OUGSME Field Trip to the Bavarian-Bohemian Geopark, October, 2010.
The Bavarian-Bohemian Geopark covers 4 rural disricts in NE-Bavaria and 2 in Bohemia, it arguably encompasses the richest and most varied selection of sites of geological interest in Central Europe. During the Variscan Orogeny, a number of micro-continents or terranes,which were parts of the Armorican Assemblage, were swept up in the collision between Laurusssia and Gondwana, to form Pangea, between the Late Devonian and Mid-Carboniferous. Three of these units form the basement rocks of the Geopark, they are: The Saxothuringian Zone, the Moldanubian Zone and the Bohemian Massif.
The Saxothuringian Zone consists of a high-grade gneissic core and lower grade Palaeozoic sediments, with extensive granite intrusions. The Moldanubian Zone represents the internal zone of the orogen, it comprises of high-grade metamorphic rocks and abundant Late Devonian to Late Carboniferous granites and migmatites. Further south, this unit forms basement inliers or windows within the Alpine nappes. Historically, due to the similarity in their constituent rocks, the Bohemian Massif and the Moldanubian Zone were considered as being one unit. Subsequently, palaeomagnetic measuremments have determined that the Bohemian Massif has rotated some 140° anti-clockwise relative to the Moldanubian Zone, strongly indicating that it was a separate terrane.
The northern boundary of the Bohemian Massif is formed by the Eger or Ohře Rift Valley, which is approximately 300 km long and 50 km wide. The rift system is part of the European Cenozoic Rift System, together with the Rhine and Auvergne Grabens. The Eger Rift evolved at the Upper Cretaceous/Tertiary boundary and reactivated the Variscan Saxothuringian/Moldanubian suture zone. The main rifting phase took place between 42-9 Ma ago, with the graben formation and intraplate alkaline volcanism. The rifting process is still active today, this results in CO2 emission at the surface in NW-Bohemia, the source of the mofettes fields, and continued uplift in some southern parts of the rift, accompanied by earthquake swarms.
Day 1, 25th October 2010, KTB, Franconian Lineament and Parkstein
Text Eileen Lawley, photos Neil Lawley.
After driving about 200km north of Munich through fields where hops and maize had grown we arrived at the buildings of the German Continental Deep Drilling Program (KTB) near Windischeschenbach where we were welcomed by the head of the KTB Geocentre Dr Frank Holzförster who was to be the guide for our visit of the facilities.The project was initiated in the 1980’s with the intention of studying continental crust and hoping to reach the Moho where the brittle continental crust becomes ductile. The site for drilling was chosen because it is a suture zone formed during the Variscan Orogeny about 380-320 Ma when the Rheic Sea between Gondwana and Laurussia closed to form the supercontinent of Pangaea. Previous geological mapping had indicated that there were 4 nappes in a vertical section.
During 1987-1989 a pilot hole was drilled to 4000m when 97% of the core was recovered for investigation. The thermal gradient was found to be 20°C/km so it was hoped that a depth of 11km to the underlying basalt layer, deduced from magnetic, seismic and gravity measurements, could be attained before the instrumentation became too hot to use. Drilling of the main borehole 200m to the east of the pilot hole began in 1990 but the thermal gradient at that spot, was found to be 30°C/km, so that drilling had to be stopped after reaching a depth of 9101 m in 1994, where the temperature was 280°C and pressure 3.8 kbar. At these temperatures and pressures the electronic instruments are unreliable. The brittle-ductile zone had been reached at a lesser depth than expected because of the higher thermal gradient.
We walked around the exhibition on the site inspecting the diamond drilling bits and samples of core retrieved and learnt that the borehole was about 30cm wide with a 12cm drill pipe.. For most of us the concept of ‘mud’ was fascinating. Having heard about this term in connection with recent attempts to stop the flow of oil from the Deepwater Horizon rig, most of us thought that the ‘mud’ used was a mixture of rock and water recovered from the borehole. In fact it seems to be a miraculous substance consisting of water and hectorite which is solid when stationary but becomes a viscous liquid when shaken or stirred. This mixture is pumped downwards through the bore hole and is used to power the motor driving the drill bit. It is also used to lubricate and cool the drill bit and returns to the surface in the space between the walls of the borehole and the drill pipe or string, carrying waste away as small fragments of rock. As only about 600m of core were recovered from the main borehole, these fragments provide valuable information about the rocks encountered.
We climbed some 18m of the 85m high drilling rig to watch a film about the changing of the drill bit and we also saw the platform and machinery where this operation took place. At the deepest depths 237 segments of drill pipe each of 39m in length had to be brought to the surface, disconnected and moved to a temporary storage area on the platform using an automated pipe handling system called an ‘iron rough neck’. After the bit had been changed the whole operation had to take place in reverse in order for drilling to commence again.
We then moved to another exhibit to examine some of the drill pipes used and the ‘business end’ of the drill which extended for several metres. We studied the sensors used to detect deviations from the vertical and redirect the drill as well as the instruments integrated into the end section.
The drilling process and instruments developed have greatly improved knowledge about deep borehole drilling and were further developed for use in the drilling industry. Continuing research takes place using magnetic, thermal and other geophysical and chemical measurements taken in the borehole and comparing these with surface measurements.
After thanking Dr Frank Holzförster for his excellent tour we then met up with Dr Andreas Peterek of Bayreuth University and the Geopark Bayern-Böhmen who was to be our guide for the rest of the geological explorations on this trip. We took a short ride to the top of the Franconian Lineament which is a fault zone bounding the western edge of the Bohemian Massif trending NW-SE for about 300 km, it is about 20km wide. This line was to play a prominent role in our other explorations of this area. We viewed the zone from a scarp that was around 200-300m high. Mesozoic sediments lie on the western side of the Lineament which shows subsidence indicating tension. Crystalline metamorphic rocks of the Bohemian Massif which formed during the Variscan Orogeny lie to the east under compression from the Alpine Orogeny. Measurements from the KTB which lies a short distance to the east of the fault have provided further confirmation of this compression and composition of the rocks.
Back on the bus we made a short trip to Parkstein which is a volcanic cone formed from a basaltic intrusion, lying a little to the west of the Franconian Lineament. The basalt intruded under water and the phreamagmatic reactions caused explosive combinations of magma and country rock forming a breccia containing large boulders up to 40-50cm in diameter. The columnar structure formed is unusual in that the columns are rather narrow, up to 20cm in diameter, and since the columns form perpendicular to the cooling surface we were able to follow the curve of this surface to visualise a crater 150-200m wide.We learnt that this basalt intruded under a lake or maar 20Ma and 200-300m of sediment has since eroded away leaving the columns and breccias that we see today. We were also able to enter some tunnels dug into the breccia to be used for storage to see other large boulders of country rock.
Day 2, 26th October, 2010, Moldanubian and Saxothuringian Tectonic Units.
Text Elisabeth d'Eyrames, photos Neil Lawley.
Our guides for Day 2 were, Dr. Andreas Peterek, (Bayreuth University and Geopark Bayern-Böhmen) and Dr. Johann Rohrmüller (Bayerisches Landesamt für Umwelt, Geological Survey Bayern).
The tour was to take us northwards, to cover the tectonometamorphic units of the North-Bavarian basement (Moldanubian Unit, Saxothuringian Unit, Münchberg Nappe Complex) including the late-Variscan granites. We followed approximately the line of the Franconian Lineament, which we had seen and discussed with Dr. Peterek on our way to Parkstein, on Day 1.
Our first stop was in the Oberbaumühle Quarry, operated by the Basalt-Actien-Gesellschaft, where we were greeted by two of the company's geo-engineers. The quarry produces aggregates from amphibolite, hornblend-gneiss and para-gneiss, which form the lower Paleozoic basement rocks of the Erbenorf-Vohenstauss Zone, 'ZEV'. The rocks have been brecciated by the intrusion of the nearby Upper Carboniferous Leuchenberger granites, during the Variscan orogeny, some 310 Ma ago. Some veins of granitic intrusions could be seen in quarry rock face, metamorphism took place in the Devonian 380 Ma ago.
The source rock for the amphibolites is lower Paleozoic 700-500 Ma basalt and volcanic tuffs. Although there is a banded structure, mainly due to the tuff deposits, it is not a gneiss as the source rock is not a plutonic one. The banded structure is called 'flaser', the white-coloured bands are plagioclase rich and the dark-coloured bands amphibole rich. Typically the amphibole here is hornblende, but green patches of chlorite and epidote are visible, brought about by low temperature mineralization, where the amphibole is in near contact with the granites.
Our second stop was at the Marienstollen quarry about 1 km north of the town of Erbendorf. Erbendorf has a history of 1,000 years of silver and lead mining in veins cutting through Permo-Carboniferous sedimentary rocks. Up to 1920 coal was also mined in the Permo-Carboniferous Rotliegendes area. The quarry is part of the northern-end of the Erbendorf -Vohenstrauss nappe complex, where the amphibolite and para-gneisses present, constitute the nappe.
The Marienstollen is an old talc quarry, the talc runs in veins through ultramafic serpentinite rocks. We were at an outcrop of the remains of an ophiolite, which had been stranded during the Devonian Variscan orogeny, some 395 Ma ago, following the closure of the Saxothuringian Ocean. The olivine and pyroxene contained in the original peridotite ultrabasic rocks have been changed into serpentine by interaction with water, at low temperatures. Two phases of metamorphism can be detected, the first is regional, which was overprinted later by contact metamorphism.
The seam of ultra mafic rocks shown in the sketch below, form the boundary between the Moldanubian tectonic unit to the south and the Saxothuringian unit to the north.
Clearly, two outcrops cannot be respresentative of a whole tectonic unit, however, those we saw in the Oberbaumühle and Marienstollen quarries are both characteristic of the high-grade metamorphic rocks associated with the Moldanubian zone. As we moved onto the Saxothuringian zone, we would expect to find lower-grade Lower Palaeozoic sediments, this is exactly what we did find.
Our third stop was at a sedimentary outcrop of grey-green coloured quarzitic schist, known as the Phycoden group, exposed in a road cutting in Kranichstein. The deposits contain trace fossils of fucoiden algen beds, for those you who have been in NW Scotland, you might remember the Cambrian fucoid beds next to Loch Assynt. The sediments display a metamorphic overpint with small-scale tectonic structures, rather than genuine metamorphism, in particular a foliation pattern of small isoclinal folds, called transposed foliation.
This low metamorphic unit was folded during Variscan orogeny, over different folding episodes. It was buried by faulting along the Franconian Lineament and later exhumed by Permo-Carboniferous and Upper Cretaceous uplift.This is not an exposure to be admired from 5 m distance, it was decidedly filigree with the structures more measurable in millimetres.
We continued the trip into the Fichtel Mountains, where we stopped at the community centre of the town of Bischofsgrün, to eat our packed lunches. Bischofsgrün is situated in the south-west corner of the Bavarian part of the Fichtel Mountains, overlooking the two highest elevations, the Schneeberg at 1,051 m and the Oxenkopf at 1,024 m. The Fichtel Mountains consist of approximatelxy 40% granites which form the topographic highs, surrounded by metamorphic basement rocks consisting of schists and marble. The Upper Carboniferous granites intruded the metamorphic units over a period of 30 to 40 Ma, some 325 to 290 Ma ago.
In 1998 there was a new investigation about the age and shape of the granite intrusions. It appears that the older one is a batholite 1to 2 km thick, that shows magma differentiation as it spreads out The younger intrusion has a different root zone and is up to 8km thick. The discussion about its shape suggests either different roots, like a bundle of plutons, as in Fig.(a). or a unique root feeding different chambers, as in Fig.(b).
The isotope signals show that it is a mixture of mantel and melted crustal material. The granites of the Fichtel Mountains are also found together with other volcanic rocks, which span from grano-diorites (Redwitzite), to diorites to gabbros, which is magma originating from the mantel.
Our next stop was at the Steinbrücke quarry, near Bad Bernek, where pillow lavas can be seen in the low metamorphic rocks of the Münchberg Complex, the quarry is positioned in a fault zone not far from the Franconian Lineament. In the Ordovician, rifting at the rim of the Gondwana, created a micro-continent. In the Devonian, the continental margin sedimentary basin was intruded with basalt forming these pillows. It is not mid-ocean ridge basalt, the setting would have looked more like the one in the figure below.
We continued our journey northwards to the Münchberg Nappe Complex, which has been enigmatic since its discovery some 200 years ago. The metamorphic gradient is reversed, with high-grade metamorphic rocks lying on top of low-grade metamorphic rocks.
There are 4 nappes in the Münchberg Complex, see figure below. The upper unit (4) was laid down under ductile thrusting conditions, at a temperature of over 400° C. The lowest level of metamorphism is found in unit (1), prasinitic phyllite, with evidence of highly brittle thrusting. The intermediate units (2) and (3) also display thrusting under low temperature, brittle conditions. Below the four nappes, there is a unit known as the 'Bavarian facies'.
The brittle to ductile transition across the thrusts is not easy to explain. The thrusts took place during different events, in different time intervals and from different directions There has been a 200 year discussion about the timing and setting of the sequences, which is still going on.
Not only was each unit transported in a different direction, they were deformed at different stages. The Münchberg Nappe is a klippe as there is no connection to the root.
We stopped at the place called Hohenknoden where the contact between the brittle and ductile thrusting is exposed.The figure below shows approximately what we could observe at this location and where we can assume the thrusts would be.
We continued to our last stop of the day at Weissenstein, where the highest grade metamorphic rocks of the Upper Unit, eclogite, are exposed at the southwest corner of the Münchberg Complex. The eclogite is composed of green clino-pyroxene (omphacite) and red pyrope garnets. It was formed at around 600°C and 20 kbar or 2 GPa, that is at a depth of about 70km. The age of the metamorphism is around 390Ma. The extremely high pressure and moderate temperature is characteristic of a subduction zone, where cold oceanic crust is subducted. Clearly, this rock was formed in a very different environment than the continental shelf sediments of the underlying Bayerische Facies of the Saxothuringian Unit.
Sample collection directly at the Geosite is not allowed, so the group walked to an exposure which was outside of the prohibited zone, where Dr. Johann Rohrmüller still had enough strength to hammer rock samples off the exposure for all participants and very impressive they are. That was the end of the day! And what a day!