VARIATIONS IN THE THICKNESS AND CONTINUITY OF THE SHALY FACIES OF THE EAU CLAIRE FORMATION, SOUTH-CENTRAL WISCONSIN

In the parts of western and southwestern Wisconsin, that are distal to the Wisconsin arch, the Eau Claire Formation includes a prominent and widespread shaly facies that forms an important regional aquitard. Along the crest of the Wisconsin arch in southern Wisconsin the shaly facies is very thin or absent, and sandstones of the Wonewoc, Eau Claire, and Mt. Simon formations form a thick undifferentiated sandstone succession. Two 15-mile-long east-west transects located in southeastern Sauk County and northeastern Iowa County demonstrate the lateral discontinuity of the shaly facies in areas proximal to the Wisconsin arch. Geologic logs along both transects show that shale is absent at the western ends, thickens toward the centers where the shaly facies is greater than 100 feet thick, and then thins toward the east. Time-domain electromagnetic (TEM) soundings were acquired at several locations along each transect to verify information shown on the geologic logs and to provide more detail on lateral variations in shale thickness. The TEM data showed gradual thickening and deepening of the shaly facies toward the center of each transect, and allowed the shale zero-edge to be precisely mapped along the west side of both transects. Beyond the shale zero-edge the aquitard in the Eau Claire Formation is absent. We interpret the laterally discontinuous nature of the shaly facies along these transects to be related to paleotopography on the Precambrian bedrock surface. Recognition of the discontinuous nature of the shaly facies will result in better geologic models of the Eau Claire Formation and more realistic groundwater flow simulations.