Geoscience Reference
In-Depth Information
is a link between evolution of soils on the continents and the deposition
of sediments in the oceans.
Cold zone: illites
and chlorites
Temperate zone: interstratified
clay minerals, vermiculites
Mediter. and trop. zones: smectites
Humid tropical and equatorial zones:
kaolinite, gibbsite
Tropical zone: smectites
Temperate zone: interstratified
clay minerals, vermiculites
Cold zone: illites
and chlorites
Fig. 3.19
Organization of the clay minerals at the world level (Millot 1979b).
During a calm period, on the geological scale, forest or other continuous
plant cover is established on the continents. Only dissolved substances
(ions) escape from soils that are protected from mechanical erosion.
Marine sedimentation, therefore, will correspond to the precipitation of
these ions in the oceans, chiefly in the form of the carbonates CaCO
3
and CaMg(CO
3
)
2
or, sometimes, fine clays. This is
biostasy
.
Then a cataclysm destroys or reduces the vegetation. The causes may
be varied and act at different scales of time and space: fire, volcanic
eruption, meteorite impact, climatic change, orogeny, etc. Then the soils
erode and contribute sands and various detrital materials to rivers and
to sedimentation zones. This is
rhexistasy
.
A typical case is represented in Fig. 3.20. We found this impressive
sight in the sedimentation zones below the geological formation called
'Sands and Clays of the Bourbonnais region' (Favrot
et al
. 1973). At
the bottom lie fine deposits (kaolinite). Above it are accumulated tree
trunks (transformed to lignite), which shows that the forest fell into the
drainage basin, for reasons unknown. Lastly, the deposits are coarser
(sands + clays + gravel + ferruginous concretions) and correspond to
the scraped soil cover of the region.
