Geoscience Reference
In-Depth Information
The anisotropy of the layers may be explained as follows. During an
episode of rain, at the start of the deposition, the mixture is dilute and
the clay platelets can get oriented and sediment parallel to each other.
Then, very gradually, the water disappears (absorption, evaporation) and
the platelets are concentrated. They are then deposited in all orientations,
leaving gaps. Hence the optical density of the layer diminishes and
it appears lighter under the microscope. But other hypotheses are
possible: more rapid sedimentation of iron oxides or certain types of
clay minerals. We have not been able to decide. But we have selectively
sampled this material from a pocket in the arène. The clay is uniform,
not mottled; it breaks naturally into flakes. It is composed of a mixture
of clay micas, lightly aluminized vermiculite and kaolinite. The total
content of HF-soluble iron is 7 per cent. After dispersion, this material
was allowed to sediment on a glass plate, drop by drop and day after
day (experiment done by L. Roger). Then the whole thing was doused
in a hardener and a thin section cut perpendicular to the plate. It was
observed under the microscope that shapes similar to those seen in the
natural environment had been produced.
More generally, the studies show that the clays translocated in the
natural environment are in the form of very fine platelets of size no
larger than 0.1 or 0.2
µ
m (Jamagne 1973). This applies to all mineralogical
types of clay as revealed by the nature of the coatings. But smectites
appear to be particularly mobile (Grossman
et al
. 1959…).
Secondary illuviation of clay is chiefly observed in the upper part of
some Albeluvisols. These are Luvisols with a light albic horizon in which
the clay is deferrified and oriented; it appears 'to flow' in tongues that
this depleted (E) horizon plunges (Fig. 8.3) into the enriched B horizon
(Jamagne and Begon 1984). These tongues or glosses correspond to cracks
opened up in dry periods or periods of cryodesiccation (cold season).
Secondary argilluviation
White deferrified oriented
material in the tongues
A
E: Residual
sands and
silts with
little clay
10-15 cm
E
Rust-coloured,
iron-rich
borders
Glossic
character
(tongues
ofEinB)
B
Vertical section
Horizontal section
Material of B
Fig. 8.3
Position of the tongues of degradation (glosses) in Albeluvisols. The beehive struc-
ture in the horizontal plane is seen only in some B horizons termed '
with fragipan
'.
