1. Illinois State Geological Survey, Champaign
Optical Ages Spanning Two Glacial-Interglacial Cycles from Deposits of the Ancient Mississippi River, North-Central Illinois
Don McKay
Dick Berg
Illinois State Geological Survey, Champaign
My topic is optical ages spanning the last 200,000 years from deposits of the Ancient Mississippi River in Illinois.
I would like to thank my coauthor, my many collaborators at the Illinois Survey, and Ron Goble and his colleagues at Nebraska for providing access to their OSL lab. Without that service, this work would not have been possible.
North-Central GSA
Evansville, IN
April 25, 2008
Funding
Illinois State Geological Survey
Illinois Dept. of Transportation

2. Middle Illinois River Valley Mapping
Batestown Mbr.
Tiskilwa Fm.
To orient you, here is a map of the Quaternary deposits of Illinois.
Pink shows the extent of Illinois Episode glacial tills and in green is the extent of Wisconsin Episode till.
The dashed line is the course of the Ancient Mississippi River through central Illinois before it was diverted to western Illinois by the late Wisconsin glacier.
The Lake Michigan lobe overrode the Ancient Mississippi Valley at least 5 times, depositing three Illinois and two Wisconsin tills west of the valley as shown in the inset map.
Geologic mapping of the Middle Illinois River valley in the area of the smaller box refocused attention on the area beginning in 2001.
Lake Michigan lobe glaciers overran the Ancient Mississippi Valley (AMV) during the Illinois and Wisconsin Episodes.

3. Quaternary Stratigraphy of Illinois
Yorkville Member
Batestown Mbr.
Tiskilwa Formation
Radnor Member
Hulick Member
Kellerville Member
Here’s the stratigraphic context for the deposits we are working with.
Colored like the previous slide, the tills in green and red are Wisconsin and Illinois Episode units, respectively.
Prior to our work, the Illinois Episode deposits were already presumed by most workers to have been deposited during Marine Oxygen Isotope Stage 6, based mainly on luminescence dates of Loveland Loess in the Missouri and lower Mississippi Valleys. Some contrary evidence in the form of a paleosol, the Pike Geosol, has been used to suggest the Illinoian spanned more than one glacial-cycle. Our work on fluvial deposits interbedded with the tills offered an opportunity to use direct dating of those deposits to test those presumed correlations.
Thick sand and gravel that virtually everywhere fills the lower hundred feet or more of the bedrock valley have been correlated to the Pre-Illinois Episode or pre-glacial Sankoty sand. Those correlations and assumptions needed to be tested.
Sources:
Hansel and McKay, 2008
Grimley et al., 2003
Curry and Follmer, 1992
Johnson, 1986
Willman and Frye, 1970

4. “Sankoty sand” Geologic mapping area 3 lines of cross section
Horberg, 1953
“Sankoty
Geologic mapping area
sand”
3 lines of
cross section
In the early 1950s, Leland Horberg defined the Sankoty sand and using available water well records mapped its distribution along the Ancient Mississippi Bedrock valley north of Peoria.
Three cross sections, along the lines shown, illustrated his concept of the unit.

5. Horberg, 1953 Wisconsin Ill. Wisc. Kan. Kansan or older Sankoty “sand”
Qsk
Sankoty “sand”
Wisc.
Ill.
Kan.
Wisconsin
Kansan or older
Horberg considered the Sankoty to be Kansan or older and his cross sections portray little heterogeneity in the unit, and show how few well records he had. We have a lot more data today.

6. Sampling and Age Determinations
Clear Creek Section
Sampling and Age Determinations
16 continuous cores
52 to 330 ft deep (eight >200 ft)
2980 ft of core
25 OSL ages (UNL)
20 from 6 cores
5 from 5 outcrops (7 more in work)
20 14C ages (conventional and AMS; ISGS and other labs)
7 from 2 cores
13 from 4 outcrops
All ages standardized as calibrated calendar years B.P. and expressed as thousand years (ka)
14C were calibrated using CalPal online: Cologne Radiocarbon Calibration Program
Wisconsin Episode till
96.1± 6.1 ka
Initially for the mapping and later for detailed study of the valley-fill deposits we drilled...
Note in the picture of the Clear Creek Section that the sand beneath Wisconsin Episode till was called Sankoty by Horberg. One of our first OSL dates was from there and dated to 96,000 BP, which generated more questions than answers.

7. Key core and outcrop locations Ancient Mississippi Valley
Condit # 1
Nauman # 1
Miller # 1
Schoepke # 1
Illinois River Valley
Ancient Mississippi Valley
Taylor # 1
This slide shows the distribution of some of our key sampling sites plotted on the left on a perspective view of ground surface and on the right on the bedrock surface.
Most of the holes, as you can see, were drilled in the modern uplands and penetrated the deep parts of the sediment filled bedrock valley.
We will look at the results from several key sites, beginning with the Nauman and Condit boreholes.
Rattlesnake Hollow
Ground surface
Bedrock surface

8. Nauman #1 Wisconsin Episode Wisconsin Episode tills 24.77 ± .25 ka 14C
Illinois Episode
Wisconsin Episode tills
25.84 ± .31 ka 14C
24.77 ± .25 ka 14C
25.60 ± .36 ka 14C
29.14 ± .83 ka 14C
Nauman # 1
Condit # 1
middle Illinois Episode
till (Hulick Mbr.)
major sand body
(Sankoty sand?)
We’ll start by comparing 14C ages in red from this, the Nauman core, with OSL ages from the Condit core. Both sites record burial of lacustrine or fluvial deposits by the Wisconsin Episode glacier just before the glacier overrode the Ancient Mississippi River.
Elevation in feet is on the left. Our lithologic log of the 250 ft core shows Wisconsin Episode tills over a lacustrine succession over the middle Illinois Episode till, interbedded with thick sand in the Sankoty sand position. Time correlation to the left reflects our present understanding.
14C results from this core are from the lacustrine succession. They show a nice sequence of ages, recording infill of a proglacial lake starting about 29,000 years ago and ending at 24,770 ± 250 years when Wisconsin till buried the site.

9. Condit #1 Wisconsin Episode tills Wisconsin Ep. no Sangamon Geosol
26.1 ± 2.3 ka
25.0 ± 1.8 ka
23.2 ± 2.5 ka
24.7 ± 2.3 ka
Wisconsin Episode tills
no Sangamon Geosol
24.8 ± 1.1 ka
Nauman # 1
Condit # 1
Moving about 6 miles westward across the bedrock valley to Condit #1, we found this succession. Thick Wisconsin Episode tills overlie 85 feet of sand and gravel, much of which matches the lithology of Horberg’s Sankoty, with its significant component of pink, well rounded quartz grains.
Four OSL analyses have these results. Clearly the deposit is not preglacial, Pre-Illinois, or Illinois Episode.
The dates are not in chronologic sequence and overlap significantly at 1 sigma.
Given that level of uncertainty, we can treat the deposit as one statistical population, and calculate a weighted mean of the four dates, which yields an age of 24,800 ± 1100 BP, virtually the same age as the top of the lake sediment at the Nauman site.

10. Condit-Nauman Cross Section
Ice flow
24.8 ± 1.1
24.77 ± .25
A cross section from the western flank of the bedrock valley eastward through these two dated cores looks like this.
Wisconsin ice flow was right to left. The base of the Wisconsin glacier rode along the solid green line, which is now the base of Wisconsin till.
Our 14C dates in black show that the ice passed over the lake at 24,770 BP and then, across the valley, overrode the channel of the river at 24,800 (OSL determined).
These results suggest consistency of the 14C and OSL. They also support the hypothesis that the 80-foot thick sand and gravel in the Condit core was in transport, its luminescence was reset, and it was deposited shortly before both sites were buried by the Wisconsin glacier.
~10km

11. Miller #1 Wisc. Ep Wisconsin Episode tills and loesses Sangamon Geosol
Illinois Episode
Wisc. Ep
middle Illinois Episode till (Hulick Mbr.)
Wisconsin Episode tills
and loesses
Sangamon Geosol
Miller #1
Now, moving to the bottom of the valley-fill succession, we will work our way upward stratigraphically through the next several cores. Two OSL age determinations on closely-space samples of sand overlying bedrock at the 325 ft depth in the Miller core date our oldest unit to between 185,000 and 193,000.
Plotting the weighted mean of those two dates over a SPECMAP marine O-isotope curve from Martinson et al (1987), shows that the unit dates to the transition between a warm Isotope Stage 7 and the cold Stage 6.
193.2 ± 16.3 ka
185.2 ± 16.4 ka
189.2 ± 11.6 ka

12. Illinois Episode
Wisc. Ep
Schoepke # 1
earliest Illinois Episode till (Kellerville Mbr.)
154.7 ± 8.1 ka
The Schoepke core recovered a thick and complete succession of Illinois Episode tills, including the oldest, Kellerville Mbr., overlying lacustrine and fluvial deposits that we interpret to be the early Illinois Episode valleytrain.
Three OSLs from the basal sand and gravel show these results. We interpret the apparent decrease in age with depth to be an artifact of randomness related to the large uncertainties of the ages.
With these three dates we have overlap at 1 sigma. A weighted mean of the three gives a value of about 155,000.
That date falls in the middle of Stage 6 during a time of maximum global ice volume.
171.1 ± 14.7 ka
150.6 ± 12.6 ka
142.7 ± 15.5 ka

13. Nauman #1 Wisconsin Episode middle Illinois Episode till (Hulick Mbr.)
= middle MIS 6
sand in Schoepke #1
Returning to the Nauman core, where we looked at 14C results from the lake deposits, we have three OSL dates. Two are from a deposit just above bedrock, and one is from sand interbedded with the middle Illinois Episode till, the Hulick Mbr.
I tend to discount the bottommost date, which is too young by a good deal.
The remaining two in black, plotted on the 18O curve, fit quite well with the results from the two previous sites, Schoepke and Miller.
158.4 ± 15.1 ka
127.7 ± 10.8 ka
193.4 ± 17.1 ka
= late MIS 7
sand in Miller #1

14. Morine #1 Wisconsin Ep. Wisconsin Episode tills and loesses
middle Illinois Episode till (Hulick Mbr.)
Wisconsin Episode tills
and loesses
Sangamon Geosol
upper Illinois Episode till (Radnor Mbr.)
Wisconsin Ep.
132.8 ± 11.4 ka
157.8 ± 12.7 ka
144.0 ± 8.5 ka
Illinois Episode
Moving up-section, at the Morine site just west of the Nauman core, we have two dates from gravelly sand that occurs between the middle and upper Illinois Episode tills. The weighted mean of the two is 144,000.
Plotted on the 18O curve, that result falls in the upper third of Stage 6.

15. Knapp # 1 Wisc. Ep 3 WI Episode tills & 2 loesses Sangamon Geosol;
no IL Episode till above sands
meandering river deposits
middle Illinois Episode till (Hulick Mbr.)
Knapp # 1
130.8 ± 5.1 ka
Wisc. Ep
124.7 ± 5.6 ka
161.8 ± 12.6 ka
123.6 ± 9.1 ka
142.6 ± 12.5 ka
117.0 ± 8.7 ka
Sangamon Episode
The last site is the Knapp core, where we sampled nearly 140 ft of sand, silty sand, silt, and silty clay containing dispersed plant debris that we interpreted as deposits of a meandering river. The deposits represent overbank, natural levee, perhaps channel-fill, and, where they coarsen at the top, splay or channel deposits. These deposits are not overlain by IL Ep till. There is a well developed Sangamon paleosol in their upper part. That, in turn, is overlain by a complete Wisconsin succession of tills and loesses. The lithology and organic content favor an origin during an interglacial. The organic matter has not yet been studied.
The four ages have an inconsistent trend with depth. The uppermost (the oldest) is a significant outlier.
The weighted mean of all four is 131,000 ka. Our interpretation of the upper coarse beds as splay deposits supports a hypothesis that those sands were eroded locally and not reset before being redeposited. Their 161,000 age matches that of a thick sand of that age we’ve documented lower in the section near this site. If we discount that uppermost (oldest) date, a weighted mean of the remaining three is 125,000 ka.
I’ve plotted that 125,000 result over the isotope curve, but either result, 125,000 or 131,000, is compatible with a Stage 5e interpretation. 5e is the warmest part of Stage 5, and is the Eemian of Europe. Ours is a reasonable result given the lithology and stratigraphy at our site and is the first 5e deposit documented in the Ancient Mississippi watershed that I’m aware of.
IL Ep

16. Correlations * * samples in work 25-75 ka West East ~144 ka 25-75 ka
Here we pull all this together in an east-west correlation, showing the units we have distinguished and dated in the boreholes and outcrops studied. From the bottom, on bedrock a widespread 190,000 year old fluvial sand, the oldest unit we have found was deposited late in the waning part of Stage 7.
This unit is overlain unconformably by a succession of Stage 6 outwash and till deposits up to 200 feet thick. An outwash unit dated about 160,000 to 155,000 BP intertongues with the lower and middle Illinois Episode tills.
That is overlain by an outwash dated about 144,000 years that occurs between the middle and upper Illinois Episode tills.
As the Illinois Episode glacier finally retreated from central Illinois, the river reoccupied the valley and incised deeply into the deposits. By about 125,000, Stage 5e, it had established a meandering pattern that probably persisted through the Sangamon Episode. Our youngest date in it is about 96,000.
Early Wisconsin outwash entered the watershed about 75,000 BP, based on one date only, and the Wisconsin Episode valleytrain occupied the valley until 24,800 BP when it was buried by the late Wisconsin glacier.
Upon retreat of Wisconsin ice, meltwater downcut a new valley and deposited outwash in the Illinois River valley. Two OSL results date Wisconsin outwash in terraces at 19,500 to 22,000 years.
Several samples are in work that may help refine these estimates.
-95 ka
~190 ka

17. Summary Illinois Episode glaciation in its type region in Illinois
occurred during a single global cold period (MIS 6)
lasted 15 to 25 ka in the latter half of MIS 6 in central Illinois
coincided with a period of massive global ice volume during MIS 6
IL Ep. glacier was present in central IL somewhat longer than late WI Episode ice was in IL.
Uncertainty in OSL ages leaves room for refinement of specific conclusions.

18. Summary and Conclusions
Systematic use of OSL to date Ancient Miss. R. fluvial sand in IL found that:
OSL results are consistent with 14C.
Individual OSL results >100 ka include significant uncertainty.
Multiple dates per unit allow statistical reduction of uncertainty (where the single-unit single-population assumption is correct).
Results fit global marine oxygen isotopic stage chronologies for late Stage 7, 6, & 5 between 190 and 125 ka.
Results allow correlation of complex subsurface fluvial successions and reconstruction of events recorded in them.
A Pre-Illinois Episode “Sankoty sand” was not found.
Earliest Sangamon Episode fluvial deposits date to Stage 5e.
Diversion of AMR occurred ~24.80 ka (OSL & 14C).
OSL should be routinely applied to mapping, stratigraphic, sedimentologic, and aquifer studies in IL.
OSL will greatly improve our understanding of Quaternary Earth history 200,000 to 50,000 yrs ago, just as 14C has improved our knowledge of the last 50,000 yrs!