Anticline

Last updated

Cross-sectional diagram of an anticline Anticline (PSF)-vector.svg
Cross-sectional diagram of an anticline
Anticline exposed in road cut (small syncline visible at far right). Note the man standing in front of the formation, for scale. New Jersey, U.S.A. NJ Route 23 anticline.jpg
Anticline exposed in road cut (small syncline 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 inverse of a anticline. A typical anticline is convex up in which the hinge or crest is the location where the curvature is greatest, and the limbs are the sides of the fold that dip away from the hinge. Anticlines can be recognized and differentiated from antiforms by a sequence of rock layers that become progressively older toward the center of the fold. Therefore, if age relationships between various rock strata are unknown, the term antiform should be used.

Contents

The progressing age of the rock strata towards the core and uplifted center, are the trademark indications for evidence of anticlines on a geologic map. These formations occur because anticlinal ridges typically develop above thrust faults during crustal deformations. The uplifted core of the fold causes compression of strata that preferentially erodes to a deeper stratigraphic level relative to the topographically lower flanks. Motion along the fault including both shortening and extension of tectonic plates, usually also deforms strata near the fault. This can result in an asymmetrical or overturned fold. [1]

Terminology of different folds

Diagram showing constituent parts of an anticline Flank & hinge.PNG
Diagram showing constituent parts of an anticline

Antiform vs anticline

An antiform can be used to describe any fold that is convex up. It is the relative ages of the rock strata that distinguish anticlines from antiforms. [1]

Elements

The hinge of an anticline refers to the location where the curvature is greatest, also called the crest. [1] The hinge is also the highest point on a stratum along the top of the fold. The culmination also refers to the highest point along any geologic structure. The limbs are the sides of the fold that display less curvature. The inflection point is the area on the limbs where the curvature changes direction. [2]

The axial surface is an imaginary plane connecting the hinge of each layer of rock stratum through the cross section of an anticline. If the axial surface is vertical and the angles on each side of the fold are equivalent, then the anticline is symmetrical. If the axial plane is tilted or offset, then the anticline is asymmetrical. An anticline that is cylindrical has a well-defined axial surface, whereas non-cylindrical anticlines are too complex to have a single axial plane.

Types

An overturned anticline is an asymmetrical anticline with a limb that has been tilted beyond perpendicular, so that the beds in that limb have basically flipped over and may dip in the same direction on both sides of the axial plane. [3] If the angle between the limbs is large (70–120 degrees), then the fold is an "open" fold, but if the angle between the limbs is small (30 degrees or less), then the fold is a "tight" fold. [4] If an anticline plunges (i.e., the anticline crest is inclined to the Earth's surface), it will form Vs on a geologic map view that point in the direction of plunge. A plunging anticline has a hinge that is not parallel to the earth's surface. All anticlines and synclines have some degree of plunge. Periclinal folds are a type of anticlines that have a well-defined, but curved hinge line and are doubly plunging and thus elongate domes. [5]

Model of anticline. Oldest beds are in the center and youngest on the outside. The axial plane intersects the center angle of bend. The hinge line follows the line of greatest bend, where the axial plane intersects the outside of the fold. Anticline.png
Model of anticline. Oldest beds are in the center and youngest on the outside. The axial plane intersects the center angle of bend. The hinge line follows the line of greatest bend, where the axial plane intersects the outside of the fold.

Folds in which the limbs dip toward the hinge and display a more U-like shape are called synclines . They usually flank the sides of anticlines and display opposite characteristics. A syncline's oldest rock strata are in its outer limbs; the rocks become progressively younger toward its hinge. A monocline is a bend in the strata resulting in a local steepening in only one direction of dip. [2] Monoclines have the shape of a carpet draped over a stairstep. [4]

An anticline that has been more deeply eroded in the center is called a breached or scalped anticline. Breached anticlines can become incised by stream erosion, forming an anticlinal valley.

A structure that plunges in all directions to form a circular or elongate structure is a dome . Domes may be created via diapirism from underlying magmatic intrusions or upwardly mobile, mechanically ductile material such as rock salt (salt dome) and shale (shale diapir) that cause deformations and uplift in the surface rock. The Richat Structure of the Sahara is considered a dome that has been laid bare by erosion.

An anticline which plunges at both ends is termed a doubly plunging anticline, and may be formed from multiple deformations, or superposition of two sets of folds. It may also be related to the geometry of the underlying detachment fault and the varying amount of displacement along the surface of that detachment fault.

An anticlinorium is a large anticline in which a series of minor anticlinal folds are superimposed. Examples include the Late Jurassic to Early Cretaceous Purcell Anticlinorium in British Columbia [1] and the Blue Ridge anticlinorium of northern Virginia and Maryland in the Appalachians, [6] or the Nittany Valley in central Pennsylvania.

Formation processes

Anticline near Ehden, Lebanon Anticline-lebanon.jpg
Anticline near Ehden, Lebanon

Anticlines are usually developed above thrust faults, so any small compression and motion within the inner crust can have large effects on the upper rock stratum. Stresses developed during mountain building or during other tectonic processes can similarly warp or bend bedding and foliation (or other planar features). The more the underlying fault is tectonically uplifted, the more the strata will be deformed and must adapt to new shapes. The shape formed will also be very dependent on the properties and cohesion of the different types of rock within each layer.

During the formation of flexural-slip folds, the different rock layers form parallel-slip folds to accommodate for buckling. A good way to visualize how the multiple layers are manipulated, is to bend a deck of cards and to imagine each card as a layer of rock stratum. [7] The amount of slip on each side of the anticline increases from the hinge to the inflection point. [2]

Passive-flow folds form when the rock is so soft that it behaves like weak plastic and slowly flows. In this process different parts of the rock body move at different rates causing shear stress to gradually shift from layer to layer. There is no mechanical contrast between layers in this type of fold. Passive-flow folds are extremely dependent on the rock composition of the stratum and can typically occur in areas with high temperatures. [4]

Economic significance

Anticlines, structural domes, fault zones and stratigraphic traps are very favorable locations for oil and natural gas drilling. About 80 percent of the world's petroleum has been found in anticlinal traps. [8] The low density of petroleum causes oil to buoyantly migrate out of its source rock and upward toward the surface until it is trapped and stored in reservoir rock such as sandstone or porous limestone. The oil becomes trapped along with water and natural gas by a caprock that is made up of impermeable barrier such as an impermeable stratum or fault zone. [9] Examples of low-permeability seals that contain the hydrocarbons, oil and gas, in the ground include shale, limestone, sandstone, and rock salt. The actual type of stratum does not matter as long as it has low permeability.

Water, minerals and specific rock strata such as limestone found inside anticlines are also extracted and commercialized. Lastly, ancient fossils are often found in anticlines and are used for paleontological research or harvested into products to be sold.

Notable examples

Asia

Ghawar Anticline, Saudi Arabia, the structural trap for the largest conventional oil field in the world.

Australia

Europe

Harpea's Cave in southern France Harpea kobazuola.jpg
Harpea's Cave in southern France

The Weald–Artois Anticline is a major anticline which outcrops in southeast England and northern France. It was formed from the late Oligocene to middle Miocene, during the Alpine orogeny.

North America

Anticlines can have a major effect on the local geomorphology and economy of the regions in which they occur. One example of this is the El Dorado anticline in Kansas. The anticline was first tapped into for its petroleum in 1918. Soon after the site became a very prosperous area for entrepreneurs following World War I and the rapid popularization of motor vehicles. By 1995 the El Dorado oil fields had produced 300 million barrels of oil. [10] The central Kansas uplift is an antiform composed of several small anticlines that have collectively produced more than 2.5 million barrels of oil. [11]

Another notable anticline is the Tierra Amarilla anticline in San Ysidro, New Mexico. [12] This is a popular hiking and biking site because of the great biodiversity, geologic beauty and paleontological resources. This plunging anticline is made up of Petrified Forest mudstones and sandstone and its caprock is made of Pleistocene and Holocene travertine. The anticline contains springs that deposit carbon dioxide travertine that help to contribute to the rich diversity of microorganisms. [13] This area also contains remains of fossils and ancient plants from the Jurassic period that are sometimes exposed through geological erosion.

The Ventura Anticline is a geologic structure that is part of the Ventura oil fields, the seventh largest oil field in California that was discovered in the 1860s. The anticline runs east to west for 16 miles, dipping steeply 30–60 degrees at both ends. Ventura County has a high rate of compression and seismic activity due to the converging San Andreas Fault. As a result, the Ventura anticline rises at a rate of 5 mm/year with the adjacent Ventura Basin converging at a rate of about 7–10 mm/year. [14] The anticline is composed of a series of sandstone rock beds and an impermeable rock cap under which vast reserves of oil and gas are trapped. Eight different oil bearing zones along the anticline vary greatly from 3,500 to 12,000 feet. The oil and gas formed these pools as they migrated upward during the Pliocene Era and became contained beneath the caprock. This oil field is still active and has a cumulative production of one billion barrels of oil making it one of the most vital historical and economic features of Ventura County. [15]

See also

Related Research Articles

Structural geology Science of the description and interpretation of deformation in the Earths crust

Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation (strain) in the rocks, and ultimately, to understand the stress field that resulted in the observed strain and geometries. This understanding of the dynamics of the stress field can be linked to important events in the geologic past; a common goal is to understand the structural evolution of a particular area with respect to regionally widespread patterns of rock deformation due to plate tectonics.

Cumberland Mountains Mountain range in the southeastern United States

The Cumberland Mountains are a mountain range in the southeastern section of the Appalachian Mountains. They are located in western Virginia, southwestern West Virginia, the eastern edges of Kentucky, and eastern middle Tennessee, including the Crab Orchard Mountains. Their highest peak, with an elevation of 4,223 feet (1,287 m) above mean sea level, is High Knob, which is located near Norton, Virginia.

Salt dome

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.

Los Angeles Basin Sedimentary basin in California

The Los Angeles Basin is a sedimentary basin located in southern California, in a region known as the Peninsular Ranges. The basin is also connected to an anomalous group of east-west trending chains of mountains collectively known as the Transverse Ranges. The present basin is a coastal lowland area, whose floor is marked by elongate low ridges and groups of hills that is located on the edge of the Pacific Plate. The Los Angeles Basin, along with the Santa Barbara Channel, the Ventura Basin, the San Fernando Valley, and the San Gabriel Basin, lies within the greater southern California region. On the north, northeast, and east, the lowland basin is bound by the Santa Monica Mountains and Puente, Elysian, and Repetto hills. To the southeast, the basin is bordered by the Santa Ana Mountains and the San Joaquin Hills. The western boundary of the basin is marked by the Continental Borderland and is part of the onshore portion. The California borderland is characterized by northwest trending offshore ridges and basins. The Los Angeles Basin is notable for its great structural relief and complexity in relation to its geologic youth and small size for its prolific oil production. Yerkes et al. identify five major stages of the basin's evolution, which began in the Upper Cretaceous and ended in the Pleistocene. This basin can be classified as an irregular pull-apart basin accompanied by rotational tectonics during the post-early Miocene.

Fold (geology) Stack of originally planar surfaces

In structural geology, a fold is a stack of originally planar surfaces, such as sedimentary strata, that are bent or curved during permanent deformation. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur as single isolated folds or in periodic sets. Synsedimentary folds are those formed during sedimentary deposition.

Upheaval Dome

Upheaval Dome is an impact structure, the deeply eroded bottom-most remnants of an impact crater, in Canyonlands National Park southwest of the city of Moab, Utah, in the United States. The crater is located in the Island in the Sky section of the park, and is approximately 42 miles (68 km) by vehicle from Moab.

Syncline

In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold (synform), termed a synformal syncline, but synclines that point upwards can be found when strata have been overturned and folded.

Joint (geology)

A joint is a break (fracture) of natural origin in the continuity of either a layer or body of rock that lacks any visible or measurable movement parallel to the surface (plane) of the fracture. Although they can occur singly, they most frequently occur as joint sets and systems. A joint set is a family of parallel, evenly spaced joints that can be identified through mapping and analysis of the orientations, spacing, and physical properties. A joint system consists of two or more intersecting joint sets.

Dome (geology) geological deformation structure

A dome is a feature in structural geology consisting of symmetrical anticlines that intersect each other at their respective apices. Intact, domes are distinct, rounded, spherical-to-ellipsoidal-shaped protrusions on the Earth's surface. However, a transect parallel to Earth's surface of a dome features concentric rings of strata. Consequently, if the top of a dome has been eroded flat, the resulting structure in plan view appears as a bullseye, with the youngest rock layers at the outside, and each ring growing progressively older moving inwards. These strata would have been horizontal at the time of deposition, then later deformed by the uplift associated with dome formation.

Vergence (geology)

In structural geology, vergence refers to the direction of the overturned component of an asymmetric fold. In simpler terms, vergence can be described as the horizontal direction in which the upper-component of rotation is directed. Vergence can be observed and recorded in folds to help a geologist determine characteristics of larger fold areas. Vergence is used to provide an overall characterization, in the symmetry of folds, and can be used to observe changes in small-scale structures in relation to the axis of a large fold. The vergence of a fold lies parallel to the surrounding surfaces of a fold, so if these surrounding surfaces are not horizontal in nature, the vergence of the fold will be inclined. For a fold, the direction, as well as the extent to which vergence occurs, can be calculated from the strike and dip of the axial surfaces, along with that of the enveloping surfaces. These calculations can be very useful for geologists in determining the overall elements of larger areas.

Petroleum 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 are the most important type of trap as they represent the majority of the world's discovered petroleum resources.

Confidence Hills

The Confidence Hills are a mountain range in the Mojave Desert, in southern Inyo County, California.

Vaqueros Formation

The Vaqueros Formation is a sedimentary geologic unit primarily of Upper Oligocene and Lower Miocene age, which is widespread on the California coast and coastal ranges in approximately the southern half of the state. It is predominantly a medium-grained sandstone unit, deposited in a shallow marine environment. Because of its high porosity and nearness to petroleum source rocks, in many places it is an oil-bearing unit, wherever it has been configured into structural or stratigraphic traps by folding and faulting. Being resistant to erosion, it forms dramatic outcrops in the coastal mountains. Its color ranges from grayish-green to light gray when freshly broken, and it weathers to a light brown or buff color.

Oklahoma City Oil Field

The Oklahoma City Oil Field is one of the world's giant petroleum fields and is located in Oklahoma City, Oklahoma in the United States of America. The field was opened just south of the city limits on December 4, 1928, and first entered Oklahoma City limits on May 27, 1930.

The Mareuil Anticline, also called Mareuil-Meyssac Anticline, is a structural high within the sedimentary sequence of the northeastern Aquitaine Basin. The northwest-southeast trending anticline was caused by tectonic movements probably starting in the Upper Cretaceous.

Detachment fold

A detachment fold, in geology, occurs as layer parallel thrusting along a decollement develops without upward propagation of a fault; the accommodation of the strain produced by continued displacement along the underlying thrust results in the folding of the overlying rock units. As a visual aid, picture a rug on the floor. By placing your left foot on one end and pushing towards the other end of the rug, the rug slides across the floor (decollement) and folds upward. Figure 1, is a generalized representation of the geometry assumed by a detachment fault.

Middle Magdalena Valley

The Middle Magdalena Valley, Middle Magdalena Basin or Middle Magdalena Valley Basin is an intermontane basin, located in north-central Colombia between the Central and Eastern Ranges of the Andes. The basin, covering an area of 34,000 square kilometres (13,000 sq mi), is situated in the departments of Santander, Boyacá, Cundinamarca and Tolima.

3D fold evolution

In geology, 3D fold evolution is the study of the full three dimensional structure of a fold as it changes in time. A fold is a common three-dimensional geological structure that is associated with strain deformation under stress. Fold evolution in three dimensions can be broadly divided into two stages, namely fold growth and fold linkage. The evolution depends on fold kinematics, causes of folding, as well as alignment and interaction of the each structure with respect to each other. There are several ways to reconstruct the evolution progress of folds, notably by using depositional evidence, geomorphological evidence and balanced restoration. Understanding the evolution of folds is important because it helps petroleum geologists to gain a better understanding on the distribution of structural traps of hydrocarbon.

Southland Syncline

The Southland Syncline is a major geological structure located in the Southland Region of New Zealand's South Island. The syncline folds the Mesozoic greywackes of the Murihiku Terrane. The northern limb of the fold is steep to overturned, while the southern limb dips shallowly to the northeast. The axial plan dips to the northeast and the axis plunges to the southeast.

Junggar Basin

Junggar Basin is one of the largest sedimentary basins in Northwest China. It is located in Xinjiang, and enclosed by the Tarbagatai Mountains of Kazakhstan in the northwest, the Altai Mountains of Mongolia in the northeast, and the Heavenly Mountains in the south. The geology of Junggar Basin mainly consists of sedimentary rocks underlain by igneous and metamorphic basement rocks. The basement of the basin was largely formed during the development of the Pangea supercontinent during complex tectonic events from Precambrian to late Paleozoic time. The basin developed as a series of foreland basins – in other words, basins developing immediately in front of growing mountain ranges – from Permian time to the Quaternary period. The basin's preserved sedimentary records show that the climate during the Mesozoic era was marked by a transition from humid to arid conditions as monsoonal climatic effects waned. The Junggar basin is rich in geological resources due to effects of volcanism and sedimentary deposition.

References

  1. 1 2 3 4 Dictionary of Geological Terms (3rd ed.). Garden City, New York: Anchor Press/Doubleday. April 11, 1984. ISBN   978-0-385-18101-3.
  2. 1 2 3 Hefferan, Kevin P. "Folds". Geology 320: Structural Geology. University of Wisconsin–Stevens Point. Retrieved December 8, 2015.
  3. Mantei, Erwin J. "Geologic Structures—Crustal Deformations". Physical Geology (GLG110). Missouri State University. Retrieved December 17, 2015.
  4. 1 2 3 Marshak, Stephen (2012). Earth: Portrait of a Planet (4th ed.). Norton. ISBN   978-0393935189 . Retrieved January 22, 2016.
  5. Roberts, Albert F. (1947). Geological Structures and Maps: A Practical Course in the Interpretation of Geological Maps for Civil and Mining Engineers. London: I. Pitman. p. 33.
  6. Monroe, James S.; Wicander, Reed (February 8, 2005). The Changing Earth: Exploring Geology and Evolution (4th ed.). Brooks Cole. ISBN   978-0-495-01020-3.[ page needed ]
  7. Earle, Steven (2015). "Folding". Physical Geology . Retrieved December 15, 2015.
  8. Riva, Joseph P. "Accumulation in reservoir beds". Encyclopædia Britannica. Retrieved December 10, 2015.
  9. Society of Petroleum Engineers Student Chapter (November 9, 2014). "Petroleum 101 – How is Petroleum Formed?". University of Waterloo. Retrieved December 10, 2015.
  10. Skelton, L.H. (1997). "The Discovery and Development of the El Dorado (Kansas) Oil Field". Northeastern Geology and Environmental Sciences. 19 (1–2): 48–53. Retrieved December 15, 2015.
  11. Baars, D.L.; Watney, W. Lynn; Steeples, Don W.; Brostuen, Erling A. (April 2001). "Petroleum: a primer for Kansas – Structure". Kansas Geological Survey, Education. p. 5. Retrieved December 9, 2015.
  12. Hart, Dirk Van (February 2003). "Gallery of Geology – Tierra Amarilla Anticline" (PDF). New Mexico Geology. 25 (1): 15. Retrieved December 15, 2015.
  13. Cron, Brandi; Crossey, Laura J.; Karlstrom, Karl E.; Northup, Diana E.; Takacs-Vesbach, Cristina (2009). "Microbial Diversity, Geochemistry and Diel Fluctuations in Travertine Mounds at Tierra Amarilla Anticline, New Mexico". Abstracts with Programs. Geological Society of America. 41 (7): 322. Retrieved December 8, 2015.
  14. O'Tousa, Jim (August 7, 2014). "Overview of the Geology of Ventura County including Seismicity, Oil and Gas Plays and Groundwater Resources" (PDF). Oil and Gas Program Informational Workshop. Ventura County Planning Commission. Archived from the original (PDF) on February 2, 2016.
  15. Jackson, Glenda (July 2013). "Oil and Ventura" (PDF). Ventura City Hall. Archived from the original (PDF) on January 31, 2017. Retrieved January 23, 2016.

Bibliography