Historical:Two Rivers Member
Ardith K. Hansel and W. Hilton Johnson
Two Rivers till (Evenson 1973a).
Two Rivers, a town along the Lake Michigan shoreline in Manitowoc County, Wisconsin.
On the basis of geomorphic and stratigraphic relationships, Evenson (1973a) concluded the upper red till that overlies the Two Creeks forest bed in the Two Rivers area was not correlative with the red till at Valders in eastern Wisconsin and, therefore, the upper red till should be treated as a separate lithostratigraphic unit. Because the term Valders till took precedent, Evenson suggested it be retained for the red till in Valders Quarry and a new name be proposed for the upper red till in the Two Rivers area. He suggested the name Two Rivers till for the new unit, which was exposed in a gravel pit in the north part of the town of Two Rivers, and he designated that area as the type locality. The Two Rivers gravel pit (later referred to as Car Dealer Section in Mickelson et al. 1984) was described as the type section of this new unit in Evenson et al. (1973).
On the basis of their coring and seismic profiles in the lake basin, Lineback et al. (1974) recognized a red till beneath Lake Michigan south of Frankfort, Michigan, that was younger than the Manitowoc Till Member. They correlated this till with the Two Rivers till of Evenson (1973a) and designated it as the Two Rivers Till Member of an unnamed formation.
Mickelson et al. (1984) classified the Two Rivers till of Evenson (1973b) as the Two Rivers Member of the Kewaunee Formation. The Two Rivers Till Member recognized by Lineback et al. (1974) beneath Lake Michigan is herein renamed the Two Rivers Member, and classified as part of the Kewaunee Formation (fig. 7).
Car Dealer Section, Two Rivers, Wisconsin; good for lithology.
Two Creeks and Kewaunee Sections, Wisconsin; both good for lithology and lower boundary. Core 17V, Lake Michigan; good for lithology and upper boundary.
The Two Rivers Member consists of the upper, red brown, loam diamicton of the Kewaunee Formation beneath Lake Michigan and in the Two Rivers moraine in Wisconsin.
Diamicton of the Two Rivers Member is redder than that of the underlying Manitowoc Member in the lake basin and it typically contains more calcite than dolomite in the clay fraction (Wickham et al. 1978). On land, Two Rivers diamicton is generally coarser grained and redder than the underlying diamicton units of the Kewaunee Formation; it contains relatively more expandable clay minerals and more calcite relative to dolomite in the clay fraction than diamicton of the Haven and Ozaukee Members (Mickelson et al. 1984). Where present, the organic-rich Two Creeks deposit provides an important marker unit below the base of the Two Rivers Member.
Extent and thickness
Lineback et al. (1974) reported thin patches (0-9 m; 0-30 ft) of the Two Rivers Member on the lake floor (or beneath the Lake Michigan Formation) between Manitowoc, Wisconsin, and Ludington, Michigan. Foster and Colman (1991) also identified the Two Rivers Member and ice-margin position on their seismic profiles. On land, the Two Rivers Member is the surface unit in the north-south trending Two Rivers moraine that extends from Two Rivers, Wisconsin, northward as far as Algoma. Mickelson et al. (1984) report an onshore width of about 10 kilometers (6 mi) and an average thickness of about 2.4 meters (8 ft).
The Two Rivers Member consists of calcareous, light to medium red brown, clay to clay loam diamicton that contains lenses of gravel, sand, silt, and clay. In places on land, its matrix contains more sand. It sometimes contains organic debris from the underlying Two Creeks forest deposit. Typically, it appears redder and browner than the other red tills of the Kewaunee Formation, and it oxidizes to orange.
Lower boundary: the contact with tongues of the Equality and Henry Formations (including the Two Creeks forest deposit), the Manitowoc, Valders, Haven, Ozaukee, or Shorewood Members of the Kewaunee Formation, older units, or bedrock. Upper boundary: the contact with upper tongues of the Equality and Henry Formations, or other surficial units.
Age and correlation
The Two Rivers Member was deposited during the Two Rivers Phase of the Michigan Subepisode in the Lake Michigan basin between about 11,800 to 11,000 radiocarbon years ago (Hansel and Johnson 1992; fig. 10). It likely correlates with drift of the Manistee moraine (Orchard Beach till, Taylor 1990) in Michigan (fig. 11).
Environments of deposition
The Two Rivers Member is made up of the subglacial and ice-marginal facies of a glacigenic sequence. The sequence was deposited during the final readvance (Two Rivers Phase) in the lake basin area and followed a retreat of the ice margin that extended north far enough for the Straits of Mackinac in Michigan to become ice-free (Larson et al. 1994). This retreat caused a lowering of lake level in the basin to an elevation near or below the present level of Lake Michigan. Deformed lake sediments beneath and within Two Rivers diamicton at the type section document subglacial deformation at that site during the Two Rivers advance.
Reclassified unit. Name changed to the Two Rivers Member, and the unit classified as part of the Kewaunee Formation. Formerly classified as the Two Rivers Till Member of an unnamed formation (Lineback et al. 1974).
EVENSON, E. B., 1973a, A reevaluation of the "Valders" limit in the Lake Michigan Basin, in E. B. Evenson, D. F. Eschman, and W. R. Farrand, editors, The "Valderan" Problem, Lake Michigan Basin: Midwest Friends of the Pleistocene, 22nd Annual Field Conference Guidebook, Ann Arbor, Michigan, p. 1-29.
EVENSON, E. B., 1973b, Late Pleistocene shorelines and stratigraphic relations in the Lake Michigan Basin: Geological Society of America Bulletin, v. 84, no. 7, p. 2281-2298.
EVENSON, E. B., D. F. ESCHMAN, and W. R. FARRAND, editors, 1973, The "Valderan" Problem, Lake Michigan Basin: Midwest Friends of the Pleistocene, 22nd Annual Field Conference Guidebook, Ann Arbor, Michigan, 59 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.
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.
LARSON, G. J., T. V. LOWELL, and N. E. OSTROM, 1994, Evidence for the Two Creeks Interstade in the Lake Huron basin: Canadian Journal of Earth Science, v. 31, p. 793-797.
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.
MICKELSON, D. M., L. CLAYTON, R. W. BAKER, W. H. MODE, and A. F. SCHNEIDER, 1984, Pleistocene Stratigraphic Units of Wisconsin: Wisconsin Geological and Natural History Survey Miscellaneous Paper 84-1, 107 p.
TAYLOR, L. D., 1990, Late Wisconsinan till stratigraphy, east shore Lake Michigan, Muskegon to Northport, Michigan: Geological Society of America Abstracts with Programs, v. 22, no. 5, p. 46.
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.
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