Stratigraphy and mammalian biostratigraphy of the Paleocene Nacimiento Formation, southern San Juan Basin

Abstract

The Paleocene Nacimiento Fonnation of the San Juan Basin is as much as 525 m thick and consists of nonmarine fluvial and lacustrine strata de posited in the Laramide San Juan Basin. South of Kutz Canyon, in the southern San Juan Bas in, 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 ye llow bentoni tic mudstones, wh ite trough-crossbedded sandstone and minor beds of lignite. It confonnably overlies, grades into and interfingers with the unde rl ying Paleocene Ojo Alamo Sandstone . The base of the Ojo Encino Member is a resistant, trough-crossbedded sandstone complex he re 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 th icker beds of trough-crossbedded sandstone characterize the overlying facavada Member, which is as much as 88 m th ick. The Cuba Mesa Member of the San Jose Fonnation unconfonnably overlies the Escavada Member of the Nacimiento Fonnation. Locally, this unconfonnity is a disconfonn ity, but across the southern San Juan Basin it is sl ightly angular. The three members of the Nacimiento Formation can be correlated on a li thologic 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 Taenio/ab is zones, are disc rete, superposed assemblage zones. Fossili ferous zones that yield Puercan and Torrej,rn ian faunas are separated by a 45 m "barren " interval. The De/ratherium and Pantolambda zones of the Torrejonian are largely success ive but overlap to so me extent. Recent ly defined biochronologic zonation of the Puercan and Torrejon ian land mammal "ages·· (Pu0Pu3, To l-To3) are based on the first appearance of key taxa and are only loosely based on biostratigraphic zonation. Biostrat igraphy of the Nacimiento Formation and correlation of Torrejonian fa unas of western North America suggest that Tetraclaenodon shou ld 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 2927, Dan ian), al though its uppennost strata may be of earl y late Paleocene age (ch ron 26 , early Thanetian). The Paleocene mammals of the Nacimiento Format ion document a significant diversificat ion of paleoplacentals during the early Paleocene and continue to provide a standard by which the earl y Cenozoic diversi fication of the Eutheria is calibrated and interpreted . 265