Calcareous plankton biostratigraphy of the Serravallian (middle Miocene) stratotype section (Piedmont Tertiary basin NW Italy)

A high-resolution study of the historical Serravallian (Middle Miocene) stratotype section located in the Serravalle Scrivia area (Tertiary Piedmont Basin, NW Italy) based on calcareous plankton content (calcareous nannofossils and planktonic foraminifers) reveals that the base of the stratotype, which is critical for defining the Langhian-Serravallian boundary, is close to the last occurrence (LO) of Sphenolithus heteromorphus, one of the most easily recognized calcareous nannofossil biohorizons, which defines the tops of Zone NN5 of Martini (1971) and CN4 of Okada and Bukry (1980). The boundary also appears to be close to a major climatic and oceanographic change in the Neogene, the so-called "Mid-Miocene event", which should facilitate its worldwide correlation. The LO of S. heteromorphus as the main criterion in locating the Global Stratotype Section and Point (GSSP) of the base of the Serravallian is consequently proposed. As the facies of the Langhian and Serravallian historical stratotypes are not suitable for accurate chronological studies, a GSSP for the base of the Serravallian should be designated elsewhere.