GIVING CONTEXT TO EARLY PALEOCENE MAMMAL EVOLUTION: CLIMATE AND LANDSCAPE RECONSTRUCTION OF THE ARROYO CHIJUILLITA MEMBER OF THE NACIMIENTO FORMATION, SAN JUAN BASIN, NEW MEXICO

To provide paleoenvironmental context for the early Puercan terrestrial ecosystems of the San Juan Basin (SJB), New Mexico, an approximately 25 m thick and 500 m laterally continuous outcrop exposure was studied focusing on sedimentologic, stratigraphic, and paleopedologic attributes. This outcrop is in the basal portion of the lower Paleocene Arroyo Chijuillita Mbr of the Nacimiento Fm located in the Bisti/De-Na-Zin wilderness area representing a significant portion of the ~one million years following the K-Pg boundary. Paleomagnetic data and Ar/Ar dates constrain the study interval to the earliest Paleocene between 65.75 and 65.45 Ma. The stratal succession consists of fluvial channel and crevasse sandstones and associated floodplain deposits, including paleosols, which were measured in six stratigraphic sections along the outcrop. The paleosols in the succession are categorized into 7 pedotypes, with each pedotype assigned a drainage and maturity index based on color and horizon attributes, with variations in characteristics interpreted to reflect landscape position. Paleosols developed in the most upland (levee) positions are relatively well-drained and poorly-developed, and formed in association with high sedimentation rates. Conversely, paleosols in distal positions are more poorly-drained and well-developed, and formed in areas with lower sedimentation rates. Eight to 14 complete or partial fluvial aggradational cycles (FACs) were documented across the outcrop and used along with associated mechanical and biological structures, and paleosol attributes to develop a stratigraphic framework for the deposits. The framework suggests that shifts in landscape position and fluvial style through time are likely primarily the result of autocyclic channel avulsion and migration. Paleoclimate reconstructions derived from paleosol geochemical proxies indicate average paleoprecipitation of 1600 mm/yr and average paleotemperature of 12-13 °C. There is no evidence for variations in climate through this interval suggesting that climate change does not account for temporal variaions in sedimentation, paleosol character, or faunal and floral changes between the Pu2 and Pu3 biochronologic horizons intersecting the study interval.