Poleo Formation

The Poleo Formation is a geologic formation in northern New Mexico. Its stratigraphic position corresponds to the late Triassic epoch.

Poleo Formation
Stratigraphic range: late Triassic
Poleo Formation exposed in fault escarpment, Youngsville, New Mexico, USA
TypeFormation
UnderliesPetrified Forest Formation
OverliesSalitral Formation
Thickness30 m (98 ft)
Lithology
PrimarySandstone
OtherConglomerate
Location
Coordinates36.173563°N 106.6534824°W / 36.173563; -106.6534824
RegionNew Mexico
CountryUnited States
Type section
Named forPoleo Mesa (now known as Mesa Montoso)
Named byHuene
Year defined1911

Description

Poleo Formation (tan sandstone beds at top) resting on Salitral and Shinarump Formations. West of Abiquiu, New Mexico.

The Poleo Formation is a cliff-forming formation composed mostly of yellowish-gray fine to medium sandstone, but with up to 20% conglomerate. The sandstone shows cross bedding and cuspate ripples. It resembles the Shinarump Conglomerate and the two formations can be difficult to distinguish where the Salitral Formation is absent. However, the Poleo Formation is finer grained and shows other distinguishing lithological features.[1] The lower contact with the Salitral Formation is sharp and scoured, while the upper contact with the Petrified Forest Formation is gradational. The Poleo Formation has an inverse thickness relationship with the Salitral that suggests the thick sections fill valleys cut into the Salitral.[2]

The formation is exposed throughout the Chama basin, the Nacimiento, and Jemez Mountains of New Mexico. It divides the lower and upper Chinle Group where present, but rapidly pinches out to the south, where the underlying Salitral Formation becomes indistinguishable from the overlying Petrified Forest Formation.[3][1][2]

The Poleo Formation is at the same stratigraphic position as the Trujillo Formation of West Texas and eastern New Mexico, the Sonsela Member of the Petrified Forest Formation in west-central New Mexico and northeastern Arizona, and the Moss Back Formation of southern Utah and southwestern Colorado. All are extensive sandstone sheets, but the Poleo Formation is distinguished by its great local thickness, grayish-yellow color, and lithology of micaceous litharenite and mixed-clast conglomerate.[2]

Fossils

The formation is almost devoid of fossils, yielding only oxidized fragments of petrified wood and indeterminate vertebrate bone that are unusable for biostratigraphy.[2]

History of investigation

Huene described the unit in 1911, giving it the name "Poleo-top-sandstone" as the caprock of Mesa Montosa (which he knew as "Mesa Poleo").[2] During their petroleum survey in 1946, Wood and Northrop formally named it the Poleo sandstone lentil of the Chinle Formation.[3] Lucas and Hunt renamed the unit the Poleo Formation in the same 1992 study in which they raised the Chinle Formation to group rank.[4]

Footnotes

  1. Stewart et al. 1972
  2. Lucas et al. 2005
  3. Wood and Northrop 1946
  4. Lucas and Hunt 1992
gollark: I was responding to gibson, of course.
gollark: Yes, it's *two* 68ks.
gollark: Not enough 68000s, but otherwise yes.
gollark: I see. It may be more suitable for inference applications.
gollark: Does it have floating point numbers?

References

  • Lucas, S.G.; Hunt, A.P. (1992). "Triassic stratigraphy and paleontology, Chama Basin and adjacent areas, north-central New Mexico" (PDF). New Mexico Geological Society Field Conference Series. 43: 151–167. Retrieved 20 May 2020.
  • Lucas, Spencer G.; Zeigler, Kate E.; Heckert, Andrew B.; Hunt, Adrian P. (2005). "Review of Upper Triassic stratigraphy and biostratigraphy in the Chama Basin, northern New Mexico" (PDF). New Mexico Geological Society Field Conference Series. 56: 170–181. Retrieved 29 April 2020.
  • John Harris Stewart; Forrest Graham Poole; Richard Farifield Wilson; R.A. Cadigan; William Thordarson; H.F. Albee (1972). "Stratigraphy and Origin of the Chinle Formation and Related Upper Triassic Strata in the Colorado Plateau Region". Retrieved 11 November 2019.
  • Wood, G.H.; Northrop, S.A. (1946). "Geology of the Nacimiento Mountains, San Pedro Mountain, and adjacent plateaus in parts of Sandoval and Rio Arriba Counties, New Mexico". Retrieved 11 November 2019.
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