REIMAGINING BEDROCK TOPOGRAPHY OF THE CHICAGO REGION: A COMPARISON OF PAST AND PRESENT MODELS

The Illinois State Geological Survey has produced a new bedrock topography model of Cook and DuPage counties in northeastern Illinois as the first phase of a multi-year project to create a 3-dimensional geologic model of Quaternary deposits. This model updates a previous version by Carrell et al. (2022) as well as previous models by using additional data to achieve a higher resolution. The main input to the Carrell et. al (2022) model was a supervised digital classification of drilling logs; we focused on quality checking interpretations and locations in that data set, adding additional interpretations not previously entered, compiling new (to us) boring reports, and collecting new horizontal-to-vertical spectral ratio soundings and electrical resistivity tomography profiles in areas where depth to bedrock data is sparse. Our working database includes 43,707 records and new records are still being added. The project boundary was conditioned by previous neighboring bedrock topography contour maps and swath geophysical surveys. We constructed the initial new grid with a 10 m nodal spacing by using ESRI’s Topo to Raster tool. This tool accommodates a variety of data types and creates a smoother interpolated surface than other available algorithms. However, the tool can be prone to artifacts. The data proved to be insufficiently dense to support 10 m grid scale regionally, although the detail is useful for preliminary analysis. We thus generalized the grid to 30 m. Further smoothing was completed with ESRI’s Focal Statistics tool. Our final map highlighted new areas of interest and resolved some discrepancies between previous models. These new areas of interest include bedrock valleys and Silurian reefs whereas the discrepancies were false bedrock highs and false cliff edges along the lakeshore. We will revise this model as new data and interpretations become available during ensuing phases of the project.

Carrell, J.E., A.C. Phillips, R. Balikian, and C.C. Abert. 2023. Bedrock Topography of Illinois [raster graphic]: Illinois State Geological Survey, US Geological Survey Great Lakes Mapping Coalition Contract Report, Award G20AC00306, 100 m grid.