Evolution of karst in Messinian gypsum (Monferrato, Northern Italy)

In the Tertiary Basin of Piedmont (Northern Italy) an important Messinian sedimentary succession crops out, composed of pre-evaporitic clays, macrocrystalline gypsum beds with marly interbeds, a microcrystalline gypsum bed, redeposited gypsum and post-evaporitic lacustrine-marine fine sediments. In the Monferrato area the entire thickness of the evaporite sequence is extremely variable, between a maximum of 70 meters up to only a few meters, due to an important erosional surface that cuts the upper part of the Messinian series. This erosional surface has formed at the end of the evaporite cycle, when freshly deposited gypsum rocks were exposed undergoing an extensive karstification both at the surface and at depth. Some of the known karst voids appear to have started forming during a short intra-Messinian phase of emersion. This hypothesis is based on three observations: 1) in many sectors of the area gypsum beds have been karstified although outcropping only rarely, most of the time composing a confined unit between Pre-Messinian and Upper Messinian-Pliocene impermeable beds of clay- and siltstones; 2) the fauna associations found in cave sediments, although partially remobilised, indicate a Burdigalian-Lower Pliocene age and would agree with a Messinian age of the voids they occupy. Their mobilisation with the Pliocene impermeable cover in place seems hard to defend; 3) the best developed 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. These voids have probably formed above local base level during the intra-Messinian uplift period.