Gravity and aeromagnetic surveys covering about 15,000 square miles in the central Colorado Plateau in Utah, Colorado, Arizona, and New Mexico were conducted to assist in determining the regional subsurface geology as it may relate to uranium, oil, and potash exploration. Precambrian rocks, either exposed on the Uncompahgre Plateau or penetrated by drill holes, are granitic, dioritic, and gabbroic bodies and complexly folded gneisses and schists. The densities of these rocks range from about 2.6 to 3.0 grams per cubic centimeter, and their magnetic susceptibilities are as much as 5xlO~3 electromagnetic unit. Paleozoic sedimentary rocks attain a maximum thickness of about 17,000-18,000 feet in the deepest part of the Paradox basin, a concealed structural feature of late Paleozoic age. Cambrian, Devonian, and Mississippian rocks consist of limestone, sandstone, shale, and dolomite. Pennsylvanian rocks consist of limestone, sandstone, shale, and, in the Paradox Member of the Hermosa Formation, evaporites. Permian rocks include sandstone, arkose, limestone, and siltstone. The limestones and sandstones of low porosity probably have an average density close to that of parts of the Precambrian basement, 2.6-2.7 grams per cubic centimeter. The evaporites are of low density, 2.2-2.35 grams per cubic centimeter. Mesozoic sedimentary rocks are dominantly clastic and attain a maximum thickness of about 5,000 feet. They are less indurated than the Paleozoic rocks, and their densities probably range from 2.3 to 2.5 grams per cubic centimeter. Thin Tertiary and Quaternary deposits, which consist of alluvium and glacial deposits, locally cover large areas, but they have little influence on the geophysical anomalies. Laccolithic intrusions of latest Cretaceous or Tertiary age form such prominent mountain groups as the Henry, La Sal, Abajo, and Carrizo Mountains, and the Sleeping Ute Mountain. These igneous masses consist of one or more central stocks from which radiate laccoliths, dikes, sills, and bysmaliths. They are composed of closely allied rocks which range in composition from microgabbro to granite, but which are predominantly diorite or monzonite porphyry. Their average densities are about 2.62 grams per cubic centimeter, and their average susceptibilities are about 0.002 electromagnetic unit. Small mafic dikes, plugs, and necks are distributed sporadically through the region, but these bodies are so small that they have little influence on the regional geophysical anomalies. Regional geologic structures may be broadly grouped, according to age, as (1) those formed in the Precambrian; (2) those formed during late Paleozoic deformation of the region, including the Paradox basin, the ancestral Uncom-MJeceased, May 1965.
