Abstract

The western United States contains three thin but remarkably widespread alluvial conglomeratic units that record episodes of large-scale tilting across the U.S. Cordilleran orogen in post-Paleozoic time. These units are: (1) the Shinarump Conglomerate of Late Triassic age exposed in northern Arizona and adjacent parts of Utah, Nevada, and New Mexico; (2) Lower Cretaceous gravel deposits that overlie the Morrison Formation throughout the Rocky Mountain region; and (3) the gravel-rich parts of the Miocene-Pliocene age Ogallala Group in western Nebraska and adjacent southeastern Wyoming. Paleoslopes of the rivers depositing these units were in the range of 10−4 to 10−3, based on paleohydraulic calculations. However, depositional thickness trends of these units are not sufficient to have generated such steep paleoslopes. Thus, long wavelength tilting of the Earth's surface must have occurred for these gravels to be transported. Although these units were deposited adjacent to large tectonic features, including an evolving and migrating continental arc, and, for the Ogallala Group, the northward-propagating Rio Grande Rift, the tilting occurred over wavelengths too broad to be directly generated by these features. These widespread gravel units attest to the interplay between the creation of subduction-related isostatic and dynamic topography and continental sedimentation. Hence, paleotopography, as determined from calculated transport gradients of sedimentary deposits, provides a means of relating constructional landforms to mantle-driven processes.

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