I sedimenti quaternari del settore meridionale della Valdelsa (provincia di Siena)

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Il Quaternario Italian Journal of Quaternary Sciences, 17(1), 2004, 33-40

Gli studi condotti sul settore meridionale della Valdelsa (Toscana meridionale) hanno permesso di caratterizzare e definire tramiteUBSU i locali depositi quaternari. Sono stati così riconosciuti sei sintemi (Sintema di Campiglia dei Foci – CDF, Sintema dell’Abbadia –ABB, Sintema di Calcinaia – CAL, Sintema del Torrente Foci – FOC, Sintema di Bellavista – BEL, Sintema di Poggibonsi – POG) chevengono riuniti sotto un unico supersintema indicato come Supersintema del Fiume Elsa.Il Sintema di Campiglia dei Foci, composto da quattro litofacies (litofacies argillosa, litofacies sabbiosa, litofacies conglomeratica, litofaciescalcarea), viene interpretato come l’espressione di un episodio palustre/lacustre datato al Pleistocene inferiore-medio.I Sintemi dell’Abbadia, di Calcinaia, del Torrente Foci e di Bellavista sono riferiti ad una successione di terrazzi tutti caratterizzati da duelitofacies (sedimenti calcarei concrezionari e sedimenti detritici) in proporzioni variabili nei vari sintemi e riferiti ad episodi di sedimentazionefluvio-palustre datati al Pleistocene superiore-Olocene sulla base di una datazione radiometrica.Il Sintema di Poggibonsi corrisponde alle alluvioni attuali ed è formato da sedimenti detritici e localmente da sedimenti calcarei tuttora informazione.Mediante l’integrazione dei dati sedimentologici-stratigrafici con quelli geomorfologici, viene proposta una ricostruzione paleogeograficadella Valdelsa meridionale dal Pleistocene inferiore-medio ad oggi.

Stratigraphic and Geomorphological researches allow to characterize the Quaternary deposits cropping out in the southern part of theValdelsa Basin and previously named “ancient travertines” and “recent travertines”. The Valdelsa Basin is a segment of a NW-SE oriented,tectonic depression extending from the Serchio Valley to the north, to the upper Tiber Valley to the south. This extensional structure,developed in Late Miocene, was filled up during Pliocene with thick terrigenous marine sediments. Afterwards, in the upper MiddlePliocene, the area, as well as most of southern Tuscany, emerged. During Quaternary the southern part of the Valdelsa Basin was characterizedby episodic calcareous sedimentation. Detailed geological investigations enable us to distinguish six Quaternary synthems.Campiglia dei Foci Synthem (CDF) – a palustrine/lacustrine synthem crops out in wide subhorizontal terraces with comparable altitudes(average quote 230 m). It is composed of four lithofacies: clayley lithofacies - grey clay and clayey silt (max 10 m thick), locally characterizedby thin, plane laminations and carbonaceous plant remains. The fossil content is represented by sporadic freshwater and terrestrialGastropods (Parmacella), stems and oogons of Charophyte and Ostracods. In the San Gimignano area, lignite-bearing beds,included in this lithofacies, were mined in the early XXth Century, up to the total exhaustion, with fossil remains of mammals of Early-Middle Pleistocene or of Galerian. This lithofacies can be referred to a palustrine environment. Calcareous lithofacies – compact, micriticlimestone, locally rather vacuolar or with bioturbation (roots). The stratification is locally enhanced by thin silty-marly beds. It is interpretedas a lacustrine environment with highly concentrated CaCO3 waters. Sandy lithofacies - poorly cemented, fine-grained, quartzcarbonatesands in beds or lenticular layers (up to 50 cm thick), with a sometimes abundant silty matrix and locally scattered centimetricpebbles occur. In some outcrops, vegetable remains (grass stems and roots) encased in carbonate crusts are present. This lithofaciescan be referred to the marginal part of the lacustrine/palustrine basin. Conglomeratic lithofacies – conglomerates and paraconglomeratesin variable amounts in the different areas of the Basin. They are characterized by a medium size, sandy matrix and a commonlymassive fabric, except for local moderate imbrication. This lithofacies is interpreted as alluvial fan deposit of small rivers.Abbadia Synthem (ABB); Calcinaia Synthem (CAL); Torrente Foci Synthem (FOC); Bellavista Synthem (BEL) – four fluvial/palustrinesynthems developed in four orders of terraces upon the recent alluvial sediments and localized along the flanks of the valleys of ElsaRiver and torrents Foci and Staggia. The deposits of these syntems are represented by two lithofacies: calcareous deposits (t1) formedby bodies of concretionary phytoclastic-phytohermal calcareous tufas associated with compact micritic limestone and occasionally darksilty clays with organic matter; detritical deposits (t2) composed of mixed, terrigenous-carbonate silty sands and lenticular beds of gravels.The calcareous lithofacies are located in precise segments of the fluvial valleys; their origin is connected to springs with highlyconcentrated CaCO3 waters, perhaps hydrothermal. A Radiometric C14 dating of 25690±180 yr BP performed on a peat layer inside theCalcinaia Synthem (CAL) implies a probable Late Pleistocene-Holocene age for these four synthems.Poggibonsi Synthem (POG) – fluvial synthem corresponding to the recent alluvial deposits and composed of sand and silty-sand withgravels. Locally calcareous tufa are still in formation. These deposits are referred to the Holocene.The collected data show that this sector of the Valdelsa Basin, during the Early-Middle Pleistocene, was characterized by an episode ofcontinental sedimentation. The initial clastic palustrine deposits (clayey lithofacies) and the following lacustrine carbonates (calcareouslithofacies) were both deposited in a probably tectonic depression. The uplifting of the area induced the rejuvenation of the local hydrographicsystem which cut deeply the Pliocene sediments and caused a relevant inversion of relief. Along this new hydrographic pattern,during the Late Pleistocene-Holocene, the interaction of tectonic uplift and climatic changes caused the formation of four terraced orders (palustrine/fluvial synthems) locally characterized by the deposition of calcareous tufas.

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