The caves in the underground quarries of Moncalvo d’Asti – Piedmont (Italy)

Many quarries for the extraction of gypsum are located in the hills of the Monferrato area (central eastern Piedmont). Close to the village of Moncalvo, Asti Province, a subterranean quarry of more than 20 km long is present. During the excavations a fracture from which water gushed at a pressure of 3 atm has been intercepted in 2005. The underground works have been suspended immediately and, after only a few hours a water flow comprised between 3000 and 4000 L s-1 has flooded the quarry tunnels filling a volume of over 60,000 m3. After more than one month of pumping the flooded areas have been made accessible again, revealing a thin rock diaphragm that separated the quarry tunnel from a natural cave, which failed under the high hydraulic pressure. Through this small gap it has been possible to access an extensive karst network that previously was completely submerged. During the following quarry operations a second natural cave has been encountered, belonging to the same system but physically divided from the first cave by some metres of physical sediments. The total development of this cave system is around 1 km. The exploration of these caves has allowed to gather an interesting set of observations that have contributed to elaborating a speleogenetic model. The first information regards the impressive amount of snottites present along the walls of the caves, and the overall thickness of gypsum rock subdued to weathering, reaching up to 30 cm. There are many morphologies that clearly demonstrate the caves being formed in phreatic conditions, such as pendants and corrosion cupola, but also flat corrosion bevels and V-shaped cross-sections, further evidences of formation in saturate conditions. The stratigraphic asset of the area surely has played a fundamental role in the formation of these karst systems. From bottom to top there is a thick shale sequence, and a thin discontinuous and extremely well karstified marly limestone bed that seemed to have enhanced the hydrological flow in the above lying gypsum beds. The principal cave systems are formed in between the first and second bed of gypsum, along a shaly finely stratified interbed rich in organic material. On the floor of the main passage there are many rather small subvertical conduits that develop up to the underlying limestone bed thus favoring the upward movement of water and the dissolution of the gypsum rocks. The subterranean excavations also have intercepted other caves, most of them of much smaller size, often reaching some cubic metres in size and partially filled with large gypsum crystals, grown by the continuous but slow feeding of slightly supersaturated waters.