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Abstract

The categories within stratigraphic classification are all closely related. All deal with the rocks of the Earth's crust, with the picture of the stratified Earth as it presently exists, and with the history of the Earth as interpreted from its rocks. 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 circumstances. Each is important for particular purposes.

Lithostratigraphic units are the basic units of geologic surface or subsurface mapping, and lithostratigraphic classification is usually the first approach in stratigraphic work in any new area. Wherever there are rocks, it is possible to develop a lithostratigraphic classification even when no other stratigraphic procedures are feasible.

Lithostratigraphic units are based primarily on the lithologic properties of rocks—sedimentary, igneous, and metamorphic. The fossil content of lithostrati-graphic units may in certain cases be an important distinguishing element in their recognition, not because of the age significance of the fossils but because of their diagnostic lithologic (physical) properties. Coquinas, algal reefs, radio-larites, oyster beds, and coal beds are good examples.

Inasmuch as each lithostratigraphic unit was formed during a specific interval of geologic time, it has not only lithologic significance but also chrono-stratigraphic significance. The concept of time, however, properly plays little part in establishing or identifying lithostratigraphic units and their boundaries. Lithologic character is generally influenced more strongly by conditions of formation than by time of origin; similar rock types are repeated

The categories within stratigraphic classification are all closely related. All deal with the rocks of the Earth's crust, with the picture of the stratified Earth as it presently exists, and with the history of the Earth as interpreted from its rocks. 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 circumstances. Each is important for particular purposes.

Lithostratigraphic units are the basic units of geologic surface or subsurface mapping, and lithostratigraphic classification is usually the first approach in stratigraphic work in any new area. Wherever there are rocks, it is possible to develop a lithostratigraphic classification even when no other stratigraphic procedures are feasible.

Lithostratigraphic units are based primarily on the lithologic properties of rocks—sedimentary, igneous, and metamorphic. The fossil content of lithostrati-graphic units may in certain cases be an important distinguishing element in their recognition, not because of the age significance of the fossils but because of their diagnostic lithologic (physical) properties. Coquinas, algal reefs, radio-larites, oyster beds, and coal beds are good examples.

Inasmuch as each lithostratigraphic unit was formed during a specific interval of geologic time, it has not only lithologic significance but also chrono-stratigraphic significance. The concept of time, however, properly plays little part in establishing or identifying lithostratigraphic units and their boundaries. Lithologic character is generally influenced more strongly by conditions of formation than by time of origin; similar rock types are repeated time and again in the stratigraphic sequence, and the boundaries of almost all lithostratigraphic units eventually cut across isochronous surfaces as they are traced laterally.

Biostratigraphic classification is also an early step in working out the stra-tigraphy of a region. But when dealing with unfossiliferous rocks, lithostratig-raphy is the main initial approach to stratigraphic classification.

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 rock strata, except that the presence or absence of fossils and the kinds of fossils present may be related to the type and lithofacies of the rocks in which they are found.

Lithostratigraphic and biostratigraphic units are fundamentally different kinds of stratigraphic units and are based on different distinguishing criteria. The boundaries of the two may locally coincide or lie at different stratigraphic horizons or cross each other. Lithostratigraphic and biostratigraphic units differ in another respect: while, as mentioned above, all rocks (sedimentary, igneous, and metamorphic) can be subdivided into lithostratigraphic units, biostratigraphic units can be recognized only in rocks containing 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 the evolutionary changes of plants and animals.

Lithostratigraphy and biostratigraphy are invaluable initial steps in working out the relative stratigraphic succession of the rocks of an area, and they are important and continuing stratigraphic disciplines in themselves. They are the fundamental, and in some areas the only, means of stratigraphic classification. 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 only be established when the diagnostic properties on which they are based and that characterize them—the presence of bounding discontinuities in the stratigraphic record and remanent magnetization—are present in the rocks. Both kinds are objective stratigraphic units which, when recognizable, provide excellent means of stratigraphic classification and correlation.

Unconformity-bounded units may include a number of other kinds of strati-graphic units (lithostratigraphic, biostratigraphic, magnetostratigraphic, and so on), both in vertical and/or lateral succession (Figure 4). Similarly, an unconformity-bounded unit may include several chronostratigraphic units. The time span of the unit may range from a substage to one or more systems. In certain cases, at a certain locality, or over a certain area, a rock body bounded by unconformities may have an overall uniform lithology or may represent a single biostratigraphic unit. The unconformity-bounded unit will then have essentially the same boundaries as a given lithostratigraphic or biostratigraphic unit.

The boundaries of other kinds of stratigraphic units included within an unconformity-bounded unit can be parallel to, correspond with, or intersect at an angle the upper and/or lower boundaries of the unconformity-bounded unit (Figure 4). Where the upper or lower boundary of an unconformity-bounded unit is an angular unconformity or where it borders a strongly onlapping or offlap-ping post-unconformity sequence, the boundary of the unconformity-bounded unit may depart markedly from the boundaries of included or adjacent stratigraphic units. If, on the other hand, the boundary is a disconformity, it may be parallel to or correspond with the boundaries of other kinds of stratigraphic units over an extensive area.

The boundaries of unconformity-bounded units are always diachronous to a lesser or greater extent, and so they never correspond with the boundaries of chronostratigraphic units which are, by definition, always isochronous.

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 in another sense different from them. Whereas lithostratigraphic and biostratigraphic units are usually geographically restricted, magnetostratigraphic polarity units are potentially of worldwide extent and, in this respect, more similar to chronostratigraphic units.

The changes from normal to reversed polarity and vice versa 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 isochronous horizons. Consequently, the body of rock strata lying between magnetic-polarity-reversal 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 approximate chronostratigraphic units, but, strictly speaking, they are not chronostratigraphic units because they are defined primarily not by the record of time but by a specific physical property, a change of the polarity of remanent magnetization, which is not instantaneous. Similarly, magnetic-polarity-reversal horizons may coincide closely with chronohorizons but are not chronohorizons.

Moreover, because of variability in the distinctness of the imprint or in the preservation of the polarity record, because of possibilities of subsequent remag-netization, because of unconformities in the section, because of the effects of bioturbation, and for other reasons, the boundaries of a polarity unit depart from isochroneity (just as in the case of biostratigraphic or lithostratigraphic units). Polarity units and polarity-reversal horizons may contribute to the definition and correlation of valuable chonostratigraphic units, and their boundaries.

In considering the time value of magnetic-polarity horizons and units, it should be noted that although these may be very useful guides to chronostratigraphic position, they have relatively little individuality (one reversal is much the same as another) and, therefore, can usually be identified only by supporting age evidence, such as paleontologic or isotopic data.

Chronostratigraphic units are defined as encompassing all rocks formed during 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 isochronous. 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 relatively objective, material units limited to the actual occurrence of certain properties or attributes of the rocks. Whereas other kinds of stratigraphic units are largely established and distinguished on the basis of observable physical features, chronostratigraphic units are identified on the basis of their time of formation—an abstract character.

All stratigraphic units are invaluable aids to developing a chronostratigraphic classification. Because of the widespread distribution of fossils in the Earth's strata and the irreversibility of biologic evolution, fossils have provided the outstanding guide for dating and time correlation of Phanerozoic sedimentary rocks. Biostratigraphic units frequently approximate chronostratigraphic units but, although biostratigraphic correlation may approach time correlation, biostratigraphic units are fundamentally not the same as chronostratigraphic units. As illustrated in Figure 15, the boundaries of a biostratigraphic zone may diverge from a time horizon for many reasons. Principal among these are changes in depositional facies, variations in conditions for fos-silization and preservation of fossils, vagaries of fossil discovery, time required for migration of animal or plant taxa, and geographic differences in evolutionary development. Biostratigraphic units cannot be recognized at all in igneous rocks and in high-grade metamorphic sequences. Even in unaltered sedimentary rocks there are many stratigraphic intervals containing few or no fossils. Even so, the contribution of biostratigraphy to chronostratigraphy has been enormous, and many of the difficulties in the use of individual biostratigraphic units as time markers can be solved by the use of vertically and laterally interlocking and/or overlapping biozones and biohorizons.

Some lithostratigraphic units or lithostratigraphic horizons may also be excellent guides to approximate time correlation over fairly extensive areas, as in the case of volcanic ash beds, but like biostratigraphic units, lithostratigraphic units are not chronostratigraphic units because they are not bounded everywhere by isochronous surfaces.

Unconformity-bounded units and magnetostratigraphic polarity units, particularly the latter, also provide valuable support for the development of chronostratigraphic classification. The boundaries of magnetostratigraphic polarity units, because they record the very rapid reversal of the Earth's magnetic field, closely approach isochronous surfaces and, if properly identified, offer a sound foundation for worldwide time correlation and chronostratigraphic classification.

Chronostratigraphic classification, which utilizes information available from all other kinds of stratigraphic classification, stands out as the basis to reach the ultimate goal of stratigraphy and to improve the knowledge and understanding of the Earth's rock bodies and their history. Chronostratigraphic units, as divisions of rock bodies based on geologic time, are in principle worldwide in extent. Chronostratigraphic units, furthermore, are 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.

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