Lithostratigraphy of the Villafranchian formations from the Barga and Castelnuovo areas

The Castelnuovo Garfagnana basin, which is investigated since 1800s for mining of lignite and quarrying of clay, is filled by Villafranchian continental deposits for long time interpreted as fluvio-lacustrine sequences (De Stefani 1889, D'Amato Avanzi & Puccinelli, 1988; Masini, 1936; Calistri, 1974; Puccinelli, 1987). A recent survey by Landi et al. (2002) provides a more detailed paleoenvironmental reconstruction, mapping four different lithostratigraphic Units. The lithostratigraphic Unit l is referred to a cohesive sediment anabranching river system developed in moist climatic conditions. This lithostratigrafic Unit, which represents the more ancient fluvial system recorded, mainly consists of spread fine clayey, silty and sandy deposits, characterized by accumulation in si tu of lignite (Overbank architectural elements ), and, subordinately, of conglomeratic deposits (Channel belt architectural elements). The lithostratigraphic Unit 2, 3 and 5 are chiefly composed of conglomeratic deposits (Channel belt architectural elements), respectively related to: (i) gravel bed-load alluvial system (lithostratigraphic Unit 2), (ii) debris flow dominated fan (lithostratigraphic Unit 3), and (iii) braided fluvial fan (lithostratigraphic Unit 5). The age of these deposits represents a matter of debate. Several previous studies suggested a generalized Villafranchian age (De Stefani, 1887, Zaccagna, 1932; Masini, 1933). Because of the occurrence of a diversified mammal assemblage (Anancus arvernensis, Lynx cf. issiodorensis, Sus cf. minor, Tapirus sp.) (De Stefani, 1887, 1889) in deposits related to the basal portion of lithostratigrafic Unit l of Landi et al. (2002), Azzaroli (1977) and Antiga (1988) proposed a Lower Villafranchian age (correlable with the Triversa Faunal Unit). In the middle portion of lithostratigraphic Unit l instead, the presence of charophyte gyrogonites, which belong to the species Nitellopsis cf. megarensis, and fresh-water molluscs (Theodoxus cf. groyanus, Prososthenia paulae and Prososthenia sp.) is indicative of a Middle Villafranchian age, corresponding to the Late Pliocene (Souliè-Marsche, 1979; Gliozzi et al., 1997; Esu & Girotti, 2001). The age of the successive lithostratigraphic Units is unclear. However, some ostracod val ves assigned to the species Cyprideis torosa have been collected in the basal portion of the lithostratigraphic Unit 2. According to Gliozzi et al. (1997), this species is commonly found in Pleistocene sediments with a stratigraphic range, which extends from Late Villafranchian to Aurelian. The different biostratigraphical setting of the lithostratigraphic Units l and 2 is consistent with the hypothesized presence of a disconformity between these two Units, as inferred by Landi et al. (2002). lithofacies that sometimes could be dominant, the Argille sabbie e conglomerati di Fornaci di Barga is characterized by variable amount of centimetric to decimentric lignite fragments , that could be very abundant and concentrate in centimetric horizons. This formation also include chunks of tree and fragments of leaves. Other fossi! remains, recognized within the fine grained intervals, are ostracods, gastropods and oogonium of Charophyta. Though with a limited latera! continuity, centimetric to decimetric paleosols are present at different levels of the formation. The Conglomerati di Barga, instead, is characterized by the dominance of conglomeratic lithofacies, whilst intercalations of sandy conglomerate and sand are subordinate. Fine grained intervals of thin and well bedded sandy and silty-clayely alternances, with very rare paleosols, are also present; in places, they could be up to 20 m thick. In this formation, micro and macro vegetable remains are rare and mainly concentrate in the silty sandy or sandy beds, however, chunks of three have been recognized even in the coarse grained intervals. Along the Apennine and Alpi Apuane borders, both formations laterally and vertically interfinger with coarse to very coarse grained clastic deposits such as Conglomerati di Colle Buvicchia (Lithostratigraphic Unit 3 of Landi et al. 2002), and Conglomerati di Mologno (Lithostratigraphic Uni t 4 of Landi et al., 2002), that in some p laces, and mainly along the Apennine side, could replace either the Argille, sabbie e conglomerati di Fornaci of Barga or the Conglomerati di Barga. West of Castelnuovo Garfagnana, the Conglomerati di Barga is laterally and vertically replaced by the Conglomerati di M. Alfonso (Lithostratigraphic Unit 5 ofLandi et al., 2002). Up to present, the Villafranchian deposits of Castelnuovo Garfagnana and Barga basins have been interpreted as sediments of fluvio-lacustrine environment (De Stefani 1989, Masini, 1936, Calistri 1974, Puccinelli 1987). Recently Landi et al. (2002) propose for these deposits a different paleoenvironmental reconstruction recognizing two main river systems (corresponding to the Argille, sabbie e conglomerati di Fornaci di Barga and the Conglomerati di Barga, respectively) flowed longitudinally to the basin axis, that were supplied by some transversal tributaries (corresponding to Litostratigraphic uni t 3,4 and 5) fed by both the Apennine and the Apuan borders.