Stratigraphy is a branch of
geology
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 Ear ...
concerned with the study of
rock
Rock most often refers to:
* Rock (geology), a naturally occurring solid aggregate of minerals or mineraloids
* Rock music, a genre of popular music
Rock or Rocks may also refer to:
Places United Kingdom
* Rock, Caerphilly, a location in Wales ...
layers (
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
) and layering (stratification). It is primarily used in the study of
sedimentary
Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentati ...
and layered
volcanic rock
Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic r ...
s.
Stratigraphy has three related subfields:
lithostratigraphy
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.
In general, strata are primarily igneo ...
(lithologic stratigraphy),
biostratigraphy
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock Stratum, strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictiona ...
(biologic stratigraphy), and
chronostratigraphy
Chronostratigraphy is the branch of stratigraphy that studies the ages of rock strata in relation to time.
The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geologica ...
(stratigraphy by age).
Historical development
Catholic priest
Nicholas Steno
Niels Steensen ( da, Niels Steensen; Latinized to ''Nicolaus Steno'' or ''Nicolaus Stenonius''; 1 January 1638 – 25 November 1686[law of superposition
The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields pertaining to geological stratigraphy. In its plainest form, it states that in undeformed stratigraphic sequences, the ...](_blank ...<br></span></div> established the theoretical basis for stratigraphy when he introduced the <div class=)
, the
principle of original horizontality
The principle of original horizontality states that layers of sediment are originally deposited horizontally under the action of gravity. It is a relative dating technique. The principle is important to the analysis of folded and tilted strata. ...
and the
principle of lateral continuity
The principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous. As a result, rocks that are otherwise similar, but are now separated by a valley or oth ...
in a 1669 work on the fossilization of organic remains in layers of sediment.
The first practical large-scale application of stratigraphy was by
William Smith in the 1790s and early 19th century. Known as the "Father of English geology",
Smith recognized the significance of
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
or rock layering and the importance of fossil markers for correlating strata; he created the first
geologic map
A geologic map or geological map is a special-purpose map made to show various geological features. Rock units or geologic strata are shown by color or symbols. Bedding planes and structural features such as faults, folds, are shown with st ...
of England. Other influential applications of stratigraphy in the early 19th century were by
Georges Cuvier
Jean Léopold Nicolas Frédéric, Baron Cuvier (; 23 August 1769 – 13 May 1832), known as Georges Cuvier, was a French natural history, naturalist and zoology, zoologist, sometimes referred to as the "founding father of paleontology". Cuvier ...
and
Alexandre Brongniart
Alexandre Brongniart (5 February 17707 October 1847) was a French chemist, mineralogist, geologist, paleontologist, and zoologist, who collaborated with Georges Cuvier on a study of the geology of the region around Paris. Observing fossil content ...
, who studied the geology of the region around Paris.
Lithostratigraphy
Variation in rock units, most obviously displayed as visible layering, is due to physical contrasts in rock type (
lithology
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. Lit ...
). This variation can occur vertically as layering (bedding), or laterally, and reflects changes in
environments of deposition (known as
facies
In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with specified characteristics, which can be any observable attribute of rocks (such as their overall appearance, composition, or condition of formatio ...
change). These variations provide a lithostratigraphy or lithologic stratigraphy of the rock unit. Key concepts in stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries imply about their original depositional environment. The basic concept in stratigraphy, called the
law of superposition
The law of superposition is an axiom that forms one of the bases of the sciences of geology, archaeology, and other fields pertaining to geological stratigraphy. In its plainest form, it states that in undeformed stratigraphic sequences, the ...
, states: in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence.
Chemostratigraphy Chemostratigraphy, or chemical stratigraphy, is the study of the chemical variations within sedimentary sequences to determine stratigraphic relationships. The field is relatively young, having only come into common usage in the early 1980s, but the ...
studies the changes in the relative proportions of trace elements and
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...
s within and between lithologic units.
Carbon
Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent
In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
and
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
isotope ratio
The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...
s vary with time, and researchers can use those to map subtle changes that occurred in the paleoenvironment. This has led to the specialized field of isotopic stratigraphy.
Cyclostratigraphy
Cyclostratigraphy is a subdiscipline of stratigraphy that studies astronomically forced climate cycles within sedimentary successions.
Orbital changes
Astronomical cycles (also known as Milankovitch cycles) are variations of the Earth's or ...
documents the often cyclic changes in the relative proportions of
mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...
s (particularly
carbonates
A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate g ...
), grain size, thickness of sediment layers (
varve
A varve is an annual layer of sediment or sedimentary rock.
The word 'varve' derives from the Swedish word ''varv'' whose meanings and connotations include 'revolution', 'in layers', and 'circle'. The term first appeared as ''Hvarfig lera'' (var ...
s) and fossil diversity with time, related to seasonal or longer term changes in
palaeoclimate
Paleoclimatology (British spelling, palaeoclimatology) is the study of climates for which direct measurements were not taken. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important to ...
s.
Biostratigraphy
Biostratigraphy or
paleontologic
Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...
stratigraphy is based on
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
evidence in the rock layers. Strata from widespread locations containing the same fossil fauna and flora are said to be correlatable in time. Biologic stratigraphy was based on William Smith's
principle of faunal succession
The principle of faunal succession, also known as the law of faunal succession, is based on the observation that sedimentary rock strata contain fossilized flora and fauna, and that these fossils succeed each other vertically in a specific, rel ...
, which predated, and was one of the first and most powerful lines of evidence for,
biological evolution
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation t ...
. It provides strong evidence for the formation (
speciation
Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
) and
extinction
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
of
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
. The
geologic time scale
The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochrono ...
was developed during the 19th century, based on the evidence of biologic stratigraphy and faunal succession. This timescale remained a relative scale until the development of
radiometric dating
Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares t ...
, which was based on an absolute time framework, leading to the development of chronostratigraphy.
One important development is the
Vail curve
The sea-level curve is the representation of the changes of the sea level throughout the geological history.
The first such curve is the Vail curve or Exxon curve. The names of the curve refer to the fact that in 1977 a team of Exxon geologist ...
, which attempts to define a global historical sea-level curve according to inferences from worldwide stratigraphic patterns. Stratigraphy is also commonly used to delineate the nature and extent of
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
-bearing reservoir rocks, seals, and traps of
petroleum geology
Petroleum geology is the study of origin, occurrence, movement, accumulation, and exploration of hydrocarbon fuels. It refers to the specific set of geological disciplines that are applied to the search for hydrocarbons (oil exploration).
Sedime ...
.
Chronostratigraphy
Chronostratigraphy is the branch of stratigraphy that places an absolute age, rather than a relative age on rock
strata
In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...
. The branch is concerned with deriving
geochronological data for rock units, both directly and inferentially, so that a sequence of time-relative events that created the rocks formation can be derived. The ultimate aim of chronostratigraphy is to place dates on the sequence of deposition of all rocks within a geological region, and then to every region, and by extension to provide an entire geologic record of the Earth.
A gap or missing strata in the geological record of an area is called a stratigraphic hiatus. This may be the result of a halt in the deposition of sediment. Alternatively, the gap may be due to removal by erosion, in which case it may be called a stratigraphic vacuity.
[Martinsen, O. J. ''et al.'' (1999) "Cenozoic development of the Norwegian margin 60–64N: sequences and sedimentary response to variable basin physiography and tectonic setting" pp. 293–304 ''In'' Fleet, A. J. and Boldy, S. A. R. (editors) (1999) ''Petroleum Geology of Northwest Europe'' Geological Society, London]
page 295
It is called a ''hiatus'' because deposition was ''on hold'' for a period of time. A physical gap may represent both a period of non-deposition and a period of erosion.
A geologic fault may cause the appearance of a hiatus.
[Chapman, Richard E. (1983) ''Petroleum Geology'' Elsevier Scientific, Amsterdam]
p. 33
Magnetostratigraphy
Magnetostratigraphy
Magnetostratigraphy is a geophysical correlation technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their '' ...
is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout a section. The samples are analyzed to determine their detrital
remanent magnetism (DRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. For sedimentary rocks this is possible because, as they fall through the water column, very fine-grained magnetic minerals (< 17
μm) behave like tiny
compass
A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with ...
es, orienting themselves with
Earth's magnetic field
Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magnetic f ...
. Upon burial, that orientation is preserved. For volcanic rocks, magnetic minerals, which form in the melt, orient themselves with the ambient magnetic field, and are fixed in place upon crystallization of the lava.
Oriented paleomagnetic core samples are collected in the field;
mudstone
Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from '' shale'' by its lack of fissility (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...
s,
siltstone
Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...
s, and very fine-grained
sandstone
Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks.
Most sandstone is composed of quartz or feldspar (both silicates) ...
s are the preferred lithologies because the magnetic grains are finer and more likely to orient with the ambient field during deposition. If the ancient magnetic field were oriented similar to today's field (
North Magnetic Pole
The north magnetic pole, also known as the magnetic north pole, is a point on the surface of Earth's Northern Hemisphere at which the Earth's magnetic field, planet's magnetic field points vertically downward (in other words, if a magnetic comp ...
near the
North Rotational Pole), the strata would retain a normal polarity. If the data indicate that the North Magnetic Pole were near the
South Rotational Pole, the strata would exhibit reversed polarity.
Results of the individual samples are analyzed by removing the
natural remanent magnetization Natural remanent magnetization (NRM) is the permanent magnetism of a rock or sediment. This preserves a record of the Earth's magnetic field at the time the mineral was laid down as sediment or crystallized in magma and also the tectonic movement o ...
(NRM) to reveal the DRM. Following statistical analysis, the results are used to generate a local magnetostratigraphic column that can then be compared against the Global Magnetic Polarity Time Scale.
This technique is used to date sequences that generally lack fossils or interbedded igneous rocks. The continuous nature of the sampling means that it is also a powerful technique for the estimation of sediment-accumulation rates.
See also
*
Assise
In geology, an assise (from the French, derived from Latin ''assidere'', "to sit beside") is two or more beds or strata of rock united by the occurrence of the same characteristic species or genera. In the hierarchy of stratigraphic units, an ...
*
Bed (geology)
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".Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds., 2005. ''Glossary of Geology'' ...
*
Conodont biostratigraphy
Conodonts are an extinct class of animals whose feeding apparatuses called ''teeth'' or ''elements'' are common microfossils found in strata dating from the Stage 10 of the Furongian, the fourth and final series of the Cambrian, to the Rhaetian st ...
*
Erygmascope (old instrument for studying strata)
*
Harris matrix
The Harris matrix is a tool used to depict the temporal succession of archaeological contexts and thus the sequence of depositions and surfaces on a 'dry land' archaeological site, otherwise called a 'stratigraphic sequence'. The matrix reflect ...
*
Important publications in stratigraphy
*
International Commission on Stratigraphy
The International Commission on Stratigraphy (ICS), sometimes referred to unofficially as the "International Stratigraphic Commission", is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigra ...
*
Key bed
Marker horizons (also referred to as chronohorizons, key beds or marker beds) are stratigraphic units of the same age and of such distinctive composition and appearance, that, despite their presence in separate geographic locations, there is no do ...
*
Sedimentary basin analysis
*
Sequence stratigraphy
Sequence stratigraphy is a branch of geology, specifically a branch of stratigraphy, that attempts to discern and understand historic geology through time by subdividing and linking sedimentary deposits into unconformity bounded units on a variety ...
*
Sadler effect
*
Tectonostratigraphy In geology, tectonostratigraphy is stratigraphy that refers either to rock sequences in which large-scale layering is caused by the stacking of thrust sheets, or nappes, in areas of thrust tectonics or to the effects of tectonics on lithostratigr ...
References
Further reading
*Christopherson, R. W., 2008. ''Geosystems: An Introduction to Physical Geography'', 7th ed., New York: Pearson Prentice-Hall.
*Montenari, M., 2016.
Stratigraphy and Timescales', 1st ed., Amsterdam: Academic Press (Elsevier).
External links
University of South Carolina Sequence Stratigraphy WebInternational Commission on StratigraphyUniversity of Georgia (USA) Stratigraphy LabStratigraphy.netA stratigraphic data provider.
Agenames.orgA global index of stratigraphic terms
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Petrology
Methods in archaeology