The Espinaso Formation is a geologic formation in New Mexico. It has a radiometric age of 34.6 to 26.9 million years, corresponding to the late Eocene through

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 the ...

epochs.

Description

The Espinaso Formation is principally debris flows and

lahar A lahar (, from jv, ꦮ꧀ꦭꦲꦂ) is a violent type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a river valley. Lahars are extreme ...

s from the

Ortiz porphyry belt The Ortiz porphyry belt is a cluster of small mountain ranges in Santa Fe County, New Mexico. The mountains are laccoliths formed by intrusion of magma into the upper layers of the Earth's crust. This took place during the late Eocene through ea ...

, with some interbedded lava flows and tuff 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 A geological contact is a boundary which separates one rock body from another. A contact can be formed during deposition, by the intrusion of magma, or through faulting or other deformation of rock beds that brings distinct rock bodies into contac ...

with the underlying Galisteo Formation but unconformably underlies the

Tanos Formation The Tanos Formation is a geologic formation in central New Mexico. It is estimated to be about 25 million years in age, corresponding to the Oligocene epoch. Description The Tanos Formation consists of a very pale brown basal conglomerate overl ...

of the

Santa Fe Group The EAC Invest A/S, formerly known as the Santa Fe Group and East Asiatic Company ( da, italic=yes, Det Østasiatiske Kompagni or ''ØK'') is a multinational holding and investment company, based in Copenhagen, Denmark. History The East Asiat ...

. 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 flow at the base of the overlying

has an age of 25.1 +/-0.6 Ma. The formation crops out in the Hagan and Galisteo Basins and the La Cienega area of New Mexico. The type section is at Arroyo del Tuerto (Arroyo Pinovetito) where the arroyo cuts a slot canyon through Espinaso Ridge. Espinaso Ridge is a hogback produced by the strongly cemented

volcaniclastics 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 ...

of the Espinaso Formation, which contrast with the less erosion-resistant beds of the underlying Galisteo Formation and overlying

. 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 and thrown back down with the opening of the Rio Grande rift. 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 Paleomagnetism (or palaeomagnetismsee ), is the study of magnetic fields recorded in rocks, sediment, or archeological materials. Geophysicists who specialize in paleomagnetism are called ''paleomagnetists.'' Certain magnetic minerals in rock ...

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