QUATERNARY GEOLOGIC MAPPING IN THE FOX LAKE/WONDER LAKE REGION OF NE ILLINOIS

Northeastern Illinois contains some of the most striking glacial depositional landforms in the Midwest and represents an ideal location for investigating the dynamics of a paleo-ice sheet that supported complex ice lateral fluctuations along the margin of the Lake Michigan trough. Reconnaissance mapping was completed using soils maps, topographic data, and aerial photographs. Field mapping (7.5' scale supported by USGS EDMAP) and stratigraphic studies were completed using shallow cores (Giddings and PowerProbe), deep borings, borehole gamma logs, and quarry outcrop descriptions.

The Fox Lake/Wonder Lake region consists of a complex terrane underlain by multiple morphostratigraphic and stratigraphic sequences including thick subglacial diamictons, ice marginal deposits of the Woodstock, Fox Lake, and Valapraiso moraines, ice contact stratified deposits, and large scale ice pitted outwash deposits of the Fox River Valley. Diamictons from at least 4 Wisconsin episode advances (Wedron Group) are preserved at the surface or subsurface of this region. From oldest to youngest, these include the Tiskilwa Formation, the Yorkville Member (Lemont Formation), sandy diamictons of the Haeger Member (Lemont Formation), and clay loam diamicton of the Wadsworth Formation. Complex bodies of outwash and lacustrine deposits (Mason Group) are also interbedded within and between the Wedron Group diamictons.

Sufficient stratigraphic data were obtained to determine a 3-D configuration of the diamicton and outwash bodies. These data are invaluable in revealing regional patterns of ice marginal and subglacial deposition as well as patterns of glacial erosion during subsequent readvances.

Ancillary studies of quarry exposures and GPR surveys within the "Ringwood Upland" west of Fox Lake revealed two sandy subglacial diamictons (Haeger members), the upper overlying, and the lower interbedded with deformed pro-glacial stratified sands and gravels. Compressional glacial tectonics produced a major asymmetrical antiform verging to the southwest with steeply-dipping overturned forelimb beds and gently dipping hinge and backlimbs. Fabric data for the upper and lower diamictons (lower rotated to pre-deformation positions) show mean ice-flow directions toward S38W, almost exactly perpendicular to the fold axes.