Reinterpretation of the Robein and Plano Silts, northeastern Illinois
Reinterpretation of till stratigraphy in northern Illinois and recently obtained radiocarbon dates suggest the Plano Silt is lithologically and stratigraphically indistinguishable from the Robein Silt. These recent data suggest colluvial silt and organic-rich sediment were deposited in most of the northern Illinois between >50,000 and 23,000 RCBP. This age range agrees with radiocarbon dates from the Mid-Wisconsinan Sidney Interstadial Soil of the Huron-Erie Lobe. Altonian (Early to Mid-Wisconsinan)-age till associated with the Lake Michigan Lobe has not yet been identified. Horberg and others recognize in northeastern Illinois a single basal Wisconsinan organic-rich zone; peat, silt and loess of the Farmdalian Substage were interpreted to overlie Sangamon Soil developed in Illinoian drift. They also interpreted a reddish-brown diamicton overlying the Plano Silt at its type section as the Capron Till Member of late Altonian age. The new data support the earlier interpretation of a single organic-rich deposit between Wisconsinan and Illinoian drifts. The Plano Silt should be abandoned as a rock-stratigraphic unit. The Robein Silt includes colluvium and organic-rich sediment not only of Farmdalian age, but also Altonian age in most of northern Illinois, similar to the rest of the upper Mississippi River Valley. The evidence of nearly continuous Robein deposition restricts possible Altonian-age glacial deposits to a narrow band on Capron Ridge in northern Illinois and southern Wisconsin.
- Research Organization:
- Illinois State Geological Survey, Champaign (USA)
- OSTI ID:
- 6646222
- Report Number(s):
- CONF-8510489-
- Journal Information:
- Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
- Country of Publication:
- United States
- Language:
- English
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