The Scaglia Toscana formation of the monti del chianti: new lithostratigraphic and biostratigraphic data

Abstract

The Scaglia Toscana Formation (Scisti Policromi Auctt.) is one of the most investigated formation of the Tuscane Nappe. The Formation is widely exposed in the Chianti Mounts and despite the number of studies in this area, some aspects remain poorly known and debated. In this paper new litho- and bio-stratigraphic data from eight key-sections distributed over the entire area are provided and discussed in order to clarify the stratigraphic relationshpis among different lithostratigraphic members, as well as the depositional ages of each member. The Formation deposited in the Cretaceous-Oligocene time interval and it can be subdivided into five lithostratigraphic members: i) the Argilliti di Brolio (wine-red shales with sporadic siliceous calcilutites and rare interbedded cherts); ii) the Marne del Sugame (red and pink marls, calcareous marls and marly limestones with interbedded calcarenitic beds and ruditic lens-shaped bodies including calcareous-siliceous clasts); iii) the Argilliti di Cintoia (grey-green to black shales, locally with manganese-rich siliceous calcilutites and cherts); iv) the Calcareniti di Montegrossi (thin beds of calcilutites and calcarenites with varicoloured shaly-marly interbeds); and v) the Argilliti e Calcareniti di Dudda (alternating thin beds of calcilutites and calcarenites with varicoloured shaly-marly interbeds). These members deposited in a marine environment and have been interpreted as deposited in a turbiditic system, in which shaly and calcareous turbiditic members have been attributed to a basin plain below the CCD, whereas the marls and marly limestones of the Marne del Sugame Member can be settled in a slope/ramp environment above or close to the CCD. Furthermore, the combination of these new data with structural informations coming from literatures allowed to a better paleogeographic reconstruction of the paleobasin. In order to better explain these data, the paper is accompanied by two geological maps realized in the past but never distributed. The two geological maps, at the scale of 1:25,000, cover the whole area from the Cintoia (south of Florence) to the San Gusme (north of Siena) villages.