STRATIGRAPHY

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>.

KEY TERMS

ABSOLUTE AGE:

The absolute age of a geologic phenomenon is its age in Earthyears. Compare with relative age.

BIOSTRATIGRAPHY:

An area of stratigraphy involving the study of fossilized plants and animals in order to establish dates for and correlations between stratigraphic layers.

CHRONOSTRATIGRAPHY:

A subdiscipline of stratigraphy devoted to studying the relative ages of rocks. Compare with geochronometry.

CORRELATION:

A method of establishing age relationships between various rock 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.

DATING:

Any effort directed toward finding the age of a particular item or phenomenon. Methods of geologic dating are either relative (i.e., comparative and usually based on rock strata) or absolute. The latter, based on such methods as the study of radioactive isotopes, usually is given in terms of actual years or millions of years.

EON:

The longest phase of geologic time, equivalent to an eonothem in the stratigraphic time scale. Earth's history has consisted of four eons, the Hadean or Priscoan, Archaean, Proterozoic, and Phanerozoic. The next-smallest subdivision of geologic time is the era.

EPOCH:

The fourth-longest phase of geologic time, shorter than an era and longer than an age and a chron. An epoch is equivalent to a series in the stratigraphictime scale. The current epoch is the Holocene, which began about 0.01 Ma (10,000 years) ago.

ERA:

The second-longest phase of geologic time, after an eon, and equivalent to an era them in the stratigraphic time scale. The current eon, the Phanerozoic, has had three eras, the Paleozoic, Mesozoic, and Cenozoic, which is the current era. The next-smallest subdivision of geologic time is the period.

EROSION:

The movement of soil and rock due to forces produced by water, wind, glaciers, gravity, and other influences.

GA:

An abbreviation meaning "giga-years" or "billion years." The age of Earth is about 4.6 Ga.

GEOCHRONOMETRY:

An area of stratigraphy devoted to determining absolute dates and time intervals. Compare with chronostratigraphy.

GEOLOGIC MAP:

A map showing the rocks beneath Earth's surface, including their distribution according to type as well as their ages, relationships, and structural features.

GEOLOGIC TIME:

The vast stretch of time over which Earth's geologic development has occurred. This span (about 4.6 billion years) dwarfs the history of human existence, which is only about two million years. Much smaller still is the span of human civilization, only about 5,500 years.

HISTORICAL GEOLOGY:

The study of Earth's physical history. Historical geology is one of two principal branches of geology, the other being physical geology.

ISOTOPES:

Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference ofmass. An isotope may be either stable or radioactive.

LAW OF FAUNAL SUCCESSION:

The principle that all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. This makes it possible to correlate widely separated strata.

LAW OF SUPERPOSITION:

Theprinciple that strata are deposited in a sequence such that the deeper the layer, the older the rock. This is applicable only or sedimentary rock, as opposed to igneous or metamorphic rock.

LITHOSTRATIGRAPHY:

An area of stratigraphy devoted to the study and description (but not the dating) of rock layers.

MA:

An abbreviation used by earth scientists, meaning "million years" or "megayears." When an event is designatedas, for instance, 160 Ma, it usually means 160 million years ago.

PALEONTOLOGY:

The study of fossilized plants and animals, or flora and fauna.

PERIOD:

The third-longest phase of geologic time, after an era; it is equivalent to a system in the stratigraphic time scale. The current eon, the Phanerozoic, has had 11 periods, and the current era, the Cenozoic, has consisted of three periods, of which the most recent is the Quaternary. The next-smallest subdivision of geologic time is the epoch.

PRECAMBRIAN TIME:

A term that refers to the first three of four eons in Earth's history, which lasted from about4,560 Ma to about 545 Ma ago.

RADIOACTIVITY:

A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei), beta particles (either electrons or subatomic particles called positrons), or gamma rays, which occupy the highest energy level in the electromagnetic spectrum.

RADIOMETRIC DATING:

A method of absolute dating using ratios between "parent" isotopes and "daughter" isotopes, which are formed by the radioactive decay of parent isotopes.

RELATIVE AGE:

The relative age of a geologic phenomenon is its age compared with the ages of other geologic phenomena, particularly the stratigraphic record of rock layers. Compare with absolute age.

SEDIMENT:

Material deposited at or near Earth's surface from a number of sources, most notably preexisting rock.

SEDIMENTARY ROCK:

Rock formed by compression and deposition (i.e., formation of deposits) on the part of other rock and mineral particles. Sedimentary rock is one of the three major types of rock, along with igneous and metamorphic.

SEDIMENTOLOGY:

The study and interpretation of sediments, including sedimentary processes and formations.

STRATA:

Layers, or beds, of rocks beneath Earth's surface. The singular form is stratum.

STRATIGRAPHIC COLUMN:

The succession of rock strata laid down over the course of time, each of which correlates to specific junctures in Earth's geologic history.

STRATIGRAPHY:

The study of rock layers, or strata, beneath Earth's surface.

UNCONFORMITY:

An apparent gap in the geologic record, as revealed by observing rock layers or strata.

WEATHERING:

The breakdown of rocks and minerals at or near the surface of Earth due to physical or chemical processes, or both.

Stratigraphy

Stratigraphy is that subarea of geology that treats the description, correlation , and interpretation of stratified Earth materials. Typically, geologists consider stratified Earth materials as layers of sediment or sedimentary rock . This definition, however, clearly encompasses other materials such as volcanic lava , ash flows, ash-fall layers, meteoritic impact ejecta layers, and soils. In fact, using this definition, any material that obeys the law of superposition during its formation could be placed in the domain of stratigraphy. Generally, internal layers within Earth (crust , mantle, and core) are not considered the type of layers studied by stratigraphers because they formed by Earth's internal differentiation processes.

Some geologists give a broader definition to the term stratigraphy. Planetary geologists sometimes view stratigraphy as if it were the study of the sequence of events on a planet or moon's surface. In addition, stratigraphy has been broadly used by some geologists who study mountain building and plate tectonics to mean the study of order of emplacement of rock units of various types, including igneous and metamorphic rocks, to which the law of superposition does not apply. In some cases, stratigraphy is used to define the study of geologic history of an area or country, but it is more correct to say that stratigraphy is the practical foundation for historical geology . In this article, the concept of stratigraphy expressed in the first paragraph is viewed as best and most correct.

Stratigraphy had its origins in the Renaissance writings of Nicholas Steno (1638–1687), who was the first to write lucidly about sedimentary strata. He observed strata exposed in the Arno River valley of Italy, and noted three axiomatic ideas, which became known as the first three "laws" of stratigraphy (Prodromus, 1669). These laws are known today as superposition, lateral continuity, and original horizontality. Superposition holds that layers are deposited so that the older layer is on the bottom. Unless strata are disturbed, this is always true. Lateral continuity holds that sedimentary layers extend laterally until they become so thin that they end at a "feather edge," abut against an obstruction, or grade laterally into other layers. Original horizontality holds that sedimentary layers are originally formed horizontally and remain so unless deformed by subsequent processes.

Steno's writings were full of common sense. In super-position, he noted the most important criterion for relative age dating. In lateral continuity, he wrote about how correlation of sedimentary layers would be possible. In original horizontality, he noted the criterion necessary for any sort of analysis of later deformation, that is, the original state of a sedimentary layer can be assumed to be horizontal.

As insightful as Steno's writings were, there is no strong evidence that they were influential beyond the Renaissance era in which he lived. Later on, during the Enlightenment, naturalists like James Hutton (1726–1797), John Playfair (1748–1819), and Charles Lyell (1797–1875) apparently independently "re-discovered" the importance of these common-sense concepts and used them in their influential writings about geology and stratigraphy. Hutton, Playfair, Lyell, and others of their time wrote books and papers, which established the foundations of modern thought about stratigraphy. Their most important contributions included promoting the concepts of actualism (understanding the past by studying modern processes) and demonstrating such key concepts as stratigraphic correlation, predictable fossil succession, and the great antiquity of Earth.

The advancement of these key concepts were given a great boost by the pioneering work of the English field engineer William Smith (1769–1839), who compiled and published the first large-scale geologic map (Wales and southern England; 1815) employing modern concepts of stratigraphic correlation and fossil succession. Smith's success inspired others to this kind of work, and was particularly important in influencing the Geological Society of London (the first geological

organization; founded 1807) to embark upon its "stratigraphical enterprise" of research in the United Kingdom. The Society and the British Geological Survey (the first geological survey, founded 1835) were important promoters of early stratigraphic studies and venues for presentation of early research. Based upon these efforts, it is fair to assert that modern stratigraphy was born in the United Kingdom during this period.

In the nineteenth century, major efforts were made by British stratigraphers and their colleagues on the European continent to develop a unified stratigraphic succession (or "geological column") for rocks in their areas. Cambridge Professor Adam Sedgwick (1785–1873) and Scottish naturalist Roderick Murchison (1792–1871) became quite famous as the preeminent "system builders" of their time. Sedgwick studied and named the Cambrian System himself and with Murchison, the Devonian. Murchison studied and named the Silurian and Permian Systems by himself. There were others who did the same during the nineteenth century, thus establishing the basis of our modern geological time scale (which has periods of the same names as those given to "systems" of rock during an era when exact ages of rock strata were unknown). This was the birth of modern chronostratigraphy , which emphasizes subdivision of geological time by studying Earth's stratigraphic record.

A Swiss geologist, Amanz Gressley (1814–1865), studied Jurassic strata in Europe in hopes of understanding what happens to sedimentary layers where they grade into other layers. He recognized that lateral continuity of layers revealed many changes, which reflected different ancient environments. To this concept, he gave the name facies, meaning an aspect of a sedimentary formation. A German stratigrapher, Johannes Walther (1860–1937), took up Gressley's ideas in his own work and became more widely known than Gressley for work with sedimentary facies. To Walther, the facies represented primary characteristics of the rock that would help him understand how and where the rock formed. He used what he called the ontological method in facies stratigraphy, which he described whimsically as "... from being, we explain becoming." This was a direct application of actualism, advocated earlier by Hutton and others, but now applied in a time of enhanced understanding of the natural world. Walter was the first naturalist to spend large amounts of time in the field studying modern environments in order to better interpret the past. His two-volume work, Modern Lithogenesis (1983; 1984), was a watershed for modern research with sedimentary facies. Accordingly, Walter is regarded as the founder of modern facies stratigraphy. Although his work was not accepted well in the United States for many years (due, in part, to anti-German feelings during the early twentieth century), it later was studied extensively for its rich descriptions of modern sedimentary environments and ancient sedimentary facies. In the latter part of the twentieth century, facies stratigraphy became much more than an academic exercise when it was realized that such knowledge could help predict the occurrence of petroleum and certain ore minerals—and facilitate more productive extraction of these materials—in host sedimentary rocks .

At the outset of the twentieth century, Austrian stratigrapher Eduard Suess (1831–1914) became the first advocate of global changes of sea level and how those changes might relate to global stratigraphy. This concept, called eustatsy, holds that global sea level rises and falls during geological history lead to the great marine transgressions and regressions noted in many sedimentary strata from locales around the world. Suess called upon subsidence of the sea floor and displacement of seawater by sediment as reasons for this global effect (today we know that gain and loss of polar ice is another contributor to sea-level change). His work stimulated much research, and strongly influenced the well-known American geologist T.C. Chamberlain (1843–1928), who perpetuated these ideas through his many well-known papers on the subject. These ideas were important in the development of a modern concept in stratigraphy called sequence stratigraphy.

Sequence stratigraphy, which holds that large bodies of sedimentary strata are bounded by interregional unconformities , formed as a result of global eustatsy. In the early 1960s, sequence stratigraphy was put forth by the American stratigrapher L.L. Sloss (1913–1996) in a series of widely read papers. During the 1970s, Sloss's student, Peter Vail (1930–), formerly with Exxon Corporation (now Exxon-Mobil Corporation), further developed these concepts while studying seismic profiles of stratigraphy from the world's continental shelves. Vail's paper's established sequence stratigraphy as one of the main subdivisions of modern stratigraphy. To recognize their contributions, sequence stratigraphy is often referred to as Sloss-Vail sequence stratigraphy in their honor.

Vail's work spawned a huge effort to produce a highly detailed, eustatic sea-level cycle chart of Earth's history based upon the vast data collection at Exxon. His work was published in 1987 in the prestigious journal Science. Sequence stratigraphy and global sea-level cycle charts are concepts used today major petroleum-company exploration laboratories all over the world.

Today, facies stratigraphy and sequence stratigraphy are not the only types of stratigraphy practiced by geologists. Modern stratigraphy includes: lithostratigraphy (naming of formations for purposes of geological mapping); biostratigraphy (correlating rock layers based upon fossil content); chronostratigraphy (correlating rock layers based upon their similar ages); magnetostratigraphy (study and correlation of rock layers based upon their inherent magnetic character); soil stratigraphy (study and mapping of soil layers, modern and ancient); and event stratigraphy (study and correlation of catastrophic events in geological history). The latter may include global or regional layers formed by asteroid or comet impacts, major volcanic events, global climate or ocean-chemistry changes, and effects of slight changes in Earth's orbital parameters (e.g., Milankovitch cycles ). Modern procedures and practices in stratigraphy are summarized in two widely read documents: the North American Stratigraphic Code, published by the American Association of Petroleum Geologists, and the International Stratigraphic Guide, 2^nd edition, published by the Geological Society of America.

Because layered Earth materials possess so much information about Earth's past, including the entire fossil record— and a sedimentary record quite sensitive to atmospheric, climatic, and oceanic changes of the past—stratigraphy is the one subarea of geology entirely focused upon retrieving and understanding that record.

See also Correlation (geology); Geologic time; Historical geology; Marine transgression and marine regression; Unconformities

Stratigraphy

Stratigraphic fundamentals

Applications of stratigraphy in historical studies

Resources

Stratigraphy is the branch of geology concerned with the description and interpretation of sequences of rock layers or strata. In most cases the layers are of sedimentary origin, but can also include sequences of volcanic ash and lava, and even the study of different layers of human occupation at an archeological site. Sediment usually forms distinct strata with the most recent layers on top. Although the strata may be folded during episodes of mountain building, interrupted by inclusions and slippages, and even metamorphosized into other forms of rock, stratigraphic analyses can still be performed in many cases. The processes of sedimentation—including the presence of certain types of fossils—provide scientists with valuable clues about Earth history. These principles are thus valuable for many different types of scientist, ranging from prospecting geologists to city planners to archaeologists and paleontologists studying human and animal history and prehistory.

Stratigraphic fundamentals

The basic principle of sedimentation—that in any given set of layers of material the most recent levels are closest to the top—were established as long ago as the seventeenth century. By the nineteenth century such early geologists as Charles Lyell (1797–1875) recognized that sediment accumulation was not necessarily regular or obvious. Weathering can also influence the stratigraphic record by introducing trace elements into the various layers. Patterns in the very layering of sediments, such as ripple marks and flumes, can introduce discontinuities. Changes in climate, which bring about changes in sea level, also create discontinuities.

Equally as important as the composition of the layers themselves are the boundaries between them, which represent breaks in time or changes in sediment accumulation. Sediments do not deposit evenly—rates of sedimentation are influenced by extraordinary events as well as everyday processes. During periods of flood, for instance, rivers can drop tons of silt on what had been working farm land, and a single storm can carry away tons of beach sand into the ocean depths. The borders marked by the beginning and ends of such events can represent as little time as a single day. Because sediments generally accumulate over long periods of time, however, the borders between different layers usually represent a long-term change in local geography.

Geologists have adopted words to describe the different types of layers based on their thickness. Sediments are generally divided between laminae and beds, with the laminae represented by an accumulation of less than one centimeter, and the beds represented by accumulations ranging from 0.4 to 47 in (1-120 cm). The beds are subdivided into very thin, thin, thick, and very thick, respectively measuring from 0.4 to 2 in (1 to 5 cm), 2 to 24 in (5 to 60 cm), 24 to 47 in (60 to 120 cm), and more than 47 in (120 cm) across. Beds are also graded on the size and type of the individual sand grains. Beds and laminae together form primary sedimentary structures, which indicate the way in which strata are laid down.

Geologists have also introduced various sub-disciplines of stratigraphy. Lithostratigraphers study changes in layers of rock. This is the type of stratigraphy most commonly seen on geological survey maps. Biostratigraphy uses microscopic fossils to determine the relative ages of rocks and helps paleontologists trace local variations in climate. Tephrostratigraphy is the study of deposits of volcanic ash, while magneto-stratigraphers trace fluctuations in Earth’s magnetic field—specifically, reversals in its polarity—over millions of years. Other useful applications of strati-graphic analysis include seismic stratigraphy, which applies the principles of acoustics (sending shock waves through the earth) to determine the positions of pockets of petroleum and other substances.

Applications of stratigraphy in historical studies

Strata can be used to study history of both Earth and, on a shorter time scale, of humankind. Anthropologists and archaeologists use stratigraphic principles to understand how and under what circumstances an archeological site was occupied, how long the people that lived there stayed, and how they lived while they were in residence. Archaeologists regularly apply microstratigraphic principles such as observation of the process of soil formation and landscape development based on weathering and sediment accumulation to their sites in order to classify and date artifacts. For example, excavators working at the Paleolithic site of Mezhirich in what is now Ukraine uncovered the bones of hundreds of mammoths, some of which had been burnt, others of which had been arranged to form houses. The scientists suggested, based on stratigraphic principles—the thinness of accumulated layers of occupation debris—that the site was not occupied year-round, but instead was a seasonal dwelling-place. Paleontologists studying the border regions between the Eocene and Oligocene periods in ancient history have studied eastern Oregon’s stratigraphy to draw conclusions about global climactic conditions. They suggest that the regularity of sediments in the area reflect, not a gradual change in sea level, but a cooling trend in which the area changed from a subtropical to a temperate climate.

Archaeologists have applied stratigraphic principles to understanding the history of the famous Roman city of Pompeii, which was buried during an eruption of the volcano known as Vesuvius in AD 79. Although the historical record of the explosion itself is quite clear, scientists use stratigraphy to help understand life in the city before the eruption. Excavators had assumed, based on the testimony of ancient written sources, that parts of Pompeii had been built as long ago as the fifth century BC and had been occupied ever since. Nineteenth and early twentieth-century archaeologists had accepted this reasoning, based on analyses of building styles and construction. Beginning in the 1930s, however, scientists began to revise their thinking using observations of the micro-stratigraphy of the site. Modern excavations suggest that most of the buildings standing at the time the volcano erupted were built in the period of Roman occupation—in other words, no earlier than the second century BC. Some finds of debris unrelated to the AD 79 eruption can be dated back to the fifth century BC, but these are not directly connected with standing houses. Stratigraphy promises to change the history of a site historians believed they knew very well.

Resources

BOOKS

Boggs, S., Jr.Principles of Sedimentology and Stratigraphy. 4th ed. Upper Saddle River, New Jersey: Prentice Hall, 2005.

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology.Upper Saddle River, New Jersey: Prentice Hall, 2004.

Kenneth Shepherd

Stratigraphy

Stratigraphy is the science of interpreting and describing layers and strata of sediments. Commonly these layers are levels of sedimentary rock , but stratigraphy can also include the study of non-ossified sediments, like those in stream beds and lake bottoms, of inclusions such as volcanic ash and lava, and even the study of different layers of human occupation. Sediment usually forms distinct strata with the most recent layers on top and, although they may be folded by continental drift , interrupted by inclusions and slippages, and even metamorphosized into other forms of rock, as long as these strata can be untangled and interpreted, scientists can perform stratigraphic analyses. The processes of sedimentation—including the presence of certain types of fossils—provide scientists with valuable clues about the age of the earth and its history. These principles are thus valuable for many different types of scientist, ranging from prospecting geologists to city planners to archaeologists and paleontologists studying human and animal history and prehistory.

Stratigraphic fundamentals

The basic principle of sedimentation—that in any given set of layers of material the most recent levels are closest to the top—were established as long ago as the seventeenth century. By the nineteenth century early geologists like Charles Lyell recognized that this accumulation was not necessarily regular nor was it obvious. Interruptions and inversions (known collectively as discontinuities) in the stratigraphic record can change the actual position of layers of sedimentation, but their apparent position remains evident to trained geologists in relation to other layers and to their contents—especially fossil contents. Weathering can also influence the stratigraphic record by introducing trace elements into the various layers. Patterns in the very layering of sediments, such as ripple marks and flumes, can introduce discontinuities. Changes in climate, which bring about changes in sea level , also create discontinuities.

Equally as important as the contents of the layers themselves, however, are the borders between them. These separations mark discontinuities, breaks between one time and another. They can mark changes in accumulation of sediment as well as changes in time. Sediments do not deposit evenly—rates of sedimentation are influenced by extraordinary events as well as everyday processes. During periods of flood, for instance, rivers can drop tons of silt on what had been working farm land, and a single storm can carry away tons of beach sand into the ocean depths. The borders marked by the beginning and ends of such events can represent as little time as a single day. Because sediments generally accumulate over long periods of time, however, the borders between different layers usually represent a long-term change in local geography.

Geologists have created terminology to describe the different types of layers based on their thickness. Sediments are generally divided between laminae and beds, with the laminae represented by an accumulation of less than one centimeter, and the beds represented by accumulations ranging from 0.4-47 in (1-120 cm). The beds are subdivided into very thin, thin, thick, and very thick—respectively measuring 0.4-2 in (1-5 cm), 2-24 in (5-60 cm), 24-47 in (60-120 cm), and more than 47 in (120 cm) across. Beds are also graded on the size and type of the individual sand grains. Beds and laminae together form primary sedimentary structures, which indicate the way in which strata are laid down.

Geologists have also introduced various subgenres of stratigraphy classified by the types of layered material. Lithostratigraphers trace changes in layers of rock. This is the type of stratigraphy most commonly seen on geological survey maps. Biostratigraphy uses microscopic fossils to determine the relative ages of rocks and helps paleontologists trace local variations in climate. Tephrostratigraphy is the study of deposits of volcanic ash, while magnetostratigraphers trace fluctuations in the earth's magnetic field—specifically, reversals in its polarity—over millions of years. Other useful applications of stratigraphic analysis include seismic stratigraphy, which applies the principles of acoustics (sending shock waves through the earth) to determine the positions of pockets of petroleum and other substances.

Applications of stratigraphy in historical studies

Because strata are deposited in layers that scientists can interpret, they can be used to study history, both the history of the earth and, on a shorter time scale, of humankind. Anthropologists and archaeologists use stratigraphic principles to understand how and under what circumstances a site was occupied, how long the people that lived there stayed, and how they lived while they were in residence. Archaeologists regularly apply microstratigraphic principles such as observation of the process of soil formation and landscape development based on weathering and sediment accumulation to their sites in order to classify and date artifacts. For example, excavators working at the Paleolithic site of Mezhirich in what is now Ukraine uncovered the bones of hundreds of mammoths, some of which had been burnt, others of which had been arranged to form houses. The scientists suggested, based on stratigraphic principles—the thinness of accumulated layers of occupation debris—that the site was not occupied year-round, but instead was a seasonal dwelling-place. Paleontologists studying the border regions between the Eocene and Oligocene periods in ancient history have studied eastern Oregon's stratigraphy to draw conclusions about global climactic conditions. They suggest that the regularity of sediments in the area reflect, not a gradual change in sea level, but a cooling trend in which the area changed from a subtropical to a temperate climate.

Archaeologists have even applied stratigraphic principles to understanding the history of the famous Roman city of Pompeii, which was buried following an eruption of the volcano known as Vesuvius in a.d. 79. Although the historical record of the explosion itself is quite clear, scientists use stratigraphy to help unwrap the city's past before the eruption. Excavators had assumed, based on the testimony of ancient written sources, that parts of Pompeii had been built as long ago as the fifth century b.c. and had been occupied ever since. Nineteenth and early twentieth-century archaeologists had accepted this reasoning, based on analyses of building styles and construction. Beginning in the 1930s, however, scientists began to revise their thinking using observations of the microstratigraphy of the site. Modern excavations suggest that most of the buildings standing at the time the volcano erupted were built in the period of Roman occupation—in other words, no earlier than the second century b.c. Some finds of debris unrelated to the a.d. 79 eruption can be dated back to the fifth century b.c., but these are not directly connected with standing houses. Stratigraphy promises to change the history of a site historians believed they knew very well.

Resources

books

Boggs., Sam, Jr. Principles of Sedimentology and Stratigraphy. 2nd edition. Englewood Cliffs, NJ: Prentice Hall, 1995.

Opdyke, Neil D., and James E.T. Channell. Magnetic Stratigraphy. San Diego, CA: Academic Press, 1996.

Waters, Michael. Principles of Geoarchaeology: A NorthAmerican Perspective. Tucson, AZ: University of Arizona Press, 1992.

periodicals

Fulford, Michael, and Andrew Wallace-Hadrill. "Unpeeling Pompeii." Antiquity, 72. (March 1998): 128-46.

Soffer, Olga, James M. Adovasio, Ninelj L. Kornietz, Andrei A. Velichko, Yurij N. Gribchenko, Brett R. Lenz and Valeriy Yu. Suntsov. "Cultural Stratigraphy at Mezhirich, an Upper Paleolithic Site in Ukraine with Multiple Occupations." Antiquity, 71. (March 1997): 48-63.

Kenneth R. Shepherd

stratigraphy
1. The branch of the geologic sciences concerned with the study of stratified rocks in terms of time and space. It deals with the correlation of rocks from different localities. Correlation methods may involve the use of fossils (biostratigraphy), rock units (lithostratigraphy), or geologic-time units or intervals (chronostratigraphy).

2. The relative spatial and temporal arrangement of rock strata.