The Nacimiento Formation is a heterogeneous nonmarine formation composed of shale, siltstone, and sandstone,[2] deposited in floodplain, fluvial and lacustrine settings,[3] and made up of sediment shed from the San Juan uplift to the north and the Brazos-Sangre de Cristo uplift to the east.[4] It was deposited mostly between ~65.7 and ~61 million years ago, during the early and middle Paleocene.[5] The climate was humid and warm to hot[6] and stable, but with a distinct dry season.[7] This unit interbeds with the underlying Ojo Alamo Formation but is separated by an unconformity from the overlying San Jose Formation.[3]
The Nacimiento Formation is divided into several subunits known as members. In outcrops in southern areas of the formation, the Puercan fauna is found in the Arroyo Chijuillita Member, the Torrejonian fauna is found in the Ojo Encino Member, and the uppermost Escavada Member lacks age-diagnostic fossils.[1] In northern outcrops, the two lower members are indistinguishable, and are called the "main body".[5] Above them are two more informal members. These preserve a younger, Tiffanian fauna.[8] The Puercan and Torrejonian faunas are further subdivided into several biostratigraphic zones.[5]
These fossils provide important clues to the impact of the Cretaceous–Paleogene extinction event on mammals[16] and to the recovery, evolution, and turnover of mammals shortly after the event. The formation and its fossils provide a particularly clear record of the To2-To3 turnover event, allowing the timing of the event to be constrained to between 62.59 and 62.47 million years ago. The event may have been associated with climate change or with the rapid development of a river system across the San Juan basin, which caused a temporary pause in sediment deposition that separates the Nacimiento Formation from the San Jose Formation.[17]
Workers in the early 1900s divided the rocks of the Nacimiento Formation into two formations, the lower Puerco Formation and the upper Torrejon Formation.[13] This was rejected on the grounds that there were no lithological differences between the two, only differences in fossil faunas, making determination of which formation was present in a given area impossible if fossils could not be found.[9] The Puerco and Torrejon were retained as zones within the Nacimiento Formation, and their faunas became the basis of the Puercan and TorrejonianNorth American Land Mammal Ages.[19]
Fassett, J.E. (1992). "Cretaceous and Tertiary rocks of the eastern San Juan Basin, New Mexico and Colorado". In Siemers, C.T.; Woodward, L.A.; Callender, J.F. (eds.). Ghost Ranch. New Mexico Geological Society Guidebook. Vol. 25. Socorro, New Mexico: New Mexico Geological Society. pp. 225–230. CiteSeerX10.1.1.527.6438.
Libed, S.A. (2005). Lithostratigraphy and mammalian biostratigraphy of the Torrejonian-Tiffanian transition in the Nacimiento Formation, northwestern San Juan Basin, New Mexico. M.A. thesis. Albuquerque, New Mexico: University of New Mexico.
Lucas, S.G. (1984). "Correlation of Eocene rocks of the northern Rio Grande Rift and adjacent areas: implications for Laramide tectonics". In Baldridge, W.S.; Dickerson, P.W.; Riecker, R.E.; Zidek, J. (eds.). Rio Grande Rift: northern New Mexico(PDF). New Mexico Geological Society Guidebook. Vol. 35. Socorro, New Mexico: New Mexico Geological Society. pp. 123–128. Retrieved 8 June 2020.
Schoch, R.M. (1981). "Taxonomy and biostratigraphy of the early Tertiary Taeniodonta (Mammalia: Eutheria)". Geological Society of America Bulletin. 92 (12): I 933-I 941, II 1982-II 2267. Bibcode:1981GSAB...92.1982S. doi:10.1130/gsab-p2-92-1982.
Silcox, Mary T.; Williamson, Thomas E. (December 2012). "New discoveries of early Paleocene (Torrejonian) primates from the Nacimiento Formation, San Juan Basin, New Mexico". Journal of Human Evolution. 63 (6): 805–833. doi:10.1016/j.jhevol.2012.09.002. PMID23084622.
Sullivan, R.M.; Lucas, S.G. (1986). "Annotated list of fossil vertebrates from the Paleocene Nacimiento Formation (Puercan-Torrejonian), San Juan Basin, New Mexico". Journal of Herpetology. 20 (2): 202–209. doi:10.2307/1563945. JSTOR1563945.
Tidwell, W.D.; Ash, S.R.; Parker, L.R. (1981). "Cretaceous and Tertiary floras of the San Juan Basin". In Lucas, S.G.; Rigby Jr., J.K.; Kues, B.S. (eds.). Advances in San Juan Basin paleontology. Albuquerque, New Mexico: University of New Mexico Press. pp. 307–332. ISBN0-8263-0554-7.
Williamson, Thomas E.; Brusatte, Stephen L.; Carr, Thomas D.; Weil, Anne; Standhardt, Barbara R. (1 December 2012). "The phylogeny and evolution of Cretaceous–Palaeogene metatherians: cladistic analysis and description of new early Palaeocene specimens from the Nacimiento Formation, New Mexico". Journal of Systematic Palaeontology. 10 (4): 625–651. doi:10.1080/14772019.2011.631592. S2CID83996185.
Williamson, T.E.; Lucas, S.G. (1992). "Stratigraphy and mammalian biostratigraphy of the Paleocene Nacimiento Formation, southern San Juan basin, New Mexico". In Lucas, S.G.; Kues, B.S.; Williamson, T.E.; Hunt, A.P. (eds.). San Juan Basin IV(PDF). New Mexico Geological Society Guidebook. Vol. 43. Socorro, New Mexico: New Mexico Geological Society. pp. 265–296. ISBN99922-2-673-0. Retrieved 26 August 2020.
