From buttes to bowls: Repeated relief inversion in the landscape of the Colorado Piedmont

Mesas and buttes of the central Colorado Piedmont are composed of at least two distinct rock types, which differ in their cohesiveness and resistance to erosion. The lower parts of the exposed stratigraphic section are poorly cemented, Upper Cretaceous to Middle Eocene sandstones of the Dawson Formation. The caprocks are composed of one or more resistant formations of Late Eocene age: the Castle Rock Conglomerate, Wall Mountain Tuff, and the conglomerate of Larkspur Butte. These formations were originally deposited in topographic lows, but due to their resistance,they now cap prominent buttes and mesas of the Colorado Piedmont. Erosion of the caprock through progressive retreat of the butte scarp produces colluvium that has a higher resistance to erosion than the poorly cemented underlying sandstone.

Once the caprock of a butte has been removed by erosion, the underlying weakly cemented Dawson Formation is readily eroded. Ultimately, the armored lower slopes of the former butte remain as a circular ridge standing as much as 100 m above the surrounding topography. This process produces a topographic low surrounded by relict faceted slopes where the flat top of the butte once stood.

Prominent alluvial fans are associated with some of these annular features, and they record the main phases of butte removal and excavation of the central part of the armored slopes. Multiple generations of alluvial fans contain coarse- and fine-grained facies that represent changes in effective stream power and record alternating phases of aggradation and erosion. The degree of soil development in the fan alluvium and height of the fan surfaces above streams indicates the oldest preserved gravel fan deposit is of late-middle Pleistocene age. The youngest luminescence (optically stimulated luminescence) dated alluvial fans were deposited during the late Pleistocene about the time of the Pinedale glacial maximum in Colorado, ca. 21,000 yr B.P.