The Mesozoic of the southern Amerini Mts. (Central Apennines, Italy): new insights from field-mapping and facies analysis

New stratigraphic-structural data are presented herein from the Amelia Ridge (Central Apennines, Italy), which is characterized by Meso-Cenozoic rocks of the Umbria-Marche-Sabina sedimentary succession. The region experienced a rifting phase during the Early Jurassic, when the vast Hettangian Calcare Massiccio carbonate platform was dismembered by normal faults, causing a complex morpho-structural patchwork of uplifted and downthrown blocks. Drowning of the benthic factories led to the development of two main sedimentary environments: pelagic carbonate platforms (PCPs) and basins. The Early Jurassic rift architecture is documented by facies and thickness variations of the Jurassic-Lower Cretaceous post-rift succession. In the southern Amerini Mts., a geological mapping project on the 1:10.000 or locally 1:5.000 scale, coupled with facies analysis and a dense network of stratigraphic/sedimentologic logs, resulted in the identification of several Jurassic structural highs (Amelia, Fornole and Foce sectors), flanked by deeper-water basins. Although the PCP-top successions are not exposed due to post-Jurassic faulting and modern erosion, highly distinctive facies associations define the escarpment margins of these platforms and the adjacent hangingwall-block successions that onlap them, which often embed gravity-driven deposits including rock-fall megaclastics. Fine-grained calcarenites made of shallow water-derived material are found embedded in the upper Pliensbachian-Bajocian part of the basin-fill succession in the eastern sector of the study area. This occurrence is unexpected as these deposits postdate the drowning of the local Calcare Massiccio carbonate platform, which suggests provenance from the Latium-Abruzzi Platform. This provides new evidence for restoring the Jurassic paleogeography of Central Apennines, and for deciphering the itineraries of resedimented carbonate sands from this relatively distant source-area. Evidence for an Early Cretaceous extensional tectonic phase, which is well-documented in the neighbouring Narni Ridge, has now been also recognized in the Amelia area. Here the Marne a Fucoidi Fm. (Aptian-Albian) rests unconformably on the Hettangian shallow-water carbonates of the Calcare Massiccio Fm., as a result of rejuvenation and erosion of the Early Jurassic margin of the Amelia intra-basinal high. Following Neogene Apenninic shortening, the region was extended in the Pliocene-Quaternary. Within this picture of polyphase deformation, the role played by Mesozoic inherited structures and by large (hill-size) olistoliths in our study area is also discussed.