stratigraphy

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Concept

Stratigraphy is the study of rock layers (strata) deposited in the earth. It is one of the most challenging of geologic subdisciplines, comparable to an exacting form of detective work, yet it is also one of the most important branches of study in the geologic sciences. Earth's history, quite literally, is written on the strata of its rocks, and from observing these layers, geologists have been able to form an idea of the various phases in that long history. Naturally, information is more readily discernible about the more recent phases, though even in studying these phases, it is possible to be misled by gaps in the rock record, known as unconformities.

How It Works

The Foundations of Stratigraphy

Historical geology, the study of Earth's physical history, is one of the two principal branches of geology, the other being physical geology, or the study of Earth's physical components and the forces that have shaped them. Among the principal subdisciplines of historical geology is stratigraphy, the study of rock layers, which are called strata or, in the singular form, a stratum.

Other important subdisciplines include geochronology, the study of Earth's age and the dating of specific formations in terms of geologic time; sedimentology, the study and interpretation of sediments, including sedimentary processes and formations; paleontology, the study of fossilized plants and animals; and paleoecology, the study of the relationship between prehistoric plants and animals and their environments. Several of these subjects are examined in other essays within this book.

Early Work in Stratigraphy

Among the earliest contributions to what could be called historical geology came from the Italian scientist and artist Leonardo da Vinci (1452-1519), who speculated that fossils might have come from the remains of long-dead animals. Nearly two centuries later, stratigraphy itself had its beginnings when the Danish geologist Nicolaus Steno (1638-1687) studied the age of rock strata.

Steno formulated what came to be known as the law of superposition, or the idea that strata are deposited in a sequence such that the deeper the layer, the older the rock. This, of course, assumes that the rock has been undisturbed, and it is applicable only for one of the three major types of rock, sedimentary (as opposed to igneous or metamorphic). Later, the German geologist Johann Gottlob Lehmann (1719-1767) put forward the theory that certain groups of rocks tend to be associated with each other and that each layer of rock is a sort of chapter in the history of Earth.

Thus, along with Steno, Lehmann helped pioneer the idea of the stratigraphic column, discussed later in this essay. The man credited as the "father of stratigraphy," however, was the English engineer and geologist William Smith (1769-1839). In 1815 Smith produced the first modern geologic map, showing rock strata in England and Wales. Smith's achievement, discussed in Measuring and Mapping Earth, influenced all of geology to the present day by introducing the idea of geologic, as opposed to geographic, mapping. Furthermore, by linking stratigraphy with paleontology, he formulated an important division of stratigraphy, known as biostratigraphy.

Areas of Stratigraphic Study

Along with biostratigraphy, the major areas of stratigraphy include lithostratigraphy, chronostratigraphy, geochronometry, and magnetostratigraphy. The most basic type of stratigraphy, and the first to emerge, was lithostratigraphy, which is simply the study and description of rock layers. Earth scientists working in the area of lithostratigraphy identify various types of layers, which include (from the most specific to the most general), formations, members, beds, groups, and supergroups.

Biostratigraphy involves the study of fossilized plants and animals to establish dates for and correlate relations between stratigraphic layers. Scientists in this field also identify categories of biostratigraphic units, the most basic being a biozone. Magnetostratigraphy is based on the investigation of geomagnetism and the reversals in Earth's magnetic field that have occurred over time. (See Geomagnetism as well as the discussion of paleomagnetism in Plate Tectonics.)

Chronostratigraphy is devoted to studying the ages of rocks and what they reveal about geologic time, or the vast stretch of history (approximately 4.6 billion years, abbreviated 4.6 Ga) over which Earth's geologic development has occurred. It is concerned primarily with relative dating, whereas geochronometry includes the determination of absolute dates and time intervals. This typically calls for the use of radiometric dating.

The Stratigraphic Column

The stratigraphic column is the succession of rock strata laid down over the course of time, each of which correlates to specific phases in Earth's geologic history. The record provided by the stratigraphic column is most reliable for studying the Phanerozoic, the current eon of geologic history, as opposed to the Precambrian, which constituted the first three eons and hence the vast majority of Earth's geologic history. The relatively brief span of time since the Phanerozoic began (about 545 million years, or Ma) has seen by far the most dramatic changes in plant and animal life. It was in this eon that the fossil record emerged, giving us far more detailed information about comparatively recent events than about a much longer span of time in the more distant past.

Relative and Absolute Dating

Precambrian time is so designated because it precedes the Cambrian period, one of 11 periods in the Phanerozoic eon. The Cambrian period extended for about 50 million years, from approximately 545 Ma to 495 Ma ago. This statement in terms of years, however inexact, is an example of absolute age. By contrast, if we say that the Cambrian period occurred at the beginning of the Paleozoic era, after the end of the Proterozoic eon and before the beginning of the Ordovician period, this is a statement of relative age. Both statements are true, and though it is obviously preferable to measure time in absolute terms, sometimes relative terms are the only ones available.

Dating, in scientific terms, is any effort directed toward finding the age of a particular item or phenomenon. Relative dating methods assign an age relative to that of other items, whereas absolute dating determines age in actual years or millions of years. When geologists first embarked on stratigraphic studies, the only means of dating available to them were relative. Using Steno's law of superposition, they reasoned that a deeper layer of sedimentary rock was necessarily older than a shallower layer.

Advances in our understanding of atomic structure during the twentieth century, however, made possible a particularly useful absolute form of dating through the study of radioactive decay. Radiometric dating, which is explained in more detail in Geologic Time, uses ratios between "parent" and "daughter" isotopes. Radioactive isotopes decay, or emit particles, until they become stable, and as this takes place, parent isotopes spawn daughters. The amount of time that it takes for half the isotopes in a sample to stabilize is termed a half-life. Elements such as uranium, which has isotopes with half-lives that extend into the billions of years, make possible the determination of absolute dates for extremely old geologic materials.

Divisions of the Stratigraphic Column

Geologic time is divided into named groupings according to six basic units, which are (in order of size from longest to shortest) eon, era, period, epoch, age, and chron. There is no absolute standard for the length of any unit; rather, it takes at least two ages to make an epoch, at least two epochs to compose a period, and so on. The dates for specific eons, eras, periods, and so on are usually given in relative terms, however; an example is the designation of the Cambrian period given earlier.

Chronostratigraphy also uses six time units: the eonothem, era them, system, series, stage, and chronozone. These time units are analogous to the terms in the geologic time scale, the major difference being that chronostratigraphic units are conceived in terms of relative time and are not assigned dates. The more distant in time a particular unit is, the more controversy exists regarding its boundary with preceding and successive units. This is true both of the geologic and the chronostratigraphic scales.

For this reason, the International Union of Geological Sciences, the leading worldwide body of geologic scientists, has established a Commission on Stratigraphy to determine such boundaries. The commission selects and defines what are called Global Stratotype Sections and Points (GSGPs), which are typically marine fossil formations. Because it is believed that life has existed longest on Earth in its oceans, samples from the water provide the most reliable stratigraphic record.

Naming of Chronostratigraphic Units

As noted, the chronostratigraphic divisions correspond to units of geologic time, even though chronostratigraphic units are based on relative dating methods and geologic ones use absolute time measures. Because attempts at relative dating have been taking place since the late eighteenth century, today's geologic units originated as what would be called stratigraphic or chronostratigraphic units. Even today the names of the phases are the same, with the only difference being the units in which they are expressed. Thus, when speaking in terms of geologic time, one would refer to the Jurassic period, whereas in stratigraphic terms, this would be the Jurassic system.

In 1759 the Italian geologist Giovanni Arduino (1714-1795) developed the idea of primary, secondary, and tertiary groups of rocks. Though the use of the terms primary and secondary has been discarded, vestiges of Arduino's nomenclature survive in the modern designation of the Tertiary subera of the Cenozoic era (era them in stratigraphic terminology) as well as in the name of the present period or system, the Quaternary. (Just as primary, secondary, and tertiary refer to a first, second, and third level, respectively, the term quaternary indicates a fourth level.)

We are living in the fourth of four eons, or eonothems, the Phanerozoic, which is divided into three eras, or erathems: Paleozoic, Mesozoic, and Cenozoic. These eras, in turn, are divided into 11 periods, or systems, whose names (except for Tertiary and Quaternary) refer to the locations in which the respective stratigraphic systems were first observed. The names of these systems, along with their dates in millions of years before the present and the origin of their names, are as follows (from the most distant to the most recent):

Periods/Systems of the Paleozoic Era/Erathem

• Cambrian (about 545 to 495 Ma): Cambria, the Roman name for the province of Wales
• Ordovician (about 495 to 443 Ma): Ordovices, the name of a Celtic tribe in ancient Wales
• Silurian (about 443 to 417 Ma): Silures, another ancient Welsh Celtic tribe
• Devonian (about 417 to 354 Ma): Devonshire, a county in southwest England
• Mississippian (a subperiod of the Carboniferous period, about 354 to 323 Ma): the Mississippi River
• Pennsylvanian (a subperiod of the Carboniferous, about 323 to 290 Ma): the state of Pennsylvania
• Permian (about 290 to 248.2 Ma): Perm, a province in Russia

Periods/Systems of the Mesozoic Era/Erathem

• Triassic (about 248.2 to 205.7 Ma): a tripartite, or threefold, division of rocks in Germany
• Jurassic (about 205.7 to 142 Ma): the Jura Mountains of Switzerland and France
• Cretaceous (about 142 to 65 Ma): from aLatin word for "chalk," a reference to the chalky cliffs of southern England and France

Within the more recent Cenozoic era, or era them, names of epochs (or "series" in stratigraphic terminology) become important. They are all derived from Greek words, whose meanings are given below:

Epochs/Series of the Cenozoic Era/Erathem

• Paleocene (about 65 to 54.8 Ma): "early dawn of the recent"
• Eocene (about 54.8 to 33.7 Ma): "dawn of the recent"
• Oligocene (about 33.7 to 23.8 Ma): "slightly recent"
• Miocene (about 23.8 to 5.3 Ma): "less recent"
• Pliocene (about 5.3 to 1.8 Ma): "more recent"
• Pleistocene (about 1.8 to 0.01 Ma): "most recent"
• Holocene (about 0.01 Ma to present): "wholly recent"

Real-Life Applications

Correlation

The geologist studying the stratigraphic record is a sort of detective, looking for clues. Just as detectives have their methods for solving crimes, geologists rely on correlation, or methods of establishing age relationships between various strata. There are two basic types of correlation: physical correlation, which requires comparison of the physical characteristics of the strata, and fossil correlation, the comparison of fossil types.

Actually, chronostratigraphic work is very similar some of the toughest cases confronted by police detectives, because more often than not the geologic detective has little evidence on which to operate. First of all, as noted earlier, only sedimentary rock can be used in making such determinations: for instance, igneous rock in its molten form, as when it is expelled from a volcano, could force itself underneath a rock stratum, thus confusing the stratigraphic record.

Potential Pitfalls

Even when the rock is sedimentary, there is still plenty of room for error. The layers may be many feet or less than an inch deep, and it is up to the geologist to determine whether the stratum has been affected by such geologic forces as erosion. If erosion has occurred, it can cause a disturbance, or unconformity (discussed later), which tends to render inaccurate any reading of the stratigraphic record.

Another possible source of disturbance is an earthquake, which could cause one part of Earth's crust to shift over an adjacent section, making the stratigraphic record difficult, if not impossible, to read. Under the best of conditions, after all, the strata are hardly neat, easily defined lines. If one observes a horizontal section, there is likely to be a change in thickness, because as the stratum extends outward, it merges with the edges of adjacent deposits.

Yet another potential pitfall in stratigraphic correlation involves one of the most useful tools available to a geologist attempting to find an absolute age for the materials he or she is studying: radiometric dating. Though this method can provide accurate absolute dates, it is quite possible that the age thus determined will be the age of the parent rock from which a sample is taken, not the age of the sample itself. The grains of sand in a piece of sandstone, for instance, are much older than the larger unit of sandstone, and for this reason, radiometric dating is useful only in specific circumstances.

Physical and Fossil Correlation

Given all these challenges, it is a wonder that geologists manage to correlate strata successfully, yet they do. Physical correlations are achieved on the basis of several criteria, including color, the size of grains, and the varieties of minerals found within a stratum. By such means, it is sometimes possible to correlate widely separated strata.

Particularly impressive feats of correlation can result from the study of fossils, whose stratigraphic implications, as we have noted, were first discovered by William Smith. Smith hit upon the idea of biostratigraphy while excavating land for a set of canals near London. As he discovered, any given stratum contains the same types of fossils, and strata in two different areas thus can be correlated.

Long before his countryman Charles Darwin (1809-1882) developed the theory of evolution, Smith conceived his own law of faunal succession, which hints at the idea that species developed and disappeared over given phases in Earth's past. According to the law of faunal succession, all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. As a result, if a geologist finds a stratum in one area that contains a particular fossil and another in a distant area containing the same fossil, it is possible to conclude that the strata are the same.

Unconformities

In discussing the many challenges facing a geologist studying stratigraphic data, the role of erosion was noted. Let us return to that subject, because erosion is a source of what are known as unconformities, or gaps in the rock record. Unconformities are of three types: angular unconformities, disconformities, and nonconformities.

Angular unconformities involve a tilting of the layers, such that an upper layer does not lie perfectly parallel to a lower one. Disconformities are more deceptive, because the layers are parallel, yet there is still an unconformity between them, and only a study of the fossil record can reveal the unconformity. Finally, a nonconformity arises when sedimentary rocks are divided from a type of igneous rock known as intrusive (meaning "cooled within Earth").

Angular Unconformities

Angular unconformities emerge as a by-product of the dramatic shifts and collisions that take place in plate tectonics (see Plate Tectonics). Sediment accumulates and then, as a result of plate movement, is moved about and eventually experiences weathering and erosion. Layers are tilted and then flattened by more erosion, and as the solid earth rises or sinks, they are shifted further. Such is the case, for instance, along the Colorado River at the Grand Canyon, where angular unconformities reveal a series of movements over the years.

Another famous angular unconformity can be found at Siccar Point in Scotland, where nearly horizontal deposits of sandstone rest atop nearly vertical ones of graywacke, another sedimentary rock. Observations of this unconformity led the great geologist James Hutton (1726-1797) to the realization that Earth is much, much older than the 6,000 years claimed by theologians in his day (see Historical Geology).

Where to Learn More

Bishop, A. C., A. Woolley, and A. Hamilton. Cambridge Guide to Minerals, Rocks, and Fossils. New York: Cambridge University Press, 1992.

Boggy's Links to Stratigraphy and Geochronology (Web site). <http://geologylinks.freeyellow.com/stratigraphy.html>.

Harris, Nicholas, Alessandro Rabatti, and Andrea Ricciardi. The Incredible Journey to the Beginning of Time. New York: Peter Bedrick Books, 1998.

Lamb, Simon, and David Sington. Earth Story: The Shaping of Our World. Princeton, NJ: Princeton University Press, 1998.

MacRae, Andrew. Radiometric Dating and the Geological Time Scale (Web site). <http://www.talkorigins.org/faqs/dating.html>.

Reeves, Hubert. Origins: Cosmos, Earth, and Mankind. New York: Arcade, 1998.

Spickert, Diane Nelson, and Marianne D. Wallace. Earth-steps: A Rock's Journey Through Time. Golden, CO: Fulcrum Kids, 2000.

Stratigraphy and Earth History—West's Geology Directory (Web site). <http://www.soton.ac.uk/~imw/stratig.htm>.

University of Georgia Stratigraphy Lab (Web site). <http://www.uga.edu/~strata/home.html>.

Web Time Machine. UCMP (University of California, Berkeley, Museum of Paleontology) (Web site). <http://www.ucmp.berkeley.edu/help/timeform.html>.

A discipline involving the description and interpretation of layered sediments and rocks, and especially their correlation and dating. Correlation is a procedure for determining the relative age of one deposit with respect to another. The term “dating” refers to any technique employed to obtain a numerical age, for example, by making use of the decay of radioactive isotopes found in some minerals in sedimentary rocks or, more commonly, in associated igneous rocks. To a large extent, layered rocks are ones that accumulated through sedimentary processes beneath the sea, within lakes, or by the action of rivers, the wind, or glaciers; but in places such deposits contain significant amounts of volcanic material emplaced as lava flows or as ash ejected from volcanoes during explosive eruptions. See also Dating methods; Igneous rocks; Rock age determination; Sedimentary rocks.

Sedimentary successions are locally many thousands of meters thick owing to subsidence of the Earth's crust over millions of years. Sedimentary basins therefore provide the best available record of Earth history over nearly 4 billion years. That record includes information about surficial processes and the varying environment at the Earth's surface, and about climate, changing sea level, the history of life, variations in ocean chemistry, and reversals of the Earth's magnetic field. Sediments also provide a record of crustal deformation (folding and faulting) and of large-scale horizontal motions of the Earth's lithospheric plates (continental drift). Stratigraphy applies not only to strata that have remained flat-lying and little altered since their time of deposition, but also to rocks that may have been strongly deformed or recrystallized (metamorphosed) at great depths within the Earth's crust, and subsequently exposed at the Earth's surface as a result of uplift and erosion. As long as original depositional layers can be identified, some form of stratigraphy can be undertaken. See also Basin; Continental drift; Fault and fault structures; Sedimentology.

An important idea first articulated by the Danish naturalist Nicolaus Steno in 1669 is that in any succession of strata the oldest layer must have accumulated at the bottom, and successively younger layers above. It is not necessary to rely on the present orientation of layers to determine their relative ages because most sediments and sedimentary rocks contain numerous features, such as current-deposited ripples, minor erosion surfaces, or fossils of organisms in growth position, that have a well-defined polarity with respect to the up direction at the time of deposition (so-called geopetal indicators). This principle of superposition therefore applies equally well to tilted and even overturned strata. Only where a succession is cut by a fault is a simple interpretation of stratigraphic relations not necessarily possible, and in some cases older rocks may overlie younger rocks structurally. See also Depositional systems and environments.

The very existence of layers with well-defined boundaries implies that the sedimentary record is fundamentally discontinuous. Discontinuities are present in the stratigraphic record at a broad range of scales, from that of a single layer or bed to physical surfaces that can be traced laterally for many hundreds of kilometers. Large-scale surfaces of erosion or nondeposition are known as unconformities, and they can be identified on the basis of both physical and paleontological criteria. See also Paleontology; Unconformity.

Most stratal discontinuities possess time-stratigraphic significance because strata below a discontinuity tend to be everywhere older than strata above. To the extent that unconformities can be recognized and traced widely within a sedimentary basin, it is possible to analyze sedimentary rocks in a genetic framework, that is, with reference to the way they accumulated. This is the basis for the modern discipline of sequence stratigraphy, so named because intervals bounded by unconformities have come to be called sequences.

Traditional stratigraphic analysis has focused on variations in the intrinsic character or properties of sediments and rocks—properties such as composition, texture, and included fossils (lithostratigraphy and biostratigraphy)—and on the lateral tracing of distinctive marker beds such as those composed of ash from a single volcanic eruption (tephrostratigraphy). The techniques of magnetostratigraphy and chemostratigraphy are also based on intrinsic characteristics, although these techniques require sophisticated laboratory analysis. Sequence stratigraphy attempts to integrate these approaches in the context of stratal geometry, thereby providing a unifying framework in which to investigate the time relations between sediment and rock bodies as well as to measure their numerical ages (chronostratigraphy and geochronology). Seismic stratigraphy is a variant of the technique of sequence stratigraphy in which unconformities are identified and traced in seismic reflection profiles on the basis of reflection geometry. See also Geochronometry; Seismic stratigraphy; Seismology.

Conventional stratigraphy currently recognizes two kinds of stratigraphic unit: material units, distinguished on the basis of some specified property or properties or physical limits; and temporal or time-related units. A common example of a material unit is the formation, a lithostratigraphic unit defined on the basis of lithic characteristics and position within a stratigraphic succession. Each formation is referred to a section or locality where it is well developed (a type section), and assigned an appropriate geographic name combined with the word formation or a descriptive lithic term such as limestone, sandstone, or shale (for example, Tapeats Sandstone). Some formations are divisible into two or more smaller-scale units called members and beds. In other cases, formations of similar lithic character or related genesis are combined into composite units called groups and supergroups.

Sequence stratigraphy differs from conventional stratigraphy in two important respects. The first is that basic units (sequences) are defined on the basis of bounding unconformities and correlative conformities rather than material characteristics or age. The second is that sequence stratigraphy is fundamentally not a system for stratigraphic classification, but a procedure for determining how sediments accumulate. See also Sequence stratigraphy.

The study of the divisions of rocks in time and of the links between similar rocks which occur in different areas. This study may be based on a variety of variables: biostratigraphy on fossil content, lithostratigraphy, or rock stratigraphy on lithological characteristics, and magnetostratigraphy on the magnetic field preserved within the rock. Aminostratigraphy measures the amount of L-amino acids and D-amino acids in a shell. The ratio of these optical isomers is a measure of the age of the shell. Chronostratigraphy, or time stratigraphy, organizes strata according to their relative ages. For stratigraphical column, See geological column.

A method of bodysection radiography.

Study of layered rock to understand the sequence of geological events. Normally, older layers are on the bottom unless the sequence has been overturned or disrupted.

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (April 2007)

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.

Contents 1 Historical development 2 Lithologic stratigraphy 3 Biostratigraphy 4 Chronostratigraphy 5 Magnetostratigraphy 6 Archaeological stratigraphy 7 See also 8 References 9 External links

Historical development

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

Rock layers were studied since the time of Avicenna (Ibn Sina), a Muslim geographer who wrote The Book of Healing in 1027. He was the first to outline the law of superposition of strata:^[1]

"It is also possible that the sea may have happened to flow little by little over the land consisting of both plain and mountain, and then have ebbed away from it. ... It is possible that each time the land was exposed by the ebbing of the sea a layer was left, since we see that some mountains appear to have been piled up layer by layer, and it is therefore likely that the clay from which they were formed was itself at one time arranged in layers. One layer was formed first, then at a different period, a further was formed and piled, upon the first, and so on. Over each layer there spread a substance of different material, which formed a partition between it and the next layer; but when petrification took place something occurred to the partition which caused it to break up and disintegrate from between the layers (possibly referring to unconformity). ... As to the beginning of the sea, its clay is either sedimentary or primeval, the latter not being sedimentary. It is probable that the sedimentary clay was formed by the disintegration of the strata of mountains. Such is the formation of mountains."

The theoretical basis for the subject was established by Nicholas Steno who re-introduced the law of superposition and introduced the principle of original horizontality and principle of lateral continuity 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 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.

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.

Chalk Layers in Cyprus - showing sedimentary layering

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.

Cyclostratigraphy documents the often cyclic changes in the relative proportions of minerals, particularly carbonates, and fossil diversity with time, related to changes in palaeoclimates.

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.

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.

Chronostratigraphy

Main article: Chronostratigraphy

Chronostratigraphy is the branch of stratigraphy that studies the relative, not absolute, age of rock strata.

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.

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

Main article: Magnetostratigraphy

Magnetostratigraphy is a chronostratigraphic 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 detrital remnant magnetism (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.

Oriented paleomagnetic core samples are collected in the field; 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. 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.

Results of the individual samples are analysed by removing the natural remnant magnetism(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.

Archaeological stratigraphy

Main article: 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.

See also

• Harris matrix
• Important publications in stratigraphy
• International Commission on Stratigraphy
• Key bed
• Sedimentary basin analysis
• Sequence stratigraphy

References

^[2]

1. ^ Munim M. Al-Rawi and Salim Al-Hassani (November 2002). "The Contribution of Ibn Sina (Avicenna) to the development of Earth sciences" (PDF). FSTC. http://www.muslimheritage.com/uploads/ibnsina.pdf. Retrieved 2008-07-01. 
2. ^ Christopherson, R.W., 2008. Geosystems: An Introduction to Physical Geography 7th ed., Prentice Hall.

External links

Wikimedia Commons has media related to: Stratigraphy

• University of South Carolina Sequence Stratigraphy Web
• Front Range stratigraphy
• International Commission on Stratigraphy
• University of Georgia (USA) Stratigraphy Lab
• Stratigraphy.net A stratigraphic data provider.
• International Commission on Stratigraphy

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