The Bell Top 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 ...

in southern

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe, New Mexico, Santa Fe , LargestCity = Albuquerque, New Mexico, Albuquerque , LargestMetro = Albuquerque metropolitan area, Tiguex , Offi ...

. Radiometric dating of surrounding tuffs indicate that it was deposited in the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but ...

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided by ...

Description

The formation consists of

tuffaceous Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...

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 ...

and minor

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 ** C ...

. These are interbedded with volcanic

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

s and flows that are assigned to the Mogollon Group. Total thickness may be as much as . In the southeastern part of the

Mogollon-Datil volcanic field The Mogollon-Datil volcanic field is a large () silicic volcanic field in western New Mexico (Mogollon Mountains-Datil, New Mexico). It is a part of an extensive Eocene to Oligocene volcanic event which includes the San Juan volcanic field in sou ...

, the upper beds of the Bell Top Formation define the break between the Mogollon Group and the Datil Group. Here the Bell Top rests on the 33.51 Ma Box Canyon Tuff, the uppermost unit of the Datil Group, and below the 28.56 Ma Vicks Peak Tuff, the lowest tuff of the Mogollon Group. In other locations, it overlies the

Palm Park Formation The Palm Park Formation is a geologic formation in southern New Mexico. It preserves fossils dating back to the Eocene epoch. Description The formation consists of reddish clastic sediments, including some beds of boulder conglomerate with ...

. The formation has been interpreted as sediments eroded from caldera rims of the Mogollon-Datil volcanic field. Oxygen isotope ratios suggest that the magma from which the caldera rims solidified had its origin in the

Earth's crust Earth's crust is Earth's thin outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The ...

as a partial melt of

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies und ...

source rock.

History of investigation

The formation was first named by F.E. Kottlowski in 1953 for exposures around Bell Top Mountain, northwest of

Las Cruces, New Mexico Las Cruces (; "the crosses") is the second-largest city in the U.S. state of New Mexico and the seat of Doña Ana County. As of the 2020 census the population was 111,385. Las Cruces is the largest city in both Doña Ana County and southern Ne ...

. His definition included both igneous beds and sedimentary beds in the formation. Steven M. Cather and his coinvestigators argued in 1994 for a revision of southwestern New Mexico stratigraphy in which the Bell Top Formation was restricted to the sedimentary beds and included in the Spears Group.

Footnotes

References

* * * * * {{cite journal , last1=Seager , first1=William R. , last2=Clemons , first2=Russel E. , year=1973 , title=Middle to late Tertiary geology of Cedar Hills-Selden Hills area, New Mexico , journal=New Mexico Bureau of Mines and Mineral Resources Circular , volume=133 , url=https://geoinfo.nmt.edu/publications/monographs/circulars/downloads/133/Circular-133.pdf , accessdate=26 August 2020 Paleogene formations of New Mexico