The Cerro Conejo Formation is a middle to

late Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene epoch (geology), Epoch made up of two faunal stage, stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million ye ...

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 ...

exposed near

Albuquerque, New Mexico Albuquerque ( ; ), ; kee, Arawageeki; tow, Vakêêke; zun, Alo:ke:k'ya; apj, Gołgéeki'yé. abbreviated ABQ, is the most populous city in the U.S. state of New Mexico. Its nicknames, The Duke City and Burque, both reference its founding in ...

. Gravel beds within the formation in the Rio Puerco valley west of Albuquerque have provided clues to the

paleogeography Palaeogeography (or paleogeography) is the study of historical geography, generally physical landscapes. Palaeogeography can also include the study of human or cultural environments. When the focus is specifically on landforms, the term pale ...

of the

Grants Grant or Grants may refer to: Places *Grant County (disambiguation) Australia * Grant, Queensland, a locality in the Barcaldine Region, Queensland, Australia United Kingdom *Castle Grant United States * Grant, Alabama *Grant, Inyo County, C ...

area since the early Miocene.

Description

The Cerro Conejo Formation is composed of very light brown to pink or yellowish-red, fine- to medium-grained, well- sorted, medium- to thick- bedded

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) ...

. The beds are tabular and locally cross-stratified and include occasional thinly bedded

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. ...

. Sandstone grains are subrounded and frosted. The formation also contains numerous white to light greenish gray

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcano, volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used t ...

beds. The total thickness is . The formation overlies the Zia Formation and interfingers with the overlying Arroyo Ojito Formation. One of the ash beds within the formation yielded a K-Ar age of 13.64 +/-0.09 million years ( Ma. The overall age range is constrained to between 14.8 and 9.5 Ma. Gravel beds within the formation in the Rio Puerco valley west of

Albuquerque Albuquerque ( ; ), ; kee, Arawageeki; tow, Vakêêke; zun, Alo:ke:k'ya; apj, Gołgéeki'yé. abbreviated ABQ, is the most populous city in the U.S. state of New Mexico. Its nicknames, The Duke City and Burque, both reference its founding in ...

have provided clues to the

of the

area since the early Miocene. These beds have been assigned to the informal Benavidez

Member Member may refer to: * Military jury, referred to as "Members" in military jargon * Element (mathematics), an object that belongs to a mathematical set * In object-oriented programming, a member of a class ** Field (computer science), entries in ...

Fossils

The formation has yielded vertebrate

fossil 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 ...

s at the Rincon and Zia quarries that are characteristic of the middle to late Miocene (

Barstovian The Barstovian North American Stage on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from 16,300,000 to 13,600,000 years BP, a period of . It is usua ...

to Clarendonian).

History of investigation

The unit was first defined by Connell and his collaborators in 1999 as the Cerro Conejo Member of the Zia Formation. It was named for exposures along Arroyo Ojito near Cerro Conejo, a small hill northwest of Albuquerque. Arroyo Ojito was also designated as the type locality (no type section defined). Tedford and Barghoon recommended that the Cerro Conejo Member be reassigned to the Arroyo Ojito Formation, since there was a significant

unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...

(corresponding to a pause in sediment deposition of 1 to 1.6 Ma) separating the Cerro Conejo from the rest of the Zia Formation. In 2007, Williams and Cole recommended that the Arroyo Ojito Formation be abandoned, because the name was being used inconsistently and because the formation straddled a significant region unconformity, since named the Rincones paleosurface. They promoted the Ceja Member, which lay above the Rincones paleosurface, to formation rank, and moved the remaining members of the Arroyo Ojito into an informal "Middle Red" formation, recalling the earliest subdivisions of the Santa Fe Group. Connell responded the next year by concurring with the promotion of the Ceja Formation (which he divided into Santa Ana Mesa and Atrisco Members), promoting the Cerro Conejo to formation rank, and retaining the middle beds as the Arroyo Ojito.

Footnotes

References

* * * * * {{cite journal , last1=Williams , first1=P.L. , last2=Cole , first2=J.C. , year=2007 , title=Geologic map of the Albuquerque 30' x 60' quadrangle, north-central New Mexico , journal=U.S. Geological Survey Scientific Investigations Map , series=Scientific Investigations Map , volume=SIM-2946 , doi=10.3133/sim2946 , hdl=2027/mdp.39015049951992 , url=https://pubs.usgs.gov/sim/2007/2946/downloads/pdf/2946_pamphlet_508.pdf , accessdate=11 August 2020, doi-access=free Neogene formations of New Mexico Fluvial deposits

Description

Fossils

History of investigation

See also

Footnotes

References