DIGITAL SOUTH-CENTRAL IOWA BEDROCK MAP OF THE UPPER AND LOWER CHEROKEE GROUP (SWEDE HOLLOW AND FLORIS FORMATIONS) BOUNDARY

The boundary between the Swede Hollow Formation and the Floris Formation is important for mapping of coal and possible coal gas resources in Iowa. The boundary was mapped in south-central Iowa, where about ninety percent of the contact in the state occurs, using several thousand wells and outcrops. To map this boundary, a digital bedrock topographic map was created in ArcGIS 9.0 using bedrock top elevation data acquired from well logs, cores and outcrops. In areas of outcrop it was adjusted to surface topography by using overlays of 7.5 minute surface topographic maps. The boundary was then identified in outcrop and core by using litholologies and microfossils (usually conodonts), as well as elevation determinations. Boundary elevations in outcrop and core were then matched to stratigraphy in well logs to identify the boundary in the well logs. Once all boundary elevations were determined, the geologic contact could be mapped onto the bedrock topographic map as a shape file. In areas of sparse outcrop and well data, the boundary was mapped using a known stratigraphic interval from overlying mapped units. Data for the overlying Marmaton/Cherokee boundary were supplied by the Iowa Geological Survey and USGS from the STATEMAP Project.