GEOLOGIC MAPPING ALONG THE WISCONSIN ARCH: RECOGNITION OF AN UNCONFORMITY WITHIN THE LATE CAMBRIAN ELK MOUND GROUP

The late Cambrian Elk Mound group includes the oldest Paleozoic sediments in Wisconsin: the Mt. Simon, Eau Claire, and Wonewoc Formations, in ascending order. In western Wisconsin, the Eau Claire Formation consists of a shaly, dolomite-cemented, fine-grained sandstone deposited near storm wave base and between the supermature, medium grained, nearshore or aeolian quartz arenites of the underlying Mt. Simon and overlying Wonewoc Formations. In central Wisconsin, the Eau Claire formation thins and becomes locally absent on the Paleozoic topographic high of the Wisconsin Arch. Previous 1:100,000-scale mapping of the Wisconsin Arch identified a fine-grained interval within the Elk Mound Group and tentatively correlated this with the Eau Claire Formation. While building a new 1:12,000-scale geologic map in the Wisconsin Dells area, we observed an approximately two-meter thick layer of silcrete clasts, sparse red silty layers, bands of iron cement, and poorly sorted, fine to very coarse, supermature quartz arenite. This layer’s characteristics, especially the silcrete clasts and a lack of trace fossils or bioturbation, suggest that it may represent a reworked exposure surface overlying an unconformity. This unconformity could be a continuation of the Sauk II-III boundary described in western Wisconsin, southern Minnesota, and Iowa. Whether and how it relates to the Eau Claire Formation remains an open question; if they are unrelated, existing geologic maps in the area will need revision.