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  Scott County Geohydrology

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Geologic Formations and Their Water-bearing Properties

Tertiary System--Pliocene Series

Ogallala formation

Character--The Ogallala formation is composed of silts, sands, and gravels and contains layers of sandstone and conglomerate, much of which is crossbedded and cemented with lime. In general, the fine-grained sediments alternate with the coarser lime-cemented beds at different horizons. The Ogallala sediments are generally buff colored to pinkish, although some of the more limy silts and sands are white to whitish gray. The coarser sediments are present at all horizons but are most prominent in the lower part of the formation, and it is this part that yields water most freely to wells. Locally, the lower part of the Ogallala formation may be composed of bentonitic clays of bright mottled colors ranging from reddish brown to olive green. These basal clays represent a local lateral change in lithology in the lower part of the Ogallala formation and may be equivalent to the Woodhouse clays of either Miocene or Lower Pliocene age of Wallace County, Kansas, described by Elias (1931, pp. 155-158).

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 (Pl. 8A). The finer-textured materials of the Ogallala formation are composed chiefly of silt that generally is intermixed with fine sand. 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. Some of the beds of silt are impregnated with lime giving them a white to light-gray color. Gradations from one lithologic type to another may take place both laterally and vertically--sometimes within relatively short distances.

Plate 8A--View of north face of gravel pit in Scott County State Park showing cross-bedding of deposits of sand and gravel of Tertiary age.

black and white photo of outcrop showing cross bedding

The coarser-textured materials of the Ogallala formation are composed of fine to very coarse gravel and they may occur in almost any part of the formation from the base to the top. 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 Beaver (Ladder) Creek 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 show 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 both sides of Beaver (Ladder) Creek in the vicinity of Scott County State Park. They are well displayed in both sides of the road cut that was constructed through the "Devil's Backbone," about 1 mile south of the south entrance to Scott County State Park (Pl. 5A). The color of this structureless part of the Ogallala ranges from buff tan to light reddish brown.

Plate 5A--Lime concretions in the Ogallala formation in a road cut through the Devil's Backbone at the south end of Scott County State Park.

black and white photo of outcrop

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. Calcium carbonate is distributed through the deposits both as fine material and in the form of small and medium-sized nodules, pipy concretions, and in irregular lenses and beds. The caliche is white to gray and generally is fairly soft. It has been used locally in the construction of roads and highways in Scott County (Pl. 6B). In many places the sedimentary materials of the Ogallala formation are so firmly cemented with calcium carbonate as to produce a series of hard ledges, interbedded with only slightly cemented beds (Pls. 5B and 6A). The hard ledges are usually unevenly cemented and form rough weathered benches and cliffs. Because of their resemblance to old mortar, these beds have long been referred to as "mortar beds."

Plate 6--A, Series of many thin parallel "mortar beds" resting on structure-less cemented sandy silt of the Ogallala formation. East side of Salt Creek in the NE sec. 24, T. 16. S., R. 31 W. B, "Caliche" of the Ogallala formation exposed in County quarry in the SW sec. 28, T. 18 S., R. 32 W.

two black and white photos; upper photo shows thin beds on horizontal contact, Ogallala below; lower photo is shallow quarry

The thickness of some of the cemented caliche beds is very irregular and ranges from a few inches to about 11 feet. In some places the Ogallala formation is capped by a rather distinctive limestone layer, referred to as the capping limestone by Smith (1940, pp. 44-45). It. is commonly massive, and weathers to a knobby, cavernous, or irregular surface, and has a maximum thickness of about 5 feet. Elias (1931, pp. 136-141) described an equivalent horizon in Wallace County and adjacent areas and referred to it as "Algal limestone" because of its peculiar concentrically banded structure. In Scott County, outcrops similar to the capping limestone described by Smith and Elias were found in which algal structure was very prominent. The most notable of these was an outcrop in the southeastern part of the county in the SW 1/4 SE 1/4 sec. 5, T. 20 S., R. 31 W. A thin development of the same bed resting directly on the Niobrara formation was noted in an exposure in the SW 1/4 SE 1/4 sec. 12, T. 20 S., R. 32 W. Two other outcrops were noted in which the algal structure was less prominent, one situated about 1 mile south and 5 miles west of Scott City near the SW cor. sec. 20, T. 18 S., R. 33 W., the other situated about 5 1/2 miles south of Modoc, in the NW 1/4 SE 1/4 sec. 16, T. 19 S., R. 34 W. The former represents a very poor exposure and consists principally of fragments of limy rubble strewn along the side slopes of Lion Creek, but the individual fragments exhibited poor algal structure. The general lithology of the Ogallala formation at the surface is indicated by the following measured sections.

Section of Ogallala formation on the east side of Lake McBride in Scott County State Park near the NE cor. NW 1/4 sec. 12, T. 16 S., R. 33 W. Thickness
(feet)
Surface covered with unconsolidated sand and gravel  
Tertiary--Pliocene
11. Silt, sandy, lime-cemented, hard, gray to white; contains scattered coarse gravel and some pebbles; grades upward into a lime-cemented silt and becomes whiter in color near top; resembles a true "caliche" 28
10. Sand, silty, lime-cemented, dirty gray, extremely hard and resistant; contains pebbles up to 1 inch in diameter and larger imbedded firmly in the coarse sandy matrix; forms distinct caprock ledge 2
9. Silt, sandy, cemented, hard, gray to buff 1
8. Sand and silt, cemented, gritty, irregular ("mortar bed"); into lighter gray structureless sandy silt 6
7. Sand, silty, lime-cemented, dirty gray; grades upward into white limy silt 5
6. Sand, silty, cemented, buff colored; contains pebbles up to 1/2-inch in diameter and larger imbedded in sandy matrix; forms resistant ledge 8
5. Sand, silty, cemented, massive, hard, somewhat siliceous, dark gray to buff; contains a seam of extremely hard dense blue chert ranging from about 1 to 8 inches in thickness at top 4
4. Sand, silty, cemented, brown to buff; forms distinct smooth-faced bench in bordering bluffs 7
3. Sand, silty, cemented, hard, gray; contains scattered coarse gravel; also contains some rather thin-bedded layers in the lower part that tend to break off in thin slabs ranging in thickness from about 1 to 3 inches 17
2. Sand, cemented, gritty, gray; contains coarse gravel and scattered pebbles; grades upward near the top to a hard cemented silty sand, brownish-buff in color; forms a resistant ledge at the top that stands out in the bluffs bordering the lake 47
1. Clay, plastic, brown to chocolate brown, mottled in part with pale-green splotches (Woodhouse clays?) 16
Total thickness of Ogallala exposed 141
CRETACEOUS (Upper)
Niobrara formation  

Section of Ogallala formation at the north end of Scott County State Park in the NW 1/4 SE 1/4 sec. 2, T. 16 S., R. 33 W. (Measured by H. T. U. Smith, H. A. Waite, and L. P. Buck.) Thickness
(feet)
Tertiary--Pliocene
11. "Limestone," sandy, cemented ("mortar bed"); weathers chunky, somewhat porous, harder near the top 11
10. Sand, cemented, calichiferous (covered interval) 9.5
9. Sand, cemented, hard, resistant ("mortar bed"); weathers very knobby and irregular; cavernous throughout 6
8. Sand and silt, cemented, gritty, irregular ("mortar bed"); slabby in lower part, cavernous in upper part; contains some pebbles and limy concretions that look like reworked caliche nodules 6
7. Gravel, cemented, sandy, gritty, structureless 4.5
6. Sand and gravel (covered interval) 35
5. Silt, sand, and gravel, cemented, hard, gritty ("mortar bed"), weathers cavernous in lower part and slabby above; contains some short vertical concretions 6
4. Caliche, massive, gritty, structureless; contains small scattered pebbles and irregular limy concretions, some having root shape - weathers irregularly knobby; contains fossil grass seeds Biorbia fossilia (middle Pliocene) 11
3. Sand, covered interval 29.5
2. Sand and gravel, crossbedded, locally cemented; fossil bone found at a point 14 feet above bottom. (Sand and gravel have been excavated at this interval for use in the State Park.) View of gravel pit in Plate 8A 20
1. Clay, buff, massive; contains some gray layers and is somewhat silty in upper part (Woodhouse clays?) 23
Total thickness of Ogallala exposed 161.5
Cretaceous (Upper)
Niobrara formation  

Distribution and thickness--Exposures of the Ogallala formation occur along Beaver (Ladder) Creek and in tributary draws in the northwestern part of the county (Pl. 1). The Ogallala formation is also exposed in the northern part of the county along the sides of draws tributary to Chalk Creek, which flows eastward across the southern part of Logan County to the north. Exposures of the Ogallala formation occur above the Cretaceous rocks in the northeastern part of the county along draws tributary to Hell Creek which flows eastward near the southeastern boundary of Logan County and along the southern boundary of Gove County. Outcrops of the Ogallala formation occur along Lion Creek from near Modoc to a point about 4 miles west of Scott City and along Rocky Draw from the west county line to a point about 5 1/2 miles south of Modoc. Isolated exposures of the Ogallala formation have been noted near the heads of draws tributary to Dry Lake in the southeastern part of the county. Most of the upland surface in Scott County is underlain by deposits of Pleistocene age. The Ogallala formation, however, is found beneath younger deposits over most of the area. It is thin to practically absent in the Dry Lake vicinity near the southeastern corner of Scott County occupied by the buried Cretaceous hill shown in Figure 2.

The Ogallala formation was not encountered in test holes 13, 16, 17, and 19 (logs 13, 16, 17, and 19), all of which are situated in the deepest part of the buried trough (Fig. 5, sections B-B' and C-C'). It is probable that the Ogallala formation was removed by erosion and in its place 'sediments of Pleistocene age were deposited. Section B-B' shown in Figure 5 shows that the Ogallala formation is very thin in the vicinity of test holes 12, 14, 15, and 18 in the eastern part of the county east of Shallow Water. In certain areas in the southeastern part of the county deposits of dune sand overlie the Ogallala formation.

The Ogallala formation ranges in thickness from a few feet to about 160 feet. The variable thickness of the Ogallala formation is shown in the three profile sections in Figure 5. The thickness of the Ogallala in the 23 test holes drilled during the investigation ranges from about 6 feet in test hole 12 in the eastern part of the county to 159 feet in test hole 1 situated about 6 miles north of Scott City. In general, the test drilling revealed that the Ogallala becomes progressively thinner from north to south and in a southeasterly direction toward the southeastern corner of the county. The thickness of the Ogallala formation ranges within wide limits because of the uneven surface on which the sediments were deposited and also in part because of the fact that all or part of the sediments were removed during post-Ogallala erosion. At one time the Ogallala formation probably was much thicker in the vicinity of the buried trough in the vicinity of Scott City and Shallow Water but most of it was removed by post-Ogallala erosion.

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 derived from Cretaceous rocks in or just west of the county. Smith (1940, pp. 85-86) describes the mode of deposition of the Ogallala formation as follows:

"The deposition of the Ogallala was mainly of the channel and floodplain type. The coarser beds of sand, gravel, and grit represent channel deposits. ... The finer materials are best interpreted as representing flood-water deposits formed by the overflow of shallow channels, perhaps approaching the character of sheet-floods locally. No recognizable deposits of eolian sand or silt have been found in the Ogallala in the area studied, but the presence of ventifacts indicates that there must have been appreciable wind action.
"The deposition of the Ogallala formation began with the change from stream degradation to aggradation. ... During the early stages of deposition, there was a topography of moderate relief. The main valleys were occupied by through-going streams from the Rocky Mountains, and the valley bottoms were mantled by normal floodplain deposits. ... Deposition probably began with the filling of stream channels, leading to more frequent overflow and thus to the up-building of the floodplains. This soon led to shifting of the channels themselves, and probably to the development of anastomosing patterns. As filling progressed, the valley flat overlapped farther and farther on the slopes of the bordering hills. ... Relief was lowered, the valley plains grew broader, and finally the divides were overtopped, and there followed overlapping and coalescing of the depositional zones of individual streams."
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 formation 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 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 intervals throughout the formation. Smith (1940, pp. 77-94) gave a much more detailed discussion of the origin of the Ogallala formation.

Age and correlation--The Ogallala formation originally was named and described by Darton (1899, pp. 732, 734) and its age was given as late 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).

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. No upper Pliocene beds were recognized in Scott County, and no attempt has been made to subdivide the Ogallala formation.

So far as is known, no vertebrate fossils have been taken from the Ogallala formation in Scott County. In Ford County, a horse tooth was recovered from the Ogallala at a depth of 113 feet during the drilling of an irrigation well in the SW 1/4 sec. 6, T. 27 S., R. 26 W. The tooth was identified by C. W. Hibbard as a right molar of Pliohippus cf. interpolatus (Waite, 1942, p. 160). 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.

Fossil grass and hackberry seeds collected from the Ogallala formation have been described by Elias (1932, pp. 333-340). 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 Scott County, notably from exposures in the bluffs just west of the dam at the north end of Lake McBride near the center sec. 2, T. 16 S., R. 33 W. (Pl. 13A) and from exposures in the bluffs on the east side of Lake McBride below the McBride monument in the NW 1/4 NE 1/4 sec. 12, T. 16 S., R. 33 W. In both localities the seeds were collected from massive layers of cemented grit and coarse sand from 5 to 10 feet in thickness that formed prominent benches along the sides of the valley. Fossil grass and hackberry seeds were also collected from exposures of the Ogallala formation in the northeastern part of the county in the SE 1/4 NE 1/4 sec. 1, T. 16 S., R. 32 W. Fragments of grass and hackberry seeds were found in test hole 23 in the southwestern corner of Scott County between depths of 103.5 and 119 feet. From comparisons with forms described by Elias, the grass seeds are believed to be Biorbia fossilia and the hackberry seeds are believed to be Celtis willistoni.

Plate 13A--Cluster of Biorbia fossilia seeds in Ogallala formation at north end of Scott County State Park in the NW SE sec. 2, T. 16 S., R. 33 W.

black and white photo; location of seeds near rock hammer; cluster is about 8 inches in diameter

Water supply--In Scott 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 in the outlying upland areas surrounding the Scott Basin, as well as many of the irrigation wells and industrial and public supply wells, derive water from the Ogallala formation. Many of the wells in the Scott Basin derive water from the undifferentiated Pleistocene deposits overlying the Ogallala formation and some of the wells penetrate these deposits as well as the Ogallala formation below and may obtain water from both sources. The Ogallala formation also supplies water to springs in the northern part of the county, notably Big Springs, Barrel Springs, and Old Steele Home Springs, all situated in the Scott County State Park (Pl. 1, springs 16, 17, 18). The yields of wells tapping the Ogallala range from several gallons a minute from small domestic and stock wells to about 2,900 gallons a minute for some of the large irrigation wells (200). 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 streams such as Beaver (Ladder) Creek in the northern part of the county and tributaries to streams in Logan and Gove counties to the north of Scott County have cut below the zone of saturation and are draining part of the water from the formation. In parts of southeastern Scott County the Ogallala and Pleistocene beds are comparatively thin and lie entirely above the water table (sec. B-B', Fig. 5). The thickness of saturated material, as shown by the profile sections in Figure 5, differs greatly. Logs of test holes indicate that a large percentage of the saturated zone in the Ogallala and the Pleistocene deposits is composed of sand and gravel, so the amount of water available is large.

In recent years there has been an increase in the number of wells tapping the Ogallala formation in Scott County, largely as a result of increased demands for irrigation and industrial supplies.

The water from the Ogallala formation is hard. The analyses of typical waters from the Ogallala formation are shown in Figure 16. Samples of water were collected from 19 wells that derived water from the Ogallala formation, from six wells that derived water from the Ogallala formation and/or undifferentiated Pleistocene deposits, and from three wells that derived water from the Ogallala and/or Niobrara formations. Analyses of the 19 samples of water collected from the Ogallala formation indicate that the hardness ranged from 162 to 319 parts per million and averaged about 221 parts. The analyses also indicate that the amount of iron contained in these samples of water from the Ogallala ranged from 0.0 to 2.9 parts per million. The fluoride content of these 19 samples ranged from 0.3 to 3.0 parts per million. Three samples of water from this group contained less than 1 part per million of fluoride, six contained more than 1 part, nine contained between 2 and 3 parts, and one contained 3 parts. Because the fluoride content of some of the water from the Ogallala formation is relatively high, some water derived from this source may be harmful to children's teeth. The analyses show also that the amount of sulfate contained in these 19 samples of water from the Ogallala ranged from 8.4 to 84 parts per million and averaged about 44 parts. The analyses indicate that the water from the Ogallala is well within the suggested safe use for irrigation. The water in the Ogallala is generally softer than the water in the overlying undifferentiated Pleistocene deposits.

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  Kansas Geological Survey, Scott County Geohydrology
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Web version March 2003. Original publication date July 1947.
URL=http://www.kgs.ku.edu/General/Geology/Scott/06_form2.html