Karst Aquifers - Hydrogeology: Objectives, Methods, Applications - page 55

level allowed stream capture by Devonian limestones within which formed
Arphidia system's galleries, which drain to the Bentia springs and fossilize
the old Aranzadi gallery drainage (Figure 28). Given this particular context
(a thick transgressive limestone layer overlying folded Paleozoic basement
rock), without the deployment of considerable geophysical means, the deep
geologic structure remains unknowable from the surface, and speleologic
exploration therefore provides irreplaceable information.

Campanian-Maastrichtian fl ysch and schist

Canyons limestone (Cenomanian to Santonian)

schisty sandstone Paleozoic basement

calcareous Paleozoic basement

Calcareous paleozoic basement aquifer

Figure 28 Withdrawal of the Saint Vincent river by the Arphidia network in the Devonian
limestone of the Paleozoic basement of the Pierre-Saint-Martin chasm system (Altantic
Pyrénées).

3 FUNCTIONAL CONSEQUENCES

3.1 Hydraulic functioning

The hydraulic consequences of this highly differentiated structure are
that:

• fl ow paths in the saturated zone have a strong horizontal component,
with drainage passageways playing an important role. Flow is
turbulent in these very open channels, resulting in the remobilisation
of sediments and a good mixing of solutions with different states of
mineral saturation;