Salt dome

from Wikipedia, the free encyclopedia

A salt dome is the best known form of geological salt structure. Salt structures in the geological sense are all deformation structures that are created by the influence of flowable salt rocks in geological time periods . Salt structures therefore include both geological formations that consist of deformed salt rocks, such as salt domes and salt pillows, as well as surrounding structures that were formed at the same time and consist of other sedimentary rocks, such as marginal depressions. Salt structures occur exclusively in the upper crust of the earth (at depths of up to 15 km) and can reach to the surface of the earth. The general theories about the formation of salt structures are summarized in the article Salt Tectonics .

The salt rock necessary for the formation of salt structures is a chemical sediment from the group of evaporation rocks ( evaporites ). It mainly consists of the mineral halite . The sedimentary formation of evaporites was formulated as early as 1877 by Carl Ochsenius using the bar or threshold theory.

Types of salt structures

Salt domes

The most striking salt structures are salt domes (also salt diapirs called English. Salt diapir ) and salt walls (Engl. Salt walls or diapiric walls ). Salt diapirs are columnar, mushroom-like or, conversely, teardrop-shaped structures that can reach heights of up to 10 km and are surrounded by younger sedimentary rocks. Salt walls are also high, but laterally extended salt structures, which mostly follow an offset at the base of the salt layer. The longitudinal section through the Northwest German Basin shown below shows some salt domes whose root zone is in the Zechstein and partly also in the Oberrotliegend (the combination of Zechstein and Oberrotliegend salt in one salt dome, which is only known from northern northern Germany, is also called "double salinar " called.)

and display information about the image


Geological profile through the Northwest German Basin with salt domes (  Zechstein-Salinar , Oberrotliegend-Salinar )

Salt pillow

Salt cushion (engl. Salt pillows ) are elevations of the salt layer, the covering layers are still relatively intact, d. H. show no significant tectonic disturbances. The term salt anticline stands for elongated salt bulges with intact overburden.

Edge sinks

Due to the displacement or migration of the salt rock into a salt structure, neighboring areas of the overburden are lowered , which is expressed on the surface by local basin structures. Such structures are referred to as peripheral edge sinks (engl. Peripheral sinks ), Randsynklinalen (engl. Rim synclines ), or mini pool (engl. Minibasins ), respectively. Younger sediments are accumulated in these structures. A distinction is made between primary edge depressions , which are formed next to salt cushions and in which the thickness of the pool filling also decreases towards the salt structure, and secondary edge depressions , which are located next to diapirs and in which the thickness of the pool filling to the salt structure increases steadily. Conversely, the amount of subsidence in secondary marginal depressions continuously decreases with increasing distance from the salt dome. Therefore, an anti-form in the overburden between two adjacent salt domes can arise as turtles structure (engl. Turtle back structure ,) is called because it resembles a tortoise shell in seismic profiles the outline. Halfway between the diapirs, the original salt layer is thickest, and it increasingly thins towards the diapirs.

Salt blankets

Salt ceiling (engl. Salt canopy ) are an advanced form of Salzdiapirismus. They arise when, after the diapir has broken through, further salt is pressed (extruded) through the overburden and flows out on the surface. In the process, an allochthonous salt layer is formed, which usually no longer has any connection to the primary ( autochthonous ) salt deposit and largely overlays the original overburden. A typical area for such salt structures is the deep part of the northern shelf of the Gulf of Mexico . While the salt cover there is covered by post-extrusive layers and is also deep under the sea, extruded salt is found directly on the surface of the earth in the Zagros Mountains in today's Iran . This phenomenon, which may occur where only due to the extremely dry climates is when salt glacier , or after the Farsi -Vokabel for salt as Namakier designated.

Formation of salt domes in the North German Basin

The Northwest German Basin is part of a larger Central European basin system and one of the world's type areas for salt structures, in particular salt domes and salt walls. The formation of the salt structures in this basin can be described as follows:

  • About 260 million years ago (in the Upper Permian , also called Zechstein in the German-speaking area ), Central Europe was in the arid climatic zone . In parts of what is now northern Central Europe and what is now the North Sea region, the earth's crust subsided and formed a large, intra-continental sedimentary basin , the surface of which sank below sea level. A rift valley between today's Norway and today's Greenland, which at that time was still directly connected to Europe, formed a connection with the ocean in the north, so that the basin was filled with seawater. This shallow sea is called the Zechstein Sea according to the deposits it left behind, and the basin in which it spreads is called the Zechstein Basin (the “Zechstein Age” is also named after these deposits).
  • An occasional interruption in the exchange of water with the ocean and the hot, dry climate led to multiple, more or less complete evaporation of the water. The resulting increase in the concentration of dissolved salts in seawater led to the precipitation and deposition of carbonates (limestone) and above all of sulphates (mainly gypsum ) and chlorides (mainly rock salt ), so-called evaporites .
  • After less than 10 million years, this interplay between evaporation of the sea water and replenishment of the basin came to an end and the salt layers, which are now 500 to 3000 meters thick due to further subsidence ( subsidence ) of the earth's crust, were replaced by deposits from the geological ages of the Triassic , the Jurassic , the Cretaceous and Cenozoic overlaid.
  • In some areas of the Zechstein Basin, stretching movements of the earth's crust occurred as early as the Early Triassic, which were due to the long-range effects of plate tectonic processes. This initiated the rise of the salt ( salt tectonics ), which reacts visco-elastically under the pressure of the increasingly powerful overburden - comparable to ice and glacier movements  - and also has a lower density than the overlying and surrounding sedimentary rocks. The salt preferentially penetrated upwards on or in these "weak zones" (salt tectonics). The cover layers were arched upwards or pushed to the side. In the vicinity of the salt domes, however, the salt migrated and peripheral sinks formed on the surface of the earth above.
  • During the late Triassic and Jurassic, the earth's crust continued to expand, which triggered the formation of salt domes in other parts of the Zechstein Basin. Already active salt domes continued their ascent.
  • During the Upper Cretaceous, convergent crust movements led to the horizontal narrowing of the salt domes, especially on the southern edge of the former Zechstein Basin.

Interactions with the terrain surface

The red sandstone rock of Heligoland was pushed to the surface of the sea by a salt dome from the underground of the North Sea

The presence of a salt dome in the subsurface is particularly noticeable if it has pushed erosion-resistant rocks upwards, so that a ridge is formed on the surface of the earth. Such ridges can be found in Germany mainly in the northern Harz foreland, z. B. the Elm or the Asse (see also →  broad saddle , →  narrow saddle ).

In the rain-rich air, the readily soluble salt forms (principally halite ) in the roof area of a tight zoom reaching to the ground surface, or even without any overlapping pending salt dome as a result of Subrosion a horizontal surface, the so-called salt mirror from. This is then overlaid by residual rock, usually dolomite , gypsum or a mixture of both (so-called gypsum hat ). In very dry climates, however, the salt that has penetrated the surface of the earth is not washed out and can flow out and form mountains or salt glaciers several hundred meters high .

distribution

Salt domes are a worldwide phenomenon that occurs as soon as the salt layer is thick enough to allow salt movement. In Central Europe they are mainly to be found in the area of ​​the former Zechstein Basin, which stretched from southern England to central Poland and from the central North Sea to central Germany.

Satellite image with outcrops of salt domes and salt glaciers (dark) in the Iranian Zāgros Mountains
Residual plaster of paris from a salt dome that has broken through to the surface of the earth (the light-colored material in the center left of the picture) in a coastal outcrop on Cape Breton Island , Nova Scotia, Canada.

Examples of salt domes or salt mines located therein are:

In the salt deposits in the Northern Limestone Alps ( Alpine Triassic ), e.g. B. the Hallstatt salt mountain, it is not a question of salt domes in the true sense, because there the salt tectonics was transformed by the alpine tectonics.

Exploration of salt structures

Salt structures are located in the upper part of the earth's crust, but rarely penetrate to the earth's surface, as the salt minerals are loosened by the entry of groundwater. The exploration of salt structures is therefore carried out indirectly with geophysical measurement methods or directly with deep boreholes.

Since salt rocks have a lower density than other sedimentary rocks, the gravity (acceleration due to gravity) is locally lower above salt structures than in the neighboring areas. This effect can be determined with gravimetric measuring methods on the surface. With these methods, the spatial extent and the height of the salt structure can be estimated. In the 1930s, for example, a large part of the salt domes in the subsurface of the North German Plain were mapped using gravimetric measurements by Hans Haalck , Rudolf Meinhold , Fritz Haalck and Gerhard Richter-Bernburg .

With the seismic reflection method , mechanical waves (“sound”) are sent through the geological subsurface, which are reflected on layers of rocks with different densities. The geometries and depths of geological layer boundaries can be determined by spatially distributed recording of the reflected waves. In relation to the exploration of salt structures, reflection seismics enables the mapping of the outline of a salt structure, the geometries of the adjacent sediment layers and, in some cases, the internal structures within a salt structure with a resolution of a few tens of meters.

More precise data on the layer boundaries, the outline of the salt structure and, in particular, the composition of the salt rock provide deep boreholes directly into the salt structure and its surroundings.

Economic importance of salt structures

Extraction of salt minerals

Salt structures and salt rock layers are primarily used for the extraction of salt minerals, such as B. rock salt ( halite ) or potassium salts significant. Rock salt is used as table salt or grit. Potash salts (e.g. carnallite , kieserite or sylvine ) are used in the production of basic chemical substances, e.g. B. used for the fertilizer industry. Since salt minerals only appear on the surface in arid climatic zones due to their high solubility, the salt structures in more humid climates are often approached by salt mines .

Oil and gas exploration

The formation of salt cushions, salt domes, etc. changes the construction of the overburden and creates structures in which fossil hydrocarbons ( crude oil and natural gas ) can collect, so-called crude oil and natural gas traps . Since salt rock is almost impermeable to fluids , hydrocarbons (crude oil, natural gas) can accumulate beneath salt rock layers or salt structures. Furthermore, due to its high thermal conductivity, salt rock ensures that heat is conductively transported from greater depths to shallower depths. As a result, neighboring sediments of salt structures reach the petroleum or natural gas window at shallow depths (temperature range between 60 ° C and 170 ° C) than sediments at a greater distance from salt structures. Therefore, in hydrocarbon exploration, special attention is paid to the surroundings of salt structures.

Some of the world's largest oil deposits are in sedimentary basins that have been influenced by salt tectonics; z. B. Gulf of Mexico , Northern Caspian Sea, Congo Delta, North Sea , Persian Gulf .

Repository for problematic waste

Salt structures are used as potential repositories for radioactive waste and other hazardous waste, since salt rock is impermeable to salt- saturated deep groundwater and has a relatively high flowability and thermal conductivity. The hope is that after some time the containers with radioactive material will be enclosed by the salt rock and sealed off from external groundwater ingress and that the heat generated when the material decays is dissipated to the outside.

The Gorleben salt dome in Lower Saxony has been intensively explored since 1973 for its suitability for the site. The Asse salt dome , in which there is a disused salt mine, has been operated as a research mine for the large-scale final storage of radioactive waste since 1965. Another former mine in a salt structure in Germany that serves as a repository for radioactive waste is Morsleben in Saxony-Anhalt.

Other partly closed potash pits are used as landfills for chemical hazardous waste, e.g. B. the former Herfa-Neurode potash mine in Hesse or the former Sondershausen salt mine in Thuringia.

Individual evidence

  1. ↑ Carl Ochsenius: The formation of rock salt deposits and their mother liquor salts with special consideration of the Douglashall flot in the Egeln'schen Mulde. CEM Pfeffer, Halle 1877.
  2. ↑ MR Hudec, MPA Jackson: The salt mine: a digital atlas of salt tectonics. (= Bureau of Economic Geology Udden Book Series. No. 5 and AAPG Memoir. Volume 99). 2011, ISBN 978-0-615-51836-7 .
  3. ↑ F. Kockel, P. Krull (project leader): Final storage of radioactive waste that generates high levels of heat in deep geological formations in Germany. Investigation and evaluation of salt formations. Federal Institute for Geosciences and Natural Resources, Hanover 1995 ( PDF 6.2 MB).
  4. ↑ F. Trusheim: About Halokinesis and its significance for the structural development of Northern Germany. In: Journal of the German Geological Society. Volume 109, 1957, pp. 111-158. (Abstract)
  5. ↑ MR Hudec, MP Jackson, DD Schultz-Ela: The paradox of minibasin subsidence into salt: Clues to the evolution of crustal basins. In: Geological Society of America Bulletin. Volume 121 (1-2), 2009, pp. 201-221.
  6. Jump up ↑ JK Warren: Evaporites: Sediments, Resources and Hydrocarbons. Springer, Berlin / Heidelberg / New York 2006, ISBN 3-540-26011-0 , pp. 375-452, chapter Salt tectonics .
  7. ↑ Debra H. Wood, Alice B. Giles: Hydrocarbon Accumulation Patterns in the East Texas Salt Dome Province. (= Geological Circular. 82-6). Bureau of Economic Geology, The University of Texas at Austin, Austin (TX) 1982. ( PDF ( Memento from March 4, 2016 in the Internet Archive ) 19 MB)
  8. ↑ R. Jrbashyan, G. Chlingarya, Y. Kagramanov, A. Karapetyan, M. Satian, Y. Sayadyan, H. Mkrtchyan: Geology of Meso-Cenozoic Basins in Central Armenia, with Comment on Indications of Hydrocarbons. (= Search and Discovery. Art.-No. 30007). 2001. (online)
  9. ↑ Fabien Favret: Up-to-date Researches and Future Trends in Underground Gas Storage Facilities: A State of the Art Review. In: Jens Hetland, Teimuraz Gochitashvili (ed.): Security of Natural Gas Supply through Transit Countries. Proceedings of the NATO Advanced Research Workshop on Security of Natural Gas Supply through Transit Countries, Tbilisi, Georgia May 20-22, 2003 (NATO Science Series, Series II: Mathematics, Physics and Chemistry, Vol. 149). Kluwer Academic Publishers, 2004, ISBN 1-4020-2076-7 , p. 178. ( PDF 21.8 MB)

Web links

Commons : Salt domes  - collection of images, videos and audio files
Wiktionary: Salt dome  - explanations of meanings, word origins, synonyms, translations