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.

Sebastien pointing out the 5 different units at the first stop
1: Sebastien pointing out the 5 different units at the first stop
Infra-cantalian basalt with many xenoliths
2: Infra-cantalian basalt with many xenoliths

Stop 1: Roadcut next to Lavessière, Murat

We stopped in front of the first expression of the volcanism, which occurred just above the basement. We could observe 5 different units as shown on photo 1. The lower (B1), 5 m thick unit, reddish in colour with prisms characteristic of a basaltic lava flow. Above is a red bed of soil (S) rich in clay, then another massive dark basalt. These two basaltic lava flows are part of the infra-cantalian volcanic activity, which was going on during the Miocene. Thanks to gravity we could have a good look to large and fresh specimen at the foot of the outcrop (photo 2). They contain xenoliths of rusty peridotite and granite as well as they olivines (rusty dots) and a zeolite, chabanite (white mineral). The orange colour of the olivines tells us that they are magnesium-rich, the rock would most probably be alcaline, thus a basanite rather than a basalt.

On top of these basaltic units are white lenses (D) of diatomite: lacustrine siliceous deposit. They are Oligocene in age. We saw more diatomates the following day in a quarry next to Murat. From where we stood, we could easily distinguish in the lenses smaller faults that are typical of compaction.

The top unit (Br) is a breccia composed of angular clasts, all with a similar colour. They are trachyandesite dated between 9.3 Ma to 8.1 Ma, and are consistent with a dome collapse breccia. They are not part of the infra-cantalian volcanism, but relate to the Paleo-Cantal stage, which lasted more than 1 Ma and during which numerous trachyandesitic domes protruded and collapsed; they were the first Cantal edifices.

Stop 2: Andelat church

We made our way to a 12-15th century church in a small village called Andelat, which was built with blocs quarried in infra-cantalian volcanics (photo 3). Except around the windows, where the grey blocs are basalts with little phenocrysts, the church is made of blocs quarried behind the village in a strombolian deposit. We could observe, layers of ash and lapilli, small bombs, angular clasts of lava, basement rock and quartz grains and calcite deposits within vesicules.

Church in Andelat; infra-cantalian volcanics; the grey blocks are massive basalt and the lighter coloured ones are strombolian deposits
3: Church in Andelat; infra-cantalian volcanics; the grey blocks are massive basalt and the lighter coloured ones are strombolian deposits
Close-up of the Andelat church wall: infra-cantalian strombolian deposits
4: Close-up of the Andelat church wall: infra-cantalian strombolian deposits

Stop 3: Le Sailhant, cascade de Barbory and infra-cantalian lava flow (10 Ma)

It was nearly lunchtime and we decided for a very scenic place: the cascade next to the small village of Le Saillant. From the central car park, we walked along a small, well-indicated path that runs at the foot of a cliff propping up a medieval castle. Next to the water pool at the foot of the cascade, we unwrapped our pique-nique.

It was also a good place to tell us about the classical structure of a large lava flow, photo 6. We could see above that little lake the top part of the colonnade and the entablature.

Lunch at the cascade de Sailhant
5: Lunch at the cascade de Sailhant
Classic lava flow picture
6: Classic lava flow picture
7: Infra-cantalian lava flow at St Flour

But now it was time for a cup of tea or coffee in Saint Flour.

Stop 4: Saint Flour, lava flow (9-10 Ma)

It was a quick stop, just to admire the upper part of a large lava flow, which has been dated 9-10 Ma with K-Ar. We took a picture, had a drink and moved on.

Neussargues lava lake
8: Neussargues lava lake

Stop 5: Neussargues, former lava lake in the Alagnon valley (5 Ma)

On our way we passed one of the most beautiful former lava lakes in the Cantal. So we had another quick stop north of the village Neussargues at the junction between the N122 and D679.

A geo-viewpoint has been built there with a few indications so that people would know what to look at. What we saw northwards was a high cliff on which we could distinguish a columnar pattern (photo 8). The 3D geometry of that pattern was evidence of lava cooling in a large depression. Although the light was not too good we still could observe ruiniform features on top of the cliff; they are the remains of scoria cone deposits that have been ejected from an edifice located further north.

To the right of the cliff, we can see layers of more volcanoclastic deposits and lava flows. We were going later that day (stop 8) to have a closer look at them. The lake and the other volcanic deposits are all dated around 5 Ma and belong to the younger events that occurred in the Cantal. Between the infra-cantalian flows and these basalt flows, many dramatic events have occurred: the formation of a large stratovolcano!

Fractured Hercynian basement
9: Fractured Hercynian basement

Stop 6: Fractures, faults and basement rock along the N122

The rock has been identified as a shistose anatexite with sillimanite, which translates into i) melting of the crust producing magma, ii) crystallisation, iii) metamorphism into schist; the presence of sillimanite indicates high-grade metamorphism.

The fractures within the basement rock of Hercynian (= Variscan) age are important feature regarding volcanism, as they control the ascent of the magma. Their direction constrains as well the shape of the 6x10 km wide caldera. Here we could measure a NW-SE trend, which is the main Hercynian trend. Sebastien mentioned that next to the town of Murat, 3 volcanic necks, remains of magma conduits, are aligned in that direction. In the Cantal area there are 3 main fault directions: i) the Hercynian NW-SE trend (more precisely N135), ii) the N45° direction related to the Alpine formation, iii) the N-S direction of the Oligocene basins, like the Limagne and the Bresse. An earlier, major Permian fault system runs on the western side of the Massif Central with N45 direction; because coal has formed in pull-apart basins along that fault system, it's named "Le Sillon Houiller". On its western side there is no Cenozoic volcanism at all.

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.

Features observed in the cliffs above Neussargues: a) bomb impact, b) ash dune formed by wind, c) palagonite, a yellow deposit resulting from alteration of basaltic glass with water, d) characteristic weathering pattern of basanite
10: Features observed in the cliffs above Neussargues: a) bomb impact, b) ash dune formed by wind, c) palagonite, a yellow deposit resulting from alteration of basaltic glass with water, d) characteristic weathering pattern of basanite

Stop 8: Top of the scoria deposits seen from Neussargues (5 Ma)

These basaltic deposits, flows and scoria, have been emplaced at the very late stage of the Cantal volcanic events and form lava plateau; they are locally named "Planeze" i.e. very flat surface.

Indeed, the top where we left the cars was a very flat plateau. From there, in the summer evening sunlight allowing a clear colourful view over the Alagnon valley, we walked along these deposits towards an outcrop Sebastien had located during his fieldwork. There we could see in the scoria deposits features like bomb impact, welded lapilli, cooked soil and wind dunes (photo 10). He also pointed out a peculiar weathering pattern in basic igneous rocks that characterizes basanite.

And on our way back we spotted a yellow deposit that is an alteration product named palagonite, resulting from interaction between water and basaltic glass. It was worth the trouble to walk across rough terrain.

Eastward view from the last stop, with the last sunshine of the day highlighting the top of the planèze
11: Eastward view from the last stop, with the last sunshine of the day highlighting the top of the planèze

Return to Trielle

After a long, very interesting day, during which we met the oldest and the youngest volcanics, both basaltic, as well as the old Hercynian basement, we headed back to La Ferme de Trielle where our evening gastronomic meal waited for us.

Text and Pictures:

Elisabeth d’Eyrames

 

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.

Breccia near St Jacques des Blats
1: Breccia near St Jacques des Blats

This breccia belongs to the lower trachyandesitic complex, formed between 9.3 to 8.1 Ma. It has a high clast proportion of up to 80%. These clasts are quite angular, and display a homogeneous dark to medium light grey colour. There is no evidence of bedding. The large size and angularity of the clasts suggests that we are near their source.

How the caldera evolved
2: How the caldera evolved

In terms of texture the clasts are vesicular, with needles and long grains of pyroxene as well as potassium feldspar and plagioclase. This shows their origin as an evolved lava. This trachyandesite can be found in many sites of the Cantal. More viscous than basalt, it can form lava flows as well as domes.

This applies here, where we are at the foot of a collapsed trachyandesite dome, a frequent scenario for the Cantal caldera where the Hercynian basement is weakened by “keyboard” type faults. These cause an accumulation of domes consisting of a lava from shallow magma chambers, and emptying magma chambers create a void.

This volcano tectonic condition is linked to the main three regional tectonic directions.

Indeed the absence of bedding structure or of any regular slope arrangement rules out a strato volcano hypothesis. Here, the tops of deposits follow the relief of collapsed “keyboard”.

Breccia at La Gazelle Bridge
3: Breccia at La Gazelle Bridge

For our second study site we headed north after the town of Murat to reach the valley of the Santoire river, on to a roadside outcrop at a locality named La Gazelle Bridge, near the small town of Ségur-les-Villas.

We are at a distance of 10 to 12 kms North of the center of the volcano, which is the origin of this 7.9 Ma old volcaniclastic breccia. The breccia is highly heterogeneous and heterometric, with clasts supported by a light colour yellowish matrix. The volume proportions are about 50:50 clast to matrix, with between 10 and 20% small pumice. The largest clast reaches nearly 10 metre in diameter. It stands high in what must have been quite a viscous groundmass. There are also some juvenile pumice from fresh magma.

This site is of particular significance as a prime example of a breccia which can be found as far as 25 kms away from the Cantal volcano centre, in all directions and, to the East, under the overlying planèzes. These breccia have been interpreted as Debris Avalanche Deposits already by early local geologists, a view reinforced in the 1980s by the Mount St Helen eruption and collapse, which had carried debris as far as 22 kms away from the source. This model then gained strength and popularity in its application to the Cantal, and the region was full of DADs (Debris Avalanche Deposits).

However, there are limits to the extent where the Mt St Helen model could apply here. First, the Mt St Helen collapse was in a single direction, which is not the case here. To apply the Mt St Helen model with breccia in all directions would require another seven volcanoes to form and collapse.

Secondly, in the situation of a massive collapse of an edifice resulting in a debris avalanche, gravity is the main factor of deposition, gases can also be involved, but water is not present. As a result, clasts interact and are very highly fragmented, again differing from what we observe now.

A discussion on the water content of a flow and its impact supports the alternative scenario of debris flow. Modelling shows the following characteristics:

> If the water content of a flow is very high (60 to 99%), conditions are fluvial, and turbulence translates into clasts interaction and fractures, which does not apply here.

> In lahar conditions (30 to 60% water) the deposit would show stratification and grain sorting, not to be observed in this outcrop.

> But with a 0 to 20 or 30% water content, i. e. a debris flow situation, clasts are dispersed, the clast percentage can be as high as 70 %, clasts are deposited en masse and are not fractured; some lenses can be seen, but no stratification.

This indeed corresponds to the local outcrop. As to where the water came from, if the Cantal volcano-tectonics result in a caldera situation, indeed visible in the overall geometry of the Cantal volcano, water will accumulate in the caldera.

So the volcano formation over several million years could have followed such a pattern: Trachytic magma, rising along existing Hercynian faults of the caldera floor, injects into the caldera which fills rising, over the edges of the caldera, the overflow incorporating clasts of all sorts including basement granites, trachyandesites, and others. The matrix acts as cement, with occasional wood fragments.

Varve in diatomite block
4: Varve in diatomite block

Diatomite

Our next station was at one of the main quarries for diatomite in France. This is located at Foufouilloux, near Murat. The formation results from lacustrine sedimentation by a unicellular silicic plankton depositing in a maar left after a magmatic eruption. The process lasted about 50 000 years and now fills a depression down to a depth of 30 meters at its maximum. Alternating layers of light and dark grey correspond to varves, layers illustrating a succession of seasons, with a light and dark pair summing up one year.

This material is extremely fine-grained and powdery, at between 5 and 50 μm, but it is also very rich in fossils of leaves and other organic materials. There are a number of other diatomite sites on the Cantal planèzes and their magmatic edifice, with frequent tension between environmental and quarrying interests: It is characteristic of such deposits to cause marshy zones which are attractive to rich and varied fauna, making them important fro conservation.

But diatomite is in great demand: After burning and sorting by granulometry, diatomites enter many sectors of industry, as different as cosmetics, pharmaceuticals, filtration processes applied to used oil recycling. It is also very important for the food and beverage industry, including filtering of wine and beer.

Passadou Dyke
5: Passadou Dyke

The Passadou Dyke

After a short drive west along the N122 we reached the village of Fraysse Haut and drove off to a side road near Maison du Buronnier. Just beyond, and under a high 19th century railway bridge, we find the Passadou Dyke, a perfect lava wall, a feeder of a basaltic planèze.

In this position, with a North 45 degrees direction, it leads up to the Plomb du Cantal and the Eastern limit of the caldera, as a feeder edge between the caldera and its external slopes.

Other dykes have been found in the two directions of N 45 and N 135 degrees, which are the main directions of the Oligocene basin.

In the more central parts, though, they consist of a more evolved magma, with more fractures, and showing more successive magma injections.

A tuff slope and lots of pumice from a plinian eruption on one edge, and fragments of a pyroclastic flow from a fragmented wall show previous volcanic activity which was cut by the dyke.

The dyke reaches a height of 1300 meters, which corresponds to the maximum height of most basaltic planèze, at 1400 meters.

Pas de Cère breccia including whitish limestone observed at an approximate distance of 3m
6: Pas de Cère breccia including whitish limestone observed at an approximate distance of 3m
Megablock at Pas de Cère, from observation post about 20m away
7: Megablock at Pas de Cère, from observation post about 20m away

Pas de Cère

Our next observation point is located further South-West in the Cère Valley, at a locality named Pas de Cère. An 8 Ma volcaniclastic breccia with very mixed clasts includes even a whitish limestone. This massive flow locally reaches a thickness of 30 meters. We are now 7 kms away from the volcano center. This corresponds to what we saw to the North East atLa Gazelle, but here we are facing a massive 100 meter long megablock in this proximal deposit.

Debris flow make up an estimated volume of 100 cubic kms in the Cantal volcano region. As base to the planèzes which form a radial pattern around the central caldera, these debris flows offered a fairly smooth surface, which resulted in the smooth relief of the planèzes. Such smooth surface again tends to disprove the theory of catastrophic avalanche debris.

Faillitoux Cascade, 2m from the closest prisms
8: Faillitoux Cascade, 2m from the closest prisms

Faillitoux Cascade

Back at Thiezac, the beautiful Faillitoux Cascade digs its way in a thick prismatic lava flow. Its prisms are perfectly shaped, many lying on the foot of the cascade. This rock is dark and very rich in wellshaped phenocrysts of olivine and augite. This is Ankaramite, formed 9 Ma ago from fractionation of a lava of alkali basalt affinity.

One of the great challenges of the Cantal volcano lies in the interpretation of its varied lava characters. Lavas of both alkaline series, typical of hot-spots, and intermediate series typical of rifting zones, can be found.

In the course of an analysis of the Cantal conducted in the mid 90s, Leeds University Professor Marjorie Wilson had taken five different samples in the Massif Central to study the magmatic mechanisms giving rise to such variations in compositions. The conclusion was a hypothesis involving different degrees of partial melting in the lower crust, in a model figuring some “hot fingers” rising from the low velocity zone, like mini mantle plumes.

In 2001, researchers from Clermont-Ferrand University offered another hypothesis involving a fragmentation of a Tethys tectonic microplate in subduction under the Alps.

The lava’s chemistry does not provide any conclusive evidence. Neither could the opening of the Atlantic in 150 Ma offer any explanation as the lavas do not match the North Atlantic Igneous Province.

We can only observe that volcanism is oldest in the Cantal, younging in the three directions of North (Chaine des Puys), South (Aubrac) and East (Forez), like a triple point, with the Sillon Houillier forming a deep-seated limit to the West.

Text and Pictures:

Brigitte Revol McDonald

 

Simplified geological map and section of the Cantal volcanic complex, Leibrandt 2011
1: Simplified geological map and section of the Cantal volcanic complex, Leibrandt 2011

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.

2: View over the valley de la Cère and the Puy Griou, 1690m

Stop1: point of view over the Cère Valley and Puy Griou

On the road from Saint Jacques des Blats towards the col de Pertus on the D317 road, we stopped at a viewpoint with orientation table to admire the Puy Griou standing up over the Cere Valley.

The valley has been shaped by glaciers and eventually by a river. Puy Griou is a phonolite dome dated at 6.60 Ma, with a little basalt on top, and trachyte pyroclastics beneath. Like within the entire central complex, the two different magma series, the intermediate (trachyte) and the alkaline (phonolite) coexist. It is also worth noticing that rocks belonging to both types of magma series can be found on the Mont Dore, the other large stratovolcano in the Auvergne.

Often it is said that the alkaline serie is associated with hot spot and continental rifting, and the intermediate or calc-alkaline serie is associated with island arc settings.

This doesn't make it easy to find a comprehensive model for the origin of the magmatism in this area; very exiting though for future studies.

Stop 2: view from the col de Pertus into the Jordanne valley

A stunning view waited for us at the Col Pertus (see picture 3 below). We first attempted to draw the landscape; the major features from left to right are first the westwards gently dipping (5°-10°) basaltic layers (β), then a pass with trachyandesite (τα), then again the Puy de Passierou with basalt (β). The Puy is followed to the right by horizontal layers of trachyandesitic volcanics, and then it is getting a little confused up to the Puy Mary (trachyte τ), and again more horizontal (τα) layers to the right.

It might not be obvious on the photo, but we could clearly see dykes on Puy Mary. In the foreground there is a ridge of trachyandesitic rocks pointing towards us, called " les roches folles"; they correspond to the flanks of former glaciers. There is no basalt in the caldera apart from very small patches (next to the Puy Griou, on top of the Plomb du Cantal). The Puy Bassierou is a feeder of the basalts flowing away from the caldera.

In the caldera the layers are not always horizontal, as a whole range of different structures exist: domes, protrusions, dome collapse breccia, flow domes, lahars… and they overlap each other.

After having spent quite some time at this scenic place, we decided that a coffee or a tea would be nice, so we headed to the little town of Mandailles.

Panoramic view from the Col de Pertus westwards into the Jordanne valley
3: Panoramic view from the Col de Pertus westwards into the Jordanne valley
Dyke cutting through volcanic tuff and trachyte, Cantal Upper Complex
4: Dyke cutting through volcanic tuff and trachyte, Cantal Upper Complex
Dome collapse breccia, Cantal Upper Complex
5: Dome collapse breccia, Cantal Upper Complex

Stop3: various deposits in the Upper Complex

From Mandailles we took the D17 road towards the Puy Mary and stopped along the road at 2 km from the top (altitude 1525 m). First we looked at a small dyke that has cut through volcanic tuff overlain by a massive trachyte (photo 4). In the trachyte we could recognise Amphibole, K-Feldspar and Biotite.

The lighter coloured tuff contains minerals, pumice and ash.

A little further down the road, 100 m, we could recognise a breccia (photo 5): grey angular clasts in a grey matrix, both are trachyandesite. A crude steep westwards layering can be observed. Although very different in age, the deposit is similar to the one we saw previously in paleo-cantalian deposits (1st stop on Tuesday). This deposit is consistent with dome collapse breccia, although it could be argued that it is a witness of a scar left by the collapse of a flank of a stratovolcanic edifice. But there is evidence of horizontal layers above and below, which rules out that hypothesis.

We went back to the cars encouraging cyclists riding up bravely to the Col de Redondet; the Tour de France passes every year through the Cantal; therefore the roads are in a perfect state.

Dyke at Col de Redondet
6: Dyke

Stop 4: Col de Redondet

We took a path to the West and 400 m further we encountered a 4 m wide dyke. In the rock we could clearly see nepheline, white and rectangular minerals. It is a feldspathoid that is typical of phonolite (φ). The alkaline magma that intruded that place is less visqueous than the calc-alkaline (=intermediate) magma that produces trachyte. This dyke is aligned in a N135 direction (Hercynian trend) with the Puy Griou, the Griounou, located just westwards to it, and the Roc d'Hoziere. They are all phonolite (φ). Usually it is possible to make the phonolite cling. But this one doesn't do the trick; it might be a little altered.

Stop 5: Col du Pas de Peyrol, alt. 1589m

The Col du Pas de Peyrol is a very touristic place; no wonder, easy access and a fantastic view by clear weather. We could see on the skyline (photo 7) a trachyandesitic (τα) lava flow next to the phonolitic (φ) Roc d'Hoziere, the col we just left and a beautifull view over the trachytic (τ) Puy Mary (1783 m). We mentioned that the Roc d'Hoziere was aligned with the Puy Griou, both are phonolitic, but there are more basement xenolites in the Roc d'Hoziere, which indicates different magma chambers. Also the dating of samples from Puy Griou gives 6.60 Ma, a younger age of than for the Roc d'Hoziere, which has been dated around 7.70 Ma. At the right side of the panorama the basaltic (β) planeze extends its flat slightly dipping (5-10°) surface westward constraining the limit of the caldera located under the Upper Complex.

Panorama from Col du Pas de Peyrol
7: Panorama from Col du Pas de Peyrol
Pyroclastic flow deposit at the Col du Pas de Peyrol, the Puy Mary stands in the background
8: Pyroclastic flow deposit at the Col du Pas de Peyrol, the Puy Mary stands in the background

Once we reached the bar at the Pass, some decided for a drink, some decided to climb first to the top of the Puy Mary (alt. 1783m).

Before we left we had a look at an outcrop situated just behind the clean public toilettes (photo 8). The clasts have the same colour than the matrix. The rock has been pulverised: there are free crystals of large K-feldspar (sanidine), biotite, amphibole baguettes, no quartz; it is typical mineral assemblage of trachyte.

Sebastien mentioned that in the Cantal the intermediate magma never evolved into rhyolite. There are some xenolites and the matrix is very friable, and easily eroded. This is a deposit of a pyroclastic flow, or nuee ardente as we like to say in French, which resulted from a dome explosion. There are also several (τ) dykes that cut through the Puy Mary. Earlier in the morning we spotted two of them from the Col du Pertus. Sebastien has dated one at 2.5 Ma, much younger than the 6.5 Ma of samples dated previously. A dyke can be seen on photo 8 on the left of the road panel. Leaving the Pas de Peyrol on the D680 road, we drove down 1.5 km into the valley of the "Petite Rhue" to our last stop and Mountain View of the day.

View over "Brèche de Roland", a notch in horizontal τα layers
9: View over "Brèche de Roland", a notch in horizontal τα layers

Stop 6: Col d'Eylac, the Puy de la Tourte and further North

At the Col d'Eylac we had an open view over the whole valley, from the pass we just left, right down the valley, out of the Cantal Volcanic complex. Standing on τ rocks, we looked into the cul-de-sac of the valley (photo 9), which is a cirque that cuts through horizontal trachyandesitic (τα) layers of the Upper complex. A notch in the middle of this formation is called "Breche de Roland". The name refers to the French historical legend of Roland who is said that he cut that breach with his sword Durandal in an attempt to destroy it after he has been defeated.

Here people pasted/copied the legend, as this action was meant to be set in the Pyrenees; the story fits so well with this notch.

Location of the edge of the caldera according to the nature of the rocks and variation in the dips
10: Location of the edge of the caldera according to the nature of the rocks and variation in the dips

On the western side of the valley, from the Col we just left, the highest point is the Puy de la Tourte with 20° Northwards dipping layers that can be distinguished in the shadow of the evening sun. To the right of the Puy de la Tourte, there are more horizontal layers, and Northwest we saw on the skyline the top of the planeze with a 10° dip. (photo 10)

Sebastien deciphered the important features we had before our eyes: the horizontal τα deposits indicate fill-up on flattened surfaces with products from dome activity; they are bound by the fault system at the edge of the caldera.

The diagram, photo 10, shows this interpretation, which argues for a proper caldera structure. The τα layers under the basaltic (β) lava flow, are overflows of the caldera which settled on the top of the large breccia flow.

After that long and exiting day, we drove to our next accomodation, the refuge de prat de Bouc, right at the foot of the highest pic in the Cantal, the Plomb du Cantal (alt. 1855 m).

The Cantal volcano conceals still many mysteries, like what is doing a small basaltic patch right at the top of the Plomb du Cantal and dated at 3.4 MA, much younger than the basaltic planezes that surround the caldera?

There is still a lot of scope to do geological studies. Then in a few years, we will have to come back.

Text and pictures:

Elisabeth d’Eyrames

View over the Plomb du Cantal, on the right side flat dipping planèze, between is the edge of the caldera, the houses are at the Col de Prat de Bouc
11: View over the Plomb du Cantal, on the right side flat dipping planèze, between is the edge of the caldera, the houses are at the Col de Prat de Bouc
The village of Apchon
1: The village of Apchon
Building stones of the church in Apchon
2: Building stones of the church in Apchon

Friday, 23 September 2016

It was a fine, sunny day again when we set out for the last time. Our first goal was the village of Apchon clustered around two hills that rise above the planèzes, one crowned with a ruined castle about to be repaired.

This is the Neck of Apchon while the other hill was simply created by a lava flow. Sébastien explained to us that despite its shape it is no dome - which requires more alkali-rich feldspar to endure for a longer period - but an eroded scoria cone containing a neck of basaltic columns that formed ~5 million years ago. An eruption must have produced a lahar visible in the building stones of the village church.

In Petite Rhue valley below Apchon we stopped at a roadside exposure where basaltic columns were forming the sides of a triangle. They formed ~5Ma ago perpendicular to the cooling. I found this too exciting and was too engrossed in marvelling to record any further explanations.

Petite Rhue valley, left side of basaltic columns
3: Petite Rhue valley, left side of basaltic columns
Petite Rhue valley, right side of basaltic columns
4: Petite Rhue valley, right side of basaltic columns
Roche moutonnée
5: Roche moutonnée

When we reached the bottom of the valley Sébastien showed us one of the few places where granites of the Cantal basement are exposed. These rocks were smoothed by a glacier to form roche moutonnée, a word derived from the french word for sheep. So, roche moutonnée could mean rocks looking like sheep lying in the grass but it was probably coined for the smoothing of the wigs fashionable in the late 18th century with mutton fat.

Quarry near Suc de Rochemonteix
6: Quarry near Suc de Rochemonteix

We visited a former quarry near Suc de Rochemonteix. Here we found different lava flows, age ~5 Ma, with flat surfaces between them and alteration colours in the bump-like lower part. It has not yet been resolved if these features represent a lava lake or stacks of lava flows. The basalts here were full of peridotites but we found also very weathered basanites.

After enjoying a coffee at Cheylade we had our more than ample picnic at Font Sainte, meaning Holy Well. Walking a litte further uphill gave us an excellent view over the elevations build by volcanism from the oldest (Plateau de Cézallier, 3-6 Ma) to the youngest at Mont Cineyre (~5000 years, probably younger). Here Sébastien explained for the last time his theory of the Cantal being a caldera and not a series of lava flows.

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.

Text and pictures:

Gisela Lunkwitz