The Apt region is world-renowned among geologists!
In 1840, the famous paleontologist Alcide d’Orbigny described the fossil-rich grey marls found between Apt, Gargas, and Saint-Saturnin-lès-Apt as the reference for a new stage in the international geological time scale: the Aptian.
Since then, numerous studies have shed light on the age of these rock formations and the story they tell — that of a sea covering the Apt area around 120 million years ago.
Historical context
In 1840, in the first volume of “Paléontologie Française” covering the Cretaceous period, d’Orbigny introduced the term “Aptian” to describe rock layers characterized by a distinctive pyritic ammonite fauna found around Apt.
“Regarding the upper layers of the Neocomian stage […] all evidence suggests linking this set to the lower Neocomian rocks, while recognizing it as a clearly distinct time period. It might even be appropriate to separate this fauna completely and give the corresponding layers a special name. I propose the name ‘Aptian,’ after the area around Apt, which is its main location. […]”
— Alcide d’Orbigny, 1840
In 1850, d’Orbigny expanded his definition by listing all species, especially ammonites, present in this stage.
Later, in 1883, Frantz Leenhardt provided the first detailed description of the Aptian section between Apt and Fort Hill, just south of the village of Gargas, in his thesis “Geological Study of the Mont Ventoux Region.”
NNE-SSW cross-section by Leenhardt (1883):
1: Urgonian limestone
2–3: Bédoulian yellow marls
4: Typical Aptian marls
5–6: Sandy marls
7: Sandy marls and sandy sandstone
C?: Yellow-greenish sandy marls
(from Moullade et al., 2009)
In 1965, M. Moullade proposed a revision of this historic cross-section during the Symposium on the Lower Cretaceous.
Since then, this cross-section has sadly nearly vanished due to urban expansion, the development of the Apt-Gargas commercial area, and the creation of a reservoir.
Today, the most significant remaining outcrops close to the original stratotype are the marl deposits around the hamlet of La Tuilière. A portion of this site has been designated as a Natural Environmental Site (ENS) and is now the subject of a management plan focused on its protection, upkeep, and enhancement.
— What is a stratotype ?
Apt is the type locality of the Aptian stage: it was in Apt that this stage was first described and established.
The grey marls of the Apt region, studied by Alcide d’Orbigny, form the Aptian stratotype — the internationally recognized reference strata.
Stratotypes like these have been essential in defining the international geological time scale.
Stratigraphical interest
The Aptian stage has been subdivided into three substages.
In 1887, Wilfrid Kilian introduced the term Gargasian to describe the “marls of Gargas.”
In 1888, Aristide Toucas coined the term Bédoulian based on the limestones from La Bédoule.
In 1947, M. Breistroffer established the Clansayesian substage, named after the locality of Clansayes (Drôme), to define the uppermost zone of the Aptian.
Recent studies (Moullade, Tronchetti, Babinot, 2009) have shown that the historic section was truncated both at its base and its top.
The original historic section no longer exists and was incomplete. In recent years, scientific research has therefore focused on other outcrops. In particular, those at La Tuilière, near Saint-Saturnin-lès-Apt, form a thick sequence over 120 meters and offer the potential to reconstruct a continuous section spanning from the Bédoulian to the Gargasian.
Paleontological interest
The grey marls around Apt were proposed by Alcide d’Orbigny as the reference for the Aptian stage due to their rich paleontological content.
About thirty different species of ammonites have been recorded in the La Tuilière sections. Belemnite rostra are very abundant in certain layers, along with gastropods and bivalves (oysters and plicatulids). These marls also contain corals, sea urchins, brachiopods, and shark teeth. Fourteen species of selachians have been described, including two new species. Microfossils (ostracods and foraminifera) are abundant and diverse, allowing for precise dating of these layers.
The entire fossil assemblage also provides valuable information on paleoecology and paleogeography.
Fossil Characteristics
After a storm or at the end of winter, the marl outcrops reveal a multitude of small fossils. Because the marly outcrops are easily eroded, fossils are visible on the surface, loose, and can be easily collected. However, excessive digging and collecting can damage the scientific value of these outcrops.
These fossils have some special features that make them especially sought after: excellent preservation and pyritization, particularly of ammonites.
Another notable characteristic of this fossil fauna is the small size of the organisms, especially the ammonites. For a long time, some authors hypothesized that this dwarfism was biological, whereas it may simply be due to the mode of fossilization and shell preservation. The ammonites found might not be entire individuals but rather their nuclei. The final chambers, more easily infiltrated by sediment, are more fragile and remain compacted within the marls before being destroyed. Ammonites from the marl-limestone Bédoulian levels show normal sizes.
As for the size of other fossils in the marls, they appear normal and consistent with the living environment of these organisms, similar to current shallow seagrass beds.
L'ESSENTIEL
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L’Aptien est un étage de l’échelle internationale des temps géologiques qui s’étend entre -125 et -113 millions d’années.
CONTACTEZ-NOUS
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60, place Jean Jaurès – 84400 Apt
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