Clays and Clay Minerals, Vol. 44, No. 4, 530-539, 1996.

GEOLOGIC CONTROL OF SEVERE EXPANSIVE CLAY D A M A G E TO A SUBDIVISION IN THE PIERRE SHALE, SOUTHWEST

D E N V E R METROPOLITAN AREA, COLORADO

J. D. GILL, l M. W. WEST, 1 D. C. NOE, 2 H. W. OLSEN, 3 AND D. K. MCCARTY 4

Michael W. West and Associates, P.O. Box 555, Morrison, CO 80465-0555 2 Colorado Geological Survey, Denver, CO 3 Colorado School of Mines, Golden, CO

4 Montana State University, Boseman, MT

Abstract--Shortly after construction of a subdivision in the southwest Denver metropolitan area in 1986, a portion of the subdivision built directly on steeply-dipping strata of the Pierre Shale began experiencing damaging differential movements, causing house foundations to fail and pavements to warp and crack. This formation is a Late Cretaceous marine clay-shale composed predominantly of fluvial mixed-layer illite/smectite and quartz. During deposition of the shale, periodic and explosive volcanism generated thin beds of bentonite, consisting initially of volcanic ash and subsequently altered to nearly pure smectite. Some of these bentonite beds were exposed in a trench adjacent to the subdivision and perpendicular to the strike of the steeply-dipping strata. The thickest bentonite beds correlated well with linear heave features that these beds parallel the bedrock strike throughout the subdivision were mapped via severely deformed pavements. Mineralogical data show the bentonite bed that correlates with the worst damage within the subdivision consists of about 62% smectite by weight with mixed-layer illite/smectite expand- ability of 92%. By comparison, a sample of the typical silty claystone, which is fluvial mixed-layer illite/smectite mixed with detrital quartz from the adjacent strata, had about 23% smectite by weight with 70% to 90% illite/smectite expandability. Geotechnical tests for swell potential show that samples of 2 bentonite beds swelled 39% to 43% compared to 2% to 8% for samples of the typical silty claystone. It is proposed that differential swell resulting from stratigraphically-controlled differences in clay mineralogy and grain-size is the primary factor controlling extreme damage for this geologic setting.

Key Words--Bentonites, Expansive clays, Pierre Shale, Smectites

I N T R O D U C T I O N

Rapid suburban growth near the foothills o f the southwest Denver metropoli tan area is causing pres- sure to build in areas where the underlying sedimen- tary bedrock formations have been deformed into steeply-dipping strata that outcrop at the ground sur- face in some areas and are covered with surficial soils elsewhere. Residential developments on one of these steeply-dipping formations, the Pierre Shale, have ex- per ienced substantially higher damage rates than sim- ilar developments near the generally fiat-lying Denver Format ion that underlies most of the Denver metro- politan area to the East (Thompson 1992). It is gen- erally recognized that damage within the Pierre Shale is caused by differential ground deformations that gen- erate linear heave features in response to development- induced changes within the natural soil cover and sub- surface hydrologic conditions.

This paper is based on an invest igat ion carried out by the authors dur ing l i t igat ion conce rn ing the cause(s) of damaging heave features for a subdivision that was constructed during 1986 on steeply-dipping strata of the Pierre Shale. Since construction, these heave features have grown continuously and become locally extreme. They have caused l ightly-loaded dril led-pier foundations to fail and pavements to crack

and buckle (Figures 1, 2 and 3). For some of the streets, the heaves exhibit over 12 in. of differential vertical displacement. For several years, construction crews have been repairing structures and pavements that have been, and are continuously affected.

The scope of the authors ' invest igation included mapping the nature and distribution o f damaging heave features throughout the subdivision, correlating these heave features with the steeply-dipping strata within the stratigraphic interval of the Pierre Shale be- neath the subdivision, correlating this stratigraphic in- terval with previous geologic studies o f the Pierre Shale and obtaining new data regarding the composi- tion and swell potential o f the strata beneath the sub- division. This paper presents the results of these stud- ies and discusses their significance concerning the convent ional design and construction approach to min- imize damage f rom expansive soils that has been em- ployed throughout the Denver metropoli tan area for several decades.

BACKGROUND GEOLOGY

The subdivision is located southwest o f Denver, CO near the foothills o f the Rocky Mountain Front Range in the southwest corner of the Denver Basin (Figure 4). The 5400 foot thick Pierre Shale strikes N 17 ~ W, is parallel to the mountain front and dips steeply 70 ~

Copyright �9 1996, The Clay Minerals Society 530

Vol. 44, No. 4, 1 9 9 6 Expansive clays in a subdivision in the Pierre Shale, Denver, Colorado 531

Figure 1. Two largest linear humps in subdivision street pavement. Note failure of street drainage.

NE (Scott 1963b) as a result of the Laramide com- pressional folding and faulting during the early to mid Eocene epoch. The outcrop width at the subdivision is 5800 ft. Stratigraphically, the subdivision is underlain by claystone and minor siltstone strata overlying the Hygiene Sandstone Member.

The Pierre Shale and geologically equivalent for- mations comprise an extensive sedimentary deposit within the western interior of the United States and Canada formed during the Cretaceous Interior Seaway between 69.5 and 81.5 m.y. ago (Cobban et al. 1994). Clay minerals dominate the constituents of the shale ranging from 50 to 75% (Schultz 1978). Mixed-layer illite/smectites make up the majority of the clay min- erals. However, smectites are concentrated within ben- tonite beds, pervasive for certain stratigraphic inter- vals.

The composition of the clay minerals and grain size within the shale vary with changes in the depositional environment. Fluvial mixed-layer illite/smectites mixed with detrital quartz derived from volcanic source terrain to the west predominate the majority of the shale. However, airborne pyroclastic material set- tled into the interior seaway during volcanic eruptions. During major eruptions, the deposition rate of volcanic air-fall sediment far exceeded that of the fluvial sedi-

ment, thereby forming discrete layers of concentrated volcanic ash with little or no detrital quartz. This ash subsequently altered into nearly pure smectite clay and thus formed bentonite beds. Some of these layers are recognizable as "air-fall" deposits with preserved phe- nocrysts, sharp contacts with the underlying shale, and gradational contacts with the overlying shale. During minor eruptions, the deposition rate of the air-fall sed- iment was slower. This allowed the air-fall sediments to be extensively transported and mixed with minor amounts of detrital quartz forming bentonitic beds that typically overlie discrete bentonite beds. These are gradational with the surrounding shale such that they contain more smectite clay and less quartz. Discrete bentonite beds represent less than a few percent of the total shale thickness. The corresponding percentage for bentonitic beds is poorly understood because they are more difficult to recognize. The discrete beds are gen- erally very thin (< 2 in.), although occasional beds having thicknesses of several inches to a foot have been reported (Gill and Cobban 1966; Schultz et al. 1980). After deposition and alteration, smectite dia- genesis, that is illitization, has not affected the Pierre Shale in Colorado (Schultz 1978), but some bentonites seem to have been almost completely replaced by cal- cite (Schultz et al. 1980).

532 Gill et al. Clays and Clay Minerals

Figure 2. Linear hump trending into driveway and house.

Figure 3. Buckled curb and gutter.

Vol. 44, No. 4, 1 9 9 6 Expansive clays in a subdivision in the Pierre Shale, Denver, Colorado 533

Figure 4. Location and geologic setting of study area. Key: pC = Precambrian core of Front Range, PPf = Pensylvanian- Permian Fountain Fm., Kp = Late Cretaceous Pierre Shale, and TKda = Cretaceous-Tertiary Dawson Arkose.

The stratigraphic section underlying the subdivision spans 1800 ft of Pierre Shale strata beginning at the top of the Hygiene Sandstone Member and continuing upward into claystones and minor siltstones. Figure 5 compares the stratigraphic relationship between this section and 2 other reference sections of the Pierre Shale. Ammonite fossils of Didymoceras nebrascense and Didyrnoceras stevensoni (Cobban 1994) found at the subdivision established the age of the underlying shale at approximately 73 to 76 m.y. old (Cobban et al. 1994). This section of shale, deposited during the Bearpaw Transgression, correlates with the upper shale unit of the Pierre of the Northern Black Hills in Mon- tana and the Unnamed shale Member of the Pierre at the Red Bird Section in Wyoming. Both sections con- tain documented bentonite beds. The Red Bird Section has 13 bentonite beds including the 36-ft thick Kara Bentonitic Member (Gill and Cobban 1966) and the northern Black Hills section has the Monument Hill Bentonitic Member (Schultz et al. 1980). The chart shows the propensity for this section of the Pierre to contain bentonite beds correlating to explosive volca-

nism from 76 to 73 + m.y. ago in the Elkhorn Moun- tains, Boulder batholith region of Montana (Gill and Cobban 1973; Robinson et al. 1968).

Figure 5 also illustrates that the section beneath the subdivision is relatively thicker than its northern coun- terparts indicating higher depositional rates for the Denver area. Higher depositional rates probably en- hanced the preservation of the air-fall deposits by al- lowing less time for the sea-floor ash deposits to be disturbed during rare, storm-generated, turbulence within the Interior Seaway.

Before site grading, Quaternary surficial geologic deposits covered bedrock within the subdivision to depths ranging from 3 to 19 ft. The predominant nat- ural soil was wind-deposited (loess) silty clay with mi- nor patches of Verdos, Slocum and Piney Creek allu- vium (Scott 1963a). During site grading, the natural soil cover was drastically changed by excavating the central portion of the subdivision, exposing bedrock, and placing the excavated soils along the east and west sides of the subdivision (Figure 6). The fill consisted of a mixture of loess and reworked claystone.

534 Gill et al. Clays and Clay Minerals

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METHODS

During litigation, the authors mapped the differen- tial heaves revealed from the street and sidewalk pave- ments. They also mapped the surficial geology and logged the bedrock stratigraphy beneath the subdivi- sion from a 25 ft-deep trench adjacent to the subdi- vision and perpendicular to bedrock strike. The strata within the trench were correlated with pavement dam- age patterns for the subdivision. A profile of relative displacement across the largest heave feature was ob- tained with a string-line method. Samples of selected strata were analyzed for mineralogy and mixed-layer illite/smectite (US) expandability, cation exchange ca- pacity (CEC) and geotechnical properties including air-dried moisture content, Atterberg limits, grain-size distribution and the percent swell upon inundation.

For mineralogy and mixed-layer illite/smectite ex- pandability, the % expandable smectite layers in US, X-ray diffraction (XRD) pattems were collected on random and oriented samples using an automated dig- ital-step scanning Siemens XRD system employing CuKa radiation, a graphite monochrometer and 1 ~ di- vergence and receiving slits. Random sample prepa-

rations were made by using a side-loading sample holder after ground bulk rock was passed through a < 74 txm sieve to ensure comparable results between samples, and to obtain representative hulk mineralogy determinations. Oriented preparations were made by using a millipore filter peel transfer onto glass slides after a < 1.0 ixm suspension was collected by centrif- ugation. After oriented mounts were X-rayed in an air- dried state, samples were glycolated within a glycol- water vapor atmosphere at 60 ~ overnight and then scanned over the same angular range. When possible, mixed-layer US expandahilities were determined by the ~ method of Moore and Reynolds (1989) utiliz- ing the I/S (001)/(002) and (002)/(003) reflections. For cases where peak interferences from other phases made this difficult, I/S expandability was determined from the correspondence of the I/S (002)/(003) peak position with calculated (002)/(003) peak positions from the NEWMOD computer program (Reynolds 1985). The given expandability values are usually within - 5 % , but because of discrete illite interference, this error could be slightly higher (McCarty and Eberl 1992).

Vol. 44, No: 4, 1996 Expansive clays in a subdivision in the Pierre Shale, Denver, Colorado 535

Figure 6. Damage distribution in pavements and site geology.

Cation exchange capacities (CEC) were measured from NH 4- saturated specimens using the ammonia electrode method of Busenberg and Clemency (1973) after cation exchange with a known weight of 1M NH4C1 solution. For selected bulk rock specimens, the weight percent of smectite was estimated using the CEC of a < 1 txm specimen from a bentonite consisting of pure smectite as a reference. We assumed that the other minerals within the sample had a CEC of 1 meq/100 g, and was a linear relation between smectite content and CEC (McCarty and Eberl 1992).

Geotechnical properties were run on bulk samples of 2 silty claystone strata and of the 2 thickest ben- tonite beds correlating to the largest heave features. The bulk samples were crushed until the material passed the number 40 sieve, spread out to a uniform depth of approximately 1A in., and allowed to air-dry at room temperature for 4 d. Then moisture contents (ASTM D-2216), Atterberg limits (ASTM D-4318), hydrometer method panicle size distribution (ASTM D-422) (Am. Soc. for Testing and Mat. 1994) and ac- tivity index (liquid limit / < 2 ~m fraction; Lambe and Whitman 1969) were determined for these mate- rials. Swell-test specimens were also prepared by com- pacting the remolded and air-dried materials to a dry density of 89 pounds per cubic foot (pcf). These spec- imens were placed into one-dimensional consolidom-

eters, loaded via steps to a surcharge of 800 pounds per square foot (psf), inundated with water and mon- itored to determine the resulting percentage of swell (ASTM D-4546).

RESULTS

Heave features within pavements mapped during 1992, are shown in Figure 6. These features follow linear trends that are parallel to bedrock strike from street to street across the subdivision. By projecting these linear trends to the trench adjacent to the sub- division, the heave features were correlated with the bentonite beds from the steeply-dipping Pierre Shale.

The locations of 37 different bentonite beds within the trench are plotted in Figures 5 and 6, The majority of the bentonite beds were very thin, less than 1 in. thick; however, a 12 in. thick, an 8 in. thick and sev- eral 3 to 4 in. thick beds were found. The bentonite beds were generally lighter in color than the surround- ing olive-gray silty shale strata (Figure 7) and ranged from light gray to yellow. Within the bentonites, fi- brous calcium carbonate and selenite crystals were commonly found within a matrix of smectite clay, which was generally soft and felt waxy (free of silt) and sometimes contained fine-grained phenocrysts. For the thinnest beds, the smectite clay was difficult or impossible to recognize by eye. For the thicker ben-

536 Gill et al. Clays and Clay Minerals

Figure 7. 12-inch thick bentonite bed "A'" correlated to pavement damage in Figure 8.

tonite beds, the smectite was typically slickensided in a tight cuspate, anastimosing manner. The slickensides did not appear to line up along a continuous plane, rather they cross-cut each other, possibly due to re- peated cycles of intense swelling, shrinking and plastic flow of the smectite.

Comparison of the linear heave trends with the ben- tonite beds exposed within the trench (Figure 6) re- veals a strong correlation of the 2 largest pavement heaves, which are traceable across 4 streets, with the 2 thickest bentonite beds. Another linear heave trend correlates with a cluster of thinner beds. Several of the thinner (less than 1 in. thick), isolated, bentonite beds did not correlate with specific pavement damage. Some minor linear heaves could not be correlated to discrete bentonite beds.

Differential displacements for the asphalt pavement across the 2 largest heaves, measured in 1994, are compared with the underlying stratigraphy correlated from the trench 150 ft along strike to the south (cross sections A-A' and B-B', Figure 6) in Figure 8. These sections differ in that the street in Section B-B' was

built directly upon excavated bedrock whereas surfi- cial soils overlie the steeply-dipping bedrock in Sec- tion A-A'. In Section B-B' the vertical differential dis- placements associated with the 2 thickest beds (A and D) exceed 12 in. and show a distinct asymmetry with the steeper side of the humps being on the west (down- section) side, and that the width of the humps can exceed that of the associated bentonite beds. Similar vertical differential displacements were not found in Section A-A' where surficial soils overlie the steeply- dipping bedrock.

The XRD and CEC data indicate differences in min- eralogy of the < 11xm fractions of the bentonite beds, bentonitic beds and the adjacent silty-claystone strata (Figure 9). This figure also shows the bentonite bed is monomineralic, the bentonitic bed shows traces of ka- olinite, and the adjacent silty claystone strata contains kaolinite and quartz. The clays in the 4 bentonite beds contained > 90% US expandabilities, whereas, clays in the adjacent silty-claystone strata generally con- tained lower percentages ranging from 70% to 90%. The CEC for the < 1.0 p,m fractions of smectite clay from 2 bentonite beds were measured at 50.5 and 53.6 meq/100 g. Using a mean value of 52 meq/100 g as a reference value relating to 100 wt% smectite, bulk- rock percent smectite was estimated for bed D and a specimen from the adjacent silty claystone (McCarty and Eberl 1992). The bulk-rock specimen from ben- tonite bed D (Figure 8) contained nearly 3 times more smectite than the silty claystone sample, having 62% compared to 23% smectite (Figure 8), derived from bulk rock CEC values of 31.8 meq/100 g and 12.7 meq/100 g, respectively.

The geotechnical test results show the bentonite beds have a much higher affinity for water and swell potential than the silty claystone (Table 1). Under equal air-dried conditions, bulk-rock samples of ben- tonite beds A and D had 13.7% to 15.1% water by weight, about 10% more than 3.6% to 5.2% found for bulk-rock samples of the surrounding silty claystone. The liquid limit (LL) and the plasticity index (PI) val- ues for the bentonite were 1.5 to 3 times higher than those for the shale. Swell tests on the bentonites re- sulted in 39% to 43% swell, which are substantially higher than values of 2% to 8% for the silty claystone.

DISCUSSION

The strong correlation of linear heave features with bentonite beds within the steeply-dipping Pierre Shale, and the relatively strong affinity of the bentonite beds for water, compared with the silty claystone strata be- tween the bentonite beds. This strongly indicates that many of the linear heave features are caused by more swelling of the bentonite beds compared with the ad- jacent strata. Steeper-to-the-west (down-section) asym- metry heave features with their widths wider than the actual bentonite beds, are attributable to the bentonite

Vol. 44, No. 4, 1 9 9 6 Expansive clays in a subdivision in the Pierre Shale, Denver, Colorado 537

Figure 8. Correlation of pavement damage with bentonite beds and respective %I/S and wt% smectite.

beds that typically have a sharp contact with the un- derlying shale and are gradational with the overlying shale. Shale immediately overlying bentonites is often bentonitic. The data presented on Figure 8 illustrates this for beds A and C by showing relatively sharp increases of deformation and % US followed by grad- uai decreases for both of these factors while progress- ing upward through the geologic section.

It follows that the dominant control on damaging vertical differential deformations in this geologic set- ring is stratigraphically-controlled differences within clay mineralogy and grain size that govern the affinity of the strata to water and hence their swell potential and magnitudes. This condition differs substantially from that in the generally fiat-lying Denver Formation that underlies most of the Denver metropolitan area to the east. For fiat-lying geologic strata, stratigraphic differences occur vertically and hence do not contrib- ute directly to differential vertical deformations. Rath- er, the dominant source of such damaging deforma- tions is development-induced changes within subsur- face moisture conditions that vary horizontally in re- sponse to infiltration and evapotranspiration changes. It needs to be recognized that, in steeply-dipping geo- logic strata, development-induced changes in subsur-

face moisture conditions are also to be expected. How- ever, the results of this study strongly suggest that the effects of this factor were overshadowed by strati- graphic differences of clay mineralogy and grain size.

The subdivision was developed with the conven- tional design and construction approach for minimiz- ing damage from expansive soils that has been em- ployed throughout the Denver metropolitan area for several decades. The essence of this approach is to avoid damaging vertical deformations by supporting light structures on drilled piers anchored at depth into claystone bedrock, and to isolate structures from as- sumed uniform swelling of surficial materials by main- taining voids beneath horizontal structural beams and structural floors.

Thompson's (1992) statistical study shows this con- ventional approach has been far less successful with steeply-dipping strata than with flat-lying strata. This study indicates a major deficiency of this approach in that it does not consider stratigraphically-controlled horizontal variations in the subgrade. He also shows the severity of damage within steeply-dipping strata decreases as the depth of overburden increases. The results of this study are consistent with his findings in that the linear heave features were substantial primar-

538 Gill et al. Clays and Clay Minerals

IS 17,ooA

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ily where the alluvial overburden had been removed and structures were constructed on the exposed clay- stone bedrock.

It is of particular interest that the bentonite beds in this study could be correlated with biostratigraphic zones that represent geologic time horizons and are mapped extensively in the western interior of the Unit- ed States and Canada (Cobban et al. 1994). This sug- gests that the biostratigraphic zones, which have al- ready been mapped for the Front Range area (Scott and Cobban 1965) can be used as guidelines for lo- cating bentonites and bentonitic layers. However, be- cause the thicknesses of the Pierre Shale and the bio- stratigraphic zones are not constant, supplementary studies will generally be necessary for defining the stratigraphy for site-specific investigations. For this purpose, trenching appears to be the best available ap- proach.

CONCLUSIONS

Residential developments on steeply-dipping Pierre Shale in the Denver metropolitan area are vulnerable to damaging differential deformations arising from stratigraphically-controlled differences within clay mineralogy and grain size. These differential defor- mations are usually expressed in terms of linear heave ridges that are parallel to strike and underlain by ben- tonite beds that are areally extensive throughout the Western Interior of the United States for rocks depos- ited into the Cretaceous Interior Seaway. At the site investigated in this study, the bentonite beds consist of more than 60% smectite compared with approxi- mately 20% smectite for the adjacent silty-claystone strata. Geotechnical tests confirmed that the bentonite beds have a much higher affinity for water and swell potential than the silty-claystone strata.

These conditions differ substantially from those for flat-lying geologic strata, such as the Denver Forma- tion that underlies most of the Denver metropolitan area to the east. Within fiat-lying geologic strata, strati-

Table 1. Geotechnical test results.

Silty claystone Bentonite

EQUILIBRIUM AIR-DRIED MOISTURE CONTENT (%)

ATTERBERG LIMITS LL P]

CLAY FRACTION (% < .002 ram) ACTIVITY INDEX SWELL INDEX TEST (REMOLDED) % SWELL ON

SATURATION (800 psf surcharge) CATION EXCHANGE CAPACITY (meq/100 g)

bulk sample, divalent cations dominant

number of specimens

number of specimens

3.6-5.2 (2) 13.7-15.1 (2)

33-52 (10) 95-103 (2) 28-47 (10) 57-65 (2)

33-52 (10) 53-68 (2) .55-.95 (12) .97-1,10 (2)

2-8 (2) 39-43 (2)

12.7 (1) 31.8 (1)

Vol. 44, No. 4, 1996 Expansive clays in a subdivision in the Pierre Shale, Denver, Colorado 539

graphic d i f ferences occur ver t ical ly and h e n c e do not con t r ibu te direct ly to dif ferent ia l ver t ical de forma- t ions. Rather, the d o m i n a n t source of such d a m a g i n g de fo rmat ions o f d e v e l o p m e n t - i n d u c e d changes is sub- surface mois tu re condi t ions tha t va ry hor izon ta l ly in response to infiltration and evapotranspirat ion changes.

It is sugges ted that the b ios t ra t igraphic zones, wh ich have a l ready b e e n m a p p e d for the F ron t R a n g e area, can be used as guidel ines for locat ing ben ton i t e s and ben ton i t i c layers wi th in the s teep ly-d ipping beds o f the Pierre Shale. However , because the th icknesses o f the Pierre Shale and the b ios t ra t igraphic zones are no t constant , supp lemen ta ry studies wil l genera l ly be nec- essa ry for def in ing the s t ra t igraphy o f s teep ly-d ipping beds for s i te-specif ic inves t iga t ions . For this purpose , t r ench ing appears to be the bes t ava i lab le approach.

R E F E R E N C E S

American Society for Testing and Materials. 1994. Moisture Content (D-2216), Atterberg limits (D-4318), Particle size distribution (D-422) and Swell (D-4546). In: Annual Book of Standards. Philadelphia, PA: ASTM. 4.08:288-307.

Busenberg E, Clemency CV. 1973. Determination of the cat- ion exchange capacity of clays and soils using an ammonia electrode. Clays & Clay Miner 21:213-217.

Cobban WA, Merewether EA, Fouch TD, Obradovich JD. 1994. Some cretaceous shorelines in the western interior of the United States. In: Caputo MV, Peterson JA, Franczyk KJ, editors. Mesozoic systems of the Rocky Mountain Re- gion, USA. p 393-413.

Cobban WA. 1994. Personal communication. Regarding fos- sil identification, United States Geological Survey, Box 25046, Mail Stop 966, Denver, CO 80225.

Gill JR, Cobban WA. 1966. In: Kier PM, editor, new echi- noid from the Cretaceous Pierre Shale of eastern Wyoming. The Red Bird section of the Upper Cretaceous Pierre Shale in Wyoming. U.S. Geological Survey Professional Paper 393-A. 73 p.

Gill JR, Cobban WA. 1973. Stratigraphy and geologic his- tory of the Montana Group and equivalent rocks, Montana,

Wyoming, and North and South Dakota. U.S. Geological Survey Professional Paper 776. 37 p.

Lambe TW, Whitman RV. 1969. Soil mechanics. New York, NY: John Wiley & Sons. 553 p.

McCarty D, Eberl D. 1992. Written communication. Miner- alogical and Chemical analyses of samples.

Moore DM, Reynolds RC. 1989. X-ray diffraction and the identification and analysis of clay minerals. New York: Ox- ford University Press. 332 p.

Reynolds RC. 1985. NEWMOD computer program for the calculation of the one-dimensional X-ray diffraction pat- terns of mixed-layer clays. Available from author, Dept. Earth Sciences, Dartmouth College, Hanover, New Hamp- shire, 03755.

Robinson GD, Klepper MR, Obradovich JD. 1968. Overlap- ping plutonism, volcanism, and tectonism in the Boulder batholith region, western Montana. In: Coats RR, Hay RL, Anderson CA, editors. Studies in volcanology--A memoir in honor of Howel Williams. Geological Society of Amer- ica Memoir. 116:557-576.

Scott GR. 1963a. Quaternary geology of the Kassler quad- rangle, Colorado. U.S. Geological Survey Professional Pa- per 421-A. 70 p.

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(Received 26 July 1994; accepted 8 November 1995; Ms. 2546)