Benthic foraminifera from the upper Collio Formation (Lower Permian, Lombardy Southern Alps): implications for the palaeogeography of the peri-Tethyan area

New palaeontological evidence points to a temporary marine transgression in the Early Permian into the Collio Basin, a major palaeogeographic feature of the present-day Southern Alps. The thick volcaniclastic succession filling the basin (Collio Formation) is widely held as deposited in alluvial to lacustrine settings. Rare calcareous foraminifers were recently found in a single sandstone interval, containing phosphate nodules, from the uppermost Collio Formation. A temporary seaway, necessary for the foraminifera to spread into a continental basin, implies that (i) the Collio Basin lake was not only an intramontane (as commonly viewed), but also a coastal lake, and (ii) its altitude did not exceed the amplitude of a first-order sea-level rise, that is, about 100 m. These constraints, along with striking similarities as to tectonic context, accumulation rates and geochemical signature, suggest that the Collio Basin was a California-type basin, resembling in particular the present-day Salton Sea (CA, USA).