SEDIMENTOLOGIC AND STRATIGRAPHIC RECONSTRUCTION OF LOWER PALEOCENE ENVIRONMENTS IN THE SAN JUAN BASIN, NEW MEXICO

Two outcrops in the basal portion of the lower Paleocene Arroyo Chijuillita Member of the Nacimiento Formation are studied with a focus on sedimentologic, stratigraphic, and paleopedologic attributes. The goal of this study is to interpret the paleoenvironmental conditions of the terrestrial ecosystems in which middle Puercan flora and faunas were evolving. The study localities, known as Mesa de Cuba and Kimbeto Wash, are separated by 85 km in the San Juan Basin, New Mexico. The stratal successions consists of channel and laterally continuous fluvial sandstones and associated floodplain fines, including numerous paleosols with variable drainage and maturity. Mean annual precipitation (MAP) and temperature (MAT) estimates from geochemical analysis of paleosols do not indicate significant climatic variability through the studied interval. Evidence for tectonically influenced sedimentation is, so far, lacking. Additionally, observed spatial and temporal sedimentologic variability is consistent with fluvial auto-cyclicity (e.g. channel migration, avulsion, progradation). Associated paleosols generally exhibit an inverse relationship between maturity and drainage. Trends in these attributes are interpreted to be a result of proximity to the paleo-channel with paleosols forming in the distal floodplain being relatively well-developed and poorly-drained, whereas paleosols forming proximal to the channel are relatively poorly-developed but well-drained. Predictably, proximal paleosols are generally coarser (f. sand-silt) than distal paleosols (silt-mud). The studied outcrops coincide with or are approximately correlative to the middle Puercan (Pu2-Pu3) North American Land Mammal Age (NALMA). To date, evidence indicating extrinsically induced environmental changes is insufficient to explain the sedimentologic variability observed. Therefore, such factors are unlikely causes of the major and, apparently, abrupt species turnover observed during this time. As such, we interpret biologically intrinsic processes (e.g. rapid speciation, migration, etc.) to be the most likely driver of biologic turnover during the middle Puercan in the San Juan Basin.