stratigraphy(redirected from stratigraphically)
Also found in: Dictionary, Thesaurus, Medical.
stratigraphy,branch of geology specifically concerned with the arrangement of layered rocks (see stratificationstratification
(Lat.,=made in layers), layered structure formed by the deposition of sedimentary rocks. Changes between strata are interpreted as the result of fluctuations in the intensity and persistence of the depositional agent, e.g.
..... Click the link for more information. ). Stratigraphy is based on the law of superposition, which states that in a normal sequence of rock layers the youngest is on top and the oldest on the bottom. Local sequences are studied, and after considering such factors as the average rate of deposition of the different rocks, their composition, the width and extent of the strata, the fossils contained, and the periods of uplift and erosion, the geological history of the sequence is reconstructed. These sequences are then correlated to those of similar age in other regions with the ultimate aim of establishing a consistent geochronology for the entire earth. Statigraphy is therefore important in the relative datingdating,
the determination of the age of an object, of a natural phenomenon, or of a series of events. There are two basic types of dating methods, relative and absolute.
..... Click the link for more information. of all types of rock. In areas where the strata have undergone folding, faulting, and erosion, stratigraphic techniques are used to determine their correct sequence. The principle of included fragments in stratigraphy states that any rock fragment included in another rock must be older than the surrounding rock. Fossils have been the most important means of correlation because, as a result of evolutionevolution,
concept that embodies the belief that existing animals and plants developed by a process of gradual, continuous change from previously existing forms. This theory, also known as descent with modification, constitutes organic evolution.
..... Click the link for more information. , rock strata of approximately equal age exhibit similar flora and fauna. Dating and correlation of stratified rocks by means of fossils is called stratigraphic paleontology. See also datingdating,
the determination of the age of an object, of a natural phenomenon, or of a series of events. There are two basic types of dating methods, relative and absolute.
..... Click the link for more information. .
See B. Kummel, History of the Earth (1961); E. W. Spencer, Basic Concepts of Historical Geology (1962); R. K. Matthews, Dynamic Stratigraphy (1974); P. C. Cattermole and P. Moore, The Story of the Earth (1985).
stratigraphy(stră-tig -ră-fee) The study of rock layers (strata). Younger geological units overlie, embay, or intrude older units. It is therefore possible to produce geological maps of the surface of a planet or satellite, and to form a stratigraphic classification to help to understand its history. Lunar stratigraphic systems include the Copernican, Imbrian, and Nectarian Systems.
in archaeology, the order in which cultural layers alternate relative to each other and to the underlying and overlying rocks and deposits. The term “stratigraphy” is also applied in English to the study of such a succession of layers. Such study is essential in the relative dating of the layers, structures, burials, and artifacts and is of particular importance in cases where the natural order of the layers has been disturbed by, for example, digging, cave-ins, landslides, or erosion.
The stratigraphy of archaeological remains is established by studying vertical sections. By means of typology and other archaeological methods, as well as methods of the natural sciences, a transition can be made from relative stratigraphic dating to absolute dating (seeARCHAEOLOGICAL DATING and GEOCHRONOLOGY). Stratigraphic conclusions based on a single remain can often be used to establish the relative dating of remains in an entire area. A relative—and, subsequently, absolute—chronology of the Stone Age was established on the basis of strati-graphic data. Stratigraphy is especially important in the study of settlements with a thick and, sometimes, greatly disturbed cultural layer—for example, primitive settlements, cities of the ancient East, Greek and Roman cities, and medieval cities. In such cases, each layer reflects a certain stage in the settlement’s or city’s history.
The term “lateral stratigraphy,” or “horizontal stratigraphy,” is applied in archaeology to the sequence of territorial growth of settlements and burial grounds.
REFERENCESAvdusin, D. A. Polevaia arkheologiia SSSR. Moscow, 1972.
Mongait, A. L. Arkheologiia Zapadnoi Evropy: Kamennyi vek. Moscow, 1973.
D. A. AVDUSIN
the branch of geology that studies the sequence of formation of geologic bodies and the initial spatial interrelations of such bodies. Stratigraphy is based primarily on the capacity to trace strata of sedimentary rocks and on the investigation of faciès changes of the rocks in basins of past geologic epochs. The fossil content of sedimentary strata reflects the irreversible development of organic life and plays a fundamental role in establishing that deposits being studied are of the same age (seeBIOSTRATIGRAPHY). For this reason, stratigraphy is closely connected with paleontology and with geochronology, which studies the age and the chronological sequence of formation of the rocks making up the earth’s crust. The rise of stratigraphy was associated with the development of geology as a science. Stratigraphy was the basis for the creation of geologic maps and the geologic time scale.
Historical survey. Some questions of the development of stratigraphy as a science were dealt with in the 17th century by the Danish scientist N. Steno (Stensen). He was the first to clearly state the law of superposition of strata. Substantiation for the law, however, was provided in the 18th century by the German scientists J. Lehmann and G. Fiichsel and the Russian scientist M. V. Lomonosov. At the turn of the 19th century, the British engineer W. Smith set forth the paleontological method, which was based on the law of faunal succession. The method was developed in the first half of the 19th century by the British scientists R. Murchison and A. Sedgwick, the Frenchmen G. Cuvier, A. Brongniart and A. d’Orbigny, the German A. Oppel, and the Russians D. I. Sokolov and N. M. Iazykov.
In the late 18th century, the constituent deposits of the earth’s crust in Western Europe were divided into Primary (now Paleozoic), Secondary (Mesozoic), and Tertiary (Cenozoic) formations. The first two terms quickly lost their importance, but the third, which was proposed by the Italian geologist G. Arduino in 1759, is still used by non-Soviet geologists. In 1829, the term “Quaternary” was proposed by the French geologist J. Des-noyers for the friable, most recent deposits [seeANTHROPOGENIC SYSTEM (PERIOD)].
In the first half of the 19th century, as a result of research carried out in various countries, stratigraphic systems were identified, and their sequence in the history of the earth was established. At the session of the International Geological Congress held in Bologna in 1881, a standard classification of the subdivisions of the geologic history of the earth was adopted, and the previously identified systems and series were given official acceptance. In 1900 the French geologist E. Reneviez proposed a composite “chronograph”; it included not only the systems and series but also stages, which were used with a number of changes and refinements. A great contribution to the creation and development of a geostratigraphic scale was made by the Russian and Soviet geologists A. P. Karpinskii, N. I. Andrusov, A. P. Pavlov, D. V. Nalivkin, A. N. Krishtofovich, and V. V. Menner, by the French scientists E. Hang and M Gignoux, by the Germans J. Walther and O. Schindewolf, by the Austrian M. Neumayr, and by the Americans C. Wolcott, C. Schuchert, and C. Dunbar. In the mid-20th century, particularly intensive study began of the oldest phases of the earth’s development; the possibility of establishing a standard system of units was raised as a result of the work of the Finnish geologist J. Sederholm, the Norwegian geologist O. Holtedahl, the American geologists A. Grabau and C. Stockwell, and a group of Soviet geologists headed by N. S. Shatskii (seePRECAMBRIAN).
Object and methods of research. The basic principle of stratigraphy is the law of superposition of strata: in a normal sequence of rock strata, each stratum is older than the one above it. An exception to this rule is observed only when tectonic deformations have altered the original sequence of strata and caused strata to be inverted. Rock strata were deposited in the basins of past geologic periods in a definite succession. By studying this sequence, it is possible to construct what is called a geologic column. Various methods are used to compare such columns. The most widely employed and reliable is the paleontológica! method, which is based on the irreversible progressive development of organic life. The paleontological method can be used only by taking into account the data of paleoecology.
Essentially all groups of fossils can be used for the purpose of stratigraphic correlation. Of especially great importance are the remains of minute organisms that are encountered in enormous quantities—such as foraminifers, radiolarians, nannoplankton, and diatoms. Even small pieces of sedimentary rocks contain hundreds or thousands of such organisms, a fact of particular importance in determining the age of rocks in cores from boreholes. Special mention should be made of spore and pollen analysis, which is used for determining the age of sedimentary strata in all subdivisions of the Phanerozoic. The paleontological method has been extensively employed throughout the Phanerozoic. In the older deposits of the Precambrian, the remains of animals are encountered extremely rarely. Traces of the vital activity of blue-green algae are encountered in very large quantities. In the 1960’s these algae came into use for the differentiation and correlation of carbonate strata of the Upper Precambrian. The paleontological method has not yet been applied to older deposits.
Radiometrie dating is acquiring primary importance for older deposits. This approach is based on the radioactive decay of various elements—such as potassium, uranium, and lead—contained in the minerals of sedimentary and magmatic rocks (seeGEOCHRONOLOGY). Information on the radiometric age of sedimentary rocks is rather meager. The potassium-argon age method makes use of very rare potassium salts (carnallite) and glauconite, which is common in sedimentary rocks. The rubidium-strontium age method is used in studying various argillaceous rocks and acidic effusive rocks. The uranium-thorium-lead age method is used to date zircons from effusive rocks.
Considerably more complete data on the age of rocks can be obtained by the indicated methods for various intrusive rocks injected into sedimentary strata. The basic difficulty here consists in tying these precise figures to the geologic column; for this purpose, the contacts of the intrusive body with the sedimentary strata are carefully studied. In many cases, the true age of intrusive bodies can be established only by radiometric dating.
Other methods are also employed for correlating sedimentary and volcanogenic stratified formations. An example is lithologic correlation. Geochemical analysis may be used to conduct a comparison with respect to the predominance of certain minerals or elements. Various geophysical methods may be employed involving the use of results from paleomagnetic (seePALEOMAGNETISM) and electric-logging determinations for the comparison of borehole logs in studied areas.
Units and scales. The use of various correlation methods made possible the compilation of a composite geologic section for the entire world and the establishment, on the basis of this section, of a strict hierarchy of time-stratigraphic, or chronostratigraphic, units. Such a system of time-stratigraphic units, or time-stratigraphic scale, was first officially adopted at the session of the International Geological Congress held in Bologna in 1881. In the mid-20th century, the eonothem, which is the largest time-stratigraphic unit and corresponds to an eon of geologic time, was added to this system. In addition, the term “erathem” replaced the term “group” in the sense of the deposits formed during an era. With these additions and changes, the accepted units are ranked as shown in Table 1, where the geologic-time unit corresponding to each time-stratigraphic unit is shown on the right.
|Table 1. Time-stratigraphic and geologic-time units in order of decreasing rank|
|Time-stratigraphic units||Geologic-time units|
Each of the time-stratigraphic units named in Table 1 corresponds to a natural stage in the development of the earth and of organic life. The units can be identified on all the continents and, as was shown by drilling conducted in the 1970’s, in the ocean basins as well. Some investigators believe that stages and zones are of only local significance. This position is valid in those cases where the stage or zone has been identified on the basis of data from isolated paleobasins, whose fauna developed separately and was not connected with the world ocean. An example is the Neogene deposits of the Black Sea-Caspian Basin. If sections of the open ocean basins are taken as the stratotype of the stages and zones, these units can be traced virtually throughout the world.
The last, or Phanerozoic, eonothem of the Soviet time-stratigraphic scale is divided into three erathems and 12 systems. [The sequence of these erathems and systems is given in GEOCHRONOLOGY, and a detailed description of specific systems and their stages and zones can be found in the articles on the systems, for example, CAMBRIAN SYSTEM (PERIOD).] For the Anthropogenic, or Quaternary, system, a system of units has been proposed that reflects the paleoclimate-based correlation methods used. In precisely the same way, special units are being introduced for the Precambrian, in which the paleontological method is of limited use. The available data indicate that series, stages, and zones cannot yet be identified in the Precambrian; the systems and erathems have a basis completely different from that for the Phanerozoic. For the Precambrian, it is more correct to speak of pro-toerathems and protosystems.
The series, stages, and zones of the geostratigraphic scale cannot always be identified with the desired precision and do not reflect local features in the structures of sections. For this reason, in many regions local stratigraphic units are the basis of the strati-graphic classification. If these units have a paleontological foundation and include deposits that change substantially in composition across the strike, then a distinction is made between horizons, which approximately correspond in scope to a stage or substage, and local zones. On the other hand, if lithologic characteristics of the rocks are the principal characteristics used in identifying a local unit, then a special system of rock-stratigraphic, or lithostratigraphic, units is employed. Table 2 gives the ranking of such units; the units accepted in the USSR are shown on the left, and the equivalent units accepted in the USA are shown on the right.
The local units may not correspond in scope to units of the time-stratigraphic scale. For example, a group may correspond to
|Table 2. Rock-stratigraphic units|
a system, series, or stage; a formation may correspond to a series, stage, or zone; and a member may correspond to a stage or zone. These rock-stratigraphic units can be traced as long as the particular lithologic characteristics of the rocks remain. The boundaries of the units are not strictly isochronous.
The identification of regional and local stratigraphic units in each country is regulated by the system of rules making up the stratigraphic code. In many countries there are officially adopted rules. For example, the stratigraphic code of the USSR, Strati-graphic Classification and Terminology, was adopted in 1965. Similar codes and rules have been worked out in Czechoslovakia, Great Britain, France, and the USA.
The principal task confronting present-day stratigraphy is the determination of the overall sequence of the constituent deposits of the earth’s crust. This task is of particular importance for the oldest Precambrian deposits. The recent (Phanerozoic) history of the earth—that is, the time from 570 million years ago to the present—has been incomparably better explained. Here too, however, work remains to be done in refining the presently accepted units and in creating global stage and zonal stratigraphic schemes. There is also the problem of constructing detailed local stratigraphic scales and correlating them with the geostrati-graphic scale.
Practical applications. Stratigraphy is the basis for regional geologic research making it possible to understand the particular features of an area’s tectonic features and to determine the direction that should be taken by mineral exploration and prospecting. This statement applies particularly to stratified deposits that are strictly confined to definite stratigraphic levels—for example, petroleum, coal, iron ores, manganese ores, phosphorites, bauxites, rock salts, potassium salts, and black uranium-containing shales. Without a detailed analysis of the geologic section, it is impossible to compile geologic maps or perform various types of engineering geology work.
In the USSR, the leading centers for stratigraphic research include the Geological Institute of the Academy of Sciences of the USSR in Moscow, the Institute of Geology and Geophysics of the Siberian Division of the Academy of Sciences of the USSR in Novosibirsk, and the All-Union Geological Institute of the Ministry of Geology of the USSR in Leningrad. Both in the USSR and abroad, stratigraphic research is carried out in virtually all major geological agencies and institutes, and in the geology departments of institutions of higher learning. The multivolume series Stratigraphy of the USSR, which generalizes regional stratigraphic works, is currently being published. The Interdepartmental Stratigraphic Committee was created in the USSR in 1955; it coordinates all stratigraphic work in the country and oversees permanent commissions that bring together specialists on particular time-stratigraphic systems.
The International Union of Geological Sciences sponsors the International Commission on Stratigraphy, which directs the work of groups devoted to various stratigraphic problems.
REFERENCESGignoux, M. Stratigraficheskaia geologiia. Moscow, 1952. (Translated from French.)
Leonov, G. P. Osnovy stratigrafii, vols. 1–2, Moscow, 1973–74.
Dunbar, C, and J. Rodgers. Osnovy stratigrafii. Moscow, 1962. (Translated from English.)
Zhamoida, A. I., O. P. Kovalevskii, and A. I. Moiseeva. Obzor za-rubezhnykh stratigraficheskikh kodeksov. Moscow, 1969.
Stratigraficheskaia klassifikatsiia, terminologiia i nomenklatura. Moscow, 1965.
Stepanov, D. L. Printsipy i melody biostratigraficheskikh issle-dovanii. (Tr. Vses. n.-i. geologorazvedochnogo in-ta, fase. 113.) Leningrad, 1958.
B. M. KELLER