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
TypeFormation
Unit ofChinle Group
Sub-unitsOjo Huelos Member
UnderliesMorrison Formation
OverliesSanta Rosa Formation
Thickness123 meters (404 ft)
Lithology
PrimaryMudstone, sandstone
OtherConglomerate, siltstone, limestone
Location
Coordinates33.89°N 106.76°W / 33.89; -106.76
RegionNew Mexico
CountryUnited States
Type section
Named forSan Pedro Arroyo
Named bySpencer G. Lucas
Year defined1991
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 laminatged 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]

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gollark: > The ES runs asynchronously on a self-timed circuit and uses thermal noise within the silicon to output a random stream of bits at the rate of 3 GHz. The ES needs no dedicated external power supply to run, instead using the same power supply as other core logic. The ES is designed to function properly over a wide range of operating conditions, exceeding the normal operating range of the processor.It isn't very specific.
gollark: https://www.intel.com/content/www/us/en/developer/articles/guide/intel-digital-random-number-generator-drng-software-implementation-guide.html
gollark: I vaguely remember reading that they or some similar system use thermal noise measured with a ring oscillator.
gollark: Really? How interesting.

See also

Footnotes
  1. Lucas 1991
  2. 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.
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