in chronostratigraphy and
An aeon or an aeonothem ( English eon or eonothem ) is the name for the highest-ranking unit in the subdivision of earth's history . In geochronology one speaks of aeons, in chronostratigraphy of aeon themes (see box).
Aeons or aeon themes always span several hundred million to well over a billion years. In the history of the earth, four aeons and aeonothems are distinguished: Hadaic , Archaic , Proterozoic and Phanerozoic .
The aeonothem represents the highest rank in the hierarchy of chronostratigraphic units of earth's history. The corresponding geochronological time span is called the aeon. In the older and popular scientific literature, “aeon” and “aeon othem” are often used synonymously, with the term “aeon” occurring much more frequently.
The boundaries of chronostratigraphic units are delimited by isochronous (contemporaneous) stratigraphic areas defined by chronostratigraphic or biostratigraphic markers, not by absolute age. On the hierarchical level of the aeonothems, however, this is only the case with the Phanerozoic . Its lower limit is chronostratigraphically defined by a GSSP . The other aeon-themes are absolutely dated and are therefore identical to the aeons.
The four aeons or aeon themes
The approximate times in millions of years = Ma correspond to the status in 2008.
|Phanerozoic||≈ 542 Ma||to||0 Ma|
|Proterozoic||2500 Ma||to||≈ 542 Ma|
|Archean||≈ 4000 Ma||to||2500 Ma|
|Hadaikum||≈ 4600 Ma||to||≈ 4000 Ma|
The Hadaic, Archean and Proterozoic are also known collectively as the Precambrian .
The Proterozoic / Archaic boundary is currently only set at an absolute value of 2500 Ma. In future, however, this limit should be defined chronostratigraphically by a GSSP . The Archean / Hadaic border and the base of the Hadaic have only been roughly established so far.
An annotated table of the eons and their subdivisions can be found in the article Geological timescale .
- Overview of the Global Boundary Stratotype Sections and Points (GSSP's) of the International Commission on Stratigraphy ( Memento from June 9, 2007 in the Internet Archive )
- Felix Gradstein, Jim Ogg, Jim & Alan Smith: A Geologic timescale. Cambridge University Press 2005, ISBN 978-0-521-78673-7
- North American Commission on Stratigraphic Nomenclature (NACSN): North American stratigraphic code. American Association of Petroleum Geologists Bulletin, 89: 1547-1591, Tulsa, Oklahoma 1983, PDF
- Fritz F. Steininger and Werner E. Piller: Recommendations (guidelines) for handling the stratigraphic nomenclature. Courier Forschungsinstitut Senckenberg, 209: 1-19, Frankfurt am Main 1999, .