Recognition and classification of evaporite karst phenomena: examples of outstanding sinkholes from Sauris (NE Italy)

Sinkholes are one of the most hazardous phenomena due to their occurrence and unpredictability. In the Friuli Venezia Giulia Region (northeast Italy), the presence of outcropping, mantled or capped evaporites is the main predisposing factor for these phenomena. Even if evaporites do not exceed 1% of the entire regional territory, their presence causes catastrophic events mainly in the Carnian alpine valleys. Chalks are included in the Bellerophon Formation (Late Permian) and in the Raibl one (Upper Carnian). These weak rocks are mainly located in the valley bottoms and not at the top of the mountains. Even if, in correspondence of the northern ridges of Sauris municipality, several sinkholes have been observed. In detail, north of Sauris, quartz sandstones and mica- siltstones belonging to the Werfen Formation (Triassic) are widely present. These rocks, capping evaporitic ones, belonging to the Bellerophon Formation, are jointed and characterized by a secondary permeability. Over time, water infiltration and weathering led to the solution of the Permian evaporites and to the progressive failure of the overlying terrigenous rocks, less soluble but more plastic. The result is the genesis and evolution of depressed landforms classifiable as sinkhol es. On the grassy meadows stretching from Sauris di Sopra to Sella Festons at 1800 m a.s.l., outstanding sinkholes can be observed at the top as well as on the slope. Since now, almost 80 sinkhole phenomena were recognized and classified in the Sauris Municipality; mo st of them are aligned along approximately E-W oriented faults. All the data were added and stored in a GIS, which represents the first sinkhole inventory related to the evaporite karst environment in Friuli Venezia Giulia Region.