The Anton Chico Formation 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

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

that was deposited in the

Anisian In the geologic timescale, the Anisian is the lower stage (stratigraphy), stage or earliest geologic age, age of the Middle Triassic series (stratigraphy), series or geologic epoch, epoch and lasted from million years ago until million years ag ...

Age Age or AGE may refer to: Time and its effects * Age, the amount of time someone has been alive or something 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

middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...

Period Period may refer to: Common uses * Period (punctuation) * Era, a length or span of time *Menstruation, commonly referred to as a "period" Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (o ...

Description

The formation consists mostly of very fine to medium-grained, red-brown to gray-red

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

with minor silty

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

and conglomerate, including a basal conglomerate bed. The total thickness of the formation is at the

type section In geology, a stratotype or type section is the physical location or outcrop of a particular reference exposure of a stratigraphic sequence or stratigraphic boundary. If the stratigraphic unit is layered, it is called a stratotype, whereas the ...

. The formation is exposed along the

Pecos River The Pecos River ( ; ) originates in north-central New Mexico and flows into Texas, emptying into the Rio Grande. Its headwaters are on the eastern slope of the Sangre de Cristo mountain range in Mora County north of Pecos, New Mexico, at an elev ...

as far south as

Santa Rosa, New Mexico Santa Rosa is a city in and the county seat of Guadalupe County, New Mexico, Guadalupe County, New Mexico, United States. The population was 2,848 at the 2010 United States Census, 2010 census. It lies between Albuquerque, New Mexico, Albuquerqu ...

. The formation is overlain by the Santa Rosa Formation and lies on the Artesia Group.

Fossils

The Anton Chico Formation was originally correlated with the Holbrook Member of the

Moenkopi Formation The Moenkopi Formation is a geological formation that is spread across the U.S. states of New Mexico, northern Arizona, Nevada, southeastern California, eastern Utah and western Colorado. This unit is considered to be a Geological unit, group ...

based on the finding of ''

Eocyclotosaurus ''Eocyclotosaurus'' (Greek ‘èoos’ = dawn, ‘kyklos’ = circle, 'ous' = ear ) is an extinct genus of mastodonsauroid temnospondyl from the Middle Triassic (Anisian). The name ''Eocyclotosaurus'' means "dawn round-eared lizard". It is chara ...

'' in the formation. The formation also contains the

trace fossil A trace fossil, also called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of part ...

'' Isopodichnus'' and traces of

actinopterygian Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of skin ...

fish in

coprolites A coprolite (also known as a coprolith) is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name i ...

. Amphibian fossils are abundant and diverse. These include fragments of paracyclotosaurid fossils, possibly related to '' Stanocephalosaurus'', and an unidentified capitosauroid. Reptile fossils are more common and diverse than in the Holbrook Member and include

erythrosuchid Erythrosuchidae (meaning "red crocodiles" in Greek) are a family of large basal archosauriform carnivores that lived from the later Early Triassic (Olenekian) to the early Middle Triassic (Anisian). Naming The family Erythrosuchidae was named by ...

s, other

archosaur Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...

rauisuchian "Rauisuchia" is a paraphyletic group of mostly large and carnivorous Triassic archosaurs. Rauisuchians are a category of archosaurs within a larger group called Pseudosuchia, which encompasses all archosaurs more closely related to crocodilians ...

s, and prolacertiforms. The formation has also yielded two incomplete skeletons of a large

and the primitive poposauroid ''

Arizonasaurus ''Arizonasaurus'' (meaning "Arizona reptile") was a ctenosauriscid archosaur from the Middle Triassic (243 million years ago) Moenkopi Formation of what is now Arizona. The taxon is known for having a large back sail formed by elongated neural sp ...

babbitti''.

History of investigation

The beds making up this formation were originally the lowest informal member of the Santa Rosa Formation, but were designated as the Anton Chico Formation by Spencer G. Lucas and Adrian Hunt in 1987. Lucas and Hunt subsequently reduced the unit to member rank within the Moenkopi Formation, but its inclusion in the Moenkopi has not been universally accepted. Some red beds previously mapped as Bernal Formation, particularly in its more northern and western exposures, are now thought to belong to the Anton Chico.

References

Bibliography

* * * * * * * {{cite journal , last1=Nesbitt , first1=Sterling J. , last2=Lucas , first2=Spencer G. , last3=Schoch , first3=Rainer R. , title=A new, large archosauriform from the Anton Chico Member of the upper Moenkopi Formation (Middle Triassic), east-central New Mexico, USA , journal=Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen , volume=239 , number=2 , year=2006 , pages= 289–311 , doi=10.1127/njgpa/239/2006/289 Triassic formations of New Mexico Sandstone formations of the United States Mudstone formations of the United States Conglomerate formations of the United States