Lithostratigraphy of the studied area: Mesozoic sediments

Lithostratigraphy, the beginning...

The opposite figure show you the ideal and simplified stratigraphy found in the "Schistes Lustrés" of northern Central Alps (from P. Probst 1980).

This one begin with a cristalline basement (Altkristallin), composed of old granitic rocks and paragneiss, both beeing pretriasic (of Primary era). These rocks are not studied in the aim of this thesis work, because they have suffered a double metamorphism, during Variscian (Primairy era) and Alpine (Tertiary era). This is a problem for datation of metarmorphism, as the determined ages are very often situated between these both major orogenesis and are so hardly interpretable (see "Argon-Argon datation").

Following lithologies are quartzites (sometimes absent or reduced) and dolomites. Thought that this rocks have suffered one single metamorphism, during the Alpine, they are not taken in account in this study, because their mineralogy do not allow a good estimation of pressure-temperature conditions achieved during a precise point of the pressure-temperature path of these rocks (see next page).

...and the most interesting levels

The first lithology of great interest for my work is so the upper Trias, or phyllitic Trias (= schists of Quarten). Generally in north Central Alps, this facies is composed of micaschists with garnet and/or amphibole porphyroblasts as in picture on the left (late crystallization, hydrothermal, of amphibole). The two first pictures below are other micaschists-type of this serie sometimes carbonate-bearing.

The major part of "Schistes Lustrés" is constituted of organic matter rich, grey to black, schists (or graphitic schists when degree of metamorphism is higher, as in the considered area). These rocks, deposited from Jurassic to Cretaceous, have suffered only one metamorphism. Numerous mineral phases are often present in equilibrium, allowing a good estimation of pressure-temperature conditions of metamorphism. An typical example of these relatively monotone lithologies is present in the two pictures at the end of this page.

These "Schistes Lustrés" are inhomogeneous, with millimetrical to decametrical alternations more or less regular of rocks, going from micaschists to limestone more or less silicaceous, without forgetting quartz- or quartzo-felsic layers.

Sample of Hornblendgarbenschiefer (schiste with fane of Hornblend, serie of Quarten) from Val Piora.

Schist with garnet (red) and plagioclase (white) porphyroblasts, muscovite-rich (figure on the left)
and biotite-rich (figure of right, both from Quarten serie, Piz Molare).

Thin section of graphitic schist with garnet porphyroblasts (polarized light on the left, polarized and analysed on the right)
Garnets, moulded by schistosity are well visible on the right (isotrop grains in black).