Mesozoic architecture and tectono-sedimentary evolution of the Mt. Cosce sector (Narni Ridge, Central Apennines, Italy)

Previously unreported stratigraphic-structural features have been identified during a geological mapping project (1:10.000 scale) in the southernmost part of the Amelia-Narni Range (Central Apennines, Italy). The study area exhibits a Meso-Cenozoic succession of the Umbria-Marche-Sabina type, with a Jurassic structural high at Mt. Cosce, flanked to the west and north by basins. While the footwall-block top condensed deposits are not preserved due to tectonics and modern erosion, the marginal escarpments are observed locally. These are marked by the onlap of basin-fill deposits, by sparse pockets of fossiliferous condensed deposits unconformably resting on the Early Jurassic substrate (Calcare Massiccio) (epiescarpment deposits), and by a peculiar diagenetic feature (silicification) at the contacts between the shallow water Calcare Massiccio Fm. and the silica-rich basinal lithologies. The silicification process also affects the huge C. Massiccio olistoliths (up to 1 km longer axis; Mt. Mandrione) embedded in the basinal successions flanking the structural high. The main stratigraphic feature discovered in this area is a sedimentary breccia (Mt. Cosce Breccia) resting through an angular unconformity on the footwall-block C. Massiccio. The breccia is a chaotic deposit with clasts of C. Massiccio, of Jurassic basinal formations and of condensed pelagites, also with white pebbly mudstones bearing calpionellids and radiolarians (Maiolica Fm.). The lack of lithoclasts younger than the earliest Cretaceous, the absence of calpionellids, and the presence of Hedbergella sp. in the matrix all suggest an Hauterivian-Barremian age for the deposit. Field relationships and geometries suggest that the breccia is a syntectonic deposit, marking an Early Cretaceous rejuvenation of the western margin of the Jurassic Mt. Cosce High. During the Miocene shortening, the Mt. Cosce horst-block was dissected by the Narni Thrust, becoming a part of its hangingwall, while the N-dipping Jurassic paleoescarpment acted as NE-striking lateral ramp. The NE-dipping forelimb of the thrust overrides an overturned footwall syncline with a thin Jurassic basinal succession (Configni-Vasciano). Our cross-sections suggest a modest shortening (few 100’s of metres), unlike previous interpretations (layer-cake models) where larger displacements were implied. The C. Massiccio olistoliths could locally develop shear contacts with the embedding units, due to the contrasting mechanical behavior of the two lithologies when subjected to folding. Last, Pliocene extensional faults apparently exploited the strikes of the rotated pre-orogenic margins.