The sirenians of Castellane

Who benefits from the collision? To the marine mammals of Castellane

A link in the history of the Alps ...

North of Castellane, there is a unique deposit in the world for its fossils of herbivorous marine mammals, the sirenians. This deposit tells a key moment in the history of the Alps: the collision.

The collision of the continental blocks of Eurasia and Apulia (microplate in northern Africa), at the origin of the Alps, occurs around -35 million years ago. The Apulia forms a sort of promontory which will mark Europe and become embedded in it. The first alpine reliefs which result from it form far to the east, at the level of the future Italy. The thrust continues to spread and the nascent chain "advances" more and more towards the west, pushing back an arm of the sea in front of it. The sea thus invades a partially flattened landscape. However, a few kilometers west of Castellane, following a line from north to south, it stumbles on reliefs too high to be crossed. North of Castellane, the coast is rocky and the seabed is irregular.

The first deposits of marine sediments fill in the hollows in the topography before becoming more homogeneous. Among these first deposits, there are limestones which contain both the bones of sirenians but also very small fossils resembling lenses of 2 to 3 mm in diameter. These small tests (kinds of shells) housed unicellular organisms, foraminifera, which lived on the seabed. These are nummulites, precious fossils used to date rocks from the first half of the Tertiary. The Paleogene is also called the Nummulitic!

On the site of the sirenians, these nummulites made it possible to attribute an age of -35 million years to the limestones and the fossils they contain and also, indirectly, to the collision.

These nummulitic limestones cover and fossilize an ancient topography made up of a limestone relief eroded by erosion and dissolution, that is to say a karstic relief. On this site, limestones of -35 million years old (Eocene) are directly superimposed on much older limestones of -145 million years old (late Jurassic). So where are the missing 110 million years between the two?

The answer lies in events that took place between -80 and -40 million years ago. It was at this time that the ensemble formed by Iberia and the Corsican-Sardinian micro-plate collided with Europe during a first phase of folding, called Pyrenean-Provençal. This phase affected Haute-Provence by forming the first reliefs there. These reliefs have undergone deep erosion, removing the softer layers over a thickness estimated at nearly 1000 meters (all of the Cretaceous), to a harder basement (the late Jurassic), attacked in turn by an intense weathering. (karstification). Then locally, the arrival of the sea partially inundated them -35 million years ago.

At that time, the sirenians lived in the sea, a few hundred meters from the dangerous rocks where the bottom sheltered meadows of algae behind reef buildings. We find their remains in the hollows formed by the karstic reliefs. These herbivorous mammals, grazers of marine algae were found there during catastrophic storms during which sediments and carcasses were thrown en masse on the coast where they remained trapped by the topography.

However, the sirenians took advantage of the place for a few million years before the alpine uplift gained ground and the marine basins were in turn filled and deformed.

... in conjunction with Les Grés d'Annot or Les Eaux Tortes

When the Tertiary Sea is driven back by the alpine uplift around -40 to -35 million years ago, it invades the depressions formed at the periphery of the Alpine arc and its first deposits are nummulitic limestones. These deposits are immediately followed by marl sets then by sandstones. Geologists speak of a “trilogy” for this characteristic succession: nummulitic limestone / marl / sandstone. This trilogy can be seen in Castellane, Annot, in Haute Ubaye as well as in the south of Mercantour. In Annot, the sandstones of this trilogy are very present in the landscape, so much so that they are called “Annot sandstone” wherever they appear.

The Eaux Tortes site offers a magnificent view of the Grande Séolane. As in Castellane, 110 million years of deposits are missing since the nummulitic limestones were deposited directly on the limestones of the Upper Jurassic. There is however a difference of size: the series was reversed by the tectonics and the nummulites passed under the Jurassic.

What we can decipher from the current landscape

The sirenian fossils in Castellane are exceptional by their number, their age and the quality of their conservation. This site, now developed by the Geological Reserve of Haute-Provence, is a real treasure for paleontologists.

The general public can discover 200 fossil bones with a multitude of ribs, vertebrae, skulls, and limb bones. It's a real puzzle to piece together!

At the exit of the small bridge which spans the stream, we walk through the ancient limestone landscape over 35 million years old and which is cleared by contemporary erosion. On the opposite bank, you can see the slightly yellow nummulitic limestones which mold themselves on the hollows and bumps of the Jurassic limestones: the old topography has been literally fossilized.

A little anecdote?

Despite some analyzes, the pink color of the sirenian bones remains a mystery. This fuels some wacky theories in favor of the kinship of sirenians and elephants.

More seriously, these two mammals are indeed very close. During evolution, only the Sirenian group has (re-) adapted to aquatic life.

The fossils from the Castellane site date from the end of the Eocene and are therefore very old for the history of the Sirenians. Such remains are rare on a worldwide scale, their study therefore provides valuable information on the evolution and transformations of skeletons and dentitions carried out from that time. For example, we know thanks to the discoveries made in Castellane, that 35 million years ago the fore legs of sirenians were already perfectly transformed into fins, while their rear legs had not yet fully regressed. In the current dugons, their descendants, however, these hind legs have completely disappeared.