Will County Geologic Mapping Update—Addressing Societal Issues
The Illinois State Geological Survey has continued its geologic mapping efforts in the suburban Chicago area, with the aim of producing large-scale, three-dimensional (3-D) geologic maps of glacial deposits in the area. The long-term objective of this mapping effort in northeastern Illinois is to provide a context for evaluating aquifer yields, estimating recharge, and developing groundwater flow models. The project addresses the overall goals of the State of Illinois, the Great Lakes Geologic Mapping Coalition, and the ISGS to map deposits both at the land surface and in the subsurface to gain a better understanding of the complex geology left behind by repeated glaciations and associated flooding events.
The project was introduced during the annual meeting of the Illinois Geologic Mapping Advisory Committee on September 6, 2012, when the ISGS presented a geologic mapping plan that included mapping in Will County. The Advisory Committee, made up of experts from private industry, academia, and local, state, and federal governments, deliberated with ISGS staff on the natural resource, economic, and environmental issues predominant in this area and voted to endorse Will County as a priority area for the next Great Lakes Geologic Mapping Coalition project. The main objective identified was to reconstruct the complex successions of ice marginal and periglacial outwash deposits by using a geomorphologic approach based on a light detection and ranging (LiDAR) digital elevation model. In most cases, these sediments and landforms, as well as other geomorphological features, are significant indicators of the extent of former ice-dammed lakes, having elevations intimately linked to well-documented outlets.
Will County encompasses urban, suburban, and rural environments, and 3-D geologic maps are necessary to guide planners and decision makers in addressing issues of sustainable economic development, water and mineral resources, and environmental protection. These include water supply management, storm water management, wetland preservation, shallow groundwater recharge, and sand and gravel quarry development. Industrial and residential growth will undoubtedly continue in Will County; thus, it is important for planners to be knowledgeable of the county’s natural assets and limitations. If limiting geologic and hydrogeologic conditions are not considered in the planning process, human activities that directly affect the physical environment could have harmful effects on the resources, industries, residential facilities, and health of the county’s citizens.
The methodology used in this study integrates maps of surficial sediments with borehole logs via GIS and a 3-D numeric geomodeling system. Will County is located at the confluence of the moraines of the Joliet sublobe, which includes the oldest moraine (Minooka Moraine) and the outlying Lake Border Moraine (the Tinley Moraine). Although such landforms are indicators of former ice-dammed lakes, their stratigraphic architecture is complex, and their subsurface extent is poorly documented. Most of the eastern portion of Will County is covered by the Valparaiso Morainic System. The western part of the county is characterized by extensive areas of flat, level topography associated with an old lake plain formed by glacial Lake Wauponsee and a few lacustrine terraces and scarps (slopes) associated with the former shorelines of this short-lived lake. Preliminary mapping, based on LiDAR, has identified two glaciogenic successions, as well as some evidence of the Tiskilwa Till Member. Ice-walled lake plains and ice-marginal lakes associated with the Tinley Moraine have also been identified. These will be characterized and may contain fossils that can constrain the age of the Tinley Moraine. The discovery of new sections should allow reassessment of the depositional environments, correlation of units with one another, and extent of the Haeger Member of the Lemont Formation (Wedron Group).