GLACIOTECTONIC DEFORMATION IN THE FOX LAKE QUADRANGLE IN NORTHEAST ILLINOIS

A surficial map and cross-sections of glacial deposits of the Fox Lake Quadrangle, located in McHenry and Lake Counties in northeast Illinois, were compiled as part of the USGS EDMAP program. Field mapping and cross-sections were completed by interpretation of topographic and soil maps, aerial photos, shallow and deep borings, ISGS and WGS sample sets, borehole gamma logs, and outcrop descriptions. Surficial data suggests at least two different local ice advances of the Lake Michigan Lobe deposited diamictons of sandy loam to loam texture (Haeger Member: Wedron Group) and silty clay to clay loam texture (Wadsworth Formation: Wedron Group). Glacigenic features such as kettles, kames, kame terrace, and kamic moraine suggest a period of ice-stagnation also influenced the geomorphology of the quadrangle.

During outcrop description of sand and gravel pits, intensive deformation of pro-glacial and sub-glacial sediments was observed. Stratified sands and gravels of the Beverly Tongue (Henry Formation) and two diamictons of the Haeger were involved in fold and thrust deformation. The upper Haeger is a horizontal diamicton that unconformably overlies a folded and faulted sequence of the lower Haeger and the Beverly Tongue. Systematic measurements of fold axial surfaces and clast fabrics in the diamictons were taken to determine fold vergences and ice flow direction. To obtain the ice flow direction for the lower diamicton the fabric data were rotated to its pre-deformation positions. Fabric orientation of the upper and lower diamicton shows an ice-flow direction to S15-20W and S45W, respectively, correlating with the fold and fault vergence to the south-southwest. This relationship suggests a glaciotectonic origin for the deformation.

Variation in the fabric orientation between both diamictons and the unconformable relationship between the upper diamicton and underlying structures suggests two local events of advance and retreat of the Harvard Sublobe (Lake Michigan Lobe). The latest ice advance overriding water-saturated sediments was responsible for the deformation of the Beverly Tongue and the lower Haeger. Further evidence includes folds that are truncated by the upper Haeger and the presence of inclusions of older diamicton at the base of the upper Haeger.