The salt surface structure is an extension of the tectonic salt formed on the Earth's surface when either diapir or salt sheet penetrates the layer above it. They can occur in any location where there is a salt deposit, ie in cratonic basin, synrift basin, passive margin and collisional margin. This is the environment in which the masses collect water and then evaporate; leaving salt and other evaporites to form a sedimentary layer. When there is a pressure difference, such as additional sediments in a particular area, the salt bed - due to the unique ability of the salt to behave as a liquid under pressure - forms into the new structure. Sometimes, these new bodies form subhorizontal structures or simply dip in on the younger stratigraphic units, called allochthonous salt bodies or salt surface structures.
Video Salt surface structures
Salt
Tectonic environment
Four key environments can facilitate salt deposition. These places allow salt-containing water to collect and evaporate, leaving precipitated sedimentary salt crystals. Below is a brief description of this environment and some examples.
- Convergent boundary - The area where two plates collide; if there is water trapped between them, there is the possibility of evaporation and deposition. The Mediterranean Sea, especially during the Messinian salinity crisis, is a prime example.
- Eliminated boundaries/passive margins - Also known as different boundaries, these areas begin as cracking basins, where the extensions separate the crust. If this rifting allows water to flood the resulting valley, salt deposition may occur. Examples include the Campos Basin, Brazil, the Kwanza Basin, West Africa, and the Gulf of Mexico.
- Chromatic basin - In continental boundaries, salt deposits can occur anywhere that can be collected by water bodies. Even away from marine sources, water is capable of dissolving and carrying ions which then settle as salt, and when the water evaporates, the salts are left behind. Examples of these basins are the South Oman Salt Basin and the Michigan Basin. In the past, there was a vast shallow sea covering most of the Great Plains region in the United States; when this ocean dries up, it creates a Strataca deposit that is now mined in Kansas, among others.
Characteristics
Salt has two key characteristics that make it unique in tectonic settings, and economically important. The first is that salt (and other evaporites) plastically damage geological time, and thus behaves as a liquid rather than a rigid structure. This allows structures with salt components to change shape more easily and have slightly different looks. Take for example, Appalachian, which contains some salt deposits, and the Rocky Mountains, which is an accretion field with little or no salt. It also allows the creation of structural traps for oil and gas, as well as the metals that make them sought after targets in the industry. The second, is the fact that evaporites are often less dense, or lighter, than the surrounding rock, which aids in its mobility and creates Rayleigh Taylor instability. This means that less dense substances will find a way to rise or move away from the denser ones. In tectonic salt, this happens in three ways; the first is the differential loading, in which the salt flows from the high pressure area to the low pressure, the second is the gravitational dispersal, in which salt spreads laterally under its own gravity, the latter is thermal convection, where it is warmer - and thus less dense - salt rises through a colder and denser salt. This is seen only in laboratory settings because it is unlikely that salt bodies with large temperature variances occur.
Maps Salt surface structures
History of evolution
In order for the horizontal bed to form an allochthonous salt initially, they must first free themselves from their geological barriers. The first basic structure can be formed in a combination of six ways:
- Reactive piercing - normal error synrift reduces the pressure above the saline layer. This causes the salt to flow into the low pressure area to maintain its balance.
- Active piercing - the salt moves through the sediment where there is no structure to use.
- Erosional piercement - the precipitate above is eroded, revealing this salt dome.
- Puncture thorns - Local thrust implements force on the salt plate following the path most resistant to fault footprints.
- Ductile piercement - not so much 'stabbing' movement, but local differential pressure forces the salt to rise through its weaker upper sediment. Occurs because of Rayleigh-Taylor instability created by low density of salt.
- Passive Passive - after the salt column initially penetrates the sediment above it, it rises accordingly or replaces the growing sedimentary layer.
From here there are three paths that the structure of the forming surface can take. Two stems from the bottom are diapir, and third from the bottom of the sheet. The sheet became a source-feeding drive, unlike the thrust, he took advantage of the local fault plane to ride. The difference between the two diapir bases, one of which, called the plug of the plug, has a sediment cap on it, preventing the salt from flowing freely until the pressure of the building forces through the cap; the other, the dipped extrusion, has no sedimentary lid and is allowed to flow freely.
Type of surface structure
Once the salt structure reaches the surface, it is referred to as one of four names; wing-salt intrusion, extrusive face, open face or thrust face. There is a certain transition rate between the four, as some processes, such as salt dissolution and removal, new sediment deposition, erosion, and thrust may alter the characteristics between them.
Disorders of the wings
Salt-wing Intrusion is technically an underground structure; found in a shortening, or compression, system, they form a radial salt wedge between separate bedding planes. However, hats on them can be eroded, revealing salt and turning it into an extrusive advance.
Extrusive forward
Extrusive progress begins once the surface of the diapir reaches the soil surface and the salt is exposed. The salt then spreads from the feeder under the pressure of gravity alone. This flow has two consequences that make up the structure. First, because the top of the salt flows faster than the bottom, there is a frontal winding along the front edge. Secondly, salt overrides any sediments stored at the same time, causing features to climb upup and prograde. Over time, some salt is dissolved, leaving layers of dirt and other sediments behind, the thickness of this roof, or the lid of the sediment, depending on the percentage of impurities in the salt and the sedimentation rate of the area.
Thrust thrust
Push back to the salt sheet as their primary base structure, and is formed because the salt provides a weak release layer for the caesarean system. When a force is applied in such a system, the buried sheet will advance along the hanging wall. There are three driving processes in this type of down payment; the gravitational pressure of both salt and sediment overlying, the spread of margins and the common tectonic plates.
Open-to-end
Open-toed progress can evolve from salt dissolution from an extrusive facial structure, or it may evolve from a push-plug thrust. They are partially buried advances where only the forward edge, called the toe, is open to flow, which is controlled by a combined force of gravity and the differential pressure of the sediments above it. There are three types of sedimentary roofs described: synclinal basins - isolated patches of consolidated sediments, prograding roofs - growing sediments, and salt breakouts - where the salt must force the way through the sediment above it.
References
Source of the article : Wikipedia
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