoids.After a hiatus related to regional uplift, at the middle lower Turonian, carbonate sedimentation was temporarily re-established on the northern part of the basin.The remaining record was alluvial and littoral, but punctuated by a single and short eustatic episode with middle upper Coniacian ammonites (Fig. 1C).

DEPOSITIONAL SEQUENCES
Stratigraphic studies have allowed us to recognize the main depositional episodes and to reconstruct the architecture of the 2 nd -and 3 rd -order sequences in the upper Cenomanian-lower Santonian of the Iberian and West Portuguese basins, recognizing and correlating the larger and most extensive sea-level oscillations, and evaluating the depositional hiatuses towards the coastal margins.
In the West Portuguese Basin, UZA-2 megasequence is also recorded by carbonates with ammonites, but the succession is incomplete due to local tectonics.UZA-2.4 is well recognized on the entire carbonate platform bearing a cephalopod assemblage with Neolobites-Angulithes.UZA-2.5 starts with an uppermost Cenomanian interval with Vascoceras-Spathites (Jeanrogericeras), but it was truncated due to regional tectonic uplift.In its upper part a middle lower Turonian ammonite assemblage with Choffaticeras (Leoniceras) is locally recognized.Finally, UZA-2.6+2.7 has only been recorded in the northern part of the basin with littoral to alluvial micaceous sandstones.Concerning the UZA-3 megasequence, the lack of carbonates with marine faunas hinders an accurate stratigraphic control, except for a middle upper Coniacian event with Hemitissotia.Nevertheless, the UZA-3.1/UZA-3.2boundary can be located into a discontinuity related with the transition to coarser siliciclastic sediments (Callapez, 1998) (Fig. 2).

AMMONOID ASSEMBLAGES
Detailed ammonite zones and assemblages of both boreal and temperate-warm affinities have been established for the upper Cenomanian-lower Santonian of the Iberian and West Portuguese basins.The ammonite zones and assemblages have been compared and correlated with those identified in South-Western Europe, North Africa and the Western Interior of USA.Systematic and biostratigraphic data provided by the specimens collected by the authors of this paper (>2500) and other researchers in the region (e.g.Karrenberg, 1935;Wiedmann, 1960;Mojica and Wiedmann, 1977;Wiedmann and Kauffman, 1978;Santamaría-Zabala, 1992;Callapez, 2003;Callapez and Ferreira Soares, 2001;Gallemí et al., 2007;Barroso-Barcenilla et al., 2009, 2011a, 2013) have been analyzed.The main part of these cephalopods do not present signs of taphonomic resedimentation or reelaboration (sensu Fernández-López, 2000), and the few that show any of these signs do not seem to have suffered notable alterations.Therefore, it has been considered that all of them maintain their respective original stratigraphic positions (Callapez, 1998;Barroso-Barcenilla, 2006;Barroso-Barcenilla et al., 2011a).
The stratal architecture and the palaeontological successions of the above mentioned 3 rd -order depositional sequences show that their Maximum Flooding Surfaces (mfs) are coincident in the Iberian and West Portuguese basins.Though the stratigraphic record in the latter basin is somewhat incomplete due to local tectonics, both show four especially rich and widespread ammonitiferous intervals.These acme-type intervals are laterally continuous in every sequence all along the main part of both basins, and successively contain numerous specimens of Neolobites vibrayeanus-Angulithes mermeti (UZA-2.4mfs), Choffaticeras (Choffaticeras) quaasi-pavillieri (UZA-2.5 mfs), Coilopoceras requienianum (UZA-3.1 mfs), and Hemitissotia ceadouroensis/celtiberica-turzoi (UZA-3.2mfs) (Figs.2; 3).

The first interval (N. vibrayeanus-A. mermeti)
coincides with the mfs (lower upper Cenomanian) of the UZA-2.4,providing specimens of these cephalopods both in the northern and central parts of the Iberian Basin (IB).It can be also recognized in the West Portuguese Basin (WPB), where the assemblage of N. vibrayeanus-A.mermeti is abundant as well.The second interval (Ch.(Ch.) quaasi-pavillieri) corresponds to the mfs (lowermost Turonian) of the UZA-2.5, containing ammonites even in southern areas of the IB (up to the Entrepeñas-Cuenca Section).In the WPB this interval coincides with the local
The morphology of these ammonoids (Fig. 3) corresponds to smooth and compressed oxycones (morphogroup 11 sensu Batt, 1989;Westermann, 1996) and, therefore, to nektonic forms, well adapted to active swimming.Then, the relationship of oxycones with mfs seems to be an adaptive response to sea-level changes (maximum depths in the basin).Hydrodynamically, a smooth and compressed form usually has a lower drag coefficient and higher swimming velocity, developing more efficient locomotion than an ornamented and depressed one (Chamberlain, 1980;Westermann, 1996).This morphological relationship can be established both in intracratonic (Iberian Basin) and active marginal (West Portuguese Basin) areas and has been suggested in other basins (e.g.Middle Jurassic of Germany: Bayer and McGhee, 1984; Upper Cretaceous of USA: Jacobs et al., 1994), relating sea-level changes with ammonoid morphologies (ecophenotypic variations : Diedrich, 2000;Wilmsem and Mosavinia, 2011) and turnovers (O'Dogherty et al., 2000;Sandoval et al., 2001Sandoval et al., , 2002;;Yacobucci, 2008).
The UZA-2.5 and UZA-3.2 are the most extensive sequences, representing the transgressive/regressive transition of two consecutive megasequences (2 nd -order).They contain, by coincidence with their respective mfs, the megasequence mfs, and show three other important particularities.Firstly, they have well-developed Transgressive Systems Tracts (TST) with abundant ammonites, containing numerous specimens of Vascoceras gamai-Spathites (Jeanrogericeras) subconciliatus (uppermost Cenomanian) in the UZA-2.5, and of Tissotiodes hispanicus-Prionocycloceras iberiense-Protexanites bourgeoisi (lower upper Coniacian) in the UZA-3.2 (Fig. 3).The presence of abundant morphologically less hydrodynamic ammonites with moderately ornamented and evolute discocones and platycones (close to morphogroups 6, 9 sensu Batt, 1989;Westermann, 1996) in the TST is interpreted here as related to sea-levels markedly lower than those of the maximum flooding stages, even during the superimposition of the highstand portions of the highfrequency cycles (4th-order).Secondly, they present, close to the mfs of these two sequences, the dark levels corresponding to the Oceanic Anoxic Event 2 (OAE-2) of the Cenomanian/Turonian transition and to the less known and more controversial Oceanic Anoxic Event 3 (OAE-3) of the Coniacian/Santonian transition (Schlanger and Jenkyns, 1976;Jenkyns, 1980;Schlanger et al., 1987;Arthur et al., 1988Arthur et al., , 1990;;Jenkyns et al., 1994) (Fig. 2).Both events are characterized by the hypoxic character of the oceanic waters and the reduced abundance and diversity of their macrofaunas (Sepkoski, 1986;Barroso-Barcenilla et al., 2011b).Thirdly, they also have well-developed and complex Highstand Systems Tracts (HST) with abundant ammonites, which can be divided in two intervals: early and late.The early HST contains numerous specimens of Choffaticeras (Leoniceras) luciae-barjonai (middle lower Turonian) in the UZA-2.5, and of Hemitissotia dullailenticeratiformis (uppermost Coniacian) in the UZA-3.2 (Fig. 3), coinciding to oxyconic, involute and moderately compressed species with hydrodynamically efficient shells (morphogroup 11 sensu Batt, 1989;Westermann, 1996), and making the characterization/differentiation of mfs and early HST difficult on the exclusive basis of the cephalopod morphologies.The late HST has less abundant ammonites, mostly representative of Mammites nodosoides (upper lower Turonian) in the UZA-2.5, and of Texanites hispanicus (lowermost Santonian) in the UZA-3.2 (Fig. 3).These ammonites correspond to coarsely ornamented planorbicones and similar (close to morphogroup 1 sensu Batt, 1989;Westermann, 1996) with  Batt, 1989;Westermann, 1996.All the figured specimens are held in the Departamento de Paleontología of the Universidad Complutense de Madrid, and taxonomical discussions on most of them were given by Barroso-Barcenilla (2006) and García-Hidalgo et al. (2012).
very low hydrodynamism.The presence of progressively less hydrodynamic ammonites in the HST is also interpreted here as related to sea-levels lower than those of the maximum flooding stages, as a result of the loss of accommodation, even during the superimposition of the highstand portions of the high-frequency cycles (4 th -order).
Among those ammonoids characterizing even lower sea-level intervals, coarsely ornamented ammonites with very low hydrodynamic efficiency predominate.A good example of this can be observed in the reduced sea-level interval of the middle Turonian (UZA-2.6+2.7).This interval is characterized by the progradation of shallower inner platform and coastal margin facies with thin levels containing bivalves and scarce heavily ornamented ammonites (e.g.Romaniceras, Fig. 3) in the northern part of the Iberian Basin (Wiedmann, 1960(Wiedmann, , 1979;;Wiedmann and Kauffman, 1978;Santamaría-Zabala, 1995;Küchler, 1998).

CONCLUSIONS
In the upper Cenomanian-lower Santonian of the Iberian and West Portuguese basins (although with incomplete record in the latter), four mfs corresponding to 3 rd -order sequences with abundant cephalopods (UZA-2.4 with N. vibrayeanus-A.mermeti in the lower upper Cenomanian; UZA-2.5 with Ch. (Ch.) quaasipavillieri in the lowermost Turonian; UZA-3.1 with C. requienianum in the lower upper Turonian; UZA-3.2 with H. ceadouroensis/celtiberica-turzoi in the middle upper Coniacian) can be identified.The morphology of these ammonites (well adapted active swimmers with smooth and compressed oxycones) is explained in this paper by their close relationship with the deeper facies of every studied sequence, corresponding to their maximum flooding surfaces.This trend is observable in the early HST as well (UZA-2.5 with Ch. (L.) luciae-barjonai, and UZA-3.2 with H. dullai-lenticeratiformis), since they also have deep facies.This relationship between morphologies and facies can additionally be observed in other basins of Western Tethys.Other system tracts representing different portions of the sea-level curve of the sequence (particularly the TST and late HST) contain abundant ammonites with less hydrodynamic morphologies.They are the TST of UZA-2.5 with V. gamai-S.(J.) subconciliatus, and of UZA-3.2 with T. hispanicus-P.iberiense-P.bourgeoisi; and the late HST of UZA-2.5 with M. nodosoides and of UZA-3.2 with T. hispanicus.These results clearly suggest that the presence or absence of ammonoids in these basins, their morphologies and, therefore, their evolutionary trends are mainly influenced by eustatic variations, and demonstrate the interest and utility of integrated studies on depositional sequences and faunal assemblages for basinal analyses and correlations.

FIGURE 1 .
FIGURE 1. A) Geographic location of the Spanish and Portuguese studied areas (red polygons) in South-Western Europe.B-C) Palaeogeographic situation of the Iberian Peninsula during the maximum transgression of the late Cenomanian-early Turonian, with B) the approximate locations of the studied basins, and C) the main composite-sections and facies distribution.Figure 1B-C is modified fromPhilip and Floquet (2000),Stampfli et al. (2001) andGelabert et al. (2002).

FIGURE 2 .
FIGURE 2. Dip cross-section of the upper Cenomanian-lower Santonian 3 rd -order depositional sequences (UZA-2.4 to UZA-3.2) in the Iberian and West Portuguese basins, showing the depositional architecture and the main ammonoid assemblages relative to the reference surfaces (Maximum Flooding Surfaces and sequence boundaries) and the systems tracts (Transgressive Systems Tracts and Highstand Systems Tracts).Ceno.: Cenomanian, S.: Santonian, L.: Lower, M.: Middle, U.: Upper.Approximate locations of the represented composite-sections can be observed in Figure 1C.

FIGURE 3 .
FIGURE 3. Sea-level cycles and characteristics of their main cephalopods, with lateral and ventral or dorsal views of some representative taxa and remarks on their characteristics.The width of the blue rhombuses represents the relative sea-level changes.TST: Transgressive Systems Tracts, mfs: Maximum Flooding Surfaces, EHST: Early Highstand Systems Tracts, LHST: Late Highstand Systems Tracts.Morphogroups sensuBatt, 1989;Westermann, 1996.All the figured specimens are held in the Departamento de Paleontología of the Universidad Complutense de Madrid, and taxonomical discussions on most of them were given byBarroso-Barcenilla (2006) andGarcía-Hidalgo et al. (2012).