Skip to Main Content

Abstract

Measurable magnetic properties of rocks, such as magnetic susceptibility and the intensity and direction of natural remanent magnetization (NRM), can be used in stratigraphic classification. NRM may indicate a number of useful properties of the magnetic field: reversals of polarity, the dipole-field-pole position (which shows apparent polar wander due to plate motions), nondipole components (secular variation), and variations in field intensity. Where any of these characters vary stratigraphically, they may be the bases for related but different kinds of stratigraphic units known collectively as magnetostratigraphic units (magnetozones).

The magnetic property most useful in stratigraphic work is the change in the direction of the remanent magnetization of the rocks, caused by reversals in the polarity of the Earth's magnetic field. Such reversals of the polarity have taken place many times during the geologic history of the Earth. They are recorded by the rocks because the rocks become magnetized in the direction of the Earth's magnetic field at the time of their formation. If it can be demonstrated that the direction of the magnetic polarity as measured today in the rocks is indeed the direction originally acquired by those rocks, rather than the result of later remagnetization, then the changes of the direction of the magnetic polarity recorded in a stratigraphic sequence can be used as the basis for the subdivision of the sequence into units characterized by their magnetic polarity. Such units are called magnetostratigraphic polarity units. A magnetostratigraphic polarity unit is present only where this property can be identified in the rocks.

Nature of Magnetostratigraphic Polarity Units

Measurable magnetic properties of rocks, such as magnetic susceptibility and the intensity and direction of natural remanent magnetization (NRM), can be used in stratigraphic classification. NRM may indicate a number of useful properties of the magnetic field: reversals of polarity, the dipole-field-pole position (which shows apparent polar wander due to plate motions), nondipole components (secular variation), and variations in field intensity. Where any of these characters vary stratigraphically, they may be the bases for related but different kinds of stratigraphic units known collectively as magnetostratigraphic units (magnetozones).

The magnetic property most useful in stratigraphic work is the change in the direction of the remanent magnetization of the rocks, caused by reversals in the polarity of the Earth's magnetic field. Such reversals of the polarity have taken place many times during the geologic history of the Earth. They are recorded by the rocks because the rocks become magnetized in the direction of the Earth's magnetic field at the time of their formation. If it can be demonstrated that the direction of the magnetic polarity as measured today in the rocks is indeed the direction originally acquired by those rocks, rather than the result of later remagnetization, then the changes of the direction of the magnetic polarity recorded in a stratigraphic sequence can be used as the basis for the subdivision of the sequence into units characterized by their magnetic polarity. Such units are called magnetostratigraphic polarity units. A magnetostratigraphic polarity unit is present only where this property can be identified in the rocks.

The positive direction of magnetization of a rock is, by definition, its “north-seeking magnetization” (it points toward the Earth's present magnetic North Pole), and a rock with positive magnetization is said to have “normal magnetization,” or “normal polarity.” Conversely, if the direction of magnetization points toward the present magnetic South Pole, the rock is said to have “reversed magnetization,” or “reversed polarity.” Magnetostratigraphic polarity units are, therefore, either normal or reversed.

The identification of the magnetic polarity can be uncertain when, due to plate motions, the north paleomagnetic pole was at one time located in the southern hemisphere, as was the case for the North American plate during much of the Paleozoic. Polarity must then be defined with respect to the apparent polar wander path (APWP) for each crustal plate. If the direction of magnetization of a rock unit indicates a paleomagnetic pole that falls on the APWP that terminates at the present North Pole, the rock unit has normal polarity. If the magnetization is directed 180° from this, it has reversed polarity. In the case of displaced terranes, it may not be possible to assign a magnetic polarity based on paleomagnetic data alone.

Magnetostratigraphic polarity units have been established in two principal and different ways: (1) by combining the determination of the orientation of the remanent magnetization of sedimentary or volcanic rocks from outcrops or cored sections with their age as determined by isotopic or biostratigraphic methods, or (2) through the use of shipboard magnetometer profiles from ocean surveys to identify and correlate linear magnetic anomalies that are interpreted as reflecting reversals of the Earth's magnetic field, recorded in the lavas of the sea floor during the sea-floor-spreading process. It has been demonstrated many times that the patterns of magnetic reversals obtained from the two different kinds of investigation are correlative and record the same causative process.

The first type may be handled using normal stratigraphic procedures. Units of the second type, currently identified by “anomaly numbers,” are less satisfactorily handled by conventional procedures of stratigraphic classification because, although the units are due to sequential variations in rock magnetic polarity, they are not measured directly on the rocks themselves. Rather, they are deduced from a remotely obtained record of the overall variations of the geomagnetic field from unseen rocks on or beneath the sea floor. For that reason, the marine magnetic anomalies are not true conventional stratigraphic units. However, they have been, and continue to be, extremely useful units, invaluable in the reconstruction of plate motions and in interpreting the geologic history of the oceanic areas. The marine magnetic anomalies also provide the most complete record of reversals of the Earth's magnetic field during the last 150 million years.

The relation of magnetostratigraphic polarity units to other kinds of stratigraphic units is discussed in Chapter 10.

Definitions

Magnetostratigraphy.

The element of stratigraphy that deals with the magnetic characteristics of rock bodies.

Magnetostratigraphic Classification.

The organization of rock bodies into units based on differences in magnetic character.

Magnetostratigraphic Unit (Magnetozone).

A body of rocks unified by similar magnetic characteristics (not only magnetic polarity) which allow it to be differentiated from adjacent rock bodies.

Magnetostratigraphic Polarity Classification.

The organization of rock bodies into units based on changes in the polarity of their remanent magnetization, related to reversals in the polarity of the Earth's magnetic field. Such changes appear to have taken place many times in the Earth's history.

Magnetostratigraphic Polarity Unit.

A body of rocks characterized by its magnetic polarity that allows it to be differentiated from adjacent rock bodies.

Magnetostratigraphic Polarity-reversal Horizons and Polarity-transition Zones.

Surfaces or very thin transition intervals separating sequences of rock strata of opposite magnetic polarity are called magnetostratigraphic polarity-reversal horizons. Where the polarity change takes place through a more gradual and substantial interval of strata, of the order of 1 m in thickness, the term magnetostratigraphic polarity-transition zone should be used. Magnetostratigraphic polarity-reversal horizons and magnetostratigraphic polarity-transition zones may be referred to simply as polarity-reversal horizons and polarity-transition zones if in the context it is clear that the reference is to changes in magnetic polarity. Polarity-reversal horizons or polarity-transition zones provide the boundaries for magnetostratigraphic polarity units.

Kinds of Magnetostratigraphic Polarity Units

The basic formal unit in the classification of magnetostratigraphic polarity units is the magnetostratigraphic polarity zone. A magnetostratigraphic polarity zone may be referred to simply as a polarity zone if in the context it is clear that the reference is to magnetic polarity. Polarity zones may be subdivided into polarity subzones and grouped into polarity superzones. If units larger than a superzone or smaller than a subzone are needed, the terms polarity megazone and polarity microzone may be used. The rank of a polarity unit may be changed when deemed appropriate.

Magnetostratigraphic polarity zones may consist of (1) rock bodies with a single polarity of magnetization throughout, (2) an intricate alternation of normal and reversed units (mixed polarity), or (3) an interval of dominantly either normal or reversed polarity, containing minor subdivisions of the opposite polarity. (Thus, a zone of dominantly normal polarity may include lesser-rank units of reversed polarity.)

Procedures for Establishing Magnetostratigraphic Polarity Units

General procedure for establishing stratigraphic units is discussed in section 3.B, and the procedure for magnetostratigraphic polarity units accords closely with that for other stratigraphic units in most respects. As in the case of other stratigraphic units, a statement of intent and a comprehensive definition should accompany the proposal of any new polarity unit or the redefinition of any already existing unit.

The matter of standards of reference and stratotypes for polarity units requires special treatment because of their nature and the history of their origin.

The best sequential record of reversals of the Earth's magnetic field for the past 150 million years is preserved in the pattern of sea-floor-spreading anomalies. This pattern of polarity reversals has been dated by extrapolation and interpolation from isotopic and paleontologic evidence. The reversal pattern observed in outcrop sequences has been correlated with the sea-floor-spreading anomaly pattern. However, because of the nature of these numbered linear magnetic-polarity anomalies from the ocean floor, it is not possible to designate satisfactory type intervals or type boundaries (conventional stratotypes) for them, in spite of their demonstrated significance as a basis for useful polarity units. Instead, the standards of reference for the marine magnetic anomalies are profiles, and the boundaries of the units are determined by model fitting.

Ideally, the standard for the definition and recognition of a magnetostratigraphic polarity unit should be a clearly designated stratotype in a continuous sequence of rock strata—a specific section showing the polarity pattern of the unit throughout and clearly defining its upper and lower limits by means of boundary-stratotypes. This procedure provides a definite tie of a polarity unit with a known stratigraphic section as well as a definite reference base from which the unit can be extended geographically and to which other workers can come to check their identification of the unit with the original concept. Such stratotypes should be designated for all polarity units for which stratigraphic sections either in outcrop or in cored holes are available.

Moreover, the stratotype of a polarity unit should ideally include both a lower boundary-stratotype and an upper boundary-stratotype. In case there is a gradual transition between two polarity zones, an arbitrary boundary within the transition may be selected, or a transition-zone between the two zones may be formally recognized and defined. The problem is much the same as with lithostratigraphic units. However, because of the difficulty of reidentifying a boundary-stratotype of a polarity unit without elaborate field and laboratory work, it is highly desirable that at the time of establishment, such boundary-stratotypes be identified by permanent artificial markers.

Procedures for Extending Magnetostratigraphic Polarity Units

A magnetostratigraphic polarity unit and its boundaries should be extended away from its stratotype or type locality only as far as the definite magnetic properties and stratigraphic position of the unit can be identified with certainty.

Naming of Magnetostratigraphic Polarity Units

The naming of magnetostratigraphic polarity units should follow the general rules for naming stratigraphic units (see section 3.B.3). The formal name of a specifically established and adequately described rock body characterized by either normal, reversed, or mixed magnetic polarity that permits it to be differentiated from adjacent rocks should be formed from the name of an appropriate local geographic feature, combined with the appropriate term for its rank—polarity superzone, zone, or subzone, etc. Where it is possible to indicate direction of polarity of a unit unambiguously, it is helpful to include the words normal, reversed, or mixed as part of the formal unit-name; for example, Jaramillo Normal Polarity Subzone. The currently well-established names derived from the names of distinguished contributors of the past to the science of geomagnetism (for example, Brunhes, Gauss, Matuyama) should not, however, be replaced. A name properly established for a formal stratigraphic unit of any other kind should be considered to be pre-empted and not available for a polarity unit and vice versa.

At present, most polarity units are unnamed or have been given number or letter designations, most commonly the numbers of the classic linear magnetic anomalies of the ocean floor, which, at the time of their establishment, were given numbers in order from youngest to oldest. This system is firmly entrenched in the literature and is being usefully employed. It should, therefore, not be discarded even though, as mentioned earlier, the ocean-floor linear magnetic anomalies are not conventional stratigraphic units and need not comply rigidly with the rules of formal stratigraphic nomenclature.

The failure to give names of geographic features to magnetostratigraphic polarity units may be due in some cases to the difficulty in finding available geographic names, in other cases to the reluctance to introduce a multitude of new names into a stratigraphic terminology already overloaded with names. Numbers and letters to name polarity units, useful as they may be as provisional designations, should, however, be considered as informal terms. In the interest of preventing confusion, the general principle of avoiding numbers and letters for formal stratigraphic units should be maintained for polarity units as well as for other stratigraphic units (see section 7.H, last paragraph).

The use of the terms “epoch,” “event,” and “interval” as previously applied to magnetostratigraphic polarity units is inappropriate and undesirable and should be discouraged. “Epoch” has long been applied to formal geochronologic units of the Standard Global Chronostratigraphic (Geochronologic) Scale, as in Late Jurassic Epoch; the word “event” expresses a happening, not an interval, either of time or of rock strata; and the use of “interval” as a formal term for time units corresponding to the time scope of magnetostratigraphic polarity units is confusing because interval is a word commonly used to refer either to a geochronologic interval or to a stratigraphic interval—either to time or to space. Appropriate terms are already available in magnetostratigraphic terminology: “chron” or “polarity chron” instead of “epoch”; “chron” or “chronozone” instead of “event,” depending on whether the term is used in a geochronologic or chronostratigraphic sense; and “chron” instead of “interval.”

The time interval represented by a magnetostratigraphic polarity zone is called a chron (superchron or subchron as necessary), a geochronologic term; for example the Gauss Chron. To refer to all the rocks anywhere in the world of the same age as a certain magnetostratigraphic polarity zone, we would use the term chronozone or subchronozone, a chronostratigraphic term; for example, the Gauss Chronozone. If it appears necessary in these examples to make clear that the name “Gauss” is derived from a polarity unit, this may be accomplished by the wording “Gauss Polarity Chron” and “Gauss Polarity Chronozone” (see Chapter 9, Chronostratigraphic Units).

Attempts to develop a special set of “magnetic time” terms should be discouraged. There is no such thing as “magnetic time,” just as there is no such thing as “biologic time,” nor any such thing as “isotopic time.” In stratigraphy, there is only one kind of time, although there are many means for determining time or position of strata with respect to time—paleontologic, magnetic, isotopic, and so forth. Consequently, there should be only one standard set of geologic-time terms, to which all methods of time-determination should be related.

Recommended terminology for magnetostratigraphic polarity units and their chronostratigraphic and geochronologic equivalents is summarized in Table 2. The terminology for magnetostratigraphic polarity units as applied to the last 3.5 million years of Earth's history is shown in Figure 12.

Table 2.

Recommended Terminology for Magnetostratigraphic Polarity Units

Magnetostratigraphic polarity unitsChronostratigraphic equivalentGeochronologic equivalent
Polarity superzoneChronozone (or superchronozone)Chron (or superchron)
Polarity zoneChronozoneChron
Polarity subzoneChronozone (or subchronozone)Chron (or subchron)
Magnetostratigraphic polarity unitsChronostratigraphic equivalentGeochronologic equivalent
Polarity superzoneChronozone (or superchronozone)Chron (or superchron)
Polarity zoneChronozoneChron
Polarity subzoneChronozone (or subchronozone)Chron (or subchron)
Figure 12.

Example of terminology for magnetostratigraphic polarity units as applied to the last 3.5 million years of Earth's history. Numerical time scale from Cande and Kent (1992).

Figure 12.

Example of terminology for magnetostratigraphic polarity units as applied to the last 3.5 million years of Earth's history. Numerical time scale from Cande and Kent (1992).

Revision of Magnetostratigraphic Polarity Units

Revision or redefinition of an adequately established polarity unit without changing its name requires as much justification and the same kind of information as for establishing a new unit and generally requires the same procedures.

Figure 12. Example of terminology for magnetostratigraphic polarity units as applied to the last 3.5 million years of Earth's history. Numerical time scale from Cande and Kent (1992).

Figures & Tables

Figure 12.

Example of terminology for magnetostratigraphic polarity units as applied to the last 3.5 million years of Earth's history. Numerical time scale from Cande and Kent (1992).

Figure 12.

Example of terminology for magnetostratigraphic polarity units as applied to the last 3.5 million years of Earth's history. Numerical time scale from Cande and Kent (1992).

Table 2.

Recommended Terminology for Magnetostratigraphic Polarity Units

Magnetostratigraphic polarity unitsChronostratigraphic equivalentGeochronologic equivalent
Polarity superzoneChronozone (or superchronozone)Chron (or superchron)
Polarity zoneChronozoneChron
Polarity subzoneChronozone (or subchronozone)Chron (or subchron)
Magnetostratigraphic polarity unitsChronostratigraphic equivalentGeochronologic equivalent
Polarity superzoneChronozone (or superchronozone)Chron (or superchron)
Polarity zoneChronozoneChron
Polarity subzoneChronozone (or subchronozone)Chron (or subchron)

Contents

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal