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Abstract

The whole Earth is stratified, in a broad sense, so that all rocks and all classes of rocks—sedimentary, igneous, and metamorphic—fall within the scope of stratigraphy and of stratigraphic classification.

Rocks have many different properties, and it is possible to classify them according to any of these properties: lithology, fossil content, magnetic polarity, electrical properties, seismic response, chemical or mineralogical composition, and many others. Rocks can also be classified according to such attributes as their time of origin or their environment of genesis.

The stratigraphic position of change for any one property or attribute does not necessarily coincide with that for any other. Consequently, units based on one property do not generally coincide with units based on another, and their boundaries commonly cut across each other. It is not possible, therefore, to express all of the different properties with a single set of stratigraphic units; different sets of units are needed (see Figure 1).

At the same time, the general unity of stratigraphy should be emphasized. While many different kinds of units are needed to express the variations in all of the many different properties and attributes of the rocks, still, these units are closely related. They concern only different aspects of the same rocks, and they are intimately involved with one another in achieving the same major goals of stratigraphy—to improve our knowledge and understanding of the Earth's rock bodies and their history.

General

The whole Earth is stratified, in a broad sense, so that all rocks and all classes of rocks—sedimentary, igneous, and metamorphic—fall within the scope of stratigraphy and of stratigraphic classification.

Rocks have many different properties, and it is possible to classify them according to any of these properties: lithology, fossil content, magnetic polarity, electrical properties, seismic response, chemical or mineralogical composition, and many others. Rocks can also be classified according to such attributes as their time of origin or their environment of genesis.

The stratigraphic position of change for any one property or attribute does not necessarily coincide with that for any other. Consequently, units based on one property do not generally coincide with units based on another, and their boundaries commonly cut across each other. It is not possible, therefore, to express all of the different properties with a single set of stratigraphic units; different sets of units are needed (see Figure 1).

Figure 1.

Illustration of the differences in position in a stratigraphic section of stratigraphic boundaries based on different properties or attributes of the rocks

Figure 1.

Illustration of the differences in position in a stratigraphic section of stratigraphic boundaries based on different properties or attributes of the rocks

At the same time, the general unity of stratigraphy should be emphasized. While many different kinds of units are needed to express the variations in all of the many different properties and attributes of the rocks, still, these units are closely related. They concern only different aspects of the same rocks, and they are intimately involved with one another in achieving the same major goals of stratigraphy—to improve our knowledge and understanding of the Earth's rock bodies and their history.

Categories of Stratigraphic Classification

Rock bodies may be classified into many different categories, each of which needs its own distinctive units. The following three kinds of formal units are the best known and the most widely used:

  1. Lithostratigraphic units—units based on the lithologic properties of the rock bodies.

  2. Biostratigraphic units—units based on the fossil content of the rock bodies.

  3. Chronostratigraphic units—units based on the time of formation of the rock bodies.

    In addition, two other kinds of stratigraphic units have become increasingly effective and formally recognized in stratigraphic work:

  4. Unconformity-bounded units—bodies of rocks bounded above and below by significant discontinuities in the stratigraphic succession.

  5. Magnetostratigraphic polarity units—units based on changes in the orientation of the remanent magnetization of the rock bodies.

Also widely used are functional, so far informal, units based on electrical properties, on seismic character, on stable isotopes, and on heavy detrital minerals. There are many others. No one can, nor need, use all possible kinds of stratigraphic units, but the way should be open to use any that promise to be useful; and it should be clear from the unit-terms to which category of classification any named unit belongs.

Though each kind of stratigraphic unit may be particularly useful in stratigraphic classification under certain conditions or in certain areas or for certain purposes, one kind—chronostratigraphic—offers the greatest promise for formally named units of worldwide application. Lithostratigraphic, biostratigraphic, and other similar kinds of stratigraphic units are restricted by the limited areal extent of the features chosen to characterize and distinguish them. Few, if any, of these features are both distinctive and present worldwide. Chronostratigraphic units, on the other hand, are based for definition on their time of deposition or formation—a universal property. In principle, they can be recognized the world over to the extent that the time-diagnostic features distinctive of the unit can be identified in the rocks.

Because chronostratigraphic units can often be recognized worldwide, they also offer the best means for international communication among stratigraphers with respect to position in the stratigraphic column. Any stratigrapher will readily understand if a colleague states that he has been studying the Jurassic, or the Miocene, or the Turanian of some area. However, if only the name of a formation, of a biostratigraphic zone, or of some other type of more local stratigraphic unit is mentioned, stratigraphers in other parts of the world may not be able to recognize even approximately the position of the unit within the stratigraphic column.

Distinguishing Terminologies for Each Category

Appropriate distinguishing terms are needed for each of the various categories of stratigraphic units. Elaborate terminologies have been developed through the years for the units of the most commonly used categories. For litho-stratigraphy and chronostratigraphy the numerous terms represent different hierarchical ranks; for biostratigraphic units they result from the recognition of various kinds of biozones and various kinds of fossils. For the units of newer or less used categories, only very simple terminologies (commonly zones of certain kinds) have been used thus far, but it may be expected that more elaborate schemes for some of these may be developed. Table 1 gives terms here recom-mended for various categories of stratigraphic units.

Table 1.

Summary of Categories and Unit-Terms in Stratigraphic

*lf additional ranks are needed, prefixes Sub and Super may be used with unit-terms when appropriate, although restraint is recommended to avoid complicating the nomenclature unnecessarily.

Chronostratigraphic and Geochronologic Units

Each rock body represents a certain interval of geologic time. Accordingly, each chronostratigraphic unit (rock body) has a corresponding geochronologic unit (interval of geologic time). Table 1 summarizes these units. Because geochronologic units are units of geologic time—an intangible property—while stratigraphic units are tangible material units composed of rocks, geochronologic units are not in themselves stratigraphic units. To illustrate the difference, a chronostratigraphic unit can be likened to the sand that flows through an hourglass during a certain interval of time, while the corresponding geochronologic unit can be compared to the interval of time during which the sand flows. It may be said that the duration of the sand flow measures a certain interval of time—an hour, for instance—but the sand itself cannot be said to be an hour.

Incompleteness of the Rock Record

Stratigraphic classification deals primarily with the Earth's rocks. However, it should be recognized that the rock record of any one area is far from continu-ous or complete. It is commonly interrupted by innumerable diastems, discontinuities, and unconformities. Short interruptions of the record may, in fact, exist in layered rocks at every bedding plane. The evidence which the rocks carry of these missing intervals is in itself a part of stratigraphy and a very important contribution to the understanding of Earth history.

Figures & Tables

Figure 1.

Illustration of the differences in position in a stratigraphic section of stratigraphic boundaries based on different properties or attributes of the rocks

Figure 1.

Illustration of the differences in position in a stratigraphic section of stratigraphic boundaries based on different properties or attributes of the rocks

Table 1.

Summary of Categories and Unit-Terms in Stratigraphic

*lf additional ranks are needed, prefixes Sub and Super may be used with unit-terms when appropriate, although restraint is recommended to avoid complicating the nomenclature unnecessarily.

Contents

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References

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