Geoscience Reference
In-Depth Information
6.7
RETURN TO THE WEATHERING-FRONT MODEL
We now have the materials for reorganizing the points made in Chapters
5 and 6 in the more general framework of the weathering-front model.
Let us return to the Kosselili sequence and diagrammatically represent
its three principal stages from the topslope to the footslope (Fig. 6.12)
keeping in mind the general scheme of desilication (Chap. 3, Fig. 3.9).
The Vertisol that is on the footslope of the sequence has undergone only
the passing of the bisiallitization front ( F1 ). The Solonetz, higher up
the slope, exhibits the passage of F1 and, at the same time, that of the
monosiallitization front F2 . In the Lixisol at the topslope, F2 has caught
up with F1 . In practical terms, smectite either does not form or has an
ephemeral existence.
F1
and
F2
mixed up
F1
and
F2
separated
F2absent
Kaolinite
Smectite
F2
Kaolinite
F1
Smectite
F2
F1
Rock
Rock
Rock
P1: Lixisol
P3: Vertisol
P2: Solonetz
Fig. 6.12 Different kinds of soil as a function of more or less advanced development in time
and space of the bisiallization front ( F1 ) and the monosiallitization front ( F2 ).
Here, the weathering-front model is not applied to the horizons in
their entirety but more minutely to the mineralogy of the horizons. We
have thus gone a little further in our concepts. This type of view and
representation has many aspects:
￿ There finally is very little difference in the genesis of the three
profiles shown in Figure 6.12. A very small increase in the speed
with which such or such clay mineral appears leads to a total
change in the nature of the soil. The uniformity of the weather-
ing process in the dry intertropical zone is translated into an
extraordinary diversity in soil profiles.
￿ Slightly different environmental conditions between the topslope
and footslope (e.g. variation in moisture) can induce, in the two
positions, specific evolutions without the materials of the top-
slope enriching the footslope.
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