Abstract

Integrated biostratigraphic and palaeoenvironmental analyses of foraminiferal, ostracod and calcareous nannofossil assemblages were carried out on the Messinian/Zanclean succession of the Moncucco quarry (Piedmont, northwestern Italy). The uppermost Messinian deposits may be correlated to the “Lago-Mare” deposits of the Mediterranean area. They yield a brackish ostracod assemblage, which belongs to the Loxocorniculina djafarovi zone of the Messinian post-evaporitic interval, indicating their deposition in oligo-mesohaline shallow waters and the influx of Paratethyan faunas. Only reworked marine microfossils are present. The boundary between the “Lago-Mare” deposits and the overlying Zanclean Argille Azzurre Fm. is marked by an irregular erosional surface, followed by a barren, 10–50 cm thick, black arenitic layer.

The Zanclean succession consists of grey to whitish clayey marls (Argille Azzurre Fm.), rich in microfossils, which document the MPl1 (Sphaeroidinellopsis acme) zone up to the MPl4a foraminiferal zone, and the MNN12 to MNN14-15 calcareous nannofossil zones. Micropalaeontological data confirm that the Pliocene flooding event was synchronous at the Mediterranean scale. Some foraminiferal bioevents recognized at the Mediterranean scale and related to precessional cycles are recorded in the studied succession : the Globorotalia scitula common occurrence (CO) (cycle 3), the occurrence of Siphonina reticulata (cycle 6), and the Globorotalia margaritae first common occurrence (FCO) (cycle 10). A short hiatus could be inferred in the lowermost Zanclean sediments for the absence of Triquetrorhabdulus rugosus and of the Reticulofenestra pseudoumbilicus paracme. Hiatuses or reduced sedimentation could also have affected the bottom during the MPl2-MPl3 zones. Foraminiferal and ostracod assemblages document a rather deep basin just from the lower Zanclean, warm surficial waters (common Globigerinoides spp.), rather high seasonal primary productivity (common phytoplankton grazer Neogloboquadrina acostaensis) and mesotrophic bottom (common Eponides pusillus, Hoeglundina elegans, Argilloecia acuminata, Krithe compressa, Paijenborchella iocosa). Bottom depths in the lower Zanclean of northwestern Italy appear to be shallower than depths suggested by coeval southernmost assemblages. A deepening of the basin during the MPl1 and MPl2 zones is suggested by the increasing diversity of microfossils and by the spreading of deep bathyal taxa (Cibicidoides robertsonianus, Paijenborchella malaiensis cymbula). Warm, oligotrophic conditions in the water column become more constant. Palaeoenvironmental conditions change progressively during the upper MPl2, the MPl3 and MPl4a zones. Cold, eutrophic planktonic foraminifers (Globigerina bulloides, N. acostaensis) become dominant, as well as benthic taxa tolerant of low oxygen content and high organic matter (Uvigerina peregrina, Brizalina spp.), suggesting an increase in eutrophic conditions. Deep benthics such as Cibicidoides kullenbergi, C. robertsonianus disappear, and many outer neritic to bathyal taxa decrease in abundance, whereas inner shelf taxa increase, suggesting a shallowing of the basin and episodes of strong winnowing, probably tectonically controlled, in the uppermost Zanclean succession.

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