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A salt dome is a type of structural dome formed when a thick bed of evaporite minerals (mainly salt, or halite) found at depth intrudes vertically into surrounding rock strata, forming a diapir. It is important in petroleum geology because salt structures are impermeable and can lead to the formation of a stratigraphic trap.
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 clastics, but it is recognised 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. 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 recognised in the geologic record, such as the Garabogazköl basin in Turkmenistan.
Over time, the layer of salt is covered with deposited sediment, becoming buried under an increasingly large overburden. The overlying sediment will undergo compaction, causing an increase in density and therefore a decrease in buoyancy. Unlike clastics, pressure has a significantly smaller effect on the density of salt due to its crystal structure and this eventually leads to it becoming more buoyant than the sediment above it. The 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).
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 glacier. In cross section, these large domes may be anywhere from 1 to 10 kilometres (0.62 to 6.21 mi) across, and extend as deep as 6.5 kilometres (4.0 mi).
Major occurrences of salt domes are found along the Gulf Coast of the USA 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.
Another example of an emergent salt dome is at Onion Creek, Utah / Fisher Towers near Moab, Utah, U.S. These two images show a Cretaceous age salt body that has risen as a ridge through several hundred meters of overburden, predominantly sandstone. As the salt body rose, the overburden formed an anticline (arching upward along its centerline) which fractured and eroded to expose the salt body.
The term "salt dome" is also sometimes inaccurately used to refer to dome-shaped silos used to store rock salt for melting snow on highways. These domes are actually called monolithic domes and are used to store a variety of bulk goods.
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.
Other uses include storing oil, natural gas, hydrogen gas, or even hazardous waste in large caverns formed after salt mining, as well as excavating the domes themselves for uses in everything from table salt to the granular material used to prevent roadways from icing over.
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- C.Michael Hogan. 2011. Sulfur. Encyclopedia of Earth, eds. A.Jorgensen and C.J.Cleveland, National Council for Science and the environment, Washington DC
- Salt dome formation
- Salt dome at Schlumberger's Oilfield Glossary