Lemont Formation

Lithostratigraphy: Wedron Group >>Lemont Formation
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

Lemont drift (Bretz 1939).

Derivation

Lemont, a village along the south side of the Des Plaines Valley in Cook County.

History/background

The Lemont drift was recognized early (Bretz 1939) and described in detail (Bretz 1955) as a distinct lithostratigraphic unit that crops out along the Des Plaines and Sag Channels southwest of the Chicago Metropolitan Area. Bretz (1955) named the Lemont a drift rather than a till because of the complex association of abundant washed sediment with till in the unit. Bretz recognized that the Lemont drift was older than the surficial clayey till in the Valparaiso, Tinley, and Lake Border Moraines in the Chicago region, but he was uncertain of its age and relationship to older units in Illinois.

Horberg and Potter (1955) described buried weathered zones in stratified sediment in the upper part of the Lemont drift. Because of the thickness (about 2 m; 6.6 ft) and character of the weathered zone at the Worth Section southwest of Chicago, they interpreted it as fossil soil correlative with the last interglacial paleosol, the Sangamon soil. Thus, they interpreted the Lemont drift to be Illinoian age. Alternatively, Frye and Willman (1960) suggested the weathering profile might correlate instead with the last interstadial paleosol, the Farmdale soil, and therefore concluded the Lemont drift could be Altonian age. Probably because of the uncertainty about its age, Willman and Frye (1970) did not give the Lemont drift formal status when they established a lithostratigraphic classification of Pleistocene units in Illinois. They suggested the Lemont drift, which they retained as an informal unit, could be Illinoian, Altonian, or Woodfordian age. Willman and Frye (1970) observed the Lemont drift was lithologically most like the Haeger Till Member of the Wedron Formation. Bogner (1973) concluded the weathered zones in the Lemont drift, which also occur in the Wadsworth Till Member, could be traced upward along joints to the modern soil. Thus, she concluded the Lemont could be Woodfordian. She correlated the Lemont drift with the Malden Till Member of the Wedron Formation, as had Kempton (Willman and Frye 1970) and Landon and Kempton (1971).

Johnson and Hansel (1985, 1989) and Hansel and Johnson (1986) agreed with Bogner's (1973) interpretation of the weathered zones within the Lemont drift as representative of an extension of the modern soil profile along joints to form secondary zones of clay accumulation below the main part of the B horizon (fig. 21). Such zones, which are leached of carbonates, can develop in stratified sediment, particularly in coarse, permeable sediment that is calcareous. Some leached zones (beta B horizons) form immediately below the main B horizon, whereas others (gamma B horizons) form below a calcareous layer but are connected by joints to the overlying main B horizon. Johnson and Hansel (1989) also agreed with Bogner's (1973) interpretation that the Lemont drift was part of the Wedron Formation, but on the basis of lithostratigraphy and sedimentological sequences in and westward of the Valparaiso Moraine, they correlated the Lemont drift with the Haeger rather than with the Malden Till Member. In 1989, Johnson and Hansel identified two glacigenic sequences within the Lemont drift in the type locality; they concluded the tongue of lacustrine sediment between the tills of the two sequences represented the first phase of ancestral Lake Michigan during the last deglaciation. They correlated the upper glacigenic sequence with the Haeger Till Member, which crops out in McHenry County, and suggested the lower sequence, although lithologically similar to the Lemont drift, may be time correlative with either the Malden or Yorkville Till Members of the Wedron Formation. Engineers in the Chicago area have commonly referred to the Lemont drift as the "Chicago hardpan" (DeLeuw-Novick 1975). Agreeing with Bretz (1955) that the Lemont drift constitutes an important lithostratigraphic unit in northeastern Illinois, as well as a unit of regional significance, Johnson and Hansel (1989) recommended the name Lemont be retained for a formation if the Wedron Formation were raised to group rank.

The Lemont Formation as proposed herein contains multiple diamicton units that vary in texture from silty clay to sandy loam. The diamicton units are interfingered with tongues of the Henry and Equality Formations of the Mason Group; they are part of several glacigenic sequences that occur stratigraphically between the Tiskilwa and Wadsworth Formations. Southwest of Chicago in the type area of the Lemont drift, the Lemont Formation is left undivided and consists of multiple diamicton units that are interfingered with tongues of sorted sediment of the Mason Group. Diamicton in the undivided Lemont Formation is predominantly gravelly silt loam that, although light gray in the subsurface, generally oxidizes to yellow brown in exposures. The diamicton contains lenses of sorted sediment, predominantly silt, sand, and gravel. Away from the type area, lateral facies changes are interpreted to occur between the locally derived dolomitic silt loam to loam diamicton of the undivided Lemont Formation and the sandy loam diamicton of the Haeger Member, silty clay diamicton of the Yorkville Member, and silt loam diamicton of the Batestown Member.

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  Figure 7 -- History of lithostratigraphic classification of the Wedron Group deposits.
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  Figure 18 -- Clayey diamicton of the Wadsworth Formation above oxidized, silty, dolomitic diamicton of the undivided Lemont Formation at the Lemont Section.
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  Figure 21 -- A gamma B horizon developed in the upper part of a tongue of the Henry Formation, which occurs beneath jointed, calcareous diamicton of the Wadsworth Formation and above diamicton of the Lemont Formation. The gamma B horizon represents an extension of the modern soil profile along joints to form a secondary zone of clay accumulation below the main B horizon.

Type section

Type location

Lemont Section, an abandoned quarry about 1 mile west of Lemont.

Reference section

Reference location

Wedron Section; Higginsville Section; Land and Lakes Landfill Section; Beverly Sand and Gravel Pit.

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  Figure 12 -- Intertongued sorted-sediment units of the Mason Group and diamicton units of the Wedron Group at Wedron Quarry pit 1. Mason Group units are shaded.
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  Figure 14 -- The Yorkville and Batestown Members of the Lemont Formation, Delavan Member of the Tiskilwa Formation, Robein Member of the Roxana Silt, and Glasford Formation at Higginsville Section.
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  Figure 19 -- Diamicton (subglacial till) of the Haeger Member of the Lemont Formation overlies the proglacial (ice proximal) sand and gravel of the Beverly Tongue of the Henry Formation at the Beverly Sand and Gravel Pit Section.

Stratigraphic relationships

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  Figure 13a -- Areal distribution of moraines and boundaries of formations and predominant members of the Wedron Group and the Trafalgar Formation in Illinois. (Names of formations and members are labeled on 13b.) Sublobe areas of the Lake Michigan Lobe and the Huron-Erie Lobe are shown on inset map: (1) Harvard; (2) Princeton, (3) Joliet; (4) Peoria; (5) Decatur; and (6) Huron-Erie (sublobe areas and moraines are modified from Willman and Frye 1970).
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  Figure 13b -- Names of Wedron Group formations and members in Illinois.
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  Figure 20 -- Unoxidized, silty, dolomitic diamicton of the undivided Lemont Formation exposed in O'hare reservoir excavation.

Extent and thickness

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Lithology

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  Figure 22 -- Sediment-flow diamicton of the undivided Lemont Formation above a coarsening-upward sand and gravel tongue of the Henry Formation. The unnamed tongue is correlative with the Beverly Tongue, which occurs beneath the Haeger Member of the Lemont Formation.

Age and correlation

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  Figure 10 -- Geochronological units, chronostratigraphic units and diachronic units in the Lake Michigan Lobe in a transect from south of Peoria, Illinois, to north of the Straits of Mackinac in Michigan (geochronologic and chronostratigraphic units are after Frye et al. 1968, as modified by Follmer et al. 1979, Curry and Follmer 1992, and Evenson et al. 1976; upper part of time-distance diagram and phases of the Michigan Subepisode are after Hansel and Johnson 1992).
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  Figure 11 Correlation of the Wedron Group formations and members in the Lake Michigan Lobe area (units in Wisconsin from Mickelson et. al. 1984, Mickelson and Syverson, in press; units in Indiana from Bleuer et al. 1983, N.K. Bleuer, Indiana Geological Survey, personal communication, 1994; units in Michigan from Monaghan and Larson 1986, Monaghan et al. 1986, Taylor 1990).

Environments of deposition

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References

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ISGS Codes

ISGS Code and Symbol

Stratigraphic Code	Geo Unit Designation
0000	l