The Nacimiento Formation is a heterogeneous nonmarine formation composed ofshale, siltstone, and sandstone, deposited in floodplain, fluvial and lacustrine settings, and made up of sediment shed from the San Juan uplift to the north and the Brazos-Sangre de Cristo uplift to the east. It was deposited mostly between ~65.7 and ~61 million years ago, during the early and middle Paleocene. The climate was humid and warm to hot. This unit interbeds with the underlying Ojo Alamo Formation but is separated by an unconformity from the overlying San Jose Formation. The Nacimiento Formation is divided into several subunits known as members. In outcrops in southern areas of the formation, the Puercanfauna 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. In northern outcrops, the two lower formations are indistinguishable, and are called the "main body". Above them are two more informal members. These preserve a younger, Tiffanian fauna. The Puercan and Torrejonian faunas are further subdivided into several biostratigraphic zones.
Fossils
Many fossils are known from the Nacimiento Formation, although bone is often altered into phosphatic concretions. Fossils belonging to a number of different organisms have been found here, including: plants, gastropods, freshwater bivalves, cartilaginous fish and bony fish, salamanders, turtles, champsosaurs, amphisbaenians, lizards, snakes, crocodilians, birds, and a variety of archaic mammals. Mammalian groups represented include multituberculates, didelphid marsupials, insectivorans, plesiadapiforms, carnivorans, taeniodonts, mesonychids, condylarths, and cimolestans.
History of investigation
Workers in the early 1900s divided the rocks of the Nacimiento Formation into two formations, the lower Puerco Formation and the upper Torrejon Formation. 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. The Puerco and Torrejon were retained as zones within the Nacimiento Formation, and their faunas became the basis of the Puercan and Torrejonian North American Land Mammal Ages.
