Chronostratigraphy - Wikipedia
Chronostratigraphy is the unifying construct that defines (ideally by international agreement) The fundamental geochronologic unit is the period – the time equivalent of a system . Dating for sediments of age less than a few hundred years. Units in geochronology and stratigraphy. Segments of rock (strata) in chronostratigraphy, Time spans in geochronology, Notes. In addition, geochronology refers to all methods of numerical dating. Chronostratigraphy would include all methods (e.g., biostrati-graphy, magnetostratigraphy.
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.
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.
Geochronology - Wikipedia
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 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.
All units of the standard chronostratigraphic hierarchy are theoretically worldwide in extent, as are their corresponding time spans. 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.
Moreover, regional units will probably always be needed whether or not they can be correlated with the standard global units.
It is better to refer strata to local or regional units with accuracy and precision 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 are governed by the same rules as are established for the units of the Standard Global Chronostratigraphic Scale. Subdivision of the Precambrian The Precambrian has been subdivided into arbitrary geochronometric units, but it has not been subdivided into chronostratigraphic units recognizable on a global scale.
There are prospects that chronostratigraphic subdivision of much of the Precambrian may eventually be attained through isotopic dating and through other means of time correlation. However, 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 various means of time correlation, predominantly isotopic dating.
Quaternary Chronostratigraphic Units The basic principles used in subdividing the Quaternary into chronostratigraphic units are the same as for other Phanerozoic chronostratigraphic units, although the methods of time correlation may have a different emphasis. As in the case of other chronostratigraphic units, those of the Quaternary require boundary definitions and designation of boundary stratotypes. Procedures for Establishing Chronostratigraphic Units.
See also section 3. 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.
For these reasons, the boundaries of a chronostratigraphic unit of any rank are defined by two designated reference points in the rock sequence. The two points are located in the boundary-stratotypes of the chronostratigraphic unit which need not be part of a single section. Both, however, should be chosen in sequences of essentially continuous deposition since the reference points for the boundaries should represent points in time as specific as possible see section 9.
Advantage of defining chronostratigraphic units by their lower boundary stratotypes. The definition of a chronostratigraphic unit places 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.
This procedure avoids gaps and overlaps in the Standard Global Chronostratigraphic Scale. For example, should it be shown that the selected horizon is at the level of an undetected break in the sequence, then the missing span of geologic history would belong to the lower unit by definition and ambiguity is avoided. Requirements for the selection of boundary stratotypes of chronostratigraphic units.
Chronostratigraphic units offer the best promise of being identified, accepted, and used globally and of being, therefore, the basis for international communication and understanding because they are defined on the basis of their time of formation, a universal property.
Particularly important in this respect are the units of the Standard Global Chronostratigraphic Geochronologic Scale. In addition to the general requirements for the selection and description of stratotypes section 4. Cboundary-stratotypes of chronostratigraphic units should fulfill the following requirements: The worst possible choice for a boundary-stratotype of a chronostratigraphic unit is at an unconformity.
Difference between chronostratigraphy and geochronology dating
Boundary stratotypes of chronostratigraphic units of local application may need to be in a nonmarine section. Permanent field markers are desirable. Procedures for Extending Chronostratigraphic Units-Chronocorrelation Time Correlation The boundaries of chronostratigraphic units are synchronous horizons by definition.
In practice, the boundaries are synchronous only so far as the resolving power of existing methods of time correlation can prove them to be so. All possible lines of evidence should be utilized to extend chronostratigraphic units and their boundaries. Some of the most commonly used are: Physical Interrelations of strata. The Law of Superposition states that in an undisturbed sequence of sedimentary strata the uppermost strata are younger than those on which they rest.
The determination of the order of superposition provides unequivocal evidence for relative age relations. All other methods of relative age determination are dependent on the observed physical sequence of strata as a check on their validity. For a sufficiently limited distance, the trace of a bedding plane is the best indicator of synchroneity. Lithologic properties are commonly influenced more strongly by local environment than by age, the boundaries of lithostratigraphic units eventually cut across synchronous surfaces, and similar lithologic features occur repeatedly in the stratigraphic sequence.
Even so, a lithostratigraphic unit always has some chronostratigraphic connotation and is useful as an approximate guide to chronostratigraphic position, especially locally. Distinctive and widespread lithologic units also may be diagnostic of chronostratigraphic position.
The orderly and progressive course of organic evolution is irreversible with respect to geologic time and the remains of life are widespread and distinctive. For these reasons, fossil taxa, and particularly their evolutionary sequences, constitute one of the best and most widely used means of tracing and correlating beds and determining their relative age.
Biostratigraphic correlation, however, is not time correlation because homotaxy between samples may result from other causes than that the samples are equal in age. Isotopic dating methods U-Pb, Rb-Sr, K-Ar, Ar-Ar based on the radioactive decay of certain parent nuclides at a rate that is constant and suitable for measuring geologic time provide chronostratigraphic data of high precision with analytical errors in the range of 0.
However, not all rock types and minerals are amenable to isotopic age determination. Isotopic dating contributes age values expressed in years and it provides the major hope for working out the ages and age relationships of Precambrian rocks.
In some circumstances, isotopic age determinations provide the most accurate or even the only basis for age determination and chronostratigraphic classification of sedimentary, volcanic and other igneous rocks. Discrepancies in age results may arise from the use of different decay constants.
It is important to geological comparisons, therefore, that the uniform sets of decay constants recommended by the IUGS Subcommission on Geochronology be used. 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.
Periodic reversals of the polarity of the Earth's magnetic field are utilized in chronostratigraphy, particularly in upper Mesozoic and Cenozoic rocks where a magnetic time scale has been developed.
Polarity reversals are, however, binary and specific ones cannot be identified without assistance from some other method of dating such as biostratigraphy or isotopic dating. Climatic changes leave imprints on the geological record in the form of glacial deposits, evaporites, red beds, coal deposits, faunal changes, etc. Their effects on the rocks may be local or widespread and provide valuable information for chronocorrelation, but they must be used in combination with other specific methods.
Paleogeography and eustatic changes in sea level. 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 are characterized worldwide by a general high or low stand of the continents with respect to sea level. The evidence in the rocks of the resulting transgressions, regressions, and unconformities can furnish an excellent basis for establishing a worldwide chronostratigraphic framework.
The identification of a particular event, however, is complicated by local vertical movements and so the method requires auxiliary help in order to identify the events correctly.
Even though a surface of unconformity varies in age and time-value from place to place and is never universal in extent, certain unconformities may serve as useful guides to the approximate placement of chronostratigraphic boundaries. Unconformities, however, cannot fulfill the requirements for the selection of such boundaries see section 9.
Crustal disturbances have a recognizable effect on the stratigraphic record. However, the considerable duration of many orogenies, their local rather than worldwide nature, and the difficulty of precise identification make them unsatisfactory indicators of worldwide chronostratigraphic correlation.