The Poleo Formation is a

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

in northern New Mexico. Its stratigraphic position corresponds to the

late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...

Description

The Poleo Formation is a cliff-forming formation composed mostly of yellowish-gray fine to medium

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

, but with up to 20%

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** C ...

. The sandstone shows

cross bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The origina ...

and cuspate

ripples Ripple may refer to: Science and technology * Capillary wave, commonly known as ripple, a wave traveling along the phase boundary of a fluid ** Ripple, more generally a disturbance, for example of spacetime in gravitational waves * Ripple (electri ...

. It resembles the

Shinarump Conglomerate The Shinarump Conglomerate is a geologic formation found in the Four Corners region of the United States. It was deposited in the early part of the Late Triassic period. Description The Shinarump Conglomerate is a highly resistant coarse-grained ...

, and the two formations can be difficult to distinguish where they are not separated by the

Salitral Formation The Salitral Formation is a Late Triassic geologic formation found in north-central New Mexico, primarily the northwestern Jemez Mountains. It is an older subunit of the Chinle Group (or formation), overlying the Shinarump Conglomerate and under ...

. However, the Poleo Formation is finer grained and shows other distinguishing lithological features. The lower contact with the Salitral Formation is sharp and scoured, while the upper contact with the

Petrified Forest Formation The Petrified Forest Member is a stratigraphic unit of the Chinle Formation in Arizona, Colorado, New Mexico, Nevada, and Utah. It preserves fossils dating back to the Triassic period. Subunits Beds (are in alphabetical order, asterisks ...

is gradational. The Poleo Formation has an inverse thickness relationship with the Salitral that suggests the thick sections fill valleys cut into the Salitral. The formation is exposed throughout the

Chama Basin The Chama Basin is a geologic structural basin located in northern New Mexico. The basin closely corresponds to the drainage basin of the Rio Chama and is located between the eastern margin of the San Juan Basin and the western margin of the Ri ...

, the

Nacimiento Mountains The Sierra Nacimiento (official name), or Nacimiento Mountains, are a mountain range in the northwestern part of the U.S. state of New Mexico. They are just west of the more prominent Jemez Mountains near the town of Cuba, and are separated ...

, and

Jemez Mountains The Jemez Mountains are a group of mountains in Rio Arriba, Sandoval, and Los Alamos counties, New Mexico, United States. Numerous Puebloan Indian tribes have lived in the Jemez Mountains region since before the Spanish arrived in New Mexico. ...

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe, New Mexico, Santa Fe , LargestCity = Albuquerque, New Mexico, Albuquerque , LargestMetro = Albuquerque metropolitan area, Tiguex , Offi ...

. It divides the lower and upper

Chinle Group The Chinle Formation is an Upper Triassic continental geological formation of fluvial, lacustrine, and palustrine to eolian deposits spread across the U.S. states of Nevada, Utah, northern Arizona, western New Mexico, and western Colorado. In Ne ...

where present, but rapidly pinches out to the south, where the underlying Salitral Formation becomes indistinguishable from the overlying Petrified Forest Formation. The Poleo Formation is at the same stratigraphic position as the

Trujillo Formation The Trujillo Formation is a geologic formation in Texas and New Mexico. It preserves fossils dating back to the upper Triassic period. It is also known as the Trujillo Sandstone. Description The formation consists mostly of gray to brown or re ...

West Texas West Texas is a loosely defined region in the U.S. state of Texas, generally encompassing the arid and semiarid lands west of a line drawn between the cities of Wichita Falls, Abilene, and Del Rio. No consensus exists on the boundary betwe ...

and eastern New Mexico, the Sonsela Member of the Petrified Forest Formation in west-central New Mexico and northeastern

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a U.S. state, state in the Southwestern United States. It is the list of U.S. states and territories by area, 6th largest and the list of U.S. states and territories by population, 14 ...

, and the

Moss Back Member The Chinle Formation is an Upper Triassic continental geological formation of fluvial, lacustrine, and palustrine to eolian deposits spread across the U.S. states of Nevada, Utah, northern Arizona, western New Mexico, and western Colorado. In ...

of southern

Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to its ...

and southwestern

Colorado Colorado (, other variants) is a state in the Mountain states, Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the wes ...

. All are extensive sandstone sheets, but the Poleo Formation is distinguished by its great local thickness, grayish-yellow color, and

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Li ...

micaceous Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...

litharenite Lithic sandstones, or lithic arenites, or litharenites, are sandstones with a significant (>5%) component of lithic fragments, though quartz and feldspar are usually present as well, along with some clayey matrix. Lithic sandstones can have a sp ...

and mixed-clast conglomerate. File:Lower Chinle Abiquiu.jpg, Poleo Formation (tan sandstone beds at top) resting on Salitral and Shinarump Formations. West of Abiquiu, New Mexico. File:Poleo Formation.jpg, Poleo Formation exposed in fault escarpment, Youngsville, New Mexico, USA

Fossils

The formation is almost devoid of fossils, yielding only oxidized fragments of

petrified wood Petrified wood, also known as petrified tree (from Ancient Greek meaning 'rock' or 'stone'; literally 'wood turned into stone'), is the name given to a special type of '' fossilized wood'', the fossilized remains of terrestrial vegetation. ' ...

and indeterminate vertebrate bone that are unusable for biostratigraphy.

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"). During their petroleum survey in 1946, Wood and Northrop formally named it the Poleo sandstone lentil of the Chinle Formation. Lucas and Hunt renamed the unit the Poleo Formation in the same 1992 study in which they raised the Chinle Formation to group rank.

Footnotes

References

* * * * {{cite web, title= Geology of the Nacimiento Mountains, San Pedro Mountain, and adjacent plateaus in parts of Sandoval and Rio Arriba Counties, New Mexico, date=1946, last1= Wood , first1=G.H., last2= Northrop , first2= S.A., url= https://ngmdb.usgs.gov/Prodesc/proddesc_5417.htm, access-date= 11 November 2019, url-status= live, df= Triassic formations of New Mexico