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 Deadwood aquifer within the study area. The map provides a tool for evaluating ground-water flow directions and hydraulic gradients in the Deadwood aquifer.
The Deadwood aquifer is comprised within the Cambrian- and Ordovician-age Deadwood Formation. The Deadwood Formation is composed primarily of brown to light-gray glauconitic sandstone, shale, limestone, and local basal conglomerate (Strobel and others, 1999). The thickness of the Deadwood Formation increases from south to north in the study area and ranges from 0 to 500 feet; the Deadwood Formation is absent in the southern part of the study area (Gries, 1975). The outcrop of the Deadwood Formation shown on the map is from Strobel and others (1999).
In the northern and north-central Black Hills, the Deadwood Formation is conformably overlain by the Whitewood Dolomite and Winnipeg Formation. The Winnipeg Formation is not present south of the approximate latitude of 44 degrees, and the Whitewood Dolomite is not present south of the approximate latitude of Nemo (DeWitt and others, 1986). South of the approximate latitude of 44 degrees, the Deadwood Formation is unconformably overlain by the Englewood Formation.
The potentiometric surface was mapped by contouring altitudes of water levels in wells completed in the Deadwood aquifer and altitudes of springs originating from the Deadwood 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 1950-98) 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. Where the Deadwood Formation is thin (less than 50 feet thick), wells are usually completed in the entire aquifer interval. However, in areas where the Deadwood Formation is thick (50 to 500 feet), wells often are completed only in the upper portion of the aquifer. No differentiation on the map was made to indicate the relative depth within the aquifer at which each well is completed. In parts of the northern Black Hills, water levels are provided, but no attempt was made to contour the data because of the complexity caused by numerous Tertiary intrusions.
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 Deadwood 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.
Carter, J.M., and Redden, J.A., 1999, Altitude of the top of the Deadwood Formation in the Black Hills area, South Dakota: U.S. Geological Survey Hydrologic Investigations Atlas HA-744-E, 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.
Gries, J.P., 1975, Mineral and water resources of South Dakota: U.S. Congress, 94th, 1st session, Interior and Insular Affairs Committee Print, 313 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, 9.5MB), Northern part of area | Sheet 2 of 2 (PDF, 10.2MB), Southern part of area
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