Early Cretaceous tectonic rejuvenation of an Early Jurassic margin at Mt. Cosce (Narni Ridge, Central Apennines, Italy)

Evidence for an extensional phase in the Early Cretaceous was detected in the western sector of Central Apennines during a geological mapping project (1:10,000 scale) in the Narni Ridge, at Mt Cosce (~100km N of Rome). A pelagic succession of the Umbria-Marche-Sabina type overlies shallow-water carbonates (Calcare Massiccio Fm., Hettangian). Jurassic sedimentation was controlled by the local architecture of the W-Tethyan rift, which dismembered the vast Calcare Massiccio platform. While tectonic subsidence forced the drowning of hangingwall-block carbonate factories around the Hettangian/Sinemurian boundary, followed by deeper marine sedimentation, benthic carbonate production survived on horsts until the early Pliensbachian, when they became sites of condensed pelagic deposition. From the Pliensbachian to Early Cretaceous, basin-fill deposits onlapped and eventually buried the inactive marginal escarpments of structural highs. A Jurassic structural high corresponds to Mt. Cosce. While its top and condensed succession are not exposed due to orogenic deformation and modern erosion, its margins are locally preserved, marked by the angular unconformity with basinal units and the diagenetic modification (silicification) of the C. Massiccio. One arresting feature at Mt. Cosce is a sedimentary breccia (Mt. Cosce Breccia), forming sparse to laterally continuous outcrops, which rests unconformably on the horst-block C. Massiccio. The polygenic breccia is chaotic, and displays: I) heterometric clasts made of rocks not younger than the Early Cretaceous, set in a greenish matrix; II) white pebbly mudstones, with radiolarian- and calpionellid-rich (Maiolica Fm.) elements. The lithoclasts were clearly fed locally, and represent formations from the C. Massiccio to the Jurassic basinal succession, as well as various Jurassic condensed facies. The youngest age detected within clasts, the absence of calpionellids and the occurrence of Hedbergella sp. in the matrix, suggest an Hauterivian-Barremian age for the breccia. The unconformity and breccia indicate in our interpretation an episode of reactivation of an Early Jurassic fault, and exhumation of a paleoescarpment tract that had been buried by the lower part of the Maiolica Fm. (with calpionellids). A purely gravitative cause must be ruled out based on the geometries and composition of the breccia: several clasts represent pre-Cretaceous units, like Middle/Late Jurassic radiolarian cherts. Their presence can only be explained by inferring that they found themselves topographically uplifted with respect to the Cretaceous basin. In our interpretation the clasts were fed from the exhumed vestiges of the Middle/Late Jurassic onlap wedge, including the peritidal substrate and the former epi-escarpment deposits (condensed facies), as a product of escarpment rejuvenation, erosion, and displacement along an Early Cretaceous fault. A non-tectonic interpretation would imply deep (100’s of m) erosion down to the Jurassic part of the basin-fill sequence along submarine canyons, for which no evidence whatsoever exists in the area. While the breccia now occupies the eastern slopes of Mt. Cosce, undoing the Neogene thrusting-related deformation (Mt. Cosce is part of the hangingwall anticline of the Narni Thrust) indicates it must in fact document a former W-facing margin as the Mesozoic paleoescarpment was rotated due to folding. Last, Quaternary extensional faults further dissected the folded and thrusted rocks embedding the Jurassic rift.