The El Paso 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 from the Permian Basin of

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 ...

and

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

to southeastern

Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...

. 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

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

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 is composed of gray cherty dolomite,

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

, and smaller amounts of siltstone. The formation often has a mottled appearance. Total thickness is . It lies unconformably on the Bliss Formation, the Coronado Sandstone, or (in the

Florida Mountains The Florida Mountains are a small long, mountain range in New Mexico. The mountains lie in southern Luna County, New Mexico, Luna County about southeast of Deming, New Mexico, Deming, and north of the Chihuahua (state), state of Chihuahua, M ...

) Precambrian basement, and is overlain by the Montoya Group or Portal Formation.

Fossils

The formation is only sparsely fossiliferous, but contains fossils of

echinoderm An echinoderm () is any animal of the phylum Echinodermata (), which includes starfish, brittle stars, sea urchins, sand dollars and sea cucumbers, as well as the sessile sea lilies or "stone lilies". While bilaterally symmetrical as ...

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

s, sponge spicules, and ''Nuia''. Rare

ostracod Ostracods, or ostracodes, are a Class (biology), class of the crustacean, Crustacea (class Ostracoda), sometimes known as seed shrimp. Some 33,000 species (only 13,000 of which are extant taxon, extant) have been identified,Brandão, S.N.; Antoni ...

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

s, and

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 ...

s are also found, as is the trace fossil '' Planolites''. Bioherms up to high are found in the McKelligon Member, built up of

siliceous sponge The siliceous sponges form a major clade of the phylum Porifera, consisting of classes Demospongiae (common sponges) and Hexactinellida (glass sponges). They are characterized by spicules made out of silicon dioxide, unlike calcareous sponges. ...

s and

receptaculitid ''Receptaculites'' is the name-bearing genus for an extinct group of conspicuous benthic marine genera, the receptaculitids (formally Receptaculitaceae or Receptaculitidae), that lived from the Early Ordovician through the Permian The Permian ...

'' Calathium''.

History of investigation

The formation was first named by

George Burr Richardson George Burr Richardson (1872–1949) was an American geologist who participated in extensive field work for the United States Geological Survey (USGS) in Alaska, Pennsylvania, California, Texas, Colorado, and Utah. In Texas, Richardson described ...

in 1904 for exposures in the Franklin and Hueco Mountains. All Ordovician beds of the Franklin Mountains were originally included in the formation. Richardson later (1908) mapped the formation into the Permian Basin and assigned the upper Ordovician beds to the Montoya Limestone. In 1965, Zeller divided the formation in southwestern New Mexico into the Sierrita and Bat Cave Members. Clemons (1991) divided the formation differently, into the Hitt Canyon, Jose, McKelligon, and Padre Members. In 1964, R.H. Flower proposed promoting the El Paso Formation to

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

rank and recommended several divisions into formations, based largely on biostratigraphy, such as the Big Hatchet Formation, the Cooks Formation, the Florida Mountain Formation, the Scenic Drive Formation, or the Victorio Hills Formation. However, this has not been widely accepted, and Greg H. Mack rejected both the promotion of the El Paso Formation to

rank and the designation of biostratigraphic zones within the El Paso as formations.

Footnotes

References

* * * * * * * * * * * {{cite journal , last1=Zeller , first1=R.A. Jr. , year=1965 , title=Stratigraphy of the Big Hatchet Mountains Area, New Mexico , journal=New Mexico Bureau of Mines and Mineral Resources Memoir , volume=16 Limestone formations of the United States Ordovician formations of New Mexico Ordovician geology of Texas Ordovician southern paleotropical deposits

Description

Fossils

History of investigation

See also

Footnotes

References