New Mexico Geological Society Annual Spring Meeting — Abstracts

Since the discovery of vertebrate fossils in the San Juan Basin of New Mexico in 1874, numerous paleontological expeditions have concentrated their efforts in the extensive, more fossiliferous localities south of the San Juan River. The Paleocene stratigraphy, sedimentology, and paleontology of the southern Nacimiento Formation have been comprehensively mapped, described, steadily collected and analyzed. But the poorer preservation, less fossiliferous and rougher terrain of the northern Animas Valley has been neglected and lacks modem geological description and analysis. Past expedition leaders allocated insufficient time or resources to explore the northwestern exposures. Indeed, it has been more than 80 years since William Granger and George Olsen of the American Museum of Natural History made the last serious scientific collection and description of the Lower Animas in 1916. Previous specimens were collected with inadequate stratigraphic data. But after the 1916 survey, Granger documented the occurrence of 14 different paleomammals in the northwestern basin: Baldwin's Neoplagiaulax molestus, Mioclaenus turgidus, M. opisthacus, Deltatheriumfundaminis, Hemithlaeus baldwini, Periptychus carinidens, P. rhabdodon var.fissus, Pantolambda cavirictus (of which six are important Torrejonian type specimens); to which Granger added Tetraclaenodon, Mixodectes, Pentacodon, Tricentes, Dldymictis haydenianus, and Claenodon. Preliminary field studies have precisely located the stratigraphic position of at least three fossil faunal horizons. While the southern Nacimiento Formation clearly differentiates into three discrete members, the contrasts dissipate in the northern lithologies. Previously referred to as "the main body" and "unnamed member", sedimentary petrological techniques are used to investigate, describe and name these less distinct northern strata and to discern their stratigraphic relationships to their southern counterparts. The northern deposits appear continuous in sedimentation throughout the
Torrejonian before being abruptly overlain by the Eocene San Jose Formation, concealing a hiatus of over five million years.