SEDIMENTOLOGIC AND STRATIGRAPHIC INDICATORS OF REGIONAL PALEOGEOGRAPHY WITHIN THE LOWER PALEOCENE NACIMIENTO FORMATION, SAN JUAN BASIN, NEW MEXICO, USA

The San Juan Basin (SJB) of New Mexico contains extensive outcrops of alluvial deposits documenting early Paleocene mammalian evolution, and corresponding paleoclimatic and paleoenvironmental records. Although the early Paleocene mammalian record is well resolved, there has been relatively little work to place it within a detailed paleoclimatic and sequence stratigraphic framework. Within the Nacimiento Formation there is a significant interval of faunal turnover in the late Torrejonian North American Land Mammal “age” (To) between biochronologic zones To2 and To3, locally identified as the transition from biostratigraphic zone Torrejonian (Tj) 5 to Tj6. In this study, sections at Kutz Canyon, Torreon West, and Torreon East were measured through the Tj5-Tj6 (late To2-To3) interval to document changes in depositional environments, paleoclimate, and/or basin evolution.

Using meter-scale, fluvial aggradational cycle (FAC) analysis, we identified 44 FACs at Kutz Canyon, 48 FACs at Torreon West, and 38 FACs at Torreon East. At Kutz Canyon, near the center of the SJB, accommodation and grain size trends are positively correlated and indicate a possible extrinsic (tectonic) control on deposition. The Torreon West section, near the southern edge of the SJB, has a high number of thin (1m) FACs that are interstratified as crevasse splay and associated overbank deposits. This trend indicates a channel-proximal to overbank transitional landscape position comparatively less influenced by tectonic overprint. Amalgamated sandstones at Torreon East, near the southern edge of the SJB, suggest a landscape position that coincided with a higher proportion of channel complexes. The different patterns of FAC stacking and grain size trends at the three locations provide information that helps characterize regional variations in paleogeography and depositional style across the SJB. Depositional environments in the center of the SJB appear to be linked to basin evolution whereas near the basin margin, depositional environments are more likely influenced by intrinsic processes related to channel migration. Future work will focus on reconstructing paleoclimate using qualitative and quantitative analyses of paleosols to help discern the differential influence of tectonics, climate, and channel migration on deposition.