The Exeter Sandstone is a geologic

formation Formation may refer to: Linguistics * Back-formation, the process of creating a new lexeme by removing or affixes * Word formation, the creation of a new word by adding affixes Mathematics and science * Cave formation or speleothem, a secondary ...

exposed in northeastern

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 poorly controlled, but it is thought to have been deposited during the

middle Jurassic The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations ...

Description

The formation consists of up to of white to pale pink

crossbedded In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The original ...

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

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 ...

. It

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 the Sheep Pen Sandstone and is in turn overlain by the

Bell Ranch Formation The Bell Ranch Formation is a Late Jurassic ( Kimmeridgian) geologic formation in eastern and northeastern New Mexico and the western Oklahoma panhandle. Fossil theropod tracks have been reported from the formation. Description At the type ...

. The Exeter Formation varies greatly in thickness, with the maximum thickness in synclinal valleys of the underlying

Dockum Group The Dockum is a Late Triassic (approximately late Carnian through Rhaetian, or 223–200 Ma) geologic group found primarily on the Llano Estacado of western Texas and eastern New Mexico with minor exposures in southwestern Kansas, eastern Color ...

and the formation being absent on some anticlinal crests of the Dockum Group. The formation has long been thought to have formed in the eastern part of the

Entrada Formation The Entrada Sandstone is a formation in the San Rafael Group found in the U.S. states of Wyoming, Colorado, northwest New Mexico, northeast Arizona, and southeast Utah. Part of the Colorado Plateau, this formation was deposited during the Juras ...

dune sea An erg (also sand sea or dune sea, or sand sheet if it lacks dunes) is a broad, flat area of desert covered with wind-swept sand with little or no vegetative cover. The word is derived from the Arabic word ''ʿarq'' (), meaning "dune field". ...

and thus to be

correlative In grammar, a correlative is a word that is paired with another word with which it functions to perform a single function but from which it is separated in the sentence. In English, examples of correlative pairs are ''both–and, either–or, nei ...

with the Entrada Formation, and

Spencer G. Lucas Spencer George Lucas is an American paleontologist and stratigrapher, and curator of paleontology at the New Mexico Museum of Natural History and Science. His main areas of study are late Paleozoic, Mesozoic and early Cenozoic vertebrate fo ...

and coinvestigators recommended demoting the Exeter Sandstone to member rank within the Entrada Formation. However, age control is poor on the Exeter Sandstone, and other investigators have retained the Exeter Sandstone at formation rank until the correlation becomes clearer.

History of investigation

The formation was first named by W.T. Lee in 1902 for exposures near the Exter Post Office.GEOLEX: "Exeter"
notes that "Exter" is the correct spelling of the town name.

Footnotes

References

* * * * * * * {{cite journal , last1=Ziegler , first1=Kate E. , last2=Ramos , first2=Frank C. , last3=Zimmerer , first3=Matthew J. , title=Geology of Northeastern New Mexico, union and Colfax Counties, New Mexico: A Geologic Summary , journal=New Mexico Geological Society Field Conference Series , date=2019 , volume=70 , issue=4 , pages=47-54 , url=https://nmgs.nmt.edu/publications/guidebooks/downloads/70/70_p0047_p0054.pdf , accessdate=1 September 2020 Jurassic formations of New Mexico

Description

History of investigation

See also

Footnotes

References