ELECTROMAGNETIC STRATIGRAPHY OF ORDOVICIAN UNITS, WINNESHIEK COUNTY, IOWA

The Groundwater and Stratigraphic Studies Section of the Iowa Geological Survey (IGS-GSS) has been refining maps of Iowa's bedrock geology. In order to aid this project, we made time-domain electromagnetic (TEM) soundings at 10 sites in Winneshiek county Iowa. Such TEM soundings give a generalized depth section at each site, showing electrical resistivity and layer thicknesses to depths of about 200 ft. Our results show that many geologic units in the study area can be distinguished by their electrical resistivities. We find the Galena limestone Group has resistivity >1000 ohm-m, the Platteville limestone Fm. has ~100 ohm-m, the St. Peter sandstone Fm. has ~30 ohm-m, and the Prairie du Chien Group has ~200 ohm-m. Further TEM soundings can now be used to help constrain layer thicknesses in the study area.