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Location and Geohydrologic Setting
[Note: The classification and nomenclature of rock units used in this report are those of the Kansas Geological Survey and differ somewhat from those used by the U.S. Geological Survey.]
Cedar Bluff Irrigation District No. 6 is north of the Smoky Hill River and east of Cedar Bluff Dam, which is about 22 miles (35 km) southwest of Hays in west-central Kansas. About 80 percent of the irrigable area in the district is on a high terrace at an average elevation of about 80 feet (24 m) above the river. In the study area, the terrace ranges from 1 to 2 miles (1.6 to 3.2 km) in width, and slopes gently to the east and south. Most of the remainder of the district is within the flood plain of the Smoky Hill River or on low discontinuous terraces adjacent to it.
The principal natural drains are stream valleys that transect the terrace at intervals of 1 to 1.5 miles (1.6 to 2.4 km). Most of the streams are intermittent, but some streams have become perennial since irrigation began. Small undrained depressions are common. The permeability of the unconsolidated sediments over most of the area is adequate to permit infiltration and deep percolation of rainfall and irrigation water.
Relatively impermeable limestone and shale form the irregular sides and bottom of the container in which unconsolidated sediments (collectively termed valley-fill deposits) were laid down by the Smoky Hill River and its tributaries. The older valley-fill deposits, mapped as undifferentiated Pleistocene, underlie the prominent terraces on which most of the irrigated acreage is located. Younger valley-fill deposits, mapped as Wisconsinan deposits and Recent alluvium, underlie the low terraces and the flood plain of the river. Variation in texture, composition, and distribution of the sediments and the configuration of the bedrock surface affect the rate and direction of flow as well as the quality of water passing through the unconsolidated sediments.
The generalized surficial geology of the area, the general configuration of the bedrock surface, and the lithologic sections, shown on plate 2, are based on information from published reports, on unpublished logs of wells, maps, and reports by the U.S. Bureau of Reclamation, and on data from wells and test holes augered during the present investigation. Land-surface elevations were obtained from detailed topographic maps of the area at a scale 1 inch (2.5 cm) equals 400 feet (120 m) made by the Bureau of Reclamation before the district was developed and from traverses made to locate wells installed during this investigation.
Differences in the quality of lithologic samples from jetted, augered, and drilled holes, differing descriptions of the material penetrated by original and replacement wells at the same site, and differences in textural classification between drillers' logs and mechanical analyses of samples were commonly found during analysis of the data. Information from closely spaced wells augered in experimental plots indicates that the lithologic characteristics of the unconsolidated sediments are quite variable and that relief on the bedrock surface is greater than shown; therefore the geologic picture presented herein probably is oversimplified. Resolution of the minor discrepancies in the presentation of geologic and hydrologic data taken from other sources was beyond the scope of this project, and of little significance in the ultimate conclusions.
Undifferentiated Upper Cretaceous Rocks
The consolidated rocks that underlie the valley and crop out on the uplands are of Cretaceous age including, in ascending order, the Greenhorn Limestone, the Carlile Shale, and the Niobrara Chalk. Individual units have not been differentiated on the geologic map. The rocks are generally impervious and do not yield significant amounts of water to wells except where highly weathered. The Greenhorn and Carlile function chiefly as a lower boundary for ground-water circulation; however, the formations contain soluble minerals that are a source of ions in the ground water.
Only the upper part of the Greenhorn Limestone, which consists of alternating beds of chalky limestone and shale, crops out and underlies part of the irrigated area. The Greenhorn is a source of calcium and carbonate for ground water in the overlying unconsolidated sediments.
The Carlile Shale, which underlies most of the irrigated area, is about 300 feet (91 m) thick. The lower part (Fairport Chalk Member) consists of alternating beds of chalky limestone and shale similar to the underlying Greenhorn Limestone. The upper part (Blue Hill Shale Member) is about 175 feet (53 m) thick. It consists of blue-gray shale containing crystals of selenite (CaSO4 · 2H2O); plates, layers, and flakes of iron oxides; and large calcareous concretions. The sulfate in the selenite may have been derived from oxidation of pyrite. Leachate (distilled water) from a sample of the shale was an acid water that contained more than 1,100 mg/l (milligrams per liter) of sulfate, nearly 300 mg/l of calcium, and 4 mg/l of chloride. The pH of the solution was 1.0. Relatively high concentrations in the sample of the nutrients; potassium (44 mg/l), nitrate (104 mg/l), and phosphate (22 mg/l) were found.
Deep weathering of the Blue Hill Shale Member of the Carlile Shale to residual clay precluded precise definition of the bedrock surface in some wells. However, the effect of the relatively impermeable buff to yellow clay on the movement of ground water is probably similar to the effect of the unaltered shale.
The Fort Hays Limestone Member of the Niobrara Chalk consists of massive chalk beds separated by thin chalky shale partings that locally yield small amounts of water to wells in the uplands north and south of the study area. Other than contributing calcium carbonate to upland runoff carried by the natural drains that traverse the area, the Fort Hays Limestone has little effect on water quality in the study area. Dissolution of fragments of the Cretaceous limestones in some stream deposits maintains high concentrations of calcium and bicarbonate in the included waters.
Undifferentiated Pleistocene Deposits
Deposits of Pleistocene age in high terrace positions fill and overlap channels eroded in a pediment on the Cretaceous bedrock. Previous investigators subdivided the Pleistocene deposits into five formations. In ascending order, these are the Grand Island, Sappa, Crete, Loveland, and Peoria Formations. The Grand Island and Crete Formations consist chiefly of arkosic sand and gravel. They are separated by the Sappa Formation, which consists chiefly of silt and sandy clay. The Grand Island and Sappa are reportedly (Leonard and Berry, 1961; Hodson, 1965) restricted to a narrow meandering ancestral channel of the Smoky Hill River north of the present channel (See section A-A', plate 2). The available subsurface information is inadequate to define the limits of the channel, if indeed a single channel exists.
The high terraces are mapped herein as undifferentiated Pleistocene deposits on the geologic map. These deposits generally coincide with the Crete and Loveland Formations mapped by Leonard and Berry (1961). In Trego County, the boundaries are nearly similar to those mapped by Hodson (1965) as terrace deposits of the Grand Island, Sappa, and Crete Formations and loess classified as the Loveland and Peoria Formations. Most of the sediment described as terrace deposits by Hodson would be included in the Crete Formation, which apparently grade upward into yellow and buff silt and sandy silt classified as Loveland by Leonard and Berry (1961). Part of the loess was deposited in or reworked by moving water. Where the terraces have not been dissected, loess and colluvial deposits obscure the upper contact with the bedrock, which is located mainly on a topographic discontinuity.
Soils developed on the silt (either Crete or Loveland) are mainly silty clay with high water-holding capacity and adequate internal drainage for good yields under irrigation. The loess deposits normally are calcareous and contain lenses and nodules of caliche (calcium carbonate), which is readily dissolved by water containing carbon dioxide to yield calcium and bicarbonate ions.
Wide lateral and vertical variability in texture and composition characterize the fluvial deposits; however, the sediments underlying the high terraces consist basically of a section of sand or sand and gravel overlain by silt and clay. Lenses of silt and clay in the coarse-grained section and of sand and gravel in the overlying section are common. In the eastern part of the area, the lower coarse-grained section contains less gravel. Most of the section probably represents the Crete and Loveland Formations as described above, but data generally are inadequate to justify placement of the formational contacts.
Relief on the bedrock surface that affected the nature and distribution of the sediments at the time of deposition, now affects to a large extent the thickness of the saturated section, the direction of movement of ground water, and the points of maximum discharge into the Smoky Hill River. Ground-water movement in the reservoir beneath the terrace, as shown in plate 1, is southward where surface or subsurface channels drain water toward the river. Where bedrock ridges retard flow toward the river, ground water moves eastward in channels beneath the terrace. Additional drilling to define the configuration of the aquifers and pumping tests to evaluate their hydraulic properties is necessary for precise mathematical modeling of the system; but collection of the data was beyond the scope of the investigation.
Recent Alluvium and Wisconsinan Deposits
As illustrated in plate 2, the present channel of the Smoky Hill River occupies only a small part of the valley eroded in the bedrock during late Pleistocene time. There is little or no surface expression of the ancestral. channel beneath the modern flood plain and low terraces. However, the ancestral and modern channels generally are parallel. Limited test drilling adjacent to the river penetrated about 60 feet (18 m) of coarse sand and gravel that generally is overlain by 10 to 20 feet (3 to 6 m) of gray silt or sandy silt. Lesser thicknesses of generally fine-grained alluvium fills the channels of tributary streams that traverse the terrace.
Most of the alluvium is lithologically similar to the fluvial deposits from which it was largely derived. Some of the coarse-grained sediment classified as early Pleistocene in test holes north of the flood plain may be younger alluvium that fills deeply incised buried tributaries. Abundant fragments of limestone and shale in the streambed of the Smoky Hill River attest to relatively recent and continuing erosion of the nearby Cretaceous bedrock. As illustrated by the map showing the general configuration of the water table (pl. 1), virtually all excess water from the irrigation district will pass through or over these generally permeable deposits. Several irrigation wells installed in the alluvium between the river and the irrigated land since 1965 are capable of sustained yields of from 800 to 1,000 gpm (50 to 63 l/s).
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Kansas Geological Survey, Geohydrology
Placed on web Nov. 2012; originally published 1975.
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