Magnetostratigraphic Polarity Units
Published:January 01, 2013
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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International Stratigraphic Guide
This reprint of the 1994 volume was produced at the request of the IUGS International Commission on Stratigraphy. The purpose of the 1994 volume was to promote international agreement on principles of stratigraphic classification and to develop an internationally acceptable stratigraphic terminology and rules of stratigraphic procedure. At the time of its first printing, this second edition was the most up-to-date statement of international agreement on concepts and principles of stratigraphic classification and a guide to international stratigraphic terminology. The first edition, published in 1976, was a significant contribution toward international agreement and improvement in communication and understanding among earth scientists worldwide. The revised, second edition updated and expanded the discussions, suggestions, and recommendations of the first edition, expansions necessitated by the growth and progress of stratigraphic ideas and the development of new stratigraphic procedures since release of the first edition.