San Pedro Arroyo Formation Explained

San Pedro Arroyo Formation
Type:Formation
Period:late Triassic
Prilithology:Mudstone, sandstone
Otherlithology:Conglomerate, siltstone, limestone
Namedfor:San Pedro Arroyo
Namedby:Spencer G. Lucas
Year Ts:1991
Region:New Mexico
Country:United States
Coordinates:33.893°N -106.75°W
Unitof:Chinle Group
Subunits:Ojo Huelos Member
Underlies:Morrison Formation
Overlies:Santa Rosa Formation
Thickness:123meters

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]

Description

The formation consists of variegated bentonitic mudstone, sandstone, conglomerate, siltstone, and minor limestone, with a total thickness of 123meters. 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 set of limestone beds near the base of the formation have been designated the Ojo Huelos Member.[1] This consists of brown-weathering limestone beds a few meters thick, interbedded with meter-thick yellowish to greenish mudstone.[2]

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.[3] However, other New Mexico stratigraphers have criticized the formation as a junior synonym for the Cooper Canyon Formation.[4]

Fossils

Fossils of amphibians and phytosaurs have been found in the formation.[1]

See also

Notes and References

  1. Lucas . S.G. . 1991 . Triassic stratigraphy, paleontology and correlation, south-central New Mexico . New Mexico Geological Society Field Conference Guidebook . 42 . 243–253 . 6 August 2020.
  2. Allen . B.D. . Love . D.W. . McCraw . D.J. . Rinehart . A.J. . Geologic Map of the Becker SW Quadrangle, Socorro County, New Mexico . New Mexico Bureau of Geology and Mineral Resources Open-file Digital Geological Map . June 2013 . OF-GM 233 . 10 . 18 November 2021.
  3. 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 . 59 . 27–50.
  4. Cather . S.M. . Zeiger . Kate E. . Mack . Greg H. . Kelley . Shari A. . Toward standardization of Phanerozoic stratigraphic nomenclature in New Mexico . New Mexico Geological Society Spring Meeting . 2013 . 12 . 10.1.1.667.3513 .