The McRae Group is a geological

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

exposed in southern

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

whose strata, including layers of the

Hall Lake Formation The Hall Lake Formation, formerly called the Hall Lake Member, is a geological formation in Sierra County, New Mexico, Sierra County, New Mexico preserving Lancian fauna, most notably dinosaurs. It is regarded as a member of the McRae Group, inc ...

and Jose Creek Formation, date to the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...

. Dinosaur remains are among the fossils that have been recovered from this unit.

Description

The group consists of a basal conglomerate interbedded with

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

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. Although its permeabil ...

and a sequence of alternating

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

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 group is present around

Elephant Butte Reservoir Elephant Butte Reservoir is a reservoir on the southern part of the Rio Grande in the U.S. state of New Mexico, north of Truth or Consequences, New Mexico, Truth or Consequences. The reservoir is the 84th largest man-made lake in the United Sta ...

, in the

Caballo Mountains The Caballo Mountains, (Spanish: "horse") are a mountain range located in Sierra and Doña Ana Counties, New Mexico, United States. The range is located east of the Rio Grande and Caballo Lake, and west of the Jornada del Muerto; the south of ...

, and under much of the

Jornada del Muerto Jornada del Muerto was the name given by the Spanish conquistadors to the Jornada del Muerto desert Endorheic basin, basin, and the almost waterless trail across the Jornada beginning north of Las Cruces, New Mexico, Las Cruces and ending sout ...

. It overlies 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

, and the upper Double Canyon Formation. The Jose Creek Formation is interpreted as

mudflow A mudflow, also known as mudslide or mud flow, is a form of mass wasting involving fast-moving flow of debris and dirt that has become liquified by the addition of water. Such flows can move at speeds ranging from 3 meters/minute to 5 meters/se ...

alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to Semi-arid climate, semiar ...

deposits emplaced in a humid tropical to semitropical environment. It includes distinctive

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

conglomerate beds. Fossil evidence firmly establishes that most of the McRae Group is late Cretaceous in age. However, it is possible that some of the uppermost beds extend into the

Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...

Fossil content

The group contains a floral assemblage that includes '' Geinitzia cf. formosa'', '' Canna magnifolia'', '' Phyllites cf. ratonensis'', ''

Salix Willows, also called sallows and osiers, of the genus ''Salix'', comprise around 350 species (plus numerous hybrids) of typically deciduous trees and shrubs, found primarily on moist soils in cold and temperate regions. Most species are known ...

'', ''

Cinnamomum ''Cinnamomum'' is a genus of evergreen aromatic trees and shrubs belonging to the laurel family, Lauraceae. The species of ''Cinnamomum'' have aromatic oils in their leaves and bark. The genus contains approximately 234 species, distributed in ...

'', '' Sabalites montana'', ''

Araucarites ''Araucarites'' is an extinct genus of conifer, used to refer to female conifer cones that resemble those of the family Araucariaceae. Species assigned to the genus lived in the Permian to Eocene The Eocene ( ) is a geological epoch (geology) ...

longifolia'', ''

Ficus ''Ficus'' ( or ) is a genus of about 850 species of woody trees, shrubs, vines, epiphytes and hemiepiphytes in the family (biology), family Moraceae. Collectively known as fig trees or figs, they are native throughout the tropics with a few spe ...

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 Ceratopsia or Ceratopia ( or ; Greek: "horned faces") is a group of herbivorous, beaked dinosaurs that thrived in what are now North America, Asia and Europe, during the Cretaceous Period, although ancestral forms lived earlier, in the Late Ju ...

frill and jaw fragments,

ankylosaur Ankylosauria is a group of herbivorous dinosaurs of the clade 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 l ...

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 b ...

femur, and the holotype specimen of '' Tyrannosaurus mcraeensis'' (found by a yachtsman in 1983.)

Turtle Turtles are reptiles of the order (biology), order Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Crypt ...

fossils have also been unearthed here. *'' Tyrannosaurus mcraeensis'' *''

Triceratops ''Triceratops'' ( ; ) is a genus of Chasmosaurinae, chasmosaurine Ceratopsia, ceratopsian dinosaur that lived during the late Maastrichtian age of the Late Cretaceous Period (geology), period, about 68 to 66 million years ago on the island ...

sp.'' *'' Sierraceratops turneri'' *''

Alamosaurus ''Alamosaurus'' (; meaning "Ojo Alamo lizard") is a genus of titanosaurian sauropod dinosaurs containing a single known species, ''Alamosaurus sanjuanensis'', from the Maastrichtian age of the Late Cretaceous period in what is now southwestern ...

sp.''

History of investigation

The group was first named as the McRae Formation 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 Assoc ...

in 1952 for

Fort McRae Fort McRae was a Union Army post, established in 1863, then a U.S. Army post from 1866 and closed in 1876, in what is now Sierra County, New Mexico. The post was named for Alexander McRae (1829–1862) a slain hero of the 1862 Battle of Valverde. ...

. 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 the basis of fossil evidence, placing the former formation in the Eocene. In 2019, Lucas and his coinvestigators proposed promoting the McRae Formation to

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