The Cowboy Spring 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 southwestern

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 ...

and southeastern

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a U.S. state, state in the Southwestern United States. It is the list of U.S. states and territories by area, 6th largest and the list of U.S. states and territories by population, 14 ...

. 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

late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

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 ...

Description

The formation is primarily

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...

cobble

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 ...

. The massive conglomerate beds are separated by thinner beds of

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 ...

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especia ...

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

, and

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 ...

. The clasts in the conglomerate are of

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

age and contain the

foraminifera Foraminifera (; Latin for "hole bearers"; informally called "forams") are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly ...

n ''Orbitolina'' and other common Cretaceous fossils. These are embedded in a red arkosic matrix. The sandstones are also mostly red and arkosic and the shale is mostly bright red. The formation interfingers with the underlying

Mojado Formation The Mojado Formation is a geologic formation in southwestern New Mexico. It preserves fossils dating back to the early Cretaceous period. Description The formation consists mostly of sandstone and shale, with some limestone, and siltstone. It re ...

, is overlain with

angular unconformity 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 ...

by the Timberlake Formation, and has a total thickness of at least . A single latite tuff bed some is exposed in the formation. The formation was deposited in the Little Hat Top basin, which was produced by

Laramide The Laramide orogeny was a time period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the ...

deformation on the southwest flank of the Hidalgo uplift.

History of investigation

The formation was first named by Zeller and Alper in 1965 for a single outcrop in the

Animas Mountains The Animas Mountains are a small mountain range in Hidalgo County, within the " Boot-Heel" region of far southwestern New Mexico, in the United States. They extend north–south for about 30 miles (50 km) along the Continental Divide,Since ...

. By 1970, it had been traced into southeastern Arizona Elston and Erb recommended merging the formation with the overlying Timberlake Formation due to lack of a clear lithological distinction, but this has not been universally accepted.

Footnotes

References

* * * * {{cite journal , last1=Zeller , first1=R.A. Jr. , last2=Alper , first2=A.M. , year=1965 , title=Geology of the Walnut Wells quadrangle, Hidalgo County, New Mexico , journal=New Mexico Bureau of Mines and Mineral Resources Bulletin , volume=84 , url=https://geoinfo.nmt.edu/publications/monographs/bulletins/downloads/84/B84.pdf , access-date=15 September 2020 Cretaceous formations of New Mexico