This map is a product of the Black Hills Hydrology Study, which was initiated in 1990 to assess the quantity, quality, and distribution of surface water and ground water in the Black Hills area of South Dakota (Driscoll, 1992). This long-term study is a cooperative effort between the U.S. Geological Survey (USGS), the South Dakota Department of Environment and Natural Resources, and the West Dakota Water Development District, which represents various local and county cooperators. This map is part of a series of 1:100,000-scale maps for the study. The maps include a hydrogeologic map, structure-contour maps (altitudes of the tops of formations) for five formations that contain major aquifers in the study area, and potentiometric maps for these five major aquifers (the Inyan Kara, Minnekahta, Minnelusa, Madison, and Deadwood aquifers).
The study area consists of the topographically defined Black Hills and adjacent areas located in western South Dakota. The Black Hills area is an elongated, dome-shaped feature, about 125 miles long and 60 miles wide, which was uplifted during the Laramide orogeny (Feldman and Heimlich, 1980). The oldest geologic units in the study area are Precambrian metamorphic and igneous rocks, which are exposed in the central core of the Black Hills. Surrounding the Precambrian core is a layered series of sedimentary rocks including limestones, sandstones, and shales that are exposed in roughly concentric rings around the uplifted flanks of the Black Hills. The bedrock sedimentary units typically dip away from the uplifted Black Hills at angles that approach or exceed 10 degrees near the outcrops, and decrease with distance from the uplift. Many of the sedimentary units contain aquifers, both within and beyond the study area. Recharge to these aquifers occurs from infiltration of precipitation upon the outcrops and, in some cases, from infiltration of streamflow (Hortness and Driscoll, 1998). Artesian conditions generally exist within these aquifers where an upper confining layer is present. Flowing wells and springs that originate from the confined aquifers are common around the periphery of the Black Hills.
The purpose of this map is to show the potentiometric surface of the Minnelusa aquifer within the study area. The map provides a tool for evaluating ground-water flow directions and hydraulic gradients in the Minnelusa aquifer.
The Minnelusa aquifer is comprised within the Pennsylvanian- and Permian-age Minnelusa Formation. The Minnelusa Formation consists mostly of yellow to red cross-stratified sandstone, limestone, dolomite, and shale (Strobel and others, 1999). In addition to sandstone and dolomite, the lower part of the formation consists of shale and anhydrite (DeWitt and others, 1986). The upper part of the Minnelusa Formation also may contain anhydrite, which generally has been removed by dissolution at or near the outcrop areas, forming collapse features filled with breccia (Braddock, 1963). The thickness of the Minnelusa Formation within the study area increases from north to south and ranges from 375 feet near Belle Fourche to 1,175 feet near Edgemont (Carter and Redden, 1999). Along the northeastern part of the central Black Hills, there is little anhydrite in the subsurface. Hence, the decreased thickness in this area is due to a change in the depositional environment. On the south and southwest side of the study area, there is a considerable increase in thickness of clastic units as well as a thick section of anhydrite. In the southern Black Hills, the Minnelusa Formation thins in areas very close to the upper contact due to leaching of anhydrite. The outcrop of the Minnelusa Formation shown on the map is from Strobel and others (1999).
The Minnelusa Formation is overlain by the Opeche Shale, which separates the Minnelusa aquifer from the Minnekahta aquifer. The Minnelusa aquifer often is hydraulically separated from the underlying Madison aquifer by shales in the lower portion of the Minnelusa Formation. However, in many areas the Minnelusa aquifer is in hydraulic connection with the Madison aquifer.
The potentiometric surface was mapped by contouring altitudes of water levels in wells completed in the Minnelusa aquifer and altitudes of springs originating from the Minnelusa aquifer. The water-level and spring altitudes shown on the map are from the ground-water database of the USGS National Water Information System and are presented in Galloway (2000). The majority of wells in the study area have a single water-level measurement that usually was obtained at the time of well completion. Some wells, especially continuous-recording wells, have numerous water-level measurements available, in which case a mean value from all measurements was calculated and used for contouring purposes. Ranges in measured water levels for continuous-recording wells generally are less than the 100-foot contour interval used; thus, in most areas the configuration of the potentiometric surface during the period of water-level data collection (approximately 1930-98) probably does not deviate substantially from that which is shown. Deviations between the mapped and actual potentiometric surfaces may be larger for areas with dashed (inferred) contours than for solid contours. Many wells are completed in the upper 200 to 300 feet of the Minnelusa Formation, but sandstone beds in the middle to lower portion of the formation are utilized locally. No differentiation was made on the map to indicate the relative depth within the aquifer at which each well is completed.
Most of the springs used in contouring are on or near the outcrop area. The actual hydraulic head in the vicinity of the springs probably is higher than the spring altitudes. In outcrop areas, stream altitudes also were considered in contouring the potentiometric surface.
In general, ground-water flow in the aquifer is radially outward from the Black Hills. Structural features in the Minnelusa Formation (Carter and Redden, 1999), such as folds and faults, may have local influence on ground-water flow directions. Therefore, structural trends also were considered in the contouring of the potentiometric surface.
Braddock, W.A., 1963, Geology of the Jewel Cave SW quadrangle, Custer County, South Dakota: U.S. Geological Survey Bulletin 1063-G, p. 217-268.
Carter, J.M., and Redden, J.A., 1999, Altitude of the top of the Minnelusa Formation in the Black Hills area, South Dakota: U.S. Geological Survey Hydrologic Investigations Atlas HA-744-C, 2 sheets, scale 1:100,000.
DeWitt, Ed, Redden, J.A., Wilson, A.B., and Buscher, David, 1986, Mineral resource potential and geology of the Black Hills National Forest, South Dakota and Wyoming: U.S. Geological Survey Bulletin 1580, 135 p.
Driscoll, D.G., 1992, Plan of study for the Black Hills Hydrology Study, South Dakota: U.S. Geological Survey Open-File Report 92-84, 10 p.
Feldman, R.M., and Heimlich, R.A., 1980, The Black Hills: K/H Geology Field Guide Series, Kendall/Hunt Publishing Company, Kent State University, Kent, Ohio, 190 p.
Galloway, J.M., 2000, Selected hydrogeologic data for the Inyan Kara, Minnekahta, Minnelusa, Madison, and Deadwood aquifers in the Black Hills area, South Dakota: U.S. Geological Survey Open-File Report 99-602, 60 p.
Hortness, J.E., and Driscoll, D.G., 1998, Streamflow losses in the Black Hills of western South Dakota: U.S. Geological Survey Water-Resources Investigations Report 98-4116, 99 p.
Strobel, M.L., Jarrell, G.J., Sawyer, J.F., Schleicher, J.R., and Fahrenbach, M.D., 1999, Distribution of hydrogeologic units in the Black Hills area, South Dakota: U.S. Geological Survey Hydrologic Investigations Atlas HA-743, 3 sheets, scale 1:100,000.
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Sheet 1 of 2 (PDF, 9.8MB), Northern part of area | Sheet 2 of 2 (PDF, 11.3MB), Southern part of area
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