1 3Facies (2015) 6112 DOI 10.1007s10347-015-0440-x.docx
SeniorSeminarPaper
1. 1
HEAVY MINERAL ANALYSIS OF LATE-EOCENE SANDSTONE OF
THE CHALKY BUTTES MEMBER EXPOSED AT SQUARE BUTTE IN
SOUTHWESTERN NORTH DAKOTA
Sauvik Chakraborty
Department of Geosciences
Spring 2016
Approved: ----------------------------------------
----------------------------------------
2. 2
ABSTRACT
Heavy mineral analysis of Late-Eocene sandstones from the Chalky Buttes Member (CBM) of
the Chadron Formation is an ongoing, long-term study directed at understanding how heavy
mineral abundances vary with grain size. The goal of this study was to provide additional heavy
mineral data on the CBM sandstone for comparison with data obtained by previous MSU
researchers on Square Butte. Heavy mineral grains (>2.85 g/cm3) from the 0.125-0.150 mm
(HM 106) and 0.180-0.212 mm (HM 103) grain size fractions were separated from the light
mineral grains in a sample using a lithium heteropolytungstate (LST) solution and placed on
round glass sides. Petrographic analyses of the heavy mineral grains were conducted and the two
grain size fractions were observed to be dominated by opaque minerals, with the 0.125-0.150
mm size fraction containing 65.7% opaque grains and the 0.180-0.212 mm size fraction
containing 66.4% opaque minerals. In the 0.125-0.150 mm size fraction (443 grains studied), the
most abundant non-opaque heavy minerals were epidote (26.0%), garnet (3.2%), and staurolite
(2.5%). The trace (<2%) minerals identified were sphene, tourmaline, and zircon. The dominant
non-opaque heavy minerals in the 0.180-0.212 mm size fraction (455 grains studied) were
epidote (22.2%), garnet (4.0%), staurolite (2.6%), and tourmaline (2.2%). Minor (<2%) non-
opaque minerals were hornblende, sphene, rutile, pyroxene, and zircon. The heavy mineral
assemblages of these two samples were very similar, with the exception of a trace percentage of
hornblende and pyroxene in the 0180-0.212 mm size fraction, but with regard to the study on the
0.25-0.30 mm size fraction studied by previous researchers, the dominant mineralogies were
apparently different owing to the higher abundances of staurolite, apatite, rutile, Al-silicates, and
lower abundances of epidote and opaque grains in the 0.25-0.30 mm size fraction. After
reanalysis of the 0.25-0.30 mm size fraction, it was found that leucoxene, an opaque heavy
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mineral, was incorrectly classified as rutile. This lowered the percentage of rutile and increased
the percentage of opaque grains and therefore, increased the degree of similarity between the
three size fractions. Calculating the absolute abundances of heavy minerals for the three grain
size fractions showed a size-shift, owing to settling equivalence, towards finer grain size
fractions when compared to the overall grain size distribution. Cluster analysis performed on 51
CBM samples placed both the size fractions studied in the same group, C, and sub-group, C4,
owing to higher abundances of epidote, which suggests a Precambrian metamorphic origin. The
0.25-0.30 mm size fraction was placed in sub-group C3 owing to comparatively lower
abundance of epidote and higher abundance of staurolite.
INTRODUCTION
Square Butte (SQB), a large-northwest trending mesa, is located in Section 8 T139N
R103W of Golden Valley County in southwestern North Dakota (Murphy et al. 1993). It is
among the many buttes or hills in the North Dakota-South Dakota-Montana tri-state area where
Tertiary strata are exposed. There are numerous isolated buttes or hills that are featured in the tri-
state area (Murphy et al. 1993). Square Butte is 400 feet higher than the surrounding landscape
and is capped by 75 feet of massive sandstone belonging to the Golden Valley Formation, which
is overlain by 10 feet of conglomeratic sandstone, the Chalky Buttes Member, and 20 feet of
claystone, the South Heart Member (Murphy et al. 1993). The Chalky Buttes Member (CBM) is
exposed at several buttes or hills in southwestern North Dakota (Figure 1). The CBM ranges
from 10-80 feet in thickness and is characterized as a light-colored, poorly to moderately
indurated, gravel bearing, cross-bedded sandstone (Murphy et al. 1993).
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Heavy Mineral Studies
Denson and coworkers (Denson et al. 1965; Sato and Denson 1967; Denson and
Chisholm 1971) studied 3000 samples and identified approximately 100 non-opaque heavy
mineral grains per sample after studying the 0.063-0.125 mm sand fraction from Tertiary strata
of the middle Rocky Mountains and the northern Great Plains. They chose to study the 0.063-
0.125 mm size fraction because Denson and Chisholm (1971) stated that it contained the greatest
diversity of heavy minerals among other size fractions for any sample procured.
Numerous ongoing studies at Minot State University (MSU) are focused on studying the
heavy mineral assemblages of various grain size fractions from samples obtained from CBM
exposures in the ND-SD-MT tri-state area. Several research initiatives were spurred by a
correlation proposed by Murphy et al. (1993) between the CBM and Medicine Pole Hills (MPH)
sandstone units on the basis of lithostratigraphic observations. Murphy et al. (1993) proposed the
correlation because both the CBM and MPH sandstone units are directly, but unconformably,
underlain by the Paleocene Fort Union Formation shown in Figure 2. Webster et al. (2015)
performed optical analysis on heavy mineral assemblages to assess the correlation suggested by
Murphy et al. (1993) between the CBM and Medicine Pole Hills (MPH) sandstone units. They
found that the MPH should not be correlated with the CBM owing to differences in heavy
mineral assemblages (Webster et al. 2015).
Another avenue of research at MSU has involved assessing the similarities or differences
between CBM samples obtained at various localities and the results of Denson and Gill (1965).
Gleich (2014) attempted to draw a comparison between samples obtained from the South Cave
Hills (SCH) sandstone and the Chalky Buttes Member (CBM) sandstones studied by Denson and
Gill (1965). Gleich (2014) observed that the heavy mineral percentages did not reflect the
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percentages obtained by Denson and Gill (1965), with the exception of a high percentage of
opaque grains. Bjornson (2014) conducted heavy mineral analysis on the South Cave Hills
(SCH) sandstone and findings obtained reflected a similarity to results of Denson and Gill (1965)
with regard to the high percentage of opaque heavy mineral grains but differed with respect to a
lack of zircon, which was abundant at several localities sampled by Denson and Gill (1965).
Fogarty (2008) conducted a study on a sample obtained from SQB and found that the heavy
mineral assemblage and abundances were unlike the Eocene sandstone data from MPH and
CBM locations.
The present study is part of an ongoing, long-term study of different CBM locations in
North Dakota directed at understanding how heavy mineral abundances vary with grain size. The
present study involved heavy mineral analysis of the CBM from SQB using petrographic
analysis. The heavy minerals analyzed for the present study were from the 0.125-0.150 mm and
0.180-0.212 mm grain size fractions. The goal of this study was to provide additional heavy
mineral data on the CBM sandstone for comparison with data obtained by Fogarty (2008) for the
0.25-0.30 mm size fraction.
GEOLOGIC SETTING
Tertiary strata are exposed at several buttes or hills in the ND-SD-MT tri-state area. Most
of these strata range from Paleocene to Eocene in age and some were deposited during the
Oligocene and Miocene epochs. The White River Group ranges from Eocene to Oligocene in
age. There are several buttes or hills in the tri-state where the White River Group is exposed. The
region that contains the White River Group lies in the southwestern part of the Williston Basin,
which as stated by Denson et al. (1965), is a large structural depression about 550 miles long and
300 miles wide. The White River Group consists of the Chadron Formation and the overlying
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Brule Formation and is unconformably overlain by the Arikaree Formation of Oligocene to
Miocene age (Murphy et al. 1993). The Chadron Formation (CF) consists of the basal Chalky
Buttes Member (CBM) overlain by the South Heart Member (SHM). The CF
is unconformably underlain by the Paleocene Fort Union Formation and this unconformity, as
stated by Murphy et al. (1993) represents at least 14 million years of non-deposition.
Square Butte is among the many buttes in southwestern ND where the CBM is exposed.
Square Butte, a large northwest-trending mesa, is located in Section 8 T139N R103W of Golden
Valley County in southwest ND (Murphy et al. 1993). Square Butte is 400 feet higher than the
surrounding landscape and is capped by 75 feet of massive sandstone belonging to the Golden
Valley Formation overlain by 10 feet of poorly-well cemented conglomeratic sandstone, the
Chalky Buttes Member and 20 feet of claystone, the South Heart Member (Murphy et al. 1993).
Murphy et al. (1993) state that igneous and volcanic rock fragments constitute the pebbles in the
conglomeratic sandstone of the CBM found at Square Butte. The volcanic rock fragments
include pink/brown porphyries, which is typical of CBM exposures in southwestern ND
(Murphy et al. 1993). The area surrounding Square Butte is dominated by the Sentinel Butte
Member of the Fort Union Formation.
In the past, the sandstone capping SQB was placed in different formations such as Fort
Union Formation, White River Group, and the Arikaree Formation by various researchers.
However, Murphy et al. (1993) identified it as the CBM of the CF of the White River Group on
the basis of the occurrence of Brontothere fossils and characteristic pebble lithologies.
Previous Researchon SQB
Fogarty (2008) studied the 0.25-0.30 mm size fraction of a CBM sample collected from
Square Butte and mineralogically identified 422 grains. Studies conducted on the 0.25-0.30 mm
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size fraction were carried out grain mounts numbered HM-14 and HM-15. Combining the two
grain mounts, some of the grains identified were rutile, which dominated the size fraction at
36.5%, pseudobrookite (12.1%), epidote (10.2%), staurolite (8.1%), apatite (7.4%), ilmenite
(5%), aluminosilicate (3.6%), tourmaline (2.6%), and garnet (2.6%) (Fogarty 2008). Sphene,
chromite, zircon, hornblende, monazite, and Mn-oxide accounted for 0.7% to 1.7%. Fogarty
(2008) also identified trace amounts (0.2%) of biotite, copper, corundum, and an Fe-Mn oxide
phase mineral. Some of the grains (537) on grain mount numbered HM-15 were later restudied
and the mineral abundances recorded (in descending order) were leucoxene, which dominated at
40.2%, pseudobrookite (13.2%), epidote (11.5%), staurolite (7.1%), ilmenite (6.5%), tourmaline
(5.6%), aluminosilicate (5.0%), apatite (4.1%), garnet (1.3%), zircon (1.3%), and Fe-Mn oxide
(1.3%) (personal communication with Dr. John Webster 2016). In the previous assessment of the
grains on HM-15, the leucoxene grains, opaque grains that are intergrowths of quartz and a TiO2
phase, were identified as rutile. There were also trace amounts (0.2% to 0.6%) of other mineral
grains such as rutile, allanite, monazite, corundum, sphene, muscovite, magnetite and chromite
(personal communication with Dr. John Webster 2016).
METHODS
Prior Work
The sample preparation work on the samples from SQB was carried out by Fogarty
(2008). The sample was transferred to a tub and distilled water was added until the tub was half
full. Three tablespoons of Calgon® water softener were added to the distilled water and the
sample was agitated and stirred thoroughly and washed through a 0.0625-mm screen to facilitate
the removal of silt and clay.
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The sample was separated into ¼ ϕ size fractions from 2.00 mm (-1 ϕ) to 0.063 mm (4 ϕ)
using 8-inch sieves and a sieve shaker. A total of 21 sieves were necessary and the sieves were
divided into 3 sets and the sample was run through a set before the next set was used. After the
entire sample had been run through all the sets of sieves, the grain size fractions were transferred
into a clear plastic bag labeled with the sample ID and grain size fraction, which had been
weighed prior to the transfer. The weight of the sand only and sand with plastic bag were
recorded.
Final Cleaning
The 0.125-0.150 mm and 0.180-0.212 mm grain size fractions chosen for this study were
cleaned thoroughly to remove any fine-grained particles adsorbed onto the surfaces of the grains.
This was achieved by placing the sand in a large beaker, filling the beaker with distilled water
and then placing the beaker in an ultrasonic bath for a few minutes. After the ultrasonic
treatment, water was decanted through a 0.063-mm sized sieve to avoid loss of sand grains.
Ultrasonic cleaning was continued until the water became clear. Any grains collected in the sieve
after decantation were transferred back into the beaker. The beaker was then placed in an oven to
dry at 60 C.
Heavy Mineral Separation
Heavy minerals were separated from the grain size fractions using a non-toxic heavy
liquid, lithium heteropolytungstate solution (LST), which had a density of 2.85 g/cm3. The
separation was carried out in 500-mL centrifuge bottles, each of which contained 200 mL of
LST. The amount of sand that was added to each bottle was 70 g. The centrifuge was run at 2500
rpm for 30 minutes and care was taken to ensure that both centrifuge bottles contained equal
weights (sand + LST).
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Heavy minerals were recovered from the bottom of the centrifuge bottles using a vacuum
extraction tube as shown in Figure 3. The light minerals were separated and remained
differentiated at the top of the bottle. During this process, heavy minerals were collected in a trap
flask after being transferred through the vacuum extraction tube. The heavy mineral grains
obtained were then vacuum filtered and rinsed with distilled water to remove the LST. The
grains recovered on the filter paper were placed into two beakers and rinsed with distilled water
several times. These beakers were placed in an oven to dry at 60 C for 72 hours. The dried
heavy mineral grains were transferred into vials, whose weights were recorded prior to transfer,
with a firm brush. After this transfer, the vials containing the dried heavy mineral grains were
weighed.
Grain Mount Preparation
There were three mounts of heavy mineral grains per size fraction that were made on
frosted, round glass slides. The glass sides were frosted uniformly with 600-grit silicon carbide.
Epoxy was applied to each slide when the slides were set on a hot plate at about 60 °C with the
frosted side up. Heavy mineral grains were sprinkled on top of the drop of epoxy. A glass cover
slip was applied on top of the epoxy and the epoxy was allowed to spread on its own. Then, gentle
pressure was applied to the cover slip to move it around to allow the epoxy and the grains to spread
out. The glass slides were left to cure for 24 hours at room temperature. An engraver was used to
label the cured glass slides. The three glass slides with 0.18-0.21 mm grains were labeled HM-
102, HM-103, and HM-104, and the three glass slides with 0.125-0.150 mm grains were labeled
HM-105, HM-106, and HM-107. The excess cover slip glass was snapped off and the grinding
wheel on the thin section machine was used to remove the rest and any glue that ends up on the
outside edge of the round slide. The grinding wheel was also be used to bevel the edge of the round
10. 10
slides and it served to continually grind away the cover slip and grind slowly into the grains. When
the grains started to appear well exposed, a stereomicroscope was used to determine how many of
the grains were exposed. The intended effect was to obtain as many exposed grains as possible,
but without grinding too far into the grains. During the use of the grinding machine, HM-104, HM-
105, and HM-107 were rendered unusable for heavy mineral identification due to excessive
grinding into and subsequent loss of heavy mineral grains. Therefore, HM-106 from the 0.125-
0.150 mm and HM-103 from the 0.180-0.212 mm grain size fraction were used to obtain results.
Preparation of Grain Maps
A 35-mm slide scanner was used to produce a high-resolution scanned image of the
sections. The scanned images of the sections were inserted into an Adobe Illustrator file, and the
image was divided into quadrants (A-D), which were further divided into sections designated by
lowercase alphabets (a-b), each of which contained 30-65 grains. The process of diving the
scanned image into quadrants and sections was done by using the polygon tool in Adobe
Illustrator. In addition, each grain was numbered to enable individual identification.
Optical Microscopy
The heavy mineral grains were identified using a polarized-light petrographic
microscope. Optical identifications were based on several characteristics such as color,
pleochroism, and birefringence. After each grain was identified, the identification as well as
characteristics of the grain was logged on a sheet of paper. The results obtained on each log entry
was tallied to obtain total and individual heavy mineral counts for comparison of the two size
fractions studied and with other studies completed on SQB. In addition, the percentages of heavy
minerals were used to determine absolute abundances and the non-opaque heavy minerals
studied, excluding muscovite and apatite, were also used for cluster analysis.
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Determination of Absolute Abundances
After optically identifying heavy minerals from each size-fraction, the grain counts for
the minerals identified were put into a Microsoft Excel spreadsheet. The grain percentages for
each heavy mineral were multiplied by the mineral’s density to get a weight for each mineral.
These values were then normalized to 100% to generate weight percentages. In order to yield the
absolute parts per million (ppm), the weight percentages obtained were multiplied by the weight
percentage of the corresponding size-fraction and the percentage of heavy minerals recovered for
the size-fraction. The absolute abundances in ppm were overlaid on top of the grain size
distribution for the entire sample obtained from SQB in order to obtain a size-shift, which is the
tendency of the mode of the heavy mineral abundances to be shifted towards smaller grain size
fractions in fluvial systems (Garzanti et al. 2008)
Cluster Analysis
Heavy mineral analyses of the size fractions involved in this study were compared with
one another and other analyses completed by previous and current MSU researchers using cluster
analysis. The degree of similarity between samples were used to construct dendrograms that
linked samples based on their m-space Euclidean distance, dij, which is given by the following
√
∑ (𝑋𝑖𝑘 − 𝑋𝑗𝑘)2𝑚
𝑘=1
𝑚
where i and j are the samples being compared and m refers to the number of minerals (Webster et
al. 2015). Cluster analysis was carried out using a program written by Dr. John Webster. The
analysis was carried out using 14 non-opaque heavy minerals after the heavy mineral counts
were normalized to 100%.
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RESULTS
Grain Size: 0.125-0.150 mm
Prior to cleaning the sample in an ultrasonic bath, the weight of the sample in the 0.125-
0.150 mm fraction was 131.3 g. After using an ultrasonic bath, the cleaned, dried sample
weighed 125.4 g. The heavy minerals recovered from 70.0 g of sample weighed 0.345 g, whereas
the light minerals weighed 69.5 g, resulting in a total weight of 69.9 g. The weight of sample lost
during the separation equaled 0.1 g.
Grain size: 0.180-0.212 mm
Initially, the 0.180-0.212 mm sample weighed 243.3g. After cleaning the sample in an
ultrasonic bath, the dried and cleaned sample weighed 237.4 g. Separation from 70.0 g of sample
in a centrifuge yielded 0.203 g of heavy minerals and 69.6 g of light minerals were recovered. In
total, a combined weight of 69.8 g were recovered and 0.2 g were lost during separation.
Optical Microscopy
Table 1 and Table 2 provide the grain counts and percentages of the heavy minerals
identified using optical microscopy. The grain mounts used for optical microscopy studies were
HM-103 (0.180-0.212 mm) and HM-106 (0.125-0.150 mm). The analyzed grains from the two
SQB size fractions were dominated by opaque heavy mineral grains, which were difficult to
identify and often appear reddish-brown around the periphery and in some cases, throughout the
grain. In HM-103, the total number of opaque heavy mineral grains, including ferruginous clay
aggregates, was 302 which was 66.4% of the 455 grains optically identified. In HM-103, the
dominant non-opaque grains analyzed were epidote (22.2%), garnet (4.0%), staurolite (2.6%),
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and tourmaline (2.2%) and the trace heavy minerals analyzed were hornblende (0.9%), zircon
(0.7%), rutile (0.4%), pyroxene (0.4%), and sphene (0.2%).
In HM-106, the opaque heavy mineral grains, including ferruginous clay aggregates,
were 65.7% of the 443 grains analyzed. The dominant non-opaque heavy mineral grains were
epidote (26.0%), garnet (3.2%), staurolite (2.5%), tourmaline (1.1%), and zircon (1.1%). One
trace non-opaques identified were sphene (0.5%).
Absolute Abundances
The absolute abundances for the 0.125-0.150 mm and the 0.180-0.212 mm size
fractions were compared to one another and the 0.25-0.30 mm size-fraction reanalyzed by Dr.
John Webster. The absolute abundances for the 0.125-0.150 mm, 0.180-0.212 mm, and 0.25-0.30
mm size fractions were 197.20 ppm, 219.0 ppm, and 72.6 ppm respectively, which are shown in
Table 5.
DISCUSSION
Comparison with Results Obtained by Previous Studies on SQB
The size fractions analyzed as part of this study including the 0.125-0.150 mm and
0.180-0.212 mm contained heavy mineral assemblages that reflected noteworthy differences
from the 0.25-0.30 mm size-fraction studied by Fogarty (2008). These differences were observed
in the percentages of staurolite, apatite, al-silicates, rutile, and epidote. There was also a
pronounced difference in the percentages of the non-opaque heavy minerals. These heavy
minerals and their respective percentages for each of the grain size fractions compared have been
shown in Table 3. However, after Dr. John Webster reanalyzed the results obtained by Fogarty
(2008), there was a remarkable decrease in the percentage of rutile. This decrease in the
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percentage of rutile after reanalysis was owing to the incorrect classification of leucoxene, an
opaque heavy mineral, as rutile by Fogarty (2008). Additionally, differences in the percentages
of staurolite, al-silicates, apatite, and epidote increased owing to the decrease in the percentage
of rutile after reanalysis. However, the opaque heavy mineral percentages were, after reanalysis,
quite similar to the opaque heavy mineral percentages of the size fractions analyzed as part of
this study. The percentages obtained after reanalysis and the major differences between the
heavy minerals are shown in Table 4. Based on the reanalyzed results, there is a high degree of
similarity between the grain size fractions involved in this study and the size-fraction previously
studied by Fogarty (2008). The absolute abundances were calculated using the results obtained
after reanalysis completed by Dr. John Webster on the 0.25-0.30 mm size fraction studied by
Fogarty (2008).
Comparison of Grain Size Yields, Results, and Absolute Abundances
The smaller grain size fraction, 0.125-0.150 mm, had a higher weight of overall mineral
grains recovered and also contained a higher proportion of heavy mineral grains than the 0.180-
0.212 size fraction. The two grain size fractions, 0.125-0.150 mm and 0.180-0.212 mm,
contained similar abundances, 34.3% and 33.6% respectively, of non-opaque heavy mineral
grains. There was a high degree of similarity among the two grain size fraction with regard to the
dominant and trace non-opaque heavy mineral assemblages. The dominant minerals identified in
both size fractions such as epidote, garnet, and staurolite were present in similar abundances.
Furthermore, the trace minerals identified such as zircon and sphene were also present in similar
abundances. The two size fractions studied only differed with regard to the presence of trace
percentages of hornblende and pyroxene in the 0.180-0.212 mm size-fraction. The opaque heavy
15. 15
mineral grains were present in similar percentages in both the size fractions, with 65.7% in the
0.125-0.150 mm size fraction and 66.4% in the 0.180-0.212 mm size fraction.
The calculation of absolute abundances of all heavy minerals recovered for the size
fractions studied and the size-fraction studied by Fogarty (2008), results later reanalyzed by Dr.
Webster, provide a method to compare the differences in heavy mineral abundances. The
absolute abundance of the 0.25-0.30 mm, reanalyzed by Dr. John Webster, was the lowest
followed by an increase in the absolute abundance for the 0.180-0.212 mm size fraction and a
comparatively lower increase than the 0.180-0.212 mm for the 0.125-0.150 mm size fraction.
Based on the absolute abundance values obtained for the three size-fractions, Figure 5 illustrates
a size-shift, which is a shift in the mode or peak of the distribution of all heavy minerals
recovered compared to the total grain size distribution. The mode was size-shifted towards the
finer grain sizes owing to the settling equivalence principle, which states that under certain
hydrodynamic conditions in fluvial systems, smaller, high density heavy minerals tend to settle
at the same velocity as larger, low density light minerals such as quartz (Garzanti et al. 2008).
Cluster Analysis
Samples obtained by MSU researchers, Denson and Gill (1965), and Denson et al.
(1965) were compared using cluster analysis on the basis of their degree of similarity determined
using the m-space Euclidean distance. There were 51 samples whose varied assemblages of the
14 heavy minerals were compared. The 51 samples were placed in groups from A to E on the
basis of they grouped on the dendrogram. Initially, the 0.125-0.150 mm and the 0.180-0.212 size
fractions were placed in group C on the basis of high abundances of epidote, garnet, and zircon
present in the samples. Furthermore, based on the relatively higher concentrations of epidote and
garnet within the C group, the two-size fractions were placed within the C4 sub-group. The
16. 16
samples in sub-group C4, on the basis of the higher abundances of epidote, garnet and relatively
lower abundances of staurolite and zircon, were from Whetstone Butte, Long Pine Hills, and
Chalky Butte. The presence of high abundances of epidote in sub-group C4 suggests a
Precambrian metamorphic source rock type (Fogarty 2008).
The 0.25-0.30 mm size-fraction studied by Fogarty (2008) was placed in the C3 group
because of the comparatively lower percentage of epidote but higher percentages of al-silicates,
staurolite, and tourmaline. The samples in group C were placed in a different group from the
other samples owing to the lower percentages or lack of zircon, biotite, and hornblende. The
samples in group C were, on the basis of Figure 4, very different from the Medicine Pole Hills
samples and the samples studied by Denson and Gill (1965).
17. 17
REFERENCES CITED
Bjornson MM. 2014. Heavy mineral analysis of the Eocene South Cave Hills sandstone using
optical and scanning electron microscopy [Senior Seminar Paper]. Minot (ND): Minot State
University. 38 p.
Denson NM, Chisholm WA. 1971. Summary of mineralogic and lithologic characteristics of
Tertiary sedimentary rocks in the middle Rocky Mountains and the northern Great Plains.
United States Geological Survey Professional Paper 750-C:117-26.
Denson NM, Gill JR. 1965. Uranium-bearing lignite and carbonaceous shale in the southwestern
part of the Williston Basin – a regional study. United States Geological Survey Professional
Paper 463. 75 p.
Denson NM, Gill JR, Chisholm WA. 1965. Uranium-bearing lignite and carbonaceous shale in
the southwestern part of the Williston Basin – a regional study. United States Geological
Survey Professional Paper 463. 75 p.
Fogarty H. 2008. Heavy mineral analysis of the Chalky Buttes Member sandstone from Square
Butte in southwestern North Dakota [Senior Seminar Paper]. Minot (ND): Minot State
University. 25 p.
Garzanti E, Ando S, Vezzoli G. 2008. Settling equivalence of detrital minerals and grain-size
dependence of sediment composition. Earth and Planetary Science Letters. 273: 138-151.
Gleich C. 2014. Heavy mineral analysis of a Late Eocene South Cave Hills sandstone located in
Harding County, South Dakota [Senior Seminar Paper]. Minot (ND): Minot State University.
29 p.
Murphy E, Hoganson J, Forsman F. 1993. The Chadron, Brule, Arikaree formations in North
Dakota. North Dakota Geological Survey Report of Investigation no. 96. 144 p.
Nelson N. 2008. Heavy mineral analysis of a Late Eocene sandstone from North Dakota using
X-ray diffraction and the Rietveld method [Senior Seminar Paper]. Minot (ND): Minot State
University. 22 p.
Sato Y, Denson NM. 1967. Volcanism and tectonism as reflected by the distribution of non-
opaque heavy minerals in some Tertiary rocks of Wyoming and adjacent states. United States
Geological Survey Professional Paper 575-C:42-54.
Webster JR, Kihm AJ, Klingbeil AA. 2015. Heavy minerals in the late Eocene sandstone of
Medicine Pole Hills, southwestern North Dakota. Rocky Mountain Geology 50:1-29.
18. 18
Figure 1. Map of ND-SD-MT tri-state area showing locations of buttes that expose strata from
the White River Group or Arikaree Formation. Heavy mineral analyses have been conducted on
samples obtained from the named localities (Webster et al. 2015)
20. 20
Figure 3. Apparatus used for heavy mineral extraction from a centrifuge flask (Nelson 2008).
21. 21
Figure 4. Dendrogram depicting cluster analysis done on the basis of m-space Euclidean
distance on samples studied by MSU researchers and Denson and Gill (1965).
22. 22
Figure 5. An illustration of the size-shift of heavy mineral abundances in ppm towards finer
grain sizes compared to the total grain size distribution, given in weight percent.
23. 23
Table 1. Heavy mineral analysis of the 0.180-0.212 mm fraction (HM-103)
Table 2. Heavy mineral analysis of the 0.125-0.150 mm fraction (HM-106)
Optical ID
Grains Grain %
Non-Opaque HM
Staurolite 12 2.6
Hornblende 4 0.9
Sphene 1 0.2
Tourmaline 10 2.2
Al-silicates 0 0.0
Apatite 0 0.0
Garnet 18 4.0
Rutile 2 0.4
Epidote 101 22.2
Pyroxene 2 0.4
Zircon 3 0.7
Opaque HM
Pseudobrookite 0 0.0
Ilmenite 0 0.0
FCA 65 14.3
Opaq 237 52.1
Total 455 100
