Bull Canyon Formation

The Bull Canyon Formation is a geological formation of Late Triassic (Norian) age in eastern New Mexico and the Texas Panhandle. It is one of several formations encompassed by the Dockum Group.

Bull Canyon Formation
Stratigraphic range: Upper Triassic, late Norian PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Unit of	Dockum Group
Underlies	Redonda Formation
Overlies	Trujillo Formation
Location
Region	New Mexico,  Texas
Country	United States

The Bull Canyon Formation preserves reptile fossils of the Revueltian "faunachron", and it is generally considered time-equivalent to the upper Cooper Canyon Formation, which crops out further south in west-central Texas.

Triassic rocks in the Tucumcari Basin of east-central New Mexico have been prospected for fossils since the 1920s. For much of the 20th century they were assumed to belong to the Chinle Formation. Kelley (1972) informally labelled a unit of fine-grained sediments in the upper part of the Triassic strata as the "upper shale member" of the Chinle Formation.^[1][2]

Lucas & Hunt (1989) introduced the name "Bull Canyon Formation" for these exposures, referring to an area of badlands near Luciano Mesa in eastern Guadalupe County, New Mexico. At the Bull Canyon badlands (the type locality), the formation preserves 95 metres (312 ft) of sediment, about 80% of which is dark reddish mudstone. Yellowish-grey or greyish-red fine quartzarenite sandstone makes up about 16% of the layers, with rare siltstone and siltstone-pebbleconglomerate.^[3] The Bull Canyon Formation lies above the Trujillo Formation, which has a much greater proportion of sandstone beds. In New Mexico, the Bull Canyon Formation is overlain by a similar geological unit, the Redonda Formation.

Relationship to the Cooper Canyon Formation

For much of the 1990s and 2000s, the Bull Canyon Formation was conflated with another geological unit in the Dockum Group: the Cooper Canyon Formation. The Cooper Canyon Formation, which is most well-exposed in Garza County, Texas, was initially named as the "Cooper Member" by Chatterjee (1986),^[4] before being raised to formation status by Lehman et al. (1992).^[5] Both formations are thick geological units with a large proportion of reddish mudstone. Lehman et al. (1992) and Lehman (1994)^[6] argued that the Bull Canyon Formation should be synonymized with the Cooper Canyon Formation. These authors suggested that the Boren Ranch Sandstone, which underlies the Cooper Canyon Formation in Garza County, is equivalent to the sandstone-rich Trujillo Formation in New Mexico and the Texas Panhandle. If this is the case, then the "Cooper Member" (and consequently Cooper Canyon Formation) would take priority as the first formal name applied to the mudstone-rich strata above the Trujillo Formation.^[6]

Lucas and his colleagues pushed back, arguing that the "Cooper Member" was an invalid name: Chatterjee (1986)'s original stratotype was too thin to be comparable with other areas, and "Cooper" was preoccupied by Cooper Marl of South Carolina. They favored the Bull Canyon Formation as the first valid formal name for the mudstone-rich strata.^[7] Carpenter (1997) reviewed both perspectives, noting various cases of noncompliance with the North American Stratigraphic Code. Carpenter eventually sided with Lehman's perspective.^[8] Despite the disagreement between these two schools of thought, most studies agreed that the two formations, as generally perceived, were probably one-to-one equivalents.^{[2][3][5][7][6][8]}

More extensive geological mapping in Texas by Martz (2008) disagreed with the idea that the Bull Canyon and Cooper Canyon formations were exact equivalents, nullifying the debate over priority. Martz notes that the sandstone beds of the Trujillo Formation are not equivalent to the Boren Ranch Sandstone, but rather to the middle part of the Cooper Canyon Formation. This would indicate that the lower-middle part of the Cooper Canyon Formation is older than the Bull Canyon Formation, and that only the upper Cooper Canyon Formation is equivalent.^[9]

Temnospondyls

Synapsids

Reptiles

Avemetatarsalians

Phytosaurs

Other archosaurs

Other reptiles

Fish

Other fossils

The Bull Canyon locality preserves a rather diverse fauna of unionid bivalves (freshwater mussels).^[15]

• List of dinosaur-bearing rock formations

• Bull Canyon Formation of Chinle Group by USGS