Geologic Formations and Their Water-bearing Properties
Permian SystemRocks of Permian age are not exposed in Scott County, but they are found beneath younger rocks in all parts of the county. They are encountered at depths ranging from about 1,100 feet in the southern part to about 1,400 feet in the northern part. Data concerning these rocks are obtained from the logs of 23 oil and gas tests that have been drilled in Scott County.
The upper part of the Permian System (Guadalupian-Leonardian Series) is chiefly of nonmarine origin and is composed of red siltstone, shale, and sandstone with lesser amounts of salt, gypsum, anhydrite, limestone, and dolomite. This upper part, which is characterized by a predominance of red beds, ranges in thickness from about 1,200 feet in the northern part of Scott County to about 1,600 feet in the southern part. The lower part, or Wolfcampian Series, of the Permian System is largely of marine origin and is composed chiefly of limestone, dolomite, and shale, but also contains some sandstone. The thickness of the Wolfcampian Series ranges from about 600 feet in the southern part of the county to about 700 feet in the northern part.
The Permian strata underlying Scott County includes representatives of all the formations recognized by the State Geological Survey of Kansas from the Towle shale to the Taloga formation.
Because of their great depth and because adequate supplies of water can be obtained from the overlying rocks, the Permian rocks have not been utilized as a source of water in Scott County. Furthermore, it is believed that the water from Permian rocks would be too highly mineralized for most uses.
Cretaceous SystemRocks belonging to the Niobrara formation of Cretaceous age are exposed at the surface in Scott County. The outcrops of these rocks are shown on Plate 1. Cretaceous rocks older than the Niobrara formation are not exposed in this area and, therefore, are known only from subsurface data. They include representatives of the Carlile shale, Greenhorn limestone, Graneros shale, and the Dakota-Kiowa-Cheyenne sequence. The Niobrara formation furnishes water to wells in parts of the county, but because of their greater depth and because adequate supplies of water can generally be found above them, the rocks lying below the Niobrara formation have not been utilized as a source of water.
The following discussion of the pre-Niobrara Cretaceous rocks of Scott County is based on the logs of oil wells drilled in the Shallow Water pool and on published reports (Elias, 1931, pp. 26-43; Landes and Keroher, 1939, pp. 10-16, 22-24; Latta, 1944, pp. 141-160; McLaughlin, 1943, pp. 116-136) that describe the geology of adjacent or nearby areas.
The oldest Cretaceous rocks present in Scott County belong to the Cheyenne-Dakota sequence or what was formerly called the Dakota group. They unconformably overlie the Permian rocks and consist of light- to dark-colored shale and sandy shale, variegated clay, and light-colored, fine- to coarse-grained sandstone. They are between 400 and 500 feet thick in this area. No attempt has been made here to differentiate these deposits into smaller units. In central and south-central Kansas they have been subdivided into three formations, which are, in ascending order, the Cheyenne sandstone, Kiowa shale, and Dakota formation.
The Cheyenne-Dakota sequence is overlain by the Graneros shale, which consists of gray noncalcareous shale and thin beds or lenses of sandstone and sandy shale. In Hamilton County, where the Graneros shale is exposed, it contains a thin bed of bentonitic clay and thin-bedded fossiliferous limestone (McLaughlin, 1943, p. 126). The thickness of the Graneros shale in Scott County is not known. It is a few feet to about 43 feet thick in Ford County (Waite, 1942, p. 148), 61 feet thick in Hamilton County (McLaughlin, 1943, p. 128), and 90 feet thick in western Logan County (Landes and Keroher, 1939, p. 24).
Approximately 100 feet of interbedded light-gray to dark-gray chalky limestone and calcareous shale of the Greenhorn limestone occurs above the Graneros shale.
The Greenhorn limestone is overlain by the Carlile shale, the lower part of which consists of chalky shale containing thin beds of chalk and the upper part consists chiefly of dark noncalcareous fissile shale. The thickness of the Carlile in this area is approximately 200 feet. The Carlile shale is composed of three members, which are, from oldest to youngest, the Fairport chalky shale member, the Blue Hill shale member and the Codell sandstone member. An oil well drilled in the SE 1/4 sec. 15, T. 20 S., R. 33 W., encountered about 10 feet of medium-gray soft sandy shale at the top of the Carlile shale that probably represents the Codell sandstone member in this area. It was not possible to recognize the other members of the Carlile in this area.
Character--The Niobrara formation is divided into two members--The Fort Hays limestone member below and the Smoky Hill chalk member above.
The Fort Hays limestone member does not crop out in Scott County, but was encountered in test hole 23 near the southwest corner of the county. The Fort Hays is composed of thick massive beds of chalky limestone and chalk separated by thin beds of chalky shale. The limestone and chalk beds range in thickness from less than 1 foot to several feet and are light to dark gray where unweathered. Weathered exposures of the Fort Hays limestone member in areas adjacent to Scott County are white, tan, or cream. The contained fossils include Inoceramus deformis, Ostrea congesta, and abundant foraminifera (Moss, 1932, p. 21).
The Fort Hays limestone member is conformably overlain by the Smoky Hill chalk member of the Niobrara formation. The Smoky Hill chalk member crops out in the northern and southeastern parts of Scott County, in the western part of the county in a small area on the south side of Rocky Draw about 512 miles south of Modoc, and was encountered below younger rocks in all of the test holes. It consists of alternating beds of soft chalky shale and chalk. Where unweathered the beds are light to dark gray or gray brown, but on weathered exposures they are white, tan, buff, or yellowish pink. It is difficult to differentiate the individual beds in the subsurface. The slight differences in the composition of different beds are brought out on weathered exposures through differential erosion so that even the thinnest beds stand out. The member also contains thin beds of bentonite and pyrite concretions. The bentonite beds are light colored when unweathered, but weather to a rusty brown.
Both vertebrate and invertebrate fossils occur in the Smoky Hill chalk. The vertebrates include birds, dinosaurs, crocodiles, mosasaurs, turtles, and fish. Inoceramus grandis and Ostrea congesta are most abundant among the larger invertebrates (Moss, 1932, p. 19). Foraminifera, chiefly Globigerina and Gumbelina, are abundant in the chalky beds, and, according to Moss (1932, p. 19), probably make up more than half of the calcareous material of the chalk.
Some of the most striking features of the Smoky Hill chalk are the tilted fault blocks that occur in the northern part of the county where the chalk lies at the surface. With a few minor exceptions, the faults are all normal faults, and are commonly marked by a slickensided calcareous gouge (Pls. 4A and 14). Russell (1929, pp. 594-604) described the stratigraphy and structure of the Smoky Hill chalk member in western Kansas and concluded that the faulting probably occurred during the Tertiary and was presumably produced by the adjustment of the brittle chalk to deformation and not to compaction.
Plate 4A--Normal fault in Smoky Hill chalk member of the Niobrara formation in the NE NE sec. 1, T. 16 S., R. 32 W. Approximate displacement 20 feet.
Plate 14--A, Small normal fault in the Smoky Hill chalk member of the Niobrara formation. Note crystalline calcite filling the fault fissure and curvature of the dark band near the fault plane as a result of drag. Displacement is 2 feet. SE NE sec. 1, T. 16 S., R. 32 W. B, Series of small step faults in the Smoky Hill chalk member. Same exposure as shown in above closeup view.
Distribution and thickness--Only the Smoky Hill chalk member of the Niobrara formation is exposed in Scott County. It is best exposed in the northern and northeastern parts of the county where streams tributary to Smoky Hill River have cut down through the plains surface into the underlying Smoky Hill chalk (Pls. 1 and 4B). Exposures of this member in the southeastern part of the county occur in short draws west and northwest of Dry Lake. Both members of the Niobrara formation are present beneath younger sediments everywhere in Scott County.
The thickness of the Niobrara formation in western Logan County, Kansas, where it is overlain conformably by the Pierre shale, is approximately 600 feet. Pre-Ogallala erosion, however, has truncated the Niobrara to the south and east so that the maximum remaining thickness of the formation in Scott County is much less. The thickness of the formation in north-central Finney County is only about 300 feet (Latta, 1944, p. 159). Only the lower part of the Smoky Hill chalk member is present in Scott County. The total thickness of the Fort Hays limestone member is present beneath the Smoky Hill chalk member in this area. In the southwestern corner of the county the Fort Hays member is 55 feet thick (log 23).
Water supply--The Niobrara is relatively unimportant as a water-bearing formation in Scott County. The Fort Hays limestone member does not supply water to any wells in this area. The beds of chalky shale and chalk that make up the Smoky Hill chalk member are relatively impervious and transmit water chiefly through fractures and solution channels. Several of the recorded wells (Nos. 154-156, 160, 162, and 166) in the southeastern part of Scott County tap the Smoky Hill chalk member. In this part of the county the overlying Pliocene and Pleistocene sands and gravels are above the water table and, therefore, are barren of water. Of the wells that are known to derive their entire supply from the Smoky Hill chalk member only well 166 was in use at the time the wells were visited in 1940.
An analysis of a sample of water collected from well 166 indicates that the water is similar to many from the overlying Pliocene and Pleistocene deposits. The water from this well had 264 parts per million of total solids, 240 parts of hardness, and 1.4 parts of iron.
Kansas Geological Survey, Scott County Geohydrology|
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Web version March 2003. Original publication date July 1947.