Ardith K. Hansel and W. Hilton Johnson
Lemont drift (Bretz 1939).
Lemont, a village along the south side of the Des Plaines Valley in Cook County.
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 (fig. 18). 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 (1) sandy loam diamicton of the Haeger Member, (2) silty clay diamicton of the Yorkville Member, and (3) silt loam diamicton of the Batestown Member (fig. 7). Diamicton of the former Malden Till Member is included in the Batestown Member (formerly, the Batestown Till Member; Johnson et al. 1971b) or the Yorkville Member (formerly, the Yorkville Till Member; Willman and Frye 1970). Diamicton of the former Snider Till Member (Johnson et al. 1971b) also is included in the Yorkville Member.
Lemont Section, an abandoned quarry about 1 mile (1.6 km) west of Lemont; good for lithology and upper boundary of the undivided Lemont Formation (fig. 18).
Wedron Section (fig. 12); good for lower contact and lithology of the Batestown Member. Higginsville Section (fig. 14); good for lower contact and lithology of the Batestown and Yorkville Members. Land and Lakes Landfill Section; good for lithology of the undivided Lemont Formation and upper contact. Beverly Sand and Gravel Pit (fig. 19); good for lithology of the Haeger Member.
The Lemont Formation of the Wedron Group is the succession of fine to coarse textured, gray diamicton units that overlies the Tiskilwa Formation and underlies the Wadsworth Formation. Three members, each part of different glacigenic sequences, have been differentiated (fig. 7): a lower member of silt loam to loam diamicton (Batestown Member), a middle member of silty clay to silty clay loam diamicton (Yorkville Member), and an upper member of gravelly, sandy loam diamicton (Haeger Member). In the type area southwest of Chicago, the Lemont Formation is not subdivided. It consists of gravelly silt loam to loam diamicton (fig. 20), much of which is derived from the local Silurian dolomite; the uppermost glacigenic sequence and in places parts of lower glacigenic sequences are represented. The Lemont Formation is not subdivided in most of the Princeton Sublobe area (fig. 13a, Lee, De Kalb, Kane, Bureau, LaSalle, and Kendall Counties), where diamicton units that are laterally contiguous and likely time equivalent with the Batestown and Yorkville Members are commonly coarser in grain size.
Diamicton of the Lemont Formation is generally grayer and more illitic than that of the underlying Tiskilwa Formation. It commonly oxidizes to olive brown or yellow brown, whereas the diamicton of the Tiskilwa Formation oxidizes to brown or red brown. Its texture varies more than that of the overlying silty clay diamicton of the Wadsworth Formation. In the Decatur Sublobe area, diamicton of the Batestown Member generally contains more silt and less sand than that of the underlying Piatt Member; where this differentiation is indistinct, the Champaign-Pesotum-Arcola moraine front is used as a vertical boundary between these members (fig. 13). In the southern part of the Peoria Sublobe area where lateral facies along moraines occur, a vertical boundary at the front of the Normal Moraine is used to separate the Lemont Formation from the underlying Tiskilwa Formation. The silty clay diamicton of the Yorkville Member is very similar to that of the Wadsworth Formation; where they are juxtaposed, the West Chicago-Wilton Center moraine front is used as a vertical boundary between the two units (fig. 13). In the Harvard Sublobe area, the Haeger Member generally is readily distinguishable from the Tiskilwa and Wadsworth Formations. Locally, diamicton of the Yorkville and Haeger Members may be red gray or red brown and the uncharacteristic redder hues and lithology have been interpreted to reflect incorporation of diamicton of the Tiskilwa Formation (Wickham et al. 1988).
Extent and thickness
The Lemont Formation consists of several wedge-shaped diamicton units that overlap the Tiskilwa Formation and pinch out beneath the Wadsworth Formation. The Lemont Formation is up to about 60 meters (197 ft) thick in some moraines and forms the surface unit in more than half the area of the Wedron Group in Illinois (fig. 13). It is volumetrically, however, not as large as the Tiskilwa Formation, which is much thicker and more extensive in the subsurface.
The Lemont Formation consists of calcareous, gray, fine to coarse textured (silty clay to sandy loam) diamicton units that contain lenses of gravel, sand, silt, and clay. The characterizing element in the matrix texture of Lemont diamicton is silt, which generally makes up about 30% to more than 50% of the matrix. Typically, the diamicton of the Lemont Formation oxidizes to brown, olive brown, or yellow brown. A coarsening-upward sand and gravel sequence (Beverly Tongue of Henry Formation) was observed beneath the Haeger Member and beneath the correlative uppermost diamicton unit of the undivided Lemont Formation in its type area (fig. 22).
Lower boundary: the contact with tongues of the Henry (fig. 22) and Equality Formations, the Tiskilwa Formation, older units, or bedrock. Upper boundary: the contact with the Wadsworth Formation (fig. 18), upper tongues of the Peoria Silt and Henry and Equality Formations, or postglacial units.
Age and correlation
The Lemont Formation was deposited during the Shelby (in the Arcola Moraine, eastern part of the Decatur Sublobe area), Putnam, Livingston, and Woodstock Phases of the Michigan Subepisode, probably between about 18,500 and 15,500 radiocarbon years ago (Hansel and Johnson 1992). Each phase represents the interval of a readvance and subsequent melting back of the ice margin (fig. 10). Fluctuations were 50 kilometers (31 mi) or more. The Lemont Formation correlates in part with the New Berlin and Horicon Members (Holy Hill Formation) of Wisconsin, the Batestown and Snider Tills of Indiana, and possibly the Ganges till of Michigan (fig. 11).
Environments of deposition
The Lemont Formation is interpreted to represent the subglacial and ice-marginal facies of several offlapping glacigenic sequences. Diamicton of the Lemont Formation is more illitic and contains fewer far-travelled crystalline erratics than that of the Tiskilwa Formation. The predominant clast lithologies consist of Paleozoic shale and carbonate. The composition of the Lemont Formation indicates predominantly a Lake Michigan basin, northern Illinois, and southeastern Wisconsin source. We attribute the fine grained matrix of Yorkville diamicton in part to reflect incorporation of proglacial lacustrine sediment that accumulated between end moraines and the glacier as the ice margin melted back and readvanced during the late Putnam and early Livingston Phases (fig. 10).
Revised unit. Elevated to formation rank; includes the Haeger Member as the uppermost unit; lower boundary extended to include the Batestown and Yorkville Members. Named the Lemont drift in 1939 and defined as a lithostratigraphic unit by Bretz (1955). Retained, but as an informal unit, in Willman and Frye (1970). Correlated with the Haeger Till Member of the Wedron Formation and recommended as a unit of formation rank by Johnson and Hansel (1989).
BOGNER, J. E., 1973, Regional relations of the Lemont drift (Pleistocene, Northern Illinois): M.S. thesis, University of Illinois at Chicago, 72 p.
BRETZ, J. H., 1939, Geology of the Chicago Region, Part I- General: Illinois State Geological Survey Bulletin 65, 118 p.
BRETZ, J. H., 1955, Geology of the Chicago Region, Part II— The Pleistocene: Illinois State Geological Survey Bulletin 65, 132 p.
DeLEUW-NOVICK ENGINEERS, 1975, Chicago-Central Area Transit Project—Monroe Line: Prepared for the Chicago-Urban Transportation District, Contract no. DOT-UT-863, 39 p.
FRYE, J. C., and H. B. WILLMAN, 1960, Classification of the Wisconsinan Stage in the Lake Michigan Glacial Lobe: Illinois State Geological Survey Circular 285, 16 p.
HANSEL, A. K., and W. H. JOHNSON, 1986, Quaternary Records of Northeastern Illinois and Northwestern Indiana: Illinois State Geological Survey Guidebook 22, 106 p.
HANSEL, A. K., and W. H. JOHNSON, 1992, Fluctuations of the Lake Michigan Lobe during the late Wisconsin Subepisode: Sveriges Geologiska Undersoekning, Series Ca 81, p. 133-144.
HORGERG, C. L., and P. E. POTTER, 1955, Stratigraphic and Sedimentologic Aspects of the Lemont Drift of Northeastern Illinois: Illinois State Geological Survey Report of Investigation 185, 23 p.
JOHNSON, W. H., D. L. GROSS, and S. R. MORAN, 1971b, Till stratigraphy of the Danville region, east-central Illinois, in R. P. Goldthwait, J. L. Forsyth, D. L. Gross, and F. Pessl, Jr., editors, Till, A Symposium: Ohio State University Press, Columbus, p. 184-216.
JOHNSON, W. H., and A. K. HANSEL, 1985, The Lemont Section, Stop 2, in Johnson, W. H., A. K. Hansel, B. J. Socha, L. R. Follmer, and J. M. Masters, Depositional Environments and Correlation Problems of the Wedron Formation (Wisconsinan) in Northeastern Illinois: Illinois State Geological Survey Guidebook 16, p. 42-52.
JOHNSON, W. H., and A. K. HANSEL, 1989, Age, stratigraphic position, and significance of the Lemont drift, northeastern Illinois: Journal of Geology, v. 97, no. 3, p. 301-318.
LANDON, R. A., and J. P. KEMPTON, 1971, Stratigraphy of the Glacial Deposits at the National Accelerator Laboratory Site, Batavia, Illinois: Illinois State Geological Survey Circular 456, 21 p.
WICKHAM, S. S., W. H. JOHNSON, and H. D. GLASS, 1988, Regional Geology of the Tiskilwa Till Member, Wedron Formation, Northeastern Illinois Illinois State Geological Survey Circular 543, 35 p.
WILLMAN, H. B., and J. C. FRYE, 1970, Pleistocene Stratigraphy of Illinois: Illinois State Geological Survey Bulletin 94, 204 p.
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