Upper Pennsylvanian strata in the Zuni Mountains, west-central New Mexico
— Karl Krainer, Spencer G. Lucas, and Barry S. Kues

In the Zuni Mountains (west-central New Mexico), at Sawyer and La Jara Springs, a thin, limestone-bearing stratigraphic interval termed Oso Ridge Member is present between the Proterozoic basement rocks and the red beds of the Abo Formation. At these localities the Oso Ridge Member is up to 12 m thick and consists of two depositional sequences, each composed of red beds that are overlain by thin limestones. Locally, immature conglomerates and sandstones of fl uvial origin fill paleorelief developed on the Proterozoic basement. The overlying siltstones and mudstones are interpreted as sheetfl ood deposits, and intercalated with thin, channel-fi ll conglomerates and sandstones. Microfacies and invertebrate fossils in the limestones indicate a shallow, open to restricted marine depositional environment with siliciclastic infl ux. The fossil assemblage indicates a Virgilian age. In central New Mexico, the stratigraphic interval underlying the Abo Formation, which is composed of alternating nonmarine clastics and shallow marine limestones and shales, is generally referred to the Bursum Formation. We remove the Oso Ridge Member from the Abo Formation and consider it a member of the Bursum Formation, representing a very thin and proximal facies of that unit.