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 .
21-605: The McRae Group is a geological group exposed in southern New Mexico whose strata, including layers of the Hall Lake Formation and Jose Creek Formation , date to the Late Cretaceous . Dinosaur remains are among the fossils that have been recovered from this unit. The group consists of a basal conglomerate interbedded with shale and siltstone and a sequence of alternating sandstone and shale . The lower conglomerates contain volcanic debris, while
42-646: Is a geologic group of formations that is spread across the U.S. states of Nevada , Utah , northern Arizona , north west New Mexico and western Colorado . It is called the Glen Canyon Sandstone in the Green River Basin of Colorado and Utah. There are four formations within the group. From oldest to youngest, these are the Wingate Sandstone , Moenave Formation , Kayenta Formation , and Navajo Sandstone . Part of
63-654: Is now Nevada and eastern Utah. As a result, the formations of the group thicken to the west. The Kayenta Formation pinches out and disappears to the north, in the Uintah Basin , and the Wingate Sandstone and Navajo Sandstone become indistinguishable. These remaining eolian beds have sometimes been mapped as simply Glen Canyon Formation, but they correlate with the Nugget Sandstone further north, and it has been recommended that they be assigned to
84-733: 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
105-490: Is separated from the underlying Chinle Formation by the regional J-0 unconformity, which represents a time of widespread erosion across western North America. The group is likewise separated from the overlying San Rafael Group by the regional J-2 conformity, representing a renewal of widespread erosion. The Glen Canyon Group was deposited in a foreland basin created by the uplift of the Sevier Mountains in what
126-921: The Colorado Plateau and the Basin and Range , this group of formations was laid down during the Late Triassic and Early Jurassic , with the Triassic-Jurassic boundary within the Wingate Sandstone. The top of the Glen Canyon Group is thought to date to the Toarcian stage of the Early Jurassic. Asterisks (*) below indicate usage by the U.S. Geological Survey . The Glen Canyon Group consists of extensive eolian deposits of latest Triassic to Early Jurassic age on
147-628: The Mesaverde Group , from which it derives much of its sediments. It is overlain by the Love Ranch Formation . The group is divided into the lower Jose Creek Formation , the middle Hall Lake Formation , and the upper Double Canyon Formation . The Jose Creek Formation is interpreted as mudflow or alluvial fan deposits emplaced in a humid tropical to semitropical environment. It includes distinctive breccia conglomerate beds. Fossil evidence firmly establishes that most of
168-768: The Moenave Formation and divided the Wingate Sandstone into the newly named Rock Point and Lukachukai members. In 1963, the upper contact was revised by Phoenix, who moved the uppermost silstone beds of the Navajo Sandstone into the Judd Hollow Tongue of the Carmel Formation. Poole and Stewart mapped the group into the Green River Basin in 1964, treating it here as a single formation. Areal extent limits were revised by Wilson and Stewart in 1967 and again by Green in 1974, who added
189-555: 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 must not be defined by fossil taxonomy. Glen Canyon Group The Glen Canyon Group
210-605: The Colorado Plateau. These form the spectacular orange canyon walls of Canyonlands National Park and Paria Canyon as well as the unflooded portions of Glen Canyon . Deposition of the Glen Canyon Group ceased in the Middle Jurassic with the transgression of the Sundance Sea , which separated deposition of the Glen Canyon Group from deposition of the overlying San Rafael Group . The Glen Canyon Group
231-578: 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
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#1732802124684252-487: The Iyanbito Member. Peterson and Pipiringos revised the upper contact and created an overview in 1979. In 1989 the age of the group was reexamined by Padian and separately by Dubiel (who also revised the lower contact). Geologic Province: Prehistoric animals from the various formations of the Glen Canyon Group include several types of dinosaurs , known from both skeletal remains and tracks. Dinosaur finds in
273-539: The McRae Group is late Cretaceous in age. However, it is possible that some of the uppermost beds extend into the Paleocene . The group contains a floral assemblage that includes Geinitzia cf. formosa , Canna magnifolia , Phyllites cf. ratonensis , Salix , Cinnamomum , Sabalites montana , Araucarites longifolia , Ficus planicostata , and Sequoia . W.T. Lee found a ceratopsian skeleton in
294-686: The Nugget Sandstone. Group rank (stratigraphic order): There is no designated type locality for this group. It was named by Gregory and Moore prior to 1928 for exposures in walls that form the Glen Canyon of the Colorado River in Coconino County, Arizona and San Juan County, Utah , though their report was not published until 1931. The name had by then been published by Gilluly and Reeside, who gave an overview of
315-583: The Wingate and Moenave formations are presently almost entirely tracks. The Kayenta Formation has a diverse skeletal fauna including the theropods "Syntarsus" kayentakatae and Dilophosaurus , the prosauropod Sarahsaurus , an unnamed heterodontosaurid , and the armored dinosaurs Scelidosaurus and Scutellosaurus . The Navajo Sandstone has body fossils of the theropod Segisaurus and an Ammosaurus -like prosauropod, and tracks. The following summarizes vertebrate fossils and tracks reported in
336-478: The area in 1905. Additional vertebrate fossil fragments have been found at twelve locations, generally along the contact between the Jose Creek and Hall Lake members, that include ceratopsian frill and jaw fragments, ankylosaur armor fragments, a sauropod femur, and the holotype specimen of Tyrannosaurus mcraeensis (found by a yachtsman in 1983.) Turtle fossils have also been unearthed here. The group
357-689: The basis of fossil evidence, placing the former formation in the Eocene. In 2019, Lucas and his coinvestigators proposed promoting the McRae Formation to group rank and adding the Double Canyon Formation as its uppermost member. The Double Canyon Formation is over 425 meters (1,394 ft) of mudstone with some sandstone and conglomerate found between Elaphant Butte Reservoir and the Fra Cristobal Mountains to
378-560: The group. In 1936, A.A. Baker reexamined the group and named the Kayenta Formation . The work was revised again in 1955 by Averitt and others. They assigned the Shurtz Sandstone Tongue (new) and Lamb Point Tongue (new) to the Navajo Sandstone , and Cedar City Tongue (new) and Tenney Canyon Tongue (new) to the Kayenta Formation. In 1957 Harshbarger and others created an overview and revision that assigned
399-400: The northeast. Group (stratigraphy) 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
420-532: The upper beds contain sparse nonvolcanic rock fragments. The shales are reddish brown to purplish while the sandstones are light gray. The sandstones are medium bedded to massive and sometimes form hogbacks . The total thickness is in excess of 3,000 feet (910 m). The group is present around Elephant Butte Reservoir , in the Caballo Mountains , and under much of the Jornada del Muerto . It overlies
441-549: Was first named as the McRae Formation by V.C. Kelley and Caswell Silver in 1952 for Fort McRae . They designated the type location as the base of Elephant Butte and the eastern shore of Elephant Butte Reservoir. H.P. Bushnell divided the formation into members in 1955. Kenneth Segerstrom and his coinvestigators argued in 1979 that the beds of the Cub Mountain Formation properly belong to the McRae Formation. Spencer G. Lucas and his coinvestigators disagreed on
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