Stratigraphic succession
The Northern Apennines are composed of structural units of European origin, which are tectonically superimposed against units belonging to the Adria and African passive paleomargin within the framework of Alpine-Himalayan orogenesis. Two regional-scale extensional tectonic phases flooded the pre-rifting Triassic-Lower Jurassic peritidal carbonatic shelf (D’Argenio and Alvarez, 1980). The main extensional tectonic phase occurred during the Triassic Period and was followed by a second extensional phase during the Early Jurassic Period.
Extension initially results in the deposition of evaporites (Anidriti di Burano Fm.) or dolomites (Dolomia Principale Fm.) during the Upper Triassic and is followed by the deposition of carbonate platform limestones (Calcare Massiccio Fm.) in the Lower Jurassic. The later extensional tectonic phase is documented by both the presence of pre-orogenic normal faults (Ancona-Anzio line, Castellarin et al., 1978) and changes in the facies and thickness of sedimentary successions deposited above the Adria passive margin (Channell et al., 1979). In fact, during this phase, the Triassic-Lower Jurassic succession was separated into persistent carbonate platform (Lazio-Abruzzo area) and pelagic (Umbria-Marche area, Northern Apennines) domains. These domains were divided by a slope-to-basin transitional domain.
In the pelagic Umbria-Marche domain, the Lower Jurassic to Lower Cretaceous pelagic syn- (Corniola and Rosso Ammonitico Fms.) and post-rift (Calcari Diasprigni to Maiolica Fms.) successions are characterized by highly variable facies and thicknesses due to differential subsidence (Fig. 2). The differential subsidence ended before the Aptian Period, when facies and thickness became uniform (Marne a Fucoidi to Scaglia Rossa Fms.). During the Cretaceous and Paleogene Periods, sedimentation evolved toward hemipelagic lithotypes (Scaglia Cinerea to Schlier Fms.) characterized by carbonate and marls with abundant chert. Sedimentation then became more terrigenous during the Miocene Period. During the Middle Miocene-Pleistocene Period, the convergence of the Africa and Eurasian plates initiated Apennine orogenesis, as indicated by the deposition of siliciclastic turbiditic systems (Laga Fm. in the outer sector of the Northern Apennines; Boccaletti et al., 1990).
Figure 2. Jurassic stratigraphy of the Umbria-Marche pelagic domain.
The succession is characterized by variable facies and thicknesses between Lower Jurassic and Aptian Periods.