PALEOCLIMATE RECORD ACROSS THE K-PG BOUNDARY FROM PALEOSOLS IN THE WEST-CENTRAL SAN JUAN BASIN, NEW MEXICO

The K-Pg boundary in the west-central San Juan Basin, NM, is at an unconformity between the Maastrichtian Naashoibito Member and the Paleocene Kimbeto Member of the Ojo Alamo Formation, which is overlain by the Paleocene Nacimiento Formation. To compare climates across the K-Pg boundary we examined the paleosols in these strata at four stratigraphic sections of the Naashoibito Member, and we measured a c. 35-m section of the lower Nacimiento Formation up from the basal contact with the Ojo Alamo Formation. The fine-grained facies of the Naashoibito Member is gray to greenish-gray and red-banded mudstones displaying varyingly developed mottling, root traces, cutans, ped fabrics, pedogenic slickensides and calcareous nodules, interpreted as Gleysols, Protosols, Argillisols, calcic Argillisols, gleyed Vertisols and Vertisols. Smectite dominates the clay mineral assemblage in the paleosols exhibiting gley features, while illite is more prominent in Vertisols. Significant differences in paleosol type between correlative strata in the Naashoibito Member reflect varied hydrologic conditions at different locations on the floodplain rather than paleoclimatic variations. Vertisols formed on the better-drained distal floodplain, while the less mature soils formed on the poorly drained floodplain proximal to the stream channels. The overall paleoclimate was consistently humid to subhumid, but strongly seasonal.

Paleosols in the lower Nacimiento Formation are similarly drab profiles, dominated by smectite except at the base, where kaolinite and illite dominate. Aside from a high chroma, potentially oxidized horizon at the formation base, paleosols are mainly low chroma, with drab gley to reddened horizons displaying varying degrees of rooting, mottling, ped development and pedogenic slickensides. These are gleyed Vertisols and Protosols, Argillisols and Vertisols, with the exception of an Oxisol at the formation base. They represent a spectrum of water table conditions due to differences in soil drainage at varying positions on the floodplain. Following a warm interval during initial deposition and formation of the Oxisol, an interval of climate stability during Nacimiento deposition was similar to that of the underlying Naashoibito, which suggests no long-term climate change across the K-Pg boundary.