Historical:Lake Michigan Member
Ardith K. Hansel and W. Hilton Johnson
Lake Michigan Formation (Willman and Frye 1970).
The Lake Michigan Formation was defined by Willman and Frye in 1970 to include the lacustrine deposits of modern lakes. Although present in most existing natural lakes, lacustrine deposits are most evident in Lake Michigan. Willman and Frye recognized the modern beach sand as the Ravinia Sand Member of the Lake Michigan Formation; they left further differentiation of the formation to other ISGS scientists, who were studying the stratigraphy beneath Lake Michigan at the same time. Thus, Lineback et al. (1970) defined, on the basis of 22 cores taken from the bottom sediments of southern Lake Michigan, five additional members in the Lake Michigan Formation. In ascending order, they are the South Haven, Sheboygan, Winnetka, Lake Forest, and Waukegan Members (fig. 9c). They were differentiated on the basis of changes in color, water content, cohesiveness, grain size, and mineralogy. The lower two members are reddish; the Sheboygan Member contains the Wilmette Bed (also defined by Lineback et al. 1970), a distinct dark clay bed that is widespread in the southern Lake Michigan basin where the water is more than 82 meters (269 ft) deep. The upper three members of the Lake Michigan Formation are dark gray to dark brown and successively coarser and darker upward. They contain thin, black beds and dark mottling.
The members of the Lake Michigan Formation were also differentiated by Lineback et al. (1974) in 50 additional cores from Lake Michigan, some of which were from the central and northern parts of the lake. Lineback et al. (1974) and Wickham et al. (1978) suggested the Carmi Member of the Equality Formation, which commonly underlies the Lake Michigan Formation and overlies the till members in the lake basin, was late glacial and the Lake Michigan Formation was postglacial.
In 55 cores collected from the southern two-thirds of Lake Michigan in 1988 and 1989, Colman and Foster (1990) were unable to consistently distinguish between the two red members nor among the three gray brown members of the Lake Michigan Formation described by Lineback et al. (1970) and Wickham et al. (1978). Instead, they informally grouped the red members into the lower Lake Michigan Formation and the gray brown members into the upper Lake Michigan Formation (fig. 9c). The two informal members are separated by the Chippewa unconformity (Hough 1955) in relatively shallow water and by a gradational color change in deeper water. Like Lineback et al. (1970, 1974), Colman and Foster (1990) recognized the Wilmette Bed in the lower Lake Michigan Formation. They attributed the thin black beds (hydrous iron monosulfides), which are most prevalent in the upper Lake Michigan Formation but are also present in the lower Lake Michigan Formation, particularly in the Wilmette Bed, to diagenesis associated with reducing conditions caused by decaying organic matter. Furthermore, 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, when traced on seismic profiles, graded laterally into stratified diamicton and sorted sediment interfingered with more massive diamicton tongues. On the basis of such relationships, they interpreted the lower Lake Michigan Formation to be a distal facies of the more ice-proximal stratified sediment of the Equality Formation and, therefore, to be part of the glacial sequence rather than the postglacial sequence. They concluded that the Chippewa unconformity, which truncates the lower Lake Michigan Formation on basin slopes and older units in the nearshore zone, marks the boundary between the glacial and postglacial sedimentary sequences and also roughly coincides with the Pleistocene-Holocene boundary. Foster and Colman (1991) suggested that the nomenclature and rank of lithostratigraphic units in the Lake Michigan basin established by Lineback et al. (1970) was inappropriate and could be replaced with two formations corresponding to the glacial and postglacial seismic sequences.
In this report, we chose to include in the Equality Formation all the fine grained, bedded sediment that underlies, interfingers with, or overlies the Wedron Group. The gray to brown, laminated clay and silt, which was classified as the upper Lake Michigan Formation by Foster and Colman (1991) and the Winnetka, Lake Forest, and Waukegan Members of the Lake Michigan Formation by Lineback et al. (1970), is reclassified in this report as the Lake Michigan Member of the Equality Formation (fig. 9c). We recommend that the Lake Michigan Member is appropriate only in the Lake Michigan basin and that the members and bed of the former Lake Michigan Formation be abandoned as formal lithostratigraphic units and used instead as informal units where they can be differentiated. The coarser grained sediment of the former Ravinia Sand Member is herein classified with the Dolton facies of the Henry Formation.
Southern Lake Michigan cross section; cores 112, 116, 117, 118, 143, 144, 145, 146, 147, and 148 taken 20 to 32 kilometers (12.5-20 mi) east of Waukegan, Illinois; good for lithology and lower contact.
Cores 5p and 9v; both good for lithology of member and boundary with undivided formation.
The Lake Michigan Member of the Equality Formation consists of the uppermost gray to brown laminated silt and clay that occurs above the Chippewa unconformity in shallow water and above a redder laminated silt and clay unit in the deep bathymetric basins beneath Lake Michigan.
On the flanks and slopes of the basins beneath Lake Michigan, the Lake Michigan Member is readily differentiated from the diamicton units and undivided Equality Formation by the Chippewa unconformity, which is evident in both seismic profiles and cores. In the deep basins where the Chippewa unconformity is absent, differentiation of the Lake Michigan Member from the rest of the formation is marked only by the transition zone between the lower red and upper gray clay.
Extent and thickness
The Lake Michigan Member forms the lake floor beneath Lake Michigan except in areas of erosion or nondeposition, mostly along the southwestern shoreline and on the midlake high (Lineback et al. 1970, Wickham et al. 1978, Foster and Colman 1992). On the basis of seismic profiles and cores, Foster and Colman (1992) mapped thicknesses greater than 14 meters (46 ft; 20 m [65.5 ft] maximum). Like Lineback et al. (1970), they show an asymmetric distribution beneath the deep basins, marginal slopes, and nearshore zone of the lake. The Lake Michigan Member is thickest in three lobes along the east side of the lake, two lobes in the southern basin, and another lobe in the eastern mid-lake basin; it generally tends to pinch out in the nearshore zones.
The Lake Michigan Member consists of gray to brown, laminated clay and silt, and overlies red, laminated clay and silt of the undivided Equality Formation in the Lake Michigan basin. Although it is separated from the formation and older units by the Chippewa unconformity in shallow water, its contact with the formation is conformable and gradational in the deep basins (Foster and Colman 1991). Locally, the Lake Michigan Member contains lenses of sand, gravel, and organic debris.
Lower boundary: the truncational contact with undivided Equality Formation on the basin slopes or older units in the nearshore zone and the conformable and gradational contact with red, laminated sediment in the deep basins. Upper boundary: the floor of Lake Michigan or the contact with tongues of the Henry Formation (Dolton facies) in the nearshore zone.
Age and correlation
The Lake Michigan Member is postglacial (Hudson Episode) and postdates the Chippewa low lake level, which resulted from the deglaciation of the isostatically depressed northern outlet at North Bay, Ontario, about 10,300 radiocarbon years ago (Larsen 1987); the member dates from postglacial to modern (appendix B2). The Lake Michigan Member replaces the gray and brown clay of the Winnetka, Lake Forest, and Waukegan Members of Lineback et al. (1970) and the upper Lake Michigan Formation of Foster and Colman (1991, 1992; fig. 9c); it is time correlative with the upper part of the Equality Formation in other parts of Illinois and unnamed units beneath adjacent Great Lakes in other states and provinces.
Environments of deposition
The Lake Michigan Member is interpreted to be postglacial lake sediment derived primarily from coastal erosion and sediment supplied by rivers flowing into Lake Michigan. The latter is likely responsible for the thick sediment wedges along the east side of the lake (Lineback et al. 1971, Foster and Colman 1992).
Revised unit. Redefined to include only the upper, gray, fine grained laminated sediment of the former Lake Michigan Formation of Willman and Frye (1970) and lowered in rank to a member of the Equality Formation; members defined by Willman and Frye (1970) and Lineback et al. (1970) are abandoned as formal lithostratigraphic units; unit is not recognized outside the Lake Michigan basin.
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.
FOSTER, D. S., and S. M. COLMAN, 1992, Maps and Seismic Profiles Showing Thickness and Distribution of Postglacial Deposits Beneath the Southern Two-Thirds of Lake Michigan: United States Geological Survey Miscellaneous Investigations Series Map I-2202, scale 1:500,000.
HOUGH, J. L., 1955, Lake Chippewa, a low stage of Lake Michigan indicated by bottom sediments: Geological Society of America Bulletin, v. 66, no. 8, p. 957-968.
LARSEN, C. E., 1987, Geological History of Glacial Lake Algonquin and the Upper Great Lakes: United States Geological Survey Bulletin 1801, 36 p.
LINEBACK, J. A., N. J. AYER, and D. L. GROSS, 1970, Stratigraphy of Unconsolidated Sediments in the Southern Part of Lake Michigan: Illinois State Geological Survey Environmental Geology Notes 35, 35 p.
LINEBACK, J. A., D. L. GROSS, and R. P. MEYER, 1974, Glacial Tills Under Lake Michigan: Illinois State Geological Survey Environmental Geology Notes 69, 48 p.
LINEBACK, J. A., D. L. GROSS, R. R. MEYER, and W. L. UNGER, 1971, High-Resolution Seismic Profiles in Southern Lake Michigan: Illinois State Geological Survey Environmental Geology Notes 47, 41 p.
WICKHAM, J. T., D. L. GROSS, J. A. LINEBACK, and R. L. THOMAS, 1978, Late Quaternary Sediments of Lake Michigan: Illinois State Geological Survey Environmental Geology Notes 84, 26 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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