Contact (geology)

Last updated October 15, 2022

A geological contact is a boundary which separates one rock body from another.^[1] A contact can be formed during deposition, by the intrusion of magma,^[2] or through faulting or other deformation of rock beds that brings distinct rock bodies into contact.^[3]

Types of contacts

Depositional

Conformable

Conformable contacts represent no time gap in the geologic record.^[3] They are usually planar, though they may have slightly irregular topography. These contacts represent continual, uninterrupted deposition and accumulation of sedimentary rocks, or represent lava flows.^[6]

A conformable contact can be abrupt, where the contact separates beds of distinct lithology. Abrupt contacts coincide with bedding planes and represent a change in depositional environment, but with only a minor hiatus in deposition with no significant erosion. A brief hiatus without erosion is known as a diastem .^[4]

Gradational contacts occur where the change in depositional environment takes place over a longer period of time. They are further divided into progressive gradual contacts where the change in lithology is more or less continuous, or intercalated contacts where thin beds of a new lithology appear at the base of the contact zone, become thicker upwards, and completely replaced the old lithology at the top of the contact zone.^[4]

Closely related to intercalated contacts is interfingering (also known as intertonguing,^[7]interdigitating, or interlocking^[8]) of laterally adjacent sedimentary rock bodies. Here the contact breaks down into a series of wedges or tongues that penetrate the adjacent rock body and pinch out individually. These record fluctuations in the location of the boundary between different depositional environments where sediments were simultaneously being laid down.^[8]

Non-conformable

Unconformities are gaps in the geologic record within a stratigraphic unit. These gaps can be caused by periods of non-deposition or by erosion.^[3] As a result, two adjacent rock units may have significantly different ages.

Intrusive

Intrusive contacts are the surfaces between host (or country) rock and an intrusive magmatic body.^[3] The older country rock is crosscut by a younger magmatic body. The nature of the intruding body depends its composition and depth. Common examples are igneous dikes, sills, plutons, and batholiths. Depending on the composition of the magma, the intrusive body may have a complex internal structure which can provide insight into its emplacement.

The country rock responds to the immense heat from the intruding body numerous ways. Chill margins are created if the magma is cooled too quickly to fully crystallise. The result is a distinct boundary of very fine grain igneous rock along the border of the country rock.^[3] The surrounding rock may be "baked" through contact metamorphism, resulting in non-foliated metamorphic rocks.^[1] Rocks that were originally limestone, quartz sandstone, and shale become marble, quartzite, and hornfels, respectively.^{[ citation needed ]}

Fault

Fault and shear zone contacts can be represented by either discrete breaks and discontinuities, or ductile deformation without a physical break in stratigraphy.^[9] Fault surface contacts show discrete breaks and have an attitude and position which describes the contact between two formations.^[3] These fault surfaces can be polished into slickensided surfaces which depict striations in the direction of the fault movement.^[3] Shear zones are different as there is no physical break displayed, but there is displacement.^[1]

Significance and application

Cross-cutting relationships between contacts and formations can be used to determine the geological history of an area. Events are in order, from oldest to youngest, A to F. Bd2i.png — Cross-cutting relationships between contacts and formations can be used to determine the geological history of an area. Events are in order, from oldest to youngest, A to F.

Identifying and understanding the relationship between contacts is important in determining relative ages of rocks and formations.^[6] Contacts are a key feature used to create geological maps. Cross-cutting relationships of these contacts can be used to determine the relative geological history of an area or of an outcrop.

Important geological contacts

The golden spikes on geologic timescales represent internationally agreed upon references for the boundaries of the stages in the geologic timescale. These contacts have been identified by the International Commission on Stratigraphy and they are known as GSSPs, or global boundary stratotype section and points. Some of these boundary points are at physical locations, while others are in ice drill core sections, or have been defined chronometrically.

The GSSP for the Danian Stage marks the end of the Cretaceous and the beginning of the Paleocene Series.^[10] Located in Tunisia, the contact is described as a reddish layer at the base of a dark clay layer. This reddish layer is the Iridium Anomaly, representative of the fallout of the major impact that resulted in the mass extinction event that ended the Cretaceous.

Related Research Articles

Geology is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth sciences, including hydrology, and so is treated as one major aspect of integrated Earth system science and planetary science.

<span class="mw-page-title-main">Sedimentary rock</span> 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 mineral or organic particles at Earth's surface, followed by cementation. 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. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, 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.

Metamorphism is the transformation of existing rock to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of 150 to 200 °C, and often also at elevated pressure or in the presence of chemically active fluids, but the rock remains mostly solid during the transformation. Metamorphism is distinct from weathering or diagenesis, which are changes that take place at or just beneath Earth's surface.

<span class="mw-page-title-main">Lithology</span> Description of its physical characteristics of a rock unit

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Examples of lithologies in the second sense include sandstone, slate, basalt, or limestone.

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.

A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exposed in a geographical region. It is the fundamental unit of lithostratigraphy, the study of strata or rock layers.

<span class="mw-page-title-main">Lithostratigraphy</span> Sub-discipline of stratigraphy

Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology.

<span class="mw-page-title-main">Rock cycle</span> Transitional concept of geologic time

The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

<span class="mw-page-title-main">Relative dating</span>

Relative dating is the science of determining the relative order of past events, without necessarily determining their absolute age. In geology, rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another. Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials. Though relative dating can only determine the sequential order in which a series of events occurred, not when they occurred, it remains a useful technique. Relative dating by biostratigraphy is the preferred method in paleontology and is, in some respects, more accurate. The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.

Chronostratigraphy is the branch of stratigraphy that studies the ages of rock strata in relation to time.

Rock microstructure includes the texture and small-scale structures of a rock. The words texture and microstructure are interchangeable, with the latter preferred in modern geological literature. However, texture is still acceptable because it is a useful means of identifying the origin of rocks, how they formed, and their appearance.

Foliation in geology refers to repetitive layering in metamorphic rocks. Each layer can be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by shearing forces, or differential pressure. The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated metamorphic rocks are typically formed in the absence of significant differential pressure or shear. Foliation is common in rocks affected by the regional metamorphic compression typical of areas of mountain belt formation.

Kambalda type komatiitic nickel ore deposits are a class of magmatic iron-nickel-copper-platinum-group element ore deposit in which the physical processes of komatiite volcanology serve to deposit, concentrate and enrich a Fe-Ni-Cu-(PGE) sulfide melt within the lava flow environment of an erupting komatiite volcano.

The Lizard complex, Cornwall is generally accepted to represent a preserved example of an exposed ophiolite complex in the United Kingdom. The rocks found in The Lizard area are analogous to those found in such famous areas as the Troodos Mountains, Cyprus and the Semail Ophiolite, Oman.

In geology, a bed is a layer of sediment, sedimentary rock, or pyroclastic material "bounded above and below by more or less well-defined bedding surfaces". Specifically in sedimentology, a bed can be defined in one of two major ways. First, Campbell and Reineck and Singh use the term bed to refer to a thickness-independent layer comprising a coherent layer of sedimentary rock, sediment, or pyroclastic material bounded above and below by surfaces known as bedding planes. By this definition of bed, laminae are small beds that constitute the smallest (visible) layers of a hierarchical succession and often, but not always, internally comprise a bed.

<span class="mw-page-title-main">Igneous intrusion</span> Body of intrusive igneous rocks

In geology, an igneous intrusion is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and compositions, illustrated by examples like the Palisades Sill of New York and New Jersey; the Henry Mountains of Utah; the Bushveld Igneous Complex of South Africa; Shiprock in New Mexico; the Ardnamurchan intrusion in Scotland; and the Sierra Nevada Batholith of California.

The geology of Hong Kong is dominated by igneous rocks formed during a major volcanic eruption period in the Mesozoic era. It made up 85% of Hong Kong's land surface and the remaining 15% are mostly sedimentary rocks located in the northeast New Territories. There are also a very small percentage of metamorphic rocks in New Territories. These are formed by deformation of pre-existing sedimentary rocks which changed its mineral assemblages (metamorphism).

This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.

A stratigraphic column is a representation used in geology and its subfield of stratigraphy to describe the vertical location of rock units in a particular area. A typical stratigraphic column shows a sequence of sedimentary rocks, with the oldest rocks on the bottom and the youngest on top.

Igneous rock, or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava.

References

1 2 3 Plummer, C.C.; Carlson, D.H.; Hammersley, L. (2015). Physical Geology, 15th ed. New York, USA: McGraw-Hill Education. ISBN 978-0-07-809610-5.
↑ Boggs, Sam (2006). Principles of sedimentology and stratigraphy (4th ed.). Upper Saddle River, N.J.: Pearson Prentice Hall. pp. 400–406. ISBN 0131547283.
1 2 3 4 5 6 7 Davis, G.H.; Reynolds, S.J.; Kluth, C.F. (2011). Structural Geology of Rocks and Regions, 3rd ed. New Jersey, USA: John Wiley & Sons, Inc. pp. 21–29. ISBN 9780471152316 . Retrieved 7 October 2020.
1 2 3 Boggs 2006, p. 401
↑ Davis and Kluth 2011, p.21
1 2 "Geologic Contacts" (PDF). University of Western Ontario. 2005.
↑ Jackson, Julia A., ed. (1997). "Intertonguing". Glossary of geology (Fourth ed.). Alexandria, Virginia: American Geological Institute. ISBN 0922152349.
1 2 Allaby, Michael (2013). "Interdigitating". A dictionary of geology and earth sciences (Fourth ed.). Oxford: Oxford University Press. ISBN 9780199653065.
↑ Twiss, R.J.; Moores, E.M. (2007). Structural Geology, 2nd ed. New York, USA: W.H. Freeman & Company.
↑ "GSSP for Danian Stage". Stratigraphy.org.

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[:0-1] 1 2 3 Plummer, C.C.; Carlson, D.H.; Hammersley, L. (2015). Physical Geology, 15th ed. New York, USA: McGraw-Hill Education. ISBN 978-0-07-809610-5.

[boggs-2006-400-406-2] Boggs, Sam (2006). Principles of sedimentology and stratigraphy (4th ed.). Upper Saddle River, N.J.: Pearson Prentice Hall. pp. 400–406. ISBN 0131547283.

[:1-3] 1 2 3 4 5 6 7 Davis, G.H.; Reynolds, S.J.; Kluth, C.F. (2011). Structural Geology of Rocks and Regions, 3rd ed. New Jersey, USA: John Wiley & Sons, Inc. pp. 21–29. ISBN 9780471152316 . Retrieved 7 October 2020.

[boggs-2006-401-4] 1 2 3 Boggs 2006, p. 401

[:1:21-5] Davis and Kluth 2011, p.21

[:2-6] 1 2 "Geologic Contacts" (PDF). University of Western Ontario. 2005.

[7] Jackson, Julia A., ed. (1997). "Intertonguing". Glossary of geology (Fourth ed.). Alexandria, Virginia: American Geological Institute. ISBN 0922152349.

[Allaby-8] 1 2 Allaby, Michael (2013). "Interdigitating". A dictionary of geology and earth sciences (Fourth ed.). Oxford: Oxford University Press. ISBN 9780199653065.

[9] Twiss, R.J.; Moores, E.M. (2007). Structural Geology, 2nd ed. New York, USA: W.H. Freeman & Company.

[10] "GSSP for Danian Stage". Stratigraphy.org.

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