THE CHALLENGE OF MAPPING WHAT CAN'T BE SEEN: A PRELIMINARY GEOLOGIC MAP OF THE BEDROCK SURFACE OF SOUTHEASTERN WISCONSIN

The Paleozoic bedrock of southeastern Wisconsin consists of a sequence of gently eastward-dipping Cambrian to Devonian sandstones, fine-grained clastics, and carbonates deposited along the western margin of the Michigan Basin. Lower and Middle Silurian units, present over half of the bedrock surface, display complex south to north lithostratigraphic transitions. Regional-scale normal faulting further complicates the entire sedimentary rock sequence. The bedrock surface is, itself, topographically complex; for example, there are many buried stream-valleys forming a dendritic pattern. Finally, all these features are covered by up to several hundred feet of Quaternary sediment.

Critical to the success of mapping the bedrock geology is the development of a detailed map of the topography of the buried surface. We used geologic and depth to bedrock information from nearly 10,000 water well, engineering, and mineral exploration samples and records collected over the past 100 years from drillers having varying levels of skill and reliability. Sparse outcrops and quarry exposures provided a basis for mapping on a local scale, but regional lithostratigraphic interpretations remain equivocal. Mapping required significant extrapolation and interpretation of data of varying quality.

As a result of mapping the geology of the buried bedrock surface in southeastern Wisconsin, we now have a better understanding of the regional distribution of complex lithostratigraphic units at the bedrock surface. This mapping establishes a geologic framework for three-dimensional representation of the nature and distribution of Paleozoic rock, within which the region’s available aggregate resources and crucial groundwater supply are contained.

Improved geologic understanding is especially beneficial because southeastern Wisconsin includes a seven-county area characterized by rapid urbanization. As population growth continues, land-use patterns change and intensify the demand on the area’s natural resources. More than 150 units of governments in the region need detailed bedrock geology information to support local and regional planning, especially for the evaluation of groundwater and aggregate resources.