A group is a lithostratigraphic unit, a part of the geologic record preserved in rock strata. Groups are generally divided into individual formations. Groups may sometimes be combined into supergroups.
Groups are useful for showing relationships between formations, and they are also useful for small-scale mapping or for studying the stratigraphy of large regions. Geologists exploring a new area have sometimes defined groups when they believe the strata within the groups can be divided into formations during subsequent investigations of the area. It is possible for only some of the strata making up a group to be divided into formations.
An example of a group is the Glen Canyon Group, which includes (in ascending order) the Wingate Sandstone, the Moenave Formation, the Kayenta Formation, and the Navajo Sandstone. Each of the formations can be distinguished from its neighbor by its lithology, but all were deposited in the same vast erg. Not all these formations are present in all areas where the Glen Canyon Group is present.
Another example of a group is the Vadito Group of northern New Mexico. Although many of its strata have been divided into formations, such as the Glenwoody Formation, other strata (particularly in the lower part of the group) remain undivided into formations.
Some well known groups of northwestern Europe have in the past also been used as units for chronostratigraphy and geochronology. These are the Rotliegend and Zechstein (both of Permian age); Buntsandstein, Muschelkalk, and Keuper (Triassic in age); Lias, Dogger, and Malm (Jurassic in age) groups. Because of the confusion this causes, the official geologic timescale of the ICS does not contain any of these names. As with other lithostratigraphic ranks, a group cannot be defined by fossil taxonomy.
- North American Stratigraphic Code, pp.1569-1570
- North American Stratigraphic Code, p.1569
- Harshbarger et al. 1957
- Peterson and Pipiringos 1979
- Jones et al. 2011
- Ogg et al. 2004
- Cohen et al. 2013
- North American Stratigraphic Code, pp.1567-1569
- Cohen, K.M.; Finney, S.C.; Gibbard, P.L.; Fan, J.-X. "International Chronostratigraphic Chart 2013" (PDF). stratigraphy.org. ICS. Retrieved 15 June 2014.
- Harshbarger, J.W.; Repenning, C.A.; Irwin, J.H. (1957). "Stratigraphy of the uppermost Triassic and the Jurassic rocks of the Navajo country". U.S. Geological Survey Professional Paper. 291. doi:10.3133/pp291.
- Jones, James V., III; Daniel, Christopher G.; Frei, Dirk; Thrane, Kristine (2011). "Revised regional correlations and tectonic implications of Paleoproterozoic and Mesoproterozoic metasedimentary rocks in northern New Mexico, USA: New findings from detrital zircon studies of the Hondo Group, Vadito Group, and Marqueñas Formation". Geosphere. 7 (4): 974–991. doi:10.1130/GES00614.1. Retrieved 15 April 2020.
- North American Commission on Stratigraphic Nomenclature (November 2005). "North American Stratigraphic Code" (PDF). AAPG Bulletin. 89 (11): 1547–1591. doi:10.1306/07050504129. Retrieved 8 August 2020.
- Ogg, James G.; Gradstein, Felix M.; Smith, Alan Gilbert (2004). A geologic time scale 2004. Cambridge, UK: Cambridge University Press. ISBN 9780521786737. Retrieved 26 October 2020.
- Peterson, Fred; Pipiringos, G.N. (1979). "Stratigraphic relationships of the Navajo Sandstone to Middle Jurassic formations in parts of southern Utah and northern Arizona". USGS Professional Paper. 1035-B: B1–B43. doi:10.3133/pp1035B.
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