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 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 the
Mesaverde Group, from which it
derives much of its sediments.[1] It is overlain by the
Love Ranch Formation.[6]
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.[9]
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 armor fragments, a
sauropod femur, and the holotype specimen of Tyrannosaurus mcraeensis (found by a yachtsman in 1983.)[9][8]Turtle fossils have also been unearthed here.[12]
The group 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.[1] H.P. Bushnell divided the formation into members in 1955.[7]
Kenneth Segerstrom and his coinvestigators argued in 1979 that the beds of the
Cub Mountain Formation properly belong to the McRae Formation.[19]Spencer G. Lucas and his coinvestigators disagreed on the basis of fossil evidence, placing the former formation in the Eocene.[20]
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 the northeast.[21]
^Dalman, S. G; Loewen, M. A.; Pyron, R. A.; Jasinski, S. E.; Malinzak, D. E.; Lucas, S. G.; Fiorillo, A. R.; Currie, P. J.; Longrich, N. R. (2024). "A giant tyrannosaur from the Campanian–Maastrichtian of southern North America and the evolution of tyrannosaurid gigantism". Scientific Reports. 13. Article number 22124. doi:10.1038/s41598-023-47011-0.
Bushnell, H.P. (1955). "Stratigraphy of the McRae formation, Sierra County, New Mexico". The Compass, Sigma Gamma Epsilon Journal of Earth Sciences. 33 (1): 9–17.
Kelley, V.C.;
Silver, Caswell (1952). "Geology of the Caballo Mountains; with special reference to regional stratigraphy and structure and to mineral resources, including oil and gas". University of New Mexico Publications in Geology. 4.
Seager, William (2004). "Laramide (late Cretaceous-Eocene) tectonics of southwestern New Mexico". In Mack, G.H.; Giles, K.A. (eds.). The geology of New Mexico. A geologic history: New Mexico Geological Society Special Volume 11. pp. 249–270.
ISBN9781585460106.