Chronostratigraphy is a discipline of stratigraphy that studies the relative time relations and ages of stratified rocks. The aim of Chronostratigraphy is to organize stratified rocks into units on the basis of their age or time of origin. A chronostrati- graphic unit is a stratified body that includes all rocks formed during a specific interval of geologic time, and only those rocks formed during that time span. There is a hierarchy of formal chronos- tratigraphic unit terms that correspond with equivalent geochronologic unit terms. The first are stratigraphic units, whereas the second are time units. The chronostratigraphic units, accord­ing to their ranking, are the following: chrono­zone, stage, series, system, erathem, and eonothem; they correspond in rank to the following geo­chronologic units: chron, age, epoch, period, era, and eon. The position within a chronostrati- graphic unit is expressed by adjectives such as basal, lower, middle, upper, and so forth. The position within a geochronologic unit is expressed by temporal adjectives such as earliest, early, middle, and late.

The boundaries of chronostratigraphic units are synchronous horizons by definition. There are several geologic methods of time correlation and dating, although their resolving power usually is greater than 20,000 years in most cases. The meth­odology used for the chronocorrelation is derived from Steno’s law of superposition, which 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. The bedding plane is the best indicator of synchroneity, but such a method usu­ally has only local validity. The identification of lithostratigraphic units always has some chronos- tratigraphic connotation, since their boundaries eventually cut across synchronous surfaces, but this lithostratigraphic method also has local validity.

The paleontological methods have greater utility because they are based on the orderly and progres­sive course of biological evolution.

The fossil record is an important source of chronostratigraphic information. Several cali­brated biochronological scales have been estab­lished from biostratigraphic data and integrated correlation. In order to attain a better biochrono­correlation, the identification of index taxa is necessary. Index taxa are fossils useful to define and identify geologic periods. The best index taxa belong to the following paleontological groups: foraminifera, calcareous nanofossils, dinoflagel­late cysts, acritarchs, ostracods, ammonites, trilo- bites, bracheopods, graptolites, and conodonts in Phanerozoic marine environments, and pollen and spores, vertebrate microfossils, charophytes, and ostracods in Phanerozoic terrestrial environ­ments. However, the paleontological methods do not provide unequivocal data, since the strata and the fossils they contain are not necessarily synchronous.

Periodic reversals of the polarity of the earth’s magnetic field are utilized in chronostratigraphy, particularly in upper Mesozoic and Cenozoic rocks, where a magnetochronological scale has been developed. Polarity reversals are binary, however, and specific ones cannot be identified without assistance from some other method of dating, mainly paleontological (biostratigraphic) methods. Biomagnetostratigraphic correlation, calibration, and dating is the best method known to date.

Radioisotopic dating methods are unique in pro­viding numeric age values, expressed in years. They are based on the radioactive decay of certain parent nuclides at a rate that is constant and suitable for measuring geologic time data with high precision with analytical errors in the range of 0.1% to 2%. Radioisotopic dating provides the best hope for working out the ages and age relationships of Precambrian rocks. However, not all rock types and minerals are amenable to radioisotopic age determination, so it can be used only on sporadic occasions.

Today, two new methods have been added for chronocorrelation and dating: cyclostratigraphy/ astrochronology and eventstratigraphy. The cyclo- stratigraphic methods try to identify evidence of the earth’s orbital fluctuations in the stratigraphic record. These fluctuations have caused climate cycles in the past, such as the quaternary glaciations, that may be recognized in the stratigraphic record from lithological, paleomagnetic, isotopic, and pale­ontological data. The best-known orbital periodici­ties are the precession (cycles of 21,000 years), obliquity (cycles of 41,000 years), and eccentricity (cycles of 100,000 and 410,000 years). Their recog­nition provides an impressive chronocorrelation and dating method (astrochronologic scale), although it needs assistance from other methods of correlation.

Chronostratigraphic units, mainly the stages, must be defined by boundary stratotypes. The boundaries of a chronostratigraphic unit of any rank are defined by two designated reference points in the rock sequence. The selected point must be a marker horizon favorable for long-dis­tance chronocorrelation; thus it must represent a global event. The identification of global events in the stratigraphic record is the aim of a new disci­pline called eventstratigraphy. In geological his­tory, the most frequent globally occurring events are: paleoclimatic changes (e.g., glacial deposits, evaporites, red beds, coal deposits, faunal changes), tectonic and eustatic changes in sea level, volcanic events, meteoritic impacts, and mass extinctions. The identification of these events and their cali­bration with paleontological, isotopic, paleomag- netic, and astrochronologic methods are the best tools for establishing the chronostratigraphic and geochronologic scales.

Three eonothems are recognized, from older to younger: Archean (3,900-2,500 million years ago [mya]), Proterozoic (2,500-542 mya), and Phanerozoic (540-0 mya), although a fourth eon is generally considered, the Hadean (4,550-3,900 mya). The informal Precambrian terms include Hadean, Archean, and Proterozoic (i.e., all time before the Cambrian). Subdivisions of the global geologic record are formally defined by their lower boundaries from a basal Global Standard Section and Point (GSSP), except for Precambrian units, which are for­mally subdivided by absolute age (Global Standard Stratigraphic Age, or GSSA). GSSP and GSSA are approved by the International Commission on Stratigraphy (ICS) and ratified by the International Union of Geological Sciences (IUGS).

The Hadean spans the time period between the earth’s formation and the age of earliest-known rocks. It has no geological record known to date, except for some zircon crystals of Hadean age. The Archean, also called Archeozoic, is defined geo­chronometrically (GSSA) and not stratigraphically. It is subdivided formally in four erathems or eras: Eoarchaean (3,900-3,600 mya), Paleoarchean (3,600-3,200 mya), Mesoarchean (3,200-2,800 mya), and Neoarchean (2,500-2,800 mya). The Proterozoic represents an eon before the first abun­dant complex life on Earth; that is, before the first period of the Phanerozoic, the Cambrian. Its geo­logic record is much better than that from the pre­ceding eonothem. The Proterozoic includes three erathems or eras: Paleoproterozoic (2,500-1,600 mya), Mesoproterozoic (1,600-1,000 mya), and Neoproterozoic (1,000-542 mya). Finally, the Phanerozoic spans the period of geologic time dur­ing which an abundant fossil record has existed. It subdivides into three erathems: Paleozoic (542-251 mya), Mesozoic (251-65 mya), and Cenozoic (65-0 mya) whose boundaries coincide with major mass extinction events: the Permian/Triassic bound­ary mass extinction event occurred between the Paleozoic and Mesozoic eras, and the Cretaceous/ Paleogene or K-T boundary mass extinction event between the Mesozoic and Cenozoic eras. The Phanerozoic erathems are subdivided into systems or periods that are widely known popularly: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian for the Paleozoic; Triassic, Jurassic, and Cretaceous for the Mesozoic; and Paleogene and Neogene for the Cenozoic. Moreover, the Paleogene is subdivided into three well-known series or epochs called Paleocene, Eocene, and Oligocene; whereas the Neogene is subdivided into Miocene, Pliocene, Pleistocene, and Holocene. Historically, the Cenozoic has been divided into Tertiary (Paleocene to Pliocene) and Quaternary (Pleistocene and Holocene), although most geologists no longer recognize these.

Ignacio Arenillas

See also Chronology; Dating Techniques; Earth, Age of; Fossil Record; Geologic Timescale; Geology; K-T Boundary; Paleontology; Permian Extinction; Stratigraphy; Synchronicity, Geological; Time, Measurements of

Further Readings

De Graciansky, P. C., Hardenbol, J., & Jacquin, T., & Vail, P. R. (Eds.). (1998). Mesozoic and Cenozoic sequence stratigraphy of European basins. Tulsa, OK: SEPM Special Publication.

Gradstein, F. M., Ogg, J. G., & Smith, A. G. (Eds.). (2004). A geologic time scale 2004. Cambridge, UK: Cambridge University Press.