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Abstract

Stratigraphy, from the Latin stratum and the Greek graphia, has traditionally been considered the descriptive science of rock strata. In the last few decades, the critical value to stratigraphy of the information provided by nonlayered rock bodies—sedimentary as well as intrusive igneous rocks and massive metamorphic rocks of undetermined origin—has become evident. Non-layered rock bodies not only are the source of geochronometric (numerical) ages determined by isotopic methods, but they also provide crucial age information through the establishment of their cross-cutting and boundary relationships with layered and/or nonlayered rocks with which they are associated. The definition of stratigraphy should, therefore, be broadened to include the description of all rock bodies forming the Earth's crust and their organization into distinctive, useful, mappable units based on their inherent properties or attributes. Stratigraphic procedures include the description, classification, naming and correlation of these units for the purpose of establishing their relationship in space and their succession in time. As such, stratigraphy is concerned not only with the original succession and age relations of rock bodies, but also with their distribution, lith-ologic composition, fossil content, and geophysical and geochemical properties—indeed, with all observed properties and attributes of rock bodies and their interpretation in terms of environment or mode of origin and of geologic history. All classes of rocks—igneous and metamorphic as well as sedimentary, unconsolidated as well as consolidated—fall within the general scope of stratigraphy and stratigraphic classification.

Certain definitions of general significance and discussions of procedures pertinent to all kinds of stratigraphic units are assembled in this chapter to avoid unnecessary repetition in the chapters that follow dealing with specific kinds of stratigraphic units.

Definitions

Stratigraphy.

Stratigraphy, from the Latin stratum and the Greek graphia, has traditionally been considered the descriptive science of rock strata. In the last few decades, the critical value to stratigraphy of the information provided by nonlayered rock bodies—sedimentary as well as intrusive igneous rocks and massive metamorphic rocks of undetermined origin—has become evident. Non-layered rock bodies not only are the source of geochronometric (numerical) ages determined by isotopic methods, but they also provide crucial age information through the establishment of their cross-cutting and boundary relationships with layered and/or nonlayered rocks with which they are associated. The definition of stratigraphy should, therefore, be broadened to include the description of all rock bodies forming the Earth's crust and their organization into distinctive, useful, mappable units based on their inherent properties or attributes. Stratigraphic procedures include the description, classification, naming and correlation of these units for the purpose of establishing their relationship in space and their succession in time. As such, stratigraphy is concerned not only with the original succession and age relations of rock bodies, but also with their distribution, lith-ologic composition, fossil content, and geophysical and geochemical properties—indeed, with all observed properties and attributes of rock bodies and their interpretation in terms of environment or mode of origin and of geologic history. All classes of rocks—igneous and metamorphic as well as sedimentary, unconsolidated as well as consolidated—fall within the general scope of stratigraphy and stratigraphic classification.

Stratum (plural = strata).

A geologic stratum is a layer (a generally tabular body) of rock characterized by certain lithologic properties or attributes that dis-tinguish it from adjacent layers. Adjacent strata may be separated by visible planes of bedding or by less perceptible boundaries of change in lithology.

Stratigraphic Classification.

The systematic organization of the Earth's rock bodies, as they are found in their original relationships, into units based on any of the properties or attributes that rocks may possess. Many different properties and attributes of the rocks may serve usefully as the bases for stratigraphic classification, and consequently there are many different categories of stratigraphic classification.

Stratigraphic Unit.

A body of rock recognized as a unit (distinct entity) in the classification of the Earth's rocks, based on any of the many properties or attrib-utes that rocks possess. Stratigraphic units based on one property will not necessarily coincide with those based on another. It is, therefore, essential that different terms be used for each so that their named units can be distinguished from each other. Clear definition of a stratigraphic unit is of paramount importance.

Stratigraphic Terminology.

The total of unit-terms used in stratigraphic classification, such as formation, stage, biozone. Stratigraphic terminology is either formal or informal.

Formal

stratigraphic terminology uses unit-terms that are properly defined and named according to an established or conventionally agreed scheme of classification; for example, Chonta Formation, Cretaceous System. The initial letter of the rank- or unit-term of named formal units is capitalized* (see section 3.B).

Informal

stratigraphic terminology uses unit-terms only as ordinary nouns without the unit's necessarily being named and without its being a part of a specific scheme of stratigraphic classification; for example, a chalky formation, the sandy zone, an oyster bed. The initial letter of an informal unit-term is printed in lowercase.

Informal terms may be appropriately used for aquifers, oil and/or gas reservoirs, coal beds, quarry layers, units based on wireline logs or reflection seismic profiles, etc. as long as it is clear that they are not formal terms, i.e., “third coal,” “B6 sandstone.” However, the informal use of formal stratigraphic terms (formation, member, biozone, series, etc.) in published documents is strongly discouraged. In the course of surface or subsurface mapping, all geologists use provisional informal terms. They are essential. But when the results of the investigation are published, it is preferable not to introduce informal terms in print unless they are formally proposed and described. Otherwise, a term intended to be informal by its originator—“the limestone at Blue Mountain,” the “Stony River granite,” or the “Victoria sandstone formation”—may sooner or later be changed to “Blue Mountain Limestone,” “Stony River granite,” or “Victoria Formation” by a subsequent author or editor. Then the intended informal term becomes formal, even though it has not been properly proposed and defined. Furthermore, in conversations and oral presentations, it is not clear if a unit-term is being capitalized or not.

If a stratigraphic unit merits a name, it merits proper definition and description. Adequate definition and description impose a discipline on a worker that reduces the risk of unsound units being created, leading to confusion in the literature.

Stratigraphic Nomenclature.

The system of proper names given to specific stratigraphic units; for example, Trenton Formation, Jurassic System, Dibuno-phyllum Range Zone.

Zone.

A stratigraphic unit in many different categories of stratigraphic classification. Thus there are many kinds of zones, depending on the stratigraphic properties under consideration—lithozones, biozones, chronozones, mineral zones, metamorphic zones, magnetic-polarity zones, and so on. When used formally, the term zone is given an initial capital letter (Zone) to distinguish it from its informal use (see section 3.A.5.b). The kind of zone being used should be clearly indicated.

Horizon.

A stratigraphic horizon is an interface indicative of a particular position in a stratigraphic sequence. In practice, the term “horizon” has often been applied to a distinctive very thin bed. There may be many kinds of stratigraphic horizons, depending on the stratigraphic properties involved—lithohorizons, biohorizons, chronohorizons, seismic horizons, electrolog horizons, and so on. Stratigraphic horizons include not only the boundaries between two stratigraphic units, but also specific markers within these units that may be particularly useful for correlation purposes.

Correlation.

To correlate, in a stratigraphic sense, is to show correspondence in character and/or in stratigraphic position. There are different kinds of correlation depending on the feature to be emphasized. Lithologic correlation (lithocorrelation) demonstrates correspondence in lithologic properties and lithostratigraphic position; a correlation of two fossil-bearing beds (biocorrelation) demonstrates correspondence in their fossil content and in their biostratigraphic position; and chronocorrelation demonstrates correspondence in age and in chronostratigraphic position.

Geochronology.

The science of dating and determining the time sequence of events in the history of the Earth.

Geochronologic Unit.

A unit of geologic time (time determined by geologic methods). It is not a body of rocks and, therefore, not a stratigraphic unit, although it may correspond to the time span of a stratigraphic unit (see section 2.D).

Geochronometry.

The branch of geochronology that deals with the quantitative (numerical) measurement of geologic time, usually in thousands or millions of years. The abbreviations ka for thousand (103), Ma for million (106) and Ga for billion (milliard or thousand million, 109) years are now generally used to express the length of time before the present (years ago), not the duration of a past geological time interval.

Facies.

In stratigraphy, the term fades can mean aspect, nature, or manifestation of character (usually reflecting conditions of origin) of rocks or specific constituents of rocks. Probably no geological term has been used more indis-criminately for a wider range of concepts than “facies.” As defined by Gressly (1838) the term “facies” was meant to express lateral change in lithologic aspect. Its use has been broadened, however, to express environment of deposition or formation (deltaic facies, marine facies, volcanic facies, shallow-water facies), lithologic composition (sandstone facies, limestone facies, red-bed facies), geographic or climatic association (Tethyan facies, boreal facies, tropical facies, Germanic facies), fossil content (graptolitic facies, shelly facies), tectonic association (orogenic facies, geosynclinal facies), and metamorphic grade. “Facies” is also used as a noun for a rock body distinctive in aspect, appearance, or char-acter. If the term “facies” is used, it is desirable to make clear the specific kind of facies to which reference is made: lithofacies, biofacies, metamorphic facies, tectonofacies, etc.

Caution Against Preempting General Terms for Special Meanings.

The preempting of general terms for special restricted meanings has been a source of much confusion and responsible for many of the controversies in stratigraphic terminology. For example, “stratigraphy” should not be confined to age relations of strata or rock bodies; “correlation” is not necessarily rime-correlation; “geochronology” should not refer exclusively to isotopic dating; “zone” can be applied to other than fossil zones; a “biozone” is not a specific kind of biostratigraphic zone; and “interval” may refer to either time or space intervals. The preferable procedure is to conserve the original general meaning of a term and to seek a more precise and less ambiguous word for the special meaning.

Procedures for Establishing and Revising Stratigraphic Units

The proposal of a new formal stratigraphic unit requires a statement of intent to introduce the new unit and the reasons for doing so. To be valid and useful, a new unit must be duly proposed and duly described. The proposal should include:

  • a clear and complete definition, characterization, and description of the unit so that any subsequent investigator can recognize it unequivocally.

  • the proposal of the kind, name, and rank of the unit.

  • the designation of a stratotype (type section) or type locality on which the definition and description of the unit is based.

The proposal should be published in a recognized scientific medium.

The revision and redefinition of a previously proposed and named unit requires a statement of intent to revise the unit, the reasons for doing so, and a discussion of the history of the unit—author, original reference, previous treatment. If necessary, it should include a comprehensive description of the unit and designation of a new type section or type locality (or revision of the old one). The revision of a unit, to be valid, must also be published in a recognized scientific medium.

Definition, Characterization, and Description.

In general, the comprehensive definition, characterization, and description of a new stratigraphic unit should include a clear account of its boundaries, diagnostic properties, and attributes. For lithostratigraphic and biostratigraphic units, emphasis should be on lithologic and paleontologic properties respectively. For unconformity-bounded units, emphasis should be placed on the description of the bounding unconformities. For chronostratigraphic units, emphasis should be placed on features bearing on age and time-correlation.

In addition, the definition, characterization, and description should include the following topics as pertinent:

Name

(see section 3.B.3).

Stratotypes and Other Standards of Reference.

Geographic and geologic designation of stratotypes (type sections) or type localities is essential in establishing a new stratigraphic unit. The geographic designation should include detailed location maps and indication of the means of access to the type section or type locality (see section 4.C.2). Also important are provisions for artificial markers (see section 4.C.3) and the relation of boundary-stratotypes to bound aries of other stratigraphic units and to other significant horizons in the section (see Chapter 4 on stratotypes and type localities).

For units of the type for which it is impractical to utilize stratotypes as standards, reliance is placed on the accurate description and illustration of those features that constitute diagnostic criteria of reference for the unit, for example, biostratigraphic units (see section 7.F).

Description of Unit at Stratotype or Type Locality.

The description is a statement summarizing the total content of a stratigraphic unit. It should include the lithologic properties, the thickness or size of the rock body, biostratigraphic character, structural attitude, geomorphic expression, unconformities or hiatuses, nature of the boundaries of the unit (sharp, transitional, unconformable, etc.), and distinguishing and identifying features characterizing the unit at the strato type or type locality.

Regional Aspects.

Geographic extent; regional variations in thickness, in lithostratigraphic, biostratigraphic, or other properties, or in geomorphic expression; regional stratigraphic relations; relations to other kinds of stratigraphic units; nature of the boundaries away from the type (sharp, transitional, uncon-formable, etc.); relations of the boundaries of the unit to the boundaries of other stratigraphic units of the same or different kind; criteria to be used in identifying and extending the unit geographically away from the stratotype or type locality.

Geologic Age.

Relative age in terms of position within the Standard Global Chronostratigraphic (Geochronologic) Scale and/or numerical age determined or inferred by isotopic or other methods.

Correlation with Other Units. Discussion of the kind of correlation and information on which it is based.

Genesis (where appropriate).

Conditions of origin of the rocks of the unit; significance with respect to paleogeography or geologic history.

References to the Literature.

Special Requirements for Establishing Subsurface Units.

Many useful stratigraphic units are based on subsurface (well, mine, or tunnel) sections, and many more subsurface units will doubtless be proposed as the sediments of offshore areas become better explored. If adequate sample information is available, such subsurface sections can be used legitimately for proposing, defining, and describing new stratigraphic units (see section 4.C.5). The same general rules of procedure used for outcrop sections apply to subsurface units defined on the basis of exposures in mines or tunnels or from sections penetrated in wells. In proposing a new subsurface unit, the well or mine in which the type section is present becomes the type locality. In well sections, stratotypes need to be designated by well depths and on well logs rather than by markers at the surface, and geological information for these stratotypes will be based largely on well samples and well logs. Subsurface parastratotypes and hypostratotypes (see section 4.B.7) may be useful in supplementing poorly exposed surface stratotypes or type localities. The following data are desirable for establishing subsurface units:

Designation of Well or Mine.

Name of type well or mine; location of the well or mine by written description, map, exact geographic coordinates, farm or lease block, or any other geographic feature suitable for location identification; name of the operating agency or individual. For mines: depth level where exposed. For wells: date of drilling, total depth, and surface elevation. If all the data needed to establish a stratotype or type locality cannot be furnished from one well, two or more wells should be used, with one being designated as the holostratotype and the others as parastratotypes or hypostratotypes.

Geologic Logs.

Lithologic and paleontologic logs of the well or wells, and maps and cross sections of the mine, in written and graphic form are needed. The boundaries and subdivisions of a new unit should be indicated clearly on logs or charts.

Geophysical Logs and Profiles.

Electrical or other wireline logs (preferably for several adjacent wells) and seismic profiles are desirable. The bound-aries and subdivisions of the unit should be marked and shown at a scale large enough to permit full appreciation of detail.

Depositories.

It is essential that sets of cores, cuttings or other samples, fossil material, logs, and so on covering the type section of a subsurface strati-graphic unit be readily available for study. Such material should be placed at geological surveys, universities, museums, or other institutions with proper curatorial facilities. The location of the depository should be given.

Naming of Stratigraphic Units.

The names of formal stratigraphic units are compound. For most categories, they should consist of a geographic name combined with an appropriate term indicating the kind and rank of the unit (La Luna Formation, Permian System, Pyongan Synthem) or descriptive term (Austin Chalk). The derivation of the geographic component of the name should be explained, and the kind and rank of the unit should be specified.

The formal name of a biostratigraphic unit is formed from the names of one or more appropriate fossils combined with the appropriate term for the kind of biostratigraphic unit (Exus albus Assemblage Zone). The choice of the fossil or fossils selected to name a biostratigraphic unit should be discussed. (For a more detailed discussion of the naming of biostratigraphic units, see Chapter 7, particularly section 7.H.)

Some chronostratigraphic units of the Standard Global Chronostratigraphic Scale bear long-established nongeographic names of diverse origin (Cretaceous System, Tertiary System, Triassic System).

The name of a new stratigraphic unit should be unique. Therefore, before attempting to establish a new formal stratigraphic unit, authors should refer to national, state, or provincial records of stratigraphic names to determine whether a name has been used previously. The many volumes of the IUGS Lexique Stratigraphique International and other appropriate national or regional lexicons constitute valuable reference sources for most countries.

Geographic Component of Names of Stratigraphic Units.
Source.

Geographic names should be derived from permanent natural or artificial features at or near which the stratigraphic unit is present. Names de-rived from such impermanent sources as farms, churches, schools, crossroads, and small communities are not entirely satisfactory but are acceptable if no others are available. Appropriate names may be selected from those shown on state, provincial, county, forest service, topographic, hydrographic, or comparable maps. If a name that does not meet this test must be used, the place from which the name is derived should be described and identified precisely and shown on a map accompanying the description of the new unit. Use of such a geographicname for a stratigraphic unit should be subject to approval by the national organization for place-names.

The geographic name after which a stratigraphic unit is named should not be changed or abbreviated (e.g., “San Cayetano” or “El Consuelo” should not be changed to “Cayetano” and “Consuelo”). On the other hand, the feature term of a geographic name, e.g., river, lake, mountain, village, should be omitted from the name of stratigraphic units, unless required to distinguish between two otherwise identical names (e.g., Redstone Formation and Redstone River Formation). Short, concise names are preferable to long compound names. A unit should not be named from the source of its components; for example, a till deposit suppos-edly derived from the Keewatin glaciation center should not be called “Kee-watin Till.” The name of a high-ranking unit may appropriately, though not necessarily, be derived from a more extensive geographic feature or area than names of its lower-ranking components.

Spelling of Geographic Names.

Spelling of the geographic component of the name of a stratigraphic unit should conform to the usage of the country that contains the geographic locality from which the name has been taken. However, a stratigraphic name repeatedly published with a spelling different from that of its geographic source should nevertheless be retained. For example, Bennett Shale, uniformly used for many years, should not be altered to Bennet Shale on the grounds that the town is named Bennet. Stratigraphic names that have been spelled variously should be made uniform by adopting the spelling used by the most authoritative local geographical and geological sources. The geographic component of a name should not be altered by translation into another language. For example, Cuchillo should not be translated to Knife, La Pena should retain the tilde, and Canyon should not be translated to Canon. It is proper, however, to translate the lithologic term or rank term. Thus, the Edwards Limestone may be called Caliza Edwards, and Formation La Casita may be called the La Casita Formation; or Redkinskaya Svita may be called Redkino Formation (but no Redkinskaya Formation).

Changes in Geographic Names.

Change in the name of a geographic feature does not entail change of the corresponding name of a stratigraphic unit. The original name of the unit should be maintained; e.g., Mauch Chunk Shale should not be changed to Jim Thorpe Shale because the former town of Mauch Chunk is now called Jim Thorpe.

Disappearance of a geographic feature does not require the elimination of the corresponding name of a stratigraphic unit. For example, Thurman Sandstone, named from a former village in Pittsburgh County, Oklahoma, does not require renaming.

Inappropriate Geographic Names.

A name that suggests some well-known locality, region, or political division should not, in general, be applied to a unit typically developed in another less well-known locality of the same name. For example, it would not be advisable to use the name “Chicago Formation” for a unit in California, or the “London Formation” for a unit in Wales, even though localities with these names do exist in California and in Wales. v.Duplication of Geographic Names. Duplication of geographic names should be avoided. A name previously applied to any unit should not later be applied to another unit unless geographic separation precludes confusion. Although the same geographic term has been applied in some cases to different categories of stratigraphic units (Oxford Clay for a lithostratigraphic unit and Oxfordian Stage for a chronostratigraphic unit), the practice should be discouraged.

Names for Subdivisions of Stratigraphic Units.

When a unit is divided into two or more formal component units, the geographic name of the original unit should not be employed for any of the subdivisions; for example, the Astoria Group should not contain an Astoria Formation, nor the Germav Formation a Germav Member.

Unit-term Component of Names of Stratigraphic Units.

The unit-term component of the name of a stratigraphic unit indicates the kind and rank of the unit. The unit-term “formation,” for example, indicates that the unit is a litho-stratigraphic unit, “taxon-range biozone” that the unit is a biostratigraphic unit, “stage” that it is a chronostratigraphic unit, etc. A stratigraphic unit-term may differ in various languages (stage, etage, Stufe, piano, piso), or it may be very nearly the same (system, systeme, sistema). If a useful term is difficult to translate into a particular language, it may be desirable to “borrow” the term from the language of its origin, for example, range zone from English. Stratigraphic terms with Greek or Latin roots are desirable because they are understood in a wide range of languages, for example, chronozone.

Relation of Names to Political Boundaries.

Stratigraphic units are not limited by international frontiers, and effort should be made to use only a single name for each unit regardless of political boundaries.

Reduction in Number of Names Through Correlation.

If correlation has established the identity of two named stratigraphic units, the later name should be replaced by the earlier, other considerations being equal, in the interest of simplicity in nomenclature.

If a named subsurface unit can be correlated with a named surface unit of the same kind, and if the characteristics of both are so similar that the two names are unnecessary, preference should generally be given to the name of the surface unit. But other factors should also be considered, such as priority of publication, usage, completeness of section, accessibility, nature of exposures in surface sections, and availability of type material from subsurface sections.

Uncertainty in Assignment.

If there is uncertainty with respect to the assignment of rock bodies to one or the other of two named units, it is always better to express this doubt rather than to make an arbitrary assignment. The following conventions may be used:

     
  • Devonian?

    = doubtfully Devonian

  •  
  • Formation Peroc-Macoa

    = doubtfully Macoa Formation

  •  
  • formation

    = strata intermediate in position (horizontally or vertically) between beds assuredly assigned to either of the two formations, which share the characters of both but which cannot be assigned decisively to either one and may eventually be made into a new formation

  •  
  • Silurian-Devonian Silurian

    = one part Silurian and one part Devonian

  •  
  • or Devonian Silurian and Devonian

    = questionably either Silurian or Devonian both Silurian and Devonian but no distinction (undifferentiated) yet possible between the two

The name of the older or lower unit, if this distinction can be made, should always come first when two units are hyphenated or used in combination.

Abandoned Names.

The name of a stratigraphic unit, once applied and then abandoned, preferably should not be revived except in its original sense. When it seems useful to refer to an obsolete or abandoned formal name, its status should be made clear by referring to it as such or by using such a phrase as “Mornas Sandstone of Hebert (1874).” To determine if a name has been abandoned or is obsolete, authors should refer to national, state, or provincial stratigraphic lexicons.

Preservation of Traditional and Weil-Established Names.

Although it is strongly urged that all new stratigraphic units be named according to the recom-mendations of this Guide, it is realized that there are many well-established and traditionally used stratigraphic units, particularly lithostratigraphic units, of long historical standing for which exception should be made. Examples are Millstone Grit, Kupferschiefer, Tea-green Marl, Belemnite Marls, and many others (see Lawson, 1979a, 1981). Such units should not be abandoned merely because they lack geographic names. Tolerance and flexibility are advised. It is suggested that national stratigraphic bodies make recommendations concerning the conservation of such units under their original names, but it is also recommended that detailed definitions, characterizations, and descriptions be published, and specific stratotypes be designated, as in the case of newly defined units.

Publication

A Recognized Scientific Medium.

Establishment of a formal stratigraphic unit, or the revision of an existing unit, requires publication of a statement of intent and an adequate description of the unit in a recognized scientific medium. It is difficult to define what constitutes “a recognized scientific medium.” The chief qualifications are scientific purpose and availability to the scientific public on request by purchase or through an accessible library. Regularly issued scientific journals meet this requirement. Many independent or irregularly issued publications also meet it, although in such cases some notice of the proposal should also appear in a widely circulated regularly issued scientific journal. Names proposed in informal or restricted media such as letters, company reports unavailable to the public, open-file releases, unpublished addresses, theses or dissertations, newspapers, and commercial or trade journals do not qualify. Publication of new stratigraphic names in abstracts issued in advance of complete reports usually does not establish these names, because the essential conciseness of abstracts does not permit adequate description. Informal reference, such as “the formation at Jonesville schoolhouse” or “the limestones cropping out near Centerville,” does not establish a new formal unit, nor does use in a table or columnar section or on a map.

Most fieldtrip guidebooks, with distribution limited to participants of the excursion, are not acceptable as a “recognized scientific medium.” However, some organizations publish and distribute widely large editions of serial guidebooks that include refereed regional papers. Although these publications do meet the test of scientific purpose and availability and, therefore, constitute valid publication, other media are preferable.

Priority.

Priority in publication of a properly proposed, named, and defined unit should be respected. However, the critical factors should always be the usefulness of the unit, the adequacy of its description, freedom from ambiguity, and suitability for widespread application. Priority alone does not justify displacing a well-established name by one not well known or only occasionally used; nor should an inadequately established name be preserved merely on account of priority.

Recommended Editorial Procedures.

This second edition of the Guide is written in the English language, and the editorial rules and procedures recommended here apply particularly to writing in that language. It is recognized that differing rules of orthography may make the recommendations inapplicable to writing in other languages.

Capitalization.

The first letters of all words used in the names of formal stratigraphic units should always be capitalized (except for the trivial terms of species and subspecies in the names of biostratigraphic units), for example, Bulimina-Bplivina Assemblage Zone, Brunswick Formation, Upper Cretaceous Series, Devonian System. Informal terms are not capitalized (except in those languages which require all nouns to be capitalized).

Hyphenation.

Compound terms for most kinds of stratigraphic units, in which two common words are joined to give a special meaning, should be hyphenated; for example, concurrent-range zone, normal-polarity zone. Exceptions are adjectival prefixes or combining forms which should generally be combined with the term-noun without a hyphen; for example, biozone, chronozone, sub-system, biohorizon, supergroup.

Repetition of Complete Name.

After the complete name of a stratigraphic unit has been referred to once in a description or discussion, part of the name subsequently may be omitted to avoid cumbersome repetition if the omission is compatible with clarity. For example, the Burlington Formation may be referred to as “the Burlington” or “the Formation” and the Oxfordian Stage as “the Oxfordian” or “the Stage.” The use of capitals for formal unit-terms when these are not coupled with a proper name is discretionary, depending on needs for clarity or emphasis.

Revision or Redefinition of Previously Established Stratigraphic Units.

Revision or redefinition of an adequately established unit without changing its name requires as much justification and the same kind of information as for proposing a new unit and generally requires the same procedures. Redefinition may be justified to make a unit more useful or easier to recognize, map, and extend throughout the area of its occurrence. Redefinition may also become desirable because of taxonomic changes of the fossil content of a biostratigraphic unit or because of errors in an earlier work. Names of long standing and common usage may be preserved even though their nomenclature may not conform to modern rules and procedures (see section 3.B.3.g).

Change in rank of a stratigraphic unit does not require redefinition of the unit or its boundaries or alteration of the geographic part of the name. Thus a stage may be raised to series rank or reduced to a substage, or a formation may be raised to a group or reduced to a member, without changing its name. For example, the Madison Formation has become the Madison Group.

The rank of any stratigraphic unit should be changed only for substantial reasons and after careful consideration. Changes in major chronostratigraphic units of international scope should be made only after consultation with appropriate stratigraphic organizations.

*Recommendations in the Guide for the capitalization of terms are made with reference to the English language. It is recognized that these may not be applicable for use in languages with different rules of orthography.

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