Parent material is the unconsolidated mass from which soil forms. Most soils in Dewey County formed in material weathered from three major geological formations: the Hell Creek Formation, the Fox Hills Formation, and the Pierre Shale Formation*. Small areas of soils formed in alluvium of Recent age and in loess.
The youngest of the three major formations is the strongly alkaline Hell Creek Formation. It is a dull gray and brown clay shale interbedded with gray sandstone and thin bands of lignite coal. The material weathered from this formation contains sodium and other salts. Many soils that formed in material from this formation have a claypan subsoil because of the sodium content in the parent material. Absher, Belfield, Dalgum, Ekalaka, and Rhoades soils are the principal soils formed in material weathered from the Hell Creek Formation.
The Fox Hills Formation is below the Hell Creek Formation and above the Pierre Shale Formation. It is olive-gray to brown sand and sandstone interbedded with silty to clayey shale. Flasher, Glenross, Reeder, and Vebar soils formed in the material weathered from the more sandy members of the Fox Hills Formation. Cabba, Lantry, and Morton soils formed in material weathered from the more silty members of this formation. Regent and Ridgeview soils formed in material from silty to clayey shales in the lower part of the Fox Hills Formation.
Pierre Shale* is the most extensive surface bedrock in the county. Four members of this formation are exposed in Dewey County (4). The uppermost member is the Elk Butte, which is underlain by the Mobridge, Virgin Creek, and Verendrye in that order. The Elk Butte is noncalcareous, light olive-gray shale and claystone. The Mobridge is a calcareous gray shale that weathers to light yellowish-brown or buff colors. The Virgin Creek is a noncalcareous or only slightly calcareous, olive-gray claystone. Soils formed in clays weeathered from the Pierre Shale Formation are those of the Chantier, Dupree, Hurley, Opal, Promise, Sansarc, and Swanboy series. Dupree soil is only in areas associated with the noncalcareous beds of the Elk Butte, Virgin Creek, and Verendrye Members of the Pierre Formation.
Soils formed in alluvium are those of the Banks, Canning, Farland, Havrelon, Lohler, Shambo, Tally, and Trembles series. Canning and Farland soils typically are on the higher and older terraces. Banks, Havrelon, Lohler, and Trembles soils, which formed in alluvium of Recent age, are on bottom land or low terraces.
Some terraces in the eastern part of the country are thinly mantled with silty loess. Agar, Lowry, and Reliance soils form in this silty material.
No glacial deposits in the county are parent materials of any soil, but scattered boulders of glacial origin are on the surface in the eastern part of the county.
Idealized Cross-Section of South Dakota
LINK TO GEOLOGY OF SOUTH DAKOTA
Northern Great Plains Aquifer System
Tatanka Wakpala: Lower Cretaceous, Upper Paleozoic, Lower Paleozoic
Lithology: Pierre Shale, Cretaceous and Paleozoic aquifers
LINK TO GROUNDWATER ATLAS OF UNITED STATES: HA 730
The lower Cretaceous aquifers are separated from the overlying upper Cretaceous aquifers by several thick shales that form an effective confining unit. The Pierre Shale and its partial equivalents, the Lewis and the Steele Shales, are the thickest and most extensive formations of the confining unit.
The Pierre Shale is as much as 3,000 feet thick in places and is the principal confining unit in North Dakota and South Dakota. Although thin sandstones are interbedded with the shales of the confining unit, the sandstones yield little water. Locally, they provide sufficient water for domestic supplies, but the water generally is highly mineralized.
Isotopic dating of water in these aquifers near their easternmost extent in South Dakota indicates that water locally entered the aquifers during Pleistocene glaciation.
The general movement of water in the lower Cretaceous aquifers is northeastward from aquifer recharge areas at high altitudes to discharge areas in eastern North Dakota and South Dakota.
The lower Cretaceous aquifers are the most widespread aquifers in the Northern Great Plains aquifer system, but contain little freshwater. Water with dissolved-solids concentrations of less than 1,000 milligrams per liter (freshwater) is only in small areas in central and south-central Montana, and north and east of the Black Hills Uplift.
Even though the water in the lower Cretaceous aquifers is slightly saline throughout most of South Dakota and a large area in southeastern North Dakota, the aquifers are the principal source of water for livestock watering and domestic supply because they are the shallowest consolidated-rock aquifers in these areas.
Paleozoic aquifers in the Northern Great Plains aquifer system are divided into upper Paleozoic aquifers and lower Paleozoic aquifers. Although the Paleozoic aquifers extend over large areas…, they are deeply buried in most places and contain little freshwater. Few water wells penetrate these aquifers, and much of what is known about them has been determined from data gathered from test holes drilled for oil and gas exploration.
Movement of Upper Paleozoic aquifers
Northeast beneath Tatanka Wakpala area
LINK TO IMAGES OF REGIONAL AQUIFERS: HA 730-I
Groundwater quality from Lower Cretaceous aquifers
Tatanka Wakpala area: moderately saline water
The concentration of dissolved solids in ground water provides a basis for categorizing the general chemical quality of the water. Dissolved solids in ground water primarily result from chemical interaction between the water and the rocks or the unconsolidated deposits through which the water moves.
Dissolved-solids concentrations in ground water generally are small in aquifer recharge areas and increase as the water moves downward into the deeper parts of the aquifers. Where the aquifers are deeply buried or where ground water is discharged at the end of long flow paths, dissolved-solids concentrations commonly exceed 3,000 milligrams per liter.
In some places, saline water or brine in deep aquifers leaks upward through confining units and contaminates fresher water in shallower aquifers or in surface-water bodies.
Total groundwater withdrawals per day (2005)
Dewey County: 0.47 million gallons per day
LINK TO ESTIMATED USE OF WATER IN SOUTH DAKOTA, 2005
Hydrologic Subbasins (surface water)
LINK TO SD SUBBASION INTERACTIVE MAP
Missouri River Basin→
Lower Lake Oahe (code: 10130105)
Surface watershed area near Tatanka Wakpala
LINK TO SURFACE WATERSHEDS IN DEWEY COUNTY