Le karst des Arbailles (Pyrénées-Atlantiques, France)

Abstract

THE MASSIF OF ARBAILLES (WEST PYRENEES). The Arbailles massif (200 -1 200 m) is located in the french north face of Pyrenees, Atlantic side. It forms a folded 165 square-kilometres unit of jurassic and cretaceous limestones under an oceanic climate of altitude (2000 mm/year). Observations jointly made on the surface and in the numerous underground galleries allow an occurate correlation of alternate surrective and karstic periods. The dismantled cavities and deposits pockets of the upper surface show two series of minerals, those from weathered marly-albian limestones and others supplied from the conglomerates pudding-stones of Mendibelza. During the Miocene, the Arbailles massif is a low area of tropical erosion on the side of the main mounts. Its surrection caused the scouring of the alterites cover, the formation of fields of karstic butts and a definitive drying of the fluviatile paleosystem. The different levels of dried valleys and the karstic hydrographie systems are successive stations of the karstic levels of origine. Paleomagnetic datations in Etxanko Zola and U/Th datations in Nébélé show that the surrection has been of about 500 m since lower Pleistocene. At the present time, water collection is made throught drainage systems without any connection to the fossil topography. Three aquifers can be distinguished : in lower cretaceous, in jurassic and in north and south limits. They are water-repellent because of more or less impermeable screens. Waters are agressive in summer and at equilibrium or lightly undersaturated the rest of the year. The modern human activities creates a recent destabilization of the environment with local erosions of grounds and an increasing turbidity of springs. An occurate study in the fail of Istaurdy allows a mesure of the effect of deforestation for the whole massif