Triassic rocks in the
Tucumcari Basin of east-central New Mexico have been prospected for fossils since the 1890s. The bulk of early fossil collecting in the formation was done by
University of Michigan paleontologist
E.C. Case (starting in the 1910s) and
Yale paleontologist
Joseph T. Gregory (starting in the 1940s). For much of the 20th century, all Late Triassic strata in New Mexico was 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.
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 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-pebble
conglomerate. Of special note is a thick brownish
litharenite sandstone bed, the
Saladito Point Bed, which is comparable to some sandstone beds in
Petrified Forest National Park,
Arizona. The Bull Canyon Formation reaches a maximum thickness of in eastern New Mexico. It 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 but generally finer-grained geological unit, the
Redonda Formation. before being raised to formation status by Lehman et al. (1992). Both formations are thick geological units with a large proportion of reddish mudstone. Lehman et al. (1992) and Lehman (1994) 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.
Carpenter (1997) reviewed both perspectives, noting various cases of noncompliance with the North American Stratigraphic Code. Carpenter eventually sided with Lehman's perspective. 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. == Paleobiota ==