The Oldest Stratigraphic Units in The
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The oldest stratigraphic units in the study area the Devonian- and Mississippian-age Englewood For- are the Precambrian igneous and metamorphic rocks mation because of the absence of the Ordovician (fig. 9), which underlie the Paleozoic, Mesozoic, and sequence. The Englewood Formation is overlain by the Cenozoic rocks and sediments. These Precambrian Madison Limestone. rocks range in age from 1.7 to about 2.5 billion years The Mississippian-age Madison Limestone is a and were eroded to a gentle undulating plain at the massive, gray to buff limestone that is locally dolomitic beginning of the Paleozoic Era (Gries, 1996). The Pre- (Strobel and others, 1999). The Madison Limestone, cambrian rocks are highly variable in composition and which was deposited as a marine carbonate, was are composed mostly of metasediments, such as schists exposed at land surface for approximately 50 million and graywackes. The Paleozoic and Mesozoic rocks years. During this period, significant erosion, soil were deposited on the Precambrian rocks as nearly hor- development, and karstification occurred (Gries, 1996). izontal beds. Subsequent uplift during the Laramide There are numerous caves and fractures within the orogeny and related erosion exposed the Precambrian upper part of the formation (Peter, 1985). The thickness rocks in the central core of the Black Hills, with many of the Madison Limestone increases from south to of the Paleozoic and Mesozoic sedimentary rocks north in the study area and ranges from almost zero in exposed in roughly concentric rings around the core. the southeast corner of the study area (Rahn, 1985) to The exposed Precambrian rocks commonly are referred 1,000 ft east of Belle Fourche (Carter and Redden, to as the crystalline core. 1999d). Because the Madison Limestone was exposed The layered series of sedimentary rocks sur- to erosion and karstification for millions of years, its rounding the crystalline core includes outcrops of the contact with the overlying Minnelusa Formation is Madison Limestone (also locally known as the unconformable. Pahasapa Limestone) and the Minnelusa Formation. The Pennsylvanian- and Permian-age Minnelusa The bedrock sedimentary formations typically dip away from the uplifted Black Hills (fig. 15) at angles Formation consists mostly of yellow to red cross- that can approach or exceed 15 to 20 degrees near the stratified sandstone, limestone, dolomite, and shale outcrops, and decrease with distance from the uplift to (Strobel and others, 1999). In addition to sandstone and less than 1 degree (Carter and Redden, 1999a, 1999b, dolomite, the middle part of the formation consists of 1999c, 1999d, 1999e). Following are descriptions for shale and anhydrite (DeWitt and others, 1986). The the bedrock formations that contain major aquifers in upper part of the Minnelusa Formation also may con- the Black Hills area. tain anhydrite, which generally has been removed by The oldest sedimentary unit in the study area is dissolution in or near the outcrop areas, occasionally the Cambrian- and Ordovician-age Deadwood Forma- forming collapse features filled with breccia (Brad- tion, which is composed primarily of brown to light- dock, 1963). The Minnelusa Formation was deposited gray glauconitic sandstone, shale, limestone, and local in a coastal environment, and dune structures at the top basal conglomerate (Strobel and others, 1999). These of the formation may represent beach sediments (Gries, sediments were deposited on top of a generally hori- 1996). The thickness of the Minnelusa Formation zontal plain of Precambrian rocks in a coastal- to near- increases from north to south and ranges from 375 ft shore environment (Gries, 1975). The thickness of the near Belle Fourche to 1,175 ft near Edgemont in the Deadwood Formation increases from south to north in study area (Carter and Redden, 1999c). In the north- the study area and ranges from 0 to 500 ft (Carter and eastern part of the central Black Hills, little anhydrite Redden, 1999e). In the northern and central Black occurs in the subsurface due to a change in the deposi- Hills, the Deadwood Formation is disconformably tional environment. On the south and southwest side of overlain by Ordovician rocks, which include the the study area, the thickness of clastic units increases Whitewood and Winnipeg Formations. The Winnipeg and a thick section of anhydrite occurs. In the southern Formation is absent in the southern Black Hills, and the Black Hills, the upper part of the Minnelusa Formation Whitewood Formation has eroded to the south and is thins due to leaching of anhydrite. The Minnelusa not present south of the approximate latitude of Nemo Formation is disconformably overlain by the Permian- (DeWitt and others, 1986). In the southern Black Hills, age Opeche Shale, which is overlain by the Minnekahta the Deadwood Formation is unconformably overlain by Limestone. 18 Hydrology of the Black Hills Area, South Dakota WYOMING SOUTH DAKOTA South Fork Castle Creek South Fork A LIMESTONE PLATEAU A' FEET Castle Creek South Fork FEET PPm OCd 7,000 7,000 MDme 6,000 6,000 Rapid Creek MDme Rapid Creek 5,000 Rapid Creek 5,000 Rapid Creek OCd Kik PPm Pmk Rapid City 4,000 Po Ju Rapid Creek QTac QTac 4,000 3,000 pCu 3,000 pCu Kps 2,000 2,000 1,000 1,000 SEA TR Ps SEA LEVEL VERTICAL EXAGGERATION X5 0 24 6 8 10 MILES LEVEL 0 281046 KILOMETERS Figure 15. Geologic cross section A-A′ (modified from Strobel and others, 1999). Location of section is shown in figure 14. Abbreviations for stratigraphic intervals are explained in figure 9. Geologic Framework 19 The Permian-age Minnekahta Limestone is a Paleozoic aquifers occurs in high-altitude outcrop areas fine-grained, purple to gray laminated limestone around the major uplifts such as the Black Hills uplift (Strobel and others, 1999), which ranges in thickness (fig. 17). from 25 to 65 ft in the study area. The Minnekahta The Cambrian-Ordovician (or Deadwood) Limestone is overlain by the Triassic- and Permian-age aquifer is contained within the sandstones of Cambrian Spearfish Formation. age (Deadwood Formation and equivalents) and lime- The Cretaceous-age Inyan Kara Group consists stones of Ordovician age (Red River Formation and of the Lakota Formation and overlying Fall River equivalents) (fig. 12). Generally, flow in the Cambrian- Formation. The Lakota Formation consists of the Ordovician aquifer is from the high-altitude recharge Chilson, Minnewaste Limestone, and Fuson Shale areas to the northeast. Discharge (fig. 17) from the members. The Lakota Formation consists of yellow, Cambrian-Ordovician aquifer is to adjacent aquifers, brown, and reddish-brown massive to thinly bedded lakes and springs in eastern North Dakota, and springs sandstone, pebble conglomerate, siltstone, and clay- and seeps where the aquifer crops out in Canada stone of fluvial origin (Gott and others, 1974); locally (Downey, 1984). Within the Great Plains region, the there are lenses of limestone and coal. The Fall River Cambrian-Ordovician aquifer contains fresh water Formation is a brown to reddish-brown, fine-grained (dissolved solids concentrations less than 1,000 mg/L sandstone, thin bedded at the top and massive at the (milligrams per liter)) only in an area surrounding the bottom (Strobel and others, 1999). The thickness of the Black Hills and in a small area in north-central Wyo- Inyan Kara Group ranges from 135 to 900 ft in the study area (Carter and Redden, 1999a). ming (Whitehead, 1996). The aquifer is a brine (dis- solved solids concentration greater than 35,000 mg/L) in eastern Montana and western and central North Ground-Water Framework Dakota (Whitehead, 1996). The Mississippian (or Madison) aquifer is con- The hydrogeologic setting of the Black Hills area tained within the limestones, siltstones, sandstones, is schematically illustrated in figure 16, and the areal and dolomite of the Madison Limestone or Group. distribution of the hydrogeologic units is shown in Generally, water in the Mississippian aquifer is con- figure 14. Four of the major aquifers in the Black Hills fined except in outcrop areas. Flow in the Mississippian area (Deadwood, Madison, Minnelusa, and Inyan Kara aquifer generally is from the recharge areas to the aquifers) are regionally extensive and are discussed in northeast. Discharge (fig. 17) from the Mississippian the following sections in the context of regional and aquifer occurs by upward leakage to the lower Creta- local hydrologic settings. A fifth major aquifer ceous aquifer in central South Dakota and eastern flow (Minnekahta aquifer) generally is used only locally, as to the Cambrian-Ordovician aquifer in eastern North are aquifers in the igneous and metamorphic rocks Dakota (Downey, 1984). Water in the Mississippian within the crystalline core area and in alluvium. In aquifer is fresh only in small areas near recharge areas some local areas, wells are completed in strata that and becomes saline to slightly saline as it moves down- generally are considered to be semiconfining and gradient. The water is a brine with dissolved solids con- confining units. centrations greater than 300,000 mg/L in the deep parts of the Williston Basin (Whitehead, 1996). Regional Aquifers The Pennsylvanian (or Minnelusa) aquifer is The major aquifers underlie parts of Montana, contained within the sandstones and limestones of the North Dakota, South Dakota, Wyoming, and Canada. Minnelusa Formation, Tensleep Sandstone, Amsden The parts of the regional aquifers in Canada are not Formation, and equivalents of Pennsylvanian age described or shown in this report. (fig. 12). Water in the Pennsylvanian aquifer moves The Paleozoic aquifers include the Cambrian- from recharge areas to the northeast to discharge areas Ordovician aquifer (Deadwood aquifer in the Black in eastern South Dakota (Downey and Dinwiddie, Hills), Mississippian aquifer (Madison aquifer in 1988). Some water discharges by upward leakage to the the Black Hills), and the Pennsylvanian aquifer lower Cretaceous aquifer (Swenson, 1968, Gott and (Minnelusa aquifer in the Black Hills).