Abstract:
The Mediterraean dessiccation theory suggests that during the Messinian the Mediterranean sea lad almost completely dried up did a thick succession of evaporites was laid down Due to dessiccation the erosional base level through the whole Mediterranean area was lowered, with the consequent development of long and deep fluviatile canyons (e.g. Nile, Rhône, Var, etc). This lowering strongly affected karst evolution This paper concerns the karst in Lombardy, around the southalpine lakes. The old evolutionary models, predating dessiccation theory, assume that the lacustine valleys were scoured by the quaternary glaciers. ln this case the karst should have been characterized by some features, like for example the altitudinal cave distribution as a consequence of the valley lowering after each glaciation. Seismic experiments through the lakes and their tributaries have shown that these valleys are deep fluviatile canyons. The study of caves has demonstrated that the caves themselves predate the entrenchment of the valleys and the glaciations. During the latter the caves were filled up and emptied several times, without any modifications of their inner morphology, including stalactites. Moreover the U/Th age determinations indicate that a great number of concretions are older than 350 ky, and that a few are older than 1.5 Ma. As a conse-quence, a general model of karst evolution can be proposed. The former karstic drainage system developed after the Oligo-Miocene emersion. Paleogeography obviously diffe-red from the present day landscape but the main valley had already been scoured. During the Messinian the dramatic lowering of base level determined major changes in karstic evolution and a reorganisation of the karst drainage system that was consequently lowered considerably. The Pliocene transgression determined a new karst evolution, after which a great number of caves were located well below the sea level base. This evolution occurred during hot and wet climate period, with seasonal high flows and relevant discharges of the karstic rivers The great caves of the Lombardian karst developed within the climatic stage.