93 research outputs found

    Impacts environnementaux sur l’érosion des sols au PlĂ©istocĂšne supĂ©rieur et Ă  L’holocĂšne dans le cratĂšre de maar du lac du Bouchet (Massif central, France)

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    Une nouvelle utilisation des donnĂ©es palynologiques, sĂ©dimentologiques et chronostratigraphiques issues de la sĂ©quence sĂ©dimentaire intracratĂ©rique du maar du Lac du Bouchet (Massif central, France) permet d’étudier les relations entre les changements environnementaux et l’érosion des sols au PlĂ©istocĂšne supĂ©rieur et Ă  l’HolocĂšne dans le cratĂšre de cet appareil volcanique. Une mĂ©thode de calcul du volume et de la masse des couches de sĂ©diments dĂ©posĂ©s Ă  l’intĂ©rieur du cratĂšre a Ă©tĂ© Ă©laborĂ©e afin d’évaluer les taux moyens d’érosion Ă  la surface du bassin versant. Ces taux ont Ă©tĂ© trĂšs Ă©levĂ©s pendant les pĂ©riodes les plus froides du dernier Ă©pisode glaciaire wĂŒrmien (Lanterne I et III, Dryas ancien), avec des valeurs comprises entre 180 et 210 t.km-2.an-1. Au contraire, durant les pĂ©riodes tempĂ©rĂ©es interglaciaires de l’EĂ©mien et de l’HolocĂšne, la vitesse de l’érosion a Ă©tĂ© plus lente, variant entre 10 et 50 t.km-2.an-1. Les analyses polliniques et sĂ©dimentologiques des dĂ©pĂŽts lacustres intracratĂ©riques permettent d’interprĂ©ter ces chiffres dans le cadre de l’évolution palĂ©oenvironnementale rĂ©gionale. Au cours des pĂ©riodes froides, l’érosion dĂ©tritique a Ă©tĂ© forte et intense dans le bassin versant du Lac du Bouchet, car la vĂ©gĂ©tation basse et ouverte Ă  caractĂšre steppique protĂ©geait moins bien le substrat des agents mĂ©tĂ©oritiques. Par contre, durant les Ă©pisodes climatiques tempĂ©rĂ©s, la vĂ©gĂ©tation forestiĂšre qui recouvrait la surface du bassin versant a induit une rĂ©duction des flux sĂ©dimentaires. D’autre part, on a pu constater une hausse des taux d’érosion dans la seconde moitiĂ© de l’HolocĂšne, Ă  partir du SubborĂ©al, en relation avec les dĂ©forestations rĂ©gionales d’origine anthropique pour l’exploitation agricole du territoire.A new use of the palynological, sedimentological and chronostratigraphical data issued from the intracrateric sedimentary filling of the “Lac du Bouchet” maar (Massif Central, France) gives an opportunity to study the relationships between the environmental changes and soil erosion in this volcanic crater from the Upper Pleistocene to the Holocene. A calculation method of the volume and mass of sediment layers deposited inside the crater were developed in order to assess the mean erosion rates in the drainage basin. These rates were very high during the coldest periods of the last WĂŒrmian glaciation (Lanterne I and III, Oldest Dryas), with values ranged from 180 to 210 t.km-2.yr-1. To the opposite, the erosion was slower for the Eemian and Holocene interglacial temperate periods, with rates varying between 10 and 50 t.km-2.yr-1. From the palynological and sedimentological studies of the intracrateric deposits, these numbers can be interpreted in the context of the regional palaeoenvironmental evolution. In the cold periods, erosion was deeply intense and coarsely detrital due to the fact that the drainage area was poorly protected by a low and open steppic vegetation. In contrast, during the temperate climatic episodes, the watershed ground surface was covered with forests, which induced a quieter morphogenesis and reduction of sedimentation fluxes. Besides, we can point out a rise in erosion rates throughout the second part of the Holocene, from the Subboreal, owing to the anthropogenic regional forest clearings for land cultivation

    Chronostratigraphy of two Late Pleistocene loess-palaeosol sequences in the RhĂŽne Valley (southeast France)

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    International audienceA sedimentological and chronostratigraphical investigation was carried out on two loess sections located in the Mediterranean area in southeast France along the Rhîne River (Lautagne) and the lower reach of a tributary of the Rhîne River (Collias). High-resolution sampling (5–20 cm) for magnetic susceptibility, grain size distribution (including non-parametric end-member modelling), colour reflectance and geochemistry was performed. The chronology was based on luminescence dating of quartz grains and radiocarbon dating of small gastropod shells, coupled with hierarchical Bayesian modelling. The Collias section (~8 m thick) records the whole last climatic cycle. It comprises a thick red basal pedocomplex S1 developed during the Last Interglacial and the Early Glacial, similar to that observed elsewhere in southern and southeastern Europe. Loess deposition occurred during the Lower (L1L2) and the Upper Pleniglacial (L1L1). It was interrupted by soil formation during the Middle Pleniglacial, of which a brown Bwk horizon has been preserved (L1S1). By contrast, the ~5 m thick Lautagne section provides a detailed record of the Upper Pleniglacial. Weakly developed hydromorphic soils are correlated with the Greenland Interstadials GI-4 to GI-2, while the main period of coarse loess sedimentation corresponds to the Greenland Stadials GS-5 to GS-2. At a regional scale, the time of loess deposition ranges between 38.5 ka and 12 ka, with a peak at ~28–24 ka, overlapping with the maximal advance of the Alpine Ice Sheet (AIS). This strongly suggests that regional glacier dynamics was the main driver of loess sedimentation

    Mesure de l'érosion post-éruptive autour des cratÚres de maars en inversion de relief dans le Massif central français / Post-eruptive erosion measurement around the inverted maar craters in the French Massif Central

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    Abstract Topographically inverted and residual maar craters form reliable morphostratigraphic and palaeogeographical landmarks that help to estimate long-term erosion. By reconstructing the initial morphometry of the crater, we can calculate the relative elevation of the pre-emptive palaeosurface that was intersected by the maar. Initial crater morphometric features were determined from the morphostructural study of thirteen well-preserved Quaternary maars. The diameter and depth of the crater, the mean angle of the crater walls and the underlying diatrĂšme, as well as the depth of the excavated crater in the country rocks have been used as morphometric criteria. By measuring the elevation of the bottom of the volcanic and/or sedimentary filling that buries the crater floor, on the one hand, and the diameter and elevation of the underlying diatrĂšme breccias, on the other hand, the initial crater diameter and relative elevation of the pre-eruptive palaeosurface were calculated. The eroded bedrock thickness around the crater was then assessed by subtracting the elevation of the current topographic level from the elevation of the pre-eruptive palaeosurface. This new calculation method is presented and applied to nineteen Neogene and Quaternary maars in the French Massif Central. Post-eruptive erosion around these maars ranges from 100 to 500 m. It occurred at mean rates ranging from less than 10 mMy' to more than 100 mMy1.RĂ©sumĂ© Les cratĂšres des maars mis en inversion de relief constituent des repĂšres morphostratigraphiques et palĂ©ogĂ©ographiques utiles pour mesurer l'Ă©rosion Ă  long terme. En reconstituant la morphomĂ©trie originelle du cratĂšre, il est possible de calculer l'altitude de la palĂ©otopographie prĂ©-Ă©ruptive sur laquelle s'est formĂ© le maar. Pour cela, les caractĂ©ristiques morphomĂ©triques initiales d'un cratĂšre de maar ont d'abord Ă©tĂ© dĂ©terminĂ©es d'aprĂšs l'analyse morphostructurale de treize Ă©difices quaternaires bien conservĂ©s. Les paramĂštres utilisĂ©s sont le diamĂštre et la profondeur du cratĂšre, l'angle moyen des parois de ce dernier et du diatrĂšme sous-jacent, ainsi que la profondeur de l'excavation du cratĂšre dans le substrat. À partir des mesures de l'altitude de la base du remplissage volcanique et/ou sĂ©dimentaire qui fossilise le fond du cratĂšre, d'une part, et du diamĂštre et de l'altitude des brĂšches de diatrĂšme sous-jacentes, d'autre part, le diamĂštre initial du cratĂšre et l'altitude de la palĂ©otopographie prĂ©-Ă©ruptive ont Ă©tĂ© calculĂ©s. L'Ă©paisseur de substrat Ă©rodĂ© autour du cratĂšre a ensuite Ă©tĂ© estimĂ©e en retranchant l'altitude actuelle du niveau topographique atteint par l'Ă©rosion de celle de la palĂ©otopographie prĂ©- Ă©ruptive. Cette nouvelle mĂ©thode de calcul a Ă©tĂ© appliquĂ©e Ă  dix-neuf maars nĂ©ogĂšnes et quaternaires du Massif central français, en supposant qu'ils possĂ©daient des geometries comparables Ă  celles des appareils quaternaires bien conservĂ©s. L'Ă©rosion post-Ă©ruptive autour des maars Ă©tudiĂ©s se situe entre 100 et 500 m et s'est effectuĂ©e selon des taux moyens variant de moins de 10 m.Ma1 Ă  plus de 100 m.Ma-'.Degeai Jean-Philippe. Mesure de l'Ă©rosion post-Ă©ruptive autour des cratĂšres de maars en inversion de relief dans le Massif central français / Post-eruptive erosion measurement around the inverted maar craters in the French Massif Central. In: GĂ©omorphologie : relief, processus, environnement, Octobre-dĂ©cembre, vol. 10, n°4. pp. 285-304

    Évolution Morphostructurale Du Plateau Volcano-SĂ©dimentaire de Gergovie Au MiocĂšne InfĂ©rieur : Implications GĂ©odynamiques Sur La Phase Tardi-Tectonique Du Rift de Limagne (Massif Central, France)

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    The Gergovie plateau is a Lower Miocene topographically inverted volcano-sedimentary system located in the monogenetic volcanic field of the Limagne rift Tertiary basin. It is composed of three east\textendashwest aligned maars partly covered by a basaltic lava flow. The eruption of the central maar (maar 1) occurred at the Oligocene\textendashMiocene transition, during the first volcanic phase. This phreatomagmatic structure was almost totally cut through by the opening of a second maar (maar 2) during the next eruptive phase. The basaltic lava flow at the summit and the eastern maar (maar 3) were placed during a third and last eruptive phase during the Middle or Upper Burdigalian (∌19\textendash16 Ma). Between these periods of volcanism, three fluvial to fluviolacustrine sedimentation episodes, separated by two erosive stages, followed one another. A bedrock thickness of 100\textendash300 m was eroded from maar 2 during the upper Aquitanian and (or) the lower Burdigalian (∌22\textendash19 Ma). This erosion is partly due to a volcano-tectonic uplift in the southern Limagne. The complex morphostructural evolution of the Gergovie plateau demonstrates the north\textendashsouth geodynamic differentiation of the Limagne rift during the Lower Miocene, since the northern part of the basin corresponded to a relatively calm lacustrine sedimentation area. More generally, the Miocene volcanic field in the South of the Limagne gives an opportunity to study interactions between volcanism, tectonics, and erosion during the late passive rifting activity phase

    Did a maar shelter the ancient port of Agde ? A volcanological perspective

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    Calcul de l'érosion à Long Terme En Région de Socle Autour de Grands AstroblÚmes Du Québec et de France

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    Résumé Une nouvelle méthode de calcul de l'érosion à long terme est proposée à partir des caractéristiques morphométriques des cratÚres d'impact complexes. Cette méthode permet d'estimer l'épaisseur de la tranche de substrat érodé autour d'un astroblÚme, forme dégradée d'un cratÚre d'impact. Pour cela, la morphométrie initiale du cratÚre est reconstituée afin d'estimer l'altitude de la topographie pré-impactique. Deux paramÚtres morphométriques sont retenus~: le diamÚtre de la structure d'impact et sa profondeur réelle, c'est-à-dire la distance entre la topographie pré-impactique et la base des brÚches d'impact polymictiques. Une relation entre ces deux paramÚtres a été déterminée à partir d'une base de données portant sur 31 cratÚres d'impact complexes terrestres bien conservés. Cette méthode est appliquée à quatre grands astroblÚmes, d'ùge dévonien à triasique, situés au Québec (Charlevoix, Manicouagan, lac à l'Eau Claire est) et dans l'ouest du Massif central français (Rochechouart). Les tranches de substrat érodé autour de ces astroblÚmes depuis leur formation sont estimées entre 100 et 2~000~m d'épaisseur. Les taux moyens d'érosion post-impactique sont relativement faibles, avec des valeurs comprises entre 1 et 10~m/Ma. Au Québec, l'érosion à long terme au cours des temps phanérozoïques semble avoir été 2 à 4 fois plus forte sur la bordure du Bouclier canadien (Laurentides) qu'à l'intérieur (Labrador). , Abstract Morphometric characteristics of complex impact craters are used in order to develop a new calculation method of long-term erosion. This one is based on an assessment of the eroded bedrock thickness around an astrobleme, a morphologically degraded impact crater. First, the initial crater morphometry is reconstructed, in order to determine the elevation of the pre-impact surface. Two morphometric parameters are used~: the diameter of the impact structure and its true depth, i.e. the height difference between the pre-impact surface and the bottom of the polymictic impact breccias. A relationship between both parameters was established from a database of 31 morphologically well-preserved terrestrial complex impact craters. This calculation method was applied to four Devonian to Triassic large astroblemes located in Québec (Charlevoix, Manicouagan, Clear Water Lake) and the western French Massif Central (Rochechouart). Bedrock thicknesses eroded around these astroblemes since their formation are evaluated between 100 and 2~000~m. Post-impact mean erosion rates are relatively low, with values ranging from 1 to 10~m/Ma. In Québec, the long-term erosion on the southeastern edge of the Canadian Shield (Laurentides) during the Phanerozoic times seems have been from 2 to 4 times higher than the one in the interior (Labrador)
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