FRACTURE ORIENTATIONS AND CLOSED DEPRESSIONS IN ST. CROIX COUNTY, WISCONSIN: EFFECTS OF KARST LANDSCAPES IN HYDROLOGY

St. Croix County in Western Wisconsin has experienced hydrological problems such as contamination of groundwater potentially linked to agricultural and development activities. Regional ground water flow is not fully understood; subsurface stratigraphic inhomogenities have frustrated groundwater modeling efforts. High nitrate levels are present in groundwater and may result from rapid transfer of surface waters to subsurface aquifers from the numerous closed depressions within the county. Accurate characterization of groundwater flow requires an understanding of subsurface stratigraphic and structural controls on water movement. St. Croix County is underlain by an 80-140m thick succession of essentially flat lying Pz sedimentary rocks, including fine to coarse grained clastic sedimentary rocks of the Cambrian Wonewoc Formation, Tunnel City Group, and Jordan Formations, and the dolomitic St. Lawrence Formation. These are overlain by the Ordovician Prairie du Chien Group, a 20-60m thick dolomitic succession with well-developed karst features. Quaternary till and unconsolidated sediments that range in thickness from < 4 m in the east to >65 m in the northwest cover the county. This investigation focused on analysis of closed depressions throughout the county to determine the origin and stratigraphic control and assess potential control on recharge rates. Analysis of topographic maps and aerial photographs was combined with a structural analysis of fractures systems to constrain the genesis and distribution of closed depressions throughout the county. The county can be divided into three regions; those where the control of closed depressions is based on bedrock characteristics and regional fracture networks, those developed as a result of glacial processes, and for which no direct control is apparent. The northwest portion of the county contains the greatest number and largest closed depressions. These depressions are glacially related and control the local topography. Depressions formed from bedrock (collapsesd karst features) are much smaller in size and primarily located in the southeastern part of the county. These commonly follow the pattern of local stream networks, suggesting a structural control in addition to that of bedrock. Closed depressions in the central portion of the county do not seem to fit either end member.