Stage 5 | Bar-le-Duc – Saint-Dié-des-Vosges / Back into geological time

Today’s stage leads the peloton through the eastern flanks and the oldest sediments of the Paris Basin. The television screen today will show green fields, but the worlds that existed when the rock formation below the wheels formed were not very green at all.

A stage of 175 km and 106 million years

The stage will start in the upper Jurassic (~145 million years old) near Bar-le-Duc, in the world of the dinosaurs, and from there the riders will travel back through geological time to the lowermost Triassic (251 million years old) near Saint-Dié. This earliest Triassic was a hot, dry world that was struggling to recover from the biggest mass extinction in geological history (the Permian-Triassic boundary). This P-T extinction was even less healthy than the Cretaceous-Tertiary boundary that wiped out the dinosaurs: more than 80% of species on earth went extinct. This mass extinction is aptly known as ‘The Great Dying”, and shows that a series of small changes in climate may eventually culminate in a gigantic catastrophe. But we will return to this later this Tour.

The Germanic Triassic

The Triassic refers to the period in the geological timescale between 251-201 million years ago, sandwiched between the Permian (299-251 Ma) and the Jurassic (201-145 Ma). The name ‘Triassic’ means tripartite, and stems from the three clearly distinguishable rock units in the stratigraphy of northwest Europe: Buntsandstein, Muschelkalk, and Keuper. This subdivision is known as the “Germanic Triassic”. Originally, these three rock formations were even used to indicate geological time, but because they cannot be recognized outside of Europe, this terminology is only used in northwestern Europe these days.

Red sandstone, white limestone, and evaporites

The brightly colored rocks of the Buntsandstein were deposited in a large basin that formed at the eastern edge in the heart of the supercontinent Pangea, in western and central Europe (from France and Spain, Belgium, the Netherlands, Germany, to Poland). The Buntsandstein (’colorful sandstone’ in German) is in France about 300 meters thick and mostly red-colored. This teint is the result of formation of iron oxides (hematite) in the rock. The sandstones formed in a hot, dry climate, whereby sand was transported from surrounding mountain ranges through large rivers and dunes, like in modern deserts. The red Triassic sandstones are very hard and resistant against weathering, and were therefore widely used for construction of important buildings (the Cathedral of Strassbourg, the castel of Haut-Koenigsbourg). 

In the middle Triassic, the basin subsided and connected in southeast Poland to the Tethys Ocean. This led to the influx of seawater, and thick formations of limestones and evaporites formed. Evaporites, like gypsum and rock salt, form by desiccation of sea water, like in the Dead Sea. These formations now seal the porous Buntsandstein. As a result, the porous sandstones became reservoirs for natural gas, and they are now used more and more for the storage of CO2, or hydrogen that is an important alternative energy resource.  

The limestones from the middle Triassic contain abundant fossil shells, and is therefore called in German ‘Muschelkalk’. In teh late Triassic, the sea started to retreat and in extensive packages of evaporates started to form, amidst red continental sandstones from rivers. These are collectively known as the Keuper.

Based on the geology, today may be a good day for a German victory!

I use the magnetic field of the Earth to date the geological record. I am especially fascinated to correlate geological successions across the globe to better understand and unravel the geodynamic and climatic process of the geological past that have continuously changed the paleoenvironmental conditions on Earth.

Wout Krijgsman