Editing Stratigraphy (section)

==Chronostratigraphy==
{{main|Chronostratigraphy}}
Chronostratigraphy is the branch of stratigraphy that places an absolute age, rather than a relative age on rock [[stratum|strata]]. The branch is concerned with deriving [[geochronology|geochronological]] data for rock units, both directly and inferentially, so that a sequence of time-relative events that created the rocks formation can be derived. The ultimate aim of chronostratigraphy is to place dates on the sequence of deposition of all rocks within a geological region, and then to every region, and by extension to provide an entire geologic record of the Earth.

A gap or missing strata in the geological record of an area is called a stratigraphic hiatus. This may be the result of a halt in the deposition of sediment. Alternatively, the gap may be due to removal by erosion, in which case it may be called a stratigraphic vacuity.<ref>{{Cite web|url=http://www.sepmstrata.org/Terminology.aspx?id=hiatus|title=SEPM Strata|website=sepmstrata.org}}</ref><ref name="Martinsen">Martinsen, O. J. ''et al.'' (1999) "Cenozoic development of the Norwegian margin 60&ndash;64N: sequences and sedimentary response to variable basin physiography and tectonic setting" pp. 293&ndash;304 ''In'' Fleet, A. J. and Boldy, S. A. R. (editors) (1999) ''Petroleum Geology of Northwest Europe'' Geological Society, London, [https://books.google.com/books?id=YTqHqJfXwYEC&pg=PA295 page 295], {{ISBN|978-1-86239-039-3}}</ref> It is called a ''hiatus'' because deposition was ''on hold'' for a period of time.<ref>Kearey, Philip (2001). ''Dictionary of Geology''  (2nd ed.) London, New York, etc.: Penguin Reference, London, p. 123. {{ISBN|978-0-14-051494-0}}.</ref> A physical gap may represent both a period of non-deposition and a period of erosion.<ref name="Martinsen" />  A geologic fault may cause the appearance of a hiatus.<ref>Chapman, Richard E. (1983) ''Petroleum Geology'' Elsevier Scientific, Amsterdam, [https://books.google.com/books?id=I8rXayF_0HIC&pg=PA33 p. 33], {{ISBN|978-0-444-42165-4}}.</ref>

===Magnetostratigraphy===
{{main|Magnetostratigraphy|Paleomagnetism}}
[[File:Oceanic.Stripe.Magnetic.Anomalies.Scheme.svg|thumb|Example of [[magnetostratigraphy]]. [[magnetic striping|Magnetic stripes]] are the result of reversals of the Earth's [[Poles of astronomical bodies#Magnetic poles|magnetic pole]]s and [[seafloor spreading]]. New oceanic crust is magnetized as it forms and then it moves away from the [[midocean ridge]] in both directions.]]

[[Magnetostratigraphy]] is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout a section. The samples are analyzed to determine their detrital [[wikt:remanent|remanent]] magnetism (DRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. For sedimentary rocks this is possible because, as they fall through the water column, very fine-grained magnetic minerals (<&nbsp;17&nbsp;[[micrometre|μm]]) behave like tiny [[compass]]es, orienting themselves with [[Earth's magnetic field]]. Upon burial, that orientation is preserved. For volcanic rocks, magnetic minerals, which form in the melt, orient themselves with the ambient magnetic field, and are fixed in place upon crystallization of the lava.

Oriented paleomagnetic core samples are collected in the field; [[mudstone]]s, [[siltstone]]s, and very fine-grained [[sandstone]]s are the preferred lithologies because the magnetic grains are finer and more likely to orient with the ambient field during deposition. If the ancient magnetic field were oriented similar to today's field ([[North Magnetic Pole]] near the [[North Pole|North Rotational Pole]]), the strata would retain a normal polarity. If the data indicate that the North Magnetic Pole were near the [[South Pole|South Rotational Pole]], the strata would exhibit reversed polarity.

Results of the individual samples are analyzed by removing the [[natural remanent magnetization]] (NRM) to reveal the DRM. Following statistical analysis, the results are used to generate a local magnetostratigraphic column that can then be compared against the Global Magnetic Polarity Time Scale.

This technique is used to date sequences that generally lack fossils or interbedded igneous rocks. The continuous nature of the sampling means that it is also a powerful technique for the estimation of sediment-accumulation rates.