Planks (geotope)
The Bohlen (also Obernitzer Bohlen) near Saalfeld - Obernitz is a 800 meter long and 100 meter high geological outcrop in the Saale valley between the villages of Köditz and Obernitz. Are open in the geological profile folded rocks of the Middle Devonian to sub-Karbon , the younger sediments of Perm discordant be superimposed. Due to the exceptionally good insight into the sequence of layers, the tectonic development and the important fossil finds , the plank was awarded as a national geotope in 2006.
location
The outcrop is located on the east side of the Saale valley 2 km south of Saalfeld on the B 85 between the districts of Köditz and Obernitz. The upper edge of the outcrop rises from northwest to southeast from 326 m NN to 340 m NN.
History of exploration
As early as 1761, the personal physician of Friedrich Karl von Schwarzburg-Rudolstadt and natural scientist Georg Christian Füchsel described the explanation in his treatise Historia terrae et maris, ex historia Thuringiae, per montium descriptionem. Actorum Academiae electoralis Moguntinae. In the second half of the 19th century, the fossil finds in the plank profile aroused scientific interest. Numerous stratigraphic and palaeontological works by Reinhard Richter contributed significantly to the understanding of the sedimentation and development history of the rock sequences on the planks. Numerous micro and macro fossils - in particular goniatites , clymenia and trilobites - were described for the first time on the planks and have their type locality here .
geology
The sequence of layers of rocks of folded Paleozoic basement in a includes thickness of about 200 meters sediments of the Middle Devonian (385 million years) to Lower Carboniferous (340 million years). The oldest exposed rocks are the dark gray to black, slate claystones of the black slate formation of the Steinach group of the upper Middle Devonian . These rocks were covered by the sediments of the Saalfeld group , beginning with the Upper Devonian Grauwacken , alternating with tuff bands , black clay slate and clay slate with limestone deposits of the shepherd's range formation .
Above this, the Upper Devonian rocks follow the plank formation , which has its type locality in this profile. The sequence of layers begins with green-gray to gray clay and marl slates, in which calcareous knot slate and Tuffitic horizons are embedded ( slab fracture subformation ). Above it are gray, small-nodular limestones of the Gositzfelsen subformation . The transition between the rocks of the Upper Devonian and Lower Carboniferous are marked by the silt slate, clay slate with limestone nodules and fine sandstone layers of the Gleitsch formation .
The subsequent sub-carbonic sequence of the Leutenberg Group is characterized by the increase in clastic rocks from the lying to the hanging wall. At the base of the lower carbon there are black-gray soot slates with phosphorite concretions ( soot slate subformation ) and blue-gray roof slate . Both sequences belong to the Lehesten group widespread in this area . The youngest sub-carbonic sediments that are exposed in the plank profile are the fine sandstone and slate of the Hasenthal formation .
During the Variscan orogeny about 338 million years ago, the rock sequences were intensely tectonically deformed. The sediment packages were unfolded and in places discarded against each other in deep disturbances . As a result of these faults, there is a doubling of the layers in the profile, especially in the middle part of the profile at the so-called Koehler and Richter fault . From a regional geological point of view, the outcrop is today on the eastern flank of the Schwarzburger saddle and on the western flank of the Ziegenrücker Mulde .
After the Variscan Mountains unfolded, they were gradually leveled in the Permian . In the shallow erosion depressions in the Rotliegend the erosion debris of the Variscan Mountains was deposited . The Rotliegend sediments are only preserved in places today, in morphologically protected positions at the top of the folded basement. During the Zechstein period , the area was flooded several times by a shallow sea . Clay, carbonate and dolomitic rocks of the Werra to Leine series (Zechstein 1 to 3) were deposited discordantly above the folded rocks, which are clearly visible on the upper edge of the plank profile.
The most recent formations are terrace deposits of the Pleistocene and the alluvial fans in the terrain cuts and the anthropogenic formations (embankments, heaps) of the Quaternary .
use
Some pieces of rock were mined in the past for the extraction of stone . In particular, the reddish colored Upper Devonian knot slate and knot limestone were sought-after natural stones. They were dismantled in the so-called Obernitz plate quarry until the Second World War . Some of the limestone slabs were processed into pavement, base and facade slabs. The higher quality parts were most recently processed and marketed in the Saalburg marble works as Fischerdorfer marble . In 1938, 1,100 cubic meters of material were extracted from the Obernitz slab quarry.
At the northwest end of the profile, the mouth of the Gottlobstollen is still preserved today. This tunnel is a relic of the attempts at mining the sequence of layers. Black and alum shale in particular have been the focus of mining activities since the Middle Ages . In addition to the extraction of alum - which was mined on a large scale in the nearby fairy grottos, silver- and lead-containing pale ores , copper, iron and cobalt ores as well as barite were brought to light here.
Geotope
The outcrop wall was designated as a geotope by the Thuringian State Institute for Environment and Geology . In 2006 the plank wall was included in the list of 77 awarded national geotopes in Germany.
natural reserve
Due to the exposed location, the different rocks in the subsurface, very differentiated site conditions and the formation of a flora and fauna typical of the site developed . The 21 hectare area around the planks was placed under nature protection on June 21, 1938 .
The plants to be found here include the shaggy flag vetch , the sand cinquefoil , the real feather grass , the pale scotland , the real rock pear , the European yew . the mountain leek , the lime blue grass and the common medlar . Specimens of the Alpine aster are less common here. A maple-linden rubble forest has developed at the foot of the slope in recent decades . On the exposed locations to form areas with Whit cloves from -Blauschwingelgras and rocks Pear bluegrass lawns, which are among the most developed plant communities in Thuringia. The increasing encroachment of the outcrop is moderately held back in coordination with the nature conservation authorities in order not to endanger the visibility of the outcrop. There is also a breeding ground for a European eagle owl in the rock face .
Trivia
Due to the special storage conditions and the excellent exposure conditions at the time, Johannes Walter chose the profile picture of Bohlens as the cover of his publication Geologie von Deutschland, published from 1910 onwards .
literature
- Horst Blumenstengel: Testimony turned to stone to a collision between two continents 330,000 years ago - the "Bohlen" near Saalfeld . In: Ernst-Rüdiger Look & Ludger Feldmann: Fascination Geology. The most important geotopes in Germany . Academy of Geosciences in Hanover eV (Ed.), E. Schweizerbart'sche Verlagsbuchhandlung (Nägele and Obermiller), Stuttgart 2006, ISBN 3-510-65219-3 , p. 72f.
- Georg Christian Füchsel: Historia terrae et maris, ex historia Thuringiae, per montium descriptionem. Actorum Academiae electoralis Moguntinae , Erfurt 1761
- Heinz Pfeiffer: The planks near Saalfeld / Thür . Geology 3, supplement 11, Berlin 1954
- Heinz Pfeiffer: The geological exploration of the Thuringian Slate Mountains from G. Ch. Füchsel to the present . Rudolstädter Naturhistorische Schriften, 6, 1994, pp. 3–20
- Otfried Wagenbreth , Walter Steiner : Geological forays. Landscape and geological history between Cape Arkona and Fichtelberg , VEB Deutscher Verlag für Grundstoffindustrie, Leipzig 1989, 3rd edition, ISBN 3-342-00227-1 , pp. 17, 124
- Luise Grundmann: Saalfeld and the Thuringian Slate Mountains: a geographical inventory in the area of Saalfeld, Leutenberg and Lauenstein. Landscapes in Germany - Values of the German homeland, Volume 62, Böhlau, Cologne / Weimar 2001, ISBN 9783412108007 , 104ff.
- K. Bartzsch, Dieter Weyer: Biostratigraphy of the Devonian / Carboniferous border in the plank profile near Saalfeld (Thuringia) . Journal of Geological Sciences, 14, 1986, pp. 147-152
- H. Meyer: The planks near Saalfeld in Thuringia , Saalfeld, 1920
- Thuringian State Institute for Environment and Geology: A National Geotope in Germany. The planks near Saalfeld / Thuringia , Flyer, Jena 2008
Web links
Individual evidence
- ↑ Horst Blumenstengel: Bohlen-Formation (ID 37). (No longer available online.) In: LithoLex (online database). BGR, May 16, 2008, formerly in the original ; accessed on December 7, 2017 . ( Page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
- ^ Otfried Wagenbreth, Walter Steiner: Geologische Streifzüge. Landscape and geological history between Cape Arkona and Fichtelberg . 3. Edition. VEB German publishing house for basic industry, Leipzig 1989, ISBN 3-342-00227-1 , p. 17 .
- ^ Otfried Wagenbreth, Walter Steiner: Geologische Streifzüge. Landscape and geological history between Cape Arkona and Fichtelberg . 3. Edition. VEB German publishing house for basic industry, Leipzig 1989, ISBN 3-342-00227-1 , p. 124 .
- ↑ a b c Thuringian State Institute for Environment and Geology (Ed.): A national geotope in Germany. The Bohlen near Saalfeld / Thuringia . Flyer. Jena 2008.
Coordinates: 50 ° 37 '53.8 " N , 11 ° 22' 58.7" E