Geoscience Reference
In-Depth Information
tropical zone with contrasting seasons: annual rainfall of the order of
1500 mm, dry period of 6 months in a year and a temperature of 28° C.
The parent rock is mostly gneiss or granite.
The transitions between horizons express a certain progression. The
vertical indentations cannot be of sedimentary origin. The characteristics
of the clays (crystallinity, crystal size, shape, iron substitution) vary
slowly from the bottom of the profile to the top. Quartz grains show
signs of etching, which increase regularly towards the top. Heavy metal
(Fe, Ti and, sometimes, Ni and Co) contents of the parent material and
of the cuirass are correlated. The cuirass is thus termed
lithodependent
(Blot
et al.
1978). The kaolinite in the surface layer also contains minerals
characteristic of the substratum (Truckenbrodt
et al
. 1991). All these lead
us to the essence of this chapter: specialists (Nahon 1976; Lucas 1989;
Tardy 1993b; Beauvais 1999; Horbe and da Costa 2005…) think that this
profile, although comprising extraordinarily contrasting horizons (red
ferruginous layers, white kaolinite layers) is formed
in situ
from a single
material, in conformity with the weathering-front model presented in
Chapter 3. We shall take it up but without hiding certain difficulties in
interpretation.
The corresponding landscapes are characteristic (Bertrand 1998). The
top cuirass forms a very solid framework that determines a plateau
morphology. At the edges, it forms cliffs seen in aerial photographs to
be considerably cut up (dissection of relief).
5.2.2
Evolution of Different Horizons
Immediately above the unweathered rock the minerals retain their
appearance, the original volume is preserved and
soil collapse
(Chap. 3)
has not started. This justifies the term 'isovolume altérite', contracted
to
isaltérite
. Some prefer to call this material
saprolite
(Greek
sapros
=
rotten;
lithos
= stone). This well indicates what the material is! More
precisely:
Differentiation of the saprolite
The material is less coherent (a knife penetrates it) than the rock;
the bulk density can drop to 0.7, especially on ultrabasic rock,
for many constituents have been eliminated.
Quartz is preserved albeit with signs of dissolution towards the
top of the saprolite; the same is true for white micas; on the other
hand, feldspars are transformed to kaolinite and the black micas
yield kaolinite as well as goethite according to the reaction
