Preliminary Structure Contour Map of the Top of the Mississippian Osagian Stage in Eastern Kansas

This report accompanies a preliminary subsurface geologic map depicting the top of the Osagian Stage in eastern Kansas. The map represents a continuation of the Mississippian subsurface mapping completed in southwestern Kansas in 2025 as a part of the U.S. Geological Survey (USGS) STATEMAP program. The map illustrates the top of the Osagian Stage (Lower Mississippian Series), represented by the undifferentiated Keokuk-Burlington Limestone. In Kansas, the Osagian is generally unconformably overlain by rocks of the Meramecian Stage. The Keokuk-Burlington Limestone, which forms the uppermost Osagian unit in the state, consists primarily of cherty, siliceous, dolomitic limestone. The data shown are composed of well tops from the Kansas Geological Survey (KGS) oil and gas database (Master List of Oil and Gas Wells). The structural map illustrates the regional distribution of the top of the Osagian across eastern Kansas. The surface deepens toward the southwest, where depths range from approximately -3600 to -4600 ft. Becoming progressively shallower toward the east, surface elevations range from approximately 500 to 800 ft. In southeastern Kansas, Mississippian strata are exposed at the surface. An accompanying isochore map depicts thickness variations within the Osagian Stage across eastern Kansas. The Osagian is generally thin (0–50 ft) adjacent to the Central Kansas Uplift (CKU) and the Nemaha Uplift (NU), where erosion and structural uplift have removed much of the Mississippian section. Thickness increases away from these uplifted areas, reaching as much as approximately 400 ft in structurally lower regions. To complement the map products, we constructed three regional structural cross sections, two oriented northsouth and one oriented east-west to illustrate the lateral continuity and vertical geometry of the Osagian Stage. The cross sections corroborate the regional structural and thickness trends observed in the top structure and isochore maps, providing a three-dimensional interpretation of the distribution of Osagian strata across eastern Kansas.