Geoscience Reference
In-Depth Information
(Chap. 5, ยง 5.4.3); Planosols will be presented in Chapter 8 and Solonetz
in Chapter 13.
INSELBERG
TRANSITIONAL ZONE
Lixisol author: leached tropical ferruginous soils
(
)
Gleyic Lixisol
Planosol
GLACIS
E
Solonetz
kaolinite
Vertisol
Gra n it e
B Horizons
k aolinite +
smectites
Fig. 6.11 Soil sequence at Kosselili (Chad) from Bocquier (1971, pub. 1973). On the foot-
slope (right of the diagram and beyond), Solonetz give place to Vertisols in a continuous
formation (not shown). These Vertisols cover tens of kilometres.
6.6.2
Interpretation and Discussion
According to Bocquier (1971), this sequence marked a lateral movement
of 2/1 clay mineral that first accumulated at the base of the profiles
and on the footslope and later invaded a greater and greater part of the
landscape. This was upstream invasion by montmorillonite . Many students
of pedology in France have been taught this concept in which the
boundary between E and B is presumed to progress uphill and upward
in the profile with time. A slightly modified model was presented a
little later by Millot, Bocquier and Paquet (1976): the lateral movement
would have occurred in the form of ions that later combined to form
montmorillonite. This is incompatible with the weathering-front model
that we support in this topic. Actually, in progressive penetration of
the horizons, the upper zone is differentiated at the expense of the
lower part the top of which is destroyed. In other words, the E/B
boundary descends in course of time when montmorillonite (more
correctly smectite) would have to give kaolinite (diagram of progressive
desilication, Chap. 3, Fig. 3.9). What disturbs us here is not that our
Bocquier's ideas
 
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