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Encyclopedia > Stratigraphy

Stratigraphy, a branch of geology, studies rock layers and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy includes two related subfields: lithologic or lithostratigraphy and biologic stratigraphy or biostratigraphy. Image File history File links Question_book-3. ... This article includes a list of works cited but its sources remain unclear because it lacks in-text citations. ... Rock redirects here. ... For other uses, see strata (novel) and strata title. ... Two types of sedimentary rock: limey shale overlaid by limestone. ... Ignimbrite is a deposit of a pyroclastic flow. ... Lithostratigraphy is the geological science associated with the study of stratum or rock layers. ... This article or section does not cite any references or sources. ...

Contents

Historical development

Engraving from William Smith's monograph on identifying strata based on fossils
Engraving from William Smith's monograph on identifying strata based on fossils

The theoretical basis for the subject was established by Nicholas Steno who introduced the law of superposition, the principle of original horizontality, and the principle of lateral continuity in an 1669 work on the fossilization of organic remains in layers of sediment. Image File history File links Size of this preview: 800 × 577 pixel Image in higher resolution (978 × 705 pixel, file size: 133 KB, MIME type: image/jpeg) Engraving of fossils from William Smiths monograph on identifying geological strata using fossils. ... Image File history File links Size of this preview: 800 × 577 pixel Image in higher resolution (978 × 705 pixel, file size: 133 KB, MIME type: image/jpeg) Engraving of fossils from William Smiths monograph on identifying geological strata using fossils. ... Nicolaus Steno. ... See here for the superposition principle of physics. ... Proposed by Nicholas Steno. ... The principle of lateral continuity states that layers of sediment initially extend laterally in all directions; in other words, they are laterally continuous. ...


The first practical large scale application of stratigraphy was by William Smith in the 1790s and early 1800s. Smith, known as the Father of English Geology, created the first geologic map of England, and first recognized the significance of strata or rock layering, and the importance of fossil markers for correlating strata. Another influential application of stratigraphy in the early 1800s was a study by Georges Cuvier and Alexandre Brongniart of the geology of the region around Paris. William Smith. ... A geologic map is a special-purpose map made for the purpose of showing subsurface geological features. ... For other uses, see strata (novel) and strata title. ... Georges Cuvier Baron Georges Léopold Chrétien Frédéric Dagobert Cuvier (August 23, 1769–May 13, 1832) was a French naturalist and zoologist. ... Alexandre Brongniart (1770 – 1847) was a French chemist and zoologist, who collaborated with Georges Cuvier. ...


Lithologic stratigraphy

Main article: Lithostratigraphy

Lithostratigraphy, or lithologic stratigraphy, is the most obvious. It deals with the physical lithologic or rock type change both vertically in layering or bedding of varying rock type and laterally reflecting changing environments of deposition, known as facies change. Key elements of stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries mean in terms of depositional environment. One of stratigraphy's basic concepts is codified in the Law of Superposition, which simply states that, in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence. Lithostratigraphy is the geological science associated with the study of stratum or rock layers. ... Petrology is a field of geology which focuses on the study of rocks and the conditions by which they form. ... The term facies was introduced by the Swiss geologist Amanz Gressly in 1838 and was part of his significant contribution to the foundations of modern stratigraphy (see Cross and Homewood 1997), which replaced the earlier notions of Neptunism. ... See here for the superposition principle of physics. ...

Chalk Layers in Cyprus - showing classic layered structure
Chalk Layers in Cyprus - showing classic layered structure

Chemostratigraphy is based on the changes in the relative proportions of trace elements and isotopes within and between lithologic units. Carbon and oxygen isotope ratios vary with time and are used to map subtle changes in the paleoenvironment This has led to the specialized field of isotopic stratigraphy. Image File history File linksMetadata Size of this preview: 800 × 536 pixelsFull resolution (1840 × 1232 pixel, file size: 1. ... Image File history File linksMetadata Size of this preview: 800 × 536 pixelsFull resolution (1840 × 1232 pixel, file size: 1. ... For other uses, see Isotope (disambiguation). ... For other uses, see Carbon (disambiguation). ... General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colorless (gas) pale blue (liquid) Standard atomic weight 15. ...


Cyclostratigraphy documents the often cyclic changes in the relative proportions of minerals, particularly carbonates, and fossil diversity with time, related to changes in palaeoclimates. For other uses, see Mineral (disambiguation). ... Ball-and-stick model of the carbonate ion, CO32− For other meanings, see Carbonate (disambiguation) In chemistry, a carbonate is a salt or ester of carbonic acid. ... Paleoclimatology is the study of climate change taken on the scale of the entire history of the Earth. ...


Biostratigraphy

Main article: Biostratigraphy

Biostratigraphy or paleontologic stratigraphy is based on fossil evidence in the rock layers. Strata from widespread locations containing the same fossil fauna and flora are correlatable in time. Biologic stratigraphy was based on William Smith's principle of faunal succession, which predated, and was one of the first and most powerful lines of evidence for, biological evolution. It provides strong evidence for formation (speciation) of and the extinction of species. The geologic time scale was developed during the 1800s based on the evidence of biologic stratigraphy and faunal succession. This timescale remained a relative scale until the development of radiometric dating, which gave it and the stratigraphy it was based on an absolute time framework, leading to the development of chronostratigraphy. This article or section does not cite any references or sources. ... Paleontology, palaeontology or palæontology (from Greek: paleo, ancient; ontos, being; and λόγος, logos, knowledge) is the study of prehistoric life forms on Earth through the examination of plant and animal fossils. ... For other uses, see Fossil (disambiguation). ... This article or section does not cite its references or sources. ... This article is about biological evolution. ... Charles Darwins first sketch of an evolutionary tree from his First Notebook on Transmutation of Species (1837) Speciation is the evolutionary process by which new biological species arise. ... For other uses, see Extinction (disambiguation). ... For other uses, see Species (disambiguation). ... // For other uses, see time scale. ... Radiometric dating (often called radioactive dating) is a technique used to date materials, based on a comparison between the observed abundance of particular naturally occurring radioactive isotopes and their known decay rates. ...


One important development is the Vail curve, which attempts to define a global historical sea-level curve according to inferences from world-wide stratigraphic patterns. Stratigraphy is also commonly used to delineate the nature and extent of hydrocarbon-bearing reservoir rocks, seals and traps in petroleum geology. Oil refineries are key to obtaining hydrocarbons; crude oil is processed through several stages to form desirable hydrocarbons, used in fuel and other commercial products. ... Petroleum geology is a term used to refer to the specific set of geological disciplines that are applied to the search for hydrocarbons (oil exploration). ...


Chronostratigraphy

Main article: Chronostratigraphy

Chronostratigraphy is the branch of stratigraphy that studies the absolute age of rock strata. Chronostratigraphy is the branch of stratigraphy that studies the age of rock strata in relation to time. ... For other uses, see strata (novel) and strata title. ...


Chronostratigraphy is based upon deriving geochronological data for rock units, both directly and by inference, so that a sequence of time relative events of rocks within a region can be derived. In essence, chronostratigraphy seeks to understand the geologic history of rocks and regions. Geochronology is the science of determining the age of rocks, fossils, and sediments. ...


The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of the Earth.


Magnetostratigraphy

When measurable magnetic properties of rocks vary stratigraphically they may be the bases for related but different kinds of stratigraphic units known collectively as "magnetostratigraphic units" ("magnetozones"). The magnetic property most useful in stratigraphic work is the change in the direction of the remanent magnetization of the rocks, caused by reversals in the polarity of the Earth's magnetic field. Such reversals of the polarity have taken place many times during geologic history. They are recorded in the rocks because the rocks may record the direction of the Earth's magnetic field at or near the time of rock formation (see paleomagnetism). The direction of the remnant magnetic polarity recorded in the stratigraphic sequence can be used as the basis for the subdivision of the sequence into units characterized by their magnetic polarity. Such units are called "magnetostratigraphic polarity units". Paleomagnetism refers to the study of the record of the Earths magnetic field preserved in various magnetic minerals through time. ...


Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic stratigraphic sections. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their Detrital Remanent Magnetization (DRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. This is possible because when very fine-grained magnetic minerals (< 17 micrometres) fall through the water column, they orient themselves with Earth's magnetic field. Upon burial, that orientation is preserved. The minerals, in effect, behave like tiny compasses.


If the ancient magnetic field was oriented similar to today's field (North Magnetic Pole near the North Rotational Pole) the strata retain a Normal Polarity. If the data indicate that the North Magnetic Pole was near the South Rotational Pole, the strata exhibit Reversed Polarity.


Sampling procedures

Oriented paleomagnetic core samples are collected in the field using a Pomeroy Drill. A minimum of three samples is taken from each sample site for statistical purposes. Spacing of the sample sites within a stratigraphic section depends on: 1) the type of depositional environment: The farther away from the orogenic front, the closer the sample spacing due to generally lower rates of deposition; and 2) the suitability of the rocks for paleomagnetic analysis. Mudstones, siltstones, and very fine-grained sandstones are the preferred lithologies because the magnetic grains are finer and more likely to orient with the ambient field during deposition. It is more likely that these samples will deliver a reliable paleomagnetic signal.


Analytical procedures

Samples are first analyzed in their natural state to obtain their Natural Remanent Magnetization (NRM). The NRM is then stripped away in a stepwise manner using thermal or alternating field demagnetization techniques to reveal the stable magnetic component. The stable component is usually interpreted to be the DRM. Natural Remanent Magnetization (Abbreviated NRM) is the permanent magnetism of a rock. ...


DRM orientations of all samples from a site are then compared and their magnetic polarity is determined with Fisher statistics. Using Watson's criteria, the statistical significance of each sample site is evaluated. The latitudes of the Virtual Geomagnetic Poles from those sites determined to be statistically significant are plotted against the stratigraphic level at which they were collected. These data are then abstracted to the standard black and white magnetostratigraphic columns in which black indicates Normal polarity and white is Reversed polarity.


Correlation and ages

Because the polarity of a stratum can only be Normal or Reversed, variations in the rate at which the sediment accumulated can cause the thickness of a given polarity zone to vary from one area to another. This presents the problem of how to differentiate different zones of like polarities between different stratigraphic sections. To overcome the possibility of confusion at least one isotopic age (or at least an age based on paleontological data) needs to be collected from each section. These are usually obtained from intercalated airfall volcanic material. With the aid of the independent isotopic age or ages, the local magnetostratigraphic column is correlated with the Global Magnetic Polarity Time Scale (GMPTS).


Because the age of each reversal shown on the GMPTS is relatively well known, the correlation establishes numerous time lines through the stratigraphic section. These ages provide relatively precise dates for features in the rocks such as fossils, changes in sedimentary rock composition, changes in depositional environment, etc. They also constrain the ages of cross-cutting features such as faults, dikes, and unconformities.


Sediment accumulation rates

Perhaps the most powerful application of these data is to determine the rate at which the sediment accumulated. This is accomplished by plotting the age of each reversal (in millions of years ago) vs. the stratigraphic level at which the reversal is found (in meters). This provides the rate in meters per million years which is usually rewritten in terms of millimeters per year (which is the same as kilometers per million years).


These data are also used to model basin subsidence rates. Knowing the depth of a hydrocarbon source rock beneath the basin-filling strata allows calculation of the age at which the source rock passed through the generation window and hydrocarbon migration began. Because the ages of cross-cutting trapping structures can usually be determined from magnetostratigraphic data, a comparison of these ages will assist reservoir geologists in their determination of whether or not a play is likely in a given trap.


Another application of these results derives from the fact that they illustrate when sediment accumulation rates changed. Such changes require explanation. The answer is often related to either climatic factors or to tectonic developments in nearby or distant mountain ranges. Evidence to strengthen this interpretation can often be found by looking for subtle changes in the composition of the rocks in the section. Changes in sandstone composition are often used for this type of interpretation.


Archaeological stratigraphy

In the field of archaeology, soil stratigraphy is used to better understand the processes that form and protect archaeological sites. The law of superposition holds true, and this can help date finds or features from each context, as they can be placed in sequence and the dates interpolated. Phases of activity can also often be seen through stratigraphy, especially when a trench or feature is viewed in section (profile). As pits and other features can be dug down into earlier levels, not all material at the same absolute depth is necessarily of the same age, but close attention has to be paid to the archeological layers. The Harris-matrix is a tool to depict complex stratigraphic relations, as they are found, for example, in the contexts of urban archaeology. In archaeology, especially in the course of excavation, stratification is of major interest and significance. ... For referencing in Wikipedia, see Wikipedia:Citing sources. ... An archaeological site is a place (or group of physical sites) in which evidence of past activity is preserved (either prehistoric or historic or contemporary), and which has been investigated using the discipline of archaeology. ... Archaeology or sometimes in American English archeology (from the Greek words &#945;&#961;&#967;&#945;&#943;&#959;&#962; = ancient and &#955;&#972;&#947;&#959;&#962; = word/speech) is the study of human cultures through the recovery, documentation and analysis of material remains, including architecture, artefacts, biofacts, human remains, and landscapes. ... Half-section through a Saxon pit In archaeology a section is a view of an excavated archaeological trench or feature showing the contents of that feature in two dimensions (vertical and horizontal) and thereby illustrating its profile and stratigraphy. ... Layer may refer to: Look up Layer in Wiktionary, the free dictionary. ... The Harris matrix or Winchester seriation diagram is a tool used to depict the temporal succession of archaeological contexts and thus the sequence of deposition on a dry land archaeological site. ... Urban archaeology is a sub discipline of archaeology specialising in the material past of towns and cities where long-term human habitation has often left a rich record of the past. ...


See also

// Foundations Principles of Geology Author: Charles Lyell Publication data: 1830–1833. ... In geology, a key bed (syn marker bed) is a widespread sedimentary layer that formed at a single time, such that it is useful for geologic correlations and dating over a large area. ... This article may be too technical for most readers to understand. ... Sedimentary basin analysis is a geologic method by which the history of a sedimentary basin is revealed, by analyzing the sediment fill itself. ... This article does not cite its references or sources. ...

External links


  Results from FactBites:
 
Stratigraphy - Wikipedia, the free encyclopedia (229 words)
One of stratigraphy's basic concepts is codified in the Law of Superposition, which simply states that, in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence.
The study of stratigraphic sequences has led to some important results and given rise to the discipline of sequence stratigraphy, such as the Vail curve, which attempts to define a global historical sea-level curve according to inferences from world-wide stratigraphic patterns.
Stratigraphy is also commonly used to delineate the nature and extent of hydrocarbon-bearing reservoir rocks, seals and traps (see petroleum geology).
Archaeology - Wikipedia, the free encyclopedia (5430 words)
Modern excavation techniques require that the precise locations of objects and features, known as their provenance or provenience, be recorded.
For example, excavation of a site reveals its stratigraphy; if a site was occupied by a succession of distinct cultures, artefacts from more recent cultures will lie above those from more ancient cultures.
There are many different styles of excavation, with some countries traditionally preferring one method over another; and some sub-disciplines like classical archaeology will often use different methods than those used for the excavation of a prehistoric site.
  More results at FactBites »

 
 

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