Brecce poligeniche a matrice argillosa alla base della Successione Epiligure della coltre della Val Marecchia (Appennino tosco-marchigiano): implicazioni tettoniche.

Middle Eocene-Upper Oligocene submarine debris flow deposits, unconformably overlying the highly deformed Ligurian allochthonous formations (Lower Cretaceous - Middle Eocene), exstensively outcrop on the Po Valley side of the Northern Apennines north-west of the "Sillaro tectonic line". These chaotic rocks show Ligurian provenances and represent the renewal of the sedimentation, in a deep water setting, on the Ligurian units previously deformed by the Ligurian tectonic phase (Middle Eocene). Due to the complex structure of the region and the widespread block-in-matrix fabric shown by the Ligurian formations at the outcrop scale, almost all of these debris flow deposits and the underlying Ligurian units were indistinctly reported in the geological literature as the "Chaotic Complex of the Northern Apennines". Accordingly, some authors hypothesized that this large complex was a stack of olistostromes and/or of "orogenic landslides". Only recently it has been recognized that the main body of the "Chaotic Complex" of the Northern Apennines is made up of broken formations or non-metamorphic tectonites. Furthermore, it has been realized that thick debris flow deposits unconformably overlie Ligurian tectonites and that these chaotic sedimentary rocks belong not to the Ligurids but to the overlying Middle Eocene - Upper Miocene epi-Ligurian Sequence.The same stratigraphic and structural framework occurs in the Val Marecchia allochthonous Ligurian sheet. In fact, at Sasso di Simone and adjoining areas polymictic shaly matrix-supported breccias outcrop in this allochthonous sheet at the base of the arenaceous and bio-calcarenitic San Marino Formation (Upper Burdigalian-Lower Langhian) of the epi-Ligurian Sequence and unconformably overlie the Val Marecchia varicoloured shales formation (Lower Cretaceous-Lower Eocene) belonging to the Ligurids.The Val Marecchia varicoloured shales formation outcropping in the studied area displays a wide range of structural styles: it looks as a large tectonic mélange, but as exotic blocks are lacking, it is here considered a dismembered formation or a unit made up of tectonites. The more evident mesoscopic structure of this unit is represented by a striking tectonic layering due to a great number of anastomosing ductile shear surfaces ranging from few meters to some hectometers. These shear surfaces split out the formation in a number of more or less large lozenge-shaped or lenticular bodies in which different deformation styles are apparent: rootless folds of different styles or isolated hinges of isoclinal folds, boudinage and pinch-and-swell are the more recurrent deformational structures affecting these bodies. The primary bedding within these bodies is rarely preserved, but where recognizable it parallelizes the main tectonic layering. The shales show a prominent primary fissility running parallel to the bedding in the coherent (i.e., bedded) units and a penetrative scaly cleavage in the incoherent ones. The wide spectrum of complex deformational structures shown by the Val Marecchia varicoloured shales formation is here ascribed to tectonic deformation (i.e., folding and shearing) of poorly lithified or unlithified sediments probably due to tectonic detachment of the originally overlying M. Morello Formation during the Ligurian tectonic phase (Middle Eocene). The penetrative scaly cleavage affecting the shales is here interpreted as deriving from a mechanism of frictional grain-boundary sliding involving the slipping of clay platelets past each other, acting on unconsolidated sediments at low effective stresses.The mesoscopic tectonic layering of the Val Marecchia varicoloured shales formation is unconformably sealed by the overlying polymictic breccias. Even if the absence of lenses and blocks of pelitic hemipelagic epi-Ligurian formations does not allow a correct definition of the age of these breccias, they can presumably be ascribed to a span of time ranging from the Middle Eocene of the youngest unit of the Ligurids to the Late Burdigalian of the San Marino Formation. Therefore, the previously described tectonic layering occurring in the Val Marecchia varicoloured shales formation can be referred to the Ligurian tectonic phase and not to the Neogene translations of the allochthonous Ligurids over Tuscan and Umbro-Marchean sequences.