H. B. Willman and John C. Frye
The Petersburg Silt was named in 1963 (Willman, Glass, and Frye) for Petersburg, Menard County.
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.
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.
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.
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. 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.
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.
|Stratigraphic Code||Geo Unit Designation|