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.

Abstract

Sedimentologic, stratigraphic, and paleopedologic attributes of an outcrop of the lower Paleocene Arroyo Chijuillita Member (Nacimiento Formation) in the Bisti/De-Na-Zin Wilderness Area (San Juan Basin, New Mexico) are studied to provide paleoenvironmental context for the Puercan North American Land Mammal Ages (NALMA). The outcrop intersects the middle and late Puercan (Pu2-Pu3) age and is ideal to assess potential causes of this turnover. Fluvial channel and laterally continuous sandstones and associated fine-grained floodplain deposits, including paleosols documenting variability in drainage and maturity, comprise the outcrop. Paleoclimatic reconstructions from paleosols indicate no change in climate, and evidence of regional tectonics effecting deposition is not observed. Spatial and temporal variability in paleopedologic and sedimentologic characteristics are interpreted to be the result of autocyclic channel avulsion/migration. These observations suggest that mammalian turnover (Pu2-Pu3) is likely the result of rapid evolution following the K-Pg boundary and/or competition from migration of mammalian species into the basin.