Formes et formations superficielles de la partie ouest du Causse de Sauveterre (Grands Causses)

Résumé En Fr

Morphology and superficial formations of the western part of the Causse de Sauveterre (Grands Causses, Aveyron and Lozère, France) In 2002, the Natural Regional Park of Grands Causses has coordinated a hydrogeological study of the western part of the Causse de Sauveterre, the northernmost of the Grands Causses. A multidisciplinary approach (geology, geomorphology, geochemistry and hydrology) was used to delineate the catchment area of the main springs and to estimate the vulnerability of karstic aquifers. The Grands Causses are situated in the southern part of the French Massif Central. The landscape is characterised by huge limestone plateaus cut by deep canyons. The morphologic study of the western part of the Causse de Sauveterre (Causse de Massegros and Causse de Sévérac), combined with analysis of superficial formations, allows us to identify the main steps of landscape evolution. The discovery of bauxite and of many outcrops of Upper Cretaceous sandstone confirm that the Coniacian ingression invaded some paleo-landscapes developed within a long period of continental evolution which was initiated at the end of the Jurassic. During the Tertiary, many residual formations form covers of the limestone plateaus. We can distinguish aIterites developed from different formations of the stratigraphie series (day with cherts from Bajocian, dolomitic sand from Bathonian and Callovian, sandy days from Cretaceous deposits) from some allochtonous deposits which can be found in some parts of the Causse de Massegros. These formations are found in association with morphological features (shelves, poljés, fluvio-karstic valleys, sinkholes) and are more or less responsible of their development. Furthermore, some volcanic rocks cut through or even reused some of them. With the deepening of canyons and the base level drop, horizontaI morphologies are preserved only where superficial formations are abundant and thick enough to maintain crypto-corrosion. Elsewhere, karst unplugging removes most of the superficial formations, and the karstic evolution tends to show a vertical development of morphologies and caves. Some springs, which benefit from a favourable lithologic, structural and hydrologie context, are more competitive and expand their catchment area at the expense of the other springs. Many superficial features express this dynamism on the plateau and allow us to determine the most sensible areas for water pollution and the most fragile ones for human activities. un plugging removes most of the superficial formations, and the karstic evolution tends to show a vertical development of morphologies and caves. Some springs, which benefit from a favourable lithologie, structural and hydrologie context, are more competitive and expand their catchment area at the expense of the other springs. Many superficial features express this dynamism on the plateau and allow us to determine the most sensible areas for water pollution and the most fragile ones for human activities.

En 2002, le Parc Naturel Régional des Grands Causses a lancé l'étude hydrogéologique de la partie ouest du Causse de Sauveterre qui constitue l'élément le plus septentrional des Grands Causses. L'analyse des morphologies karstiques et la caractérisation des formations superficielles ont permis de préciser la vulnérabilité des aquifères karstiques et d'appréhender les différents bassins d'alimentation karstique de ce plateau. Ce travail a également permis de caractériser la morphogenèse caussenarde et ses liens avec les différentes formations superficielles identifiées. Les résultats confirment l'existence d'une paléotopographie karstique en prélude à la transgression du Crétacé supérieur. Puis, au gré de la structuration tectonique des causses, les formations résiduelles crétacées, celles provenant de l'altération de la série jurassique (argiles à chailles, grésou dolomitique) mais aussi les formations allochtones originaires des zones de socle, sont intervenues plus ou moins directement dans la genèse du modelé caussenard. Des formes anciennes héritées du Crétacé côtoient des morphologies plus récentes qui précèdent ou accompagnent le creusement des gorges du Lot et du Tarn. Localement, des manifestations volcaniques ont également recoupé voire réutilisé certaines de ces morphologies karstiques. La conservation différentielle de ces anciennes couvertures s'accorde relativement bien avec les grands traits de la structure géologique.