GIS-BASED SURFICIAL GEOLOGY MAP OF EAU CLAIRE COUNTY, WEST-CENTRAL WISCONSIN, USA

Eau Claire County is located in west-central Wisconsin, immediately north of the unglaciated Driftless Area. A GIS-based 1:250,000-scale surficial geology map of Eau Claire County has been produced as part of a Wisconsin Geological and Natural History Survey [WGNHS] project to compile a 1:500,000-scale surficial geology map of Wisconsin.

Multiple data sources were used to map the 1670 km² study area. A Pleistocene geology map of the adjacent county to the north (Chippewa County, Syverson, 2007) provided a good geologic framework. LiDAR DEMs, USDA digital soil survey shapefiles, and WDNR well logs were compiled in ArcGIS. Landforms on 7.5’ topographic maps also were used to draw geologic contacts. Unit thicknesses were determined from the WDNR well logs.

The land surface of Eau Claire County ranges in elevation from 281– 435 m asl and is underlain by stream-dissected glacial sediments and Paleozoic bedrock. The Paleozoic bedrock contains the Mt. Simon, Eau Claire, and Wonewoc formations which are well exposed in the southern half of the county. The Wonewoc Formation has been mined for industrial sand in eastern Eau Claire County.

The River Falls Formation outwash sediment is ~16 to 30 m thick in northwestern Eau Claire County. The lithology and highly weathered nature of the River Falls Formation suggests it was deposited by ice flowing out of the Lake Superior lowland during the Illinoian Glaciation. This eroded stream sediment is associated with dissected Paleozoic bedrock highlands. Till of the Merrill Member of the Copper Falls Formation is ~4.5 to 17 m thick in northeastern Eau Claire County, and displays a rolling, hilly topography. Outwash of the Late Wisconsinan Copper Falls Formation is ~12 to 40 m thick and is found throughout Eau Claire County along the Chippewa and Eau Claire river valleys and major tributaries.

Sand dunes modify Late Wisconsinan outwash terraces, as observed on LiDAR DEMs. The dunes indicate northwesterly winds. Well logs across western Eau Claire County reveal silty pre-Illinoian lacustrine sediment of the Kinnickinnic Member of the Pierce Formation in the subsurface.

This surficial geology map of Eau Claire County will be useful for regional planning purposes. The WGNHS will also incorporate this map information in the compilation of an updated statewide surficial geology map.