No evidence for conformable contact between the Ojo Alamo Sandstone and the Nacimiento Formation

The San Juan Basin of northwestern New Mexico is a Laramide basin that accumulated sediments through the Cretaceous-Paleogene (K-Pg) boundary. However, the precise age of the Ojo Alamo Ss which lies near the K-Pg boundary in the San Juan Basin, remains controversial. Determining its age is crucial to understanding the tectonic and biotic history near the K-Pg boundary in this area. Based on study of these strata in the Bisti/De-na-zin Wash area (BDNZ) near Barrel Springs, previous workers have argued that the top of the Ojo Alamo Ss and the base of the Nacimiento Fm are interfingering and therefore are conformable. Based on this interpretation, most workers have regarded the Ojo Alamo Ss (here regarded in the restricted sense; = Kimbeto Member) in the BDNZ to be early Paleocene in age. The base of the Nacimiento Fm has been correlated to C29n and produces early Paleocene mammals. This provides a minimum age for the underlying Ojo Alamo Ss. Given this minimum age constraint workers have generally taken one of two positions for the precise age of the Ojo Alamo Ss: (1) the Ojo Alamo Ss. spans the upper part of C29r, or (2) it is restricted to only the lower part of C29n. To resolve this question and to determine a more precise age for the Ojo Alamo Ss, we examined the stratigraphic relationships between the Ojo Alamo Ss and the overlying Nacimiento Fm in the BDNZ. Contrary to previous interpretations, we find that this formational contact is not conformable. Rather, what other workers have assumed to be the “upper part of Ojo Alamo Sandstone” that interfingers with basal parts of the Nacimiento Fm is actually a remnant of a lenticular channel sandstone of the Nacimiento Fm. This reinterpretation suggests that the formational contact between the Nacimiento Fm and the Ojo Alamo Ss may represent a significant hiatus of up to ~ 0.5 Ma separating the Ojo Alamo Ss from the base of the Nacimiento Fm in the BDNZ area. Future work should focus on more accurately determining the age and refining the duration of the hiatus at the Ojo Alamo – Nacimiento formational contact.