The Chamita 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 north-central

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 ...

. It preserves unique

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...

dating back to the

Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...

period Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

. The presence of

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcano, volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used t ...

beds in the formation, which can be

radiometrically dated 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 the absolute age of the fossils, which is valuable for establishing the geologic time scale of the Neogene.

Description

The formation is mostly coarse

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...

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) ...

with some coarse gravel. It varies greatly in thickness, likely due to extensive

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is distin ...

. There are two prominent

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 c ...

zones used by Galusha and Blick for regional

correlation 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 statistics ...

. Each is about thick and they are separated by of tan sediments.

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 ...

shows that the age of the Upper Chamita Tuffaceous Zone is 6.93 ±0.05 million years at its base and 6.78 ±0.03 million years at its top, essentially identical with the Peralta Tuff, which it also chemically resembles. This helps establish the absolute date for the late

Hemphillian The Hemphillian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 10,300,000 to 4,900,000 years BP. It is usually considered ...

, since a fossil assemblage of that

stage Stage or stages may refer to: Acting * Stage (theatre), a space for the performance of theatrical productions * Theatre, a branch of the performing arts, often referred to as "the stage" * ''The Stage'', a weekly British theatre newspaper * Sta ...

is found in the zone. The Lower Chamita Tuffaceous Zone has a radiometric age of 7.7 ±0.3 million years. The age range for the complete section has been estimated as 9.5 to 5.8 million years. However, more recent work has yielded an age range of 12 to 13 Ma for a Lower Coarse White Ash Zone towards the base of the formation, which is thickest northeast of 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 ...

and has been interpreted as ash fall from early silicic

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

activity from vents now buried under later flows. The base of the formation contains sediments resembling the underlying Ojo Caliente

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 Tesuque Formation. This is interpreted as reworking of sediments in a changed sedimentation environment rather than

interfingering A geological contact is a boundary which separates one rock body from another. A contact can be formed during deposition, by the intrusion of magma, or through faulting or other deformation of rock beds that brings distinct rock bodies into contac ...

. In other words, the lower part of the formation contains sediments eroded from the Ojo Caliente Member, rather than recording a fluctuating depositional environment. Members of the formation include the Cejita and Cuarteles Members west of the modern

Rio Grande River The Rio Grande ( and ), known in Mexico as the Río Bravo del Norte or simply the Río Bravo, is one of the principal rivers (along with the Colorado River) in the southwestern United States and in northern Mexico. The length of the Rio G ...

and the Pilar Mesa, Vallito, and Hernandez Members on both sides of the Rio Grande.

Fossil content

The Round Mountain Quarry has produced numerous fossils from the Chamita Formation. The

artiodactyl The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...

Osbornoceros ''Osbornoceros'' is an extinct artiodactyl genus of the family Antilocapridae.Frick 1937 All antilocaprid species are extinct except for the pronghorn. ''Osbornoceros osborni'' is the only known species of the genus ''Osbornoceros''. ''Osbornoc ...

osborni'' was first located in the formation in the Lyden area, as was the only known specimen of the taxideine badger, ''

Chamitataxus ''Chamitataxus'' is a prehistoric badger genus. ''Chamitataxus avitus'' is the only known species of the genus. ''Chamitataxus'' lived during the Late Miocene, around 6 million years ago in what is now North America. Out of the three taxid ...

''.

Mammals

Carnivorans

Lagomorphs

Proboscideans

Rodents

Ungulates

History of investigation

The beds making up the unit were originally included by Bryan and McCann in 1937 in the Middle Red member of the

Santa Fe Formation The Santa Fe Group is a group of geologic formations in New Mexico and Colorado. It contains fossils characteristic of the Oligocene through Pleistocene epochs. The group consists of basin-filling sedimentary and volcanic rocks of the Rio Gr ...

. This became the Tesuque Formation in 1956. In 1971, Galusha and Blick split the uppermost beds from the Tesuque Formation as the Chamita Formation based on differences in

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. Lit ...

and fossil assemblage.

Footnotes

References

* * * * * * {{cite journal , last1=McIntosh , first1=William C. , last2=Quade , first2=Jay , year=1995 , title=40Ar/39Ar geochronology of tephra layers in the Santa Fe Group, Espanola Basin, New Mexico , journal=New Mexico Geological Society Guidebook , volume=46 , pages=279–287 , url=https://nmgs.nmt.edu/publications/guidebooks/downloads/46/46_p0279_p0287.pdf , accessdate=10 August 2020 Neogene formations of New Mexico