TABLELAND EVOLUTION IN THE CENTRAL GREAT PLAINS

In the central Great Plains, many interfluves are tablelands, characterized by broad, low-relief summits and steep, highly dissected side slopes. Tableland summits often have low drainage density, and closed depressions (playas) are common. In most cases, low-relief tableland summits can be explained as an inheritance from low-relief fluvial surfaces. Playas and associated lunettes represent ubiquitous eolian modification of the original fluvial surfaces (and possibly groundwater processes). These similarities in form and origin conceal divergent geomorphic histories, with implications for inferences about long-term stream incision and landscape evolution. Along the northwestern and northern margins of the High Plains, in Colorado, Wyoming, and western Nebraska, tablelands formed largely through late Cenozoic stream incision into the Miocene Ogallala Group and underlying rocks. The Ogallala and other erosion-resistant rocks or coarse fluvial gravels preserve undissected tableland summits over long time periods. At least some of the tableland summits may represent remnants of a High Plains surface present at the end of Ogallala deposition, and can be used to estimate the minimum extent of subsequent stream incision and isostatic response. In central Nebraska, some superficially similar tablelands are covered with thick Quaternary loess, and as a result, now stand up to 100 m above adjacent stream valleys where the loess was not preserved. This local relief is about half of that produced by stream incision into bedrock near the Nebraska-Wyoming border, but much of it results from preferential preservation of loess on interfluves, rather than fluvial erosion. Loess-capped tableland surfaces are not remnants of old landscapes, since more than half of the loess accumulated within the last 25,000 yr, and they may be destroyed by erosion much more rapidly than bedrock-controlled landforms. Other tablelands in central Nebraska have acted as surfaces for eolian sand transport, locally capped with sand sheets or patchy dunes but with little long-term accumulation of sediment above the Ogallala Group or overlying Plio-Pleistocene sands and gravels. The modest local relief from these sand-mantled surfaces to adjacent valleys implies quite limited late Cenozoic incision.