THE TORREJONIAN-TIFFANIAN TRANSITION IN THE NACIMIENTO FORMATION, NORTHWESTERN SAN JUAN BASIN, NEW MEXICO: LITHOSTRATIGRAPHY AND MAMMALIAN BIOSTRATIGRAPHY

A new lithostratigraphy and mammalian biostratigraphy provides first evidence of the Torrejonian-Tiffanian transition in the northwestern San Juan Basin of New Mexico. Previously, the stratigraphically highest mammal fossils from the Nacimiento Formation were early Paleocene/late Torrejonian (~61.5Ma) and occurred 60 m below the base of the lower Eocene San Jose Formation (T_SJ). The lowest Eocene fossils derived 30 m above the contact, or approximate Paleocene-Eocene boundary. Farther north, in the San Juan Basin of Colorado, the type Tiffanian fauna is estimated late Paleocene/middle Tiffanian “Ti 4” (~57.3Ma). But, in New Mexico, intervening Tiffanian-aged faunas were unknown. Thus, a minimum 90 m stratigraphic, and 4.2 my temporal, gap persisted in the San Juan Basin. New data gathered in the northwestern basin reveal less of a hiatus than to the south. A minimum of seven successive fossiliferous intervals include the youngest Torrejonian biostratigraphic zone, with the highest zone now less than 22 m below the T_SJ. Over 150 new fossil localities yield more than 300 vertebrate specimens, including over 130 mammalian specimens. A minimum of 28 mammalian genera and 36 species are represented, including at least two new genera and six new species. While dominated by Torrejonian taxa, the new Aztec local fauna is assigned a late Paleocene and earliest Tiffanian age based on four lines of evidence: (1) stratigraphic superposition above the previously youngest, type Torrejonian fossil interval; (2) the derived condition of several Torrejonian forms; (3) new transitional taxa antecedent to earliest Tiffanian species; and (4) occurrence of the Tiffanian index genus Plesiadapis. As the type area and original basis for the Puercan, Torrejonian, and Tiffanian North American land mammal “ages”, the San Juan Basin, its Paleocene faunas, and their stratigraphic succession, remain pivotal to biochronological correlation of western North American Paleocene strata. The new San Juan Basin fossiliferous zones and assemblages preserve a previously unknown temporal interval, augment this relatively continuous biostratigraphic framework, begin to bridge a significant morphological, stratigraphic, and temporal gap, and deepen our knowledge of the Paleocene-Eocene faunal succession.