Hunsrück slate

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As Hunsrückschiefer become weak metamorphic overprinted shales from the western Rhenish mountains , in particular the Hunsrück , the Taunus and the Eifel referred to as marine sediments in the era of Devon have been deposited. The easily split slate was previously widely used as roofing slate , and this use continues to this day in a few companies.

In geology often be distinguished Hunsrückschiefer in the strict sense , the only sediments from the chronostratigraphic stage of Emsium , or only its earlier especially the reference Brachiopodenfauna distinct biostratigraphic sublevels elm (or elm and wins ) would include the elderly Unteremsium and a Hunsrückschiefer in a broader sense , which would also include all rocks of similar facies from adjacent lower stages. The Hunsrück slate in the narrower sense includes in particular the world-famous fossil deposit of the roof slate pits near Bundenbach , Gemünden and Herrstein in the Middle Hunsrück , to which the name is often directly related. Other authors completely reject a stratigraphic use, for them the Hunsrück schist is only a lithological and facial designation.

distribution

The Hunsrück slate forms a broad band, strongly articulated by folding, which runs through the Hunsrück on the left of the Rhine and the south-west Taunus on the right-hand side of the Rhine, north of the city of Bingen am Rhein , and also, separated from it, a belt in the southern Eifel between the Moselle in the south and the city of Mayen in the north. It is the sediment filling of a former trough-like sea basin, which is over five kilometers thick in places in trough axes and around one kilometer thick in the Middle Hunsrück. The sediment trough, interrupted and covered by younger sediments, continues to the northeast at least over the Lahn-Dill area to the Harz Mountains . The facially similar slate rocks like the Wissenbach slate are no longer referred to as Hunsrück slate. Stratigraphically, the typical Hunsrück slate is underlain by Taunus quartzite or other quartzites such as Gilsbach quartzite and overlaid by so-called porphyroids, volcanic tuffs. However, one or both of them are missing in parts of the distribution area.

Emergence

In the Lower Devonian, the region of today's Rhenish Slate Mountains formed a section of an elongated sea basin, then located near the equator, which is known as the rheno-hercynian basin (also rheno-hercynian zone, rheno-hercynian ocean). This was a marginal sea of ​​the Rheic Ocean , from which it was separated by a threshold, possibly also a peninsula or chain of islands (an earlier continental zone probably disappeared in the Unteremsium and was replaced by a submarine threshold). The Rhine Ocean separated a northern continent, called "Old Red" or Laurussia , which was formed shortly before by merging several crustal blocks or terranos , Baltica , Avalonia and Laurentia , and a southern continent, called Gondwana . The interpretation of the Rhenohercynian Basin is not entirely clear in geology. While some geologists assume a passive continental margin , many assume an active continental margin with a separate subduction zone . In both cases, a relatively narrow (250 to 300 kilometers), but elongated (more than 2000 kilometers) sea basin in the southwest-northeast extension is assumed. The Hunsrück slate emerged from the sediment filling of the rhenohercynian basin. The basin, which is divided into several lower basins separated by thresholds, sank sharply , presumably at an active rift as far as the Lower Emsium, so that sediment thicknesses of a total of 10 kilometers resulted. The coastline of the Old Red continent is located roughly on the line of today's cities of Aachen and Leverkusen.

The sediments of the Hunsrück slate were brought into the basin either from the north, from the Old Red continent, or from the threshold region in the south (“Central Hunsrück Sill” as part of the Central German Sill). Depending on the proximity to the coast, it was fine-sandy to clayey sediments that were deposited in a shelf sea , presumably at a depth of no more than about 200 meters. Coarser sandy or gravelly (conglomerate) sediments are completely absent here. In some cases, delta formations influenced by fluvial sediments are to be developed. Unstably stored sediments could suddenly slide again, for example during seismic activity, and form submarine turbid currents. These presumably led to the good preservation of the fossils.

Later, in the course of the Variscan mountain formation, the continents Laurussia and Gondwana collided, with the Rhine Ocean and its adjacent sea disappearing. The sediments were unfolded and metamorphically embossed, creating today's Hunsrück slate.

rock

The layer joints of the slate plates follow the foliation of the rock, so they were only created in the course of the metamorphosis and do not correspond to the old layer boundaries of the sediment. Therefore, well-preserved fossils can only be found in the deposits in which foliation and sediment layers run approximately parallel to each other, otherwise the fossils disintegrate when the slate plates split. In the case of a clear angle between foliation and sediment sequence, the slate plates are striped (called "Knappstein").

Roofing slate

Hunsrück slate was, and is still partially extracted today, for the extraction of slate as roofing slate. Only slates that can be split easily and straight into slab-like form with a small amount of extraneous additions are suitable, especially iron sulfides ( pyrite and marcasite ), calcite and organic, carbonaceous additions. Slate that breaks in splinters is mined locally to a lesser extent for road construction. The roofing slates of the Hunsrück are colored monochrome black. The layers suitable for roofing slate are usually not very thick and only last a few hundred meters, resulting in a large number of mostly small pits. Mining was carried out both in open-cast and underground tunnels. The roofing slate was previously split and trimmed by hand with split iron and wooden hammers. On the left bank of the Rhine, in the Hunsrück and the Eifel, 22 roof slate pits were still being mined in the Hunsrück slate in the immediate post-war period. Mining ceased in the 1960s because cheaper imported slate and synthetic "artificial slate" pushed the material off the market. At times it was still in demand, especially for the restoration of historical buildings.

The only roof slate mine in the Hunsrück slate that will be working in 2020 is the Altlayer slate mine in the northern Hunsrück , which was newly opened in 1984 . No fossils are found in the Altlayer slate quarry. An important mining area for roofing slate in the Hunsrück slate was the so-called Moselle slate with the mines Grube Katzenberg near Mayen and Margareta near Polch in the Eifel. The dismantling at Mayen was stopped in 2019. In the approximately 3 kilometers thick series of slates are embedded four series that were suitable as roofing slate. In Hesse, roofing slate used to be extracted from Hunsrück slate in the Wispertal between Bad Schwalbach and Lorsch, the mining was only of local importance and was discontinued a long time ago.

The former roof slate pits near Bundenbach in the Hunsrück are famous as fossil deposits, the finds of which are shown in museums around the world. The last dismantling by Johann & Backes was stopped here in 1999. The Herrenberg mine, which works underground, was made accessible to tourists as a visitor mine. The slate layers in the last working opencast mine Eschenbach were almost vertical. At the end of mining, scientists in 1997 in the "Nahecaris project" (named after the Nahecaris , a fossil genus of the Phyllocarida subclass of the higher crustaceans ) excavated a rock in a scientific excavation in order to elucidate the stratigraphy of the fossil-bearing layers in detail. Since most of the fossils had been found by quarry workers while preparing the plates, this was until then insufficiently known.

Fossils

Bundenbacher slate

The roofing slate deposits near Bundenbach show the best preserved fossils of the Hunsrück slate, so that in international specialist literature the name Hunsrück slate is often only used for this fossil deposit. In the Bundenbacher Schiefer there are also similar faunas as in other sites of the Hunsrück Schiefer as a whole, but these in the activated rocks of other facies, i.e. not together with the roofing slate fossils. The special preservation of the roof slate fossils is explained by the fact that during submarine landslides (turbid currents) in slope areas of the shelf, entire communities were abruptly showered with sediment and thus escaped normal decomposition. Contrary to previous assumptions, the seabed here was probably not anoxic and hostile to life. The special preservation of the fossils in the Bundenbach slate, which are often preserved in soft tissue, is based on the fact that the fossils are pyritized, i.e. the entire former living being, including parts of the soft body, has been replaced by the iron mineral pyrite . This also makes it possible to visualize fossils that are still completely embedded in the rock by X- raying them; this technique was re-developed since the 1950s by the physicist Wilhelm Stürmer for the investigation of fossils using the Hunsrück slate. In addition, some fossils have been partially replaced by silica and phosphates.

In the Bundenbacher schist, entire communities of the seabed have been preserved in fossil form and allow an insight into the Devonian fauna that is almost unique worldwide. However, the vertebrates are rather poorly represented. There are seven species of tankfish here , but most of them are only preserved in fragments. There is only one isolated sting of the spiny sharks ( Acanthodii ). Well-preserved shellfish of the genera Gemuendina ( Rhenanida ) and Drepanaspis ( Pteraspidiformes ) were flattened, bottom -living forms that resembled recent angel sharks , they reached a maximum body length of about 60 centimeters to one meter.

Individual evidence

  1. ^ A b Hans-Georg Mittmeyer (1980): On the geology of the Hunsrück slate. In: Wilhelm Stürmer, Friedemann Schaarschmidt, Hans-Georg Mittmeyer (editor): Petrified life in X-rays. Kleine Senckenberg series no. 11. Verlag Waldemar Kramer, Frankfurt am Main 1980. ISBN 3-7829-1078-8 , pp. 26-33.
  2. ↑ so also as "Hunsrück slate, elm lower stage" in the general legend of the geological overview map of Germany 1: 200,000, Federal Institute for Geosciences and Natural Resources, Hanover 2015.
  3. ↑ a b c d e Gabriele Kühl, Christoph Bartels, Derek Briggs, Jes Rust: Fossils in the Hunsrück slate. Edition Goldschneck in Quelle & Meyer Verlag, Wiebelsheim 2012. ISBN 978-3-494-01483-8 .
  4. ↑ Jürgen Gad (2006): What actually is Hunsrück slate? Annual reports and communications from the Upper Rhine Geological Association NF 88: 53-65.
  5. ↑ a b J.Stets & A. Shepherd: The Siegenian delta - land-sea transitions at the northern margin of the Rhenohercynian Basin. In: P.Königshof (editor): Devonian Change. Case Studies in Palaeogeography and Palaeoecology. Geological Society Special Publication no.314. published by the Geological Society, London 2009. ISBN 978-1-86239-273-1 .
  6. ↑ a b T. Schindler, OE Sutcliffe, C. Bartels, M. Poschmann, M. Wuttke (2002): Lithostratigraphical subdivision and chronostratigraphical position of the middle Kaub Formation (Lower Emsian, Lower Devonian) of the Bundenbach area (Hunsrück, SW Germany). Metalla (Bochum) 9 (2): 73-88.
  7. ↑ a b c Peter Königshof, Raph Thomas Becker, Sven Hartenfels (2016): The Rhenish Massif as a part of the European Variscides. Münster research on geology and palaeontology 108: 1-13.
  8. ↑ H. Wolfgang Wagner (2018): Roof and wall slate - a traditional building material in Central Europe. Publications of the “Stones in the City” network, issue 1. 31 pages.
  9. ^ Hermann Hommer (1966): Slate mining in the Hunsrück. Messages from Pollichia, 3rd row, 13: 142-144.
  10. ^ German slate mine Altlay in the Hunsrück. Nikolaus Theis Nachf. Böger GmbH
  11. ↑ Jens Albes: Glück auf: Does slate mining have a future? , Article, www.welt.de, February 18, 2020.
  12. ↑ Wolfgang Wagner (1990): Roof slate deposits in Rhineland-Palatinate with special consideration of the Mayen roof slate sequence (deposit name: Mosel slate) (Rheinisches Schiefergebirge). New Yearbook for Geology and Paleontology - Monthly Issues Vol. 1990 Issue 1 (1990): 54-64.
  13. ^ Hessian State Office for Environment and Geology (publisher): Raw material security concept Hessen. Technical report natural stones and natural stones. Processing status 11/20/2006.
  14. ↑ Herrenberg mine for visitors in Bundenbach . Museum portal Rhineland-Palatinate.
  15. ↑ Michael Wuttke, Thomas Schindler, Markus Paschmann (2002): Nahecaris project: Deciphering Devonian Palaeo ecosystems from the Hunsrück slate of Bundenbach. Metalla 9 (2): 59-138.
  16. ↑ a b Hans Jahnke, Christoph Bartels: The Hunsrück slate and its fossils, Lower Devon. In Dieter Meischner (editor): European fossil deposits. Springer Verlag, Berlin and Heidelberg 2000. ISBN 978-3-642-62975-4 .
  17. ^ Wouter H. Südkamp (2007): An atypical fauna in the Lower Devonian Hunsrück Slate of Germany. Paleontological Journal 81: 181–204.
  18. ↑ Wilhelm Stürmer: X-rays explore prehistoric times. In: Wilhelm Stürmer, Friedemann Schaarschmidt, Hans-Georg Mittmeyer (editor): Petrified life in X-rays. Kleine Senckenberg series no. 11. Verlag Waldemar Kramer, Frankfurt am Main 1980. ISBN 3-7829-1078-8 , pp. 3-18.
  19. ↑ Thomas Fletcher, John Altringham, Jeffrey Peakall, Paul Wignall, Robert Dorrell (2014): Hydrodynamics of fossil fishes. Proceedings of the Royal Society B 281: 20140703. doi: 10.1098 / rspb.2014.0703