Distribution and Character of Loess and Loess Soils in Southeastern Idaho

70s

Distribution and Character of Loess and Loess Soils in Southeastern Idaho

Glenn C. Lewis’ and Maynard A. Fosberg’

ABSTRACT

The depth of loess deposits over most of the area of south-central and southeastern Idaho is between 50 to 100 centimeters thick. The thickest loess deposits are south and east of the Snake River where depths of loess up to 36 meters have been measured. These deposits are parallel to and immediately adjacent to the Snake River.

The particle size of loess decreases with distance from the eastern margin of the Snake River Plain as indicated by a decrease in very fine sand and coarse silt and an increase in medium and fine silt. It appears that the Snake River Plain is a major source of the loess in this area. Minimum deviations to the overall trend suggest that some loess was derived from more local sources such as localized flood plains.

The calcium carbonate equivalent of the loess in the Snake River Plain varies from place to place and within a loess section. Large areas in the central Snake River Plain are underlain by a lime-silica duripan which suggests deposition and removal of loess with intervening periods when calcium was leached downward and deposited as calcium carbonate within a loess body. In the loess on the eastern side of the plain, there is no consistent relationship between the thickness of loess or annual precipitation and the calcium carbonate equivalent at the maximum zone of accumulation or in the unaltered loess.

In general, the amount of clay in the first and second clay maxima of loess sections east and south of the Snake River Plain increases with altitude and distance from the plain; however, there is no consistent relationship of the depth in the loess to the increase of clay. The presence of at least two clay maxima in most loess sections indicates more than one period of loess deposition. One loess section, described in detail, contains a buried soil, which has

‘Plant and Soil Sciences Department, University of Idaho, Moscow, Idaho 83843.

been named the Fort Hall Geosol, and two additional buried soils deeper in the section.

Species of clay mineral found in the coarse clay fraction are similar throughout loess deposits in southern Idaho. They consist primarily of smectite, illite, and kaolinite with lesser amounts of vermiculite and occasionally a trace of chlorite. Quartz and feldspars in the coarse clay fraction, along with high illite throughout the loess deposit, indicate relatively little weathering has taken place.

INTRODUCTION

Loess deposits in excess of 6 meters thick cover several thousand square kilometers of south-central and southeastern Idaho. Although these silty deposits constitute one of the major soil parent materials in this area, they have received little study. Most investigations of loess have been a product of agronomic, soil, geological, and ground-water surveys.

In his early work in Idaho, Russell (1902) made note of fine, yellowish white silts resembling the celebrated loesses of China and the Mississippi River Valley. He recognized that these materials were not derived from the underlying rocks and that the older lava flows are covered with fine dust whereas the younger flows are free of dust.

Early soil surveys in the southeastern Idaho area recognized eolian silt deposits (Lewis and Peterson, 1921; Baldwin and Youngs, 1925; Youngs and others, 1925; Poulson and Thompson, 1927; Youngs and others, 1928). Deposits in the Portneuf area were described as fine, floury, wind-borne material, much of which had been carried long distances and derived from a wide variety of rocks (Lewis and Peterson, 1921). Practically all of the soils of the area were thought to be modified to some extent by deposition of wind-blown material. Small areas were covered by coarser sandy material which has blown short distances, mostly from dry stream channels or exposed lava beds. In the vicinity of Burley in south-

706 Cenozoic Geology of Idaho

central Idaho (Youngs and others, 1928) the soils were recognized as being derived from fine dustlike, wind-borne materials over 6 meters thick in places.

Mansfield (1920) identified extensive deposits in the Fort Hall Indian Reservation in southeastern Idaho as volcanic ash, hill wash, etc., forming rounded hills and sloping benches. These same deposits are identified as loess in the soil survey of the Fort Hall Reservation (McDole, 1977).

Poulson and others (1943) in the soil survey of the Blackfoot-Aberdeen area in southeastern Idaho, stated that the wind-borne loess has an indefinite origin, principally from sources to the west, giving them a mixed mineralogy. These soils were described as fairly uniform with excellent moisture-holding properties.

McDole (1968, 1969) wrote a masters and doctoral thesis on the loess deposits of southeastern Idaho. His work was in the area east of Fort Hall and southwest of Pocatello. He studied some of the physical, chemical, and mineralogical properties of the loess. Some of his data is included in this paper.

The main objective of this paper is to show the aerial extent, thickness, and some properties of the loess deposits in southeastern Idaho.

GEOLOGY OF AREA

Most of southeastern Idaho lies in the northeastern part of the Basin and Range physiographic province. The mountain ranges and intervening valleys trend approximately north-south and apparently result from crustal extension and associated normal faulting during the last lo-20 million years (Williams and others, 1982 this volume; K. L. Pierce, written communication, 1981). The eastern Snake River Plain transects the trend of these ranges and basins at a high angle. Along the axis of the eastern Snake River Plain, Quaternary basaltic shield volcanoes have built up its central part, leaving the edges of the plain, lower with drainages such as the Snake River.

In Pleistocene time, the basins and the Snake River Plain have periodically been the site of both glacial and nonglacial gravel deposition (Pierce and Scott, 1982 this volume;l. Much of the loess in southeastern Idaho appears to have been derived by deflation of silty sediment from these flood plains and active alluvial fans.

Times of loess deposition were apparently associated with the rigorous climatic conditions of Pleistocene glaciations. Some of the ranges on the south side of the plain and most of the ranges on the north side supported sma.11, valley glaciers (Pierce

and Scott, 1982 this volume). A large ice field covered the Yellowstone-Teton area at the head- waters of the Snake River (U. S. Geological Survey, 1972). Quaternary climatic change resulted in repeated glaciation as well as other geomorphic changes. In the Yellowstone area, the last glaciation (Pinedale) occurred between 40,000+ years and 11,000 years ago; the next to last glaciation (Bull Lake) is dated at about 140,000 to 150,000 years old (Pierce and others, 1976). Although the sequence of glaciations older than the Bull Lake Glaciation has not been well dated in the Rocky Mountains, the marine, oxygen-isotope record shows that glaciations tens of thousands of years long have occurred at intervals of about every 100,000 years for more than the last 700,000 years (Shackleton and Opdyke, 1973). On the average, Quaternary environments were considerably different from the present; 90 percent of the last half a million years was more glacial than present (Emiliani, 1972).

DISTRIBUTION AND THICKNESS OF LOESS

Aerial extent of loess deposits in southern Idaho was compiled by several methods. Much of the information was derived from reports prepared by the University of Idaho and the Soil Conservation Service (SCS) for the Idaho Water Resource Board (J. C. Chugg and others, 1967-68, Special Soil Sur- veys of 13 Idaho Counties. Reports No. 1 through 13, U. S. Department of Agriculture (USDA), SCS, and University of Idaho). Soil survey data from pub- lished (Daniels and others, 1969; McDole, 1977) and unpublished USDA SCS Soil Surveys (Gooding County, 1969; Lincoln County, 1969; Elmore County, 1971) and unpublished field data supplied by local soil scientists from the SCS were used as major sources of data in this study. Areas not covered by soil survey literature were completed with the aid of field reconnaissance. Observations were made and field data collected on depths of the loess and nature of the underlying material throughout the study area.

A generalized map of loess thickness (Figure 1) categorizes loess. deposits: shallow, 25 to 50 centimeters; moderately thick, 50 to 175 centimeters; and very thick, over 175 centimeters (Lewis and others, 1975). A minimum thickness of 25 centimeters of silty material with characteristic loess appearance was necessary before the deposit was classified and mapped as loess.

The loess depth over much of the southwestern part of the Snake River Plain and Owyhee Uplands

Lewis and Fosberg-Distribution and Character of Loess 707

IO 0 IO 20 30 40mlles

z 2~~~Oktitoms+ar

I 0 LOESS 25 cm. or absent El LOESS 25-50cm.

4 Ulll LOESS 50- 175cm. $j I LOESS OVER 175cm. w -, _ BOUNDARIES

e _-

____ APPROX. BOUNDARIES i +RlVERS AND LAKES #-Ml1

NEVADA I 114” UTAH

Figure I. Area1 distribution and thickness of Ioess in south-central and southeastern Idaho.

is less than 25 centimeters and was not shown on the map. Thus, the western limit of mapped loess (Figure 1) is in the vicinity of the Bruneau River in the Mountain Home area. Although there are small isolated areas of loess west of this boundary thicker than 25 centimeters, they are not large enough to be included on the map. These isolated areas of loess are found mostly adjacent to the Snake River in Owyhee, Canyon, and Payette Counties. There are also sizeable areas of loess deposition in western Wyoming and southwest Montana; both areas are adjacent to the Idaho deposits.

Shallow loess deposits are primarily on the area of younger basalt flows in the central part of the Snake River Plain and on the more gentle slopes in the mountain ranges adjacent to the plain. The area of loess cover may be larger than that depicted on the map in Figure 1, because the discontinuous and spotty occurrence of loess on the irregular surface of this basaltic plain made an exact boundary difficult

to establish. The depth of loess indicated in Figure 1 represents the most representative loess thickness. Undoubtedly all of these older lava flows were covered with loess. In contrast, Holocene lava flows have only traces of wind-blown materials.

The thickness of loess over much of the southern Idaho study area is between 50 and 100 centimeters. This loess mantle occurs on a variety of materials including volcanic rocks, alluvium, and lacustrine sediments. In much of the western and central portion of the mapped area, the loess thinner than l-2 meters commonly has a lime-silica duripan in the C horizon of the soil profile.

The loess mantle in the mountain valleys in the extreme southeastern part of the study area ranges from 50 to 100 centimeters thick. These north-south- trending valleys are separated from the Snake River by mountains, some of which reach elevations of 2,500 to 2,900 meters. Bright (1963) identified and named Niter loess in Gem Valley in Caribou


County, about 65 kilometers southeast of Pocatello, and suggested the source was the nearby flood plain of the Bear River.

The thickest loess deposits are east of the Snake River from the vicinity of Twin Falls (in the south- central part of the state) and extend to the northeastern corner of the study area about 100 kilometers northeast of Idaho Falls (Figure 1). One significant area lies northeast of Twin Falls on the north side of the Snake River.

Most of the very thick loess indicated in Figure 1 is considerably thicker than 175 centimeters. Well log information was used to establish loess thickness in the area between Twin Falls and Burley in south- central Idaho. Depths in this area range from 5 meters near Twin Falls to 36 meters, in lee positions, 32 kilometers east of Twin Falls. Near Burley (about 38 kilometers farther east) depths again decreased to a maximum of about 12 meters.

Maximum depth of the deposits in several areas between Raft River and the area northeast of Idaho Falls, on the eastern side of the Snake River Plain, is largely unknown. Maximum depth of loess in a section located a few kilometers southwest of Pocatello was found to be 8.5 meters. Other sites examined a few kilometers north of Pocatello showed depths of 12, 13, and 16 meters.

CHAKACTEK OF LOESS AND LOESS SOILS

PARTICLE SIZE DISTRIBUTION

The sampling procedure for studying the particle size distribution, as well as for other characteristics of the loess and associated soil profiles, was as follows: soils were sampled from the genetic horizons of the modern surface soil at each site. Composite samples were taken in intervals of 25 to 50 centimeters from the rest of the section. These intervals were arbitrarily selected except where distinct changes in color or other characteristics indicated natural breaks in the section.

Particle size distribution was determined on carbonate-removed samples by the pipette method as described by Kilmer and Alexander (1949).

No consistent area1 pattern of particle size distribution in the loess deposits on the Snake River Plain is presently evident, either in the direction of the present prevailing winds or as a function of distance from the Snake River. Evidence suggests that this may be due to at least two factors: continuous mixing by the removal and redeposition of the loess, and multiple sources of the loess. The

mineralogy of the coarse silt fraction also suggests that the wind direction was occasionally from the northwest, not from the Snake River. We are continuing our study of particle size distribution in the loess deposits on the Snake River Plain.

The particle size of loess decreases with distance from the Snake River in the terrain along the eastern margin of the Snake River Plain (Figure 1, Table 1). This will be discussed with data from loess sections along four traverses (Figure 2, Table 1).

Traverse A

Traverse A starts near the eastern margin of the Snake River Plain in the vicinity of Fort Hall and continues southeast to the vicinity of Grace. There is a decrease in very fine sand and coarse silt and an increase in medium and fine silt with increased distance from the Snake River Plain (Table 1). The decrease in particle size with distance from the plain suggests that the flood plain of the Snake River was at least part of the source of the loess.

There are some minor deviations to the overall trend. Very fine sand should be lower in section 0301 and 1504 than in 0611 since the latter is closer to the plain. Also, fine silt did not consistently increase with distance from the plain. This may suggest that the loess in traverse A was not derived wholly from the Snake River Plain. McDole (1969) postulated that the loess in sections 0611, 0301, and 1504 was derived from three different sources: the Snake River Plain and two local sources, Ross Fork Creek and Portneuf River drainages.

Traverse B

Changes in altitude as well as in distances from the Snake River Plain from Blackfoot to Garden Peak complicate the interpretations of loess particles in traverse B. Traverse B covered a distance of 8 kilometers from the plain with an elevation increase of 400 meters. The distribution of very fine sand and silts is shown in part B of Table 1. There was a small decrease in very fine sand and coarse silt and a small increase in medium and fine silt with the increase in elevation and distance from the plain. Thus, it appears that as both distance and altitude above the Snake River increase, particle size decreases.

Traverse C

Traverse C runs from west of Pocatello on the Snake River Plain to near Malad. Loess section 3611 (Figure 2) at the southern end of the traverse is

Lewis and Fosberg-Disrribulion and Charocier of LOPS 709

just a few miles north of the shore line of pluvial Lake Bonneville. There was a decrease in very fine sand and coarse silt and an increase in medium and fine silt with distance from the plain. This suggests that the plain was the source of the loess. Because loess section 3611 is close to the location of Lake Bonneville it could be argued that the lake constituted the source but it seems unlikely that the sources are different, since section 3611 is at the southern end of a body of loess which is continuous from the Snake River Plain.

Traverse D

Data on particle size distribution were available on four sites near the northeastern margin of the

Snake River Plain (traverse D). Two of the sites are near the edge of the plain near Rexburg, and the other two sites are east near the Idaho-Wyoming border (Figure 2).

The two loess sections (3302, 3306) nearest the major rivers on the Snake River Plain had a higher content of very fine sand than those (W2015, 4101) away from the plain. The coarse silt content was also higher in the two sites nearer to the plain than at site W2015 to the east. No breakdown of the silt fractions was available at site 4101. Section 3306 is closer to the St. Anthony sand dunes than section 3302, and section 3302 is near an outwash fan from the South Fork of the Snake River, which may account for the higher amounts of very fine sand in section 3306.

Table I. Average sand and silt size* particle distribution along four traverses on the eastern side from the Snake River Plain

‘CaCO, removed; calculated on clay-free basis **Measured from western edge of loess deposit in the vicinity of Rexburg


UTAH Figure 2. Location of traverses and sampling sites.

CALCIUM CARBONATE EQUIVALENT

The calcium carbonate equivalent of the loess varies both within and between loess sections. Soils of large areas in the central Snake River Plain have a lime-silica duripan which suggests cycles of loess deposition and subsequent partial removal by deflation during which calcium and silica were leached downward in the soil. In the Snake River Plain, calcium carbonate equivalents are as much as 50 percent in the zone of accumulation, although values of 20 to 25 percent are more common. In unweathered loess, carbonate content consistently ranges between 15 and 20 percent.

Data for calcium carbonate equivalent of the upper loess deposit are given in Table 2 for the four traverses. The data include the thickness of the leached zone and the carbonate content in the zone of maximum accumulation and in the unaltered loess below. These values are compared to annual precipitation, altitude, distance from Snake River Plain, and thickness of the upper deposit.

The thickness of the leached zone and the depth of maximum carbonate increases with increased precipitation as expected. The leached zone may

contain some carbonates, particularly at lower elevations.

There is no consistent relationship between thickness of loess or annual precipitation and the calcium carbonate equivalent at the maximum zone of accumulation or in the unaltered loess, with the exception of section W2015. The relatively high annual precipitation of 76 centimeters at this site was apparently enough to leach out part of the carbonates during and following deposition. There did not appear to be any unaltered loess in the relatively thin deposits at sections 1504, 1523, or 3611.

More work is needed to understand the carbonate chemistry of the southern Idaho loess deposits. This discussion serves to indicate amounts present and to show the complexity of the loess deposits.

CLAY MAXTMA

Changes in clay content with depth in a loess section might indicate buried soils separating different periods of loess deposition. There is a clay maximum in the B horizon of the modern soil in all four traverses (Table 3). Below this surface soil at least one additional clay maximum is in the loess sections of traverses A, B, and C. A second clay maximum was found in section W2015 but not in the other sections in traverse D. Interpretation of the upper two clay maxima in the loess deposits of southeastern Idaho have been made by Pierce and others (1982 this volume).

The clay maximum in the modern soil can be interpreted as normal soil development. There does not appear to be a consistent relationship in traverse A between the clay content of the modern soil and the distance from the Snake River Plain. This is also true of the second clay maximum along the traverse. Also, the depth to the clay maximum is variable along the traverse. The magnitude of the second maximum is similar to the first, except for sections 1521 and 1523, where they were higher than the first.

For traverse B, clay content of the modern soil increases with distance from the Snake River Plain (Table 3). The magnitude of the second clay maxima is slightly greater than in the modern soil except for the section farthest away from the plain. Depth to the first clay maximum is variable, but the three sections farther away from the plain have somewhat similar depths to the second clay maximum.

The clay maxima in the modern soils in traverses C and D increase with altitude in the direction away from the Snake River Plain as does the second maxima in traverse C. In traverse C, the clay content of the second maxima is higher in the two sites (3905

Lewis and Fosberg- Disrribufion and Character of Loess 711

and 3611) than in the modern soil. In traverse D, only section W2015 has a second clay maximum.

These loess sections reveal that, in general, the amount of clay increases with altitude and distance from the Snake River Plain. There is no consistent relationship of the depth to the clay along the transects. It may not be possible to compare a second clay maximum since they may not represent the same buried soil or depositional phase. This will become evident when the properties of buried soils are discussed.

CLAY MINERALOGY

X-ray diffraction patterns were made on the coarse clay fraction (0.2 to 2 pm) of samples following magnesium saturation and treatment with ethylene glycol (Jackson, 1956). In order to differentiate between vermiculite and chlorite and substantiate the presence of kaolinite, X-ray diffraction patterns were made on potassium-saturated

samples. The potassium-saturated slides were heated to 500°C and rerun to differentiate between the chlorite and kaolinite. Interpretation of the X-ray patterns was based on Whittig (1965).

The clay mineral species found in the coarse clay fraction are similar throughout all the loess sections examined. They consist primarily of expanding lattice minerals (smectite), illite, and kaolinite with lesser amounts of vermiculite and occasionally a trace of chlorite. Quartz and feldspars are also found in the coarse clay fraction of most samples. Changes in amounts of these principal clay-size minerals with depth in the loess section will be discussed later.

A complete clay mineral analysis should ideally include both qualitative identification and quantitative estimation of the clay minerals present. The presence of one or more of the following characteristics in natural clay mixtures (Keller, 1962) may make rigorously quantitative expresssions of clay mineral percentages or ratios misleading: (1) inconsistencies in the chemical compositions of the clay minerals, (2) variation in the amount of amorphous (to X-ray)

Table 2. Calcium carbonate equivalent in the zones of maximum accumulation and in the unaltered loess, and thickness of leached zone as related to precipitation. altitude, distance from Snake River Plain, and thickness of upper loess.

(I) No apparent unaltered loess present


material from specimen to specimen, (3) variability in ratios of mixing in clays that show random mixed layering, and (4) differences in the degree of postdepositional weathering of samples.

Although it is recognized that a quantitative expression of the amounts of clay minerals present in the loess is not possible, an estimate of relative amounts can be achieved by using illite equivalents (Lewis and White, 1964). Converting peak heights to illite equivalents was accomplished by dividing the smectite peak height by 4, vermiculite peak by 2, and kaolinite peak by 4, summing these along with the illite peak height, making this equal to 100 percent, and determining the contribution of each clay mineral in terms of percent,

An estimate of the relative urooortions of 1

smectite, vermiculite, illite, and kaohnite are shown in Table 4. The traverses run from the eastern edge of the Snake River Plain to the southeast, to the south, and to the east.

appears to be decreasing with increasing distance from the Snake River Plain. Smectites occur in larger quantities in soils that have high amounts of soluble salts present, a condition more prevalent in the Snake River Plain than at higher elevations where there is more precipitation and leaching. The presence of quartz and feldspar in the coarse clay fraction, along with high illite content throughout the loess deposit, would indicate that relatively little weathering has taken place. The kaolinite content does not change significantly along the transects or between the loess sections studied. The kaolinite is probably detrital and has not changed or formed in this environment since this would require strong weathering.

PROPERTIES OF BURIED SOILS

The expanding lattice component (smectite) Much work remains before the stratigraphy of the

southern Idaho loess deposits can be interpreted. It

Table 3. Depth and clay content of modern and upper buried soil in loess sections.

Lewis and Fosberg-Distribution and Characrer of Loess 713

Table 4. Estimated amounts of clay minerals in coarse clay fraction.

has been observed that buried soils are present in many of the sections studied. Stratigraphic units in the loess deposits of southeastern Idaho are discussed by Pierce and others (1982 this volume).

McDole and others (1973) reported buried soils in seven of eleven sections comprising three transects traversing the loess deposits adjacent to the Snake River flood plain near Pocatello. However, some sections did not extend through the entire depth of loess. Section 0694 gives the best expression of the greatest number of buried soils in this group. Figure 3 shows three distinct zones where maxima in calcium carbonate equivalent suggest the presence of buried soils at depths of 7, 10, and 11.5 meters. These three maxima in carbonates are stronger than the maximum present in the Cca horizon of the modern soil. Approximately coinciding with these three zones of carbonate accumulation are maxima in soluble salts (E.C.) and increases in total clay fraction. In the original auger samples, the B horizons were difficult to recognize, for they were white and enriched with secondary carbonate and other salts. The carbonate, clay, and soluble salts in these zones show magnitudes at least twice as high as the rest of the section.

There is an anomaly in the characteristic horizon sequence of these buried soils. This is the occurrence of the soluble salt maxima coincident with or above the carbonate and clay maxima. This sequence is contrary to expected horizon development as ex- hibited by the modern soil in the upper part of the section (Figure 3). In usual soil development, the

genetic soil horizons occur with a maximum in clay nearest the surface, followed by a maximum in carbonates at a slightly greater depth and the maximum in soluble salts deeper than the carbonate maximum. Thus, since the accumulations of soluble salts occur above or coincident with the clay and carbonate maxima, it must be assumed that the accumulation of the more soluble materials was subsequent to the event that created the carbonate and clay maxima. This implies that the three buried soils in the section are each complex or polypedo- genie. This is further verified by secondary CaCO, accumulation in the B horizons of buried soils in the exposed head walls of the Gay phosphate mine near the middle of traverse A, Figure 2 (Ken Pierce, 1981, personal communication).

Each of these polypedogenic buried soils reflects at least two different sets of soil-forming factors. The initial set of factors responsible for each of the buried soils formed a soil profile reflected by the accumulation of clays and carbonates. The resultant soil profile included a weakly developed B horizon and a strong Cca horizon. This soil profile reflects a degree of development at least as strong or stronger than the modern soil profile. The subsequent set of soil-forming factors, which included new deposition of materials, produced movement of more readily soluble salts and carbonates, perching these latter materials on top of the previously accumulated clay and carbonates.

Particle size analyses show the modern soil surface and buried soil zones to be higher in coarser


sands (greater than 100 pm) than the rest of the section (0694). The higher sand content is possibly the result of deflation of some of the finer loess particles during the interval of soil development. A geosol is a buried soil, having physical characteristics that are easily recognized, which can be used as a stratigraphic marker to correlate between different sites (Morrison, 1968).

The first (uppermost) buried soil with development equal to or greater than the surface soil was named the Fort Hall Geosol by McDole (1969) and McDole and others (1973). This geosol was found in seven out of ten sections in the Pocatello area. The relative increase in clay, the amount of fine clay, clay mineral types, calcium carbonate equivalent, silt, sand fractions, and total salts served to correlate this buried soil from one section to another. The Fort Hall Geosol finds its best expression in section 0694 (Figures 3 and 4) and is described as follows: The Fort Hall Geosol has 50 to 100 percent more total clay than the surface soil profile or other buried soil in the section and an increase in the fine clay fraction which is about double the increase in the coarse clay fraction. The lesser buried soils show only slight increases in both fine and coarse clay fractions.

Calcium carbonate equivalent of 32 to 34 percent in samples from the lower part of the Fort Hall Geosol is twice that of the unaltered parent loess. The calcium carbonate equivalent of the geosol also represents an accumulation of carbonates about twice the magnitude of the surface soil profile and lesser buried soils.

A slight decrease in very coarse silt occurs in the Fort Hall Geosol. The most striking anomaly in the silt fractions is seen in the amounts of coarse and medium silts. In the upper loess unit, above the Fort Hall Geosol, the amount of coarse silt is distinctly higher than the amount of medium silt. Coincident with the geosol, the relative amounts of these two fractions converge and maintain approximately equal values throughout the remainder of the sampled section.

Another identifying criterion of the Fort Hall Geosol is based on changes in the amount of very fine sand (100 to 50 pm) and coarser sands (1.0 to 0.1 millimeter) fractions (Figure 4). The very fine sand ranges from 12 percent at the surface of the geosol to a maximum of 20 percent within the geosol. The coarser sands fraction reaches a maximum of 1.3 percent in the geosol compared with less than 0.5 percent above and below the geosol.

Very fine sand reaches a maximum of 19 percent

Figure 3. Calcium carbonate equivalent, soluble salts and clay in section 0694.

Figure 4. Sand and silt fractions in section 0694.

near the surface of the modern soil and then decreases to about 14 percent until the geosol is reached. Coarser sands reach a maximum of 2.8 percent in the upper 56 centimeters of the modern soil and then decrease to about 0.5 percent. The increase in sands in the modern soil can be explained as the removal of finer materials by water or wind erosion of the surface, leaving the coarser materials behind. The increase in sands of the Fort Hall Geosol and lesser buried soils also supports pedologic origin of these zones in the sections.

These identifying characteristics imply the nature of events which took place to form these buried soils. The initial event in the formation of the buried soils resulted in accumulations of relatively insoluble carbonates and clays. During the period of soil development deflation removed some of the finer particles from the surface of the soil and increased the percentage of sand-size particles. The Fort Hall Geosol represents a much stronger set of soil- forming factors than those responsible for the formation of the existing modern soil or the other buried soils in the section.

Lewis and Fosberg-Distribution and Character of Loess 715

REFERENCES

Baldwin, Mark and F. 0. Youngs, 1925, Soil survey of the Twin Falls area, Idaho: U. S. Department of Agriculture, Bureau of Soils, 52 p.

Bright, R. C., 1963, Pleistocene Lakes Thatcher and Bonneville, southeastern Idaho: University of Minnesota Ph.D. dissertation, 363 p.

Daniels, D. M., H. L. Hansen, T. W. Priest, and W. G. Perrin, 1969, Soil survey, Teton area, ldaho- Wyoming: U. S. Department of Agriculture, Soil Conservation Service, 95 p.

Emiliani, Cesare, 1972, Quaternary paleotempera- tures and the duration of the high temperature intervals: Science, v. 178, p. 398-401.

Jackson, M. L., 1956, Soil chemical analysis, advanced course: Department of Soils, Univeristy of Wisconsin, 894 p.

Keller, W. D., 1962, Clay minerals in the Morrison Formation of the Colorado Plateau: U. S. Geological Survey Bulletin 1150, 90 p.

Kilmer, V. J. and L. T. Alexander, 1949, Methods of making mechanical analysis of soils: Soil Science, v. 68, p. 15-24.

Lewis, G. C. and J. L. White, 1964, Chemical and mineralogical studies on slick spot soils in Idaho: Soil Science Society of America, Proceedings, v. 28, p. 805-808.

Lewis, G. C., M. A. Fosberg, R. E. McDole, and J. C. Chugg, 1975, Distribution and some properties of loess in south-central and southeastern Idaho: Soil Science Society of America, Proceedings, v. 39, p. 1165-1168.

Lewis, H. G. and P. P. Peterson, 1921, Soil survey of the Portneuf area, Idaho: U. S. Department of Agriculture, Bureau of Soils, 52 p.

Mansfield, G. R., 1920, Geography, geology and mineral resources of the Fort Hall Indian Reservation, Idaho: U. S. Geological Survey Bulletin 713, 152 p.

McDole, R. E., 1968, Some properties of loess in the Bannock Peak transect in southeastern Idaho: University of Idaho M.S. thesis, 162 p.

-1 1969, Loess deposits adjacent to the Snake River flood plain in the vicinity of Pocatello, Idaho: University of Idaho Ph.D. dissertation, 231 p.

-, 1977, Soil survey of Fort Hall area, Idaho: U. S. Department of Agriculture, Soil Conserva- tion Service, 97 p.

McDole, R. E., G. C. Lewis, and M. A. Fosberg, 1973, Identification of paleosols and the Fort Hall Geosol in southeastern Idaho loess deposits:

Soil Science Society of America, Proceedings, v. 37, p. 611-616.

Morrison, R. B., 1968, Means of time-stratigraphic division and long-distance correlation of Quater- nary successions, in R. B. Morrison and H. E. Wright, Jr., editors, Means of Correlation of Quaternary Successions, v. 8: Proceedings, VII Congress, International Association for Quater- nary Research, University of Utah Press, Salt Lake City, Utah, p. 113.

Pierce, K. L., J. D. Obradovich, and Irving Friedman, 1976, Obsidian hydration dating and correlation of Bull Lake and Pinedale Glaciations near West Yellowstone, Montana: Geological Society of America Bulletin 87, p. 703-710.

Pierce, K. L. and W. E. Scott, 1982, Pleistocene episodes of alluvial-gravel deposition, southeastern Idaho, in Bill Bonnichsen and R. M. Brecken- ridge, editors, Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26.

Pierce, K. L., M. A. Fosberg, W. E. Scott, G. C. Lewis, and S. M. Colman, 1982, Loess deposits of southeastern Idaho: age and correlation of the upper two loess units, in Bill Bonnichsen and R. M. Breckenridge, editors, Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26.

Poulson, E. N. and J. A. Thompson, 1927, Soil survey of the Jerome area, Idaho: U. S. Department of Agriculture, Bureau of Chemistry and Soils Series 1927, no. 16, 22 p.

Poulson, E. N., L. B. Nelson, and A. E. Poulson, 1943, Soil survey Blackfoot-Aberdeen area, Idaho: U. S. Department of Agriculture, Bureau of Plant Industry in cooperation with University of Idaho, ser. 1937, no. 6, 111 p.

Russell, I. C., 1902, Geology and water resources of the Snake River Plain of Idaho: U. S. Geological Survey Bulletin 199, 192 p.

Shackleton, N. J. and N. D. Opdyke, 1973, Oxygen isotope and paleomagnetic stratigraphy of equa- torial Pacific core V28-238-oxygen isotope temp- eratures and ice volumes on a 105 and 106 year scale: Quaternary Research, v. 3, p. 39-55.

U. S. Geological Survey, 1972, Surficial geologic map of Yellowstone National Park: U. S. Geological Survey Miscellaneous Geologic Investigations Map I-710.

Whittig, L. D., 1965, X-ray diffraction techniques for mineral identification and mineralogical composition, in C. A. Black, editor, Methods of Soil Analysis, Part l-Physical and Mineralogical Properties: American Society of Agronomy Monograph 9, p. 671-678.


Williams, P. L., H. R. Covington, and K. L. Pierce, 1982, Cenozoic stratigraphy and tectonic evolu- tion of the Raft River basin, Idaho, in Bill Bonnichsen and R. M. Breckenridge, editors, Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26.

Youngs, F. O., A. W. Kern, and E. N. Poulson, 1925, Soil survey of the Soda Springs-Bancroft area, Idaho: U. S. Department of Agriculture, Bureau of Chemistry and Soils, ser. 1925, no. 6, 33 p.

Youngs, F. O., Mark Baldwin, A. J. Kern, and G. R. McDole, 1928, Soil survey, Minidoka area, Idaho: U. S. Department of Agriculture, Bureau of Chemistry and Soils, ser. 1923, no. 27, p. 589-902.

Documents

Distribution and Character of Loess and Loess Soils in Southeastern Idaho