La successione dell'Unità Due Ponti (Unità Liguri Interne, Appennino Settentrionale): evidenze du un'area sorgente carbonatica nelle successioni torbiditiche cretaciche della Tetide Occidentale

Stratigraphy and petrography of arenites from the Canale Fm. (Due Ponti Unit; Internal Liguride units) has been investigated. The sedimentary succession, more than 400m thick, is characterized by a pelitic-arenaceous turbidite sequence not older than Late Santonian. Facies analysis and sedimentological features indicate a deposition in a basin plain and/or lobe fringe setting. In the uppermost part of the sequence several metric intercalations of pebbly mudstone (olistostrome Auctt.) made up of Palombini Shale-derived clasts are present. The pebbly mudstones are sometimes associated to structureless arenitic beds characterized by the same composition. On the basis of composition, a/p ratio and sedimentological features, two different kinds of beds have been recognized in the turbidite succession of the Canale Fm.: "lithoarenitic beds" (forming more than 80% of the turbidite sequence) and "calcareous beds". The framework of lithoarenitic beds includes not younger than Early Cretaceous platform derived carbonate grains (mainly oolitic- and peloidal-grainstones), acidic volcanic fragments (mainly rhyolite and rhyodacite) and crystalline s.l. grains (mainly derived from plutonic and Low grade metamorphic rocks). The "lithoarenitic beds" are characterized by a lithoarenitic composition (Q36-F14-L+C50) that indicate a sharp difference with the siliciclastic composition of the Gottero Sandstone (Q47-F34-L+C19). Comparison of modal analysis from Canale Fm. arenites and Gottero Sandstone suggests the presence of multiple source areas for Late Cretaceous internal liguride turbidite deposits, that supplied different fan systems characterized by different compositional features. Petrographic data from lithoarenitic beds of the Canale Fm. suggest a "European continental margin" derived source area showing strong affinity with the Briançonnese and Pre-Piemontese domains outcropping in Corsica and in the Sestri-Voltaggio Zone (cf. Trias-Lias Unit). The presence of Palombini Shale-derived pebbly mudstones at the top of the investigated sedimentary succession indicate a sharp change in facies associations and petrographic composition that can be recognized in several Internal Liguride units (e.g. Bocco Shale; Colli/Tavarone Fm. and Lavagnola Fm.). This change represents a regional event indicating a crucial point during the paleogeographic and paleotectonic evolution of the Internal Liguride Domain related to the presence of the Eoalpine accretionary wedge. This morphotectonic element can be considered as the main source area for the upper part of the Internal Liguride successions.