- Geological account of the Beaujolais region
Geological account of the Beaujolais region
Regional geology, irrespective of the size of the area in question, from a simple vineyard plot to an entire natural region, is inextricably linked to geology on a wider scale. Understanding the geology of a region involves studying neighbouring territories and rock formations - formations that transcend the usual climactic, topographic, biological and administrative borders.
The term "account," conveys an idea of continuity, a concept that sits awkwardly with geology as the geology of a region is revealed to geologists through rocks. Rocks provide information about specific points in time, about when they were formed, or any transformations they may have undergone. Linking these different pieces of information results in an incomplete account, with numerous gaps.
The Beaujolais region is no exception; the rocks that you encounter here are all relics from ancient times, offering a partial geological account which alternates between very detailed episodes and genuine geological silences.
The oldest rocks in the Beaujolais region are roughly 500 million years old. In terms of human evolution and even the animal kingdom, this seems a very long time ago but when compared to the history of the Earth, 500 million years is relatively recent.
If we consider the geology of the earth, and all the continents, the Beaujolais region is classified as a young land. The region's subsoil provides evidence of a short but very eventful history.
We can divide this history into nine major stages separated by gaps and chronological vacuums about which there is much uncertainty.
In the south of the Beaujolais region, you'll find numerous distorted, rippled rocks just visible above the ground. One type of rock, called mica schist, disintegrates easily into thin sheets; another type, called gneiss, splinters into grains of sand.
These rocks are the oldest in the region and were formed roughly 500 million years ago. When exposed to extreme variations in temperature and pressure, over time - and 500 million years is a certainly a good length of time - these rocks transform from one type of rock into another. This process is called metamorphism.
The old rocks in the Beaujolais region - those that are 310 million years or older - have all undergone metamorphism. The original unmetamorphosed rock is called the "protolith." By way of analogy, bread dough is the protolith of bread which, once baked differs in texture and structure from the initial dough. The protolith of most of the gneiss that you find in the Southern Beaujolais region is granite; while the protolith of mica schist, that you'll find in the vicinity of gneiss, is a type of sedimentary rock.
All these clues help us imagine the environment in which these rocks were formed, in this case, a site with both granite and sedimentary rocks as well as a geographical phenomenon that caused their transformation.
During this period (Cambrian Period), the "primitive" Beaujolais region was probably located on a continental shelf, an area of ocean that suddenly drops off, becoming deeper. The phenomenon known as subduction (where one tectonic plate slides under another) occurs at the level of the continental shelves, generating stresses that are conducive to metamorphosis. In fact, this type of environment brings together a combination of certain rocks and stresses associated with the formation of gneiss and mica schist.
Our knowledge about the movement of continents enables us to model the position and morphology of continents over the last 600 million years. During the Cambrian Period, these models place Beaujolais not far from the South Pole.
Ephemeral Sea (-370 Ma)
The first part of our story begins 250 million years ago. Rocks from the Beaujolais region provide us with no information about this period but we do know, after examining rocks from neighbouring regions, that at that time in Western Europe, there were a series of collisions between micro-continents. We also know that a process of subduction was occurring, setting two continents on a certain collision course.
During the Devonian Period, the various lands that would later become the Beaujolais region, were located behind a subduction zone. As happened with Andes, the subduction zone created a very active volcanic arc above the ground, the remains of which is visible today in the Morvan mountains. The Beaujolais region also witnessed volcanic activity, but it was rather different. The phenomenon known as subduction results in the stretching of continents. Like pizza dough that has been over-stretched, continents break apart, cracks appear which are filled with water, creating mini-oceans. The term"rift," is used to describe these embryonic seas;
and the "Brévenne rift" in the case of the Beaujolais sea created by subduction. At the bottom of these seas was an area of permanent volcanic activity. As you can see by the lava pillows visible on the roads around L'Arbresle. Lava pillows are formed by the rapid cooling of magma when in contact with water.
Like all oceans and seas, the Brévenne rift was prone to sedimentation. Rivers most likely flowed into this sea, bringing deposits of sand and clay. You'll find thick sediment accumulations, in the form of shale, around Ternand and Létra. Some shale even contains limestone lenses (similar to marble).
The Devonian period also saw the formation of rocks that would contribute to the richness of the Beaujolais region's subsoil. Water flowing through rocks on the earth's crust created natural chimneys on the sea bed. As it flowed, the water would heat up (we refer to hydrothermal water) and pick up different minerals which it would then deposit in these chimneys, called "black smokers." 370 million years later, Man would extract these metals (copper and iron, in particular) from mines in the Beaujolais region.
Pillow-lavas de l'Arbresle
Ash and water (-335 Ma)
During the Carboniferous Period, 300 to 346 million years ago, the subduction of the previous period concluded with the meeting of two continents and a series of continental collisions, forming a "primitive" version of today's Europe. The merging of these two continents resulted in the closure of the Brévenne sea, the formation of a mountain range known as the "Hercynian belt" or the "Variscan belt," and the onset of volcanic activity, typical at plate collision zones.
During the early Carboniferous period, the landscape of the Beaujolais region resembled the shores of large lakes, or inland seas, bordered by nascent hills and volcanoes. These lakes or inland seas, like all natural reservoirs, were filled with sediments. The north and west of the Beaujolais region are partly made up of accumulations of sand and silt deposits (consolidated into rocks, over time.) Some layers even contain traces of vegetation from this period, in the form of fossilized leaves and coal.
Carboniferous sediments extend over a significant part of the Beaujolais region. They are still hidden, covered by a layer of of volcanic rocks, from the volcanoes bordering the great lakes.
Collision volcanism involves very acidic (acidic meaning rich in quartz), and therefore, very viscous, magma. Existing volcanoes that release this type of magma give us some idea of the violent eruptions that occurred during the Carboniferous Period. Eruptions are triggered by an increase in pressure in the magma chamber until it reaches a certain level, causing the volcanic edifice to eventually explode. It then throws out into the atmosphere significant quantities of ash and other burning particles. When these particles fall back down, depending on the residual heat, this pyroclastic (ash, volcanic bombs, lapilli etc.) debris may become welded together. Over a period of several million years, pyroclastic fallout from Beaujolais volcanoes accumulated and solidified to create deposits that are over 300 m thick. We use the term "pyroclastic tuffs" to describe these deposits.
High mountains & plutons (310 Ma)
During the Carboniferous Period, the continents continued to move closer and volcanic activity subsided. Pressure continued to build between the continents, resulting in a thickening of the Hercynian belt, which then formed a vast mountain range similar to today's Himalayas.
Deep below the earth's surface, masses of magma slowly solidified. Geologists call these large pockets of magma trapped in the Earth's crust, "plutons." It took hundreds of thousands, or even several million years, for the plutons to cool completely. The rock formed within these plutons was granite. During this long cooling phase, the plutons began to affect the surrounding rock. With temperatures approaching 1000°C, they released a halo of heat, baking the surrounding rocks. The term used to describe this is contact metamorphism. Such high temperatures can cause the rocks to restructure and create new minerals. Garnet is one of the minerals formed when certain rocks are exposed to heat.
The rocks which made up the Hercynian belt, and particularly the rocks buried several kilometres deep, underwent radical transformations as a result of the high pressure generated by the continental collision. Most of the ancient rocks in the Beaujolais region (pre-dating the Carboniferous Period) underwent metamorphism during the Hercynian orogeny (the term "orogeny" refers to the process of forming mountain ranges.)
The plutons, acting like genuine reactors, triggered the circulation of "hydrothermal" fluids (literally heated water). These fluids, which contained water and quartz, circulated through the plutons and neighbouring rocks, picking up chemical elements en route. These fluids permeated the dense network of cracks, created by the plutons and the process of orogenesis, coating them with quartz deposits. All these cracks were eventually filled with quartz and other minerals, including barite and fluorine, creating seams which have been extracted by Beaujolais miners since ancient times.
Erosion (-250 à -201 Ma)
The next stage of Beaujolais' geological history is some 70 million years after the creation of the great Hercynian massif; during those intervening 70 million years, the agents of erosion (rain, wind, ice, etc.) had worn down the mountains. The thick layers of hardened sand and sandstone serve as a reminder that it only takes a few dozen million years to reduce one of Earth's largest mountain ranges to grains of sand.
Sandstone, which can be up to 20 metres thick, is a sedimentary rock containing grains of sand cemented together. In the Beaujolais region, you come across a wide range of sandstone which varies depending on its grain size and cement composition (limestone, flint, clay etc.) All the region's sandstone, irrespective of type, was deposited in a coastal or estuarine environment (perhaps a delta). Coarse sandstone (with large grains) suggests strong hydrodynamic forces (current area, breaking waves) while finer grains indicate weaker hydrodynamic conditions. You can sometimes find prints of protodinosaurs in fine-grained sandstones, prints that have been covered - and preserved - by another layer of sand.
We can confirm the location of the coastal deposit area thanks to the recurring presence of "phantom" salt crystals. When puddles of water evaporate from wet sand, salt crystals (sodium hydroxide) form. Over time, these crystals are replaced by limestone, silica and other elements, leaving the outline of the salt crystal visible.
Ebb and flow (-201 à -160 Ma)
From the Early Triassic until the Mid Triassic Period, sea levels fluctuated; sometimes they rose, other times they fell. There were two possible causes for these changes in eustatic levels (another term for sea levels): climactic and geodynamic. In the short time, in other words over a period of a few million years, climactic factors were the most obvious cause. Land ice melted, increasing the volume of the oceans, leading to an overall increase in sea levels. These fluctuations in level were probably in the order of around 100 metres (maximum). This phenomenon of marine transgression, caused by melting ice, is accentuated by the thermal ocean expansion (oceans expand slightly when it's warm). In the longer term, several dozens of millions of years, geodynamic factors came into play causing rises in sea levels of up to 300 metres. Such significant variations in sea levels are made possible by the shape of the sea bed. Ocean beds are covered with ridges, dynamic mountain ranges where the ocean crust is created. These mountain ranges are responsible for the opening and widening of oceans. A so-called "fast-spreading" ridge, implying that an ocean is opening more quickly, indicates a much larger large mountain range than a "slow" ridge. These differences in size, and therefore the speed at which oceans open, affect the overall sea levels.
During the Early Jurassic Period, the North Atlantic Ocean opened up quickly, causing an increase in sea levels. The Jurassic rocks found in the Beaujolais region, demonstrate variations in the horizontal sedimentary layers, for example oolite limestone, typical of shallow areas, and marls, usually formed in deeper zones.
During the Triassic Period, the Beaujolais region was a coastal environment. The Jurassic Period, which followed on from the Triassic Period, marked the transition to a more overtly marine environment, producing a wide range of limestone and layers of marlstone.
During the Sinemurian Stage, (195 million years ago), one of the Early Jurassic Epochs, the Monts d'Or and part of the Beaujolais region were covered in limestone mud, rich in gryphaea shells (early types of oyster.) The swollen structure of the limestone is indicative of the turbulence produced by the waves. From these marks, we can place these deposits at an estimated depth of between 5 and several dozen metres.
Over time, the mud consolidated, producing solid limestone (the process whereby sediment is transformed to sedimentary rock is called diagenesis). The trend for limestone slabs containing gryphaea combined with its hard-wearing qualities make it a popular material for landings and staircase in old buildings.
Rocks from the next stage, which are mainly marls, suggest a significant marine transgression. The Pliensbachian Stage (186 million years ago) left around sixty metres of grey marls. The fine sediment that ended up on the marls (a mix of limestone and clay) suggest that the deposits were undisturbed, and not exposed to either waves or currents. Only deep environments would satisfy these conditions which gives us an estimated depth of between 100 and 200 metres.
Although some geological periods were characterized by marine transgressions, others demonstrated a return to a more coastal environment. This is the case for the Aalenian Stage (174 million years ago), a geological stage during which the region's famous "pierre dorée" (ochre-coloured limestone) was formed.
"Pierre dorée" is a type of limestone, mostly comprising entrochites, star-shaped or circular remains of animals. The term used is "entrochal limestone." The prevalence of bioclasts (fragments from organisms) suggests that the sediment was subject to a very turbulent environment, with sufficient force to break shells. The stratified layers visible in this limestone indicate the presence of strong currents. The stratifications are often oblique, truncated and, in turn, covered by further layers of oblique strata. These strata are typical of underwater dunes, shaped by the currents.
Silence (-157 à -35 Ma)
The geological history of the Beaujolais region alternates between offering abundant evidence of past eras and stratigraphic silences. After the youngest Jurassic rocks, there's a gap of 120 million years from which there are no rocks to provide information about the Beaujolais landscape. Two possible hypotheses are presented by way of explanation: either the region stood above sea level during this period (and as a result no sediment was deposited) or it was underwater but any sediment has since disappeared.
During the Cretaceous Period, the second period of the Secondary Era, overall sea levels rose again. There were significant reductions in sea level but generally the trend was towards rising levels. Despite this rise in sea levels, involving sedimentary deposition, Beaujolais retains no trace of these sediments.
The absence of sedimentary rock from the Cretaceous Period, can be explained by erosion. The overall sea level started to drop 65 million years ago, when dinosaurs became extinct. Sediment accumulated during the Cretaceous Period was exposed to the air and, consequently, subject to erosion. In the space of just a few dozen million years, agents of erosion would destroy all the sediment deposited during the Cretaceous Period.
Reverberations in the Alps
During the Quarternary Period, Europe experienced alternating cold or 'glacial' periods and warm 'inter-glacial' periods. During glacial periods, climactic conditions favoured glacial expansion and, specifically, an increase in ice-cover.
When Neanderthal man inhabited the region of Beaujolais (between 130,000 and 300,000 years ago) he would have negotiated a landscape that was periodically covered with ice and bordered by a large lake. A vast lake, covering several thousand km2 extended from the Monts d'Or as far as the Dijon region. This expanse of flood water was formed by a natural dam: a glacier that flowed down from the Alps, blocking the Saône by the Monts d'Or.
The land in the Beaujolais region retains the memory of this glacial episode. The sub-soil found in certain villages that lie within the Saône plain reveals fine layers of sediment (deposited at the bottom of the lake). In other villages, which are higher up, the ground contains large blocks of rock, particularly sandstone, which have been transported several kilometres from the Beaujolais hills. Their presence, far from their original environment, suggests the flow of glaciers.
Glacial Periods (-3 Ma à -10 000 ans)
During the Quarternary Period, Europe experienced alternating cold or "glacial" periods and warm "inter-glacial" periods. During glacial periods, climactic conditions favoured glacial expansion and, specifically, an increase in ice-cover.
When Neanderthal man inhabited the region of Beaujolais - between 130,000 and 300,000 years ago, he would have negotiated a landscape that was periodically covered with ice, and bordered by a large lake. A vast lake, covering several thousand km2 extended from the Monts d'Or as far as the Dijon region. This expanse of flood water was formed by a natural dam: a glacier that flowed down from the Alps, blocking the Saône by the Monts d'Or.
The land, in the Beaujolais region, retains a memory of this glacial episode. The sub-soil found in certain villages that lie within the Saône plain, reveals fine layers of sediment (deposited at the bottom of the lake). In other villages, which are higher up, the ground contains large blocks of rock, particularly sandstone, which have been transported several kilometres from the Beaujolais hills. Their presence, far from their original environment, suggests the flow of glaciers.