Tesuque Formation

The formation is primarily siltstone and fine sandstone, with some boulder conglomerate beds in eastern exposures close to the Sangre de Cristo Mountains. It is interpreted as coalesced alluvial fans with highly variable lithology. There is significant crossbedding and lenticular bedding, and some sandstone beds are well cemented with calcite. The formation dips 10 degrees to the west. Faulting makes an accurate estimate of its thickness uncertain, and it was originally estimated to be over 2,000 meters (6,600 feet) thick.[2] However, a careful reconstruction taking into account faults gives a thickness of 1,370–1,460 meters (4,490–4,790 feet).[3]

The Nambe Member is pinkish to reddish coarse-grained alluvial fan deposits resting on basement rock of the Sangre de Cristo Mountains. The member is named for Nambé Pueblo and has a total thickness of about 125 meters (410 feet).[4] An ash bed withing the member has an aged based on Ar-Ar dating of 25.52 ± 0.07 Ma. Its age range is estimated to be from 25.6 to 16.1 Ma.[5]

The Skull Ridge Member is separated from the Nambe Member by a distinctive ash bed. The member has numerous other ash beds, which result in relatively strong cementing that makes this a cliff-forming member. The upper portion is much less resistant to erosion. The member is up to 230 meters (750 feet) thick. Galusha and Blick did not identify the geographical location for which the member was named, and the place name "Skull Ridge" is unknown in New Mexico.[6] An ash bed in the Skull Ridge Member has an Ar-Ar age of 15.45±0.06 Ma.[7]

The Pojoaque Member is separated from the Skull Ridge Member by a greenish-gray conglomerate bed interpreted as lag gravel on an erosional surface. It is named for the Pojoaque Bluffs, also known as Los Barrancos. The formation disappears into the subsurface under the Rio Grande River floodplain but is at least 550 meters (1,800 feet) thick.[8] Its age range is estimated to be from 14.9 to 12.5 Ma.[9]

The Chama-El Rito Member is found only in the northwestern portion of the Espanola Basin. It correlates with the Skull Ridge and Pojoaque Members, but contains conglomerate lenses with distinctive volcanic clasts likely derived from the San Juan volcanic field.[10]

• 
  Chama-El Rito Member north of Mesa Vista School

The Ojo Caliente Member is 140 meters (460 feet) of soft eolian sandstone overlying the Chama-El Rito member. It is named for the village of Ojo Caliente.[11]

In addition to the traditional lithostratic units, the Tesuque Formation is divided into two lithosomes. Lithosome A is granitic material deposited by a network of westward-flowing streams off the Sangre de Cristo Mountains. Lithosome B is richer in clay and Paleozoic clasts and was deposited by south- to southwest-flowing rivers coming from the Penasco area. The two can be distinguished in the field by the redder coloration of Lithosome A compared with the tan to gray color of Lithosome B.[12]

• 
  Ojo Caliente Member of the Tesuque Formation at Embudo Station
• 
  Concretions in an outcrop of the Ojo Caliente Member on the Lemitas Trail

G.K. Gilbert visited San Ildefonso Pueblo with the Hayden Survey in 1873 and found fossil mammal bones characteristic of the Pliocene. Some of these were sent to Othniel Marsh. Marsh's bitter rival, Edward Drinker Cope, arrived at San Ildefonso the next year and collected a number of Miocene reptile, bird, and mammal fossils.[13]

Childs Frick sent an expedition into the Tesuque area in 1924, and immediately recognized the paleontological potential of the Santa Fe beds. The Fricks Laboratory (merged with the Department of Vertebrate Paleontology of the American Museum of Natural History in 1968) carried out field work through 1972. Work prior to 1940 was careless about identifying exact source strata, though greater care was taken thereafter.[14] Most of the fossils came from the Pojoaque Member of the Tesuque Formation and were almost entirely found within thin (0.5–3 m) maroon-red to pale green claystone to fine-grained siltstone beds of lithosome B. These are interpreted as small lacustrine deposits.[15]

Fossils found in the Tesuque Formation include the canids Hemicyon and Carpocyon webbi, the antilocaprids Cosoryx, Merycodus, and Ramoceros, chiroptera from the Vespertilionidae and Antrozoinae, the turtles Glyptemys valentinensis and Kinosternon pojoaque,[16] and mastodonts.[17][18]

The beds making up the unit were originally described by Bryan and McCann in 1937 as the Middle Red member of the Santa Fe Formation.[19] The formation was named by Baldwin in 1956 at the same time that the Santa Fe Formation was promoted to group status. The formation is named after exposures around the town of Tesuque, New Mexico.[1]

In 1971, Galusha and Blick removed the uppermost beds from the Tesuque Formation as the Chamita Formation and divided the Tesuque Formation into members. These were, in ascending stratigraphic order, the Skull Ridge Member, the Nambe Member, Pojoaque Member, the Chama-El Rito Member, and the Ojo Caliente Member.[17] Manley added the Cejita Member in 1977.[20]

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

• Baldwin, Brewster (1956). "The Santa Fe Group of north-central New Mexico" (PDF). New Mexico Geologic Society Field Conference Series. 7: 115–121. Retrieved 13 May 2020.
• Bryan, Kirk; McCann, Franklin T. (November 1937). "The Ceja Del Rio Puerco: A Border Feature of the Basin and Range Province in New Mexico: I. Stratigraphy and Structure". The Journal of Geology. 45 (8): 801–828. Bibcode:1937JG.....45..801B. doi:10.1086/624608. S2CID 128563355.
• Bourque, Jason R. (January 2012). "An extinct mud turtle of the Kinosternon flavescens group (Testudines, Kinosternidae) from the middle Miocene (late Barstovian) of New Mexico". Journal of Vertebrate Paleontology. 32 (1): 68–81. doi:10.1080/02724634.2012.626824. S2CID 84889382.
• Galusha, Ted; Blick, John C. (1971). "Stratigraphy of the Santa Fe Group, New Mexico" (PDF). Bulletin of the American Museum of Natural History. 144 (1). Retrieved 13 May 2020.
• Koning, Daniel J. (May 2002). "Geologic Map of the Española Quadrangle, Rio Arriba and Santa Fe Counties, New Mexico". New Mexico Bureau of Geology & Mineral Resources Open Map Series. 54. Retrieved 13 May 2020.
• Kues, Barry S.; Lewis, Claudia J.; Lueth, Virgil W. (2014). A brief history of geological studies in New Mexico : with biographical profiles of notable New Mexico geologists (First ed.). New Mexico Geological Society. ISBN 978-1-58546-011-3.
• Manley, K.A. (1977). "Geologic map of the northeastern part of the Espanola Basin, New Mexico showing the Cejia Member (new name) of the Tesuque Formation". U.S. Geological Survey Miscellaneous Field Studies Map. MF-877. doi:10.3133/mf877.
• McIntosh, William C.; Quade, Jay (1995). "40Ar/39Ar geochronology of tephra layers in the Santa Fe Group, Espanola Basin, New Mexico" (PDF). New Mexico Geological Society Guidebook. 46: 279–287. Retrieved 10 August 2020.
• Williamson, Garrett R. (2016). "The stratigraphic position of fossil vertebrates from the Pojoaque Member of the Tesuque Formation (middle Miocene, late Barstovian) near Española, New Mexico". Electronic Theses and Dissertations. 41.