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 paleoecological data from the upper Messinian to Lower Pliocene succession exposed in the Moncucco quarry (Torino Hill, Tertiary Piedmont Basin). The foraminifer, ostracod and calcareous nannofossil assemblages have been studied in detail and correlated with classical Mediterranean successions. 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 latters are in turn followed by marine sediments of the Argille Azzurre Fm. (Lower Pliocene) 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 Lower Pliocene sediments has been observed. The ostracod assemblages recognised in the sediments just below the black layer are referable to the Loxocorniculina djafarovi Zone (upper Messinian post-evaporitc interval). They indicate oligo-mesohaline shallow-water conditions and show the influx of Paratethyan faunas. Foraminifers and calcareous nannofossils in this interval are reworked. The microfaunas and the calcareous nannofossils found in the sediments just above the black layer testify the MPl1 Foraminifera Biozone and the MNN12 calcareous nannofossils Zone. However, the absence of the first sinistral coiling shift of Neogloboquadrina acostaensis and of Triquetrorabdulus rugosus in the lowermost Pliocene samples suggest a short hiatus, further confirmed by the presence of the omission surface at the top of the black layer. The recognition of classical biostratigraphic markers along the section allow to identify the MPl2, MPl3 and MPl4a Foraminifera Biozones and MNN13 and MNN14-15 calcareous nannofossil Zones. The occurrence of Agrenocythere pliocenica within the MPl2 Biozone, confirm the biostratigraphic importance of this taxon at the scale of the Mediterranean. Foraminifer and ostracod palaeoecological data suggest an upper epibathyal depositional paleoenvironment 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, this data suggest that also in the Tertiary Piedmont Basin the marine refilling after the Messinian Salinity Crisis occurred in the Early Pliocene, as suggested by several multidisciplinary studies on Miocene/Pliocene boundary in the Mediterranean area.