NEW DIGITAL BEDROCK GEOLOGIC MAP OF IOWA

The Iowa Geological and Water Survey has recently completed a new digital map of the bedrock geology of Iowa. Production of this map, the first new statewide map in over 40 years, utilized all available data including exposure studies, well and core information, soils maps, and other information to produce a map of the elevation of the bedrock surface (50’ c.i.) and identify and determine the elevation of the geologic contacts to be mapped. The mapping, which took over 12 years to complete, includes 37 geologic units from Precambrian through Cretaceous, revising and refining previous interpretations. Since Iowa bedrock is virtually everywhere overlain by glacial drift, the map was drawn to represent the density of available data, with areas of shallow overburden showing greater detail than areas controlled by widely-spaced wells. This map is the first to delineate the major fault zones in Iowa, including a newly identified major fault. Mapping in eastern Iowa has clarified the western extension of the Plum River Fault Zone (PRFZ) and its relationship to the Amana Fault Zone (AFZ), and has identified a new sub-parallel fault, the Iowa City – Clinton Fault Zone (ICCFZ). The PRFZ is shown to join the AFZ, which trends to central Cedar Co. The down-to-south displacement of the AFZ displays a scissor fault relationship with the down-to-north PRFZ, the zero point in SW Linn Co. The ICCFZ parallels the PRFZ, about 15 miles to the south, and extends from south-central Johnson Co to east-central Clinton Co. In southwest Iowa the Thurman-Redfield Structural Zone is mapped from the Nebraska border to Story Co in central Iowa. This fault demonstrates Phanerozoic reactivation of the southern bounding fault of the Midcontinent Rift Iowa Horst and is evident in the juxtaposition of numerous Pennsylvanian units in the area. The fault that delineates the outer margin of the Manson Impact Structure is also mapped as a much more circular feature than previous interpretations. The map also shows a much greater area of Cretaceous bedrock in southwest Iowa than previous maps and many more Cretaceous outliers in the eastern part of the state. The Pennsylvanian Cherokee Group was subdivided for the first time, and the map is enhanced with a shaded relief overlay of the bedrock elevation.