The Miocene/Pliocene boundary and the Early Pliocene micropalaentological record: new data from the Tertiary Piedmont Basin (Moncucco quarry, Torino Hill, Northwerstern Italy)

This paper reports new integrated biostratigraphic and palaeoecological data from the upper Messinian to Zanclean succession exposed in the Moncucco quarry (Torino Hill, Tertiary Piedmont Basin, Northwestern Italy). The foraminifer, ostracod, and calcareous nannofossil assemblages have been studied in detail. In the Moncucco quarry the Vena del Gesso Formation is followed by post-evaporitic chaotic deposits (Valle Versa Chaotic Complex) and by continental and brackish water sediments, correlatable to the Lago-Mare deposits of the Mediterranean area. The latter are in turn followed by marine sediments of the Argille Azzurre Fm. (Zanclean) through an irregular surface that is overlaid by a 10-50 cm thick black arenitic layer very rich in organic matter. At the top of the bed an omission surface, evidenced by a network of firm ground burrows filled by the Zanclean sediments, has been observed. The ostracod assemblages recognised in the sediments just below the black layer are referable to the Loxocorniculina djafarovi biozone (upper Messinian post-evaporitic interval). They indicate oligo-mesohaline shallow-water conditions and show the influx of Paratethyan faunas. Foraminifers and calcareous nannofossils in this interval are reworked. Microfaunas and calcareous nannofossils found in the sediments just above the black layer testify the MPl1 foraminifer biozone and the MNN12 calcareous nannofossil biozone. However, the absence of the first sinistral coiling shift of Neogloboquadrina acostaensis and of Triquetrorhabdulus rugosus in the lowermost Pliocene samples suggests a short hiatus, further confirmed by the presence of the omission surface at the top of the black layer. The recognition of biostratigraphic markers along the section allows to identify the MPl2, MPl3, and MPl4a foraminifer biozones and MNN13 and MNN14-15 calcareous nannofossil biozones, even if partially documented by the sedimentary record. The occurrence of Agrenocythere pliocenica within the MPl2 biozone, confirms the biostratigraphic importance of this taxon at the Mediterranean scale. Foraminifer and ostracod palaeoecological data suggest an upper epibathyal depositional palaeoenvironment in the MPl1 biozone, a further deepening in the MPl2 biozone and a progressive reduction of the water depth in the MPl3 and MPl4a biozones. On the whole, these data suggest that also in the Tertiary Piedmont Basin the termination of the Messinian salinity crisis was abrupt, followed by the deep-sea Zanclean flooding event.