Enhancement of the Bedrock-Surface-Elevation Map Beneath the Ogallala Portion of the High Plains Aquifer, Western Kansas

Abstract

Development of tailored ground-water-management plans for the Ogallala portion of the High Plains aquifer in western Kansas calls for a more accurate assessment of the water table and bedrock-surface-elevation data used to calculate saturated thickness. Until this most recent effort, bedrock-surface elevation beneath the Ogallala aquifer in western Kansas was known only from a map based on data collected up through about 1980. Since then, new data have become available for the thousands of wells that have been drilled for water and hydrocarbons in western Kansas and in the adjacent townships of Nebraska, Colorado, and Oklahoma. Out of more than 40,000 records examined, a total of 22,535 data points were selected for inclusion in the bedrock-surface-elevation database for this project. Depth-to-bedrock information in Kansas was determined from interpretation of driller and gamma-ray logs or was provided from the WIZARD database. Data also were included from the surrounding states that border the Kansas Ogallala region. The data were entered into an ArcGIS geographic-infomation system from spreadsheets. A land-surface digital-elevation model was used to estimate a surface elevation and with depth-to-bedrock data, to estimate bedrock-surface elevation for each well location. Bedrock-surface elevations were plotted and manually contoured to produce the latest version (2004) of the bedrock-surface-configuration map.

Overall, bedrock-surface elevation across the Ogallala region is approximately 1 ft (0.3 m) lower than the previous map version, which is well within the error of the data used to create the updated map. Locally, the magnitude of the change in the bedrock-surface configuration is greatest in southwest Kansas, and the size of the areas where the change is greatest is larger in this region of the state than elsewhere. A visual comparison of the two map versions clearly shows that the increase in data used to produce the updated map has resulted in a concomitant increase in the level of topographic detail that can be shown.

A significant part of the process of producing the final updated bedrock map (Plate lA and 1B) involved investigation of the Crooked Creek-Fowler and Bear Creek faults that appear on earlier bedrock-surface-elevation maps. These features have previously been interpreted as continuous-fault zones associated with dissolution of evaporites in the Flower-pot Shale rather than dissolution fronts characterized by sinkholes, coalesced subsidence basins, and local faulting with minor displacement. The top of the Blaine Formation was chosen as a structural-mapping horizon to evaluate shallow geologic structure, and comparisons were made to the better-understood Hutchinson Salt Member dissolution zone in central Kansas. Results of the investigation suggest that the dissolution-front interpretation is more likely than the continuous-fault interpretation. Consequently, the fault traces on the bedrock were removed from the updated map, and the bedrock-surface-elevation data were re-contoured to reflect the dissolution-front hypothesis.

The updated map also provides insight into the Cenozoic drainage development of western Kansas in response to uplift of the Rocky Mountains to the west, isostatic rebound attributed to erosion of significant sediment thickness on the western Great Plains, and Quaternary climatically driven cycles of erosion, deposition, and stability. Pre-Ogallala drainage that included the ancestral Arkansas River entered Kansas from Colorado and exited the state approximately where the present-day Cimarron River enters Oklahoma. A southward-flowing tributary drainage followed the axis of the Scott-Finney depression to the main stem in southern Finney County. An incised paleovalley extending from eastern Gray, Ford, and Edwards counties seems to represent a post-Ogallala segment of the eastward-flowing ancestral Arkansas River drainage.