Messinian karst in Monferrato gypsum areas (North Italy).

The discovery of large caves in underground gypsum quarries and the formation of a sinkhole in a Messinian evaporite sequence in Piedmont (N-Italy) has induced the mining company Fassa Bortolo S.p.A. to conduct detailed geomorphological and hydrological investigations in the exploitation areas. A series of observations lead to propose the hypothesis of an intra-Messinian epigenic karst cycle that started the cave development, followed by a hypogenic stage still active today: 1) in many sectors of the area gypsum beds have been extensively karstified although outcropping on very small areas only and at present being sandwiched between Pre-Messinian and Upper Messinian-Pliocene impermeable clay- and siltstones; 2) the partially remobilised Burdigalian-Lower Pliocene fauna associations in cave sediments agree with a Messinian age of the voids they occupy. Their mobilisation with the Pliocene impermeable cover in place is highly improbable; 3) most karst conduits in gypsum have been discovered well below the present valley bottoms, and their genesis is difficult to explain in a phreatic situation between two impermeable beds; 4) at present all investigated waters, except for those at the surface, are supersaturated with respect to both calcite and gypsum, and cannot dissolve any gypsum. It is likely these voids have at least started forming during the intra-Messinian uplift periods, when they were above local base level and not yet covered by impermeable Messinian post-evaporitic and Pliocene sediments