View source for Earth:Abiquiu Formation

{{Short description|A geologic formation in New Mexico}}
{{Infobox rockunit
| name           = Abiquiu Formation
| image          = Abiquiu Formation.jpg
| caption        = Abiquiu Formation at Plaza Blanca, north of Abiquiu, New Mexico
| type           = Formation
| age            = [[Earth:Oligocene|Oligocene]] to Miocene<br/>{{fossilrange|25|18.9}}
| period         = Oligocene
| prilithology   = [[Earth:Sandstone|Sandstone]], [[Earth:Siltstone|siltstone]]
| otherlithology = [[Earth:Conglomerate (geology)|conglomerate]], mudstone
| namedfor       = Abiquiu, New Mexico
| namedby        = H.T.U. Smith 
| year_ts        = 1938
| region         = New Mexico
| country        = {{wipe|United States}}
| coordinates    = {{coord|36.240|N|106.298|W|display=inline}}
| unitof         = [[Earth:Santa Fe Group (geology)|Santa Fe Group]]
| subunits       = Pedernal Chert
| underlies      = [[Earth:Tesuque Formation|Tesuque Formation]]
| overlies       = El Rito Formation
| thickness      = {{convert|400|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 = 36.240
  | lon_deg = -106.298
  | mark = Orange pog.svg
  | marksize = 12
  }}
}}
| map_caption    =
}}
[[File:From the White Place Georgia O'Keeffe.jpg|thumb|right|upright=1.2|''From the White Place'', painting by Georgia O'Keefe depicting the Abiquiu Formation]]

The '''Abiquiu Formation''' is a geologic formation found in northern New Mexico. [[Physics:Radiometric dating|Radiometric dating]] constrains its age to between 18 million and 27 million years, corresponding to the late Oligocene to Miocene [[Earth:Epoch (geology)|epochs]].{{sfn|Maldonado|Kelley|2009|p=6}}

The formation is composed mostly of volcanic debris from the Latir volcanic field, and records the early stages of the opening of the Rio Grande rift in northern New Mexico.{{sfn|Church | Hack |1939}}{{sfn|Maldonado | Kelley |2009}}

Artist Georgia O'Keeffe drew inspiration from her study of the formation at Plaza Blanca.<ref name="ftwp">{{Cite web|url=http://www.georgiaokeeffe.org/from-the-white-place/|title=From the White Place by Georgia O'Keeffe}}</ref>

==Description==
The Abiquiu Formation  consists of light-gray to yellowish-gray, locally crossbedded, thin to thick [[Earth:Bed (geology)|beds]] of tuffaceous [[Earth:Sandstone|sandstone]], pebbly sandstone, and [[Earth:Siltstone|siltstone]]. There are also a few gravel beds and lenses of mudstone. The clasts are mostly of volcanic rock, including Amalia Tuff and [[Earth:Trachyandesite|trachyandesite]] and trachydacite possibly also from the Latir volcanic field. The formation is exposed in a broad belt from the southwest flank of the Tusas Mountains to the Jarosa area in the northwest [[Earth:Jemez Mountains|Jemez Mountains]].{{sfn|Vazzana | Ingersoll|1981|p=2467}}{{sfn|Maldonado | Kelley |2009|p=5}} The formation reaches a maximum thickness in excess of {{convert|400|meters|feet|abbr=off}} near Cañones. The age of the formation is approximately bracketed by a lava flow near its top dated to 18.9 Ma and a 25 Ma flow near its base.{{sfn|Maldonado | Kelley |2009|p=6}}

Both the lower [[Earth:Contact (geology)|contact]] of the formation with the [[Earth:Ritito Conglomerate|Ritito Conglomerate]] and the upper contact with the Chama-El Rito Member of the [[Earth:Tesuque Formation|Tesuque Formation]] are gradational. The transition to the Ritito Conglomerate is characterized by thick [[Earth:Chert|chert]] beds, informally designated the Pedernal chert for outcrops around Cerro Pedernal ({{coord|36.163|N|106.512|W|type:mountain|display=inline}}).{{sfn|Maldonado | Kelley |2009|p=6}}

Individual beds in the formation thicken across the Canones fault zone, which separates the [[Earth:Colorado Plateau|Colorado Plateau]] from the Rio Grande rift. This indicates that the formation was deposited after rifting began within the Rio Grande rift.{{sfn|Maldonado | Kelley |2009|p=6}}

The formation correlates with the Cordito Member of the [[Earth:Los Pinos Formation|Los Pinos Formation]].{{sfn|Maldonado | Kelley |2009|p=5}} It can be divided into three intervals based on the [[Earth:Provenance (geology)|source]] of its sediments. The first interval derives its sediments from basement rock of the Tusas Mountains and from the [[Earth:San Juan volcanic field|San Juan volcanic field]]. The second interval is rich in eroded Amalia Tuff and is interpreted as erosion of the edge of the Amalia outflow sheet. The third interval is entirely composed of debris of the Latir volcanic field. These intervals reflect development of the early Rio Grande rift, with the final interval showing that tilting to the east in the San Luis basin was initially slow enough that debris from the Latir field filled the basin and overflowed to the southwest.{{sfn|Smith|1995}}

==History of investigation==
The Abiquiu Tuff was first named by H.T.U. Smith in 1938 for exposures near the town of Abiquiu, New Mexico. Smith designated only a type area{{sfn|Smith|1938}} and no type locality has been defined.<ref name="geolex-abiquiu">GEOLEX: [https://ngmdb.usgs.gov/Geolex/Units/Abiquiu_6308.html Abiquiu]</ref> Fermor S. Church and John T. Hack recognized almost at once that the unit consisted of a lower [[Earth:Conglomerate (geology)|conglomeratic]] member and an upper tuffaceous member separated by a chert horizon (the Pedernal chert member).{{sfn|Church|Hack|1939}} L.A. Woodward and R.S. Timmer first referred to the unit as the Abiquiu Formation in 1979.{{sfn|Woodward|Timmer|1979}} In 2009, Florian Maldonado and Shari A. Kelley [[Earth:Stratigraphy|correlated]] the lower conglomerate beds with the [[Earth:Ritito Conglomerate|Ritito Conglomerate]] and removed them from the formation, retaining Pedernal Chert as an informal name for the chert beds of the transition between the [[Earth:Ritito Conglomerate|Ritito Conglomerate]] and Abiquiu Formation.{{sfn|Maldonado|Kelley |2009|}}

==Cultural importance==
Georgia O'Keeffe painted her landscape "From the White Place" in 1940 based on her study of the Abiquiu Formation at Plaza Blanca, and her studio at Abiquiu commanded a view of the Plaza Blanca outcrops to the north.<ref name="ftwp"/>

==Footnotes==
{{Reflist}}

==References==
* {{cite journal |last1=Church |first1=Fermor S. |last2=Hack |first2=John T. |title=An Exhumed Erosion Surface in the Jemez Mountains, New Mexico |journal=The Journal of Geology |date=August 1939 |volume=47 |issue=6 |pages=613–629 |doi=10.1086/624816|bibcode=1939JG.....47..613C |s2cid=128546688 }}
* {{cite journal |last1=Maldonado |first1=Florian |last2=Kelley |first2=Shari A. |title=Revisions to the stratigraphic nomenclature of the Abiquiu Formation, Abiquiu and contiguous areas, north-central New Mexico |journal=New Mexico Geology |date=February 2009 |volume=31 |issue=1 |pages=3–8 |url=https://geoinfo.nmt.edu/publications/periodicals/nmg/31/n1/nmg_v31_n1_p3.pdf |accessdate=12 May 2020}}
* {{cite journal |last1=Smith |first1=Gary A. |year=1995 |pages=261–270 |title=Paleogeographic, volcanologic and tectonic significance of the upper Abiquiu Formation at Arroyo del Cobre, New Mexico |journal=New Mexico Geological Society Field Conference Series |volume=46 |url=https://nmgs.nmt.edu/publications/guidebooks/downloads/46/46_p0261_p0270.pdf |accessdate=24 September 2020}}
* {{cite journal |last1=Smith |first1=Harold T. U. |title=Tertiary Geology of the Abiquiu Quadrangle, New Mexico |journal=The Journal of Geology |date=October 1938 |volume=46 |issue=7 |pages=933–965 |doi=10.1086/624710|bibcode=1938JG.....46..933S |s2cid=140678910 }}
* {{cite journal |last1=Vazzana |first1=M. E. |last2=Ingersoll |first2=R. V. |title=Stratigraphy, Sedimentology, Petrology, and Basin Evolution of the Abiquiu Formation (Oligo-Miocene), North-Central New Mexico |journal=GSA Bulletin |date=December 1981 |volume=92 |issue=12_Part_II |pages=2401–2483 |doi=10.1130/GSAB-P2-92-2401|bibcode=1981GSAB...92.2401V }}
* {{cite journal |last1=Woodward |first1=L.A. |last2=Timmer |first2=R.S. |year=1979 |title=Geology of the Jarosa quadrangle, New Mexico |journal=New Mexico Bureau of Mines and Mineral Resources Geologic Map |volume=47 |url=https://ngmdb.usgs.gov/Prodesc/proddesc_73100.htm |accessdate=10 August 2020}}

[[Category:Neogene formations of New Mexico]]

{{Sourceattribution|Abiquiu Formation}}