Mont-Cenis Plateau, Lanslebourg-Mont-Cenis, Val-Cenis, Saint-Jean-de-Maurienne, Savoie, Auvergne-Rhône-Alpes, Francei

Regional Level Types
Mont-Cenis Plateau	Plateau
Lanslebourg-Mont-Cenis	Commune (Former)
Val-Cenis	Commune
Saint-Jean-de-Maurienne	Arrondissement
Savoie	Department
Auvergne-Rhône-Alpes	Region
France	Country

Fairly flat area (ca. 1900-2100 m) to the south of Mont Cenis Pass (Col du Mont-Cenis in French, Colle del Moncenisio in Italian) in which is located Lac du Mont-Cenis, an artificial reservoir constructed by enlarging in different years (1912, 1920-24 and 1963-68) the already existing homonymous lake. The water level (once at 1921 m) can reach its highest value at 1971 m, but it seasonally varies covering and uncovering some areas.

The Lac du Mont-Cenis depression is mostly carved in a thick belt of Triassic gypsum (anhydrite in depth) and cargneules, which divides two masses of the Schistes Lustrés complex (the Signal du Petit Mont-Cenis - Pointe du Lamet - Rocciamelone mass and the Corne Rousse - Pattacreuse mass). The Triassic evaporitic belt continues to the southeast up to the southern foot of Rocciamelone (Susa Valley); to west of the lake, passing through Vallon de Savalin and Col du Petit Mont-Cenis, it continues towards Bramans, where it connects outcrops of the so-called Maurienne gypsum nappe or "Nappe des Gypses" formation (of which it appears to be a southeastern branch). The lansdscape in the Col du Mont-Cenis - Vallon de Savalin area and in the lake area is marked by numerous sinkholes, described by various authors (e.g. De Saussure, 1796; Piolti, 1889; Dainelli, 1907).

The Triassic evaporitic belt consists of a sedimentary sequence of evaporites (mainly anhydrites), marine carbonates (dolomite), and clay layers (micaschists). Anhydrite rock is the most abundant rock of the sequence. Sedimentary bedding is underlined in white anhydrite by numerous pale gray layers that are commonly folded. Due to massive recrystallization, alpine foliations are most of the time invisible. Marine carbonates consist of centimeter to meter scale beds of grey dolomite. As these carbonate beds have been strongly folded and stretched by Alpine tectonics, they form various scale blocks more or less rounded and elongated within the main anhydrite layering, called "boudins". A secondary white dolomite, in which no sedimentary structures are visible, is frequently observed around marine carbonate blocks. Sedimentary clay layers have been transformed into micaschist rich in phengite by Alpine metamorphism.

Quartz crystals from the evaporitic sequence contain fluid inclusions (Barré et al., 2020).

Boudins of greenish schists (metapelites) are observed in the Mont-Cenis gypsum hillock. These boudins contains "metamorphic veins", mainly composed of albite and iron-bearing dolomite, with accessory pyrite, hematite, clinochlore and other minerals, in which are present narrow fissures hosting a Ti-Th-U-REE mineralisation (De Ascenção Guedes et al., 2008; De Ascenção Guedes, 2013).

Rocks of the surrounding Schistes Lustrés complex mainly consist of calcschists. A small strip of serpentinite, lying between gypsum and calcschists, crops out on the western shore of the lake at the eastern foot of Cime du Laro. The specimen of asbestos described by Barelli (1835) probably originated from this outcrop.

Twinned rutile crystals from schists (generic) were described by Lacroix (1888). In more recent times, rutile, as red prismatic crystals, often in the form of geniculate twins, or as mesh-like networks of tiny golden crystals (var. sagenite of Saussure), is reported from fissures in the greenish schist (metapelite) boudins embedded in the evaporitic sequence at the Mont-Cenis gypsum hillock (Piccoli et al., 2007; De Ascenção Guedes et al., 2008).

In the first half of the 19th century, peat from wetland near the lake was mined for local uses (De Mortillet, 1858).

Fossils

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References