3.4 Magnetostratigraphical units
3.4.1 General properties, magnetopolarity units
Magnetostratigraphical classification is based on the characteristic remanent-magnetic properties of rocks and superficial deposits. Four basic types of palaeomagnetic phenomena can be determined or inferred from remanent magnetism: (a) polarity, (b) dipole-field-pole position (including apparent polar wander), (c) the non-dipole component (secular variation), and (d) field intensity.
Many palaeomagnetic signatures in a rock reflect earth magnetism at the time the rock was formed. Physical and chemical changes in the rock may have led to one or more of the remanent-magnetic components in the rock reproducing the magnetic field of the Earth at a time subsequent to when the rock was formed. To be capable of interpretation the palaeomagnetic properties must therefore be relatable to the geological history of the rock.
In accordance with NACSN (1983), only magnetopolarity units are here considered to belong to this category. Magnetopolarity is the "imprint" in the rock or superficial deposit of changes and variations in the polarity history of the Earth's magnetic-dipole field. A magnetopolarity unit is defined on the basis of a type section in which the lithostratigraphical and/or biostratigraphical units are specified (see Section 2.4).
Definition of a magnetopolarity unit does not require knowledge of the time at which the unit acquired its remanent magnetism; this magnetism may be primary, dating from when the rock was formed, or secondary. Demonstration of the primary origin of the magnetopolarity properties entails the use of petrographical criteria and of correlations using biostratigraphy and numerical age determinations. Magnetopolarity units can therefore not be used as an independent basis for chronostratigraphical classification, even though their boundaries are considered to be synchronous (see Sections 4.5 and 4.6).
The fundamental magnetopolarity unit is the polarity zone.