Chronostratigraphy


Chronostratigraphy

Chronostratigraphy is the branch of stratigraphy that studies the age of rock strata in relation to time.

The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of the Earth.

The standard stratigraphic nomenclature is a chronostratigraphic system based on palaeontological intervals of time defined by recognised fossil assemblages (biostratigraphy). The aim of chronostratigraphy is to give a meaningful age date to these fossil assemblage intervals and interfaces.[citation needed]

Contents

Methodology

Chronostratigraphy relies heavily upon isotope geology and geochronology to derive hard dating of known and well defined rock units which contain the specific fossil assemblages defined by the stratigraphic system. However, it is practically very difficult to isotopically date most fossils and sedimentary rocks directly, and thus inferences must be made in order to arrive at an age date which reflects the beginning of the interval.

The methodology used is derived from the law of superposition and the principles of cross-cutting relationships.

Because igneous rocks occur at specific intervals in time and are essentially instantaneous on a geologic time scale, and because they contain mineral assemblages which may be dated more accurately and precisely by isotopic methods, the construction of a chronostratigraphic column will rely heavily upon intrusive and extrusive igneous rocks.

Metamorphism, often associated with faulting, may also be used to bracket depositional intervals in a chronostratigraphic column. Metamorphic rocks can occasionally be dated, and this may give some limits to the age at which a bed could have been laid down. For example, if a bed containing graptolites overlies crystalline basement at some point, dating the crystalline basement will give a maximum age of that fossil assemblage.

However, 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

Chronostratigraphic units, with examples:[1]

Differences between chronostratigraphy and geochronology

It is important not to confuse geochronologic and chronostratigraphic units.[2] Chronostratigraphic units are geological material, so it is correct to say that fossils of the species Tyrannosaurus rex have been found in the Upper Cretaceous Series.[3] 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 that Tyrannosaurus rex lived during the Late Cretaceous Epoch.[4]

Chronostratigraphy is an important branch of stratigraphy because the age correlations derived are crucial to drawing accurate cross sections of the spatial organization of rocks and to preparing accurate paleogeographic reconstructions.

See also

References

  1. ^ Julia Jackson: Glossary of Geology, 1987, American Geological Institute, ISBN 0-922152-34-9
  2. ^ David Weishampel:The Evolution and Extinction of the Dinosaurs, 1996, Cambridge Press, ISBN 0-521-44496-9
  3. ^ Smith, J.B., Lamanna, M.C., Lacovara, K.J., Dodson, P. Jnr., Poole, J.C. and Giegengack, R. 2001. "A Giant Sauropod Dinosaur from an Upper Cretaceous Mangrove Deposit in Egypt." Science, 292: 1704-1707. [1]
  4. ^ International Commission on Stratigraphy. "Chronostratigraphic Units." International Stratigraphic Guide. Accessed 14-DEC-2009. [2]

Wikimedia Foundation. 2010.

Look at other dictionaries:

  • chronostratigraphy — noun The interpretation of geologic history by means of the determination of the ages, and time sequence of rock strata …   Wiktionary

  • chronostratigraphy — …   Useful english dictionary

  • Stratigraphy — Stratigraphy, a branch of geology, studies rock layers and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy includes two related subfields: lithologic or lithostratigraphy and… …   Wikipedia

  • Geochronology — A schematic depiction of the major events in the history of our planet. Geochronology is the science of determining the age of rocks, fossils, and sediments, within a certain degree of uncertainty inherent to the method used. A variety of dating… …   Wikipedia

  • Geologic time scale — This clock representation shows some of the major units of geological time and definitive events of Earth history. The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga.[1] Other… …   Wikipedia

  • Lithostratigraphy — is a sub dicipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general a stratum will be primarily igneous or sedimentary… …   Wikipedia

  • Chronozone — For articles dealing with the fundamentals of geologic chronology, see: Biostratigraphy#Concept of zone, chronology (geology), and geologic time scale e  h Units in geochronology and stratigraphy …   Wikipedia

  • System (stratigraphy) — NOTOC A system in the natural sciences and stratigraphy is an idealized composite unit of the geologic record made up of a succession of rock layers that were laid down together within a certain corresponding geological time span, and are used in …   Wikipedia

  • Geologic record — The layer cake like appearance of concordant strata laid down over hundreds of thousands of years …   Wikipedia

  • List of geochronologic names — This is a list of official and inofficial names for time spans in the geologic timescale and units of chronostratigraphy. Since many of the smallest subdivisions of the geologic timescale were in the past defined on regional lithostratigraphic… …   Wikipedia


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.