Carboniferous continental margin deposits in southern Tuscany, Italy: results from geological mapping of the geotopes Farma Valley and San Antonio Mine Area

Abstract

Relics of the Carboniferous shelf, slope and basin margin section occur as intensely tectonized slices within the middle Tertiary Monticiano-Roccastrada metamorphic core complex in southern Tuscany. These sedimentary successions are interpreted as predominantly siliciclastic second-order sequence stratigraphic cycles, deposited in a successor basin (Farma Basin) established near the Southern Convergence Front of the westmediterranean Hercynides. The units developed as highstand systems tracts above a downlap surface and buried pelagites of a starved, moderately deep-water basin (Risanguigno Basin). The Risanguigno Basin was characterized by dys-and anoxic conditions and underwent partial tectonic inversion during the early Carboniferous. Detailed mapping has revealed that Carboniferous turbidites deposited on the slope, base of slope and Farma Basin margin are present in the eastern part of the Farma Valley and in the San Antonio Mine Area. Coeval lithologies-storm influenced deposits of the inner shelf and lower shoreface-crop out in the western part of the Farma Valley. Both units, considered to be related to storm-triggered offshore bottom currents bypassing the outer shelf, are covered by offlapping shallow marine and littoral clastics probably of Permian age. Subsequent tectonic uplift and partial inversion of the Farma Basin resulted in subaerial exposure, erosion of the upper part of the basin fill and deposition of the terrestrial Verrucano Group (middle Triassic), which sealed these early Carboniferous to late Permian sequences considered to represent a late Palaeozoic preligurian failed rift.

UPb and 40Ar39Ar geochronology of Palaeozoic units in the northern Apennines: determining protolith age and alpine evolution using the Calamita Schist and Ortano Porphyroid

“…In contrast in the southern portion (southern Tuscany and Elba Island), the Palaeozoic formations testify to an area of almost continuous Early Carboniferous–Late Permian marine‐littoral sedimentation that records a late to post‐Variscan evolution. According to palinspastic restoration proposed by Vai (2001) this area corresponded to the external zone of Variscan chain along the northern margin of Gondwana (Engelbrecht, 2008).…”

“…In particular, a correlation can be proposed with the Farma Fm. of Bashkirian‐Moscovian age (Cocozza et al , 1974), interpreted as a Variscan flysch deposit consisting of a monotonous siliciclastic sequence of greywackes, siltstone and pelites that are deposited in extensional or transtensional basins (Engelbrecht, 2008). These newly proposed correlations allow us to delineate a different distribution of the Palaeozoic basement formations in the northern Apennines.…”

“…Moving southward Early to Late Carboniferous and Permian formations that recording a late to post‐Variscan evolution are dominant. Such a distribution of Palaeozoic formations could be related to a different evolution of the Palaeozoic basement throughout the Tuscan domain during the Late Palaeozoic (Engelbrecht, 2008).…”

“…(Visean), Carpineta Fm (Visean–?Moscovian) and Farma Fm. (Bashkirian–Moscovian), which are exposed along the Farma valley in the Mid Tuscan ridge (Engelbrecht, 2008).…”

“…Pisani and along the Mid Tuscan ridge (Montagnola Senese) in southern Tuscany (south of the Arno River). This group mainly consists of siliciclastic sedimentary formations with minor carbonate rocks with ages spanning from the Late Carboniferous (Westphalian) to the Permian (Bagnoli et al , 1979; Conti et al , 1991a; Engelbrecht, 2008). This group also includes Early to Late Carboniferous deposits, namely the Poggio al Carpino Fm.…”