EVALUATION OF CHANGES IN DEPOSITIONAL ENVIRONMENTS AND THEIR IMPACTS ON THE EARLY PALEOCENE (MIDDLE PUERCAN) MAMMALIAN FOSSIL RECORD IN THE NACIMIENTO FORMATION, SAN JUAN BASIN, NEW MEXICO

The Paleocene Nacimiento Formation located in the San Juan Basin, New Mexico, has long been studied for its diverse mammalian faunas; the type faunas of the Puercan and Torrejonian North American Land Mammal “ages” (NALMAs). The Nacimiento is a thick (up to 500m) succession of cyclically interstratified fluvial deposits and paleosols. Within the Nacimiento, mammalian fossils are often found in discrete beds that are separated by “barren zones” devoid of fossils. To date, studies that link the faunal record to the depositional environments in the Nacimiento are lacking. Evaluation of this link is important to better understand why mammalian fossils are restricted to discrete beds and will potentially help explain the factors that caused this restriction. This project focuses on determining what, if any, environmental changes are observed in the lowest portion (~15-25m) of the Nacimiento Formation. The study interval contains the lowest Nacimiento Formation vertebrate fossil horizon which yields the middle Puercan (Pu2) faunas. Six sections were measured across a ~500m continuous outcrop exposure, and include detailed documentation of sedimentological and pedological features of both the Pu2 horizon and the bracketing successions. Alluvial deposits and paleosols are organized within 6-13 (depending on measured section) thin (~2m) “fluvial aggradational cycles” (FAC). Sandstones within each FAC were sampled to establish mineralogic composition for provenance analysis, and paleosols weathered into FACs were described and sampled to determine relative maturity, drainage and paleoclimate conditions. Detailed description of the one- and two-dimensional distribution of FACs and associated paleosols allows reconstruction of the depositional, geomorphic and paleoclimatic conditions within a time-stratigraphic framework. This evolving history, in turn, is compared to the biological record to detect any changes in depositional and/or climatic conditions that may account for the variable preservation of mammalian fossils.