The Rincon Valley 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 the Rincon Valley of

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 ...

. It preserves

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 ...

dating back to the

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 ...

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 b ...

and records a time when the valley was a

closed basin Closed may refer to: Mathematics * Closure (mathematics), a set, along with operations, for which applying those operations on members always results in a member of the set * Closed set, a set which contains all its limit points * Closed interval, ...

, just before being integrated into the ancestral

Rio Grande River The Rio Grande ( or ) in the United States or the Río Bravo (del Norte) in Mexico (), also known as Tó Ba'áadi in Navajo, is one of the principal rivers (along with the Colorado River) in the Southwestern United States and in northern Me ...

Description

The formation consists of pink to reddish-brown gypsiferous

claystone Mudrocks are a class of fine-grained siliciclastic sedimentary rocks. The varying types of mudrocks include siltstone, claystone, mudstone and shale. Most of the particles of which the stone is composed are less than and are too small to ...

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 ...

; conglomerate; and conglomeratic

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 ...

. The thickness is . The formation is exposed in the

badlands Badlands are a type of dry terrain where softer sedimentary rocks and clay-rich soils have been extensively eroded."Badlands" in '' Chambers's Encyclopædia''. London: George Newnes, 1961, Vol. 2, p. 47. They are characterized by steep slopes, ...

bordering the

Rio Grande The Rio Grande ( or ) in the United States or the Río Bravo (del Norte) in Mexico (), also known as Tó Ba'áadi in Navajo language, Navajo, is one of the principal rivers (along with the Colorado River) in the Southwestern United States a ...

valley north of

Las Cruces, New Mexico Las Cruces (; ; lit. 'the crosses') is the List of municipalities in New Mexico, second-most populous city in the U.S. state of New Mexico and the county seat, seat of Doña Ana County, New Mexico, Doña Ana County. As of the 2020 United States ce ...

. It grades below into the Hayner Ranch Formation and

unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

underlies the

Camp Rice Formation The Camp Rice Formation is a geologic formation in west Texas and southern New Mexico. It preserves fossils of the Pliocene-Pleistocene. These include the distinctive Tonuco Mountain Local Fauna. Description The formation consists of poorly cem ...

. It is interbedded with flows of the Selden

Basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

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 ...

with a radiometric age of 9.6 million years. The formation consists of two

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...

representing different depositional environments. The first is a fine-grained gypsiferous slope-forming facies interpreted as playa deposits in a closed basin. This contains gypsum beds up to thick. The second facies, which forms the upper of the formation, is a more resistant poorly sorted conglomerate interpreted as

piedmont Piedmont ( ; ; ) is one of the 20 regions of Italy, located in the northwest Italy, Northwest of the country. It borders the Liguria region to the south, the Lombardy and Emilia-Romagna regions to the east, and the Aosta Valley region to the ...

deposits,

eroded 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 disti ...

from the uplifts that bordered the basin. The formation is interpreted as the final filling of the closed basin. During its deposition, tectonic deformation continued along 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 ...

, of which the basin was a part. Some of displacement occurring along the

Caballo Mountains The Caballo Mountains, (Spanish: "horse") are a mountain range located in Sierra and Doña Ana Counties, New Mexico, United States. The range is located east of the Rio Grande and Caballo Lake, and west of the Jornada del Muerto; the south of ...

border faults on the west side of the basin. The Sierra de las Uvas and Dona Ana Mountains to the south and southeast were first thrown during deposition of the formation, with a displacement of about on the bounding fault of the Sierra de las Ulvas block. Study of this and other

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

formations in the region has helped provide evidence for four episodes of block faulting in the Rio Grande rift in the last 35 million years.

Fossils

The formation is largely devoid of fossils. However, a single

carpal bone The carpal bones are the eight small bones that make up the wrist (carpus) that connects the hand to the forearm. The terms "carpus" and "carpal" are derived from the Latin carpus and the Greek καρπός (karpós), meaning "wrist". In hu ...

of the rhinoceros ''

Teleoceras ''Teleoceras'' (Greek: "perfect" (teleos), "horn" (keratos)) is an extinct genus of rhinocerotid. It lived in North America during the Miocene and Pliocene epochs during the Hemingfordian to the end of Hemphillian from around 17.5 to 4.9 million ...

fossiger'' has been recovered from the formation, of

Hemphillian The Hemphillian North American Stage on the geologic timescale is a 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 t ...

age, in agreement with the age of the formation from radiometric dating.

History of investigation

The formation was first defined by W.R. Seager and coinvestigators in 1971, and assigned to the Santa Fe Group.

Footnotes

References

* * * {{cite journal , last1=Seager , first1=W.R. , last2=Hawley , first2=J.W. , last3=Clemons , first3=R.E. , date=1971 , title=Geology of San Diego Mountain area, Dona Ana County, New Mexico , journal=New Mexico Bureau of Mines and Mineral Resources Bulletin , volume=97 , url=https://geoinfo.nmt.edu/publications/monographs/bulletins/downloads/97/Bulletin97.pdf , accessdate=14 August 2020 Neogene formations of New Mexico Sandstone formations of the United States Mudstone formations of the United States Conglomerate formations of the United States