Litostratigrafia ed assetto strutturale delle Unità Liguri Interne nel settore dell'alta Val Trebbia ed alta Val d'Aveto (Appennino ligure)

Differently from previuos statement, the presence of three new tectonic units located between the Mt. Gottero and the Mt.Antola Units has been revealed by a comprehensive structural and geological field-mapping and the detailed lithostratigraphical, biostatigraphical and petrographical analysis of the upper part of the Lavagna, Trebbia and Aveto Valleys (Genoa Province). These units will be hereafter referred as: Due Ponti unit, Vermallo unit and Portello unit (from the lowest to the highest) . The Due Ponti unit, which directly overlies the M.Gottero Unit, contains a sequence made of a mixed calcareous-siliciclastic turbidites younger than Santonian age, which is here referred as Canale formation. The Canale formation consists of a 300 m thick turbidite sequence characterized by the presence of carbonatic-platform derived fragments. The uppermost part of the Canale formation is distinguished by the presence of lateral change phase made up of debris-flow deposits mainly derived from the Argille a Palombini formation. The Due Ponti unit is in turn overlain by the Vermallo unit. The Vermallo unit shows a thickness ranging from a few meters up to 200m and is characterized by a sedimentary sequence younger than Maastrichtian (probably Paleocenic) here referred as Cassingheno formation. The Cassingheno formation consists of siliciclastic thin-medium turbidites interfingering with minor debris-flow deposits. Widespread synsedimentary deformation structures and extensive pebbly mudstone deposits characterize this typical slope-base of slope deposits. The Portello unit is sandwiched between the Vermallo and the Mt.Antola units; it is here referred as the uppermost internal Liguride unit. The stratigraphic sequence includes hemipelagic shales interbedded with thin calcareous turbidites (Argille a Palombini formation) of Santonian age and a thick calcareous-turbiditic sequence (Ronco Formation Auct.). The Ronco Formation consists of a “not older than” Campanian thin marly-calcareous turbidites topped by a Paleocenic ophiolitic-bearing slope deposits here referred as Lavagnola formation. Both Lavagnola and Cassingheno formations (Vermallo unit) can be related to the Tertiary deposits of the Internal Liguride units outcropping in the southernmost areas (Bocco Shales and Colli/Tavarone Formation). The pre-Oligocene structural evolution recognized in the Due Ponti, Vermallo and Portello units is characterized by strong similarities with the deformation history recognized in the southernmost Internal Liguride units. The structural analysis reveals three pre-Oligocene folding phases which history was probably developed during a late Cretaceous-Paleocene subduction and an Eocene-early Oligocene exhumation in a subduction zone setting. The D1 phase consists of non cylindrical isoclinal to sub-isoclinal folds (axis trend N110E-N160E) with approximately similar geometry and an originally SW vergence. The F1 folds are characterized by a penetrative axial plane foliation well developed in the shaly beds, where it can be classified as slaty-cleavage. The second folding phase D2 is characterized by overturned parallel folds (axis trend N120E-140E) associated with a crenulation cleavage. The NE vergence of the D2 phase can be clear detected and the tectonic boundaries between the different tectonic units formed during the D1 phase are folded by the D2 folding phase. A following D3 phase is distinguished by gentle, concentric folds with steep axial plane, which show N170E axis trend and NE vergence. The lithostratigraphical, petrographical and structural analysis indicate the strong resemblance of the Due Ponti, Vermallo and Portello units to the southernmost Internal Liguride units (Bracco/Val Graveglia, Colli/Tavarone and Mt.Gottero Units). However, some lithostratigraphic and petrographic features seem to indicate important differences in the stratigraphic sequence probably related to the source areas of the Campanian-Paleocene turbiditic deposits. The presence of such important differences suggests a regional importance for the Due Ponti, Vermallo and Portello units and offers new interesting constraints to the Late Cretaceous paleotectonic reconstruction of the western Tethys.