The Early Zanclean succession of Piedmont (Northwestern Italy): new data and perspectives about the paleogeographical setting of the region

Recent researches, still in progress, are adding a great amount of data about the Messinian/Zanclean succession of the central part of Piedmont (Northwestern Italy). Until few years ago, the Messinian/Zanclean boundary and the Early Zanclean basal interval were very poorly documented in the region. The M/Z boundary was only briefly described by Sturani (1978) in the Narzole (Alba, CN) borehole. The lowermost Zanclean deposits seemed missing in the Pliocene succession, that was widely studied in many other sections, allowing the identification of a marine sedimentation that spans the MPl2 - MPl4 foraminiferal zones. New micropaleontological and stratigraphical analyses have documented the M/Z boundary in the central Piedmont, from the Turin Hill (Moncucco T.) to the Astigiano. The M/Z boundary, marked by a black decimetric arenitic layer intercalated between the Upper Messinian Lago-Mare sediments and the Zanclean Argille Azzurre Formation, was recently recovered also in the succession cropping out along the Tanaro river (Albese). In the Moncucco quarry outcrop and borehole, as well as in the Narzole borehole, quantitative micropaleontological analyses were carried out on foraminifers, calcareous nannofossils and ostracods. In both these sites, many bioevents recognized at the Mediterranean scale were progressively recorded in the Piedmont Early Zanclean succession at stratigraphic distances from the M/Z boundary similar to the ones recorded in Sicily, Western and Eastern Mediterranean basins: a basal abundance peak of Globigerina nepenthes, one sinistral coiling shift of Neogloboquadrina acostaensis, the Globorotalia scitula common occurrence (CO), the re-immigration of Siphonina reticulata, the first influx of NADW (North Atlantic Deep Water) forms as Cibicidoides robertsonianus and the Globorotalia margaritae first common occurrence (FCO). Rare specimens of Ceratholithus acutus have been recorded at the very base of the Zanclean, whereas no specimens of Triquetrorhabdulus rugosus are recorded. Assemblages were characterized by a very high P/(P+B) ratio, by the presence of mesopelagic planktonic foraminifers as Sphaeroidinellopsis spp., common to frequent deep cosmopolitan benthic foraminifers (Cibicidoides pseudoungerianus, Sphaeroidina bulloides, Uvigerina peregrina), common bathyal ostracods (A. kissamovensis, A. acuminata, P. jocosa), rare discoasterids and ceratholiths, tropical open marine taxa (C. acutus, Amaurolithus primus, A. delicatus). Foraminiferal and ostracod assemblages documented an epibathyal basin just from the Early Zanclean. Moreover, a deepening of the basin during the MPl1, from about 500 m to about 1000 m depth was suggested by the increasing diversity of benthic foraminifers and of deep bathyal ostracods. All paleobiological data suggest open sea conditions and active paleocirculation in this sector of Northwestern Italy during the MPl1-MPl2 Early Pliocene zones. The nearly contemporaneous basin-wide diffusion of both open marine planktonic taxa and bathyal benthic “Lazarus” forms in Northwestern Italy suggests the existence of direct deep connections between this basin and the western Mediterranean area during the Early Zanclean. References Sturani C. (1978). Messinian facies in the Piedmont Basin. Mem. Soc. Geol. It., 16: 11-25.