Stratigraphy, magnetic susceptibility, and mineralogy of loess-paleosol sequences in southwestern Illinois and eastern Missouri
Grimley, David A.
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https://hdl.handle.net/2142/19274
Description
Title
Stratigraphy, magnetic susceptibility, and mineralogy of loess-paleosol sequences in southwestern Illinois and eastern Missouri
Author(s)
Grimley, David A.
Issue Date
1996
Doctoral Committee Chair(s)
Johnson, W. Hilton
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Geology
Agriculture, Soil Science
Language
eng
Abstract
Magnetic susceptibility (MS) zones, controlled by silt-size magnetite concentrations, have been delineated within oxidized, C horizon Peoria and Loveland Silts along the Illinois and Mississippi Valleys. Lower Peoria and lower Loveland Silts (MS = 55-80 $\times\ 10\sp{-5}$ SI) and Roxana Silt (MS = 60-90 $\times\ 10\sp{-5}$) have high MS probably reflecting greater contributions from Superior Lobe outwash. Middle Peoria and upper Loveland Silts (MS = 20-55 $\times\ 10\sp{-5}$) and upper Peoria Silt (MS = 40-60 $\times\ 10\sp{-5}$) have low MS, probably reflecting dilution by Lake Michigan Lobe outwash upon maximum advance of glaciers in Illinois. Feldspar/quartz and kaolinite/illite X-ray diffraction peak height ratios also increase in zones of high MS. Bulk and magnetic grain size distributions are generally uniform across MS zones at sections studied. Peoria Silt MS zones are particularly striking in loess proximal to the Illinois (Ancient Mississippi) Valley because of source area shifts caused by Mississippi River diversion at about 20.5 ka. MS zones are less pronounced along the Mississippi Valley south of St. Louis. Alteration of magnetite in gleyed loess can occur but is not thought to cause MS zones observed because minor increases in illite and dolomite in low MS zones are more consistent with a source area hypothesis. Jules and Gardena (interstadial) Geosol occur in upper portions of middle and lower Peoria Silt and are estimated to be 15-16 ka and 20-20.5 ka, respectively. Clay beds, deposited during peak glacial floods (16-20 ka), occur within middle Peoria Silt.
Ultrafine magnetite and maghemite (0.01-0.1 $\mu$m), authigenically formed, are responsible for high MS in upper solums of modern soils and paleosols, where they are preserved and can accumulate under oxidizing conditions. Highly substituted superparamagnetic and single domain magnetite probably forms extracellularly to bacteria in soils and later alters to maghemite. Soil firing can be locally important in causing high MS. Bt horizon thicknesses, clay content increases, feldspar weathering, and amounts of neoformed ferrimagnetics and mixed-layered kaolinite-expandables all indicate considerably greater time for development of the Yarmouth Geosol compared to the Sangamon Geosol. In the study area, Yarmouth Geosol is estimated to represent oxygen isotope stages 7-11 and the Sangamon Geosol stage 5 and portions of stage 4. Loveland Silt was probably deposited during stage 6 only. Crowley's Ridge Silt, into which Yarmouth Geosol formed, is estimated to have been deposited during stage 12. Bridgeton geosol (new) may represent stages 13-15 and Bonfils silt (new), stage 16.
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