Le karst du massif des arbailles (pyrenees occidentales, france). Controle tectonique, climatique, hydrogeologique et antropique de la morphogenese
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Abstract EN:
The "arbailles" massif constitutes a folded area of jurassic and lower cretaceous limestones. This massif is characterized by a top karst surface. The tertiary paleosurface of alteration is symbolised by the virtual surface linking the cone karst summits. The paleosurface was dug by an hydrographic system which is represented today by a network of inherited dry valleys. Old karst deposits are associated to the erosion top surface. Large speleothems which can currently be seen on the residual cones, indicate the erosion of a thick limestome layer. Some detritic deposits associated with the eroded paleocaves, are composed of quartz and cuirasse gravels. Alterite pockets show elements of crust and nodular ferruginous cuirasse and corroded fibrous quartz mixed with clays and thin quartz sands. These ferruginous cuirasses indicate an evolution by saturation level beat in the hydromorphic depressions. The clay and heavy minerals in karst and alteritic infillings are of two origins : 1) weathering of the marlous limestone and flysch cover; 2) weathering of alluvia coming from triassic sandstones and ophites and cenomanian mendibelza conglomerates. The old karst deposits and paleoalterites, situated on the high surface, can be attributed to a prequaternary period when the massif was at a lower altitude and in a hot and wet climate. The top erosion surface and the present base level show a total neotectonic uplift of 1000 m divided into two major phases by the main intermediary fluviokarst level : the first uplift attributed to pliocene, the second uplift attributed to pleistocene. During the middle tertiary, the "arbailles" massif constituted a weathering surface situated at a low altitude. This surface was in relation upstream with mendibelza conglomerates and downstream with upper cretaceous flyschs. During the miocene, a weathering surface developed together with karstification under alterite cover. A well identified hydrographic system occured at the start of the uplift in the late miocene. During the pliocene uplift, a cone karst was formed by the drying-up of the hydrographic network and the evolution of interfluve elements in residual cones. During the pleistocene, karst landforms were under periglacial actions and strong uplift. The "arbailles" constitutes an example of tertiary chemical weathering surface on carbonated rock, transformed into fluviokarst, dr
Abstract FR:
Le massif basque des arbailles (165 km2) forme l'unite geologique et geomorphologique bien individalisee. C'est une structure plissee de calcaires du jurassique et du cretace inferieur. Les plis, orientes wnw-ese, sont structures en quatre unites. Le controle geodynamique est sous la dependance de plusieurs phases tectoniques. Le relief polygenique associe des formes fluviokarstiques et karstiques. Les phases de karstification tertiaires sont attestees par des paleocavites demantelees qui permettent de preciser les modalites de la pedogenese au cours de l'evolution geomorphologique mio-plio-quaternaire. L'etude des reseaux souterrains permet de mettre en evidence les stades d'enfoncement successifs du reseau hydrographique soulignant ainsi la part majeure jouee par la surrection plio-quaternaire. Le climat, oceanique montagnard, est favorable a la vegetation et a la pedogenese, donc a la dissolution. Huit sources karstiques ont ete choisies pour l'etude du fonctionnement hydrochimique. Les recherches prouvent l'existence d'une dizaine de grands systemes karstiques. Quatre types d'aquiferes karstiques sont separes par les ecrans marneux. Les reserves dynamiques sont generalement faibles. Les arbailles represente un chateau d'eau naturel. Les amenagements actuels visent essentiellement a alimenter en eau potable les villages et a desservir les cayolars et abreuvoirs d'altitude. Actuellement, des amenagements sont menes par les commissions syndicales pour eviter l'abandon de la montagne. Mais le karst est un milieu fragile. Le surpaturage, les feux pastoraux encore intensement pratiques et les coupes forestieres sont responsables d'une erosion des sols. En conclusion, une synthese de l'evolution geomorphologique et une chronologie generale est donnee.