The Timberlake 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

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 ...

of 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 ...

. Its age is estimated to be

late Paleocene The Thanetian is, in the ICS Geologic timescale, the latest age or uppermost stratigraphic stage of the Paleocene Epoch or Series. It spans the time between . The Thanetian is preceded by the Selandian Age and followed by the Ypresian Age (part ...

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

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 embedded in a red arkosic matrix or a poorly sorted limestone conglomerate matrix. In some locations, the formation contains boulders of limestone conglomerate over in diameter that appear to be derived from the underlying

Cowboy Spring Formation The Cowboy Spring Formation is a geologic formation in southwestern New Mexico and southeastern Arizona. It preserves fossils dating back to the late Cretaceous period. Description The formation is primarily limestone cobble conglomerate. The m ...

. The formation is separated from the Cowbow Springs Formation, which it strongly resembles, 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 ...

. Its maximum thickness is . The formation was deposited in the Little Hat Top basin, a downwarp on the southern flank of the Hidalgo uplift. It is overlain by the Bluff Creek Tuff, which has been

radiometrically dated Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to Chronological dating, date materials such as Rock (geology), rocks or carbon, in which trace radioactive impurity, impurities were selectively incorporat ...

as 35.1 million years old. This constrains the Timberlake Formation to not be younger than Eocene. The formation is interpreted as a

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 fills ...

produced by erosion of

early Tertiary The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...

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 ...

uplifts.

History of investigation

The formation was first named by Zeller and Alper in 1965 for outcrops near Cowboy Spring in the

. Elston and Erb recommended merging the formation with the underlying Cowboy Spring 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 Neogene formations of New Mexico