The Ranzano unit boundaries in the type area: Lower Oligocene events in the epi-Ligurian Succession (northern Apennines, Italy)

In the Enza Valley area (northern Apennines) the occurrence of two angular unconformities within the lower Oligocene deposits of the epi-Ligurian Succession provides the opportunity to redefine the Ranzano unit in the type area. The calcareous nannofossil biostratigraphy allows us to chronologically constrain the stratigraphic events corresponding to these unconformities. The lower unconformity, which represents the basal boundary of the Ranzano unit, overlaps the middle-upper Eocene deposits of the epi-Ligurian Succession and the Ligurian Units (upper Campanian-Maastrichtian Helminthoid Flysch). This unconformity corresponds to an important erosional event encompassing the Eocene-Oligocene boundary (MNP21a Subzone) which can be related with the progressive uplift, exhumation and erosion of the Ligurian Phase thrust-belt. The upper unconformity directly overlaps a thrust system, affecting the middle Eocene-lower Oligocene deposits of the epi-Ligurian Succession, which can be related to a Rupelian (MNP23 Zone) thrusting phase. According to our data the Ranzano unit of the type area can be considered as the part of epi-Ligurian Succession bracketed between these two major unconformities, thus corresponding to a Rupelian unconformity-bounded lithostratigraphic unit. The regional occurrence of these events, recognized in several sections of the epi-Ligurian Succession and in the eastern Tertiary Piedmont Basin, provides new constraints for the reconstruction of the epi-Ligurian depositional basin.