Note illustrative della Carta Geologica d'Italia alla scala 1:50.000. Foglio 194 "Acqui Terme"

The n. 194 "Acqui Terme" sheet of the Geological Map of Italy is located in the "Alto Monferrato" geographical region, that corresponds to the south-eastern sector of the geological domain known as Tertiary Piedmont Basin (TPB). The TPB consists of a sedimentary succession of mainly terrigenous deposits that unconformably overlies non-metamorphic Ligurian units as well as tectono-metamorphic units of the Ligurian Alps. The stratigraphic succession of the "Acqui Terme" sheet spans in age since Early Oligocene to Quaternary and rests onto a metamorphic basement made up of metaophiolites and relative metasediments (Voltri tectonometamorphic Unit) whcih includes tectonic slices of polymetamorphic continental crust (Valosio tectonometamorphic Unit). Within the stratigraphic succession, nine regional unconformities (D1 to D8) are recognized that allow to subdivide it into eight synthems. The synthem I is introduced by continental and shallow water coarse-grained sediments of the Molare formation (Lower Oligocene), that are followed by slope fine-grained and silica-rich deposits of the Rigoroso formation and of the siliceous member of Montechiaro d'Acqui Formation (Oligocene - Lower Burdigalian). The synthem II consists of shallow water bioclastic ramp deposits followed by outer ramp glauconitic sediment and emipelagic slope marls (Visone Formation) of Burdigalian age that pass toward basin to slope silica-rich marls (Montechiaro d'Acqui Formation - calcareous marly member). The synthem III is introduced by turbidite deposits of Upper Burdigalian and Lower Langhian age (Serole and Cortemilia formation), that are followed by Lower Langhian slope sediments (Bistagno Formation) and by outer shelf to slope sediments (Cessole formation). The synthem is closed, to the east, by Serravallian shelf deposits (Serravalle formation), to the west by Serravallian-Tortonian turbidite sediments (Cassinasco formation) and by slope sediments of the lower part of the marne di Sant'Agata Fossili (Tortonian). The synthem IV is made up of Tortonian chaotic sediments (Complesso caotico di Rocca Grimalda), intra-slope resedimented carbonates (calcarenitic member of the marne di Sant'Agata Fossili; Tortonian), followed by- and heteropically grading into slope sediments of the upper part of the marne di Sant'Agata Fossili Formation (Tortonian-Lower Messinian) and is closed by the euxinic and evaporite slope deposits of the Nizza Monferrato member (Messinian). The synthem V consists of Upper Messinian chaotic sediments (complesso caotico della Valle Versa). The synthem VI consists of shallower Upper Messinian fluvio-deltaic deposits (conglomerati di Cassano-Spinola). The synthem VII consists of Lower Pliocene bathyal sediments (lower portion of Argille Azzurre). The synthem VIII consists of Lower Pliocene slope and outer shelf marine sediments (middle and upper portion of Argille Azzurre) followed by shallow water sands (sabbie di Asti) and finally by tide-dominated delta front and delta plain sediments of Middle Pliocene age (sabbie di Ferrere). In the Acqui Terme sheet Quaternary continental sediments, referred to Early Pleistocene-Holocene, widely occur. These consist both of vertically stacked, laterally continuous fluvial deposits and terraced fluvial sediments of ancient rivers, incised by present-day watercourses. The structural setting of the "Foglio 194 Acqui Terme" is characterized in its southern part by the Voltri and Valosio Alpine tectono-metamorphic units, that are gently folded and locally thrust onto the Molare and Rigoroso formations by NW-SE transpressive fault systems that run for several kilometers and depict a mosaic-like fault pattern. These steep faults and their associated tight to open folds, induced a remarkable deformation of the synthem I. The synthem II to IV are affected by transtensive and strike-slip faults, with local remarkable offsets, while the synthems V to VIII are affected only by very poor brittle deformation, consisting of localized individual faults whose lenght rarely is more than one kilometer.