Two Rivers Member

Lithostratigraphy: Wedron Group >>Kewaunee Formation >>Two Rivers Member
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

Two Rivers till (Evenson 1973a).

Derivation

Two Rivers, a town along the Lake Michigan shoreline in Manitowoc County, Wisconsin.

History/background

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.

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  Figure 7 -- History of lithostratigraphic classification of the Wedron Group deposits.

Type section

Type location

Car Dealer Section, Two Rivers, Wisconsin; good for lithology.

Reference section

Reference location

Two Creeks and Kewaunee Sections, Wisconsin; both good for lithology and lower boundary. Core 17V, Lake Michigan; good for lithology and upper boundary.

Stratigraphic relationships

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. 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. 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) 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 and an average thickness of about 2.4 meters.

Lithology

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.

Contacts

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.

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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

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 in the lake basin area following retreat of the ice margin.

Remarks

Reclassified unit.

References

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ISGS Codes

ISGS Code and Symbol

Stratigraphic Code	Geo Unit Designation
0000	--