Water Bearing Formations
Water-bearing Formations, continued
Sanborn FormationIn 1981 Elias (pp. 168-181) described unconsolidated Pleistocene deposits, consisting mostly of silt--or loess--in northwestern Cheyenne County, Kansas, and named these deposits the Sanborn formation from the town of Sanborn, located just north of the type area in Nebraska. Beds of similar character in Nebraska have been studied and described by Lugn (1935, pp. 128-168). Elias (1937, p. 7) briefly described the occurrence of this formation in Rawlins and Decatur counties, and the stratigraphy, fossil mollusks, and fossil vertebrates have recently been studied (Leonard and Frye, 1943; Hibbard, Frye, and Leonard, 1944).
Character--In north-central Kansas, the Sanborn formation generally consists of a basal sand and gravel that ranges in texture and composition from grains and pebbles of chalk and shale to boulders of igneous rocks; this is overlain by heterogeneous deposits of clay, silt, sand, and gravel that are light gray to red-brown. A distinct soil zone is present at the top of these heterogeneous deposits and separates them from the overlying beds. The beds above the soil zone consist of massive silt--or loess--containing some zoned of fine to very fine sand (Hibbard, Frye, and Leonard, 1944). In Thomas County, which was probably a part of an upland divide area during Sanborn time, the prominent soil zone and lower beds of the Sanborn seemingly were not formed. At a few places very coarse sand and gravel containing cobbles several inches in diameter have been observed overlying the Ogallala at the base of the Sanborn formation, but for the most part the Sanborn of this area consists of massive silt containing a thin sandy zone at the base (see well logs, and cross-sections in figs. 3 and 4).
Along the Rock Island Railroad near Levant (SW NW sec. 8, T. 8 S., R. 35 W.) more than 80 feet of this massive silt, or loess, is exposed in the nearly vertical side of a cut (pl. 6C). Samples were collected from this cut at vertical intervals of 5 feet and mechanical analyses of them were made by Ada Swineford and Carrie B. Thurber. The results of these analyses are given in table 7. The uppermost sample, no. 7, was collected from the upper 8 inches of soil.
The high degree of sorting displayed by this material is indicated in table 7 by the fact that more than 50 percent by weight of six of the seven samples was included in the single fraction 0.062-0.081 mm. Russell (1944, p.3) stated concerning the definition of loess: "In the writer's experience the single fraction 0.01-0.05 mm. ordinarily constitutes at least 50 percent (by weight) of a sample and in many cases amounts to 75 percent." Although these exact units were not used in the analyses reported here, Ada Swineford (personal communication) determined graphically the percentage of each sample included in this fraction, and found that from 59 to 70 percent of the material was included in the fraction 0.01-0.05 mm. indicating a degree of sorting comparable to that required by Russell for loess. The sorting and major grades of these samples are comparable to those of other samples of loess from the High Plains and of wind blown dust reported by Swineford and Frye (1945).
The origin of loess deposits has been discussed at length by Russell (1944) who concluded that loess is generally not formed by eolian processes but is produced by a process he called loessification, operating on water deposited material. Frye (1945, p. 86) discussed the problems of the loess in northwestern Kansas and pointed out that although Russell's hypothesis of origin seems to explain adequately some of the deposits of massive silt along terraces in central and southern Kansas and some of the valley-side or slope deposits in northwestern Kansas, it cannot be accepted for the extensive loess--or silt--deposits that comprise the upper part of the Sanborn formation. The cross sections (figs. 3 and 4) show that this deposit blankets the highest part of the uplands to depths of more than 40 feet and therefore no parent material seems to be available for the loessification process. There may be a question, based on Russell's discussion, as to whether or not an eolian deposit such as this should be called "loess."
Distribution, thickness, and surface form--As shown on plate 1, the Sanborn formation and associated slope deposits underlie the surface of most of Thomas County. The logs of test holes included with this report indicate that the Sanborn formation attains a maximum thickness of more than 80 feet and commonly is 35 to 40 feet thick in the county. In the upland-divide areas the surface developed on the Sanborn formation is a flat to gently rolling plain (pl. 8B). In some places low bluffs (pl. 3C) were formed along valley sides, and along the major valleys these deposits merge with slope deposits to produce gentle well-rounded valley slopes.
Age and correlation--The Sanborn formation is separated from the underlying Pliocene Ogallala formation by a prominent disconformity, which is evidence of an important erosion interval after the deposition of the "Algal limestone" and before deposition of the lowermost beds of the Sanborn. This stratigraphic relationship and the fossil mollusks and fossil vertebrates that have been described from these beds in northwestern Kansas (Leonard and Frye, 1948; Hibbard, Frye, and Leonard, 1944) indicate a middle and upper Pleistocene age for the Sanborn formation; but the uppermost part may be Recent.
Definite correlation with the extensive Pleistocene formations of southwestern Kansas has not been made. It is my opinion that age equivalents to the lower part of the Meade formation are absent in this area, but the Sanborn may be in part equivalent to the upper part of the Meade formation and the Kingsdown silt. To the north of this area beds of similar character have been described as the Loveland and Peorian formations separated by a prominent soil zone (Lugn, 1935). In Thomas County the extensive tan loess, or silt, of the Sanborn formation may be equivalent in part to the Peorian of western Nebraska, but as the soil zone was not observed in this area correlation of the lower part of the Sanborn with the Nebraska section is not attempted.
Water supply--In most parts of Thomas County the Sanborn formation lies wholly above the water table and hence is dry. None of the wells visited in the county and reported in table 9 obtain water from this formation. The local deposits of sand and gravel at the base of the formation would constitute good water-bearing material if they were saturated with water. The primary importance of the Sanborn with respect to water supply is its retarding effect on recharge.
Slope DepositsIn Elias' (1931, pp. 179-180) original description of the Sanborn formation he stated that only the loess on the divides should be considered Pleistocene in age and a part of the Sanborn formation.
"The loess of the valley slopes, which is reworked loess of the divides and must not be called Sanborn formation, attains a thickness of 50 feet, but approaches that thickness in only a few places, usually along a narrow zone high on the slopes. This loess is usually distinctly stratified, which is due to the interbedded layers composed of fragments of locally outcropping rocks (chiefly Ogallala) mixed with loess. The valley-bottom loess is more evenly distributed and is usually 10 to 15 feet thick. It passes downward into alluvial sands and gravel and it also must be regarded as a part of the alluvial deposits."Slope deposits such as those described by Elias are extensive in Thomas County. They mantle the slopes of large and small valleys alike and in some places, where the parent material consists entirely of loess of the Sanborn formation, they are virtually indistinguishable from the Sanborn. Deposits of a comparable origin along major valleys south of this area have been described by Frye and Smith (1942, p. 220) who stated:
"Along certain segments of the valleys of such streams as the Smoky Hill and Cimarron rivers, both of which head in the Great Plains province and have a relatively small flow, it seems that lateral planation by the larger streams is of very limited effectiveness as an erosional process. Valley flats are typically narrow, and the graded slopes into which they merge are characteristically broad. Slope processes and side stream work are largely responsible for the lowering of the land surface. The trunk streams to which these processes are graded exhaust their energies in transportation, and have little power to erode, save in comparatively brief interludes following rejuvenation.
"The conditions summarized above are comparable to those of more arid regions where pediments are widespread and constitute the dominant landform. The latter, also, are formed by the work of slope processes and of many small streams, graded either to a through-going trunk stream or to the advancing edge of alluvial fill in an enclosed basin. In such regions, the erosive work of any through-going streams that are present generally lags far behind that of the slope processes, and is commonly limited to a comparatively narrow belt."The slope deposits observed in Thomas County differ from those described by Frye and Smith primarily because of the different nature of the beds that underlie the valleys. The massive silt of the Sanborn formation that underlies the uplands of Thomas County has little resistance to processes of slope erosion so that rounded shoulders were formed where the Sanborn and slope deposits merge (pl. 3A). The more resistant Tertiary and Cretaceous formations Underlying the valleys studied by Frye and Smith, on the other hand, produced distinct escarpments at the crests of the valley walls so that the slope deposits constitute a wedge of material which thickens down slope toward the main channel.
Down-slope migration of unconsolidated deposits is not unique to semiarid regions. Mass movement of thick soils from deeply weathered crystalline rock in the Piedmont region of South Carolina has been described by Ireland, Sharpe, and Eargle (1939, pp. 20-24). They stated that the A and B horizons migrate down slope and that this movement produces a sharp line of demarcation between the B and C horizons with a concentration of rock fragments, a "stone line," near or at the base of the B horizon. The mechanics of this soil migration seem to have much in common with the movement of the slope deposits described here--the slope deposits being comparable to the A and B horizons and the unconsolidated parent material below the slope deposits being in the same relative position as the C horizon of the southern Piedmont region. There are several important points of difference, however. The slope deposits attain a greater thickness, have a greater range in thickness, formed more rapidly and during a much shorter span of geologic time, probably migrate down slope at a greater rate of speed, and the character of the deposits has been altered but little from that of its source material.
In an attempt to discover the differences in mechanical composition between the silt of the Sanborn formation and the slope deposits that so closely resemble it in appearance, a sequence of samples was collected along a roadside cut that ascends the south valley wall of South Sappa Creek in the west-central part of the county. Mechanical analyses of these samples which were collected 18 to 20 inches below the surface are given in table 8. Sample 1 was collected at the level of the flood plain, and samples 2 to 9 at 5-foot vertical intervals up the valley wall. Sample 10 was collected 10 feet higher up the slope than sample 9, and sample 11, 10 feet above sample 10. These last two samples were collected from above the rounded shoulder of the valley wall and are believed to represent loess of the Sanborn that has not been moved by slope processes. The next several samples (9, 8, 7, 6, 5, and 4) seem to be derived mostly from silt of the Sanborn. Sample 3 contains an admixture of coarser sediments and may be the highest sample that contains an appreciable amount of material derived from the Ogallala formation, although the top of the Ogallala (obscured by the slope deposits) is believed to occur at a higher level.
These analyses indicate that where the slope deposits are derived from the upper massive silt of the Sanborn formation they cannot be distinguished from it on the basis of grain size and sorting. The loess-like character of the slope deposits seems to extend some distance down the slope below the top of the Ogallala formation and the failure to differentiate this material from the Sanborn would result in an erroneous idea of the thickness of the Sanborn. On the geologic map (pl. 1) the slope deposits were mapped with the Sanborn formation where they were of sufficient thickness and continuity to obscure the underlying beds even along road cuts and sides of gullies. Where these deposits mantle the Ogallala thinly or intermittently, they were ignored in the mapping and the material is mapped as the Ogallala formation.
As pointed out above, Russell's (1944) recent discussion of loess raises some question as to the application of that term to the massive silt of the Sanborn formation and to the slope deposits of similar character. Elias (1931) applied the term loess to both the massive silt and the slope deposits that were largely derived from it. Russell stated concerning loess (1944, pp. 4, 5):
"The definition should include the following essential characteristics: loess is unstratified, homogeneous, porous, calcareous silt; it is characteristic that it is yellowish or buff, tends to split along vertical joints, maintains steep faces, and ordinarily contains concretions, and snail shells. From the quantitative standpoint at least 50 percent, by weight, must fall within the grain size fraction 0.01-0.05 mm. and it must effervesce freely with dilute hydrochloric acid."The massive silt of the Sanborn formation fills all the requirements of this definition. In some places the slope deposits also fill these requirements with the exception of lack of stratification, for most exposures of this material exhibit at least an indistinct stratification. In this report the term loess is not applied to the slope deposits.
The slope deposits are quite young and probably all reached their present position during the Recent epoch. Actually, the processes that gave rise to these deposits are probably still in operation and this material is slowly migrating down slope, being added to from the underlying beds and modified in character. The slope deposits on continuous slopes between the uplands and the stream channels are sediments in transit, even though the movement is imperceptible. These deposits are intimately related to the alluvium along the narrow valley bottoms and in fact the part of the sheet of slope deposit nearest the stream channel is acted upon by stream processes during flood periods and by slope processes during intervening periods.
AlluviumGeneral features--Alluvial deposits occur along the bottoms of all the major valleys of the county. In most places the alluvium is relatively fine-textured and consists of the materials supplied to the channel by the lower edge of the sheet of slope deposits. The streams for the most part are not actively eroding the valley sides, but are serving merely as transportation lines for the sediment furnished to them. The general distribution of alluvium is shown on the geologic map (pl. 1.). The field mapping of the alluvium was more or less arbitrary in some places, for there is not a distinct boundary line between the slope deposits and alluvium.
Along a few of the major valleys where the valley floor is some distance below the top of the Ogallala formation, the alluvium includes a relatively large percentage of sand and gravel derived from the Ogallala.
Water supply--Along some of the valleys in Thomas County the alluvium is above the water table and therefore is dry except during rainy periods. In some other valleys the alluvium does not yield appreciable quantities of water to wells because of the fine texture and poor sorting of the material. Along parts of South Sappa Creek, Prairie Dog Creek, and the Saline River near the east county line, the alluvium is derived largely froth the Ogallala formation and is below the water table; therefore, it yields some water to wells. In these areas, however, the alluvium is so thin that wells should be drilled into the underlying Ogallala to obtain a permanent supply of water.
Kansas Geological Survey, Thomas County Geohydrology|
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Web version Nov. 2001. Original publication date Dec. 1945.