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Table of Contents

Abstract

Introduction

Geography

Geology

Ground Water

Water Bearing Formations

Well Records

Well Logs

References

Plates

Water-bearing Formations, continued

Tertiary System

Ogallala Formation

Character--The Ogallala formation consists of clay, silt, sandy silt, cross-bedded sand and gravel (pl. 6B), and locally sandy limestone. In areas adjacent to Thomas County it also contains beds of volcanic ash, diatomaceous marl, bentonitic clay, and hard silicified beds that resemble chert or quartzite. The generalized character of the Ogallala is given by the logs of test holes included in this report. As there are no exposures that permit a continuous examination of any considerable thickness of these strata in Thomas County, the beds were examined along canyon walls in southern Rawlins County and also in northeastern Wallace County. The following stratigraphic section was measured 1.4 miles north of the Thomas County line in eastern Rawlins County.

Measured section of the Ogallala formation, SE 1/4 sec. 30, T. 5 S., R. 32 W., Rawlins County. (Measured by John C. Frye and August Lauterbach.)
Bed No.	Description	Thickness (feet)
13.	Mostly covered, capped by nodular mortar bed, irregular, hard, gray,	6.0
12.	Mortar bed, coarse gravel to sand cemented by calcium carbonate; weathers irregularly cavernous	5.5
11.	Silt, sand, and gravel; massive; poorly sorted; red-tan	7.0
10.	Mortar bed, sand and some fine gravel; loosely cemented by calcium carbonate; friable; gray	1.5
9.	Silt, sand, and gravel; massive; poorly sorted; red-tan	17.8
8.	Mortarbed, hard, massive, gray	4.0
7.	Sand, fine, and silt; tan to red-buff, partly covered	7.5
6.	Mortarbed, hard, gray	3.5
5.	Sand, fine to medium, tan to buff, partly covered	6.0
4.	Mortarbed, sand and coarse gravel; hard; gray	2.2
3.	Sand and gravel; tan	9.5
2.	Caliche and mortar bed; massive; having thin lenses of clay and silt interbedded; light gray to light gray-tan	8.0
1.	Silt and fine sand; massive; gray; covered in lower part	13.7
Total thickness of beds measured		92.2

A section of the Ogallala formation was measured along a creek bank and road cut in northwestern Thomas County. The total thickness is much less than that in the measured section in Rawlins County but the general character of the beds is about the same.

Measured section in the SW corner sec. 26, T. 6 S., R. 31 W., Thomas County. (Measured by John C. Frye.)
Bed No.	Description	Thickness (feet)
Sanborn formation
10.	Silt and fine sand; gray; containing concretions of calcium carbonate	20.0
Ogallala formation
9.	Sand, silt and caliche; buff	10.0
8.	Mortarbed of sand and gravel; gray	0.4
7.	Clay and silt; blocky; gray with yellow along some bedding planes; containing some carbonaceous material	0.6
6.	Sand and silt containing some soft caliche; light gray	1.8
5.	Silt and sand; tan; containing a few pebbles and caliche	5.2
4.	Sand and some gravel; cemented; gray	0.5
3.	Silt, sand, and gravel; containing some stringers of caliche; red-tan and gray	12.4
2.	Mortar bed of sand, gray	1.5
1.	Silt and sand; red-tan and gray; containing some caliche	4.8
Total thickness of beds measured		57.8

In order to determine the grain size and sorting, the percentage of soluble material, and the permeability of the several beds, samples were collected from seven of the beds described in the stratigraphic section measured in Rawlins County. Analyses of these samples were made by Ada Swineford and Carrie Thurber and are reported in table 6. The solubility was determined by digesting a known quantity of sample in dilute hydrochloric acid, the spent acid and dissolved material were then washed from the sample and the residue was caught on filter paper, dried, and weighed. A mechanical analysis, or sieve analysis, was made of the insoluble fraction of each sample by placing a known weight of dry sample in a set of sieves and agitating the sieves for 20 minutes in a mechanical shaker. The fraction caught on each sieve was then weighed and the percentage calculated. The analyses of loess and slope deposits reported in tables 7 and 8 were made by the same method supplemented by pipette determinations of grade sizes less than 0.062 mm.

A comparison of the analyses in table 6 with the descriptions of the beds given in the measured section in Rawlins County indicates that the caliche and mortar beds have a high percentage of calcium carbonate, which represents the soluble fraction. In these two beds (2 and 4) the percentage of soluble material was 23 and 38. The insoluble fraction of the mortar beds corresponds closely to the sand and gravel beds that contain a smaller amount of soluble material. It will be noted that none of the samples is very well sorted. The highest percentage of material in any one grade is 36.91 in the fine-sand fraction of the sample from bed 7. The Ogallala formation consists essentially of partly discontinuous and interconnected lenticular bodies of stream-deposited sand and gravel interspersed through an enclosing mass of poorly sorted stream-deposited clay, silt, sand, and gravel. Locally where the top of the formation has not been eroded away, there is a capping layer of sandy ("Algal") limestone. The logs and measured sections give the general character of the formation but because of its manner of deposition the details of lithology range widely in short horizontal distances in some places, whereas they may be quite persistent in other places.

Distribution and thickness--The Ogallala formation underlies all of Thomas County and rests directly upon the Pierre shale. As shown in plate 1, it is overlain in most parts of the county by the Sanborn formation, slope deposits, or alluvium. The thickness of the Ogallala under Thomas County is shown by the 29 logs of test holes to range from 77 feet in test hole 7 to 266 feet in test hole 14. The average thickness in the 29 test holes is 165 feet. In general the Ogallala is thick under northwestern, central, and eastern Thomas County and thin under the southwestern and north-central parts.

Age and correlation--The beds in the type area of the Ogallala formation near Ogallala, Nebraska, have been studied by Elias (1931) and the fossil vertebrates have been described and discussed (Hibbard, 1983; Hesse, 1935; Stirton, 1985, p. 444). A committee report on the nomenclature and correlation of the North American continental Tertiary (Wood, et al., 1941, pl. 1, pp. 12, 27) placed the Ogallala of Kansas and eastern Colorado in the Blancan, Hemphillian and Clarendonian, and the Ogallala of Nebraska and eastern Wyoming in the lowermost Blancan, the Hemphillian, Clarendonian, and upper part of the Barstovian, North American provincial age zones, which they correlated with the upper, middle, and lower Pliocene and upper Miocene, respectively. They listed the characteristic fossil vertebrates for each of these provincial age zones. Lugn (1939) presented a classification of the Tertiary formations of Nebraska in which he considered the Ogallala as a group containing the following formations in ascending order: Valentine, Ash Hollow, Sidney gravel, and Kimball, all of Pliocene age (p. 1266). Elias (1931, pp. 149-153; 1932; 1942) described plant remains from the Ogallala of the High Plains region and correlated certain zones over wide areas. The Ogallala is generally agreed to be of Pliocene age.

During the progress of field work in Thomas County, I collected from the Ogallala formation (middle North line sec. 18, T. 8 S., R. 36 W.) one fossil tooth (Kansas University Museum of Vertebrate Paleontology no. 6550) that has been identified by C. W. Hibbard of the University of Kansas as ?Pliohippus. Allan Graffham collected fossil vertebrates from a gravel pit south of Brewster (sec. 29, T. 8 S., R. 36 W.). One fossil from the Ogallala in the lower part of the pit has been identified by C. W. Hibbard as Pliohippus cf. ansae (Matthew and Stirton).

For the purpose of this paper the Ogallala is classed as a formation rather than as a group as used by Lugn, and definite correlation is not made with the stratigraphic units widely used in Nebraska. A sample of limestone from an outcrop just northwest of Brewster was submitted to M.K. Elias of the Nebraska Geological Survey, who identified it as a typical specimen of the "Chlorellopsis limestone" (personal communication dated September 22, 1944). The presence of this bed, which marks the top of the Ogallala of Nebraska, indicates that the Thomas County section includes stratigraphic equivalents of the upper part of the Ogallala of Nebraska. Although the limits of the several stratigraphic units have not been drawn in this area, age equivalents of Lugn's Kimball, Sidney gravel (?), and Ash Hollow are probably included within the Ogallala, but it is my opinion that most of the Ogallala in this area is of Ash Hollow age. The lower part of the Ogallala formation is not exposed in Thomas County and it may include beds equivalent in age to the Valentine or even older beds in Nebraska.

A clay zone having the appearance of a bentonitic clay is present in some of the test holes between the base of the Ogallala and the top of the Pierre shale. This clay is tan to light gray and seems to grade downward into typical blue-gray or black Pierre shale, thus probably representing the product of weathering of the Pierre prior to the deposition of the Ogallala. It is possible, however, that in some places this may represent the Woodhouse clay described by Elias (1931, pp. 155-158) as occurring at the base of the Ogallala in Wallace County.

Water supply--The sand and gravel of the Ogallala formation is the primary source of ground water in Thomas County. Nearly all the wells in the county obtain all or part of their water from this formation. The finer materials of the formation generally are porous and hold considerable water but are not permeable enough to yield water freely. The coarser materials, the gravels in particular, commonly yield abundant supplies of water.

Mechanical analyses and coefficients of permeability of samples from a measured stratigraphic section of the Ogallala are given in table 6. The permeability of a water-bearing material is its capacity for transmitting water under pressure. The coefficient of permeability, as determined in the field or laboratory, is expressed by O.E. Meinzer as the number of gallons of water a day at 60 deg. F. that is conducted laterally through each mile of the water-bearing bed under investigation (measured at right angles to the direction of flow) for each foot of thickness of the bed, and for each foot per mile of hydraulic gradient (Stearns, 1927, p. 148). The coefficients of permeability given in table 6 were determined by means of a portable apparatus designed by V. C. Fishel of the Federal Geological Survey. It will be noted that the coefficient of permeability of five samples ranged from 107 to 609.

Concerning the relation of the permeability of water-bearing material to the yield of wells, Wenzel (1942, p. 11) stated:

"Although there are many water-bearing materials of low permeability, most formations that are sufficiently water-bearing to be utilized by wells have coefficients that are whole numbers of two or more figures when expressed in Meinzer's units--that is, above 10. The yields of wells depend, of course not only on the permeability of the formations they tap but also on the thickness of the formations, the drawdown of the water level, and the diameter and construction of the wells. For many places in the United States the physical and economic conditions are such that wells with moderate to high yields--100 gallons a minute or more--generally penetrate materials with coefficients of permeability of 100 or more."

The Ogallala formation is a large underground reservoir that is only partly filled with water. The thickness of saturated material is shown in the cross sections in figures 3 and 4. Logs and test holes indicate that a large percentage of the saturated zone of the Ogallala is composed of sand and gravel; therefore the amount of water available is large.

All but one of the analyses in table 5 are of water pumped from the Ogallala formation; thus the section on quality of water refers especially to this formation.

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