Integrated stratigraphy at the Bartonian−Priabonian transition: Correlation between shallow benthic and calcareous plankton zones (Varignano section, northern Italy)

The Varignano section (Trento province, northern Italy) provides an exceptional opportunity for a direct correlation between shallow benthic (SB) zones and standard calcareous plankton zones at the Bartonian–Priabonian transition (middle–late Eocene). This transition has attracted great attention by biostratigraphers in the last decades in searching for a boundary stratotype section. The Alano di Piave section (NE Italy) is the leading candidate for the base Priabonian Global Stratotype Section and Point (GSSP). However, at Alano, larger foraminifera-bearing resedimented levels occur exclusively well below the critical interval. Conversely, the Varignano section, located ∼80 km west of the Alano section, preserves several coarse bioclastic levels rich in larger foraminifera throughout the section. These levels are intercalated with basinal marlstones, crystal tuff layers, and an organic-rich interval. The Varignano section spans planktic foraminiferal Zones E10 and E11 to lower E14, calcareous nannofossil Zones MNP16Bc to MNP18 and Chrons 18n to 17n.2n. The main calcareous plankton events recently proposed as primary base-Priabonian correlation tools, i.e., the last occurrence of the genus Morozovelloides and the Base common (= acme beginning) of Cribrocentrum erbae occur, respectively, within C17n.3n and C17n.2n. We correlate prominent crystal tuff layers exposed at Varignano with those outcropping at Alano, including the Tiziano bed, whose base has also been proposed as the GSSP level. The Varignano section spans the upper SB17 and the lower SB18 Zones, with the zonal boundary marked by the first occurrence of the genus Pellatispira. This event occurs in the lower part of Zones E13 and MNP17A within C18n, well below all the potential criteria to identify the GSSP that also includes Chron C17n.1n base. We point out that the usage of shallow-water biostratigraphers in placing the base of the Priabonian at the base of Zone SB19 is inconsistent with the proposed plankton events.