CLIMATIC AND DEPOSITIONAL HISTORY OF THE LOWER PALEOCENE UPPER NACIMIENTO FORMATION, SAN JUAN BASIN, NEW MEXICO

The upper Nacimiento Formation of the San Juan Basin, northwest New Mexico, contains extensive outcrops of alluvial deposits documenting early Paleocene mammalian evolution. Through the Nacimiento, there are intervals of high faunal turnover, one of which occurs in the Tj5 and Tj6 local biozones (~late To2 and To3 biochrons) of the Torrejonian (To) North American Land Mammal “age” (NALMA).This study constructs a detailed climatic and environmental record for these deposits at three locations across the basin (Torreon East, Torreon West, and Kutz Canyon) utilizing paleosol macromorphology and proxies from bulk geochemical analyses to investigate the role of climate and/or environmental change driving the Tj5-Tj6 mammalian turnover.

Paleosols are divided into 7 pedotypes and their relationship to depositional environments. The Torreon West section has thin interstratified crevasse splay and overbank deposits that suggest a channel-proximal to transitional landscape position with continual sedimentation. The Kutz Canyon and Torreon East sections contain amalgamated sandstones that suggest a landscape position near channel complexes. These results indicate that although there was variability in depositional environments across the basin, there were no major changes during the Tj5-Tj6 interval. Analyses of bulk organics in paleosols and fine grained strata document two large (3-4‰) negative shifts in δ¹³C values, suggesting “hyperthermals” in the upper Nacimiento. Temperature reconstructions using the BU-PPM paleosol bulk geochemistry proxy record 3-4°C warming events following the peak of each isotopic excursion. No significant changes in precipitation are documented. The lower carbon isotope excursion and warming event occurs in the upper part of the Tj5 biozone and the later excursion and warming event occurs at the end of polarity chron C27n, and likely corresponds to the Latest Danian Event recognized in the marine record. The timing of the warming events relative to the isotopic excursions suggests that the bulk organic excursions are truncated in comparison with the marine record. These environmental and climatic reconstructions suggest that the Tj5-Tj6 faunal turnover was not caused by depositional environment changes but rapid, short-term warming events require further investigation.