Geologic reconnaissance of the southwest flank of the Zuni Uplift, New Mexico

Abstract

Pre-Cambrian crystalline rocks of the core of the Zuni Uplift are overlain successively by the Agua Fria formation (Pennsylvanina?), Abo sandstone, Yeso formation, and San Andres formation (Permian), Chinle formation and Wingate sandstone (Triassic), Carmel formation, Entrada sandstone, Cow Springs sandstone, and Morrison formation (Jurassic), and the Dakota sandstone, and Mesa Verde formation (Cretaceous). Sedimentary rocks on the southwest flank generally dip from 1°-15° southeast except where cut by longitudinal monoclines and faults; along these, dips steepen to vertical, and may even be overturned. The largest monocline is the Nutria Monocline with 2500 feet displacement. Displacements up to 1300 feet occur along the McGaffoy, Oso Ridge, and Temaja Monoclines. Gentle folds trend parallel to the monoclines. Fractures occur in conjugate sets, but the trends are not very consistent. A structural analysis of the fracture pattern, folds, and monoclines suggests that the Zuni Uplift may have been formed by compressional rather than vertical forces. Known uranium ore deposits occur in the Dakota sandstone along the Nutria Monocline. Uranium prospects and radioactive anomalies are also found in the Chinle formation and Mesaverde formation. Copper and fluorite deposits occur locally in the pre-Cambrian crystalline complex; one copper deposit is in the Agua formation, a coarse-grained arckose which overlies the pre-Cambrian crystalline complex. Favorable formations and structures suggest that sizable uranium deposits could be found on the southwest flank of the Zuni Uplift. However, rock units and structures associated with uranium deposits on the northeast flank are not present on the southwest flank.