Geology Reference
In-Depth Information
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The fissured layer (Fig. 4) is generally characterized by fresh (hard) rock
cut by a dense horizontal fissuring in the first few metres and a depth-
decreasing density of subhorizontal and subvertical fissures (Cho et al.,
2003; Maréchal et al., 2003; Maréchal et al., 2004; Wyns et al., 2004).
Several processes such as cooling stresses in the magma, subsequent
tectonic activity or lithostatic decompression were invoked to explain the
origin of these fissures. However, it has been demonstrated that fissuring
results from the weathering process itself (Cho et al., 2003; Dewandel et
al., 2006; Wyns et al., 2004). Swelling of certain minerals results in a
local increase of volume that favours cracks and fissuring. In granitic
rocks, the most sensitive mineral to swelling is biotite. Where the rock
texture is relatively isotropic (in granite for example), the generated fissures
are orthogonal to the lower constraint vector (
3 ), and thus subparallel to
the topographic surface contemporaneous with the weathering process
(Fig. 4 left). In highly foliated rocks (i.e., gneisses or schists) the orientation
of the fissures can be also controlled by the rock structure. The
intensification of this horizontal fissuring at the top of the layer constitutes
the overlying laminated layer.
The fissured layer mainly assumes the transmissive function of the global
composite aquifer and is drawn from most of the wells drilled in hard
rock areas. However, the covering saprolite layer may have been partially
or totally eroded, or may be unsaturated. In these cases, the fissured layer
assumes also the capacitive function of the composite aquifer; e.g., in
French Brittany 80 to 90% of the groundwater resource is located in the
fissured layer (Wyns et al., 2004).
- The fresh unfissured basement is permeable only locally, where tectonic
fractures are present. The hydraulic properties of such fractures have been
investigated in various studies, particularly in details when the purpose of
these studies is the storage of nuclear waste (Neuman, 2005). Even if
these tectonic fractures can be as permeable as the fissures induced by the
weathering processes described here above, in most of the geological
contexts, their density with depth is much more lower than within the
fissured layer (Cho et al., 2003). At the catchment scale, and for water
resources applications, the fresh basement can then be considered as
impermeable and of very low storativity (Maréchal et al., 2004).
In addition to rock mineralogy, the development of such thick weathering
profiles requires specific climatic conditions: mainly significant rainfall, in
order to ensure mineral hydrolysis and, on the second order, quite high mean
temperatures to favour the kinetics of the process. Its development also
requires long periods of time under stable tectonic conditions (a few millions
to a few tens of millions years), the latest duration leading to profiles a few
tens of metres thick. In addition, relatively flat topography is required to
avoid the erosion of weathering products (saprolite), and also to favour
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