The contribution of calcareous nannofossil biostratigraphy in solving geological problems: the example of the Oligocene-Miocene foredeep of the Northern Apennines (Italy).

In the Northern Apennines fold and thrust belt of Italy, the post-collisional stages are characterized by the Oligocene-Pliocene progressive shortening of the foredeep basins, resulting from the eastward migration of the orogenic front toward the belt foreland represented by the Adria plate continental margin. The foredeep deposits were involved in the tectonic stack as thrust sheets detached from their substrate and overthrust by the Ligurian Units derived from the Ligurian-Piedmont oceanic basin. The aim of this paper is to discuss the decisive contribution of the calcareous nannofossil biostratigraphy in the determination of the space-time evolution of the Oligocene-Miocene foredeep basins of the Northern Apennines. We present synthetically the results of the analysis of the nannofossil content of thousands of samples collected in several stratigraphic sections in the foredeep deposits, which are represented by thick sandstone turbidites interposed between shale and marl deposits marking the starting and the closure of the sedimentation. The nannofossil data indicate that the development of the Oligocene-Miocene foredeep is comprised between the Rupelian, the age of the oldest successions, and the lower Messinian of the more external basin. These data also highlight the diachronous evolution of the foredeep basins associated with their eastward migration; in particular, the closure age data show that, starting from the Oligocene, the tectonic activity of the Northern Apennines is arranged in a continuous series of events, during which new basins were successively opened and closed, and that the emplacement of the Ligurian Units over the foredeep basins seems to be an uninterrupted process rather than due to distinct tectonic stages.