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 .
6-682: The Lockport Group is a geologic group in the Appalachian Basin and Michigan Basin in the northeastern United States and Canada . This unit makes up the Niagara Escarpment . Its most famous feature is Niagara Falls . The unit outcrops in New York , Ontario , Michigan , Wisconsin , and Illinois . The Lockport Group is made up of four formations. From base to top: Gasport Formation , Goat Island Formation , Eramosa Formation and Guelph Formation . The entire unit
12-661: Is composed of dolomite , with the exception of the Gasport which can be limestone , as well as occasional chert nodules. The Guelph Formation is at the top of the Lockport Group. It has been called the "Vernon Transition Zone" due to thin beds and laminations between carbonates, claystones and evaporates. Its base is delimited by a stromatolite bed. Below the Guelph is the Eramosa Formation. This formation
18-436: Is typically a thinly laminated packstone and grainstone . There are stromatolite beds as well as corals preserved in this formation. Near the base the unit becomes vuggy . Again the top of this formation is marked by a sharp change between grainstone and a stromatolite bed. Group (geology) Groups are useful for showing relationships between formations, and they are also useful for small-scale mapping or for studying
24-586: 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
30-866: 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
36-468: 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)
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