Mineralogical and grain size composition of clays developing calanchi and biancane erosional landforms

Clayey terrains outcropping in many parts of Italy are frequently affected by accelerated erosion processes producing landforms known as calanchi and biancane. The calanchi are intensely dissected landscapes, presenting rill and gully landforms and a dense dendritic drainage network. The biancane are landforms characterized by bare cones and hummocks, separated by flatter areas of surface wash deposits. The factors controlling the development of these two landforms are still poorly understood. This study was undertaken to identify the material properties of calanchi and biancane. The data suggest: (a) biancane tend to develop in very fine sediments, such as silty clays with a very high clay content (65–70%); (b) calanchi tend to occur in relatively coarser sediments, such as clayey silts and sandy clayey silts having a notable sand fraction (6–18%); (c) both biancane and calanchi may develop in intermediate textures, such as silty clays and clayey silts, made up of approximately equal quantities of silt and clay; (d) no strong trend in clay mineralogy can be correlated with the landforms; the swelling phyllosilicate contents are roughly equivalent in the parent materials of biancane and calanchi; (e) pore water chemistry seems to play a very important role in the development of these landforms; the biancane have high concentrations of sodium relative to other cations (essentially calcium and magnesium) in pore water; all the analysed calanchi pore waters have low concentrations of sodium relative to divalent cations.