Biostratigraphic and paleoenvironmental analyses at the Messinian/Zanclean boundary in the Moncucco quarry (Piedmont, Northwestern Italy)

Integrated biostratigraphic and paleoenvironmental analyses of foraminifer, ostracod and calcareous nannofossil assemblages have been carried out on the sediments encompassing the Messinian/Zanclean boundary exposed in the Moncucco quarry (Piedmont, Northwestern Italy), where a Tortonian to Pleistocene succession is preserved, even if marked by severe tectonics accidents and by omission surfaces. The uppermost Messinian deposits ("Marne a Congeria") consist of clayey marls (about 6 m thick), correlatable to the "Lago-Mare" deposits of the Mediterranean area (Dela Pierre et al., 2007). They yield rare brackish water mollusks (Limnocardium sp., Dreissena (=Congeria) sp.) and a brackish ostracod assemblage referable to the Loxocorniculina djafarovi Zone (Gliozzi et al., 2006) of the upper Messinian post-evaporitc interval. This assemblage testifies the deposition in oligo-mesohaline, shallow water conditions and the influx of Paratethyan faunas. No evidences of marine conditions have been found in these sediments. Actually, the marine microfossils found in the “Lago-Mare” sediments are reworked. Foraminifers are represented by long-ranging or Messinian taxa as Globorotalia nicolae, which LO is dated to 6.72 Ma. Calcareous nannofossil assemblage yields Oligocene and Miocene taxa. The boundary between the “Lago-Mare” deposits and the overlying Zanclean Argille Azzurre Fm. is marked by an erosional surface, followed by a 10-50 cm thick black arenitic layer very rich in organic matter, barren of microfossils. The black layer top corresponds to an omission surface, evidenced by a network of firm-ground burrows infilled by the Argille Azzurre Fm. sediments and suggests the occurrence of a short hiatus across the Miocene/Pliocene boundary. The Argille Azzurre unit consists of an about 26 m thick succession of grey to whitish clayey marls, rich in planktonic foraminifers and ranging from the MPl1 (Sphaeroidinellopsis acme) zone to the MPl4a foraminiferal zone. Palaeoecological data suggest a deposition in the upper epibathyal zone in the MPl1 zone. The increasing specific diversity during the upper MPl1 zone and the following MPl2 zone and the occurrence of deep water benthic foraminifers, of typical NADW (North Atlantic Deep Water) taxa as Cibidoides robertsonianus, suggests a further deepening of the basin and allow the comparison of Northwestern Italy Zanclean deposits with the coeval ones of the Southern Mediterranean region (Sgarrella et al., 1999; Pierre et al., 2006). Upwards in the section (MPl3 and MPL4a zones) the reduction in diversity and abundance of deep water taxa and the higher frequency of inner neritic forms suggest a water depth shallowing and an increase of turbiditic transport.