A NEAR-SURFACE GEOPHYSICAL INVESTIGATION OF SINKHOLE FORMATION, NACHUSA GRASSLANDS, NORTHERN ILLINOIS

Abstract

The Nachusa Grasslands, located in Franklin Grove, IL, is owned and operated by The Nature Conservancy. A sinkhole, 40 m in diameter and 6.5 m deep, exists in an area of the Grasslands. In this location, Ordovician age dolomite subcrops beneath St. Peter sandstone and several meters of unconsolidated sand, gravel and clay. A geophysical investigation was conducted for hydraulic conduits responsible for the sinkhole’s formation and possible future growth. The investigation included EM conductivity profiles, two-dimensional (2D) resistivity profiles and soundings, and ground-penetrating radar (GPR) profiles. These surveys were supplemented by conductivity logs, soil cores, and tree cores. Resistivity soundings and conductivity logs indicate bedrock depth averages 5.0 m near the sinkhole rim. Soil core analysis has shown no evidence of glaciation. Tree cores and growth patterns indicate the sinkhole has remained essentially unchanged during the last 100 yrs. The water table was not visible, suggesting it lies at least several meters below the bedrock surface. Two-dimensional (2D) resistivity profiles show subsurface features within the sinkhole, but have not detected any anomalous features outside of sinkhole. GPR profiles at 50 and 100 MHz provided the highest resolution of the subsurface. These profiles show possible conduits in and around the sinkhole as diffraction hyperbolas 1-2 m below the surface within the sinkhole. GPR profiles also show a distinct termination of the bedrock reflector near the sinkhole rim, suggesting this is a possible collapse feature. Future work will continue GPR surveys to confirm the sinkhole is, in fact, a collapse karst feature.