New sedimentoogical data from the Villafranchian deposits of Castelnuovo Garfagnana and Barga basins (Lucca, Tuscany, Italy)

The Castelnuovo Garfagnana and Barga basins are located about 25 km north of Lucca along the intramontane valley of the Serchio river. They are about 5 Km wide, 15 and IO Km respectively long and are separated by a structural high, known in literature as "Soglia di Monte Perpoli" (Puccinelli, 1987). These basins constitute a tectonical extensional depression, elongated in a NW-SE direction, delimited southwest by the Alpi Apuane and northeast by the Apennines. The geologica! history of the Castelnuovo Garfagnana and Barga basins, as the other depressions formed along the Tyrrhenian margin of the Apennine chain, is related to the Tyrrhenian rifting, active since late Tortonian, and the counter-clockwise migration of the chainforedeep- foreland system (Patacca et al., 1990). The two basins, studied since 1800 for mining of lignite and quarrying of clay, are filled with a remarkable thickness of continental deposits, mainly of Villafranchian age, generically interpreted in literature as fluvio-lacustrine (Masini, 1936; Calistri, 1974). This study aims at offering a more detailed reconstruction of the depositional history of these deposits. A geologica! survey 1/ 10.000 scale, together with sedimentological and compositional analysis and paleontological data, allow to locate five informai litostratigraphic units, whose geometrica! relationships are shown in Fig. l. Within each unit, a various number of lithofacies can be seen (Miall, 1996 and, Sanchez-Moya et al., 1996 lithofacies code are used) and their association allows to define some architectural elements (Miall, 1996). Their features and their geometrica! relationships give rise to a more detailed and complex reconstruction of the depositional history of the Vi llafranchian deposits. Sixteen conglomeratic, heterolithic, sandy and silty-clay lithofacies were distinguished and grouped in nine architectural elements belonging to channel belt elements and overbank elements. The channel belt elements consist of conglomeratic deposits (isolated channels !C, grave! bar GB, debris flow deposit DF), heterolithic and sandy elements (heterolithic bar HB and sandy bar SB), while swamp (SP), pond (PO), levee (L) and muddy-floodplain (M) constitute the fine-grained overbank elements. On this basis four alluvial depositional systems can be recognized. The basai fluvial system, corresponding to lithostratigraphic Uni t l , li es in disconformity o n the substratum made by the metamorphic and nonmetamorphic "Falda Toscana" units and subordinate "Ligurian" units. !t is composed mainly of fine- 54 grained deposits, related to overbank elements with intercalation of conglomeratic and sandy lithosomes (Channel belt elements). Within overbank elements SP deposits are characterized by widespread presence of lignite (C lithofacies) and subordinate intercalation of clays (Fm lithofacies). Tree trunks in life-wise position have been observed. Vegetai remains of G/yptostrobus sp., are consistent with humid-subtropical climatic conditions. PO element is characterized by rhythmical alternations of clay and silt (Fh, Fsm and Fm lithofacies) containing freshwater taxa like ostracodes (Candona spp. 1/yocypris gibba), gastropods (Me/anoides curvicosta, Prososthenia paulae Theodoxus sp. Laminifera vi/lajranchiana), teeth of fish (Tinca sp., Leuciscus cepha/us), together with charophytes oogonia and leaves. L and M elements consist respectively of rhythmical alternations of fine sand, silt, clay (Sh, Fsm, Fm lithofacies) and clay and silty-clay (Fm, Fsm lithofacies). Widespread presence of hydromorphic paleosols (P lithofacies) containing land taxa (e.g. Helix sp. , Pomatias e/egans, Discus rotundatus) is observed mainly in M element. Channel belt elements consist of isolated channels (IC) and heterolithic bar (HB). IC element shows a ribbon geometry and frequently "ailes d' etalement" (sensu Friend et al. , 1979). Width-depth ratio is consistent with stable channels. They are composed of polygenic, grain-supported conglomerate (Gp, Gt and Gcm lithofacies), frequently organized in fining upward trend. HB element shows a plane-convex upward geometry and consists of alternations between sand and grave! (Ep, Et, St, Sp lithofacies) organized in fining upward trend. They represent bars migration within the channel. Compositional data and paleocurrent analysis (northwest to southeast direction) are consistent with a single longitudinal (sensu Miall, 1996) fluvial course flowing through the two basins. The whole acquired data is consistent with the characteristic of cohesive sediment anabranching river system, organo-clastic sub-type (sensu Nanson & Knighton, 1996). The second alluvial system recognized corresponds to lithostratigraphic Uni t 2 (Fig. l). Unlike the underlying system, Unit 2 is mainly composed of conglomerates (channel belt elements), while finegrained deposits (overbank elements) are subordinate. GB and SB elements constitute channel belt elements. GB element is made up of openwork and matrix-filled conglomerates (Gp and Gt lithofacies), frequently organized in fining upward trend. Reactivation surfaces are frequently observed. SB element consists of medium to coarse-grained sand (Sp and St lithofacies) sometimes containing clay soft clasts. GB and SB associations are interpreted to represent bars migration within the channel. L and M elements constitute the overbank elements. They are respectively characterized by fine to medium-grained sand (Sh and Sr lithofacies) and by drapes of clay (Fm lithofacies), while immature paleosols (P lithofacies) are common in M element. L and M deposits show strong affinities with the actual overbank deposits of Serchio river. Deposits belonging to this unit are related to a gravel bed load braided system (sensu Sanchez-Moya et al., 1996). Compositional and paleocurrent analysis point out more or less the same longitudinal fluvial course, flowing through the two basins, observed for the underlying cohesive anabranching river system organo-clastic sub-type. Along the Castelnuovo Garfagnana and Barga margins, spatially limited conglomeratic lithosome, showing latero-vertical relationships with Unit l and 2 (Fig. l) are mapped. Lithostratigraphic Unit 3 crops out along the Apennine margin of the two basins. !t consists of channel belt elements of very coarse massive monogenic conglomerate (Gcm and Gmm lithofacies), exclusively derived from the surroundings "Macigno" F ormation. The same textural characteristics were observed in the litostratigraphic Unit 4 (Fig. l) cropping out in a small area of the southwest margin of the Barga basin. Besides "Macigno" clasts, the conglomerate is also composed of deriving "Falda Toscana" and Methamorphic units clasts. The close proximity of source area and textural features are consistent with a debris flow dominated fan (sensu Stanistreet & Mc Carthy, 1993) developed transversally to the basi n' s axis. The lithostratigraphic Unit 5 crops out in the southwest margin of the Castelnuovo Garfagnana basin and consists of channel belt elements. Lithofacies Gp, Gt e Sm, (polygenic, clast supported conglomerate and medium-coarse grained sand) representing the composition of surrounding area are associated in GB element. Paleocurrent analysis shows a radiai pattem. This unit is interpreted as a braided fluvial fan alluvial system (sensu Stanistreet & McCarthy, 1993) developed transversally to the basin 's axis. The whole acquired data lead to delineate the depositional history of villafranchian deposits cropping out in the Castelnuovo Garfagnana and Barga basins. The results of this study can be summarized in two principal points: i) onset, in a humid sub-tropical climate phase, of a first alluvial system, flowing through the two basins and related to a cohesive sediment anabranching river (sub-type organo-clastic) characterized by a prevalent finegrained overbank organic-rich sedimentation. At the same time coalescent debris flow dominated fan, mainly derived from Apennines margin, forms transversally to the basins axis. ii) Development of a gravel bed load braided alluvial system, characterized by prevalent gravelly channel belt deposits, flowing through the two basins. The erosive contact between this system and the underlying one, is an abrupt facies change inferring a possible disconformity surface. At the same time debris flow dominated fans derived from the Apennines margin continue to develop. Although the proposed reconstruction involves the necessity of a disconformity surface also within these deposits, the spatially limited outcrops and the frequently weathering do not permit to evaluate this hypothesis. During this depositional phase a braided fluvial fan derived from the Apuan Alps develops transversally in the Castelnuovo Garfagnana basin. This palaeoenvironment reconstruction implies that Monte Perpoli high plays a different role compared to that proposed by other authors. In our interpretation, according to the sedimentological and statistica! compositional data (Sodini M. et al., 2002), this area acted as a sedimentary by-pass since the opening (Lower Villafranchian) of the Castelnuovo Garfagnana and Barga basins. This interpretation is consistent with most recent data about Alpi Ar uane uplift times (Molli G., 2001).