Stage 2 | Meaux-Provins / Riding the Oligocene wheel of Brie

Today’s stage takes us 100m up a flat plateau, a bit like climbing up on a famous Brie cheese (Fig 1). But how does this relate to geology? After yesterday’s warm and wet ride through the subtropical Eocene, our time traveling riders move up to the next younger, and completely different geologic epoch.

Dramatic global climate cooling: From greenhouse to icehouse

The Oligocene started abruptly 34 million years ago with a dramatic global climate cooling from greenhouse to icehouse conditions. The opposite of today’s warming: CO2 levels went down and decreased the greenhouse effect, average temperatures cooled down 5-10°C and the huge Antarctic ice sheet suddenly formed. This completely changed the world. With so much water trapped in the ice sheet, sea levels dropped by 70 meters exposing vast expanses of new land. Lush evergreen forests turned into cold arid deserts with small bushes and pine trees covered mountains. Asian animals, including the largest mammals that ever existed (Fig 2), crossed the newly formed steppa over Eurasian to invade Europe.

Reaching the Oligocene via the Côte de Tigeaux

To reach the Oligocene, the bikers will rise up a stack of sedimentary layers in the biggest climb of the day, the 135m of the “côte de Tigeaux” at km 15.9. Their time travel speed will be about 1 million year younger every 10m up. After leaving below the Eocene strata, they will go through the ‘Calcaire de Brie’, a hard layer deposited in an ancient giant lake extending from Paris to Epernay. Then they will reach the upper crust of the cheese made of the beautiful ‘Sables de Fontainebleau’. These blond and pure sands are from ancient dunes, beaches and sand banks formed when the sea came back into an immense bay larger than the Baie du Mont Saint Michel.

A major fault in the Côte de Tigeaux

The “côte de Tigeaux” also corresponds to a major fault, a huge scar deep in the Earth crust. It was left after a major continental collision between the ancient tectonic plates of Laurussia and Gondwana, forming the supercontinent Pangaea around 400-300 millions years ago. This fault is now all covered by the Oligocene sedimentary layers. It is like a big split that used to separate 2 parts of the table, now joined below the cheese. In the Oligocene, other tectonic plates were moving. The collision of the African and Iberian tectonic plates with Europe formed the Alps and Pyrennes while also opening large gashes (grabens) in the cheese crust (Fig 3). But this did not affect our region much. The deposited layers were not faulted, broken or folded. They remained flat and fortunately are still flat lying today. In fact, the cyclists are riding on the same surface left since the Oligocene time, only the climate has become even colder today and there are very different animals around, some of them riding bikes!

I focus on understanding how geodynamics, climate and life interact during major changes of our Earth evolution. Check the Geo-TdF-team-2022.

Guillaume Dupont-Nivet