Geology Reference
In-Depth Information
an "ancient" one
which deeply affects hard rocks (up to more than 50 m)
and results in a typical weathering profile, similar to those observed in
other regions of the world (Africa, South America, Europe, etc.); from
bottom to top, it comprises:
o the
unweathered hard rock
,
o the
fissured horizon
, and,
o above, the
saprolite
in metamorphic rocks and
grus
in granites, including
a laminated horizon in the lower part. The upper part of the original
weathering profiles (mottled horizon and iron crust) has not been
observed. As a consequence of its position at the top of the original
profile, it has probably been removed by erosion,
a more recent one
which only affects the rocks (and the ancient weathering
profile) within their first metres below the topographic surface. It is clearly
posterior to the previous weathering phase(s) as it crosscuts all the horizons
of the previously described weathering profile. It is probably quite recent
as it clearly follows the actual topographic surface.
This recent weathering phase didn't develop a thick fissured layer. It
is thus of low hydrogeological interest.
In
southern India
(Dewandel et al., 2006), the non-laminated saprolite is
very thin (1-3 m) and is almost constant in thickness while it should be
thicker in the plateau areas than in the valleys if the weathering had occurred
earlier than the erosion that shaped the present topography. The thickness of
the laminated layer (10-15 m) is disproportionate compared to that of the
non-laminated saprolite, while in the classical weathering profile (Fig. 2) the
laminated layer occupies only one third to half of the entire saprolite layer.
The thickness of the fissured layer is small compared to that of the saprolite
(ratio of ~1 instead of ~2). Moreover, the laminated layer presents preserved
fissures, which are usually not observed in the classical model.
In fact, the weathered zone appears to be composed of an old, probably
Mesozoic, weathering profile (Fig. 6a), where only a part of the fissured
layer has been preserved. An erosion phase, probably due to regional uplift,
thus caused the erosion of the entire saprolite layer and a part of the fissured
layer (Fig. 6b). Then, at least one more recent weathering phase, the latest
being probably still active, is responsible for the saprolitisation of this truncated
profile and explains the development of 1-3 metres non-laminated saprolite
locally capped by an iron crust, and the lamination of a large part of the
ancient fissured layer (Fig. 6c). The profile structure is thus controlled by a
multiphase weathering process that was induced by the geodynamic history
of the Indian Peninsula.
This (or these) more recent weathering phase(s) was (were) efficient enough
to hydrogeologically rejuvenate the old weathering profile through the
apparition of new permeable weathering induced fissures.
