Peoria Silt

Lithostratigraphy: Mason Group >>Peoria Silt
Chronostratigraphy: Cenozoic Erathem >>Quaternary System >>Pleistocene Series

Primary source

Hansel, Ardith K., and W. Hilton Johnson, 1996, Wedron and Mason Groups: Lithostratigraphic Reclassification of Deposits of the Wisconsin Episode, Lake Michigan Lobe Area: Illinois State Geological Survey Bulletin 104, 116 p.

Contributing author(s)

Ardith K. Hansel and W. Hilton Johnson

Name

Original description

Peorian soil and weathering interval (Leverett 1899).

Derivation

Peoria, a city along the Illinois Valley in Peoria County.

History/background

The term Peorian was introduced by Leverett (1899) for a soil that he inferred to have formed during an interglacial interval. Later, Alden and Leighton (1917) applied the term to loess deposits previously called Iowan. Kay and Leighton (1933) restricted usage of the term Peorian to loess beyond the limit of the Shelbyville till (the Wedron Group of this report). The name was modified to Peoria loess by Frye and Leonard (1951) for use as a lithostratigraphic unit in Kansas, and that usage was introduced in Illinois by Frye and Willman in 1960. In 1970, Willman and Frye designated separate formations for the unit below, beyond, and above the diamicton deposits of the last glacial episode. They are the Morton, Peoria, and Richland Loesses, respectively. At the glacial limit, the Morton and Richland Loesses were separated from laterally equivalent silt of the Peoria Loess by arbitrary vertical boundaries.

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  Figure 3 -- Diagram shows Willman and Frye's (1970) use of arbitrary vertical boundaries to avoid intertonguing of till members with sorted-sediment units in classification of the deposits of the Wisconsinan Stage.

We have retained the Peoria Silt as a lithostratigraphic unit at formation rank. The former Morton and Richland Loesses are not retained at formation rank; instead, they are treated as tongues of the Peoria Silt that underlie and overlie the Wedron Group, respectively.

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  Figure 9 -- History of the litho-stratigraphic classification of the Mason Group deposits with respect to the Wedron Group deposits: (a) Peoria and Roxana Silts, (b) Henry Formation, and (c) Equality Formation. Vertical lines indicate part of section missing due to erosion.

Locally, the Peoria Silt is interfingered with well-sorted (eolian) sand. Where at the surface, this sand was formerly classified as the Parkland Sand.

Type section

Type location

Tindall School Section in the west bluff of the Illinois Valley south of Peoria; good for lithology and lower boundary.

Reference section

Reference location

Athens North Quarry, Bellefontaine Quarry, and Cottonwood School Sections; Danvers, Gardena, and Farm Creek Sections.

Stratigraphic relationships

The Peoria Silt consists of a silt unit that overlies the Roxana Silt or other units and interfingers with diamicton units of the Wedron Group and sorted sediment units of the Henry and Equality Formations of the Mason Group.

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  Figure 29 -- Loess of the Peoria and Roxana Silts above the Sangamon Geosol developed in the Teneriffe Silt (Illinois Episode) at the Pleasant Grove School Section. The modern soil is developed in the upper part of the Peoria Silt.

Extent and thickness

The Peoria Silt is an extensive unit that occurs throughout the midcontinent region, thickest along major river valleys and thinning away from them.

Lithology

The Peoria Silt generally is light yellow tan to gray silt that grades from sandy silt near major valleys to clayey silt in upland areas. It is dolomitic below the weathered zone and typically lacks strong bedding structures.

Contacts

Lower boundary: gradational contact with the Roxana Silt or older units. Upper boundary: contact with overlying sediments or land surface.

Age and correlation

The Peoria Silt was deposited adjacent to major meltwater channels during the Michigan Subepisode.

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  Figure 4a -- Lobe and sublobe boundaries in Illinois during the last glaciation (from Willman and Frye 1970). Locations of the ancient Mississippi and Iowa Rivers added.

Environments of deposition

The Peoria Silt is interpreted as proglacial loess derived from glacial meltwater channels, with minor contributions from eolian sand and colluvial processes.

Remarks

Revised unit.

References

(unchanged)

ISGS Codes

ISGS Code and Symbol

Stratigraphic Code	Geo Unit Designation
0100	p