The Miocene/Pliocene boundary in Piedmont (North-Western Italy): subsurface (Narzole corehole) and outcrop (Moncucco quarry) data.

Almost thirty years ago a corehole carried out in Narzole (Langhe Region, Central Piedmont) by Sturani (1975) intercepted the boundary between lacustrine-brackish ipohaline deposits referable to "Facies a Congeria" (correlated with well-known post-evaporitic Messinian "Lago-mare" deposits), and marine hemipelagic Lower-Pliocene deposits. This boundary was described as extremely sudden and characterized by the presence of a black arenitic bed (Sturani, 1976). Afterwards the untimely death of Sturani some months after the Narzole drilling, the study of the corehole was never completed. The renewed interest for the Messinian Salinity Crisis had led to new researches in the Piedmont region (North-Western Italy), where the preliminary study of micropalaeontological assemblages pointed out that hemipelagic sediments overlaying messinian post-evaporitic deposits are referable to the Zanclean MPl1 biozone. Moreover, biostratigraphical and sedimentological studies are in progress on the Miocene-Pliocene boundary outcropping at Moncucco quarry (Turin Hill, Northern Piedmont), where the boundary is marked by a 0.7-1 meter thick black arenitic bed separating Messinian post-evaporitic deposits (with typical ostracoda of "Lago-mare" facies) from Lower-Pliocene marls (Argille Azzurre Fm.), whose lower portion is also referable to the MPl1 biozone. Both at Moncucco quarry and in the Narzole corehole, micropalaeontological content of the "Congeria" beds consists of Tortonian and Lower Messinian planktonic foraminifers (Globorotalia conomiozea, Gt. suterae) and rare deep shelf to bathyal benthic species (Uvigerina rutila), all interpreted as reworked; brackish ostracods (Loxochonca djaffarovi) are very rare. The black arenitic bed, observed for the first time in the Northern Piedmont, is generally barren, very rich in organic matter and is composed of a terrigenous fraction (quartz and lithic fragments of metamorphic rocks) with subordinate intrabasinal grains (glaucony and phosphates). At the top of the bed, a network of firm ground burrows, infilled by the overlying Pliocene marls, has been observed and corresponds to an omission surface (sensu Bromley, 1990). Foraminiferal assemblages of the Pliocene marine succession are rich and diversified. Both in the outcrop and in the corehole samples, the marls basal part contains rare to common Sphaeroidinellopsis dehiscens. Planktonic specimens are frequent (P/(P+B=60-70%), deep outer neritic to bathyal benthic foraminifers are common; ostracods are rare, with typical open-marine taxa (Bythocypris obtusata). It is noteworthy that also in Piedmont the Miocene-Pliocene boundary appears characterized by the black arenitic bed described in many circum-mediterranean areas (Cita et alii 1978; Roveri et alii, 2004); this boundary corresponds to the facies change usually related with the end of the Messinian Salinity Crisis (Iaccarino et alii, 1999).