Geoscience Reference
In-Depth Information
Since we are fortunate to have available a series of soils manifesting
a chronological evolution, it is possible to make some observations in
line with our model of pedogenesis. This model is fully confirmed
by the studies of M. Bornand. The weathering fronts are many and
progress downwards. Figure 9.8, summarized from the publications
cited, presents the fronts corresponding to the disappearance of coarse
fragments.
The other principal weathering fronts are linked to transformation of
the fine particles and correspond to the horizon boundaries. This gives
us an opportunity to describe in detail the nature of the boundaries in
the most evolved stage of the sequence (Fig. 9.9).
Weathering-front model
Evolution of coarse elements
and fine elements
Fronts
E
100% of
residual
coarse
elements
Total depletion
Variegated
clay
B21
Degradation
Yellow
clay
B22
Warping
Q
Fe-clay
B23
Argillization
Gravels: Q + G
E
2
G
Depletion of CaCO
3
C
ca
Carbonates
Gravels:
Q + Ca + G
Enrichment in CaCO
3
Ca
C
Fig. 9.9
Presentation of the horizon-differentiation fronts in the Villafranchian soils of the
Rhône valley (Bornand 1978). The relative thickness of the various horizons is not adhered
to. Coarse elements: Ca = limestone; G = igneous rock; Q = siliceous.
There is decarbonatation at the base of the soil (E2 horizon). The
calcium carbonate released is partly removed in soluble form. But part
is immediately reprecipitated below in a Cca horizon that is temporarily
enriched in calcium before being itself caught up by the decarbonatation
front. After decarbonatation, argillization of igneous rocks occurs. This
provides fine material (clay and bound iron) in a very porous system.
