A natural salt marsh: salt or gypsum?

A link in the history of the Alps…. 

During the first part of the secondary era, in the Triassic, the single continent, Pangea, cracks more and more. Its physiognomy has changed a lot since the end of the primary era, since erosion has flattened it. The low areas are regularly invaded by the sea. Shallow seas, rich in fauna, occupy entire regions in the Middle Triassic (-247 to -237 Ma).

Under the influence of a still very hot climate, with long periods of aridity, marine evaporation is intense.

In semi-desert environments, expanses of seawater give way to over-salty lagoons. Under the scorching sun, brines are formed.

Indeed, in closed seas or in certain large lakes, under the action of evaporation, different types of salts precipitate, in the form of sodium, calcium, potassium sulphates, in the form of borates, etc.

The rocks thus formed (halite, gypsum, anhydrite, etc.) are called evaporites. Around Digne-les-Bains, there are large masses of gypsum which are hydrated calcium sulphate. These rocks bear witness to a low-altitude sub-desert environment, with large areas that can occasionally be flooded by marine incursions and runoff water laden with ions from the surrounding rocks.

Is salt now on the surface in the Alpes de Haute-Provence? Well no, because this one is too soluble. When it reaches the surface, it is immediately dissolved and washed away by runoff. However, some springs are salty by infiltration and then by the circulation of rainwater infiltrated into the massifs. When they encounter masses of salt, they dissolve it, take charge of it and thus arrive salted on the surface.

Other sources are sulphated, such as the thermal waters of Digne-les-Bains: they dissolved gypsum during their underground journey.

… In connection with 22 other geosites

Gypsum is a particularly fragile rock. It can be scratched on the nail, not very dense and soluble. These properties are responsible for the particular behavior of gypsum (and other evaporites) and its role in tectonics.

During the Jurassic and Cretaceous (-205 to -66 Ma), the Triassic gypsum were covered by a very large thickness of rocks, mainly limestones, harder and denser. Crushed under their weight, gypsum plays the role of "soap" layers during tectonic thrusts. The piles of rocks located above slide and peel off their old substrate, dragging all or part of the gypsum in their movement. The latter are therefore very often located at the base of the thrust slicks, as can be observed in Digne-les-Bains, in Esclangon around the Vélodrome, or above the Verdaches site.

What we can decipher from the current landscape

When pure, gypsum is white. But in the Triassic, sediments, and particularly clays, were transported by runoff to the bottom of the over-salty areas where gypsum was formed. Mixed with clays and polluted by other elements, these gypsum adopt different colors, red, salmon, yellow or gray… This is how Champourcin gypsum presents such a wide variety of colors. It is the same for the argillites (lithified clays, transformed into compact rock) of the Upper Triassic which are easily recognized with their "variegated" colors, pink, wine lees, yellow or green.

A little anecdote?

For local use and structural work, the gypsum was not sorted. Loaded with impurities, en particular clay, it gave after firing, the pink plaster typical of old low-alpine houses.

White gypsum was reserved for the production of quality plaster, and even for export.

Very finely crystallized pure gypsum is called gypsum alabaster. In Italy, the most famous alabaster, that of the quarries of Tuscany, is a limestone alabaster called onyx. It was popular with sculptors from Antiquity and the Renaissance for the delicacy of its “grain”.