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McRae Formation
Stratigraphic range: Maastrichtian (Lancian)
~70–66Ma
McRae Formation.jpg
McRae Formation near its type location, Elephant Butte Reservoir, New Mexico, USA. The light bands are "bathtub rings" from stands of the reservoir.
Type Geological formation
Sub-units Hall Lake & Jose Creek Members
Overlies Mesaverde Group
Thickness 3,000 ft (910 m)
Lithology
Primary Sandstone, shale, conglomerate
Other Tuff
Location
Coordinates 33°11′49″N 107°10′01″W / 33.197°N 107.167°W / 33.197; -107.167Coordinates: 33°11′49″N 107°10′01″W / 33.197°N 107.167°W / 33.197; -107.167
Approximate paleocoordinates 40°24′N 85°42′W / 40.4°N 85.7°W / 40.4; -85.7
Region New Mexico
Country  United States
Type section
Named for Fort McRae
Named by Kelley & Silver
Year defined 1952

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 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 formation 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.

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 magnifolia, Phyllites cf. ratonensis, Salix, Cinnamomum, Sabalites montana, Araucarites longifolia, Ficus planicostata, and Sequoia.

Vertebrate paleofauna

W.T. Lee found a ceratopsian skeletion 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 an incomplete jaw of Tyrannosaurus rex (found by a yachtsman in 1983.)

History of investigation

The formation was first named 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 the basis of fossil evidence, placing the 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 the northeast.

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