From buttes to bowls: Repeated relief inversion in the landscape of the Colorado Piedmont
Published:January 01, 2008
Matthew L. Morgan, Vincent Matthews, Francisco Gutiérrez*, Jon P. Thorson, Richard F. Madole, Paul R. Hanson, 2008. "From buttes to bowls: Repeated relief inversion in the landscape of the Colorado Piedmont", Roaming the Rocky Mountains and Environs: Geological Field Trips, Robert G. Raynolds
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Mesas and buttes of the central Colorado Piedmont are composed of at least twodistinct rock types, which differ in their cohesiveness and resistance to erosion. Thelower parts of the exposed stratigraphic section are poorly cemented, Upper Cretaceousto Middle Eocene sandstones of the Dawson Formation. The caprocks arecomposed of one or more resistant formations of Late Eocene age: the Castle RockConglomerate, Wall Mountain Tuff, and the conglomerate of Larkspur Butte. Theseformations were originally deposited in topographic lows, but due to their resistance,they now cap prominent buttes and mesas of the Colorado Piedmont. Erosion of thecaprock through progressive retreat of the butte scarp produces colluvium that has ahigher resistance to erosion than the poorly cemented underlying sandstone.
Once the caprock of a butte has been removed by erosion, the underlying weaklycemented Dawson Formation is readily eroded. Ultimately, the armored lower slopesof the former butte remain as a circular ridge standing as much as 100 m above thesurrounding topography. This process produces a topographic low surrounded byrelict faceted slopes where the flat top of the butte once stood.
Prominent alluvial fans are associated with some of these annular features, andthey record the main phases of butte removal and excavation of the central part of thearmored slopes. Multiple generations of alluvial fans contain coarse- and fine-grainedfacies that represent changes in effective stream power and record alternating phasesof aggradation and erosion. The degree of soil development in the fan alluvium andheight of the fan surfaces above streams indicates the oldest preserved gravel fandeposit is of late-middle Pleistocene age. The youngest luminescence (optically stimulatedluminescence) dated alluvial fans were deposited during the late Pleistoceneabout the time of the Pinedale glacial maximum in Colorado, ca. 21,000 yr B.P.
Keywords: Colorado Piedmont, talus flatiron, talus flatiron ring, inverted topography.
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Roaming the Rocky Mountains and Environs: Geological Field Trips
Prepared following the 2007 GSA Annual Meeting in Denver, Colorado, these 15 guides illustrate the latest geological and archeological thinking on a variety of current research themes. Regional-scale topics include landscape responses to dynamic processes of volcanism and uplift in Yellowstone and western Colorado, geomorphic evolution along the Front Range of Colorado and on the High Plains of South Dakota, and geoarchaeological research in central Colorado and western Nebraska. A series of papers illustrates tectonic and stratigraphic processes through time and space, with discussions of Precambrian structures in western Colorado, Jurassic deposition in south-central Colorado, and near-shore stratigraphic patterns in the Cretaceous strata of the Book Cliffs. One paper reviews potential seismic signatures in Cretaceous and Early Tertiary strata in northern Wyoming and Montana, and another discusses patterns of extension in southern Nevada and adjacent portions of California. Other topics in this well-rounded volume include the history of volcanism and gold mineralization at Cripple Creek, development of coalbed methane resources in the Powder River Basin, and a long-lived subsurface coal fire in western Colorado. Follow in the footsteps of these field trips, and see for yourself the patterns and evidence discussed.
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