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Geologic Formations And Their Water-bearing Properties
Permian System
Undifferentiated Redbeds
General description--Undifferentiated redbeds of Permian age underlie the Cretaceous rocks in Pawnee and Edwards Counties. They do not crop out in this area; hence their lithologic character is known only from a few well cuttings. The redbeds consist primarily of red siltstone and sandstone containing gypsum and anhydrite (calcium sulfate with and without water of crystallization, respectively).
Water supply--Several wells and test holes in this area have encountered salt water under strong artesian pressure. A deep well near Frizell was reported to have a yield of nearly 700 gallons of salt water a minute. Darton (1905) reported that two wells at Larned, drilled to depths of 743 and 756 feet, yielded salt water at the rate of 400 and 250 gallons a minute, respectively. Water from the shallower well had a temperature of 650 F. and rose to a level 50 feet above the land surface. Test hole 28 at the NW cor. sec. 32, T. 21 S., R. 18 W., encountered salt water under artesian pressure at a depth of about 400 feet. An analysis of the water is shown in Table 9 and Figure 10.
Cretaceous System
Cheyenne Sandstone
Character--The Cheyenne sandstone does not crop out in Pawnee and Edwards Counties, hence little is known of its lithologic character in this area except as determined by the cuttings from five test holes (Pl. 3). The formation has been described in detail by Latta (1946) at the type locality near Belvidere, which is about 20 miles south of the south line of Edwards County. Latta (1946, p. 235) states:
The Cheyenne consists chiefly of light-colored fine- to medium-grained friable cross-bedded sandstone and lenses of sandy shale and conglomerate. Minor amounts of clay, selenite crystals, iron nodules, and pyrite occur in different parts of the formation. . . . Sandstone is by far the most dominant type of rock in the Cheyenne. The most common colors of the sandstone are white, light gray, and tan, but in some places iron staining has produced beautiful shades of yellow, red, purple, and brown along bedding and lamination planes or in irregular splotches. The brightly colored zones are most common in the upper half of the formation. The texture of the sandstone ranges from flour-like material of silt and clay size to fine gravel, but fine- to medium-grained sandstone is most common. The material in general is well sorted although the degree of assortment varies from one part of the formation to another and from one locality to another.
The materials encountered in test holes drilled by the State and Federal Surveys is primarily fine- to medium-grained sandstone containing gray and gray-green shale and a little siltstone.
Distribution and thickness--The Cheyenne sandstone probably underlies all or most of Pawnee and Edwards Counties (Pl. 3) as well as large areas of southwestern Kansas, except in some of the counties in the southern tier. The formation crops out in very few places in southwestern Kansas. The principal localities of outcrops are in the Belvidere area in southeastern Kiowa County and in adjacent areas in Comanche and Barber Counties. Sandstone in outcrops in Clark County is of undetermined age but may belong to the Cheyenne sandstone.
The thickness of the Cheyenne sandstone has been reported by Latta (1946 to range from 32.5 to 94 feet in the Belvidere area and to average about 45 feet. The thickness of the formation in Pawnee and Edwards Counties, as determined by test drilling, ranged from about 19 feet in test holes 28 and 88 to 47 feet in test hole 95. The average thickness was about 27 feet.
Age and correlation--The exact age of the Cheyenne sandstone is not, known. Berry (1922, p. 226) reports that. the flora is post-Trinity and pre-Woodbine, which would place the Cheyenne in the Fredericksburg or Washita. Bullard (1928, p. 53) states that the Cheyenne is pre-Washita or may represent a part of the most basal Washita. The Cheyenne probably is also equivalent to the lower sandstone member of the Purgatoire formation of eastern Colorado and the Oklahoma Panhandle. During investigations of the geology and ground-water resources of most of the counties in southwestern Kansas, members of the State and Federal Surveys have traced the Cheyenne sandstone and Kiowa shale, by means of extensive test drilling, from the type localities in Kiowa County to the Kansas-Colorado State line in Morton and Stanton Counties. There is little doubt, therefore, that these beds are equivalent, to the near-by Purgatoire formation.
Water supply--The Cheyenne sandstone is an important potential source of ground water but is unexploited in Pawnee and Edwards Counties, owing to its considerable depth and to the availability of larger supplies of potable water in the overlying formations.
Kiowa Shale
Character--The Kiowa shale does not crop out in Pawnee and Edwards Counties and its lithologic character is known only from cuttings from test holes and from its exposures in adjacent areas particularly in the type locality near Belvidere in Kiowa County.Latta (1946, p. 244) describes the Kiowa shale in the vicinity of the type locality as follows:
The Kiowa shale consists dominantly of thinly laminated dark-gray to black shale in the lower part grading upward into gray, tan, mottled tan, red, and brown clay and clay shale. The shale in the lower part generally is black and has been called a paper-shale because it is so thinly laminated. A conspicuous feature of the formation, especially of the lower part, is the presence of thin beds of shell limestone. . .
Latta states that large lenses of sandstone occur at the top of the formation in some places and that thin lenses Of sandstone occur throughout the formation.
The material encountered in test drilling through the Kiowa shale in Pawnee and Edwards Counties consisted principally of light-to dark-gray and black clay shale containing thin beds of sandstone. Thin layers of pyrite were encountered in several of the test holes. A hard layer of limestone that may be equivalent to one of the beds of shell limestone was encountered in test hole 28 at a point 78 feet above the base of the formation. Small fragments of limestone were noted in some of the other test holes.
Distribution and thickness--The Kiowa shale crops out in small areas in Kiowa, Clark, Comanche, and Barber Counties and over a wide area in central Kansas. The formation underlies all of Pawnee and Edwards Counties and much of southwestern Kansas, where it is in most places concealed beneath Cretaceous, Tertiary, or Quaternary rocks.
The maximum reported thickness of the Kiowa shale is 293 feet in the Belvidere area (Latta, 1946). The formation has a maximum thickness of 100 to 125 feet in central Kansas and about 135 feet in southwestern Kansas. The thickness of the formation in Pawnee and Edwards Counties, as determined by test drilling, ranged from 133 feet to 222 feet and averaged about 180 feet.
Age and correlation--The Kiowa shale is Comanchean, but its position within the Comanchean has been in dispute for many years. Discussions concerning the age of the Kiowa have been summarized by Latta (1946, p. 248), who concluded that:
The available evidence indicates, therefore, that the Kiowa shale is equivalent in age to the Washita division and possibly in part to the Fredericksburg division of the Texas section.
As stated in the section on the Cheyenne sandstone, the Kiowa shale has been traced by means of test drilling from the type locality to the Kansas-Colorado State line; hence, the formation probably is equivalent to the upper shale member of the Purgatoire formation of southeastern Colorado and the Oklahoma Panhandle.
Water supply--No wells in Pawnee and Edwards Counties obtain water from the Kiowa shale. Small quantities of water may be available from beds of sandstone within the Kiowa but larger quantities of potable water are available from overlying formations.
Dakota Formation
Character--The lithology of the Dakota formation in the Pawnee-Edwards area was determined from the study of a few small outcrops (Pl. 6) and of the cuttings from more than 100 test holes that penetrated the formation. The formation consists principally of buff, yellow-brown, and brown sandstone and varicolored clay and sandy clay. Where the formation is exposed the sandstone may be thin-bedded to massive but generally is strongly ripple-marked and cross-bedded. Where the sandstone is well cemented, as at Larned, it forms steep bluffs. In other areas, however, the beds of the formation are poorly cemented and form low, smooth hills having a thick cover of soil. For this reason the outcrops in many areas are far apart or are absent, and the contacts between the Dakota and younger formations are difficult to map.
Plate 6--Outcrops of the Dakota formation. A, SW corner SE sec. 28, T. 22 S., R. 20 W., B, Along highway about 1.5 miles west of Burdett. C. Along Pawnee River in sec. 2, T. 2 S., R. 17 W.
In a few places in Pawnee County there are zones of hard ironstone and very hard quartzitic sandstone. These deposits weather to large rounded boulders and hard outcropping ledges (Pl. 7).
Distribution and thickness--The Dakota formation crops out in several isolated areas in Pawnee County (Pl. 1), particularly in the vicinity of Larned and of Burdett. The Dakota underlies all of Pawnee County except perhaps local areas in the eastern part of the county, and it underlies all but the southeastern part of Edwards County (Pl. 3).
The Dakota is reported to attain a maximum thickness of about 275 feet in north-central Kansas (Plummer and Romary, 1942, p. 330). None of the test holes in Pawnee and Edwards Counties penetrated the entire thickness of the Dakota formation but it is believed that the average thickness of the Dakota in this area is about 200 feet (Pl. 3). Test hole 61, which entered the formation at a point below the base of the Graneros shale, penetrated 183 feet of the Dakota formation before entering the Kiowa shale.
Age and correlation--The Dakota formation, as defined by the Kansas Geological Survey, includes the Cretaceous strata from the top of the Kiowa shale, below, to the base of the Graneros shale, above. The Dakota of this area is in the lower part of the Upper Cretaceous and is equivalent to the Dakota formation of adjacent areas.
Water supply--The Dakota formation yields small to moderate quantities of water to many domestic and stock wells in Pawnee and Edwards Counties and to public-supply wells at Larned. Most of the domestic and stock wells have small diameters and yield only small quantities of water from the Dakota, but the large-diameter public-supply wells at Larned yield quantities ranging from 125 to 150 gallons a minute.
The Dakota supplies water to wells primarily in the areas where it is overlain by relatively impermeable materials such as those in the Graneros shale, Greenhorn limestone and Carlile shale. These areas lie north of Arkansas River and include nearly half of Pawnee County and the northwestern part of Edwards County. Where the Dakota is overlain by coarse water-bearing materials such as in the Pawnee and Arkansas Valleys and in the large dune-sand area south of Arkansas River, wells generally obtain an adequate supply of suitable water in the overlying materials. In the Arkansas Valley, however, where the water in the alluvium is very hard, a few domestic wells have been drilled into the Dakota formation in order to obtain softer water.
Water from the Dakota formation generally contains a large amount of dissolved solids but is comparatively soft, owing to natural softening by the base-exchange silicates within the formation (p. 71 and Fig. 10). As indicated by the analyses in Tables 9 and 10 water from the Dakota formation generally contains fluoride in quantities sufficient to cause slight to severe mottling of the enamel on children's teeth.
Graneros Shale
General description--The lithology of the Graneros shale is variable. In some places it consists entirely of dark-gray to black fissile argillaceous shale, whereas in other places it consists of shale, sandy shale, and sandstone. The formation is soft in most localities and forms a gentle slope between the Greenhorn limestone and the Dakota formation. Because of the poor and scattered exposure of the Graneros shale in Pawnee and Edwards Counties, it was not possible to map the formation separately; hence, the Greenhorn and Graneros were mapped as a unit. The distribution of the Graneros and Greenhorn formations in this area is shown on Plate 1.
The thickness of the Graneros shale in Pawnee and Edwards Counties is not known but is believed not to exceed about 35 feet. Moss (1932) observed a maximum thickness of 36 feet in Hodgeman County but stated that the thickness in that area was variable and in most places was less than 30 feet.
The Graneros shale yields little or no water to wells in the Pawnee-Edwards area, inasmuch as much larger supplies are available from the underlying Dakota formation.
Greenhorn Limestone
Character--The Greenhorn limestone consists largely of a succession of thin chalky to crystalline limestones interstratified with thicker beds of gray calcareous shale (Pl. 7). The shales contain thin beds of bentonitic clay. The formation has been divided into four members in this area, including the Pfeifer shale at the top, the Jetmore chalk, the Hartland shale, and the Lincoln limestone at the base. The base of the formation is marked by an abrupt change from the calcareous beds of the Greenhorn to the non-calcareous shale and sandy shale of the Graneros. The top of the formation is marked by the "Fencepost" limestone, which is a prominent bed of relatively hard chalky limestone that is quarried extensively in north-central Kansas for use as fence posts.
Plate 7--Outcrops of the Greenhorn limestone and Dakota formation. A, Bluffs formed by resistant beds of the Greenhorn limestone in the NE sec. 21, T. 22 S., R. 20 W. B, Upper part of the Greenhorn limestone in sec. 10, T. 20 S., R. 18 W. C, Residual boulders of ironstone in the Dakota formation in the SW SE sec. 15, T. 21 S., R. 17 W.
Distribution and thickness--The Greenhorn limestone is poorly exposed in Pawnee and Edwards Counties. There are a few places in the area where several beds in the formation are exposed, but generally the outcrops are indicated only by occasional fragments of limestone that have been turned up in plowed fields. Accurate mapping of the formation, therefore, was impossible. The distribution of the formation as shown on Plate 1 was based on the few small outcrops, on the small fragments in plowed fields, and on the topographic expression, and is subject to considerable error.
The formation crops out in the upland areas in Pawnee County that lie north of Arkansas River and in small areas in northwestern Edwards County (Pl. 1). It underlies the area of outcrop of the Carlile shale in northern Pawnee County and that of the Tertiary formations in part of northwestern Edwards County (Pl. 3).
The thickness of the Greenhorn limestone in Pawnee and Edwards Counties is not known because a complete section of the formation is not exposed in the area and because none of the test holes penetrated the entire formation. Inasmuch as the thickness of the formation is moderately uniform, it is believed that the thickness in this area is about the same as in adjacent counties to the west, where a thickness of about 125 feet was measured by Moss (1932).
Water supply--The Greenhorn limestone yields small quantities of water to a few dug domestic and stock wells in northern Pawnee County. The water, which is derived from cracks and fissures in the thin beds of limestone, is replenished by local rainfall; hence, wells in this formation may become dry during long periods of drought. Water from the Greenhorn generally is very hard, as indicated by the analyses in Table 9.
Carlile Shale
Character--The Carlile shale consists of the Codell sandstone member at the top, the Blue Hill shale member, and the Fairport chalky shale member at the base. The basal Fairport chalky shale member, which is the only part of the Carlile exposed in the Pawnee-Edwards area, consists of thick beds of chalky shale containing flat concretions and a few beds of bentonite and alternating with thin beds of chalky limestone. The thin beds of chalky limestone are harder and more numerous near the base of the member. The lower, more resistant beds form small terraces, whereas the upper beds erode to smooth, rounded, soil-covered hills.
Distribution and thickness--The Carlile shale, like the Greenhorn limestone, is very poorly exposed in Pawnee County and, hence, accurate mapping is not possible. The formation crops out only in the northern part of Pawnee County, as indicated by Plate 1. Inasmuch as only a part of the formation is exposed in this area, the total thickness of the Carlile in Pawnee County is not known. It is believed that nowhere in the county does it exceed 100 feet.
Water supply--The Carlile shale yields small quantities of water to a few shallow dug wells in the northern part of Pawnee County. The water is obtained from the joints and bedding planes of the, beds of chalky limestone in the lower part of the formation. The upper shaly part of the formation probably would yield little or no water to wells.Water from the Carlile shale is hard (Fig. 10) but it can be used for domestic and stock purposes. (See analyses in Table 9.)
Tertiary System--Pliocene Series
Ogallala Formation
General description--The Ogallala formation consists mainly of silt, sand, and gravel containing caliche (Pl. 8). It crops out principally in northwestern Edwards County but also in a few scattered areas in Pawnee County (Pl. 1). At many places in Pawnee County there are thin patches of algal limestone overlying the Dakota formation, which probably are equivalent to the algal limestone that marks the top of the Ogallala formation in parts of western Kansas. The Ogallala is 66 feet thick at test hole 86 but it may be nearly 100 feet thick at the Edwards-Hodgeman county line. Moss (.1932) reports a maximum thickness of 100 feet in Hodgeman County, and Waite (1942) stated that its thickness may be as much as 250 feet in Ford County.
The Ogallala formation yields small quantities of water to a few domestic and stock wells in the northwestern part of Edwards County. The formation overlies high areas of Cretaceous rocks and is, therefore, largely drained of water.
Plate 8--Views of the Ogallala formation and alluvium. A, Outcrop of caliche (Ogallala) in road ditches on Edwards-Hodgeman county line. B and C, Terrace deposits in the foreground and the flood plain of the Arkansas River in the distance (darker areas).
Quaternary System--Pleistocene Series
Meade Formation
The Meade formation was recognized and described by Cragin (1896, p. 53) as the Meade gravels. The name was proposed for to lowest of three "terranes" in the vicinity of the old Vanhem post office in sec. 13, T. 30 S., R. 23 W., Clark County (Hubbard, 1944, p. 709). In addition, he gave the name Pearlette ash to the deposits of volcanic ash in that region. Smith (1940, pp. 100-111) described the Pleistocene Odee formation, Equus niobrarensis beds, and Jones Ranch beds in Meade County and adjacent areas in 1940. Frye and Hibbard (1941, pp. 411-419) redefined the Meade formation to include Cragin's Meade gravels and Pearlette ash; Smith's Odee formation, Equus niobrarensis beds, and Jones Ranch beds; and all other beds of Pleistocene age above the Rexroad formation and below the Kingsdown silt.
Additional geological studies made by members of the State and Federal Geological Surveys have shown that these beds are distributed widely in southwestern Kansas. Latta (1948) recognized these beds in Kiowa and Stafford Counties, which border Edwards County on the south and west. Fossil vertebrates collected from these beds in Kiowa and Stafford Counties were identified by Claude W. Hibbard and indicate that they belong to the Meade. No fossil remains were found in these deposits in the Pawnee-Edwards area, inasmuch as they are covered by younger deposits of dune sand, but test drilling indicates that they are continuous with the Meade formation of Kiowa and Stafford Counties.
Character--Inasmuch as the Meade formation does not crop out in the Pawnee-Edwards area, its lithologic character is known only from test-hole cuttings. Test holes drilled in this area indicate that the formation consists predominantly of coarse sand and gravel containing beds of fine sand, silt, and clay. The beds generally are poorly consolidated but in some places the sand and gravel is cemented with calcium carbonate to form hard ledges known as "mortar beds."
Distribution and thickness--The Meade formation is distributed widely throughout southwestern Kansas. It underlies parts of Hamilton, Stanton, Morton, Kearny, Grant, Stevens, Finney, Haskell, Seward, Gray, Ford, Meade, Clark, Kiowa, and Stafford Counties, Kansas, as well as parts of Texas and Beaver Counties, Oklahoma. In much of this area, however, the formation is overlain by younger beds, such as silt (Kingsdown) and dune sand. In Pawnee and Edwards Counties it underlies only the area south of the Arkansas Valley.
The Meade formation in Pawnee and Edwards Counties ranges in thickness from about 50 feet to more than 300 feet. Test hole 119, south of Trousdale in Edwards County, encountered the base of the formation at a depth of 288 feet. A test hole drilled near the Kiowa-Edwards county line as a part of the ground-water study of Kiowa County (log 132) penetrated more than 300 feet of materials composing the Meade formation. In general, the formation thickens southward from the Arkansas Valley through Pawnee, Edwards, and northern Kiowa Counties and becomes thinner in central and southern Kiowa County.
Age and correlation--Claude W. Hibbard, of the University of Michigan Museum of Vertebrate Paleontology, has been collecting fossils from the Meade formation of southwestern Kansas since 1936, during which time he has collected the Cudahy, Borchers, and Cragin Quarry faunas (Hibbard, 1938, 1940, 1941, and 1943). These faunas have definitely established the age of the Meade formation as Pleistocene.
The beds in this area that have been assigned to the Meade formation may be equivalent in part to the Pleistocene McPherson formation of south-central Kansas (Williams and Lohman, 1949), but sufficient studies have not been made in the intermediate area to justify correlation with those beds.
Water supply--The Meade formation yields water to all wells in Pawnee and Edwards Counties south of the Arkansas Valley. The yields of these wells range from a few gallons a minute in most domestic and stock wells to more than 1,000 gallons a minute in some of the irrigation wells. The Meade formation is the most extensive and potentially the most important water-bearing formation in the two-county area.
There is little danger of the overdevelopment of this aquifer by pumping for irrigation, except locally, because of the relatively rapid rate of recharge from precipitation and from undrained depressions, and because of the widespread distribution of areas of dune sand in which the soil and topography generally are unsuitable for the development of irrigation. Large supplies of water could be obtained in this area for industrial use, however.
Water from these beds is moderately hard but is suitable for most uses (Tables 9 and 10, Fig. 10).
Terrace Deposits
Deposits consisting primarily of silt and clay overlie the bedrock in large areas in Pawnee and Edwards Counties. The origin of these deposits is not understood entirely, inasmuch as they do not resemble the coarse-grained terrace deposits of the major streams in southwestern Kansas. They probably were derived primarily from fine-grained sedimentary rocks in areas to the west but their origin may be in part colluvial-that is, by soil creep.
Character--The terrace deposits consist principally of light-tan to brown clay and silt containing some caliche and interbedded with fine to coarse sand and a little gravel. The silt and clay, which generally are brown or buff, may have a variety of colors ranging from white to bright green and blue. The clay generally is blocky and the silt is poorly consolidated except where it is in part cemented by calcium carbonate. The sand and gravel is poorly sorted and generally occurs at the base of the formation. The sand may consist primarily of grains of quartz or may contain fragments of sandstone and limestone. The gravel pebbles consist principally of limestone, sandstone, and ironstone, which probably were derived from the Greenhorn limestone, Carlile shale, and Dakota formation.
Locally the terrace deposits may consist of coarser material, such as along the lower reaches of Ash Creek on the north side of the Arkansas Valley northeast of Larned. Here the deposits contain sufficient sand and gravel to warrant the development of irrigation from wells.
Distribution and thickness--Moderately thick soils have been developed on the terrace deposits in this area, making them difficult to map. The area was mapped primarily on the basis of the topographic expression of the deposits and with the aid of the accurate soils map prepared by the Soil Conservation Service. The terrace deposits underlie the areas adjacent to the Pawnee and Arkansas Valleys as well as smaller areas along Ash Creek and Little Walnut Valleys (Pl. 1 and 3).
The topographic expression of these deposits is in places very prominent (Pls. 8 and 9). Two terraces can be observed in many places adjacent to the Pawnee Valley, but in some areas they are difficult to recognize because of dissection by streams tributary to Pawnee and Arkansas Rivers. The lowermost terrace, which is the most prominent, is about 20 feet above the flood plain of Pawnee River and the upper terrace is about 50 or 60 feet above the flood plain. Latta (in press) observed three terraces along Arkansas River in Barton County, one of which was underlain by coarse sand and gravel. It may be that the coarse materials underlying the low terrace northeast of Larned represent a third terrace equivalent to the third terrace in Barton County.
The thickness of the terrace deposits in Pawnee and Edwards Counties ranges from a featheredge near its contact with the Cretaceous bedrock to 145 feet in Ash Creek Valley north of Larned (Pl. 3). The average thickness of these deposits, as determined by the cuttings from 58 test holes, was slightly more than 60 feet.
Plate 9--Terrace deposits in Pawnee County. A, Low terrace in foreground and Pawnee Valley flood plain in distance. (View north toward Rozel.) B, High terrace in foreground (dark area); low terrace and Pawnee Valley in distance. (View north from a point 0.6 mile east of the SW sec. 17, T. 22 S., R. 19 W.) C, Low terrace in foreground (darker plowed area) and Pawnee Valley flood plain in distance. (View northwest from a point 0.2 mil north of the SE sec. 33, T. 21 S., R. 19 W.)
Water supply--Inasmuch as the terrace deposits consist primarily of clay and silt, the formation yields only small to moderate quantities of water to wells. Generally, sufficient water for most domestic and stock uses can be obtained from the thin beds of sand and gravel. In the few places where the formation contains much sand and gravel the beds will yield adequate water for irrigation. In general, however, wells in these beds will not yield enough water for irrigation. In the vicinity of Rozel a farmer drilled several test holes on the lower terrace in an attempt to find a suitable site for an irrigation well, but only silt and clay and a little gravel were encountered. As a last resort the well was drilled in the alluvium of Pawnee Valley and the water was pumped to the first terrace, where it was used for irrigation.
In a few localities the clay in this formation yields more water than would be expected from a material of such seemingly low permeability. Moderate quantities of water have been obtained from these beds in Little Walnut Creek Valley, where they are known locally as "water clay." The joints in the blocky clay seem to remain open when the beds are saturated, thus allowing the relatively free movement of water. One well in these beds yielded more than 50 gallons of water a minute.
Water from the terrace deposits is moderately hard but otherwise is of good quality (Fig. 10 and Tables 9 and 10).
Quaternary System--Pleistocene and Recent Series
Alluvium
Character--The alluvium of the Pawnee and Arkansas Valleys consists of sand, gravel, silt, and clay. The alluvium of the Pawnee Valley, however, differs in several respects from that in the Arkansas Valley. In the Pawnee Valley the upper part of the alluvium consists predominantly of clay containing some silt and sand. This zone ranges in thickness from about 15 feet to 50 feet and has an average thickness of about 30 feet. The clay makes possible the development of an excellent, soil but it retards the recharge of the underlying sand and gravel from local precipitation and from Pawnee River. Beneath the clay there is, in most places, a thick deposit of sand and gravel that yields large quantities of water to wells in the valley.
The alluvium of Arkansas Valley contains no thick deposit of clay or silt in the upper part, although thick beds of these materials may occur within the formation. In Arkansas Valley the sandy soil may be underlain by thick beds of sand and gravel; hence, recharge from precipitation and from streams probably is much greater than in Pawnee Valley.
Distribution and thickness--Alluvium underlies the bottomland of the Pawnee and Arkansas Valleys and of some of the larger tributaries to these valleys. The southern limit of the alluvium on the south side of Arkansas River is not known because of the overlap of dune sand. Below Larned it may extend several miles south of the alluvium-dune sand contact but its southern limit could not be determined by test drilling, owing to the lithologic similarity of the alluvium and the Pleistocene materials underlying the dune sand in the southern part of the area.
The thickness of the alluvium in the Pawnee Valley as determined by 15 test holes ranged from 65 to 138 feet and averaged 105 feet. In the Arkansas Valley the thickness, as determined by 16 test holes, ranged from 18 to 135 feet and averaged about 61 feet. The thickness of the alluvium in the Pawnee Valley is moderately uniform, whereas in the Arkansas Valley there are areas where the alluvium is very thin and others where there is a deep channel. This condition is well illustrated by test holes 97, 110, and 111 on Plate 3.
Age--The alluvium in the Pawnee and Arkansas Valleys has been deposited in channels cut into Cretaceous, Tertiary, and Pleistocene sediments. The age of the alluvium, therefore, probably is late Pleistocene and in part Recent. There may be older terraces of Arkansas River beneath the cover of dune sand toward the south, but their presence could not be determined by test drilling.
Water supply--The alluvium of the Arkansas and Pawnee valleys yields large quantities of water to wells. Most of the domestic and stock wells and all the irrigation wells in the valleys obtain water from the sand and gravel of this formation. The water in the alluvium is hard but generally is suitable for most domestic, stock, and irrigation uses. In general, the water in the Pawnee Valley alluvium is softer and contains less dissolved solids than the water in the alluvium of the Arkansas Valley. Some of the water in the alluvium of the Arkansas Valley contains enough dissolved solids to be harmful to plants. (See Tables 9 and 10 and Fig. 10.)
Dune Sand
Almost all the area lying south of Arkansas River in Pawnee and Edwards Counties is underlain by dune sand. The dune sand contains uniform-grained, moderately well rounded fragments of quartz, as well as lesser amounts of silt and clay. Inasmuch as the dune sand overlies the alluvium of Arkansas River in some places, it probably is largely Recent, but it may be in part Pleistocene.
Two types of topography may be recognized in the dune-sand areas south of Arkansas River (Pl. 1) which are reflections of the stage or phase of erosion of the sand dunes. The first type is characterized by typical sand-dune topography wherein the dunes are grass-covered, moderately steep, irregular hills between which are small undrained basins. This type is best exposed in the areas near the Arkansas Valley. The second type of topography comprises broad subdued swells and swales having a thicker, heavier soil, which is cultivated extensively.
As a result of his studies of sand dunes, Smith (1940, pp. 159-165) described an ideal dune cycle consisting of two phases: (1) an eolian or active phase during which the dune is built up, and (2) an eluvial or passive phase during which the vegetation prevents further growth and the dune is subdued by weathering and creep. He divides the eluvial phase into stages of youth, maturity, and old age. In the youthful stage the soil zone is formed and the slopes are reduced. The dune becomes mature when its profile is smooth and regular and when its soil becomes thicker and more stable. Old age is reached when the dune form is indistinguishable. He states that the eluvial phase in any stage may be interrupted by rejuvenation.
Most of the sand dunes in Pawnee and Edwards Counties are in the eluvial phase of the dune cycle, although a few dunes are in the eolian or active phase. The types of dune sand mapped in the Pawnee-Edwards area are (1) those that produce a typical sand dune topography and which are in the youthful and mature stages of the eluvial phase of the dune cycle, together with those that are in the eolian phase of the cycle, and (2) those that produce a relatively flat topography and which are in the old-age stage of the eluvial phase of the dune cycle. The boundaries between the two types of dunes and between the dune sand and other formations are indistinct in many places; hence they are shown on Plate 1 by dashed lines.
The thickness of the dune sand is variable and ranges from a featheredge to about 40 or 50 feet. Where the dunes have reached the old-age stage they are very thin and where they are in the eolian phase and in the youthful stage of the eluvial phase they attain their maximum thickness.
The dune sand lies above the water table and, hence, yields no water to wells, but the areas of dune sand form ideal catchment areas for rainfall and facilitate the relatively rapid recharge of the underlying formations.
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Kansas Geological Survey, Pawnee and Edwards Geology and Groundwater
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