The Glencairn 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 ...

found in

Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...

and

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 characteristic of the Albian

Age Age or AGE may refer to: Time and its effects * Age, the amount of time someone or something has been alive or has existed ** East Asian age reckoning, an Asian system of marking age starting at 1 * Ageing or aging, the process of becoming older ...

of the

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 th ...

Period.

Description

The Glencairn Formation consists of dark gray

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, especial ...

and buff

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, p ...

. It disconformably overlies the Lytle Formation, underlies the

Dakota Group The Dakota is a sedimentary geologic unit name of formation and group rank in Midwestern North America. The Dakota units are generally composed of sandstones, mudstones, clays, and shales deposited in the Mid-Cretaceous opening of the Western Int ...

, and varies in thickness from . The formation is present from central Colorado to the valley of the Dry Cimarron in northeastern New Mexico. The formation locally contains

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywall. ...

veins and gypsum-filled desiccation cracks. The exposures at the valley of the Dry Cimarron include a basal sandstone bed, the Long Canyon Sandstone Bed, that is up to thick, is heavily bioturbated, and contains an abundant late Albian invertebrate fossil fauna. This is interpreted as infilling of a drainage system preceding the Kiowa-Skull Creek

transgression Transgression may refer to: Legal, religious and social *Sin, a violation of God's Ten Commandments or other elements of God's moral law *Crime, legal transgression, usually created by a violation of social or economic boundary **In civil law ju ...

. It is likely the lateral equivalent of the Tucumcari Shale.

Fossils

The lower beds of the formation are heavily bioturbated and contain abundant fossils of the gryphaeid oyster ''Texigryphea''. The upper beds locally contain petrified plant material. The formation also contains

ammonoid Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

s, including ''Goodhallites'', ''Idiohamites'', and ''Engonoceras uddeni'', and associated solitary corals, bivalves, and gastropods

History of investigation

The formation was first named as the Glencairn shale member of the now abandoned

Purgatoire Formation The Purgatoire Formation is an abandoned (1987) Cretaceous period geologic formation classification. The classification was used in Colorado, New Mexico, and Oklahoma, with mentions in older geologic literature in neighboring states.Griggs and Re ...

by G.I. Finlay in 1916, for exposures near Lytle, Colorado. Waage subsequently traced the unit into northeastern New Mexico, where it has been raised to formation rank.

References

{{reflist, 30em Cretaceous Colorado Cretaceous formations of New Mexico