I SEDIMENTI QUATERNARI DEL SETTORE MERIDIONALE DELLA VALDELSA (PROVINCIA DI SIENA)

Abstract

Capezzuoli E. & Sandrelli F., The Quaternary sediments of the Southern part of Valdelsa basin (Province of Siena). (IT
ISSN 0394 – 3356, 2004).
Stratigraphic and Geomorphological researches allow to characterize the Quaternary deposits cropping out in the southern part of the
Valdelsa 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 Middle Pliocene, the area, as well as most of southern Tuscany, emerged. During Quaternary the southern part of the Valdelsa Basin was characterized by 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 characterized by thin, plane laminations and carbonaceous plant remains. The fossil content is represented by sporadic freshwater and terrestrial Gastropods (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 EarlyMiddle Pleistocene or of Galerian. This lithofacies can be referred to a palustrine environment. Calcareous lithofacies – compact, micritic limestone, locally rather vacuolar or with bioturbation (roots). The stratification is locally enhanced by thin silty-marly beds. It is interpreted as a lacustrine environment with highly concentrated CaCO3 waters. Sandy lithofacies - poorly cemented, fine-grained, quartzcarbonate sands in beds or lenticular layers (up to 50 cm thick), with a sometimes abundant silty matrix and locally scattered centimetric pebbles occur. In some outcrops, vegetable remains (grass stems and roots) encased in carbonate crusts are present. This lithofacies can be referred to the marginal part of the lacustrine/palustrine basin. Conglomeratic lithofacies – conglomerates and paraconglomerates in variable amounts in the different areas of the Basin. They are characterized by a medium size, sandy matrix and a commonly massive 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/palustrine
synthems developed in four orders of terraces upon the recent alluvial sediments and localized along the flanks of the valleys of Elsa
River and torrents Foci and Staggia. The deposits of these syntems are represented by two lithofacies: calcareous deposits (t1) formed
by bodies of concretionary phytoclastic-phytohermal calcareous tufas associated with compact micritic limestone and occasionally dark silty 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 highly concentrated CaCO3 waters, perhaps hydrothermal. A Radiometric C14 dating of 25690±180 yr BP performed on a peat layer inside the Calcinaia 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 with
gravels. 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 of
continental sedimentation. The initial clastic palustrine deposits (clayey lithofacies) and the following lacustrine carbonates (calcareous
lithofacies) were both deposited in a probably tectonic depression. The uplifting of the area induced the rejuvenation of the local hydrographic system 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.