Field trip to the Cantal
Monday was the first day out it the field. But as usual we all gathered with our Leader, Sébastien Leibrandt, the day before around a gastronomic meal at La Ferme de Trielle in Thiezac. We stayed in this lovely place for the following four nights. Sébastien presented us with a programme designed so that by the end of the week we would have a good understanding of the morphology of the largest volcanic complex in Europe and of the eruptive model he proposes; this new model is largely debated, but is supported by extensive field evidence. We were going to see a large selection of these outcrops and discuss them all along this week.
But first a few figures about the Cantal volcanic complex: it is twice as large as the Etna, (50x70 km), its volume reaches 380 km3, the geographical centre is the Puy Griou, the highest is the Plomb du Cantal (1855m alt.), volcanic activity in the Cantal lasted from 13 Ma to 2 Ma, the activity of the Cantal Massif stricto sensu lasted 6 Ma that is 12 times the usual lifespan of stratovolcanoes.
Monday, 19 September 2016
The plan was to look at basement rocks and volcanics that are the oldest in the Cantal. They only outcrop at the margins of the volcanic complex, where the basement is not too elevated, yet they can be massive lava flows. They are prior to the building of the large Cantal stratovolcano, as such they are referred to as Infra-cantalian (13 Ma - 9.3 Ma). As our daytrip took us close to younger basalts (5 Ma), we went to see them too; it saved some driving.
Stop 7: More Basement rock
On our way to Joursac off the N122 road, we had a quick stop at another basement outcrop to have a look at leucogranite and to keep the Hercynian fans happy. Leucogranite is light coloured granite, containing mainly feldspar, quartz and some muscovite. It is granite that has been crystallised from evolved magmas often involving crustal melting, and is found in areas of continental collision.
Yet the interesting point is that this Hercynian granite, on which the Cantal volcanism has spread, is at 840 m in altitude. In the Northern part of the Cantal, the basement rock is at 1100 m in altitude. The volcanic activity in the Cantal has thus taken place on a high plateau.
Tuesday, 20 September 2016
At Trielle, a clear sunny day begins with a generous hand-warming bowl of coffee or tea, after which we headed for our first station: a Paleo Cantal trachyandesitic unit near St Jacques des Blats in the Cère Valley.
Thursday, 22 September 2016
On the first day of the trip, we had a look at the Variscan basement rocks and at the infra-Cantal basaltic flows that erupted prior to the building of the large Cantal volcanic complex. On Wednesday we went southwards to look at the Cantal basaltic flows that formed the planèzes, a local name for basaltic plateau (blue on the map). On Tuesday we looked at the lower trachyandesitic complex or paleo-Cantal (brown on the map) and at the large breccia deposits above it (orange on the map).
Now on Thursday, we headed towards the exiting central high relief: the Upper Complex (green on the map). We left the ferme of Trielle with luggage, in a clear sunny morning; the weather was spot on, as we were mostly going to look at Mountain Views.
Our last stop had nothing to do with geology: we visited a cheese farm to buy local cheese varieties produced from the cows of the Salers breed. Cheese produced during summer from cows grazing on upland pastures is called Salers while cheese produced at other times it is called Cantal. While the farmer's cats were not interested in visitors, the donkey brayed loudly as if to call attention.
Since we were back early, some more adventurous among us chose whichever hill above the gite they favoured and climbed it. A final generous dinner ended this splendid day and thereby, sadly, the trip.
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