OCCURRENCE OF HYPOGENIC CAVES IN A KARST REGION - EXAMPLES FROM CENTRAL ITALY

The caves of the Umbria and Marche regions in central Italy are made u p of three-dimensional maze systems that display different general mor phologies due to the various geological and structural contexts. At th e same time, the internal morphologies of the passages, galleries, and shafts present some similarity, with solutional galleries characteriz ed by cupolas and blind pits, anastamotic passages, roof pendants, and phreatic passages situated at different levels. Some of these caves a re still active, as is the case for Frassassi Gorge, Parrano Gorge, an d Acquasanta Terme, with galleries that reach the phreatic zone, where there is a rising of highly mineralized water, rich in hydrosulfydric acid, and with erosion of limestone walls and the formation of gypsum . Elsewhere there are fossil caves, such as Monte Cucco and Pozzi dell a Piana, where large speleothems of gypsum are present 500 m or more a bove the regional water table. In all of these important karst systems it is possible to recognize basal input points through fracture and i ntergranular porosity networks at the base of the oxidizing zone in th e core of the anticline, where mineralized water rises up from the Tri assic evaporitic layers in small hydrogeological circuits. Different u nderground morphologies can derive from the presence of a water table related to an external stream or from the confined setting of the carb onate rocks, underlying low permeable sedimentary cover, where artesia n conditions can occur.