BEDROCK GEOLOGIC MAPPING AND GEOCHEMISTRY OF TREMPEALEAU COUNTY, WISCONSIN

Trempealeau County, in west-central Wisconsin, is at the center of the Midwest’s frac sand resources. However, the current understanding of this natural resource is generalized. Industry, government, and citizens in the county request detailed information regarding the distribution of this resource, as well as the potential effects of sand mining on surface and groundwater quality. We are undertaking a two-year bedrock mapping project of Trempealeau County at the 1:100,000 scale, with a focus on developing mapping techniques for the Driftless Area in the first year, and the geochemistry of actively mined frac sand units in the second year. Map units identified include, from oldest to youngest, the Mount Simon Fm., Eau Claire Fm., Wonewoc Fm., Lone Rock Fm., Trempealeau Group (Jordan and St. Lawrence Fms.), Prairie du Chien Group, and, possibly, the St. Peter Fm. One result of the first year is that acquisition of continuous drill core is necessary for accurately determining the elevation of map unit contacts, which rarely outcrop in the study area, and constraining thickness changes in map units, which is imprecise with outcrops alone. Another outcome of mapping in the first year was the successful application of a Topographic Position Index method to derive a basic landform map from a 5-foot resolution DEM. This landform map was used to model a high-resolution bedrock topographic surface onto which bedrock geology was mapped, and saved a considerable amount of manual editing time given the complex dendritic incisions characteristic of the Driftless Area topography. In the second year of the project, we are undertaking geochemical and mineralogical analyses to ascertain potential natural sources of trace metals observed in surface and groundwater regionally. We recognize trace metal-bearing sulfides within the frac sand mined units with origins from both Cambrian seawater and MVT mineralization. In general, MVT sulfides have higher trace metal concentrations. The presence of these sulfides has important implications for the quality of groundwater in private wells and surface water at industrial (frac) sand mines. This map and associated data will provide useful techniques and information for further mapping of the Driftless Area and making informed decisions about natural resource extraction.