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 Succ ession provides the opportunity to redefine the Ranzano unit in the type ar ea. 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 Success ion and the Ligurian Units (upper Campanian-Maastrichtian Helminthoid Flysc h). This unconformity corresponds to an important erosional event encompass ing the Eocene-Oligocene boundary (MNP21a Subzone) which can be related wit h the progressive uplift, exhumation and erosion of the Ligurian Phase thru st-belt. The upper unconformity directly overlaps a thrust system, affectin g the middle Eocene-lower Oligocene deposits of the epi-Ligurian Succession , which can be related to a Rupelian (MNP23 Zone) thrusting phase. Accordin g to our data the Ranzano unit of the type area can be considered as the pa rt of epi-Ligurian Succession bracketed between these two major unconformit ies, thus corresponding to a Rupelian unconformity-bounded lithostratigraph ic unit. The regional occurrence of these events, recognized in several sec tions of the epi-Ligurian Succession and in the eastern Tertiary Piedmont B asin, provides new constraints for the reconstruction of the epi-Ligurian d epositional basin.