Petersburg Silt

Lithostratigraphy: Petersburg Silt
Chronostratigraphy: Cenozoic Erathem >>Quaternary System >>Pleistocene Series

Primary source

Willman, H. B., and John C. Frye, 1970, Pleistocene Stratigraphy of Illinois: Illinois State Geological Survey Bulletin 94, 204 p.

Contributing author(s)

H. B. Willman and John C. Frye

Name

Original description

Petersburg Silt (Willman, Glass, and Frye 1963).

Derivation

The Petersburg Silt was named for Petersburg, Menard County.

Other names

History/background

Type section

Type location

The type section is a roadcut (supplemented by an auger boring) at the south edge of the city and is called the Petersburg Section (table 6), NW NW NE Sec. 23, T. 18 N., R. 7 W.

Table 6 -- Stratigraphic Sections (partial) The following 21 stratigraphic sections describe exposures in Illinois and illustrate many of the aspects of Pleistocene stratigraphy. These sections contain the type localities for 21 rock-stratigraphic units, 4 soil-stratigraphic units, and 3 time-stratigraphic units and include paratypes for several other units. The sample numbers preceded by "P" are the numbers used in the Illinois State Geological Survey collections. Analytical data on many of these samples are on file at the Survey. The sections are arrange alphabetically by name.
Table 6 -- Stratigraphic Sections Malden South Section (cont.), New Salem Northeast Section, and Petersburg Section

Type author(s)

Type status

Reference section

Reference location

Reference author(s)

Reference status

Stratigraphic relationships

The silt rests on the Yarmouth Soil and is overlain by till of the Glasford Formation of Illinoian age.

The Petersburg Silt was previously included in the Loveland Loess (Leighton and Willman, 1950; Wanless, 1957). In this report it is separated from the Loveland by a vertical cuttoff at the outer limit of till in the Glasford Formation. This relationship is shown diagrammatically in figure 7.

Fig. 7 -- Diagrammatic cross section showing the relations of formations and members of Illinoian age in western Illinois.

The Petersburg Silt is described in the Petersburg Dam, Pryor School, Rock Creek Township, and Rushville (2.4W) Sections (table 7), and in other stratigraphic sections.

Table 7 -- Described Stratigraphic Sections Previously Published (414 sections from 78 counties are arranged alphabetically, with citation to publication source).

Extent and thickness

Lithology

The formation consists of gray to yellow-tan to purplish tan, calcareous silt with some fine sand and clay. The upper part is generally massive and loess-like, but the lower part is distinctly bedded in places.

Core(s)

Photograph(s)

Contacts

Well log characteristics

Fossils

It commonly contains fossil snail shells.

Age and correlation

The Petersburg Silt is in the early part of the Liman Substage of the Illinoian Stage. It is a pro-glacial deposit of the advancing earliest Illinoian glacier and includes outwash, loess, and some locally derived sediment.

Environments of deposition

Economic importance

Remarks

References

LEIGHTON, M. M., and H. B. WILLMAN, 1950, Loess formations of the Mississippi Valley: Journal of Geology, v. 58, no. 6, p. 599-623. (Reprinted as Illinois State Geological Survey Report of Investigations 149)
WANLESS, H. R., 1957, Geology and mineral resources of the Beardstown, Glasford, Havana, and Vermont Quadrangles: Illinois Geological Survey Bulletin 82, 233p.
WILLMAN, H. B., H. D. GLASS, and J. C. FRYE, 1963, Mineralogy of Glacial Tills and Their Weathering Profiles in Illinois, Part 1—Glacial Tills: Illinois State Geological Survey Circular 347, 55 p.

ISGS Codes

Stratigraphic Code	Geo Unit Designation
0990	pb