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

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 Termearea (Southern Tuiscany, Northern Apennines) in order to contribute to the reconstruction of the depositional setting and palaeogeographical features of a secotr of the Tuscan Dornain 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 deply 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 Camp 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 calcerenites (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 (Marie siltose and ed argillii marnose position with the Marie del Sugame described in the Chiantu Mts The thickness does not excceed 10 m, Early-Middle Eocene 3)Argilliti di Brolio consisting of three lithofacies BRLa - manganesiferous radiolarites (thickness from 5 t 10 m). BRLb - siliceous siltistones (thickness of about 10 m); BRLc - red marly clays (thickness from 6 to 8 m), Albian-Turonian The boundary separating the Argilliti di Biolio from the overlying Marne siltose ed marnose rosse Podere Le Rossole is representative of a time gap about 7Ma lone (from 40 to 47Ma) According to CANUTI & MARCUCCI ( I 967) we exclude the emersion during this time span, but we support for 1) the reduction of the sedimentation anchor, 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 Argillit, di Broho, and the sedimentation of the Marne siltose ed marnose rosse di Podere Le Rossole started diachronically The sedimentary environment of the Manic siltose ed argillut marnose rosse di Podere Le. Rossole is pelagic-hemipelagic, characterised by Foraminifera bearing many 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 Argilluti e calcareniti, di Dudda with interbedded Nummulites bearing calcarenites and calcirudites (Calcerniti di Montegrosst) 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 argillitu marnose notice di Podere Le Rossole The sed mentological eat tires of the Argalaue calcawurri di Dud suggest a radical change of the sedimentary environment In fact, the hemipelagic sediments were substituted by the turbichte systems 'Flue tut bichtes are characterized by graded beds containing Eocene planktonic Forammifera indicating an intrabasinal reworking of penecontemporaneous sediments Contrarily. the calcaremtes of the upper part of the succession are characterized by Cretaceous planktonic Forammifera being buiclasts indicating an extrabasinal origin This is indicative for the development of a carbonate turbichte system fed by Cretaceous sediments The uppermost part of the succession is newly typified by the occurrence of calcaremte beds with intrabasinal Eocene planktonic Forammilera Such a litholoeical assemblage and palaeontological contamts became a recurrent feature for the overlying Numinulites bearing calcaremtes and calciruchtes ( Nurm-ntiliticcm, Calcarelim di Mouregrossi) Furthermore, these latter are characterized by Eocene bent ionic Foraminifera suagesting the development of one or more lobes of a submarine fan channels fed by carbonate platforms The Nummulite hearing calcaremtes are characterized by a trend thinning-fining upward, and gradually pass to the Ari,///iti e ccdcareurti di Durkin, indicating the deactivation of the lobe fan system Such a succession passes to the Macigno siItctclatic sandstone (Macigno Fm) In sum, we hyphotesise that the embliyonal foredeep system of the Northern Apennines could be developed since the Middle-Late Eocene with the activation of a carbonate turbichte system (Argilliti it calcarewri di Dudda plus the Nummulites bearing calcarenues) successively evolved in a siliciclastic turbichte system represented by the Macigno Fm The sedimentary gap recorded within the Scaelia Toscana succession could indicate the records of the beemnine of the tectonic actwity which affected the Tuscan Domain for a long time, related to die development of the Northern Apennines foredecp system