Permian sporomorphs from upper Palaeozoic succession of Southern Tuscany (Italy): new constraints for the stratigraphy and palaeogeographic setting of the Tuscan Domain

Recent biostratigraphic, sedimentological and petrological studies in the inner Northern Apennines (Italy) permit to refine the upper Palaeozoic successions of southern Tuscany, allowing new hypothesis to frame these formations in the Permian palaeogeographical scenario of the western Mediterranean domain. The Tuscan pre-Triassic deposits, belonging to the Monticiano-Roccastrada Unit, are generally barren or scarce in term of biomineralized fossiliferous content. They were mostly affected by HP-LT to LP-HT metamorphism that, together with the limited distribution of deposits, made difficult their stratigraphic correlation.

The present study is focused on two metamorphic units (i.e. Filladi e metacalcari di Fosso della Falsacqua Formation and Filladi e quarziti del Torrente Mersino Formation) which age attribution and correlation was strongly debated in the literature.

The Filladi e metacalcari di Fosso della Falsacqua Formation (minimum estimated thickness of about 150 m), cropping out in Monte Leoni area, is mainly characterized by black to dark-grey phyllite, metasiltstones and metasandstones with dark limestone intercalations. Due to the lack of biomineralized fossil content, by lithostratigraphic correlation with other formations cropping out in Tuscany, this formation was differently assigned to late Carboniferous-early Permian or Devonian.

The Filladi e quarziti del Torrente Mersino Formation (minimum estimated thickness of about 200 m) crops out in the Boccheggiano mining area and mainly consists of black to dark-grey quartz-phyllite, quartz metaconglomerates, light-grey quartzites, green phyllites and quartzites and light-grey phyllites. This formation resulted barren of fossil content and has been differently assigned to Ordovician-Silurian, Silurian-Devonian, late Carboniferous-Permian and Triassic by lithostratigraphic correlation with other Tuscan-Sardinian successions.

In the present study, the first finding of a middle Permian well-preserved microflora adds more constrains to the age attribution of these studied formations. This new age assignment permits to correlate the investigated formations with the coheval ones belonging to southern Tuscany (i.e. Farma Formation) and Elba Island (Rio Marina Formation) characterized by a similar microfloral content. Moreover, the occurrence of Gondwana-related sporomorphs, in all the studied formations, points to a new palaeogeographic scenario of the upper Palaeozoic successions from the northern Gondwana margin. The results of this integrated study inclines to consider the fragmentation of the northern margin of Gondwana as a result of several transtensional (pull-apart) basins where different laterally-related depositional environments leaded the sedimentation of these Tuscan middle Permian formations.