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RESEARCH POSTER PRESENTATION DESIGN © 2015
www.PosterPresentations.com
The goal of this study is to test our hypothesis that sand dunes in the White
River Badlands (WRB), South Dakota, northern Great Plains, formed
during one or more prolonged prehistoric drought events. WRB dune
fields are located on tabletops north of the White River in southwestern
South Dakota. These parabolic dunes have maximum relief of 30 m and
open to the northwest, consistent with the dominant wind direction for the
region. Currently, dunes are stabilized by a drought-adapted, mixed grass
prairie community.
Sand dunes form when 1) prolonged drought kills prairie vegetation,
allowing wind to erode and transport sediment, forming sand dunes, or 2)
local stream sediment supplies increase, causing dune formation in
nondrought conditions. These scenarios can be distinguished based on
quantitative and qualitative comparisons of dune and stream sand
compositions. If mineralogical composition of WRB dune sand is
equivalent to compositions of White River sand, then the origin of the
dunes is related to increased sand supply, not regional drought.
In the summers of 2015 and 2016, investigators collected
representative samples from the WRB dunes and the White River channel.
Here investigators analyze the compositions using two
methodologies. First, using a petrographic microscope, investigators
identify 200 randomly chosen grains in thin section from each sample to
produce a statistically valid modal composition based on quartz, feldspar,
and rock fragment content. Second, using the petrographic microscope,
investigators identify distinct mineralogical suites in each sample for
comparison. The presentation will include results of these quantitative and
qualitative analyses.
Abstract Introduction
Five samples were collected from the field area (Figure 7), which included
a sample from the Cheyenne River, White River, and three samples from
the sand dunes. The samples were washed to remove silt or clay. Thin
sections were made from these five samples. Each of the thin sections were
polished to 30 microns thickness. The thin sections were scanned to
produce a digital image. Digital image analysis software (JMIcroVision)
was used to determine the mineralogical composition's of each sample. For
the point counting methodology, a random grid was superimposed on the
image. The software advanced a pointer (Figure 5) through the grid. At
each selection, the investigator identified the grain composition as either 1)
clear (quartz or feldspar), 2) colored (accessory minerals), and 3) opaque
grains (rock fragments). After each identification, the pointer was
advanced until 200 points were identified. The results are plotted in Figure
5.
Materials and Methods
Results
Conclusions
Results of our experiments indicate dune sand compositions are neither
equivalent to White River nor Cheyenne River sand. However, it is
possible that the dune sands came from the rivers but was mechanically
and chemically weathered, reducing rock fragments and enriching quartz
and feldspar. We expect that with wind transport of river sediment that
rock fragments will decrease and the relative composition of clear grains
will increase. This suggests that either the White River or the Cheyenne
River sand could be a source with compositional differences between river
and dune sands due to distance of transport and weathering of sand (Figure
8).
While, no definitive match was made either with the qualitative
comparison or quantitative studies, the weight of evidence including the
shape of the sand dunes and the modal compositions similarity to the
Cheyenne River favor a drought activation of the dunes. Further research
is needed to identify unique compositional differences that would uniquely
determine the source of the WRB dunes sands.
Acknowledgements This project was supported in part by an NSU President’s Faculty Research
and Development Grant 335392. Thanks as well to our collaborators Pat
Burkhart at Slippery Rock University and Paul Hanson at University of
Nebraska,
Sand dune activity is a sensitive indicator of climate change. When
plant communities die during droughts wind erodes soils and creates
sand dunes. During periods of greater moisture, soils are again
stabilized by vegetation and dunes become inactive (Figure 3). Because
dunes in our field area in the northern Great Plains are stabilized by
drought tolerant plant communities, sand dunes are only active during
especially severe and prolonged droughts. Alternatively, the dunes may
become active during increased sand supply in local rivers, in this case
the White River.
To determine the source of the sand in the dunes, we have analyzed
the mineralogic composition of sands from dunes and nearby rivers. A
strong northwest seasonal wind and the shape of the dunes suggest that
the source of the sand may be the Cheyenne River valley north of the
field area. Alternatively, the southern end of the White River borders the
dune fields to the south, making it another possible source of aeolian
sediment. Because the Cheyenne River and the White River drain
regions with distinctly different rock types, we predicted that the
composition of the sand would be an indicator of the source.
Hypothesis: If mineralogical composition of WRB dune sand is
equivalent to compositions of White River sand, then the origin of
the dunes is related to increased sand supply, not regional drought.
Nova Southeastern University
Halmos College of Natural Sciences and Oceanography
Department of Marine and Environmental Sciences
Megan O’Connor and Paul Baldauf, Ph.D.
Mineralogical Analysis of Aeolian Dune Deposits, White River Badlands,
South Dakota
White River
Cheyenne River
Sand Dunes
0%
5%
10%
15%
20%
25%
30%
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Op
aqu
e R
ock
Fra
gmen
ts
Quartz and Feldspar
Modal Grain Composition
Quartz
Figure 8: Indicates the 200 points that were randomly selected during the point counting
process. Evolution plots stabilized at 200 grains counts per slide. White River had the greatest
composition of rock and colored grains, Cheyenne was next highest in this category. Dune sands
were distinctly lower in rock fragments and colored grains but enriched in clear grains (quartz
and feldspar).
Figure 1: The above image shows the field area, rivers, and Black Hills uplift. The Black
Hill uplift is essential to our hypothesis because it shows that the Cheyenne will collect
all the sediments before they are able to reach the White River.
Figure 7: Student collecting sand dune samples during the Summer of 2016.
Figure 2: Image showing the parabolic dunes that are developed from strong northwestern
winds. Red arrows show the regional dominant wind direction.
Figure 5: Digital image of the White River thin section seen in JMicroVision software. The
above image shows the point counting methodology and how the different grains were classified
based on mineralogical composition.
Figure 3: Stabilized dunes covered in mixed grass prairie vegetation.
Cuny Table.
Figure 6: Plot of weight percent sand retained versus grain size. Green
(Cheyenne River), blue (dune sand), yellow (Miami Beach sand).
These show that grain size distributions are distinctly different in these
different environments.
References
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Miami
Beach
sand
Cheyenne
River
Dune sands
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