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.
Ardith K. Hansel and W. Hilton Johnson
Roxana silt (Frye and Willman 1960).
Roxana, a village in Madison County, southwestern Illinois.
Frye and Willman (1960) named the Roxana Silt for a succession of distinctive silt units that occur above a well-developed buried soil, the Sangamon Geosol, and below the Peoria Silt. The Roxana Silt was previously called the Farmdale loess (Leighton and Willman 1950) or late Sangamon loess (Leighton 1931, 1933, Smith 1942, Wascher et al. 1948). The unit was formalized as a formation in 1970 (Willman and Frye), and three members were defined within it: the Markham Silt, a sandy and commonly weathered unit at the base; the McDonough Loess, a gray to tan unit; and the Meadow Loess, a thick upper unit that locally contains three color zones: pink tan at the base, gray tan in the middle, and pink tan at the top. Although no changes in these members are made in this report (except removal of genetic terms from their names), the stratigraphic significance of the Markham and McDonough Members was questioned by McKay (1979b), who was not able to identify them on stable upland landscape positions. The Markham Member was recognized on more sloping landscape positions (Frye et al. 1974), and Curry and Follmer (1992) recognized the Markham and Meadow Members, but not the McDonough Member, as being valid. Where the Roxana Silt is thin (less than 1 m [3.3 ft] thick), appreciable sand commonly occurs throughout, and it has been referred to as the sandy silt facies (Johnson et al. 1972). In this revision, the Roxana Silt is retained as a formation, and the former Robein Silt, which is lithologically related to the Roxana Silt, is classified as the uppermost unit, the Robein Member, within the formation (fig. 9a).
Pleasant Grove School Section (fig. 29), located in the bluff of the Mississippi Valley 4 miles (6.4 km) southeast of Roxana; good for lithology and lower contact.
No longer available, has been mined out for fill.
Arenzville, Athens North Quarry, Bellefontaine Quarry, Cottonwood School, Farm Creek, and Glendale School Sections; all good for lithology and contacts.
The Roxana Silt is a unit dominated by silt, which occurs above the Sangamon Geosol and below the Peoria Silt (fig. 29), the Henry and Equality Formations, or the Wedron Group. The Farmdale Geosol commonly occurs in the top of the Roxana Silt.
Although the Roxana Silt is lithologically distinct from diamicton of the Glasford Formation, its lower contact with the Glasford Formation is commonly gradational with the Sangamon Geosol as a result of pedogenic processes during its deposition. In well and poorly drained situations, this contact usually occurs in the upper soil horizon(s) of the Sangamon Geosol. The contact is drawn where (1) silt dominates the texture, (2) pedogenic characteristics are less evident and less developed, and (3) relatively unweathered minerals occur in the sediment. The same criteria for determining the contact are used where the Roxana Silt overlies silt formations (Loveland Silt, Teneriffe Silt) of the Illinois Episode, except in such cases texture is less useful. Although the contact between the Roxana and Peoria Silts is generally gradational, the Roxana Silt can usually be distinguished from the Peoria Silt by color. The Roxana Silt is more red brown, whereas the Peoria Silt is more yellow brown. Also, the upper portion of the Roxana Silt is commonly weathered and contains pedogenic character related to the Farmdale Geosol. The Roxana Silt can be readily distinguished from the diamicton formations of the Wedron Group.
Extent and thickness
The distribution of the Roxana Silt is widespread in the midcontinent region; it occurs from Minnesota and Wisconsin south to Mississippi and Louisiana, and its distribution is clearly related to major river valleys. It is thick (5-15 m; 16-49 ft) only along the middle and southern portions of the Illinois Valley (ancient Mississippi Valley, Fehrenbacher et. al 1986), and south along the Mississippi Valley. It is thin along other valleys. Variation in thickness of the Roxana Silt along the ancient Mississippi Valley can be explained by variations in valley relief, width, and orientation and distance from source valley (Johnson and Follmer 1989, Leigh 1991).
The Roxana Silt is dominated by silt and is commonly brownish red in oxidized occurrences and gray where it is water saturated. The basal portion and, if thin, the entire unit contains appreciable sand (15%-30%) and clay (15%—20%) where it overlies weathered sandy materials. In many exposures it appears massive with abundant biopores and root traces. The Roxana Silt is usually leached of carbonates, except where the unit is thick and its central portion is dolomitic; color strata and pulmonate gastropods are common in thick occurrences. The upper part, usually weathered and noncalcareous, represents the Farmdale Geosol.
Lower boundary: the contact with the Glasford or Winnebago Formations, Loveland Silt, Pearl Formation, or older units. Upper boundary: the contact with the Peoria Silt, the Henry, or Equality Formations, or the Wedron Group. Both upper and lower boundaries are usually gradational rather than distinct.
Well log characteristics
Age and correlation
The Roxana Silt was deposited during the Alton Phase of the Athens Subepisode of the Wisconsin Episode (fig. 10). Estimates on the age of the lower boundary have varied from about 75,000 radiocarbon years to 45,000 or 55,000 radiocarbon years (Willman and Frye 1970, McKay 1979b, Curry and Follmer 1992, Leigh and Knox 1993); the minimum age on the upper boundary is about 27,000 radiocarbon years (Leigh and Knox 1993). The Roxana Silt correlates with the Gilman Canyon Formation of Nebraska (Reed and Dreeszen 1965), the Pisgah Formation of Iowa (Bettis et al. 1990), the Roxana Formation of Wisconsin (Leigh and Knox 1994), and part of the Atherton Formation of Indiana (Wayne 1963).
Environments of deposition
The Roxana Silt is interpreted to be predominantly loess (Smith 1942, Wascher et al. 1948, Frye and Willman 1960, McKay 1979b). Locally, portions of it are colluvial (Frye et al. 1974); much of it has been pedogenically modified. The loess was derived primarily from the floodplain of the ancient Mississippi River, and most geologists suggested that it is related to glacially derived valley train sediment (Wascher et al. 1948, Willman and Frye 1970, McKay 1979b, Johnson and Follmer 1989, Leigh 1991, 1994). Other origins were suggested by Ruhe and Olson (1978), Winters et al. (1988), and Norton et al. (1988).
Revised unit. Upper boundary locally extended to include materials formerly included in the Robein Silt, which is reclassified as the Robein Member of the Roxana Silt.
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