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Geology of the Rapolano Hot Springs area, Siena (Northern Apennines)

Anna Maria Bambini, Andrea Brogi, Gianluca Cornamusini, Armando Costantini, Luca Maria Foresi and Antonio Lazzarotto
Geologia dell'area di Rapolano Terme in provincia di Siena (Appennino settentrionale)
Italian Journal of Geosciences (October 2010) 129 (3): 457-495


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 morphotectonic 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 Tithonian 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 Tithonian yellow and red limestones and marly limestones. This formation gradually passes to the "Maiolica" Fm that occurs in an about 100 m 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.

ISSN: 2038-1719
EISSN: 2038-1727
Serial Title: Italian Journal of Geosciences
Serial Volume: 129
Serial Issue: 3
Title: Geologia dell'area di Rapolano Terme in provincia di Siena (Appennino settentrionale)
Translated Title: Geology of the Rapolano Hot Springs area, Siena (Northern Apennines)
Affiliation: Universita di Siena, Dipartimento di Scienze della Terra, Siena, Italy
Pages: 457-495
Published: 201010
Text Language: Italian
Summary Language: English
Publisher: Societa Geologica Italiana, Rome, Italy
References: 147
Accession Number: 2011-048201
Categories: Areal geology
Document Type: Serial Map
Bibliographic Level: Analytic
Illustration Description: illus. incl. strat. cols.
N42°19'60" - N43°19'60", E10°00'00" - E12°00'00"
Country of Publication: Italy
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute.
Update Code: 201127
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