Upper Cretaceous and Paleogene stratigraphy and sedimentation adjacent to the Nacimiento uplift, southeastern San Juan Basin
— Larry N. Smith

Uppermost Cretaceous and Paleogene terrestrial strata in the San Juan Basin record the local disruption and partitioning of the Cretaceous foreland basin by Laramide-aged uplifts. The importance of unconformities within the stratigraphic section, which includes the Fruitland Formation, Kirtland Shale and Ojo Alamo Sandstone, and the nature of the Cretaceous/Tertiary boundary has been controversial, especially in the southeastern San Juan Basin near the Nacimiento uplift. Surface and subsurface mapping has recently identified an unconformity-bound sand-rich fluvial unit, here referred to as "unit B," between the Fruitland Kirtland and Ojo Alamo formations, which may explain some of the earlier mapping problems. "Unit B" on Mesa Portales contains distinct channel sandstones and interbedded siltstones and mudrocks that show mottling and oxidation profiles typical of a high energy fluvial system with well-drained floodplains. Paleocurrent measurements clearly show that deposition of much of "unit B" was related to north-to-south paleoslopes. Detrital lithologies indicate that sandstones of "unit B" and the Ojo Alamo Sandstone were derived from similar bedrock lithologies. North-to-south directed channels, parallel to the Nacimiento uplift, were responsible for depositing "unit B," the Ojo Alamo Sandstone, the upper Nacimiento Formation and the Cuba Mesa Member of the San Jose Formation. Maintenance of the position of channels parallel and adjacent to the Nacimiento uplift represents continued syntectonic sedimentation in this region from the Late Cretaceous through the early Eocene.