Where Did They Come From? – The Search for the Slackwater Sediments of the Savanna Terrace
Shortcut URL: https://serc.carleton.edu/59968
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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Figure 2: Red and gray sediments from the slackwater terrace from core SVS-26. (Photo by author.) Details
Description
As geologists, we sometimes travel to exotic locales for research. Sometimes, we work a little closer to home. In my case, I work very close to home. While investigating the materials that make up the Savanna Terrace along the Mississippi River, my field area was located an hour north of where I grew up! In essence, I got the joy of studying my own backyard. In the case of the Savanna Terrace, I wanted to know where certain slackwater terrace materials came from.
The Savanna Terrace along the Mississippi River in northwestern Illinois (Figure 1) was formed due to drainage of the Laurentide Ice Sheet (LIS) during the Wisconsin Episode Deglaciation. Meltwater from the LIS seasonally 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 and resulting in the deposition of alternating layers of red and gray clayey sediments (Figure 2).
Due to the incision of the Mississippi River and its nearby tributary the Apple River, there were several levels of terraces in the field are that are above the modern floodplain: the clayey slackwater terrace of the Equality Formation (which is the Savanna Terrace itself), two fluvial sand terraces of the Henry Formation (one 20 ft. lower in elevation than the other and therefore younger), and one alluvial terrace of the Cahokia Formation (Figure 3). According to reverse morphostratigraphy, the terraces that are the highest in elevation are the oldest. The uppermost fluvial sand terrace and the clay terrace were at approximately the same elevation (~15 m above the modern Mississippi), and prevailing winds had reworked some of the fluvial sand into eolian dunes, most of which were only a few feet in elevation. While many of these dunes were encroaching on the western edge of the slackwater terrace and a few dunes are located on the slackwater terrace in the north, the majority of the terrace is flat, with the slackwater deposits making up the uppermost sediments. Why is this information about the multiple terraces and the location of the sand dunes important? In Illinois, there isn't much in the way of outcrops that can be used for stratigraphic studies. Everything is covered by thick deposits of glacial or glacial-related materials. We have to make do with exposures along streambanks (when present) or take sediment cores. Although Rush Creek bisects the field area, the steep slopes leading down to the stream are covered with trees and underbrush. The only way to do stratigraphic work is with sediment cores. Coring can be expensive and, since I was most interested in the sediments of the clayey Savanna Terrace, I needed to use the location of the sand dunes to find the margin between the clayey slackwater terrace and the fluvial sand terraces. Once I read the landscape, I could select my coring sites more intelligently.
My original hypothesis was that the gray sediments in the slackwater terraces were the result of drainage from Lake Agassiz into the Mississippi whereas the red sediments came from drainage from glacial Lake Superior/Lake Duluth. Drainage from Lake Agassiz would have started around 11,700 14C yr B.P. and drainage from Lake Superior would have begun around 11,000 14 C yr B.P. However, radiocarbon dating provided ages of ~13,000 14C yr B.P., which predates both proglacial lakes (lakes on the margin of the glacier). However, Curry and Grimley (2006) researched similar terraces on the Mississippi near the St. Louis Metro East area and stated that such terrace deposits were the result of meltwater drainage from glacial lobes. With the radiocarbon age in mind, the hypothesis was revised to try to link the red sediments to the Superior Lobe and the gray sediments to the Des Moines Lobe, both of the LIS.
X-Ray Diffraction (XRD) and trace element analysis were used to establish source area "fingerprints" to compare against. Sediment samples were collected using Dual Tube-22 sample cores taken with a Geoprobe direct push rig. Source materials used for comparison included Lake Agassiz and Lake Superior sediments as well as the local Peoria Silt loess, which was added to rule out local influences. (It should be noted that radiocarbon age was not known until sample analysis was well underway. As a result, samples were tested against Lake Agassiz and Lake Superior material. The melting of the Des Moines Lobe helped to create Lake Agassiz, so those two source materials would hopefully be similar.)
For the XRD analysis, the clay minerals of illite, kaolinte, chlorite, and the expandable clays (i.e. smectite) were used for comparison. Lake Agassiz samples were rich in expandable clays whereas Lake Superior samples were rich in kaolinite. The field samples displayed similar characteristics as the hypothesized source areas: the gray field samples had more expandable clays and the red field samples had more kaolinite. To be sure, the next step was to compare the trace elements of the samples to those of the source areas.
Samples from Lake Superior were shown to be rich in Fe, Cu, and Y whereas samples from Lake Agassiz were rich in Se and Cd. Red samples from the field area tended to cluster around the samples from Lake Superior, particularly in the Cu/Se and Y/Cd comparisons (Figure 4). In the Fe/Se comparison, there was a similar clustering of gray field samples around Lake Agassiz source materials. However, there was little correlation between the gray field samples and Lake Agassiz samples for other trace element pairings. No field samples correlated with the Peoria Silt samples taken from the surrounding uplands, suggesting there was no significant input from local loess sources in terrace materials.
So, this research answered one question: the red sediments in the Savanna Terrace likely came from the region of the Superior Lobe, which occupied the same geographic location as the subsequent Lake Superior. However, while there were some similarities between the gray field sediments and Lake Agassiz, it wasn't a close match. In later research, a student obtained samples of Des Moines Lobe material from the Twin Cities area of Minnesota and looked at the trace elements. Surprisingly, there wasn't a close correlation between the lobe material and the gray field sediments. The search continues.....
The Savanna Terrace along the Mississippi River in northwestern Illinois (Figure 1) was formed due to drainage of the Laurentide Ice Sheet (LIS) during the Wisconsin Episode Deglaciation. Meltwater from the LIS seasonally 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 and resulting in the deposition of alternating layers of red and gray clayey sediments (Figure 2).
Due to the incision of the Mississippi River and its nearby tributary the Apple River, there were several levels of terraces in the field are that are above the modern floodplain: the clayey slackwater terrace of the Equality Formation (which is the Savanna Terrace itself), two fluvial sand terraces of the Henry Formation (one 20 ft. lower in elevation than the other and therefore younger), and one alluvial terrace of the Cahokia Formation (Figure 3). According to reverse morphostratigraphy, the terraces that are the highest in elevation are the oldest. The uppermost fluvial sand terrace and the clay terrace were at approximately the same elevation (~15 m above the modern Mississippi), and prevailing winds had reworked some of the fluvial sand into eolian dunes, most of which were only a few feet in elevation. While many of these dunes were encroaching on the western edge of the slackwater terrace and a few dunes are located on the slackwater terrace in the north, the majority of the terrace is flat, with the slackwater deposits making up the uppermost sediments. Why is this information about the multiple terraces and the location of the sand dunes important? In Illinois, there isn't much in the way of outcrops that can be used for stratigraphic studies. Everything is covered by thick deposits of glacial or glacial-related materials. We have to make do with exposures along streambanks (when present) or take sediment cores. Although Rush Creek bisects the field area, the steep slopes leading down to the stream are covered with trees and underbrush. The only way to do stratigraphic work is with sediment cores. Coring can be expensive and, since I was most interested in the sediments of the clayey Savanna Terrace, I needed to use the location of the sand dunes to find the margin between the clayey slackwater terrace and the fluvial sand terraces. Once I read the landscape, I could select my coring sites more intelligently.
My original hypothesis was that the gray sediments in the slackwater terraces were the result of drainage from Lake Agassiz into the Mississippi whereas the red sediments came from drainage from glacial Lake Superior/Lake Duluth. Drainage from Lake Agassiz would have started around 11,700 14C yr B.P. and drainage from Lake Superior would have begun around 11,000 14 C yr B.P. However, radiocarbon dating provided ages of ~13,000 14C yr B.P., which predates both proglacial lakes (lakes on the margin of the glacier). However, Curry and Grimley (2006) researched similar terraces on the Mississippi near the St. Louis Metro East area and stated that such terrace deposits were the result of meltwater drainage from glacial lobes. With the radiocarbon age in mind, the hypothesis was revised to try to link the red sediments to the Superior Lobe and the gray sediments to the Des Moines Lobe, both of the LIS.
X-Ray Diffraction (XRD) and trace element analysis were used to establish source area "fingerprints" to compare against. Sediment samples were collected using Dual Tube-22 sample cores taken with a Geoprobe direct push rig. Source materials used for comparison included Lake Agassiz and Lake Superior sediments as well as the local Peoria Silt loess, which was added to rule out local influences. (It should be noted that radiocarbon age was not known until sample analysis was well underway. As a result, samples were tested against Lake Agassiz and Lake Superior material. The melting of the Des Moines Lobe helped to create Lake Agassiz, so those two source materials would hopefully be similar.)
For the XRD analysis, the clay minerals of illite, kaolinte, chlorite, and the expandable clays (i.e. smectite) were used for comparison. Lake Agassiz samples were rich in expandable clays whereas Lake Superior samples were rich in kaolinite. The field samples displayed similar characteristics as the hypothesized source areas: the gray field samples had more expandable clays and the red field samples had more kaolinite. To be sure, the next step was to compare the trace elements of the samples to those of the source areas.
Samples from Lake Superior were shown to be rich in Fe, Cu, and Y whereas samples from Lake Agassiz were rich in Se and Cd. Red samples from the field area tended to cluster around the samples from Lake Superior, particularly in the Cu/Se and Y/Cd comparisons (Figure 4). In the Fe/Se comparison, there was a similar clustering of gray field samples around Lake Agassiz source materials. However, there was little correlation between the gray field samples and Lake Agassiz samples for other trace element pairings. No field samples correlated with the Peoria Silt samples taken from the surrounding uplands, suggesting there was no significant input from local loess sources in terrace materials.
So, this research answered one question: the red sediments in the Savanna Terrace likely came from the region of the Superior Lobe, which occupied the same geographic location as the subsequent Lake Superior. However, while there were some similarities between the gray field sediments and Lake Agassiz, it wasn't a close match. In later research, a student obtained samples of Des Moines Lobe material from the Twin Cities area of Minnesota and looked at the trace elements. Surprisingly, there wasn't a close correlation between the lobe material and the gray field sediments. The search continues.....
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.
- Curry, B.B. & Grimley, D.A. (2006). Provenance, age, and environment of mid-Wisconsinan slackwater lake sediment in the St. Louis Metro East area, USA. Quaternary Research 65, p. 108-122.
- 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., 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