The Terni “corridor”: new stratigraphic constraints for the reconstruction of the Jurassic paleotopography of Central Apennines.

This paper is an outcome of a field-mapping project encompassing the Sabini, Martani and Narnesi-Amerini Mts. Here, Meso-Cenozoic rocks referable to the Umbria-Marche-Sabina succession are unconformably overlain by Neogene- Quaternary marine-to-continental and volcanoclastic deposits. During field-work, our attention was focused on the analysis of Pelagic Carbonate Platform (PCP)/Basin systems, in particular on the identification of Jurassic submarine paleoescarpments and of facies associations, these being the key tools for understanding the original Jurassic-Early Cretaceous architecture of the local depositional system and its tectono-sedimentary evolution. The Terni area is crucial for appreciating the influence of inherited structures on younger deformation (Neogene- Quaternary). The present-day geological setting of the Terni basin is that of an intermontane basin, bounded by Plio- Pleistocene normal faults and filled with lacustrine-to-continental deposits of the Tiberino basin. Surrounding it, however, the Mesozoic inherited elements and their original (Jurassic-to-Early Cretaceous) relationships are well-preserved. As a result of the Early Jurassic rifting stage, a huge shallow-water carbonate platform (Lazio-Abruzzi Platform) characterized the eastern sector of the study area (present-day coordinates), passing west-ward to a slope and basinal setting with intra-basinal highs (respectively, Sabina and Umbria-Marche-Tuscany Basins). Abundant neritic material was resedimented in the deeper-water environments, and the dispersion patterns mirrored the sea-bottom rift topography. The widest PCP recognized in the area is the Sabina Plateau, a N-S trending fault-bounded high passing east- and north-ward to the Sabina Basin via steep paleoescarpments. In particular, the N or NW-dipping escarpment of the Sabina Plateau is exposed near Terni, facing the Terni plain. Opposite to this across the Plain, the circa S-dipping Jurassic paleoescarpment of the Poggio Cisterne Vecchie PCP is well-exposed on the southern slopes of the Martani Mts. These Jurassic paleo-structures imply the existence of a Jurassic basin in the subsurface of the Terni Plain, as marked by: i) unconformable contacts between the horst-block Calcare Massiccio (silicified) and the Jurassic-Lower Cretaceous olistolith-bearing hangingwall-block pelagites; ii) neptunian dykes. The existence of an arm of the Sabina Basin branching westwards (the Terni “corridor”) is constrained by the occurrence of shallow-water material embedded in Lower-to-Upper Jurassic basin-fill pelagites exposed in the Amelia Ridge. This post-lower Pliensbachian resedimented material implies a productive carbonate platform (Lazio-Abruzzi Platform) other than the Sabina Plateau, which was already drowned, as the source-area. The Narni Ridge, in contrast, during the Jurassic was effectively shielded by the huge Sabina Plateau from the influx of shallow-material shed by the Lazio-Abruzzi Platform.