The Espinaso 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 ...

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ke ...

. It has a radiometric age of 34.6 to 26.9 million years, corresponding to the late

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

through Oligocene

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 Espinaso Formation is principally

debris flow Debris flows are geological phenomena in which water-laden masses of soil and fragmented rock rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. They generally ...

s and lahars from the Ortiz porphyry belt, with some interbedded lava flows and

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 ...

beds. The upper part of the formation is dominated by upward-fining sequences, where sediments are coarser near the base of the sequence and finer towards the top. This suggests waning volcanic activity and decreasing topographic relief. The formation intertongues with the underlying Galisteo Formation but

unconformably 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 ...

underlies the Tanos Formation of the Santa Fe Group. Radiometric dating gives an age range of 34.3 +/-0.8 million years ( Ma) near the base of the formation, 34.6 +/-0.7 Ma near the middle, and 26.9 +/-0.6 my near the top. A

basalt Basalt (; ) is an 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 surface of a rocky planet or moon. More than 90 ...

flow at the base of the overlying Tanos Formation has an age of 25.1 +/-0.6 Ma. The formation crops out in the

Hagan Hagan may refer to: __NOTOC__ Places Iran * Hagan, Iran, a village in Hamadan Province Norway * Hagan, a village in Harstad * Hagan, a village in Akershus United States * Hagan, Georgia, a city * Hagan, Minnesota, an unincorporated community * ...

and

Galisteo Basin The Galisteo Basin is a surface basin and a closely related groundwater basin in north-central New Mexico. Its primary watercourse is the Galisteo River or Galisteo Creek, a perennial stream, for part of its course, that flows from the ea ...

s and the La Cienega area of

. The

type section A stratotype or type section in geology is the physical location or outcrop of a particular reference exposure of a stratigraphic sequence or stratigraphic boundary. If the stratigraphic unit is layered, it is called a stratotype, whereas the stan ...

is at Arroyo del Tuerto (Arroyo Pinovetito) where the arroyo cuts a

slot canyon A slot canyon is a long, narrow channel or drainageway with sheer rock walls that are typically eroded into either sandstone or other sedimentary rock. A slot canyon has depth-to-width ratios that typically exceed 10:1 over most of its length and ...

through Espinaso Ridge. Espinaso Ridge is a hogback produced by the strongly cemented volcaniclastics of the Espinaso Formation, which contrast with the less

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...

-resistant beds of the underlying

Galisteo Formation The Galisteo Formation is a geologic formation in New Mexico. It contains fossils characteristic of the Bartonian stage of the Eocene epoch, Duchesnean in the NALMA classification.Tanos Formation. The formation is also found in the subsurface in the southern Espanola Basin, where it was deposited on the eroded surface of a now-buried block of crust that was uplifted during the

Laramide orogeny The Laramide orogeny was a time period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the ...

and thrown back down with the opening 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 Chihuahu ...

. The formation records the style and composition of volcanic activity in the Ortiz porphyry belt, which is otherwise obscured by erosion of the volcanic centers. The paleomagnetism of the formation has been closely studied, with one study indicating that the block of crust on which the formation was deposited subsequently experienced a counterclockwise rotation of 17.8° ± 11.4°, due to opening of the Rio Grande Rift, but another showing no such rotation.

History of investigation

The formation was first described by C.E.Stearns in 1943, who credited the name "Espinaso Volcanics" to an unpublished manuscript by Kirk Bryan and J.E. Upson. P.F. Kautz and coinvestigators recognized that it was primarily alluvial and designated it as the Espinaso Formation in 1981.

Footnotes

References

* * * * * * * {{cite journal , last1=Stearns , first1=C.E. , title=The Galisteo formation of north-central New Mexico , journal=Journal of Geology , date=1943 , volume=51 , issue=5 , pages=301–319 , doi=10.1086/625156, s2cid=129038565 Paleogene formations of New Mexico