As the midwestern United States is covered largely by Paleozoic sedimentary rocks, geologic studies of the basement complex are based primarily upon extrapolation of trends from the Laurentian shield and samples from approximately 50 basement wells in Illinois, Indiana, Ohio, and Michigan. Age determinations and lithologic studies of samples from these basement tests suggest an extension of the Grenville province from Ontario into the area of the Indiana-Ohio platform. Geophysical surveys also have revealed structural and lithologic patterns in the basement complex that can be

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associated with certain geologic provinces. Recent gravity and magnetic studies suggest a possible structural relationship between the Lake Superior syncline and the Michigan basin and suggest the presence of basalt flows along the Cincinnati arch similar to the Keweenawan flows of Michigan.

Configuration of the basement surface is conventionally interpreted from well data, aeromagnetic surveys, and projection of dips. Recent development of the continuous velocity log, however, has led to a resurgence of spot correlation seismic surveys. The technique, based on widely spaced seismic shot points, has been successfully applied in Illinois, Indiana, and Ohio as a low-cost method of regionally mapping the basement. In general, preliminary seismic results and gravity and magnetic studies show close relation between the major sedimentary structures of the midwestern United States and the regional configuration of the basement surface.

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