San Pedro Arroyo Formation
The San Pedro Arroyo Formation is a geologic formation in south-central New Mexico. It preserves fossils dating back to the late Triassic period.[1]
| San Pedro Arroyo Formation Stratigraphic range: late Triassic | |
|---|---|
| Type | Formation |
| Unit of | Chinle Group |
| Sub-units | Ojo Huelos Member |
| Underlies | Morrison Formation |
| Overlies | Santa Rosa Formation |
| Thickness | 123 meters (404 ft) |
| Lithology | |
| Primary | Mudstone, sandstone |
| Other | Conglomerate, siltstone, limestone |
| Location | |
| Coordinates | 33.893°N 106.750°W |
| Region | New Mexico |
| Country | United States |
| Type section | |
| Named for | San Pedro Arroyo |
| Named by | Spencer G. Lucas |
| Year defined | 1991 |
  San Pedro Arroyo Formation (the United States)  San Pedro Arroyo Formation (New Mexico) | |
Description
The formation consists of variegated bentonitic mudstone, sandstone, conglomerate, siltstone, and minor limestone, with a total thickness of 123 meters (404 ft). The sandstones are purple to red in color and are crossbedded, laminar, ripple laminated and micaceous. They are interbedded with conglomerates composed of mudstone and calcrete clasts. The formation rests on the Santa Rosa Formation and underlies either the Morrison Formation or, where that is not present, the Mesa Rica Formation or Dakota Group.[1]
A limestone bed near the base of the formation has been designated the Ojo Huelos Member.[1]
History of investigation
The formation was defined by Lucas et al. in 1991 to resolve an inconsistency in the stratigraphy of south-central New Mexico. Previous work had separated the Shinarump Formation from the Chinle Formation, but the Shinarump is a member of the Chinle Formation on the Colorado Plateau.[1] Lucas further rationalized the stratigraphy by promoting the Chinle to group rank in New Mexico in 1993.[2]
Fossils
Fossils of amphibians and phytosaurs have been found in the formation.[1]
Footnotes
- Lucas 1991
- Lucas 1993
References
- Lucas, S.G. (1991). "Triassic stratigraphy, paleontology and correlation, south-central New Mexico" (PDF). New Mexico Geological Society Field Conference Guidebook. 42: 243–253. Retrieved 6 August 2020.
- Lucas, S.G., 1993. The Chinle Group: revised stratigraphy and biochronology of Upper Triassic Nonmarine strata in the western United States. Museum of Northern Arizona Bulletin, v. 59, pp. 27–50.