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 secondar ...

exposed in

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 ...

that was deposited in the

Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Triassic Ep ...

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

middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...

Period.

Description

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

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) ...

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 (parallel layering).Blatt, H., and R.J. Tracy, 1996, ''Petrology. ...

and

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 ** Co ...

, including a basal conglomerate bed. The total thickness of the formation is at the type section. The formation is exposed along the

Pecos River The Pecos River ( es, Río Pecos) 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 ...

as far south as Santa Rosa, New Mexico. 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 group in Arizona. Part ...

based on the finding of ''

Eocyclotosaurus ''Eocyclotosaurus'' is an extinct genus of mastodonsauroid temnospondyl from the Middle Triassic (Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from millio ...

'' in the formation. The formation also contains the trace fossil '' Isopodichnus'' and traces of

actinopterygian Actinopterygii (; ), members of which are known as ray-finned fishes, is a class of bony fish. They comprise over 50% of living vertebrate species. The ray-finned fishes are so called because their fins are webs of skin supported by bony or ho ...

fish in coprolites. Amphibian fossils are abundant and diverse. These include fragments of paracyclotosaurid fossils, possibly related to ''

Stanocephalosaurus ''Stanocephalosaurus'' (stano from Greek stenos, meaning “narrow”, cephalo from Greek kephalē, meaning “head”) is an extinct genus of large-sized temnospondyl amphibians living through the early to mid Triassic. The etymology of its na ...

'', 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 b ...

s, other

archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avian d ...

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

prolacertiform Protorosauria is an extinct polyphyletic group of archosauromorph reptiles from the latest Middle Permian (Capitanian stage) to the end of the Late Triassic (Rhaetian stage) of Asia, Europe and North America. It was named by the English anatomis ...

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

and the primitive poposauroid ''

Arizonasaurus ''Arizonasaurus'' was a ctenosauriscid archosaur from the Middle Triassic (243 million years ago). ''Arizonasaurus'' is found in the Middle Triassic Moenkopi Formation of northern Arizona. A fairly complete skeleton was found in 2002 by Sterling ...

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 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 fossils ...

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