Precambrian Basement Rocks of the Colorado Plateau

Rocks of Precambrian age are exposed in scattered locations around the margin of the Colorado Plateau. Ancient rocks are well exposed in the Grand Canyon, uplifts in southwestern Colorado, and in the core of the Uinta Range. Rocks of Precambrian age underlie the entire Colorado Plateau, but stream erosion has exhumed these rocks but in a few isolated locations in the region.

The oldest rocks are complex metamorphic massifs that date to Archean Era (prior to 2.5 billion years). Rocks of Archean age are known to occur in the northwestern Wyoming region (Wind River Mountains) with only small area of Archean rocks crop out in northern Colorado (Sims and others, 2001). Through plate-tectonic activity, these rocks were highly deformed, faulted, and replaced by younger metamorphic alteration and igneous intrusion (plutonism), or were buried by sediments and volcanic rocks in the following Proterozoic Era. The Owiyukuts Complex of the Uinta Mountains region has been described as Archean in age.

Rocks of Proterozoic age (2.5 to 0.8 billion years) are more abundant and widespread. Parts of the Colorado Plateau are underlain by Proterozoic-age granite (or it metamorphic equivalent, granite gneiss), schist, and other crystalline rocks. An example of these most ancient rocks is the Vishnu Basement Rocks exposed in the Inner Gorge of the Grand Canyon. These rocks formed probably from ancient marine sediments and volcanic rocks that were deeply buried and subjected to great heat and pressure typical of mid crustal depths (10 to 20 miles). They have been highly deformed and have been intruded by igneous material numerous times in their history. These rocks demonstrate that the region experienced extensive igneous intrusion and volcanism in the ancient past. Examples of ancient granitic rocks include exposures in Colorado National Monument and in the Canyon de Chelly region. However, sedimentary rocks of Proterozoic age occur in great belts of rock on the margins (and underneath the surface) of the Colorado Plateau. The Grand Canyon Supergroup consists of a series of sedimentary rock formations and lava flows. The Grand Canyon Supergroup is subdivided into two groups of rock formations. The older Unkar Group includes, in ascending order: The Bass Limestone, Hakatai Shale, Dox Sandstone, the Shinumo Quartzite, and the Cardenas Lava (basalt). The younger Chuar Group includes the Nankoweap, Galeros, Kwagunt, and Sixtymile formations. All formations represent materials deposited in a variety of marine, transitional, and terrestrial sedimentary environments. These rock formations crop out beneath the Great Unconformity (an ancient erosional surface boundary beneath Cambrian-age sedimentary rocks) exposed throughout the Inner Gorge of the Grand Canyon.

Another massive belt of Proterozoic-age sedimentary rocks are exposed throughout the core of the Uinta Mountains in Flaming Gorge National Recreation Area and in the Canyon of Lodore in Dinosaur National Monument. This sequence of sedimentary deposits (mostly sandstone converted to quartzite) is called the Uinta Mountain Group. It probably underlies much of the northern Colorado Plateau. It consists of sediments deposited in an ancient intercratonic basin and is many tens of thousands of feet thick at its maximum extent.

Early Proterozoic-age crystalline rocks (granite, gneiss, and schist) that are probably equivalent in age to the Vishnu Basement Rocks crop out in the Nevada-California border region south of Las Vegas (McCullough Range and New York Mountains region). Late Proterozoic-age sedimentary rocks are also locally exposed in the southern Great Basin-Mojave Desert region. In Death Valley National Park and surrounding region, sedimentary rocks exposed in the Amargosa Ranges including the Late Proterzoic Pahrump Group (which includes the Crystal Spring, Beck Spring and Kingston Peak formations), and sedimentary rocks of latest Proterozoic age include the Noonday Dolomite, Ibex Formation, Johnnie Formation, and Stirling Quartzite, in ascending order respectively. The Stirling Quartzite is conformable with the overlying Early-Cambrian-age Wood Canyon Formation.

References

Hewett, D.F., 1956, Geology and mineral resources of the Ivanpah Quadrangle, California and Nevada: U.S. Geological Survey Professional Paper 275, 172 p.

Sears, J.W., Graff, P.J. and Holden, G.S., 1982, Tectonic evolution of lower Proterozoic rocks, Uinta Mountains, Utah and Colorado: Geological Society of America Bulletin, v. 93, no. 10, p. 990-997.

Tweto, Ogden, 1987, Rock units of the Precambrian basement in Colorado, IN Geology of the Precambrian basement in Colorado: U.S. Geological Survey Professional Paper, 1321-A, p. A1-A54, (incl. geologic map, scale 1:1,000,000).

Sims, P.K., Bankey, V., and Finn, C.A., 2001, Preliminary Precambrian basement map of Colorado-A geologic interpretation of an aeromagnetic anomoly map: U.S. Geological Survey Open-File Report 01-0364 <http://pubs.usgs.gov/of/2001/ofr-01-0364/>.