A NEW STATEWIDE QUATERNARY MAP OF ILLINOIS: CURRENT PROGRESS AND NEW FINDINGS

A new Quaternary (surficial geology) map is being developed for Illinois, to update and replace the 1979 Quaternary deposits map (J.A. Lineback). Remapping of Illinois’ Quaternary deposits is a 5-year project, currently in its third year. Contact lines are being drawn digitally at 1:80,000 to 1:100,000 scale with the intention of final map publication at 1:250,000 or 1:500,000 scale. To follow current USGS standards, a geological mapping schema (GeMS) is being implemented that tracks data sources used to inform the map, confidences in contact line locations and certainty of polygon unit identification. With four decades of mapping since publication of the Lineback Quaternary map, many updates to map unit boundaries and stratigraphic classifications are being made. Data sources include USDA-NRCS soil survey maps, historical field notes (early- to mid-20^th century), aerial photography, digital elevation maps, detailed surficial geology maps, archived sample sets/cores, geophysical data, subsurface boring data, and the 1979 Lineback map.

One significant update from earlier mapping in northeastern Illinois is the addition of previously unrecognized, last glacial, ice-walled lake deposits. Radiocarbon dating of macrofossils within these lake deposits is helping to improve the chronology of glacial moraines and till units. A significant change in south-central Illinois is that many hills, formerly mapped to contain ice-contact deposits, have been found to be bedrock-controlled hills. Based on shallow drillholes and archived samples descriptions, these hills are mainly cored by Pennsylvanian sandstone. Across wide areas of the Illinois Episode till plain in southern Illinois with shallow bedrock and thin loess cover, glacial lineations (from < 0.5 to 8 km long) are notable on digital elevation or LiDAR surface elevation maps. These lineations are indicated on the Quaternary deposit map, along with loess thickness contour lines and moraine crests. Another new addition is a 10-mile buffer zone into surrounding states, thus displaying the full width of the Wabash, Ohio, and Mississippi River valleys. This buffer zone will help to facilitate collaboration with adjacent state geological surveys to resolve lithostratigraphic correlations and to help achieve the long-term goal of a seamless national map.