Abstract

Regional geologic mapping in Marathon County, Wisconsin by the Wisconsin Geological and Natural History Survey has revealed that the general geology consists of a sequence (or sequences) of volcanic and metasedimentary rocks suggesting a “greenstone belt”. However, these rocks have been extensively segmented by deformation and intrusion of a myriad of plutonic rocks of varied composition and age. A large (± 1500 m.y. old) batholith intrudes this complex on the east.

At least six sub-parallel northeast-trending zones of sheared rocks up to a mile wide and separated by 4-6 miles of relatively unsheared rocks transect the area over a width of at least 40 miles perpendicular to the strike of the unsheared rocks. The largest zone has been traced almost continuously for 35 miles along strike. Rocks within the mylonitic zones resemble regionally metamorphosed rocks, but their linear distribution and the fact that they are separated by unsheared rocks seems to rule out a regional metamorphic origin.

A number of these shear zones are topographic lineaments traceable for nearly 100 miles as alignments of stream valleys. A strong parallelism of river valleys is evident even in areas of thick glacial cover, suggesting that bedrock lineaments may have influenced glacial movements.

Regional aeromagnetic surveys underway suggest that this geologic pattern covers thousands of square miles in Wisconsin, and thus must be of crustal dimensions. This structural belt (whatever it is) lies on the west edge of a large 1500 m.y. old batholith, and is roughly parallel to the major trend of the mid-continent gravity high. The relationship between these features is, of course, conjectural.