THE ARBAILLES KARST
(Western Pyrénées, France).Tectonic, climatic, hydrogeological
and anthropic controls of the morphogenesis
Nathalie VANARAUniversité Michel de Montaigne - Bordeaux 3
JURY : AUDRA P. : Maître de Conférence à l'Université de Sophia-Antipolis - Nice
CANEROT J. :Professeur à l'Université Paul-Sabatier - Toulouse III
DELANNOY J.-J. :Professeur à l'Université de Savoie - Chambery
MAIRE R. : Directeur de Recherche au C.N.R.S. - Bordeaux
MUDRY J. : Professeur à l'Université de Franche Comté - Besançon
PEULVAST J.-P. : Professeur à l'Université de Paris IV - Sorbonne
SALOMON J.-N. : Professeur à l'Université Michel de Montaigne - Bordeaux III
Soutenance publique de thèse de doctorat
Janvier 1998
Basque Arbailles mountains, of 165 km2 in superficy constitute a calcareous bastion, which rises 1 000 metres above pyrenean piedmont. They form the best individualized geologic and geomorphologic unit of all the north pyrenean zone.
Since the Miocene, the morphologic evolution is under dependance of surrection periods and bioclimatic factors directly propitious to karstification (wet oceanic climate, forest cover).
During the Holocene and historic period, the human impact has had a notable role on morphogenesis, especially on scouring of crypto-lapiés and probably on dissolution intensity as a consequence of deforestation.
GEOLOGICAL CONTROL
Arbailles mounts form a folded structure of Jurassic and cretaceous limestone beginning with the calcareo-dolomitic facies of lower Lias. Limestones of Dogger (100 - 150 m), urgo-aptian of upper Aptian (250 m), marly albian (100 - 300 m) form a carbonated ossature thicker than 500 m, appropriate to karstification.
There are four WNW-ESE folds : 1/the brachyanticlinal of Haute-Bidouze notched down to Stephanian in Hosta weald and Bidouze blind valley ; 2/ the intermediary folds of the high central karst subsisting in residual outliers and dried valleys ; 3/ the Apanicé synclinal strongly notched by the Apoura and Ilhounatzé valleys ; 4/ the cizean karst, west periclinal and of Apanicé synclinal with its smooth karstic forms. The geodynamic evolution depends on several tectonic periods, roughly distensive in the Mesozoïc and essentially compressive in the Cenozoïc. During the lower cretaceous and following the opening of Atlantic, a distensive N-S move (rifting) induces cutting blocks out of the hercinian substratum with a rocking toward the south and generation of diapiric wrinkles. During the middle Lutecian, a compressive period for the Pyrenees, the Arbailles area is folded and the ancient normal faults turn into reverse faults more or less overthrusting with a north vergency. At the same time a long period of erosion scours the flysch cover. But it is only from the Neogene and Pleistocene on, that Arbailles mounts find their present individuality thanks to a powerful epirogenic surrection.
A FLUVIATILE PALEOSYSTEM DRIED BY UPLIFT
Arbailles have a complex polygenic relief associating dried fluviatile erosion forms disorganized by water infitration (fluviokarst) and forms only due to dissolution (karst).
Longitudinal profiles of valleys clearly show the successive stages of depression corresponding to the surrections of the massif. There are several stages of levels of dried valleys. The 700 m level characterized by the big Ithé and Elsarré valleys is the most obvious. It is supposed to correspond to a halt of base level between two important phases of surrection.
The karstic relief itself is remarkable for its high degree of evolution discernable in the fullness of figures. For instance, the upper surface shows fragments of plateaus hollowed by series of deep dolines (Guillembero) and residual interfluves which constitute the field of butts of the high central karst.
The megadolines, up to 1 000 m diameter (Sihigue, Belchou, Zaboze) and the dolinary valleys are the "negative" forms which individualized the interfluves. A large part of chemical corrosion is of a crypto-karstic nature due to the alterites cover resulting from the deterioration of marly albian limestones.
PALEOKARSTS AND DISMANTLED PALEOCAVITIES
The mesozoïc karstification phases give us proof of a local instability during the Jurassic and Cretaceous after formation of the Atlantic : karstifield bauxites of the end of the Jurassic and breccia d'Arhansus indicating an emersion during the aptian period (upper Gargasian) and an intra-aptian discordance detected in the 300 m pit of Apanice swallowhole confirming a emersion phase (rifting) at the south border of the carbonated plateform.
The tertiary karstification periods are witnessed by dismantled paleocavities located on the upper surface and by associated ancient fillings which indicate a hot climate with contrasted seasons (hardpans, speleothems) and a paleotopography which is no more existing. The stalagmitic concretions, the breccia and indurated fluviatile deposits attest to a direct relation with the High Range during the Neogene.
The studies of alterites pockets and hardpans give precisions about the pedogenesis during the mio-plio-quaternary geomorphologic evolution : hot climate and surrective regime.
THE MORPHOSEDIMENTARY SUBTERRANEAN MARKERS
The studies of the distribution in space and altitude of 600 cavities and of their morphologies and filling offer a classification : 1/ the ancient leakages of the south border dried following the isolation of the massif (Yéti, Apanicé, Otxolatzé and Bienvenu) ; 2/ the disconnected cavities of upper perched small valleys (Landanoby) ; 3/ the watering systems marking the dried valleys of Ithé and Etchaltia (Azaléguy, Etxanko Zola and Nébélé)
The studies of forms, of levels and of fillings of these systems give precisions about the sucessive stages of the hydrographic system driving in, underlining the major role of the plioquaternary surrection.
THE KARSTIC DESSOLVING AT THE PRESENT TIME
The calcareous Arbailles mounts are under oceanic mountainous climate (1 500 - 2 000 mm/year) propitious to vegetation and pedogenesis, i.e. to dissolving (biogenic CO2). Eight big karstic sources have been selected for studies of the hydrochemical process. Chemical analyses show three families of sources : calcic bicarbonated, calcic bicarbonated lightly sulfated and calcic bicarbonated and sulfated waters. The mineralization is in opposition to yield, common rule for a mountaneous karst (by dissolution process). Waters are agressive in summers with lowest water levels (biogenic CO2) and at equilibrium or lightly undersaturated the rest of the year (high waters).
The present specific dissolution rate, about 94 m3/km2/year is typical of a wet karst of mid high mountains. As 90 % of dissolution happen at a depth of 0 to 100 m, the section of concerned rocks (karstic denudation) tends to be global in the long time.
THE KARSTIC SYSTEMS AND THEIR DRAINAGE BASSINS
The drainage basins of karstic systems of Arbailles are difficult to delimitate because of their complex geologic structure. Speleological explorations and eighteen tracings bring proof of the existence of more than 10 karstic systems, some of them still hardly known (Garaybie, Camou). A synthetical hydrogeologic map on a scale of 1/50 000e adapted from the geologic view indicates big and small sources, leakages (more than 200), tracings, main underground stream systems and locates the 600 known cavities.
Four main types of karstic aquifers separated by Bedoulian and Oxfordian marly screens can be discerned : aquifers of marly albian limestones, of urgo-aptian, of Dogger and of Dogger marly limestones. The hydrodynamic functionning of the sources is characterized by a quick transfer of flood waters (few hours) and a rather small buffer accumulation. Dynamic reserves are scanty (some tens of thousands m3) except at the big Cent Sources emergency which probably drains a quarter of the massif.
Thirteen big karstic basins can be classified into six geologic types : 1/ the synclinal systems of albian mearly limestones in the south (Arhanzéta) and south-west (Etcheberriborda), 2/ the anticlinal system of Haute Bidouze - Hosta north-north-east (Garaybie), 3/ the perched systems of urgo-aptian limestone in center (Grande and Petite Bidouze),, 4/ the faulted urgo-aptian systems of the periclinal versant south-east,, 5/ the synclinal systems of the west border (Abotecoborda and Espila), 6/ the jurassic systems (Zahagui, Urondoa, Ur Belcha, Uthurbietta)..
WATER RESSOURCES : ESPLOITATION AND VULNERABILITY
Arbailles mounts are a natural water tower of which abundant resources only appear at the massif periphery. The modern equipments aim essentially at supplying villages with drinking water and providing cayolars and thoughs in altitude. As all calcareous massifs, Arbailles are sensitive to the development of roads, agriculture, forest and touristic industries. The bacteriologic quality of water of emergencies is not satisfying for what treatment stations are put in service for all tappings in the whole massif.The difficulties to find more reliable resources, a recent deterioration as turbidity increases, pollution and drying of rivers show the fragility of the environnement. Only an agreement among the different customers will allow a lasting arrangement to find an equilibrium between the necessities of economic exploitation and the respect of nature.
ANTHROPIC IMPACT ON RECENT MORPHOGENESIS
Recently important developments are carried out by local "Comices" to avoid desertification of the mountain. Development of the various resources of karstic Arbailles by men is ancient. There is an exceptionnal concentration of remains of different human sociological integration which shows a permanent occupation from middle Paleolithic up to this day. But with time, the use of possibilities offered by nature has varied diversely and originally. Nowadays large investments made by the local syndicates have avoided abandonning these mountains and succeeded in keeping the Basque farmers : roads building, sheep-folds building, running water distribution and various helps. But karst is a fragile environment. Over-pasture, burn-beats still frequent and deforestation are at the origin of slopes erosion. In many places, rocks appear in the form of crypto-lapiés, which shows a recent destabilization of the environment. Karstic waters are very sensitive to equipment as shown by the increasing turbidity of some sources. Nevertheless, in comparison with the mediterranean regions, ground erosion is still moderate in Arbailles, and vegetation can recover as soon as pastoral pressure decreases. In most administrations the awareness of karstic fragility to environment is now present, but concretization in facts remains difficult.
SYNTHESIS OF GEOMORPHOLOGY AND GENERAL CHRONOLOGY
1/ During the cretaceous period, first orogenic moves come with diapirism and formation of a special karstification with gypsum dissolution ;
2/ The main Pyrenean orogenesis takes place during the mid Eocene. It comes with a long period of erosion and scouring of geologic covers (flyschs) which goes on up to the Middle Tertiary ;
3/ Middle and upper Miocene was a period of stability and of feeble surrection. Ancient speleothems cut through by erosion of the upper surface are attribuated to this period under tropical climate. A fluviatile system originally traced on the flyschs cover becomes encrusted into limestones being in hydrologic relation with the help of upper water from Mendibelza impermeable area ;
4/ At the end of the upper Miocene, the pyrenean range goes through a long period of surrection which puts upright the karstic system and emphasizes karstic and fluviokarstic forms (perched upper valleys) ;
5/ During the lower Pleïstocene, a second surrection definitely isolates the large dried valleys of Ithé and Elsarré from upper Mendibelza by capture of the heads of valleys by regressive erosion (Apoura, Ilhounatze). These large dried valleys become dolinary valleys during the quaternary ;
6/ At the present time, drainage is made by new underground systems without any connection to the fossile topography.
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