BEDROCK GEOLOGIC MAP OF MITCHELL COUNTY, IOWA

The Bedrock Geologic Map of Mitchell County represents the culmination of three years of geologic mapping conducted as part of the Iowa Geological Survey’s (IGS) ongoing participation in the United States Geological Survey’s (USGS) National Cooperative Geologic Mapping Program (STATEMAP). Recent flooding in the region and an increased demand on bedrock aquifers for irrigation has generated significant interest from local government and conservation groups which highlighted the need for a better understanding of the geologic setting of the region.

Mitchell County occupies approximately 470 square miles of primarily agricultural land situated in north-central Iowa. The mapping area is within the northern portion of the Devonian Iowa Basin, a region of thickened shelf carbonate and shale that was deposited from the late Eifelian through early Frasnian stages. Due to its stratigraphic completeness, rich fossil fauna, and hydrogeologic significance, the stratigraphy of the Devonian Iowa Basin has been intensively studied. However, detailed geologic mapping has not been done. The most recent bedrock geologic map of Iowa published in 2010 mapped the Devonian bedrock in north-central Iowa at the group level.

Creating the Bedrock Geologic Map of Mitchell County incorporated all available data, including existing geologic maps from Iowa and Minnesota, applicable publications, soil survey maps, over 100 bedrock exposures, and more than 900 borehole records. The resulting bedrock geologic map illustrates the refined topography of the bedrock surface as well as the distribution of bedrock units at the formation level. The Middle and Upper Devonian strata were subdivided into their respective stratigraphic formations (in ascending order), the Spillville, Pinicon Ridge, Little Cedar, Coralville, Lithograph City, and Shell Rock. The Bedrock Geologic Map of Mitchell County, Iowa and its accompanying Summary Report will serve as a valuable tool to educate and inform the citizens of Mitchell County of the complexity of the underlying bedrock geology and will assist in better management of groundwater resources, planning for urban and agricultural development, and identification of potential aggregate resource areas.