| Madison Group, Williston Basin
The development of karst, sink holes, caves, and solution breccias is common in the Madison Group, modifying the permeability. Movement along major faults and lineaments may have affected permeability over a large area and through time. Madison permeability is secondary to fracture permeability (Downey, 1984, 1986). Flow-net analysis assumes that a steady-state flow condition exists and no leakage is occurring from or to adjacent aquifers. The transmissivity is about 0,013 ft2/s (Konikow in Downey, 1984, 1986). Other studies suggest that some leakage from overlying rocks may be present and that the transmissivity value should be less than 0,013 ft2/s. According to Downey (1984, 1986), the variation in transmissivity values reported by several scientists is the result of local conditions and may not reflect the average on a regional scale. As a result of his analysis, Downey found that transmissivity values are related to porosity (fracturing porosity). A simulated transmissivity map by Downey (1986) was therefore added to the GIS data base.
Downey, J. S., 1986, Geohydrology of bedrock aquifers in the northern Great Plains in parts of Montana, North Dakota, South Dakota and Wyoming: U.S. Geological Survey Professional Paper 1402-E, 87 p.
Downey, J. S., 1984, Geohydrology of the Madison and associated aquifers in parts of Montana, North Dakota, South Dakota and Wyoming: U.S. Geological Survey Professional Paper 1273-G, 47 p.
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