Abiquiu Formation

The Abiquiu Formation consists of light-gray to yellowish-gray, locally crossbeded, thin to thick beds of tuiffaceous sandstone, pebbly sandstone, and siltstone. There are also a few gravel beds and lenses of mudstone. The clasts are mostly volcanic clasts, including Amalia Tuff and trachyandesite and trachydacite possibly also from the Latir volcanic field. The formation is exposed in a broad belt from the southwest flank of the Tusas Mountains to the Jarosa area in the northwest Jemez Mountains.[1][2] The formation reaches a maximum thickness in excess of 400 meters (1,300 feet) near Canon. The age of the formation is approximately bracketed by a lava flow near its top dated to 18.9 Ma and a 25 Ma flow near its base.[3]

Both the lower contact of the formation with the Ritito Conglomerate and the upper contact with the Chama-El Rito Member of the Tesuque Formation are gradational. The transition to the Ritito Conglomerate is characterized by thick chert beds, informally designated the Pedernal chert for outcrops around Cerro Pedernal (36.163°N 106.512°W / 36.163; -106.512).[3]

Individual beds in the formation thicken across the Canones fault zone, indicating the unit was deposited after rifting began within the Rio Grande rift.[3]

The Abiquiu Tuff was first named by H.T.U. Smith in 1938 for exposures near the town of Abiquiu, New Mexico.[4] Church and Hack recognized almost at once that the unit consisted of a lower conglomeratic member and an upper tuff member separated by a chert horizon (the Pedernal chert member).[5] Woodward and Timmer first referred to the unit as the Abiquiu Formation in 1979.[6]. In 2009, Maldonado and Kelley identified the lower conglomerate beds with the Ritito Conglomerate and removed them from the formation, retaining Pedernal Chert as an informal name for the chert beds of the transition between the Ritito Conglomerate and Abiquiu Formation.[7]

Georgia O'Keeffe painted her landscape "From the White Place" in 1940 based on her study of the Abiquiu Formation at Plaza Blanca, and her studio at Abiquiu commanded a view of the Plaza Blanca outcrops to the north.[8]

• List of fossiliferous stratigraphic units in New Mexico
• Paleontology in New Mexico

• Church, F.S., and Hack, J.T., 1939, An exhumed erosion surface in the Jemez Mountains, New Mexico: Journal of Geology, v. 47, no. 6, p. 613-629.
• Maldonado, Florian; Kelley, Shari A. (February 2009). "Revisions to the stratigraphic nomenclature of the Abiquiu Formation, Abiquiu and contiguous areas, north-central New Mexico" (PDF). New Mexico Geology. 31 (1): 3–8. Retrieved 12 May 2020.
• Smith, H.T.U., 1938, Tertiary geology of the Abiquiu quadrangle, New Mexico: Journal of Geology, v. 46, no. 7, p. 933-965.
• Vazzana, M.E., and Ingersoll, R.V., 1981, Stratigraphy, sedimentology, petrology, and basin evolution of the Abiquiu Formation (Oligo-Miocene), north-central New Mexico; summary: Geological Society of America Bulletin, v. 92, no. 12, pt. 1, p. 990-992.
• Woodward, L.A., and Timmer, R.S., 1979, Geology of the Jarosa quadrangle, New Mexico: New Mexico Bureau of Mines and Mineral Resources Geologic Map, 47, scale 1:24,000