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.

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.

History and geology

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.

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), 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.

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. 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.

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.

Paleobiota

Several microvertebrate assemblages are known from the Bull Canyon Formation.

Temnospondyls

Synapsids

Reptiles

Avemetatarsalians

Phytosaurs

Other pseudosuchians

Other reptiles

Fish

Scales, coprolites, and other fragmentary fish fossils are common in the Bull Canyon and Revuelto Creek areas. Most of these likely belong to actinopterygians.

Other fossils

The Bull Canyon Formation preserves a rather diverse fauna of freshwater mollusks. Unionid bivalves (freshwater mussels) are locally abundant, primarily '' Unio'' (''U. arizonensis'' and at least six unnamed species) and '' Antediplodon'' (''A. dockumensis'' and one unnamed species). A hydrobiid snail, '' Triasamnicola pilsbryi'', is also common. Ostracods and coiled ''Spirorbis''-like structures (probably tiny snail shells) round out the invertebrate body fossils known from the formation.

Trackways of both vertebrates and invertebrates are known to occur in the Bull Canyon Formation. Narrow '' Acripes'' tracks were probably emplaced by notostracan crustaceans (tadpole shrimp) crawling in shallow temporary pools. A vertebrate ichnotaxon, '' Barrancapus cresapi'', was named for vertebrate footprints found near Barranca Creek.

A few plant fossils have been found in the formation, including charophyte green algae, bennettitalean leaves ('' Zamites powellii''), large horsetail stems ('' Neocalamites'' sp.), and foliage of an enigmatic shrubby plant ('' Sanmiguelia'' sp.). Root casts and fossil wood are not uncommon, sometimes showing signs of fungus damage.

See also

* List of dinosaur-bearing rock formations

References

{{reflist

External links

Bull Canyon Formation of Chinle Group
by USGS

Triassic formations of New Mexico
Triassic geology of Texas
Norian Stage
Mudstone formations of the United States
Sandstone formations of the United States
Siltstone formations of the United States
Conglomerate formations