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 areintensely dissected landscapes, presenting rill and gully landforms and a dense dendriticdrainage network. The biancane are landforms characterized by bare cones and separated by flatter areas of surface wash deposits. In spite of an extensive research, thegenetic processes differentiating calanchi and biancane are still poorly understood. Thepresent study was undertaken with the aim to extend the collection of data on the properties of their parent material and to possibly establish which property plays an important role in their differentiation. Research has focused on the calanchi and biancane of Tuscany, which has a widespread occurrence of these landforms and where most of the earlier studies have been conducted. In this region, calanchi and biancane are generally developed in the clays of the Neo-autochtonous Complex, particularly in the fine-grained Pliocene sediments, which were deposited in tectonic depressions developed after the emplacement of the allochtonous Ligurian and Tuscan nappes.Among the Neogene tectonic depressions recognized by geologists, Volterra and Siena basins may be regarded as two of the most important, both for their extension and for the thickness of their Neo-autochtonous deposits. Within these basins, a frequent occurrence of calanchi and biancane has been reported . This study focused on landforms developed in these two basins on Pliocene blue clays of the Piacenzian facies. In these areas, the morphological survey was followed by sampling of parent material of selected examples of calanchi, biancane and intermediate forms (bialanchi). The data collected 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 materialsof 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.