# Menefee Formation

The Menefee Formation is an upper Santonian to lower Campanian geologic formation found in Colorado and New Mexico, United States.

## Description

The Menefee Formation consists of fluvial sandstone, shale, and coal. Based on ammonite biostratigraphy, the age of the Menefee Formation can be constrained to 84.2-79 million years (Ma), based on the presence of Baculites perplexus in the overlying Cliff House Sandstone, and ammonites from the late Santonian in the underlying Point Lookout Sandstone.
Named members include a lower Cleary Coal Member and an upper Allison Member.
The Mesaverde Group in the San Juan Basin records a marine regression-transgression sequence of the western margin of the Western Interior Seaway. The Menefee Formation was deposited at the peak of the regression as coastal river delta and swamp sediments, and includes numerous coal beds.
The formation is exposed at Chaco Canyon National Park, where many of the coal beds have been burned to produce distinctive red cinder outcrops.

## Fossils

The Menefee Formation includes fossils of turtles, fish and crocodiles and fragmentary evidence of hadrosaurs, ankylosaurs, and ceratopsian dinosaurs. Plant fossils include leaf impressions of palms, conifers, laurels, witchhazel, and camellia. The flora are suggestive of a moist subtropical environment.

### Vertebrate fauna

Several vertebrates have been recovered from the Menefee Formation, including intermediate remains of baenids, trionychids, and dromaeosaurids.
Dinosaurs reported from the Menefee Formation :
| Genus | Species | Stratigraphic position | Material | Notes | Images |
| Ankylosauria | Indeterminate | Allison Member | Numerous partial osteoderms. | Indeterminate Ankylosaur remains. | Dynamoterror Invictarx Menefeeceratops Ornatops |
| Dynamoterror | D. dynastes | Allison Member | Frontals, fragmentary vertebral centra, fragments of dorsal ribs, metacarpal, supraacetabular crest of an ilium, unidentifiable fragments of long bones and phalanxes. | A tyrannosaurid tyrannosaurine known from fragmentary remains. | Dynamoterror Invictarx Menefeeceratops Ornatops |
| Hadrosauridae | Indeterminate | Allison Member | A proximal femur, a distal metatarsal, jaw fragments, a radius, an ulna, caudal vertebrae and a distal tibia. | Indeterminate hadrosaurid remains. | Dynamoterror Invictarx Menefeeceratops Ornatops |
| Invictarx | I. zephyri | Allison Member | Dorsal rib fragments, dorsal vertebrae, distal end of humerus, distal end of ulna, proximal ends of radii, incomplete metacarpal, and numerous incomplete and complete osteoderms. | A nodosaurid, similar to Glyptodontopelta from the Ojo Alamo Formation. | Dynamoterror Invictarx Menefeeceratops Ornatops |
| Menefeeceratops | M. sealeyi | Allison Member | A partial premaxilla, nearly complete postorbital horncore, squamosals, an incomplete parietal, jugal, predentary, dentary, a cervical vertebra, dorsal vertebrae, sacral vertebrae, dorsal ribs, ilium, radius, the proximal and distal portions of an ulna, metatarsal, femur, and the distal end of a fibula. | The oldest recognized centrosaurine ceratopsid. | Dynamoterror Invictarx Menefeeceratops Ornatops |
| Ornatops | O. incantatus | Allison Member | A partial premaxilla, postorbital, squamosal, quadrates, skull roof, braincase, partial dorsal vertebrae, dorsal rib, ossified tendons, scapula, proximal end of a humerus, ulna lacking the proximal end, radius lacking the proximal end, metacarpals, and incomplete pubis and ischium. | The first brachylophosaurin reported from New Mexico and the southernmost occurrence of the clade. | Dynamoterror Invictarx Menefeeceratops Ornatops |
| cf. Saurornitholestes | cf. S. sp | Allison Member | A fragmentary tooth. | A saurornitholestine dromaeosaurid represented by a single, isolated tooth. | Dynamoterror Invictarx Menefeeceratops Ornatops |
| Tyrannosauridae | Indeterminate | Allison Member | A scapula, metatarsal, shaft of anterior thoracic rib, postorbital and lateral tooth. | Indeterminate tyrannosaurid remains. | Dynamoterror Invictarx Menefeeceratops Ornatops |
Crocodilians reported from the Menefee Formation :
| Genus | Species | Stratigraphic position | Material | Notes | Images |
| Crocodylia | Indeterminate | Allison Member | A jaw fragment, teeth and scutes. | Indeterminate crocodylian remains. | Brachychampsa Deinosuchus |
| Brachychampsa | B. sealeyi | Allison Member | A partial skull, associated partial right mandible, partial ramus of the left mandible, and a nearly complete osteoderm. | Mandible preserves bite marks which may have been inflicted by another alligatorioid. | Brachychampsa Deinosuchus |
| Deinosuchus | D. sp | Allison Member | [Six] osteoderms, caudal vertebrae, and a fragmentary tooth. | One of the earliest occurrences of the genus on the Laramidian continent and all of North America. | Brachychampsa Deinosuchus |
Turtles reported from the Menefee Formation :
| Genus | Species | Stratigraphic position | Material | Notes |
| Baenidae | Indeterminate | Allison Member | Carapace fragments and plastron fragments. | Indeterminate baenid remains. |
| Testudines | Indeterminate | Allison Member | A very fragmentary partial carapace and plastron. | Indeterminate turtle remains. |
| Trionychidae | Indeterminate | Allison Member | A nearly complete costal, carapace fragments and plastron fragments. | Indeterminate trionychid remains. |

## Economic geology

The Menefee Formation has been extensively mined for coal since the early 20th century. The Monero field in Rio Arriba County, New Mexico, was mined from the 1880s into the early 1920s to support the Denver and Rio Grande Western Railroad, but while the coal is of good quality, the coal beds are relatively thin and the terrain is rugged. Remaining reserves are around 13.5 million tons, inadequate for economic exploitation in the 21st century.

## History of investigation

The Menefee Formation was first described by W.H.Holmes in 1877 during the Hayden Survey as the "Middle Coal Group" of the Mesaverde Formation. A.J. Collier redesignated this unit in 1919 as the Menefee Formation and raised the Mesaverde Formation to group rank.