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Formation and horizon: types of boundaries and their relationship
Russian Geology and Geophysics 48 (2007) 668–674 www.elsevier.com/locate/rgg
Formation and horizon: types of boundaries and their relationship V.S. Tsyganko * Institute of Geology, Komi Science Center, Uralian Branch of the RAS, 54 ul. Pervomaiskaya, Syktyvkar, 167982, Russia Received 25 January 2006
Abstract In dividing supracrustal strata, formation and horizon have been and are basic stratigraphic units. Stratigraphic boundaries of a formation, a natural geologic body, are drawn mostly on the basis of its composition. Paleontological remains constrain the formation in time and spatially locate it in the Earth’s crust. Boundaries between formations can be of three types: strictly stratigraphic, parastratigraphic, and allostratographic. The stratigraphic interval can range from a fraction of a horizon or chronozone to several stages. At the boundary between two systems the adjacent parts of the formation can relate to both systems. The main stratigraphic characteristics for recognizing horizons are paleontologic (biostratigraphic) features, revealed by zonal, paleoecosystemic (ecostratigraphic), bioeventual, and other methods to make a basis for their immanent signature. Horizon can be characterized by boundaries of only two types: strictly stratigraphic and allostratigraphic. The stratigraphic interval of a horizon can vary from a single chronozone to a stage. Boundaries of neighboring horizons at the contact between two stages or systems should coincide with the latter. The stratigraphic units of the International Stratigraphic Chart, in contrary to formation and horizon, are characterized by borders of only one type — strictly stratigraphic. © 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Formation; horizon; stratigraphic unit; boundary; grade
Introduction Any stratigraphic inferences are known to originate from the study of particular records of the Earth’s sedimentary sequence in a certain region of the planet. Thus, all possible operations in stratigraphy are based on the data on regional stratigraphy. This was first postulated by Nikitin and Chernyshev (1889) and, in our time, was stressed by Sokolov (1971, 1987) believing that regional stratigraphy is the basement for stratigraphy in general. According to Sokolov, all stratigraphic models are derivatives of a real regional stratigraphy, including the composite construction of the International Stratigraphic Chart. Considering relationships between regional and global stratigraphic divisions, Sokolov (1971) argued that a “regional stratigraphic division” should cover any real stratigraphic subdivisions — from narrow local units to basins. Their actual areal extent is bearing, in each particular case, on the character of the stratigraphic units themselves specified by diverse factors of the geologic history of the given territory. At present, when dividing supracrustal structures in the course
* Corresponding author. E-mail address: [email protected] (V.S. Tsyganko)
of medium- and large-scale geological surveys and search for ores, Russian geologists use the main stratigraphic units formation and horizon. As there is no consensus on their definitions and estimation of their role in dividing supracrustal sequences, it is necessary to characterize each of these stratigraphic units.
Formation Russian geologists began to use the term formation (suite) in the second half of the 19th century. In 1881, the Second International Geological Meeting in Bologna adopted the nomenclature of main stratigraphic subdivisions making the base of the modern International Stratigraphic Chart. The Russian delegation headed by A.A. Inostrantsev offered to subdivide stage into formations and complexes and into still smaller subunits, beds. This proposal, however, was rejected, and until 1956 the term formation was used loosely. In 1952, an Ad hoc Stratigraphic Commission was set up at the VSEGEI, which two years later published a brochure entitled “Stratigraphic and geochronological subdivisions: principles, content, terminology, and rules of the use” (Librovich et al., 1954). Formation was considered there a stratigraphic unit of local (regional) scale. Its definition is very close to the modern
1068-7971/$ - see front matter D 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.200 7. 07.002
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one. At a meeting devoted to general problems of stratigraphic classification, which was held at VSEGEI in 1955, a commission was organized, headed by A.P. Rotai, to summarize all the proposals on this problem. The results were discussed and published by the Interdepartmental Stratigraphic Committee as an ad hoc instruction (Rotai, 1956, 1960). In this Instruction, formation was first (Rotai, 1956) considered the basic unit of auxiliary local divisions and then (Rotai, 1960), of regional stratigraphic divisions. The USSR Stratigraphic Code published in 1977 changed the rank and interpretation of formation. In this Code formation was ranked a local stratigraphic unit. The same was in the second edition of the Stratigraphic Code (Bekker et al., 1992), where formation was defined as a sum of deposits that occur within a certain geologic area, differ from over- and underlying deposits in having specific lithological, facies, and paleontological (given remains of organisms) features, and are characterized by the same composition and structure and specific character of boundaries. At present, most geologists are positive toward formation, but this does not mean that they are unanimous in interpretation of formation itself as well as the character of its boundaries (Krasnov, 1980; Vereshchagin, 1980; Zhamoida, 1980). There are all reasons to share the anxiety of geologists about the intense recent “creation of formations” (Karogodin, 2003). The problems bearing on the interpretation of the concept formation and its bounds were discussed in my previous publications (Tsyganko, 1994, 2002). I came to quite the same conclusion which was made in the Stratigraphic Code (Bekker et al., 1992): The age sliding of boundaries is one of the most specific features of formation. Its manifestation and extent are governed by the specific character of the geologic history of a region partly or as a whole. Thus, diachronous boundaries can be considered a universal property of formations, series, and other local stratigraphic units, whose immanent signature is their lithology. However, the available methods often fail in detecting the diachronous character of the boundaries. Zubakov (1978, p.19) was quite decisive on this issue: “Any stratigraphic boundary that seems to be isochronous at a given detail of study becomes diachronous as soon as methods of more accurate diagnosis are applied.” Many stratigraphers, however, are proponents of the opposite point. Thus, Vereshchagin (1980, p. 134) wrote: “Formation should be characterized by the same time of origin, hence its lower and upper boundaries delimiting its full extent should be isochronous.” This opinion is shared by Meyen (1989, p. 62): “... if a boundary is shown to be diachronous it automatically loses its stratigraphic significance.” Earlier, all the boundaries used in stratigraphy were divided into three main types (Meyen, 1989). The first type embraces the boundaries mentioned by Vereshchagin and Meyen, i.e., isochronous. They were named stratigraphic sensu stricto. The second type boundaries are parastratigraphic. These boundaries are sliding, including lateral limits of adjacent local stratigraphic units in the zone of mutual transitions. This type of boundaries was characterized by Zhamoida (1980, p. 42) as follows: “Lateral boundaries of the stratigraphic unit are of
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facies nature and of the same type as the boundaries of the lithological body... Undistinguishable or difficult-to-distinguish stratigraphic and lateral boundaries of formation gave rise to the phrase “boundary sliding”... Actually we deal with the “sliding” of lateral boundaries of separate beds of straton. By “sliding boundary” is meant a certain resultant between boundary elements of two types.” Cases are known in geological practice when during the formation of a geologic body the sedimentation regime changed considerably to nondeposition (“nil sedimentation” according to N.B. Vassoevich) or the formed body was subsequently eroded partly or completely. In this case, the younger geologic body will have predominantly distinct boundaries with the older body. Most of these boundaries are, additionally, of “sliding” nature. Sometimes they have a more intricate “relief”. The boundaries of this kind are supposed to be called allostratigraphic (Tsyganko, 1994). Analysis of views of many geologists who share the above opinion and my own observations suggests that the three types of stratigraphic boundaries — stratigraphic s.s., parastratigraphic, and allostratigraphic — are pertinent to formation. The nature is many-sided. Therefore, particular manifestation and degree of development of each of the boundary types are controlled by the specific conditions under which the formation developed. On the basis of our analysis, we offer the following definition of formation: Formation is a total sum of deposits with specific lithology developed within a geologic area. It is separated from neighboring coeval and/or younger or older deposits (formations) by boundaries of three types — stratigraphic s.s., parastratigraphic, and allostratigraphic. Paleontological remains (if present) supplement the lithological signature of the formation and locate it in the general stratigraphic scale. The formation-related problems that remain to be solved concern the limits of their vertical (age) and lateral spreading. As regards the former, many authors report considerable fluctuations in the age range of formations. In the Precambrian its maximum reaches half an eonotheme (Antsygin et al., 1993). In the Phanerozoic, where paleontological data more accurately constrain the age range of formations, it varies from some part of chronozone or horizon to several stages (Koren’ et al., 1933) (Fig. 1). It should be noted that formation-by-formation division of sections may become more detailed as the geology of the area comes to be better understood or new methods come into use. An example is the Lower Devonian deposits of the Elets structure-facies zone in the northern Urals. In the range corresponding to the Pragian and lowermost Emsian, Pershina (1960) recognized the Filippchuk Formation, a regressive series of carbonate-terrigene deposits. As a result of detailed lithological studies, Shcherbakov (1977) distinguished the topmost carbonate-free and largely red-colored part of the formation as a new unit, the Pristan’ Formation, with the lower part having retained its former name. Then I considered both parts of the Philippchuk Formation as subformations modified by geographical names (Tsyganko, 1997). At present, the sections of the formation are studied well enough to rank its subformations two
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nant lithotype (Miklyaev and Belyaev, 1994); in some cases, the “deviating” sections of the formation are called facies modified by lithological or geographic terms. In recent time, to classify “deviating” sections of formations, the term gradation is used, which usually characterizes the change of formations in space (Eliseev, 1978; Eliseev et al., 1984, Khvorova, 1961). Now we are sure that the so-called “facies variability” of immanent characteristics is pertinent to all formations. The level of this variability and criteria for recognizing formation’s parts distinguished by some features are presently regulated by the experience and skill of a geologist and the available tools. The procedure of distinguishing of lateral sites of formations can be ordered after the sufficient experience has been acquired and generalized. Therefore, it is reasonable to carry out this procedure to substantiate new formations as well as to revise the existing formations already now. This will reduce their quantity on dividing the supracrustal sequences. As to the name for the distinguished lateral sites of formation, most proposed terms are of loose use, and gradation is considered part of formation. Therefore, a part of formation is recommended to be named grade. Grade is the part of formation that characterizes its lateral change; it differs from the typical set of formation characteristics by a change in their ratio or by the appearance of new attributes inferior in significance to the main ones; the latter can embrace the whole stratigraphic range of formation or its part (Fig. 2). Each grade should be accompanied with a geographic reference of the type section of the distinguishable part of formation, e.g., Silovaya (after river) grade of the Put’yu Formation, Kara (after river) and Mareishor (after brook) grades of the Padei Formation, and so on. Fig. 1. Age sliding of the lower boundary of the Tamasha Formation in southern Fergana, after: Koren’ et al., 1993. 1–3 — limestones: 1 — crystalline-granular, 2 — clastic, 3 — siliceous; 4 — radiolarian cherts and radiolarites; 5 — sponge cherts and spongolites; 6 — phthanites; 7 — shales, siltstones; 8 — siliceous siltstones; 9 — conglomerates and sandstones; 10 — turbidites. Solid line marks straton composed of reef limestones.
independent formations, Rybatskaya and Pristan’, with the Filippichuk Formation ranked group. The question on the extent of lateral distribution of formations is rather controversial. To answer it is easier where the formation developed within a geologic district or its part is delimited by disjunctives or sites with complete denudation of its deposits followed by deposits of different lithology. Where deviations from its typical sections are revealed on the basis of their typification, it is necessary to estimate the taxonomic level (rank) of these deviations. For this purpose we must clear up to what extent the typical set of lithotypes of the formation is subject to qualitative and quantitative changes. For instance, whether or not new lithotypes may appear within its limits. Their quantity critical for a given formation is presently just guessed by geologists. As a result, two scenarios are possible: Either new elements are simply added to the description of the formation, or new sections are established, modified by a geographic name or by a predomi-
Horizon The term “stratigraphic horizon” was introduced into geology by N.A. Golovkinsky. In his thesis “On the Permian formation in the central Kama-Volga watershed”, he argued that “... it is necessary to be careful in distinguishing concepts of chronological, stratigraphic, petrographic, and paleontological horizons” assuming that all the above horizons can be covered by the general term “geologic horizon”: “... by geologic horizon are meant the directions that connect those parts of formation which are similar according to one of the attributes” (Golovkinsky, 1869, p. 400). Golovkinsky clearly distinguished stratigraphic horizon from the rest — chronological, petrographic, and paleontological horizons. Thus, he noted that “even as a bed wedges out its stratigraphic horizon continues, whereas petrographic horizon naturally disappears” (Golovkinsky, 1869). An important inference follows from this definition: Horizon is a unit that combines different facies types of deposits. Of course, it is impossible to correlate coeval deposits of various facies without using a paleontological method, which in turn necessitates horizon to be considered a 3D body. Later on, in the absence of any rules and norms with regard
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Fig. 2. Schematic relationship of formations with stratigraphic units of different ranks. 1 — zones (chronozones); 2 — boundaries of zones (chronozones): a — proven, b — supposed; 3 — interval matching stage; 4 — stage boundaries: a — proven, b — supposed; 5 — isochronous or diachronous (parastratigraphic) sedimentation boundaries of formations; 6 — boundaries of stratons (formations) accompanied by sedimentation gap (allostratigraphic); 7 — formation X, grades: a — X′, b — X′′; 8 — formation Y, grades: a — Y′, b — Y′′, c — Y′′′ 9 — boundaries of biozones based on different groups of fauna; 10 — nondeposition.
to stratigraphic nomenclature the term stratigraphic horizon long used in Russia as a loose term. In 1874, estimating the state of the art in terminology and nomenclature of stratigraphic division, Karpinsky (1874, p. 95) wrote: “As regards the nomenclature of sedimentary formations, there is some discrepancy in geological literature which would be desirable to eliminate.” An attempt to solve this problem was undertaken at the Second International Geological Congress in Bologna (1881), where the nomenclature of the largest stratigraphic units was accepted. Until now, the accepted units — group (era), system (period), series (epoch), stage (age) — make the base of the International Stratigraphic Chart. However, with respect to subdivision more detailed than stage the Congress accepted only one unit termed by the French word assise. At present, in the French Stratigraphic Code the term assise corresponds to member. In Russia, the term assise after the 2nd IGC was applied to deposits referred to as horizon. At that time the term horizon was not officially supported by most of the Russian Commission on Unification of Stratigraphic Nomenclature, whose members were A.A. Inostrantsev, V.I. Meller, A.P. Karpinsky, I.V. Mushketov, and others. The main objection was that horizon is commonly understood to be linear rather than volumetric: either line or plane but not a body. Nevertheless, the term horizon was used by many Russian geologists. As a unit equal to one or more zones but more detailed than stage, horizon was used by Laguzen (1883), Pavlov (1886), Nikitin (1885), Chernyshev (1898–1902),
Andrusov (1897, 1899, 1906), and others. But given no clear criteria for its separation, horizon was a loose term until the middle 20th century. An illustrative example is the Geological Map of the Ukhta-Izhem region at a scale of 1:500,000 compiled by Tikhonovich, Anosov, and Kompanets (Tikhonovich et al., 1932), where Devonian horizons are distinguished by fauna or lithology or both. In the Soviet Union, B.M. Keller was the first to pay attention on the principles and methods of division, terminology, and nomenclature of stratigraphic units, which was dictated by widely deployed postwar prospecting. In his work “Stratigraphic division” (Keller, 1950) he placed emphasis on the terms horizon and zone. He considered horizon the main division of the local stratigraphic scale based on paleontological findings. According to Keller (1950, p.8), horizon is a subunit of stage and “can spatially embrace several coeval zones that belong to different facies... or several heterochronous zones lying upon one another in a vertical succession.” It means that horizon can cover deposits of different facies types. In an above-mentioned work (Librovich et al., 1954) horizon was classified as an auxiliary stratigraphic unit. It was defined eclectically, which is usual for loose terms. The authors proposed to name horizons according to typical fossils (e.g., Aucella horizon) or to lithology (e.g., limestone horizon), or to geographical names of the localities where they are situated. This interpretation of horizon was considerable regress as compared with the definition of stratigraphic horizon proposed by N.A. Golovkinsky decades before.
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In 1956, in the “Stratigraphic Classification and Terminology” supervised by A.P. Rotai as well as in its second edition (Rotai, 1960) stratigraphic horizon was interpreted closely to its initial definition: “Horizon is a subsidiary unit of regional scale comprising horizontally (in area) several coeval formations (or their parts)... Horizon is recognized by a combination of paleontological, facies-lithological, paleoclimatic, and other features” (Rotai, 1960, p. 26). Subsequently, a similar definition of horizon was also given in stratigraphic codes edited by A.I. Zhamoida (Bekker et al., 1992; Zhamoida, 1977). The only difference is a higher rank of horizon: In the stratigraphic codes it is considered a major taxonomic unit of regional stratigraphy. The areal spread of horizons is delimited by paleobiogeographic areas or paleobasins of sedimentation. Horizons can be used in medium- and small-scale geological mapping. Horizons should have stratotypes. The above landmarks in the prehistory of the concept horizon seemingly gave the green light to the broadest use of this straton in geological practice and, first of all, for detailed division and correlation of the Phanerozoic sections. However, the use of the concept stratigraphic horizon in its modern interpretation provided little, if any, progress. I believe that its failure largely stems from the dualism of the concept horizon itself: In the Stratigraphic Code it is a regional straton situated between local (formation, group, etc.) and general (stage, system, etc.) stratigraphic units. Most horizons have stratotypes and, therefore, geographic names in common with formations or groups while the same age of their deposits and isochronous boundaries make them related to stages and systems. Formation, the main straton among local units, is considered above. Here I note only that one of its typical features is the age sliding of most of its boundaries. In general, formation and other local stratons, whose immanent feature is substance composition, are universally characterized by a possibility of development of boundaries of the three types: stratigraphic s.s., parastratigraphic, and allostratigraphic. In practice, the above differences between horizons and formations (groups) are often ignored. As a result, the volumes of horizons and corresponding formations are mostly identical in the correlation stratigraphic schemes of Russia’s regions.
Moreover, the boundaries between vertically adjacent formations corresponding to different horizons are everywhere shown by a continuous horizontal line to stress its isochronous character. This “levelling” hinders us from specifying the geologic history of a region or its parts, with corresponding practical implications. The similarity of horizons with stratons of the Phanerozoic stratigraphic scale is based, first of all, on the use of the biostratigraphic method and its derivative — division of records into chronozones. This approach implies that in continuous records the horizon’s boundaries coincide with boundaries of systems, stages and, under certain conditions, substages. At the same time, horizon bears an imprint of its regional rank. It is for this reason that the Stratigraphic Code notifies that recognized horizons may or may not cover the whole stratigraphic section exposed in the region. This brings up some questions: — What to do in this case with medium- and small-scale mapping involved with a section subdivided uniformly throughout? — What to do with hiatuses in the sections owing to gaps in sedimentation? Surveyors and cartographers try to answer the first question. The answer to the second question is given in Fig. 3, a, which schematically shows a fragment of the continuous section divided into horizons A, B, and C meeting the requirements of the Stratigraphic Code. Figure 3, b shows the case when some part of the record is missing because of sedimentation gap. The hiatus is delimited in the reconstructed section by diachronous surfaces one of which belongs to the underlying and the other to overlying part of the section. I refer such boundaries to the allostratigraphic type (Tsyganko, 1994). If biostratigraphic data evidence that the corresponding section’s intervals belong to different horizons, adjacent (B′ and C′) as in Fig. 3, b or with one (B′, D′) and more (B′, E′, etc.) horizons missed, the considered intervals may and must be distinguished as independent valid horizons. But taking into account that these horizons have only one isochronous (stratigraphic s.s.) boundary, the other being allostratigraphic, and that they have reduced volumes, we propose to consider them incom-
Fig. 3. Types of horizons depending on their completeness. 1, 2 — boundaries: 1 — isochronous (stratigraphic s.s.), 2 — allostratigraphic; 3 — hiatus. A-(E′) — horizons: A, B, C, A′ — complete; B′, C′, (D′, E′) — incomplete; a, b — see text for explanation.
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plete horizons (B′, C′, D′, E′, etc.). The boundaries of complete horizons (A, B, C, A′...) are both isochronous. Thus, analysis of many stratigraphic schemes, including those published recently, and inspection of sections in the field evidence that horizon is a specific stratigraphic unit very important for correlating deposits of different facies. Depending on their structure, two types can be distinguished among horizons. Type I — complete horizons with lower and upper isochronous boundaries. A possibility is in sight that some of these horizons can be considered regional stages. Type II — incomplete horizons, which has only one isochronous boundary while the other, upper or lower, is allostratigraphic. Thus, the following emended definition of stratigraphic horizon can be proposed. Horizon is a regional (subregional) stratigraphic unit that correlates different facies deposits of other regional and local stratons or their parts. In volume it fits chronozone(s) or its (their) parts and has, at least, one boundary of strictly stratigraphic type. It matches a sedimentary basin or its part. The stratigraphic unit horizon is absent from codes of other nations. The International Stratigraphic Code (International..., 2002) also misses it. The USSR and Russia followed their own path. The future of the term horizon depends on how accurately and efficiently this straton will be used in geological practice.
Conclusions Thirty-five years ago, the patriarch of Soviet and Russian stratigraphy B.S. Sokolov (1971) indicated that stratigraphy is lacking in irrefutable logic, a steady system of axioms, and well-proven ideas of principles of stratigraphic classification. For the past period stratigraphy as a science made considerable progress: Its general principles were formulated, problems of stratigraphic division were solved with respect of their types, substantiation, spatial extent, and methods of correlation. Considerable progress was noted with respect to the most urgent problem — stratigraphic boundaries. For most supracrustal shell of the Earth’s crust is has been methodologically solved, which is reflected in the International Stratigraphic Chart for some Precambrian and Phanerozoic systems. Boundaries of stages are fixed by a “gold nail” — the Global Boundary Stratotype Section and Point (GSSP). Nevertheless, despite evident progress, stratigraphy still faces many problems. This paper reports some approaches to their solution. First of all, it deals with the typification of stratigraphic units on the basis of types of their boundaries (Tsyganko, 1994). To consider formations and horizons from this point of view is dictated by the practical interest to these stratons of all geologists engaged into division and correlation of sedimentary, volcanic and, partly, metamorphic rocks. At first glance, methodological approach to the problem of boundaries of formations is rather simple: The problem is reduced to search for a distinctly observable change in
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lithology, facies change, sedimentation and denudation gaps, and, in some cases, for drastic changes in faunal and floral associations. Virtually, three basic types of boundaries that characterize formations have been established: stratigraphic s.s., parastratigraphic, and allostratigraphic. It is most difficult to diagnose allostratigraphic boundaries of formations where stratigraphic boundaries are hidden. A certain problem is also to reveal formations in localities with anomalous facies change of their lithological characteristics. As a rule, these are intermontane depressions and mountainous lands. In the mountains well-expressed primary facies change is supplemented by tectonic convergence of sections. My proposal to document formations well deviated from type sections by distinguishing grades is my reaction to this procedure practiced under different names (facies, gradation, etc.) and, to a certain extent, an invitation to discuss this problem. Horizons are derivatives of formations (or their parts) they correlate but, unlike the latter, they are based first of all on biostratigraphic correlation criteria. Laterally, they can cover considerable areas of ancient basins. When the sole of horizon is diachronous because the basin gradually transgressed onto the uplifting eroded surface of land or because the roof of the straton’s sediments was subjected to denudation, the main correlative is its other boundary — upper in the first case and lower in the second. The use of these intervals of sections as horizons allows the correlative potential of this stratigraphic unit to be used to a greater extent in division and correlation of sections, first of all, on the territories with a complex geologic history. I thank I.V. Budnikov and E.A. Yolkin for valuable advice and criticism improving the manuscript.
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stratigraphic classification, in: Stratigraphic Classification [in Russian]. Nauka, Leningrad, pp. 135–146. Librovich, L.S., Alikhova, T.N., Krasny, L.I., Krishtofovich, A.N., Kulikov, M.V., Luppov, N.P., Markovsky, B.P., Muzylev, S.A., Rotai, A.P., Sergievsky, V.M., Fomichev, V.D., Chernysheva, N.E., Chikhachev, P.K., Yakovleva, S.V., 1954. Stratigraphic and Geochronological Units (Principles, Content, Terminology, and Rules of Use) [in Russian]. Gos. Nauch.-Tekh. Izd. Lit. Geol. Okhr. Nedr, Moscow. Murphy, N.A., Salvador, A., Gladenkov, Yu.B. (Eds.), 2002. Concised International Stratigraphic Guide [Russian translation]. GEOS, Moscow. Meien, S.V., 1989. Introduction to Stratigraphy Theory [in Russian]. Nauka, Moscow. Miklyaev, A.S., Belyaev, A.A., 1994. Upper Devonian and Lower Carboniferous deposits of the shale zone of southeastern Pai-Khoi, in: Boundaries of Stratigraphic Units [in Russian]. Syktyvkar, pp. 27–34. Nikitin, S.N., Chernyshev, F.I., 1889. International Geological Congress and its last sessions in Berlin and London. Gorny Zhurnal 1, 115–150. Pershina, A.I., 1960. Stratigraphy and Paleogeography of Devonian Deposits of Right Bank of Srednyaya Pechora River and Southern Chernyshev Ridge [in Russian]. Izd. AN SSSR, Moscow-Leningrad. Rotai, A.P., Bobkova, N.N., Librovich, L.S., Menner, V.V., Ovechkin N.K., Radchenko, G.P., Ruzhentsev, V.E., Sokolov, B.S., Shantser, E.V., 1956. Stratigraphic Classification and Terminology. Gosgeoltekhizdat, Moscow, 28 pp. Rotai, A.P., Bobkova, N.N., Librovich, L.S., Menner, V.V., Ovechkin N.K.,
Radchenko, G.P., Ruzhentsev, V.E., Sokolov, B.S., Shantser, E.V., 1960. Stratigraphic Classification and Terminology, second ed. Gosgeoltekhizdat, Moscow, 59 pp. Shcherbakov, E.S., 1977. Terrigenous Devonian of western slope of Northern Urals [in Russian]. Nauka, Moscow. Sokolov, B.S., 1971. Biochronology and biostratigraphic boundaries, in: Problems of General and Regional Geology [in Russian]. Nauka, Novosibirsk, pp. 155–178. Sokolov, B.S., 1987. Model of biostratigraphic boundary, in: Historical Geology. Totals and Perspectives [in Russian]. Izd. MGU, pp. 17–25. Tikhonovich, N.N., Anosov, A.A., Kompanets, B.R., 1932. Geological Map of Ukhta-Izhem Region [in Russian]. Geokartprom, Moscow. Tsyganko, V.S., 1994. Problems of Classification of Regional Stratigraphic Units [in Russian]. Syktyvkar. Tsyganko, V.S., 1997. Stratigraphy and correlation of terrigenic deposits of Lower Devonian of Near-Polar Urals and southern Chernyshev Ridge, in: Geology of European North of Russia [in Russian]. Syktyvkar, pp. 54–68. Tsyganko, V.S., 2002. Horizon in stratigraphy: theoretical and practical aspects. Vestn. Int. Geol. Komi. Nauch. Tsentra UrO RAN 1, 14–16. Vereshchagin, V.N., 1980. Formation as an important stratigraphic unit, in: Stratigraphic Classification [in Russian]. Nauka, Leningrad, pp. 130–135. Zhamoida, A.I., Kovalevsky, O.P., Moiseeva, A.I., Yarkin, V.I. (Eds.), 1977. Stratigraphic Code of the USSR [in Russian]. VSEGEI, Leningrad. Zhamoida, A.I., 1980. Essence and relationship of main stratigraphic units, in: Stratigraphic Classification [in Russian]. Nauka, Leningrad, pp. 32–63.
Formation and horizon: types of boundaries and their relationship V.S. Tsyganko * Institute of Geology, Komi Science Center, Uralian Branch of the RAS, 54 ul. Pervomaiskaya, Syktyvkar, 167982, Russia Received 25 January 2006
Abstract In dividing supracrustal strata, formation and horizon have been and are basic stratigraphic units. Stratigraphic boundaries of a formation, a natural geologic body, are drawn mostly on the basis of its composition. Paleontological remains constrain the formation in time and spatially locate it in the Earth’s crust. Boundaries between formations can be of three types: strictly stratigraphic, parastratigraphic, and allostratographic. The stratigraphic interval can range from a fraction of a horizon or chronozone to several stages. At the boundary between two systems the adjacent parts of the formation can relate to both systems. The main stratigraphic characteristics for recognizing horizons are paleontologic (biostratigraphic) features, revealed by zonal, paleoecosystemic (ecostratigraphic), bioeventual, and other methods to make a basis for their immanent signature. Horizon can be characterized by boundaries of only two types: strictly stratigraphic and allostratigraphic. The stratigraphic interval of a horizon can vary from a single chronozone to a stage. Boundaries of neighboring horizons at the contact between two stages or systems should coincide with the latter. The stratigraphic units of the International Stratigraphic Chart, in contrary to formation and horizon, are characterized by borders of only one type — strictly stratigraphic. © 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Formation; horizon; stratigraphic unit; boundary; grade
Introduction Any stratigraphic inferences are known to originate from the study of particular records of the Earth’s sedimentary sequence in a certain region of the planet. Thus, all possible operations in stratigraphy are based on the data on regional stratigraphy. This was first postulated by Nikitin and Chernyshev (1889) and, in our time, was stressed by Sokolov (1971, 1987) believing that regional stratigraphy is the basement for stratigraphy in general. According to Sokolov, all stratigraphic models are derivatives of a real regional stratigraphy, including the composite construction of the International Stratigraphic Chart. Considering relationships between regional and global stratigraphic divisions, Sokolov (1971) argued that a “regional stratigraphic division” should cover any real stratigraphic subdivisions — from narrow local units to basins. Their actual areal extent is bearing, in each particular case, on the character of the stratigraphic units themselves specified by diverse factors of the geologic history of the given territory. At present, when dividing supracrustal structures in the course
* Corresponding author. E-mail address: [email protected] (V.S. Tsyganko)
of medium- and large-scale geological surveys and search for ores, Russian geologists use the main stratigraphic units formation and horizon. As there is no consensus on their definitions and estimation of their role in dividing supracrustal sequences, it is necessary to characterize each of these stratigraphic units.
Formation Russian geologists began to use the term formation (suite) in the second half of the 19th century. In 1881, the Second International Geological Meeting in Bologna adopted the nomenclature of main stratigraphic subdivisions making the base of the modern International Stratigraphic Chart. The Russian delegation headed by A.A. Inostrantsev offered to subdivide stage into formations and complexes and into still smaller subunits, beds. This proposal, however, was rejected, and until 1956 the term formation was used loosely. In 1952, an Ad hoc Stratigraphic Commission was set up at the VSEGEI, which two years later published a brochure entitled “Stratigraphic and geochronological subdivisions: principles, content, terminology, and rules of the use” (Librovich et al., 1954). Formation was considered there a stratigraphic unit of local (regional) scale. Its definition is very close to the modern
1068-7971/$ - see front matter D 2007, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.200 7. 07.002
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one. At a meeting devoted to general problems of stratigraphic classification, which was held at VSEGEI in 1955, a commission was organized, headed by A.P. Rotai, to summarize all the proposals on this problem. The results were discussed and published by the Interdepartmental Stratigraphic Committee as an ad hoc instruction (Rotai, 1956, 1960). In this Instruction, formation was first (Rotai, 1956) considered the basic unit of auxiliary local divisions and then (Rotai, 1960), of regional stratigraphic divisions. The USSR Stratigraphic Code published in 1977 changed the rank and interpretation of formation. In this Code formation was ranked a local stratigraphic unit. The same was in the second edition of the Stratigraphic Code (Bekker et al., 1992), where formation was defined as a sum of deposits that occur within a certain geologic area, differ from over- and underlying deposits in having specific lithological, facies, and paleontological (given remains of organisms) features, and are characterized by the same composition and structure and specific character of boundaries. At present, most geologists are positive toward formation, but this does not mean that they are unanimous in interpretation of formation itself as well as the character of its boundaries (Krasnov, 1980; Vereshchagin, 1980; Zhamoida, 1980). There are all reasons to share the anxiety of geologists about the intense recent “creation of formations” (Karogodin, 2003). The problems bearing on the interpretation of the concept formation and its bounds were discussed in my previous publications (Tsyganko, 1994, 2002). I came to quite the same conclusion which was made in the Stratigraphic Code (Bekker et al., 1992): The age sliding of boundaries is one of the most specific features of formation. Its manifestation and extent are governed by the specific character of the geologic history of a region partly or as a whole. Thus, diachronous boundaries can be considered a universal property of formations, series, and other local stratigraphic units, whose immanent signature is their lithology. However, the available methods often fail in detecting the diachronous character of the boundaries. Zubakov (1978, p.19) was quite decisive on this issue: “Any stratigraphic boundary that seems to be isochronous at a given detail of study becomes diachronous as soon as methods of more accurate diagnosis are applied.” Many stratigraphers, however, are proponents of the opposite point. Thus, Vereshchagin (1980, p. 134) wrote: “Formation should be characterized by the same time of origin, hence its lower and upper boundaries delimiting its full extent should be isochronous.” This opinion is shared by Meyen (1989, p. 62): “... if a boundary is shown to be diachronous it automatically loses its stratigraphic significance.” Earlier, all the boundaries used in stratigraphy were divided into three main types (Meyen, 1989). The first type embraces the boundaries mentioned by Vereshchagin and Meyen, i.e., isochronous. They were named stratigraphic sensu stricto. The second type boundaries are parastratigraphic. These boundaries are sliding, including lateral limits of adjacent local stratigraphic units in the zone of mutual transitions. This type of boundaries was characterized by Zhamoida (1980, p. 42) as follows: “Lateral boundaries of the stratigraphic unit are of
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facies nature and of the same type as the boundaries of the lithological body... Undistinguishable or difficult-to-distinguish stratigraphic and lateral boundaries of formation gave rise to the phrase “boundary sliding”... Actually we deal with the “sliding” of lateral boundaries of separate beds of straton. By “sliding boundary” is meant a certain resultant between boundary elements of two types.” Cases are known in geological practice when during the formation of a geologic body the sedimentation regime changed considerably to nondeposition (“nil sedimentation” according to N.B. Vassoevich) or the formed body was subsequently eroded partly or completely. In this case, the younger geologic body will have predominantly distinct boundaries with the older body. Most of these boundaries are, additionally, of “sliding” nature. Sometimes they have a more intricate “relief”. The boundaries of this kind are supposed to be called allostratigraphic (Tsyganko, 1994). Analysis of views of many geologists who share the above opinion and my own observations suggests that the three types of stratigraphic boundaries — stratigraphic s.s., parastratigraphic, and allostratigraphic — are pertinent to formation. The nature is many-sided. Therefore, particular manifestation and degree of development of each of the boundary types are controlled by the specific conditions under which the formation developed. On the basis of our analysis, we offer the following definition of formation: Formation is a total sum of deposits with specific lithology developed within a geologic area. It is separated from neighboring coeval and/or younger or older deposits (formations) by boundaries of three types — stratigraphic s.s., parastratigraphic, and allostratigraphic. Paleontological remains (if present) supplement the lithological signature of the formation and locate it in the general stratigraphic scale. The formation-related problems that remain to be solved concern the limits of their vertical (age) and lateral spreading. As regards the former, many authors report considerable fluctuations in the age range of formations. In the Precambrian its maximum reaches half an eonotheme (Antsygin et al., 1993). In the Phanerozoic, where paleontological data more accurately constrain the age range of formations, it varies from some part of chronozone or horizon to several stages (Koren’ et al., 1933) (Fig. 1). It should be noted that formation-by-formation division of sections may become more detailed as the geology of the area comes to be better understood or new methods come into use. An example is the Lower Devonian deposits of the Elets structure-facies zone in the northern Urals. In the range corresponding to the Pragian and lowermost Emsian, Pershina (1960) recognized the Filippchuk Formation, a regressive series of carbonate-terrigene deposits. As a result of detailed lithological studies, Shcherbakov (1977) distinguished the topmost carbonate-free and largely red-colored part of the formation as a new unit, the Pristan’ Formation, with the lower part having retained its former name. Then I considered both parts of the Philippchuk Formation as subformations modified by geographical names (Tsyganko, 1997). At present, the sections of the formation are studied well enough to rank its subformations two
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nant lithotype (Miklyaev and Belyaev, 1994); in some cases, the “deviating” sections of the formation are called facies modified by lithological or geographic terms. In recent time, to classify “deviating” sections of formations, the term gradation is used, which usually characterizes the change of formations in space (Eliseev, 1978; Eliseev et al., 1984, Khvorova, 1961). Now we are sure that the so-called “facies variability” of immanent characteristics is pertinent to all formations. The level of this variability and criteria for recognizing formation’s parts distinguished by some features are presently regulated by the experience and skill of a geologist and the available tools. The procedure of distinguishing of lateral sites of formations can be ordered after the sufficient experience has been acquired and generalized. Therefore, it is reasonable to carry out this procedure to substantiate new formations as well as to revise the existing formations already now. This will reduce their quantity on dividing the supracrustal sequences. As to the name for the distinguished lateral sites of formation, most proposed terms are of loose use, and gradation is considered part of formation. Therefore, a part of formation is recommended to be named grade. Grade is the part of formation that characterizes its lateral change; it differs from the typical set of formation characteristics by a change in their ratio or by the appearance of new attributes inferior in significance to the main ones; the latter can embrace the whole stratigraphic range of formation or its part (Fig. 2). Each grade should be accompanied with a geographic reference of the type section of the distinguishable part of formation, e.g., Silovaya (after river) grade of the Put’yu Formation, Kara (after river) and Mareishor (after brook) grades of the Padei Formation, and so on. Fig. 1. Age sliding of the lower boundary of the Tamasha Formation in southern Fergana, after: Koren’ et al., 1993. 1–3 — limestones: 1 — crystalline-granular, 2 — clastic, 3 — siliceous; 4 — radiolarian cherts and radiolarites; 5 — sponge cherts and spongolites; 6 — phthanites; 7 — shales, siltstones; 8 — siliceous siltstones; 9 — conglomerates and sandstones; 10 — turbidites. Solid line marks straton composed of reef limestones.
independent formations, Rybatskaya and Pristan’, with the Filippichuk Formation ranked group. The question on the extent of lateral distribution of formations is rather controversial. To answer it is easier where the formation developed within a geologic district or its part is delimited by disjunctives or sites with complete denudation of its deposits followed by deposits of different lithology. Where deviations from its typical sections are revealed on the basis of their typification, it is necessary to estimate the taxonomic level (rank) of these deviations. For this purpose we must clear up to what extent the typical set of lithotypes of the formation is subject to qualitative and quantitative changes. For instance, whether or not new lithotypes may appear within its limits. Their quantity critical for a given formation is presently just guessed by geologists. As a result, two scenarios are possible: Either new elements are simply added to the description of the formation, or new sections are established, modified by a geographic name or by a predomi-
Horizon The term “stratigraphic horizon” was introduced into geology by N.A. Golovkinsky. In his thesis “On the Permian formation in the central Kama-Volga watershed”, he argued that “... it is necessary to be careful in distinguishing concepts of chronological, stratigraphic, petrographic, and paleontological horizons” assuming that all the above horizons can be covered by the general term “geologic horizon”: “... by geologic horizon are meant the directions that connect those parts of formation which are similar according to one of the attributes” (Golovkinsky, 1869, p. 400). Golovkinsky clearly distinguished stratigraphic horizon from the rest — chronological, petrographic, and paleontological horizons. Thus, he noted that “even as a bed wedges out its stratigraphic horizon continues, whereas petrographic horizon naturally disappears” (Golovkinsky, 1869). An important inference follows from this definition: Horizon is a unit that combines different facies types of deposits. Of course, it is impossible to correlate coeval deposits of various facies without using a paleontological method, which in turn necessitates horizon to be considered a 3D body. Later on, in the absence of any rules and norms with regard
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Fig. 2. Schematic relationship of formations with stratigraphic units of different ranks. 1 — zones (chronozones); 2 — boundaries of zones (chronozones): a — proven, b — supposed; 3 — interval matching stage; 4 — stage boundaries: a — proven, b — supposed; 5 — isochronous or diachronous (parastratigraphic) sedimentation boundaries of formations; 6 — boundaries of stratons (formations) accompanied by sedimentation gap (allostratigraphic); 7 — formation X, grades: a — X′, b — X′′; 8 — formation Y, grades: a — Y′, b — Y′′, c — Y′′′ 9 — boundaries of biozones based on different groups of fauna; 10 — nondeposition.
to stratigraphic nomenclature the term stratigraphic horizon long used in Russia as a loose term. In 1874, estimating the state of the art in terminology and nomenclature of stratigraphic division, Karpinsky (1874, p. 95) wrote: “As regards the nomenclature of sedimentary formations, there is some discrepancy in geological literature which would be desirable to eliminate.” An attempt to solve this problem was undertaken at the Second International Geological Congress in Bologna (1881), where the nomenclature of the largest stratigraphic units was accepted. Until now, the accepted units — group (era), system (period), series (epoch), stage (age) — make the base of the International Stratigraphic Chart. However, with respect to subdivision more detailed than stage the Congress accepted only one unit termed by the French word assise. At present, in the French Stratigraphic Code the term assise corresponds to member. In Russia, the term assise after the 2nd IGC was applied to deposits referred to as horizon. At that time the term horizon was not officially supported by most of the Russian Commission on Unification of Stratigraphic Nomenclature, whose members were A.A. Inostrantsev, V.I. Meller, A.P. Karpinsky, I.V. Mushketov, and others. The main objection was that horizon is commonly understood to be linear rather than volumetric: either line or plane but not a body. Nevertheless, the term horizon was used by many Russian geologists. As a unit equal to one or more zones but more detailed than stage, horizon was used by Laguzen (1883), Pavlov (1886), Nikitin (1885), Chernyshev (1898–1902),
Andrusov (1897, 1899, 1906), and others. But given no clear criteria for its separation, horizon was a loose term until the middle 20th century. An illustrative example is the Geological Map of the Ukhta-Izhem region at a scale of 1:500,000 compiled by Tikhonovich, Anosov, and Kompanets (Tikhonovich et al., 1932), where Devonian horizons are distinguished by fauna or lithology or both. In the Soviet Union, B.M. Keller was the first to pay attention on the principles and methods of division, terminology, and nomenclature of stratigraphic units, which was dictated by widely deployed postwar prospecting. In his work “Stratigraphic division” (Keller, 1950) he placed emphasis on the terms horizon and zone. He considered horizon the main division of the local stratigraphic scale based on paleontological findings. According to Keller (1950, p.8), horizon is a subunit of stage and “can spatially embrace several coeval zones that belong to different facies... or several heterochronous zones lying upon one another in a vertical succession.” It means that horizon can cover deposits of different facies types. In an above-mentioned work (Librovich et al., 1954) horizon was classified as an auxiliary stratigraphic unit. It was defined eclectically, which is usual for loose terms. The authors proposed to name horizons according to typical fossils (e.g., Aucella horizon) or to lithology (e.g., limestone horizon), or to geographical names of the localities where they are situated. This interpretation of horizon was considerable regress as compared with the definition of stratigraphic horizon proposed by N.A. Golovkinsky decades before.
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In 1956, in the “Stratigraphic Classification and Terminology” supervised by A.P. Rotai as well as in its second edition (Rotai, 1960) stratigraphic horizon was interpreted closely to its initial definition: “Horizon is a subsidiary unit of regional scale comprising horizontally (in area) several coeval formations (or their parts)... Horizon is recognized by a combination of paleontological, facies-lithological, paleoclimatic, and other features” (Rotai, 1960, p. 26). Subsequently, a similar definition of horizon was also given in stratigraphic codes edited by A.I. Zhamoida (Bekker et al., 1992; Zhamoida, 1977). The only difference is a higher rank of horizon: In the stratigraphic codes it is considered a major taxonomic unit of regional stratigraphy. The areal spread of horizons is delimited by paleobiogeographic areas or paleobasins of sedimentation. Horizons can be used in medium- and small-scale geological mapping. Horizons should have stratotypes. The above landmarks in the prehistory of the concept horizon seemingly gave the green light to the broadest use of this straton in geological practice and, first of all, for detailed division and correlation of the Phanerozoic sections. However, the use of the concept stratigraphic horizon in its modern interpretation provided little, if any, progress. I believe that its failure largely stems from the dualism of the concept horizon itself: In the Stratigraphic Code it is a regional straton situated between local (formation, group, etc.) and general (stage, system, etc.) stratigraphic units. Most horizons have stratotypes and, therefore, geographic names in common with formations or groups while the same age of their deposits and isochronous boundaries make them related to stages and systems. Formation, the main straton among local units, is considered above. Here I note only that one of its typical features is the age sliding of most of its boundaries. In general, formation and other local stratons, whose immanent feature is substance composition, are universally characterized by a possibility of development of boundaries of the three types: stratigraphic s.s., parastratigraphic, and allostratigraphic. In practice, the above differences between horizons and formations (groups) are often ignored. As a result, the volumes of horizons and corresponding formations are mostly identical in the correlation stratigraphic schemes of Russia’s regions.
Moreover, the boundaries between vertically adjacent formations corresponding to different horizons are everywhere shown by a continuous horizontal line to stress its isochronous character. This “levelling” hinders us from specifying the geologic history of a region or its parts, with corresponding practical implications. The similarity of horizons with stratons of the Phanerozoic stratigraphic scale is based, first of all, on the use of the biostratigraphic method and its derivative — division of records into chronozones. This approach implies that in continuous records the horizon’s boundaries coincide with boundaries of systems, stages and, under certain conditions, substages. At the same time, horizon bears an imprint of its regional rank. It is for this reason that the Stratigraphic Code notifies that recognized horizons may or may not cover the whole stratigraphic section exposed in the region. This brings up some questions: — What to do in this case with medium- and small-scale mapping involved with a section subdivided uniformly throughout? — What to do with hiatuses in the sections owing to gaps in sedimentation? Surveyors and cartographers try to answer the first question. The answer to the second question is given in Fig. 3, a, which schematically shows a fragment of the continuous section divided into horizons A, B, and C meeting the requirements of the Stratigraphic Code. Figure 3, b shows the case when some part of the record is missing because of sedimentation gap. The hiatus is delimited in the reconstructed section by diachronous surfaces one of which belongs to the underlying and the other to overlying part of the section. I refer such boundaries to the allostratigraphic type (Tsyganko, 1994). If biostratigraphic data evidence that the corresponding section’s intervals belong to different horizons, adjacent (B′ and C′) as in Fig. 3, b or with one (B′, D′) and more (B′, E′, etc.) horizons missed, the considered intervals may and must be distinguished as independent valid horizons. But taking into account that these horizons have only one isochronous (stratigraphic s.s.) boundary, the other being allostratigraphic, and that they have reduced volumes, we propose to consider them incom-
Fig. 3. Types of horizons depending on their completeness. 1, 2 — boundaries: 1 — isochronous (stratigraphic s.s.), 2 — allostratigraphic; 3 — hiatus. A-(E′) — horizons: A, B, C, A′ — complete; B′, C′, (D′, E′) — incomplete; a, b — see text for explanation.
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plete horizons (B′, C′, D′, E′, etc.). The boundaries of complete horizons (A, B, C, A′...) are both isochronous. Thus, analysis of many stratigraphic schemes, including those published recently, and inspection of sections in the field evidence that horizon is a specific stratigraphic unit very important for correlating deposits of different facies. Depending on their structure, two types can be distinguished among horizons. Type I — complete horizons with lower and upper isochronous boundaries. A possibility is in sight that some of these horizons can be considered regional stages. Type II — incomplete horizons, which has only one isochronous boundary while the other, upper or lower, is allostratigraphic. Thus, the following emended definition of stratigraphic horizon can be proposed. Horizon is a regional (subregional) stratigraphic unit that correlates different facies deposits of other regional and local stratons or their parts. In volume it fits chronozone(s) or its (their) parts and has, at least, one boundary of strictly stratigraphic type. It matches a sedimentary basin or its part. The stratigraphic unit horizon is absent from codes of other nations. The International Stratigraphic Code (International..., 2002) also misses it. The USSR and Russia followed their own path. The future of the term horizon depends on how accurately and efficiently this straton will be used in geological practice.
Conclusions Thirty-five years ago, the patriarch of Soviet and Russian stratigraphy B.S. Sokolov (1971) indicated that stratigraphy is lacking in irrefutable logic, a steady system of axioms, and well-proven ideas of principles of stratigraphic classification. For the past period stratigraphy as a science made considerable progress: Its general principles were formulated, problems of stratigraphic division were solved with respect of their types, substantiation, spatial extent, and methods of correlation. Considerable progress was noted with respect to the most urgent problem — stratigraphic boundaries. For most supracrustal shell of the Earth’s crust is has been methodologically solved, which is reflected in the International Stratigraphic Chart for some Precambrian and Phanerozoic systems. Boundaries of stages are fixed by a “gold nail” — the Global Boundary Stratotype Section and Point (GSSP). Nevertheless, despite evident progress, stratigraphy still faces many problems. This paper reports some approaches to their solution. First of all, it deals with the typification of stratigraphic units on the basis of types of their boundaries (Tsyganko, 1994). To consider formations and horizons from this point of view is dictated by the practical interest to these stratons of all geologists engaged into division and correlation of sedimentary, volcanic and, partly, metamorphic rocks. At first glance, methodological approach to the problem of boundaries of formations is rather simple: The problem is reduced to search for a distinctly observable change in
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lithology, facies change, sedimentation and denudation gaps, and, in some cases, for drastic changes in faunal and floral associations. Virtually, three basic types of boundaries that characterize formations have been established: stratigraphic s.s., parastratigraphic, and allostratigraphic. It is most difficult to diagnose allostratigraphic boundaries of formations where stratigraphic boundaries are hidden. A certain problem is also to reveal formations in localities with anomalous facies change of their lithological characteristics. As a rule, these are intermontane depressions and mountainous lands. In the mountains well-expressed primary facies change is supplemented by tectonic convergence of sections. My proposal to document formations well deviated from type sections by distinguishing grades is my reaction to this procedure practiced under different names (facies, gradation, etc.) and, to a certain extent, an invitation to discuss this problem. Horizons are derivatives of formations (or their parts) they correlate but, unlike the latter, they are based first of all on biostratigraphic correlation criteria. Laterally, they can cover considerable areas of ancient basins. When the sole of horizon is diachronous because the basin gradually transgressed onto the uplifting eroded surface of land or because the roof of the straton’s sediments was subjected to denudation, the main correlative is its other boundary — upper in the first case and lower in the second. The use of these intervals of sections as horizons allows the correlative potential of this stratigraphic unit to be used to a greater extent in division and correlation of sections, first of all, on the territories with a complex geologic history. I thank I.V. Budnikov and E.A. Yolkin for valuable advice and criticism improving the manuscript.
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