Quaternary Deposits of the Colorado Plateau

Most of the spectacular landscape features visible today on the Colorado Plateau are a result of erosional processes (the removal of material from the region by water and wind). However, Quaternary age deposits in various forms occur throughout the Colorado Plateau. Mass wasting, stream and wind erosion, and soil development processes are responsible for Quaternary age deposits that cover much of the landscape, particularly along river bottoms and on broad, flat upland areas. Volcanism occurred during Quaternary time along the eastern and western margins of the greater Colorado Plateau. Large (mappable) alluvial, eolian, and landslide deposits occur throughout the region, and their occurrence is related to both bedrock geology and the changing climatic conditions throughout the Quaternary Period.

Aeolian Deposits: Sand, silt, and dust deposits are abundant throughout the Colorado Plateau, particularly along the southwestern expanses of upland surfaces that are typically void of river canyons and forested areas. In most areas, dunes are stabilized by grasses and other vegetation. Perhaps the largest and best known are the sand dunes of Coral Pink Sand Dune State Park in Utah. Other areas include dune accumulations in Arches National Park and the upland region west of Canyonlands National Park in the vicinity of Hanksville, Utah. Aeolian deposits blanket many elevated stream terrace deposits throughout the Colorado River Canyons and their tributaries, particularly in broad river bottom areas. However, sandy soil and associated eolian sand deposits occur throughout all parts of the Colorado Plateau, particularly where massive sandstone formations crop out, providing a constant sediment supply to keep pace with erosion.

Stream Terrace Deposits: Accumulations of alluvial gravels, sand, silt, and mud occur along stream bottoms and valleys throughout the Colorado River canyons. Some of the largest and most extensive occur along the valley of the Green River (in the broad, flat region both north and south of Green River, Utah). Alluvial gravels and gravel bars throughout the Colorado River Basin were targets for gold exploration in the late 19th century. Elevated gravel deposits reflect periods of alluvial deposition during the Quaternary when stream base-level was higher. Possibly during wetter periods during the Quaternary Period river discharge was greater. With greater flow, rivers broadened their floodplains. Isolated remnants of once greater floodplain deposits are all that remain in many places as erosion continues to wear away the landscape.

Pediment Gravels: In some areas, particularly on high mesas and upland flat areas, the ground surface is littered with weathered stream-rounded rock material (gravel, cobbles, and boulders). These are remnants of ancient alluvial fan and pediment surfaces that spread across parts of the Colorado Plateau in Late Tertiary through Quaternary time. The lands surface was blanketed with gravels deposited in migrating braided-stream channels draining from upland area on and around the Colorado Plateau. Ongoing down cutting by streams and the expansion of canyon systems have removed most of these ancient alluvial fan deposits that covered many parts of the Colorado Plateau in the past. Durable rock fragments, particularly chert, quartz, quartzite, and basalt, are typically all that have survived the long-term effects of surface weathering. In wind-swept areas these rock fragments preserve the effects of long-term wind abrasion (producing "ventifacts" - rocks with wind-faceted surfaces).

Landslide Deposits: Landsides, earthflows, rockfalls, and other mass-wasting features are common in all areas of relief on the Colorado Plateau. Massive rock falls are common in areas where massive sandstone units overlie softer shale (such as where the Wingate Sandstone overlies the soft shales of Chinle Formation). Landslides are common in the outcrop belts of the Morrison Formation and the Mancos Shale. Landsliding is responsible for upland landscape of the Aquarius and Paunsaugunt plateau regions in central-southern Utah, including parts of Bryce Canyon and Cedar Breaks parks.

Glacial Deposits: Evidence of Quaternary glaciation can be found in regions typically above 8,000 feet in the mountainous regions in the Colorado Plateau. Small cirques (glacial-carved bowl-shaped valleys) can be seen along the eastern flank of Boulder Mountain between Torrey and Boulder, Utah. Upland natural ponds on the Aquarius Plateau are evidence of glaciation.

Colluvium: Broken-down rock materials cover the surface throughout the Colorado Plateau region. Remnants of great talus slopes consisting of weathered rock and soil cover the lower slopes along river canyons. These deposits reflect wetter (or faster weathering) periods in the past. In many areas these areas have eroded away since the last Ice Age of the Pleistocene (ending about 15,000 years ago). In the past, a wetter climate supported vegetation and soil development that stabilized colluvium covered slopes. The modern climate on the Colorado Plateua consists of drought periods followed by summer monsoonal flooding or occasional winter storms. This modern climate variation prevents the development of colluvial accumulations relative to the faster rate of erosion by windstorms and flash flooding, particularly in area of sparce vegetation.

Alluvium: Nearly every river bottom on the Colorado Plateau is host to alluvial sediments deposited during high-standing water. Infrequent storms produce flash floods and debris flows that carry vast quantities of sediments into the Colorado River and its tributaries. These sediments accumulate at the mouths of canyons (forming rapids in many locations by blocking the greater more gentle flow of the larger rivers). With time, these sediments are carried downstream and locally accumulate as sorted sediments (sand bars, gravel bars), and today in the modern reservoirs, large accumulations of mudflats.

References

Billingsley, George H.. Hamblin, W. Kenneth, Wellmeyer, Jessica L., and Dudash, Stephanie L., 2001, Geologic Map of Part of the Uinkaret Volcanic Field, Mohave County, Northwestern Arizona: U.S. Geological Survey Miscellaneous Field Studies 2368. [Geologic map scale: 1:31,680 and text (39 p.)] - <http://geopubs.wr.usgs.gov/map-mf/mf2368/>.

Cooke, R. Warren, A., and Goudie, A., 1993, Desert geomorphology: London, UCL Press, 526 p.

Ford, Richard L. and Gillman, Shari L., 2000, Geology of Coral Pink SandDune State Park, Kane County, Utah: In Geology of Utah's Parks and Monuments, Sprinkel, D.A., Chidsey, T.C., and Anderson, P.B., editors, Utah Geological Association Publication 28., p. 365-389.

Hereford, Richard, 2000, Recent Alluvial History of the Southern Colorado Plateau: U.S. Geological Survey, Earth Surface Processes Team, SW Climate Impacts Project website: http://climchange.cr.usgs.gov/info/sw/scpalluvial/>.