Geologia dell'area di Rapolano Terme in Provincia di Siena (Appennino Settentrionale) [Geological setting of the Rapolano Terme area (Siena, Northern Apennines)]

This paper deals with the geological setting of the Rapolano Terme area (Siena) located in southern Tuscany, inner Northern Apennines. The Rapolano area is part of an important morpho-tectonic feature NNW-SSE oriented, ranging from the Chianti Mts to the Cetona Mt, separating the Siena-Radicofani and the Valdichiana Basins in the western and eastern sides, respectively. In the study area the Late Triassic-Early Miocene succession of the Tuscan Nappe, as well as the Eocene succession belonging to the Morello Unit (external Ligurian Unit) and the Pliocene-Pleistocene post-orogenic marine to continental deposits are broadly exposed. The oldest cropping out Tuscan Nappe formation consists of the «Calcari e marne a Rhaetavicula contorta» Fm. It is mainly composed of dark limestone beds with decimeter thick grey marls interbedded. The occurrence of Triasina hantkeni MAIZON allows referring this formation to the Rhaetian. The underlying formation (the base of the Tuscan Nappe), represented by the «Formazione anidritica di Burano» was encountered at depth (900 m below the ground level) by the Rapolano 1 borehole, and was partially drilled for 114 m. The «Calcare massiccio» Fm (Early Lias) overlies on the «Calcari e marne a Rhaetavicula contorta» Fm. This formation broadly crops out in the study area. It is mainly composed of grey massive limestones, often dolomitic, containing meter thick lenses of sin-sedimentary breccias formed by centimeter to decimeter carbonate clasts. The «Calcare massiccio» Fm is overlain by the «Calcare selcifero» Fm. Such a formation consists of bedded grey cherty limestones (Late Hettangian-Domerian) . The «Calcare selcifero» Fm is overlain by the «Calcare Rosso Ammonitico» Fm, giving rise to an anomalous stratigraphic succession with respect to that described for the Tuscan Nappe exposed in the western Tuscany. The «Calcare Rosso Ammonitico» Fm is composed of Toarcian-Aalenian red and yellow nodular limestones and marly limestones, with heteropic relationships with the uppermost part of the «Calcare selcifero» Fm and with the lower part of the «Marne a Posidonomya» Fm. The «Marne a Posidonomya» Fm is mainly composed of Toarcian-Callovian meter thick beds consisting of red, grey and yellow marly limestones and marls, often interlayered with red and grey siltstones. Such a formation is discontinuously exposed in the northern part of the study area (mainly between Rapolano and Serre di Rapolano villages), where it directly overlies the «Calcare selcifero» Fm. The «Marne a Posidonomya» Fm is characterized by a thin succession occurring in the southern part of the study area. In few areas such a formation is substituted by the «Calcare Rosso Ammonitico» Fm. The «Marne a Posidonomya» Fm and the «Calcare selcifero» Fm are overlain by the «Diaspri» Fm, mainly composed of centimeter beds of Late Callovian-Early Titonian red, yellow and green radiolarites with interbedded very thin claystones levels. The «Diaspri» Fm gradually passes to the overlying succession represented by the «Calcari ad Aptici» Fm. Such a formation is mainly composed of thin bedded Titonian yellow and red limestones and marly limestones. This formation gradually passes to the «Maiolica» Fm that occurs in an about 100m thick succession, unusual with respect to that documented for the southern Tuscany. The «Maiolica» Fm consists of thin bedded white and grey cherty calcilutites (Berriasian-Aptian). Nodular cherts are very abundant, often black or red in color. The « Maiolica» Fm is overlain by the Albian-Late Eocene (Priabonian) Scaglia Toscana Group which is composed of, from the bottom to the top: i) the «Argilliti di Brolio» Fm; ii) the «Marne siltose ed argilliti marnose rosse di Pod. Le Rossole» correlatable with the «Marne del Sugame» Fm described for the Chianti Mts; iii) the «Calcareniti di Montegrossi» Fm containing a megabreccias with volcanic blocks; iv) the «Argilliti e calcareniti di Dudda» Fm. The «Macigno» Fm is the topmost formation of the Tuscan Nappe. This is mainly composed of a turbidite succession broadly exposed in the northern part of the study area. The «Macigno» Fm consists mainly of arkosic sandstones with interbedded micaceous siltstones and rare thin carbonate levels. The «Macigno» Fm can be subdivided in three main depositional units: i) the lowermost depositional unit is mainly composed of sandstones facies referred to distal or intermediate-distal lobe; ii) the middle depositional unit is characterized by coupled siltstones and sandstones beds, referred to a very distal depositional environment; iii) the topmost depositional unit is mainly composed of siltstones with subordinate sandstones suggesting the migration of the foredeep system. The «Macigno» Fm is referred to the Late Oligocene-Early Miocene. The Ligurian Units are represented by the Morello Unit only composed of the «Monte Morello Fm». Such a formation is exposed in small outcrops located in the southern part of the study area, near Pod. S. Bernardino. In the attached geological map the Morello Unit has been erroneously attributed to the «Scaglia Toscana» Group (labeled as Mc). The «Monte Morello» Fm consists of brown to grey marls and subordinate calcilutites and marly-limestones, with local intercalation of centimeter and decimeter beds of ophiolite bearing turbidite sandstones. The age is Early-Middle Eocene. The Tuscan Nappe and the Morello Unit are unconformably overlain by the Pliocene marine deposits filling the Neogene Siena Basin. The Pliocene deposits consist, from the top to the bottom (see the attached geological map): i) «Argille e argille sabbiose grigie, talvolta fossilifere» consisting of clays and sandy-clays; ii) «Sabbie talvolta argillose ed arenarie poco cementate gialle a luoghi arrossate, Arenarie ben cementate gialle» consisting of sands and clayey-sands; iii) «Conglomerati e ciottolami poligenici, non classati, saltuariamente con fori di Litodomi» consisting of polygenic conglomerates and pebbles with borings of lithophagid bivalves. The lower part of this succession is characterized by the absence of Globorotalia puncticulata. The overlying part is characterized by the occurrence of Bulimina marginata and Discoaster pentaradiatus. On the whole, marine deposits can be ascribed to the Piacenzian even though we cannot exclude the lowest Gelasian at least for the uppermost part of the succession. The Quaternary deposits unconformably overlie both the Pliocene deposits and the pre-Neogene formations. They are exposed from 190 m to 375 m above the sea level. These deposits consists of broad Middle-Late Pleistocene and Holocene travertine deposits, mainly exposed in the quarries close to Serre di Rapolano and Rapolano Terme villages, and the alluvial deposits of the Piano del Sentino, Piano della Bestina and Borgo ai Piani. The sedimentological and stratigraphic features, as well as the lithological association coupled with the rare fossil remains, allow to refer such deposits to a fluvio-lacustrine depositional environment. The tectonic setting is characterized by superposed deformational events developed during the structural evolution of the Northern Apennines. The structures related to the different deformational events are, from the youngest: i) Late Pliocene-Late Pleistocene oblique to strike-slip faults, E-W to NE-SW striking, giving rise to hydrothermal circulation, as well as the occurrence of thermal springs and gas emissions (mainly CO2); ii) Early-Middle Pliocene normal faults, NNW-SSE and N-S oriented, interfering with the Pliocene sedimentation and driving the architecture of the eastern side of the Siena Basin; the most important structure belonging to this fault system is the Rapolano normal fault, N-S striking and west-dipping, which separates the pre-Neogene successions from the Pliocene deposits; iii) extensional detachments with top-to the east sense of shear giving rise to significant tectonic elisions within tectonic units forming the Chianti Mts-Cetona Mt. ridge (serie ridotta Auctt); iv) east-verging folds with N-S and NNW-SSE axial trend; they are the most representative contractional structures in the whole study area (see the geological map) and developed after the emplacement of the Ligurian Units on the Tuscan Nappe; v) thrust and related minor contractional structures (folds and reverse faults) developed during the Late Oligocene-Early Miocene stacking of the tectonic units. In the last part of the paper, the relationships between tectonic activity and travertine deposition and hydrothermal circulation are discussed.