Salt surface structures

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Schematic showing concave folded beds pierced by salt structures. Lower image shows a cross section of a possible sub-surface structure. Salt Surface Structures.jpg
Schematic showing concave folded beds pierced by salt structures. Lower image shows a cross section of a possible sub-surface structure.

Salt surface structures are extensions of salt tectonics that form at the Earth's surface when either diapirs or salt sheets pierce through the overlying strata. They can occur in any location where there are salt deposits, namely in cratonic basins, synrift basins, passive margins and collisional margins. These are environments where mass quantities of water collect and then evaporate; leaving behind salt and other evaporites to form sedimentary beds. [1] When there is a difference in pressure, such as additional sediment in a particular area, the salt beds – due to the unique ability of salt to behave as a fluid under pressure – form into new structures. Sometimes, these new bodies form subhorizontal or moderately dipping structures over a younger stratigraphic unit, which are called allochthonous salt bodies or salt surface structures. [1] [2]

Salt tectonics study of salt-controlled structures (like salt domes), mechanisms, and tectonic deformation involving salt or other evaporates

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 due both to the low density of salt, which does not increase with burial, and its low strength.

Diapir A type of geologic intrusion in which a more mobile and ductily deformable material is forced into brittle overlying rocks

A diapir 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 range from idealized mushroom-shaped Rayleigh–Taylor-instability-type structures in regions with low tectonic stress such as in the Gulf of Mexico to narrow dikes of material that move along tectonically induced fractures in surrounding rock. The term was introduced by the Romanian geologist Ludovic Mrazek, who was the first to understand the principle of salt tectonics and plasticity. The term "diapir" may be applied to igneous structures, but it is more commonly applied to non-igneous, relatively cold materials, such as salt domes and mud diapirs.

Rift A linear zone where the Earths crust is being pulled apart, and is an example of extensional tectonics

In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.

Contents

Salt

Image of example environments for salt deposition. Areas of likely deposition are shown in lavender. Deposition environments.jpg
Image of example environments for salt deposition. Areas of likely deposition are shown in lavender.

Tectonic environments

Four key environments can facilitate salt deposition. These places allow salt-bearing water to collect and evaporate, leaving behind bedded deposits of solidified salt crystals. Below are short descriptions of these environments and a few examples.

  1. Convergent boundaries – Areas where two plates collide; if there is water trapped between the two, there is the possibility of evaporation and deposition. The Mediterranean Sea, [3] particularly during the Messinian salinity crisis, is a prime example.
  2. Rifted boundaries/passive margins – Also known as divergent boundaries, these areas begin as rift basins, where extension is pulling apart the crust. If this rifting allows water to flood the resulting valley, salt deposition can occur. Examples include the Campos Basin, Brazil, Kwanza Basin, West Africa, [4] and the Gulf of Mexico. [5]
  3. Cratonic basins – Within continental boundaries, salt deposition can occur anywhere that bodies of water can collect. Even away from ocean sources, water is capable of dissolving and carrying ions that can later precipitate as salts, and when the water evaporates, the salts are left behind. Examples of these basins are the South Oman Salt Basin [6] and the Michigan Basin. In the past, there was a great shallow sea covering most of the Great Plains region of the United States; when this sea dried up, it created the Strataca deposit now mined in Kansas, among others.

Characteristics

Salt has two key characteristics that make it unique in a tectonic setting, and important economically. The first is that salt (and other evaporites) deform plastically over geologic time, and thus behaves as a fluid rather than a rigid structure. [7] This allows structures with salt components to deform more easily and have a slightly different appearance. Take, for example the Appalachians, which contain some salt deposits, and the Rocky Mountains, which is an accretionary terrain with little to no salt. This also allows for the creation of structural traps for oil and gas, as well as metals [8] which makes them sought after targets in industry. The second, which is the fact that evaporites are often less dense, or more buoyant, than the surrounding rock, which aids in its mobility and creates a Rayleigh Taylor instability. This means that the less dense substance will find a way to rise through or away from the more dense one. In salt tectonics, this occurs in three ways; the first is differential loading, where the salt flows from an area of high pressure to lower pressure, the second is gravitational spreading, where the salt spreads out laterally under its own gravitational weight, the last is thermal convection, where warmer – and thus less dense – salt rises through colder and more dense salt. [9] This is only seen in laboratory settings due to the unlikely occurrence of salt bodies with great enough temperature variance.

Geology of the Appalachians

The geology of the Appalachians dates back to more than 480 million years ago. A look at rocks exposed in today's Appalachian Mountains reveals elongate belts of folded and thrust faulted marine sedimentary rocks, volcanic rocks and slivers of ancient ocean floor – strong evidence that these rocks were deformed during plate collision. The birth of the Appalachian ranges marks the first of several mountain building plate collisions that culminated in the construction of the supercontinent Pangaea with the Appalachians and neighboring Little Atlas near the center. These mountain ranges likely once reached elevations similar to those of the Alps and the Rocky Mountains before they were eroded.

Geology of the Rocky Mountains

The geology of the Rocky Mountains is that of a discontinuous series of mountain ranges with distinct geological origins. Collectively these make up the Rocky Mountains, a mountain system that stretches from Northern British Columbia through central New Mexico and which is part of the great mountain system known as the North American Cordillera.

Accretion (geology) process by which material is added to a tectonic plate or a landmass

Accretion, in geology, is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts, or other igneous features.

Evolution histories

Illustration of the six piercement types; black arrows show the forces acting on the salt layer, white arrows show the reaction of the salt to these forces. Peircemnt 2.jpg
Illustration of the six piercement types; black arrows show the forces acting on the salt layer, white arrows show the reaction of the salt to these forces.

In order for originally horizontal beds to form the allochthonous salts, they must first break free of their geological restraints. The first base structure can be formed in a combination of six ways: [1]

  1. Reactive piercement – a normal fault synrift relieves pressure above the salt layer. This causes the salt to flow into the area of lower pressure to maintain its equilibrium. [10]
  2. Active piercement – salt moves through sediments where there are no structures to take advantage of. [10]
  3. Erosional piercement – overlying sediments are eroded away, revealing the present salt dome.
  4. Thrust piercement – local thrust faults apply force to salt sheets which follow the path of least resistance up the footwall of the fault.
  5. Ductile piercement – not so much a 'piercing' movement, but local differential pressure force the salt to rise through weaker overlying sediments. Occurs due to the Rayleigh-Taylor instability created by salt's low density.
  6. Passive piercement – after the salt column has initially pierced the overlying sediments, the rate it rises matches or supersedes the growing sediment layers. [10]

From here there are three paths that a forming surface structure can take. Two stem from a diapir base, and the third from a sheet base. The sheet becomes a source-fed thrust, not unlike the thrust piercement, it takes advantage of local fault planes to rise. The difference between the two diapir bases, is that one, termed a plug-fed thrust, has a sediment cap over the top, preventing the salt from freely flowing until building pressure forces it through the cap; the other, a plug-fed extrusion, lacks the sediment cap and is allowed to flow freely. [2]

Types of surface structures

Once the salt structure has reached the surface, it is termed one of four names; salt-wing intrusions, extrusive advance, open-toed advance or thrust advance. [1] [2] There is a certain level of transition between the four, as some process, such as the dissolution and removal of salt, deposition of new sediment, erosion and thrusting can shift the characteristics between them.

Salt-wing intrusion Salt Wing Intrusion.jpg
Salt-wing intrusion

Salt-wing intrusions

Salt-wing intrusions are technically underground structures; found in shortening, or compressional, systems, they form radial salt wedges between detached bedding planes. However, the caps on them can be eroded away, revealing the salt and transforming it into an extrusive advance. [1] [11]

Extrusive advance showed in 3D Extrusive Advance 3d.jpg
Extrusive advance showed in 3D

Extrusive advance

Extrusive advances begin once the diapir reaches the ground’s surface and the salt is exposed. The salt then spreads from the feeder under gravitational pressure alone. [1] This flowing has two consequences that form the structure. First, as the top of the salt flows faster than the bottom, there is a frontal roll along the leading edge. Second, the salt overrides any sediment being deposited at the same time, causing the feature to climb upsection and prograde. Over time, some of the salt is dissolved away, leaving a layer of impurities and other sediments behind, the thickness of this roof, or sediment cap, depends on the percentage of impurities in the salt and the sedimentation rate of the area. [1] [11]

Thrust or close-toed advance in 3D Close toed advnance.jpg
Thrust or close-toed advance in 3D

Thrust advance

Thrust advances return to salt sheets as their primary base structure, and form because salt provides a weak detachment layer for faulting systems. When force is applied in such systems, the buried sheet will advance along the hanging wall. There are three driving processes in this type of advance; gravitational pressure of both the salt and overlying sediments, spreading of the margin and general plate tectonics. [1] [11]

Open-toed advance

Open-toe advance in 3D. Wide arrows show the direction of motion, thin arrows indicate salt motion. Open-toe advance 3d.jpg
Open-toe advance in 3D. Wide arrows show the direction of motion, thin arrows indicate salt motion.

Open-toed advances can either evolve from the dissolution of salts from an extrusive advance structure, or it could have evolved from a plug-fed thrust. They are partially buried advances where only the advancing edge, called the toe, is open to flow, which is controlled by a combination of gravitational forces and differential pressure of the overlying sediments. There are three described sediment roof types: synclinal basins – isolated patches of consolidated sediments, prograding roof – a growing sheet of sediments, and salt breakout – where the salt had to force its way through the overlying sediments. [1] [11]

Related Research Articles

Sedimentary rock Rock formed by the deposition and subsequent cementation of material

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particles and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Evaporite A water-soluble mineral sediment formed by evaporation from an aqueous solution

Evaporite is the term for a water-soluble mineral sediment that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean deposits, and non-marine, which are found in standing bodies of water such as lakes. Evaporites are considered sedimentary rocks and are formed by chemical sediments.

Niger Delta Basin (geology)

The Niger Delta Basin, also referred to as the Niger Delta province, is an extensional rift basin located in the Niger Delta and the Gulf of Guinea on the passive continental margin near the western coast of Nigeria with suspected or proven access to Cameroon, Equatorial Guinea and São Tomé and Príncipe. This basin is very complex, and it carries high economic value as it contains a very productive petroleum system. The Niger delta basin is one of the largest subaerial basins in Africa. It has a subaerial area of about 75,000 km2, a total area of 300,000 km2, and a sediment fill of 500,000 km3. The sediment fill has a depth between 9–12 km. It is composed of several different geologic formations that indicate how this basin could have formed, as well as the regional and large scale tectonics of the area. The Niger Delta Basin is an extensional basin surrounded by many other basins in the area that all formed from similar processes. The Niger Delta Basin lies in the south westernmost part of a larger tectonic structure, the Benue Trough. The other side of the basin is bounded by the Cameroon Volcanic Line and the transform passive continental margin.

Salt dome geological structure

A salt dome is a type of structural dome formed when a thick bed of evaporite minerals 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.

Nappe A large sheetlike body of rock that has been moved a considerable distance above a thrust fault

In geology, a nappe or thrust sheet is a large sheetlike body of rock that has been moved more than 2 km (1.2 mi) or 5 km (3.1 mi) above a thrust fault from its original position. Nappes form in compressional tectonic settings like continental collision zones or on the overriding plate in active subduction zones. Nappes form when a mass of rock is forced over another rock mass, typically on a low angle fault plane. The resulting structure may include large-scale recumbent folds, shearing along the fault plane, imbricate thrust stacks, fensters and klippe.

The Lewis Overthrust is a geologic thrust fault structure of the Rocky Mountains found within the bordering national parks of Glacier in, Montana, United States and Waterton Lakes in Alberta, Canada. They were created as a result of a collision of tectonic plates about 170 million years ago that drove several miles thick wedge of rock 50 mi (80 km) eastwards, causing it to overlie softer Cretaceous age rock that is 400 to 500 million years younger.

Salt glacier

A salt glacier is a rare flow of salt that is created when a rising diapir in a salt dome breaches the surface of the Earth. The name ‘salt glacier’ was given to this phenomenon due to the similarity of movement when compared with ice glaciers. The causes of these formations is primarily due to salt’s unique properties and its surrounding geologic environment. A rising body of salt is referred to as a diapir; which rises to the surface and feeds the salt glacier. Salt structures are usually composed of halite, anhydrite, gypsum and clay minerals. Clays may be brought up with the salt, turning it dark. These salt flows are rare on earth. In a more recent discovery, scientists have found that they also occur on Mars, but are composed of sulfates.

Décollement

Décollement is a gliding plane between two rock masses, also known as a basal detachment fault. Décollements are a deformational structure, resulting in independent styles of deformation in the rocks above and below the fault. They are associated with both compressional settings and extensional settings.

Accretionary wedge The sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary

An accretionary wedge or accretionary prism forms from sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the downgoing slab of oceanic crust, but in some cases the wedge includes the erosional products of volcanic island arcs formed on the overriding plate.

Geology of the Pyrenees

The Pyrenees are a 430-kilometre-long, roughly east–west striking, intracontinental mountain chain that divide France, Spain, and Andorra. The belt has an extended, polycyclic geological evolution dating back to the Precambrian. The chain's present configuration is due to the collision between the microcontinent Iberia and the southwestern promontory of the European Plate. The two continents were approaching each other since the onset of the Upper Cretaceous (Albian/Cenomanian) about 100 million years ago and were consequently colliding during the Paleogene (Eocene/Oligocene) 55 to 25 million years ago. After its uplift, the chain experienced intense erosion and isostatic readjustments. A cross-section through the chain shows an asymmetric flower-like structure with steeper dips on the French side. The Pyrenees are not solely the result of compressional forces, but also show an important sinistral shearing.

The salt tectonics off the Louisiana gulf coast can be explained through two possible methods. The first method attributes spreading of the salt because of sedimentary loading while the second method points to slope instability as the primary cause of gliding of the salt. The first method results in the formation of growth faults in the overlying sediment. Growth faults are normal faults that occur simultaneously with sedimentation, causing them to have thicker sediment layers on the downthrown sides of the faults. In the second method both the salt and the sediment are moving, making it more likely to migrate.

Growth fault

Growth faults are syndepositional or syn-sedimentary extensional faults that initiate and evolve at the margins of continental plates. They extend parallel to passive margins that have high sediment supply. Their fault plane dips mostly toward the basin and has long-term continuous displacement. Figure one shows a growth fault with a concave upward fault plane that has high updip angle and flattened at its base into zone of detachment or décollement. This angle is continuously changing from nearly vertical in the updip area to nearly horizontal in the downdip area.

Gulf of Mexico basin

The formation of the Gulf of Mexico, an oceanic rift basin located between North America and the Yucatan Block, was preceded by the breakup of the Supercontinent Pangaea in the Late-Triassic, weakening the lithosphere. Rifting between the North and South American plates continued in the Early-Jurassic, approximately 160 million years ago, and formation of the Gulf of Mexico, including subsidence due to crustal thinning, was complete by 140 Ma. Stratigraphy of the basin, which can be split into several regions, includes sediments deposited from the Jurassic through the Holocene, currently totaling a thickness between 15 and 20 kilometers.

Geology of the southern North Sea

The North Sea basin is located in northern Europe and lies between the United Kingdom, and Norway just north of The Netherlands and can be divided into many sub-basins. The Southern North Sea basin is the largest gas producing basin in the UK continental shelf, with production coming from the lower Permian sandstones which are sealed by the upper Zechstein salt. The evolution of the North Sea basin occurred through multiple stages throughout the geologic timeline. First the creation of the Sub-Cambrian peneplain, followed by the Caledonian Orogeny in the late Silurian and early Devonian. Rift phases occurred in the late Paleozoic and early Mesozoic which allowed the opening of the northeastern Atlantic. Differential uplift occurred in the late Paleogene and Neogene. The geology of the Southern North Sea basin has a complex history of basinal subsidence that had occurred in the Paleozoic, Mesozoic, and Cenozoic. Uplift events occurred which were then followed by crustal extension which allowed rocks to become folded and faulted late in the Paleozoic. Tectonic movements allowed for halokinesis to occur with more uplift in the Mesozoic followed by a major phase of inversion occurred in the Cenozoic affecting many basins in northwestern Europe. The overall saucer-shaped geometry of the southern North Sea Basin indicates that the major faults have not been actively controlling sediment distribution.

Angola Basin

The Angola Basin is located along the West African South Atlantic Margin which extends from Cameroon to Angola. It is characterized as a passive margin that began spreading in the south and then continued upwards throughout the basin. This basin formed during the initial breakup of the supercontinent Pangaea during the early Cretaceous, creating the Atlantic Ocean and causing the formation of the Angola, Cape, and Argentine basins. It is often separated into two units: the Lower Congo Basin, which lies in the northern region and the Kwanza Basin which is in the southern part of the Angola margin. The Angola Basin is famous for its "Aptian Salt Basins," a thick layer of evaporites that has influenced topography of the basin since its deposition and acts as an important petroleum reservoir.

Hormuz Formation

The Hormuz Formation, Hormuz Series, Hormuz Evaporites or Hormuz Group is a sequence of evaporites that were deposited during the Ediacaran to Early Cambrian, a period previously referred to as the Infra-Cambrian. Most exposures of this sequence are in the form of emergent salt diapirs within anticlines of the Zagros fold and thrust belt. As a result of their involvement in post-depositional salt tectonics, the internal stratigraphy of the sequence is relatively poorly understood. They are the lateral equivalent of the evaporite-bearing Ara Group in the South Oman Basin.

Lusitanian Basin

The Lusitanian Basin is located on both mainland and continental shelf of the west-central coast of Portugal. It covers a 20,000 km2 area and extends from south of Lisbon, the capitol of Portugal, to Porto. This north-south oriented Atlantic margin rift basin is approximately 130 km wide and 340 km long and belongs to a family of periatlantic basins such as the Jeanne d'Arc Basin. To the east of the Lusitanian Basin lies the Central Plateau of the Iberian Peninsula and a marginal horst system lies to the west. The Alentejo and Algarve Basins connect to the southern end of the Lusitanian Basin. In the north, it is connected to both the Porto and Galicia Basins via a basement ridge.

Salt deformation

Salt deformation is the change of shape of natural salt bodies in response to forces and mechanisms that controls salt flow. Such deformation can generate large salt structures such as underground salt layers, salt diapirs or salt sheets at the surface. Strictly speaking, salt structures are formed by rock salt that is composed of pure halite (NaCl) crystal. However, most halite in nature appears in impure form, therefore rock salt usually refers to all rocks that composed mainly of halite, sometimes also as a mixture with other evaporites such as gypsum and anhydrite. Earth's salt deformation generally involves such mixed materials.

References

  1. 1 2 3 4 5 6 7 8 9 Hudec, Michael R.; Jackson, Martin P.A. (2007). "Terra infirma: Understanding salt tectonics". Earth-Science Reviews. 82 (1): 1–28. Bibcode:2007ESRv...82....1H. doi:10.1016/j.earscirev.2007.01.001.
  2. 1 2 3 Hudec, Michael R.; Jackson, Martin P.A. (2006). "Advance of allochthonous salt sheets in passive margins and orogens". AAPG Bulletin. 90 (10): 1535–1564. doi:10.1306/05080605143.
  3. ↑ Huguen, C; Chamot-Rooke, N.; Loubrieu, B.; Mascle, J. (March 2006). "Morphology of a pre-collisional, salt bearing, accretionary complex: The Mediterranean Ridge (Eastern Mediterranean)". Marine Geophysical Research. 27 (1): 61. Bibcode:2006MarGR..27...61H. doi:10.1007/s11001-005-5026-5.
  4. ↑ Rouby, D; Raillard, Stéphane; Guillocheau, François; Bouroullec, Renaud; Nalpas, Thierry (2002). "Kinematics of a growth fault/raft system on the West African margin using 3-D restoration". Journal of Structural Geology. 24 (4): 783. Bibcode:2002JSG....24..783R. doi:10.1016/S0191-8141(01)00108-0.
  5. ↑ Prather, BE (May 2000). "Calibration and visualization of depositional process models for above grade slopes: a case study from the Gulf of Mexico". Marine and Petroleum Geology . 17 (5): 619. doi:10.1016/S0264-8172(00)00015-5.
  6. ↑ Amthor, JE (2005). "Stratigraphy and sedimentology of a chert reservoir at the Precambrian-Cambrian Boundary: the Al Shomou Siliclyte, South Oman Salt Basin". GeoArabia. 10 (2): 89.
  7. ↑ Weijermars, D.M.; Jackson, M.P.A.; Venderville, B. (1993). "Rheological and tectonic modeling of salt provinces". Tectonophysics. 217 (217): 143. Bibcode:1993Tectp.217..143W. doi:10.1016/0040-1951(93)90208-2.
  8. ↑ Warren, J. (1999). Evaporites: Their Evolution and Economics. Oxford. p. 438. ISBN   978-3-540-26011-0.
  9. ↑ Jackson, M.P.A.; Talbot, C.J. (1986). "External shapes, strain rates an dynamics of salt structures". Geological Society of America Bulletin. 97 (3): 305. Bibcode:1986GSAB...97..305J. doi:10.1130/0016-7606(1986)97<305:ESSRAD>2.0.CO;2.
  10. 1 2 3 Vendeville, BC; Jackson, MPA (August 1992). "The rise of diapirs during thin-skinned extension" (PDF). Marine and Petroleum Geology. 9 (4): 331–354. doi:10.1016/0264-8172(92)90047-I.
  11. 1 2 3 4 Fossen, Haakon (2010-07-15). Structural Geology. Cambridge University Press. p. 388. ISBN   978-1-139-48861-7.
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