The Onate 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 ...

that is exposed in most of the highlands of south-central

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 ...

. It preserves

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

dating back to the

middle Devonian In the geological timescale, the Middle Devonian epoch (from 393.3 ± 1.2 million years ago to 382.7 ± 1.6 million years ago) occurred during the Devonian period, after the end of the Emsian age. The Middle Devonian epoch is subdivided into two ...

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

At the type section, the Onate Formation consists of about of orange yellow-weathering dolomitic

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. Although its permeabil ...

with

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

s and

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 ...

. Elsewhere the thickness is more typically . The base of the formation is a profound regional unconformity, so that the formation rest on either the Fusselman Formation or the

Montoya Group The Montoya Group is a group of geologic formations in westernmost Texas and southern New Mexico. It preserves fossils dating back to the late Ordovician period.Richardson 2008Pope 2004 Description The group consists mostly of dolomitePray 1953 ...

. It is overlain by the Sly Gap Formation and thins to the north and south. The formation is interpreted as having been deposited on a shallow shelf environment deepening to a euxenic basin to the south.

Fossils

The formation contains the fossil

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear e ...

Spirifer ''Spirifer'' is a genus of marine brachiopods belonging to the order Spiriferida and family Spiriferidae. Species belonging to the genus lived from the Middle Ordovician (Sandbian) through to the Late Triassic (Carnian) with a global distribut ...

acuminatu'', as well as

crinoid Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are ...

s and

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...

ns. The brachiopod assemblage includes 34 genera and 41 species. Conodonts are rare but a few tabulate corals,

ichnofossil 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 ...

s, and the receptaculid '' Sphaerospongia'' is present. The receptaculids provided a solid substrate for colonization by the

rugose Rugose means "wrinkled". It may refer to: * Rugosa, an extinct order of coral, whose rugose shape earned it the name * Rugose, adjectival form of rugae Species with "rugose" in their names * ''Idiosoma nigrum'', more commonly, a black rugose tra ...

coral '' Tabulophyllum traversensis''. The fossil assemblage gives an age of late

Givetian The Givetian is one of two faunal stages in the Middle Devonian Period. It lasted from million years ago to million years ago. It was preceded by the Eifelian Stage and followed by the Frasnian Stage. It is named after the town of Givet in ...

. The formation is highly bioturbated.

History of investigation

The beds assigned to the formation were previously correlated with the Canutillo Formation of west Texas. However, F.V. Stevenson questioned the correlation, and designated a new Onate Formation including the beds in 1945. In 1985, G.A. Cooper and J.T. Dutro, Jr., conducted the first thorough study of the fossil assemblage of the formation. The fossils were further characterized by James E. Souraf in 1987.

Footnotes

References

* * * * * * {{Use dmy dates, date=April 2017 Devonian formations of New Mexico Devonian southern paleotemperate deposits Siltstone formations of the United States Shale formations of the United States Sandstone formations of the United States

Description

Fossils

History of investigation

See also

Footnotes

References