The categories within stratigraphic classification are the rocks of the Earth’s crust, with the picture of the stratified Earth, and with the history of the Earth as interpreted from its rock bodies. Each category, however, is concerned with a different property or attribute of the rocks and a different aspect of Earth history. The relative importance of the different categories varies with the circumstances. Each is important for particular purposes.
Lithostratigraphic units are the basic units of geologic mapping. Wherever there are rocks, it is possible to develop a lithostratigraphic classification. Lithostratigraphic units are based on the lithologic character of rocks.
Fossils may be an important distinguishing element in their recognition, but only because of their diagnostic lithologic characterization.
Inasmuch as each lithostratigraphic unit was formed during a specific interval of geologic time, it has chronostratigraphic significance. The concept of time, however, plays little part in establishing or identifying lithostratigraphic units and their boundaries.
Lithologic character is influenced more strongly by conditions of formation than by time of origin; nearly identical rock types are repeated time and again in the stratigraphic sequence, and the boundaries of almost all lithostratigraphic units cut across synchronous surfaces when traced laterally.
Biostratigraphic classification is also an early step in working out the stratigraphy of a region. Biostratigraphic units are based on the fossil content of the rocks.The selection and establishment of biostratigraphic units are not determined by the lithologic composition of the strata, except that the presence or absence of fossils and the kind of fossils present, may be related to the type and lithofacies of the rocks in which they are found.
Biostratigraphic units are distinct from other kinds of stratigraphic units in that the organisms whose fossil remains define them show evolutionary changes through geologic time that are not repeated in the stratigraphic record.
This makes the fossil assemblages of any one age distinctive from any other.
Lithostratigraphic and biostratigraphic units are fundamentally different kinds of stratigraphic units based on different distinguishing criteria.
Their boundaries may coincide locally, but commonly they lie at different stratigraphic horizons or cross each other.
Whereas lithostratigraphic classification is possible for any body of rock, biostratigraphic classification is possible only for fossiliferous rocks that bear identifiable fossils.
Both lithostratigraphic and biostratigraphic units reflect the environment of deposition, but biostratigraphic units are more influenced by, and indicative of, geologic age.They are also less repetitive in character because they are based on irreversible evolutionary change.
Lithostratigraphic and biostratigraphic units are indispensable objective units, essential in picturing the lithologic constitution and geometry of the rocks of the Earth’s crust and the development of life and past environments on the Earth.
Unconformity-bounded units and magnetostratigraphic polarity units, like biostratigraphic units, can be established only when the diagnostic properties on which they are based are present in the rocks.
Unconformity-bounded units may include a number of other kinds of stratigraphic units, both in vertical and lateral succession.Similarly, an unconformity-bounded unit may represent all or parts of several chronostratigraphic units. In special cases, the boundaries of an unconformity-bounded unit may coincide with the boundaries of other kinds of stratigraphic units.
However, the boundaries of unconformity-bounded units are always diachronous to a lesser or greater extent, and so never correspond with the boundaries of chronostratigraphic units.
Magnetostratigraphic polarity units while similar to lithostratigraphic and biostratigraphic units in that they are based only on a directly determinable property of the rocks, their magnetic polarity, are different from them because magnetostratigraphic polarity units are potentially recognizable globally and, in this respect, they are similar to chronostratigraphic units.
The changes in magnetic polarity are the result of very rapid worldwide reversals of the Earth’s magnetic field, generally occurring through a time span of no more than about 5,000 years. The magnetic-polarity-reversal horizons produced as a result of these events do not, therefore, constitute synchronous horizons.
Consequently, the body of rocks lying between magnetic-polarity reversals horizons produced by two successive polarity reversals constitutes a polarity unit containing everywhere strata representing essentially, but not exactly, the same time span. Such units may closely approximate chronostratigraphic units, but they are not chronostratigraphic units because they are defined primarily not by the record of time but by a specific physical character, the change in the polarity of remanent magnetization, which is not instantaneous.
Moreover, because of the variability in the distinctness of the imprint or in the preservation of the polarity record, because of unconformities in the section, because of the effects of bioturbation, or because of possibilities of subsequent remagnetization, or for other reasons, the boundaries of a polarity unit depart from synchroneity.
Although magnetostratigraphic-polarity horizons and units may be useful guides to chronostratigraphic position, they have relatively little individuality, one reversal looks like another, and can usually be identified only by supporting age evidence, such as paleontologic or isotopic data.
Chronostratigraphic units are defined as encompassing all rocks formed within certain time spans of Earth history regardless of their compositions or properties.
By definition, these units everywhere include rocks of only a certain age and their boundaries are everywhere synchronous. This is in contrast with lithostratigraphic units that can be objectively recognized wherever there are rocks, and with biostratigraphic, magnetostratigraphic polarity, and unconformity-bounded units that are limited by the occurrence of specific properties or attributes of the rocks. Whereas other kinds of stratigraphic units are distinguished, established, and identified on the basis of observable physical features, chronostratigraphic units are distinguished, established, and identified on the basis of their time of formation-an abstract character-as interpreted from these observable properties.
Biostratigraphic units may approximate chronostratigraphic units even over wide areas, but the boundaries of biostratigraphic units may diverge from those of a chronostratigraphic unit for many reasons. Principal among these are changes in depositional facies, variations in conditions for fossilization and preservation of fossils, vagaries of fossil discovery, and biogeographic differences. Biostratigraphic units cannot be recognized in rocks where there are no fossils.
Some lithostratigraphic units are excellent guides to approximate time correlation over large areas, as in the case of volcanic ash beds, but they, like biostratigraphic units, are not chronostratigraphic units because they are not bounded everywhere by synchronous surfaces.
Unconformity-bounded units and magnetostratigraphic polarity units also provide valuable support for the development of chronostratigraphic classification.
Especially, the boundaries of magnetostratigraphic polarity units because they record the very rapid reversal of the Earth’s magnetic field, approach synchronous surfaces closer than any other kind of objective stratigraphic unit.
If properly identified, they offer a sound foundation for global time correlation and chronostratigraphic classification.
Chronostratigraphic classification stands out as the basis to reach the ultimate goal of stratigraphy. Chronostratigraphic units, as divisions of rock bodies based on geologic time, are in principle worldwide in extent, and important in providing a worldwide basis for communication and understanding.
The above-mentioned kinds of stratigraphic units and their corresponding fields of stratigraphic investigation are the most commonly used.
However, there are many other fruitful lines of stratigraphic endeavor and many other kinds of stratigraphic units which, under appropriate circumstances and for certain objectives, are useful.
Thus, we may find it useful to recognize stratigraphic units or horizons based on electric-log characters, seismic properties, chemical changes, stable isotope analyses, or any of many other properties of rock bodies. No one can or need use all the possible kinds of stratigraphic tools or units that are potentially available, but the way should be kept open within the definition and scope of stratigraphy to apply any that give promise of being useful.