Sedimentologic and paleontologic obsrvations on the Castelnuovo Garfagnana Villafranchian basin (Lucca, Tuscany, Italy)

The Castelnuovo Garfagnana Basin is located along the intramontane valley of the Serchio River. This basin extends approximately for 15 Km in length and it is separated from the contiguous Barga Basin by a structural high, currently known as "Soglia di Monte Perpoli" (Puccinelli, 1987). From a regional point of view, the two basins represent a tectonic extensional depression delimited southwest by the Apuan Alps and northeast by the Apennines. The geologica! investigation of the Castelnuovo Garfagnana Basin began in the 1800s because of the presence of abundant lignite deposits. The sedimentary succession of this basin consists of continental deposits, mainly of Villafranchian age, for long time related to a generi c fluvio-lacustrine environment (see e. g., D'Amato Avanzi & Puccinelli, 1988; Puccinelli, 1987). A recent geologica! and sedimentological survey of this area (Landi et al., 2002) argues about the presence of four different litostratigraphic Units, which were deposited by two river systems and some tributaries that flowed respectively, longitudinally an d laterally to the axis of the basin. The litostratigraphic Unit l , bere discussed in detail from both a sedimentological and a paleontological point of view, represents the earliest fluvial system recorded irt this area, which is mainly characterized by often fossiliferous, fine-grairted, deposits. By contrast, the other litostratigraphic Units prevalently consist of sterile conglomeriltes. The sedimentary succession of the litostratigraphic Unit l lies in disconformity on metamorphic and non-metamorphic Falda Toscana and Ligurian Units. The thickness of the entire succession, which measures about 200m., was estimated on both seismic and geologica! evidences (Cancelli et al., 2002). The succession described by Landi et al. (2002) mainly consists of fine-grained deposits, related to overbank elements (swamp, pond, levee and muddy-flood plain, coded respectively as SW, PO, L and M in Landi et al., 2002), with intercalations of conglomerates and sand lithosomes. The latter are irtterpreted as channel belt elements (simple channel fill and heterolithic bar, coded respectively as CH and HB in Landi et al., 2002). The overbank elements L, M, SW and PO are represented by silty, sandy or muddy deposits. M and L deposits, mainly constituted by silty, clayey and sandy lithofacies, show some intercalations of hydromorphic paleosoils containing remains of land molluscs (e.g., Helix sp., Pomatias elegans, Discus rotundatus). Freshwater fossi! taxa, such as charophytes (Chara vulgaris, Lychnothamnus barbatus), ostracods (Candona neglecta, Candona sp., Ilyocypris gibba), molluscs (Bithynia sp., Prososthenia paulae, Theodoxus cf. groyanus), and fishes (Barbus cf. plebejus, ? Chondrostoma sp., Leuciscus souffia, Rutilus aff. rubilio, Scardinius erythrophthalmus), are also commonly found in these sediments. From a sedimentological point of view, these elements are clearly indicative of a floodplain depositional environment (Landi et al., 2002) and this interpretation is consistent with the paleontological assemblage. SW elements, which were deposited in swamp environments, are characterized by the widespread presence of both Glyptostrobus trunk remains, which lack of any evidence of transport, and leaves. PO deposits are represented by a rhythmic altemation of laminated clays and silts, containing abundant fossi! remains and sideritic glaebulae (Landi et al., 2002). A more detailed paleontological study was carried out in these deposits. The fossi! assemblage consists of charophyte gyrogonites (Chara vulgaris, Lychnothamnus barbatus, Nitellopsis cf. megarensis), plant remains, freshwater ostracods (Candona spp., Ilyocypris gibba) , gastropods (Melanoides curvicosta, Theodoxus cf. groyanus), and fish remains (Scardinius erythrophthalmus). The paleontological characters confirm that the PO elements were deposited in smalllakes (Lychnothamnus barbatus and Nitellopsis cf. megarensis; Bathia et al. , 1998) and temporary ponds (Chara vulgaris; Soulié-Marsche, 1993), characterized by anoxic conditions at bottom (Flores, 1981) as supposed in Landi et al. (2002). The occurrence of abundant fish remains (bones and teeth) is consistent with the information derived from facies analysis and charophyte gyrogonites. As discussed by Smith et al. (1988) delicate fish bone remains in floodplain environments are common only in fine-grained deposits related to swamps, ponds or smalllakes.