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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.

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