Asalt dome is a type ofstructural dome formed when salt (or otherevaporite minerals) intrudes into overlying rocks in a process known asdiapirism. Salt domes can have unique surface and subsurface structures, and they can be discovered using techniques such asseismic reflection. They are important inpetroleum geology as they can function aspetroleum traps.
Stratigraphically, salt basins developed periodically from theProterozoic to theNeogene. The formation of a salt dome begins with the deposition of salt in a restrictedbasin. In these basins, the outflow of water exceeds inflow. Specifically, the basin loses water throughevaporation, resulting in the precipitation and deposition of salt. While the rate ofsedimentation of salt is significantly larger than the rate of sedimentation ofclastics, 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 the formation of saltdiapirs, indicating that a sustained period of episodic flooding and evaporation of the basin must occur.[1]
Over time, the layer of salt is covered with depositedsediment, becoming buried under an increasingly largeoverburden. Previously, researchers believed that the compaction of overlying sediment and subsequent decrease inbuoyancy led to salt rising and intruding into the overburden due to itsductility, thereby creating a salt diapir. However, after the 1980s, the primary force that drives the flow of salt is considered to be differential loading.[2]
Differential loading can be caused by gravitational forces (gravitational loading), forced displacement of salt boundaries (displacement loading), or thermal gradients (thermal loading).[2] The flow of the salt overcomes the strength of the overburden as well as boundary friction aided by overburdenextension, erosion,thrust faults, ductile thinning, or other forms of regional deformation. The vertical growth of salt formations creates pressure on the upward surface, causingextension andfaulting.[3] Once the salt completely pierces the overburden, it can rise through a process known aspassive diapirism where the accumulation of sediments around the diapir contribute to its growth and eventually form into a dome.[2]
Some salt domes can be seen from Earth's surface. They can also be located by finding uniquesurface structures and surrounding phenomena. For instance, salt domes can contain or be nearsulfur springs andnatural gas vents.[4] Some salt domes have salt sheets that extrude from the top of the dome; these are referred to as salt plugs. These plugs can coalesce to form salt canopies, which can then be remobilized by roof sedimentation, with the most prominent example in the northernGulf of Mexico basin. Another structure that can form from salt domes aresalt welds. These occur when the growth of a dome is prevented by an exhausted supply of salt, and the top and bottom contacts merge.[2]
Salt domes have also been located usingseismic refraction andseismic reflection. The latter was developed based on techniques from the former and is more effective. Seismic refraction usesseismic waves to characterize subsurface geologic conditions and structures. Seismic reflection highlights the presence of a stark density contrast between the salt and surrounding sediment. Seismic techniques are particularly effective as salt domes are typically depressed blocks of crust bordered by parallel normalfaults (graben) that can be flanked by reverse faults.[5] Advances in seismic reflection and the expansion of offshorepetroleum exploration efforts led to the discovery of numerous salt domes soon afterWorld War II.[6]
Salt domes are the site of many of the world'shydrocarbon provinces.[6] The rock salt of the salt dome is mostly impermeable, so, as it moves up towards the surface, it penetrates and bends existing rock along with it. Asstrata of rock are penetrated, they are, generally, bent upwards where they meet the dome, forming pockets andreservoirs ofpetroleum andnatural gas (known aspetroleum traps).[2] In 1901, an exploratory oil well was drilled intoSpindletop Hill nearBeaumont, Texas. This led to the discovery of the first salt dome, revealed the importance of salt to the formation of hydrocarbon accumulations, and produced enough oil for petroleum to become an economically feasible fuel for the United States.[4][6] Several countries usesolution mining to formcaverns for holding large amounts of oil or gas reserves.
Thecaprock above the salt domes can contain deposits of nativesulfur (recovered by theFrasch process). They can also contain deposits of metals,sodium salts,nitrates, and other substances, which can be used in products such as table salt and chemicalde-icers.[6]
Salt domes occur in many parts of the world where there is a sufficiently thick layer of rock salt developed.
In the Middle East, the upperNeoproterozoic salt of theHormuz Formation is associated with widespread salt dome formation in most parts of thePersian Gulf and onshore in Iran, Iraq,United Arab Emirates, andOman. The thicker salt is found in a series of basins: the Western Gulf, the Southern Gulf, and the Oman salt basins.[7]
Pennsylvanian age salt of theParadox Formation forms salt domes throughout theParadox Basin in the US, which extends from easternUtah, through southwesternColorado into northwesternNew Mexico.
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, predominantlysandstone. As the salt body rose, the overburden formed ananticline (arching upward along its center line) which fractured and eroded to expose the salt body.[8]
Offshore northern Norway in the southwesternBarents Sea, thickUpper Carboniferous–Lower Permian salt was deposited, forming salt domes in theHammerfest andNordkapp basins.
In northwest EuropeUpper Permian salt of theZechstein Group has formed salt domes over the central and southernNorth Sea, extending eastwards into Germany.
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 conjugateNova Scotiamargin.
.jpg)
The Gulf Coast is home to over 500 salt domes formed fromMiddle JurassicLouann Salt.[4] This region is home to most of theUS Strategic Petroleum Reserve.Avery Island was formed by a salt dome.[9]
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.
During theMessinian salinity crisis (Late Miocene), thick salt layers were formed as theMediterranean Sea dried out. Later deposition, once the sea refilled, triggered the formation of salt domes.
