In geology, a group is a lithostratigraphic unit consisting of a series of related formations that have been classified together to form a group. Formations are the fundamental unit of stratigraphy. 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 The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

, 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 The Vadito Group is a group of geologic formations that crops out in most of the Precambrian-cored uplifts of northern New Mexico. Detrital zircon geochronology and radiometric dating give a consistent age of 1700 Mya for the group, correspondi ...

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 The Zechstein (German either from ''mine stone'' or ''tough stone'') is a unit of sedimentary rock layers of Middle to Late Permian (Guadalupian to Lopingian) age located in the European Permian Basin which stretches from the east coast of Englan ...

(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 The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochronol ...

of the

ICS ICS may refer to: Computing * Image Cytometry Standard, a digital multidimensional image file format used in life sciences microscopy * Industrial control system, computer systems and networks used to control industrial plants and infrastructu ...

does not contain any of these names. As with other lithostratigraphic ranks, a group must not be defined by fossil taxonomy.

Footnotes

References

* * * * * * {{Authority control Stratigraphy Geological units