Thrust-top lacustrine-lagoonal basin development in accretionary wedges: late Messinian (Lago-Mare) episode in the central Apennines (Italy)

We analysed in detail the geometry, sedimentology and palaeontology (both m ollusc and ostracod assemblages) of a small lacustrine-lagoonal basin locat ed at about 1100 m a.s.l. in the Marsica region of the Apennines, at Le Vic enne locality (L'Aquila, Abruzzo, central Italy). The structure of the basi n infill deposits outlines an open syncline with an E-W axis. Polygenic con glomerates interbedded with clay levels and sandy marls, which constitute r oughly coarsening-upward progradational sequences, are the dominant litholo gies. These sequences are interpreted as lacustrine to coastal lagoon depos its with episodic fan delta supplies. This sedimentological interpretation is also supported by mollusc and ostracod assemblages, which suggest that t he sedimentary environment was characterized by a mesohaline closed water b ody, with salinity probably ranging from 10 parts per thousand to 13 parts per thousand. Both mollusc and ostracod assemblages show a clear Paratethya n influence. In the ostracod assemblage ten species out of the fourteen col lected forms belong to the Loxoconcha djaffarovi Zone as defined by Carbonn el (1978). Thus, it is possible to refer the siltitic-clayey intercalations of Le Vicenne to the top of the Messinian (post-salinity crisis). The pres ence of a strong angular unconformity at the base of the basin, together wi th the structural setting of its deposits, suggesting a light compressive d eformation, are common features in all the thrust-top basins recognized in the central Apennines. Taking into account the regional data, the age of th e youngest deposit involved in the substratum (siliciclastic turbidites whi ch recorded the Messinian salinity crisis) and the palaeobiological indicat ions given by the mollusc and ostracod assemblages, it is possible to const rain the origin of the Le Vicenne basin to the Messinian Lago-Mare-Early Pl iocene tectono-sedimentary event of the Apennines. The recognition in the M arsica range of this thrust-top basin allows us to attempt a palinspastic r econstruction locating the Marsica region in the latest Miocene-Early Plioc ene active thrust zone of the Apennine accretionary wedge. (C) 1999 Elsevie r Science B.V. AU rights reserved.