The Oliete Sub-basin is located in the link zone between the Iberian and the Catalonian Coastal Ranges (Teruel Province, Spain). The palynological samples have been collected in the Upper Aptian–lower Albian rocks of the Escucha Formation, which present an organic-rich sedimentary succession deposited in a variety of continental and coastal environments. Four detailed sections have been studied in order to establish the stratigraphical framework to perform the palynological study. The rocks of these sections contained abundant and well-diversified palynomorph assemblages. Their study allowed the identification of 78 taxa or taxonomic groups (dinoflagellate cysts, acritarchs, phycomes of prasinophytes, algae, bryophytes, lycophytes, pteridophytes, gymnosperms, and primitive angiosperms). The quantitative percentual and the multivariate analysis performed upon the supplied palynological data supports the hypothesis of subtropical palaeoenvironments controlled by non-uniform conditions. The assemblages are comprised of parautochthonous and allochthonous elements, which reflect the existence of coniferous forests and wetlands.
Le sous-bassin d’Oliete est situé dans une zone de transition située entre les cordillères Ibérique et prélittorale Catalane (province de Teruel, Espagne). Les échantillons polliniques ont été prélevés sur du matériel daté de l’Aptien supérieur–Albien inférieur de la formation Escucha, qui présente une succession riche en matière organique, déposée en divers environnements paraliques et continentaux. Quatre sections détaillées ont été étudiées de façon à établir le cadre stratigraphique nécessaire à l’élaboration de l’étude palynologique. Le matériel de ces sections a révélé des associations palynologiques riches et diversifiées. Soixante-dix-huit taxa ou groupes taxonomiques ont pu être déterminés (kystes de dinoflagellés, acritarches, phycomes de prasynophytes, algae, bryophytes, lycophytes, pteridophytes, gymnospermes et angiospermes primitives). Les résultats apportés par les analyses quantitatives classiques et multivariées réalisées confortent l’hypothèse d’un paléoenvironnement subtropical non uniforme. Les associations sont constituées par des éléments parautochtones et allochtones, qui reflètent l’existence de forêts dominées par les conifères et d’environnements plus humides.
Palynology, Palaeoecology, Upper Aptian–lower Albian, Oliete Sub-basin, SpainPalynologie, Paléoécologie, Aptien supérieur–Albien inférieur, Sous-bassin d’Oliete, EspagnepresentedWritten on invitation of the Editorial BoardIntroduction
The Early Cretaceous, and especially the Aptian–Albian interval (125–100 Ma), are considered as key periods in the evolution of modern ecosystems [15], [16] and [32]. Exceptional palaeoenvironmental conditions characterized by globally warm temperatures [12], [20] and [59], high atmospheric CO2 levels [10] and [48], active tectonic [37] or large-scale sea level changes [23] and [25] have been proposed as possible controls of the observed changes in palaeovegetation. Besides biological factors [19], a possible low equator-to-pole temperature gradient, inferred by geochemical data [30], climate simulations [6] and supported by palaeontological evidences [27], [51] and [54] may also account for the poleward expansion of tropical vegetation including sensitive organisms such as primitive angiosperms. In this context, palaeogeography turns out to be as a major factor controlling the migration of organisms.
Situated between Eurasia and Gondwana, the Iberian Peninsula may prove to be ideally located to perform palaeobotanical studies. Among other appropriate sedimentary basins, the Oliete Sub-basin (Escucha Formation) is particularly interesting to carry out a palaeoecological analysis, since the area has been the object of a previous promising palynological studies establishing the precise taxonomical and temporal landmarks [40], [41] and [49]. We propose here a detailed quantitative palynological study of materials from the Escucha Formation, which includes a multivariate statistical analysis in order to infer the palaeoenvironmental conditions of the region.
Geological setting
The Oliete Sub-basin is located in the Iberian Range (eastern Central Spain, Figs. 1(a) and 1(b)). This extensional sub-basin was active from the Early Barremian to the Early–Middle Albian and it was infilled with sediments deposited in a variety of palaeoenvironments from continental to marine realms [35] and [50]. The stratigraphic record of this sub-basin (Fig. 1(c)) includes the Escucha Formation that has been dated back to the Early–Middle Albian in the Oliete Sub-basin based on ammonoids and palynological data [49]. However, new palynological data from this unit in the Oliete Sub-basin permits to consider an Upper Aptian–Lower Albian age for these deposits [41].
The Escucha Formation
The Escucha Formation is a heterolithic unit that lies on limestones and marls of the Urgonian Platforms (Aptian), and is covered by sandstones of the Utrillas Formation (Late Albian–Early Cenomanian) [3], [11], [39] and [44]. This unit has been interpreted as deposited in a deltaic [39] or deltaic-estuarine [44] setting. However, in the Oliete Sub-basin, at the lower part of this unit, more transgressive sedimentary systems have been recognized as barrier island systems with backbarrier marshes and flood-tidal deltas [47]. The formation can be divided into three sub-units named E1, E2, and E3 from base to top (Fig. 2).
Sub-unit E1 is composed of siltstones and mudstones usually organic-rich, some sandstone beds, and interbedded coal seams. It was deposited in a wide spectrum of coastal environments with strong marine influence, as evidenced by the presence of foraminifers and dinoflagellate cysts (Fig. 3: 1, 2, 4, 5).
E2 is a predominantly sandy sub-unit with scarce siltstone levels. The marine influence is only present as sandy tidal deposits outcropping at the southern border of the sub-basin. In other areas, E2 shows a clear continental influence and only restricted low-energy shallow coastal sub-environments, such as restricted lagoons and coastal ponds locally developed. In these sub-environments, grey organic-rich siltstones were deposited.
Sub-unit E3 is composed of variegated mudstones and siltstones with common pedogenic features all over the sub-basin. Only a local black organic-rich bed, probably deposited in a confined pond, has been found.
Material and methodsMethod of preparation and microscopy
Four stratigraphic sections of the Escucha Formation have been studied in the southern sector of the Oliete Sub-basin for palynological studies. Nine organic-rich beds from the three sub-units have been sampled for quantitative study (Fig. 2). Each sample has been prepared following the standard palynological techniques including acid treatments (HCl, HF, HNO3) at high temperatures [60]. Microscopic analysis of residues, mounted in glycerine jelly, has been performed by a Leica Laborlux D microscope, and they produced pollen counts of at least 600 per sample. Problematic specimens and/or biostratigraphic key forms has been later analysed and imaged by a Biorad 1024 confocal laser scanning microscope driven by Leica Confocal Scanware (Centro de Microscopía y Citometría, Universidad Complutense de Madrid, Spain). VGStudio Max 1.2 (Volume Graphics GmbH, Heidelberg, Germany) was employed to process the image stacks and generate three-dimensional reconstructions of the whole grains.
Multivariate analysis
In order to extract the greatest variance of the pollen data set, multivariate analysis methods have been performed. Largely employed in Quaternary palaeoecology [42], the use of multivariate analytical approach remains scarce in Mesozoic studies [14], [29], [31] and [34]. To analyse the main gradients of changes of the levels according to changes in their taxa, a correspondence analysis [9] was performed based on the levels × taxa data matrix. The correspondence analysis (CA) has been successfully employed in many palynological [43] and palaeoecological studies [24] and [52]. Later, the chi-squared distance matrix was subjected to Q- and R-mode hierarchical cluster analysis using the Ward's method [58] as agglomerative algorithm, in order to group levels and taxa according to their affinities. Eventually, to determine the grade of recruitment of groups of taxa among the different group of levels, a synthetic matrix (Fig. 7) has been elaborated based on frequencies of the groups of taxa in each group of levels. The statistical analyses were carried out with the computer programs PC-ORD 4.0 [33] and STATISTICA 6.0 (StatSoft Inc., 2001).
Results
The palynological study has yielded assemblages with high diversity of taxa. Terrestrial and marine palynomorphs (level M1), including dinoflagellate cysts (Fig. 3: 1-2, 4-5), acritarchs and foraminifer linings, have been observed [41]. Concretely, 72 terrestrial palynomorphs (Table 1) have been identified here: 44 related to cryptogams, 21 to gymnosperms and seven to primitive angiosperms.
The palynological assemblages of the levels M1 to M5 corresponding to the sub-unit E1, are dominated by pollen of gymnosperms such as Classopollis, Araucariaceae, Taxodiaceae-Cupressaceae and Alisporites (Fig. 4). By contrast, the levels U3, N1, N2 and BC, corresponding to the sub-units E2 and E3, are characterized by the high representation (levels N1 and N2) or dominance (levels U3 and BC) of the spores of vascular cryptogams. The level BC stands out by the predominance of the genera Deltoidospora and Dictyophyllidites, which account for 94% of the total palynomorph sum. The Schizaeaceae (Appendicisporites, Cicatricosisporites, Costatoperforosporites, Ischyosporites, Nodosisporites) are particularly well diversified, especially in the levels of the Unit E2.
The presence of pollen grains of primitive angiosperms (Fig. 3: 3, 6–8) is relevant in all the studied samples, except in the level BC. The tectate monosulcate pollen attributed to monocots and chloranthoid dicots are the most abundant. Clavatipollenites is the best represented, but pollen grains of the genus Liliacidites and Brenneripollis–Pennipollis complex [18], [21], [22] and [28] have been also recorded but in lower amounts. Scarce tricolpate grains (Tricolpites) have been noticed along the series. In addition, Afropollis and Stellatopollis, two genera related to angiosperms with Gondwanan affinities, have been identified in various samples.
A first CA ordination carried out with the nine levels reveals a major direction of variation accounting for 55.5% of the total variance, and confirms the original pollen composition of the level BC compared with the other levels.
Omitting the level BC, a second CA presents a variance of 82.5% in the first two dimensions (Fig. 5). The first axis (48.1% of variance) explains a gradient between Schizaeaceae, Cycadales/Ginkgoales/Bennettitales, Lycophyta, Alisporites spp., Polypodiaceae, Gleicheniaceae and Osmundaceae in the negative extreme, and Classopollis spp., angiosperms and Cyatheaceae/Dicksoniaceae/Dipteridaceae in the positive one. In this dimension, the most representative levels are U3 against M4, M3, and M2. The second axis (34.4% of variance) explains a gradient between Taxodiaceae/Cupresaceae and Pinaceae/Podocarpaceae in the negative extreme, and Schizaeaceae, Lycophyta, Briophyta and Cyatheaceae/Dicksoniaceae/Dipteridaceae in the positive one. The most representative levels are M5 and M1 in the negative, and U3 (but also N1 and N2) in the positive extreme. Cluster analysis reveals four groups of levels (Fig. 6(a)) and five groups of taxa (Fig. 6(b)). The matrix of Fig. 7 represents the percentages for each taxa group in every level-group.
Discussion and conclusions
Reconstructions of Early Cretaceous environments based on palynological studies are complex since fossil plants do not present modern counterparts. In these conditions, actualistic studies remain difficult to undertake.
The palynological analysis indicates the existence of highly diversified assemblages often dominated by spores attributed to different groups of vascular cryptogams. These plants are assumed to have grown under warm conditions (subtropical to tropical) in moist environments, although some inhabited drier locations [1] and [56]. High percentages of spores encountered in the levels corresponding to the sub-units E2 and E3 (Fig. 4) condition the composition of taxon-groups A and B (Fig. 6). Taxon-group A, including Bryophyta, Lycophyta, Gnetophyta and Polypodiaceae, may be associated with hygrophilous places related to understorey vegetation. Taxon-group B includes taxa that could be riparian such as Osmundaceae, Schizaeaceae and Cycadales/Ginkgoales/Bennettitales, and ferns of families Gleicheniaceae and Matoniaceae that were adapted to sunnier and drier places [17] and [56]. The palynomorphs that constitute Taxa groups A and B were mainly produced by local and parautochthonous elements, transported only over a small distance toward the deposition site [8]. Very likely, the spore-producers of taxon-groups A and B grew in back swamps, riverbanks or other freshwater-related environments.
Taxon group C is a miscellaneous group recruiting spores of Cyatheaceae/Dicksoniaceae/Dipteridaceae and pollen grains of Classopollis and angiosperms (Fig. 6). It is well represented in levels constituting the sub-unit E1, except in the level M5 (Fig. 4 and Fig. 7). Classopollis was produced by extinct anemophilous conifers of the Cheirolepidiaceae family. Traditionally, taxa attributed to this family are assumed drought-resistant and adapted to various environments, including saline ones [4], [38] and [53]. On the contrary, representatives of the fern families Cyatheaceae, Dicksoniaceae, and Dipteridaceae are generally associated with wet places. Nevertheless, several taxa attributed to Dicksoniaceae and Dipteridaceae present a noteworthy protection of sporangia as well as coriaceous fronds, which may indicate adaptation to stressful environments like brackish areas [56]. Palaeoecological interpretations of fossil angiosperms [13], [19], [45] and [46] support the hypothesis of pioneer herbaceous plants or shrubs capable of colonizing frequently disturbed environments including riverbanks, floodplains and back swamps. Hence, the miscellaneous taxon-group C could integrate miospores of plants occupying both coastal forests with dry conditions and wetter environments, such as riverbanks, temporary ponds, and periodically flooded plains. Taxon-group C includes local and extra-local miospores of parautochthonous and allochthonous elements.
Taxon-groups D and E are constituted by pollen grains of different types of anemophilous gymnosperms (Fig. 6). Concretely, taxon-group D only includes bisaccate pollen grains and is well-represented in level M5 of the sub-unit E1 (Fig. 4). These pollens are related to pteridosperms (Alisporites and Vitreisporites) and conifers such as Pinaceae and Podocarpaceae. It appears highly probable that all these pollen grains were produced from diverse areas, since conifers inhabited environments of varied ecological conditions including dry, saline, or upland areas [2] and [55], whereas seed-ferns are assumed to grow in moist lush vegetation, mangroves and tidally influenced environments [26] and [57]. In a similar way, taxon-group E includes inaperturate forms such as pollen grains of Araucariaceae as well as pollen grains attributed to the genera Inaperturopollenites and Cerebropollenites. Pollen of this group is well represented along the succession, except in sub-unit E3 (Fig. 4). Following an actualistic approach, it appears reasonable to attribute to this group environments of very diverse ecological conditions including saline coastal locations, but the exclusion of floodplains cannot be discarded. Contrarily, Cerebropollenites, pollen grains of uncertain filiation could be related to pioneer plants [1] and [36]. Hence, the components of taxon-groups D and E include local, extra-local, regional and surely extra-regional pollen of allochthonous elements. The grouping of these components may reflect a taphonomic bias.
The quantitative study presented here permits to precise the environmental conditions of the successive levels. According to CA ordination (Fig. 5) and pollen diagram (Fig. 4), levels from sub-unit E1 (M1 to M5) are mainly characterized by the high representation of gymnosperm pollen grains. To the contrary, the cluster analysis (Fig. 6(a)) clearly separates the level M5 (level-group 2 of Fig. 6(a)) from levels M1 to M4, bunched together (level group 1 of Fig. 6(a)). The pollen diagram (Fig. 4) and the synthetic matrix (Fig. 7) indicate that level M5 differs from the group formed by levels M1 to M4 (level-group 1 of Fig. 6(a)) by low percentages of spores of Cyatheaceae and pollen grains of Classopollis and angiosperms (taxon-group C of Fig. 6(b)) and a high representation of bisaccate pollen grains (taxon-group D of Fig. 6(b)). This could indicate the existence of forest communities widely distributed in the zone. Assemblages dominated by Classopollis pollen grains would represent forests integrated by Cheirolepidiaceae growing in more stressed conditions, while assemblages dominated by Araucariaceae, Taxodiaceae/Cupressaceae, and bissacate pollen grains (especially Alisporites, Vitreisporites), would correspond to forests occupying riverbanks or mangroves [1], [2], [4], [13] and [38]. The availability of water could be one cause explaining the existence of these two different forest communities. Nevertheless, this hypothesis must be contrasted by taphonomic considerations, already mentioned concerning the wind-dispersed pollen grains that belong to taxon-groups D and E (and especially Pinaceae/Podocarpaceae).
The predominance of these pollen grains in levels from sub-unit E1 indicates open environments of sedimentation, which can be subject to marine influence (level M1). This evidence is probably related to the wide palaeoenvironmental differentiation characterizing the palaeogeography of the area during the deposition of the lower part of the Escucha Formation [47].
The levels U3, N1 and N2 from sub-unit E2 are characterized by the high representation of spores (Fig. 4 and Fig. 7). Considering the cluster analysis (Fig. 6(a)), levels N1 and N2 appear bunched up and separated from level U3. This separation may be explained by the sedimentological characteristics of the considered levels. Level U3 arises from organic-rich sediments deposited under tidal conditions, whereas levels N1 and N2 come from grey organic-rich siltstone deposited in confined low-energy areas developed in more continental environments [47].
The high representation of spores of Bryophyta, Lycophyta and Polypodiaceae (taxon-group A) as well as of Osmundaceae and Schizaeaceae (taxon-group B) in level U3 (Fig. 4 and Fig. 7) suggests the existence of a local humid vegetation. Levels N1 and N2 differ from level U3 by the higher representation of extra-local elements such as Classopollis and Taxodiaceae/Cupressaceae pollen grains (Fig. 4). It could reflect humid environments recruiting parautochthonous and allochthonous elements such as opened ponds and back swamps. Nevertheless, the representation of wind-dispersed pollen grains is lower in sub-unit E2 than in the sub-unit E1. The sedimentological data clarified elsewhere [47] indicate that materials from sub-unit E2 have been deposited in more restrictive and continental environments.
The low diversity of the assemblage exhibited in level BC, and especially its unusual percentage of fern spores (Fig. 4) could be related to very poorly diversified communities growing in stressed habitats. The palynological assemblage of level BC (sub-unit E3) differs drastically from all other studied associations and supports original palaeoenvironmental conditions of deposition yet inferred by sedimentological evidences [47]. These ferns communities could be a possible case of recovery vegetation after crisis or represent a vegetation of replacement of more complex and diverse palaeoecosystems .
Acknowledgments
This research work was financed by projects CGL2005-07445-C03-03, CGL2005-00046/BTE and CGL2005-01765/BTE of the ‘Ministerio de Educación y Ciencia’ (Spain), as well as by the ‘Basler Stiftung für biologische Forschung’ (Switzerland). We would like to thank Drs. Luis Miguel Alonso Colmenar and Alfonso Cortés Peña (Centro de Microscopía y Citometría, Universidad Complutense de Madrid, Spain) for help with confocal microscopy.
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(a) Geological map of the Iberian Peninsula. (b) Geological map of the Iberian Range, showing the location of the Oliete Sub-basin. (c) Chronostratigraphic chart of the Lower Cretaceous of the Oliete Sub-basin.
Fig. 1. (a) Carte géologique de la péninsule Ibérique. (b) Carte géologique de la cordillère Ibérique, avec le sous-bassin d’Oliete. (c) Tableau chronostratigraphique du Crétacé inférieur du sous-bassin d’Oliete.
Stratigraphic correlation of studied sections showing location of samples. The Escucha Formation is divided in three sub-units: E1, E2 and E3.
Fig. 2. Corrélation stratigraphique des sections étudiées et localisation des échantillons. La formation Escucha est divisée en trois sous-unités : E1, E2 et E3.
1. Subtilisphaera perlucida (Alberti) Jain & Millepied, 1973. Confocal microscopy, maximum of projections, level M1. 2. Cyclonephelium compactum Deflandre & Cookson, 1955. Confocal microscopy, 3D-reconstruction, level M1. 3. Brenneripollis/Pennipollis. Confocal microscopy, 3D-reconstruction, level M4. 4. Cribroperidinium orthoceras (Eisenack) Davey, 1969. Confocal microscopy, 3D-reconstruction, level M1. 5. Same specimen as in 4. Wide-field microscopy. 6. Same specimen as in 3. Confocal microscopy, section of the grain showing the non-columellate nature of the exine. 7. Clavatipollenites hughesii Couper, 1958. Confocal microscopy, maximum of projections, level M4. 8. Stellatopollis dejaxii Ibrahim, 2002. Wide-field microscopy, level N1. Scale bar: 10 μm.
Fig. 3.1. Subtilisphaera perlucida (Alberti) Jain & Millepied, 1973. Microscopie confocale, maximum de projections, niveau M1. 2. Cyclonephelium compactum Deflandre & Cookson, 1955. Microscopie confocale, reconstruction 3D, niveau M1. 3. Brenneripollis/Pennipollis. Microscopie confocale, reconstruction 3D, niveau M4. 4. Cribroperidinium orthoceras (Eisenack) Davey, 1969. Microscopie confocale, reconstruction 3D, niveau M1. 5. Même spécimen qu’en 4. Microscopie optique traditionnelle. 6. Même spécimen qu’en 3. Microscopie confocale, section du grain montrant la nature non columellée de l’exine. 7. Clavatipollenites hughesii Couper, 1958. Microscopie confocale, maximum de projections, niveau M4. 8. Stellatopollis dejaxii Ibrahim, 2002. Microscopie optique traditionnelle, niveau N1. Barre d’échelle : 10 μm.
Miospore percentage diagram of the Oliete Sub-Basin, illustrating the main groups of taxa classified according to their botanical affinities and abundance.
Fig. 4. Diagramme pollinique (en pourcentages) du sous-bassin d’Oliete, illustrant les principaux groupes de miospores, classifiés selon leurs affinités botaniques.
Representation of the position of the levels and taxa in the two first dimensions of the correspondence analysis. Level BC was extracted from the analysis.
Fig. 5 Représentation de la position des niveaux et des taxa dans les deux premières dimensions de l’analyse factorielle des correspondances. Le niveau BC a été retiré de l’analyse.
Dendrograms of the Q-mode (a) and R-mode (b) cluster analysis using the Ward's method and the chi-squared distance matrix.
Fig. 6. Dendrogramme des modes Q (a) et R (b) de la classification ascendante hiérarchisée utilisant l’algorithme de Ward et la matrice de distance du chi2.
Reduced data matrix calculated from the main clusters of the dendrograms in Fig. 5. Representation < 15%:○, 15–30%: ●, 30–50%: Δ and > 50%: ▴.
Fig. 7. Matrice réduite calculée à partir des principales catégories des dendogrammes représentés sur la Fig. 5. Représentation < 15 % :○, 15–30 % : ●, 30–50 % : ▴ et > 50 % : ▴.
List of palynomorphs identified in the Upper Aptian–Lower Albian of the Oliete Sub-basin, modified from [41]
Tableau 1 Liste des palynomorphes identifiés dans l’Aptien supérieur–Albien inférieur du sous-bassin d’Oliete, modifié d’après [41]