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Cretaceous Research 52 (2015) 292e312

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Cretaceous Research

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Palynology of Aptian and upper Albian (Lower Cretaceous)amber-bearing outcrops of the southern margin of theBasque-Cantabrian basin (northern Spain)

Eduardo Barr�on a, *, Daniel Peyrot b, Juan Pedro Rodríguez-L�opez c, d, Nieves Mel�endez c,Rafael L�opez del Valle e, María Najarro a, Idoia Rosales a, Mª Jos�e Comas-Rengifo f

a Instituto Geol�ogico y Minero de Espa~na (IGME), Ríos Rosas 23, E-28003 Madrid, Spainb Robertson (UK) Ltd., Tyn-y-Coed, Pentywyn Road, Llandudno, Gwynedd LL30 1SA, UKc Departamento de Estratigrafía, Facultad de Ciencias Geol�ogicas, Universidad Complutense de Madrid, Jos�e Antonio Novais 2, E-28040 Madrid, Spaind Saudi Arabian Oil Company, Dhahran, Saudi Arabiae Museo de Ciencias Naturales de �Alava, Siervas de Jesús 24, E-01001 Vitoria-Gasteiz, Spainf Departamento de Paleontología, Facultad de Ciencias Geol�ogicas, Universidad Complutense de Madrid, Jos�e Antonio Novais 2, E-28040 Madrid, Spain

a r t i c l e i n f o

Article history:Received 11 July 2014Accepted in revised form 2 October 2014Available online

Keywords:PalynologyAmberAptianUpper AlbianBasque-Cantabrian basinSpain

* Corresponding author. Tel.: þ34 913495883.E-mail address: e.barron@igme.es (E. Barr�on).

http://dx.doi.org/10.1016/j.cretres.2014.10.0030195-6671/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The Lower Cretaceous deposits of the southern margin of the Basque-Cantabrian basin (northern Spain)are characterised by continental deposits interbedded with amber-bearing marine-influenced facies.These facies crop out in various localities and have yielded well-preserved palynological assemblages.The palynoflora is dominated by gymnosperm pollen grains, and shows relatively diversified sporecontent but scarce dinoflagellate cysts. The palynofloral evidence and regional geological setting indicatethat the studied successions are dated as Aptian (Montoria-La Mina outcrop) and late Albian(Pe~nacerrada 1 and 2 and Salinillas de Burad�on outcrops, and the Pancorbo site). Angiosperm pollen doesnot constitute a significant part of the Aptian assemblages but becomes diversified and numericallyabundant in those dated as late Albian. Although broadly similar to contemporaneous palynofloras fromeastern Spain, the Aptian assemblages of Montoria-La Mina do not yield tricolpate angiosperm pollen.Conversely, the inferred late Albian assemblages show a high content in polyaperturate angiospermpollen grains, as occurs in other localities in Portugal, Western Europe and North America. The studiedpalynoflora shows significant differences from published assemblages located further north, in westernFrance and Canada.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The Lower Cretaceous outcrops of the southern margin of theBasque-Cantabrian basin (northern Spain) are renowned for theirexceptional content in amber (Alonso et al., 2000; Delcl�os et al.,2007; Najarro et al., 2009, 2010). Over the past 10 years,numerous descriptions of insect taxa and microorganisms includedin amber have been published (Pe~nalver and Delcl�os, 2010 andreferences therein). However, the stratigraphic and palae-oecological context associated with these amber-bearing outcropshas received little research attention. To date, only two palyno-logical studies with a local emphasis have been published (Barr�onet al., 2001; García-Blanco et al., 2004). Thus, it becomes

necessary to carry out an integrated analysis of these sites in orderto better elucidate the structure and development of such unusualpalaeoenvironments.

Coinciding with the initiation of a generalised and widespreadperiod of warmth (Wilson and Norris, 2001; Jenkyns, 2010), amberdeposits extended from 27�N to near 70�N during the mid-Cretaceous (Delcl�os et al., 2007; Najarro et al., 2009). During thisperiod, the Iberian Peninsula was situated at low latitudes, underprevalent megathermal climatic conditions characterised byorbitally-paced climate episodes (Chumakov et al., 1995; Tiraboschiet al., 2009; Rodríguez-Lop�ez et al., 2012). Located between the Eu-ropean and African plates, the Iberian plate was an authentic centreof biotic interchange between the Gondwanan and Boreal realms(Heimhofer et al., 2007; Villanueva-Amadoz et al., 2010). The plantmicro- and macro-remains indicate the predominance of tropical/subtropical vegetation extending from the equator to mid-latitudes

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(Vakhrameev, 1991; Spicer et al., 1994; Morley, 2003), while arid tohumid palaeoenvironments developed locally in the Iberian Penin-sula (Villanueva-Amadoz et al., 2011; Heimhofer et al., 2012).

The present paper reports a comprehensive study of the palyno-logical content of Lower Cretaceous sedimentary rocks includingamber-bearing material from the southern area of the Basque-Cantabrian basin. Since the age of these deposits remaineduncertain, special attention was paid to the biostratigraphic signifi-canceof the components constituting the recovered assemblages. So,a critical review of previously published palynological studies wasconducted, focussing particularly on other Lower Cretaceous paly-nofloras fromWesternEuropeandNorthAmerica. Toourknowledge,the present study is the first to describe the palynological content ofan entire array of amber-bearing Lower Cretaceous outcrops.

2. Geological setting

The study area (Fig.1) is located in the Cantabrian range, which isan EeWtrending narrowmountain belt that constitutes thewesternextension of the southern Pyrenees. The evolution of the Basque-Cantabrian basin is related to the kinematic relationships between

Fig. 1. Geographical and geological setting with location of the st

the Iberian and European plates and to the opening of the NorthAtlantic ocean and the Bay of Biscay (Martín-Chivelet et al., 2002).The basin developed on thinned continental crust between the Eu-ropean and Iberian plates during the Cretaceous Period. The studiedoutcrops form part of fault-related folds associated with a thrustsystem constituting part of the South-Pyrenean Frontal Thrust(Capote et al., 2002). The regional stratigraphy of the studied areaincludes Triassic, Jurassic, Cretaceous and Tertiary units (Martínez-Torres et al., 2003). The Triassic is represented by reddish andgreenish mudstones and gypsums related to the Keuper facies. TheJurassic includes carbonate units. Someof themshowalternations oflimestones and marlstones with ammonites and other bioclasts,indicating open marine conditions, whereas others show shallowercarbonate facieswithoolites and shallow-water bioclasts (Martínez-Torres et al., 2003). The Cretaceous is represented by the EscuchaFormation, the Utrillas Group (sensu Rodríguez-L�opez, 2008) andUpper Cretaceous carbonate formations. The Palaeogene andNeogene are represented in the Basque-Cantabrian region by a va-riety of sedimentary rocks. These Cenozoic deposits appear asdisconnected outcrops associated with WNWeESE and WeEtrending thrust fault-related synclines (Alonso-Zarza et al., 2002).

udied sections. Modified after Martín-Chivelet et al. (2002).

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This paper focuses on the outcrops of the Escucha Formationand the Utrillas Group (Aptianelower Cenomanian) in the southernarea of the Basque-Cantabrian Basin. Both units occur on thesouthern margin of the basin and are mainly comprised of silici-clastic deposits that accumulated in continental to shallow marineenvironments (e.g. García-Mond�ejar et al., 2004).

3. Material and methods

Forty six samples were collected from five surface sectionsincluding amber-bearing outcrops: Montoria-La Mina, Pe~nacerrada

Fig. 2. Stratigraphic succession of the Aptian in the Montoria-La Mina outcrop (MIN), showspecimens; C Abundant >30 specimens.

1, Pe~nacerrada 2, Salinillas de Burad�on and Pancorbo (Fig. 1).Although sampling primarily focused on amber-bearing strata, amore extensive collection was carried out at each outcrop,depending on exposure conditions (Figs. 2e6).

The collected material was processed following standard paly-nological techniques (Traverse, 2007) based on acid treatment (HCl,HF) at high temperature. The residues were sieved through 120 and10-mm-mesh sieves. A short oxidation with HNO3 (“nitric wash”)was performed on some residues when required. Slides were pre-pared by mounting on glycerine jelly. In order to include rare taxawith potential biostratigraphic value, between 500 and 1000

ing the distribution of palynomorphs. � Present 1e10 specimens; B Common 11e30

Fig. 3. Stratigraphic succession of the late Albian in Pe~nacerrada 1 (IP) and Pe~nacerrada 2 (IIP) outcrops, showing the distribution of palynomorphs. � Present 1e10 specimens; B Common 11e30 specimens; C Abundant >30specimens (see the legend in Fig. 2).

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Fig. 4. Stratigraphic succession of the late Albian in the Salinillas de Burad�on outcrop (Sb), showing the distribution of aquatic palynomorphs and vascular cryptogam spores. � Present 1e10 specimens; B Common 11e30 specimens; CAbundant >30 specimens (see the legend in Fig. 2).

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Fig. 5. Stratigraphic succession of the late Albian in the Salinillas de Burad�on outcrop (Sb), showing the distribution of gymnosperm and angiosperm pollen. � Present 1e10 specimens; B Common 11e30 specimens; C Abundant >30specimens (see the legend in Fig. 2).

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Fig. 6. Stratigraphic succession of the late Albian at the Pancorbo site (PAN), showing the distribution of palynomorphs. � Present 1e10 specimens; B Common 11e30 specimens; C Abundant >30 specimens (see the legend in Fig. 2).

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palynomorphs were identified per sample. Microscopic analysis ofthe palynological slides was performed with an Olympus BX51microscope incorporating a ColorView IIIu camera. All studiedslides are housed in the deposits of the Geomining Museum(Instituto Geol�ogico y Minero de Espa~na, Madrid). A list of identi-fied taxa accompanied by their occurrences in each outcrop is givenin Appendix A.

Different approaches were employed to make stratigraphic in-ferences. The most recent biostratigraphic studies on Europeanmid-Cretaceous terrestrial successions were performed on Portu-guese material by Heimhofer et al. (2005, 2007) and Hochuli et al.(2006), and a similar methodology involving a comparison of di-versity patterns and abundance of monoaperturate and tri-aperturate angiosperm pollen was used in the present study.However, these data analyses remain highly facies-dependent, andthus a traditional approach involving the range of selected taxawasalso carried out by means of a literature review.

4. Results

4.1. Stratigraphy

Two stratigraphic units, the Escucha Formation and the UtrillasGroup (sensu Rodríguez-L�opez, 2008; Rodríguez-L�opez et al., 2009),were differentiated and correlated. These units are separated by aprominent unconformity, the geological significance of which willbe discussed in the following sections.

4.1.1. The Escucha FormationThis stratigraphic unit crops out near the village of Montoria

(�Alava province) (Montoria-La Mina outcrop, MIN). The EscuchaFormation is >120 m thick and rests on underlying Jurassic car-bonates (Martínez-Torres et al., 2003). Three main intervals can bedifferentiated from base to top in the studied outcrop (Fig. 2): (i) abasal heterolithic interval characterised by a thick development ofgrey mudstones with sandstone layers. The interbedded sand-stones are medium-to coarse-grained or bioclastic; (ii) a medianinterval comprising carbonates which contain rounded quartziteclasts with abundant bioclasts including badly preserved forami-nifera (miliolids and orbitolinids) and oyster fragments; and (iii) anupper unit comprising fine-grained and very well-sorted yellowsandstones with large-scale cross-bedding. Palynomorphs occur ingreymudstones of the lowest 35m of the succession correspondingto the interval (i).

4.1.2. The Utrillas GroupThe overlying Utrillas Group mainly comprises thick siliciclastic

deposits including siltstones, mudstones and fine-grained sand-stones (Rodríguez-L�opez, 2008; Rodríguez-Lop�ez et al., 2009). TheUtrillas Group is generally overlain by younger Late Cretaceousplatform carbonates or, more rarely, by Cenozoic units associatedwith the Alpine Orogeny (Martínez-T�orres et al., 2003). From thisgroup, four sites with amber-bearing levels were considered in thisstudy; Pe~nacerrada 1 and 2 and Salinillas de Burad�on outcrops, andthe Pancorbo site.

Pe~nacerrada 1 and 2 outcrops (henceforth termed IP and IIP,respectively) were previously assigned to the Escucha Fm. byDelcl�os et al. (2007). They are located approximately 30 km south ofthe city of Vitoria, near the village of Pe~nacerrada (�Alava province)and correspond to a narrow band of outcrops of about 5 km long by2 km wide, extending in an NeS direction (Fig. 1).

The IP outcrop (today covered) comprises three stratigraphicintervals (Martínez-Torres et al., 2003; Fig. 3): the lower interval isformed by interbedded mudstones, fine-to coarse-grained sand-stones, and coal seamswith bivalve and coal fragments; themedian

interval is made up of stacked tabular strata of coarse-grainedsandstones with carbonaceous fragments and macrofloral re-mains with encrusted surfaces towards the top; and lastly, theupper interval is formed by fine-grained and very well-sortedsandstones covered by a heterolithic interval of mudstones andcm-to m-thick sandstone levels. Mudstones contain macrofloralremains.

Overlying the IP deposits, the IIP succession (20 m thick) dis-plays a variety of lithologies (sandstones, siltstones and mixedcarbonates) (Fig. 3). Mixed carbonates appear at the base of thesuccession, and consist of ochre-coloured tabular strata formed byOrbitolina-rich sandy limestones interbedded with heterolithicsuccession of alternating sandstones and grey siltstones. Organic-rich sandstone and siltstone intervals occur locally (Alonso et al.,2000). These deposits contain pollen grains and have a highsulphur content. Thick aeolian sandstones appear covering theunderlying organic-rich deposits. These sandstones are fine-grained, very well-sorted and non-cemented.

The Salinillas de Burad�on outcrop (Sb) is located in the �Alavaprovince (Fig. 1), very close to the administrative border of the re-gion of La Rioja. The deposits (over 50 m thick) comprise alter-nating sandstones, conglomerates, siltstones and organic-richmudstones (Figs. 4e5). The lower part of the succession consists ofcoarse-grained sandstones that display sharp-flat surfaces indi-cating a low erosional capacity of the flow. The organic-rich in-tervals are formed by heterolithic (sandstones-mudstones)deposits with a high sulphur content similar to that of thosedescribed in the IIP outcrop. Tabular intervals of grey siltstonesshow lenticular bedding. The top of the lower part of the successiondisplays a thrust fault that placed the siliciclastic Cretaceoussequence inmechanical contact with Upper Triassic gypsums of theKeuper facies.

The uppermost part of the Salinillas de Burad�on outcrop islocated 5 km north of the village of Haro (region of La Rioja). Itcontains amber lumps that were extracted from a 20 m thick suc-cession displaying heterolithic (sandstones/mudstones) depositsaffected by a series of conjugated faults. Three topologically suc-cessive levels (SFB, SFD, SFF) were studied (Figs. 4e5) in thisoutcrop.

At the Pancorbo site (Burgos province) (Figs.1, 6), amber-bearingdeposits (PAN) were found in two well-delimited areas herereferred as the Pancorbo anticline and Pancorbo road outcrops. ThePancorbo anticline outcrop displays a 20 m thick siliciclastic in-terval constituted by sandstones and subordinate mudstones andsiltstones. It can be subdivided into threemain parts: the lower partcomprises mottled mudstones, the middle part consists of alter-nating well-sorted fine-grained to poorly-sorted coarse-grainedsandstones, and the upper part is formed by grey siltstones. Thesandstones beds show tabular stratification with superimposedcross-bedded sets. In this unit, the coarser-grained intervals displayerosive bases that cut down into the underlying well-sorted in-tervals. Here, only one amber-bearing level was identified andsampled (PAN0). The top of the stratigraphic succession (Pancorboroad outcrop) is formed by 15 m of fractured heterolithic sand-stones, in which fine-grained, well-sorted intervals with wavy andflaser bedding are interbedded with organic-rich mudstones. Twosamples were collected in successive levels of organic-rich andamber-bearing mudstones (PAN1 and PAN2).

4.2. Palynological study

The recovered assemblages showedahighdiversityand includeda total of 243 types of palynomorphs (Appendix A), mainly con-sisting of dinoflagellate cysts (22 taxa identified), cryptogam spores(104 taxa), gymnosperm pollen grains (41 taxa) and angiosperms

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(68 taxa). A selection of characteristic palynomorphs identified inthis study is illustrated in Figs. 7e11. Marine palynomorphs, essen-tially consisting of dinocysts, never comprised more than a smallportion of the palynological assemblages in all the studied outcrops.Foraminiferal test linings, acanthomorph acritarchs, prasinophytes

Fig. 7. Light photomicrographs of selected vascular cryptogam spores (I), scale bar ¼ 20 mm.De�ak 1963, level SFD (Salinillas de Burad�on outcrop). (C) Biretisporites potoniaei Delcourt andIIP.6 (Pe~nacerrada 2 outcrop). (E) Baculatisporites comaumensis (Cookson 1953) Potoni�e 195focus on the distal side, level IP.1 (Pe~nacerrada 1 outcrop). (G) Distaltriangulisporites m(Pe~nacerrada 1 outcrop). (I) Acritosporites cf. kyrtomus Juh�asz 1979, level IP.8, (Pe~nacerradAppendicisporites sp. A, (K) focus on the proximal side, level Sb-3 (Salinillas de Burad�on outcrlaesurae, focus on proximal side, level IP.3 (Pe~nacerrada 1 outcrop). (N) Appendicisporites tcosisporites apicanalis Paden-Phillips and Felix 1972, focus on proximal side, level Sb-5 (Sali(Pe~nacerrada 1 outcrop).

and freshwater algae (e.g. Fig. 10AL, AN) were also sporadicallyrecorded. In contrast to the miospores, palynomorphs of marineoriginwere generally poorly preserved. The colour of themiosporeswas relatively homogeneous; the pollen grains were usually yellowto pale brown and the spores, slightly darker.

(A) Deltoidospora sp., level IIP.2 (Pe~nacerrada 2 outcrop). (B) Cicatricosisporites venustusSprumont 1955, level Sb-7 (Salinillas de Burad�on outcrop). (D) Trilobosporites sp., level6, level IP.8 (Pe~nacerrada 1 outcrop) (F) Distaltriangulatisporites irregularis Singh 1971,utabilis Singh 1971, level IP.1 (Pe~nacerrada 1 outcrop). (H) Murospora sp., level IP.3a 1 outcrop). (J) Antulsporites? sp., level Sb-10 (Salinillas de Burad�on outcrop). (KeM)op), (L) same specimen, focus on distal side, (M) specimen with smaller lips around thericornitatus Weyland and Greifeld 1953, level IP.1 (Pe~nacerrada 1 outcrop). (O) Cicatri-nillas de Burad�on outcrop). (P) Costatoperforosporites triangulatus De�ak 1962, level IP.1

Fig. 8. Light photomicrographs of selected vascular cryptogam spores (II), scale bar ¼ 20 mm. (AeB) Cicatricosisporites venustus De�ak 1963, (A) distal focus, level SFB (Salinillas deBurad�on outcrop), (B) proximal focus, level Sb-5 (Salinillas de Burad�on outcrop). (CeD) Cicatricosisporites cf. ludbrookiae Dettmann 1963, level MIN.5 (Montoria-La Mina outcrop),(C) focus on proximal side, (D) same specimen, focus on distal side; (EeF) Appendicisporites potomacensis Brenner 1963, level MIN.11d (Montoria-La Mina outcrop). (G)

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The assemblages are generally dominated by gymnospermpollen (Figs. 2e6), and the most abundant species recorded wereInaperturopollenites dubius (Fig. 9C) and Classopollis major (Fig. 9J),except for the MIN outcrop where the latter taxon was replaced byClassopollis cf. obidosensis (Fig. 9I). Other frequently identifiedgymnosperm taxa included Araucariacites australis (Fig. 9F), Exesi-pollenites tumulus and Perinopollenites halonatus (Fig. 9Q). Bisaccatepollen grains generally represent a minor component of the as-semblages, but were common to abundant in the MIN, IP and IIPoutcrops. Spores were well diversified in all the studied outcropsand occasionally reached sub-dominance in the IP and Sb outcrops.Themost frequently observed species included Cyathidites australis,Cicatricosisporites venustus (Figs. 7B, 8AeB) and Patellasporitestavaredensis (Fig. 8TeV). Angiosperm pollen grains were identifiedin all the studied outcrops. Together with monoaperturate grains,triaperturate forms were recorded, including Cupulifer-oidaepollenites parvulus (Fig. 9U) and abundant Tricolpites minutus(Fig. 9VeW), T. micromunus (Fig. 9XeY) and Striatopollis trochuensis(Fig. 9ACeAD). However, the composition of the assemblages var-ied greatly according to the outcrop.

4.2.1. The Montoria-La Mina outcropThirteen horizons were sampled, of which eight were produc-

tive while the other four were palynologically barren (Fig. 2). Therecovered palynofloras consist of marine and terrestrial paly-nomorphs. The phytoplankton included poorly diversified andbadly preserved dinoflagellate cysts and acritarchs and reachedmaximum abundance and diversity patterns in the middle part ofthe succession (level MIN.9a, Fig. 2), reflecting a short episode ofmore pronounced marine influence (García-Blanco et al., 2004).The spores constitute the palynological group with the highestdiversity (42 taxa identified) and include Cyathidites australis,Patellasporites tavaredensis, Pilosisporites trichopapillosus (Fig. 8R)and Cicatricosisporites spp. (e.g. Fig. 8CeD). Although representedby a relatively low number of taxa (19 taxa), gymnosperm pollenwas numerically dominant, and the genera Classopollis, Inapertur-opollenites and Araucariacites were especially well represented, aswere bisaccate pollen grains (e.g. Fig. 9H). Angiosperm pollen (6taxa) occurred in low diversity and number. Monoaperturate taxaattributed to Clavatipollenites spp., Retimonocolpites sp. and Lil-iacidites sp. were the most frequently observed. Zonasulculatepollen of Schrankipollis microreticulatus (Fig. 9OeP) was occasion-ally observed throughout the succession, being more frequentlyrecorded in level MIN.5.

4.2.2. The Pe~nacerrada outcropsNine horizons were collected in the IP outcrop, of which six are

palynologically productive (Fig. 3). They included both marine andterrestrial palynomorphs. Cryptogam spores are diverse (55 taxaidentified) and relatively common in all the productive samples.Patellasporites tavaredensis and Cyathidites australis occasionallyreached high abundance. The Schizaeaceae were represented byseveral genera, including Appendicisporites, Cicatricosisporites, Cos-tatoperforosporites and Distaltriangulisporites (Fig. 7FeG, MeN, P).With 30 taxa identified, gymnosperms were fairly diverse and

Cicatricosisporites sprumontii D€oring 1965, focus on radial exinal thickenings, level SB10 (Sal(Salinillas de Burad�on outcrop). (IeJ) Gregussosporites orientalis Juh�asz and Smirnova 1985,(Salinillas de Burad�on outcrop). (K) Crybelosporites pannuceus (Brenner, 1963) Srivastava 1977level Sb-5 (Salinillas de Burad�on outcrop). (M) Clavifera sp., level IP.1 (Pe~nacerrada 1 outcroForaminisporis asymmetricus (Cookson and Dettmann 1958) Dettmann 1963, level IP.8 (Pe~noutcrop). (Q) Pilosisporites brevipapillosus Couper 1958, level Sb-1 (Salinillas de Burad�on o(Montoria-La Mina outcrop). (S) Taurucosporites segmentatus Stover 1962, level MIN.11e (Mfocus on verrucae forming the cingulum, level IP.8 (Pe~nacerrada 1 outcrop), (U) focus on plobosporites purverulentus (Verbitskaya 1958) Dettmann 1963, level MIN.11g. (X) Laevigatos

quantitatively dominant, and the most frequently observed taxawere Araucariacites australis, Classopollis major, Inaperturopollenitesdubius and Alisporites spp. Angiosperms were rare, and are repre-sented by tricolpate pollen grains attributed to Senectotetraditesvarireticulatus (Fig. 10AM), Tricolpites cf. parvus (Fig. 10NeO) andTricolpites spp. A total of eleven dinocysts were identified. Thelower part of the succession showed a higher diversity and Oli-gosphaeridium complex, Cyclonephelium chabacca (Fig. 11C) andChichaouadinium vestitum (Fig. 11B, E) are present.

Although seven organic-rich levels were sampled in the IIPoutcrop (Fig. 3), only two of themwere productive (IIP.2, IIP.6). Theoverall species diversities for both terrestrial and marine compo-nents were comparatively lower than those recorded at IP. At IIP,gymnosperm pollen grains (16 taxa) comprise about two-thirds ofthe recovered palynoflora, the remainder being mainly constitutedby spores (30 taxa). Angiosperms (12 taxa) were not recorded inhigh numbers and showed a preferential distribution according tothe horizon considered. Monoaperturate grains (Clavatipollenitesspp., Dichastopollenites sp., Stellatopollis cf. barhoornii (Fig. 10AK)and Retimonocolpites spp.) were frequent in both levels, and tri- andtetracolpate forms (Rousea spp., Striatopollis trochuensis, Tetra-colpites spp. [e.g. Fig. 10AI]) were better represented in level IIP.6than in IIP.2. On the basis of the abundance and diversity of therecovered phytoplankton, level IIP.2, characterised by abundantPalaeoperidinium cretaceum, showed a more pronounced marineinfluence than level IIP.6.

4.2.3. The Salinillas de Burad�on outcropThe Sb succession can be divided into three palynological in-

tervals (Figs. 4e5). The lower interval, up to Sb-4 (inclusive), wascharacterised by a high proportion of spores and gymnospermpollen, and a reduced number of angiosperm specimens (less than10% of the total palynomorphs) and taxa (30). With 53 taxa, thespore component showed the highest diversity of the studiedoutcrops Cicatricosisporites spp., Cyathidites australis and Patellas-porites tavaredensis are the most abundant spores. Gymnosperms(26 taxa) were mainly represented by Araucariacites australis, Ina-perturopollenites dubius and Classopollis spp. Angiosperms (30 taxa)comprised a small number of triaperturate species that includedCupuliferoidaepollenites parvulus, Phimopollenites pannosus(Fig. 10AG), Striatopollis trochuensis, Rousea cf. miculipollis, Tricol-poroidites bohemicus, Tricolpites spp. and the obligate tetrad Virgoamiantopollis (Fig. 9AE).

The Sb-5eSb-7 interval is characterised by a lower diversity ofspores (46 taxa) and a higher proportion (>25%) of angiosperms. Inthis interval, the observed increase in the abundance of angio-sperms is parallelled by a significant augmentation in diversity (55taxa), mainly marked by the appearance of new triaperturate formsincluding Gemmatricolpites gemmatus (Fig. 9AAeAB), Rhoipites spp.,Tricolpites cf. vulgaris, T. cf. interangulus (Fig. 10JeL) and T. cf. sagax(Fig. 10CeD). Polyporate pollen grains attributed to Cretacaeiporitesmulleri (Fig. 10AH) and the obligate tetrad Senectotetradites varir-eticulatus were also recorded at this interval. Scarce operculatepollen grains attributed to Tucanopollis crisopolensis were observedin level Sb-7 (Fig. 9Z). The presence of a single specimen of

inillas de Burad�on outcrop). (H) cf. Ruffordiaspora sp., focus on proximal side, level SFB(I) distal focus, level Sb-1 (Salinillas de Burad�on outcrop), (J) proximal focus, level Sb-3, sectional focus, level PAN1 (Pancorbo site). (L) Camarozonosporites insignis Norris 1967,p). (N) Undulatisporites sinuosis Groot and Groot 1962, level PAN0 (Pancorbo site). (O)acerrada 1 outcrop). (P) Neoraistrickia robusta Brenner 1963, level IP.8 (Pe~nacerrada 1utcrop). (R) Pilosisporites trichopapillosus Delcourt and Sprumont 1955, level MIN.11gontoria-La Mina outcrop). (TeV) Patellasporites tavaredensis Groot and Groot 1962, (T)roximal side, level PAN2 (Pancorbo site), (V) same specimen on distal focus. (W) Tri-porites sp., level IP.3 (Pe~nacerrada 1 outcrop).

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Elaterocolpites castelainii at Sb-5 (Fig. 9B) is of interest as it suggeststhat the area was under Gondwanan influence (Dino et al., 1999).

The upper interval of the Sb outcrop (levels SFBeSFF) is char-acterised by a less diversified angiosperm association (35 taxa), andincludes the first occurrence of Rousea georgensis, Foveotricolpitesconcinnus (Fig. 10W) and Nyssapollenites nigricolpus (Fig. 10Y). Inthis interval, Cupuliferoidaepollenites parvulus, Pennipollis cf. retic-ulatus and Tricolpites minutus were notably more frequent than inthe underlying strata. In addition, the gymnosperms Equisetospor-ites spp. and Perinopollenites halonatus (Fig. 9Q) were commonlyobserved in the upper part of the succession.

The phytoplankton component of the entire succession of the Sboutcrop is not quantitatively significant (<2%), and includes Palae-ohystrichophora infusorioides.

4.2.4. The Pancorbo siteFifty-three spores and sixty-six pollen grains (27 of gymno-

sperms and 39 of angiosperms) were identified from the PAN site(Fig. 6). The most abundant spores are Cicatricosisporites spp.,Cyathidites australis and Patellasporites tavaredensis (Fig. 8UeV).Gymnosperms are mainly represented by Araucariacites, Class-opollis, Exesipollenites, Inaperturopollenites and Perinopollenites.Other common forms included Cycadopites spp., Monosulcitesminimus and Equisetosporites spp. Angiosperms are diverse andtriaperturate pollen (Cupuliferoidaepollenites parvulus, Striatopollistrochuensis, Tricolpites micromunus and T. minutus [Fig. 9VeW]) isbetter represented than monoaperturate forms (Clavatipollenitesminutus, Pennipollis spp. and Transitoripollis similis). A single spec-imen of the monoaperturate operculate pollen grain Tucanopolliscrisopolensis is also recorded. The dinocyst assemblages recoveredfrom the PAN site show a poor diversity, reflecting a reduced ma-rine influence. Cyclonephelium chabacca is the only dinocystrecorded in high numbers.

5. Discussion

5.1. Palynomorph-based biostratigraphy

The stratigraphic resolution of Cretaceous miospore-bearingsuccessions is often hampered by the lack of associated age-diagnostic marine micro- or macrofossils. This is especially truewith successions deposited under marginal marine, tidally-influenced or fluvio-lacustrine depositional settings, where dino-flagellate cysts may be absent or under-represented.

Together with selected spores and gymnosperm pollen, theangiosperm record provides valuable correlatable events and hasbeen used successfully to date Cretaceous terrestrial successions(Brenner, 1963; Doyle and Robbins, 1977; Schrank, 1992; Hochuli

Fig. 9. Light photomicrographs of selected gymnosperm and angiosperm pollen grains, scaleMIN.9a (Montoria-La Mina outcrop). (B) Elaterocolpites castelainii Jardin�e and Magloire 1965Venitz 1932) Thomson and Pflug 1953, level Sb-3, (Salinillas de Burad�on outcrop). (D) Callialoutcrop). (E) Equisetosporites barghoornii Pocock 1965, level Sb-5 (Salinillas de Burad�on outcr(G) Rugubivesiculites sp., level IP.3 (Pe~nacerrada 1 outcrop). (H) Podocarpidites sp., level MIN.MIN.3 (Montoria-La Mina outcrop). (J) Classopollis major Groot and Groot 1962, level IP.1Eucommiidites minor Groot and Penny 1960, level MIN.11g (Montoria-La Mina outcrop). (M)Singh 1983, level IP.8 (Pe~nacerrada 1 outcrop). (OeP) Schrankipollis microreticulatus (BrenneMina outcrop). (Q) Perinopollenites halonatus Paden Phillips and Felix 1971, level SFD (Salini(Salinillas de Burad�on outcrop). (S) Pennipollis cf. peroreticulatus (Brenner, 1963) Friis, Pedreticulatus (Brenner, 1963) Friis, Pedersen and Crane 2000, level Sb-10 (Salinillas de Burad�onlevel Sb-10 (Salinillas de Burad�on outcrop). (VeW) Tricolpites minutus (Brenner, 1963) Dettequatorial view. (XeY) Tricolpites micromunus (Groot and Penny 1960) Burger 1971, level Sb-7view. (Z) Tucanopollis crisopolensis Regali 1989, level Sb-7 (Salinillas de Burad�on outcrop)outcrop), (AA) specimen in oblique view, focus on gemmate ornamentation, (AB) specimespecimen in oblique-equatorial view, level Sb-10 (Salinillas de Burad�on outcrop), (AD) speci(Srivastava 1977) Ward 1986, level Sb-1 (Salinillas de Burad�on outcrop).

et al., 2006; Eisawi and Schrank, 2008). A major radiation phaseof flowering plants occurred during this time, which is reflected bythe first appearance of pollen with complex morphologies in thefossil record (Heimhofer et al., 2005). Accordingly, particularemphasis is placed on the highly diversified angiosperm pollencontent from the studied successions.

5.1.1. The Aptian recordAt the MIN outcrop, the occurrences of Pilosisporites trichopa-

pillosus (Figs. 8R, 12) and Schrankipollis microreticulatus (Figs. 9OeP,12) suggest an age no younger than Aptian. The extinction datum ofP. trichopapillosus occurs in the late Aptian in North Africa (Eisawiet al., 2012) and in contemporaneous deposits from accuratelydated sections from western North America (Bujak and Williams,1978). The record for the genus Schrankipollis is restricted to theAptian in northern Gondwana (Schrank and Mahmoud, 1998;Regali and Santos, 1999; see also discussion in El Beialy et al.,2010; pag. 221). Schrankipollis microreticulatus, however, mayrange into the early Albian in northern latitudes (Hochuli et al.,2006), although independently dated records supporting this ageassignation are still lacking. An Aptian age is also well supported bythe angiosperm component, which lacks polyaperturate pollengrains (García-Blanco et al., 2004). The assemblages from MIN arecorrelatable with those from the lower part of the North Americanpalynological Subzone I of the Potomac Group, described byBrenner (1963) and dated as Aptian (Doyle, 1992; Hochuli et al.,2006).

5.1.2. The upper Albian recordUpdating a preliminary palynological study performed on a

reduced part of the assemblages from the IP outcrop (Barr�on et al.,2001), the present study reveals marine associations characterisedby the presence of Chichaouadinium vestitum (Figs. 11B, E, 12) andPalaeohystrichophora infusorioides (Figs. 11I, 12). The record of thesedinocysts suggests a late Albian age. According to Williams et al.(2004), C. vestitum does not occur in rocks younger than the lateAlbian. The presence of P. infusorioides permits a more precisebiostratigraphic interpretation, as the oldest accepted occurrenceof this species is in the “latest” Albian (Costa and Davey, 1992) andin the upper part of the Dispar ammonite Zone in Europe (FoucherandMonteil, 1998). The terrestrial palynoflora also included severalage-diagnostic species that provide further support for this ageassignment: Distaltriangulisporites mutabilis (Figs. 7G, 12), Sen-ectotetradites varireticulatus (Figs. 10AM, 12) and Rugubivesiculitesspp. (Figs. 9G, 12), recorded in the IP succession, have not beenobserved in older deposits (Singh, 1971; Doyle and Robbins, 1977;Ward, 1986; Burden and Hills, 1989; Villanueva-Amadoz et al.,2011). The presence of Callialasporites dampieri (Figs. 9D, 12)

bar ¼ 10 mm. (A) Cerebropollenites macroverrucosus (Thiergart 1949) Schulz 1967, level, level Sb-5 (Salinillas de Burad�on outcrop). (C) Inaperturopollenites dubius (Potoni�e andasporites dampieri (Balme 1957) Dev 1961 emend. Norris 1969, level IP.3 (Pe~nacerrada 1op). (F) Araucariacites australis Cookson 1947, level Sb-1 (Salinillas de Burad�on outcrop).5 (Montoria-La Mina outcrop). (I) Classopollis cf. obidosensis Groot and Groot 1962, level(Pe~nacerrada 1 outcrop). (K) Monocolpopollenites sp., level PAN0 (Pancorbo site). (L)Phyllocladidites sp., level PAN2 (Pancorbo site). (N) Dichastopollenites cf. dunveganensisr, 1963) Doyle, Hotton and Ward 1990, (O) level MIN.5, (P) level MIN.11d (Montoria-Lallas de Burad�on outcrop). (R) Transitoripollis similis G�ocz�an and Juh�asz 1984, level Sb-8ersen and Crane 2000, level Sb-5 (Salinillas de Burad�on outcrop). (T) Pennipollis cf.outcrop). (U) Cupuliferoidaepollenites parvulus (Groot and Penny 1960) Dettmann 1973,mann 1973, level PAN0 (Pancorbo site), (V) specimen in polar view, (W) specimen in(Salinillas de Burad�on outcrop), (X) specimen in polar view, (Y) specimen in equatorial. (AAeAB) Gemmatricolpites gemmatus Pierce 1961, level Sb-8 (Salinillas de Burad�onn in polar view. (ACeAD) Striatopollis trochuensis (Srivastava 1977) Ward 1986, (AC)men in polar view, level Sb-7 (Salinillas de Burad�on outcrop). (AE) Virgo amiantiopollis

Fig. 10. Light photomicrographs of selected angiosperm pollen grains and aquatic palynomorphs, scale bar ¼ 10 mm. (AeB) Asteropollis trichotomosulcatus (Singh, 1971) Singh 1983,(A) level IP.8 (Pe~nacerrada 1 outcrop), (B) level Sb-5 (Salinillas de Burad�on outcrop). (CeD) Tricolpites cf. sagax Norris 1967, (C) specimen in equatorial view, level Sb-11 (Salinillas deBurad�on outcrop), (D) specimen in polar view, sectional focus, level Sb-7 (Salinillas de Burad�on outcrop). (EeF) Tricolpites cf. vulgaris (Pierce 1961) Srivastava 1961, (E) specimen in

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polar view, level PAN0 (Pancorbo site), (F) specimen in equatorial view, level PAN0 (Pancorbo site). (G) Retitrescolpites sp. A, level Sb-6 (Salinillas de Burad�on outcrop). (HeI)Tricolpites amplifissus (Laing 1975) Ward 1986, (H) sectional focus, level Sb-7 (Salinillas de Burad�on outcrop), (I) superficial focus, level PAN2 (Pancorbo site). (JeL) Tricolpites cf.interangulus Newman 1965, (J) superficial focus, level Sb-10 (Salinillas de Burad�on outcrop), (K) sectional focus, level Sb-7 (Salinillas de Burad�on outcrop), (L) specimen in equatorialview, level Sb-10 (Salinillas de Burad�on outcrop). (MeO) Tricolpites cf. parvus Stanley 1965, (M) specimen in equatorial view, level Sb-7 (Salinillas de Burad�on outcrop), (N) specimenin polar view, superficial focus, level IP.6 (Pe~nacerrada 1 outcrop), (O) same specimen, sectional focus. (P) Fraxinoipollenites? sp. A, level SFD (Salinillas de Burad�on outcrop). (Q)Fraxinoipollenites? sp. B, PAN0 (Pancorbo site): (R) Fraxinoipollenites? constrictus (Pierce 1961) Chlonova 1976, PAN1 (Pancorbo site). (S) Tricolpites sp. cf. Retitricolpites varireticulatusBrenner 1968, level PAN1 (Pancorbo site). (TeU) Rousea georgensis (Brenner, 1963) Dettmann 1973, (T) specimen in polar view, (U) specimen in equatorial view, sectional focus, levelPAN2 (Pancorbo site). (V) Rousea cf. miculipollis Srivastava 1977, level Sb-6 (Salinillas de Burad�on outcrop). (W) Foveotricolpites concinnus Singh 1971, level SFB (Salinillas de Burad�onoutcrop). (X) Margocolporites sp., level Sb-5 (Salinillas de Burad�on outcrop). (Y) Nyssapollenites nigricolpus Singh 1983, level SFD (Salinillas de Burad�on outcrop). (Z) Nyssapollenitestriangulus (Groot, Penny and Groot 1961) Singh 1983, level PAN0 (Pancorbo site). (AA) Nyssapollenites? sp. A, level SFB (Salinillas de Burad�on outcrop). (AB) Tricolporoidites sp. A,level SFD (Salinillas de Burad�on outcrop). (AC) Tricolporoidites spp., level Sb-7 (Salinillas de Burad�on outcrop). (AD) Tricolporoidites bohemicus Pacltov�a 1971, level Sb-5 (Salinillas deBurad�on outcrop). (AE) Rhoipites spp., level PAN2 (Pancorbo site). (AF) Nyssapollenites sp., level PAN0 (Pancorbo site). (AG) Phimopollenites pannosus (Dettmann and Playford 1968)Dettmann 1973, level Sb-11 (Salinillas de Burad�on outcrop). (AH) Cretacaeiporites mulleri Herngreen 1973, level Sb-8 (Salinillas de Burad�on outcrop). (AI) Tetracolpites reticulatusSrivastava 1966, level IIP.6 (Pe~nacerrada 2 outcrop); (AJ) Tetracolpites spp. cf. T. pulcher Srivastava 1969, level SFD (Salinillas de Burad�on outcrop); (AK) Stellatopollis cf. barghoorniiDoyle in Doyle, Van Campo and Lugardon 1975, level IIP.2 (Pe~nacerrada 2 outcrop). (AL) Schizosporis reticulatus Cookson and Dettmann 1959, level IP.6 (Pe~nacerrada 1 outcrop);(AM) Senectotetradites varireticulatus Dettmann 1973, level IP.8 (Pe~nacerrada 1 outcrop). (AN) Chomotriletes minor (Kedves 1961) Pocock 1970, level SFD (Salinillas de Burad�onoutcrop). (AO) Spiniferites ramosus (Ehrenberg 1838) Mantell 1854, level IP.6 (Pe~nacerrada 1 outcrop).

Fig. 11. Light photomicrographs of selected dinoflagellate cysts, scale bar ¼ 20 mm. (A) Palaeoperidinium cretaceum (Pocock 1962) Lentin andWilliams 1976, level IP.1 (Pe~nacerrada 1outcrop). (B, E) Chichaouadinium vestitum (Brideaux 1971) Bujak and Davis 1983, (B) level IP.2 (Pe~nacerrada 1 outcrop), (E) level IP.3 (Pe~nacerrada 1 outcrop). (C) Cyclonepheliumchabaca Below 1981, level IP.3 (Pe~nacerrada 1 outcrop). (D) Chlamydophorella? sp., level IP.3 (Pe~nacerrada 1 outcrop). (F) Oligosphaeridium pulcherrimum (Deflandre and Cookson1955) Davey and Williams 1966, level IP.2 (Pe~nacerrada 1 outcrop). (G) Ovoidinium sp., level IIP.2 (Pe~nacerrada 2 outcrop). (H) Palaeohystrichophora cheit (Below 1981) Mahmoud1998, level IIP.2 (Pe~nacerrada 2 outcrop). (I) Palaeohystrichophora infusorioides Deflandre 1935, level IP.1 (Pe~nacerrada 1 outcrop).

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Fig. 12. Composite range chart showing selected species of biostratigraphic interest identified in the Lower Cretaceous amber-bearing outcrops of the southern margin of theBasque-Cantabrian basin. L ¼ lower; M ¼ middle; U ¼ upper.

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suggests an age no younger than late Albian, since this taxon doesnot show a consistent record in younger strata (Batten and Uwins,1985; Burden and Hills, 1989; Schrank and Mahmoud, 1998). It ishowever of interest to note that this species has been sporadicallyrecorded in Cenomanian shales from the Outer Western Carpa-thians in Moravia (Svobodov�a et al., 2004).

The palynoflora from the IIP outcrop did not show significantdifferences with the IP outcrop in terms of composition of the as-semblages and/or content of biostratigraphic markers, and thus itseems reasonable to assume the absence of a major stratigraphicgap between both outcrops. Based on the available data, a lateAlbian age is attributed to the sedimentary rocks deposited in theoutcrops of Pe~nacerrada.

The assemblages from the lower interval of the Salinillas deBurad�on outcrop (Sb1eSb4) shared common characteristics withthe palynoflora described at Pe~nacerrada. Various spores, includingAcritosporites cf. kyrtomus (Fig. 7I), Costatoperforosporites triangu-latus (Fig. 7P), Distaltriangulisporites irregularis (Fig. 7F) and Gre-gussosporites orientalis (Fig. 8IeJ) were recorded in both outcrops.The record of Cupuliferoidaepollenites parvulus and Rugubivesiculitesspp. (Fig. 12) in samples Sb-3 and Sb-4 indicates an age no olderthan late Albian (Doyle and Robbins, 1977; Bujak and Williams,1978; Hochuli et al., 2006) for this interval.

The interval Sb-5eSb-7 showed a significant increase in angio-sperm pollen and a high content in morphotypes with tricolpor-oidate apertural morphologies (e.g. T. bohemicus [Figs. 10AD; 12]).These grains relate the Spanish outcrop with the upper Albianangiosperm assemblages from the Kiowa Formation (USA)described by Ward (1986). The interval Sb-5eSb-7 also containeddiverse tricolpate grains (Fig. 12), including abundant Cupulifer-oidaepollenites parvulus (Figs. 9U, 12), Tricolpites micromunus

(Figs. 9XeY, 12) and T. minutus (Fig. 12). These features suggest acorrelation with assemblages from the North American SubzoneIIC, characterised by common to abundant T. micromunus andT. minutus (Doyle and Robbins,1977). Other common characteristicsbetween the Iberian and North American palynoflora include stri-ate pollen grains (i.e. Striatopollis trochuensis) and the presence ofbisaccate grains attributed to the genus Rugubivesiculites, these twobiostratigraphic markers are observed in assemblages presumed tobe late Albian (Doyle and Robbins, 1977) or independently dated assuch (Norris, 1967; Reinson et al., 1994) from the western USA andCanada, respectively.

5.1.3. Upper Albian/?lower Cenomanian assemblagesThe uppermost interval (SFDeSFF) from the Sb outcrop pre-

sented the same overall biostratigraphic characteristics as the un-derlying strata but also included Nyssapollenites nigricolpus(Figs. 10Y, 12), a species that has only been recorded in lowerCenomanian deposits elsewhere (Agasie, 1969; Az�ema et al., 1972;Singh, 1983). The appearance of this taxon suggests that the up-per part of the Sb succession may be lower Cenomanian. However,given the rarity of this species in AlbianeCenomanian successions(i.e. its non-consistent biostratigraphic record), a tentative lateAlbian/?early Cenomanian age was considered for this interval.

The PAN microflora, which is very similar to that of the upperpart of the Sb succession, suggests a similar age. Of particular in-terest is the record of tricolporate grains attributed to Nyssa-pollenites triangulus (Fig. 10Z). El-Beialy et al. (2010) consideredN. triangulus to be a Cenomanian marker in West Africa alsoappearing in the lower Cenomanian of Egypt. However, this speciesis relatively widespread further north and its occurrence is recor-ded from the late Albian onwards (Singh, 1983 and references

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therein). Given these conflicting interpretations, the geographiclocation of the study area and the limitations imposed by themainly terrestrial (limited) fossil record, an inconclusive lateAlbian/?early Cenomanian age is also postulated for this outcrop.

The occurrence of grains of Tucanopollis crisopolensis at the Sboutcrop (Figs. 9Z, 12) and the PAN site is interpreted to reflectreworking from lower Albian or older deposits (Doyle et al., 1977;Regali, 1989; Heimhofer and Hochuli, 2010).

5.1.4. The regional angular unconformity between the EscuchaFormation and the Utrillas Group

The Escucha Formation and the Utrillas Group have recentlybeen re-defined by Rodríguez-L�opez (2008) and Rodríguez-L�opezet al. (2009) for the Iberian basin. These units have been corre-lated and recognised from Teruel to Soria provinces (Rodríguez-L�opez et al., 2009, 2010) and are now also recognised and corre-lated in the Basque-Cantabrian basin. In the Iberian basin, located350 km south of the Basque-Cantabrian basin, the Escucha For-mation and the Utrillas Group are separated by the same angularunconformity that separates both units in the Basque-Cantabrianbasin. In the Iberian basin, this unconformity is associated withsyn-sedimentary extensional tectonics, erosion and sedimentarybypass (Rodríguez-L�opez, 2008; Rodríguez-L�opez et al., 2009, 2010,2012, 2013). In the Iberian basin, this angular unconformity sepa-rates upper Aptianelower Albian deposits of the Escucha Forma-tion below from lower Albian to lower Cenomanian deposits of theUtrillas Group above. Thus, this surface developed in that basinduring early Albian times. By contrast, and taking into account thenew biostratigraphic data presented here, the same regional un-conformity in the Basque-Cantabrian basin separates Aptian de-posits of the Escucha Formation below from upper Albian/lowerCenomanian deposits of the Utrillas Group above (Fig. 13). Thissuggests the presence of an erosional and/or non-depositional hi-atus in the southern margin of the Basque-Cantabrian basin thatspans at least from the latest Aptian to the early late Albian. Thisfinding is in part consistent with previous studies of other time-equivalent marine units from the central part of the Basque-Cantabrian basin where, based on ammonites and benthic fora-minifera, the presence of a hiatus spanning at least the middleAlbian and probably also the earliest late Albian has been reported(e.g. L�opez-Horgue et al., 2009). Therefore, the unconformity rec-ognised in Teruel, Zaragoza and Soria provinces in the Iberian basinseems to be associated with a minor hiatus compared to thatobserved in the southern Basque-Cantabrian basin, where the hi-atus associated with the unconformity is larger, spanning fromAptian to late Albian times.

5.2. Palaeoenvironmental considerations

The studied outcrops showed similar characteristics, including:i) dominant but poorly diversified gymnosperm associations, andii) generally, a highly diversified, sub-dominant (MIN, IP, IIP and thelower interval of Sb) to common (PAN and the two upper intervalsof Sb) spore component. Angiosperms were rare or subordinate(MIN, IP, IIP) to sub-dominant (PAN, Sb). The diversity pattern ofangiosperms was more or less coincident with their abundance.The assemblages with few specimens were also poorly diversified(i.e. small number of species). Angiosperm abundance and diversitywere determined by the age of the outcrop (Heimhofer et al., 2007;Hochuli et al., 2006), but also depended on the characteristics of thefacies. The upper Albian palynoflora of the IP and IIP outcropsshowed a more reduced angiosperm diversity and number ofspecimens than at the Sb outcrop and the PAN site (Fig. 13). Inter-estingly, the abundance and diversity of dinocysts at the PAN sitewere distinctly lower than at the IP and IIP outcrops. The lower

representation of angiosperms and, more specifically of poly-aperturate grains, in assemblages with a marked marine influence(i.e. with abundant microforaminiferal linings and more diversifieddinocyst associations) may be the consequence of distinct palae-oecological behaviours. In contrast to the monocots and/or mag-noliids (i.e. monoaperturate grains), early eudicots includednumerous entomophilous species (i.e. insect-pollinated) with verylow pollen production and dispersal (Hu et al., 2008; Taylor and Hu,2010; Friis et al., 2011). Accordingly, the diversified angiospermassociations of the Sb outcrop and the PAN site may be represen-tative of hinterland vegetation, while the “poorly” diversified as-semblages of the IP and IIP outcrops may be a partial,taphonomically biased, reflection of vegetation characterised byunder-represented eudicot content. However, this hypothesis willrequire additional evidence from different fossil groups to be fullyaccepted.

When the entire array of the studied outcrops is considered, thediversity and abundance of the three main groups of terrestrialplants (angiosperms, gymnosperms and spore-producing plants)displayed a trend that is consistent with the evolution of thevegetation in the Iberian Peninsula (Di�eguez et al., 2010) andelsewhere, in middle to high latitudes (Lidgard and Crane, 1990)during the interval considered. In this context, the palynoflora ofthe MIN outcrop represents a good example of Mesophytic floracharacterised by highly diversified spore-producing plant com-munities. Conversely, the PAN site and the Sb outcrop successionsare representative of Cenophytic vegetation composed of diverseand abundant angiosperms.

The picture of the Aptian vegetation of the southern Basque-Cantabrian basin corresponds to coastal forests containing repre-sentatives of the conifer families Cheirolepidiaceae, Cupressaceae,Araucariaceae and Pinaceae/Podocarpaceae. Coastal environmentsmay also have included mangrove-type vegetation, including Alis-porites-producers such as Corystospermales. Angiosperms wererare and presumably may have included a significant proportion ofherbaceous forms. In Aptian assemblages, their co-occurrence withnumerous spores suggests common palaeoenvironmental re-quirements. It seems reasonable to assume that spore-producersand angiosperms inhabited themoister habitats in the studied area.

Compared to the Aptian vegetation, the diversity of conifers(especially Pinaceae/Podocarpaceae) in coastal (forested?) areaswas limited during the late Albian. Conversely, angiosperms weremore diversified and constituted a much more significant compo-nent of the forest vegetation. They probably included both herba-ceous and arborescent taxa. The higher abundance and diversityobserved in the palynological assemblages could also reflectbroader palaeoecological tolerances. Late Albian pteridophytecommunities were only slightly less diversified than during theAptian and still represented a relatively significant component ofthe vegetation. Assuming similar palaeoecological requirements forthis group, the presence of plants inhabiting wetter habitats such asmarshes, riparian zones and forest understories may be inferredfrom the palynological assemblages.

5.2.1. Resin productionAmber was found associated with levels containing organic

matter in the IP, IIP and Sb outcrops and at the PAN site. In theEscucha Formation, amber deposits are always associated with coallayers that are more abundant in the middle member of the for-mation (Martínez-Torres et al., 2003).

Botanical affinities of Lower Cretaceous amber-producers arestill poorly understood. Preliminary chemotaxonomic studies per-formed on ambers from the IP and IIP outcrops suggested a possiblerelationship with the modern genus Agathis (Alonso et al., 2000;Chaler and Grimalt, 2005; Delcl�os et al., 2007). However, recent

Fig. 13. Stratigraphic correlation of studied sections.

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research has indicated multiple affinities for amber-producers,including the extinct family Cheirolepidiaceae (Menor-Salv�anet al., 2010). This lack of consensus clearly reflects an incompleteknowledge in spite of the considerable interest generated by fossilresins.

Major uncertainties also surround the causes that triggered theproduction of resin by conifers during the later part of the EarlyCretaceous. Various hypotheses have been proposed, includingabrupt changes in temperature, local salt resurgence, fires, damageinflicted by megaherbivores, insects or pathogens and storage ofunwanted metabolic substances (Martínez-Delcl�os et al., 2004;Najarro et al., 2009). Tiraboschi et al. (2009) suggested that hightemperatures and drastic and repeated changes in the precipitationregime were key factors in the deposition of lower and upperAlbian black shales from the eastern part of the Tethys. The pro-duction of resin by certain conifers from coastal associations mayalso have been a response to such unusual, widespread, palae-oclimatological conditions, and may represent the terrestrialcounterpart of the black shales associated with Albian oceanicanoxic events. However, this hypothesis remains extremely spec-ulative. Although the potential relationship between resin pro-duction and global palaeoenvironmental factors (e.g. thosecontrolling the record of OAE1b-d) is attractive, the lack of perti-nent age determination involving marine faunal and/or floralmarkers with a broader distribution and a consistent record willprobably prevent any further fine calibration. Recent studies(Tiraboschi et al., 2009; Trabucho-Alexandre et al., 2011) suggestthat the depositional settings of ‘black shales’ aremuchmore variedthan previously thought and may have been the result of severalprocesses acting on a local rather than global scale. In this context,the further study and correlation between coeval amber-bearingterrestrial and marine successions of the Basque-Cantabrian basinmay help to clarify the relative role of local and global palae-oenvironmental forcing in the origin of these deposits.

5.3. Comparison of assemblages

The palynological assemblages from the MIN outcrop comparewell with the upper Aptian/lower Albian palynoflora from theOliete sub-basin (Iberian Ranges, E Spain) (Peyrot et al., 2007a,2007b) and the Lusitanian basin (Mendes et al., 2013). They sharecommon features such as highly diversified, common spores,poorly diversified but numerically dominant gymnosperms andrare, mainly monoaperturate angiosperms. The presence of tricol-pate pollen grains in the Oliete sub-basin, the Lusitanian basin andthe northwestern margin of the Basque-Cantabrian basin suggeststhat, contrary to the situation observed in the MIN outcrop, a sig-nificant part of the Aptian vegetation from the Iberian Peninsulaalready included basal eudicots (Peyrot et al., 2007a; Najarro et al.,2011; Mendes et al., 2013). This is supported by the meso- andmacrofossil evidence (Friis et al., 2010), and suggests that the lateAptianeearly Albian vegetation of the Iberian Peninsula showed arelatively scattered, non-homogeneous distribution of eudicots.

The IP and IIP successions exhibit a low diversity of angiospermassociations and a conspicuous spore and dinocyst content. The lownumber of angiosperm pollen grains in the inferred late Albianmicroflora suggests a long distance from the source area and moredistal depositional settings. These characteristics, not oftenobserved in presumed coeval assemblages from the IberianPeninsula, become impossible to conduct any detailed compari-sons. In contrast, the assemblages from the Sb outcrop and the PANsite were characterised by diversified polyaperturate grains, andshowed similarities with the assemblages of Foz de Folc~ao andFolc~ao-Magoito, in Portugal (M�edus and Berthou, 1980; Berthouet al., 1981). A renewed study of these Portuguese successions

would, however, be opportune in order to carry out a more thor-ough palynological comparison.

The palynoflora from the Aliaga and Oliete sub-basins of theIberian basin (eastern Spain) (Villanueva-Amadoz et al., 2010,2011), interpreted to be middle Albianeearly Cenomanian in age,showed relatively diversified spore content and several tri-aperturate angiosperm taxa. These features and the presence ofabundant gymnosperms (including specimens of Elaterosporites)relate these assemblages to the Sb outcrop and the PAN sitepalynofloras.

A comparison of the Spanish palynoflora and northerncontemporaneous assemblages is far more speculative, but mayprovide interesting examples of eudicot dispersal patterns. UpperAlbian-middle Cenomanian assemblages from the amber-bearingoutcrop of Charentes, northwestern France (Peyrot et al., 2005),and the lower Cenomanian assemblages from the upper Shaftes-bury and Dunvegan Formations, Alberta (Singh, 1975, 1983; Lidgardand Crane, 1990), contained angiosperm associations with lowerdiversity than those at the PAN site and Sb outcrop. These differ-ences may be partially explained by the incompleteness of thesuccessions, at least in the case of the Canadian ones (Bhattacharyaand Posamentier, 1994). Polyaperturate-producing angiospermsare thought to have first evolved in the early Aptian or late Barre-mian from northern Gondwana and, subsequently, to have spreadpoleward during the Albian (Brenner, 1976; Doyle et al., 1977;Doyle, 1992; Hochuli et al., 2006). The hypothesis of a latitudi-nally diachronous dispersal (Brenner, 1976; Hickey and Doyle,1977), although not completely supported by the palynologicaland mesofossil records (Hughes and McDougall, 1990; Jud andHickey, 2013), would match well with the results obtained inSpain and with the cited palynoflora located further north.

6. Conclusions

The palynological study of five Cretaceous outcrops with amberdeposits reveals diverse microfloral assemblages containing 106spores, 42 gymnosperms and 68 angiosperm pollen grains.Although dominated by miospores, the recovered assemblages alsocontain marine palynomorphs, including dinoflagellate cysts,acanthomorph acritarchs, prasinophytes and foraminiferal testlinings. Spores were diverse but mainly represented by few taxa.Gymnosperms are poorly diversified and constitute the dominantcomponent of the palynological assemblages. Angiosperms are rareto sub-dominant in the studied outcrops. Their distribution in thepalynological assemblages has been interpreted to be age- andtaphonomically controlled. The age of the outcrops ranges from theAptian to the late Albian, not excluding the early Cenomanian. At aregional scale, lower tomiddle Albian materials would appear to beabsent (eroded or non-deposited). This stratigraphic hiatus mayreflect a regional angular unconformity, clearly identifiablethroughout the southern area of the sedimentary basin.

The biostratigraphy presented here, although partially con-strained by non-marine palynological markers, has made itpossible to integrate the amber-bearing outcrops into a broaderstratigraphic context and to construct palaeoenvironmental andevolutionary interpretations. The contemporaneous developmentof amber-bearing outcrops in several areas of the Basque-Cantabrian basin and in other basins suggests that a commonpalaeoenvironmental factor may have triggered the production ofamber. The structure and composition of the vegetation thatinhabited the southern area of the Basque-Cantabrian basin un-derwent significant changes during the Aptianelate Albian timespan. The Aptian flora included extended conifer forests andincorporated diversified free-sporing plants (mosses, ferns) andcomparatively few angiosperms. Conversely, the late Albian

E. Barr�on et al. / Cretaceous Research 52 (2015) 292e312 311

vegetation included much more diversified angiosperm commu-nities with numerous eudicots, and less significant free-sporingplant and gymnosperm communities. The changes in the compo-sition and structure of the Lower Cretaceous vegetation that havepreviously been described at a global scale, have now been evi-denced locally in the vegetation of the southern area of the Basque-Cantabrian basin.

Acknowledgements

This work is a contribution of the DGI projects CGL2011-23948,CGL2011-23717, CGL2011-24546 and CGL2011-25894 financed bythe Spanish Ministry of Economy and Competitiveness. DanielPeyrot publishes with the approval of the board of Robertson (UK)Ltd. We thank Xavier Delcl�os (University of Barcelona), EnriquePe~nalver and Pepa Torres (IGME), Jim Fenton (Robertson), Paulo R.Trinc~ao (Botanical Garden of the University of Coimbra) and JesúsAlonso (Natural History Museum of �Alava) for their inestimableassistance. Wewish also to thank Dr. E. Koutsoukos (Editor in Chief)and the two anonymous referees who provided valuable sugges-tions for the improvement of the manuscript.

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Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.cretres.2014.10.003.