Abiquiu Formation
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The Abiquiu 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 ...
found in northern
New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...
.
Radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to Chronological dating, date materials such as Rock (geology), rocks or carbon, in which trace radioactive impurity, impurities were selectively incorporat ...
constrains its age to between 18 million and 27 million years, corresponding to the
late Oligocene The Chattian is, in the geologic timescale The geologic time scale or geological time scale (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the pro ...
to
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
epochs 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 ...
. 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 The Rio Grande rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift extends from central Colorado in the north to the state of Chihuah ...
in northern New Mexico. Artist
Georgia O'Keeffe Georgia Totto O'Keeffe (November 15, 1887 March 6, 1986) was an American Modernism, modernist painter and drafter, draftswoman whose career spanned seven decades and whose work remained largely independent of major art movements. Called the "M ...
drew inspiration from her study of the formation at Plaza Blanca.


Description

The Abiquiu Formation consists of light-gray to yellowish-gray, locally crossbedded, thin to thick
beds A bed is a piece of furniture that is used as a place to sleep, rest, and relax. Most modern beds consist of a soft, cushioned mattress on a bed frame. The mattress rests either on a solid base, often wood slats, or a sprung base. Many be ...
of
tuffaceous Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...
sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
, pebbly sandstone, and
siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...
. There are also a few gravel beds and lenses of
mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...
. The
clast Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks b ...
s are mostly of volcanic rock, including Amalia Tuff and
trachyandesite Trachyandesite is an extrusive igneous rock with a composition between trachyte and andesite. It has little or no free quartz, but is dominated by sodic plagioclase and alkali feldspar. It is formed from the cooling of lava enriched in alkal ...
and
trachydacite Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava (or shallow intrusions) enri ...
possibly also from the Latir volcanic field. The formation is exposed in a broad belt from the southwest flank of the
Tusas Mountains The Tusas Mountains are a mountain range in northern New Mexico, extending slightly into southern Colorado. They are considered the southeasternmost part of the San Juan Mountains. Grouse Mesa, , is the highest peak in the range. The mountains a ...
to the Jarosa area in the northwest
Jemez Mountains The Jemez Mountains (, Tewa: ''Tsąmpiye'ip'įn'', Navajo: ''Dził Łizhinii'') 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 Moun ...
. The formation reaches a maximum thickness in excess of near Cañones. The age of the formation is approximately bracketed by a
lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...
near its top dated to 18.9 Ma and a 25 Ma flow near its base. Both the lower
contact Contact may refer to: Interaction Physical interaction * Contact (geology), a common geological feature * Contact lens or contact, a lens placed on the eye * Contact sport, a sport in which players make contact with other players or objects * C ...
of the formation with the
Ritito Conglomerate The Ritito Conglomerate is a Formation (geology), geologic formation in northern New Mexico dating to the Oligocene Epoch (geology), epoch. Description The formation is composed of orange-pink to reddish-brown arkosic Conglomerate (geology), c ...
and the upper contact with the Chama-El Rito Member of the Tesuque Formation are gradational. The transition to the Ritito Conglomerate is characterized by thick
chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...
beds, informally designated the Pedernal chert for outcrops around Cerro Pedernal (). Individual beds in the formation thicken across the Canones
fault zone In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic f ...
, which separates the
Colorado Plateau The Colorado Plateau is a physiographic and desert region of the Intermontane Plateaus, roughly centered on the Four Corners region of the Southwestern United States. This plateau covers an area of 336,700 km2 (130,000 mi2) within w ...
from the
Rio Grande rift The Rio Grande rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift extends from central Colorado in the north to the state of Chihuah ...
. This indicates that the formation was deposited after
rifting In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics. Typical rift features are a central linear Fault (geology), downfaulted depression, called a graben, or more commonly ...
began within the
Rio Grande rift The Rio Grande rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift extends from central Colorado in the north to the state of Chihuah ...
. The formation correlates with the Cordito Member of the Los Pinos Formation. It can be divided into three intervals based on the
source Source may refer to: Research * Historical document * Historical source * Source (intelligence) or sub source, typically a confidential provider of non open-source intelligence * Source (journalism), a person, publication, publishing institute ...
of its sediments. The first interval derives its sediments from
basement rock In geology, basement and crystalline basement are crystalline rocks lying above the mantle and beneath all other rocks and sediments. They are sometimes exposed at the surface, but often they are buried under miles of rock and sediment. The baseme ...
of the Tusas Mountains and from the
San Juan volcanic field The San Juan volcanic field is part of the San Juan Mountains in southwestern Colorado. It consists mainly of volcanic rocks that form the largest remnant of a major Composite Volcanoes, composite volcanic field that covered most of the southern ...
. 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.


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 and no type locality has been defined.GEOLEX
Abiquiu
/ref> Fermor S. Church and John T. Hack recognized almost at once that the unit consisted of a lower conglomeratic member and an upper tuffaceous member separated by a chert horizon (the Pedernal chert member). L.A. Woodward and R.S. Timmer first referred to the unit as the Abiquiu Formation in 1979. In 2009, Florian Maldonado and Shari A. Kelley
correlated In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistic ...
the lower conglomerate beds with the
Ritito Conglomerate The Ritito Conglomerate is a Formation (geology), geologic formation in northern New Mexico dating to the Oligocene Epoch (geology), epoch. Description The formation is composed of orange-pink to reddish-brown arkosic Conglomerate (geology), c ...
and removed them from the formation, retaining Pedernal Chert as an informal name for the chert beds of the transition between the
Ritito Conglomerate The Ritito Conglomerate is a Formation (geology), geologic formation in northern New Mexico dating to the Oligocene Epoch (geology), epoch. Description The formation is composed of orange-pink to reddish-brown arkosic Conglomerate (geology), c ...
and Abiquiu Formation.


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.


Footnotes


References

* * * * * * {{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 Neogene formations of New Mexico Paleogene formations of New Mexico Sandstone formations of the United States Siltstone formations of the United States Conglomerate formations of the United States Mudstone formations of the United States