Geologic Formations and their Water-bearing Properties
Paleozoic RocksPermian System--Lower Permian Series
The Permian rocks of Sedgwick County are a part of the Sumner Group of the Lower Permian Series. The rocks of the Sumner Group that crop out at the surface include parts of the Wellington Formation and the Ninnescah Shale. The Wellington and Ninnescah are covered by younger sediments in most of the County and surface exposures are scarce. However, they are easily recognized in the subsurface by their distinctive color, the Wellington being gray to grayish-green and the Ninnescah reddish-brown. The distribution of the Permian rocks in the County is shown on the geologic map (Pl. 1).
Character--The lithology of the Wellington Formation has been described by Bass (1929), Ver Wiebe (1937), Norton (1939), and Swineford (1955). The part of the Wellington that crops out in Sedgwick County consists mostly of calcareous gray and blue-gray shale containing several thin beds of impure limestone, and thin beds of gypsum and anhydrite. Some beds of maroon and gray-green shale occur near the top of the Formation. The gypsum beds in the Wellington are most common in the lower part and crop out at a few places east of the Arkansas Valley. Near the surface, solution of the gypsum has taken place, and the Wellington is characterized by local deformation that has resulted from differential compaction in solution areas. These small-scale structures are best exposed in cuts along the Kansas Turnpike northeast of Wichita. A thick salt bed containing some shale and anhydrite is present in the subsurface near the middle of the Wellington Formation. The salt has been removed by solution in the area now occupied by the Arkansas Valley and to the east. However, it underlies the area west of the valley and attains a thickness of about 350 feet near the west county line (Kulstad, 1959). The Milan Dolomite Member (Norton, 1937) forms the top of the Wellington Formation in southern Kansas, and it is exposed in a creek bank in the center of the west side sec. 9, T 28 S, R 3 W. At the stratigraphic position of the Milan the gray color of the Wellington gives way to the red-brown of the overlying Ninnescah Shale. The color change from gray to red is considered by most workers to mark the top of the Wellington Formation.
Distribution and thickness--The Wellington Formation forms the bedrock under about the eastern four-fifths of the County and underlies the Ninnescah Shale in the remainder of the County. Where the Wellington Formation is near the surface east of the Arkansas Valley, the relatively soft shale and evaporates yield easily to erosion, and as a result the topography is gently rolling and rounded. In most of the County younger sediments cover the Wellington and surface exposures are scarce. The Formation ranges in thickness from about 80 feet along the east county line to about 550 feet along the west county line where the full thickness of the salt bed is present.
Water Supply--The Wellington Formation is the only source of usable ground water in that part of the area where it is present and is not overlain by younger water-bearing beds. Most domestic and stock wells in the area east of the Arkansas Valley and many wells in the south-central part of the County derive water from the Formation. The quantity and quality of the water available differs with both the location and the method of construction of a well. In most cases the water is very hard or has other objectionable properties. Large-diameter dug wells finished in the weathered zone of the Wellington are common east of the Arkansas River. These wells are subject to failure during prolonged droughts, and when water is available from them, a high nitrate content in the water is commonly a problem. Where gypsum and anhydrite beds are present at shallow depths along parts of the east county line, solution has been active, and the Wellington contains water under artesian pressure. Moderately large well yields are obtainable from this zone (see wells 27-2E-13ccc 1, 2, 3, 4, Table 13), and a few large springs issue from it where exposed along small streams. The water from the evaporate zone is quite hard and has a high sulfate content, but it is used for domestic and stock purposes. The westward extent of the evaporite solution zone in the lower part ot the Wellington is not known. However, meager information obtained from interviews with local residents indicates that as the evaporates become more deeply buried to the west, solution has been less active, and enough salt is present in parts of the shale to make the water from this zone unusable.
West of the Arkansas Valley where thick salt beds near the middle of the Wellington are present in the subsurface, an active solution zone along the east margin of the salt contains saturated brine. The extent of the solution zone or the quantity of brine available are not known, but in the past the brine has been used by one company as a raw material for the manufacture of chemicals.
Character--The Ninnescah Shale was named by Norton (1939) from exposures on the North and South Forks of the Ninnescah River in Reno and Kingman counties. The formation conformably overlies the Wellington Formation and is composed of alternating beds of brownish-red silty shale and siltstone. A few thin beds of gray-green silty shale occur in the lower part. Some gypsum is present in the Ninnescah and occurs as thin intersecting veins that were deposited secondarily in the red and green shales.
Distribution and thickness--The Ninnescah Shale forms the bedrock under about the western one-fifth of the County. In a part of its outcrop area the Ninnescah is covered by younger sediments, but it is exposed in the valley walls of North Fork Ninnescah River and its tributaries. The formation ranges in thickness from a featheredge along the eastern edge of the outcrop area to about 175 feet along parts of the west county line.
Water supply--The Ninnescah Shale yields water to many stock and domestic wells in its outcrop area where it is not overlain by younger water-bearing beds. Large yields from wells are not known to be available from the formation. The water-bearing properties of the formation are not well understood, but most of its water is probably obtained from the weathered surface zone. There is some evidence, though unconfirmed, that solution openings or fractures containing water occur at depths of 100 feet or more in the Ninnescah. The water from the weathered part of the Ninnescah is of generally good chemical quality although it commonly has large concentrations of nitrate. Water from deeper parts of the formation, though usuable, usually contains a large concentration of dissolved solids with sulfate being the most objectionable constituent.
Kansas Geological Survey, Geohydrology of Sedgwick County|
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Web version April 1998. Original publication date Dec. 1965.