Historical:Equality Formation

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Wedron and Mason groups: lithostratigraphic reclassification of deposits of the Wisconsin Episode, Lake Michigan Lobe area
Series Bulletin 104
Author Ardith K. Hansel and W. Hilton Johnson
Date 1996
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Lithostratigraphy: Mason Group >>Equality Formation
Chronostratigraphy: Cenozoic Erathem >>Quaternary System >>Pleistocene Series

Authors

Ardith K. Hansel and W. Hilton Johnson

Name

Original description

Equality Formation (Willman and Frye 1970).

Derivation

Equality, a village in Gallatin County, southeastern Illinois.

History/background

The Equality Formation was defined by Willman and Frye (1970) to include lake sediment that occurs at the surface or underlies loess or Holocene deposits. Tongues of equivalent lake sediment interfingered with till units of the Wedron and Winnebago Formations were separated from the Equality Formation by vertical boundaries and classified as part of the till units (fig. 3). In addition to deposits of ice-contact, kettle, and proglacial lakes, Willman and Frye (1970) also included deposits of large slackwater lakes in back-flooded valleys tributary to the major meltwater drainageways in the formation. They classified similar fine grained deposits that occur stratigraphically below the Sangamon Geosol as the Teneriff Silt (Illinoian Stage). <br.

Willman and Frye (1970) divided the Equality Formation into two members, the Carmi and Dolton, which constitute sedimentary facies. They included the relatively deep-water, finer grained sediment consisting predominantly of silt and clay in the Carmi Member and the near-shore, coarser grained sediment consisting predominantly of sand and gravel in the Dolton Member. The members are not differentiated where neither type of sediment dominates.

In this report, the Equality Formation is redescribed to (1) exclude generic criteria from its definition, (2) include laterally equivalent tongues of sediment that interfinger with diamicton units of the Wedron Group or other sorted-sediment units of the Mason Group, and (3) include laminated red and gray clay beneath Lake Michigan that formerly was classified as the Lake Michigan Formation. As redescribed, the Equality Formation consists of relatively fine grained, stratified sediment of predominantly silt and clay, overlies the Sangamon Geosol, and intertongues with units of the Wedron and Mason Groups or surficial units (fig. 9c). The redescription excludes the coarser grained Dolton Member (Willman and Frye 1970) as part of the Equality Formation. Instead, the stratified sand and gravel formerly classified as the Dolton Member is included in the Henry Formation as an informal facies to which the name "Dolton" is applied. When the Equality Formation is restricted to the finer grained facies, it is similar in concept to the Carmi Member; therefore, the latter name is abandoned.

Two former formations, the Lake Michigan (Willman and Frye 1970) and the Peddicord (Willman et al. 1971), are classified as part of the Equality Formation: the former as a member and the latter as a tongue. On the basis of seismic profiles and 55 cores collected from the southern two-thirds of Lake Michigan, Colman and Foster (1990), Foster and Colman (1991), and Colman et al. (1994) found that the laminated red clay of the lower Lake Michigan Formation grades laterally into stratified diamicton and sorted sediment interfingered with more massive diamicton tongues of the Wedron Group. They concluded that the lower red part of the former Lake Michigan Formation was a distal facies of the more ice-proximal stratified sediment of the Equality Formation. Herein, the upper gray to brown part of the former Lake Michigan Formation is classified as the Lake Michigan Member of the Equality Formation, whereas the underlying red laminated sediment, which Colman and Foster found to be separated from the upper part by a gradational contact in deep basins or the Chippewa unconformity in shallow water, is classified as undivided Equality Formation.

Originally (Willman et. al. 1971), the Peddicord Formation was defined to include gray and pink silt that had accumulated in a lake confined to the buried Ticona drainage system. Because Willman et al. (1971) correlated organic-rich silt above the gray and pink silt of the Peddicord Formation with the Robein Silt of the Farmdalian Substage in the type area (Morris North Section), they also classified the Peddicord Formation as part of the Farmdalian Substage. New exposures at Wedron, the type section of the Peddicord Formation, indicate the above correlation was in error (Johnson et al. 1985a); the laminated silt and clay facies classified as the Peddicord Formation stratigraphically overlies the Robein Silt (fig. 12 [herein classified as the Robein Member of the Roxana Silt]). In the type area, the Peddicord silt and clay is similar in clay-mineral composition to diamicton of the Tiskilwa Formation.

The fine grained stratified sequence found in the buried Ticona drainage system near Wedron is in the same stratigraphic position as similar sediment that occurs predominantly in buried bedrock valleys or other low places in the pre-Michigan Subepisode landscape. Where this laminated silt and clay unit extends as a tongue beneath the Tiskilwa Formation, it is classified as the Peddicord Tongue of the Equality Formation (fig. 9c).

Type section

Saline River Section, an exposure at a bridge excavation site, 4 miles (6.4 km) southwest of Equality; no longer exposed.

Reference section

Core 9V, Lake Michigan; good for lithology and lower boundaries of Lake Michigan Member and undivided formation. Wedron and Charleston Sections; good for boundaries and lithology of Peddicord Tongue (figs. 12, 15).

Stratigraphic relationships

The Equality Formation consists of gray to red silt and clay, generally shows evidence of bedding structures, and occurs above the Sangamon Geosol. It intertongues with diamicton units of the Wedron Group, other formations of the Mason Group, particularly the Henry Formation, and some surficial units. Beneath Lake Michigan, the upper, gray part of the Equality Formation that is above the Chippewa unconformity and/or red clay and silt of the undivided formation is recognized as the Lake Michigan Member. The lower tongue of the formation that extends beneath the Delavan Member or undivided Tiskilwa Formation is recognized as the Peddicord Tongue.

Where the Equality Formation contains distinct bedding structures, it is readily distinguishable from the more massive diamicton units of the Wedron Group and Winnebago and Glasford Formations. Where bedding structures are less apparent, the Equality Formation is sometimes similar to the finer grained diamicton units (e. g., the Yorkville Member of the Lemont Formation and the Wadsworth and Kewaunee Formations), but it is usually softer and better sorted. Although its contacts are sometimes gradational and interfingered, the Equality Formation is generally distinct from deposits of the other Mason Group formations, which are coarser grained or lack bedding structures. The presence of the Sangamon Geosol in the upper part of the Teneriffe and Loveland Silts and the Pearl Formation helps to distinguish these older sorted-sediment units from the Equality Formation.

Extent and thickness

Like the Henry Formation, the Equality Formation is present in nearly all of the counties inundated by the Lake Michigan Lobe during the Michigan Subepisode, but its distribution is patchy and discontinuous (plate 1). It is most extensive (1) beneath Lake Michigan, (2) beneath plains between end moraines or adjacent to Lake Michigan in northeastern Illinois, and (3) in low-lying plains adjacent to and in tributaries of the Wabash, Ohio, and Mississippi Rivers in southern Illinois and the Green and Illinois Rivers in western Illinois. In the subsurface, tongues and lenses of the Equality Formation are present in some buried valleys and basins; they tend to pinch out in the up-ice direction beneath the Henry Formation or diamicton units of the Wedron Group. Locally, the Equality Formation is interfingered with or grades into the Henry Formation. The Equality Formation is extremely variable in thickness. Willman and Frye (1970) reported thicknesses ranging from less than 2 meters (6.6 ft) to more than 20 meters (65.6 ft) for the Equality Formation (former Carmi Member), but it is much thicker beneath Lake Michigan, where Foster and Colman (1991) infer thicknesses (i. e., the combined thickness of the former Equality and Lake Michigan Formations) greater than 50 meters (164 ft) in seismic profiles.

Lithology

The Equality Formation consists predominantly of brown to gray to red bedded silt and clay (fig. 31) that is similar in lithology to the silt- and clay-size fractions of diamicton units of the Wedron Group. Lonestones (isolated stones) and lenses of gravel, sand, diamicton, organic debris, and wood are present locally in the silt and clay. Bedding structures in the unit range from distinct rhythmites to fine indistinct laminae to more massive beds that locally contain lonestones but exhibit little apparent bedding.

Contacts

Lower boundary: the contact with tongues of the Wedron Group, Peoria Silt, Henry and Roxana Formations, or the Loveland Silt, Pearl, Glasford, and Winnebago Formations, or older units. Upper boundary: the contact with the Wedron Group, Peoria Silt, and Henry Formation, or other units. The contact with the Henry Formation is commonly gradational and/or interfingered.

Age and correlation

The Equality Formation was deposited during the Wisconsin and Hudson Episodes. Most of it probably ranges from about 26,000 radiocarbon years to modern, although some of it is older (appendix B2). It correlates with part of the Atherton and Martinsville Formations in Indiana (Wayne 1963) and unnamed units in Wisconsin, Michigan, Iowa, and Missouri.

Environments of deposition

The Equality Formation is interpreted to be predominantly fine grained lacustrine sediment deposited in glacial and postglacial lakes. The sediment of former proglacial lakes records ice-distal and ice-proximal facies, and in some cases the rhythmites may reflect annual couplets (varves). These deposits in lakes and former lakes record the glacial to postglacial transition.

Remarks

Redescribed unit. Formation redescribed to remove lithogenetic concepts from its definition and recognized as intertonguing with diamicton units of the Wedron Group as well as other sorted-sediment units of the Mason Group; former Lake Michigan Formation lowered in rank to member of unit; former Peddicord Formation recognized as a formal tongue of unit.

References

COLMAN, S. M., R. M. FORESTER, R. L. REYNOLDS, D. S. SWEETKIND, J. W. KING, P. GANGEMI, G. A. JONES, L. D. KEIGWIN, and D. S. FOSTER, 1994, Lake-level history of Lake Michigan for the past 12,000 years— The record from deep lacustrine sediments: Journal of Great Lakes Research, v. 20, no. 1, p. 73-92.
COLMAN, S. M., and D. S. FOSTER, 1990, Stratigraphy, Descriptions, and Physical Properties of Sediments Cored in Lake Michigan: United States Geological Survey Open File Report 90-478, 92 p.
FOSTER, D. S., and S. M. COLMAN, 1991, Preliminary Interpretation of the High-Resolution Seismic Stratigraphy Beneath Lake Michigan: United States Geological Survey Open File Report 91-21, 42 p.
JOHNSON, W. H., A. K. HANSEL, B. J. SOCHA, and L. R. FOLLMER, 1985a, The Wedron Section, Stop 1, in W. H. Johnson, A. K. Hansel, B. J. Socha, and J. M. Masters, Depositional Environments and Correlation Problems of the Wedron Formation (Wisconsinan) in North-eastern Illinois: Illinois State Geological Survey Guidebook 16, p. 13-42.
WAYNE, W. J., 1963, Pleistocene Formations of Indiana: Indiana Geological Survey Bulletin 25, 85 p.
WILLMAN, H. B., and J. C. FRYE, 1970, Pleistocene Stratigraphy of Illinois: Illinois State Geological Survey Bulletin 94, 204 p.
WILLMAN, H. B., A. B. LEONARD, and J. C. FRYE, 1971, Farmdalian Lake Deposits and Faunas in Northern Illinois: Illinois State Geological Survey Circular 467, 12 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation
0220
e