Geoscience Reference
In-Depth Information
regularly distributed degradation tongues, so that, when observed in
horizontal section after scraping off the surface horizons, the prisms
have hexagonal shape (10-20 cm) and are separated by white or grey
boundaries (1-2 cm) filled with clayey or silty deposit having reddish
brown edges (see Fig. 8.3).
Fragipans correspond to the diagnostic
fragic
horizon of the WRB.
Their exceptional bulk density is explained by a bimodal particle-size
distribution such that the smaller particles, with a diameter about ten
times less than that of the larger particles, have filled the voids between
the latter (Chrétien 1986). In the dry season, the material develops cracks
and hexagonal prisms form. In the wet season, under the constraint of the
weight of the horizons above it, the clay slips between the silt particles
and the bulk density of the mixture increases. This is
hydroconsolidation
.
It can be experimentally produced (Attou and Bruand 1998). But the
residual porosity is 30 per cent. Thus we are still far from complete
filling of all the voids!
Fragipans are often linked to regions that have experienced periglacial
climates, but are also found in the tropical zone (Boulet
et al
. 1998). It
then affects certain Podzols and depleted tropical soils (Ultisols of Soil
Taxonomy). In our view, the fragipan is linked to a behaviour of the
materials and not to a soil type determined by pedogenesis.
Frequently we see clayey B horizons that are replicated and appear
in successive brown bands vertically superposed in the profile (
clay
lamellae
or
clay laminae
). We have demonstrated that the different
bands are contemporaneous (Legros 1975, 1976). They are formed by
lateral circulation of solutions or suspensions. To understand them it
is necessary to find quarries where the soil can be studied in three
dimensions (Fig. 8.10). This is the case in France in the Pilat massif
(Ardèche-Loire).
These profiles are observed on more or less strong slopes. The water
circulates laterally in the altérites on the discontinuities slanted relative
to the horizontal. These discontinuities may have various origins:
stratification of the loess, slipping along the compartments of granitic
arènes or layering of screes. There are soils showing bands of ferruginous
nature, particularly in some Podzols. But in some cases, fluctuation of
the water table could be involved.
Banded B horizons
