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

The Bartonian–Priabonian transition has attracted great attention by biostratigraphers in the last decades in searching for a boundary stratotype section. The Alano di Piave section is the present candidate for Global Stratotype Section and Point (GSSP) of the base of the Priabonian. At Alano, resedimented levels including larger foraminifera are very rare and occur exclusively well below any of the biotic and magnetostratigraphic criteria proposed to correlate the base of the Priabonian, namely the base of the crystal tuff layer called Tiziano Bed, the extinction of the genus Morozovelloides and the base of magnetochron C17n.1n. The Varignano section (Trento province) crops out in a quarry, now inactive, ca 80 km west of the Alano section and provides a unique opportunity for direct correlation between Shallow Benthic Zones (SBZ) and standard calcareous plankton zones at the Bartonian–Priabonian transition. This section preserves indeed several coarse bioclastic levels rich in larger foraminifera that are quite evenly distributed across the critical interval, and are intercalated with basinal marls, crystal tuff layers and sapropel-like levels. The Varignano section was deposited in a middle-upper bathyal setting of the Lombardian Basin, near the western margin of the Lessini Shelf, the paleogeographic unit source of the larger-foraminiferal-rich turbidites. There is no evidence of subaerial exposure and erosion of the Lessini Shelf shallow-water carbonates during the Bartonian/Priabonian interval, thus supporting the reliability of the larger foraminiferal events. Our record shows that all the primary and secondary calcareous plankton bioevents are recorded at Varignano in the same order and stratigraphic position as in Alano. The Varignano section spans the planktic foraminiferal Zones E10-11 to lower E14, the calcareous nannofossil Zones MNP16Bc to MNP18 and the Chron 18n to 17n.2n. Biomagnetostratigraphic data allow us to correlate a prominent crystal-tuff layer outcropping at Varignano with the Tiziano bed in the Alano section. Our results demonstrate that the Varignano section spans the upper SBZ 17 and the lower SBZ 18, with the zonal boundary marked by the first occurrence of the genus Pellatispira. The latter occurs ca 2 m below the extinction of Morozovelloides. This is in contrast with the common usage of shallow-water biostratigraphers, who consider the base of the Priabonian corresponding to the base of SBZ 19. Biotic events from shallow-water environments are rarely directly correlated with those from deep-water setting. The results from the Varignano section, supported by the magnetostratigraphic data, give an exceptional chance to verify the current biostratigraphic integrated schemes and allow us to validate the calcareous plankton correlation, whereas correlations with SBZ need to be revised