17 research outputs found
Le karst du synclinal de Villefranche, enregistreur de lâincision de la vallĂ©e de la TĂȘt
International audienceEl sistema kĂĄrstico de Villafranca de Conflent se ha formado en de las calizas devĂłnicas del sinclinal deVillafranca, en la margen norte de la fosa tectĂłnica neĂłgena del Conflent. EstĂĄ compuesto por una decena deniveles epifreĂĄticos horizontales, escalonados sobre un desnivel de unos 900 m, asociados a pĂ©rdidas de unpaleo Tet, cuyos aluviones se conservan en las galerĂas. Solamente los niveles inferiores alcanzan el kilĂłmetroo decenas de kilĂłmetros de longitud y los grandes complejos de En Gorner, Lachambre, Canaletes nos ofrecenun modelo para interpretar los vestigios muy limitados de los niveles superiores. Este sistema escalonadopermite una reconstituciĂłn de la historia de la incisiĂłn del valle del Tet desde el Mioceno.Le systĂšme karstique de Villefranche-de-Conflent sâest formĂ© dans les calcaires dĂ©voniens du synclinal deVillefranche, en bordure nord du fossĂ© tectonique nĂ©ogĂšne du Conflent. Il se compose dâune dizaine deniveaux Ă©piphrĂ©atiques subhorizontaux Ă©tagĂ©s sur prĂšs de 900 m de dĂ©nivelĂ©e et associĂ©s Ă des pertes dâunepalĂ©o TĂȘt, dont ils conservent les alluvions. Seuls les niveaux infĂ©rieurs atteignent des longueurs kilomĂ©triquesĂ dĂ©cakilomĂ©triques et les grands rĂ©seaux dâEn Gorner, Lachambre, Canalettes fournissent un modĂšle pourinterprĂ©ter les vestiges trĂšs limitĂ©s des hauts niveaux. Ce systĂšme Ă©tagĂ© permet de reconstituer lâhistoire delâincision de la vallĂ©e de la TĂȘt depuis le MiocĂšne
De la difficulté de dater les réseaux karstiques étagés
Les rĂ©seaux de galeries karstiques Ă©tagĂ©es contenant des alluvions fluviatiles quartzeuses peuvent enregistrer les taux dâincision des vallĂ©es. Selon un schĂ©ma conceptuel simple dâĂ©volution des cavitĂ©s souterraines, dans lequel on suppose une mise en charge alluviale des galeries suivie de leur abandon dĂ©finitif par la riviĂšre, la dĂ©termination de la durĂ©e dâenfouissement du sĂ©diment piĂ©gĂ© est possible par une mesure des concentrations de deux nuclĂ©ides cosmogĂ©niques radioactifs de pĂ©riodes diffĂ©rentes (communĂ©ment 26Al et 10Be). Lâapproche permet une datation valable pour une plage de temps variant de ~0.1 Ă ~5 Ma
De la difficulté de dater les réseaux karstiques étagés
Les rĂ©seaux de galeries karstiques Ă©tagĂ©es contenant des alluvions fluviatiles quartzeuses peuvent enregistrer les taux dâincision des vallĂ©es. Selon un schĂ©ma conceptuel simple dâĂ©volution des cavitĂ©s souterraines, dans lequel on suppose une mise en charge alluviale des galeries suivie de leur abandon dĂ©finitif par la riviĂšre, la dĂ©termination de la durĂ©e dâenfouissement du sĂ©diment piĂ©gĂ© est possible par une mesure des concentrations de deux nuclĂ©ides cosmogĂ©niques radioactifs de pĂ©riodes diffĂ©rentes (communĂ©ment 26Al et 10Be). Lâapproche permet une datation valable pour une plage de temps variant de ~0.1 Ă ~5 Ma
De la difficulté de dater les réseaux karstiques étagés
Les rĂ©seaux de galeries karstiques Ă©tagĂ©es contenant des alluvions fluviatiles quartzeuses peuvent enregistrer les taux dâincision des vallĂ©es. Selon un schĂ©ma conceptuel simple dâĂ©volution des cavitĂ©s souterraines, dans lequel on suppose une mise en charge alluviale des galeries suivie de leur abandon dĂ©finitif par la riviĂšre, la dĂ©termination de la durĂ©e dâenfouissement du sĂ©diment piĂ©gĂ© est possible par une mesure des concentrations de deux nuclĂ©ides cosmogĂ©niques radioactifs de pĂ©riodes diffĂ©rentes (communĂ©ment 26Al et 10Be). Lâapproche permet une datation valable pour une plage de temps variant de ~0.1 Ă ~5 Ma
De la difficulté de dater les réseaux karstiques étagés
Les rĂ©seaux de galeries karstiques Ă©tagĂ©es contenant des alluvions fluviatiles quartzeuses peuvent enregistrer les taux dâincision des vallĂ©es. Selon un schĂ©ma conceptuel simple dâĂ©volution des cavitĂ©s souterraines, dans lequel on suppose une mise en charge alluviale des galeries suivie de leur abandon dĂ©finitif par la riviĂšre, la dĂ©termination de la durĂ©e dâenfouissement du sĂ©diment piĂ©gĂ© est possible par une mesure des concentrations de deux nuclĂ©ides cosmogĂ©niques radioactifs de pĂ©riodes diffĂ©rentes (communĂ©ment 26Al et 10Be). Lâapproche permet une datation valable pour une plage de temps variant de ~0.1 Ă ~5 Ma
De la difficulté de dater les réseaux karstiques étagés
Les rĂ©seaux de galeries karstiques Ă©tagĂ©es contenant des alluvions fluviatiles quartzeuses peuvent enregistrer les taux dâincision des vallĂ©es. Selon un schĂ©ma conceptuel simple dâĂ©volution des cavitĂ©s souterraines, dans lequel on suppose une mise en charge alluviale des galeries suivie de leur abandon dĂ©finitif par la riviĂšre, la dĂ©termination de la durĂ©e dâenfouissement du sĂ©diment piĂ©gĂ© est possible par une mesure des concentrations de deux nuclĂ©ides cosmogĂ©niques radioactifs de pĂ©riodes diffĂ©rentes (communĂ©ment 26Al et 10Be). Lâapproche permet une datation valable pour une plage de temps variant de ~0.1 Ă ~5 Ma
Analyse 3D de la conservation du remplissage détritique de la grotte Roche (Vercors, France)
International audience3D analysis of the conservation of the Detrital Deposit of the grotte roche (vercors, france). An important excavation (1993-2008) has been digged in the âgrotte Rocheâ (Vercors) which led to the discovery of new galleries (1608m) and a major stratigraphic proïŹle. This proïŹle displays interesting sedimentary forms such as progradation, crisscross deposits, erosional surfaces⊠and interest is as much aesthetic as scientiïŹc or educational. A non-destructive analysis, based on a series of laserscanning surveys and ïŹeld observations, allowed a better understanding of the dynamics of such a deposit and the ways of its erosion associated with excavation and current visitors frequentation. The numerical results show that while the impact of caving practice is not negligible, it should be balanced with the discovery of the underground object and its scientiïŹc and educational value. This type of study shows that today it is important to carry on further assessment on the knowledge and the preservation of the underground environments, including their deposits.La grotte Roche (Vercors) a fait lâobjet dâune importante dĂ©sobstruction spĂ©lĂ©ologique (1993-2008) qui a permis de mettre au jour 1 608 m de nouvelles galeries ainsi quâune importante coupe stratigraphique au droit du chantier de dĂ©sobstruction. Celle-ci prĂ©sente des formes sĂ©dimentaires intĂ©ressantes : progradation, dĂ©pĂŽts entrecroisĂ©s, surfaces dâĂ©rosion⊠et son intĂ©rĂȘt est autant esthĂ©tique que scientiïŹque ou pĂ©dagogique. Une analyse, non destructive, basĂ©e sur une sĂ©rie de relevĂ©s lasergrammĂ©triques et sur des observations de terrain, a permis de mieux comprendre la dynamique de ce dĂ©pĂŽt et les modalitĂ©s de son dĂ©mantĂšlement associĂ© Ă la dĂ©sobstruction et Ă la frĂ©quentation actuelle. Les rĂ©sultats chiffrĂ©s montrent que si lâimpact de la pratique spĂ©lĂ©ologique est loin dâĂȘtre nĂ©gligeable, elle doit ĂȘtre mise en regard de la mise au jour de lâobjet souterrain et de son intĂ©rĂȘt scientiïŹque et pĂ©dagogique. Ce type dâĂ©tude montre quâil importe aujourdâhui de pousser plus loin encore la rĂ©ïŹexion sur la connaissance et la conservation des milieux souterrains et notamment de leurs dĂ©pĂŽts
Capturing the timing and rates of valley incision through cave dating in the Eastern Pyrenees : geodynamic implications
PosterTectonic uplift in active mountain belts narrowly controls the rate and tempo of valley incision, with additional modulation effected by climatic change. There is no consensus over whether the Pyrenees still is an active orogen, even though surface uplift rates based on modern land levelling techniques register values of up to 1 mm/yr. Clarifying matters for the longer term should rely on opportunities to measure and date the vertical displacement of landforms of known initial geometry. Fluvial terrace systems in the Pyrenees have so far not yielded any dated sequences that reach further back in time than the latest Middle Pleistocene. Subterranean karstic networks, however, provide a fruitful alternative, with potential for obtaining coupled 26Al and 10Be burial ages for fluvial sediment which became trapped in limestone cavities while being conveyed through the catchment during valley incision. Suitable subhorizontal, gravel-filled phreatic cave galleries cross-cut by limestone canyon sidewalls occur in the TĂȘt valley. These can effectively be treated as a bedrock straths correlatable with subaerial terrace treads situated further up- and downstream. Such markers also provide information about karstic base levels, which in this setting connect directly to the regional marine base level. The TĂȘt valley exhibits a succession of cave levels spanning 300 m of vertical relief, two of which have been dated. Quartz-rich sediment samples were collected from of each system but also replicated among sand-sized and gravel-sized clasts, dated separately. Initial results reveal a continuous process of canyon incision throughout the entire Pliocene and Quaternary, with relatively steady mean incision rates. Nuclide inheritance in the samples provides added information about catchment-wide mean denudation rates. These rates are found to triple after 2 Ma, clearly reflecting a climatic signal linked to the onset of the Pleistocene glacial-deglacial cycle
Cave levels as proxies for measuring post-orogenic uplift: Evidence fromcosmogenic dating of alluvium-filled caves in the French Pyrenees
International audienceThe rates and chronology of valley incision in mountain ranges have been studied in various parts of the globe, but the causes of river incision are often blurred because tectonic, climatic, and sea level-related forcing signals are difficult to distinguish from one another. The TĂȘt River limestone gorge in the Eastern Pyrenees, which displays multiple cave levels containing datable alluvial deposits, provides an opportunity for clarifying this debate. Horizontal epiphreatic passages in limestone can be used as substitutes for fluvial terraces because they correspond to former valley floors and, therefore, also record the position of former local base levels. In the TĂȘt canyon, the passages are filled with quartz-rich sand and gravel sequences that can be dated by 26Al/10Be burial dating. The canyon has cut into a Middle Miocene pediment systemânow forming a raised plateau at 1250â1500 mâand displays nine cave levels over a vertical height of 1 km. One alluvial fill sequence in a cave at + 270 m above datum (i.e., the local river bed) yielded a weighted mean age of 5.14 ± 0.41 Ma; another, situated at + 110 m above datum, yielded weighted mean ages of 2.23 ± 0.230 Ma and 1.20 ± 0.286 Ma. The data convert to a mean incision rate of ~ 52 m·Maâ 1 since the beginning of the Pliocene, and involved an acceleration to 92 m·Maâ 1 during the Quaternary. Pre-burial catchment denudation rates range from 35 to 7 m·Maâ 1, and these also doubled during the early Quaternary. It is concluded that: (i) valley incision into the Miocene pediment has been occurring since 5, probably 10 Ma; (ii) there is no evidence of a Messinian canyon in the Villefranche gorge, strongly suggesting through various additional indicators that interference of the Messinian Salinity Crisis with the canyon incision history was minimal; (iii) valley deepening was not a steady process, and recorded periods of stability around 1â2 Ma and perhaps 6â5 Ma; and (iv) the terraced network of epiphreatic cave levels is primarily explained by tectonic uplift. It follows that the elevated erosion surfaces of the Pyrenees, such as the Miocene pediment directly situated above the canyon edge, were not shaped at high elevations, e.g., by âaltiplanationâ; they formed, instead, close to base level and were uplifted in successive stages by tectonic processes. The study emphasizes the more general proposition that tectonic signals (as opposed to climatic or eustatic) in valley-incision chronologies are best singled out at locations situated among the outer ranges of mountain belts, i.e., in canyons such as the TĂȘt, that respond immediately to base level changes relative to the adjacent foreland. In the inner ranges, fluvial incision is more likely to be affected by the interference of climatic factors (e.g., glaciers), or to be delayed by bedrock impediments to upstream-propagating knickpoints
Capturing the timing and rates of valley incision through cave dating in the Eastern Pyrenees : geodynamic implications
PosterTectonic uplift in active mountain belts narrowly controls the rate and tempo of valley incision, with additional modulation effected by climatic change. There is no consensus over whether the Pyrenees still is an active orogen, even though surface uplift rates based on modern land levelling techniques register values of up to 1 mm/yr. Clarifying matters for the longer term should rely on opportunities to measure and date the vertical displacement of landforms of known initial geometry. Fluvial terrace systems in the Pyrenees have so far not yielded any dated sequences that reach further back in time than the latest Middle Pleistocene. Subterranean karstic networks, however, provide a fruitful alternative, with potential for obtaining coupled 26Al and 10Be burial ages for fluvial sediment which became trapped in limestone cavities while being conveyed through the catchment during valley incision. Suitable subhorizontal, gravel-filled phreatic cave galleries cross-cut by limestone canyon sidewalls occur in the TĂȘt valley. These can effectively be treated as a bedrock straths correlatable with subaerial terrace treads situated further up- and downstream. Such markers also provide information about karstic base levels, which in this setting connect directly to the regional marine base level. The TĂȘt valley exhibits a succession of cave levels spanning 300 m of vertical relief, two of which have been dated. Quartz-rich sediment samples were collected from of each system but also replicated among sand-sized and gravel-sized clasts, dated separately. Initial results reveal a continuous process of canyon incision throughout the entire Pliocene and Quaternary, with relatively steady mean incision rates. Nuclide inheritance in the samples provides added information about catchment-wide mean denudation rates. These rates are found to triple after 2 Ma, clearly reflecting a climatic signal linked to the onset of the Pleistocene glacial-deglacial cycle