Professional Paper 1698 Energy Resource Studies, Northern Front Range, Colorado

Edited by Neil S. Fishman

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Abstract

Saline soils along the Front Range north of Denver occur in specific geomorphic and hydrologic settings where ground water enriched in dissolved solids is close to the soil surface for at least part of the year. More than 200 saline soil areas were mapped in a five-quadrangle area extending northward from Northglenn to Johnstown and then westward to Longmont. Saline soils occur on:

1. Upland areas underlain by loess deposits (47 percent of mapped occurrences);

2. Upland areas underlain by residual soils formed on shale (28 percent);

3. Areas along flood plains of minor streams where the upland parts of the drainage basin are dominated by loess deposits (16 percent);

4. Areas of eolian sand immediately adjacent to wetlands or lakes (9 percent).

Saline soils are rare on the flood plains of larger streams that flow eastward across the study area from the mountains to the South Platte River.

Some saline soils occur close to oil production tank batteries, but only two of the tank battery sites examined show clear indications of saltwater leakage from pits or tanks. Soils near these two sites contain chloride-dominant salts in soil leachates, which contrasts with the sulfate-dominated salt assemblages at all other sampled localities.

The mapped saline soils show a preferred geographic distribution, lying primarily in a belt that extends from the city of Loveland southeastward to the northern Denver suburbs. The northern part of this belt is underlain by the Upper Cretaceous Pierre Shale. Numerous depressions have formed on this unit, probably as deflation basins, during the late Pleistocene and early Holocene. Saline soils are rare to absent in areas east of the main area of depressions. These deflation basins appear to be a source for the thick (as much as 2–3 meters) clayey loess that blankets upland areas downwind and may also be a source for windblown salts. This is consistent with a dominantly northwesterly wind direction indicated by linear orientations of dunes and dune axes in the east-central part of the study area and with prevailing paleowind directions documented in sand-dune areas farther to the east. Sulfur isotopic composition of dissolved sulfate and of sulfate salts in soils in the area of Pierre Shale bedrock is mimicked in saline soils closer to the Denver suburbs despite a transition to other bedrock units. We conclude that the Pierre Shale is a primary source of salts in saline soils of the study area.

Introduction

In many semiarid areas of the Western United States and Canada, farmers and ranchers contend with saline soils that lead to reduced yields on crop and pasture lands (U.S. Department of Agriculture–Natural Resources Conservation Service, 1992). Within the Front Range Infrastructure Resources Project (FRIRP) area (fig. 1), saline soils are common from the northern Denver suburbs north to the latitude of Greeley, an area of about 1,400 km². Surface water and shallow ground water associated with these saline soils and nearby wetlands commonly exceed 5,000 microsiemens per centimeter at 25° Celsius (μS/cm) specific conductance compared to an 838-μS/cm northern Front Range regional average (Gaggiani and others, 1987) and a range of 100 to 500 μS/cm for water in irrigation ditches and reservoirs fed directly from streams exiting the mountains (Otton and Zielinski, unpub. data, 2001).

Areas of saline soils are characterized by stunted or missing vegetation, the presence of salt-tolerant plant species, and, commonly, “white alkali” salt crusts. In some places, the salt crusts are sufficiently abundant to form small (a few square meters) to locally large (a few hectares) patches of white soils completely barren of vegetation.

The FRIRP area has many potential sources of soil salinity, including natural salts in bedrock or surficial deposits, fertilizer, oil and gas produced-water spills and leaks, coal-mine waste piles, and runoff from feedlots and roads. Development of saline soils may be enhanced near irrigation ditches and reservoirs where seepage from these features raises the water table and leaches additional salts from the local soils or bedrock.

Surface expressions of saline soils can form seasonally in semiarid areas during periods of high ground-water levels that typically occur in the spring after snowmelt, following significant rainstorms, and during seasonal irrigation of fields. Shallow ground water is transported through the capillary fringe toward the surface. The thickness of this capillary fringe is dependent on the texture of the soil. In coarse, permeable, sandy soils the capillary fringe may only be 15 cm thick, whereas in less permeable, silty and clayey soils it may be 1 m or more (Henry and others, 1987). Water within the capillary fringe becomes more saline as it moves toward the surface because part of the water is continuously removed through evapotranspiration and the dissolved solids are concentrated in the remaining water. Where the concentration of dissolved solids exceeds the solubility limits of certain minerals, those minerals precipitate in the soil profile. Salt crusts may form at the soil surface if the water table is high enough to cause the capillary fringe to reach the ground surface.

Unfortunately, high water-table conditions usually occur in the early spring as crops are germinating. If the salinity exceeds critical levels in the root zone, germination does not occur or seedlings are severely stunted. Later in the growing season, the water table declines in most soils, the surface accumulations of salt are cut off from replenishment, and rainfall flushes salts from the surface or wind blows the salts away. Where the salinity in the root zone decreases to certain tolerance levels, some late-germinating, salt-tolerant forbs may then establish themselves on the saline soil sites.

Saline wetlands have formed in parts of the study area. They occur along stream drainages where saline ground waters are discharged to the surface or where evaporation along the drainage has concentrated salts. Such saline wetlands can be recognized by salt crusts that form on the stems of wetland plants and by the presence of salt-tolerant wetland plant species.

During preliminary springtime field work in the FRIRP study area north of Denver, we noted numerous areas of saline soils, some adjacent to oil and gas production operations (figs. 2A, B, C, D) or coal-mine waste piles. As part of a study of the effects of energy development on the infrastructure of the FRIRP area, one of our goals was to determine whether releases of saline produced water from the oil and gas operations or leaching of salts from coal-mine waste piles was partly responsible for the saline soils. Conversations with some local landowners about saline soils and oil and gas operations on their property indicated that the development of saline soils preceded the oil and gas operations. Moreover, mapping showed that most saline soil areas were not near oil and gas production operations. Aqueous leachates of variably saline soils, including soils near possible produced-water-affected sites, were analyzed for anion compositions. Leachates of natural nonsaline soils were dominated by carbonate and bicarbonate anions; the saline soils were dominated by sulfate; and some soils and waters near oil and gas production pits and tanks were dominated by chloride (Otton and Zielinski, 1999a; and fig. 3, this chapter). Soils affected by chloride-dominated produced-water salts were devoid of vegetation but did not have obvious surface accumulations of white sulfate salts. We conducted a detailed study of one site where both natural salts and produced-water salts leaking from a brine pit were documented (Zielinksi and Otton, 2001). A detailed study of a site surrounding a coal-mine waste pile (Zielinski and others, 2001) showed that sulfate and nitrate were leaching from the coal-mine waste into an adjacent wetland, but most salts in the drainage downstream from the wetland were derived from soils underlying adjacent irrigated fields.

The purpose of this report is to document further the geologic origins of sulfate-dominated saline soils of the study area. Data collection included (1) saline soil mapping, sulfur-isotope geochemistry, and mineralogy of saline soils in most of a five-7.5-minute-quadrangle area to establish the location and characteristics of saline soils in the landscape (figs. 4, 5, 6); and (2) compilation of the surficial and bedrock geology in a multiquadrangle area beyond the five-quadrangle saline-soil-mapping area to establish the geologic setting for the saline soils (figs. 5, 6).

Study Methods

Saline soils were mapped by visual observation during road and foot traverses throughout the five-quadrangle study area, primarily in the late winter–early spring when white saline soil crusts show maximum development. The dimensions of the saline soil areas were established by pace and compass surveys or by visual comparison with features of known extent such as houses, roads, topographic features, and fence lines.

The saline soils as mapped represent a minimum for their extent in the study area. Some ranchers have installed drain-tile systems under fields where saline soils occur. We did not attempt to locate such systems or to map them, although we found a few during the course of this study. Many slightly saline soil areas, characterized by subtle variations in plant health or the presence of salt-tolerant species, were not mapped.

The bedrock geology was compiled from maps by Colton (1978), Trimble and Machette (1979), and Scott and Cobban (1965) (table 1, fig. 6). The map of loess and eolian sand deposits, residual soils on bedrock, and soils developed on flood plains (fig. 5) was derived from county soil surveys for Adams, Boulder, Larimer, and Weld Counties (Sampson and Baber, 1974; Moreland and Moreland, 1975; Moreland, 1980; Crabb, 1980) by assigning specific soil map units to either a loess or sand geologic map designation following Madole (1995). Other units were assigned to the flood plain or residual soil units based on the soil map unit descriptions. Some soil map units are formed on erosional remnants of Pleistocene alluvium that occur in upland areas. Because these erosional remnants typically have a thin surface horizon of loess, they were assigned to the loess geologic map unit. Surficial geologic map areas showing hydrologic features and roads were compiled on a digital base map at 1:150,000 scale.

Soil profile samples were collected by augering holes with a 5-cm-diameter soil auger at approximately 15-cm intervals. We also collected a single Pierre Shale sample from the wall of a deep agricultural trench. Water samples were collected in 250-mL bottles prerinsed with the sample. Leachates of the soil and rock samples were prepared by air-drying the sample at 40°C and passing it through a ceramic-plate jaw crusher with a 3-mm opening to disaggregate it. Most of the sample (200 g) was weighed into a plastic beaker and 200 mL of deionized water added. Each 1:1 weight mixture was stirred vigorously, allowed to stand overnight at room temperature, and again stirred vigorously prior to pouring into two 250-mL centrifuge bottles. The slurries were centrifuged at 8,000 rpm for 40 minutes. Clear supernatant was decanted and filtered (0.45 μm). Concentrates of efflorescent salts were collected from the undisturbed surface of saline soil crusts by scraping and lifting with a stainless-steel spatula or knife blade at 23 sites across the study area. Samples were analyzed within a few days by powder X-ray diffraction (XRD) using CuKα radiation generated at 40 kilovolts and 25 milliamperes. Salt samples also were dissolved in deionized water for sulfur isotope analyses.

Soluble sulfate in surface water, soil leachates, and dissolved efflorescent salt samples was precipitated as barium sulfate by adding barium chloride to filtered sample solutions. Total sulfur in soil samples was extracted from powdered samples by Eschka fusion and recovered as barium sulfate following the procedure of Tuttle and others (1986). The barium sulfates were combusted in an elemental analyzer to form SO₂ gas. The sulfur isotopic composition of the gas was determined using a Micromass mass spectrometer (Micromass UK Limited, Manchester, UK) by a continuous-flow method modified from Giesemann and others (1994). Sulfur isotopic composition is reported in terms of a δ³⁴S value:

δ³⁴S (per mil, ‰) = (R_sample/R_standard – 1) * 1,000

where R is the atomic ³⁴S to ³²S ratio and the standard is Canon Diablo troilite (CDT). Reproducibility was ± 0.2‰ (parts per thousand). Analysis of the National Bureau of Standards NBS 127 seawater standard averaged 21.0‰ (n=5) compared with the value of 20.99‰ reported by Rees and others (1978) for the modern oceans.

Geologic and Geomorphic Setting

Bedrock Geology

The study area is underlain by middle to Upper Cretaceous and lower Tertiary sedimentary rocks of the northern Denver Basin (Colton, 1978; Trimble and Machette, 1979) (table 1, fig. 6). These beds dip gently southeast across the area toward the center of the basin southeast of Denver. The Upper Cretaceous Pierre Shale underlies the northwest part of the study area including most of the Berthoud and Johnstown 7.5-minute quadrangles and the northwest part of the Gowanda 7.5-minute quadrangle (figs. 4, 6). The Pierre Shale is composed primarily of marine shale, siltstone, sandstone, and minor limestone (Scott and Cobban, 1965). To the southeast, the formation is overlain by nonmarine sedimentary rocks of the Upper Cretaceous Laramie Formation and Fox Hills Sandstone and by the Cretaceous-Tertiary Denver and Arapahoe Formations. These rocks consist variously of shale, siltstone, sandstone, conglomerate, and coal. Coal locally forms substantial seams in the Laramie Formation that have been mined by underground and open-pit methods (Roberts, this volume).

Surficial Geology

Major streams that head in the mountains to the west traverse the study area from west to east, eroding the relatively soft sedimentary bedrock of the area and forming broad valleys along which alluvium of various ages has accumulated (fig. 5). The alluvium is composed primarily of clasts of Precambrian igneous and metamorphic rocks. Smaller streams head within the sedimentary rock terrane of the study area or areas immediately to the west, and their stream valleys are underlain by locally derived alluvium. Of the smaller streams only Big Dry Creek, however, has substantial alluvial deposits (figs. 5, 6). Loess and eolian sand form sheetlike deposits that cover much of the uplands between the stream valleys, although such sand deposits are commonly missing where the topography is steep (fig. 5). The dominant paleowind direction is from the northwest.

Eolian sand deposits are common on the downwind side of the larger stream valleys, indicating that sand may be largely derived from alluvium in those valleys (Reheis, 1980; Muhs and others, 1996). The largest area of eolian sand is southeast of Saint Vrain Creek downstream from its confluence with Boulder Creek (fig. 5). Eolian sand also occurs in smaller accumulations on the downwind side of upland areas and along the Big Dry Creek drainage (fig. 5).

Irregular topographic depressions occur in the northwestern part of the study area. Most are underlain by the lower, shale-rich part of the Pierre Shale (fig. 6). The depressions are semicircular and some appear to be overlapping. Larger depressions are used by irrigation companies for water storage. Typically, the companies have enhanced the storage capacity by building dams at low places on the perimeter. These features are believed by Colton (1978) to be deflation basins formed by the wind or perhaps the action of buffalo or cattle. Holliday and others (1996) concluded that depressions occupied by playas in the southern High Plains formed primarily through wind erosion.

Distribution of Saline Soils

In the five quadrangles of the mapped area (figs. 4, 5, 6), saline soils occur most commonly in four geologic settings:

1. Upland areas underlain by loess deposits (47 percent), sited typically in low swales in the landscape, occasionally on hillslopes;

2. Upland areas underlain by residual soils formed on shale (28 percent);

3. Areas along flood plains of minor streams where the upland parts of the drainage basin are dominated by loess deposits (16 percent); and

4. Areas of eolian sand immediately adjacent to wetlands or lakes (9 percent).

Saline soils show a preferred geographic distribution, lying primarily in a belt that extends from the city of Loveland southeastward to the northern Denver suburbs. The northwest part of this belt is underlain by the Pierre Shale, on which the numerous depressions mentioned previously have formed. Saline soils are missing or sparse in the region east of the main area of depressions. This latter area is underlain by upland loess deposits that occupy much of the Johnstown 7.5-minute quadrangle (figs. 4, 5). Saline soils also are rare on the flood plains of the major streams (South Platte River, Saint Vrain Creek, Little Thompson River, Big Thompson River, fig. 5). Only two saline soil areas were mapped—one adjacent to the south margin of the flood plain of Big Thompson River and one west of I–25 near the south edge of the flood plain of Saint Vrain Creek. Saline soils also are rare in the large area of eolian sand southeast of Saint Vrain Creek downstream from its confluence with Boulder Creek (fig. 6).

Wind Direction

The prevailing wind direction during deposition of eolian deposits in northeastern Colorado was evaluated by Madole (1995) and Muhs (1996) from the orientation of linear features in sand-dune areas east of the FRIRP study area. Madole (1995, fig. 10) shows wind directions of about S.40°E. for the area east and northeast of Platteville (just east of the study area). Linear features in dune areas can be observed on the Platteville 7.5-minute quadrangle soil map (Crabb, 1980) for the area immediately east of the north-trending South Platte River. The median value for the orientation of nine well-developed linear features is S.42°E., which is in close agreement with Madole (1995). Within the present study area, dune features also formed in the area of eolian sand in the northern part of the Frederick 7.5-minute quadrangle and the southeastern part of the Gowanda 7.5-minute quadrangle. The orientation of linear features is portrayed in figure 6. The median value for the orientation of the 11 linear features is S.52°E.

Mineralogy of Salt Crusts

Sodium-, magnesium-, and calcium-sulfate minerals, showing varying degrees of hydration, dominate the salt crusts (table 2), in agreement with previous studies of northern Great Plains salts (Keller and others, 1986; Skarie and others, 1987; Mermut and Arshad, 1987; Kohut and Dudas, 1993). The sodium-sulfate salt, thenardite, dominates most assemblages we studied. The relative abundances of the salts varied from location to location even where locations were within a few hundred meters of one another. Variations in the sulfate-mineral assemblages are probably related to subtle differences in the availability and relative concentration of cations in the soil pore water and changes in temperature and moisture content within the soil profiles. The chloride mineral halite is commonly present in trace amounts, but carbonate minerals are absent.

The sulfate minerals initially form as patches of delicate efflorescent coatings on the soil surface, usually favoring features standing above the surface of the substrate. These coatings eventually coalesce into thicker, more continuous coatings of minerals. The freshly formed crusts are highly susceptible to wind erosion.

Sulfur Isotopes

Sulfur isotopes were measured in salts from surface crusts, surface water, soil leachates, weathered bedrock leachates, and coal-mine spoil at 27 sites across the study area (see figure 6 for numbered locations, table 3 for δ³⁴S values, and figure 7 for comparison of values). Values of δ³⁴S from salts in surface soils across the entire study area ranged from –16.7 to +4.3 with a median value of –4.3. The only positive value is from salt in disturbed saline soils that lie in depressions associated with coal-mine collapse features (site 16, fig. 6 and table 3). Values of δ³⁴S in salts from residual or loessal soils, where the marine Pierre Shale (Kpl and Kptz in fig. 6) is the underlying bedrock (sites 20–22, 24–27, fig. 6), range from –13.6 to –2.3 (median value –8.5, “C” fig. 7). The median value compares favorably with a δ³⁴S value of –7.9 for a weathered Pierre Shale sample taken from a hillslope cut (site 23, fig. 6). Salts from surface soils where nonmarine sedimentary rocks are the underlying bedrock (1–10, 12–19, fig. 6, table 3) range from –16.7 to –0.4 (excluding the one positive δ³⁴S value mentioned above) with a median value of –4.7 (letter D in fig. 7).

Values of δ³⁴S in leachates of samples from two coal-mine waste piles were –0.2 and +6.6 (sites 11 and 14). At both coal-mine sites, the waste consists mostly of shale and coal derived from the underlying Laramie Formation. Additional sulfur isotope measurements were made on surface soil leachates, seep and surface waters, and coal-mine spoil leachates at an abandoned coal-mine site (Zielinski and others, 2001) at site 14 (fig. 6). δ³⁴S of soluble sulfate in 11 samples of the coal-mine spoil and surface water leaching from the coal-mine spoil pile into an adjacent wetland ranges from –2.3 to +6.9 with a median value of +4.2 (letter A in fig. 7). In contrast, sulfur isotopes derived from the leachates of a nearby thick loess soil profile and a contact seep range from –5.0 to –6.3 (letter B in fig. 7).

In a study of sulfur isotopes in the Pierre Shale and other marine shales of the northern interior of the United States (Colorado, North Dakota, Montana), Gautier (1986) showed wide-ranging but generally negative δ³⁴S values ranging from +16.7‰ to –34.7‰ CDT (mean –19.7‰, n=50) with all but three values being negative. The most negative values were found in organic-carbon-rich, laminated shales deposited in an offshore, anoxic, restricted marine setting (δ³⁴S values range from –25 to –36 CDT, mean –31 ‰, n=9). The fewest negative values were found in bioturbated silty shales (δ³⁴S values range from +16.7‰ to –34.58‰ CDT, mean –12.4 ‰, n=25). The mean δ³⁴S for this study (–5.7) is closest to that of the silty shales of Gautier (1986). Scott and Cobban (1965) suggested that laminated, organic-rich shale is absent in the Pierre Shale northeast of Boulder, where the section is silt- and sand-rich, and thus the range of sulfur-isotope values for salts is expected to be similar to the silty shale subset reported by Gautier.

Discussion

Saline soils are present throughout much of the study area, preferentially in (1) upland areas underlain by loess deposits; (2) upland areas underlain by residual soils formed on shale; and (3) areas along flood plains of minor streams. However, they seem to be concentrated in a belt that extends southeast from the area of depressions on the Pierre Shale south of Loveland to the northern suburbs of Denver. Saline soils are rare on the flood plains of the major streams that originate in the mountains to the west and on the large area of eolian sand south of Saint Vrain Creek. On the flood plains of the major streams, the ground-water chemistry may be much less saline because of the influence of low-salinity waters in the streams exiting the mountains. Saline soils may be less likely to form in sandy to gravelly soils because the water table must persist much closer to the land surface (within 15 cm or so) for salts in the capillary fringe to reach the surface.

Saline soils across the study area are dominated by sulfate salts that yield predominantly negative δ³⁴S values. The southern part of the study area shows a shift toward less negative values when compared to the northern part of the study area (compare letters C and D values, fig. 7). Nevertheless, in the southern part of the study area, the predominantly negative δ³⁴S signature in soils contrasts with the largely positive δ³⁴S signature of the underlying nonmarine, locally coal-bearing Laramie Formation bedrock. This contrast and the southerly shift toward less negative values indicates that the overlying loess probably is derived from the local bedrock and outcrop areas of the marine Pierre Shale found to the northwest. This interpretation is consistent with (1) wind directions inferred from the orientation of dunes in the central part of the study area and in areas just to the east (however, it must be noted that the dune features are largely younger than the loess); and (2) the observations of Muhs and others (1999), who noted a westward increase in the clay content of loess in areas east of the FRIRP study area and speculated that the Pierre Shale was the contributor. As suggested by Muhs and others (1999), it seems likely that the Pierre Shale depressions mapped by Colton (1978) are sources for much of the clay fraction of the loess.

Sulfate salts with a negative sulfur isotope signature that occur in soils in the southern part of the study area could be derived from leaching of the clayey loess by infiltrating precipitation. However, salts in the eolian sand area, where clay and silt form only a small fraction of the transported material, also show negative isotopic values. We suggest that, in addition to transport of clay from weathered Pierre Shale, salts were also transported downwind with the fine sediment.

The process of eolian transport of salts can be observed on a large scale for the major playa lake basins of the Southwestern United States, including Owens Lake (Reheis, 1997) and Mono Lake (Mono Lake Committee, 2002). Both lakes have served as major sources of atmospheric dust in the Southwestern United States since water diversion projects lowered the lake levels, exposing mud and salt flats. Reheis (1997) showed that the soluble salt content of Owens Lake-derived dust is as high as 30 percent. At Mono Lake, as of 1989, efflorescent salts covered 65 percent of the exposed lakebed. According to studies at Mono Lake as reported by Mono Lake Committee (2002):

“These efflorescent salt deposits vary—both seasonally and diurnally—in their susceptibility to wind erosion, with wet conditions and warm and dry conditions causing high resistance to wind erosion. Warm and dry conditions favor the formation of a strongly cemented crust, which is prevalent through most of the summer.

“Cool salt deposit temperatures and low surface moisture levels favor the development of powdery noncrystalline salts highly susceptible to wind erosion. Powdery deposits usually form during spring or after fall rains. Daily fluctuations occur, especially in spring and fall, when deposits are wet at night and dry during the day.”

Our observations support the idea that depressions in the Pierre Shale in the northwest part of the study area provided salts and sediment to downwind locations during dry periods in the late Pleistocene. Although most of these depressions are used for irrigation storage and thus water levels are artificially maintained, one is fed only by runoff and ground water (site 24, fig. 6). During 2000, 2001, and early 2002, the size of the lake in this depression has become progressively smaller. In the winter of 2000–2001, the lake was shallow enough that a fishkill occurred. Although we have not observed wind transport of salt and sediment from this depression, salt and mud flats have become progresssively more exposed around the shrinking perimeter of the lake. We have also observed the wind picking up salt and sediment from saline fields in the study area and from playa lakes in the Arkansas River Valley in southeastern Colorado.

The paucity of saline soils in the northeastern part of the study area contrasts with their abundance in the southern part of the area, despite the similar extent of mapped loess (Sampson and Baber, 1974; Crabb, 1980). In contrast to the southern part of the study area, however, depressions upwind from the northeast part of the study area are limited in number and size.

Windblown sediment from deflation areas as a source for loess in the saline soils is consistent with work on the origins of loess throughout the rest of northeastern Colorado. Recent investigations of the source materials for loess deposits elsewhere in northeastern Colorado cite alluvial sediment from the major stream valleys (South Platte River, Saint Vrain Creek, and Cache La Poudre River) and bedrock exposures of the White River Group as primary sources based on age, sedimentology, and Pb-Pb ages of mineral grains in these deposits (Muhs and others, 1999; Aleinikoff and others, 1999). In particular, loess may have been derived from the stream alluvium source during those times when glaciers provided large quantities of fine-grained sediment to these valleys. This is consistent with radiocarbon ages of the loess, which overlap the last (Pinedale) glacial period in the Front Range (Muhs and others, 1999). The areas studied by these authors are distant from Pierre Shale outcrops, which are restricted to within a few kilometers of the Front Range. However, Muhs and others (1999) noted that the clay content of loess increases from less than 10 percent at sites near the northeast corner of Colorado to about 30 percent at sites 20 km east of the South Platte River (about 25 km east of the FRIRP study area) and inferred that a clay-rich source of sediment such as the Pierre Shale was necessary. They (Muhs and others, 1999) also noted that Colton (1978) mapped several deflation basins in outcrop areas of the Pierre Shale to the northwest of the loess deposits and inferred that these deflation basins were sources for the clay; the results of our study support these inferences.

Saline soils that occur elsewhere in the semiarid Western United States in areas downwind from outcrops of the Pierre Shale and other marine shales may have some salinity contributed by clay and salt derived from such rocks. In the glaciated terrain of the northern Great Plains, large areas of clayey till derived from marine shale may also be sources of clay and salt in downwind areas of loess deposition. The presence of deflation basins on marine shale outcrops and till areas could be an indicator that downwind saline soils may be present.

References

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