A seismic reflection study of the precambrian basement along the Illinois-Wisconsin state line

Includes bibliographical references.Includes illustrations and maps.The Precambrian surface beneath southern Wisconsin and northern Illinois is interpreted from a synthesis of semicontinuous seismic reflection and borehole data. Seismic profiles were shot along the Illinois-Wisconsin state line in order to better define the framework of the deeper aquifers in the northern midwest. A structure map of the Precambrian surface shows elevations ranging from more than 3000 feet below sea level beneath the Lake Michigan shoreline to +800 feet in southern Wisconsin. Elevations reflect the position of the region with respect to the Illinois and Michigan Basins and the Wisconsin Arch. A geologic cross-section based upon the seismic and borehole data shows the configuration of the Precambrian surface and the thickness of the Mt. Simon Formation, a Cambrian sandstone aquifer resting unconformably on the basement. The location and strike of proposed basement faults agrees with previously published faults postulated on various tectonic maps and with an orthogonal joint system observed in the bedrock of northern Illinois. It is proposed that an orthogonal fault network, with orientations defined by the Sandwich Fault Zone in northern Illinois and the faults crossing the state line, is responsible for a network of orthogonal bedrock joints described in previous publications. The age and cause of the basement faulting is not known with certainty; however, the age of faulting postdates the Late Precambrian and pre-dates the Pleistocene.M.S. (Master of Science