This report 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. The map in this report is part of a series of 1:100,000-scale maps that are being produced for the study. Other maps include 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 artesian springs that originate from confined aquifers are common around the periphery of the Black Hills.
The map in this report shows outcrops of stratigraphic units for the northern (sheet 1) and southern (sheet 2) parts of the Black Hills area. Generalized sections (sheet 3) showing the subsurface occurrence of these units also are presented. The map was compiled at 1:100,000 scale from other published and unpublished geologic maps that ranged in scale from 1:24,000 to 1:500,000, but mostly at scales equal to or larger than 1:100,000. The primary sources for mapping the distribution of the hydrogeologic units were supplemented by additional site specific maps (see index to sources of geologic data) and field work that focused on the smaller areas covered by the smaller scale maps.
The stratigraphic units provide the basis for designating the hydrogeologic units, which are based on similarity in hydraulic properties. The hydrogeologic units include aquifers, semiconfining units, and confining units. Aquifers are units that commonly are used as sources of ground water. Semiconfining units contain some layers with low permeability. Confining units consist mainly of layers with low permeability. Both semiconfining and confining units may contain rocks that are local aquifers, but these generally are not of regional importance.
The unconsolidated units include alluvium, colluvium, gravel deposits and wind-blown deposits of Quaternary age, and gravel deposits of Tertiary age. All have the potential to be local aquifers where they are saturated. The White River aquifer consists of various discontinuous units of sandstone and channel sands along the eastern flanks of the Black Hills, which may produce water where saturated. The Tertiary intrusive units are restricted to areas in the northern Black Hills, and generally are relatively impermeable. Intrusive sills can increase bedding plane fracturing and often are associated with "perched" ground water.
Rocks of Mesozoic age include the Cretaceous-sequence confining unit, Inyan Kara aquifer, Jurassic-sequence semiconfining unit, and the upper part of the Spearfish confining unit. The Cretaceous-sequence confining unit mainly includes shales of low permeability, such as the Pierre Shale and Carlile Shale, but may contain minor aquifers, such as the Newcastle Sandstone. The Inyan Kara aquifer consists of sandstones in the Fall River and Lakota Formations and is a major aquifer in the Black Hills. The Jurassic-sequence semiconfining unit consists of shales and sandstones of the Morrison Formation, Unkpapa Sandstone, Sundance Formation, and Gypsum Spring Formation. The overall unit is semiconfining because of the low permeability of the interbedded shales; however, minor local aquifers do exist. The Spearfish confining unit consists of sandy shale, sandstone, and siltstone, with gypsum and local limestone layers. Local aquifers may exist where gypsum and anhydrite have been dissolved.
Rocks of Paleozoic age include the lower part of the Spearfish confining unit, Minnekahta aquifer, Opeche confining unit, Minnelusa aquifer, Madison aquifer, Ordovician-sequence semiconfining unit, and Deadwood aquifer. The Paleozoic units include four of the five major aquifers for the Black Hills area. The Minnekahta Limestone consists of a laminated limestone. Although not particularly thick (25 to 65 feet), it is a reliable source of water around the perimeter of the Black Hills uplift. The Opeche confining unit is a silty to sandy shale that generally impedes ground-water movement between the Minnekahta and Minnelusa aquifers. The Minnelusa Formation is a thick (375 to 1,175 feet) unit of sandstone, limestone, dolomite, and shale. Sandstones in the upper part of the formation constitute a regionally important aquifer with relatively consistent yields. Larger yields can be obtained where secondary permeability results from fracturing or solution enhancement. The lower portion of the Minnelusa Formation contains interbedded limestone and shale, and is a lower confining zone within the unit. However, for mapping purposes, the entire unit was considered an aquifer.
The Madison aquifer is most productive in the karstic upper part of the Madison Limestone. Fractures and solution openings in the limestone and dolomite increase the secondary permeability of the aquifer. The Englewood Formation, a limestone at the base of the Madison Limestone, is included in this unit because of the hydraulic connection between the two units and relatively minor thickness of the Englewood Formation in outcrop areas. The lower portion of the Madison Limestone and the Englewood Formation generally may be confining zones.
The Ordovician-sequence semiconfining unit consists of the Whitewood Formation (Red River Formation) and Winnipeg Formation. The Whitewood Formation, where present, may contain a local aquifer, but seldom is used because of more reliable sources in the adjacent Madison or Deadwood aquifers. The shale layers of the Winnipeg Formation, where present, are confining zones in the unit. The Deadwood Formation, which consists of sandstone, limestone, basal conglomerate, and shale, is fractured in areas intruded by igneous sills and is missing in areas with massive igneous intrusions. Thus, the Deadwood aquifer is discontinuous in parts of the northern Black Hills where the maximum thicknesses also occur. The unit thins to the south, which limits its use in the southern Black Hills.
Igneous and metamorphic rocks of Precambrian age are exposed throughout the central Black Hills and consist of the Harney Peak Granite, other igneous rocks, and a variety of metamorphosed igneous and sedimentary rocks. These rocks are structurally complex and have a large variety of hydraulic properties. The Precambrian rocks constitute a lower confining unit to overlying sedimentary aquifers. However, fracturing and faulting has increased porosity and permeability in many outcrop areas. Numerous residents in the central Black Hills rely on these rocks for their water supply.
The six generalized sections illustrate the thicknesses and structural configuration of the hydrogeologic units in the Black Hills. Section A-A' was selected to show the northern flank of the Black Hills. Section B-B' extends across the northern portion of the Black Hills and illustrates the relations of hydrogeologic units in the Spearfish area, across the Precambrian igneous and metamorphic units and the Vanocker laccolith, and along the eastern flank of the Black Hills. Section C-C' was selected to show the relation of the hydrogeologic units in the Boxelder Creek drainage basin. Section D-D' extends from the western flank of the Black Hills, locally known as the Limestone Plateau, across the Precambrian igneous and metamorphic units, to the eastern flank of the Black Hills. Section E-E' extends from the southwest portion of the Black Hills area, across the Precambrian igneous and metamorphic units, to the eastern flank of the Black Hills. Section F-F' extends southward from the Precambrian igneous and metamorphic units in the central Black Hills to the southern flank of the Black Hills. These cross sections were constructed from published map information and well logs. Data to delineate the configuration of the Precambrian units, Tertiary intrusive units, White River aquifer, and unconsolidated units generally are very limited. These sections are provided mainly to illustrate the approximate conditions at depth. They should not be used to predict depth to aquifers at specific locations.
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