PRECAMBRIAN TERRANES IN SOUTHERN WISCONSIN AND NORTHERN ILLINOIS: WHAT'S HIDING IN THE BASEMENT

Recently completed aeromagnetic mapping in southern Wisconsin enables an "x-ray view" beneath glacial and Paleozoic sedimentary-rock cover. Wisconsin's Precambrian geology is unique in that there is the conjunction of seven or more ages of tectonic components (at least two Archean, Penokean, ca.1760Ma, ca.1630Ma, ca.1500Ma, and Keweenawan). Complexity and sparse Precambrian exposure in southern Wisconsin continue to foster various opinions on the tectonic evolution of the region south into northern Illinois. A current hypothesis is that metamorphic cooling ages indicate the major influence of Mazatzal (ca.1600Ma) orogenic foreland activity. The lithology of available basement samples, geophysical data, and, metamorphic/deformational relationships may also offer a different perspective. The long-term episodic character of anorogenic magmatism in the region suggests the significant developmental role of thermal influx from 1760Ma through the ca.1100Ma Keweenawan rifting.

Specific geophysical features of interest include large-wavelength regional gravity anomalies, clustered patterns of circular to ovoid anomalies, and certain fine-scale magnetic patterns suggesting the presence of dikes, fault systems and folding of Baraboo interval metasedimentary units. This additional detail is useful in helping to further constrain opinion. It may be that the apparent conflict over interpretation of this region has arisen from fragmented perspectives, emphasizing only some aspects of the very complicated puzzle. A better integrated approach that takes all forms of pertinent data into consideration, should be more fruitful.