{{short description|Geologic formation in northern New Mexico, United States}} {{Infobox rockunit | name = Poleo Formation | image = Mesa Montosa New Mexico.jpg | caption = Poleo Formation at its type location, forming the tan beds capping Mesa Montosa, near [[Coyote, Rio Arriba County, New Mexico]] | type = [[Formation (stratigraphy)|Formation]] | period = late Triassic | age = {{Geological range|late Triassic}} | prilithology = [[Sandstone]] | otherlithology = [[Conglomerate (geology)|Conglomerate]] | namedfor = Poleo Mesa (now known as Mesa Montoso) | namedby = Huene | year_ts = 1911 | region = [[New Mexico]] | country = [[United States]] | coordinates = {{coord|36.173563|N|106.6534824|W|display:inline}} | unitof = [[Chinle Group]] | subunits = | underlies = [[Petrified Forest Formation]] | overlies = [[Salitral Formation]] | thickness = {{convert|30|m|feet|abbr=on}} | extent = | area = | map = {{Location map+ | United States#New Mexico | relief = 1 | width = 250 | float = center | places = {{Location map~ | United States#New Mexico | lat_deg = 36.1736 | lon_deg = -106.6535 | mark = Purple pog.svg | marksize = 12 }} }} | map_caption = }}
The '''Poleo Formation''' is a [[formation (geology)|geologic formation]] in northern New Mexico. Its stratigraphic position corresponds to the [[late Triassic]] [[Epoch (geology)|epoch]].
==Description== The Poleo Formation is a cliff-forming formation composed mostly of yellowish-gray fine to medium [[sandstone]], but with up to 20% [[Conglomerate (geology)|conglomerate]]. The sandstone shows [[cross bedding]] and cuspate [[Ripple marks|ripples]]. It resembles the [[Shinarump Conglomerate]], and the two formations can be difficult to distinguish where they are not separated by the [[Salitral Formation]]. However, the Poleo Formation is finer grained and shows other distinguishing lithological features.{{sfn|Stewart|Poole|Wilson|Cadigan|1972}} 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.{{sfn|Lucas|Zeigler|Heckert|Hunt|2005}}
The formation is exposed throughout the [[Chama Basin]], the [[Nacimiento Mountains]], 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.{{sfn|Wood|Northrop|1946}}{{sfn|Stewart|Poole|Wilson|Cadigan|1972}}{{sfn|Lucas|Zeigler|Heckert|Hunt|2005}}
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 Member]] 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 [[Mica|micaceous]] [[Lithic sandstone|litharenite]] and mixed-clast conglomerate.{{sfn|Lucas|Zeigler|Heckert|Hunt|2005}} <gallery> 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 </gallery>
==Fossils== The formation is almost devoid of fossils, yielding only oxidized fragments of [[petrified wood]] and indeterminate vertebrate bone that are unusable for biostratigraphy.{{sfn|Lucas|Zeigler|Heckert|Hunt|2005}}
==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").{{sfn|Lucas|Zeigler|Heckert|Hunt|2005}} During their petroleum survey in 1946, Wood and Northrop formally named it the Poleo sandstone lentil of the Chinle Formation.{{sfn|Wood|Northrop|1946}} Lucas and Hunt renamed the unit the Poleo Formation in the same 1992 study in which they raised the Chinle Formation to group rank.{{sfn|Lucas | Hunt |1992}}
==Footnotes== {{Reflist}}
==References== * {{cite journal |last1=Lucas |first1=S.G. |last2=Hunt |first2=A.P. |year=1992 |title=Triassic stratigraphy and paleontology, Chama Basin and adjacent areas, north-central New Mexico |journal=New Mexico Geological Society Field Conference Series |volume=43 |pages=151–167 |url=https://nmgs.nmt.edu/publications/guidebooks/downloads/43/43_p0151_p0172.pdf |accessdate=20 May 2020}} * {{cite journal |last1=Lucas |first1=Spencer G. |last2=Zeigler |first2=Kate E. |last3=Heckert |first3=Andrew B. |last4=Hunt |first4=Adrian P. |title=Review of Upper Triassic stratigraphy and biostratigraphy in the Chama Basin, northern New Mexico |journal=New Mexico Geological Society Field Conference Series |date=2005 |volume=56 |pages=170–181 |url=https://nmgs.nmt.edu/publications/guidebooks/downloads/56/56_p0170_p0181.pdf |accessdate=29 April 2020}} * {{cite web|title= Stratigraphy and Origin of the Chinle Formation and Related Upper Triassic Strata in the Colorado Plateau Region|date=1972|first1= John Harris |last1=Stewart|first2=Forrest Graham |last2=Poole|first3=Richard Farifield |last3=Wilson|first4= R.A. |last4=Cadigan|first5=William |last5=Thordarson|first6= H.F. |last6=Albee|url= https://pubs.er.usgs.gov/publication/pp690|access-date= 11 November 2019}} * {{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}}
[[Category:Triassic formations of New Mexico]] [[Category:Sandstone formations of the United States]] [[Category:Conglomerate formations of the United States]]