Castillon lake

Observe the rhythms of the Barremian sea

A link in the history of the Alps….

During the Lower Cretaceous (-145 to -100 million years ago), the Alpine Ocean continued to expand slowly in places, at a rate of 1 centimeter per year, or even less.

The Atlantic Ocean expands to the west and especially to the south where it begins to separate South America from Africa, which is rising northwards in a turning movement. As a result, Africa's relative movement blocks, and even reverses, the expansion of the Alpine Tethys.

The south-east of France, still bathed by the sea, is gradually changing its face. The Vocontian basin opens onto the ocean to the east. It is bordered to the southwest and to the east by shallow platforms where reefs develop, in particular during the Barremian (-130 to -125 Ma).

Around Lake Castillon, the geological layers have recorded the history of this basin and well beyond. Connected to the ocean, it also recorded what was happening on a planet scale!

At the bottom of the sea, the deposits were made almost continuously. Also, the layers alternate in regular, shallow banks: a limestone bank, a marl bank (limestone mixed with clay), a limestone bank, a marl bank, etc. These rhythmic alternations are the result of variations in climatic cyclics.

Tropical environments are subject to long dry spells, even arid spells, which follow wet spells. Under these conditions, the phenomena of alteration and erosion of rocks have an impact on sediment inputs at the bottom of the sea and on the availability of nutrients for marine organisms.

The marly deposits indicate the arrival of clays of continental origin in the sea by erosion phenomena. Limestone deposits are a sign of the production of sediments of essentially biological origin linked to the presence of marine plankton.

These climatic, and therefore sedimentary, rhythms are the consequence of cyclical astronomical variations in the Earth's orbit around the Sun and the Earth's axis of rotation on itself. These variations are called Milankovich cycles, named after their discoverer. This phenomenon is very complex because there are as many nested cycles between them as there are periods. Each period corresponds to a cycle: 19, 000, 23, 000 and 41 years.

Cyclical variations in the environment also have an influence on the evolution of living organisms. Moreover, ammonites are a good model to study these phenomena.

… In connection with 22 other geosites

In the Barremian, ammonites are very diversified with a significant biodiversity. Each stratum includes dozens of different species. There are some very particular forms with a non-contiguous spiral called "unrolled ammonites", typical of the Cretaceous.

While they are of the same age (Barrémien), the site of Castillon is very different from that of the gorges of Oppedette. In Castillon, the sediments were deposited by alternating limestone beds and marl in a marine basin. In Oppedette, the very thick limestones in which the gorges are dug, were built by organisms called rudists, in an environment of shallow sea in which ammonites were absent. The rudists were shellfish whose accumulation formed real reefs.

What we can decipher from the current landscape

From the level of Lake Castillon up to the top of the slope, one can admire the impressive sedimentary pile with increasingly important and thick limestone layers towards the top and majority marly banks towards the bottom.

The limestone bar which forms the summit is dated from the Upper Barremian. It is more resistant to erosion than the thick marly formation which succeeds it over time (on a scale of several million years) and which has disappeared here: the limestones then form a small hillock ended by a steep slope. .

On a landscape scale, we often notice that thick marl sets are surmounted by alternating marl-limestone, then are capped by a limestone bar. Near Castillon, this pattern is repeated up to the upper Cretaceous layers of the mountain of Chamatte. These repetitions correspond to very long sedimentary cycles, on a scale of tens of millions of years.

A little anecdote?

The Barrémien takes its name from the village of Barrême since it is in this sector of the Alpes de Haute-Provence that the geological level was defined for the first time in 1862 by the geologist Henri Coquand.

However, it was not until 1963 that a reference site (stratotype), making it possible to describe layer after layer of the entire floor, was defined near Lake Castillon along the road to Angles. The name of the stage, the Barrémien, should normally have been renamed Anglésien, but this was not the case and Barrême kept its international notoriety!