thakns but it is difficoult; no volcanic morphologies anywhere in the study area and in the surrounding. Anyway thank a lot for your answer.

Davide

I found some outflow channels and i conclude that are thermokarst depressions, after discarded all other origin.

My proposal is: sinkhole, a karstlike phenomenon, if the size (you did not mention) allows. The reasoning is: if a material can be precipitated by a líquid, it can also have been solved later by that liquid.

thanks i'll think about your sugestions

Warm regards