Microfossil assemblages of Zanclean diatomaceous silts from the Monferrato (Piedmont, Northwestern Italy): new paleoenviromental data

Massive whitish silty-clayey to sandy silts, yielding mixted calcareous (mainly foraminifers, ostracods, echinoid fragments) and siliceous (sponge spicules, diatoms and radiolarians) were discontinuously recovered in an area of about 40-45 km2 of the Monferrato (Piedmont, Northwestern Italy). The diatomaceous silts are local members of the Argille Azzurre (Blue Clay) Formation (Zanclean, Lower Pliocene) (Dela Pierre et al., 2003). Micropaleontological assemblages (diatoms, foraminifers, calcareous nannoplancton, ostracods and radiolarians) were quantitatively studied in the natural section of Castelcebro and La Torretta and in an abandoned quarry near Calliano (Asti). On the basis of foraminifers, they have been dated to the MPl3 biozone (Zanclean) and therefore are the first documentation of Pliocene mixted calcareous and siliceous assemblages in the Piedmont, where only Miocene diatomites or diatomaceous silts were previously described (Bonci et al., 1990; Bonci et al., 1991). The occurrence of rare Nitzschia jouseae both at La Torretta and Castelcebro allows the correlation to the Nitzschia jouseae diatom biozone (upper Early Pliocene to Late Pliocene). Calcareous and siliceous microfossils show often opposite abundances and suggest conditions of moderate, probably seasonal upwelling on shelf bottoms. Planktonic foraminifers are mainly represented by phytofagous, eutrophic taxa as Globigerina bulloides and Neogloboquadrina acostaensis; the infaunal, opportunistic Bulimina spp., Cassidulina carinata, well adapted to abundant trophic resources and to dysaerobic bottoms, are dominant. Diatoms are more frequent and well preserved at La Torretta, meroplanktonic forms (Paralia sulcata, Thalassionema group) and Chaetoceros resting spores are common to abundant. Stress tolerant taxa are common to dominant also between ostracods (Costa edwarsi) and radiolarians (Dictyocorynids, Rhopalastrum zitteli). Within the calcarous nannofossils, the absence of Discoaster spp. is inferred to be due to high nutrient availability in surface waters. The quantitative micropaleontological study documents rather high productivity episodes and enlights an Early Pliocene palaeoenvironment more complex and diversified than previously known in the region, where only calcareous assemblages are documented. References Bonci C., Clari P., Ferrero Mortara E., Ghibaudo G., Pirini C., Ricci B., Valleri G. & Violanti D. (1990) - The diatomites of Marmorito (Western Monferrato, northern Italy). Mem. Sc. Geol., v. 67, pp. 189-226 Bonci M.C., Fanucci F., Pirini Radrizzani C., Rizzi A. e Tedeschi D. (1991) - I livelli diatomitici della successione miocenica di Cappella Montei (area di Serravalle Scrivia, Bacino di Alessandria). Boll. Soc. Paleont. It., 30(3): 381-301. Dela Pierre F., Balestro G., Bicchi E., Boano P., Clari P., De Luca D., D’Atri A., Fioraso G., Forno M.G., Morelli M., Piana F., Ruffini R. e Violanti D. (2003) - Note illustrative della Carta Geologica d’Italia alla scala 1:50.000, Foglio 157 Trino. APAT Dipart. Dif. Suolo: 1-147.