Nuovi dati litostratigrafici e biostratigrafici sulla Scaglia Toscana nella Toscana meridionale (area di Rapolano Terme)

Anna Maria Bambini(*), Andrea Brogi(*), Gianluca Cornamusini(*), (**), Armando Costantini (*), Antonio Lazzarotto(*) & (***)
(*) Dipartimento di Scienze della Terra, Università degli Studi di Siena, Via Laterina, 8 - 53100 Siena. (**) CGT Centro di GeoTecnologie, Università degli Studi di Siena, Via Vetri Vecchi, 34 - 52027 San Giovanni Valdarno, Arezzo. (***) Corresponding author: Antonio Lazzarotto, e-mail: lazzarotto@unisi.it.

The Scaglia Toscana represents a stratigraphic unit belonging to the Tuscan Domain of the Northern Apennines. Such a stratigraphic unit results lithologically heterogeneous, and is representative of a very long time span (from the Early Cretaceous to the Oligocene) during which very important geological events took place, such as the convergence and collisional events giving rise to the edification of the Northern Apennines orogene. We have studied several key outcrops of the Scaglia Toscana exposed in the Rapolano Terme area (Southern Tuscany, Northern Apennines) in order to contribute to the reconstruction of the depositional setting and palaeogeographical features of a sector of the Tuscan Domain from the Cretaceous to the Early Miocene, and to hypothesise a possible geodynamic evolution. The Scaglia Toscana exposed in the Rapolano Terme area has been deeply investigated through lithostragraphic and biostratigraphic analyses mainly carried out in three key areas indicated in the fig. 1: 1) the Podere Cetinaia-Podere Monte Petroso section; 2) the Podere Le Rossole section; and 3) the Modanella-Podere Campo d'Aia section. From the top (represented by the overlying Macigno Fm) to the bottom (Maiolica Fm) we have recognized three litostratigraphic units: 1. Claystones and calcarenites (Argilliti e calcareniti di Dudda), containing lenses of nummulite bearing calcarenites and calcirudites («Nummulitico», Calcareniti di Montegrossi). The thickness ranges from 50 to 100 m; Middle-Late Eocene. 2. Silty marls and claystones (Marne siltose ed argilliti marnose rosse di Podere Le Rossole) comparable for the age and stratigraphic position with the Marne del Sugame described in the Chianti Mts. The thickness does not exceed 10 m; Early-Middle Eocene. 3. Argilliti di Brolio consisting of three lithofacies: BRLa - manganesiferous radiolarites (thickness from 5 to 10 m); BRLb - siliceous siltstones (thickness of about 10 m); BRLc - red marly clays (thickness from 6 to 8 m); Albian-Turonian. The boundary separating the Argilliti di Brolio from the overlying Marne siltose ed argilliti marnose rosse di Podere Le Rossole is representative of a time gap about 7Ma long (from 40 to 47Ma). According to Canuti & Marcucci (1967) we exclude the emersion during this time span, but we support for: i) the reduction of the sedimentation and/or; ii) submarine erosion induced by tectonic instability. Such possible hypotheses can be confirmed by the fact that the gap affects different stratigraphic horizons of the Argilliti di Brolio, and the sedimentation of the Marne siltose ed argilliti marnose rosse di Podere Le Rossole started diachronically. The sedimentary environment of the Marne siltose ed argilliti marnose rosse di Podere Le Rossole is pelagic-hemipelagic, characterised by Foraminifera bearing marly and pelitic sediments. The occurrence of thin bedded calcarenites indicates the formation of a turbidite system, mainly carbonate. Such a turbidite system typifies the overlying succession: the Argilliti e calcareniti di Dudda with interbedded Nummulites bearing calcarenites and calcirudites (Calcareniti di Montegrossi). The stratigraphic contact between the Argilliti e calcareniti di Dudda and the Marne siltose ed argilliti marnose rosse di Podere Le Rossole lithostratigraphic units is gradual and continuous, and is characterized by the frequency and thickness increase of the turbite beds, as well as by the progressive appearance of the reddish color, moreover characterizing the overlying Marne siltose ed argilliti marnose rosse di Podere Le Rossole. The sedimentological features of the Argilliti e calcareniti di Dudda suggest a radical change of the sedimentary environment. In fact, the hemipelagic sediments were substituted by the turbidite systems. The turbidites are characterized by graded beds containing Eocene planktonic Foraminifera indicating an intrabasinal reworking of penecontemporaneous sediments. Contrarily, the calcarenites of the upper part of the succession are characterized by Cretaceous planktonic Foraminifera being bioclasts indicating an extrabasinal origin. This is indicative for the development of a carbonate turbidite system fed by Cretaceous sediments. The uppermost part of the succession is newly typified by the occurrence of calcarenite beds with intrabasinal Eocene planktonic Foraminifera. Such a lithological assemblage and palaeontological containts became a recurrent feature for the overlying Nummulites bearing calcarenites and calcirudites («Nummulitico», Calcareniti di Montegrossi). Furthermore, these latter are characterized by Eocene benthonic Foraminifera suggesting the development of one or more lobes of a submarine fan channels fed by carbonate platforms. The Nummulite bearing calcarenites are characterized by a trend thinning-fining upward, and gradually pass to the Argilliti e calcareniti di Dudda, indicating the deactivation of the lobe fan system. Such a succession passes to the Macigno siliciclastic sandstone (Macigno Fm). In sum, we hyphotesise that the embryonal foredeep system of the Northern Apennines could be developed since the Middle-Late Eocene with the activation of a carbonate turbidite system (Argilliti e calcareniti di Dudda plus the Nummulites bearing calcarenites) successively evolved in a siliciclastic turbidite system represented by the Macigno Fm. The sedimentary gap recorded within the Scaglia Toscana succession could indicate the records of the beginning of the tectonic activity which affected the Tuscan Domain for a long time, related to the development of the Northern Apennines foredeep system.

Tuscan Domain, Northern Apennines, Scaglia Toscana, lithostratigraphy, biostratigraphy