2008 Joint Annual Meeting (5-9 Oct. 2008): Preferential Flow Features of the Madison Aquifer, White Creek Canyon, Wyoming

The Madison Aquifer is a critical groundwater resource for Wyoming and surrounding states. This study investigated preferential flow features (fractures, karst, and paleokarst) in the Madison Formation in White Creek Canyon, WY. Over 1,500 fractures and solution openings were documented in the 800-ft-thick Madison Formation during this study. Orientation of fractures and solution openings was determined by Brunton compass. Fracture length and spacing were measured using a survey tape, scanline, and orthorectified digital images. GPS was used to identify feature locations and to orthorectify images. Fractures were plotted on equal-area stereonets to identify fracture sets and their relation to solution openings, paleokarst, and fold axes.

We identified a primary fracture set with a mean orientation of tr. 352° pl. 15.9°. This orientation corresponds with the principal stress direction of the Laramide Orogeny. These fractures were at least 100 m in length with a mean fracture spacing of 28.0 m, which appears to be independent of location within the monocline exposed in White Creek Canyon. A zone of paleokarst features was identified at the base of the Mission Canyon Member. This paleokarst was infilled by collapse breccia, calcite, and speleofluvial deposits. Open karst features were discontinuous, dry, and displayed an apparently random orientation. Our observations suggest that groundwater flows primarily through fracture networks and not paleokarst in this aquifer. This conclusion is supported by similar studies conducted in Trapper Creek Canyon south of this study area. The large fracture spacing and lack of relationship between spacing and structure may explain the highly variable yield of wells completed in the Madison locally. We are hopeful that continuing investigation of White Creek Canyon will reveal a contemporary karst system interconnected with fractures that could be used as an aquifer for nearby residents.