The Tewa Group is a

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic ide ...

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 ...

s exposed in and around the

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. T ...

of northern

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ker ...

Radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares t ...

gives it an age of 1.85 million to 72 thousand years, corresponding to the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

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 ...

Geology

The Jemez Mountains lie on the intersection of the western margin 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 Chihua ...

and the

Jemez Lineament The Jemez Lineament is a chain of late Cenozoic volcanic fields, long, reaching from the Springerville and White Mountains volcanic fields in East-Central Arizona to the Raton-Clayton volcanic field in Northeastern New Mexico. The lineament ...

. Here

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

produced from the fertile rock of an ancient

subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

has repeatedly found its way to the surface along faults produced by

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 downfaulted depression, called a graben, or more commonly a half-graben ...

. This has produced a long-lived

volcanic field A volcanic field is an area of Earth's crust that is prone to localized volcanic activity. The type and number of volcanoes required to be called a "field" is not well-defined. Volcanic fields usually consist of clusters of up to 100 volcanoes ...

, with the earliest eruptions beginning at least 13 million years ago in both the northern ( Polvadera Group) and southern ( Keres Group) portions of the volcanic field. High-

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

eruptions of the Tewa Group began about 1.85 million years ago ( Ma) and continued almost to the present day. The Tewa Group is a sequence of mostly

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

pyroclastic Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

flows and domes that underlies much of the

Valles Caldera Valles Caldera (or Jemez Caldera) is a wide volcanic caldera in the Jemez Mountains of northern New Mexico. Hot springs, streams, fumaroles, natural gas seeps and volcanic domes dot the caldera floor landscape. The highest point in the caldera i ...

and forms extensive

outflow sheet A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

s surrounding the

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

. It has a maximum thickness of . It overlies the Santa Fe,

Keres In Greek mythology, the Keres (; Ancient Greek: Κῆρες), singular Ker (; Κήρ), were female death-spirits. They were the goddesses who personified violent death and who were drawn to bloody deaths on battlefields. Although they were pre ...

, and Polvadera Groups to the east and south of the caldera and older rocks ranging in age from

Paleoproterozoic The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6 Ga), is the first of the three sub-divisions (eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...

to the west and north. It forms the upper surfaces of the

Pajarito Plateau The Pajarito Plateau is a volcanic plateau in north central New Mexico, United States. The plateau, part of the Jemez Mountains, is bounded on the west by the Sierra de los Valles, the range forming the east rim of the Valles Caldera, and on the ea ...

east of the caldera and the Jemez Plateau west of the caldera. There is geochemical evidence that the Tewa Group originated through rejuvenation of intrusions associated with the Paliza Canyon Formation of the Keres Group.

Formations

From oldest to youngest, the formations recognized within the Tewa Group are the

Bandelier Tuff The Bandelier Tuff is a geologic formation exposed in and around the Jemez Mountains of northern New Mexico. It has a radiometric age of 1.85 to 1.25 million years, corresponding to the Pleistocene epoch. The tuff was erupted in a series of at ...

, the Cerro Toledo Formation, and the Valles Rhyolite. However, the Pueblo Canyon

Member Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...

of the Cerro Toledo Formation was deposited between the Otowi Member and Tsherige Member of the Bandelier Tuff. The ''Bandelier Tuff'' is a sequence of rhyolitic pyroclastic flows erupted in three caldera eruptions, at 1.85 Ma (La Cueva Member), 1.62 Ma (Toledo event; Otowi Member), and 1.25 Ma (Valles event; Tsherige Member). These form a vast outflow sheet surrounding the Jemez Mountains. The Cerro Toledo Formation consists of rhyolitic domes erupted in the Toledo Embayment, a structural feature in the northeast caldera rim, between the Toledo and Valles events. It also includes ring fracture dome remnants of the Toledo event and mixed

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular vol ...

and sediment beds separating the Otowi and Tsherige Members (Pueblo Canyon Member). Ages range from 1.54 Ma for the west Los Posos dome to 1.20 Ma at Pinnacle Peak. File:Pueblo Canyon Member.jpg, Pueblo Canyon Member north of Los Alamos The Valles Rhyolite consists of rhyolite domes and flows associated with caldera resurgence and with the ring fracture of the Valles event. It includes the most recent eruptions in the Jemez Mountains, ending with the Banco Bonito flow. The earliest members, the Deer Canyon Member and Redondo Creek Member, are associated with resurgence, with argon-argon ages of 1.23 to 1.28 Ma. The Cerro del Medio Member (1.169-1.229 Ma), Cerros del Abrigo Member (0.973 Ma), Cerro Santa Rosa Member (0.787-0.936 Ma), Cerro San Luis Member (0.800 Ma), Cerro Seco Member (0.800 to 0.77 Ma), San Antonio Mountain Member (0.557 Ma), and South Mountain Member (0.52 to 0.53 Ma) are individual ring fracture dome complexes that show a progression in age counterclockwise along the presumed ring fracture of the Valles caldera. The El Cajete, Battleship Rock, and Banco Bonito flows are the youngest flows in the Jemez volcanic field, erupting from 77.4 to 68.3 ka. File:Deer Canyon Rhyolite.jpg, Exposures of Deer Canyon Member on the east flank of Redondo Peak File:Redondo Creek Rhyolite.jpg, Redondo Creek Member at Thompson Ridge File:El Cajete Bandelier Wilderness.jpg, Beds of unconsolidated El Cajete Pumice in western Bandelier National Monument File:Banco Bonito.jpg, East face of Banco Bonito flow at El Cajete vent The

quartz latite A quartz latite is a volcanic rock or fine grained extrusive rock composed mostly of alkali feldspar and plagioclase with some quartz. It forms from the rapid cooling of magma of intermediate composition but moderately enriched in alkali metal ...

of Cerro Rubio, originally included in the Tewa Group, has been reassigned to the Tschicoma Formation of the Polvadera Group based on radiometric dating and geochemistry.

History of investigation

The group was first defined by R.L. Griggs in 1964 his study of groundwater resources near the town of

Los Alamos, New Mexico Los Alamos is an census-designated place in Los Alamos County, New Mexico, United States, that is recognized as the development and creation place of the atomic bomb—the primary objective of the Manhattan Project by Los Alamos National Labora ...

. The group was named for the Tewa Mountains, an obsolete name for the Jemez Mountains. R.L. Smith, R.A. Bailey, and C.S. Ross adopted Grigg's nomenclature in 1969 as part of their work establishing the stratigraphy of the Jemez Mountains, but proposing some further subdivisions of formations and members. The stratigraphy was further refined by Gardner and collaborators in 2010.

Footnotes

References

* * * * * * * * * * * * {{cite journal , last1=Zimmerer , first1=Matthew J. , last2=Lafferty , first2=John , last3=Coble , first3=Matthew A. , title=The eruptive and magmatic history of the youngest pulse of volcanism at the Valles caldera: Implications for successfully dating late Quaternary eruptions , journal=Journal of Volcanology and Geothermal Research , date=January 2016 , volume=310 , pages=50–57 , doi=10.1016/j.jvolgeores.2015.11.021, bibcode=2016JVGR..310...50Z , doi-access=free Geologic groups of New Mexico Quaternary formations of New Mexico