Early Cretaceous tectonic rejuvenation of an Early Jurassic margin in the Central Apennines: The “Mt. Cosce Breccia”

Abstract

Evidence for an extension-dominated tectonic phase in the late Early Cretaceous has been identified for the first time at Mt. Cosce in the Narni Ridge (60 km N of Rome, Central Apennines) as a result of geological mapping. There, the Umbria-Marche-Sabina pelagic succession overlies shallow-water carbonates (Calcare Massiccio Formation, Hettangian). The Mt. Cosce area represents a Jurassic pelagic carbonate platform-basin system, the inheritance of Early Jurassic Tethyan rifting, hosting a remarkable stratigraphic unit named the “Mt. Cosce Breccia”. This is a sedimentary breccia, bearing clasts of Calcare Massiccio and of pelagic units and forming sparse to laterally continuous outcrops, which rests unconformably on the horst-block Calcare Massiccio. The polygenic breccia is chaotic and displays heterometric clasts made of rocks not younger than the Early Cretaceous, white pebbly mudstones with radiolarian- and calpionellid-rich (Maiolica Formation) elements and a Maiolica-type matrix. The lithoclasts were clearly sourced locally and represent formations from the Calcare Massiccio to the Jurassic basinal and condensed pelagic carbonate platform units. The age of the polygenic breccias is assigned to the “middle” Barremian due to the youngest age detected within the clasts, the occurrence of Hedbergella cf. H. luterbacheri in the matrix, and calcareous nannofossils. The unconformity and breccia indicate an episode of exhumation of an original Jurassic paleoescarpment tract that had by then been buried by the lower part of the Maiolica Formation. The clasts were sourced from the exhumed vestiges of the Jurassic onlap wedge (hangingwall-basin succession), the peritidal pre-rift substrate and its condensed drape (pelagic carbonate platform-type deposits), as a product of escarpment rejuvenation, erosion, and displacement along an Early Cretaceous fault.