MF 2417: Abstract
The 1:24,000-scale geologic map of the Pinedale 7.5' quadrangle lies in the western part of the Grants uranium mineral belt, which was mapped and studied under a cooperative agreement between the USGS and the U.S. Department of Energy. A spectacular panoramic view of the southern half of the Pinedale quadrangle is obtained looking northward from Interstate Highway 40, particularly from the New Mexico State travelers' rest stop near the Shell Oil Company's Ciniza Refinery, 28.5 kilometers (17.8 miles) east of Gallup. A west-trending escarpment, 200 meters high, of massive red sandstone, rises above a broad valley, its continuity broken only by a few deep and picturesque canyons in the western half of the quadrangle. The escarpment is formed by the eolian Entrada Sandstone of Late Jurassic age. The Entrada unconformably overlies the Chinle Formation of Late Triassic age, which occupies the valley below. The Chinle Formation consists of cherty mottled limestone and mudstone of the Owl Rock Member and underlying, poorly consolidated, red to purple fluvial siltstone, mudstone, and sandstone beds of the Petrified Forest Member. The pinyon- and juniper-covered bench that tops the escarpment is underlain by the Todilto Limestone. A quarry operation, located just north of the Indian community of Iyanbito in the southwestern part of the quadrangle, produces crushed limestone aggregate for highway construction and railroad ballast.
Beyond the escarpment to the north and rising prominently above it, is the northwest-trending Fallen Timber Ridge. Near the west side of the quadrangle lie the peaks of Midget Mesa, and Mesa Butte, the latter of which has the highest altitude in the area at 2,635 meters (8,030 feet) above sea level. The prominences are capped by buff-colored resistant beds of the Dakota Sandstone of Late Cretaceous age, containing some interbedded coal. These beds unconformably overlie the uranium-bearing Morrison Formation, which consists of red, green, and gray shale, orange feldspathic sandstone, and green tuffaceous mudstone, deposited in ancient lakes, alluvial fans, and rivers during the Upper Jurassic Period. Thick, crossbedded, white beds of the Cow Springs Sandstone, derived from ancient windblown desert sands, underlie the Morrison. In the northern part of the quadrangle, the Dakota Sandstone is overlain by gray Mancos Shale and yellowish-gray Two Wells and Gallup Sandstones that were deposited in Late Cretaceous seas. Unconsolidated deposits of Quaternary age are found throughout the quadrangle in talus, slope wash, fans, valley alluviums, pediments, and as windblown sands in dunes and blanket deposits.
The strata conform to the regional dip of about three degrees to north, except where they are down-folded some 200 meters along the Pinedale monocline, whose limbs follow a sinuous west-northwest trend across the northern half of the quadrangle. The monocline is beautifully exposed at Pinedale, where it shows as much as 20 degrees dip on the pine-studded bare rock slope of the Two Wells Sandstone. A north-plunging broad anticline and accompanying syncline is developed in the east-central part of the quadrangle but dies out against the monocline. A minor fault, with barely 3 meters of vertical displacement, extends several kilometers westward across the Todilto Limestone bench. A large landslide mass, 1.5 kilometers long by 0.7 kilometers wide occurs in the Mancos Shale west of Pinedale. Exploration drilling for uranium in the Morrison Formation has been extensive in the quadrangle, particularly north of the monocline, which adjoins the Old Church Rock mine area, west northwest of the quadrangle.
