Mesozoic palaeogeography and tectono-stratigraphic features of the northern Amerini Mts. (Central Apennines, Italy): new constraints on their Jurassic and Cretaceous evolution

A geological mapping project was performed on the 1:10,000 scale in the northern Amerini Mts. (Narni–Amelia Ridge, Central Apennines), coupled with facies analysis and multidisciplinary outcrop characterisation. This project was focused on the Jurassic-Lower Cretaceous succession, in order to reconstruct the Mesozoic palaeogeography and tectono-sedimentary evolution of the study area. This sector of the Apenninic Chain (i.e. Umbria-Marche-Sabina palaeogeographic domain) experienced the Early Jurassic rifting phase, which dismembered the vast Calcare Massiccio carbonate platform. The development of a rugged submarine topography, coupled with drowning of the benthic factories, were the main effects of this normal faulting. The complex submarine physiography, made of structural highs and lows, is highlighted by facies and thickness variations of the Jurassic and Lower Cretaceous deposits. The hangingwall blocks hosted thick (hundreds of metres) pelagic successions, with variable volumes of admixed gravity-flow deposits. These successions onlapped the horst blocks along escarpments, rooted in the rift faults, where the pre-rift Calcare Massiccio was exposed. The tops of footwall blocks (Pelagic Carbonate Platforms or PCPs) were capped by thin (few tens of metres or less), fossil-rich and chert-free, condensed pelagic successions. This rift architecture was evened out at a domain scale in the Early Cretaceous. Successively, Miocene orogenic and Plio-Pleistocene extensional faulting caused uplift and exhumation of the Mesozoic rocks.

In the study area, geothematic mapping associated with the analysis of basin-margin unconformities and successions revealed a narrow and elongated Jurassic structural high (Mt. Croce di Serra - Mt. Alsicci structural high), surrounded by Jurassic basinal pelagites. The PCP-top condensed succession is not preserved. The chert-rich basinal units rest on the horst-block Calcare Massiccio through unconformity surfaces (palaeoescarpments), as marked by the silicification of the (otherwise chert-free) shallow-water limestone. The onlap successions embed megablocks of Calcare Massiccio (hundreds of metres across), detached from their parent palaeoescarpments. Very thin, condensed deposits form discontinuous veneers on the olistoliths of Calcare Massiccio (epi-olistolith deposits) and are onlapped by younger basin-fill pelagites. The beds surrounding the olistoliths are characteristically bent due to differential compaction, as their (newly acquired) strikes mimic the outline of the stiff objects they were burying.

Indirect evidence for a Toarcian, post-rift, tectonic pulse can be locally mapped, and is documented by angular unconformities between the Pliensbachian and Toarcian pelagites, as well as by mass-transport deposits found in the Rosso Ammonitico (Toarcian).

The same goes for millimetric to centimetric neptunian dykes made of Maiolica pelagites cross-cutting the Corniola Fm. (Sinemurian-Pliensbachian). These dykes, coupled with the occurrence of unconformities between Aptian-Albian pelagites (Marne a Fucoidi Fm.) and Lower Jurassic rocks (Calcare Massiccio and Corniola formations), provide evidence for a further Early Cretaceous tectonic phase, recently reported from the southern sectors of Narni-Amelia ridge.