Pleistocene Series

Chronostratigraphy: Cenozoic Erathem >>Quaternary System >>Pleistocene Series

Primary source

Willman, H. B., Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, and Jack A. Simon, 1975, Handbook of Illinois Stratigraphy: Illinois State Geological Survey Bulletin 95, 261 p.

Contributing author(s)

John C. Frye and H. B. Willman

Name

History/background

The term "Pleistocene" was introduced by Lyell in 1839 (p. 616-621) as a replacement for "Newer Pliocene" (Lyell, 1833, p. 52-53) to apply to the marine strata of the Mediterranean region, in which more than 70 percent of the fossils represent living species. In 1846 Forbes applied "Pleistocene" to the "glacial epoch" in the British Isles, and in 1925 Wilmarth defined the Pleistocene in North America as including glacial and contemporaneous deposits of the "Great Ice Age." A report to the International Geological Congress in 1952 suggested worldwide adoption of a definition of the base of the Pleistocene as the base of the Villafranchian or of the equivalent marine Calabrian as exposed in Italy. However, as correlations of this proposed type area with marine strata in North America are not firmly established, the Pleistocene Series in Illinois and the rest of the continental interior continues to be defined as the glacial sequence plus the contemporaneous deposits laid down beyond the limit of continental glaciation. The Pleistocene Series includes the Holocene (Recent), which is classified as a stage rather than as a separate series, the usage accepted by many others.

Stratigraphic relationships

The stratigraphic relations of Pleistocene deposits in Illinois are complex and contrast sharply with the more regular and uniform succession of predominantly marine Paleozoic rocks that they overlie. The complexity is caused by the different modes of deposition, the multiple sources of sediment, and the lack of regional continuity of many of the stratigraphic units. The loesses and the soil-stratigraphic units have the greatest regional continuity, although some of the glacial till units are traceable for more than 100 miles. Outwash and alluvial deposits, in general, occur along present and buried valleys, lacustrine deposits are found along segments of ponded valleys and on low areas of the till plain surfaces, and colluvium and gravity deposits are restricted to localized areas along valley margins. An idealized representation of stratigraphic relations of deposits of the Illinoian Stage in west-central and western Illinois is shown by figure Q-7, and another of Wisconsinan deposits from northeastern to west-central Illinois by figure Q-8.

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  Fig. Q-7 -- Diagrammatic cross section showing the relations of the formations and members of Illinoian age in central and western Illinois (Willman and Frye, 1970).
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  Fig. Q-8 -- Diagrammatic cross section showing the relations of the formations and members of Wisconsinan age in northern and western Illinois (Willman and Frye, 1970).

Pleistocene deposits in Illinois are classified in four basic hierarchies-- time-stratigraphic, rock-stratigraphic, soil-stratigraphic, and morphostratigraphic. Time-stratigraphic and rock-stratigraphic classifications are used for Pleistocene deposits in the same manner they are used in the rest of the stratigraphic sequence (fig. 16) and will not be redescribed here. The chart in figure Q-4 shows all time-stratigraphic, rock-stratigraphic, and soil-stratigraphic terms currently in use in Illinois and this report. The surface distribution of the dominantly till formations and members is shown in figure Q-5. Figure Q-6 shows graphically the historical development of the time-stratigraphic classification of the Wisconsinan Stage in Illinois.

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  Fig. 16 -- Development of the stratigraphic classifications used in Illinois. The references include reports that introduced changes in the classification and others that show typical usage.
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  Fig. Q-4 -- Classification of the Pleistocene Series.
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  Fig. Q-5 -- Areal distribution of the dominantly till formations and members of Illinois (after Willman and Frye, 1970; Johnson et al., 1972).
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  Fig. Q-6 -- Development of classification of the Wisconsinan Stage in Illinois.

Soil-stratigraphic and morphostratigraphic units are virtually unique to the Pleistocene. Soils form at the land surface as a result of the effects of weathering and of plants and animals on the sediments that underlie the surface. In most places soil forms in situ, that is, by the progressive downward alteration of sediments below a relatively stable surface. In some places, however, soils have formed by slow accumulation of fine sediment on the surface, with or without organic material. Soils buried by younger sediments are part of the stratigraphic sequence. Soil-stratigraphic classification, consisting of only one rank (soil), is a means of naming, describing, and correlating the soils in the sediment sequence. Soils do not replace the rock-stratigraphic units they are developed on; commonly, each soil is developed on several units. Only the upper surface of a soil presents a definable stratigraphic plane, because the lower part of a soil is gradational. The physical and mineral characteristics of a soil profile are generally useful in its recognition and correlation.

Morphostratigraphic units, as the name implies, are bodies of rock that are recognized and named on the basis of their surface form. Glacially produced morainic ridges, the materials in which are called drifts, and dissected alluviated surfaces, called alluvial terraces, are the two types of morphostratigraphic units recognized in Illinois. Only the drifts are formally named. Drifts are recognized not only in the surface areas but also wherever they can be identified in buried sequences. Such units are particularly useful in classification and mapping of glacial deposits in areas where a single rock-stratigraphic unit, such as glacial till, includes several morainic ridges.

Extent and thickness

Deposits of Pleistocene age are the surficial materials in virtually all of Illinois. Nearly 80 percent of the state was covered at least once by continental glaciers that left characteristic deposits (drift). Both glaciated and unglaciated areas were largely covered by wind-deposited silt (loess), by lacustrine deposits, or by deposits from streams. The only areas that were not covered by glaciers are the extreme northwestern corner, the southernmost part, and relatively small areas along the western border.

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  Fig. Q-1A -- Wisconsinan Richland Loess (R), Delavan Till Member of the Wedron Formation (Wd), Morton Loess(M), Robein and Roxana Silts (RR), and the Sangamon Soil (S) on till of the Illinoian Glasford Formation(G); in a railroad cut a mile east of Farmdale, Tazewell County, near the classic Farm Creek Section (Willman and Frye, 1970, p. 183).
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  Fig. Q-1B -- The Wisconsinan Wedron (W-interbedded till and outwash) and Peddicord (P-lake sediments) Formations overlying till of the Illinoian Glasford Formation (G) in the Wedron Silica Company pit at Wedron, LaSalle County-the type sections of the Wedron and Peddicord Formations (Willman and Payne, 1942, p. 148; Willman and Frye, 1970, p. 190; Willman et al., 1971).
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  Fig. Q-1C -- Wisconsinan Peoria Loess (P) overlying the Roxana Silt (Rm-Meadow Loess Member, Rmc-McDonough Loess Member) and the Illinoian Teneriffe Silt (T) in the Pleasant Grove School Section in the Mississippi River bluffs northwest of Collinsville, Madison County (Willman and Frye, 1970, p. 187).
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  Fig. Q-1D -- Illinoian Glasford Formation (Hagarstown Member) showing steeply dipping sand and gravel (crevasse deposit) in a mound in the Kaskaskia Ridged Drift area, 2 miles southwest of Lakewood, Shelby County.
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  Fig. Q-1E -- Wisconsinan Roxana Silt (R), Sangamon Soil developed in the Illinoian Loveland Silt (SL) and Yarmouth Soil developed in till of the Kansan Banner Formation (YB); in a roadcut at Independence, 5 miles south of Pittsfield, Pike County (Frye et al., 1964, p. 27).
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  Fig. Q-1F -- Harkness Silt Member of the Kansan Banner Formation (Bh) overlying the Afton Soil developed on gravel of the Nebraskan Enion Formation (AE); in a roadcut at the Zion Church, 2 miles southeast of Marblehead, Adams County (Willman and Frye, 1970, p. 191).

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  Fig. Q-2 -- Glacial map of Illinois (after Willman and Frye, 1970).

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  Fig. Q-3 -- Generalized thickness of the Quaternary deposits of Illinois (after Piskin and Bergstrom, 1967; Willman and Frye, 1970).

Lithology

The mineral composition of Pleistocene deposits strongly reflects the source areas of the sediments. Source areas of colluvial and gravity deposits are close at hand, but glacial deposits reflect sources hundreds of miles away.

Fossils

Fossils are not abundant in most Pleistocene deposits of Illinois, although mollusks and vertebrate remains occur locally.

Environments of deposition

Pleistocene deposits include glacial tills, outwash, loess, lacustrine sediments, and colluvium, reflecting diverse depositional environments associated with glaciation and post-glacial processes.

Remarks

The Nebraskan and Kansan Stages are now considered to be obsolete.

References

FORBES, EDWARD, 1846, On the connection between the distribution of the existing fauna and flora of the British Isles...
LYELL, CHARLES, 1833, Principles of geology...
LYELL, CHARLES, 1839, Elements of geology...
WILMARTH, M. G., 1925...

ISGS Codes

ISGS Code and Symbol

Stratigraphic Code	Geo Unit Designation
0020	Ps