Magnetostratigraphy

Magnetostratigraphy is a geophysical correlation technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their characteristic remanent magnetization (ChRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. This is possible because volcanic flows acquire a thermoremanent magnetization and sediments acquire a depositional remanent magnetization, both of which reflect the direction of the Earth's field at the time of formation. This technique is typically used to date sequences that generally lack fossils or interbedded igneous rock. It is particularly useful in high-resolution correlation of deep marine stratigraphy where it allowed the validation of the Vine–Matthews–Morley hypothesis related to the theory of plate tectonics.

When measurable magnetic properties of rocks vary stratigraphically they may be the basis 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. The direction of the remnant 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" or chrons.

If the ancient magnetic field was oriented similar to today's field (North Magnetic Pole near the Geographic North Pole) the strata retains a normal polarity. If the data indicates that the North Magnetic Pole was near the Geographic South Pole, the strata exhibits reversed polarity.

A polarity chron, or in context chron, is the time interval between polarity reversals of Earth's magnetic field. It is the time interval represented by a magnetostratigraphic polarity unit. It represents a certain time period in geologic history where the Earth's magnetic field was in predominantly a "normal" or "reversed" position. Chrons are numbered in order starting from today and increasing in number into the past. As well as a number, each chron is divided into two parts, labelled "n" and "r", thereby showing the position of the field's polarity. Chrons are also referred by a capital letter of a reference sequence such as "C". A chron is the time equivalent to a chronozone or a polarity zone.

It was called a "polarity subchron" when the interval is less than 200,000 years long, although the term was redefined in 2020 to an approximate duration between 10,000 and 100,000 years and polarity chron for an approximate duration between 100,000 years and a million years. Other terms used are Megachron for a duration between 10⁸ and 10⁹ years, Superchron for a duration between 10⁷ and 10⁸ years and Crytochron for a duration less than 3×10⁴ years.

The nomenclature for the succession of polarity intervals, especially when changes are of short durations, or not universal (the earth's magnetic field is complex) is challenging, as each new discovery has to be inserted (or if not validated, removed). The two standardised marine magnetic anomalies sequences are the "C-sequence" and "M-sequence" and cover from the Middle Jurassic to date. Accordingly, the main C polarity chrons series extend backwards from the current C1n, commonly termed Brunhes, with the most recent transition at C1r, commonly termed Matuyama, at 0.773 Ma which is the Brunhes–Matuyama reversal. The C (for Cenozoic) sequence ends in the Cretaceous Normal Superchron termed C34n which on age calibration occurred at 120.964 Ma and lasted to Chron C33r at 83.650  Ma that defined the Santonian geologic age. The M series is defined from M0, with full label M0r, at 121.400 Ma, which is the beginning of the Aptian to M44n.2r which is before 171.533 Ma in the Aalenian.

Subdivisions in the sequencies also have specific nomenclature so C8n.2n is the second oldest normal polarity subchron comprising normal-polarity Chron C8n and the youngest cryptochron, the Emperor cryptochron, is named C1n-1. Certain terms in the literature such as M-1r to describe a postulated brief reversal at about 118 Ma are provisional.

Oriented paleomagnetic samples are collected in the field using a rock core drill, or as hand samples (chunks broken off the rock face). To average out sampling errors, a minimum of three samples is taken from each sample site. Spacing of the sample sites within a stratigraphic section depends on the rate of deposition and the age of the section. In sedimentary layers, the preferred lithologies are mudstones, claystones, and very fine-grained siltstones because the magnetic grains are finer and more likely to orient with the ambient field during deposition.