Regional geophysical investigations in the central Colorado Plateau

Abstract

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.

Mineral resources of the Behind the Rocks Wilderness Study Area, Grand and San Juan Counties, Utah

“…In the region of the wilderness study area itself, the magnetic field trends about east-west, and the magnetic values increase smoothly northward, despite the northward increase in depth of the Precambrian basement. As discussed at length by Case and Joesting (1972), these circumstances indicate that the regional magnetization of the Precambrian basement must increase substantially toward the north and northeast. A similar conclusion was drawn by Johnson (1983) from the widely spaced NURE data.…”

“…Triassic Moenkopi Formation, the Lower Permian Cutler Formation, the Middle and Upper Pennsylvanian Hermosa Formation, including the Paradox Salt Member and the Molas Formation, the Lower Mississippian Leadville Limestone, and a section of lower Paleozoic limestone, sandstone, and shale. The entire section is 5,000-6,000 ft thick and overlies a lithologically heterogeneous Precambrian crystalline basement (Case and Joesting, 1972).…”

“…Diameter of the body is about 3 mi. Available drill-hole data indicate that the body also lies partly on a northeasttrending basement structural high (Case and Joesting, 1972). An associated gravity high is also present in this area, indicating that the body is probably mafic in composition.…”

Mineral resources of the Indian Creek, Bridger Jack Mesa, and Butler Wash Wilderness Study Areas, San Juan County, Utah

“…A gravity high of about 6 mGals or more occurs in the south over the northern nose of the Monument uplift. Part of the high is due to structural relief of the uplift, but, as discussed by Case and Joesting (1972), part of the high is due to fairly dense rocks within the Precambrian basement. In the extreme southwest corner of the area, a gravity low of more than 10 mGals occurs even though the elevation of the top of the Precambrian surface is about the same as at the site of the gravity high.…”

“…North that have diameters of 3-5 mi (Case and Joesting, 1972). The southern parts of two highs are located at the extreme north edge of the map area in figure 4.…”