Geoscience Reference
In-Depth Information
2.3.7 Concept of Paleosols
The term Paleosol is vague and generic. Actually two principal kinds
must be distinguished.
Polyphasic soils or Vetusols
. The term Vetusols was recently
proposed (Busacca and Cremaschi 1998). They have the ancient features
that do not correspond to present-day environmental conditions.
Vegetation, climate and hydrologic regime have changed over time. But
these soils continued to evolve till now and, therefore, also have features
in equilibrium with the present conditions.
Fossil soils
are generally buried and their evolution has been blocked
abruptly. This could be related to various phenomena:
aeolian
(deposition of loess or sands on an existing soil); fossil
soils are found in the centre of sand dunes;
fluvial
(covering of alluvial soils by new additions of sediments);
it is a very classic case, especially in the mountains because the
buried organic materials are not mineralized, which enables later
identification of the ancient surface of the soil(s); there will be
nearly 1200 superposed Paleosols in the Eocene formations of
Wyoming!
volcanic
(fossilization under a basalt flow). Figure 2.9 explains the
mechanism and Figure 2.10 presents an example of soil preserved
by this mechanism.
Stage 1
: Open valley where soils (shown in black) are formed
Basalt
Stage 2
: The
valley is filled by
a basalt flow that
solidifies there,
baking and
burying the soils
Stage 3
: As basalt is resistant to erosion,
there is a gradual inversion of relief; the
fossil soils are seen on the exposed edges
of the flow.
Fig. 2.9
Mechanism of fossilization of a soil by a basalt fl ow.
