Chapter 8. Magnetostratigraphic polarity units
A. Nature of Magnetostratigraphic Polarity Units
When measurable magnetic properties of rocks 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 geologic history.They are recorded in the rocks because the rocks become magnetized in the direction of the Earth's magnetic field at the time of their formation. The direction of the remanent magnetic polarity recorded in the 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 the rock is said to have "normal agnetization", or "normal polarity".Conversely, if it points to 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.
A problem arises because the north paleomagnetic pole is believed to have crossed the geographic equator in Paleozoic time, so that for some lower Paleozoic and older rocks it is unclear which is the direction of the North Pole and which the South Pole.Polarity must in these cases be defined with respect to the apparent polar wander path (APWP) for the crustal plate where it is found. 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 degrees from this, it has reversed polarity.
Magnetostratigraphic polarity units have been established in two ways:
1. combining the determination of the orientation of the remanent magnetization of sedimentary or volcanic rocks from outcrops or cored sections with their age determined by isotopic or biostratigraphic methods;
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 lava of the sea floor during the sea-floor-spreading process. It has been shown that the two kinds of investigation are correlative and record the same causative process.
The first type may be handled by using normal stratigraphic procedures.
Units of the second type, currently identified by "anomaly numbers", are deduced from a remotely obtained record of the overall variations of the geomagnetic field from unseen rocks on or below the sea floor.Marine magnetic anomalies are, thus, not true conventional stratigraphic units.
However, they are useful units in the reconstruction of continental plate motions and in the interpretation of the geologic history of the ocean basins.
The relation of magnetostratigraphic polarity units to other kinds of stratigraphic units is discussed in Chapter 10.
B. Definitions
1. Magnetostratigraphy. The element of stratigraphy that deals with the magnetic characteristics of rock bodies.
2. Magnetostratigraphic classification. The organization of rock bodies into units based on differences in magnetic character.
3. Magnetostratigraphic unit (magnetozone). A body of rocks unified by similar magnetic characteristics which allow it to be differentiated from adjacent rock bodies.
4. 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.
5. Magnetostratigraphic polarity unit. A body of rocks characterized by its magnetic polarity that allows it to be differentiated from adjacent rock bodies.
6. Magnetostratigraphic polarity-reversal horizons and polarity-transition zones.
Magnetostratigraphic polarity-reversal horizons are surfaces or thin transition intervals across which the magnetic polarity reverses.
Where the polarity change takes place through a 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 polarity-transition zones provide the boundaries for magnetostratigraphic polarity units.
C. Kinds of magnetostratigraphic polarity units
The basic formal unit in magnetostratigraphic polarity classification is the magnetostratigraphic polarity zone, or simply polarity zone. Polarity zones may be subdivided into polarity subzones and grouped into polarity superzones.
Magnetostratigraphic polarity zones may consist of bodies of strata unified by:
1. a single polarity of magnetization;
2. an intricate alternation of normal and reversed polarity of magnetization;
3. having dominantly either normal or reversed polarity, but with minor intervals of the opposite polarity.
D. Procedures for establishing magneto-stratigraphic polarity units. See section 3.B.
Standards of reference and stratotypes for polarity units require special treatment. The standard of reference for the definition and recognition of a magnetostratigraphic polarity unit for land-based units is a designated stratotype in a continuous sequence of strata that shows its polarity pattern throughout and clearly defines its upper and lower limits by means of boundary stratotypes. These are marked with artificial permanent markers to facilitate restudy.
The standard of reference of marine-based units is a designated profile along a designated traverse with all instrumental and guidance conditions specified.
This pattern of polarity reversals from the ocean floor should be dated by extrapolation and interpolation from isotopic and paleontologic information.
E. Procedures for Extending Magnetostratigraphic Polarity Units
A magnetostratigraphic polarity unit and its boundaries may be extended away from its type locality or stratotype only as far as the magnetic properties and stratigraphic position of the unit can be identified.
F. Naming of Magnetostratigraphic Polarity Units. See section 3.B.3.
The formal name of a magnetostratigraphic polarity unit is formed from the name of an appropriate geographic feature combined with a term indicating its rank and direction of polarity, e.g. Jaramillo Normal Polarity Zone.
The currently well-established names derived from the names of distinguished contributors to the science of geomagnetism (for example, Brunhes, Gauss, Matuyama) should not be replaced.
Numbered or lettered units may be used informally, but this is not recommended as a general practice.
However, the classic linear magnetic anomalies of the ocean floor are excepted, because of their historical importance and dominance in the literature.
The time interval represented by a magnetostratigraphic polarity unit is called a chron (superchron or subchron if necessary). Chronozone is the term used to refer to the rocks formed anywhere during a particular magnetostratigraphic polarity chron (Table 2).
G. Revision of Magnetostratigraphic Polarity Units. See Section 3.B.5.