Geoscience Reference
In-Depth Information
Consequently,
As the diffusion of water across the clayey walls is slow, at
the same time the water content in the soil can double along a
horizontal line arbitrarily drawn within the profile (Jaillard and
Cabidoche 1984).
The micro-depressions are the wettest zones (Kovda
et al.
1996),
richer in carbon, darker, more decarbonated (Nordt
et al.
2004);
salts do not accumulate in the first one metre; iron oxides are
often present.
The micro-bumps, on the contrary, are drier. Greater accumulation
of salts is seen for a given depth than in the soils of the
depressions. Their pH is higher.
The intermediate zone is undoubtedly that which suffers the
widest variations in water content. It is here that the most
developed slickensides are seen.
It is difficult to model the hydric functioning of these soils particularly
where the rapid infiltration in the cracks is concerned (Ruy
et al.
1999).
But modelling of the opening of the cracks on drying has been done
(Chertkov 2002).
6.4 GENESIS
The smectites of Vertisols result in all regions of the world from
inheritance or, in the intertropical zone alone, by neoformation (Pédro
1976).
Inheritance comes in particular from the decarbonation of calcareous
rocks because smectites easily precipitate in marine environment with
calcium carbonate. Locally, this inheritance comes from the concentration
of fine clays of alluvial origin (sedimentation basins).
Neoformations are related to weathering
in situ
of rocks. In Togo,
on gneiss, careful mineralogical studies have shown the following
transformations (Kounetsron
et al.
1977):
Origin and formation of clay minerals
Plagioclases yield smectites directly by neoformation.
The micas evolve by direct transformation to vermiculite, which
in turn gives a smectite; but the transformation requires external
addition of silica, whence the presence of this substance in the
soil environment.
