Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 266

The diffi culty of studying the Triassic is locally increased by frequent
variations in thickness and facies, by the development of plastic clayey marl
horizons implicated in the tectonics, and by the existence, throughout the
depression lining the Keuper, of Quaternary colluvial fi ll (Plans de Grasse
and Plans de Saint-Martin). The geometric summary was therefore based on
the structural analysis of peripheral regions, on the systematic exploitation
of aerial photographs, and on specifi c surveys undertaken (geophysical
surveying by the electric method, core drilling, and destructive drilling).
The rough characteristics of the local structure reveal an intense cutting-
up of the rock mass by a tight network of vertical faults, principally aligned
N 0 to N 30 and N 90 to N 130, and its dissection into a mosaic of relatively
lowered (grabens and synclines) and uplifted (horsts and anticlines) units.
The contacts between units are frequently offset in a stair-step fashion or
relayed by fault zones, which makes it diffi cult to differentiate the relative
roles played by brittle and ductile deformation (Figure 128). The most
notable features are, to the northwest (town of Grasse), the Aspres anticline-
horst forming a hump of Muschelkalk carbonate overlooking the Keuper
surrounding it, and, in the middle region (town of Mougins), the narrow
Touramy anticline-horst (Muschelkalk limestone) separating two Keuper
syncline-grabens. It is within one of these lower regions that the collapses
occurred.
From a hydrogeologic point of view, the gneissic basement rock and the
Werfenian sandstone make up a continuous impermeable base, supporting
the region's main aquifer. This karst aquifer is contained within the
Muschelkalk carbonate reservoir. It drains to its southwestern edge, through
the Foux de Mouans-Sartoux at an elevation of 69 m and the Carimaï spring
at an elevation of 32 m (Figure 127).
The overlying Keuper formation also represents an important regional
impermeable unit, but contains permeable horizons (dolomite, gypsum,
cellular dolomite) containing small reserves of water and feeding scattered
springs. The deep drainage for the gypsic and dolomitic lenses is, however,
relayed to the underlying Muschelkalk aquifer, along lithologic or tectonic
contacts, which explains the calcareous sulfated character of the water at
its outlets.
3.4.3 Local characteristics
The 1998 collapses occurred in the Saint-Martin graben, the detailed structure
of which is illustrated in Figure 128. Below the superfi cial colluvium, the
Triassic fi ll consists of multi-colored clayey marl, then of gypsic masses 10
to 30 meters thick resting directly on Muschelkalk carbonate. The hydraulic
connection between the two formations was verifi ed thanks to tracer tests,