GSA Today - Chronostratigraphy and geochronology: A proposed realignment
Relative dating by stratigraphic correlations, employing bio-, magneto-, or isotope -stratigraphy, provides a relative time scale between specific. Meaghan has a Bachelors in Biology and a Ph.D. in Geology. She taught Chronostratigraphy is the process of using dating methods to determine rock ages. The units of geologic time during which chronostratigraphic units were formed are . which will serve as a standard scale of reference for the dating of all rocks .
Both parallel sets of units are retained, although there remains the option to adopt either a single i. Geochronology can also qualify rock bodies, stratified or unstratified, with respect to the time interval s in which they formed e. In addition, geochronology refers to all methods of numerical dating. Chronostratigraphy would include all methods e. Both hierarchies would remain available for use, as recommended by a formal vote of the International Commission on Stratigraphy in Geological context helps determine the appropriate usage of the component units.
International Stratigraphic Guide
Manuscript received 16 July ; accepted 13 Dec. Study of these rocks has yielded the 4. Stratigraphy is the means of analyzing and ordering these phenomena, with chronostratigraphy and geochronology dealing explicitly with the relations of rock and time. The most familiar of these units are the geological periods of geochronology, sensu stricto, or, more simply, of time e.
Historically, the systems were built from, or subdivided into, series and stages; the periods, epochs, and ages were then used to refer to the intervals of time in which the strata encompassed were deposited. Many of these units were originally set up as and remain fundamentally relative time-rock units.
These are typically of the last half billion years the Phanerozoic Eonwhere there are good fossil assemblages i. Wherever feasible, additional tools, such as magnetostratigraphy, chemostratigraphy, sequence stratigraphy, cyclostratigraphy, and radiometric dating are employed e.
These projects will result in a Precambrian time scale that likely will be very different from that presently used. At the other end of the geologic time scale, the recognition of long oceanic successions with effectively complete Milankovitch signatures has led to the revival of the unit-stratotype concept Hilgen et al.
Neogene stages Zanclean and Piacenzian with upper and lower boundaries defined by GSSPs in the same section have within them all significant biostratigraphic and magnetostratigraphic signals for the time encompassed and numerical ages that are integrated and precisely dated at high resolution through astronomical tuning.
Here geologic events are observed, recorded, and dated as they occur using human time year, month, day, hour. Superposition in deposits analyzed at such high time resolution may commonly be compromised, for example, by the blurring effects of bioturbation cf.
Early versions of the GTS were created, and functioned effectively, in the days before radiometric dating e. Today, considerable effort is expended to calibrate the GTS with numerical ages.
Nevertheless, it remains more common to convey geological time information in terms of GTS units rather than by numbers of years. This is partly because of the familiarity and convenience of the units to geologists at least and partly because it is usually easier and more useful to establish relative correlations than to establish the numerical ages of rock phenomena.
More importantly, however, the rocks formed during a time unit often encompass and record distinctive, time-constrained global environments e. They provide a con-venient and practical method of reference to the events and time intervals they represent, just as with human history, when terms are used for a distinctive time interval e. Even informal terms, such as Caledonian and Grenvillian, are widely used in the same way in geology.
For circumstances in which global units are difficult to apply, regional ones have been established see Gradstein et al. While traditionally chronostratigraphic units consist of rocks, whereas geochronologic units are spans of time, there has been debate over the necessity of retaining a dual and parallel time scale with the same formal names.
This debate represents subtle but distinct perspectives on the stratigraphic record. In a formal ballot following the workshop, the ICS voting members recommended overwhelmingly 15 yes, 2 no, 0 abstain to maintain the dual usage. Next, we consider the definition and application of these terms and of their units, discuss their proper usage, and provide examples and explanations of good practice.
Figure 1 Diagram illustrating the relation of time and rock. Locality B shows a succession with sporadic gaps that includes the GSSP of the succeeding time unit near the top. Each of the GSSPs is precisely located at its type section, but there is uncertainty shown as the gray shading in locating each away from its respective type section. The relation of chronostratigraphic units to other kinds of stratigraphic units is discussed in Chapter The element of stratigraphy that deals with the relative time relations and ages of rock bodies.
The organization of rocks 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.
- Stratigraphy - Key terms
A body of rocks that includes all rocks formed during a specific interval of geologic time, and only those rocks formed during that time span. Chronostratigraphic units are bounded by synchronous 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.
A stratigraphic surface or interface that is synchronous, everywhere of the same age. Kinds of Chronostratigraphic Units 1. 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. Position within a chronostratigraphic unit is expressed by adjectives indicative of position such as: 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.
The stage includes all rocks formed during an age. A stage is normally the lowest ranking unit in the chronostratigraphic hierarchy that can be recognized on a global scale.
It is a subdivision of a series. A stage is defined by its boundary stratotypes, sections that contain a designated point in a stratigraphic sequence of essentially continuous deposition, preferably marine, chosen for its correlation potential. 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.
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 vary in time span, but most range between 2 and 10 million years. The thickness of the strata in a stage and its duration in time are independent variables of widely varying magnitudes. The name of a stage should be derived from a geographic feature in the vicinity of its stratotype or type area.
In English, the adjectival form of the geographic term is used with an ending in "ian" or "an". The age takes the same name as the corresponding stage. Substage and Superstage A substage is a subdivision of a stage whose equivalent geochronologic term is subage. Adjacent stages may be grouped into a superstage. Names of substages and superstages follow the same rules as those of stages.
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Series and Epoch a. The series is a chronostratigraphic unit ranking above a stage and below a system. The geochronologic equivalent of a series is an epoch. The terms superseries and subseries have been used only infrequently. Series are defined by boundary stratotypes see section 9.
The time span of currently accepted series ranges from 13 to 35 million years. A new series name should be derived from a geographic feature in the vicinity of its stratotype or type area. The names of most currently recognized series, however, are derived from their position within a system: Names of geographic origin should preferably be given the ending "ian" or "an". The epoch corresponding to a series takes the same name as the series except that the terms "lower" and "upper" applied to a series are changed to "early" and "late" when referring to an epoch.
The use of the term "series" for a lithostratigraphic unit more or less equivalent to a group should be discontinued. System and Period a. A 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. Occasionally, the terms subsystem and supersystem have been used. The boundaries of a system are defined by boundary-stratotypes see section 9. The time span of the currently accepted Phanerozoic systems ranges from 30 to 80 million years, except for the Quaternary System that has a time span of only about 1.
The names of currently recognized systems are of diverse origin inherited from early classifications: Likewise, they bear a variety of endings such as "an", "ic", and "ous".
There is no need to standardize the derivation or orthography of the well-established system names. The period takes the same name as the system to which it corresponds. An erathem consists of a group of systems. The geochronologic equivalent of an erathem is an era. The names of erathems were chosen to reflect major changes of the development of life on the Earth: Paleozoic old lifeMesozoic intermediate lifeand Cenozoic recent life.
Eras carry the same name as their corresponding erathems. An eonothem is a chronostratigraphic unit greater than an erathem. The geochronologic equivalent is an eon. Three eonothems are generally recognized, from older to younger, the Archean, Proterozoic and Phanerozoic eonothems.
The combined first two are usually referred to as the Precambrian.
The eons take the same name as their corresponding eonothems. Nonhierarchical formal chronostratigraphic units - the Chronozone.
A chronozone is a formal chronostratigraphic unit of unspecified rank, not part of the hierarchy of conventional chronostratigraphic units. It is the body of rocks formed anywhere during the time span of some designated stratigraphic unit or geologic feature. The corresponding geochronologic unit is the chron. The time span of a chronozone is the time span of a previously designated stratigraphic unit or interval, such as a lithostratigraphic, biostratigraphic, or magnetostratigraphic polarity unit.
It should be recognized, however, that while the stratigraphic unit on which the chronozone is based extends geographically only as far as its diagnostic properties can be recognized, the corresponding chronozone includes all rocks formed everywhere during the time span represented by the designated unit. For instance, 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 Figure 8.
Chronozones may be of widely different time spans. The designation of the boundaries of a chronozone and of its time span can be done in several ways depending on the nature of the stratigraphic unit on which the chronozone is based.
If the unit has a designated stratotype, the boundaries and time span of the chronozone can be made to correspond either to those of the unit at its stratotype or to the total time span of the unit, which may be longer than that at the stratotype.
In this second case, the boundaries and time span of the chronozone would vary with increasing information concerning the time span of the unit. If the unit on which the chronozone is based is of the type which cannot appropriately have a designated stratotype, such as a biostratigraphic unit, its time span cannot be defined either because the time span of the reference unit may change with increasing information see section 7.
The geographic extent of a chronozone is, in theory, worldwide, but its pplicability is limited to the area over which its time span can be identified, which is usually less. A chronozone takes its name from the stratigraphic unit on which it is based, e. 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.