Étude des cyclicités de dépôt dans un environnement lacustre aquitanien carbonaté à bioconstructions

Wattinne Aurélia, Vennin Emmanuelle, Wever Patrick de, Montenat Christian. Étude des cyclicités de dépôt dans un environnement lacustre aquitanien carbonaté à bioconstructions. In: Documents des Laboratoires de Géologie, Lyon, n°156, 2002. STRATI 2002. 3ème congrès français de stratigraphie. Lyon, 8-10 juillet 2002. p. 235

Environmental changes around the Oligocene/Miocene boundary in the Limagne graben, Massif Central, France

Continental environments are very sensitive to climatic variations. A unique opportunity to study the climate changes around the Oligocene/Miocene boundary is offered by the Limagne graben Basin (France) where this stage boundary is well constrained by fossils. Indeed, some localities of the Limagne Graben Basin are so rich in mammal remains that they have become a European reference for mammal biostratigraphy. The dominant sedimentary facies of the lacustrine deposits in the northern part of the Limagne Graben Basin are composed of poorly cemented marls and calcarenites containing various plants and animals remains (e.g. algae, fish bones and teeth, gastropods, ostracods, mammals, birds and reptiles remains) associated with stromatolites. Mammal remains, well described in this area from the literature, were used to constrain the chronostratigraphic context of this lacustrine basin, with refinement thanks to new carbon and oxygen isotope measurements, palynological and sedimentological data. In this work, the available information obtained from a classical paleoecological study has been refined by new carbon and oxygen isotope analyses of carbonates and fish teeth retrieved from the washed sediment residues. The results of this study confirm that the Oligocene/Miocene boundary in Europe was a general period of aridity, associated with a cooler climate, which could be the cause of the decline in mammal paleodiversity observed during this period. The gradual evolution from brackish to fresh waters is indicated by the presence of ostracods and gastropods, and the major development of caddisflies. This period is associated to humid climatic conditions while a gradual increase in temperature took place throughout the second part of the Aquitanian. These environmental changes were driven by strong variations of temperatures and a contrasted seasonality

Occurrence of tosudite in the Guezouman, Tarat and Tchirezrine 2 formations, hosts of uranium deposits in Niger (Tim Mersoï basin)

International audienceTosudite, a regularly interstratified chlorite-smectite, crystallizes as an alteration mineral of several preexisting Al-bearing silicates (feldspars, kaolin minerals, chlorites) present in arkosic sandstones hosted in uranium deposits in Niger. X-ray diffraction patterns show a sharp superstructure at 29–29.6 Å for an air-dried state and a peak at 30.8–31.6 Å following ethylene glycol solvation. The 060 reflection at 1.507–1.509 Å indicates an overall dioctahedral character, and the very low coefficient of variation of the d00l reflections for the solvated mineral (0.03–0.13) permits validation of the regular interstratification justifying its identification as tosudite. Microprobe analysis allowed specification of the component layers of this mixed-layer mineral. The chlorite is a di-trioctahedral type analogous to sudoite (Si3Al4Mg2(OH)8), and the smectite component is a low-charge montmorillonite type (Si4Al1.67Mg0.33M+0.33(OH)4). Tosudite is characterized by large Al2O3 and MgO contents and small Fe content; its composition corresponds approximately to the formula ((Si7Al)O20(Al4.5Mg2.3Fe3+0.2) M+0.3(OH)10), where octahedral occupancy is ∼7. Scanning electron microscope (SEM) observations show that tosudite is closely associated with some uranium minerals: tosudite crystallization occurred during a late alteration event which post-dates burial diagenesis and during which uranium was remobilized by Mg-rich oxidizing fluids

Changements environnementaux survenant à la limite Oligocène/Miocène du bassin des Limagnes (Massif central, France).

16 pagesInternational audienceContinental environments are very sensitive to climatic variations. A unique opportunity to study the climate changes around the Oligocene/Miocene boundary is offered by the Limagne graben Basin (France) where this stage boundary is well constrained by fossils. Indeed, some localities of the Limagne Graben Basin are so rich in mammal remains that they have become a European reference for mammal biostratigraphy. The dominant sedimentary facies of the lacustrine deposits in the northern part of the Limagne Graben Basin are composed of poorly cemented marls and calcarenites containing various plants and animals remains (e.g. algae, fish bones and teeth, gastropods, ostracods, mammals, birds and reptiles remains) associated with stromatolites. Mammal remains, well described in this area from the literature, were used to constrain the chronostratigraphic context of this lacustrine basin, with refinement thanks to new carbon and oxygen isotope measurements, palynological and sedimentological data. In this work, the available information obtained from a classical paleoecological study has been refined by new carbon and oxygen isotope analyses of carbonates and fish teeth retrieved from the washed sediment residues. The results of this study confirm that the Oligocene/Miocene boundary in Europe was a general period of aridity, associated with a cooler climate, which could be the cause of the decline in mammal paleodiversity observed during this period. The gradual evolution from brackish to fresh waters is indicated by the presence of ostracods and gastropods, and the major development of caddisflies. This period is associated to humid climatic conditions while a gradual increase in temperature took place throughout the second part of the Aquitanian. These environmental changes were driven by strong variations of temperatures and a contrasted seasonality.Les environnements continentaux sont très sensibles aux variations climatiques. Une opportunité unique d’étudier les variations climatiques intervenant autour de la limite oligo-miocène est fournie par le bassin des Limagnes, car cette limite y est bien identifiée par les fossiles. En effet, certaines localités du bassin sont si riches en restes de mammifères qu’elles sont devenues des références pour la biostratigraphie mammalienne européenne. Les différents faciès sédimentaires présents dans les dépôts lacustres de la partie nord du bassin sont peu indurés. Il s’agit de marnes et de calcarénites qui ont livré de nombreux fossiles de plantes et d’animaux (algues, restes de poissons, gastéropodes, ostracodes, mammifères, oiseaux et reptiles), associés à des stromatolites. Les mammifères fossiles ont été utilisés par le passé pour contraindre le contexte chronostratigraphique du bassin lacustre ; ces données ont été affinées dans ce travail par une étude palynologique et sédimentologique. Les informations obtenues lors de cette étude paléoécologique classique ont été complétées par l’analyse en isotopes stables des carbonates et des dents de poissons issues des lavages de sédiments. La présence d’une période d’aridité probablement à l’échelle de l’Europe à la limite oligo-miocène est confirmée par cette étude. Cette période d’aridité, associée à un climat plus froid, comme l’atteste la présence de pollens spécifiques, est probablement responsable de l’appauvrissement de la diversité mammalienne également observée durant cette période. Puis, le passage du milieu saumâtre à un milieu d’eau douce se met en place, à partir de l’Aquitanien, attesté par la présence d’ostracodes et de gastéropodes et le développement exceptionnel des phryganes (insectes Trichoptères). Ce passage est associé à des conditions climatiques de nouveau plus humides. La température, quant à elle, augmente graduellement dans la seconde partie de l’Aquitanien. Ces changements environnementaux ont eu lieu sous un climat à fortes variations de températures et une saisonnalité contrastée

Changements fauniques en Limagne à la limite Oligocène-Miocène : les mammifères, oiseaux et ostracodes des différents niveaux de Billy-Créchy (Allier)

Hugueney Marguerite, Berthet Didier, Bodergat Anne-Marie, Escuillié François, Mourer-Chauviré Cécile, Wattinne Aurélia. Changements fauniques en Limagne à la limite Oligocène-Miocène : les mammifères, oiseaux et ostracodes des différents niveaux de Billy-Créchy (Allier). In: Documents des Laboratoires de Géologie, Lyon, n°156, 2002. STRATI 2002. 3ème congrès français de stratigraphie. Lyon, 8-10 juillet 2002. p. 129

Oligo-Miocene lacustrine microbial and metazoan buildups from the Limagne Basin (French Massif Central)

International audienceThe Limagne Basin (French Massif Central) is an extensive continental lacustrine system accommodating microbialand metazoan buildups from Chattian to Aquitanian age. A description of these buildups and theirassociated biotic components in Grand Gandaillat and Crechy quarries provides insights into their spatio-temporaldistribution patterns. Flats, cauliflowers, domes, cones and coalescent columnar morphologies have beenidentified with a main laminated mesofabric and laminated, columnar, filamentous and caddisfly-coated microfabrics.Two low-gradient margin models emerged based on the changes in the distribution, morphology andsize of the microbial and metazoan-rich deposits through time: the first model applies to the Chattian deposits inthe Grand Gandaillat quarry and the second to the Aquitanian deposits in the Crechy quarry.This study emphasizes the heterogeneity of marginal lacustrine carbonate-siliciclastic cycles. The lacustrine/palustrine cycles of the Limagne Basin record (i) lacustrine deposits composed of microbial and metazoanbuildups and organic matter-rich marls indicating periods of high accommodation, and (ii) palustrine depositscomposed of mudstone and clayey paleosoils, indicative of periods of low accommodation. The cycles differ withthin and symmetrical deposits that dominate the Chattian cycles whereas thicker and asymmetrical depositsmark the Aquitanian cycles. In addition, the Chattian buildups are exclusively microbial and only a few centimetersthick whereas the Aquitanian ones are multiple meters in height and are composed of both microbes andmetazoans. Climatic, tectonism, volcanic and local parameters are involved in the deposition of buildups andtheir organization in each different cycle. Climate may control the long-term arid/humid sedimentary succession,tectonic may explain the difference in the deposit thickness between two cycles of different ages andlocalizations as well as the specific distribution of the buildups throughout the basin, volcanism may control thechemical lacustrine conditions and local physiography may impact the soil removal ability of the lake margin

Le bassin des Limagnes : une excursion à travers les dépôts microbiens modernes et fossiles.

35 pagesInternational audienceThe Limagne Basin (Massif Central, France) originated during a major, European-scale, extensive event (European Cenozoic Rift System), which led to the formation of several rift systems in the foreland of the Alps between the Upper Eocene and Pliocene. A fluvio-lacustrine system emplaced in the basin and resulted in a mixed carbonate-siliciclastic sedimentation in which microbial and metazoan buildups occupy an important place. However, microbial deposits are not exclusive to the Cenozoic history of the Limagne Basin; nowadays, in the basin, they still form in association with thermal spring systems. A fieldtrip was carried out in the Limagne Basin as part of the Microbialites: formation, evolution and diagenesis (M-Fed) meeting (October 2019). The objective of this excursion was to assess the diversity of modern and fossil (Chattian to Aquitanian) microbial sediments and structures in three prime locations (the Jussat and Chadrat outcrops and the Grand Gandaillat quarry). A detailed description of the morphologies and fabrics of the buildups and their associated biotic components can be used to discuss the spatio-temporal distribution pattern. Different basin margin models are proposed based on the changes in the distribution, morphology and size of the microbial and metazoan-rich deposits through time. The Jussat outcrop offers novel perspectives to unravel the evolution of the lacustrine/palustrine cycles over time and to establish a long-term paleoenvironmental history of the western margin of the basin during the Aquitanian. These cycles are composed of (i) lacustrine sedimentation comprising microbial and metazoan buildups and organic matter-rich marls reflecting a period of high accommodation, and (ii) palustrine deposits made of mudstones and clayey paleosoils, indicative of a period of low accommodation. It is suggested that climatic, tectonic, volcanic and local parameters (physiography, substrate) control the deposition of the buildups in each of the different cycles. In addition, the modern microbial mats of the Sainte-Marguerite and La Poix outcrops offer an opportunity to constrain the controlling processes at the origin of the mineralization involved in the formation of the microbialites and their preservation in the fossil record.Le bassin des Limagnes (Massif Central) est issu de la mise en place d’une phase de rifting liée à la géodynamique Ouest-Eropéenne (Rift Ouest Européen) entre l’Eocène Supérieur et le Pliocène. Il représente un lieu privilégier pour étudier les bioconstructions à microbialites et métazoaires qui s’inscrivent dans une sédimentation mixte carbonate-terrigène fluvio-lacustre. Ce secteur bénéficie par ailleurs de la présence de tapis microbiens modernes. L’excursion organisée en octobre 2019, à l’occasion du colloque Microbialites: formation, evolution and diagenesis , dans cette région permet d’observer et d’approcher la diversité et la structuration de sédiments microbiens modernes et fossiles. Une description détaillée de la morphologie, des fabriques des bioconstructions chattiennes et aquitaniennes et de leurs composants biotiques, des affleurements de Jussat et Chadrat et dans la carrière de Grand Gandaillat, fournit un éclairage concernant leur organisation spatio-temporelle. Les changements de distribution, de morphologie et de taille des bioconstructions à microbialites et métazoaires à travers le temps permettent de proposer plusieurs modèles de rampe lacustre. Les cycles palustre/lacustre ont fait l’objet de publication récente, toutefois le site de Jussat offre des conditions d’affleurement exceptionnelles qui soulignent le rôle du climat et de la tectonique dans la mise en place de cette cyclicité. Ces cycles sont organisés en une succession de dépôts : (1) lacustres composés de bioconstructions à microbialites et métazoaires et de marnes riches en matière organique qui indiquent des périodes de forte accommodation et (2) de dépôts palustres boueux et argileux à paléosols soulignant des périodes de faible accommodation. Le climat, la tectonique, le volcanisme mais également des paramètres locaux tels que la physiographie des lacs et le substrat sont impliqués dans la formation des bioconstructions et leur organisation au sein des différents cycles sédimentaires. De plus, les tapis microbiens modernes de Sainte-Marguerite et de La Poix viennent offrir une opportunité de comprendre les processus de minéralisation à l’origine de la formation des microbialites et de leur préservation dans le registre fossile