The McRae Formation is a geological formation exposed in southern New Mexico whose strata date back to the Late Cretaceous. Dinosaur remains are among the fossils that have been recovered from the formation.

Description

The formation consists of a basal

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** Co ...

interbedded with

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...

and

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, p ...

and a sequence of alternating sandstone and

. The lower conglomerates contain volcanic debris, while 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 . The formation is present around Elephant Butte Reservoir, in the

Caballo Mountains The Caballo Mountains, ( Spanish: "horse") are a mountain range located in Sierra Sierra (Spanish for "mountain range" and "saw", from Latin '' serra'') may refer to the following: Places Mountains and mountain ranges * Sierra de Juárez, a m ...

, and under much of the

Jornada del Muerto The name Jornada del Muerto translates from Spanish as "Single Day's Journey of the Dead Man" or even "Route of the Dead Man, though the modern literal translation is closer to "The Working Day of the Dead". It was the name given by the Spanish ...

. It overlies the Mesaverde Group, from which it derives much of its sediments. It is overlain by the Love Ranch Formation. The formation is divided into the lower Jose Creek Member and upper Hall Lake Member. The Jose Creek member 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 the McRae Formation is late Cretaceous in age. However, it is possible that some of the uppermost beds extend into the Paleocene.

Fossil content

The formation contains a floral assemblage that includes '' Geinitzia cf. formosa'', ''

Canna Canna may refer to: Places * Canna, Western Australia, a locality in the Shire of Morawa, Australia * Canna, Calabria, a ''comune'' in the Province of Cosenza, Italy * Cannae, a ''frazione'' in the Province of Barletta-Andria-Trani, Apulia, Ita ...

magnifolia'', '' Phyllites cf. ratonensis'', '' Salix'', '' Cinnamomum'', ''

Sabalites ''Sabalites'' is an extinct genus of palm. Species belonging to the genus lived in the late Cretaceous to Miocene and have been found in South America, North America, Europe, and Asia. The genus is characterized by its costapalmate leaves, which ...

montana'', '' Araucarites longifolia'', '' Ficus planicostata'', and '' Sequoia''.

Vertebrate paleofauna

W.T. Lee found a ceratopsian skeleton in 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 Ankylosauria is a group of herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms, similar to turtles. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. ...

armor fragments, a

sauropod Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their bo ...

femur, and an incomplete jaw of '' Tyrannosaurus rex'' (found by a yachtsman in 1983.) Turtle fossils have also been unearthed here. *'' Tyrannosaurus rex'' *'' Triceratops sp.'' *'' Torosaurus sp.'' *''

Alamosaurus ''Alamosaurus'' (; meaning "Ojo Alamo lizard") is a genus of opisthocoelicaudiine titanosaurian sauropod dinosaurs, containing a single known species, ''Alamosaurus sanjuanensis'', from the late Cretaceous Period of what is now southern North Am ...

sp.''

History of investigation

The formation was first named by V.C. Kelley and

Caswell Silver Caswell Silver (June 25, 1916 – October 18, 1988) was an American geologist and entrepreneur who was President of Sundance Oil Company from 1960 to 1984. In addition to the business of oil and gas exploration, he was active in the American Ass ...

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 Spencer George Lucas is an American paleontologist and stratigrapher, and curator of paleontology at the New Mexico Museum of Natural History and Science. His main areas of study are late Paleozoic, Mesozoic and early Cenozoic vertebrate fossils ...

and his coinvestigators disagreed on 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 of mudstone with some sandstone and conglomerate found between Elaphant Butte Reservoir and the Fra Cristobal Mountains to the northeast.

References

Bibliography