Tectonic movements occurred with the re-opening of Straits of Gibilterra, that allowed the end of the salinity crisis in the Mediterranean; at the beginning of Pliocene era, Italy was characterized by reappearance of rich Atlantic origin sea fauna as well as subtropical-like luxuriant flora.
As time goes by during Pliocene a general decrease in temperature occurred, so that a lot of species that was typical of hot climate areas became extinct or migrated to southern latitudes.
“Complesso Villafranchiano”
During Upper Pliocene continuous Orogenic processes pushed to the surface the Alps chain and, thanks to mountain erosion, the primitive Padan plane was at a good stage of formation, from West side to East side.
The shoreline was made up of a wide fluvial and lacustrine strip, whose sediments has been set down between 4.5 and 1 mya, and leaded to the formation of “Complesso Villafranchiano”, which is a stratigraphic layer that owes its name to the outcropping in Villafranca d’Asti.
Pliocene sea series is divided into Zoncleano, Piacenziano and Gelasiano, that can be roughly matched respectively with a Early, a Middle and a Late phase.
In Alba territory the first two levels of sea sedimentation are present, while Upper or Late Pliocene is here evolved into the Complesso Villafranchiano, that is made up of continental sediments.
Piacenziano (from 3.5 to 2.5 mya)
This period is characterized by the sedimentation of muddy and sandy sediments in the circalittoral sea plane, from 50 to 200 meters of depth.
These sediments caused the formation of faint and solid clays, barely permeable, which contain fossils (shells in particular) characterized by an excellent status of preservation.
On the contrary, over 50 meters of depth, yellow sand has been deposited and today these constitute Astiana facies, so called because of the high presence of these type of sediments in Asti territory.
“Sabbie di Asti” show in particular a lot of fossil shells, typical of littoral waters, but frequently there are the aforementioned phyllites as well as fossils belonging to other animals.
Zancleano
Pliocene first part is generally characterized by deposition of grey-blue faint clays, which sedimented in rather deep flat waters.