Search

The Eocene-Oligocene Transition (EOT) marks the onset of the Antarctic glaciation and the switch from greenhouse to icehouse climates. However, the driving mechanisms and the precise timing of the EOT remain controversial mostly due to the lack of well-dated stratigraphic records, especially in continental environments. Here we present a cyclo-magnetostratigraphic and sedimentological study of a ∼7.6 Myr-long lacustrine record spanning the late Eocene to the earliest Oligocene, from a drill-core in the Rennes Basin (France). Time-series analysis of natural gamma-ray (NGR) log data shows evidence of Milankovitch cycle bands. In particular, the 405 kyr stable eccentricity is expressed with strong amplitudes. Astronomical calibration to this 405 kyr periodicity yields duration estimates of Chrons C12r through C16n.1n, providing additional constraints on the middle–early Eocene timescale. Correlations between the orbital eccentricity curve and the 405 kyr tuned NGR time series and assumptions on their phase relationships, enable to test previously proposed ages for the EO boundary, indicating that 33.71 and 34.10 Ma are the most likely. Additionally, the 405 kyr tuning calibrates the most pronounced NGR cyclicity to a period of ∼1 Myr matching the g1-g5 eccentricity term. Such cyclicity has been recorded in other continental records, pointing to its significant expression in continental depositional environments. The record of g1-g5 and sometimes g2-g5 eccentricity terms in previously acquired sedimentary facies proxies in CDB1 core led us to hypothesize that the paleolake level may have behaved as a lowpass filter for orbital forcing. Two prominent changes in the sedimentary facies were detected across the EOT, which are temporally equivalent to the two main climatic steps, EOT-1 and Oi-1. Combined with previously acquired geochemical (δ15Norg, TOC), mineralogical (Quartz, clays) and pollen assemblage proxies from CDB1, we suggest that these two facies changes reflect the two major Antarctic cooling/glacial phases via the hydrological cycle, as significant shifts to drier and cooler climate conditions, thus supporting the stepwise nature of the EOT. Remarkably, a strongly dominant obliquity expressed in the latest Eocene corresponds in time to the interval from the EOT precursor glacial event till the EOT-1. We interpret the obliquity dominance as reflecting preconditioning phases for the onset of the major Antarctic glaciation, either from its direct impact on the formation/(in)stability of the incipient Antactic Ice Sheet (AIS), or through its modulation of the North Atlantic Deep Water production given the North Atlantic coastal location of the CDB1 site.

International audience; Borehole CDB1 (675.05 m) crosses the deepest Cenozoic sedimentary basin of the Armorican Massif, the Rennes Basin, to reach the underlying basement at a depth of 404.92 m, made up of the Late Neoproterozoic to Early Cambrian Brioverian Group, weathered down to 520 m depth. The basin's Cenozoic deposits are divided into seven formations, ranging from Early–Middle Bartonian to Late Pliocene in age. Coastal sediments at the very base, along with a thick Priabonian lacustrine episode, imply a major revision of the regional palaeogeography, whilst a very steady and low-energy lacustrine-palustrine environment throughout the Priabonian and Early Rupelian argue for an aggradational system associated with uniform subsidence. Palynological assemblages attest to environmental and climatic changes through the Eocene and Early Oligocene, in accordance with regional and global trends (Eocene–Oligocene Transition).

International audience; The uppermost Cretaceous to early Palaeogene is a period of major deformations of the western part of the Eurasian plate with prominent basin inversions starting from the Coniacian onwards. These deformations occur in a complex geodynamic setting within both the context of the Africa-Eurasia convergence and the North Atlantic opening. While Mesozoic graben inversions have been extensively studied, particularly in Eastern Europe and the North Sea, more gentle deformations that affect thicker crust areas (intracratonic basins and emerged lands) are not as well documented. The objective of this study is to constrain the exact timing, type and magnitude of the early Palaeogene deformations affecting the intracratonic Paris basin and to integrate them at the Western European scale. Relatively gentle deformations are attempted through a high-resolution reconstitution of its stratigraphic record based on outcrops and well-dated wells, and a high number of well-logs that are correlated using the "stacking pattern" sequence stratigraphic technique. Two orders of sequences are identified (third- and fourth-order) and correlated throughout the basin. Basin geometric and palaeogeographic reconstitutions are based on sediment thickness and facies analysis. Two-dimensional accommodation space measurements were taken in order to quantify the magnitude of the deformations. Three phases of deformation were recognized. 1. An intra-Maastrichtian-pre-Thanetian (59 Ma) deformation, with major uplift and erosion of the Cretaceous strata with two sub-periods of deformation: Maastrichtian-pre-middle-Danian and Upper Danian-pre-Thanetian long wavelength deformations. This period of major deformation is coeval with Upper Cretaceous-pre-Danian compressive deformations linked to the Africa-Eurasia convergence in southern France and with volcanic activity from the North Atlantic to Massif Central and the Rhenish Shield during the Palaeocene; 2. an early Ypresian (55.1-54.3 Ma) medium wavelength deformation (× 10 km), here reported to be a stress rearrangement related to the onset of the North Atlantic opening; 3. an uppermost Ypresian (49.8 Ma) long wavelength deformation (× 100 km), contemporaneous with flexural compressive deformations in the Aquitaine Basin (Pyrenean deformation), and related to the Iberia-Eurasia convergence.

For the first time the Carnivora and the Perissodactyla from Aubenas-les-Alpes are studied. Five taxa are identified: two Carnivora, Nimravus intermedius and a medium-sized species of amphicyonid and three Perissodactyla Ronzotherium romani , Molassitherium albigense and Cadurcotherium cayluxi . The microflora shows a subtropical to Mediterranean climate with a marked dry season and indicates an age near the Rupelian-Chattian boundary for the site.

The climatic transition between Eocene and Oligocene (EOT) is marked by the change from a greenhouse to an icehouse climatic mode that affects both the marine and terrestrial realms. Terrestrial ecosystems are more sensitive to climate changes but suffer from the lack of continuous records. Recently, the BRGM retrieved a 340 m-long core from the Rennes Basin, which includes a lacustrine sedimentary series that covers the EOT. In order to decipher the climatic evolution during the EOT and its impacts on ecosystems, we analyzed molecular biomarkers and their isotopic composition. Indices calculated from branched GDGT concentrations allowed us estimating the evolution of mean annual air temperature. The paleohydrological reconstruction relied on the dD of n-alkanes and the Average Chain Length (ACL) of n-alkanes. The evolution of temperature records is marked by two cooling episodes (one during the late Early Priabonian and a second during the Early Rupelian) and warming intervals observed during the Early Priabonian and the EOT. From the Late Priabonian to the Early Rupelian, recurrent ACL and n-alkanes dD values variations indicate changes in relative humidity. One of these variations occurs during the EOT and is interpreted as the transition from humid to dry conditions. The evolution of vegetation is unraveled through specific biomarkers of ferns (fernenes), gymnosperms (diterpenes) and angiosperms (triterpene ketones) and compared with palynological results. The latter shows an increase in mesothermal species from the Late Eocene to the Early Oligocene, in agreement with the global cooling that characterizes this interval. However, the evolution of biomarkers does not show any global trend but high frequency changes from the Late Eocene to the Early Oligocene. These high frequencies are consistent with the rhythm of ACL values. The evolution of continental ecosystems thus appears mainly paced by changes in relative humidity.

On behalf the CINERGY project, a 675m-long borehole was cored through the sedimentary column (405 m) and the basement (270 m). The studied interval encompasses 3 lithostratigraphic units, from 66m to 405m depth. The chronostratigraphic framework of the sedimentary series relies on benthic foraminifera and palynology. Palaeoenvironmental reconstruction is based on sedimentology, pollen analysis and clay mineralogy. 95% of the series deposited continuously from the Early-Middle Bartonian to the Rupelian, including 290 m-thick lacustrine shaly organic deposits, alternating open and closed system depending on the floatting vegetal mats extension. Climate evolution is illustrated by palynological content. The Eocene climate evolves from a from warm and humid condition to a more contrasted seasons. the EOT is marked by a short drier and fresher period, the Rupelian is characterized by disappearance of megathermic taxa but back to more humid condition. Clay mineralogy changes more abruptly at the E/O boundary (~200 m depth), from an almost pure kaolinitic content to a kaolinite-smectite dominated assemblage. Whether the sudden mineralogical change across de E/O boundary is related to a change in source material or to a change in the hydrolysis in soil clay production, a climatic origin is very likely.

International audience; A unique high resolution record of climate and vegetation changes during the EOT. The CDB1 core, Rennes Basin, France. Climate and Biotic Events of the Paleogene

International audience; C4 plants are widely successful in the grass-dominated ecosystems of tropical, subtropical, and warm-temperate regions, largely as a result of their ability to limit photorespiration and improve water-use efficiency. A widely held paradigm is that low (

International audience

This study presents the results of a palynological analysis (dinocysts, spores and pollen) that are discussed with those obtained from stable isotopes geochemistry (δ13C and δ18O) on Miocene coastal and continental deposits in the southern area of the Digne Valensole Basin. Palynology study has been conducted on dark levels while isotopic analysis was performed on paleosol carbonate nodules.

Resume Des grains de pollen d'un paletuvier du genre Avicennia , typique des mangroves neogenes d'Europe, ont ete observes dans des facies marins de la serie deltaique du Miocene moyen de Châteauredon (Sud-Est de la France). Le cadre stratigraphique conduit a proposer pour ces niveaux un âge entre 15,8 et 16,5 Ma. L'âge et la position de ces grains dans des facies transgressifs sont en accord avec le developpement connu de la mangrove en Mediterranee occidentale, rapportee a l'intervalle N8 et NN4 pro parte , et associee au haut niveau marin du Langhien. Cette mangrove, a une paleolatitude de 42°N, est un temoin plus septentrional que celle connue au Miocene moyen en Provence. Elle implique un gradient latitudinal climatique plus faible que l'Actuel. Pour citer cet article : J.-J. Châteauneuf et al., C. R. Geoscience 338 (2006).

Dans le cadre du projet CINERGY, un forage carotté de 675 m a recoupé l'ensemble de la série sédimentaire remplissant ce bassin, puis la partie altérée du socle sur plus de 100 m et le socle sain sur 150 m. La série sédimentaire carottée a fait l'objet d'une première étude intégrant biostratigraphie, minéralogie, sédimentologie de faciès et analyse séquentielle, visant à établir le cadre chronostratigraphique, l'évolution des paléoenvironnements et du climat. L'intervalle étudié (66-405 m) comprend 4 unités lithostratigraphiques : - les Marnes à Natica (66-85 m) correspondent à des dépôts laguno-marins argilo-calcaires. La malacofaune et les associations de dinokystes sont typiquement rupéliennes ; - les Sapropèles inférieurs (85-206 m) sont caractérisés par des alternances d'argiles laminées et massives à traces de racines. Le Rupélien inférieur est attesté par la présence du pollen B. hohli. La limite E/O est localisée dans l'intervalle 189-206 m ; - les Argiles à Mohria (206-375 m) ont des faciès similaires, mais particulièrement riches en matière organique. Ces deux formations témoignent d'un environnement lacustre à palustre. Les associations polliniques sont d'âge Priabonien ; - la Formation basale argilo-sableuse (375-405 m) correspond à l'apparition des premiers sables. Les environnements de dépôts fluctuent entre fluvio-lacustre et fluvio-marin. Ces dépôts ont pu être datés par les pollens et les foraminifères du Lutétien supérieur au Bartonien. L'analyse séquentielle de cette succession montre deux MFS majeures, l'une dans les dépôts côtiers vers 380 m, l'autre dans les dépôts très organiques lacustres vers 240 m. A l'échelle de la séquence élémentaire, deux types d'unités génétiques sont définies selon l'environnement lacustre ou lagunaire. La comparaison du bassin de Rennes avec celui de Saffré indique une subsidence nettement plus importante pendant le Priabonien à Rennes. La comparaison avec d'autres bassins ouest-européens place les bassins de Rennes et Saffré dans une dynamique intermédiaire entre les bassins d'effondrement très subsidents comme le fossé rhénan et les bassins intracratoniques parisien, belge ou anglais. L'étude palynologique montre un changement important de la végétation à la fin de l'Eocène avec l'augmentation des Pinacées, puis un changement plus profond du climat à l'Oligocène avec le développement des Herbacées, attestant d'une saison sèche plus marquée, et l'apparition de la chlorite-smectite. A l'échelle ouest-européenne, on constate la simultanéité de ce refroidissement fini-éocène. Une étude à plus haute résolution de cet intervalle clé, particulièrement bien enregistré à Rennes, fournira des réponses sur ce changement global.

International audience; Le système de l'ECRIS correspond à une phase de rifting intracontinental attribuéeà l'Eocène supérieur-Oligocène, et couramment rapporté à une extension oligocène. La dynamique de l'ouverture de ces bassins dans le contexte compressif lié à la convergence Afrique-Eurasie restent cependant controversée. Plusieurs hypothèses sont évoquées concernantles mécanismes, dont (1) une activité mantellique, (2) une extension surla plaque européenne liée à la subduction alpine (slab pull ou slab roll-back), et (3) une transtension liée à un mouvement décrochant entrainé par les contraintes N-S pyrénéennes et la convergence Apulie-Eurasie. Ladynamique de l'ouverture de ces fossésest ici étudiée par une analyse du remplissage sédimentaire couplée à une étude structurale des bordures. Nous présenterons ici, les résultats acquis sur le fossé rhénan (partie française). Cette étude nous a permis d'identifierquatre grandes étapes : (1) Lutétien-Bartonien : un premier stade d'ouvertures'enregistre par la mise en place de bassinsà sédimentation lacustre d'extension spatiale limitée. Ces bassins, limités par des failles de direction N060° et N010-020° héritées de l'orogenèse varisque,s'ouvrent en décrochement (senestre ?)dans un contexte compressif N-S. (2) Priabonien: la subsidence est plus homogène avec l'apparition d'un seul bassindont l'extension géographique est plus large que les limites actuelles du bassin.La structuration de ce bassin est essentiellement contrôlée par les failles normales de direction N010-020°, plus nombreuses à l'inverse des failles de direction N060°. Cette période enregistre un changement progressif des contraintescompressives N-S à des contraintes extensives E-W. (3) Rupélien inférieur : la subsidence semble se généraliser à l'échelle du bassin. Trois grands compartiments structuraux du nord au sud sont identifiés et limités par des failles de transfert reprenant les directions initiées au premier stade (1). Chacun présente une asymétrie E-W avec, dans les zones Nord et Sud, un jeu plus important le long de la bordure Est, alors que dans la zone médiane, un jeu plus important est identifié sur la bordure Ouest. La tectonique cassante, moins active est essentiellement caractérisée par les failles normales de direction N010-020° suggérant des contraintes extensives E-W. (4) A partir de l'Oligocène supérieur et du Miocène, le contrôle structural s'intensifie et s'enregistre par de grands décrochements contemporains de la collision continentale de l'orogène alpin. Pendant la période dite « syn-rift », il est montré que le contrôle structural et l'amplitude des mouvements verticaux restent faibles en comparaison à la période dite « post-rift ».