The Upper Miocene basin of La Cerdaña (eastern Pyrenees, Spain) is characterised by siliciclastic sedimentation in alluvial, fluvial, deltaic, and lacustrine depositional settings. The lacustrine deposits in the southwestern part of the basin show intercalations of levels rich in organic matter linked to plant accumulations. The differential accumulation of miospores in this part of the basin is due to the intrinsic features of the miospores themselves and of their producers, and extrinsic factors such as the palaeoecology of the miospore producers and the characteristics of the depositional setting. The features of the miospore assemblages and the factors that produced their accumulation are analysed. The proportions of the different miospores in the studied assemblages showed that their accumulation occurred differently in the various lake sediments; this agrees well with a NE–SW running Neves effect. This latter effect is mainly a result of the type of transport the miospores underwent, which was influenced by the vegetation and climate, the structure of the lake, and other phenomena such as fires. The sediments from swampy areas were characterised by high percentages of local and extra-local miospores of parautochthonous and allochthonous nature, whereas those from deepwater areas showed mainly extra-local and regional miospores. The means by which the miospores were preserved in the different lithologies of the La Cerdaña basin are analysed.
Le bassin de Cerdagne (Est des Pyrénées, Espagne), daté du Miocène supérieur, est caractérisé par une sédimentation silicoclastique où dominent les dépôts alluviaux, fluviatiles et deltaïques. Les dépôts lacustres, riches en matière organique et incluant des restes végétaux, se sont déposés au sud-ouest du bassin. Une accumulation différentielle des miospores est essentiellement due à la coexistence de facteurs intrinsèques (caractéristiques de la miospore et de son producteur) et extrinsèques (paramètres environnementaux). Les différents assemblages de miospores fossiles ainsi que les facteurs qui ont déterminé leur accumulation dans les sédiments lacustres du Miocène supérieur de Cerdagne sont analysés dans ce travail. La concentration pollinique des différents niveaux étudiés révèle une accumulation différentielle dans les sédiments lacustres, en accord avec l’effet Neves, orienté NE–SW. Cet effet est lié au type de transport des miospores, à la végétation et au climat, à la structure du lac et à la fréquence d’événements tels que les incendies. Ainsi, les sédiments provenant de zones marécageuses sont caractérisés par des pourcentages élevés en miospores locales et extra-locales à caractère para-autochtone et allochtone, alors que les sédiments provenant d’eaux profondes révèlent principalement des miospores extra-locales et régionales à caractère allochtone. La préservation des miospores provenant des différentes lithologies rencontrées dans le bassin de Cerdagne a également été analysée.
A fossil plant assemblage is an accumulation of plant parts derived from one or more individuals entombed within a volume of sediments laid down under essentially common conditions
The pollen and spore walls of plants are composed of sporopollenin, a complex polymer of carotenoids and carotenoid esters
Lake bottoms with anoxic waterlogged sediments are one of the best environments for miospore preservation
Owing to its richness in preserved phytocoenoses, the palaeobotany of the lacustrine rocks of the La Cerdaña basin has been known since the 19th Century
The main aim of the present work was to describe the different miospore accumulations (thanatocoenoses;
The La Cerdaña basin is one of the small basins that formed during the Neogene in connection with a series of NE–SW- and east–west-running fractures
This paper concentrates on the analysis of the miospore associations identified in the Lower Unit materials (400–800 m), a thick succession of alluvial, fluvial, deltaic, and lacustrine sediments
The Puigcerda sub-basin, situated to the north (
The sub-basin of Bellver, situated to the south, is dominated by a very uniform lacustrine sedimentation of highly laminated diatomites and lutites rich in organic materials. At certain points, there is a well-preserved palaeofauna, including insects
Twenty-two samples from the Sanavastre open cast lignite mine were examined, along with 31 samples from the Sampsor open cast lignite mine (
Samples were prepared following the standard palynological technique
To study the different accumulations of spores and pollen in the Vallesian rocks of the La Cerdaña basin, nine palynological groups of miospores were taken into account according to their bio- and ecological characteristics: (
The palynological study of 22 levels of the Sanavastre mine led to the identification of 19 assemblages with more than 300 specimens per sample
The Sampsor mine (
In this mine, the absolute miospore concentrations did seem to be related to the lithology. The greatest miospore concentration was found in lutites and the lowest in sands. SAN7 was the richest sample, with 1864 × 103 grains/g of rock, while the poorest sample was SAN14, with 112 × 103 grains/g of rock
The Sanavastre mine and the middle section of the Sampsor mine showed similarities in terms of the proportions of palynomorph groups in each level; the mid section of the latter mine showed a stratigraphic organization similar to that seen in the Sanavastre mine
The rocks of the western lacustrine sector are formed from diatomites and mudstones. Sixteen samples showed a statistically representative number of palynomorphs (
The comparative analysis of the assemblages from the lignite mines and of those from the lacustrine outcrops clearly reflects the Neves effect
Six outcrops of diatomites and mudstones were studied in the eastern sector of the palaeolake (
The miospores incorporated into the thanatocoenoses of the La Cerdaña palaeolake clearly represent a mixture of components recruited from incoming water and the air. Surface water appears to have been the main vector of miospore transport to the lake
The anemophilous pollen of trees that grew near the lakeshore (RIP taxa, e.g.,
The composition of miospore assemblages was also affected by the Ferguson–Spicer effect
As in extant mesophilous communities
Other factors can affect the recruitment of miospores in a lacustrine basin, such as the morphology of the lake basin, the dominant winds of the region, the area's topography, the ecological and spatial structure of the plant communities
Another phenomenon that can distort the composition of thanatocoenoses is resedimentation
The miospore assemblages of the alluvial sediments of both mines were mainly represented by local and extra-local elements (
In the present assemblages, extra-regional pollen deposited far from the area of its production was recorded, including a very small number of pollen grains of the palm
Taking into account the different miospore transport processes possible, the thanatocoenoses of both the alluvial and the deep lake environments of the La Cerdaña basin might be termed allocoenoses (
The lack of a relationship between the proportion of different palynological groups and the lithology of the levels has no satisfactory explanation, although the small size of the delta may have conditioned this. Miospores produced closest to the shore would be deposited in similar amounts in both lignites and lutites. The palaeobotanical data clearly indicate that the delta area was covered by swamp vegetation – riparian alder forests, with a fern undergrowth of Osmundaceae. There are no conclusive studies confirming the presence of
Miospore preservation in the deltaic deposits of the La Cerdaña basin is however related to the type of rock. The lignites showed pollen grains with semi-destroyed exines, whereas spores were well preserved. This differential preservation is due to the high resistance of spores to diagenesis in organic matter-rich deposits (see
The sands preserved the miospores less well than either the lutites or lignites (see
The diatomitic sediments of the deep lacustrine facies showed the best-preserved miospore assemblages of the entire basin, a consequence of their rapid burial in an anoxic environment. All levels were characterised by the occurrence of
The variation in the proportions of the palynological groups in the miospore assemblages does not allow the differentiation of palynofacies 3 and 4 described by Martín-Closas et al.
In the western sector, the contrasting proportions of the palynological groups shown by levels BEL2 and BEL3, along with their high absolute miospore concentrations, may indicate environmental conditions that favoured M and RIP over BIS (
We would like to thank Dr. D. Néraudeau for inviting us to publish in this issue, as well as for his interesting opinions. We acknowledge the expert assistance of Dr C. Martín-Closas; his contribution greatly improved this paper. We also thank Dr C. Diéguez, Dr C. Arias and D. Peyrot for their constructive comments, and Dr A. Moguilevsky and A. Burton for their language review.
Geological context of La Cerdaña basin modified from Roca
Fig. 1. Contexte géologique du bassin de Cerdagne, modifié d’après Roca
Proportions of the different palynological groups in the La Cerdaña basin at (1) the Sanavastre mine (SAN), (2) the Sampsor mine (SAM).
Fig. 2. Pourcentages des différents groupes palynologiques définis pour le bassin de Cerdagne : (1) mine à ciel ouvert de lignite de Sanavastre (SAN), (2) mine à ciel ouvert de lignite de Sampsor (SAM).
Proportions of the different palynological groups in the La Cerdaña basin. (1) Western lacustrine sector. BEL: Bellver de Cerdanya outcrop, BAR: Barranc de Salanca outcrop, TOV: Torrent de Vilella outcrop. (2) Eastern lacustrine sector. COL: Coll de Saig outcrop, PRAT: Prats outcrop, RIU: Riu de Santa María outcrop, BAL: Baltarga outcrop, BED: Beders outcrop, TOB: Torrent de la Bavosa outcrop.
Fig. 3. Pourcentages des différents groupes palynologiques définis pour le bassin de Cerdagne. (1) Secteur lacustre ouest. BEL : Affleurement de Bellver de Cerdanya ; BAR : affleurement de Barranc de Salanca ; TOV : Affleurement de Torrent de Vilella. (2) Secteur lacustre est. COL : affleurement de Coll de Saig ; PRAT : affleurement de Prats ; RIU : affleurement de Riu de Santa María ; BAL : affleurement de Baltarga ; BED : affleurement de Beders ; TOB : affleurement de Torrent de la Bavosa.
(1) Photograph of the Sampsor mine taken in 1995. (2) Frontal view of the diatomiferous outcrop of Coll de Saig. (3) Tetrad of
Fig. 4. (1) Aspect général de la mine de lignite à ciel ouvert de Sampsor, en 1995. (2) Vue frontale du gisement à diatomites de Coll de Saig. (3) Tétrade d’