Rapport entre karst et glaciers durant les glaciations dans les vallées préalpines du Sud des Alpes

Résumé En

Karst and glaciations in the Southern pre-alpine valleys. At least 13 glaciations occurred during the last 2.6 Ma in the Southern prealpine valleys. The glaciers scouring alpine and pre-alpine valleys had all the same feature, being valley temperated glaciers. Their tracks and feeding areas were always the same, just like the penological contents of their deposits. Contrary to previous assumptions until a few years ago, the origin of these valleys and of the lakes occupying the floor of some of them (Orta, Maggiore, Como, lseo, Garda Lakes ) is due to fluvial erosion related to Messinian marine regression. The valley slopes modelling is Messinian in age, too, while most caves are older. As a general rule, glaciers worked on valley slopes just as a remodelling agent, while their effects were greater on valley floors. The karstic evolution began as soon as the area was lifted above sea level (upper Oligocene -lower Miocene), in a palaeogeographical environment quite different from the present one, although the main valley floors were already working as a base level. During Messinian age, the excavation of deep canyons along pre-existing valleys caused a dramatic lowering of the base level, followed by a complete re-arrangement of the karstic networks, which got deeper and deeper. The Pliocene marine transgression caused a new re-arrangement, the karst network getting mostly drowned under sea level. During these periods, the climate was hot-wet tropical, characterized by a great amount of water circulating during the wet season. At the same time, tectonic upliftings were at work, causing breaking up of the karst networks and a continuous rearrangement of the underground drainage system. In any case, karstic networks were already well developed long before the beginning of Plio-Quaternary glaciations. During glaciations, karst systems in pre-alpine valleys could have been submitted to different drainage conditions, being : a) isolated, without any glacial water flowing ; b) flooded, connected to the glacier water-filled zone ; c) active, scoured by a stream sinking at glacier sides or in a sub-glacial position. The stream could flow to the flooded zone (b), or scour all the unflooded system long down to the resurgence zone, the latter being generally located in a sub-glacier position. The glacier/karst system is a very dynamic one : it could get active, flooded or isolated depending on endo-and sub-glacial drainage variations. Furthermore, glaciers show different influences on karstic networks, thus working with a different effect during their advance, fluctuations, covering and recession phases. Many authors believe, or believed, the development of most surface and underground karst in the Alps is due to glaciations, with the last one held to be mostly responsible for this. Whatever the role of glaciers on karstic systems, in pre-alpine valleys caves, we do not have evidence either of development of new caves or of remarkable changes in their features during glaciations. It is of course possible some pits or galleries could have developed during Plio-Quaternary glaciations, but as a general rule glaciers do not seem to have affected karstic systems in the Southern pre-alpine valleys with any remarkable speleogenetic effects : the glaciers effects on them is generally restricted to the transport of great amounts of debris and sediments into caves. The spotting of boulders and pebbles trapped between roof stalactites shows that several phases of in-and out-filling of galleries occurred with no remarkable changing in pre-dating features , including cave decorations. The presence of suspended karst systems does not prove a glacial origin of the valleys, since most of them pre-date any Plio-Quaternary glaciation, as shown by calcite cave deposits older than 1,5 Ma. The sediments driven into caves might have caused a partial or total occlusion of most galleries, with a remarkable re-arrangement of the underground drainage system. In caves submitted to periglacial conditions all glaciations long, we can find deposits coming from weathered surface sediments, sharp-edged gelifractbn debris and, more rarely, alluvial deposits whose origin is not related to the circulation of the glacial meltwater. In caves lower than or close to the glaciers limit we generally find large amounts of glacier-related deposits, often partly or totally occluding cave galleries. These sediments may be directly related to glaciers, i.e. carried into caves by glacial meltwaters, resulting from surface glacial deposit erosion. They generally show 3 dominant fades : A) lacustrine deposits B) alluvial deposits and C) debris flow deposits fades. The only way of testing the soundness of the forementioned hypothesis is to study the main characters and spreading of cave sediments, since they are the only real data on connection of glaciers to endokarst networks.