Stratigraphy (geology)

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Aeonothem Arathem system Age
( mya )
Phanerozoic
Duration: 541 Ma 		Cenozoic
Modern Earth
Duration: 66 Ma 		quaternary 0
⬍
2.588
Neogene 2,588
⬍
23.03
Paleogene 23.03
⬍
66
Mesozoic
Middle Ages
Duration: 186.2 Ma 		chalk 66
⬍
145
law 145
⬍
201.3
Triad 201.3
⬍
251.9
Paleozoic
Paleozoic
period: 288.8 Ma 		Perm 251.9
⬍
298.9
Carbon 298.9
⬍
358.9
Devon 358.9
⬍
419.2
Silurian 419.2
⬍
443.4
Ordovician 443.4
⬍
485.4
Cambrian 485.4
⬍
541
P
r
ä
k
a
m
b
r
i
u
m
Length: 4,059 Ma 		Proterozoic
Duration: 1,959 Ma 		Neoproterozoic
Young Proterozoic
Duration: 459 Ma 		Ediacarium 541
⬍
635
Cryogenium 635
⬍
720
Tonium 720
⬍
1000
Mesoproterozoic
Middle Proterozoic
Duration: 600 Ma 		Stenium 1000
⬍
1200
Ectasium 1200
⬍
1400
Calymmium 1400
⬍
1600
Paleoproterozoic
Ancient Proterozoic
Duration: 900 Ma 		Statherium 1600
⬍
1800
Orosirium 1800
⬍
2050
Rhyacium 2050
⬍
2300
Siderium 2300
⬍
2500
Archean
Period: 1,500 Ma 		Neo-Archaic
Duration: 300 Ma 		2500
⬍
2800
Mesoarchean
Duration: 400 Ma 		2800
⬍
3200
Paleoarchean
Duration: 400 Ma 		3200
⬍
3600
Eoarchic
Duration: 400 Ma 		3600
⬍
4000
Hadaikum
Duration: 600 Ma 		4000
⬍
4600

The stratigraphy (from Latin stratum "layer" and -graphy ) is a branch of geology , the important methods of correlation and relative dating particularly from fossil-bearing sedimentary rocks but also from fossil free volcanics represents (lava flows, volcanic ash) are available. Today it is divided into a number of sub-disciplines whose combined use (correlation) in connection with geochronology enables the relative and absolute age determination of rocks and thus a reconstruction of the earth's history . For the area of ​​Germany the German Stratigraphic Commission u. a. responsible for coordinating the correlation of the various stratigraphic units or systems of units.

aims

The aim of stratigraphy is to temporally order rock bodies based on the organic and inorganic features they contain and also to relate spatially distant rock units to one another in time (correlation). Based on local and regional sequences, these should be inserted into the internationally used, global chronostratigraphic or geochronological units.

meaning

As a branch of historical geology, stratigraphy is the basis for the reconstruction of the history of the earth and the history of life on earth. However, it also often serves to solve general geological questions.

In the 19th century it was recognized that this method could also be applied to completely different layers, including the elements contained in them. With this, stratigraphy was also transferred to archeology .

Stratigraphic principle

The stratigraphic principle (also known as the “basic stratigraphic law” or storage rule) is the basis of stratigraphy: sediment layers in the horizontal (“below”) are older than sediment layers in the hanging wall (“above”). This principle recognized already Nicolaus Steno in 1669 . However, tectonic processes , unusual deposits and intrusive bodies can break this rule in some cases. Georg Christian Füchsel and Johann Gottlob Lehmann were among the early stratigraphy pioneers in Germany in the 18th century .

Methods of stratigraphy

Sub-disciplines

  • Chronostratigraphy : The relative determination of time using time stamps in rock. These time stamps can be the first appearance or extinction of certain fossils , event horizons, geochemical markers and also polarity changes in the earth's magnetic field (see GSSP ). Chronostratigraphy is on the one hand interdisciplinary, on the other hand it forms the framework into which the stratigraphic schemes, which were obtained by the various other methods of stratigraphy listed below and are still being developed today, can be linked and thus correlated with one another.
  • Biostratigraphy : The relative determination of time or the structure of a sequence and the correlation of different sequences with the help of fossils . Primarily the lifespan and the first appearance or extinction of individual animal species are used here, and more rarely also of fauna communities. Further distinctions:
    • Orthostratigraphy : structure and correlation through binding, globally distributed index fossils
    • Parastratigraphy : structure and correlation through index fossils that only occur regionally or that are tied to a specific facies
  • Lithostratigraphy : classification and correlation according to distinguishable rock units. It is primarily used to map and display rock units on a geological map.
  • Sequence stratigraphy : structure and correlation with the help of sedimentation units (sequences) that were generated by fluctuations in the relative sea level or their isochronous interfaces
  • Eventstratigraphie : Structure and correlation using special marker layers each (due to certain individual events Engl . Events ), have emerged. Such events include storms (see tempestite ), tsunamis (see tsunamite ) and volcanic eruptions (see volcanic ash - the structure and correlation of sedimentary sequences with the help of volcanic ash layers is also known as tephrostratigraphy ), but also biological events such as frequency maxima in the occurrence of a certain species (English acme occurences ) or change of fauna .
  • Pedostratigraphy : structure and correlation using fossil soils
  • Magnetostratigraphy : structure and correlation using the polarity of the earth's magnetic field handed down in the rock at the time of its formation (normal or inverse)
  • Isotope stratigraphy : structure and correlation using anomalies of the isotope ratios in sedimentary rocks, usually isotopes of the elements oxygen ( δ 18 O ), carbon ( δ 13 C ) and sulfur ( δ 34 S )
  • Allostratigraphy : classification of rocks based on quasi-isochronous events

Connection with geochronology

The geochronology deals with the absolute time determining the geological past. In principle, it is a discipline that is independent of stratigraphy, but the dating of rocks only makes sense in connection with stratigraphy.

Two examples of geochronological methods:

  • The dendrochronology allowed under certain circumstances a year exact dating of the investigated woods. However, it can only be used in the geologically very short period of the last 12,000 years (i.e. only in the Holocene ). It is an important tool for dating the strata , especially in archaeological stratigraphy .
  • Ideally, the warven chronology also enables age to be determined precisely to the year. In Sweden it is limited to the last 10,000 years, in the Eifel to the last 23,000 years. In individual lakes, however, dating up to 76,000 years ago has been successful.
  • The Tephrochronologie is closely related to Tephrostratigraphie.

Geological timescale

The various methods of stratigraphy have now resulted in a very detailed relative time scale of earth's history . The methods of stratigraphy are not sufficient to classify these relative time periods on an absolute time scale. The researchers of the late 18th and 19th centuries for the first time pushed through the idea that the earth did not form within a few thousand years. At first, however, they could only estimate the time periods in which the earth and life on it actually came into being.

In the 20th century, the discovery of radioactive decay processes by Henri Becquerel and the radiometric age determinations derived from them made a method of absolute-temporal determination of rocks possible. The various methods of radiometric age determination developed in the meantime allow a relatively precise absolute age determination of the events of the earth's history. However, the accuracy decreases with increasing age of the rocks. The relative time scale created by the various methods of stratigraphy continues to be valid, however, since the various stratigraphic methods allow a much more precise structure and subdivision of the earth's history than geochronology alone.

Individual evidence

  1. ^ Press and Siever, 1995.

literature

  • H. Franke: Methods of Geochronology. Springer-Verlag, 1969
  • H. Murawski, W. Meyer: Geological dictionary. Ferdinand Enke Verlag, 1998
  • F. Press, R. Siever: General geology. Spectrum Academic Publishing House, 1995
  • J. Rey: Geological Age Determination. Ferdinand Enke Verlag, 1991
  • KA Gradstein, JG Ogg: A Geologic Time Scale 2004. Cambridge University Press, 2005
  • North American Commission on Stratigraphic Nomenclature (NACSM): North American stratigraphic code. American Association of Petroleum Geologists Bulletin, 67: 841-875, Tulsa, Oklahoma 1983, ISSN  0149-1423
  • Smolka, Peter Paul (1993) Quantitative Stratigraphy. The Earth Sciences; 11, 10-11; 385-391; doi : 10.2312 / geosciences . 1993.11.385 .

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