The Picuris 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 ...

exposed in the eastern flank of the

Rio Grande rift The Rio Grande rift is a north-trending continental rift zone. It separates the Colorado Plateau in the west from the interior of the North American craton on the east. The rift extends from central Colorado in the north to the state of Chihuah ...

in northern

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 was deposited from the

late Eocene The Priabonian is, in the International Commission on Stratigraphy, ICS's geologic timescale, the latest age (geology), age or the upper stage (stratigraphy), stage of the Eocene epoch (geology), Epoch or series (stratigraphy), Series. It spans ...

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

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 b ...

Description

The formation consists mostly 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 cont ...

conglomerate 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 ...

with a thickness of more than . The formation is divided into three informal

members 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 ...

. These are a lower conglomerate member, a middle tuffaceous member, and an upper

volcaniclastic Volcaniclastics are geologic materials composed of broken fragments (clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it ...

member. The lower member is a sandy or silty conglomerate containing mostly

Hondo Group The Hondo Group is a group of geologic formations that crops out in most of the Precambrian-cored uplifts of northern New Mexico. Detrital zircon geochronology gives a maximum age for the lower Hondo Group of 1765 to 1704 million years ( Mya), ...

quartzite Quartzite is a hard, non- foliated metamorphic rock that was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tecton ...

rock fragments (

clasts Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks b ...

) ranging in size from to over in diameter. The conglomerate is greenish to pale yellow in color, poorly

sorted Sorted may refer to: * ''Sorted'' (TV series), a BBC television series *"Sorted for E's & Wizz", a 1995 Pulp song * ''Sorted'' (The Drones album), 1999 * ''Sorted'' (DJ? Acucrack album) * ''Sorted'' (film), a 2000 British thriller film *'' Sorted'' ...

, and moderately to well rounded. Ar-Ar

geochronology Geochronology is the science of Chronological dating, determining the age of rock (geology), rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, ...

indicates an age range from before 34.5 million years ( Ma) to less than 28.3 Ma. The middle member is a tuffaceous fine to pebbly sandstone. This is buff to yellowish in color, generally poorly cemented, and poorly sorted. However, some beds are highly silicified. The member contains some well-developed normal grading sequences a few to tens of centimeters thick. There are some interbeds and channel fills of conglomerate containing local

Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...

clasts, with more conglomerate towards the top. Some

ash Ash is the solid remnants of fires. Specifically, ''ash'' refers to all non-aqueous, non- gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, ash is the ...

layers are bioturbated. Ash beds from the Amalia Tuff eruption have been identified within the member, with ages of 23 and 27 Ma, interpreted as pre- and post-

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of extremely vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicula ...

s. This member is at least thick. Based on Ar-Ar geochronology, this member was deposited from 28.3 to less than 23 Ma. The upper member is red to purple volcaniclastic conglomerate and sandstone, typically carbonate cemented. Individual bed

contacts Contact lenses, or simply contacts, are thin lenses placed directly on the surface of the eyes. Contact lenses are ocular prosthetic devices used by over 150 million people worldwide, and they can be worn to correct vision or for cosmetic ...

are abrupt and the lower bed is often scoured. A

basalt Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...

clast from the base of the member has an age of 19.78 +/- 0.24 Ma, and the member as a whole spans the age range of less than 19.8 Ma to about 15 Ma. This member is overlain by the Dixon Member of the

Tesuque Formation The Tesuque Formation is a geologic formation in north-central New Mexico, United States. The formation provides an unusually complete record of the evolution of mammals during the Miocene epoch. Description The formation is primarily siltstone ...

. Cabot included tuffaceous beds just north of Santa Fe in the Picuris Formation, but the

correlation In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistics ...

of this Bishop's Lodge Member remains controversial, with some geologists keeping it with the Picuris Formation, others assigning it to the Tesuque Formation, and others more recently assigning it to the

Espinaso Formation The Espinaso Formation is a Formation (geology), geologic formation in New Mexico. It has a radiometric dating, radiometric age of 34.6 to 26.9 million years, corresponding to the late Eocene through Oligocene Epoch (geology), epochs. Descripti ...

. The formation is of unusual interest in that it spans the transition from Laramide crustal shortening to

crustal extension.

History of investigation

The formation was first named by E.C. Cabot in 1938, likely for exposures in the

Picuris Mountains The Picuris Mountains are a mountain range in northern New Mexico. They are considered a subrange of the Sangre de Cristo Mountains. Geography The mountains are located to the east of Dixon and surround Picuris Pueblo to the west, north, and ...

north of

Vadito, New Mexico Vadito is a census-designated place (CDP) in southern Taos County, New Mexico, United States. It is on the scenic High Road to Taos. The population was 270 at the 2010 census. Geography According to the United States Census Bureau, the CDP h ...

Footnotes

References

* * * * * * {{cite journal , last1=Read , first1=Adam S. , last2=Rodgers , first2=John B. , last3=Ralser , first3=Steven , last4=Ilg , first4=Brad R. , last5=Kelley , first5=Shari , last6=Koning , first6=Daniel J. , title=Geologic map of Seton Village Quadrangle, Santa Fe County, New Mexico , journal=New Mexico Bureau of Geology and Mineral Resources Open-file Geologic Map , volume=OF-GM 023 , date=February 2013 , url=https://geoinfo.nmt.edu/publications/maps/geologic/ofgm/downloads/23/SetonVillageReport.pdf , accessdate=31 August 2020 Neogene formations of New Mexico Conglomerate formations of the United States Sandstone formations of the United States