تتابع طبقي زمني

(تم التحويل من Chronostratigraphy)

تتابع طبقي زمني Chronostratigraphy هي فرع من التتابع الطبقي الذي يدرس عمر طبقات الصخور بالنسبة للزمن.

والتتابع الطبقي يكون خلال فترة زمنية محددة من الزمن البيولوجي، ويتحدد بحدود معرفة زمنيا. وتشمل الوحدات الرئيسية لمقياس الزمن الطبقي، مثل: صخور الدهر eonothem وصخور الحقبة erathemK، والنظام system، والنسق series، والمرحلة stage، وأخيرا نطاق زمني chronozone.[1]

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. In practice, as it is very difficult to isotopically date most fossils and sedimentary rocks directly, 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 relies 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 limitations to the age in 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.

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:[2]

Differences from geochronology

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 species Tyrannosaurus rex have 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 that Tyrannosaurus rex lived during the Late Cretaceous Epoch.[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.

طالع أيضاً

المصادر

  1. ^ عبد الجليل هويدي، محمد أحمد هيكل (2004). أساسيات الجيولوجيا التاريخية. مكتبة الدار العربية للكتب.
  2. ^ Jackson, Juia A., ed. (1997). Glossary of Geology (4th ed.). Alexandria, Virginia: American Geological Institute. ISBN 978-0-922152-34-6.
  3. ^ Schoch, Robert M. (1989). Stratigraphy: Principles and Methods. New York: Van Nostrand Reinhold. p. 351. ISBN 978-0-442-28021-5.
  4. ^ Fastovsky, David E.; Weishampel, David B. (1996). The Evolution and Extinction of the Dinosaurs. Cambridge, England: Cambridge University Press. pp. 22–23. ISBN 978-0-521-44496-5.
  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. ISSN 1095-9203. PMID 11387472. S2CID 33454060.
  6. ^ Salvador, Amos, ed. (1994). "Chapter 9. Chronostratigraphic Units". Stratigraphic Guide (Second (abridged) ed.). International Commission on Stratigraphy. Archived from the original on 12 June 2020.
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