QUATERNARY DEPOSITS OF ILLINOIS: MAP UPDATE

We are remapping the surficial deposits of Illinois at 1:500,000 scale, replacing J. A. Lineback’s (1979) edition. This effort is a five-year project, currently in year three. Contacts are drawn digitally typically 1:80,000 to 1:100,000. To follow current USGS standards, the geological mapping schema (GeMS) tracks data sources as well as confidence levels in contact line locations and polygon unit identification. We will adopt Hansel and Johnson’s (1996) diachronic classification for the entire Quaternary stratigraphic column. We will also follow their convention for changing unit names, if needed, to emphasize physical characteristics rather than assumed genesis (e.g., Peoria Loess is now Peoria Silt). Most of our effort to date has entailed remapping geological contacts by interpreting landform-sediment associations guided by shaded relief DEMs of LiDAR data. Contact location and schema are informed by traditional means (USDA-NRCS soil survey maps, field notes, aerial photography, scanned surficial geology maps (including Lineback’s map), archived sample sets and cores, geophysical data, subsurface boring data, and outcrops). Compared to work on the 1979 map, the biggest game-changers are LiDAR DEM base-maps and digital GIS working environment.

Significant changes to Lineback’s map follow the findings of recently published STATEMAP surficial maps. For example, DEM maps reveal that moraines of the last glaciation are festooned with low-relief (<10 m) pancake-shaped lenses of rhythmically bedded fossiliferous silt and other sediment. Mapped as a facies complex, these deposits are interpreted as ice-walled lake deposits. South of the last-glacial boundary, many loess-covered hills of gravelly sand are mapped now with cores of Paleozoic sandstone. We plan to show long positive glacial lineations (< 0.5 to 8 km long) that occur across wide areas of the Illinois Episode till plain. Another new feature on our map is a 10-mile buffer zone into surrounding states, thus displaying the full width of state-bounding river valleys. The buffer zone has already facilitated collaboration with adjacent state geological surveys causing us to reconsider chronological and lithostratigraphic correlations. These discussions are important precursors to the long-term goal of a national, digital, seamless geologic map.