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Abstract

Chronostratigraphic units are bodies of rocks, layered or unlayered, that were formed during a specified interval of geologic time. The units of geologic time during which chronostratigraphic units were formed are called geochronologic units.

The relation of chronostratigraphic units to other kinds of stratigraphic units is discussed in Chapter 10.

Nature of Chronostratigraphic Units

Chronostratigraphic units are bodies of rocks, layered or unlayered, that were formed during a specified interval of geologic time. The units of geologic time during which chronostratigraphic units were formed are called geochronologic units.

The relation of chronostratigraphic units to other kinds of stratigraphic units is discussed in Chapter 10.

Definitions

Chronostratigraphy.

The element of stratigraphy that deals with the relative time relations and ages of rock bodies.

Chronostratigraphic Classification.

The organization of the rocks of the Earth's crust into units on the basis of their age or time of origin.

The purpose of chronostratigraphic classification is to organize systematically the rocks forming the Earth's crust into named units (chronostratigraphic units), corresponding to intervals of geologic time (geochronologic units), to serve as a basis for time-correlation and a reference system for recording events of geologic history. Specific objectives are as follows:

To Determine Local Time Relations.

Local time-correlation of strata and the simple determination of the relative age of strata in local sections or areas are important contributions to local or regional geology, regardless of any scheme of organization of strata into named chronostratigraphic units of world- wide application.

To Establish a Standard Global Chronostratigraphic Scale.

This scale is a complete and systematically arranged hierarchy of defined and named chronostratigraphic units, of both regional and worldwide application. Such a hierarchy serves as a standard framework for expressing the age of rock bodies and for relating all rocks to Earth history. The named units composing each rank in this Standard Global Chronostratigraphic Scale encompass, as a whole, the entire stratigraphic sequence, without gaps and without overlaps.

Chronostratigraphic Unit.

A body of rocks that includes all rocks forming during a specific interval of geologic time and only those rocks formed during that time span. Chronostratigraphic units are bounded by isochronous horizons. The rank and relative magnitude of the units in the chronostratigraphic hierarchy are a function of the length of the time interval that their rocks subtend, rather than of their physical thickness.

Chronostratigraphic Horizon (Chronohorizon).

A stratigraphic surface or interface that is isochronous, everywhere of the same age. Although a chronohorizon is theoretically without thickness, the term chronohorizon has been commonly applied also to very thin and distinctive intervals that are essentially isochronous over their whole geographic extent and thus constitute excellent time-reference or time-correlation horizons. Chronohorizons have also been called markers, marker horizons, marker beds, key beds, key horizons, datums, levels, time-surfaces, and so on. Examples of horizons that may have strong time significance include many biohorizons, bentonite beds resulting from volcanic ash falls, tonsteins, phosphorite layers, horizons of magnetic polarity reversal, coal beds, some electric-log markers, seismic reflectors, etc., but these are not chronostratigraphic horizons. The geochronologic equivalent of a chronohorizon is a moment (or an instant, if it has no resolvable time duration on a geologic scale).

Kinds of Chronostratigraphic Units

Hierarchy of Formal Chronostratigraphic and Geochronologic Unit-terms.

The Guide recommends the following formal chronostratigraphic terms and geochronologic equivalents to express units of different rank or time scope (Table 3).

Table 3.

Conventional Hierarchy of Formal Chronostratigraphic and Geochronologic Terms

ChronostratigraphicGeochronologic
EonothemEon
ErathemEra
System*Period*
Series*Epoch*
StageAge
SubstageSubage or Age
ChronostratigraphicGeochronologic
EonothemEon
ErathemEra
System*Period*
Series*Epoch*
StageAge
SubstageSubage or Age

*If additional ranks are needed, the prefixes sub and super may be used with these terms.

Several adjacent stages may be grouped into a superstage (see Section 9.C.3).

Position within a chronostratigraphic unit should be expressed by adjectives indicative of position, such as basal, lower, middle, upper, uppermost; whereas position within a geochronologic unit requires a temporal adjective, such as earliest, middle or medial, late, latest.

Stage (and Age).

The stage has been called the basic working unit of chronostratigraphy because it is suited in scope and rank to the practical needs and purposes of intraregional chronostratigraphic classification. Furthermore, it is the smallest unit in the standard chronostratigraphic hierarchy that can be recognized at a global scale.

Definition.

The stage is a chronostratigraphic unit of relatively minor rank in the conventional hierarchy of formal chronostratigraphic terms, representing a relatively minor interval of geologic time. Its geochronologic equivalent is known as an age.

Stages may be subdivided into substages and are subdivisions of a series (see section 9.C.3).

Boundaries and Stratotypes.

A stage is defined by its boundary-stratotypes (the Global Boundary Stratotype Section and Points, or GSSPs, of Cowie et al., 1986) (see section 9.H).

The boundary-stratotypes of a stage should be within sequences of essentially continuous deposition, preferably marine (except in cases such as the stages based on mammalian faunas in regions of nonmarine Tertiary sequences or the Quaternary glacial stages). Both should be associated with distinct marker horizons, such as biozone boundaries or magnetic polarity reversals, that can be readily recognized and widely traced as time-significant horizons. The stage boundaries as they are extended away from the boundary-stratotypes are in principle isochronous. In attempting to determine and extend such isochronous surfaces it is desirable to use as many indicators of time-correlation as possible. For example, it may be desirable to utilize not one but several biostratigraphic zones.

If major events in the geological development of the Earth can be identified at specific points in sequences of continuous deposition, these may constitute desirable points for the boundary-stratotypes of stages. The selection of the boundaries of the stages of the Standard Global Chronostratigraphic Scale deserves particular emphasis because such boundaries serve to define not only the stages but also chronostratigraphic units of higher rank, such as series and systems of which stages are components.

Time Span.

The lower and upper boundary-stratotypes of a stage represent specific moments in geologic time, and the time interval between them is the time span of the stage. Currently recognized stages are variable in time span, but most range from 2 to 10 million years as indicated by isotopic age determinations. The thickness of any one stage may vary from place to place, depending on the local rate of rock accumulation and the degree of its preservation; stages may range from a few meters to many thousands of meters.

Name.

The name of a stage should preferably be derived from a geo-graphic feature in the vicinity of its stratotype or type area. Conventionally, most stages have been given geographic names. Many currently used stages have names derived from those of the lithostratigraphic units on which they were originally based in their type areas; others have been given names unrelated to other stratigraphic units. In English, the adjectival form of the geographic name is used with an ending in “ian” or “an”; for example, Burdigalian Stage, Cenomanian Stage, Famennian Stage, Zanclean Stage. An age takes the same name as the corresponding stage.

Substage and Superstage.

The substage is a subdivision of a stage. Some stages have been divided completely into formally named substages; others have had only certain parts designated as substages. The geochronologic equivalent of a substage might be termed a subage, but preferably is called simply an age. A substage is defined by boundary-stratotypes.

Several adjacent stages may be grouped into a superstage. Restraint is rec-ommended, however, in creating both substages and superstages to avoid complicating the nomenclature unnecessarily. Subject to the proper international sanction, it is often preferable to subdivide the original stage into two or more new stages and, if the need for the original larger unit remains, to make the original stage into a series including the new stages.

Names of substages and superstages follow the same rules as those of stages.

Series (and Epoch)

Definition.

The series is a unit in the conventional chronostratigraphic hierarchy, ranking above a stage and below a system. The geochronologic equivalent of a series is an epoch.

A series is always a subdivision of a system; it is usually but not always broken up into stages. Many systems have been divided into three series, but the number varies from two to six. Series commonly include from two to six stages. The terms superseries and subseries have been used only infrequently; for example, Senonian Subseries (or Superstage) of the Upper Cretaceous Series. Most series can be recognized worldwide, although some as yet have had only more restricted application.

Boundaries and Boundary-stratotypes.

Series are defined by boundarystratotypes (see section 9.H). If a series has been completely divided into stages, its boundaries should be the lower boundary of its oldest stage and the upper boundary of its youngest stage, or the lower boundary of the lowermost stage of the overlying series (see section 9.H.2). If stage subdivisions are not available, the series may be defined independently by its own boundary-stratotypes.

Time Span.

Most currently accepted series vary in time span from 13 to 35 million years. The time span of a series, if divided fully into stages, is the sum of the time span of its component stages.

Name.

A new series name should preferably be derived from a geographic feature in the vicinity of its stratotype or type area. Nevertheless, the names of currently recognized series which are of other origins should not be changed. The names of most series are derived from their position within a system (lower, middle, upper)—for example, Middle Devonian Series, Upper Cretaceous Series; other names have Greek word derivations—for example, Miocene; while others have geographic derivations—for example, Tournaisian Series. Many of the currently recognized series names of geographic origin have been given the ending “an” or “ian”; for example, Visean Series, Chesterian Series.

The epoch corresponding to a series takes the same name as the series except that the terms lower, middle, and upper applied to a series are changed to early, middle, and late when referring to an epoch. In both cases, these terms are capitalized when referring to a formal unit—for example, Lower Devonian, Early Devonian; but not for informal reference to chronostratigraphic or geochronologic position—for example, “in the lower part of the Devonian” and “early in Devonian time.”

Misuse of “Series.”

The term series has been frequently used incorrectly as a lithostratigraphic term more or less equivalent to a group and consisting of an alternation of lithologic types. This usage must be discontinued.

System (and Period)

Definition.

The system is a unit of major rank in the conventional chronostratigraphic hierarchy, above a series and below an erathem. The geochronologic equivalent of a system is a period.

Special circumstances have suggested the occasional need for subsystems and supersystems; for example, the Mississippian and Pennsylvanian Subsystems of the Carboniferous System.

Boundaries and Boundary-stratotypes.

As in the case of stages and series, the boundaries of a system are defined by boundary-stratotypes (see section 9.H). If a system is divided into series and stages, its lower boundary-stratotype is that of its oldest series or stage and its upper boundary-stratotype is that of the base of the overlying system (see section 9.H.2).

The boundaries of most recognized systems were at one time ill defined, uncertain, and controversial in varying degrees. This was due to inexact original definitions, to the discovery of gaps and overlaps at the level of what were previously thought to be the boundaries of adjacent systems, and to the lack of a universally accepted concept of what a system is and how its boundaries should be defined. During the last 30 years, ad hoc Working Groups of the IUGS International Commission on Stratigraphy have been engaged in the selection of stratotypes for the boundaries between the systems of the Standard Global Chronostratigraphic Scale and their component series and stages. Several have been selected, submitted to the Commission, and approved. The rest are under investigation (see sections 9.D and 9.H).

A primary step in refining the definition of a system is to decide just what stages and series are to be included in the system. The definition of these component stages and series then automatically defines the system and its boundaries.

The procedure of extending system boundaries geographically away from the type is the same as that for extending other chronostratigraphic horizons (see section 9.1).

Time Span.

The time span of a system can most readily be defined as the time span of the sum of its component series or component stages. The time spans of the currently accepted Phanerozoic systems range from 30 to 80 million years. An exception is the Quaternary System that has a time span of only 1.6 to 1.64 million years.

All of the generally accepted systems have a time span sufficiently great so that they serve as worldwide chronostratigraphic reference units. In fact, of all units of the chronostratigraphic scale, systems are probably the most widely recognized and the most widely used to indicate general chronostratigraphic position.

Name.

The names of currently recognized systems are of diverse origin. Some are indicative of chronologic position (e.g., Tertiary and Quaternary); others have a lithologic connotation (e.g., Carboniferous, Cretaceous); others are tribal (e.g., Ordovician, Silurian); and still others are geographic (e.g., Permian, Devonian). Likewise, they also bear a variety of endings, such as “an,” “ic,” and “ous.” There is no need to standardize the derivation or orthography of system names. The period takes the same name as the system to which it corresponds.

Certain stratigraphic units in parts of the world distant from western Europe are locally called “systems,” although they do not coincide with the so-called standard global systems and are somewhat larger in scope. Such are the Karoo “System” of South Africa (now called the Karoo Sequence) and the Gondwana “System” of India.

Erathem (and Era).

An erathem (from the Greek roots era and them, “the deposit of an era”) is the unit in the chronostratigraphic hierarchy above a system. The interval of geologic time corresponding to an erathem is an era.

Erathems have traditionally been named to reflect major changes in the development of life on the Earth: Paleozoic (old life), Mesozoic (intermediate life), and Cenozoic (recent life). Eras carry the same name as their corresponding erathems.

Eonothem (and Eon).

The term eon has been used for a geochronologic unit immediately above an era in rank. Logically, the chronostratigraphic equivalent would be an eonothem. Three eonothems are now generally recognized (see Table 4). One is the Phanerozoic Eonothem (time of evident life), which encompasses the Paleozoic, Mesozoic, and Cenozoic erathems. The other two eonothems are, from older to younger, the Archean and the Proterozoic. They correspond to what has been commonly called “Precambrian.” The eons take the same name as the corresponding eonothems.

Table 4.

Major Units of the Standard Global Chronostratigraphic (Geochronologic) Scale (1)

(1) A number of more detailed chronostratigraphic or geochronological scales have been published in the last 10 to 15 years including those of Palmer (1983) and Harland et al. (1982, 1990), referenced below, and the 1989 Global Stratigraphic Chart of the International Commission on Stratigraphy (Episodes, v. 12, no. 2).

(2) Palmer, A. R., 1983, The Decade of North American Geology 1983 Geologic Time Scale.

(3) Snelling, N. J., 1987, Measurement of geological time and the Geological Time Scale.

(5) In North America, in place of a Carboniferous System, two systems have been recognized: Mississippian System (older) and Pennsylvanian System (younger). These are also sometimes known as subsystems of the Carboniferous System.

Nonhierarchical Formal Chronostratigraphic Units—The Chronozone

Definition.

The chronozone is a formal chronostratigraphic unit of unspecified rank, not part of the hierarchy of chronostratigraphic units (eonothem, erathem, system, series, stage, substage). A chronozone is the body of rocks formed anywhere in the world during the time span of some designated stratigraphic unit or geologic feature. The corresponding geochronologic unit is the chron.

Time Span.

The time span of a chronozone is the time span of a previously designated stratigraphic unit or interval, such as a lithostratigraphic unit, a biostratigraphic unit, a magnetostratigraphic polarity unit, or any other body of rocks. For example, a formal chronozone based on the time span of a biozone includes all strata equivalent in age to the total maximum time span of that biozone regardless of the presence or absence of fossils diagnostic of the biozone (see Figure 13).

Figure 13.

Relation between Exus albus Chronozone and Exus albus Biozone. (Distribution of specimens of Exus albus shown by dot-pattern.)

Figure 13.

Relation between Exus albus Chronozone and Exus albus Biozone. (Distribution of specimens of Exus albus shown by dot-pattern.)

The chronozone based on the range of a certain taxon should, however, be clearly distinguished from the biozone based on the range of that same taxon (taxon-range zone). The loose use of the unqualified word “zone” for both has been the source of much confusion. The difference between the concepts of biozone and chronozone is illustrated in Figure 13. The Exus albus Biozone (a range zone) is limited in extent to the strata in which specimens of Exus albus occur. The Exus albus Chronozone (a chronostratigraphic unit) includes all strata, everywhere, of the same age as that represented by the total vertical range of Exus albus, regardless of whether specimens of Exus albus are present.

Chronozones may be of widely different time spans. Thus we may speak of the chronozone of the ammonites, which would include all strata formed in the long time interval during which the ammonites existed, regardless of whether the strata contained ammonites; or we may speak of the chronozone of Exus albus, a species of very limited time range; or we may speak of the chronozone of the São Tomé volcanic rocks, a unit of very local development but representing a relatively long interval of Cenozoic time, which would include all strata formed anywhere during this interval of time whether or not it is represented by São Tomé volcanic rocks.

If the unit on which the chronozone is based is of the type that has a designated stratotype (e.g., a lithostratigraphic unit), the time span of the chronozone may be defined in either of two ways: first, it may be made to correspond to the time span of the unit at its stratotype; in this case the time span of the chronozone would be permanently fixed. Second, the time span of the chronozone may be made to correspond to the total time span of the unit (which may be longer than that of the stratotype); in this case the known time span of the chronozone would vary with increasing information concerning the development of the unit. Where there is an appreciable difference between the time span of the stratigraphic unit at its stratotype and the total known time span of the unit, the definition of the chronozone should be explicit in designating one or the other as the reference; for example, chronozone of the type Barrett Formation or chronozone of the Barrett Formation. This is important because while one of the boundaries of a chronozone based on the stratotype of a stratigraphic unit might coincide with one of the boundaries of a stage or substage, the boundaries of a chronozone based on the total time span of a unit will vary in position with changes in information on the time span and diachronism of the unit and hence will not necessarily continue to coincide with the boundaries of the stage or substage although originally made to do so.

If the unit on which a formal chronozone is based is a unit of the type which cannot appropriately have a designated stratotype (such as a biostratigraphic range zone: taxon-range zone, concurrent-range zone), its time span cannot be defined permanently because the time span of the reference unit may change with increasing information about the time range of the diagnostic taxon or taxa on which the unit is based.

Geographic Extent.

The geographic extent of a chronozone is, in theory, worldwide, but its applicability is limited to the area over which its time span can be identified in the strata, which is usually less.

Name.

The chronozone takes its name from the stratigraphic unit on which it is based; for example, Exus albus Chronozone (derived from the Exus albus Range Zone), Barrett Chronozone (derived from the Barrett Formation). A chron takes the same name as its chronozone.

THE STANDARD GLOBAL CHRONOSTRATIGRAPHIC (GEOCHRONOLOGIC) SCALE

Concept.

A major goal of chronostratigraphic classification is the establishment of a hierarchy of chronostratigraphic units of worldwide scope, which will serve as a standard scale of reference for the dating of all rocks everywhere and for relating all rocks everywhere to world geologic history (see section 9.B.2).

All units of the standard chronostratigraphic hierarchy are theoretically worldwide in extent, as is their corresponding time span. However, the effective worldwide recognition of chronostratigraphic units and their boundaries decreases with increasing distance from their stratotypes or areas of definition because of limitations in the resolving power of long-range time correlation.

Present Status.

Table 4 shows the Standard Global Chronostratigraphic (Geochronologic) Scale of common current usage to which are added numerical ages taken from three of the most recent and more often used geologic time scales (Palmer, 1983; Snelling, 1987; Harland et al., 1990). Only the major units for which there is general agreement are shown. Agreement is not as prevalent concerning the names of chronostratigraphic and geochronologic units smaller than those shown in Table 4—series (epochs), and stages (ages)—even though considerable progress has been made since the publication in 1976 of the first edition of the International Stratigraphic Guide. Much of this progress should be credited to the work of the Subcommissions and Boundary Working Groups of the IUGS International Commission on Stratigraphy. But there is still much more to be done: the boundaries between some systems are still in dispute; some of the systems have been subdivided into units which some consider to be series and others consider to be stages; and the Precambrian (Archean and Proterozoic), representing a time span much greater than the entire Phanerozoic, has not yet been subdivided into worldwide chronostratigraphic units.

REGIONAL CHRONOSTRATIGRAPHIC SCALES

The units of the Standard Global Chronostratigraphic (Geochronologic) Scale are valid only as they are based on sound, detailed local and regional stratigraphy. Accordingly, the route toward recognition of uniform global units is by means of local or regional stratigraphic scales, particularly with respect to stages, and series. Moreover, regional units of this rank will probably always be needed whether or not they fit exactly into the standard global units. It is better to refer strata with accuracy to local or regional units rather than to strain beyond the current limits of time-correlation in assigning these strata to units of a global scale. Local or regional chronostratigraphic units should adhere to the same rules established for the units of the Standard Global Chronostratigraphic Scale.

Subdivision of the Precambrian

The Precambrian has been subdivided into arbitrary geochronometric units, direct subdivisions of geologic time without corresponding rock sequences to which they may be referred (Plumb, 1991), but it has not yet been subdivided into globally recognized chronostratigraphic units.

The largest of these geochronometric units are, from older to younger, the Archean and the Proterozoic, with their boundary conventionally placed at 2500 Ma. They are established, and rocks are assigned to them, on the basis of any method that permits numerical dating, principally isotopic age determinations. There is much less agreement concerning the subdivision of the Archean and the Proterozoic. Following Plumb (1991), the Archean and Proterozoic are shown in Table 4 as eons corresponding in rank to the Phanerozoic Eon. The Precambrian is considered a general term for that part of geologic time that preceded the Cambrian and the rocks formed during that time. The name “Precambrian” seems to have developed from the repeated use of “pre-Cambrian,” and for that reason is not a very appropriate term. It is, however, by far the most widely used name for time and rocks older than Cambrian.

There are prospects that chronostratigraphic subdivision of much of the Precambrian may eventually be attained through isotopic dating and through such other means as lithologic sequence, stromatolites, paleomagnetic signature, relation to volcanic or plutonic episodes, orogenic cycles, major climatic changes, geochemical events, and major unconformities. The basic principles to be used in subdividing the Precambrian into major chronostratigraphic units should be the same as for Phanerozoic rocks, even though different emphasis may be placed on the various means used for time correlation.

As in the Phanerozoic, the definition of chronostratigraphic units in the Precambrian as intervals between designated points in the rock sequence (boundary-stratotypes) leaves the way open to use all methods of time correlation; and although in the Precambrian heavy reliance will be placed on isotopic dating, there should remain the fixed standard of definition in the rocks for these units—one that will allow the use of all methods known at present, as well as new ones to come, for the extension and identification of the units.

In the Precambrian, as in the Phanerozoic, the logical procedure is first to build up local chronostratigraphy in appropriate areas, utilizing all possible guides to local time-correlation; then to proceed geographically from local to regional to continental to worldwide scope, as the means and evidence justify. Local chronozones of whatever rank, defined by boundary-stratotypes, will furnish units useful for local Precambrian history regardless of any worldwide scheme. They will also constitute the best possible foundation for regional, continental, and worldwide units if, as, and when these can be established with reasonable assurance.

Such a chronostratigraphic subdivision of the Precambrian may be usefully supplemented by a scheme of geochronometric units based on isotopic dating. However, such units may vary with corrections or changes in isotopic data and the development of new geochronometric techniques and cannot be considered to have as stable a base as chronostratigraphic units defined by boundary-stratotypes designated in the rocks.

Quaternary Chronostratigraphic Units

The basic principles to be used in dividing the Quaternary into chronostratigraphic units should be the same as for the older Phanerozoic rocks, although different emphasis may be placed on the various means (climatic, magnetic, isotopic, etc.) used for time-correlation. Carbon-14 dating has been particularly useful in the late Quaternary, the last 50 to 70 thousand years of Earth history.

Quaternary chronostratigraphic units, as other Phanerozoic chronostratigraphic units, are best defined and characterized as the intervals between designated boundary-stratotypes.

Procedures for Establishing Chronostratigraphic Units

See also section 3.B.

Boundary-stratotypes as Standards.

The essential part of the definition of a chronostratigraphic unit is the time span during which the unit described was formed. Since the only record of geologic time and of the events of geologic history lies in the rocks themselves, the best standard for a chronostratigraphic unit is a body of rocks formed between two designated instants of geologic time.

Such a body of rocks could be defined and characterized by a reasonably complete, designated outcrop through the entire unit, a unit-stratotype. Unfortunately such complete rock exposures are rare even for chronostratigraphic units of low rank, such as stages and substages. In addition, means of time-correlation are not good enough at present to make sure that no gaps or overlaps in time occur between the geographically distant unit-stratotypes of successive chronostratigraphic units.

As an example, a stage may have its type locality in one area and the adjacent under- or overlying stages may have their type localities in other areas (lefthand side of Figure 14). In such instances, there is a problem in making certain that the upper boundary of the unit-stratotype of one stage corresponds exactly with the lower boundary of the unit-stratotype of the next younger stage. Correlation of the boundary between two successive stages from the type area of one stage to that of the other may not be good enough to prevent gaps or overlaps between the type limits of the two successive stages.

Figure 14.

Advantage of defining stages by lower boundary-stratotypes, where type localities are widely separated, rather than by unit-stratotypes.

Figure 14.

Advantage of defining stages by lower boundary-stratotypes, where type localities are widely separated, rather than by unit-stratotypes.

For these reasons it is preferable to define and characterize a chronostratigraphic unit, of any rank, by two designated reference points in the rocks, the lower and upper boundary-stratotypes of the unit (righthand side of Figure 14).

The two boundary-stratotypes of a chronostratigraphic unit need not be part of a single section, nor need they be in the same locality. Both, however, should be chosen in sequences of essentially continuous deposition, even if this means establishing them within individual beds, since the reference points for the boundaries should represent points in time as specific as possible (see section 9.H.3).

The boundary-stratotypes between stages are selected so that certain ones serve also as the boundary-stratotypes between larger units (series, systems, etc.). The procedure lends itself to a hierarchical scheme of chronostratigraphic divisions with no gaps and no overlaps.

Advantage of Defining Chronostratigraphic Units by Their Lower Boundary-stratotypes.

If the stratotype of the boundary between two chronostratigraphic units (boundary-stratotype) could indeed be selected with assurance in a stratigraphic sequence of essentially continuous deposition, a single common (mutual) boundary stratotype would serve as the standard for the top of the lower chronostratigraphic unit and that of the base of the successive, younger unit.

However, since assurance of continuous deposition is unusual, it has been suggested (Geological Society of London, 1967; McLaren, 1977) that the definition of a chronostratigraphic unit should place emphasis in the selection of the boundary-stratotype of its lower boundary; its upper boundary is defined as the lower boundary of the succeeding unit. It has been pointed out (McLaren, 1977, p. 20) that in this manner, “should it subsequently be shown that the selected horizon is at a level of an undetected time break or hiatus, unrepresented by sedimentation in the section, then the time missing would, by definition, belong to the [lower unit].” The lower boundary-stratotypes of successive chronostratigraphic units define without ambiguity, therefore, their time spans.

Requirements for the Selection of Boundary-stratotypes of Chronostratigraphic Units.

Chronostratigraphic units, because they are defined on the basis of their time of deposition or formation, a universal property, offer the best promise of being recognized, accepted, and used worldwide and of being, therefore, the basis for international communication and understanding. Particularly important in this respect are the units of the Standard Global Chronostratigraphic (Geochronologic) Scale.

The definition of chronostratigraphic units is best accomplished, as discussed above, by the selection of boundary-stratotypes of their lower boundaries. These boundary-stratotypes are specific and representative sequences of strata in specific geographic localities that contain unique and specific points that are the record of unique instants of geologic time. They define unequivocally a standard against which other sequences can be correlated. The term “Global Boundary Stratotype Section and Point (GSSP)” has been proposed for these standard boundary-stratotypes between units of the Global Chronostratigraphic Scale (Cowie et al., 1986; Cowie, 1986). They are being selected and described with great care by units of the IUGS International Commission on Stratigraphy.

In addition to the general requirements for the selection and description of stratotypes (see section 4.C), boundary-stratotypes of chronostratigraphic units should fulfill the following requirements.

  • The boundary-stratotypes must be selected in sections representing essentially continuous deposition. The worst possible choice for a boundary-stratotype of a chronostratigraphic unit is at an unconformity; it not only does not represent a sharp point in time but tends to change in age laterally.

  • The boundary-stratotypes of Standard Global Chronostratigraphic Units should be in marine, fossiliferous sections without major vertical lithofacies or biofacies changes. However, the boundary-stratotypes of chronostratigraphic units of local application may need to be selected in nonmarine sections.

  • The fossil content preferably should be abundant, distinctive, well preserved, and representing a fauna and/or flora as cosmopolitan and diverse as possible.

  • The section should be well exposed and in an area of minimal structural deformation or surficial disturbance, metamorphism, and diagenetic alteration (for example, extensive dolomitization). An ample thickness of strata below, above, and laterally from the selected boundary-stratotype is desirable.

  • Boundary-stratotypes of units of the Standard Global Chronostratigraphic (Geochronologic) Scale should be selected in sections of easy access that offer reasonable assurance of free study, collection, and long-range preservation. Marking in the field, hopefully of permanent nature, is desirable.

  • The selected section preferably should be well studied and collected, the results of the investigations published, and the fossils collected from the section securely stored and easily accessible for study.

  • The selection of the boundary-stratotypes of chronostratigraphic units of the Global Chronostratigraphic Scale, where possible, should take account of historical priority and usage and should approximate the traditional boundaries.

  • To insure its acceptance and use over wide areas, preferably worldwide, a boundary-stratotype should be selected to contain as many specific good marker horizons or other features favorable for long-distance time correlation (chronocorrelation) as possible. Examples are significant biohorizons characterized by distinctive marine, cosmopolitan fossils; magnetic-polarity reversals; and stratigraphic intervals favorable for accurate numerical dating by isotopic or other geochronometric methods. Reliable ties with sections representative of different lithofacies and biofacies are also advantageous. This may be accomplished by the selection and designation of regional reference or auxiliary sections that illustrate the extension of time-correlation away from the section containing the boundary-stratotype.

The IUGS International Commission on Stratigraphy is the body responsible at present for coordinating the selection and approval of boundary-stratotypes (GSSPs) of the units of the Standard Global Chronostratigraphic (Geochronologic) Scale. The requirements for the submission to the Commission of proposed boundary-stratotypes are discussed by Cowie et al. (1986) and Cowie (1986).

Procedures for Extending Chronostratigraphic Units—Chronocorrelation (Time Correlation)

Only after the limits of a chronostratigraphic unit have been established at the boundary-stratotypes can the unit be extended geographically beyond the type section. The boundaries of a chronostratigraphic unit are by definition isochronous surfaces, so that the unit includes everywhere rocks representing the same time span. In practice, the boundaries are isochronous only so far as the resolving power of existing methods of time correlation can prove them to be so. Generally, an ideal of isochroneity can be approached only with diminishing accuracy as the chronostratigraphic boundaries are followed away from their boundary-stratotypes. Therefore, all possible lines of evidence should be utilized for time correlation (chronocorrelation): distribution of fossils of many kinds, trace of beds, sequence of beds, lithology, isotopic dating, electric-log markers, unconformities, transgressions and regressions, volcanic activity, tectonic episodes, paleoclimatologic data, paleomagnetic signature, and so on. Nevertheless, the isochonous boundaries of chronostratigraphic units are inherently independent of all other kinds of stratigraphic boundaries, except as these may serve as local guides to time correlation.

Physical Interrelations of Strata.

The simplest and most obvious clue to the relative age or chronostratigraphic position of rock strata lies in their physical interrelations. The classic Law of Superposition of Strata states that in an undisturbed sequence of sedimentary strata the uppermost strata are younger than those on which they rest.

The order of superposition of strata provides the most unequivocal indicator available for relative age relations. All other methods of age determination, both relative and numerical, are dependent, directly or indirectly, on observed physical sequence of strata as a check and control on their validity. For a sufficiently limited distance, the trace of a bedding plane is the best indicator of isochroneity.

Difficulties arise, however, when strata are severely disturbed, overturned, or overthrust; when a younger igneous rock is emplaced within a sequence of older strata; when a relatively mobile sedimentary rock like shale, salt, or gypsum is diapirically injected into younger strata or flows over them; or, perhaps most importantly, when continuous exposures are lacking and there are discontinuities caused by lateral variability, overlap, unconformities, faulting, intrusion, and so on. Even in such difficult situations, correlation by means of the physical interrelationships of strata and their stratigraphic sequence is frequently helpful in relative age determination.

Lithology.

At one time many of the systems and their subdivisions were primarily lithostratigraphic divisions whose distinctive lithology was supposed everywhere to characterize the rocks generated during certain intervals of geologic time. It was soon recognized, however, that lithologic properties are commonly influenced more strongly by environment than by age, that the boundaries of all lithostratigraphic units eventually cut across isochronous surfaces and vice versa, and that lithologic features are repeated time and again in the stratigraphic sequence. Even so, a lithostratigraphic unit such as a formation always has some chronostratigraphic connotation and is useful as an approximate guide to chronostratigraphic position, at least locally. Individual bentonites, volcanic ash beds, tonsteins, limestone beds or phosphate beds, for example, may be excellent guides to approximate time-correlation over extensive areas. Distinctive and widespread lithologic units also may be diagnostic of general chronostratigraphic position.

Paleontology.

Fossils constitute one of the best and most widely used means of tracing and correlating sedimentary sequences and thus determining their relative age. Because the orderly and progressive course of organic evolution is irreversible with respect to geologic time and the remains of life are widespread and distinctive, fossils have also constituted the best means for worldwide relative dating and approximate long-distance time correlation throughout the Phanerozoic and have largely made possible the development of a Global Chronostratigraphic Scale for Phanerozoic strata.

Although biostratigraphic correlation is not necessarily time correlation, it has been and continues to be one of the most useful approaches to time correlation if used with discretion and judgment. Biostratigraphic methods are continually being refined to make them increasingly effective. Two fossil-bearing intervals at widely separated localities may have large differences in general fossil content because of lithofacies changes, but subtle paleontological discrimination may show that there is a time correlation between them. On the other hand, two superficially similar fossil assemblages may similarly be shown to be of quite different ages.

Although no individual biozone has either a lower or an upper boundary that is everywhere of exactly the same age, the use of several interlocking biozones, laterally interfingering and replacing each other, may often provide a reasonably accurate indication of time correlation. Such a system of interlocking biozones can be particularly helpful in providing a tie across major lateral changes in depositional environment. An example is the use of the land-to-ocean progression of terrestrial animals and plants, pollen, benthonic marine organisms and planktonic marine organisms in the correlation between continental and marine deposits. Another example is the use of overlapping plant and animal zones in correlation from tropical to temperate to polar environments.

Another effective paleontological key to long-range time correlation is through the interpretation of evolutionary sequences of fossil forms. Numerous statistical techniques have been developed for this purpose.

A realization of the problems to be faced in paleontological time correlation may be gained from consideration of the variety of life-environments of the Earth at the present time and the great lateral variation in living forms. With the added complexities due to fluctuating environments of the past, continental drift, diagenetic changes in strata, metamorphism, the vagaries of fossil preservation, the time required for migration, accidents of collection, and other factors (see Figure 15), it is understandable that along with its great value, long-range paleontological time correlation also has serious limitations. Moreover, Precambrian rocks, constituting a large part of the Earth's crust and corresponding to 85 percent of geologic time, do not usually contain usable fossils, and even in the Phanerozoic not all strata are fossiliferous, and fossils where present yield only a relative age, not an exact age measured in multiples of years.

Figure 15.

Possible causes of local variation in the relation of both the original upper limit occurrence of a graptolite taxon and the upper limit of present known occurrences of the taxon to an isochronous horizon (chronostratigraphic horizon).

Figure 15.

Possible causes of local variation in the relation of both the original upper limit occurrence of a graptolite taxon and the upper limit of present known occurrences of the taxon to an isochronous horizon (chronostratigraphic horizon).

Isotopic Age Determinations.

Isotopic dating methods based on the radioactive decay of certain parent nuclides at a rate that is constant and suitable for measuring geologic time provide an additional key to chronostratigraphy. The most commonly used methods (U-Pb, Rb-Sr, K-Ar, Ar-Ar) produce data of high precision. Analytical errors are commonly in the range of 0.1 to 2 percent.

Isotopic dating is almost unique in its potential for contributing age values expressed in years, millions, or billions (109) of years. It has furnished the first quantitative evidence of the length of geologic time, suggesting that the age of the oldest known rocks of the Earth's crust is at least 3,960 million years. Isotopic dating has also provided the major hope of working out to some extent the ages and age relationships of the great mass of Precambrian rocks, where fossils are of little use in age determination and where structural complication and metamorphism frequently inhibit direct observation of the original stratal se-quences. It is not uncommon now to see zircon U-Pb ages expressed at the 1-million-years level of precision. Likewise, for Phanerozoic rocks, isotopic age determinations now provide useful data on ages and durations in years, as well as valuable checks on relative age determinations by other methods. Under certain circumstances, isotopic age determinations of intrusive or extrusive igneous rock bodies may provide the best, or even the only, basis for age determination and chronostratigraphic position of sedimentary sequences.

Discrepancies in age results may arise from the use of different decay constants. For geochronometric comparisons it is, therefore, important that uniform sets of decay constants be used in the computation of ages. In common use now are those recommended in 1976 by the IUGS Subcommission on Geochronology.

Isotopic methods may be applied both to whole rock samples and to minerals separated from the rocks. The age significance of isotopic data depends on a variety of geological parameters, and the use of isotopic methods in chronostratigraphy generally requires geological interpretation. The various isotope systems in different mineral and rock samples may reflect a specific response to varying conditions of pressure, temperature, or other vicissitudes which they may have experienced. Thus it may be necessary to decide whether the age obtained is that of metamorphism or other subsequent alteration—rather than the true age of formation of the rocks. Likewise, ages determined for detrital minerals derived from older sources may cause erroneous conclusions on the age of formation of a rock stratum. Finally, an important limitation to use is that not all rock types are amenable to isotopic age determination.

A method of age determination through radioactivity differing from those mentioned above is that based on the proportion of the radiocarbon isotope (14C) to normal carbon in the organic matter of sediments. This method has been extremely valuable but is limited in application to the dating of upper Quaternary strata.

Geomagnetic Polarity Reversals.

Periodic reversals of the polarity of the Earth's magnetic field are utilized importantly in chronostratigraphy, particularly in upper Mesozoic and Cenozoic rocks, where a more detailed magneticpolarity scale has been developed. The magnetic-polarity scale has also played an important role in determining the chronostratigraphy of the rocks of the oceanic regions. Polarity reversals are, however, binary in character, and specific ones cannot be identified without assistance from other methods such as bio-stratigraphy or isotopic dating.

Paleoclimatic Changes.

Climatic changes leave a conspicuous imprint on the geologic record in the form of glacial deposits, evaporites, red beds, coal deposits, paleontological changes, and such. Since many climatic changes appear to have been regional or worldwide, their effects on the rocks provide valuable information for chronocorrelation. The extent of their effects is complicated, however, by normal variations in climate due to latitude, elevation, oceanic circulation, plate movements, and other factors.

Paleogeography and Eustatic Changes in Sea Level.

Alternating transgressions and regressions of the sea and the resulting unconformities have classically provided the basis for local and regional division of stratal sequences, and many of the chronostratigraphic units of Western Europe originated in this way. As a result of either epeirogenic movements of the land masses or eustatic rises and lowerings of the sea level, certain periods of Earth history seem to have been characterized worldwide by a general high or low stand of the continents with respect to sea level. The evidence in the rock sequence of these eustatic changes can furnish an excellent basis for establishing a worldwide “natural” chronostratigraphic framework. However, local vertical movements of the Earth's crust may have been so great and so variable geographically that the record in the rocks may be difficult to interpret locally.

Unconformities.

Many of the geologic systems were originally defined as representing the rocks lying between certain major unconformities because these marked natural breaks in lithology, paleontology, and other features of the rocks. It is known, however, that a surface of unconformity inevitably varies in age and in time-value from place to place and that it is never universal in extent. Moreover, unconformities frequently result from very slow epeirogenic movements taking place over long periods of geologic time. Hence, while unconformities frequently serve as useful guides to the approximate placement of chronostratigraphic boundaries, they cannot in themselves fulfill the requirements of such boundaries (see section 9.H.3.a).

Although unconformity surfaces are not isochronous and continually cut across time horizons, major regional unconformities obviously have very important, though broad, time significance. Likewise, unconformity-bounded units—syn-thems—form a class of stratigraphic units which, though not chronostratigraphic, have great significance in chronostratigraphy (see Chapter 6).

Orogenies.

A classic, but now discredited, concept of historical geology is that periodic worldwide orogenies have furnished “natural” worldwide dividing lines in Earth history and that these can be identified in the rocks by their effects on sedimentation, erosion, igneous activity, and rock deformation. This is valid only for certain regions and is reflected in the usage of such terms as Caledonian, Hercynian, Laramide, and Alpine orogenies. In the Precambrian, chronostratigraphic classification has been attempted on the basis of worldwide orogenic cycles and cyclic periods of crustal metamorphism. However, the considerable duration of many orogenies, their local rather than worldwide nature, their lack of coincidence with classic system or series boundaries, and the difficulty of identifying them closely make them generally unsatisfactory indicators of worldwide chronostratigraphic correlation.

Other Indicators.

Many other lines of evidence may under limited circumstances be helpful as guides to time correlation and as indicators of chronostratigraphic position. For example, certain invertebrates may provide a valuable clue to chronostratigraphic position because they show a decreasing number of daily growth increments per year caused by the slowing of the Earth's rate of rotation in response to tidal impedance.

Various mineralogical, geochemical, and geophysical features of rock strata provide means of approximate time-correlation over considerable distances. Detrital heavy-mineral assemblages are valuable in time correlation and in determining relative time of formation. Varves and seasonal bands in sediments are indicators of age and duration of stratigraphic intervals. Probable rates of sedimentation are indicators of the time required for the formation of sedimentary sequences. Seismic profiles and electrical and nuclear logging of boreholes provide very useful means of time correlation and detailed evidence of relative chronostratigraphic position.

Several special numerical methods not mentioned above have been developed for dating very young sediments. Various dating methods have been tried utilizing thermoluminescence, fission tracks, pleochroic halos, and other forms of radiation damage. Many other means could be mentioned, and it is expected that many totally new methods will be developed.

Many of the above-mentioned contributors to time correlation, though of only limited accuracy, can be useful in working out the time-relations of strata under the right circumstances. Some are more used than others, but none should be rejected. Even with the help of all, time correlation to extend the boundaries of chronostratigraphic units geographically away from their boundary-stratotypes never achieves complete isochroneity.

Naming of Chronostratigraphic Units

A formal chronostratigraphic unit should be given a binomial designation— a proper name plus a term-word—and the initial letters of both should be capitalized; for example, Cretaceous System. The geochronologic equivalent of a chronostratigraphic unit should use the same proper name combined with the equivalent geochronologic term; for example, Cretaceous Period. The proper name of a chronostratigraphic unit may be used alone where there is no danger of confusion; for example, “the Aquitanian” in place of “the Aquitanian Stage.”

Conventions for the names of individual kinds or ranks of chronostratigraphic units are discussed under the headings of the units. Chronostratigraphic nomenclature follows the general rules for stratigraphic nomenclature given in sections 3.B.3 and 3.B.4.

Revision of Chronostratigraphic Units

Much confusion concerning the scope of specific chronostratigraphic units has arisen due to inadequate definition of the units at the time they were proposed. To make these units more useful, it is strongly urged that originally inadequate definitions of units now in common use should be revised to accord with recommended procedures. It is also urged that any new chronostratigraphic unit for which formal status is desired should be adequately proposed and defined according to the procedure outlined in sections 3.B and 9.H.

Figures & Tables

Figure 13.

Relation between Exus albus Chronozone and Exus albus Biozone. (Distribution of specimens of Exus albus shown by dot-pattern.)

Figure 13.

Relation between Exus albus Chronozone and Exus albus Biozone. (Distribution of specimens of Exus albus shown by dot-pattern.)

Figure 14.

Advantage of defining stages by lower boundary-stratotypes, where type localities are widely separated, rather than by unit-stratotypes.

Figure 14.

Advantage of defining stages by lower boundary-stratotypes, where type localities are widely separated, rather than by unit-stratotypes.

Figure 15.

Possible causes of local variation in the relation of both the original upper limit occurrence of a graptolite taxon and the upper limit of present known occurrences of the taxon to an isochronous horizon (chronostratigraphic horizon).

Figure 15.

Possible causes of local variation in the relation of both the original upper limit occurrence of a graptolite taxon and the upper limit of present known occurrences of the taxon to an isochronous horizon (chronostratigraphic horizon).

Table 3.

Conventional Hierarchy of Formal Chronostratigraphic and Geochronologic Terms

ChronostratigraphicGeochronologic
EonothemEon
ErathemEra
System*Period*
Series*Epoch*
StageAge
SubstageSubage or Age
ChronostratigraphicGeochronologic
EonothemEon
ErathemEra
System*Period*
Series*Epoch*
StageAge
SubstageSubage or Age

*If additional ranks are needed, the prefixes sub and super may be used with these terms.

Several adjacent stages may be grouped into a superstage (see Section 9.C.3).

Table 4.

Major Units of the Standard Global Chronostratigraphic (Geochronologic) Scale (1)

(1) A number of more detailed chronostratigraphic or geochronological scales have been published in the last 10 to 15 years including those of Palmer (1983) and Harland et al. (1982, 1990), referenced below, and the 1989 Global Stratigraphic Chart of the International Commission on Stratigraphy (Episodes, v. 12, no. 2).

(2) Palmer, A. R., 1983, The Decade of North American Geology 1983 Geologic Time Scale.

(3) Snelling, N. J., 1987, Measurement of geological time and the Geological Time Scale.

(5) In North America, in place of a Carboniferous System, two systems have been recognized: Mississippian System (older) and Pennsylvanian System (younger). These are also sometimes known as subsystems of the Carboniferous System.

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