DECODING CRYPTIC TOPOGRAPHY IN THE HIGH PLAINS OF COLORADO

The cause of uplift of the High Plains in the central United States is a complex and stubborn mystery. Rather than testing a single overall explanation, we approach the problem incrementally by removing topographic contributions that can be identified in surface geology. This allows us to present the truly unexplained topography. In order to constrain the amount of this unexplained, or cryptic, topography we have begun to quantify uplift that has resulted from measurable events. Although eventually we hope to study the whole of the High Plains, here we consider the area in the vicinity of Denver. The Dakota Group was deposited near sea level during the late Cretaceous. Therefore, these rocks serve as a reference from which we have studied subsequent geologic events and their effects on surface topography. If removing all known effects restored the Dakota to its depositional elevation, no further explanation would be necessary. Alternatively, any difference between the restored Dakota and its depositional elevation would reveal our cryptic topography. First, we constrain the thickness and density of the sediments that overlay the Dakota in eastern Colorado and western Kansas from well log records. From these records we calculate a load estimate, which we use in flexural deflection calculations. The load on the western end of the High Plains, the Rocky Mountains, will affect deflection calculations. Estimating this load is challenging because thrust faulting and other geologic deformation create uncertainties in reconstructing this orogen. We create 2D, cross-sectional models based on published cross-sections to infer the characteristics of the Rocky Mountain load and to calculate the flexural response of the crust to this load. With these models we are able to compare the tradeoffs between these parameters to describe uncertainties in our assumptions. These parameters can then be used in our ongoing analysis of the uplift of the High Plains. Our results are most robust in the High Plains, where it appears that ~900-1000m of topography from 105° to 102°W (i.e., the Colorado High Plains to the Kansas border) require some subsurface load. We present preliminary results of these models for several east-west sections across the Front Range and High Plains in Colorado and eastern Kansas.