Unconformities, neptunian dykes and mass-transport deposits as an evidence for Early Cretaceous syn-sedimentary tectonics. New insights from the Central Apennines

This work presents the results of a geological mapping project performed on the Narni-Amelia Ridge (Central Apennines, Italy). Fieldwork was aimed at defining the rift and post-rift architecture of the Jurassic to Cretaceous part of the Umbria-Marche-Sabina basin succession, which is characterised by Jurassic rift-related pelagic carbonate platform (PCP)/basin systems. While the effects of the Early Jurassic rifting are well documented in the regional literature, a phase of post-rift extensional tectonics in the late Early Cretaceous is a relatively under-investigated subject. This normal faulting was apparently widespread at a super-regional scale, and the Narni-Amelia Ridge carries direct and indirect evidence for this episode of revived deformation: (i) the occurrence of limestone megabreccias ("Mt. Cosce Breccia") resting unconformably, through an erosional surface, on a Jurassic (and Cretaceous) horst-block. Stepped unconformities are associated with "spur and groove" geometries of the escarpment, suggesting topographic backstepping due to rockfall processes; (ii) millimetre to decametre-scale neptunian dykes and sills made of "Mt. Cosce Breccia" and of Maiolica pelagites. These fractures cut the footwall-block of Cretaceous faults, made of Calcare Massiccio Fm (Hettangian), sub-orthogonal to its masterbedding. Dykes are associated with tension gashes filled with chert (silicification of fracture zones). The steep walls of those neptunian dykes filled with Maiolica-type deposits also exhibit a thin chert band; (iii) the onlap of Aptian-Albian Marne a Fucoidi Fm on the Hettangian Calcare Massiccio Fm. This is an unicum in the whole UMS Domain; (iv) the presence of mass-transport deposits (slumps and debris flows) in the uppermost part of the Maiolica Fm and lower part of the Marne a Fucoidi Fm. These stratigraphic "anomalies" are collectively interpreted as the products of normal faulting, which caused the retreat of the original Jurassic margins of the PCPs, rejuvenating the submarine palaeotopography. A "middle" Barremian age for this phase of synsedimentary extension is indicated by the nannofossil assemblage of Maiolica-type facies sampled from the "Mt. Cosce Breccia" and from the neptunian dykes.