GLACIAL TECTONIC DEFORMATION IN THE CHAIN O' LAKES REGION OF NE ILLINOIS

The western margin of the Lake Michigan trough in NE Illinois contains some of the most striking glacial landforms. The "Ringwood Upland" west of Fox Lake is an undulating plateau underlain by two sandy subglacial diamictons (the upper and lower Haeger members of the Wedron group) interbedded with pro-glacial stratified sands and gravels (Beverly tongue of the Henry formation). The upper Haeger is a horizontal diamicton (at the surface) that unconformably overlies a deformed sequence of interbedded lower Haeger diamicton and the Beverly sediments. Compressional tectonics produced a series of anticlines, and thrust faults partially truncated in angular unconformities with the upper Haeger diamicton. Inclusions of lower Haeger and Beverly sediments were also documented at the base of the upper Haeger. Systematic measurements of fold axial surfaces (45 measurements) and diamicton fabrics (10 stations) revealed fold vergences and ice-flow directions. Three folding domains were observed including: 1) short wavelength (15-50m) recumbent folds with steeply-dipping overturned limbs, 2) large-amplitude/short wavelength recumbent folds developed in fine sands, and 3) vertical and sub-horizontal folds in the lowest exposed part of the Beverly tongue. Recumbent, asymmetric folds in a thrusted sequence which brought the lower/middle Beverly sections over the upper Beverly were also documented. The overall structure represents a major asymmetrical antiform verging to the southwest with steeply-dipping overturned forelimb beds and gently dipping hinge and backlimb. Total depth of deformation varies between 30 and 50m with total lateral shortening between 15 and 35%. Fabric data for of the upper and lower diamictons (lower rotated to pre-deformation positions) show ice-flow directions toward S27W to S48W, respectively, correlating with the fold and fault vergence to the south-southwest. Fold axial surface orientations average N45W dipping 15NE, almost exactly perpendicular to the lower diamicton fabric and up to 21 degrees from the perpendicular to the upper diamicton fabric.