EVOLUTION OF RELIEF ON THE COLORADO FRONT RANGE

Although the timing, rate, and magnitude of the Laramide orogeny (~80–55 Ma) are widely debated, clues to the evolution of the orogen are preserved in the stratigraphy, sedimentology, and paleontology of synorogenic Laramide basins, specifically the Middle Park and Denver Basins. Today, these basins show ~750 m of relative elevation difference, but in the Late Cretaceous they were below sea level. The Denver Basin preserves the regression of the Western Interior Seaway from ~69–68 Ma, with marine sediments giving way to beach sands, followed by coastal swamp deposits. The first major pulse of westward-derived synorogenic sediment appears on an unconformity above the coastal swamp. Clast composition in this deposit shows that by ~68 Ma more than 3 km of Phanerozoic cover had been eroded from the Front Range and that Precambrian basement was incised. By the Early Paleocene, the Denver Basin shows a strong longitudinal and possibly orographically driven plant-diversity gradient that increases toward the mountain front. Leaf-based estimates of MAT from Middle Park and Denver Basin floras show that difference in surface relief between these two basins was significant at ~64 Ma—a conclusion corroborated by a marked offset in oxygen isotope ratios of authigenic clays between the basins. These data, combined with apatite fission track ages, indicate that the Front Range had been eroded to near present-day depths by the Early Paleocene. Following this episode of uplift, the topography between the Front Range and flanking basins equilibrated through a combination of tectonic and erosive processes that led to the formation of the Rocky Mountain Surface—a low-relief upland that projects across basins and ranges alike. Uniformly low oxygen isotope ratios obtained from Late Eocene mammal fossils support a spatially widespread low-relief surface. The Wall Mountain Tuff (36.7 Ma), erupted on this surface, shows that little differential uplift has occurred between the Denver Basin and the Front Range since latest Eocene time. Miocene and younger epiorogenic uplift has led to basin exhumation, renewed bedrock incision by the headward erosion of the tributaries to the Mississippi River system, deposition of the Ogallala Group, and exposure of the early Cenozoic topography on the Front Range.