The Sierra Ladrones 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 near the

Rio Grande Valley The Lower Rio Grande Valley ( es, Valle del Río Grande), commonly known as the Rio Grande Valley or locally as the Valley or RGV, is a region spanning the border of Texas and Mexico located in a floodplain of the Rio Grande near its mouth. The ...

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

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 ...

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

age.

Description

The formation consists of three

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 ...

representing different depositional environments. These are

piedmont it, Piemontese , population_note = , population_blank1_title = , population_blank1 = , demographics_type1 = , demographics1_footnotes = , demographics1_title1 = , demographics1_info1 = , demographics1_title2 ...

slope and alluvial fan deposits, typically composed of light-brown to light-reddish-brown

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

fanglomerate Conglomerate () is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contains a matrix of finer-grained sediments, such as sand, silt, or clay, which fill ...

; axial stream deposits, which are composed of light-gray to light-yellowish-brown fine- to medium-grained sand and sandstone with

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

cross-bedding and cut-and-fill channels; and interbedded

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 ...

flows with a K-Ar age of 4.5 +/-0.1 million years ( Ma. The total thickness is in excess of . The formation

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 ...

overlies or is in fault contact with the

Popotosa Formation The Popotosa Formation is a geologic formation in New Mexico. It preserves fossils dating back to the Neogene period. These include the Socorro flora, notable for its fine preservation of plant reproductive structures. Description The Popotos ...

or older formations. Its age is early Pliocene to middle Pleistocene (2 Ma to 5 Ma.) The formation is interpreted as fanglomerates shed from the flanking uplifts 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 channel and

floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge.Goudi ...

deposits of the ancestral

Rio Grande 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 ...

Fossils

The formation has yielded abundant fossils of

Irvingtonian The Irvingtonian North American Land Mammal Age on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), spanning from 1.9 million – 250,000 years BP. ...

age at Tijeras Arroyo, south of

Albuquerque International Airport Albuquerque International Sunport is the primary international airport serving the U.S. state of New Mexico, the Albuquerque metropolitan area, and the larger Albuquerque– Santa Fe–Las Vegas combined statistical area. It handles around 5 ...

. These include ''

Hypolagus ''Hypolagus'' is an extinct genus of lagomorph, first recorded in the Hemingfordian (early to middle Miocene) of North America. It entered Asia during the early Turolian and spread to Europe not much later, where it survived until the Middle Plei ...

'', ''

Equus Equus may refer to: * ''Equus'' (genus), a genus of animals including horses, donkeys and zebras * ''Equus'' (play), a play by Peter Shaffer * ''Equus'' (film), a film adaptation of the Peter Shaffer play * Equus (comics), a comic book characte ...

'', ''

Mammuthus A mammoth is any species of the extinct elephantid genus ''Mammuthus'', one of the many genera that make up the order of trunked mammals called proboscideans. The various species of mammoth were commonly equipped with long, curved tusks and, ...

'', and ''

Hesperotestudo ''Hesperotestudo'' ("Western turtle") is an extinct genus of tortoise native to North and Central America from the Oligocene to the Late Pleistocene. Species of ''Hesperotesudo'' varied widely in size, with the largest unnamed species from El Sal ...

''.

History of investigation

The formation was defined by M.N. Machette in 1978 for exposures in the Sierra Ladrones, a range of low foothills of the Ladron Mountains, in the

Sevilleta National Wildlife Refuge The Sevilleta National Wildlife Refuge is a protected area of New Mexico managed by the United States Fish and Wildlife Service as part of the National Wildlife Refuge System. It is located in the Chihuahuan Desert, 20 miles north of Socorro, New ...

. The formation was subsequently mapped into the lower

Rio Puerco The Rio Puerco is a tributary of the Rio Grande in the U.S. state of New Mexico. From its source on the west side of the Nacimiento Mountains, it flows about ,Calculated in Google Earth generally south to join the Rio Grande about south of B ...

valley and as far north as the Santo Domingo basin.

Footnotes

References

* * * * * {{cite journal, title=Geology of the Santo Domingo Pueblo and Santo Domingo Pueblo SW 7.5-minute quadrangles, Sandoval County, New Mexico , last1=Smith , first1=G.A. , last2=Kuhle , first2=A.J. , journal=New Mexico Bureau of Mines and Mineral Resources Open-File Map Series , volume=OF-GM 15/26 , year=1998 , url=https://ngmdb.usgs.gov/Prodesc/proddesc_73218.htm , accessdate=18 May 2020 Neogene formations of New Mexico

Description

Fossils

History of investigation

Footnotes

See also

References