The Numidian Flysch: a guide formation for the reconstruction of the paleogeography and tectono-sedimentary evolution of Southern Apennines

The Numidian Flysch shows constant lithological features from the strait of Gibraltar to central Italy. It is characterized by quartzarenites showing grains of monocrystalline, rounded and frosted quartz, and by kaolinitic mudstones. This research has pointed out that in the southern Apennines 1) the Numidian Flysch was deposited exclusively in the Campania-Lucania carbonate platform and in the Lagonegro-Molise basin, both located on the Apulian continental margin, and never is present in tectonic units originated from the oceanic area located west of that margin; 2) in the axial zone of the Lagonegro basin it stratigraphically follows a formation consisting of varicoloured clays (Argille Varicolori Auct.); 3) its age is limited to the early-middle Langhian, that is to say, it begins to sediment about 7 million years later than in the Maghrebian chain and deposited for a time span limited to 1-1.5 Ma. The thickness of the Numidian Flysch gradually decreases towards the north from about 600-1,000 meters to a few tens of meters and in some of the northeastern outcrops it is represented only by some layers of quartzarenites. This is accompanied by a decrease in size of the particles becoming more and more finer. In addition, northwards and frequently in the same section, a lower mineralogical and textural maturity (from quartzarenites to litharenites, and presence of abundant matrix, sub-angular, polycrystalline and deformed quartz grains) is well recognizable. In the Campania-Lucania carbonate platform the Numidian Flysch evolves to pelagic marly-clayey deposits, followed by mineralogically immature turbidite sandstones of Serravallian age. In the Lagonegro basin the Numidian Flysch replaces Cretaceous-lower Miocene turbidite deposits, consisting of limestones and red marls, on the western side of the basin, variegated clays in the axial zone and calcareous turbidites or variegated clays in the eastern side. Since the late Langhian, it evolves to pelagic sediments followed by lower Tortonian immature turbidite sandstones. In the successions of the Molise basin the Numidian Flysch is interbedded in a succession consisting of calcareous turbidites and pelagic limestones and marls, reaching the Messinian. The lithological features and the age of the Numidian Flysch in central-southern Apennines, therefore, point out an evolution different from that of the Numidian Flysch of the Maghrebian chain. During the early Miocene, a paleogeographic barrier or other unknown obstacles prevent Numidian sands from reaching the south-Apenninic domains. In the early Langhian, the disappearance of these obstacles allows sands to reach the deep basins located on the Apulian margin. In the late Langhian the Numidian sedimentation is canceled and replaced by mainly pelagic sediments, which will evolve to foredeep deposits in the Serravallian-Messinian time span. In addition, the significant presence of feldspathic and lithic grains testifies a double detrital supply: polycyclic quartzose sands and kaolinitic mudstones from the African craton and metamorphic and plutonic grains from the Hercynian or older rocks of the internal units of the southern Apennines. The Numidian Flysch of the southern Apennines allows to assign the tectonic units in which is present to the Campania-Lucania carbonate platform or to different zones of the Lagonegro-Molise basin and therefore is of great importance in the reconstruction of both the Mesozoic-Cenozoic paleogeography and a tectono-sedimentary evolution very difficult to decipher, given the convergence of sedimentary facies in the Apenninic deep basins since Cretaceous to Miocene, the presence of several tectonic phases and of out of sequence and back-thrusts.