El Rito Formation

The El Rito Formation is a geologic formation in New Mexico dating to the Eocene epoch.

El Rito Formation
Stratigraphic range: Eocene
El Rito Formation near Arroyo del Cobre, New Mexico
TypeFormation
UnderliesRitito Conglomerate
OverliesMorrison Formation, Dakota Formation
Thickness70 m (230 ft)
Lithology
PrimarySandstone
OtherMudstone
Location
Coordinates36.276°N 106.302°W / 36.276; -106.302
RegionNew Mexico
CountryUnited States
Type section
Named forEl Rito Creek
Named byH.T.U. Smith
Year defined1938
El Rito Formation (the United States)
El Rito Formation (New Mexico)

Description

The formation is composed of brick-red well-consolidated sandstone, conglomerate, and breccia. The pebbles are predominantly quartzite, with no volcaniclastics. The formation crops out over a narrow belt from the southern Tusas Mountains[1] to upper Coyote Creek.[2] In the southern Tusas Mountains, it fills narrow paleovalleys eroded in basement quartzite. Further south, the rock grades from heterogeneous breccia to a poorly sorted conglomerate. The breccia is interpreted as regolith and talus, while the conglomerate is interpreted as a high-energy stream deposit. The distinctive red color is found penetrating joints in the underlying quartzite to depths of several meters. Further south and west, the formation transitions to lighter-colored sandstone. Maximum thickness is 70 meters (230 feet).[1] A characteristic feature of the sandstone is the presence of muscovite.[2]

The base of the formation is an angular unconformity with much older beds of Jurassic to Cretaceous age, while the upper contact is an erosional disconformity with the Abiquiu Formation.[1][2]

Age control is poor due to an almost complete lack of fossils. However, the formation was deposited in the waning phase of the Laramide orogeny under arid conditions. Paleocurrents and conglomerate clast provenance show that the source region was the Brazos uplift to the north and northeast. The formation was deposited in an asymmetrical basin formed in response to compression of the Laramide orogeny,[3] truncated to the west along the Canones fault zone.[4] The El Rito basin likely was continuous with the Galisteo Basin before the opening of the Rio Grande rift separated the two basins starting in the Miocene. The El Rito Formation thus likely correlates with the Diamond Tail and Galisteo Formations.[3]

History of investigation

The formation was named in 1938 by Harold T.U. Smith for exposures along El Rito Creek.[1]

Footnotes

References

  • Lawrence, R.; Kelley, S. A.; Rampey, M. (2004). "Preliminary geologic map of the Cerro del Grant 7.5-minute quadrangle, Rio Arriba and Sandoval Counties, New Mexico". New Mexico Bureau of Geology and Mineral Resources, Open-file Geologic Map. 87. Retrieved 26 August 2020.
  • Kelley, S.A.; Kempter, K.A.; McIntosh, W.C.; Maldonado, F.; Smith, G.A.; Connell, S.D.; Koning, D.J.; Whiteis, J. (2013). "Syndepositional deformation and provenance of Oligocene to Lower Miocene sedimentary rocks along the western margin of the Rio Grande Rift, Jemez Mountains, New Mexico". In Hudson, M.R.; Grauch, V.J.S. (eds.). New Perspectives on Rio Grande Rift Basins: From Tectonics to Groundwater. Geological Society of America Special Paper 494. doi:10.1130/2013.2494(05). Retrieved 25 August 2020.
  • Logsdon, Mark J. (1981). "A preliminary basin analysis of the El Rito Formation (Eocene), north-central New Mexico". Geological Society of America Bulletin. 92 (12): 968. doi:10.1130/0016-7606(1981)92<968:APBAOT>2.0.CO;2.
  • Smith, Harold T. U. (October 1938). "Tertiary Geology of the Abiquiu Quadrangle, New Mexico". The Journal of Geology. 46 (7): 933–965. doi:10.1086/624710.
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