RELATIONSHIPS BETWEEN PALEOSOLS AND PALEOENVIRONMENTS IN THE PALEOCENE (DANIAN–SELANDIAN) FLUVIAL SEDIMENTARY ROCKS OF THE SAN JUAN BASIN, NEW MEXICO, USA

Stratigraphic position, relationship to adjacent sedimentary packages, pedogenic features, and geochemical characteristics of paleosols preserved in the Paleocene Ojo Alamo Formation (OAFm) and Nacimiento Formation (NFm) in the western San Juan Basin (SJB), New Mexico, provide insight into the environments and climates present during deposition of this ~1 km-thick package of sediments. While mudstones dominate the lithologic composition of these formations and most red, green, grey, or black mudstones in fluvial systems are often interpreted as paleosols, our study shows that significant pedogenic alteration (i.e., producing horizonation or other physical pedogenic features preserved in the geologic record) occurred in less than 10% of OAFm and NFm mudstones. Even so, the stratigraphic position and degree of pedogenic development of these paleosols suggest that the deposition of the OAFm and NFm was punctuated by periods of non-deposition during which pedogenesis occurred and that the duration and environmental conditions of these periods changed through the Paleocene. The relatively thin (10 to 70 m thick) and sandy OAFm contains no well-developed paleosols. The ~500 to ~1000 m thick, lithologically varied NFm displays predominately vertic paleosols in the basal Arroyo Chijuillita Member, rare vertic and alfic paleosols in the middle Ojo Encino Member, and alfic paleosols and silcretes in the upper Escavada Member. The effects of climate, sedimentation, basin dynamics, and landscape position on pedogenesis are often difficult to differentiate from one another; here we present a reconstruction of Paleocene environmental conditions to investigate those relationships.