Middle Triassic carbonate lithostratigraphy of the Southern Briançonnais (Cottian Alps, Italy) and comparison with the surrounding areas

In the Southern Briançonnais Zone (Cottian Alps), the Middle Triassic carbonate succession has been investigated. The most significant and continuous sections are located at Colle del Preit and Cima Fauniera, between the Stura and Maira river valleys. The Colle del Preit Middle Triassic section is 276 m thick and lies over the Permian?-Early Triassic “Verrucano Brianzonese” and “Werfenian quartzites” siliciclastics. Carbonate deposits are arranged in two transgressive-regressive sub-cycles: the inferred depositional environments are from the sabkha-tidal flat to the midouter shelf transition. The general small-scale erratic stacking pattern of the lithofacies implies mild, continue tectonic instability. Only in the middle part of the section well-defined peritidal cycles (“Calcaires rubanés” lithofacies) suggest an ephemeral tectonic quiescence with temporary predominance of autogenic control factors on sedimentation. Thin pyroclastic levels in the lower part of the carbonate succession indicate a close volcanic activity. The Middle Triassic Colle del Preit section is unconformably overlain by the basal breccias of the carbonatic “Dogger à Mytilus” Auct. At Cima Fauniera, three short sections have been analyzed. The most representative, nearly 30 m thick, is correlatable with the Colle del Preit section and, specifically, can be identified as its upper prosecution. It shows the upper evolution of the Middle Triassic carbonates: from shallow inner-shelf, high to low energy environments to the sabkhas of the “Cargneules supérieures”, early Late Triassic in age. For the Southern Briançonnais Middle Triassic carbonate succession the informal name of Rocca la Meja Fm is proposed and a homoclinal ramp geometry is inferred. Comparisons with the next sub-domains (Middle Triassic Briançonnais s.s. to the north and Ligurian Briançonnais to the south) have been performed: they confirm strict affinities, but also slightly different timings of the Middle Triassic transgressive-regressive events and a variable influence of synsedimentary tectonics through time.