Sedimentary evolution of a migrating thrust-belt-foredeep system: Physical stratigraphy and sedimentology of the Macigno Fm. of Southern Tuscany (Northern Apennines) [Evoluzione deposizionale di un settore del sistema catena-avanfossa dell'Appennino settentrionale: stratigrafia fisica e sedimentologia del Macigno della Toscana meridionale]

The Macigno deep-sea turbidite unit was part of the diachronically migrating foredeep system characterizing the collisional phases of the apenninic orogen at the transition between Oligocene and Miocene time. The Macigno Fm. stratigraphically rests above the Scaglia toscana, a pelitic and calcareous unit of basin plain environment, while it is superimposed by the ligurian and subligurian thrust sheets that formed the accretionary prism in the internal margin of the foredeep. The nappe was responsible, with its advancing, of the migration of the depocenter basin and of the progressive reduction and closure of the sedimentation. The turbidite deposition gradually moved from the inner Tuscan domain (Macigno clastic wedge) to the outer Tuscan domain (Cervarola-Falterona clastic wedge), and from here to the outermost Umbro-Marchean domain (Marnoso-arenacea clastic wedge), as fully explained in several papers. Here I have studied the lesser known southern Macigno outcrops of the southern Tuscany, along a transversal chain transect from the inner outcrops of the Piombino-Baratti area to west, to the outer outcrops of the Chianti region to east, through the outcrops of the Sassetta area in the inland Marittime Tuscany, and of the Poggio Ritrovoli area in the Colline Metallifere region. The aim is to contribute for a better knowledge of the depositional evolution of the Northern Apennines foredeep system. The methods of research have been a detailed 1:10.000 geological mapping and stratigraphical-sedimentological analyses aimed at reconstruction of the architecture of the depositional systems. The geological mapping has allowed to recognize the relationships between Macigno and others formations, and to define the tectonic units stack. The vertical distribution of nine sedimentological facies (of genetic type) has allowed defining five facies associations indicative of specific turbidite environments. The facies analysis shows for the innermost Baratti succession, multiple stages of progradingaggrading lobe system moved until to a channel proximal to the slope stage that is previous to the closure of the basin through the allochtonous ligurian thrust sheet. Eastward, in the outer succession of Sassetta, the turbidite system is characterised by progradingaggrading lobe system in turn replaced by starved basin deposits in the upper part. Olistostromes coming from the internal slope are embedded in the top part. The allochthonous thrust sheet closed the depositional system. Eastward again, the Poggio Ritrovoli succession has got the same features of the previous, with lesser evident thickening and coarsening upward trends for the middle part, and with peculiar presence of several carbonaceous turbidite bedsets interlayered to the siliciclastic deposits. Also in this succession, the normal-feeding turbidite system is superimposed by a thick interval of starved basin deposits topped by several olistostromes coming from the advancing nappe, and then closing it. The outermost succession, located in the southern sector of the Chianti region, has shown very developed facies and facies associations typical of distal lobe environment; carbonaceous turbidite beds are also recognized. The whole data suggest a complicated framework of the foredeep turbidite system at the Oligo-Miocene transition. Two main types of turbidite systems belonging to the same foredeep system have been distinguished. The internal and oldest succession of Macigno costiero reveals turbidite proximal conditions, and typical features of low-efficiency turbidite system, with feeding transversal to the basin coming from storage basins close to the Corso-Sardinian Massif. To follow the foredeep depositional system migrated toward outer position with changes in the turbidite fed, longitudinal to the basin, from farther source like western alpine crystalline terrain. The turbidite system changed to high-efficiency, migrating on space and on time, with progressively deactivation of the sedimentation, due to the approaching of the allochtonous thrust sheet, as showed by replacing of starved sediments and olisthostromes onto lobe turbidite sedimentation. In conclusion, the research has allowed to identify two turbidites system for the Macigno in Northern Apennines, the oldest and innermost, of low-efficiency type, transversal to the foredeep structural chain system deposited in open piggyback or inner complex foredeep basin, and the other, younger and wider long-lived migrating high-efficiency turbidite system deposited in wide foredeep basins.