Surficial Geology of Parts of the Forsyth, Argenta, Long Creek, and Decatur 7.5 Minute Quadrangles, Macon County, IL

Detailed geologic mapping was conducted in central Illinois to help constrain the stratigraphy of Quaternary units overlying the proposed CO₂ sequestration site near Decatur, Illinois. This project is part of a larger effort led by the Illinois State Geological Survey to determine the feasibility of long-term CO₂ sequestration in deep saline reservoirs within the Illinois Basin. This project focused on parts of the Forsyth, Argenta, Long Creek, and Decatur Quadrangles. The methodologies for this project included field observations, compilation of existing surficial and subsurface data, shallow seismic surveys and drilling. No bedrock units are exposed in the area. Subsurface data indicates that Quaternary units range in thickness from 100-300 feet, and Pennsylvanian strata of the Mcleansboro, Kewanee, and McCormick Formations comprise all of the bedrock in the area. The elevation of the bedrock surface ranges from 350-600 feet. Several Quaternary units occur in the area. The oldest is the Banner Formation. It is comprised of alternating sand, gravel, silt, and clay deposits and is interpreted to be a Pre-Illinoian glacial deposit. Above this are the Glasford Formation and Teneriffe Silt which ranges in thickness from 10-25 feet, and is comprised of multi-colored tills including yellow, brown, olive, and reddish-brown. These formations are interpreted to be Illinoian-aged glacial deposits. The Tiskilwa Formation of the Wedron group and the Peoria Silt, Henry Formation and Equality Formation of the Mason group intertounge and represent the youngest Quaternary units observed. They range in thickness from 50-300 feet thick and are comprised of sand, gravel, silt, and clay deposits. They are interpreted to be Wisconsinan-aged glacial deposits.