OUGSME Field Trip to the Bavarian-Bohemian Geopark, October, 2010.

Day 3, 27th October 2010, the Cheb Basin.

Text Elisabeth d'Eyrames, photos David Kopsch.

Day 3 was to take us into the Tertiary sedimentary Cheb Basin, the yellow coloured region marked 3 on the geological map below. The basin is intracontinental, its sediments were predominately deposited during the Neogene. It is situated at the south-western end of the Eger Rift Valley, or Graben, where it is delineated from the Eger Graben by a major shear fault, known as the Mariánské Lázně or Marienbad fault. A number of small volcanoes are located along the western fringe of the basin, they were described as 'pocket' volcanoes by Dr. Peterek. (Eger is the German name for the Czech town of Cheb, the Czech name of the River Eger is Ohře') The map also shows the position of the KTB and the Franconian Lineament which we visited on Day 1 and the Duppauer Complex which was visited on Day 4.

Geological map of  the Eger Graben.
Geological map of the Eger Graben. © Geopark Bayern-Böhmen.

We started Day 3 by driving from Windischeschenbach to Neualbenreuth, where we stopped some 50 m from the Czech/Bavarian border, which we crossed by foot in the bright, misty morning light, into a nature reserve. The old border post, about the size of a garden shed, was still standing next to the foot-path. The path took us to a cinder or scoria  cone, formed during the middle to late Pleistocene, 300,000 to 500,000 years ago, according to radiometric dating. However, some volcanologists think that it is no older than 100,000 years. Either way, it is the youngest volcano of North Bohemia: and is known as Železná hůrka or Eisenbühl (Iron Hill).

Železná hůrka or Iron Hill scoria.
Železná hůrka or Iron Hill scoria.

The study of the porosity of the scoria hints towards a first stage of pyroclastic events, followed by phreato-magmatic or hydroclastic events involving ground water and less gases. As such, less porosity is visible in the phreato-magmatic scoria than that produced during the pyroclastic events. It was followed by explosive events, creating a large crater surrounded by scoria, called a 'Schlake' in German. The diameter of the volcano is approximately 200 m, and the material that has been thrown out is mantel basalt.

When this area was geologically mapped, more volcanic material was discovered 1km from the crater and  a depression nearby was also identified by using magnetic and gravity anomaly measurement, as a maar. This name relates to the outcome of a phreato-magmatic eruption when the magma on its way up encounters groundwater. The distribution pattern of the volcanic material lead geologists to consider that there are more volcanic pipes aligned in a trend parallel to the Marienbad fault.

Komorní hůrka or Kammerbühl.
Komorní hůrka or Kammerbühl.

We drove past Cheb and stopped next at a small, but very famous volcano, the  Komorní hůrka or Kammerbühl. It forms a gentle hill, some 50m high, on the Cheb basin which has an altitude of around 450m. Komorní hůrka is a  cinder cone of lapilli, which are lava fragments of a few millimetres to a few centimetres in size, depending on the classification. When they are larger they are called scoria. The eruption started at the end of the Tertiary to the beginning of the Quaternary, with strombolian activity and lasted into Pleistocene 115,000 to 15,000 years ago. The local wind pattern blew most of the pyroclastics to the east. At the final stage, lava filled in the chimney and the crater, eventually flowing over the cone towards the south-west. Some of the last outbreaks happened during Holocene, that is less than 10,000 years ago. The lava is a nephelinite, quite a rare basalt in Europe. The lapilli and the basaltic lava flow have been quarried since the Middle Ages; the black tower of the castle of Eger is made of it.

The importance of this site is historical. It was at the heart of a controversy in the nineteenth century, when  naturalists started to question the origin of this hill. The Neptunists (Neptune is the Roman god of the sea) thought that the basalt precipitated from the water as a sedimentary process. The Plutonists (Pluton is the Roman god on the interior of the Earth) defended the theory that basalt is the product of melting of  brown coal, which erupted on the surface.

J.W. von Goethe, was not only a romantic poet, but also a passionate naturalist, and as such visited the site in 1808. He first felt that the Plutonists had a point, but later he changed his mind. He persuaded  Kašpar Šternberk, one of the founders of the National Museum in Prague, to take part in a research project, digging a gallery through the hill to prove that it is a volcano.  They started in 1826, excavated  300 m of galleries and  found the chimney, but by that time Goethe had died. The galleries have since collapsed, but the portal of the old tunnel with its epigraphs can still be seen.

Františkovy Lázně or Franzensbad.
Františkovy Lázně or Franzensbad.

From Komorní hůrka we drove to Františkovy Lázně or Franzensbad, a spa town some 10 km North of Cheb, still at the western margin of the Cheb sedimentary basin. We tasted  the waters from different wells: Nový Kostelni, Glauber III and Glauber IV. Glauber IV is the most mineralised water in the Cheb Basin (ca. 21g/l), it has a carbon dioxyde content of 1,5g/l and flows at a rate of 0,9 l/min from a depth of 93 m. Extremely healthy if not the tastiest of waters!

We visited some other mineral springs in the park; there are more than 20 listed wells in Franzensbad, why is there so much healthy water? The answer to that question is related to the tectonic structure of the sedimentary basin. An active magma reservoir in the upper mantel, at a depth of about 30km, releases carbon dioxide and mantle fluids which get trapped in the crust at a depth of 5km to 10km. Above this, in the upper crust, water-rock interaction  releases minerals into the water, and eventually peat adds the extra taste.

After having taken the waters and had a walk through the town, we made our way to Soos, 5km to the north-east of  Franzensbad. Soos is a nature reserve in the central part of the basin. The basin is asymetrical in depth, which increases eastwards, to around 350 to 400m at its eastern margin along the Marienbad fault. See  Fig. 2 below.

A cross-section through the Eger Graben.
A cross-section through the Eger Graben.

It was lunchtime and the Sun was warm enough for us to sit in the park-like entrance area to eat our packed lunches. We then took a pleasant stroll on a boardwalk, through the nature protected area of 'Soos'. This consists a partly drained swamp, displaying up to 8 m thick diatomites* from the early Holocene and spectacular mofettes with intense CO2 emanation, deriving from the upper mantle (30km). A mofette or mofet is a gas emitting hole.

* The Oxford Dictionary of Earth Sciences defines a diatomite as: 'A diatom- or bacillariophyceae-algae-rich sediment which has been laid down in a lacustrine or deep-sea environment. The diatom cell wall is made of silica, therefore, the sediment is siliceous.

Mofettes in the Soos nature reserve.
Mofettes in the Soos nature reserve.

In the whole of the Cheb Basin, there is a high concentration of degassing, aligned with the trend of the Marienbad fault; the gases find their way up along minor faults cutting through the sedimentary layers. How do we know the source of the gases? Measurements of the abundance of helium isotopes are made, helium3 has its origin in the mantel whereas the crust is richer in helium4. Although the helium content of the gases is very small, the ratio of He3 to He4 is a good indicator of the source. When the He3/He4 ratio increases, strong mantel degassing is indicated. The Cheb basin has the highest He3/He4 ratio in Central Europe, similar to what is found in the region of Etna. The question of continued subsidence of the basin is still under discussion; there might be some, but it is insufficient for further deposition, hence some erosion is taking place.

Our next stop was to Nový Kostel, the epicentral area of the NW-Bohemian swarm earthquakes, related to the Marienbad fault. Records of seismic activity go back to the fourteenth century, although not a lot of damage is associated with these earthquakes. After the tremors up to 4.6 on Richter scale during  winter 1985-86, intensive investigation of the area was launched. Amongst other techniques used is paleoseismology, which  consists in digging trenches to study soil disturbances. In 2008, 40,000 events were recorded over a period of 2 months. 3D plotting of the centre of the seisms shows a curved fault plane. As the stresses in the crust are not released in a major event, but through thousands of minor quakes over several weeks, the phenomenon is called  an earthquake swarm. Usually earthquake swarms occur next to a boundary where two tectonic plates meet, but here, that is not the case. It is more likely to be continuing magma injections into the crust, due to continuing rifting, that increases the pressure of the carbon dioxyde trapped in the rocks, which triggers the earthquakes at a depth of 6 to 14 km.

Bublak, the bubbling spring.
Bublak, the bubbling spring.

The highlight of the day was the Big Mofette south of Milhostov. This is the most beautiful and largest mofette of the Cheb Basin, it is called Bublak or the bubbling spring. It is located in a swampy area in the valley of the Plesna River, which, according to Dr. Peterek  flows along an active fault, oriented NW-SE. A related fault scarp is indicated by the difference in the height of the flanks of the valley and asymmetry of the valley cross section.

Twice a month the gasses are sampled and the isotopes checked. The idea is to get data that relates the variations of the isotopic ratio with the seismic activity, implying sampling before and after the tremors. Since direct measurements can only be done in wellington boots and wading through the swamp it is difficult to carry out. Remote measurements are now made, but the results todate are not very satisfactory. Around this area, dry mofettes also exist. They can be located where the carcasses of dead animals are found, a dangerous place to hang about.

To finish the day, we took a walk on the Litov mine tip close to Chlum Svaté Maří or Maria Kulm, with samples of brown-coal and tempered shales (Porzellanite, Porcellanite) resulting from subsurface fires. The tip also provided a good view across the Eger Graben with the Krušné hory'or Erzgebirge to the north and the Slavkovský les or the Kaiserwald to the south.

Day 4, 28th October 2010, Loket and Surroundings in the Eger Rift Valley.

Text and photos Fred Owen, North West England Branch.

Twinned plagioclase phenocrysts
Twinned plagioclase phenocrysts in Karlsbad Granite

A short, brisk walk from the hotel over the bridge crossing the R Eger took us to a grand exposure of the basement Karlsbad Granite exhibiting huge plagioclase phenocrysts, some clearly displayed twinning.

Loket castle.
Loket castle.

Below us, on the river bank, was an open air auditorium and opposite stood Loket castle, perched atop the same granite. It had been built inside the loop of a huge meander cut about 100 m into the granite. How had it managed to cut so deeply into such tough rock instead of the softer sediments?

How the River Eger incised the granite
Fig 1: How the River Eger incised the granite.

Andreas explained that this could only happen if the R Eger originally cut through the soft sediments into the granite beneath. Subsequent faulting of the granite and erosion of the soft sediments left the river in its present granite bed, as shown in Fig. 1.

Družba Open Cast Lignite (brown coal) Pit, Nové Sedlo.
Družba Open Cast Lignite (brown coal) Pit, Nové Sedlo.

We moved on a short distance to the lookout point over the massive Družba Open Cast Lignite (brown coal) Pit, Nové Sedlo shown below. Production started in 1642 and reached its peak output of 23 million tonnes/yr in 1983. The mine covers about 50 sq km and is easily identified on satellite images.

The Tertiary deposits lie in a West-East basin bounded to the North and South by tectonic faults. The lignite lies beneath 200 m of overburden (seen in the spoil heaps at the close of Day 3) comprising clay and lake sediments of the Cypris Lake Formation, containing Late Miocene Ostracod angulata, and volcanic ash of the Chodov Formation. Parts of both these sediments are used as raw materials for making such products as ceramics and absorbents.

There are two main lignite beds of Oligocene age, the deepest being the 22 m thick 24 Ma Josef seam, whilst the upper 24 m thick 22 Ma Antonine seam, is the one being worked. The whole sedimentary pile is underlain by granite and igneous extrusives. Mining operators take great care not to penetrate the water table below which feeds water to the Karlsbad health spas. Production is planned to continue for another 20 yrs then the whole mine will be flooded to form a massive leisure facility.

Fossil cinnamon leaf cast.
Fossil cinnamon leaf cast.

Our journey continued to the Erika Sand Quarry, Lomnice. This extensive quarry comprises 22 m thick beds of the Staré Sedlo Formation, a sequence of Tertiary, fluvial and shallow lake, sediments showing both laminar and trough cross-bedding, channels and graded-bedding. Erika is an internationally recognized locality for Late Eocene flora. Numerous excellent leaf and plant stem fossils were found like the cinnamon leaf shown below. They represent the flora of a subtropical environment similar to that found in present day Vietnam, comprising laurel, palm fans and the precursor to oak. Goethe collected many plant fossils here.

These Late Eocene sediments are the oldest in the Eger Graben, but are topographically higher than the youngest lignite bed seen earlier due to a vertical

400 m fault displacement post-dating the Cypris Formation. They contain no carbonaceous material. This was a period of global warming due to the CO2 produced by the major volcanic activity contemporaneous with forming the Deccan Traps at the end of the Cretaceous.

The Erika sand quarry
The Erika sand quarry.

The sediments derived from the Teplá Barrandian metamorphic belt in central Bohemia to the SE and from further south, being carried north via the Leipzig Basin and on to the palaeo-North Sea coast in large braided, meandering rivers, many running parallel to the original one. From May to August the deposits are the home of a colony of sand martins that bore tunnels deep into the quarry faces to nest.

Castle ruins on Andělská Hora or Angel’s Mountain.
Castle ruins on Andělská Hora or Angel’s Mountain.

After a short drive, we climbed a steep outcrop of quartz/plagioclase-rich rhyolite to reach a Middle Age castle atop Andělská Hora (Angel’s Mountain). This is one of several rhyolite complexes to the south and southeast of Karlovy Vary (Karlsbad) associated with extension during the Eger Graben evolution. To the NE a major source of magma was Doupov (Duppauer Gebirge), a volcano similar in size to Mt Etna, active 35 to 25 Ma to the east of Karlsbad and the second largest stratovolcano** in  Europe. Doupov lies within the Eger Graben and these rhyolitic exposures are scattered between the ENE-WSW rift boundary faults of the Graben.

** A recent paper (in English) says that the current indications are that the volcanic complex is made up of a cluster of shield volcanoes rather than a single stratovolcano.
Ref: Petrology and geochemistry of the Tertiary alkaline intrusive rocks at Doupov, Doupovské hory Volcanic Complex (NW Bohemian Massif) by Frantisek V. Holub,
Vladislav Rapprich, Vojtech Erban2, Zoltán Pécskay3, Bedrich Mlcoch, Jitka Míková in the Journal of Geosciences, 55 (2010), 251-278. This can be read at: www.jgeosci.org/content/jgeosci.074_2010_3_holub.pdf

South dipping planation surface.
South dipping planation surface.

We were looking down on a southward, gently dipping planation surface, which truncates against the Slavkovský les or Kaiserwald to the south. The Erzgerbirge range forms the northern margin of the Eger Graben, with the highest peak, Keilberg, reaching 1244 m.  The western margin of the Eger Graben abuts the NWN-SES Mariánské Lázně Lineament seen on Day 3. So we were looking at the landscape in the north western corner of the Eger Graben. The picturesque view from the castle is shown through one of its remaining arches.

Tepla Monastery.
Tepla Monastery.

We were privileged to take lunch in the surroundings of Teplá Monastery, founded in 1193, by a nobleman who was beatified in 1897. The monastery houses a famous library of rare 15 th century books and a substantial collection of minerals. The Czechoslovak People’s Army occupied it for 28 yrs from 1950 during which time its condition deteriorated badly. It is now undergoing extensive renovation.

Augen gneiss in a stone wall, Tepla Monastery
Augen gneiss in a stone wall, Tepla Monastery

Whilst touring the grounds I couldn’t resist taking the photo shown of a stone of augen gneiss in the wall near to the main gate entrance. It wasn’t discussed but presumably it is from the Teplá Barandian metamorphic belt.

Dr. Andreas Peterek with Goethe.
Dr. Andreas Peterek with Goethe.

The Mariánské Lázně or Marienbad Geopark was to be our final geological locality of the trip with Andreas; an ideal place to acquire an image of Andreas and his friend Goethe.

The geopark lies on the southwestern slopes of Zizkov Hill and forms part of the Marianbad Museum. Along a 1.5 km circular walk there are numerous exhibits of large, cut and polished boulders of a wide variety of igneous and metamorphic rocks representing the nearby geology. All show clearly the features selected for each location, be it the crystalline fabric of the rock or its structure. Alongside each rock is an explanation board describing the formation and source of the rock and features to be examined.

Marienbad, water-music display.
Marienbad, water-music display.

After taking spa water at the famous colonnade and a relaxing coffee in the square we watched and listened to the singing fountain display to water music.

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