Evidence for extended Hercynian basement and a preserved Jurassic basin-margin tract in Northern Calabria (Southern Italy): The Longobucco Basin

Abstract

In the Longobucco Basin (northeastern Calabria, Southern Italy) a Mesozoic succession covers the Hercynian basement, documenting the evolution of a continental margin during the Early Jurassic extensional tectonic phase correlated with the Western Tethys rift. The basin evolved from continental red beds around the Rhaetian/Hettangian boundary to shallow marine and eventually deep-sea siliciclastic turbidites in the late Pliensbachian/Toarcian, to dominantly pelagic deposits (Middle Jurassic onwards). Around the Sinemurian/Pliensbachian boundary, following a prominent phase of normal faulting, sedimentation took different paths in either hangingwall-block or footwall-block settings. This study, based on geological mapping, focuses on the southwestern boundary of the Longobucco Basin and in particular on the mutual relationships existing between the deeper-water basin-fill units (Fiume Trionto and Fosso Petrone Formations), the basement, and the shallow-water limestone of the Caloveto Formation which forms a narrow strip running parallel to the rift shoulder. The contacts among the above-mentioned units are generally all stratigraphic (unconformities of various types: onlap, downlap, nonconformity). These characteristics separate the study area from the depocentral parts of the basin in terms of vertical stratigraphy, nature of contacts among stratigraphic units (unconformable vs conformable), and impact of synsedimentary tectonics. The signature of Early Jurassic extension occurs in the form of (1) laterally continuous clastic bodies interpreted as products of catastrophic basin margin collapse and avalanching, (2) exhumation of the pre-rift units, which are otherwise deeply buried, and (3) neptunian dykes of at least two generations.Our new data provide evidence for the presence of a 20 km long, NW-SE trending preserved paleoescarpment, where the basinal units onlap either the basement or the Caloveto Formation which forms a fringing carbonate body with intriguing facies and bio-sedimentological features. The paleoescarpment was the submarine morphological expression of a Sinemurian/Pliensbachian fault, as testified by the age of the clastic body that seals it. Another pulse of synsedimentary extension took place in the latest Pliensbachian/Toarcian, overprinting the existing rift architecture and reviving the production of megaclastic deposits.