Water-bearing Formations, continued
Pliocene SeriesOgallala Formation
Character--The Ogallala formation is composed of structureless silt and fine sand, together with some coarse sand and gravel, and ranges in color from buff to gray to white. The coarser sediments are present at all horizons but are most prominent in the lower part of the formation, the part that yields water most freely to wells.
Sand constitutes the principal material of the Ogallala formation and ranges in texture from fine- to coarse-grained, some of the coarser material containing scattered pebbles and thin beds of pebbles. The pebbles and grains are composed chiefly of granite, but there are a few pebbles of chert, limestone, and basic igneous rock. Quartz and feldspar generally make up about 95 percent of the mineral content of the sediments.
The finer materials of the Ogallala are composed chiefly of silt and generally contain only minor amounts of clay. Lenses or beds of sandy silt occur in all parts of the formation but principally in the upper part. The color of the silt ranges from gray to buff to light tan.
The gravels of the Ogallala comprise mainly pebbles of limestone sandstone, and ironstone, but also contain pebbles of quartz and quartzite. The coarse gravel near the base of the Ogallala in the Arkansas valley in Ford County comprises mainly water-worn pellets of Greenhorn limestone but contains some pebbles of sandstone and ironstone that probably were derived from the Dakota formation. The occurrence of pebbles of this type indicates that locally at least some of the Ogallala sediments were derived from the erosion of Cretaceous bedrock in nearby areas. Smith (1940, pp. 42, 43) described two distinct facies of gravel in the Ogallala formation one composed principally of sandstone, ironstone and quartzite, and the other made up mainly of crystalline igneous and metamorphic rocks. The former facies occurs at the base of the formation and the latter is widespread along the Arkansas valley in outcrops above the base of the formation. The sandstone-ironstone-quartzite facies is composed of material similar to that found in the Dakota formation and other Cretaceous formations; whereas, the granite facies that occurs above the base of the formation is composed of granite, feldspar, quartzite, quartz, felsite and other crystalline igneous and metamorphic rocks.
The Ogallala formation is characterized by lenticular bedding; thus, individual beds of sand or gravel are not continuous over wide areas, but generally are discontinuous lenses that may grade laterally into finer materials such as silt or clay, in some places within relatively short distances. The deposits range from those that exhibit definite bedding to those that show no bedding whatever. The structureless layers are commonly found in the upper part of the formation, are rather fine-grained, and contain some silt and small amounts of clay and lime. Irregular limy concretions are abundant in these layers, typical exposures of which occur in the bluffs on the north side of U.S. Highway 154, about one-half mile east of Fort Dodge. The color of this structureless part of the Ogallala ranges from gray to buff to light tan.
In many places the deposits are consolidated by calcium carbonate, forming beds of caliche. In some places the cemented beds resemble true limestone; elsewhere they may consist of sand and pebbles imbedded in a lime matrix. The cemented beds of the Ogallala have long been called "mortar beds," and in many places the "mortar beds" are highly cross-bedded (pl. 15). Cemented beds occur in the Ogallala at many places, but are most common in the upper part of the formation.
The character of the Ogallala formation as revealed by drilling is illustrated by the logs of wells and test holes, pages 206 to 243. Although some of the individual beds have been described in the logs as clay, only minor amounts of clay are known to occur in the Ogallala formation. Some of the beds of silt are described by drillers as sand or clay or as combinations of both, and some beds of different texture are described simply as sand. The finer sands, sandy silts, and limy clays have been lumped together and called "magnesium and clay" by some drillers. The term "magnesia rock" appears frequently in drillers' logs and generally implies a hard, dense, lime-cemented bed. The terms "gyp," "gyp and sand," and "gyp rock" also have been used by some drillers in describing the more resistant lime-cemented beds encountered in the Ogallala. The upper unsaturated part of the formation consisting of fine sand, sandy silt, and some clay is described by some drillers as "dry sand" or as "fine packed sand." When coarser grains are imbedded in the matrix the material has been described by some as "coarse packed sand." The terms "sand rock" and "cemented sand" given in some of the drillers' logs refer to beds of sand and gravel that have been more or less cemented with calcium carbonate.
Subdivisions--In much of western Kansas the top of the Ogallala formation is formed by the top of the "Algal limestone," which marks the top of the middle Pliocene (Elms, 1931, pp. 136-143; Smith, 1940, pp. 44-46; and Frye and Hibbard, 1941, pp. 404, 405). In the Meade basin adjoining the southwestern corner of Ford County, however, beds that have yielded upper Pliocene fossils (Hibbard, 1938, p. 241) occur above the middle Pliocene beds. Smith (1940, p. 95) named these beds the Rexroad formation, from exposures on Rexroad ranch in south-central Meade County from which Hibbard collected the Rexroad fauna. Frye and Hibbard (1941, p. 407), however, have pointed out that in the central part of the Meade basin these beds conformably overlie middle Pliocene deposits with no apparent break, and generally are indistinguishable lithologically from the middle Pliocene deposits of that area. For these reasons they have classified the deposits as the Rexroad member of the Ogallala formation. Deposits of sand and gravel and interbedded silt and clay, encountered in test drilling in southern Ford County, at the northern end of the Meade basin, occur at the same stratigraphic position as the Rexroad member of Meade County and may be equivalent, in age. Fresh-water mollusks from some of these beds (described below) indicate, however, that some of the beds may be of Pleistocene age.
Distribution and thickness--The Ogallala formation is exposed at or near the surface over most of the county north of Arkansas River in Ford County, the top of the formation being essentially the surface of the High plains (pl. 1). No exposures have been noted south of Arkansas River where the Ogallala formation is mantled by the Kingsdown silt. In certain areas deposits of dune sand overlie both the Kingsdown and the Ogallala. In much of the Arkansas valley west of the city of Ford, the Ogallala formation is covered with alluvium. East of Ford the Ogallala has been largely removed in the Arkansas valley, and the alluvium rests unconformably on the Dakota formation. The Ogallala is thin or entirely absent in the north-central part of the county in places where dissecting streams such as Sawlog Creek and its tributaries have cut beneath the plains surface into the underlying Cretaceous bedrock.
The Ogallala formation ranges in thickness from a few feet to about 250 feet, and its maximum thickness is probably attained under the uplands north and west of Dodge City. The maximum thickness of the formation was not encountered in any of the test holes but is inferred from the geologic cross-sections (pl. 15). The thickness of the Ogallala in the 21 test holes drilled during the investigation ranged from about 22 feet in test hole 1 in the northeastern part of the county to 150 feet in test hole 6 in the Arkansas valley 1 mile west of Dodge City. The test drilling indicated that the Ogallala becomes progressively thinner toward the east and northeast.
Origin--As pointed out under Geologic history the sediments comprising the Ogallala formation were deposited by heavily-laden streams that flowed from the Rocky Mountain region. The pebbles of igneous and metamorphic rocks in the gravels and abundance of quartz and feldspar in the sands are believed to have been derived from the Rocky Mountains. Locally, the basal gravels also contain some reworked material from less distant sources, including water-worn fragments of Greenhorn limestone and pebbles of sandstone and ironstone from the Dakota formation. Much of this material probably was derived from Cretaceous rocks in or just west of the county.
The origin of the abundant calcium carbonate in the Ogallala has been discussed by Smith (1940, p. 79) who concluded that-
"the transported calcareous matter in the Ogallala originated mainly, if not only, in the Rocky Mountain area from weathering of Paleozoic limestone and calcic minerals in the crystalline rocks. ... Additional lime may have been provided also by weathering in situ after deposition."Smith also suggested that the silt and clay of the Ogallala probably were derived from soils and weathering products in the mountain area and to a lesser extent from the wearing down of coarser materials in transit. The coarser beds of sand and gravel represent channel deposits and the finer materials represent floodwater deposits formed by the overflow of shallow channels that perhaps approached the character of sheet-floods locally.
The sandstone and conglomerate in the Ogallala represent beds of sand and gravel that have been cemented by underground waters. Deposits of sandy silt, cemented with calcium carbonate, and often referred to as caliche, probably are a product of surficial calichification formed during a relatively long pause in deposition, at a time when streams had shifted to some other part of the region. The concentration of calcium carbonate, in the soil zone, by surficial processes was accomplished during such periods, and was halted by recurring periods of deposition to give rise to caliche zones at varying 4 intervals throughout the formation.
There are at least 3 different hypotheses to account for the origin of the capping limestone at the top of the Ogallala. Elias (1931, p. 41) suggested a lacustrine origin and stated that the limestone was deposited on the nearly flat bottom of a very large and very shallow lake at the close of Ogallala time.
Gould and Lonsdale (1926, pp. 29-33) advanced the caliche hypothesis for the capping limestone in the Oklahoma area. Smith (1940, pp. 91, 92) suggested that:
"Possibly both hypotheses are partly right, and both caliche and lacustrine limestone are present in different parts of the region, possibly gradational into one another."Theis (1936) suggested that the capping limestone was deposited, either by inorganic or by organic agencies, in unconnected pools that resulted from the flooding of shallow depressions by a rising water table under conditions of a cooling climate and probably increasing precipitation associated with the approach of Pleistocene time. For a much more detailed discussion of the origin of the Ogallala formation the reader is referred to the report by Smith (1940, pp. 77-94).
Age and correlation--The Ogallala formation originally was named and described by Darton (1899, pp. 732, 734) and its age was given as hate Tertiary, or Pliocene (?). Darton later (1920, p. 6) designated the type locality near Ogallala station in western Nebraska. The conclusions of later workers regarding the age and correlation of the Ogallala formation in western Kansas have been summarized by Smith (1940, pp. 73-74).
The Ogallala formation is classified as Pliocene by the United States Geological Survey. Smith (1940, pp. 75, 76) concluded that the Ogallala of southwestern Kansas, insofar as it is represented by exposures at the surface, may be assigned to middle Pliocene age. According to this definition, the top of the Ogallala formation is marked by the top of the capping limestone. Smith considered the overlying beds that have yielded upper Pliocene fossils as a separate formation, which he called the Rexroad formation (see p. 95). For reasons stated earlier all of the deposits below the Kingsdown silt and above the Cretaceous bedrock are included in the Ogallala formation, the upper (late Pliocene) part of the formation being segregated as the Rexroad member.
Few vertebrate fossils have been collected from the Ogallala formation in Ford County. A horse tooth that had been recovered from the Ogallala at a depth of 113 feet during the drilling of an irrigation well (402, see log 61) in the SW 1/4 sec. 6, T. 27 S., R. 26 W., was given by me to C.W. Hibbard who identified it as a right molar of Pliohippus cf. interpolatus. Schoff (1939, pp. 61, 62) reported that large collections of vertebrate remains taken from excavations in the vicinity of Optima and Guymon, Texas County, Oklahoma, were considered by Stovall to be middle Pliocene in age. According to Schoff, the fossils were found in the upper 100 feet of the formation.
Elias (1932, pp. 333-340) described fossil grass and hackberry seeds that were collected mostly from typical "mortar beds" of the Ogallala formation. In a later paper (Chancy and Elias, 1936) it was shown that certain of the fossil seeds are of widespread occurrence and have a short vertical range, making them useful as guide fossils. Fossil grass and hackberry seeds were collected by me from the Ogallala formation in Ford County, notably on the east side of Five-Mile Creek in the NW 1/4 NW 1/4 sec. 30, T. 25 S., R. 23 W., and on the east side of Spring Creek in the SW 1/4 sec. 28, T. 25 5., R. 23 W. In both localities the seeds were collected from hard massive layers of cemented grit and coarse sand from 8 to 10 feet in thickness that formed abrupt benches along the sides of the valleys. At the first locality mentioned, the seeds were collected from beds about 15 feet above the top of the Dakota formation. From comparisons with forms described by Elias, the grass seeds are believed to he Biorbia rugosa and the hackberry seeds are believed to be Celtis willistoni.
The following fresh-water mollusks recovered from a test hole in the Rexroad member of the Ogallala formation were identified by Dr. Calvin Goodrich of the University of Michigan.
These forms have been described from the Meade formation of Pleistocene age which overlies the Rexroad member in Meade County (Frye and Hibbard, 1941, pp. 413-415) so it is possible that some of the beds below the Kingsdown silt encountered in test holes in southwestern Ford County are of Pleistocene age. Due to the fact that beds believed to be equivalent to the Rexroad member of the Ogallala formation and of the Meade formation are nowhere exposed in Ford County; however, it is impossible to make any definite correlation on the basis of the data available.
Water supply--In Ford County, as in many other parts of the High Plains, the Ogallala formation is the most important water-bearing formation. Most of the domestic and stock wells on the uplands, many of the irrigation wells, and all of the large industrial and public-supply wells derive water from the Ogallala formation. Even in the Arkansas valley, where water is available at shallow depth in the alluvium, many well owners have drilled their wells deeper to tap the Ogallala formation. All of the irrigation wells south of the Arkansas valley in Ford County obtain water from this formation.
The yields of wells tapping the Ogallala range from several gallons a minute from small domestic and stock wells to about 1,700 gallons a minute for some of the large industrial wells (369, 370 and 371). The largest yields from the Ogallala are obtained from the coarser materials, generally in the lower part of the formation.
The beds of the Ogallala formation once extended from the Rocky Mountains eastward to perhaps as far as the eastern third of Kansas, but they have been removed by erosion from much of the territory they once occupied. A much greater thickness of the formation may have been saturated at one time, but dissection by streams such as Arkansas River and Sawlog Creek have cut below the zone of saturation and are draining part of the water from the formation.
In recent years there has been an increase in the number of wells tapping the Ogallala formation in the Arkansas valley in Ford County, largely as a result of increased demands for irrigation and industrial supplies.
The water from the Ogallala is hard. The analyses of typical waters from the Ogallala formation are shown in figure 20. Analyses of 45 samples of water collected from the Ogallala formation indicate that the hardness ranged from 171 to 276 parts per million and averages about 228 parts. The analyses also indicate that the amount of iron contained in these samples of water from the Ogallala ranges from 0.0 to 4.7 parts per million, but about half had less than 0.1 part per million. The fluoride content of these 45 samples ranged from 0.1 to 3.6 parts per million--28 samples of water from this group contained 0.5 parts per million or less of fluoride and 13 contained more than 1 part. The analyses indicate that the water from the Ogallala is well within the suggested safe limits for use in irrigation. The water in the Ogallala is generally softer than the water in the overlying alluvium.
Kansas Geological Survey, Ford Geohydrology|
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Web version April 2002. Original publication date Dec. 1942.