Geoscience Reference
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average case, the results in the Central African Republic for various
cuirassed assemblages (Beauvais 1989).
We see from the bottom to the top the relative concentration of iron
and the approximate stability of the Al/Si ratio (diagram on the left).
Thus there is desilication and, at the same time, dealumination. Kaolinite
is destroyed (diagram on the right). Oxides of iron are concentrated, first
as goethite and then, higher in the profile, as haematite.
SiO
2
Kaolinite
Legend
Lithomarge
Mottled zone
50
50
50
50
Cuirass
Al
23
Fe
23
50
Haematite
50
Goethite
Fig. 5.5
Chemical and mineralogical composition of a typical weathering profi le with basal
lithomarge and top cuirass (Beauvais 1989, simplifi ed).
In most profiles of tropical soils, there are no micromorphological
features that could lead us to presume that clay has moved vertically
in large quantities. This has posed a problem for American classifiers. It
was necessary to create, in addition to the
argillic
horizon rich in coatings
characteristic of temperate soils, the
kandic
horizon. It characterizes the
tropical B horizons, clayey but without clay coatings. We shall return
to this problem in Chapter 8.
Vertical transport of clay
Some scientists consider the cuirasses to be allochthonous (Bertrand
1998; Bourman and Ollier 2002; Brown
et al.
2003). Actually, iron is
mobile when it is reduced and can be transported towards low points
in the landscape. In this concept, the cuirasses would have been formed
in low plains before being taken to higher position by inversion of
relief (Taylor and Howard 1999). But the phenomenon is by no means
general.
Possible allochtony of ferruginous cuirasses
