Silurian and its surroundings in the inner nappes of Sardinian Variscides: lithostratigraphical evidence from metamorphosed deposits

In the tectonic units of the Inner Nappe Zone of the Sardinian Variscides, due to metamorphism and deformation, the Silurian System and its lower and upper boundaries are badly defined. Apart the occurrence of black, graphite bearing phyllites and schists, no sedimentary and fossil record survived. Only in Nurra (northwestern Sardinia) a unique finding of Orthoceras-made black coquina allows attributing, even broadly, the thick, monotonous black phyllites outcrops to the Silurian System. Here, in addition, the metasediments lying at the Ordovician-Silurian transition exhibit high variability both in composition and texture: they include glaciomarine diamictite, oolitic ironstones, phosphorites, sub-aerial conglomerate and epiclastite. This succession is also characterized by the occurrence of alkaline metabasites. Different Variscan blocks, from Maures to eastern Alps, share similar litho-stratigraphic features possibly testifying glacial environment, emergence and rifting at northern Gondwana between Late Ordovician and early Silurian.