In geology, agroup is alithostratigraphicunit consisting of a series of relatedformations that have been classified together to form a group. Formations are the fundamental unit of stratigraphy. Groups may sometimes be combined intosupergroups.[1]
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.[2]
An example of a group is theGlen Canyon Group, which includes (in ascending order) theWingate Sandstone, theMoenave Formation, theKayenta Formation, and theNavajo Sandstone. Each of the formations can be distinguished from its neighbor by itslithology, but all were deposited in the same vasterg.[3] Not all these formations are present in all areas where the Glen Canyon Group is present.[4]
Another example of a group is theVadito Group of northernNew Mexico. Although many of its strata have been divided into formations, such as theGlenwoody Formation, other strata (particularly in the lower part of the group) remain undivided into formations.[5]
Some well known groups of northwestern Europe have in the past also been used as units forchronostratigraphy andgeochronology. These are theRotliegend andZechstein (both ofPermian age);Buntsandstein,Muschelkalk, andKeuper (Triassic in age);Lias,Dogger, andMalm (Jurassic in age) groups.[6] Because of the confusion this causes, the officialgeologic timescale of theICS does not contain any of these names.[7] As with other lithostratigraphic ranks, a group must not be defined by fossil taxonomy.[8]
