Tectono-stratigraphic evolution of Jurassic Tethyan rift basins in the Early Cretaceous: a comparison between the Central Apennines and the Southern Alps

Abstract

Evidence for the Early Jurassic Western Tethys rifting is in the Alps and the Apennines. This normal faulting phase dismembered a vast carbonate platform (Calcari Grigi and Calcare Massiccio platform) and produced a complex paleogeographic setting made of intrabasinal morphostructural highs and deeper-water basins (i.e. pelagic carbonate platform or PCP/basin system). Basin-fill pelagites and their geometrical and facies variations highlight this complex architecture. There are growing clues for Early Cretaceous, post-rift, synsedimentary normal faults affecting and displacing the margins of PCP, as in the Umbria-Marche-Sabina Domain and the Trento Plateau-Lombardy Basin. Typical stratigraphic and sedimentological features of this syndepositional tectonic phase include: i) the back-stepping of PCP margins, ii) the deposition of calciclastic bodies, iii) the occurrence of neptunian dykes, and iv) the development of angular unconformities. Geological mapping in the Narni-Amelia Ridge allowed to identify field-evidence for this Cretaceous normal faulting. A polygenic breccia (“Mt. Cosce Breccia”) made of clasts of lithologies not younger than the Early Cretaceous, dispersed in a matrix of Maiolica-type facies, rests through an erosional surface on a Jurassic horst-block of Calcare Massiccio. Comparable clastic deposits are embedded in the uppermost part of the Maiolica in hangingwall-basin successions, associated with soft-sediment deformation. Lower Cretaceous pelagites (Maiolica+Marne a Fucoidi) are locally seen onlapping the unroofed Calcare Massiccio, as well as infilling neptunian dykes. Impressive similarities were recognized along the Jurassic western margin (Ballino escarpment) of the Trento Plateau, albeit at a larger scale. Here, a Lower Cretaceous polygenic breccia (Ballino, Pregasio and Prabione breccias) bearing heterometric (up to >20 m across) blocks of Jurassic-Lower Cretaceous deposits referable to the Calcari Grigi Group, the basinal Lombardy Succession and the horst block-top Venetian Succession, occurs. The clasts are dispersed in a Maiolica-type matrix. The breccia rests unconformably on the Calcari Grigi Group and on several Jurassic units of the Lombardy basin-margin succession, and is accompanied by neptunian dykes made of Maiolica facies. Where preserved by Neogene Alpine deformations, Cretaceous normal faults were locally observed, sealed by the calciclastics and the uppermost part of Maiolica. Field-evidence suggests a phase of tectonic instability during the late Early Cretaceous, coeval with the deposition of the uppermost part of Maiolica, affecting PCP-basin systems in both Central Apennines and Southern Alps. Cretaceous faults locally re-activated Jurassic ancestors, locally had a higher angle than Jurassic faults, locally developed as new faults. The result was the displacement and rejuvenation of inherited Early Jurassic escarpments, or the backstepping of PCP-margins producing exhumation of Jurassic structural highs.