UPPER PENNSYLVANIAN STRATA IN THE ZUNI MOUNTAINS, WEST-CENTRAL NEW MEXICO

In the Zuni Mountains (west-central New Mexico), at Sawyer and La Jara Springs, a thin limestone-bearing sequence termed Oso Ridge Member occurs 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 fluvial origin fill up the paleorelief. The overlying siltstones and mudstones are interpreted as sheet flood deposits, intercalated are thin channel fill conglomerates and sandstones. Microfacies of the limestones indicate a shallow, open to restricted marine depositional environment with siliciclastic influx. The fossil assemblage, particularly brachiopods, bivalves and conodonts, indicates a Virgilian age. As the sequence underlying the Abo Formation, which is composed of alternating nonmarine clastics and shallow marine limestones and shales, in central New Mexico is generally referred to the Bursum Formation, the Oso Ridge Member is assumed to be a member of the Bursum Formation, representing a very thin and proximal facies of that formation. Biota and microfacies indicate that the limestones of sequence 1 formed in a shallow marine environment of normal salinity with strong clastic influx. A regressive event caused deposition of fine-grained red beds, which sharply overlie the limestone horizon. The limestone horizon on top of depositional sequence 2 represents a short transgressive event and was deposited in a shallow, restricted to open marine environment. A further regression caused deposition of immature red beds of the Abo Formation.