Field trip, stromatolites and volcanism

[marguy]

Since the spring my wife and I have been very busy with the garden and the orchard to maintain, the canned vegetables from our production, and we decided last Thursday to take a break for a geological excursion south of Clermont-Ferrand. The goal was twofold, to search for stromatolites and to take advantage of the volcanic context of the region.

The detailed geological map of this area is unavailable but we had serious research ideas for our prospecting.

The day began with a visit to a magnificent village that immediately immersed us in the context: Saint Saturnin.

The walls of houses often combine basalt and limestone from the surrounding area.

Usually tourists look at the fountain or the castle from a distance, but some are more original ...

So, let's look at some examples in the walls of the houses in the village.

[marguy]

I can't resist the pleasure of showing you the castle and the fountain; and at the base of the castle we can see the front of a lava flow coming from a volcano located 12 km away, dating back a little over 8000 years with a mixture of huge boulders and rock fractured during the advanced basalt flow.

A (very rare) exceptional outcrop (2 meters in length) shows the contact with the ground of the flow time which was organized in prisms under the effect of the heat of the basalt flow above.(prismated paleosoil)

After this volcanic episode, we will try to find outcrops of the Oligocene with stromatolitic formations in the vicinity of the village. It's not obvious in this type of landscape ...

[marguy]

But with perseverance, we find a path whose pavement presents some promising pebbles, then at the edge of a cultivated field a few blocks of limestone ...

In the plowed field there are some pebbles without signs of impact (but many are degraded by agricultural machinery)

Further on, on the edge of the field, a superb plaque of encrustation around stems (missing) is visible but must remain in place (it probably weighs 60kg and we are almost a km from the car! The photo will suffice for our good memories)

After the lunch break, we go to a nearby village where the retaining wall of an embankment shows a fine example of concentric structures, and for a neighboring house it is the rocks that delimit the car parking area that catch our eye.

[marguy]

In the surrounding countryside, we spot a rocky slope well sheltered from view by dense and thorny vegetation ... we will do as the wild boars to approach ... it is difficult to take pictures due to lack of perspective and because of the shadows of the vegetation, but the decor is magnificent.

There is still one site we want to explore, near a village called ‘La roche blanche’ (= the white rock), I wonder why the name!

[marguy]

Unfortunately on this site we will only be able to see the stromatolite layers from afar because landslides have occurred and it is forbidden to walk towards these outcrops, but safety is essential; we see in a meadow a metric block (fallen from the cliff) that has traveled 400 meters before stopping there!

In the pavement of the path, very strange stromatolites… in fact, roots of poplars!

during the way back to home, some volcanic observations, such as this plateau in the landscape showing an inversion of relief: a volcanic flow solidified in a valley, then the basaltic rock resisted erosion better than the surrounding rocks, and this basalt became the dominant relief.

[marguy]

A passage on the Gergovie plateau bearing a Gallic archaeological site

And on the way back, a view of the Puy de Dôme and the volcanic chain of Puys before a final stop at the entrance of a quarry showing a dyke that has withstood the quarry's operating devices and explosives!

In conclusion, it was a busy hot and sunny day.

[FranzBernhard]

Great report, with great landscape, fossils and cultural heritage. Thanks for sharing your tightly packed day with us!

 

8 hours ago, marguy said:

It's not obvious in this type of landscape ...

Looks rather like my preferred hunting ground - there are outcrops, as you have discovered youself .

Franz Bernhard