Paleocene climate change in the San Juan Basin, New Mexico: A paleosol perspective

The fluvial Nacimiento Formation in the San Juan Basin, New Mexico, is host to abundant paleosols of early Paleocene age (Danian stage, Puercan/Torrejonian NALMA). Paleosols with vertic properties are common, with less common horizonated clay-rich paleosols and siliceous sandy paleosols. Since paleosols represent under the long-term average state of climate conditions over 10²-10⁵ years of pedogenesis, San Juan Basin paleosols are among the most useful proxies for understanding the history of local and regional climate conditions and change in the Paleocene. Here we present geochemical and paleoclimatological interpretations of paleosols from the Nacimiento Formation in the western and southern San Juan Basin; discuss their significance on interpretation of basin sedimentology; and seek possible modern analogues. An interesting and seemingly paradoxical problem in the Paleocene San Juan Basin is the presence of vertic paleosols (typically associated with subhumid to semiarid climates) in close geographic and stratigraphic proximity to well-horizonated or base-poor paleosols and Arecales and Crocodilia fossils (all associated with tropical to subtropical humid climates). Preliminary data show that some San Juan Basin vertic paleosols formed under mean annual temperatures (MAT) of ~12° ± 4.4° C and mean annual precipitation (MAP) amounts of ~1,100 mm. The mineral composition and presence of kaolinite in more horizonated paleosols suggest they formed under warmer and wetter conditions. This study also investigates the silcrete-bearing paleosols of the Nacimiento Formation. Pedogenic silcretes typically form in stable landscapes with subtropical to tropical variable moisture climates. Unlike other Laramide basins where Paleogene hyperthermals often are represented by reddened paleosols, it is possible that hyperthermals are represented by silcretes in the San Juan Basin. Regardless of the pedogenic representation of hyperthermals in the San Juan Basin, the paleosols there record significant climatic variation through the Paleocene.