Zechsteinmeer

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Expansion of the Zechstein Basin approx. 255 million years ago (outlined in red) compared to the current geography of Central Europe
The Dead Sea in the border area Israel-Jordan-West Bank, an over-salted body of water in a hot desert. It could have looked something similar from time to time over 250 million years ago on the shores of the Zechstein Sea.

The Zechstein Sea was a flat Epikontinentalmeer that in the late Permian ( Lopingium ) about 258-250 million years ago in what is now Central Europe was. It extended with an area of ​​approximately one million square kilometers, i.e. more than twice the area of ​​the Black Sea , from what is now England to what is now the Baltic States and from what is now the northern North Sea region to what is now southwest Germany. The sedimentary basin , which was covered in large part from the Zechstein Sea is Zechsteinbecken called. It represents the earliest phase in the history of the Germanic Basin .

Formation of the Zechstein Sea

Outcrop with Rotliegend sediments, which are overlaid by the lowest layers of the Werra formation ("mother seam", copper slate and basal Werra carbonate), in Schiefergasse in Gera-Milbitz, Thuringia
"Lot's Wife" ( " Lots wife"), a surf pillars in Marsden Bay near Newcastle , northeast England. The rock consists of the dolomite of the British stratigraphic counterpart of the Staßfurt carbonate. It is an alternating layer of very fine-grained, small- thickness micrites and coarser-grained, thicker layers of calcareous biide .

After the Variscan mountain range in the Upper Carboniferous, the erosion of the Variscan mountain range began. The corresponding sediments ( molasses ), which were deposited between the Upper Carboniferous and Middle Perm in the northern foothills and in smaller basins in the interior of the mountains, reddish sandstones and conglomerates, are called Rotliegend . In the Upper Permian, large areas of the mountain range sank and, together with the former foreland, formed an extensive plain, the Germanic Basin or its forerunner, the Zechstein Basin . More or less at the same time, a rift was formed between Norway and Greenland, then immediately adjacent because the North Atlantic was only to emerge more than 150 million years later . Through this the sea penetrated from the north to Central Europe and flooded the plain. This sea invasion has been handed down as a Zechstein transgression in the sequence of rocks in Central Europe . The earliest deposits of the Zechstein Sea come to lie on Rotliegend sediments as well as directly on folded pre-Permian rocks of the Variscan Mountains. In the latter case one speaks of the Zechstein discordance .

The name of the sea and the basin is derived from the deposits left behind or contained in it, which lithostratigraphically are summarized under the name Zechstein . They consist largely of gypsum and rock salt (halite), which belong to the evaporite sediment class , and are now widespread in the subsurface of Central Europe. The evaporite-rich deposits of Zechstein are also known as Zechstein saline . In contrast, there are clastic deposits of the coastal plain of the Zechstein Sea ( marginal facies ) that do not contain any evaporites.

Zechstein transgression and copper slate

Traditionally, the so-called Zechstein conglomerate , a sediment that was created by the processing (erosion and redeposition) of older (pre-Upper Permian) rocks in the area of ​​the advancing coastline of the Zechstein Sea, was regarded as the first geological evidence of the Zechstein transgression. However, since there were already minor sea invasions in Central Europe during the deposition period of the Rotliegend and a Zechstein conglomerate where Rotliegend deposits merge into Zechstein deposits is difficult to distinguish from underlying Rotliegend conglomerates, the copper shale is now formally considered to be the oldest deposit in the Zechstein Sea . Copper slate, a black clay stone best known for its ore and fossil management, represents a phase of rapid sea level rise ( maximum flooding ) with very low sedimentation rates (i.e. it took a relatively long time to produce a relatively small amount was deposited on sediment). Only after the rise in sea level slowed down and the Zechstein salinar began to be deposited did the sedimentation rates increase e.g. T. drastically.

Creation of the Zechstein Salinar

The evaporite deposits were able to form because the region of the Zechstein Basin was very dry and warm (→  arid climate ). Due to the high evaporation rates and the narrow connection between the sea and the ocean in the north, the concentration of the salts dissolved in the sea ​​water gradually increased . These salts precipitated according to their solubility, first the poorly soluble carbonates ( limestone , presumably subsequently converted diagenetically to dolomite ) and gypsum (diagenetically converted into anhydrite ), then the predominantly sodium chloride (as halite) and finally the potassium and magnesium chlorides and - sulfates (commonly referred to as potash salts or precious salts ). Every now and then the sea water experienced z. B. due to a temporary weakening of the dryness, a sweetening and the progressive precipitation (precipitation from sparingly soluble to easily soluble) reversed (recessive precipitation: from easily soluble to sparingly soluble) or stopped completely, only to start again when the dryness increased again . The period between the onset and interruption of precipitation or the corresponding rock sequence is called the evaporation cycle . Through the continuous, gradual lowering of the basin floor ( subsidence ) and the alternation between drier and wetter periods, a sequence that is more than 1500 meters thick in places, in which several evaporation cycles are documented, was created over millions of years. Since they have the most extreme educational conditions, the layers of potash salt ( potash seams ) , which are particularly sought after as a raw material, are no thicker than three to eight meters. The deposits of two evaporation cycles are ideally separated from each other by clay sediments, which are assigned to the more recent evaporation cycle and were probably deposited in particularly strong sweetening phases.

Generalized, schematic profile of the Zechstein in the northern Harz foreland. Note that the Werra sequence (Werra formation) only contains anhydrite and no rock or potash salt and that cycles z5-z7 do not contain any significant amounts of evaporite.

Classically, four main evaporation cycles, represented by four saline sequences, are distinguished:

Especially in the subsurface of northern Germany and the North Sea, three more salinar cycles can be detected: Ohre cycle / formation (z5 / zO), Friesland cycle / formation (z6 / zFr) and Fulda cycle / formation (z7 / zFu), which in terms of thickness and evaporation do not come close to the four main cycles / formations. The cycle z7 was previously considered to be represented by the Mölln sequence. In the meantime, the latter has been integrated into the upper part of the Friesland Formation as the Mölln Subformation and thus part of the 6th cycle (the lower part of the Friesland Formation is called the Eider Subformation). The 7th cycle is now represented by the Fulda Formation, which in turn is largely identical to the Bröckelschiefer series of older literature.

As the extensive restriction of the episodes z5-z7 to northern Germany and the North Sea region shows, the Zechstein series is not equally powerful or similar everywhere in the Zechstein Basin. Furthermore, during the first evaporation cycle, the precious salt phase is only found in the Werra Basin (hence also known as the Werra Formation or Werra Cycle) in today's Hesse and Thuringia, in the Lower Rhine Basin (North Rhine-Westphalia) and in the so-called Peribaltic Basin reached in the area of ​​the Russian exclave Kaliningrad . In contrast, in the following cycles (Staßfurt and Leine), potash salts are no longer deposited in these sub-basins, but in extensive areas of the central Zechstein basin. The reason for this could be that, on the one hand, these three sub-basins were separated from the rest of the Zechstein Sea by a relatively pronounced threshold during the Werra cycle, where the concentration of dissolved substances was apparently lower, and on the other hand, that during the subsequent cycles the subsidence in the three sub-basins was lower, so that they had largely silted up before the potash salt phase was reached. In the early phases of the evaporation cycles, carbonates (partly with reefs ) formed mainly in the shallowest parts of the sea, near the basin edges and in the threshold areas.

Life in the Zechstein Sea

Wodnika (live reconstruction), a species of shark that also lived in the Zechstein Sea

Multicellular organisms only existed in the Zechstein Sea during the phases in which the sea water was not extremely over-salted. Fossils therefore only occur in the lowest layers of the evaporation cycles, in the clay and carbonate rocks.

A large number of vertebrate fossils occur in the copper schist (lowest Werra formation) in particular, including both fish that lived in the Zechstein Sea (e.g. Palaeoniscum freilebeni ) and terrestrial vertebrates that live on the dry land of the Zechstein Basin have lived and were washed into the sea after her death (z. B. the Pareiasaurier Parasaur geinitzi or early Diapside protorosaurus speneri ).

In the carbonates are found in z. A large number of brachiopods (e.g. the spiny form Horridonia horrida ) and bryozoa (especially in the reef complexes of the Werra formation, e.g. the genus Fenestella ).

During the precipitation phases of gypsum, rock salt and potash salt, only extremophile single-cell organisms are likely to have survived in the water of the Zechstein Sea. The interactions of the metabolic products of some of these unicellular cells with the earth's atmosphere could have contributed to the Permian Triassic mass extinction.

meaning

Influence on topography and geology

The deposits of the Zechstein Sea are in fact the only sediments in the geological history of Central Europe that contain large amounts of rock salt. If this rock salt is subjected to high pressure due to the load on the surface layers, it behaves plastically, it begins to flow to where the pressure of the overlying layers is lowest ( halokinesis ). Where the salt migrates ultimately arise salt domes . The overlying overburden is raised and, insofar as the raised rocks are more resistant to erosion than the surrounding rocks, mountains or entire ridges are formed there, such as B. the Elm near Braunschweig. The red sandstone rock of Heligoland has also been pushed out of the subsoil through younger cover layers through the action of a salt dome to the surface of the earth. Where the salt migrates, the overburden is lowered, creating so-called edge depressions . The halokinesis of the Zechstein Salinar had a lasting influence on the history of sedimentation in northern Germany from around the second half of the Mesozoic era through the creation of storage areas (marginal depressions) or delivery areas (mountain ranges). The so-called saxonic tectonics is strongly influenced by halokinetics, at least in northern Germany.

The rise of the salt domes has also created geological structures in the subsurface of northern Germany (especially in Lower Saxony) and the North Sea region, in which deposits for crude oil and natural gas could form (so-called crude oil and natural gas traps ), which, if exploited, still have a certain amount today Contribute to meeting the needs in Germany.

raw materials

Historical

Due to the rock salt storage of the Zechstein basin, northern Germany is the most saline area in Europe. Where salt deposits reach far to the surface of the earth (e.g. in the form of salt domes ), salt was already extracted in the Middle Ages. Since salt was a very popular commodity, it brought prosperity to the mining areas, such as B. Lueneburg . The medieval extraction and trade in salt can still be seen today in German place names such as Salzwedel , Halle , Salzdetfurth , Salzelmen or Salzuflen .

Industrial promotion

Large halite crystals in typical cubic form in the Merkers show mine , grown in a natural cavity of the Werra rock salt.
Gypsum crystals (here the transparent variety Marienglas), grown in a natural cavity in the Werra anhydrite. Marienglashöhle Friedrichroda , Thuringia.
Active stone (blue) and potash salt mines (pink) in Germany. The symbols of those mines that are built in the Zechstein Salinar have a light blue border.

Two processes are currently used in Germany for the industrial extraction of rock and potash salt from the Zechstein saline. Firstly, the borehole solution , in which the reservoir is drilled and then hot fresh water is pumped into it through the outer pipe of two pipes lying one inside the other. The salt solution (brine) that forms in the process rises to the surface of the earth in the inner pipe and is forwarded from there to the processing plant (e.g. in Ohrensen near Harsefeld in the Stade district, where the brine obtained in this way is piped directly into chlorine- Alkali electrolysis systems are pumped). This process is primarily used to extract rock salt, but is only worthwhile for particularly powerful deposits, e.g. B. Salt domes. The German rock salt deposits are estimated at 100,000 cubic kilometers. Around Staßfurt thicknesses of one kilometer were determined.

Potash salt, on the other hand, is mostly extracted in large mines with the help of heavy machinery.

The rock salt is mainly obtained for the production of table salt and road salt and as a raw material for the chemical industry ( industrial salt ). The potash salts are processed into artificial fertilizers, among other things.

The Zechstein's gypsum and anhydrite deposits are also being dismantled. The plaster serves u. a. as a raw material for the building materials industry, for porcelain and ceramic production and for the industrial production of sulfuric acid.

Until the beginning of the 1990s , copper slate, the ore-bearing black clay stone horizon in the deepest part of the Werra Formation, was mined in the Mansfelder Land , before that also in the Richelsdorf Mountains and other regions in Germany, for the extraction of various metals. Today copper shale is only mined in Lower Silesia , in the Lubin-Sieroszewice mining area.

Others

Subterranean cavities that are created by the solution of salt deposits, so-called caverns , are used as underground storage for crude oil and natural gas. For example, at the locations in Heide (Schleswig-Holstein), Burglesum (Bremen), Sottorf and Wilhelmshaven-Rüstringen (both Lower Saxony), Nord-West Kavernen GmbH (NWKG) produces around 15 million tons of crude oil, heating oil and other mineral oils in 58 caverns saved. Each cavern measures 30 to 35 meters in diameter and 250 to 450 meters in height. The volume of a cavern thus corresponds to the volume of a single super tanker. As of December 31, 2011, there were 205 individual cavern storage facilities for natural gas in the Zechstein Salinar in Germany with a total volume V n of 13 billion cubic meters of working gas (under standard conditions), of which a maximum of 10 billion cubic meters could be used for effective injection and withdrawal, with a total natural gas consumption V n in Germany of 86 billion cubic meters of working gas in 2011 (or 99 billion cubic meters in 2010).

Open cavities in the salt are also used to dispose of hazardous waste . In Germany there are five in operation ( Herfa-Neurode , Zielitz, Sondershausen ) or disused ( Thiederhall ) underground landfills for non-radioactive “toxic waste” in the Zechsteinsalinar. The Asse II mine near Wolfenbüttel, which was initially designed as an “experimental mine ” for the storage of radioactive waste , is now a de facto repository . Radioactive waste that primarily originates from the operation of power and research reactors in the GDR and others Hazardous waste is stored in the Morsleben salt dome (→  Morsleben repository ). A mine was built in Gorleben im Wendland to investigate the Gorleben-Rambow salt dome with regard to its suitability as a repository for highly radioactive waste. Mineral waste (e.g. fly ash ) is sometimes used to fill excavated cavities in salt mines. Since this can stabilize the mine, which would otherwise have to be done with other materials, this is referred to as waste recycling .

The therapeutic efficacy of brine applications has meant that in many places near salt deposits spa town businesses have emerged. Many of these places have this reference in their names, such as Bad Salzdetfurth , Bad Salzuflen , Salzgitter-Bad or Bad Salzungen .

Web links

literature

  • Hans Füchtbauer : Sediment-Petrology , Part 2: Sediments and sedimentary rocks , Schweitzerbart, 4th edition 1988, ISBN 978-3510651382
  • Museum for Natural History of the City of Gera, Geraer Minerals and Fossil Friends e. V .: The fossil life of the Gera Zechstein lagoon 255 million years ago. Brochure for the special exhibition September 1, 2006 to March 31, 2007 online (PDF; 5.6 MB)
  • Peter Ziegler: Geological Atlas of Western and Central Europe , Den Haag 1990, ISBN 90-6644-125-9 .

Individual evidence

  1. ↑ Reinhard E. Gast: Cornberg outcrops revisited (Hessen, Germany): The depositional environment of its saurian tracks and Weissliegend Sandstones. Meyniana. Vol. 46, 1994, p. 67 (Fig. 3)
  2. ^ Bernard Cooper: A Classic Southern North Sea Analogue. GeoExPro, Vol. 7, 2010, No. 5 online (HTML version)
  3. ↑ a b c M. Menning, B. Schröder, E. Plein, T. Simon, J. Lepper, H.‐G. Röhling, C. Heunisch, K. Stapf, H. Lützner, K.‐C. Käding, J. Paul, M. Horn, H. Hagdorn, G. Beutler, E. Nitsch: Resolutions of the German Stratigraphic Commission 1991–2010 on Permian and Triassic of Central Europe. Journal of the German Society for Geosciences, Vol. 162, 2011, No. 1, pp. 1–18, DOI: 10.1127 / 1860-1804 / 2011 / 0162-0001 ( alternative download [PDF; 2.2 MB] of the manuscript from GFZ Potsdam)
  4. ↑ Josef Paul: Weißliegend, Grauliegend and the Zechstein conglomerate: the Rotliegend / Zechstein border. In: German Stratigraphic Commission (ed .; coordination and editing: H. Lützner and G. Kowalczyk for the sub-commission Perm-Trias): Stratigraphie von Deutschland X. Rotliegend. Part I: Innervariscan Basin. Series of publications by the German Society for Geosciences, Vol. 61, 2012, pp. 707–714 ( abstract )
  5. ↑ Frank Becker, Thilo Bechstädt : Sequence stratigraphy of a carbonate-evaporite succession (Zechstein 1, Hessian Basin, Germany). Sedimentology. Vol. 53, 2006, No. 5, pp. 1083-1120, DOI: 10.1111 / j.1365-3091.2006.00803.x
  6. ^ Gerhard Richter-Bernburg: Stratigraphic structure of the German Zechstein. Journal of the German Geological Society. Vol. 105 (born 1953), 1955, pp. 843–859 ( abstract )
  7. ↑ Gerhard Best: The Zechstein / Buntsandstein border in northwest Germany according to borehole measurements. Journal of the German Geological Society. Vol. 140, 1989, pp. 73-85 ( abstract )
  8. ↑ Linda A. Tsuji, Johannes Müller: A re-evaluation of Parasaurus geinitzi , the first named pareiasaur (Amniota, Parareptilia). Canadian Journal of Earth Sciences, Vol. 45, 2008, No. 10, pp. 1111-1121 DOI: 10.1139 / E08-060
  9. ↑ Annalisa Gottman-Quesada, P. Martin Sander: A redescription of the early archosauromorph Protorosaurus speneri Meyer, 1832, and its phylogenetic relationships. Palaeontographica, Section A (Paleozoology, Stratigraphy), Vol. 287, 2009, No. 4-6, pp. 123-220
  10. ↑ L. Weissflog, NF Elansky, K. Kotte, F. Keppler, A. Pfennig Dorff, CA Long, E. Putz, LV Lisitsyna: Late permian changes in conditions of the atmosphere and environments Caused by halogenated gas. Doklady Earth Sciences, Vol. 425, 2009, No. 1, pp. 291-295, DOI: 10.1134 / S1028334X09020263
  11. ↑ ExxonMobil Production Germany GmbH (Ed.): The production of natural gas. Hanover, 2009, online (PDF; 880 kB)
  12. ↑ Economic Association for Petroleum and Natural Gas Production e. V. (Ed.): Natural gas and oil from Germany. Hanover, 2008, online ( Memento from June 27, 2013 in the Internet Archive ) (PDF; 480 kB)
  13. ↑ Mining and saline sites for rock salt and potash salt mining sites in Germany. Website of the Association of the Potash and Salt Industry e. V. (VKS)
  14. ↑ Caverns for crude oil, petroleum products and liquid gas. Table from the 2003 yearbook of the European energy and raw materials industry, 110th year, Verlag Glückauf GmbH, Essen, 2002, p. 261 ; Course reserves of the University of Duisburg-Essen (PDF; 660 kB)
  15. ↑ Oil stocks in Germany Presentation at the autumn conference of the Austrian Society for Petroleum Sciences (ÖGEW) and the German Scientific Society for Petroleum, Natural Gas and Coal , October 11 and 12, 2007, Salzburg (PDF; 530 kB)
  16. ↑ The formation of the Etzel salt dome and the economic importance of the salt dome ; source-based page on Nordwestreisemagazin.de
  17. ↑ Underground gas storage in Germany. Oil, natural gas, coal. Volume 128 (2012), No. 11, pp. 412–423, online (PDF; 1.5 MB)