Chronostratigraphy
Chronostratigraphyis the branch ofstratigraphythat studies the ages of rockstratain relation totime.
The ultimate aim of chronostratigraphy is to arrange the sequence ofdepositionand the time of deposition of all rocks within ageologicalregion, and eventually, the entire geologic record of theEarth.
The standard stratigraphicnomenclatureis a chronostratigraphic system based onpalaeontologicalintervals of time defined by recognisedfossilassemblages (biostratigraphy). The aim of chronostratigraphy is to give a meaningfulage dateto these fossil assemblage intervals and interfaces.[1]
Methodology[edit]
Chronostratigraphy relies heavily uponisotope geologyandgeochronologyto derive hard dating of known and well defined rock units which contain the specific fossil assemblages defined by the stratigraphic system. In practice, as it is very difficult toisotopicallydate most fossils andsedimentary rocksdirectly, inferences must be made in order to arrive at an age date which reflects the beginning of the interval.
Themethodologyused is derived from thelaw of superpositionand the principles ofcross-cutting relationships.
Becauseigneousrocks occur at specific intervals in time and are essentially instantaneous on a geologic time scale, and because they containmineralassemblages which may be dated more accurately and precisely by isotopic methods, the construction of a chronostratigraphic column relies heavily uponintrusiveandextrusiveigneous rocks.
Metamorphism,often associated withfaulting,may also be used to bracket depositional intervals in a chronostratigraphic column. Metamorphic rocks can occasionally be dated, and this may give some limitations to theagein which a bed could have been laid down. For example, if a bed containinggraptolitesoverliescrystallinebasementat some point, dating the crystalline basement will give a maximum age of that fossil assemblage.
This process requires a considerable degree of effort and checking of field relationships and age dates. For instance, there may be many millions of years between a bed being laid down and an intrusive rock cutting it; the estimate of age must necessarily be between the oldest cross-cutting intrusive rock in the fossil assemblage and the youngest rock upon which the fossil assemblage rests.
Units[edit]
Chronostratigraphic units, with examples:[2]
- eonothem –Phanerozoic
- erathem –Paleozoic
- system –Ordovician
- series – Upper Ordovician
- stage – Ashgill
Differences from geochronology[edit]
It is important not to confuse geochronologic and chronostratigraphic units.[3][4]Chronostratigraphic units are geological material, so it is correct to say that fossils of the speciesTyrannosaurus rexhave been found in the Upper Cretaceous Series.[5]Geochronological units are periods of time and take the same name as standard stratigraphic units but replacing the terms upper/lower with late/early. Thus it is also correct to say thatTyrannosaurus rexlived during theLate CretaceousEpoch.[6]
Chronostratigraphy is an important branch of stratigraphy because the age correlations derived are crucial in drawing accurate cross sections of the spatial organization of rocks and in preparing accurate paleogeographic reconstructions.
See also[edit]
- Biostratigraphy
- Chronozone
- Geochronology
- Geologic record
- Geologic time scale
- List of geochronologic names
- Tectonostratigraphy
References[edit]
- ^Subcommission on Quaternary Stratigraphy (2002)."Chronostratigraphy".International Commission on Stratigraphy(ICS).Archivedfrom the original on 28 July 2018.
- ^Jackson, Juia A., ed. (1997).Glossary of Geology(4th ed.). Alexandria, Virginia: American Geological Institute.ISBN978-0-922152-34-6.
- ^Schoch, Robert M. (1989).Stratigraphy: Principles and Methods.New York: Van Nostrand Reinhold. p. 351.ISBN978-0-442-28021-5.
- ^Fastovsky, David E.; Weishampel, David B. (1996).The Evolution and Extinction of the Dinosaurs.Cambridge, England: Cambridge University Press. pp. 22–23.ISBN978-0-521-44496-5.
- ^Smith, Joshua B.; Lamanna, Matthew C.; Lacovara, Kenneth J.; Dodson, Peter Jr.; Poole, Jason C.; Smith, Jennifer R.; Giegengack, Robert; Attia, Yousry (2001)."A Giant Sauropod Dinosaur from an Upper Cretaceous Mangrove Deposit in Egypt"(PDF).Science.292(5522): 1704–1706.Bibcode:2001Sci...292.1704S.doi:10.1126/science.1060561.ISSN1095-9203.PMID11387472.S2CID33454060.
- ^Salvador, Amos, ed. (1994). "Chapter 9. Chronostratigraphic Units".Stratigraphic Guide(Second (abridged) ed.). International Commission on Stratigraphy.Archivedfrom the original on 12 June 2020.
