Trip Report

(Lynn Everson)

Part 1

The story of our trip to the French Jura is one of limestone, water and wine. Elisabeth D’Eyrames put together a six-day programme of trips radiating from Arbois, her home town, which illustrated the interconnectedness of these three themes. Each trip had a new leader and so the week was one of immense variety. Elisabeth not only arranged the trips but also acted as translator for those amongst us whose French had got a bit rusty. And she arranged great evening meals and barbecues for the whole group in her garden. I imagine that at the end of the week she probably felt the need to hibernate for the rest of the year!

Since the Jura is the type region for the Jurassic, it should not be surprising to hear that 3 out of the 6 days were almost entirely devoted to exploring limestone.

Our leader on the first day was the distinguished Prof. P Chauve, the author of a geological guide of the Jura. He is a specialist in the structural geology of limestone.

We started out at Le Cirque du Fer à Cheval near Reculée des Planches. Like so many of the terms we were introduced to, I don’t know the English word for ‘reculée’ (dead-end valley). A reculée forms when a limestone layer overlies a layer of marl. A stream will erode and remove the limestone, eating back an arena-shaped area in the upper layer so that cliffs surround the flat floor of underlying impermeable marl. Then the whole arena structure is known as a reculée.

For the rest of the day the professor took us on a journey across the short axis of the Jura. He started on the small scale, by leading us through a geological mapping exercise of the fault graben around a village called Montrond, near Champagnole. As we travelled further he showed us different tectonic styles and everlarger geological structures until we reached the grand finale overlooking the great kilometre-scale mega-cliffs of the Flumen Gorge at Roche Blanche. I seem to remember spontaneous applause breaking out as these came into view, but maybe it was just that I personally felt like applauding, for the sheer dramatic effect.

 

Picture 1: Roche Blanche

Mapping the graben at Montrond had another significance, apart from demonstrating how the smaller structures relate to the larger ones they are part of. It was an introduction to the concept of a 'faisceau’ (fasces or bundle in English), which was a word new to me that cropped up time and again throughout our wanderings. Rather than meaning a sedimentary facies, I think it would be better translated as a large-scale folded sedimentary complex. The down-faulted block of this graben at Montrond was part of the Faisceau of Syam. After the initial, extensional, graben-forming stage the block had been compressed and folded. Then a third event had seen the graben sliced through and offset laterally by a strike-slip fault.

The Jura region is roughly banana-shaped, surrounded by depressions. Our journey took us from northwest to south east across the short axis of the banana so that we encountered the three longitudinal sub units of the Jura region – the outermost 'faisceau’ (furthest from the centre of the orogeny), the middle plateau and the innermost folded subalpine high Jura.

 

Picture 2: Road cutting

From Montrond, we moved on to investigate a large fold structure in a road cutting by a tunnel near Sirod. This structure explained more about the formation of the Faisceau de Syam. We were told a story of faults that formed in the Oligocene and were reactivated in the opposite sense during the mid Pliocene by a second event that pushed the Jura over into the Plain of Bresse. I’m sorry to say that the details of this story escaped me, but I have to mention it as Annette has a good photo! Several of the geological sites had interpretation boards and an amusing one was to be found at ‘Le Chapeau de Gendarme’. The gendarme’s hat was the name for a local beauty spot, a cross section through an anticline in the shape of a hat, visible beside a waterfall. It seemed quite imposing as we stood close by, but at the end of the afternoon it looked very tiny and was only just visible when we viewed from Roche Blanche the grand sweep of the hillside of which it formed part.

 

Picture 3: Chapeau de Gendarme

Picture 4: Chapeau de Gendarme

On the second day, too, we concerned ourselves with limestone, but this was more ‘noses on the rocks’, upclose geology. Prof. Davaud of the University of Geneva had us crawling over limestone scree on the Prapont hillsides, along the Samine river valley near St. Germain de Joux, to inspect the reef and lagoonalperitidal carbonates from the upper Kimmeridgian and the lower Berriasian of the western Jura.

It seemed so strange to hear the names Kimmeridgian, Purbeckian, Oxfordian etc., so far from their English counterparts. We have become accustomed to vast geological timescales as they appear in the shallow vertical record, but often forget the enormity of the lateral extent of some formations. However, it appears that the English and French definitions of the terms do differ.

We looked for the development and eustatic control of an Upper Jurassic reef complex. Here were fine examples of transgressive cycles, maximum flooding surfaces and sequence boundaries; great for those of us who have studied S338 or it’s successor S369.

 

Picture 5: group on limestone scree

The hillside was composed of core reefs and flanks. The first reef we saw shallowed up into beach deposits. At the top of this reef, pore spaces caused by shrinkage and evidence for mixed brackish and salt water suggested emergence. Overlying this there was a black pebble conglomerate topped by microbial mats, the base of which is believed to represent the new sequence boundary (according to a study in our handouts by Dr. E. Fookes, University of Geneva). That afternoon we moved laterally from the reef facies into the lacustrine, coccolith mudstone. There were no other fossils preserved in life position, but there was up to 10% organic matter in some levels and it is the decomposition of this that had made the water anoxic. The environment of deposition for these muds has been interpreted as seasonal lacustrine brines that became oxic in winter, when there was less organic stuff entering the system.

This facies is likely to have been a protected lagoon, bordered all around by reefs, like in an atoll. We split open the rocks, looking for plant remains, which were dark brown and easy to see against the pale mudstone. There were occasional fish remains too. That night we stayed in a ski lodge called La Fauconière near Giron, in the high Jura, not far from Geneva. The day had been hot and sunny like the day before, 29 °C, and so the first thing I did when we arrived was to plunge into the hotel’s outdoor swimming pool. Bliss! No time to linger, though, as it was closing before dinner, in a few minutes time.

Then the third day dawned sunny and bright again. This time we were led by Christian Gourrat, a friend of Peter Skelton, so familiar to many of us from our OU studies. The first stop was beneath the Cirque Glaciale Fauconière, in the Oyonnax valley, where we searched for beautiful ammonites and other fossils that were plentiful in the bed of a stream.

After that we moved on to the entrance of a working quarry. There were 20 miles of tunnels in the quarry, but we stayed outside and inspected more corals and other reef fossils in the Upper Jurassic strata at the entrance. The limestone here was 99.99% pure calcium carbonate, with very little iron content and it used to be added to white bread, but is now used for toothpaste, paint, concrete, etc.

Then Christian took us to a small exposure of a reef in the woods near Marchon, a short drive from Oyonnax. He and Peter Skelton are researching the rudist bivalves that lived in the corals here. The shells of the rudists were not all the same shape – some had shells like ice-cream cones and some were twisted. However, the shape was not characteristic of different species. They were all oriented in the same direction and this gives an indication of the current direction. They were suspension feeders. The limestone here was peloidal, rather than oolitic.

We had lunch on the patio at Christian’s house in Marchon and drank wine and kir while munching our food. Then Christian showed us his workroom, which housed a superb collection of fossils of all sorts and sizes. He also demonstrated the stages in the preparation of fossils for study or display. These involved using a wooden chisel to hammer gently at the surface of the fossil to chip away the matrix and then using a pneumatic drill to expose the fine detail. I think that the final stage involved the use of a highly diluted hydrochloric acid solution – but please don’t try any of this unless you have confirmation from another source that it is correct, because I didn’t write it down and don’t remember the details, I’m afraid.

After this we took our leave of Christian and started our return journey to Arbois, stopping on the N78 at Pont de Poitte to gaze at the ‘marmites’ (cooking pots) in the bed of a river. These were depressions that had been excavated by water arriving in a fast narrow stream, then spreading out over a shallower area and expending its energy by stirring stones around in whirlpools, thereby excavating the pots. It was a beautiful scene, with people swimming in the shallow water and fishing and enjoying the sunshine.

Next was a stop near Montmorot to view a panorama above Lons-le-Saunier. We could see the main features of the tectonic style of part of the outer chain of the Jura (the Faisceau le Donien in Revermont). This stretches from Orgelet to Lons. During the Tertiary Period the faisceau was carried for 7km along a decollement surface over the crystalline basement towards the Plain of Bresse. In the group photo taken here you can see another new geological interpretation board that has been put in place by students at the local agricultural college.

 

Picture 6: group photo Isabelle Barnier, Elisabeth D'Eyrames, who organised the trip, Sylviane Niot, Jo Brown, Mike Molloy, Ole Nielsen, Stuart Fairley, Jane Hickman, Jo Lakeland, Brigitte Revol-McDonald, and kneeling in front, Hans Kimmich.

Our last stop of the day was at the salt wells known as Les Salines in the town of Salins les Bains. Large drops of rain were starting to fall, forewarning us of the weather to follow over the next two days. We descended into the system of vaults where the pumps are housed. Inside Les Salines the temperature is only 14°C. Natural brine is pumped, using the power of an underground river called La Furieuse, from a depth of 246m up to a boiler house where it used to be evaporated to extract salt. The wells were in operation from the 6^th century until 1962, when it was found to be too expensive to buy coal for the boilers.

The source of the brine is rainwater, which passes through the permeable layers in the limestone, dissolving the salt. Pressure brings the water back to near the surface, carrying 340g of salt in every litre. In the early days the brine was brought up from springs & wells by hand buckets, then horse, then paternoster. Now, a great wheel, turned by the river water drives a 5-horse power beam and pivot pump that that lifts 5,000 litres per hour.

The river water turning the wheel has only 30g salt per litre as the brine gets diluted on its way up. The water is no longer stored in the wells, though. Dilution of the water put back into the river means that there is no pollution from the wells. The salt would all end up in the river anyway. It’s all balanced.

Picture 7: wheel at les Salines

There were 35 different boilers in the middle ages – big iron pans. Vapours from the boilers went back into the boiler room, making it very hot and humid. In the 18^th century life expectancy for the workers was 35 years. Children used to heat rivets and crawled under boilers to take them for repairs.

In 1628 Salins les Bains was the 4th largest town in France and the wells were a walled town within a town because the salt was so valuable. It was needed to preserve food. For a first theft of salt in those days the thief was whipped. For a second offence, he was hung! In the past there were about 60,000 people in Salins, but now there are only 3,300. Our guide was a biochemist who could find no other work in the town. It is no longer known why there is such a large system of stone vaults. All evidence has been lost in fires and invasions. The building stone was quarried in Angoul, 17km away.

There was too much to tell about our visit to the Jura to include it all in a single newsletter article, so a description of the last three days and the other main themes - water and wine (and the soil which makes French wines so special) will follow in due course.

By Lynn Everson