Island and Coastal Aquifers - Hydrogeology: Objectives, Methods, Applications - page 69

establishment of a water current, necessary for the excavation of conduits
(see chap. A.8), depends on the hydraulic gradient and the fractured
permeability of the rock. Water fl ows from recharge to discharge zones so
as to minimise the loss of head, that is to say, along the most direct path,
dependent on the geometry of the aquifer's fractures. The principal drains
are therefore generally placed in the upper portion of the aquifer and have
a sub-rectilinear profi le, close to the piezometric slope. However, since the
system may have evolved during the past, with the creation of drains with
a greater cross-sectional area, if the water table rises above these, the head
loss will be lower in these conduits than in the network of non-karstifi ed
fractures, and drainage will occur at least partially through them.
Paleokarsts are known around the edges of the Mediterranean
Sea, dating back to at least the Cretaceous (Rousset, 1968). The current
karstifi cation, unaffected or little-affected by tectonic activity, probably
dates from the Oligo-Miocene for relatively tectonically inactive regions, and
from the Pliocene, after the last alpine phase, for other regions (subalpine
ranges). This heritage is important, and the drainage of a karst aquifer can,
as a result, have a sometimes-disconcerting geometry. Upstreal of coastal
springs, speleologic explorations reveal more and more the presence of
inherited karst passages, developing signifi cantly below the lowest levels
of Quaternary glacio-eustatic variations. The most convincing hypothesis
is that these systems were put into place during the Messinian, when the
Mediterranean Sea dried up, before the opening of the Strait of Gibraltar.
Nevertheless, the role of tectonics must not be underestimated, as countries
such as Greece show very high rates of uplift, currently exceeding 1 mm
per year.

Table 5 Depth of Passages in A Few Karst Springs.

Spring Country Depth below

sea level

Hypothesized origin

Rio el Mante Mexico 300 m hyper eustatism in the Gulf of Mexico

Port Miou France 179 m Messinian hyper eustatism

Vaucluse France 224 m Messinian hyper eustatism

Almyros Greece > 100 m Messinian hyper eustatism

Pozzo el Merro Italy 300 m Messinian hyper eustatism

The impressively deep Fontaine de Vaucluse, nevertheless located
150 km away from the coast, as well as several examples in Ardèche and
in the Causses of the Montpellier region, demonstrate that this mechanism
affected the spatial organisation of karst systems very far from the coast
(Audra et al. , 2003).