The Use of Geophysical Analysis to Identify the Extent of Slackwater Terrace Materials
Shortcut URL: https://serc.carleton.edu/59967
Location
Continent: North America
Country: United States of America
State/Province:Illinois
City/Town: Savanna
UTM coordinates and datum: none
Setting
Climate Setting: Humid
Tectonic setting: Craton
Type: Stratigraphy
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Description
The Savanna Terrace along the Mississippi River in northwestern Illinois was formed due to drainage of the Laurentide Ice Sheet (LIS) during the Wisconsin Episode Deglaciation (Figure 1). Meltwater from the LIS periodically drained down the Mississippi River from glacial lobes to the north, sending large volumes of water downstream. This water backed up into tributaries, creating slackwater lakes. At this location, the field area was bisected by a tributary of the Mississippi called Rush Creek, which flowed roughly from north to south across the Savanna Terrace before entering the Mississippi. Slackwater terrace materials are found on both the east and west sides of the creek, and the terrace is surrounded on three sides by Silurian dolomite bluffs.
One of the research goals at this location was to determine the extent and thickness of the clayey slackwater terrace. Determining the surface extent was easy – all that was necessary was to look around and use the shape of the land surface as a guide. When the slackwater lakes were generated up the Rush Creek valley, the sediments were deposited on flat floodplains. Subsequent fluvial erosion incised through these floodplains, but the tops of the new terraces remained untouched. Terraces to the west of the slackwater terrace were composed of fluvial sands, from which subsequent winds whipped up dunes. For the most part, determining the surface extent of the slackwater terrace was a matter of determining where the rolling dunes left off and the flat terrace surface picked up (Figure 2).
But, what about depth? How thick were the slackwater materials? Knowing that streams flow downslope, it is logical to assume that the slackwater materials would be thicker closer to the confluence of Rush Creek and the Mississippi. Taking sediment cores could prove this, but coring is time-consuming and expensive. Geophysical surveys, on the other hand, provide a fast, low-cost alternative to get the information that was needed. The few sediment cores that were taken for this research were used to provide "ground truth" to test the effectiveness of the geophysical data.
A multivariate approach was used with geophysical surveys to account for the high clay content of the slackwater terrace. Clay particles attract other ions, which creates a double layer effect that results in rapid attenuation of the radar waves of geophysical methods such as ground penetrating radar (GPR). The radar wave loses strength quickly, so researchers cannot "see" as deeply.
One thing that worked in favor of the researchers: the slackwater materials were sitting on a bed of fluvial sand. There were also some thin layers of sand within some of the clayey deposits. The sand acted as a reflector, so while details within the slackwater sediments may not visible, it was still possible to determine their total thickness.
GPR surveys were conducted using 50 and 100 MHz antennae with 1 and 2 m spacings between the transmitter and receiver (Figure 3). A common midpoint (CMP) velocity test provided a ground wave velocity of 0.07 m/ns that was used to estimate depths on the GPR sections. Using this at the southern end of the terrace, closer to the confluence of Rush Creek and the Mississippi, reflections that appeared on the GPR profile revealed a reflector approximately 3.3 m deep, which corresponded to a thin layer of silty material in the clayey materials. A second reflection, though fainter, marked the base of the clayey materials. These reflectors matched up with a sediment core taken at the same location, proving the usefulness of the method, though because of the attenuation of the radar wave due to the clays, it was difficult to pick up anything after the first reflector.
Resistivity soundings were also used to locate the boundary between slackwater materials and fluvial sands. While they could pick up the boundary of the fluvial sands, they could not resolve any finer details within the slackwater materials. Also, since the sounding curves only picked up the uppermost major reflector, they tended to undervalue the depth to the fluvial sands. So, the soundings were not as precise as other geophysical methods, but using them along survey lines perpendicular to the stream provided data as to the location of the original location of Rush Creek at the time the slackwater sediments were deposited. This data puts the paleochannel approximately 0.25 km to the east of the present one, farther away from the modern channel of the Mississippi River
Electrical conductivity used in two locations of the field area was useful in confirming the boundary between the slackwater materials and the fluvial sands that was seen with GPR. It also provided detailed information about changes in sediment size in the fluvial sands. Clay particles are more conductive and sands are more resistive, so sediments that are fining upward are displayed as a change in conductivity. Because of this, sequences of channel bars were identified in the fluvial sands (Figure 4).
When the geophysical data was combined with the few sediment cores that were collected, the data supported that the slackwater deposits were thinner upstream on Rush Creek and progressively thickened downstream (Figure 5). The thickness of the deposits thickened eastward, indicating that the original location of Rush Creek was closer to the dolomite bluffs in the past (Figure 6). Finally, the fact that the slackwater sediments do not thicken in the direction of the Mississippi supports the hypothesis that the clayey terrace was the result of water backing up into Rush Creek rather than the Mississippi itself depositing these materials.
One of the research goals at this location was to determine the extent and thickness of the clayey slackwater terrace. Determining the surface extent was easy – all that was necessary was to look around and use the shape of the land surface as a guide. When the slackwater lakes were generated up the Rush Creek valley, the sediments were deposited on flat floodplains. Subsequent fluvial erosion incised through these floodplains, but the tops of the new terraces remained untouched. Terraces to the west of the slackwater terrace were composed of fluvial sands, from which subsequent winds whipped up dunes. For the most part, determining the surface extent of the slackwater terrace was a matter of determining where the rolling dunes left off and the flat terrace surface picked up (Figure 2).
But, what about depth? How thick were the slackwater materials? Knowing that streams flow downslope, it is logical to assume that the slackwater materials would be thicker closer to the confluence of Rush Creek and the Mississippi. Taking sediment cores could prove this, but coring is time-consuming and expensive. Geophysical surveys, on the other hand, provide a fast, low-cost alternative to get the information that was needed. The few sediment cores that were taken for this research were used to provide "ground truth" to test the effectiveness of the geophysical data.
A multivariate approach was used with geophysical surveys to account for the high clay content of the slackwater terrace. Clay particles attract other ions, which creates a double layer effect that results in rapid attenuation of the radar waves of geophysical methods such as ground penetrating radar (GPR). The radar wave loses strength quickly, so researchers cannot "see" as deeply.
One thing that worked in favor of the researchers: the slackwater materials were sitting on a bed of fluvial sand. There were also some thin layers of sand within some of the clayey deposits. The sand acted as a reflector, so while details within the slackwater sediments may not visible, it was still possible to determine their total thickness.
GPR surveys were conducted using 50 and 100 MHz antennae with 1 and 2 m spacings between the transmitter and receiver (Figure 3). A common midpoint (CMP) velocity test provided a ground wave velocity of 0.07 m/ns that was used to estimate depths on the GPR sections. Using this at the southern end of the terrace, closer to the confluence of Rush Creek and the Mississippi, reflections that appeared on the GPR profile revealed a reflector approximately 3.3 m deep, which corresponded to a thin layer of silty material in the clayey materials. A second reflection, though fainter, marked the base of the clayey materials. These reflectors matched up with a sediment core taken at the same location, proving the usefulness of the method, though because of the attenuation of the radar wave due to the clays, it was difficult to pick up anything after the first reflector.
Resistivity soundings were also used to locate the boundary between slackwater materials and fluvial sands. While they could pick up the boundary of the fluvial sands, they could not resolve any finer details within the slackwater materials. Also, since the sounding curves only picked up the uppermost major reflector, they tended to undervalue the depth to the fluvial sands. So, the soundings were not as precise as other geophysical methods, but using them along survey lines perpendicular to the stream provided data as to the location of the original location of Rush Creek at the time the slackwater sediments were deposited. This data puts the paleochannel approximately 0.25 km to the east of the present one, farther away from the modern channel of the Mississippi River
Electrical conductivity used in two locations of the field area was useful in confirming the boundary between the slackwater materials and the fluvial sands that was seen with GPR. It also provided detailed information about changes in sediment size in the fluvial sands. Clay particles are more conductive and sands are more resistive, so sediments that are fining upward are displayed as a change in conductivity. Because of this, sequences of channel bars were identified in the fluvial sands (Figure 4).
When the geophysical data was combined with the few sediment cores that were collected, the data supported that the slackwater deposits were thinner upstream on Rush Creek and progressively thickened downstream (Figure 5). The thickness of the deposits thickened eastward, indicating that the original location of Rush Creek was closer to the dolomite bluffs in the past (Figure 6). Finally, the fact that the slackwater sediments do not thicken in the direction of the Mississippi supports the hypothesis that the clayey terrace was the result of water backing up into Rush Creek rather than the Mississippi itself depositing these materials.
Associated References
- Blackhawk Quadrangle Map, 7.5 minute series (1953). United States Department of Interior Geological Survey, 1:24,000 scale. (Photorevised in 1975.) Downloaded from http://www.isgs.uiuc.edu/nsdihome/ on Sept. 17, 2011.
- Johnson, B.A. (2009). Provenance of Slackwater Sediments in the Savanna Terrace, Northwestern Illinois. Ph.D. Dissertation, Northern Illinois University, DeKalb, 304 p.
- Johnson, B.A. & Carpenter, P.J. (In Submission). Geophysical response of slackwater and sandy terrace deposits near Savanna, northwestern Illinois. Submitted to Environmental Earth Sciences.
- Johnson, B.A., Stravers J., Konen M., & Wright M. (2008). Quaternary Geologic Map of the Blackhawk Quadrangle (Carroll and Jo Daviess Counties, Illinois and Jackson County, Iowa). US Geological Survey map, EDMAP Series, http://www.isgs.illinois.edu/maps-data-pub/isgs-quads/b/blackhawk-ed.shtml Blackhawk Quadrangle Map, 7.5 minute series (1953). United States Department of Interior Geological Survey, 1:24,000 scale. (Photorevised in 1975.) Downloaded from http://www.isgs.uiuc.edu/nsdihome/ on Sept. 17, 2011.
- Johnson, B.A., Stravers J., Konen M., & Wright M. (2008). Quaternary Geologic Map of the Blackhawk Quadrangle (Carroll and Jo Daviess Counties, Illinois and Jackson County, Iowa). US Geological Survey map, EDMAP Series, http://www.isgs.illinois.edu/maps-data-pub/isgs-quads/b/blackhawk-ed.shtml