Revisiting and exploring Middle-Late Triassic vertebrate tracksites in Ardèche (southern France)

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Controls of bifurcation geometry on bed deposition patterns during experimental channel abandonments

International audienceAbandoned channels are major features of fluvial systems as their distribution and sedimentary fills control the architecture of alluvial plains and the connectivity between fluvial reservoirs. The extent and geometry of the bodies formed by bedload deposition in abandoned channel, e.g., sand-plugs and sand-bars, depend on a number of factors, one of which is the geometry at the bifurcation point. In this study we build on existing modeling experiments, which focused on the global repartition of water or sediment between two active channels. We present a novel series of experiments designed to test the conditions for channel abandonment by modifying the bifurcation angle between channels, the flow incidence angle and differential slope. The surface evolution of disconnecting channels was monitored to constrain construction processes and geometry of sedimentary bodies. For a given bifurcation geometry, three scenarios were tested: a free evolution, a simulated levee breach-i.e. opening of a branch after channel aggradation in the other one-and a rise of the free water surface in one branch. We find that: (i) sand-plug extension is inversely proportional to the flow incidence angle, which we were able to quantify experimentally for the first time. (ii) The flow incidence angle has a greater influence on disconnection potential than the absolute value of the bifurcation angle, and we quantify thresholds above which disconnection becomes possible. (iii) The few sediment fluxes we tested have limited impact on channel fills geometry, although they increase sediment filling rates. (iv) Independently of their final length-scale, sand-plugs develop in very similar patterns from one scenario to another. In contrast, their length and distribution vary with the forcing scenario. Analyses of sand-plug and sand-bars chronology further show that they form at different stages of the channel abandonment process. In the future this may be compared to field data gathered on the geomorphic evolution of abandoned channels

Experimental evidence for bifurcation angles control on abandoned channel fill geometry

International audienceThe nature of abandoned channels' sedimentary fills has a significant influence on the development and evolution of floodplains and ultimately on fluvial reservoir geometry. A control of bifurcation geometry (i.e., bifurcation angle) on channel abandonment dynamics and resulting channel fills, such as sand plugs, has been intuited many times but never quantified. In this study, we present a series of experiments focusing on bedload transport designed to test the conditions for channel abandonment by modifying the bifurcation angle between channels, the flow incidence angles and the differential channel bottom slopes. We find that disconnection is possible in the case of asymmetrical bifurcations with high diversion angle (≥30∘) and quantify for the first time an inverse relationship between diversion angle and sand plug length and volume. The resulting sand plug formation is initiated in the flow separation zone at the external bank of the mouth of the diverted channel. Sedimentation in this zone induces a feedback loop leading to sand plug growth, discharge decrease and eventually to channel disconnection. Finally, the formation processes and final complex architecture of sand plugs are described, allowing for a better understanding of their geometry. Although our setup lacks some of the complexity of natural rivers, our results seem to apply at larger scales. Taken into account, these new data will improve fluvial (reservoir) models by incorporating more realistic topography and grain size description in abandoned channels

Infill dynamics and depositional patterns in gravel-bed chute cutoffs channels: the Ain River, France

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Inventory and conservation of Triassic vertebrate tracks in the Monts d’Ardèche UNESCO global Geopark, France.

13 pagesInternational audienceThe Regional Natural Park (RNP) of the Monts d'Ardèche, located in south-eastern France, became the Monts d’Ardèche UNESCO Global Geopark in September 2014. This territory possesses significant geological structures, including numerous and rich Middle-Late Triassic vertebrate tracksites. The UNESCO Global Geopark label helped to formalize a long-standing partnership for the study of this ichnological patrimony between the RNP of the Monts d'Ardèche and the University of Burgundy. Developing scientific research, protection, training and outreach are the main lines of the agreement signed in 2015. The strategy is to make a detailed and sedimentologically contextualized inventory of vertebrate tracks in the Geopark and its surroundings, establish conservation and protection priorities, set up geosites for the public and involve local people in all these activities. To date, 15 tracksites and nearly a thousand individual tracks have been inventoried. The locality “Le Sartre” stands out as being the first recognized geosite amongst the remarkable tracksites in Ardèche. After four years of scientific investigations and fitting out works, it was inaugurated in October 2020. This geosite is the first equipped locality in France to show Late Triassic dinosaur tracks and to be freely accessible to the public. This success was made possible by the intellectual, logistic and social involvement of local people throughout, by acknowledgement of these contributions, and by the capacity of all actors to collaborate efficiently with each other. With more than 1000 fossil vertebrate tracks still awaiting to be valorized in Ardèche, the designation as a UNESCO Global Geopark clearly marked a turning point from academic-only to articulated research, conservation and geotouristic initiatives involving local communities

Tracking dinosaurs in coarse-grained sediments from the Upper Triassic of Ardèche (southeastern France).

14 pagesInternational audienceCoarse-grained sediments deposited in high-energy environments are usually considered unfavorable to the preservation of fossil tracks. Here we report dinosaur footprints showing good physical preservation, despite being found in coarse-grained sandstones of alluvial origin from the Upper Triassic of Ardèche, southeastern France. The ichnoassemblage, dominated by Grallator isp., raises questions about the processes leading to the formation and preservation of tracks in coarse-grained sediments. The track-bearing surface is a medium- to coarse-grained quartz arenite that is microconglomeratic locally. The tracking surface grain size ranges from 0.2 to 2 mm and numerous pebbles are present. It is overlain by a succession of thin, intercalated layers of claystones and siltstones, themselves covered by a mix of siltstones and coarse-grained sandstones. We interpret this succession as a progressive decrease in energy due to channel migration culminating in channel abandonment, and the establishment of a lower energy setting where the tracking surface formed. Sedimentological and taphonomic observations indicate that the trackmakers walked on fine-grained layers (clay, silt) in which true tracks formed. The passage of the animals along the tracking surface deformed the older coarse-grained sand layers and resulted in the formation of the transmitted undertracks. The fine-grained layer helped record the pedal anatomy of the trackmakers and contributed to protecting the transmitted undertracks from destruction. Overall, we suggest that the fossil footprints were preserved by abiotic processes only, the main factor being the lithological contrast between successive sedimentary layers. The exceptional preservation of those relatively high quality undertracks in coarse-grained deposits contrasts sharply with the prevailing models of true track formation involving fine-grained sediments and microbial mats present in low-energy environments. This mode of undertrack formation may have been relatively frequent elsewhere but potentially overlooked in previous studies

Dynamique sociétale et évolutions des paysages fluviatiles de la Seine : exemples de paléo-chenaux sur le territoire de la Petite-Seine

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Dynamique sociétale et évolutions des paysages fluviatiles de la Seine : exemples de paléo-chenaux sur le territoire de la Petite-Seine

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Étude des traces de pas de vertébrés du Permien et du Trias d'Ardèche. Enjeux et perspectives.

8 pagesInternational audienceLe département de l'Ardèche estremarquable car il offre un accès privilégié auxroches sédimentaires permiennes et triasiques danslesquelles abondent les traces de vertébrés fossiles.Une vingtaine de localités et plus de 1500empreintes de pas ont déjà été recensées le longd'une succession sédimentaire affleurant entrePrivas et Les Vans. Ce patrimoine ichnologique,reconnu internationalement, documente le passagede grands groupes de reptiles en terres ardéchoises,incluant essentiellement des représentants basauxde la lignée des crocodiles (pseudosuchiens) et de lalignée des oiseaux (dinosauriformes et dinosaures),ainsi que quelques membres de la lignée des lézards/serpents (lépidosauromorphes) et de lalignée des reptiles mammaliens/mammifères( s y n a p s i d e s ) . C e p a t r imo i n e i c h n o l o g i q u eexceptionnel ouvre des perspectives passionnantesde recherche, notamment en macroévolution, entaphonomie et pour la reconstruction des paléopaysages.Il lance aussi de grands défis en termes deconservation et de valorisation, défis qui nécessitentla mobilisation coordonnée d'une multituded'acteurs