Micropalaeontological analyses of the Narzole core: biostratigraphy and palaeoenvironment at the Late Messinian and Early Zanclean of Piedmont (Northwestern Italy)

Integrated biostratigraphic and palaeoenvironmental analyses of foraminiferal, ostracod and calcareous nannofossil assemblages are here presented for the Late Messinian/Early Zanclean succession of the Narzole bore-hole (Albese area, Piedmont, Northwestern Italy), briefly described by Sturani (1978) but never studied after his death. The Narzole core is made by about 20 m of marine sediments and is presently stored in the collections of the Torino University, deposited at the Torino Regional Museum of Natural Sciences and still now represents the only documentation of the Messinian/Zanclean boundary in the Albese region. The uppermost Messinian "Lago-Mare" deposits yield reworked marine microfossils and a brackish ostracod assemblage representative of the Loxocorniculina djafarovi Zone, indicating their deposition in oligo-mesohaline shallow waters and the influx of Paratethyan faunas. The boundary between the post-evaporitic Messinian sediments and the overlying Early Zanclean Argille Azzurre Fm. is marked by an about 0.50 m thick barren arenitic layer, dark brown to black in its middle part, well correlateble to the black level recognized at the M/Z boundary in the nearby Moncucco quarry outcrop (Torino Hill). The Early Zanclean succession yields abundant microfossils, which document the MPl1 (Sphaeroidinellopsis acme) and the MPl2 foraminiferal zones, and the MNN12 calcareous nannofossil zone. Many bioevents recognized at Moncucco and at the Mediterranean scale are recorded in the studied succession: one sinistral coiling shift of Neogloboquadrina acostaensis, the Globorotalia scitula common occurrence (CO), the re-immigration of Siphonina reticulata, the Globorotalia margaritae first common occurrence (FCO), the first influx of NADW forms as Cibicidoides robertsonianus. Henryhowella asperrima and Oblitacythereis mediterranea firstly occurred in the MPl1 zone, like in the Moncucco section, while in southern Italy and Mediterranean Pliocene sections they occurred at the base of MPl 2 biozone. Foraminiferal and ostracod assemblages document a rather deep epibathyal basin just from the Early Zanclean, fluctuations in water temperature and productivity are suggested by quantitative changes of warm water, oligotrophic surficial taxa (Globigerinoides) versus intermediate water, eutrophic and phytoplankton grazers (N. acostaensis). A deepening of the basin during the MPl1 is suggested by the increasing diversity of benthic foraminifers and of deep bathyal ostracods. All palaeobiological data suggest open marin circulation patterns in this sector of the Northwestern Italy during the MPl1-MPl2 Early Pliocene zones. This palaeoenvironmental interpretation is corroborated by the very high P/(P+B) ratio, the presence of mesopelagic planktonic foraminifers as Sphaeroidinellopsis spp., common to frequent deep cosmopolitan benthic foraminifers (Cibicidoides pseudoungerianus, Sphaeroidina bulloides, Uvigerina peregrina etc.), common bathyal ostracods (A. kissamovensis, A. acuminata, P. jocosa, P. dimorpha, O. mediterranea, Xestoleberis prognata etc.), the rare discoasterids and ceratholiths (C. acutus, Amaurolithus primus, A. delicatus), typical tropical open marine taxa, usually very rare in the Mediterranean Zanclean.