Geology Reference
In-Depth Information
-
The
saprolite or alterite, or regolith
(Fig. 3), a clay-rich material, derived
from prolonged in situ decomposition of bedrock, a few tens of metres
thick (where this layer has not been eroded). The saprolite layer can be
divided into two sub-units: the alloterite and the isalterite.
The alloterite is mostly a clayey horizon where, due to the volume
reduction related to mineralogical weathering processes, the structure
of the mother rock is lost.
In the underlying isalterite, the weathering processes only induce slight
or no change in volume and preserve the original rock structure; in
most of the cases this layer takes up half to two thirds of the entire
saprolite layer. In plutonic rocks, such as granites, the base of the
isalterite is frequently laminated; this layer is thus named the 'laminated
layer'. It is constituted by a relatively consolidated highly weathered
parent rock with coarse sand-size clasts texture and a millimetre-scale
dense horizontal lamination crosscutting the biggest minerals (e.g.,
porphyritic feldspars), but still greatly preserving the original structure
of the rock.
Because of its clayey-sandy composition, the saprolite layer can reach
a quite high porosity, which depends on the lithology of the parent rock
(bulk porosity is mainly between 5 and 30%; Compaore et al., 1997;
Wyns et al., 2004), and displays generally a quite low permeability,
about 10
-6
m/s. Where this layer is saturated, it mainly constitutes the
capacitive function of the global composite aquifer.
Figure 3.
Saprolite (grus and laminated grus) in granites
(Jeonju area, South Korea).
