Chronostratigraphy

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 stratigraphie 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 chronostratigraphic 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 chronostratigraphic connotation, since their boundaries eventually cut across synchronous surfaces, but this lithostratigraphic method also has local validity. [Page 190]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 biochrono-correlation, 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, dinoflagel-late 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.

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