The Daluis gorges

After Carboniferous black, Permian red

A link in the history of the Alps….

Pangea constitutes, at the end of the primary era, in the Permian, a single continental block, lying north-south. The Varisque chain is level, the landscape is generally flat at low altitude.

The Permian, -299 to -253 million years ago, marks the end of the Varisque adventure and the beginning of that of the Alps.

The history of the Alps begins with a tectonic activity that will gradually disrupt Pangea and create new landscapes.

The Permian also marks a major climate change which is reflected, in particular, by the formation of red earth: the landscapes change color.

Far from the ocean coasts, in the center of Pangea, vast expanses are desert. Other regions benefit from more favorable climatic conditions for life, similar to a humid equatorial belt. In some areas where the subtropical climate has contrasting seasons, wet and arid episodes alternate. It is in this latter type of environment that the red lands of Daluis were formed.

Daluis  is remarkable for its 270 meter thick Permian sedimentary pile (about -253 to -900 million years ago). These layers were deposited in a vast floodplain with expanses of lake, alternating between periods of floods and droughts. The rocks thus formed are composed of more or less fine-grained sands (sandstone) to very fine (pelites) as well as clays (argillites).

But how to erode, transport and deposit such volumes of sediment (900 meters thick) when the landscape was level?

The mechanism that will dislocate Pangea is launched! The continent is subject to tectonic forces which stretch it. They also crack its surface causing the appearance of faults. Collapse ditches form at the foot of higher and sometimes tilted areas.

Thus, Daluis sediments accumulate over millions of years in one of these vast areas which sag very very slowly over time. This phenomenon is called subsidence.

Within the red pelites, we find thin layers of volcanic ash transformed into rocks, the cinerites. They testify to the very intense volcanic activity that reigns in the Permian. This volcanism is also the announcement of the fragmentation of Pangea.

… In connection with 22 other Argentera and Verdaches geosites

The Permian red of Daluis is younger than the Carboniferous Black of Verdaches, and much more so than the granites and other crystalline rocks of Argentera-Mercantour.

 Logically, these older rocks should be visible at the lower layers. Despite the deep erosion of the Var at the bottom of the gorges, they have not been brought to light.

On the other hand, towards the northern exit of the gorges, the white bar formed by quartzites of the more recent Triassic (base of the secondary era, around -250 million years ago) "normally" succeeds the wine-red layers of the Permian. .

Today, all of these layers are no longer horizontal but slightly inclined towards the north and plunge towards the Var.

Unlike the “normal” succession of layers at Daluis, we observe in the Verdaches clue that the Triassic quartzites lie directly on the Carboniferous. The Permian is absent there. To explain it, two hypotheses are possible: either it was not deposited in Verdaches, or it was deposited but was eroded before the Triassic ...

What we can decipher from the current landscape

The red color of sedimentary rocks generally reflects a continental origin and contrasting climatic conditions (alternating wet and arid periods) and hot.

For example, red soils are forming today in tropical Africa. Comparison with current processes allows us to reconstruct the history of Daluis. Under a hot climate and plant cover, the iron that results from the weathering of certain rocks changes from a reduced state to an oxidized state in the presence of oxygen and water linked to the rainy seasons. Newly formed minerals, such as iron hydroxides, are red.

Subsequently, during significant rainfall events, the red soils rich in iron oxides are washed away by erosion towards the lowest areas, as here in Daluis.

The gorges of Daluis, for their part, were dug late. The alpine uplift and its folds carried the entire sedimentary series aloft. Then, the erosion released the red layers and the Var started them in a spectacular way.

A little anecdote?

During the primary era, life was extremely diversified, great stages of evolution were taken and all environments were colonized by species.

The end of the Permian is marked by the largest known mass extinction of life on Earth ...

The responsibles ? The configuration of Pangea as a single land mass, its climate, the scarcity of shallow seas and, above all, the very intense volcanism and its consequences on land, in the atmosphere and in the oceans. Chain reactions have led to the release of very large quantities of greenhouse gases.

This episode is far from being an anecdote!