Geologic framework model of a glacially-filled bedrock valley in McHenry County, Illinois

Jason F. Thomason, Donald A. Keefer, Jodi A. Lau, Timothy Larson, Ahmed Ismail

Research output: Chapter in Book/Report/Conference proceedingConference contribution

Abstract

A detailed geologic framework model (GFM) has been developed for a glacially-filled bedrock valley in western McHenry County, Illinois. The bedrock valley has likely served as the major route for glacial meltwater discharge throughout the Quaternary. The valley is filled primarily with pre-Wisconsin Episode glacial deposits, and though the valley has no modern surface expression, it controlled routing of late-Wisconsin Episode meltwater. High-resolution continuous core drilling and coupled geophysical techniques (seismic, electrical resistivity, borehole) have provided details of sediment distribution and character that were previously unknown or poorly understood. The valley is developed largely in fractured and karstified carbonate rock with as much as 60 meters of relief along its walls. Erosion and deposition events may have occurred a number of times during glaciation. However, the sediment record locally includes at least one ice-advance sequence including ice-distal lake sediments, ice-proximal outwash, and subglacial till interpreted to have been deposited during the pre-Wisconsin Episode. This sequence is overlain by a thin cap or veneer of Wisconsin Episode outwash up to 15 meters thick. The GFM suggests that several regional aquifers are present together within the valley to form an aquifer system that includes the coarse valley fill and underlying fractured bedrock. Preliminary analysis of water-level observation data has been conducted for a limited number of observation wells throughout the valley. The results show a downward gradient along the northern valley margin, and a small upward gradient in the middle of the valley, near the modern stream channel. Analysis of tritium isotope concentrations in recently collected groundwater samples indicates that all the water sampled is pre-bomb in age. The heavy reliance on groundwater for irrigation of sod farms in a portion of the valley and the potential long-term competition for shallow groundwater throughout this rapidly growing county underscore the importance of understanding this complicated hydrogeologic system.
Original languageEnglish (US)
Title of host publicationGeological Society of America Abstracts with Programs
PublisherGeological Society of America (GSA), Boulder, CO, United States
Pages171
Volume43
StatePublished - 2011

Keywords

  • ISGS

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