Climate and landscape reconstruction of the Arroyo Chijuillita Member of the Nacimiento Formation, San Juan Basin, New Mexico: Providing environmental context to early Paleocene mammal evolution

The sedimentologic, stratigraphic, and paleopedologic attributes of a laterally continuous outcrop exposure of the basal portion of the lower Paleocene Arroyo Chijuillita Member of the Nacimiento Formation located in the Bisti/De-Na-Zin Wilderness Area was examined to provide paleoenvironmental context for the Puercan North American Land Mammal Age (NALMA) terrestrial ecosystems of the San Juan Basin, New Mexico. The studied outcrop spans the middle and late Puercan (Pu2-Pu3) NALMA, and thus is an ideal location to assess the potential causes of faunal turnover between Pu2 and Pu3. The stratal succession consists of fluvial channel and laterally continuous sandstone and associated fine-grained floodplain deposits, including paleosols that document variability in drainage and maturity. Quantitative and qualitative reconstructions of paleoclimate from paleosols, supplemented by analyses of fossil floras, indicate no change in climate through the studied interval. Further, there is no evidence that regional tectonics relate to changes in depositional environments. Given the lack of evidence for climate change or regional tectonics, and the characteristics of the fluvial facies and the paleosols across the study locality, we interpret the spatial and temporal variability in paleopedologic and sedimentologic characteristics to be the result of autocyclic channel avulsion and migration. Additionally, the stable climatic conditions and lack of tectonic influence between the Pu2 and Pu3 biozones suggest that mammalian turnover is unlikely to have been caused by extrinsic environmental changes (i.e. climate or tectonics) and is instead probably the result of rapid evolution following the K-Pg boundary and/or competition from the migration of new mammalian species into the basin.