APPLICATIONS FOR QUATERNARY THICKNESS MAPS DERIVED FROM NEW STATEMAP-SUPPORTED MAPPING OF ELEVATION OF THE BEDROCK SURFACE IN IOWA

The Iowa Geological Survey is currently completing a six-year STATEMAP-supported project to produce a new digital map of the Bedrock Geology of Iowa. Each year a multi-county project area was mapped. A key initial activity in the construction of these maps was the production of a new map displaying the elevation of the bedrock surface. These maps of bedrock elevation were produced by compiling depth-to-bedrock information from water wells, resource exploration wells, research wells, and cores, road and bridge construction data, quarries and various exposure information from current and historical sources, and from detailed soil maps and parent-material information. These data were entered into digital databases that were accessed to produce a digital map displaying the elevation of the bedrock surface. On-screen mapping was conducted at multiple scales, contingent on the detail of the data, with scales as large as 1:2,000 utilized in areas of high-relief bedrock exposure. The use of all available sources of bedrock elevation information allowed the production of the most detailed and accurate map of bedrock topography possible. Derivative maps displaying the thickness of Quaternary materials overlying the bedrock surface were produced for several regions by gridding the bedrock elevation data, then subtracting grid values from a digital elevation model of the land surface. This map of Quaternary thickness, when used in association with other maps and data, has proven to be a valuable tool for such applications as exploration for new aggregate production sites, assisting water well drillers in estimating costs for new wells, and delineating source-water protection areas for public water supplies. Depth-to-bedrock information is also critical for various regulatory parameters, including siting, engineering, and evaluating sanitary landfills, municipal waste-water treatment lagoons, animal waste lagoons, and other regulated land-use issues.