BEDROCK GEOLOGIC MAPPING AND HYDROSTRATIGRAPHY OF LOWER PALEOZOIC STRATA IN PIERCE AND ST. CROIX COUNTIES, WEST-CENTRAL WISCONSIN

Recent groundwater modeling efforts in Pierce and St. Croix counties were hampered by a lack of data on the hydrogeology of shallow bedrock units. To close this data gap, the Wisconsin Geological and Natural History Survey has begun a three-year project to map the bedrock geology of the two counties at a scale of 1:100,000. The work will include a detailed hydrostratigraphic framework for Lower Paleozoic strata in the area built on recent work by the Minnesota Geological Survey in nearby southeastern Minnesota. First-year mapping focused on the northern two-thirds of St. Croix County, where Lower Ordovician dolomites of the Prairie du Chien Group (PdC) subcrop thick Pleistocene deposits throughout a large upland area. Understanding the nature and distribution of solution features and fractures in the PdC in this area is critical to evaluating the unit's role in recharging deeper sandstone aquifers. Significant drilling difficulties were encountered in attempting to core the PdC at two widely spaced locations in the area; only three to four incomplete 10-ft core runs were recovered from the unit in each hole. Fluid circulation was lost immediately upon entering the PdC and only broken pieces of rock were recovered. At one of the locations a 6-ft-deep solution cavity was encountered. Although complete core records through the PdC were not obtained, we gained invaluable qualitative information on the highly broken nature of the PdC, a feature that is likely characteristic of the unit throughout the area.