SURFICIAL GEOLOGIC MAP OF THE LEONORE 7.5 MINUTE QUADRANGLE, LASALLE COUNTY, ILLINOIS

The Leonore 7.5' Quadrangle is located in LaSalle County, IL. It occupies the area between 880 52'30" and 890 00'west longitude, and 410 07'30" and 410 15' north latitude. A surfical geologic map of the Leonore Quadrangle was constructed using soil maps, surficial geologic observations, borehole logs, and geophysical data. This work was supported by a USGS EDMAP research grant. The LaSalle Anticlinorium locally influences the bedrock. The anticlinorium, which is strongly asymmetric, trends N20W, and extends from south-west Indiana to north-central Illinois. Although the western limb dips as much as 35 degrees locally, the study area occurs just to the east of the axis, resulting in less than a 2 degree dip of the bedrock. Bedrock within this area is exposed along the northwest flowing Vermilion River and its tributaries. The bedrock is Pennsylvanian in age and is part of the Carbondale Formation. The Carbondale Formation is composed of sandstone, shale, and limestone. The Ticona Channel (buried bedrock valley) occurs in the north-central part of the quadrangle. The Ticona Channel indicated by the absence of bedrock and presence of diamicton where the Vermilion River cuts across it. The valley is filled with sand and gravel and may provide a source of groundwater for the surrounding communities. The bedrock is unconformably overlain by diamicton of the Lemont Formation. This formation is part of the Wedron Group and was deposited during the Wisconsinan Stage glaciation. The diamicton is a grey silty-clay to clay loam and is variable is thickness. Weathered surfaces of this unit oxidize to an olive brown color. Diamicton of the Lemont Formation is covered by the Peoria Silt (Mason Group) over the majority of the mapping area. However, the loess was mapped in areas only where its thickness exceeded 5 feet (1.5 meters). It is a light gray to yellowish brown silt loam. Sand and gravel deposits, as well as stratified sandy loam deposits also occur throughout the area. They were classified as either Mason Group or Cahokia Alluvium based on stratigraphic position in relation to the Lemont Formation.