Stratigraphy and mammalian biostratigraphy of the Paleocene Nacimiento Formation, southern San Juan Basin
— Thomas E. Williamson and Spencer G. Lucas

The Paleocene Nacimiento Formation of the San Juan Basin is as much as 525 m thick and consists of nonmarine fluvial and lacustrine strata deposited in the Laramide San Juan Basin. South of Kutz Canyon, in the southern San Juan Basin, we divide the Nacimiento Formation into (in ascending order) the Arroyo Chijuillita, Ojo Encino and Escavada Members. The Arroyo Chijuillita Member is as much as 134 m thick and consists mostly of drab gray, olive and yellow bentonitic mudstones, white trough-crossbedded sandstone and minor beds of lignite. It conformably overlies, grades into and interfingers with the underlying Paleocene Ojo Alamo Sandstone. The base of the Ojo Encino Member is a resistant, trough-crossbedded sandstone complex here named the Penistaja Bed. The Penistaja Bed is as much as 60 m thick and is overlain by as much as 122 m of Ojo Encino Member strata—mostly variegated red, green and black bentonitic mudstones and troughcrossbedded sandstones. Thin (up to 50 cm) beds of silcrete and thicker beds of trough-crossbedded sandstone characterize the overlying Escavada Member, which is as much as 88 m thick. The Cuba Mesa Member of the San Jose Formation unconformably overlies the Escavada Member of the Nacimiento Formation. Locally, this unconformity is a disconformity, but across the southern San Juan Basin it is slightly angular. The three members of the Nacimiento Formation can be correlated on a lithologic basis across the southern San Juan Basin in surface measured sections and in the subsurface by geophysical well logs. These correlations also demonstrate that the two fossil mammal zones that yield Puercan faunas, the Ectoconus and Taeniolabis zones, are discrete, superposed assemblage zones. Fossiliferous zones that yield Puercan and Torrejonian faunas are separated by a 45 m "barren" interval. The Deltatherium and Pantolambda zones of the Torrejonian are largely successive but overlap to some extent. Recently defined biochronologic zonation of the Puercan and Torrejonian land mammal "ages" (PuO–Pu3, Tol–To3) are based on the first appearance of key taxa and are only loosely based on biostratigraphic zonation. Biostratigraphy of the Nacimiento Formation and correlation of Torrejonian faunas of western North America suggest that Tetraclaenodon should not be used to define the base of To2. Fossil mammals and magnetostratigraphy document that most of the Nacimiento Formation is of early Paleocene age (chrons 29-27, Danian), although its uppermost strata may be of early late Paleocene age (chron 26, early Thanetian). The Paleocene mammals of the Nacimiento Formation document a significant diversification of paleoplacentals during the early Paleocene and continue to provide a standard by which the early Cenozoic diversification of the Eutheria is calibrated and interpreted.