Thrust-top lacustrine-lagoonal basin development in accretionary wedges: late Messinian (Lago-Mare) episode in the central Apennines (Italy)
Abstract We analysed in detail the geometry, sedimentology and palaeontology (both mollusc and ostracod assemblages) of a small lacustrine–lagoonal basin located at about 1100 m a.s.l. in the Marsica region of the Apennines, at Le Vicenne locality (L'Aquila, Abruzzo, central Italy). The structure of the basin infill deposits outlines an open syncline with an E–W axis. Polygenic conglomerates interbedded with clay levels and sandy marls, which constitute roughly coarsening-upward progradational sequences, are the dominant lithologies. These sequences are interpreted as lacustrine to coastal lagoon deposits with episodic fan delta supplies. This sedimentological interpretation is also supported by mollusc and ostracod assemblages, which suggest that the sedimentary environment was characterized by a mesohaline closed water body, with salinity probably ranging from 10‰ to 13‰. Both mollusc and ostracod assemblages show a clear Paratethyan influence. In the ostracod assemblage ten species out of the fourteen collected forms belong to the Loxoconcha djaffarovi Zone as defined by Carbonnel (1978) . Thus, it is possible to refer the siltitic–clayey intercalations of Le Vicenne to the top of the Messinian (post-salinity crisis). The presence of a strong angular unconformity at the base of the basin, together with the structural setting of its deposits, suggesting a light compressive deformation, are common features in all the thrust-top basins recognized in the central Apennines. Taking into account the regional data, the age of the youngest deposit involved in the substratum (siliciclastic turbidites which recorded the Messinian salinity crisis) and the palaeobiological indications given by the mollusc and ostracod assemblages, it is possible to constrain the origin of the Le Vicenne basin to the Messinian Lago-Mare–Early Pliocene tectono-sedimentary event of the Apennines. The recognition in the Marsica range of this thrust-top basin allows us to attempt a palinspastic reconstruction locating the Marsica region in the latest Miocene–Early Pliocene active thrust zone of the Apennine accretionary wedge.