Ardith K. Hansel and W. Hilton Johnson
Tiskilwa Till Member (Willman and Frye 1970).
Tiskilwa, a village in Bureau County, northern Illinois.
The Tiskilwa Till Member was originally defined by Willman and Frye (1970) and described as the pink till member of the Wedron Formation. The Tiskilwa Till Member has become a fundamental unit in describing and interpreting the geology in the area of the Lake Michigan Lobe. It is readily recognized by its red or pink hues. It is the thickest and volumetrically the most extensive unit of the Wedron in Illinois (Wickham and Johnson 1981, Wickham et al. 1988).
The relationship of the lower tills of the Wedron Group across the Decatur-Peoria Sublobe boundary area stimulated further study in the 1970s, probably in part because Willman and Frye (1970) did not subdivide the Wedron Formation in the Decatur Sublobe area (fig. 4a). With the exception of Chamberlin (1883, 1894), Leverett (1899), and Leverett and Taylor (1915), many geologists (Leighton et al. 1948, Horberg and Anderson 1956, Anderson 1955, 1957, Leighton 1960, Frye et al. 1965, Willman and Frye 1970, Frye and Willman 1973, Dreimanis and Goldthwait 1973) attributed the drift of the Decatur Sublobe area to a more eastern source than the Lake Michigan basin. This interpretation was mainly based on moraine configurations. Studies of the tills across the sublobe-boundary area, however, indicate lithologic similarities between tills of the Peoria and Decatur Sublobes (Wascher and Winters 1938, Newell 1954, Kempton et al. 1971). McKay (1975) and Moore (1981) argued that till members are continuous across the sublobe boundary and were deposited by the Lake Michigan Lobe. A Lake Michigan Lobe source for the till members in the Decatur Sublobe area is also indicated from recent provenance studies (Bleuer 1975, Johnson et al. 1986).
In the decade that followed the publication of Pleistocene Stratigraphy of Illinois (William and Frye 1970), six till members and one nontill member were differentiated within the Wedron Formation in the area of the Decatur Sublobe. They include the Glenburn, Batestown, and Snider Till Members (Johnson et al. 1971b); the Oakland and Fairgrange Till Members and the Ashmore Member (Ford 1973); and the Piatt Till Member (Wickham 1979a). All of these except the Glenburn and Oakland Till Members and the Ashmore Member were recognized on the state Quaternary map compiled by Lineback in 1979 (fig. 5).
In 1976, Johnson correlated the Glenburn, Oakland, and Fairgrange Till Members of east-central Illinois (Decatur Sublobe) with the Tiskilwa and Delavan Till Members of central and northern Illinois (fig. 7). He suggested these units formed a lower, medium-textured group of Wedron Formation till members. (The Lee Center Till Member of northeastern Illinois was also included in this group, but it has since been interpreted to underlie the Sangamon Geosol [Follmer and Kempton 1985]).
On the basis of the preceding work and in an attempt to simplify the classification system that evolved for the lower part of the Wedron Group, we elevate the Tiskilwa Member to a formation that includes two lithologically related, mappable subunits (figs. 7, 13). They are (1) the Delavan Member, proposed by Willman and Frye (1970), which is expanded in concept and regional extent to include the former Glenburn, Oakland, and Fairgrange Till Members of the Decatur Sublobe area; and (2) the Piatt Member, proposed by Wickham (1979a). The Ashmore Member, proposed by Ford (1973), is recognized herein as the Ashmore Tongue of the Henry Formation.
Where undivided, the Tiskilwa Formation consists predominantly of red and pink loam to clay loam diamicton. Diamicton of the Delavan Member is more gray and considered a lithologic variant of the main diamicton of the Tiskilwa Formation. It occurs stratigraphically below (in the Princeton, Harvard, and northern part of the Peoria Sublobe areas) or replaces (in the southern part of the Decatur and Peoria Sublobe areas) the main pink diamicton of the Tiskilwa Formation (fig. 13). The Piatt Member, a sandier, grayer facies of the Tiskilwa Formation, occurs stratigraphically above the Delavan Member in part of the Decatur Sublobe area (figs. 7, 13).
The Oakland Till Member proposed by Ford (1973) is replaced herein by the Oakland facies, a lithologic variant of the Delavan Member or the undivided Tiskilwa Formation. Diamicton of the Oakland facies is browner, siltier, more abundant in expandable clay minerals, and generally more variable than typical diamicton of the Tiskilwa Formation (Johnson et al. 1972, Ford 1973, Johnson 1976). It is a discontinuous, basal facies of the Tiskilwa Formation; thicknesses up to about 4 meters were reported from exposures and cores in east-central Illinois (Johnson et al. 1972, Ford 1973, Johnson 1976). Ford (1973) attributed the browner color and distinct clay-mineral composition of the Oakland diamicton to glacial incorporation of the Roxana Silt, including the organic-rich Robein Member.
Buda East Section, located in a roadcut 5 miles (8 km) east of Tiskilwa. No longer exposed.
Wedron Section (fig. 12); good for upper and lower boundaries and lithology. Danvers Section; good for lower boundary and lithology. Higginsville Section (fig. 14); good for upper and lower boundaries and lithology.
The Tiskilwa Formation is the lowermost sequence of red to gray diamicton units of the Wedron Group. Two grayer diamicton members (a lower Delavan Member and an upper Piatt Member) are differentiated from the main body of red gray diamicton (fig. 7).
Diamicton of the Tiskilwa Formation is readily distinguishable from sorted sediment of the Mason Group. In places in the subsurface, the contact with the organic-rich Robein Member of the Roxana Silt is a distinctive marker for recognizing the base of the Tiskilwa Formation. Where the Tiskilwa Formation overlies older diamicton units, the units can usually be differentiated on the basis of lithology. Where present, the Sangamon Geosol helps to differentiate these older diamicton units from those of the Tiskilwa Formation. Lenses of the underlying units may be within the Tiskilwa Formation, particularly the basal part, but they are usually accompanied by evidence of subglacial deformation. Locally, the Oakland facies may be present as a basal diamicton facies. Diamicton of the Tiskilwa Formation generally contains more clay and is redder than that of the overlying Batestown Member of the Lemont Formation. In some areas where the Delavan or Piatt Members are in contact with the Batestown Member, however, the lithologies of these units are quite similar, and arbitrary boundaries along the outer margins of moraines may be necessary to separate the units for mapping purposes.
Extent and thickness
The Tiskilwa Formation forms a wedge-shaped deposit that pinches out beneath the Lemont Formation to the north and east (fig. 13). It is volumetrically the largest formation of the Wedron Group in Illinois. It forms the surface unit in the outermost moraines of the Harvard, Princeton, Peoria, and Decatur Sublobe areas in Illinois, where in some places it reaches thicknesses up to 90 meters (295 ft) (Wickham et al. 1988).
The Tiskilwa Formation consists of calcareous, red gray to gray, medium textured (clay loam to loam) diamicton that contains lenses of gravel, sand, silt, and clay. Typically, it oxidizes to red brown, brown, or yellow brown.
Lower boundary: the contact with the Ashmore Tongue of the Henry Formation, the Peddicord Tongue of the Equality Formation, the Morton Tongue of the Peoria Silt, the Robein Member or undivided Roxana Formation (in which the Farmdale Geosol is developed), or older units. Upper contact: the contact with the Batestown, Yorkville, or Haeger Members of the Lemont Formation, upper tongues of the Peoria Silt and Henry and Equality Formations, or postglacial units.
Age and correlation
The Tiskilwa Formation was deposited during the early part of the Michigan Subepisode (Marengo and Shelby Phases) between about 26,000 and 18,500 radiocarbon years ago (Hansel and Johnson 1992). The Lake Michigan Lobe advanced to its maximum position in the Harvard Sublobe area (Marengo Moraine) about 25,000 radiocarbon years ago; it reached its maximum position in the Princeton, Peoria, and Decatur Sublobe areas (Bloomington and Shelbyville Morainic Systems) about 20,000 radiocarbon years ago before it wasted back about 50 kilometers (31 mi; fig. 10). The Tiskilwa Formation correlates with the Tiskilwa Member of the Zenda Formation in Wisconsin (fig. 11) and the Fairgrange Till in Indiana.
Environments of deposition
The Tiskilwa Formation consists of part(s) of one or multiple glacigenic sequences. Over much of Illinois the glacigenic sequences represented by the Tiskilwa Formation and the interfingered tongues of the Peoria Silt and Henry and Equality Formations appear fairly complete (e. g., Wedron Section), suggesting that deposition clearly dominated over erosion in the marginal areas of the Lake Michigan Lobe during the Michigan Subepisode (Johnson and Hansel 1990).
Revised unit. Elevated in rank from the Tiskilwa Till Member (Willman and Frye 1970); lower boundary extended to include the Delavan Member, originally defined as a separate till member of the Wedron Formation (Willman and Frye 1970); upper boundary extended to include the Piatt Member, originally defined as a member of the Wedron Formation (Wickham 1979a).
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