Structure and Hydrogeology of Deformed Sedimentary Bedrock Aquifers, Western Summit County, Utah

The Snyderville basin, near Park City in western Summit County, Utah, has experienced significant water shortages coupled with a 50 percent growth rate in the past 10 to 15 years. Recent development rests directly on complexly folded and fractured sedimentary bedrock aquifers in the hanging wall of the Mount Raymond thrust. Detailed geologic and fracture scanline mapping coupled with structural analyses in the Pinebrook subdivision, one site within the Snyderville basin demonstrating abrupt hydrogeologic changes, provide a clearer picture of the local hydrogeologic setting. The dominant map-scale structure is the Twomile Canyon anticline. Several macroscopic faults cut this fold, including the Toll Canyon fault, a backthrust off the Mount Raymond thrust. Fracture orientations and densities vary within meters across the Twomile Canyon anticline as a function of lithology and position relative to macroscopic faults. Exposures of the Toll Canyon fault show that the width and lithologic composition of the fault core and related damaged zone are a function of lithology, and the fault strongly controls fracture permeability. Damage zones in limestones and sandstones with high fracture intensities may be regions of enhanced permeability, whereas shale smears and clay gouge adjacent to the fault core act as barriers to fluid flow. A conceptual model of the subsurface in the Pinebrook study area has been developed, and several test well sites have been proposed based on this model and field observations. The target formation, structural position, fracture intensity, local hydrogeology, and accessibility were factors considered in locating these wells.