A GEOPHYSICAL VIEW OF THE PRECAMBRIAN BASEMENT OF SOUTHERN WISCONSIN

Recent high resolution aeromagnetic and gravity surveys in southern Wisconsin have produced maps showing a high level of detail of variations in the potential fields in areas where Precambrian rocks are covered by a thin veneer of Paleozoic strata. The new data, along with about 200 drill holes that reached basement and scattered erosional windows into basement, have been used to make a broad interpretation of Precambrian features in southern Wisconsin. The geology revealed by the new data is substantially different than that exposed in the northern part of Wisconsin. Further interpretation of this high resolution data promises to improve our understanding of poorly known Late Proterozoic crustal evolution of the mid-continent North America. Newly mapped features include: 1) the apparent southern limit of Archean rocks along an east-west lineament interpreted to be a major fault, 2) extensive areas of “Baraboo type” (Early Proterozoic) quartzite infolded with slightly older rhyolite, 3) numerous granitic plutons, some apparently composite bodies with variable magnetic susceptibility, 4) two sets of dikes, probably diabase, an E-W set apparently with normal magnetic polarity and a NE set apparently with reversed magnetic polarity. Also apparent on the magnetic map is a set of ENE trending linear features across which there are prominent changes in the magnetic signature. These must be major faults. Prominent on the gravity map is the “Central Wisconsin Gravity Low”, an enigmatic feature that occupies about one third of the area of the state. The new data suggest that the low is caused, at least in part, by granitic plutons ranging from very large bodies to clusters of smaller plutons.