The Palomas 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 ...

exposed in the southern

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 Chihu ...

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe, New Mexico, Santa Fe , LargestCity = Albuquerque, New Mexico, Albuquerque , LargestMetro = Albuquerque metropolitan area, Tiguex , Offi ...

. 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

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Quaternary.

Description

The formation is composed of beds of poorly cemented

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** C ...

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 ...

, 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.Blatt ''et al.'' 1980, ...

, with a total thickness of . The formation is better cemented to the west, where it resembles the

Gila Formation The Gila Group is a Group (geology), group of Formation (geology), geologic formations found along the upper tributaries of the Gila River in Arizona and New Mexico. Radiometric dating of lava flows within the group yields an age of Miocene to ...

. The conglomerate is gray to brown, poorly to moderately sorted, and contains clasts of

tuff 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 ...

basalt Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90% of a ...

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 min ...

, and

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

up to long. The siltstone is well- sorted and trough- crossbedded. The formation may

correlate 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 statisti ...

with the

Sierra Ladrones Formation The Sierra Ladrones Formation is a geologic formation exposed near the Rio Grande Valley in New Mexico. It preserves fossils of Pliocene to Pleistocene age. Description The formation consists of three facies representing different depositional e ...

to the north and 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 ceme ...

to the south. The age is estimated as 5 million years ( Ma) to 0.4 Ma. The formation includes both piedmont and axial river

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

. The piedmont facies represents

alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to semiarid climates, but a ...

s and other sediments deposited at the feet of the mountains surrounding the original depositional basin. The axial river facies represents deposition near the main channel of the ancestral

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 ...

. The lower part of the formation contains two nested paleovalleys near

Truth or Consequences, New Mexico Truth or Consequences (often abbreviated as T or C) is a city in New Mexico, and the county seat of Sierra County. In 2020, the population was 6,052. It has frequently been noted on lists of unusual place names for having chosen to rename itsel ...

, recording climate fluctuations that

incised Incision may refer to: * Cutting, the separation of an object, into two or more portions, through the application of an acutely directed force * A type of open wound caused by a clean, sharp-edged object such as a knife, razor, or glass splinter * ...

and then backfilled the valleys. The first was incised and backfilled between 5.1 and 4.87 Ma, while the second dates to 4.87 to 4.5 Ma. The lack of axial river facies in the Palomas Formation between the Palomas and Mesilla Basins may be due to confinement of flow to narrow paleovalleys in this area.

Fossils

The lower part of the formation contains fossil mammals and reptiles, including turtles, lizards, snakes, rabbits, rats, gophers,

mastodons A mastodon ( 'breast' + 'tooth') is any proboscidean belonging to the extinct genus ''Mammut'' (family Mammutidae). Mastodons inhabited North and Central America during the late Miocene or late Pliocene up to their extinction at the end of the ...

, horses, and deer. Paleomagnetic dating gives an age for this bed of 4.05 to 4.20 Ma.

History of investigation

The unit was first named the Palomas Gravel by C.H. Gordon in 1907. It was renamed the Palomas Formation and assigned to the upper

Santa Fe Group The EAC Invest A/S, formerly known as the Santa Fe Group and East Asiatic Company ( da, italic=yes, Det Østasiatiske Kompagni or ''ØK'') is a multinational holding and investment company, based in Copenhagen, Denmark. History The East Asiat ...

by Lozinsky and Hawley in 1986. However, Reppening and May regard it as post-Santa Fe.

Footnotes

References

* * * * * {{cite journal , last1=Repenning , first1=C.A. , last2=May , first2=S.R. , year=1986 , title=New evidence for the age of the lower part of the Palomas Formation, Truth or Consequences, New Mexico , journal=New Mexico Geological Society Field Conference Guidebook , volume=37 , page=257 , url=https://nmgs.nmt.edu/publications/guidebooks/downloads/37/37_p0257_p0260.pdf , accessdate=31 July 2020 Neogene formations of New Mexico

Description

Fossils

History of investigation

See also

Footnotes

References