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 Inyan Kara aquifer within the study area. The map provides a tool for evaluating ground-water flow directions and hydraulic gradients in the Inyan Kara aquifer.
The Inyan Kara aquifer is composed of sandstones within the Cretaceous-age Inyan Kara Group, which includes the Lakota and Fall River Formations and ranges in thickness from 135 to 900 feet in the study area (Carter and Redden, 1999). The Lakota Formation consists of a yellow, brown, and reddish-brown massive to thinly bedded sandstone, pebble conglomerate, siltstone, and claystone of fluvial origin (Gott and others, 1974). The Fall River Formation is a brown to reddish-brown, fine-grained sandstone, thin bedded at the top and massive at the bottom (Strobel and others, 1999). The outcrop of the Inyan Kara Group shown on the map is from Strobel and others (1999). Away from the outcrop, as much as 4,300 feet of mostly Cretaceous-age shales form a confining unit overlying the Inyan Kara aquifer in the study area (Kyllonen and Peter, 1987). The aquifer is separated from underlying major aquifers by a semiconfining unit of Jurassic-age rocks including the Gypsum Spring Formation, Sundance Formation, Unkpapa Sandstone, and Morrison Formation (Strobel and others, 1999).
The potentiometric surface was mapped by contouring altitudes of water levels in wells completed in the Inyan Kara aquifer and altitudes of springs originating from the Inyan Kara 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 1935-98) 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. Wells completed in either the Lakota or Fall River Formations, or both, were used in preparing the map.
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 Inyan Kara Group (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
Inyan Kara Group in the Black Hills area, South
Dakota: U.S. Geological Survey Hydrologic Investigations Atlas HA-744-A, 2 sheets,
scale 1:100,000.
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.
Gott, G.B., Wolcott, D.E., and Bowles, C.G., 1974, Stratigraphy of the Inyan Kara Group and localization of uranium deposits, southern Black Hills, South Dakota and Wyoming: U.S. Geological Survey Professional Paper 763, 57 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.
Kyllonen, D.P., and Peter, K.D., 1987, Geohydrology and water quality of the Inyan Kara, Minnelusa, and Madison aquifers of the northern Black Hills, South Dakota and Wyoming, and Bear Lodge Mountains, Wyoming: U.S. Geological Survey Water-Resources Investigations Report 86-4158, 61 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, 9.1MB), Northern part of area | Sheet 2 of 2 (PDF, 11.5MB), Southern part of area
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