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in the
Zagros Mountains The Zagros Mountains ( fa, کوه‌های زاگرس; ku, چیاکانی زاگرۆس, translit=Çiyayên Zagros;) are a long mountain range in Iran, northern Iraq and southeastern Turkey. This mountain range has a total length of . The Zagros moun ...
in
Bushehr Province Bushehr Province ( fa, استان بوشهر, ''Ostān-e Būshehr'' ) is one of the 31 provinces of Iran. It is in the south of the country, with a long coastline onto the Persian Gulf. Its center is Bushehr, the provincial capital. The province ha ...
,
Iran Iran ( fa, ایران ), also called Persia and officially the Islamic Republic of Iran ( fa, جمهوری اسلامی ایران ), is a country in Western Asia. It is bordered to the northwest by Armenia and Azerbaijan, to the north by ...
(the white area in the middle) ,
Iran Iran ( fa, ایران ), also called Persia and officially the Islamic Republic of Iran ( fa, جمهوری اسلامی ایران ), is a country in Western Asia. It is bordered to the northwest by Armenia and Azerbaijan, to the north by ...
A salt dome is a type of Dome (geology), structural dome formed when a thick bed of
evaporite evaporated from the Dead Sea, Israel Evaporite () is the term for a water-soluble mineral sediment that results from concentration and crystallization by evaporation from an aqueous solution.Jackson, Julia A., 1997, Glossary of Geology 4th editi ...
minerals (mainly salt, or
halite Halite ( or ), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride (Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, p ...
) found at depth intrudes vertically into surrounding rock
strata (Argentina). , Canada. These are Middle Cambrian marine sediments. This formation covers over half of Nova Scotia and is recorded as being 8,800 m (29,000 ft) thick in some areas. In geology and related fields, a stratum (plural: strata) is a l ...
, forming a
diapir A diapir (; French, from Greek ''diapeirein'', to pierce through) is a type of geologic intrusion in which a more mobile and ductily deformable material is forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can r ...
. It is important in
petroleum geology Petroleum geology is the study of origin, occurrence, movement, accumulation, and exploration of hydrocarbon fuels. It refers to the specific set of geological disciplines that are applied to the search for hydrocarbons (oil exploration). Sediment ...
because salt structures are impermeable and can lead to the formation of a
stratigraphic trap In petroleum geology, a trap is a geological structure affecting the reservoir rock and caprock of a petroleum system allowing the accumulation of hydrocarbons in a reservoir. Traps can be of two types: stratigraphic or structural. Structural traps ...
.


Formation

The formation of a salt dome begins with the deposition of salt in a restricted marine basin. Because the flow of salt-rich seawater into the basin is not balanced by outflow, much to all water lost from the basin is via evaporation, resulting in the precipitation and deposition of salt evaporites. The rate of sedimentation of salt is significantly larger than the rate of sedimentation of
clastic of a clast (sand grain), derived from a basalt scoria. Vesicles (air bubbles) can be seen throughout the clast. Plane light above, cross-polarized light below. Scale box is 0.25 mm. Clastic rocks are composed of fragments, or clasts, of pre-exi ...
s, but it is recognized that a single evaporation event is rarely enough to produce the vast quantities of salt needed to form a layer thick enough for salt diapirs to be formed. This indicates that a sustained period of episodic flooding and evaporation of the basin must occur, as can be seen from the example of the Mediterranean
Messinian salinity crisis The Messinian salinity crisis (MSC), also referred to as the Messinian event, and in its latest stage as the Lago Mare event, was a geological event during which the Mediterranean Sea went into a cycle of partial or nearly complete desiccation thr ...
. At the present day, evaporite deposits can be seen accumulating in basins that merely have restricted access but do not completely dry out; they provide an analogue to some deposits recognized in the
geologic record forces near San Sebastián, Spain The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata. That is, deposits laid down by volcanism or by deposition of sediment derived from ...
, such as the
Garabogazköl The Garabogazköl Aylagy or Kara-Bogaz-Gol ( tk, Garabogazköl, ) is a shallow water-filled depression in the northwestern corner of Turkmenistan. It forms a lagoon of the Caspian Sea and has a variable surface area, being about . The Caspian Sea p ...
basin in
Turkmenistan Turkmenistan ( or ; tk, Türkmenistan, ;), also known as Turkmenia, is a sovereign country in Central Asia, bordered by Kazakhstan to the northwest, Uzbekistan to the north, east and northeast, Afghanistan to the southeast, Iran to the south a ...
. Over time, the layer of salt is covered with deposited
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and ...
, becoming buried under an increasingly large overburden. The overlying sediment will undergo compaction, causing an increase in
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its mass per unit volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter '' ...
and therefore a decrease in
buoyancy . Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...
. Unlike clastics, pressure has a significantly smaller effect on the density of salt due to its
crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns that ...
and this eventually leads to it becoming more buoyant than the sediment above it. The
ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stress befo ...

ductility
of salt initially allows it to plastically deform and flow laterally, decoupling the overlying sediment from the underlying sediment. Since the salt has a larger buoyancy than the sediment above—and if a significant faulting event affects the lower surface of the salt—the salt can begin to flow vertically, forming a salt pillow. The vertical growth of these salt pillows creates pressure on the upward surface, causing extension and faulting (see
salt tectonics upright=1.7 Salt tectonics, or halokinesis, or halotectonics, is concerned with the geometries and processes associated with the presence of significant thicknesses of evaporites containing rock salt within a stratigraphic sequence of rocks. This is ...
). Possible forces that drive the flow of salt are differential loading on the source layer and density contrasts in the overburdening sediment. Forces that resist this flow are the mass of the roof block and the block's inherent resistance to faulting, ''i.e.'', strength. To accommodate common density contrast between the overburden sediment and the salt, beginning active diapirism, the diapir height must be more than two-thirds to three-quarters the thickness of the overburden. If the diapir is narrow its height must be greater. Eventually, over millions of years, the salt will pierce and break through the overlying sediment, first as a dome-shaped, and then a mushroom-shaped, fully formed salt diapir. If the rising salt diapir breaches the surface, it can become a flowing
salt glacierupright=1.35, Salt domes (hills) and salt glaciers (dark areas) in the Zagros Mountains of southern Iran ">Iran.html" style="text-decoration: none;"class="mw-redirect" title="Zagros Mountains of southern Iran">Zagros Mountains of southern Iran ...
. In cross section, these large domes may be anywhere from across, and extend as deep as .


Structure

Typical structures of active diapirism are a central crestal
graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel faults. Etymology ''Graben'' is German for ''ditch'' or ''trench''. The plural form is either graben or grabens. The German plural is ''Gräben' ...
flanked by flaps that rotate upward and outward. Reverse faults can separate the flaps from the overburden. Normal faults create the crestal graben and propagate downward. New faults form farther outward as the dome arch becomes more intense. These structures occur beneath the surface and are not necessarily associated with the dome at the surface. Emergence of the dome will not occur unless the dome is very wide or tall relative to the overburden's thickness.


Recognizing salt domes in seismic data

If a salt dome has not pierced the surface they can be found located beneath the surface in various ways. The unique surficial structures can be observed as indicating the salt dome beneath the surface. Salt domes can also be interpreted from
seismic reflection Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismic ...
where the stark
density The density (more precisely, the volumetric mass density; also known as specific mass), of a substance is its mass per unit volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter '' ...
contrast between the salt and surrounding sediments outlines the salt structures. Salt domes can also be associated with sulfur springs and natural gas vents.


Occurrence

Salt domes occur in many parts of the world where there is a sufficiently thick layer of rock salt developed. Stratigraphically, salt basins developed periodically from the
Proterozoic The Proterozoic () is a geological eon spanning the time from the appearance of oxygen in Earth's atmosphere to just before the proliferation of complex life (such as trilobites or corals) on the Earth. The name Proterozoic combines the two forms o ...
to the
Neogene The Neogene ( ) (informally Upper Tertiary or Late Tertiary) is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago (Mya) to the beginning of the present Quaternary Period Mya. The ...
.


Hormuz salt

In the Middle East, upper
Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1,000 million to 541 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran Periods. It is pre ...
salt of the
Hormuz Formation320px, anticlinal_crests_in_the_Zagros_Mountains.html" style="text-decoration: none;"class="mw-redirect" title="halite">Salt glaciers (dark grey areas) derived from Hormuz halites, extruding from anticline">anticlinal crests in the Zagros Moun ...
is associated with widespread salt dome formation in most parts of the
Persian Gulf The Persian Gulf ( fa, خلیج فارس, translit=Xalij-e Fârs, lit=Gulf of Fars, ) is a mediterranean sea in Western Asia. The body of water is an extension of the Indian Ocean (Gulf of Oman) through the Strait of Hormuz and lies between Iran ...
and onshore in Iran, Iraq,
United Arab Emirates The United Arab Emirates (UAE; Arabic: الإمارات العربية المتحدة '), sometimes simply called the Emirates (Arabic: الإمارات '), is a country in Western Asia located at the eastern end of the Arabian Peninsula. It b ...
and Oman. The thicker salt is found in a series of basins, the Western Gulf, Southern Gulf and Oman salt basins.


Paradox basin

Lateral view of emergent salt dome from ridge of remnant of displaced overburden Pennsylvanian age salt of the
Paradox Formation In geology, the Paradox Formation Is a Pennsylvanian age formation which consists of abundant evaporites with lesser interbedded shale, sandstone, and limestone. The evaporites are largely composed of gypsum, anhydrite, and halite. The formation is ...
forms salt domes throughout the
Paradox Basin The Paradox Basin is an asymmetric foreland basin located mostly in southeast Utah and southwest Colorado, but extending into northeast Arizona and northwest New Mexico. The basin is a large elongate northwest to southeast oriented depression formed ...
in the US, which extends from eastern
Utah Utah ( , ) is a state in the Mountain West region of the United States. It is bordered by Colorado to the east, Wyoming to the northeast, Idaho to the north, Arizona to the south and Nevada to the west. It also touches a corner of New Mexic ...
, through southwestern
Colorado Colorado (, other variants) is a state in the Mountain West region of the United States. It encompasses most of the Southern Rocky Mountains as well as the northeastern portion of the Colorado Plateau and the western edge of the Great Plains. ...

Colorado
into northwestern
New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Greater Albuquerque , OfficialLang = English, Spanish , Languages = Navajo, Keres, Zuni , Governor = , Lieutenan ...
. An example of an emergent salt dome is at Onion Creek, Utah / Fisher Towers near Moab, Utah. A Paradox Formation salt body that has risen as a ridge through several hundred meters of overburden, predominantly
sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones make up about 20 to 25 percent of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates) be ...
. As the salt body rose, the overburden formed an
anticline visible at far right). Note the man standing in front of the formation, for scale. New Jersey, U.S.A. In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the ...

anticline
(arching upward along its center line) which fractured and eroded to expose the salt body.


Barents Sea

Offshore northern Norway in the southwestern
Barents Sea The Barents Sea ( , also ; no, Barentshavet, ; russian: Баренцево море, Barentsevo More) is a marginal sea of the Arctic Ocean, located off the northern coasts of Norway and Russia and divided between Norwegian and Russian territorial ...
, thick
Upper Carboniferous Upper may refer to: * Shoe upper or ''vamp'', the part of a shoe on the top of the foot * Stimulant, drugs which induce temporary improvements in either mental or physical function or both * ''Upper'', the original film title for the 2013 found foot ...
Lower Permian salt was deposited, forming salt domes in the
Hammerfest Hammerfest (; sme, Hámmerfeasta) is a municipality in Troms og Finnmark county, Norway. Hammerfest is the northernmost town in the world with more than 10,000 inhabitants. The administrative centre of the municipality is the town of Hammerfest. ...
and
Nordkapp), North Cape, Norway, other uses, North Cape (disambiguation) Nordkapp ( en, North Cape; sme, Davvinjárga or ; fkv, Kappa or ) is a municipality in Troms og Finnmark county, Norway. The administrative centre of the municipality is the town of Honnin ...
Basins.


Zechstein basin

In northwest Europe
Upper Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous period million years ago (Mya), to the beginning of the Triassic period 251.902 Mya. It is the last period of the Paleozo ...
salt of the
Zechstein Group The Zechstein (German either from ''mine stone'' or ''tough stone'') is a unit of sedimentary rock layers of Middle to Late Permian (Guadalupian to Lopingian) age located in the European Permian Basin which stretches from the east coast of England ...
has formed salt domes over the Central and Southern North Sea, extending eastwards into Germany.


Morocco–Nova Scotia

Upper Triassic salt forms salt domes in the Essaouira Basin onshore and offshore Morocco. An equivalent salt sequence, the Argo Formation is associated with salt dome formation on the conjugate Nova Scotia passive margin, margin.


Gulf of Mexico

The Middle Jurassic Louann Salt of the Gulf of Mexico has formed many salt domes in both the US and Mexican parts of the gulf. Major occurrences of salt domes are found along the Gulf Coast of the US in Texas and Louisiana. One example of an island formed by a salt dome is Avery Island in Louisiana. At present ocean levels it is no longer surrounded by the sea but it is surrounded by bayous on all sides. The Gulf Coast is home to over 500 currently discovered salt domes.


South Atlantic salt basins

During the break-up of the South Atlantic, Aptian (Lower Cretaceous) age salt was deposited within the area of thinned crust on both the Brazilian and conjugate Angola/Gabon margins forming many salt domes.


Messinian salt

During the
Messinian salinity crisis The Messinian salinity crisis (MSC), also referred to as the Messinian event, and in its latest stage as the Lago Mare event, was a geological event during which the Mediterranean Sea went into a cycle of partial or nearly complete desiccation thr ...
(Late Miocene), thick salt layers were formed as the Mediterranean Sea dried out. Later deposition once the sea refilled, triggered the formation of salt domes.


Commercial uses

The rock salt that is found in salt domes is mostly impermeable. As the salt moves up towards the surface, it can penetrate and/or bend strata of existing rock with it. As these strata are penetrated, they are generally bent slightly upwards at the point of contact with the dome, and can form pockets where petroleum and natural gas can collect between impermeable strata of rock and the salt. The strata immediately above the dome that are not penetrated are pushed upward, creating a dome-like reservoir above the salt where petroleum can also gather. These oil pools can eventually be extracted, and, indeed, form a major source of the petroleum produced along the coast of the Gulf of Mexico.Salt dome
at Schlumberger's Oilfield Glossary
The first salt dome was discovered in 1900 when an exploratory oil well was drilled into Spindletop Hill near Beaumont, Texas. The caprock above the salt domes is sometimes the site of deposits of native sulfur, which is recovered by the Frasch process. Other uses include storing Strategic Petroleum Reserve (United States), oil, natural gas, hydrogen gas, or even Waste Isolation Pilot Plant, hazardous waste in large caverns formed after salt mining, as well as excavating the domes for uses in everything from Sodium chloride, table salt to the De-icing#Chemical de-icers, granular material used to prevent roadways from icing over.


See also

* Plasticity (physics) * Salt tectonics * Strategic Petroleum Reserve (United States), Strategic Petroleum Reserve * Underground hydrogen storage * Structural trap


References


External links

{{Authority control Salt domes, Salt production Economic geology Evaporite